, the international laboratory and instrumentation show. 

It is reported that the annual gathering of the analytical industry is once again demonstrating the sector's amazing growth, not only in the Middle East, but also globally. Now in its 28th year, 

 has grown from just 40 exhibitors to over 700 of the industry's leading companies from around the globe. 

 focuses on laboratory technology, biotechnology, nanotechnology, environmental sciences, robotics and automation, analytical instrumentation and measurement and testing. The show is reported to attract some 10000 visitors from more than 75 countries, and covers all of the important growth markets of the Middle East, China, the Indian sub-continent, South-east Asia and Africa. 

 is not only a showcase that draws buyers, specifiers, end-users and distributors from all six continents; the event also provides a forum for industry leaders involved in medical, petroleum and environmental research and analysis, laboratory automation and data process management. All the industry's foremost companies will be exhibiting — and, at the 2011 show, more national groups than ever before will be participating. 

According to the organizers of the event, the show has enjoyed consistent annual growth in recent years, and the exhibitor attendance for 2011 has increased by more than 20%, including many companies and national groups who are exhibiting for the first time. 

Complementing the exhibition itself is the packed seminar programme — MediLab, BioLab, EnviroLab and PetroLab — which offers opportunities to gain information about all the very latest developments in these fields. 

The latest information about the show is available online at 

 — the annual Conference and Exposition on laboratory science — includes a wide-array of symposia, short courses, poster sessions, networking opportunities and, of course, an exhibition featuring leading analytical and chromatography suppliers. 

For those seeking to develop new skills or learn more about chromatographic techniques, an attractive feature of Pittcon is the informative short courses. There are about 61 short courses associated with liquid chromatography and gas chromatography. Several of these short courses are new to Pittcon this year and include topics such as 

LC–MS Strategies for the Identification of Impurities, Degradants and Metabolites

LC/MS: Method Development and Applications to Small Molecules; and New Concepts of TOF-MS for Fast GC and GC

The Conference itself includes 53 invited symposia which showcase nationally and internationally recognized scientists. As well as the invited symposia, there are an additional 200 papers that are related to chromatography in the organized contributed, and poster sessions and workshops.  

 attracts nearly 20000 attendees from industry, academia and government from 90 countries worldwide. Each year thousands of scientists take advantage of the opportunity to see the latest in instrumentation all on one exhibition floor. At this year's Exposition, 1000 exhibiting companies, including leaders in chromatography instrumentation and accessories, are expected to attend. 

A complete list of exhibitors is available on the Pittcon website at 

2nd Kuwait Lab Tech Conference and Exhibition,

				Advanced Analytical Separation Science Techniques to Achieve Higher Throughput and Leverage Efficiencies				

				Hotel Palacio Estoril, Estoril, Portugal  			

Spring Symposium: High Throughput High Speed Separations

				Budapest Congress and World Trade Centre, Budapest, Hungary			

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Capillary electrophoresis (CE) is well suited to the analysis of simple inorganic anions such as chloride, nitrate and sulphate. These analytes are water-soluble and highly mobile. Indirect UV detection is required as the analytes possess limited UV-absorbing properties. Electrolyte systems also require considerable optimization to accommodate the negative charge(s) on the anions. The methods are robust and rugged and have been applied to a variety of areas including pharmaceuticals, forensics and clinical applications.

What are the separation mechanisms used to analyse inorganic ions?

Separation of inorganic anions is achieved based on their electrophoretic mobilities; by their molecular weights and number of negative charges. Their mobilities are highly negative and oppose the normal electroendosmotic flow (EOF) direction. This situation would lead to very long analysis times and poor peak shapes and sensitivity. The EOF direction is therefore reversed to make it identical to the migration direction of the small anions, thereby reducing analysis times. A range of cationic surfactants have been employed at low mM concentrations, including hexamethonium [1,6-bis-(trimethylammonium)hexane], tetraethylenepentaamine, 3 mM 1,3,5-benzenetricarboxylic acid and 15 mM TRIS in 20% (v/v) methanol at pH 8.4 in reference 1 and tetradecyltrimethylammonium bromide in reference 2. Figure 1 shows that the positively cationic surfactant binds onto the negatively charged capillary wall. A bi-layer of surfactant then forms which generates a positive charge on the capillary wall which reverses the EOF direction. A negative voltage is then used for analysis as both the EOF and the anions migrate in the same direction.  

Figure 1: Reversal of EOF direction with cationic surfactant bilayer.			

How is detection of inorganic anions achieved?

Simple anions such as chloride and sulphate have no chromophores so cannot be detected using UV detection. Detection of non-chromophoric metal ions (and those with limited chromophores) is therefore achieved by indirect UV detection. In this process a UV-absorbing species is added to the electrolyte which gives a high background signal to almost full scale (FS) deflection. When the peak moves along the capillary it displaces the UV-absorbing species which leads to a decreased concentration and a dip in UV signal.  

A high-mobility species is incorporated into the electrolyte to give the background UV signal. The background signal is reduced when the sample ion moves passes through the detector and the area of the peak generated is linearly related to the sample concentration. The migration speed of the UV-absorbing ion must closely match that of the ion being detected otherwise the analyte peak shape is severely distorted and poor sensitivity is obtained. Chromate

  and 1,2,4,5 benzene tetracarboxylic acid (pyromellitate)

  are popular choices as they give optimal peak shape and sensitivity for small anions such as chloride and sulphate. Species similar to benzoate are preferred for medium-sized anions such as citrate, phthalate and carbonate. Species similar to sorbate give optimal performance for larger inorganic anions such as alkyl sulphonates. 

Figure 2 shows separation of a range of common inorganic anions using pyromellitate as the UV absorber. Pyromellitate may be preferred to chromate, as chromate is potentially toxic and requires more handling care.  

Figure 2: Separation of a range of inorganic anions by CE. 2.25 mM pyromellitic acid, 6.5 mM NaOH, 0.75 mM hexamethonium hydroxide, 1.6 mM triethanol-amine, pH 7; Negative polarity 465 V/cm, 9 mA; Capillary dimentions 56 Ã 64.5 cm i.d. 50 Î¼m; Injection 4 sec at 50 mbar, Temp = 25 Â°C; Indirect detection signal: 350.60 nm, ref: 245.10 nm.			

Electrolyte concentrations are kept low to generate low currents in the region of 5–10 μA. Alternatively zwitterionic buffers such as TRIS are used which do not carry current. Higher currents lead to an increase in temperature in the capillary which causes refractive index changes and excessively noisy baselines. 

Chemistry kits are available from a number of capillary electrophoresis (CE) equipment vendors and consumable suppliers that are optimized for the analysis of inorganic anions by CE. The reagents supplied include capillary rinsing solutions and an optimized electrolyte solution. Operating conditions are recommended for parameters such as temperature, detection wavelengths and applied voltage. The kits also contain the capillary which has a length and diameter which give optimized performance using the recommended operating parameters. The kits may also contain test mixes of standard anions. 

Typically, sample is injected into the capillary by dipping the capillary into the sample vial and applying a pressure which forces a few nanolitres of sample solution into the capillary. Alternatively it is possible to use voltage to selectively inject charged components from the sample solution into the capillary end. This is called electrokinetic injection and is achieved by dipping the injection end of the capillary into the sample solution. Small ions rapidly move into the capillary when the voltage is applied which results in preferential loading and improved sensitivity. For example, detection limits can be improved 10-fold for small inorganic anions compared with hydrodynamic injection.

  Typical sample loadings are achieved employing 30 second injections at –5 kV. Electrokinetic injection can be further optimized using sample additives such as octanesulphonate and have been used to monitor ppb levels of anions in nuclear power plant feed water. Good linearities and acceptable precisions can be achieved by electrokinetic injection, however the amounts loaded are highly dependent on the sample matrix which makes pressure injection more applicable to routine analysis. 

CE has been used for a number of applications covering several industries and acceptable method validation has been reported with methods being routinely used. 

Several samples of mineral water were analysed by CE and ion chromatography (IC) for chloride, sulphate, nitrate and fluoride.

  Table 1 shows the good agreement between the two techniques. Monitoring of the quality of the feedwater used in nuclear power plant is essential to avoid fouling of catalysts.

  Contamination occurs with ppb levels of anions and a CE method has been shown to be appropriate for monitoring a range of anions at that level.

Table 1: Anion content in various water samples by CE and IC.5			

CE has been widely used in the pharmaceutical analysis for anion determinations and the applications described would be appropriate in other industries such as agrochemicals. Basic drugs are often prepared as salts, with counter-ions, chloride or sulphate, manufactured to alter their physiochemical properties such as solubility. The counter-ion may typically constitute 5–30% of the weight of the output drug substance batch. Therefore it is essential that accurate and precise analytical methods are used to quantify counter-ion content.  

Table 2: Levels of anionic counter-ions in drug substance batches determined by CE and microanalysis.3			

Table 2 shows the data obtained by CE for the analysis of the chloride counter-ion content of a basic drug.

  The CE data agrees well with the expected theoretical content and the values obtained by a manual titration method. The method was fully validated (Table 3) and employed in routine analysis.  

Table 3: Method validation data for a chloride assay using nitrate as an internal standard.3			

Figure 3 shows the analysis of two different batches of a basic drug manufactured as a chloride salt. The analysis shows that one of the batches contained the expected level of chloride but also a range of other inorganic anionic contaminants.

  The CE method therefore offered a higher data quality than a simple test such as a titration.  

Figure 3: Analysis of two batches of a chloride drug substance showing different levels of inorganic contaminants.3			

CE has been used for both forensic analysis and profiling for 20 years. Tagliaro et al. have recently produced an excellent review article on the subject.

  CE is seen as a valuable alternative to IC particularly for the analysis of inorganic-based explosives because of its low running costs and its simplicity of use. 

Early work by the FBI included profiling of explosive residues

  — the pattern of the inorganic anions in bomb residues is indicative of the ingredients used in bomb manufacture. CE that offers fast, selective and sensitive analysis of inorganic anions is compulsory for the identification of explosives in post-blast or environmental samples. 

A CE method has been recently been reported for the simultaneous analysis of ten anions (chloride, nitrite, nitrate, thiosulphate, perchlorate, chlorate, thiocyanate, carbonate, sulphate and phosphate) which can be found in post-blast residues.

  The method also allows quatification of the azide anion which is often present in the composition of detonators. Formate is used as the internal standard and gives a 20 min total runtime. Intermediate precisions were 2.11% for normalized areas and 0.72% for normalized migration times. Limits of detection close to 0.5 ppm for all anions were obtained with the use of preconcentration techniques. Simple sample preparation was used, allowing the analysis of a large variety of matrices with the generic CE method. The method was applied to the analyses of real post-blast residues and detonator extracts. The CE results were compared with those obtained with the routine IC method and showed excellent correlation. 

Profiling of inorganic anions can be used for diagnosing and monitoring critical conditions for metabolic disorders. On-site monitoring of urine anions has been reported using a microfluidic chip-capillary electrophoresis device.

  This allows simple bed-side rapid analysis of the highly variable anionic metabolites in urine samples, with samples injected neat or after simple dilution. A complete assay for four samples can be achieved within an hour for 15 anions commonly found in urines. Recoveries of more than 94% were obtained, with CE results agreeable with results obtained by a conventional ion chromatography method.

Monitoring of trace impurities in electroplating baths are required to meet EU regulations for WEEE and RoHS and for quality control of electrodeposits. A CE method using a buffer containing 1.5 mM tetraethylenepentaamine, 3 mM 1,3,5-benzenetricarboxylic acid and 15 mM TRIS in 20% (v/v) methanol at pH 8.4 gave satisfactory repeatability with baseline separation for 14 organic and inorganic anions within 14 min.

  Good repeatability for migration time [0.32–0.57% Relative standard deviation (RSD)], satisfactory peak area and peak height (2.9–4.5 and 3–4.7% respectively), low detection limit (

 = 2, 20–150 ppb) and wide working ranges (0.1–100 ppm) were obtained. The CE method was used to identify anion impurities which had been causing problems in aged tin baths. 

A CE method has been reported for the simultaneous determination of inorganic anions, aliphatic and heterocyclic organic acids in various processed samples (commercial softwood and hardwood kraft lignin and two red wine samples of Pinot Noir grapes).

  The analytes were determined simultaneously in 10 min using an electrolyte containing 20 mM 2,3-pyrazine dicarboxylic acid, 65 mM tricine, 2 mM BaCl,

  0.5 mM cetyltrimethylammonium bromide and 2 M urea at pH 8.06. Linear plots for the analytes were obtained in the concentration range of 2–150 mg/L. RSDs of peak areas during a 3-day analysis period varied from 5.5% for glycolate to 9.5% for oxalate. RSDs of migration times varied between 0.4% and 1.1%. The detection limit (at 

 = 3) was 1 mg/L for all the analytes studied.  

Ion exchange chromatography (IEC) is widely used to determine inorganic anions. IEC columns are expensive and may have a short column life. IEC eluent purchase and disposal costs can be high. Specific IEC equipment is also required. In comparison, CE uses inexpensive fused-silica capillaries, low volumes of reagents and metal ion analysis is conducted on standard CE equipment, so no additional expense or training is required if a CE kit is already available. The set-up time for CE analysis is also very rapid — especially if kits are used, as there is no requirement with prepare any reagents. Compared with techniques such as gravimetric analysis and titration; CE offers advantages of improved selectivity and the highly automated system which allows unattended operation and automated results calculations. 

CE is also able to tolerate injection of quite complex samples as the capillary can be aggressively cleaned between injections. 

The simple and robust nature of the methods used for inorganic anion determination by CE has ensured that this has become a popular application. Standard separation conditions have been developed which give optimized performance. These standard conditions have allowed kits to be developed which greatly simplifies routine operation. Methods have been routinely employed in a number of application areas including forensic, clinical and pharmaceutical determinations. 

 is an associate director in the pharmaceutical development department at GlaxoSmithKline. He is editor of CE Currents in 

. Direct correspondence about this column to 

 editor Alasdair Matheson, Advanstar Communications, Bridgegate Pavilion 4A, Chester Business Park, Wrexham Road, Chester CH4 9QH, UK, or e-mail at 

2. S. Rovio, A. Kalliola, H. Siren and T. Tamminen, 

3. K.D. Altria, M.M. Rogan and D.M. Goodall, 

6. F. Tagliaro, J. Pascali, A. Fanigliulo and F. Bortolotti, 

CE Currents

CE is suitable for the analysis of simple inorganic ions, such as chloride, nitrate and sulphate. This method is robust and rugged.

Analysis of Inorganic Anions by Capillary Electrophoresis

Gas chromatographers are using hydrogen carrier gas more frequently as an inexpensive and a readily available substitute for helium, as well as to gain some performance improvements. In this instalment, John Hinshaw investigates the influence that choosing hydrogen carrier has on column selection and operation with direct-interface gas chromatography–mass spectrometry.

Recently, the use of hydrogen carrier gas in gas chromatography (GC) has been discussed, reviewed, analysed and praised numerous times in this magazine

  as well as many other publications. With the prospects of shortages and everincreasing costs for helium, hydrogen offers some desirable economic and logistical advantages, as well as performance changes for columns. The latter topic is the main focus of this instalment, but first let's review some of the economic and logistical benefits and concerns associated with hydrogen use in the laboratory. 

 First, hydrogen as a replacement carrier gas is less expensive than helium. Switching to hydrogen also offers significant logistical advantages because it makes it possible to reduce the number of tanks of compressed gas in the laboratory, by installing a gas generator instead. In laboratories where multiple GC systems equipped with split inlets and flame ionization detection (FID) systems together consume large quantities of hydrogen gas, the deployment of hydrogen generators is an attractive proposition: The high rate of consumption means an early break-even point on the initial cost of the generators. Also, gas generators allow safety personnel to sleep better knowing that the amount of stored flammable gas is limited and that there are fewer high-pressure gas cylinders chained to the walls. It can be argued that the presence of any high-pressure gas cylinders represents a greater hazard than that imposed by a self-generated hydrogen carrier. This argument comes to a logical conclusion with the installation of zeroair generators, in addition to hydrogen generators, that clean up compressed air for use with FID systems. Thus, switching from helium to hydrogen means that high-pressure cylinders can be eliminated for routine GC–FID applications as well as for gas chromatography–mass spectrometry (GC–MS) systems that use electron ionization (EI). Chemical ionization (CI) does require a cylinder of collision gas such as methane or ammonia, neither of which are particularly amenable to 

 Although switching to hydrogen carrier can improve safety by making it possible to reduce the number of high-pressure gas cylinders in the laboratory, hydrogen does pose safety risks. Reasonable precautions should be taken to prevent and to detect the accumulation of hydrogen from multiple split vent flows or other significant sources, whether using tanks or gas generators. The built-in leak detection features of modern electronic pressure control (EPC) pneumatic systems along with appropriate awareness and procedural training for those who use hydrogen in the laboratory can reduce the likelihood of an accident to low levels. The best practice is to install a hydrogen sensor high up in each laboratory. You may also want to consider installing small dedicated hydrogen sensors in the GC ovens, especially if not using an EPC system with hydrogen-leak shutdown. 

 Beyond the economic and logistical gains to be obtained from gas generators and hydrogen, it has some chromatographic performance advantages over helium carrier. There is less dependency of column performance on the average carrier gas linear velocity with hydrogen compared with helium. The range of linear velocities over which column efficiency lies close to the optimum for any particular solute is broader than with helium. This comparison has been made many times before. Two years ago "GC Connections"

 = 10.0 on a 50 m × 0.25 mm column at 100 °C had an optimum average linear velocity range of 18–45 cm/s for helium versus 25–65 cm/s for hydrogen, where "optimum" was defined as the region in which the plate height lies within 25% of its minimum. This wider optimum range gives chromatographers more latitude in setting average velocities with hydrogen carrier. The optimum range for hydrogen covers faster linear velocities too, which can yield higher analysis speeds because peaks can be eluted in less time without sacrificing as much performance as when pushing helium carrier to the same higher speeds. Hydrogen also has the advantage of requiring lower inlet pressures to deliver a given average velocity, due to its reduced viscosity. 

All is not so simple, however, when moving an existing method from helium to hydrogen carrier either with conventional or MS detectors; there are several key considerations that GC and GC–MS users should understand. 

The effects of switching to hydrogen carrier with the column outlet at atmospheric pressure are well known. Figure 1 shows a family of curves that relate the inlet system gauge pressure (the EPC pressure setting or readout) to column length for a series of column inner diameters with a 40 cm/s average linear velocity at 50 °C. Figure 2 shows a similar plot for the same series of columns with hydrogen carrier gas: The overall decrease in the pressure drop required to reach 40 cm/s is evident. The pressure needed to drive the carrier gas at 40 cm/s through an example 25 m  × 0.25mm column decreases from 16 psig (110 kPa) for helium to 7.3 psig (50 kPa) for hydrogen, while the column flow decreases from 1.9 to 1.5 mL/min. (Flow-rates have been adjusted to 20 °C for these examples. Some EPC systems use a 0 °C reference temperature and will display slightly different pressures, velocities and flowrates. For simplicity when considering multiple column diameters, the film thickness was set to 0.0.) 

Figure 1: Plots of inlet pressure against column length, with helium carrier gas and vacuum compensation off. Column inner diameters (mm): (a) 0.20, (b) 0.25, (c) 0.32 (d) 0.53; column temperature: 50 Â°C; average linear velocity: 40 cm/s. 

EPC systems make it easy to establish the same initial velocity or flow in this situation; I prefer to keep the starting velocity constant. Consider that maintaining the flow at 1.9 mL/min while changing to hydrogen carrier will cause the initial average velocity to increase from 40 cm/s up to 50 cm/s, at a pressure drop of 8.9 psig (61 kPa). This increased velocity in turn may shift relative peak positions with temperature-programmed elution to a greater degree than is desirable. For better control of the resulting separation, try using Method Translation Software (Agilent Technologies, Santa Clara, California, USA) or a similar utility. This application also considers the column temperature programme when suggesting adjustments to a GC method being moved to hydrogen carrier gas — as well as for many other situations including switching to an MS detector. In this case, it suggests that a slightly higher temperature programme rate plus a shorter initial hold time would preserve peak elution order. In all cases, any time that a major change is made to an existing method the identities and quantitative behaviour of all solutes must be revalidated to be sure that the new method produces comparable results. 

Figure 2: Plots of inlet pressure against column length, with hydrogen carrier gas and vacuum compensation off. Column inner diameters (mm): (a) 0.20, (b) 0.25, (c) 0.32 (d) 0.53; column temperature: 50 Â°C; average linear velocity: 40 cm/s.  

Considerations for Direct-Interface GC–MS

In direct-interface GC–MS, the end of the GC column is inserted directly into the ion source. The column exit sits in the source vacuum and this low pressure strongly influences the movement of carrier gas and solutes through the column compared to when the outlet is at atmospheric pressure. In addition to influencing the chromatographic separation, all of the carrier gas enters the mass spectrometer's source and must be pumped away by the MS vacuum system. The type of carrier gas — helium or hydrogen — significantly influences the vacuum characteristics of the source as well and may also affect solute ionization. 

Carrier Gas Flow Under Vacuum Outlet Conditions

For those readers who are interested, here is some theory that pertains to the effects of placing the column exit at vacuum on the movement of carrier gas through the column.  

The pressure drop across a column is the difference between the inlet and outlet absolute pressures:  

When the outlet of the column is at the ambient atmospheric pressure then the pressure drop is equal to the control point of a split-inlet pressure controller that maintains its set pressure in reference to the atmospheric pressure. For example, if the absolute inlet pressure were 44.1 psia (303 kPa) and the outlet pressure were 14.7 psia (101 kPa) then the pressure drop would be the difference, 29.4 psig (202 kPa), and an EPC split inlet controller would read this value for the inlet pressure. This is the normal situation when using FID or most other non-MS detection methods.  

 across a column produces an average linear gas velocity 

 through the column that can be expressed as follows:  

 is the column length, η is the carrier gas viscosity at the column temperature, and 

This factor, termed "j-prime," adjusts for the influence of carrier gas compressibility on the flow of gas through the column, specifically its influence on the average velocity. Don't confuse it with the "j" factor that relates the average velocity to the outlet velocity on the basis of the volumetric expansion of gas as it transits the column from the inlet to the outlet pressure. The variable 

In the example above with the column outlet at atmospheric pressure, 

 equals 0.9231. At this modest pressure ratio the influence of j-prime on the average velocity predicted from the pressure drop is less than 10%, which means that about 10% higher pressure would be required to achieve the desired velocity than predicted without this factor. This influence exceeds 10% as the pressure ratio goes higher than 4.0, and it is incorporated into EPC systems that calculate the apparent average linear velocity of a column from the pressure drop and other variables. 

When the column exit is placed under vacuum, the pressure drop across the column is essentially equal to the absolute inlet pressure: The pressure drop in our example case increases from 29.4 psi (202 kPa) to 44.1 psi (303 kPa). Most EPC pressure controller displays — and all mechanical controller readouts — will continue to show 29.4 psig (202 kPa), however, because the pressure controller operates relative to ambient pressure and not in relation to a vacuum. EPC systems determine the absolute pressure drop by adding the ambient atmospheric pressure (from a separate sensor) to the controller's pressure level and then use it to estimate the column average linear velocity and flow using similar equations. Any discrepancies between the actual column dimensions and the values entered into the EPC system will cause the flow and velocity readouts to deviate from the true values as measured by independent means such as with a flow meter or from the unretained peak time. 

The pressure ratio approaches infinity as the outlet pressure approaches zero, and the limit of j-prime approaches 0.75:  

This equation is used to calculate the average velocity under vacuum conditions.  

Now consider what happens when the column outlet pressure is reduced to near zero in a direct-interface MS ion source while the inlet pressure and carrier gas type are left unchanged: The increased pressure drop across the column causes both the velocity and flow to increase. For the example 25 m × 250 μm column at 40 cm/s, the average velocity increases to 60 cm/s and the flow-rate increases to 2.5 mL/min when the absolute outlet pressure drops to zero. 

The resulting increased flow and velocity are not necessarily desirable in this situation. Most bench-top MS detectors can pump 2–4 sccm of helium carrier gas, and somewhat less with hydrogen, depending on the capacity of the turbomolecular or diffusion pump. Higher flow-rates easily can exceed the pumping capacity, and the consequent higher source pressures may reduce EI efficiency, reduce the mass resolution and produce unwanted fragments from extra collisional effects that may degrade library search results. On the peak separation side, changes in the velocity or flow with temperatureprogrammed elution may cause peaks to shift relative to each other or even to be coeluted or to reverse their elution order — the same kind of effects seen when modifying a method where the column outlet remains at atmospheric pressure. 

Therefore, adjustments to the columninlet pressure settings are also in order when a separation is moved from a detector that operates near atmospheric pressure to an MS detector, or when changing an existing MS detection method from helium to hydrogen. Chromatographers can use built-in EPC calculations to establish the inlet-pressure setting that produces the same constant flow or average velocity at the starting oven temperature with a vacuum outlet, as was discussed earlier for a configuration without a vacuum outlet. For example, the above column at 50 °C with helium carrier requires about 5.9 psig (41 kPa) for the same average velocity of 40 cm/s at vacuum outlet conditions as was produced by a 16-psig (110 kPa) pressure with the outlet at atmospheric pressure. At the lower outlet pressure, the column flowrate drops to 1.1 mL/min at the reduced inlet pressure set point. 

Here too, keeping the initial velocity the same while changing from atmospheric to vacuum outlet will help preserve peak elution order. If the same constant flow-rate of 1.9 mL/min was required in translation, an inlet pressure of around 12.3 psig under vacuum outlet conditions would yield the same flow as did the original 16.1-psig inlet pressure with the outlet at atmospheric pressure. This does, however, increase the initial average velocity to 53 cm/s, which may affect the separation more than desired. Again, a method translation utility will help to establish the closest-matching conditions. 

When working with a direct GC–MS interface, chromatographers should be aware that many common combinations of column diameter and length lead to the EPC system demanding a negative, subatmospheric inlet pressure. The situation occurs when the required pressure drop from the column inlet to the vacuum outlet is less than atmospheric pressure. The principal symptom of this situation is the EPC channel not becoming ready, the inlet pressure at or slightly below zero, and a positive flow and velocity readout.  

This "underwater" condition of negative inlet pressures arises readily within our collection of example columns. Figure 3 shows what happens to the inlet pressures when the outlet pressure goes to vacuum with helium carrier gas at 40 cm/s. The graphed data suggest that a 530 μm i.d. column will not be useful at all for direct-interface GC–MS, while 320 μm columns need a minimum length of 40 m, 250 μm columns should be longer than 25 m, and 200 μm columns should be longer than 15 m. These column lengths were selected by keeping in mind that the inlet system needs to operate at a pressure above ambient: Zero inlet gauge pressure is not usable. Flow-rates under these conditions run between about 1.0 and 2.2 mL/min and should not be problematic for the MS vacuum system, although the longer 320 μm columns would operate better with a higher capacity vacuum pump that can remove up to 4 mL/min of carrier gas. The higher capacity pump is also better suited to hydrogen carrier gas. 

Figure 3: Plots of inlet pressure and column length, with helium carrier gas and vacuum compensation on. Column inner diameters (mm): (a) 0.20, (b) 0.25, (c) 0.32 (d) 0.53; column temperature: 50 Â°C; average linear velocity: 40 cm/s. The blue shaded area designates negative inlet pressures. 

With hydrogen carrier at 40 cm/s this underwater zone eliminates a larger portion of the column collection, as shown in Figure 4. Here, only 200 μm i.d. columns of 30 m and greater length or 50 m × 250 μm columns are usable, while none of the selected 320 μm or 530 μm columns call for positive inlet pressures under these conditions. If the average velocity is increased to 60 cm/s then the set of useful columns for hydrogen carrier is much the same as with helium carrier at 40 cm/s, but in that case a translation from helium conditions to hydrogen conditions that yield a similar peak separation may prove difficult to achieve. Simply operating wider-bore columns at a low positive inlet pressure will certainly create very high average linear velocities that result in drastically reduced separation efficiency, and also such a set-up can provide too much carrier gas flow for the mass spectrometer's vacuum system to pump away effectively. For example, even a 50 m × 530 μm column when operated with just 3.0 psig (20 kPa) of hydrogen carrier pressure and a vacuum outlet will produce an average velocity of 165 cm/s and a flow of almost 18 mL/min, far above the pumping capacity of benchtop mass spectrometers. 

Figure 4: Plots of inlet pressure against column length, with hydrogen carrier gas and vacuum compensation on. Column inner diameters (mm): (a) 0.20, (b) 0.25, (c) 0.32  (d) 0.53; column temperature: 50 Â°C; average linear velocity: 40 cm/s. The blue shaded area designates negative inlet pressures. 

There are at least two alternatives to a direct GC–MS interface that help alleviate the underwater effect: an opensplit interface or a vacuum restrictor arrangement. In an opensplit interface the column exit remains close to atmospheric pressure while additional make-up gas flow surrounds it. A restrictor tube transfers a fixed flow of column effluent and make-up gas to the MS source from the opensplit interface while any remaining gas is vented. This arrangement keeps the column exit close to atmospheric pressure and enables most opentubular columns to work with an MS detector. The portion of the column effluent that enters the MS source depends both on the column flow and make-up flow, however, so constant-flow operation is preferred. Some fraction of the solutes does not enter the source, which reduces the sensitivity in proportion. An open-split interface is more difficult to maintain, but as a plus it does enable a column change without having to vent the MS detector. 

It is also possible to connect a restrictor at the column outlet with a zero-dead-volume connector and then pass the restrictor through a direct interface into the MS source. The restrictor length and inner diameter need to be chosen according to the column flow-rate, so that the column outlet pressure will be high enough to bring the inlet pressure above atmospheric levels. This arrangement is comparatively simple, but it will cause EPC display of the column flow and velocity to be in error. The absolute pressure of the column exit depends on the column flow-rate: Higher flows will increase the outlet pressure. This behaviour is not built into EPC systems that assume the column outlet remains at either atmospheric pressure, zero pressure or a fixed operator-entered pressure. This restrictor arrangement does work well with 0.53 mm i.d. columns that receive carrier gas from a directly connected mass flow controller, as long as the flow-rate does not exceed the pumping capacity of the mass spectrometer's vacuum system. 

Reasons for switching from helium to hydrogen carrier gas include cost-savings and safety. The choice of generating hydrogen carrier gas on-site is attractive in many cases, and it has the added benefit of not requiring users to store, manage and handle gas cylinders. Hydrogen safety can be managed with the right combination of procedures, education and equipment.  

Several issues arise when switching to hydrogen carrier. For all situations the effects of changes in the column flow and velocity profile on the separation must be evaluated carefully and then accommodated. Method revalidation is usually necessary. When using an MS detector, the carrier gas pumping capacity of the detector and the effect of vacuum outlet operation on column inlet pressures as well as on the flow and velocities must be accounted for. A direct-interface GC–MS interface will work well for narrowerbore columns but an opensplit interface may be a better solution for wider-bore columns. 

 is senior research scientist at Serveron Corp., Hillsboro, Oregon, USA and a member of 

's editorial advisory board. Direct correspondence about this column to 

GC Connections

A look at the effect of choosing hydrogen carrier gas on column selection and when using direct-interface GC–MS

Hydrogen Carrier Gas and Vacuum Compensation

The historical development of porous layer open tubular (PLOT) gas–solid chromatography columns along with the technology and applications is reviewed from a personal perspective by one of the pioneers in this field: Jaap de Zeeuw. This instalment of "Column Watch," discusses PLOT columns from a stationary phase basis and provides practical hints on how to get the best separation power out of PLOT columns by providing a number of application examples.

Gas chromatography (GC) had its beginnings in the early 1950s with the famous Nobel Prize winner A.J.P. Martin and his colleague A.T. James demonstrating the feasilibility of GC.

  They provided the guidelines for separations in the gas phase but GC had its development via very smart scientists like Desty,

  and many others, who had a remarkable power of combining technologies with creativity to foresee unique solutions for separation challenges. 

Initially, the work started with chromatographic separations using packed-bed columns, but the level of separation power moved to another magnitude by using the capillary columns. In the early days of capillary GC, most of the work was done using metal columns, which gave many issues in efficiency and inertness, but when glass capillaries were used (Figure 1), it was possible to make highly inert and efficient columns. The internal glass surfaces could be modified relatively easily to coat nonpolar as well as polar phases using surface roughening and silane-deactivation techniques.  

Despite the excellent chromatographic characteristics of glass, the practical issues of column installation were always a big problem. Because both of the column ends had to be inherently straight for 10–12 cm to fit into injector and detector. This was a challenge for every chromatographer who had to install a glass capillary column and was a problem not only for the user, but also for the manufacturer. I remember in the early days of our column manufacturing operation at Chrompak, our biggest problem was to find a suitable way to package the glass capillary columns to ship them all over the world while keeping the straightened ends in shape for future installation. Cutting off a piece of the column, as well as straightening the ends, was an art. However, some users found a way around that problem. For certain brands of GC columns, there were already cassette systems in which the glass capillary columns were mounted in a fixed frame. At the time, this was a very practical way to solve the shipping problem and even today, such a system would be highly appreciated. 

To prevent having to straighten the ends, many columns were supplied with platinum-iridium tubing. This metal tubing is relatively inert and could be melted onto the ends of the glass capillary columns, creating flexible ends on both sides. However, it was also a challenge to straighten the platinum–iridium capillary tubing because it always tended to bend. Another challenge was to find the correct ferrule and get a good gas-tight seal. The outer diameter of the Pt/Ir tubing was 0.4–0.5 mm, considered very small in those days. Graphite ferrules had to be deformed heavily with all the risks of cold flow and resultant leaks.  

Another approach to packaging was the coating of a paraffin-film around the column. The glass column was doped in liquified "candle wax" before shipping. Of course, an application sheet was sent with the column instructing the user to first put the column into hot water to remove the candle wax. As might be expected, we received some interesting phone calls from customers who exclaimed that their GC instrument was "smoking." Indeed, they had forgotten to take off the candle wax before placing the column in the hot oven!  

The big change came in 1979 with the invention of fused silica.

  In just a few years, the glass capillary was completely replaced by the fused-silica columns, which solved a lot of the problems very simply because it was easy to use. The fused-silica column was flexible, so there was little risk for breakage. It was very easy to install as the ends were self-straightening. When a column was contaminated, a piece could be cut off. Columns also could be coupled, and the surface was high-purity silica, so few impurities were present to interact with analytes. This application of fused silica was the start of a revolution in capillary GC, and the technique grew exponentially. 

A great deal of work has been done to explore the scientific applications and limitations of capillary GC. I had the opportunity to work with the former research group at the University of Eindhoven, The Netherlands,

  who did a tremendous job in laying out new concepts and foundations for high-speed capillary GC-applications.  

In addition to the fused-silica capillary advancements the stationary phase chemistry developed very fast, initially with the coating techniques, but also with the development in the bonding and cross-linking of different types of phases. Most of these phases were polymeric and were based on siloxane chemistry. Even today, this chemistry is still playing an important role in the development of new phases.  

In my research work, I was personally interested in separation of volatile compounds. To separate volatile compounds using polysiloxane liquid phases, we always ended up making very thick films, which lose their efficiency rapidly because of the slow stationary phase mass transfer (that is, the resistance in the liquid phase when the component is dissolved and tries to get out). Due to their higher capacity, adsorption on solid sorbents coated onto the capillary tubing walls can separate volatile compounds without having to use thick layers. 

My interest in adsorbents started with packed-column applications. From the beginning of GC, volatile components were analysed mainly using particulate adsorbents such as molecular sieves, porous polymers, carbon, silica gel and alumina as stationary phases. On occasion, a combination of solids and liquids was used by coating active adsorbent surfaces with thin films of liquid phases. The adsorbents were packed into glass or metal tubes with outer diameters ranging from 1/16 to 1/4 in. (1–5.3 mm i.d.) for analytical work. Adsorption on these solid materials resulted in strong analyte interactions but packed columns were much slower and less efficient compared with the liquid coated open tubular capillary columns.  

Figure 2: PLOT column, with typical "yellow" appearance due to white solid coating. 

To mimic the packed columns, these solid adsorbents had to be coated onto the internal surface of the fused-silica capillary tubing. The coating processes for adsorbents are significantly different from the polysiloxane type phases used in gas–liquid chromatography. These new adsorbent-coated capillary columns were (and still are) referred to as porous layer open tubular (PLOT) capillary columns. Physically, the PLOT columns look different as the coating of a white or black "powder" will give the column a unique appearance, as can be seen in Figure 2. The chromatography using these adsorbent solids is also remarkably different from gas–liquid phases. For instance, column overloading in gas–liquid chromatography results in a leading peak, while overloading in adsorption chromatography results in a more tailing peak as can be noted in Figure 3. Compared to liquid-coated phases, adsorbents are known to be very stable over time and they are difficult to "destroy." Liquid phase–coated columns can develop activity by hydrolysis or oxidation. In the case of adsorbents, the driving separation mechanism is already an active surface. Any additional activity created by hydrolysis or oxidation will generally have minor impact.  

Figure 3: Peak shape of overloaded component in gasâliquid and gasâsolid chromatography. 

However, adsorbents have high retention for organic substances and any contamination in injector or carrier gas will accumulate on an adsorption column. For example, if PLOT columns are left overnight in flowing carrier gas steams at room temperature, in the morning they will need extensive conditioning as septum bleed and gas impurities are all trapped on the adsorbent surface.  

In looking at the challenge for light hydrocarbon separations, early on, alumina showed very interesting selectivity. However, the coating of alumina "particles" instead of soluble polymers was a new challenge for making GC capillary columns. Deposition of solid layers of adsorbents for PLOT column production required very stable suspensions. A very frequent problem for making such columns was always the plugging of PLOT columns after they were coated. After the adsorbent layer was deposited, it had to be dried and activated while the particles remained in place. 

The first alumina PLOT columns made showed very high retention and far too much activity. To reduce the activity, the sorbent had to be wetted with water. One way to accomplish this wetting was to have the carrier gas deliver the water in a controlled manner. A nice solution to this problem was found by using copper sulphate crystals placed in a filter cartridge. By keeping this cartridge under constant pressure and temperature, the carrier gas was "humidified" in a constant and controllable way. This gave beautiful separations for hydrocarbons, but the setup could only be used under isothermal conditions, which limited the upper hydrocarbon range. Also, the slightest variation of laboratory temperature resulted in a change of water level and thus a change of retention. 

The solution to this problem was not all that difficult: change the water for a more permanent deactivating molecule, a procedure that was accomplished based upon the work of Schneider, Frohne and Brudereck,

  who did the work on glass capillary columns. Our group had to transfer this process from glass to fused silica. Initially, the dynamic coating process was very challenging to obtain sufficient layer thickness. After optimizing the thixotropic behaviour of suspensions, this coating was also successful on fused-silica columns. 

Instead of water, the columns were permanently deactivated with potassium chloride. As a result, the alumina PLOT column could be used in temperature-programmed analysis up to 200 °C, thus allowing hydrocarbons to be eluted up to decane. We also found that column polarity could be changed by using different deactivation salts. Sodium sulphate as well as sodium molybdate could also be used, which created more polar alumina surfaces. Later, this technology was also applied in smaller internal diameter columns. At Restek (Bellefonte, Pennsylvania, USA), we went down to 0.18 mm diameter with Rt-Alumina Bond/KCl (Figure 4). However, with smaller internal diameter columns, the chromatography became very challenging due to minimal loadability.  

Figure 4: Light hydrocarbons separated on Rt Alumina Bond/KCl. Column dimensions: 30 m Ã 0.18 mm; oven temperature: 100 Â°C. Peaks: methane, ethane, ethylene, propane, cyclopropane, propylene, acetylene, propadiene, isobutane, butane, trans-3-butene,1-butene, isobutene, cis-2-butene, isopentane, prpyne, pentane, 1,3-butadiene. 

Regretfully, until now, the alumina PLOT columns have been applied mainly to hydrocarbon separations. If water, CO

 or any other polar compounds are present in the sample or carrier gas, they will be adsorbed by the alumina and the retention times for hydrocarbons will change. After commercialization of the first fused-silica alumina column, workers in the hydrocarbon field appreciated the selectivity of the alumina for hydrocarbons and this type of column became available from different commercial suppliers. Investigations were started in "other" adsorption-type columns. Alumina also was investigated for use in other application fields, like the chlorofluorocarbon (CFC) ozone precursors. Alumina separates these components very well, but the activity of the column does not allow measurement of every CFC component. Some compounds showed in-column decomposition, which is not unexpected considering the high activity and the high temperatures applied. By more intense deactivation, the reactivity can be reduced, but not completely eliminated.

The next challenge in PLOT capillary column technology was the application of molecular sieves targeting the separation of permanent gases. We initially looked at the zeolite chemistries and were surprised by their immense possibilities. The growing of zeolites 

 under autoclave conditions was possible, and we investigated this technology. The experiment failed as the column exploded, leaving a GC oven full of dust along with a terrible smell caused by burning the polyimide outside coating on the fused-silica capillary column! 

For the zeolite column, there was a lot of early work done by adsorption specialist Manfred Mohnke, who was working at the University of Leipzig, East Germany. At that time, the Cold War was still on and it was not easy to get in contact with scientists in the DDR. Dr Mohnke was the developer of many adsorption columns in the early days of GC.

  However, a successful collaboration between our two labs was initiated with the goal of investigating different coating techniques for making molecular sieve (molsieve) columns. The challenge with molsieve compared with alumina is that the layers required are 10× thicker to get sufficient retention for permanent gases at temperatures above ambient (for example, 50 μm in 0.53 mm i.d. capillaries). 

Molsieve columns were developed successfully but we ran into testing challenges. To detect permanent gases with capillary columns, we also needed detection systems that could be used. The only systems available were the thermal conductivity detection (TCD) systems that were used for packed columns but those systems had huge cell volumes and minimum flow was 10–20 mL/min. Because the molsieve PLOT columns used only 2–3 mL/min, we needed to add make up gas, but lost sensitivity as a result. GowMac (Madison, New Jersey, USA) was one of first instrument companies who made micro TCD systems and we successfully used one for our work. Later, Hewlett-Packard (now Agilent Technologies, Santa Clara, California, USA) developed their flipflop TCD system for capillary columns. Still, compared with flame ionization detection (FID), TCD is 50–100× less sensitive, which meant that the molsieve column needed to possess a higher loadability. For this reason, the use of smaller bore molsieve columns for laboratory applications had been limited. Things changed when the chip TCDs were developed. Those systems have detection limits approaching 1 ppm and they can work with flow rates produced by 0.15–0.25 mm columns. 

Working with molsieves also opened a new separation mechanism in GC that is based upon size exclusion. Permanent gases are separated because they have access to the enormous surface inside the zeolite pores. Molecules that cannot enter the pores will only be retained by the outside surface, which has a much smaller surface area. Components that cannot enter the pore will be eluted faster. For example, isobutane is eluted before methane, having a retention factor of approx 3 at 30 °C. While 

-butane, because it is linear, can enter the pore and shows huge retention with a retention factor >>100. 

Molsieves are used widely, but have very limited application range. Similar to alumina, water and CO

 are adsorbed and need very high temperature to elute them from the zeolite PLOT columns. It would be nice to see the development of some hydrophobic molsieve materials. 

The porous polymers were widely known under the names Porapak (Waters, Milford, Massachusetts, USA) and Hayesep (Hayes Separations, Bandera, Texas, USA) in the packed-column era. The base material is a porous structure of polydivinylbenzene. With this polymeric material, different functional groups can be incorporated to create various selectivities. Most widely known is the "Q" type polymer. The porous polymers have interesting characteristics as they have high retention, are highly inert, and elute both polar and nonpolar compounds. Also, the porous polymers can be coated onto the fused-silica capillary columns, but the coating process becomes more challenging. Porous polymers have the habit of developing an electrical potential, making the particles quite static, especially when particle-size has to be reduced. Once the particles are in suspension, this is not an issue but after the solvent is removed, when particles are isolated where they can move, one can expect trouble. If such effects occur inside the PLOT capillary column, the coatings can become unstable and the particles can be eluted and plug the columns. When columns are transported, used in valve switching or placed a vibrating environment, the polymer particles can be released. 

To prevent particulates from reaching the detector, where they can produce spikes, or to switching valves that can be damaged, a simple solution was developed that was called a "particle trap." Such particle traps are 1–2 m long pieces of normal polydimethylsiloxane (PDMS)-coated fused-silica capillary column coupled between the column and the valve or between the column and the detector. The PDMS phase in the particle trap will act as "glue," trapping any particles that are released. The extra retention generated by the PDMS capillary column can be neglected because the retention of the adsorbent is several magnitudes higher than the liquid phase. This solution works quite well if the particle release is moderate. If particles are released by the thousands, the particle trap will also start to plug. 

There has been ongoing research in stabilization of porous polymers using stabilizers, 

 gluing but the stability still has not been at a comparable level to bonded liquid phases. When column switching is to be used, it is recommended to use the flow (so-called Deans) switching approach, in which pressure shocks are not as big as seen with valves. Because the porous polymer PLOT columns can always have some free-particle formation during transport, it is recommended to precondition fresh columns at a flow rate 50% higher than what will be used for analysis. To build in long-term security, one can always add particle traps or other screens between the column and switching device. 

Chemically, the porous polymer PLOT columns are highly inert and they separate polar volatiles like alcohols and aldehydes as well as hydrocarbons — Figure 5 shows the use of an Rt-U-Bond column (Restek Corp.) to separate polar volatiles. Porous polymers can be modified by including vinyl-pyridine or acrylate groups to create higher polarity. As with the liquid phases, the maximum operation temperatures of porous polymers drops when the polarity of the phase is increased.  

Figure 5: Polar volatiles on porous polymer PLOT column. Column: 30 m Ã 0.53 mm fused-silica Rt-U-Bond, df = 20Î¼m; oven temperature: 100 Â°C; carrier gas: H2; injection: split; detection: TCD. Peaks: 1 = N2 + O2 + CO, 2 = CO2, 3 = formaldehyde, 4 = water, 5 = methanol. 

Silica gel is not used widely in GC, as only few applications have been developed on this material. It remains an interesting material because it has high retention for hydrocarbons and it also can be used to separate a range of semipolar compounds. Silica gel is the perfect precolumn if low parts-per-million oxygen has to be measured in hydrocarbon stream. Normally, porous polymers could be used, but trace oxygen gets removed by the porous polymer, not by the silica gel. Commercialization of silica-coated columns like Gaspro (Supelco, Bellefonte, Pennsylvania, USA) and SilicaPLOT (Agilent Technologies),

  revealed that selectivity wise, the silica did very well in the C

 isomers tend to be eluted closer as can also been seen with porous polymers. An important characteristic of silica is that this material separates sulphur compounds at parts-per-billion levels. The sulphur compounds are well resolved from hydrocarbons, which made the parts-per-billion analysis of H

S and COS in propylene–propane possible, as can be seen in Figure 6. Silica-coated columns are also much less sensitive to moisture, a big advantage over alumina. However, the poor selectivity for C

 hydrocarbon isomers limits its use for light hydrocarbons. As well as the sulphur compounds, chloro-fluoro compounds and ethers can be separated on silica PLOT columns. One of the big challenges using silica is, similar to high performance liquid chromatography (HPLC) packings, that the manufacture of reproducible silica surfaces is difficult. One of the outcomes of this challenge is the apparent variation of trace sulphur response that seems to be difficult to control among columns as well as among suppliers.  

Figure 6: Separation of trace sulphur compounds in propylene using a silica PLOT column. Column: 30 m Ã 0.32 mm CP-Silica PLOT; oven temperature: 50 Â°C, 1 min to 120 Â°C, 10 Â°C/min; detection: Antek SCD. Peaks: 1 = H2S, 34 ppb; 2 = COS, 108 ppb. (Varian application note 1293 at www.varianinc.com/cgi-bin/scanweb/scanview.) 

Active carbon is also of interest because of its low polarity and huge surface area. Like the porous polymers, it is very nonpolar, but with much higher retention and significantly lower retention compared to zeolite molecular sieves. With carbon, CO

 is eluted as a sharp peak and if layers can be built to 10 μm or thicker, CO can be resolved from air. However, thick layers of carbon are not easy to make. Also, working with these powder suspensions is very challenging because any contact will immediately give black spots (fingers as well as labware). 

There are different ways to make carbon-type PLOT columns, suspensions, pyrrolysis or carbon nanotubes. It's all about getting enough surface. A unique application of carbon PLOT columns is the separation of acetylene from the main ethylene matrix. If impurity analysis is the goal, it is usually preferable to have the impurity eluted before the main component. The carbon PLOT column shows ideal selectivity because carbon materials are very nonpolar as can be seen in Figure 7, showing the analysis of 3 ppm acetylene in ethylene using a CarboBond (Agilent Technologies) column.  

Figure 7: Separation of impurities in ethylene. Column: 25 m Ã 0.53 mm CP CarboBond, df = 10 Î¼m; oven temperature: 35 Â°C, 1 min to 180 Â°C, 10 Â°C/min;sample: 3-ppm acetylene in ethylene.; Peaks: 1 = methane, 2 = acetylene, 3 = ethylene, 4 = ethane, 5 = propane. (Varian application note 1433 at www.varianinc.com/cgi-bin/scanweb/scanview.) 

 values, the use of packed columns remains an interesting option. Recently, packed columns have been miniaturized, by filling 0.53 mm deactivated stainless steel tubing with adsorbents like carbon, molecular sieves and porous polymers.

  Such columns show promise for simple fast separations that can be performed in any instrument. Figure 8 shows an example on a 1 m × 0.53 mm MXT Shincarbon (Restek Corp) micropacked column. Such columns can be coiled in small diameters and operate at flow rates of 2–4 mL/min.  

Figure 8: Separation of CO and CO2 using carbon 0.53-mm i.d. micropacked column. Column: 1 m Ã 0.53 mm MXT, Shincarbon 80/100 mesh (Restek); oven temperature: 100 Â°C, carrier gas: He, 165 kPa; split: 1:30; detection: micro-TCD. Peaks: 1 = hydrogen, 2 = oxygen + nitrogen, 3 = CO, 4 = methane, 5 = CO2. 

Column Watch

A practical guide on how to get the most from porous layer open tubular gas–solid chromatography columns.

The Development and Applications of PLOT Columns in Gas–Solid Chromatography

The environmental analysis of phthalates is established and widespread with most classical methods based on gas chromatography mass spectrometry (GC–MS). Attention has recently turned to the analysis of di-isononylphthalate (DiNP) and di-isodecylphthalate (DiDP) but their analysis by GC results in an unresolved cluster of peaks that all predominantly fragment to the phthalic anhydride ion at 

 = 149. Consequently, quantification of DiNP and DiDP using the most abundant ion is impossible and the same sensitivity as for the single isomer phthalates cannot be reached. GC combined with atmospheric pressure chemical ionization (APCI) was used to analyse the high-molecular-weight phthalates. DiNP and DiDP were successfully ionized in proton transfer mode yielding spectra that contained the molecular ion as the base peak. The method was also applied to measure DiNP and DiDP in a sediment extract at relevant levels for environment samples.

The analysis of phthalates in environmental samples is an important application because some phthalates are considered to be potential endocrine-disrupting chemicals (EDCs).

  Most classical environmental methods focus on the analysis of the dialkyl esters of phthalic acid ranging from dimethylphthalate (DMP) to dioctylphthalate (DOP),

  among which di-isobutylphthalate (DiBP), dibutylphthalate (DBP) and bis(2-ethylhexyl)phthalate (DEHP) are the most important single isomer phthalates found in environmental samples.

  These solutes are single isomer compounds and relatively easy to analyse by GC–MS because only one peak for each compound is obtained by chromatographic analysis.

Using electron impact (EI) ionization, the phthalic acid diesters strongly fragment, the most abundant ion being a phthalic anhydride ion at 

 = 149, common for all saturated alkyl phthalates (except DMP). This ion can be used for trace analysis in selected ion recording (SIR) mode, provided that the target solutes are well separated by GC.

More recently, attention has been paid to the analysis of DiNP and DiDP. These compounds are synthesized from phthalic acid and a mixture of C

-alcohols, respectively. Consequently, the GC analysis of these solutes results in a cluster of peaks corresponding to different branched isomers. 

In the case of DiNP and DiDP determination, the C

-isomers present in commercial DiNP samples partially overlap with the C

-isomers present in DiDP. No column or temperature programme conditions could be found to completely separate C

-isomers, making quantification of DiNP and DiDP using the most abundant ion at 

 =149 impossible. To differentiate DiNP and DiDP, another ion should be measured. This can be done using ions 293 and 307 respectively, corresponding to the M-alkyl ion.

  These ions are, however, relatively low in abundance and consequently the same sensitivity as for the single isomer phthalates cannot be reached. 

Possible solutions are offered by the use of soft ionization techniques, such as chemical ionization (using ammonia), or by LC–MS. Recently, the combination of GC with atmospheric pressure chemical ionization (APCI) became commercially available

  and this technique was tested for the analysis of the high-molecular-weight phthalates. The method was also applied to measure DiNP and DiDP in a sediment extract from The Netherlands. 

) were obtained from BASF (Ludwigshafen, Germany). Individual stock solutions at 1000 ppm were prepared in cyclohexane. Calibration solutions containing DiNP and DiDP at 1–10 ppm level were prepared by dilution in cyclohexane. 

A sediment sample, obtained during an environmental study,

A novel and sensitive approach for analysing high-molecular-weight phthalates is described.

LCGC Europe-01-01-2011

Event News

Analysis of Inorganic Anions by Capillary Electrophoresis

Hydrogen Carrier Gas and Vacuum Compensation

The Development and Applications of PLOT Columns in Gas–Solid Chromatography

Determining High-molecular-weight Phthalates in Sediments using GC–APCI-ToF-MS

Selectivity in Reversed-Phase LC Separations (Part 2): Solvent-Strength Selectivity

Electrostatic Repulsion-Hydrophilic Interaction Chromatography: Using One Mode to Tune Retention from a Second Mode