Liquid Chromatography (LC/HPLC)

The first of a two-part series addresses HPLC pumps, sample introduction systems, and columns for nanoLC-MS.

Separation and retention of both polar and nonpolar compounds by the same stationary phase can be a useful approach for analyses of complex samples with a broad range of chemical properties. Typical stationary phases are designed for retention of either polar or nonpolar compounds so that multiple steps are required when designing a separation strategy. Hydride silica–based stationary phases are new materials with properties that allow for the simultaneous retention of both polar and nonpolar compounds over a range of aqueous–organic mobile phase compositions. Adjustment of the aqueous–organic ratio will determine whether polar or nonpolar compounds have greater retention.

Part 2 of the column explores using the right balance for the right job in the right way.

This article assesses the extent to which some familiar HPLC detectors can provide reliable quantitative analytical measurements when no suitable primary reference standard exists. The advantages and limitations of two candidate detector schemes are discussed ? evaporative light-scattering detection (ELSD) and chemiluminescent nitrogen detection (CLND). It was found that when the ERETIC (Electronic REference To access In-vivo Concentrations) method in proton NMR was used as a "gold standard" reference procedure the ability of CLND to provide reliable single calibrant quantification is superior to ELSD. Furthermore, ELSD showed bias towards underestimation of chromatographically-resolved impurities, resulting in an overestimation of analyte purity.

It would help to have a restricted set of chromatographic systems (CS) that together serve as potential starting points in method development.

Check valves are the number one problem area for most LC pumps.

Purification of synthetic, natural and biological compounds in any quantity usually requires the use of preparative HPLC.

A more practical parameter for system suitability would be to use the peak width for a reference peak...

LCGC North America and its companion publication, LCGC Europe, are targeted for chromatographers in the United States and Europe, respectively. Sometimes I wonder whether or not the same types of problems are encountered by workers in liquid chromatography (LC) on both sides of the Atlantic. As I write this installment of "LC Troubleshooting," I have just completed three weeks of teaching LC short courses in Europe. I can assure you that the content of the questions I get in these classes is the same, no matter where they are taught. This column is based upon questions gleaned from the most recent courses - you can see that there are no national borders for LC problems.

The minimum peak size for reportable results for a liquid chromatographic (LC) method depends upon the application and the signal-to-noise ratio (S/N).

Higher productivity and faster analyses are two of the driving forces for continued improvement in high performance liquid chromatography (HPLC) column technology.

No column lasts forever, but in this month's installment of "LC Troubleshooting," John Dolan offers some tricks to help extend a column's life.

Once the visual problems have been eliminated as problem sources, you are left with problems related to system performance.

Because of progress in liquid chromatography/time-of-flight mass spectrometry (LC/TOF-MS) instrumentation, data processing and reporting, the measurement of compounds' accurate masses is becoming routine practice in screening analysis based on target databases. As such databases of monoisotopic masses can be easily updated with recent data from the literature; rapid characterization of new compounds and metabolites is possible without the need for primary reference standards. This approach has already been established in comprehensive toxicological urine screening and in analysis of drugs-of-abuse in seized street drug samples. Currently, a mass accuracy within 5 ppm can be routinely achieved, and confirmation via a numerical isotopic pattern match (SigmaFit) is provided by a new generation LC/TOF-MS instrument.

Analytical chemists are concerned with the quality of their methods and results. An important question in this context is whether the precision of a newly developed and validated method is up to standard. In other words: is the precision of the newly developed method comparable to what could be expected? This article looks at how the Horwitz equation can answer this. It also describes the results of an extensive study involving 10000 laboratories which indicates that the relative reproducibility approximately doubles for every 100-fold decrease in concentration and that, surprisingly, it does not depend on the type of material or method.

A problem that is encountered occasionally in liquid chromatography (LC) separations is the presence of unusually broad peaks in the chromatogram. This problem is seen most often in isocratic separations, but it can occur with gradients as well. This month's instalment of "LC Troubleshooting" will cover some techniques to help determine the reason these wide peaks are seen.

The authors describe a new laser-induced flourescence detector for application to both capillary electrophoresis (CE) and high-performance liquid chromatography (HPLC).

Ron Majors discusses advances in multidimensional chromatography and comprehensive multidimensional chromatography.

This month's "LC Troubleshooting" is aimed at those who are just starting out in the liquid chromatography business, as John Dolan presents some helpful tips.

HPLC 2005 was held in Stockholm, Sweden, June 26-30. In this month's installment of "Column Watch," Ron Majors covers this year's hot topics including proteomics, column technology - especially monoliths - microfluidics, and multidimensional and high-throughput separations.

The use of a simple Excel-based "Golf Score Card" tool can facilitate assessments and instrument acquisistion decisions by collating and weighting the relative importance of the many criteria that impact the final purchase decision. The authors describe this process.

Some simple practices will help to make problems with gradient methods rare rather than frequent. this month's installment of "LC Troubleshooting" presents a baker's dozen of these tips.

Sometimes the mobile phase conditions selected for reversed-phase and ion-pairing methods seem either arbitrary or mysterious.

The implementation and validation of a new site-wide chromatography data system for a major active pharmaceutical ingredient custom manufacturer provided the opportunity to map and optimize the GMP and ISO business processes to use electronic signatures effectively. This article describes the process optimization and how it integrates with the validation activities of the system. This overall approach provides substantial business benefits from the use of electronic signatures as evidenced by the savings resulting from process improvement and by the fact that the non-GMP laboratories implemented a similar process to analyse and approve results electronically.

The authors explain why sample diluent is more than just a solubilizing agent and describe a systematic process for diluent selection when developing an HPLC assay of an active pharmaceeutical ingredient.

In this month's "LC Troubleshooting," we look at two questions submitted by readers that relate to mobile phase.

The established kinetic plots clearly show that monolithic columns are suited for large plate number separations whereas in the small plate number range the 3 ?m packed bed system is to be preferred.

Although there are other possible causes, the most common source of fronting peaks is a disturbance in the column packing bed. The fix is straightforward: replace the column.

A good pretreatment system combines reverse osmosis and electrodeionization technologies.

The authors look at three column performance problems encountered frequently in today's laboratory.