4th International Conference on Polyolefin Characterization (ICPC)

The 4th International Conference on Polyolefin Characterization (ICPC)

 covers different aspects on polyethlyene (PE) and polypropylene (PP) molecular architecture characterization, including molar mass distribution (GPC/SEC), comonomer incorporation and distribution, long chain branching, crystallinity and rheological properties. 

ICPC is targeted at researchers typically working at polyolefin producers and processors, contract research organizations, government laboratories, academia, analytical and testing laboratories and instrumentation developers. 

, a thriving downtown in the forest that has become a regional destination for business, culture and entertainment. 

The technical programme will consist of lectures and posters in separation and fractionation, molecular structure and properties, morphology, thermal analysis, rheology and spectroscopy.  

A short course on analytical and preparative separation techniques is scheduled for 21 October. This one-day course complements the meeting by providing new researchers an opportunity to engage the world's experts to share their knowledge and experiences of polyolefin characterization. Topics covered are introduction to polyolefin microstructure, GPC/SEC practical considerations and applications, chemical composition distribution (TREF, CRYSTAF and CEF), cross fractionation, high temperature HPLC and preparative fractionation. 

Previous conferences have been held in Houston, Texas, USA, in 2006, in Valencia, Spain, in 2008 and in Shanghai, China, in 2010. In this way the event has covered the whole world map and become a truly international conference on the field of polyolefin characterization. With attendees from over 20 countries and from the leading polyolefin players, ICPC unifies the participation of researchers from both industry and academia throughout the continents. 

The ICPC technical committee is represented by Dr Benjamin Monrabal (Polymer Char, Spain), Dr Colin Li Pi Shan (The Dow Chemical Company, USA) and Prof João Soares (University of Waterloo, Canada), who founded it together. Later, in 2008, the committee was pleased to welcome Prof Minoru Terano (JAIST, Japan Advanced Institute of Science and Technology, Japan) and Prof Dujin Wang (ICCAS, Institute of Chemistry, Chinese Academy of Sciences, China), in 2010. 

13th Fera/JIFSAN Symposium 'Omics' Technologies in Food Science

High Performance Liquid Phase Separations and Related Technologies (HPLC 2012)

ExTech 2012, 14th Symposium on Advances in Extraction Technologies

 Luigi Mondello, Chromaleont, University of Messina, Italy

SFC 2012 – 6th International Conference on Packed Column SFC

6th International Symposium on the Separation and Characterization of Natural and Synthetic Macromolecules (SCM-6) 

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Comprehensive two-dimensional chromatography necessitates additional nomenclature over and above that used in classical one dimensional (1D) chromatography. The use of two (or more) columns requires various parameters on these separate dimensions to be adequately identified. This is achieved by use of a superscript preceding the respective symbol. Also, the process of coupling different dimensions leads to specific terminology, which we report here. A number of recent examples where specific nomenclature related to retention indices, estimation of first dimension retention times, and the definition of modulation ratio, are included. This article and its predecessor aims to establish a basis for suitable nomenclature and conventions in this rapidly advancing area.

 (1) defining various nomenclature and conventions in comprehensive multidimensional chromatography. This paper was the outcome of a session organized to discuss the terminology and nomenclature used in this emerging field, during the First International Symposium on Comprehensive Multidimensional Gas Chromatography (GC×GC), held on 6–7 March in Volendam, the Netherlands.

	
	This first symposium was the progenitor to eight further symposia so far, with the 9th GC×GC scheduled for Riva del Garda, Italy, in May 2012. Many of the listed conventions and nomenclature were based on practices that had appeared in the literature up to that time – and admittedly this was largely in the field of GC×GC. Perhaps not surprisingly, unanimous agreement was reached on many of these definitions, terms and symbols, and the need to ensure that establishing a basic ‘set of conventions’ was appreciated. Most of the terms were logical and this aided acceptance and understanding among chromatographers.

	
	At that time, there were relatively few reports on LC×LC or other comprehensively coupled separation methods. Since this first meeting, many LC×LC developments have been introduced, with a number of different implementation formats, and the review of Stoll 

. in 2007 should be referred to (2). In addition, the 2003 paper included some proposals for nomenclature and conventions that were recommended even though they had not been used in the literature. According to Scopus, that paper has been cited almost 90 times in subsequent literature (becoming the most highly cited paper in 

), and so has been accorded good acceptance within the chromatography community.

	
	It is time to update the earlier review with various terms and conventions that have been described since that time in this rapidly advancing field of chromatographic analysis. In addition, comments on methods for derivation of various parameters for comprehensive multidimensional chromatography experiments will be included. The more extensive literature on LC×LC in recent years has resulted in a range of new terminologies being added to the lexicon of comprehensive twoâdimensional chromatography and this justifies the addition of terms peculiar to that experimental arrangement.

	
	For completeness, the Tables of the earlier review will be reproduced here and additions included where appropriate.

	There still remains some lingering confusion or inconsistency over the use of the words “comprehensive” and “orthogonal,” and of the multiplex sign (×) as a short notation for comprehensive two-dimensional (2D) separations. Hence, as in the 2003 paper, the background to, and further comment on, these aspects will be included.


	An important discussion surrounded the argument of when a 2D separation can be called comprehensive. Three criteria were established, based essentially on the definitions formulated earlier by Giddings (3). There is a need to clearly distinguish between classical (heart-cut) 2D separations and comprehensive 2D separations. Also, some practitioners use the term ‘comprehensive GC analysis’ to mean ‘analysing everything’.

	
	However, when a specific part of the sample is removed prior to (by selective sample preparation) or during the analysis (for example, solvent vent or a split-flow arrangement), it is still possible to refer to the separation as comprehensive. In that instance it is good practice to specify the fraction of the sample that is analysed. For example, a comprehensive 2D separation of a specific plant extract will refer to theselected extract, which acknowledges the inherent limitations of extraction selectivity (not all the components may be extracted equally). In such an instance the sample can be redefined as that well-defined fraction of the initial material to be analysed. Thus the extract (and not the plant) is the sample to be analysed by comprehensive 2D chromatography. Similarly, the sample can be a serum obtained from whole blood, a distillation fraction obtained from crude oil, or a specific fraction obtained from a chromatographic separation, etc. Therefore, the guiding principle (3) is that a 2D separation can be called comprehensive if:

		Every part of the sample is subjected to two different (independent) separations;

		Equal percentages (either 100% or lower) of all sample components pass through both columns and eventually reach the detector; and

		The separation (resolution) obtained in the first dimension is essentially maintained.

	
	The third criterion can never be met completely, but chromatographers have learned to live with arbitrary distinctions in similar contexts (for example, the extra-column contribution to peak broadening must be kept negligible). Clearly Giddings understood that criterion 3 was a key to differentiate the heart-cut process from what he envisioned as the proper comprehensive separation experiment, but he will not have conceived how the column chromatography experiment could be conducted to ensure the 1D separation was fully maintained. Blumberg and Klee (4) have recently revisited this definition and re-cast it as “An n-dimensional (nD) analysis is one that generates n-dimensional displacement information”.

	One way that the qualifier “essentially maintained” (point 3) may be translated into practical terms is to specify that a reduction in the apparent 

D resolution (observed by projecting a specific peak pair in the comprehensive 2D chromatogram on the 

D axis) should not exceed some fixed limit (for example, 10%). However, this has not been paid due attention in the literature. The most important aspect of preserving resolution in the first dimension is the number of 

D chromatograms recorded across the width of a 

D peak, with three or four normally referred to as the minimum target number. It should be noted that the desire to “essentially maintain” the separation achieved in the firstâdimension may not coincide with the optimum conditions of analysis. For example, Horie 

(6) independently came to the conclusion that around two “cuts” per 

D peak is optimal for achieving efficient LC×LC separations.

	
	In the earlier paper, the second criterion was reported to give rise to much discussion. Most delegates felt that the term “comprehensive” should not be reserved for instances in which every analyte is completely (100%) transferred to the 

D column and then recorded at the detector - provided that the “split factor” 

 (%) is equal for all components. If (apart from a single intensity factor of 

) the obtained chromatogram is identical to the one obtained with 100% transfer, then the term comprehensive may be used. In simple words, a “faithful representation” of the sample must be obtained in truly comprehensive 2D chromatography. In practice, it is not easy to obtain such a faithful representation if the transfer factor is not 100%. By selecting time fractions for transfer to the 

D column (for example, sending the effluent to waste during 90% of the time), the fractions of individual peaks will vary. This may result in the comprehensive 2D chromatography experiment being a qualitative process, rather than a quantitative method. When flow splitters are used, gas chromatographers hardly need to be reminded of the danger of discrimination such devices entail. If in doubt (i.e., when some discrimination may occur), it is better to speak of nearâcomprehensive 2D separations.

	
	The sub-sampling interfaces (i.e. those that sample less than 100% of the injected material and can sample or modulate each solute to a different amount) include diaphragm and early (differential) flow modulators in GC, and instances where the volume transferred to, for example, dual sampling loops in LC exceed the break-through volume of the loop.

	
	A final point worth making is that “comprehensive gas (or liquid) chromatography” is not the same as “comprehensive two-dimensional gas (or liquid) chromatography.” The former is an imprecise usage of language that should be avoided. Shorter, unambiguous abbreviations are discussed below. Likewise, it is noted that sometimes the literature includes terms such as “comprehensive GC×GC chromatography”. This is also an imprecise use and should be avoided.

	To abbreviate the term “comprehensive 2D” the multiplex sign (×) may be used (7). This is a clear distinction from conventional “linear” or “heartâcut” 2D separations, which are usually abbreviated with a hyphen (or n-dash) (for example, GC–GC, LC–GC). The multiplex sign can be used as an abbreviation for a number of different comprehensive 2D separation techniques. Some examples are listed in Table 1. Hyphens are conventionally used to indicate the coupling of a chromatographic system to any kind of detector: as in GC with flame-ionization detection (GC–FID), GC coupled with Fourier-transform infrared spectroscopy (GC–FTIR), or GC with atomic-emission detection (GC–AED), although usage such as GC/MS is often noted), and to indicate on-line coupled (“hyphenated” or “coupled-column”) systems (such as on-line solid-phase extraction in combination with LC, SPE–LC). This illustrates that hyphens are used in several different ways and that the meaning of the symbol is not unambiguously defined. However, in all three instances the hyphen indicates that the systems are coupled on-line.

	
	Other variants to coupled instrument notations include GC(FID) for “simple” (single-channel) detectors and LC/MS or LC//MS for off-line combinations, and even variations within abbreviations for GC with time-of-flight mass spectrometry, such as GC–TOF/MS, GC–TOF–MS or GC–TOF MS for this instrumental arrangement are noted. A detailed discussion of such notations and obtaining some consensus in their use is beyond the scope of the present paper. For a discussion on the notation of chromatography–MS systems see the glossary of terms proposed by Murray (8).

	
	Because MS is a technique to separate ions generated on-line with the separation step, one can argue that GC×MS and LC×MS are comprehensive 2D separation methods. This argument is most realistic when a soft ionization method is used, which results in minimal fragmentation of parent ions. This has been used, for example, for GC×FIMS (field ionization MS), or other similarly soft ionization methods, with data displayed on largely independent axes of retention and ion mass (which predominantly generate molecular ions).
	Every analyte component is then indeed separated in two dimensions. If MS acquisition takes place sufficiently fast to meet the requirements of comprehensiveness and if discrimination at the MS inlet is avoided, the ‘×’ sign may arguably be used. For conventional gas chromatography–mass spectrometry with electron ionization, a hyphen is more appropriate. The abbreviation GC×2GC is suggested for a comprehensive two-dimensional gas chromatography system with two parallel second-dimension (2D) columns (9). According to the guidelines given above for use of the word “comprehensive,” this is applicable to such a system, provided that the sample is distributed across the two 2D columns without any discrimination. An example of a comprehensive three dimensional separation (GC×GC×GC) has been noted (10).

	
	A point that may have been noted by authors in recent years is the lack of consistency in the expression used for the abbreviation GC×GC. Of 15 journals checked, eleven used GC×GC and four used GC × GC. Other infrequent and inaccurate uses include GCXGC, and GCxGC (that is, using the normal font as either capital or lower case, instead of the multiplex sign). There appears to be consensus that GC×GC without spaces should be used.


	“Orthogonality” remains one of the terms used most inconsistently in the literature on 2D separations. This is quite unlike the precise definitions in mathematics and statistics. In the chemical literature, orthogonality concepts are often associated with multidimensional instrumental chemical analysis and especially with separation science, where it has been used informally to indicate merely different separations (coupled separation dimensions) or mechanisms (hyphenated with detection), rather than those that are “perpendicular” or “independent.”

	
	In the Volendam discussion, no reasons were found for such a broad and peculiar interpretation of the word. A sensible definition for “orthogonality” in separation science is listed together with definitions from mathematics and statistics in Table 2. The analytical definition implies that completely independent analysis processes are used, or in separation science that independent retention mechanisms apply in the two dimensions.

	
	Whether or not orthogonal separation mechanisms (orthogonal separations) lead to good separations depends on the sample, its components and the specific physical-chemical manner in which the retention mechanisms are exploited. For example, if 

D separates according to boiling point, while 

D is only able to separate according to stereo-selectivity, then only stereoâisomers in the sample will exhibit an orthogonal 2D separation. Sometimes it is noted that authors refer to the ‘degree of orthogonality’ in a separation. For instance, LC×LC comprising different LC mechanisms – such as, reversed-phase and normal phase; size-exclusion and reversedâphase should meet this criterion. Note in contrast with the GC×GC experiment both dimensions have an underlying analyte boiling point mechanism. The presumed ‘orthogonal’ column set comprising a non-polar 

D stationary phase has led some authors to refer to a polar×non-polar stationary phase column set as non-orthogonal. There should be no justification for this designation. There is still scope for a rational basis and agreement for determining the ‘extent of orthogonality’.

	
	In the following section, some recent studies that support certain concepts in GC×GC are further exemplified. The following sections refer to the nomenclature and terms reported in Table 3.

D data in GC×GC, and the experiment can be conducted in a number of ways. 

D data are best acquired by using a ‘split T’ before the 

) recorded with an FID. However, such an arrangement must lead to an incorrect assessment of the actual 

 value due to added retention on the short transfer line that connects the 

D column to the FID. Others have used the retention time of the largest modulated peak (or the modulation time of this peak) in the GC×GC result as a surrogate for the 

 value, and then used this for deducing the 

 in GC was reported by using a suite of alkanes injected periodically through the first injector, which requires them to pass through the entire first column (12). The interference caused by co-injected solvent was a drawback to the method. A solid-phase micro-extraction (SPME) method with appropriate alkane sampling was then described (9), with again injection into the first injector. The constraint of passing alkanes through the 

D column was overcome by delivering alkanes directly to the end of the

D column just prior to the modulator, so that any alkane could be introduced to the 

D column at any time. The use of a second injector made this possible, with SPME again used for sampling the alkanes so that no solvent was introduced.

	
	In all cases, the sequentially injected alkanes describe a so-called ‘isoâvolatility’ plot within the 2D separation space, which refers to the observation that the injected component defines an exponential reduction in 

 as temperature increases. Interpolation (or sometimes extrapolation) between successive alkanes in the pseudoâisothermal 2D analysis gives the 

 value. The method is not straightforward and demands some attention to the details of the experiment. However, one can predict that once the 2D space has been defined by suitable alkane elution patterns to derive an equivalent 

 value of any solute should be easily deduced.


	The modulated peaks that are generated from the GC×GC experiment can be used to estimate the apparent 

,app because it is a derived value rather than a directly measured quantity. A model describing the resulting distribution of 2D peaks, based on a 1D Gaussian peak shape, can be fitted to the 

 also has to include measurement of the solute hold time in the modulator, which we choose to call th, and must also account for the 

 value is not simply the same as the modulation time or modulation period (

; in some literature, this has been proposed as the second dimension cycle time 

). The total retention time of a modulated peak in the experiment should take into account all retention contributions to the total. It might be thought that a simple formula such as:

	
	will account for the different contributions to the total. However, an accurate 

 value cannot be easily defined for a single modulated peak – due to the manner in which the first peak is sampled in the experiment. There is no specific 

D peak that is sampled, but rather a range of retention times for the segment of the sampled peak. However, 

values can be estimated by applying a Gaussian or exponentially modified Gaussian (EMG) function to the distribution of modulated peaks for the component, and deriving the peak maximum for this reconstructed 


	The modulation ratio was introduced (15) as a means to quantify the relationship between 

, to the peak width at baseline on the first column (

). In many cases the time available for the 

. In some cases (for example, when using gradient-elution LC for 

D in the LC×LC experiment) the available time may be shorter. The obvious question to ask is how does one decide upon the appropriate 

 to use? Note here that in LC, the time available to the 

D separation has been termed 2tc, the second dimension cycle time. LC normally does not involve a focusing ‘modulation’ step (nor a timeâcompression effect as in flow modulation in GC), so modulation period 

. The modulation ratio is approximately equal to the number of second-dimension chromatograms (“cuts” or “slices”) recorded for each 

D peak, the modulation time will be shorter than the peak width at base if 

 are or should be known and this can aid in method set-up (16). The number of modulated peaks recorded (the modulation number, 

) depends on the magnitude (injected mass) of the component, the modulation phase, the modulation period relative to the peak width and the sensitivity of detection of the 2D peaks for the compound. Thus, 

	
	A number of experimental parameters can be set based on the desired 

 determines the maximum value of 2tR that can be recorded without wrap-around. Under nonâprogrammed conditions 


	In Table 3 a number of other relevant definitions are collected. Several of these are specifically derived from the field of GC×GC, but their use need not be exclusive for this technique. For example, the word modulator may equally well be used to describe the intermediate stage in a number of other comprehensive multidimensional chromatography methods. One result from the Volendam discussion was that a modulator does not necessarily need to incorporate a focusing effect; in fact there is no rationale for insisting that focusing is necessary according to the definitions of comprehensive multidimensional chromatography. However, GC×GC is most readily accomplished in practice with the aid of focusing. The latter is easily achieved by thermal modulation in GC×GC, but the increasingly popular method of flow modulation does not rely on a focusing effect in space. However, it does have a compression effect in time. Focusing is also not usually encountered in the comprehensive 2D combination of two liquid-phase separation methods.


	Table 4 lists a number of symbols that are recommended for use in comprehensive 2D separations in general and GC×GC in particular. The use of the superscript prefix 1 or 2 is suggested to distinguish between the 

D columns. This table has a number of more recent examples of symbols that have been reported for GC×GC. Table 5 reports a selection of nomenclature proposed for aspects of LC×LC according to a recent paper (17), in which the authors state that they endeavour as much as possible to base their nomenclature on that used in the 2003


	Comprehensive 2D separations are increasingly important for the separation of complex mixtures. In this paper, a number of suggestions for definitions, nomenclature and symbols are presented that originated from the community involved with comprehensive 2D gas chromatography (GC×GC), but may be equally applicable for other comprehensive 2D separations. Some examples of studies that illustrate determination of specific parameters in the GC×GC experiment are outlined.

	
	The use of clear, unambiguous and well-defined terms and symbols may assist in better mutual communication between separation scientists, and between separation scientists and the broader scientific community.

		P.J. Schoenmakers, P.J. Marriott, J. Beens, 

		D.R. Stoll, X. Li, X. Wang, P.W. Carr, S.E.G. Porter, S.C. Rutan, 

		K. Horie, H. Kimura, T. Ikegami, A. Iwatsuka, N. Saad, O. Fiehn, N. Tanaka, 

		G. Vivo-Truyols, S. van der Wal, P.J. Schoenmakers, 

		N.E. Watson, W.C. Siegler, J.C. Hoggard, R.E. Synovec, 

		Z.L. Cardeal, P.P. de Souza, M. da Silva, P.J. Marriott, 

		J.L. Adcock, M. Adams, B.S. Mitrevski, P.J. Marriott, 

		W. Khummueng, J. Harynuk, P.J. Marriott, 

		L.W. Potts, D.R. Stoll, X. Li, P.W. Carr, 

		P.J. Marriott, R.M. Kinghorn, R. Ong, P. Morrison, P. Haglund, M. Harju, 

Coupling Matters

A review of nomenclature and conventions used in comprehensive multidimensional chromatography. An update of the popular paper on this topic from 2003.

Nomenclature and Conventions in Comprehensive Multidimensional Chromatography – An Update

The present state-of-the-art in chiral capillary gas chromatography (GC) is reviewed in this article. The article provide a short historical overview of phase development followed by a comparison of GC and high performance liquid chromatography (HPLC) chiral separations. After a short discussion of mechanisms of the most popular cyclodextrin phases, practical recommendations on how to select the best stationary phase and then how to use it to optimize chiral GC separations are described. Examples of elution order reversal of racemates, injection of large volumes of diluted sample using retention gaps and other applications are also provided. 

The origins of chiral gas chromatography (GC) technology can be found in an article by Volker Schurig and Leslie S. Ettre entitled "Emanuel Gil-Av and the Separation of Enantiomers on Chiral Stationary Phases by Chromatography" (1). For GC, there were two methods described, the direct approach utilizing chiral stationary phases with or without achiral derivatization and the indirect approach requiring derivatization of the racemate with a chiral reagent and separating the diastereoisomers on conventional achiral GC stationary phases. In today's world, the former is the preferred method given the extensive validation problems of chiral derivatization and the increased stability, reliability and diversity of available chiral stationary phases for capillary GC. In 1966, Gil-Av, Feibush and Sigler (2) achieved the first ever GC chiral separation using GC with 

-trifluoroacetylLisoleucine lauryl ester chiral stationary phase (CSP) and separated 

-trifluororacetyl alkyl esters of (±)-natural amino acids. Other stationary phases based on other amino acid or diamide derivatives have been developed and there are examples of chiral metal complexes that can interact with analytes through coordination complexation but none have been as successful or expansive as the cyclodextrin technology (3). 

Chiral GC has seen a dramatic explosion of applications in many fields of study including natural products, asymmetric synthesis, biological studies, environmental contaminants, agriculture, food, flavour and fragrance with the evaluation of essential oils. See reference 3, in which 185 papers on chiral applications were reviewed using CSPs in capillary GC.  

In this article, I attempt to review the types of columns that are currently available for chiral separations by GC, citing the advantages and disadvantages of each type. I also review the options available to solve a particular separation problem and the approaches to developing a separation. Optimization and troubleshooting tips are also included. 

GC offers a number of advantages over high performance liquid chromatography (HPLC) for the separation of enantiomers including the ease of generating theoretical plate numbers ranging from 12000 to 60000, depending on column length, resulting in the separation of more pairs of enantiomers for each column; high speed of analysis meaning higher sample throughput; and with flame ionization detection (FID) as close to a universal detection method as you can get, so a lack of aromaticity or functionality of the target molecules are not negatives. The downside is that injected samples need to be quite clean of extraneous material (precolumns, so called retention gaps, can help here), have a vapour pressure below 250 °C (achiral derivatization can help here), and be thermally stable and not racemize (there are techniques that can help here such as lowering elution temperature with proper achiral derivatives). Please note that elution temperatures are related to analyte vapour pressure, not boiling point. There are also other advantages, such as short column equilibration time, ease of connection to mass spectrometry (MS) and ease of trace impurity quantitation.  

Selectivity of other detection methods such as electron capture, the techniques of headspace extraction and the more routine use of two dimensional GC (GC×GC) have made the technique ideal for the analysis of enantiomers in complicated matrices like environmental, biological and agriculture samples. Development time and optimization are also faster than for HPLC. Increased thermal stability, high resolution, and increased peak capacity of current GC capillary columns make these tools ideal for the analysis of complex mixtures commonly encountered in samples from biological or natural sources. With current technological advances in CSPs there are often multiple choices for a particular racemate to maximize speed and increase efficiency leading to lower limits of detection. It is also easier to reverse elution order — that will be explained in more detail later. Also, despite the great success by the use of chiral LC, there are no chiral LC columns that can routinely separate a variety of volatile, nonaromatic enantiomers. And, of course, the biggest advantage is no waste solvent. The biggest drawbacks are the lack of preparative capabilities, the fact that the final analyte must be volatile and the inability to analyze thermally labile compounds. 

Although HPLC has a wider variety of stationary phases (for example, π-complex, crown ethers, macrocyclic antibiotics and proteins, among others), cyclodextrin (CD) technology has been ubiquitous in the area of chiral stationary phases for capillary GC. CDs are cyclic, chiral, torus-shaped macromolecules composed of six or more D(+)-glucose residues bonded through α-(1–4) glycosidic linkage. The CDs containing six, seven and eight glucose units are the most common, best characterized and commercially available. There are now more than 16 different cyclodextrin chemistries on α-, β- or γ-cyclodextrin sold as chiral stationary phases for capillary GC (Figure 1). If you add in the differences in cyclodextrin that was derivatized, carrier type and concentration, bonded or unbonded that number increases to 45 choices. Unlike in HPLC, however, only a few of these columns serve the vast majority of required separations. There is one popular noncyclodextrin CSP that is widely used, Chirasil-Val, which is supplied by Agilent Technologies (Santa Clara, California, USA). This phase was based on the work of E. Bayer's group (4) and was introduced more than three decades ago. It was one of the earliest commercially available CSPs and is still most popular for the separation of D, L amino acids but has limited capabilities in other areas. It comes as either the L-valine or D-valine version. For the L-antipode, the D-amino acid is eluted first, which can be reversed using the D-valine CSP. For determining optical purity you can then choose the appropriate column so that the minor component elutes first, yielding the lowest possible detection limit. Using this column, a typical capillary GC separation of amino acids isolated by solid-phase extraction from green coffee beans that were derivatized as their 

-ethoxycarbonylheptafluorobutyl esters is depicted in Figure 2 (5). As might be expected, roasted coffee beans showed almost no amino acids in their extract because they can't survive the roasting temperatures. 

The mechanisms of separation of CD technologies fall into two basic categories: surface interactions vs. inclusion. It is now known that certain CD derivatives are largely a surface interaction, being more efficient and with higher capacity while others involve the inclusion phenomena usually associated with cyclodextrin technology. Because the internal cavity of a cyclodextrin molecule tends to be more hydrophobic than its exterior, most nonpolar molecules (or molecules with nonpolar moieties) prefer to reside in the cyclodextrin cavity. This inclusion phenomenon in capillary chiral GC is controlled by the type and number of derivatives linked to the 3-position hydroxyl as opposed to the 2-position of the glucose units that make up the CD structure (see Figure 3). This article addresses these differences and how to use them as an advantage for a particular separation. 

It should be noted that enantioselectivity is rare at temperatures above 200 °C. Temperature plays a significant role in chiral separations on GC — the lower the elution temperature the greater the selectivity differences. Thus, a compromise must be reached between retention time and selectivity enhancement in the separation of enantiomers. 

Thermodynamic relationships and sample loading studies can be used to indicate the dominate chiral recognition mechanism. Enantiomers that have large 

° values also show low loading capacity. Sample capacity can be determined by injecting increasingly more sample on the column and monitoring the peak efficiency. As a rule of thumb, when the peak efficiency falls 10% from its underloaded value, the column is said to be overloaded. This suggests that inclusion plays a role in the enantioselective mechanism. Enantiomers that have smaller 

° values show a higher sample loading capacity. This implies that inclusion plays a less dominant, or negligible, role in the enantioselective mechanism. It is difficult to imagine that this is a black and white situation, but it is indicative of a situation that can prove advantageous. The second phenomenon that has been observed is the efficiency of peaks is much higher with those derivatives that indicate a surface mechanism as opposed to an inclusion mechanism. This is consistent with what has been observed with cyclodextrin derivatives in different solvent modes for HPLC. 

The list of derivatives in Table 1 does not include column lengths or capillary diameters, only differences in chemistries and cyclodextrin type. Of the 45 choices in derivatives there are basically two chemical types, the alkylated version: methyl, ethyl, butyl or pentyl; and the polar type: acetyl, propionyl or the patented trifluoroacetyl. When more polar groups are placed in the 3-hydroxyl position of the glucose unit of the cyclodextrin the chiral interaction does not appear to involve the inclusion phenomena. These types of CSPs typically have higher efficiency, faster separations and capacities as high as 10–50 times greater than those relying on the inclusion phenomena. There is a second classification one can make in the choice between the coated versions and the derivatives bonded to polysiloxane. This bonding process increases the temperature stability only modestly, but there is a recognizable loss of selectivity in the bonding process (6). It is a consideration to be made based on the process needs, not in the method development scheme. Therefore, for screening purposes it is useful to have one version of each chemical type. The two this author recommends is the 2,3-di-

-butyldimethylsilyl β-CD and the 2,6 dipentyl-3-trifluoroacetyl γ-CD. Further improvements in the method may come from the use of some of the other closely related derivatives. 

Table 1: List of manufacturers with cyclodextrin-based columns for capillary GC. 

There are two prime conditions for operating CSPs in GC. First, the recognition that the lower the analysis temperature, the greater the energy differences between the diastereomers; therefore, you want to operate at the lowest temperature possible. It is also possible to miss the window of separation by running at a temperature above the optimum elution temperature. Second, to achieve the best chiral selectivity in the shortest amount of time, it is advantageous to run at high linear velocities of carrier gas while using as low an elution temperature as possible. This is opposed to manipulating the temperature itself. Linear velocities of 100 cm/s and 60 cm/s are ideal for hydrogen and helium, respectively. To exploit high linear velocities when using temperature programming to find the window of separation, shallow ramp rates of 1–5 °C/min should be used if the elution temperature is below 130 °C and 5–10 °C/min if it is above 130 °C. It is important to use a good oxygen scavenger for all the CD phases especially when you exceed 110 °C. Oxygen, when present at the high temperatures used in GC, rapidly causes oxidation of stationary phases, resulting in column bleed, shortened lifetime and potential selectivity and retention changes. 

Available CSPs for Chiral GC Separations 

Every major supplier that has sold capillary GC columns in the past now has some version of a cyclodextrin CSP for GC. The leading CD derivative in the field from an application standpoint has been the 2, 3-di-

-butyl dimethyl silyl β-CD. This has largely replaced the straight 2-3-6-tri-

-methyl version. The reason for this is the 

-butyl group allows for possible orientation of the CD in the polysiloxane carrier leading to greater selectivity and more efficient peaks. Comments in the literature indicating that the 

-butyl group blocks the entrance to the lower opening of the CD cavity has not been substantiated by experimentation. Here is a little background: the trimethyl β-CD derivative is a crystalline material and, as such, in a gas environment has poor mass transfer leading to poor peak efficiency and resulting in very broad peaks and high retention times. It has to be diluted with a polysiloxane carrier to make it more efficient to the tune of 20–30% CD to carrier. The 

butyl group in the 6-position has little effect on chiral selectivity and therefore the site is derivatized to provide greater selectivity of the CD in the carrier. A number of different polysiloxane carriers are used, but no significant difference in selectivity has been noted from these changes. The isothermal operating temperature of this phase is usually the highest of all the CD phases at 220–230 °C, but when using a temperature ramp the upper temperature limit can be extended to 250 °C for short periods. This product has been shown to provide ~60% of all chiral separations possible on capillary GC phases. As in all of these types of cases, there are a few separations that still work better on the tri-

-methyl version, but it appears to be less than 10%. Furthermore, there are differences in the type, purity and concentration of carriers used by different manufacturers as well. As one would expect, as the elution temperature of the analyte gets lower the greater the influence of the carrier in retention of the analyte; so when working with low temperature analytes it is better to employ other types of derivatives that do not require a carrier. The differences from manufacturer to manufacturer for these phases are minimal for most separations. The main problem comes from those analytes that are eluted at low temperatures where the differences in concentration and purity of the carrier become significant. Table 2 lists classes of racemates suitable for the 2,3-di-

Table 2: Racemates suitable for separation by the two best chiral phases in GC. 

For application breadth, the next CD derivative that could handle a good portion of the remaining 40% of the applications is the patented 2,6-di-

-pentyl-3-trifluoroacetyl γ-CD produced by Sigma-Aldrich/Supelco (Bellefonte, Pennsylvania, USA). The mechanism for this phase is a surface dipole–dipole interaction and as such covers a broader area using the γ-CD as opposed to the β-CD. This phase is highly selective for oxygen-containing analytes such as alcohols, ketones, acids, aldehydes, diols, epoxides, esters, phthalides, sulfoxides and lactones. This phase is also highly selective for halogenated compounds (alkanes) and amines. It separates not only the widest variety of enantiomers, but also the greatest number within certain classes. It has two limitations: its sensitivity to moisture and its isothermal upper temperature limit of 180 °C. This limit can be exceeded by 25 °C for short periods during temperature programming. The best defense to avoid hydrolysis is to use excess trifluoroacetic anhydride (TFAA) in your sample and to use a retention gap both after the injector and before the detector. A 5-m retention gap as a precolumn and 1-m to protect the column from the high temperature of the detector are recommended. Reference 7 demonstrates the application of this technique to monitor aqueous enzyme reactions. In a homologous series of alkane enantiomers, identical alpha values are observed regardless of chain length or branching indicating, only one or two carbons may be contributing to chiral recognition. Note that the sources of moisture that can effect this phase include moisture in the sample, carrier gas, or from air if not stored properly. 

The versatility of Chiraldex G-TA can be seen in Figures 4 and 5. Separations of racemic mixtures of various classes of compounds are depicted in these figures. 

When a carbonyl is present from a trifluoroacetyl, acetyl or propionyl group bonded to the 3-position and the 2-position is either a methyl, ethyl or pentyl group, the chiral interaction of analyte favours the surface interaction mechanism of the CD as opposed to cavity inclusion mode. Separations are typically faster and more efficient. The added advantage is the capacities are generally higher for this mechanism as opposed to those derivatives that promote an inclusion interaction. The trifluoroacetyl is the more effective of the three possible derivatives based on a review of the applications followed by acetyl then propionyl. The inclusion mechanism recognizes analyte size, shape and functionality and, therefore, is useful for certain target analytes. The other derivatives that are available offer incremental improvements over the types already stated. 

Approaches to Obtaining Chiral Selectivity 

The easiest approach in selecting a CSP is to take advantage of the many free laboratory services offered by column manufacturers. Turnaround time is usually short in comparison to HPLC and you are typically guaranteed to get the best column available for the job at hand. Many of the manufacturers will also consult with you on the phone to give you some direction for column selection and method development. The second, more time-consuming, approach is to search the many databases available on the web or from the manufacturers. If your molecule of interest is new and not closely related to what has been reported then search the list of class categories typically supplied by the manufacturers. For example, the dipentyl derivatives of α-, β-, and γ-CD demonstrate selectivity differences based on size, shape and functionality. α-CD is useful for simple epoxides, cyclic ethers and linear substituted amines; β-CD is useful for heterocyclic amines; and γ-CD is useful for the naphthyl analogs. The derivatives are typically listed with that kind of description. However, you now have 25 possible choices to study. Also there is the extensive literature that is available in the various fields where this technology has been used. A number of application reviews can help here (3,8). Finally, you can take my recommendations of the two most widely applied CD columns and run temperature ramps as recommended above. 

There are three basic methods that have been used with this chiral column stationary phase technology to reverse elution order. The most effective is to switch the size of the cyclodextrin, for example β to γ or vice versa (see Type I in Figure 6). The second method is to change the mechanism, going from a surface interaction phase like 3-trifluoroacetyl-2-6-di-

-pentyl γ-CD to an inclusion mechanism phase like 2,6-di-

-pentyl-3-butyl γ-CD (see Type II in Figure 7). Finally, but least effective, is to change the derivatizing agent, for example, from trifluoroacetyl to acetyl for an amine chiral site (Type III in Figure 8). Be aware that this approach can also change selectivity so it is not always appropriate. Reversal of elution order methods becomes valuable when you are developing a trace analysis separation and want to have the impurity peak eluted first so that the column can be loaded with sample and the impurity will not interfere with the smaller trace peak. If the smaller impurity peak appears on the tail of a very large peak, there is a chance that it will appear to be unresolved or not be seen. This can also result in the integrator not defining an accurate peak area or retention time and poor quantitation results. It has also been useful here to use a retention gap of up to 5 m to load the sample onto the separation column. Here, a slow injection of a large volume of sample (up to 5 µL) is performed at a temperature below the elution temperature of the sample, but above the vapour pressure of the solvent. 

Examples of three basic methods (Types I–III) for reversal of racemate elution order can be seen in Figures 6, 7 and 8. An example of the use of a 5-m retention gap to load a diluted sample onto a GC chiral column is shown in Figure 9. The use of the retention gap prevents localized overloading of the stationary phase and allows the analytes to be widely separated from the large ethanol solvent peak. 

There are a number of influences on chiral selectivity in this technology. Achiral derivatization of the racemate influences chiral selectivity. It is therefore necessary to investigate several types of derivatives to determine ultimate success. Usually, the larger the analyte, the smaller derivative size is most effective. For small molecules the reverse may be true. In addition, elution order can be affected by the type of derivative chosen and can be used to affect elution of the trace enantiomer first to maximize sensitivity. See Table 3 for a list of successful derivatizing agents and preferences. 

It is suggested that work at the lowest possible elution temperature and using an increased carrier gas flow will reduce retention time. Usually a 30-m column is considered the workhorse but a 10-m scout column can be a handy, quick guide to find the required window of separation. Remember that both moisture and oxygen can affect selectivity as well as the column and analyte stability. Therefore, it is necessary to use good drying and oxygen traps for continued optimal performance. 

Table 3: Achiral derivatization methods. 

Three carrier gases have been used in this technology: nitrogen, hydrogen and helium. Nitrogen offers low cost and high efficiency. The disadvantage of nitrogen is the optimal linear velocity is 12 cm/s and efficiency is rapidly lost with increasing linear velocity. Helium is more expensive, but the optimum linear velocity is 20 cm/s and the slope of HETP vs. linear velocity is flatter. Hydrogen has an optimum linear velocity of 40 cm/s, has the flattest HETP vs. linear velocity of all and the lowest viscosity, but workers often question its safety. Helium, therefore, is a good compromise. As stated above, differences between diastereometric complexes need to be established first through the use of low elution temperatures; the use of hydrogen will result in more efficient peaks. There are cases where highly volatile racemates may be investigated and in those cases nitrogen would be the preferred carrier gas. 

In this instalment, I have discussed the current status of the separation of enantiomeric compounds by GC. All in all, the CD phases have achieved the greatest success, and, unlike HPLC, only a few phases are required to cover the gamut of volatile, derivatized or nonderivatized enantiomeric compounds. I have discussed retention mechanisms and provided recommendations for column selection as well as suggested operating conditions. Some application examples of the use of chiral stationary phases were shown including their versatility, ability to reverse racemate elution order, the separation of derivatized amino acids in green coffee beans and the use of retention gaps to inject large volumes of diluted chiral compounds. 

This article was originally published in "Column Watch" in the August 2011 issue of LCGC North America.

A review of the historical development and latest trends in phase development in the field of chiral capillary gas chromatography. A range of novel applications in chiral capillary GC are also described.

The State of the Art in Chiral Capillary Gas Chromatography

The 2012 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy (Pittcon) was held 11–15 March, in Orlando, Florida, USA, at the Orange County Convention Centre. The 63rd annual occurrence of this venerable analytical chemists' symposium was only slightly less attended than the conference's 2010 southern sojourn at the same location, with 15754 registrants. I'm comparing the attendance to that at the previous Orlando venue rather than Atlanta from 2011 because the attendance always has been somewhat lower when the conference heads south. The amount of exhibitor booth space also declined by a similar margin, driven by many companies' decisions to attend with a smaller footprint, although the total number of exhibiting companies only dropped by about 2%. 

Significantly absent this year were Agilent and PerkinElmer, which are both engaging in post-conference seminar tours and on-line product displays. Both companies exhibited at Analytica in Münich, Germany, 17–20 April, 2012. Several companies opted for nearby hotel suites to meet key customers at rather than hosting an in-exhibition booth. To an aisle wanderer such as myself, the overall impression was one of reduced acreage accompanied by a larger Technology Park area and, although I did not measure them, perhaps slightly wider corridors. Smaller though it may have been, the exhibit floor experience was almost relaxed, with more time to visit each exhibitor of interest, unlike some earlier years where I often had to choose between visiting a booth or attending a session. 

The conference organizers' emphasis on the technical programme was evident in the consistent number of poster sessions that were mounted in two large areas of the exhibition floor, as well as in the continuing high count of oral presentations, symposia and workshops. Short-course attendance was up 16% from last year.  

Among the conferees, 36% listed liquid chromatography (LC) and 34% gas chromatography (GC) among their selected scientific specialties; these were the top two categories at the conference. Spectroscopic specialties accounted for the majority of the remaining mentions; ion chromatography was cited by 11%, and other chromatography techniques were cited by 7%. 

Pittcon 2013 will be held in Philadelphia for the first time, 17–21 March, at the Pennsylvania Convention Centre. The conference is scheduled to return to Chicago (2014, 2017), New Orleans (2015) and Atlanta (2016), before returning to Orlando in 2018. 

This instalment is our annual review of GC instrumentation and accessories shown at this year's Pittcon or introduced during the previous year. For a review of new GC and LC chromatography columns and related accessories, please see Ron Majors' "Column Watch" in the April and May 2012 issues of 

The information presented here is based on manufacturers' replies to questionnaires, as well as on additional information from manufacturers' press releases, websites and product literature, and not upon my actual use or experience. During Pittcon, I took time to stroll around the convention aisles and see some of the new products firsthand, as well as discover a number of items that weren't covered by the questionnaires. Every effort has been made to collect accurate information, but because of the preliminary nature of some of the material 

 cannot be responsible for errors or omissions. This column cannot be considered a complete record of all new GC products introduced this year at Pittcon or elsewhere because not all manufacturers chose to respond to the questionnaire, nor is all of the submitted information necessarily included here because of the limited available space and the editors' judgment as to its suitability.  

Another new crop of GC instruments appeared in 2012, which demonstrates once again the ongoing resilience of the technique and also emphasizes a continuing shift toward mass-selective detection since what used to be topend capabilities have become more affordable. Nearly half of the newly introduced instruments are tightly associated with a mass spectrometric (MS) detector. Most were more about the MS detector than the GC itself. See Table 2 for the details of each new product. 

Shimazdu's GCMS-QP2010 SE system is an upgraded version of the company's QP2010 S system that features high-performance ion optics and a quadrupole mass filter along with lab-friendly energy- and timesaving features. While not seen at Pittcon, PerkinElmer introduced the Clarus SQ 8 GC/Mass Spectrometer system with high sensitivity, tool-free source maintenance and a rapid scanning speed earlier this year. Bruker Daltonics highlighted its highend Scion TQ GC–MS-MS system with 180° optics that enables multiple ionselective scan modes for resolution of complex samples and matrices. This triplequadrupole GC–MS system, introduced in July 2011, won the 2012 Pittcon Editors' Silver Award. All of these new GC–MS systems operate in tandem with the companies' respective existing gas chromatograph offerings. 

From Torion, the Tridion-9 GC-TMS system is a gas chromatograph–toroidal mass spectrometer system that is aimed at the rapid screening of chemicals such as environmental volatile organic compounds (VOCs) and semivolatile organic compounds (SVOCs), explosives, chemical threats and hazardous chemicals. The unit is billed as a "PersonPortable" GC–MS system that can easily be carried to remote sites. It uses a conventional 100-µm i.d. capillary column bundled with a low-thermal-mass resistive heater and helium carrier gas. Sample injection is accomplished via the company's Custodion solid-phase microextraction (SPME) syringe. 

Thermo Fisher Scientific came to Pittcon with the 1300-Series GC system, a completely new frame. With a unique system of "instant-connect" modular interchangeable inlet and detector options, the 1300-Series system allows end users to quickly exchange or replace either or both inlets or detectors as pluggable modules. Each module includes pneumatic and electronic connections that are engaged by putting them in place and tightening a couple of screws. 

PAC introduced the AC Analytical Controls CNS SIMDIS for Crude Oil analyzer. The system simultaneously determines the boiling range distribution of carbon, nitrogen and sulphur in crude oils and final products, using simulated distillation and a combination of flame ionization detection (FID) and chemiluminescence detection. 

A unique portable GC instrument was on display at the 2012 Pittcon. The Frog-4000 system from Defiant Technologies is a miniaturized GC system with photoionization detection (PID). Based on micromachined preconcentrator and GC designs, the Frog-4000 system targets field analysis of VOCs in water and soil.  

This year saw strong growth in new automated headspace GC (HSGC) samplers. Together with Agilent's 7697A headspace sampler that was introduced at Pittcon 2011, this burgeoning of HSGC is indicative of the ongoing demand for volatiles analysis in solid, semisolid and liquid matrices such as for pharmaceutical, food or flavour products. Table 3 lists the salient details. 

Table 3: Sampling systems and accessories. 

The new offerings bring a wide range of sizes and capabilities. The SHS40 headspace sampler from Bruker Daltonics is a multivial equilibrium headspace autosampler for the company's 400-series GC systems as well as the Scion Series GC–MS system. It accommodates up to 40 vials and supports a range of modes and capabilities. The Master SHS static headspace sampling system from Dani is designed to work with the company's Master GC system, but it also interfaces with most other GC systems. The Master SHS system has a 120-vial capacity and a range of options such as a bar-code reader. The Versa automated headspace vial sampler from Teldyne-Tekmar is a compact device that holds up to 20 vials and works with most GC systems as well.  

A related application-specific headspace sampler, EST Analytical's Centurion RSK50, performs US EPA RSK 175 analysis for dissolved methane, ethane and ethylene in water, including sample preparation of the requisite 40-mL volatile organics analysis (VOA) vial and sampling of the evolved headspace gases onto nearly any model of GC system. 

Table 3: Sampling systems and accessories (continued).

Two new cannister sampling devices were on-hand. Markes International introduced its CIA Advantage canister sampling system in 2011, after previewing it at last year's Pittcon. The cryogen-free sampler handles multiple cannisters and is fully compliant with US EPA TO-15. From Entech, the 7200 Automated Preconcentrater uses a three-stage trapping procedure to preconcentrate gaseous samples from a variety of sources, add internal standards or spikes and like the Markes unit it works with most GC systems. 

Two new liquid GC autosampler systems were shown this year. HTA s.r.l. displayed its model HT3100A autoinjector, HT3000A autosampler and HT3200A top-class autosampler. Together, the three models deliver a wide range of capabilities to almost any GC system. From Thermo Fisher Scientific, the TriPlus RSH autosampler is a robotic 

 axis sample handling system and autoinjector with all of the flexibility and versatility that comes with this type of device. One TriPlus RSH autosampler can address up to two GC systems with sample preparation, derivatization and micro-volume injection among many sampling options. 

Finally, Gerstel introduced two accessories for its robotic MultiPurpose Sampler (MPS). The mVAP MultiPosition Evaporation Station concentrates up to six samples at userdefined temperatures and vacuum levels. Solvent exchange, solid-phase extraction and filtration are a few of the possible sample preparation techniques. Gerstel's mVORX vortex and shaker accessory for its MPS system agitates up to eight sample vials in the horizontal plane to eliminate vialcap contamination and over-mixing of microsized samples. 

I would like to thank the manufacturers and distributors that kindly furnished the requested information before, during and after Pittcon 2012, allowing a timely report on new product introductions. For those manufacturers who did not receive a preconference questionnaire this year and would like to receive one and be considered for early inclusion into Pittcon 2013 coverage, please send the name of the primary company contact, the mailing address, fax number and e-mail address to Laura Bush, Editorial Director, 

, Advanstar Communications, 485F US 1 South, Iselin, New Jersey 08830, USA, Attn: 2013 New GC Products. 

 is senior research scientist at BPL Global, Ltd., Hillsboro, Oregon, USA, and a member of

 editorial advisory board. Direct correspondence about this column should go to 

 editor, Alasdair Matheson, 4A Bridgegate Pavilion, Chester Business Park, Wrexham Road, Chester, CH4 9QH, UK, or email 

GC Connections

In this instalment John V. Hinshaw reviews gas chromatography instruments and accessories that were newly presented at Pittcon 2012 or were introduced to the marketplace in the preceding year.

New Gas Chromatography Products 2012

A common liquid chromatography (LC) problem that I get e-mails about from readers of this column relates to unexpected and unwanted peaks that appear in the chromatogram. This month's discussion centres around an e-mail discussion I had with a reader of the electronic version of 

 (available on-line in countries where the paper editions are not available) about a problem they were experiencing. Because this problem relates to a pharmaceutical product and contains proprietary information, I've exercised some obfuscation privileges to hide some of the specifics, but retain the key information. 

The problem relates to the analysis of an antibiotic for impurities. It is a fairly simple method, comprising a phosphate–acetonitrile gradient on a reversed-phase column. The method had been validated and worked quite well to determine a mixture of eight antibiotics and impurities. After successful use in the development laboratory, it was transferred to another laboratory for routine analysis. For identification, I'll refer to these as the R&D laboratory and the production laboratory.  

When the method was transferred to the production laboratory, it appeared to work well, with one exception — a broad peak appeared at a retention time that overlapped the elution of antibiotic 2. In Figure 1, I've shown a section of the baseline including peaks for antibiotics 1 (13.4 min) and 2 (15.6 min) from a chromatogram obtained in the R&D laboratory. This was a typical result for the injection of these two compounds. When the same sample was run in the production laboratory, the results of Figure 2(a) were observed. The peak for antibiotic 1 appears as normal, but there is a broad peak that all but obscures antibiotic 2. 

An extra peak, that is not related to the sample itself, in a gradient run typically arises from one of three sources:  

late elution from a previous chromatogram

contamination of the mobile phase (ghost peaks).

Many people call all three kinds of peaks ghost peaks, but I like to distinguish between them because, although they may look similar in the chromatogram, their sources are different, and so the process of eliminating them will be different. Let's look briefly at my definitions of each of these peak types. 

 of a peak from a previous chromatogram results when you don't allow enough time for all the peaks to be eluted following sample injection. As a result, the problem peak continues to travel through the column, but shows up in a later chromatogram, instead of the one in which it belongs. 

 from a previous injection is caused by sample that is actually injected, but its injection is unintentional. For example, you make an injection and get a normal run, then you make a blank injection with no analyte present, but the peak appears anyway, usually much smaller than the original. 

 tend to be unique to gradient elution, whereas late elution and carryover are common for isocratic separations, as well. Ghost peaks appear when a contaminant is present in the mobile phase (usually the weak, A-solvent), is concentrated on the column and then is released as the gradient runs. Ghost peaks will appear in sample injections when a blank is injected, and even when a gradient is run with no injection at all. 

What do you do when an unexpected peak appears in the chromatogram?

LCGC Europe-05-01-2012

Event News

Nomenclature and Conventions in Comprehensive Multidimensional Chromatography – An Update

The State of the Art in Chiral Capillary Gas Chromatography

New Gas Chromatography Products 2012

Extra Chromatographic Peaks — A Case Study