Application of capillary gas chromatography to automated on-line quantitative analysis of solvent vapors

Solvent vapors in air may be measured with capillary gas chromatographic columns. By using large diameter columns and sample loops of approximately the same internal diameter, the column may be connected directly to the gas sampling valve. This approach eliminates the use of a splitter or cryogenic trapping and allows low levels to be measured. By operating the column at high velocity, column efficiency is sacrificed for increase in speed.     

Improvement of the Detection Limit in Capillary Gas Chromatographic Trace Analysis of C2–C4 Hydrocarbons by Post Column Cryogenic Trapping

A method for the improvement of signal-to-noise ratios and hence detection limits in capillary gas chromatographic trace analysis of C₂–C₄ hydrocarbons was developed. It is based on the post column cryogenic trapping of the separated hydrocarbons using liquid nitrogen as coolant and its fast reinjection into the detector by rapid heating of the capillary tubing used for trapping. The improved signal-to-noise ratios were applied to decrease the sample volume and/or to lower the detection limit 10 to 27 times. The concentrations of these hydrocarbons in an urban air sample determined without and with post column cryogenic trapping were in good agreement but the precision was better when applying the cryogenic trapping mode.     

A new configuration for coupling purge-and-trap/cryogenic focusing/capillary column gas chromatography with splitless injection mode

A new configuration for coupling a purge-and-trap unit to a capillary column gas chromatograph via a cryogenic focusing interface has been developed. In this configuration, the precolumn of the cryogenic focusing interface was inserted through the septum of a split/splitless injection port where it served as both sample transfer and carrier gas supply lines. The injection port of the gas chromatograph was modified by plugging the carrier gas and the septum purge lines. This configuration allowed for the desorption of analytes at high flow rates while maintaining low, analytical-column flow rates which are necessary for optimum capillary column operation. The capillary column flow rate is still controlled by the column backpressure regulator. Chromatograms of purgeable aromatics exhibited improved resolution, especially for early eluting components compared to those obtained by direct liquid injection using the normal splitless injection mode. Quantitative sample transfer to the analytical column afforded excellent linearity and reproducibility of compounds studied.     

Dual Adsorber-Capillary Column System for Gas Chromatographic Analysis of Air Samples

Abstract

An interface which allows thermal desorption and subsequent capillary gas chromatographic analysis of air samples is described. A small solid-sorbent trap is positioned between the sampling tube and capillary column. A sample thermally released from the sampling tube is transferred by a carrier gas at high flow rate to the trap and retained. From there it is again thermally released and transferred to the capillary column by carrier gas at a low flow rate, as required by capillary GC. The transfer and injection steps are effected by means of externally placed solenoid valves. The performance of the system depends on the desorption temperature and time allowed for transfer of the sample between the two adsorbers and the column. These parameters are programmable and can be changed to suit the requirements of a particular analysis. The system allows the analysis of sub-parts-per-billion concentrations of organic compounds in a comparatively simple and reproducible manner. Operation of the system does not require cryogenic cooling of either the trap or the GC oven. Chromatograms of a variety of air samples are presented and discussed.     

Sensitive determination of styrene and related compounds in human body fluids by headspace capillary gas chromatography with cryogenic oven trapping

Summary A simple and sensitive method is presented for determination of styrene, toluene, ethylbenzene, isopropylbenzene andn-propylbenzene in human body fluids by capillary gas chromatography (GC) with cryogenic oven trapping. After heating a blood or urine sample containing each compound andp-diethylbenzene (internal standard, IS) in a 7.0-mL vial at 60°C for 20 min, 5 mL of headspace vapor was drawn into a glass syringe and injected into a GC. All vapor was introduced into an Rtx-Volatile middle bore capillary column in splitless mode at oven temperature of 20°C to trap entire analytes, and the oven temperature then programmed to 280°C for GC measurements by flame ionization detection. The present conditions gave sharp peaks of each compound and IS, and low background noises for whole blood or urine samples.     

Enhancement of Signal-to-Noise Ratios in Capillary Gas Chromatography by Using a Longitudinally Modulated Cryogenic System

A Longitudinally Modulated Cryogenic System (LMCS) was evaluated for its use in detection enhancement in capillary gas chromatography. The mechanism for chromatographic re-elution for the LMCS is substantially different to other cryogenic devices. The cooled region of the capillary column in which chromatographic bands may be focused is heated by the surrounding oven temperature either by moving the trap along the column, or by moving the column out of the trap. By continually modulating the LMCS at the detector end of the capillary column, signal-to-noise ratios of routine chromatograms can be readily increased by a factor of ten, thus enhancing chromatographic detection. Base widths of peaks, which are often about 2–3 s or more can be easily reduced to 0.3 s when the LMCS is employed in the detection enhancement mode, thus offering a simple avenue to improved peak height sensitivity in capillary gas chromatography.     

Headspace analysis with cryogenic focusing: A procedure for increasing the sensitivity of automated capillary headspace analysis

Summary A convenient procedure has been described for enhancing the sensitivity of equilibrium headspace gas chromatography. The technique involves the cryogenic focusing of headspace vapors at the head of a 0.32 mm fusedsilica capillary column which has been coupled to a packed column injection port. Cryofocusing is accomplished automatically by cooling the GC oven and results in enhanced sensitivity with improved resolution. With this simple focusing technique no sample splitting is required which gives at least a 10-fold increase in sensitivity. A Multiple Headspace Injection (MHI) technique has been described which allows several rapid headspace injections to be made at the start of a single chromatographic run. Cryofocusing causes the injections to be superimposed, thus increasing the sensitivity even further. These procedures have been demonstrated for flavor and aroma analyses of toothpaste and soft drink samples. Presented at the Symposium on Headspace GC, National American Chemical Society Meeting, Chicago, Ill., September 8–13, 1985     

On-line multidimensional chromatography using packed capillary liquid chromatography and capillary gas chromatography

The introduction of selected fractions from a liquid chromatograph into a gas chromatograph has been described; however, analyses were performed by off-line experiments requiring collection and reinjection of the separate fractions or by on-line procedures where disadvantageously, only a fraction of the separated peak or a well resolved component in a mixture could be introduced into a gas chromatograph. This disadvantage is overcome by the apparatus and method described in this paper, which utilizes a multidimensional chromatography system employing a high efficiency, packed capillary LC column coupled on-line to a capillary gas chromatograph. The liquid chromatograph (so designed) can act as a highly efficient clean-up or chemical class fractionation step prior to introduction into the gas chromatograph, significantly reducing sample preparation times in many applications. Thus minor components in a complex matrix can be determined without prior sample clean-up, an example of which is the determination of polychlorinated biphenyls in a complex hydrocarbon matrix.     

Studies on cryogenic trapping of solutes during chromatographic elution in capillary gas chromatography

Cryogenic trapping of solutes leads to narrowing of the chromatographic band. By placing the trap at the end of a capillary column, it is possible to study the effectiveness of the trap in terms of producing a sharpened elution profile. The trap may be heated by supplementary heating, but here convective heating from the GC oven is employed simply by turning off the cryogenic coolant. It is estimated that it takes about 50 s for the trap to heat up sufficiently to allow trapped solute to be remobilized, although this depends upon the oven temperature and thermal mass of the trap. It can also be shown that the more volatile solutes mobilize faster from the trap in this particular mode of operation. The recovery of trapped components shows that there is essentially quantitative trapping, and the solutes are trapped just at the leading edge of the trap.     

Time-resolved cryogenic modulation reveals isomer interconversion profiles in dynamic chromatography.

The dynamic chromatographic study of interconversion of E and Z forms of oximes has been investigated by using a novel cryogenic modulation method in a two-dimensional gas chromatographic array. The primary column is a conventional capillary GC column on which the molecular interconversion proceeds. In this case, the molecular dynamical process leads to a peak profile describing the kinetics and thermodynamics of the interconverting molecules during its chromatographic elution. Thus an interconversion region intercedes the elution of the individual stereoisomers of the reaction. Since the molecules are isomers, classical molecular identification methods such as gas chromatography-mass spectrometry are unable to study the individual instantaneous amounts of each of the compounds. Hence the infinitesimal profiles of interconversion along the entire column have never been experimentally observed; rather the total profile is normally subjected to mathematical modelling studies in order to match experiment with theory, and to gain the kinetic parameters of the process. In the present study, an instantaneous ratio of the individual isomers can be found during the chromatographic elution by direct measurement. This is achieved by using a cryogenic zone focussing process, with rapid longitudinal modulation of a cold trap and continual pulsing of collected zones into a fast-analysis high-resolution capillary column on which isomer interconversion is minimized. The data can be displayed as a two-dimensional contour plot to demonstrate the individual isomer profiles. The two-dimensional analysis also allows easy measurement of the peak ratios of the two isomers which is an indicator of the extent of interconversion that has taken place. Two model systems, acetaldoxime and butyraldoxime, were chosen to illustrate the use of the cryogenic modulation procedure. It is anticipated that the procedure could be applied to other molecules which exhibit gas-phase isomerizations or reactions.     

Some practical experiences in the use of a solventless injection system for packed column supercritical fluid chromatography

The operating characteristics of a solventless injector for packed column supercritical fluid chromatography are described. Successful operation depends on the difference between the volatilities of the analytes and the solvent or matrix being sufficient for their separation by gas purging in a thermostatted precolumn, and also on the existence of an effective re-focusing mechanism at the head of the analytical column for the sample dissolved in liquid or supercritical fluid carbon dioxide. The latter is easily achieved in density programmed operation by stationary phase trapping at low fluid densities or phase ratio trapping by changing the temperature at low to moderate fluid densities. The solventless injector can easily accommodate sample volumes from 1–100 μl and is easily adapted for in-line derivatization using reagents which can, after reaction, be eliminated by gas purging. For general purposes the solventless injector can be used to overcome most of the problems encountered when rotary valve loop injectors are used with small bore packed columns, and will probably replace this type of injector as the injector of choice for supercritical fluid chromatography with mobile phases of low polarity.     

Cold trapping of volatile organic compounds on fused silica capillary columns

A 30 m, 0.25 mm ID, fused silica capillary column at temperatures from –60 to –100°C has been shown to be a quantitative trap for organic compounds with volatilities ranging from that of 1, 1 -dichloroethene that of chlorobenzene. This type of “whole column cryotrapping” provided sharp peaks (peak width approximately 4–7 seconds) for all compounds at a trapping temperature of –80°C and with high carrier gas pressures and linear velocities (30 psi and 110 cm/s, respectively). Whole column cryotrapping possesses great simplicity, chromatographic efficiency (no trapping loop connections), and a built-in indicator of quality assurance for trapping efficiency (i.e., peak shape). These advantages are extremely attractive and are indicative of the fact that the potential of this approach has not yet been fully appreciated.     

Development of a Dual Capillary Column GC Method for the Trace Determination of C2–C9 Hydrocarbons in Ambient Air

An analytical method based on a dual capillary gas chromatographic technique combining the advantages of GasPro PLOT and a non polar narrow bore WCOT column was developed for the analysis of air samples containing C2–C9 NMHCs. A refocusing step was not required due to the fast heating rate of the sample preconcentration trap and the resolving power of the PLOT column for C2 and C3 NMHCs. Water had to be removed from the air samples to avoid plugging of the columns if the initial GC oven temperature was below ambient temperature. To dry air samples, a scrubber and a cryogenic technique were employed. The interferences caused by carbon dioxide were reduced by purging the loaded sample preconcentration trap with helium. The dual column system was compared to a method employing a refocusing device and a single narrow bore WCOT column. Both systems provided a high degree of precision. However, the dual column approach was superior to the single column system due to better resolution of low molecular weight components.     

Sensitive analysis of alkyl alcohols as decomposition products of alkyl nitrites in human whole blood and urine by headspace capillary GC with cryogenic oven trapping

The abuse of alkyl nitrites is becoming a serious social problem worldwide. In this report, a simple and sensitive method is presented for the determination of n-butyl alcohol, isobutyl alcohol, and isoamyl alcohol as decomposition products of alkyl nitrites in human whole blood and urine samples using capillary gas chromatography (GC) with cryogenic oven trapping. After heating a whole blood or urine sample containing each alkyl alcohol and t-butyl alcohol [the internal standard (IS)] in a 7-mL vial at 55 degrees C for 15 min, 5 mL of the headspace vapor is drawn into a gas-tight syringe and injected into a GC inlet port. The vapor is introduced into an Rtx-BAC2 medium-bore capillary column in the splitless mode at 0 degrees C oven temperature in order to trap the entire analytes, and then the oven temperature is programmed up to 240 degrees C for the GC measurements by flame ionization detection. These conditions give sharp peaks for each compound and the IS and low background noise for whole blood or urine samples. The detection limits of the analytes are 10 ng/mL for whole blood and 5 ng/mL for urine. Linearity and precision are also tested to confirm the reliability of this method. Isobutyl alcohol and methemoglobin could be determined from the whole blood samples of three male volunteers who had sniffed isobutyl nitrite.     

Applications of heart cutting from packed to capillary columns

The power of capillary gas chromatography can be enhanced by selective fractionation of the sample. One way of doing this is heart cutting from a packed gas chromatographic column. This paper describes modification of a commercial instrument for such separations. Components of the system include an automated Deans switch and an electrically heated trap. The benefits of this arrangement are illustrated with chromatograms of naphtha, urinary aromatic acids, and wine volatiles.     

变径毛细管柱GC／FTIR联用技术的研究

本文介绍了使用新型变径毛细管柱的气相色谱/傅里叶变换红外光谱联用技术(GC/FTIR)。以正构烷烃的混合物为样品,研究了载气速流、色谱进样量及柱后死体积对联用效果的影响。分别在变径毛细管柱GC/FTIR和普通毛细管柱GC/FTIR联用系统上分析了烷烃芳烃混合物和合成汽油样品,并利用软件系统提供的程序和红外标准谱库对所得光谱进行了检索。实验结果表明,变径毛细管柱GC/FTIR系统具有色谱进样量大,可鉴定组分多,分离能力强和检测灵敏度高等显著优点。     

Automatic system for rapid analysis of volatile compounds by purge-and-cold-trapping/capillary gas chromatography

A system for automatic analysis of volatile compounds by purge-and-cold-trapping/capillary gas chromatography is described. It is suitable for analysis of volatile compounds in a wide variety of samples, such as water, food products and environmental samples. Possibilities and limitations of the system are evaluated in relation to several parameters. The efficiency of different types of cryogenic trap (open tubular, coated, packed) is also investigated; it depends on purge-flow rate, temperature of trapping, and total purge volume. Examples of the analysis of volatile compounds in foods and water are given.     

Use of a recently developed thermal modulator within the context of comprehensive two‐dimensional gas chromatography combined with time‐of‐flight mass spectrometry: Gas flow optimization aspects

The present research is based on the use of a recently developed comprehensive two‐dimensional gas chromatography thermal modulator, which is defined as solid‐state modulator. The transfer device was installed on top of a single gas chromatography oven, while benchtop low‐resolution time‐of‐flight mass spectrometry was used to monitor the compounds exiting the second analytical column. The solid‐state modulator was first described by Luong et al. in 2016, and it is a moving modulator that does not require heating and cooling gases to generate comprehensive two‐dimensional gas chromatography data. The accumulation and remobilization steps occur on a trapping capillary, this being subjected to thermoelectric cooling and micathermic heating. In this study, the effects of the gas linear velocity on the modulation performance were evaluated by using two different uncoated trapping capillaries, viz., 0.8 m × 0.25 mm id and 0.8 m × 0.20 mm id. Solid‐state modulator applications were carried out on a standard solution containing n‐alkanes (C_9, C_10, C₁₂), and on a sample of diesel fuel. The results indicated that the type of trapping capillary and gas velocity have a profound effect on modulation efficiency.     

Temperature requirements for thermal modulation in comprehensive two-dimensional gas chromatography

Temperature requirements for trapping and release of compounds in a cryogenic gas loop-type GC x GC modulator were determined. Maximum trapping temperatures on the uncoated, deactivated modulator capillary were determined for compounds from C4 (bp -0.5 degrees C) to C40 (bp 522 degrees C). The liquid-nitrogen cooled gas flow rate was reduced from a high of 15.5 to 1.5 SLPM over the range to achieve the required trapping temperature. Excessive cold jet flow rates caused irreversible trapping and peak tailing for semi-volatile compounds above C26. Alternate cold jet coolants were investigated. An ice water-cooled jet was able to trap compounds with boiling points from C18 (bp 316 degrees C) to C40 and a room temperature air-cooled jet was able to trap compounds from C20 (bp 344 degrees C) to C40. The hot jet produced launch temperatures approximately 40 degrees C hotter than the elution temperature with heating time constants of 8 to 27 ms. Modulated compound peaks were symmetrical with half-height peak widths of 43 to 56 ms for compounds with little second column retention, and 70 to 75 ms for compounds with more second column retention. The liquid nitrogen-cooled loop modulator with gas flow programming was used to produce a GC x GC chromatogram for a crude oil that contained compounds from C7 to C47.     

Micropreparative system for enrichment of capillary GC effluents

An automated system for preparative gas chromatography with capillary columns is described. The effluet from the capillary column is switched to the FID detector or to the traps by means of a Live-T switching device. The pneumatics is controlled by a microprocessor so that repetitive sampling can be performed over a period of days in order to enrich sufficient amount of material for NMR or other spectroscopic methods. The effluent containing the compounds is collected in glass tubes filled with column packing material (e.g. Chromosorb coated with 3% OV - 101, crosslinked). The trap temperatue can be adjusted from + 20°C to ? 80°C, depending on the trapping material and volatility of trapped substances. The analysis of enriched substances or chromatographic fractions can be performed by thermal desorption of the same traps or by solven elution. The recovery of enriched substances is higher than 90%. High capacity and resolution for enrichment of trace components are obtained with the aid of a double column-double oven system. Examples of such applications are given.