Carel A. Cramers

Microcolumn liquid chromatography: Instrumentation, detection and applications

This review discusses many different aspects of microcolumn liquid chromatography (LC) and reflects the areas of microcolumn LC research interest over the past decades. A brief theoretical discussion on a number of major issues, such as column characterisation, chromatographic dilution effects and extracolumn band broadening in microcolumn LC is given. Recent progress in column technology and the demands and developments of instrumentation and accessories for microcolumn LC are also reviewed. Besides that, the developments in a large number of established and also more recent detection techniques for microcolumn LC are also discussed. The potential of hyphenation of microcolumn LC with other techniques, more particularly of multidimensional chromatography and microcolumn LC coupled to mass spectrometry is reviewed. Finally, the perspective of microcolumn LC separation methods is stressed by a number of relevant applications.

Group-Type Identification of Oil Samples Using Comprehensive Two- Dimensional Gas Chromatography Coupled to a Time-of-Flight Mass Spectrometer (GC×GC-TOF)

In this work a comprehensive two-dimensional system (GC x GC) was coupled to a time-of-flight mass spectrometer (TOF/MS) for the analysis of oil samples. Group-types like the alkanes and saturated cyclic compounds (naphthenes), which are present in oil, are shown separately by selecting their unique masses. On selecting appropriate ion fragments, this method also permits the determination of sulfur-and oxygen-containing species in oil. Former results obtained by FID detection could be confirmed. After proper selection of unique ions in GC x GC-TOF both selectivity and sensitivity increase.

Trace level analysis of micropollutants in aqueous samples using gas chromatography with on-line sample enrichment and large volume injection

In this review article an overview of methods for the analysis of aqueous samples with capillary gas chromatography is presented. The methods can be classified into two approaches: a direct approach in which water is directly introduced onto the GC column, and an indirect approach in which water is eliminated, i.e., by liquid-liquid extraction or solid-phase extraction, prior to introduction of the analytes onto the GC column. The emphasis is on methods in which sample preparation and GC analysis are combined on-line. An outline of methods for large volume injection is also presented as the use of on-line systems often involves the introduction of large volumes of sample extract.

Optimization of temperature-programmed gas chromatographic separations I. Prediction of retention times and peak widths from retention indices

A numerical method is described to predict retention times and peak widths of a mixture containing components with known identities in capillary gas chromatography. The procedure is based on extracting thermodynamic values (enthalpy and entropy terms) from Kovats retention indices. Next, a numerical procedure is developed that uses these data to calculate retention times and peak widths on any capillary column containing the same stationary phase but with a different phase ratio. The estimations are based on a sound theoretical basis. The predictions can be performed either in the isothermal or temperature-programmed (single- or multi-ramp) mode. In the temperature programs, which cover a broad temperature range, isothermal plateaus are allowed. Errors in the predictions of retention times are generally less than 4%. Prediction of peak widths under the same conditions can be performed with errors of about 10%. An attractive feature of the approach is, that once the thermodynamic values of the solutes of interest are known, future optimizations can be performed without the need to perform experimental input runs. This indicates that the concept can be used for complete off-line simulations and/or optimizations of gas chromatographic separations.

HPLC-FID with superheated water as the eluent: improved methods and instrumentation

In the current study, an HPLC-FID system using superheated water as the eluent is constructed and further improved. Signal stability during temperature programmed operation was improved by using separate thermostatting of the 50 μm capillary restrictor. Operating the restrictor at 75°C prevented the superheated water from sputtering in the restrictor. Conventional octadecyl-modified silica, thermostable polymeric reversed phase and carbon type HPLC columns were used. The feasibility of the system is demonstrated by the analysis of a variety of compounds such as alcohols and aldehydes. It is shown that increased temperatures decrease elution times and, in addition to this, frequently improve peak shapes and column efficiencies. The separation of lower aldehydes is performed at 175°C with a detection limit in the low μg/ml range. The possibility of temperature programming is demonstrated for the separation of a range of alcohols. Furthermore, it is shown that temperature programming can be used for sample enrichment. Large volume injection with low temperature enrichment resulted in good peak shapes and a significant improvement of the detection limit. In this way, detection limits of 0.2 μg/ml (ppm) could be obtained for the aldehydes studied. Finally, the possibilities of using mobile phase additives as, e.g., buffers were briefly examined.

Retention model for sorptive extraction-thermal desorption of aqueous samples : application to the automated analysis of pesticides and polyaromatic hydrocarbons in water samples

In this report, an automated method for sorptive enrichment of aqueous samples is presented. It is based on sorption of the analytes of interest into a packed bed containing 100% polydimethylsiloxane (PDMS) particles followed by thermal desorption for complete transfer of the enriched solutes onto the GC column. Compared to other solvent-less sample preparation techniques for water samples, several improvements can be noted of which the most obvious are an enhanced sensitivity and improved blanks. Moreover, degradation products formed from the PDMS material can easily be identified with the use of a mass spectrometric detector. As these products contain silicone, they do not interfere with the target solutes (pesticides, PAHs). In this report a theory model is derived which allows calculation of breakthrough volumes from octanol–water partitioning coefficients (KO/W). Alternatively, the KO/W value required for complete retention can be calculated using only the sample volume and trap specific parameters. For a sample volume of 10 ml, theory predicts a required log KO/W of 1.77 for the trap used here which was found to be in good agreement with experimental results. For the most apolar solutes, with a log KO/W in excess of seven, poor recoveries were found. This is most likely due to adsorption of these apolar solutes in the system. With the current set-up, detection limits are in the order of 10 ng/l using mass spectrometric detection in the full scan mode.

Use of an open-tubular trapping column as phase-switching interface in on-line coupled reversed-phase liquid chromatography—capillary gas chromatography

Abstract The applicability of open-tubular traps for phase switching in coupled RPLC—GC was studied. The phase-switching process involves sorption of the analytes of interest from a methanol—water mobile phase into the stationary phase of an open-tubular column, removal of the aqueous phase by purging the trap with nitrogen and desorption of the analytes with hexane. Water elimination carried out in this manner appears to be highly efficient. In the sorption step the sampling flow-rate and the capacity factors of the analytes in the trap are critical parameters. Using a 2 m × 0.32 mm I.D. trap with a swollen 5-μm stationary phase at flow-rates not exceeding 100 μl/min, polycyclic aromatic hydrocarbons are trapped quantitatively from 300 μl of aqueous phases containing up to 65% (v/v) of methanol. For desorption 70–125 μl of hexane are needed. These volumes are easy to handle in solvent elimination carried out using a PTV injector prior to transfer of the analytes to a GC column.

Possibilities and limitations of fast temperature programming as a route towards fast GC

One possible way to speed up a gas chromatographic analysis is the application of fast temperature programming by using resistive heating techniques. With this heating technique programming rates up to 20° per second can be reached. A relative standard deviation of retention times better than 0.2% is obtained. Using fast temperature programming the analysis-times of a mineral oil sample, an industrial oligomer sample, and toxic compounds in diesel fuel have been reduced 5 to 20 times, compared to a standard temperature programmed analysis. In most cases resistive heating cannot be applied to reduce the analysis time of a complex sample. The use of fast temperature programming is preferable to the use of short columns and columns operated at above-optimum carrier gas velocities.

Sorption Tubes Packed with Polydimethylsiloxane: A New and Promising Technique for the Preconcentration of Volatiles and Semi-Volatiles from Air and Gaseous Samples

This paper is concerned with the determination of volatile and semi-volatile organic components in air and gaseous (headspace) samples, focusing primarily on polar analytes. Samples were analyzed by preconcentration on different (ad)sorbents followed by thermal desorption and analysis by capillary gas chromatography. The performance of a cartridge filled with 100% polydimethylsiloxane (PDMS) particles was compared to that of adsorbents like Tenax TA and Carbotrap 300. Though the PDMS phase is non-polar it showed adequate retention for both polar and non-polar components. The blank runs of the PDMS trap were significantly better than those of most adsorbents and did not deteriorate, as was the case with all the conventional adsorbents investigated. With respect to trapping efficiencies, the PDMS phase performed better for most of the analytes under investigation compared to the adsorbents. For a range of analytes including triethylamine, butanone, diacetyl, nicotine, and acetic acid the PDMS phase performed exceptionally well whereas all adsorbents showed unsatisfactory performance. The packed PDMS traps were employed for the determination of organic acids, PAHs and nitro-PAHs in air and for the analysis of the headspace of cacao and hop.

Large-volume injection in gas chromatographic trace analysis using temperature-programmable (PTV) injectors

The use of programmed-temperature vaporising (PTV) injectors for large-volume injection in capillary gas chromatography is briefly reviewed. The principles and optimisation of large-volume PTV injection are discussed. Guidelines are given for selection of the PTV conditions and injection mode for specific applications. Relevant examples from the recent scientific literature serve as illustrations.