GC injection

High performance liquid chromatography (HPLC) is an excellent technique for sample preseparation prior to GC injection since the separation efficiency is high, analysis time is short, and method development is easy. An LC-GC system could be fully automated and the selectivity characteristics of both the mobile and stationary... 

An on-line supercritical fluid chromatography-capillary gas chromatography (SFC-GC) technique has been demonstrated for the direct transfer of SFC fractions from a packed column SFC system to a GC system. This technique has been applied in the analysis of industrial samples such as aviation fuel (24). This type of coupled technique is sometimes more advantageous than the traditional LC-GC coupled technique since SFC is compatible with GC, because most supercritical fluids decompress into gases at GC conditions and are not detected by flame-ionization detection. The use of solvent evaporation techniques are not necessary. SFC, in the same way as LC, can be used to preseparate a sample into classes of compounds where the individual components can then be analyzed and quantified by GC. The supercritical fluid sample effluent is decompressed through a restrictor directly into a capillary GC injection port. In addition, this technique allows selective or multi-step heart-cutting of various sample peaks as they elute from the supercritical fluid... 

To determine secondary alkanesulfonates in sewage wastewaters, solid phase extraction (SPE) and a single-step procedure which combines elution and injection port derivatization for analysis with GC-MS were developed [36]. Again a tetrabutylammonium ion pair reagent was employed both to elute the secondary alkanesulfonates as their ion pairs from CI8-bonded silica disks and to derivatize sulfonate ion pairs under GC injection port conditions. Secondary alkanesulfonates were effectively recovered from samples of raw sewage (>92%) and from primary (>98%) and secondary (>85%) effluents. No... 

Direct GC Injection of Amines. Morpholine, 2,6-dimethyl-morpholine, or pyrrolidine in methanol (2.6 to 2.8 g/ml) solutions were injected directly into the GC inlet at 210 C. Portions of these solutions were exposed to ultraviolet radiation (366 nm) for 16 hours prior to injection. 

The concentrations of nitrosamines were reduced to undetectable levels by ultraviolet treatment of the amine solutions and were not increased by addition of 2 ppm NaN02> indicating that the nitrosamines were present originally in the amines and were not formed in the GC injection port. Similar concentrations were found when the amine samples were analyzed using the column extraction method. Direct injection is appropriate for analysis of relatively simple mixtures, if adequate precautions are taken ( ), but can result in significant artifact formation in more complex systems (42). 

As the sensitivity and selectivity of the above GC/MS methods are for many analytes around 1 pg injected into the GC system, cleanup by SiOa fractionation can be omitted when larger sample sizes (25-100 g) are possible. For difficult dry (e.g. hops, pharmaceutical herbs) or oily (e.g. rape seed, fat, liver) materials which start with smaller sample sizes (5-10 g) and tend to overload the chromatographic cleanup systems, however, cleanup is still an important requirement as the GC injection system is vulnerable when the ratio of co-extracted material to analyte is too high. 

For a set of 10 samples, the analytical method can be completed within 16 laboratory hours from the time of sample weighing to GC injection. 

There are basically three methods of liquid sampling in GC direct sampling, solid-phase extraction and liquid extraction. The traditional method of treating liquid samples prior to GC injection is liquid-liquid extraction (LLE), but several alternative methods, which reduce or eliminate the use of solvents, are preferred nowadays, such as static and dynamic headspace (DHS) for volatile compounds and supercritical fluid extraction (SFE) and solid-phase extraction (SPE) for semivolatiles. The method chosen depends on concentration and nature of the substances of interest that are present in the liquid. Direct sampling is used when the substances to be assayed are major components of the liquid. The other two extraction procedures are used when the pertinent solutes are present in very low concentration. Modem automated on-line SPE-GC-MS is configured either for at-column conditions or rapid large-volume injection (RLVI). 

Cold split Column independent 100-fold increase in sensitivity with respect to conventional GC injection Reproducible and accurate sampling Controlled evaporation No discrimination on basis of b.p. Rapid transfer of sample to column Low volume of CIS liners (2-3 xL)... 

Principles and Characteristics Although early published methods using SPE for sample preparation avoided use of GC because of the reported lack of cleanliness of the extraction device, SPE-GC is now a mature technique. Off-line SPE-GC is well documented [62,63] but less attractive, mainly in terms of analyte detectability (only an aliquot of the extract is injected into the chromatograph), precision, miniaturisation and automation, and solvent consumption. The interface of SPE with GC consists of a transfer capillary introduced into a retention gap via an on-column injector. Automated SPE may be interfaced to GC-MS using a PTV injector for large-volume injection [64]. LVI actually is the basic and critical step in any SPE-to-GC transfer of analytes. Suitable solvents for LVI-GC include pentane, hexane, methyl- and ethylacetate, and diethyl or methyl-f-butyl ether. Large-volume PTV permits injection of some 100 iL of sample extract, a 100-fold increase compared to conventional GC injection. Consequently, detection limits can be improved by a factor of 100, without... 

GC injection port) may well be. This was clearly demonstrated by a comparison between cryotrapping/direct injection, cryotrapping/SPME, and solid-phase (Tenax-GC) extraction for sampling of odorous sulfur compounds [71], Thermally labile compounds are likely to break down in the GC injection port/column/transfer line. Instead of SPME-GC, the recently developed SPME-HPLC [72] might be more applicable to analysis of such thermally unstable compounds. 

Figure 11.1 shows the pyrogram of lead white pigmented linseed oil paint obtained at 610 °C with a Curie-point pyrolyser, with on-line methylation using 2.5% methanolic TMAH. The pyrolyser was a Curie-point pyrolysis system FOM 5-LX, specifically developed at FOM Amolf Institute (Amsterdam, the Netherlands), to reduce cold spots to a minimum. This means that the sample can be flushed before pyrolysis in a cold zone, and it also ensures optimum pressure condition within the pyrolysis chamber, thus guaranteeing an efficient transport to the GC injection system [12]. 

Riggin and Howard (1982) reported that 1,2-diphenylhydrazine "...instantaneously decomposes to azobenzene in the GC injection port." The authors further stated that ... 

Finally, a few miscellaneous compounds which were identified in the Delaware River and which have not been previously reported as water contaminants will be discussed Chloro (trifluoromethyl) aniline and chloro (trifluoromethyl) nitrobenzene (no. 55 and 56) were identified in the water, they had maximum concentrations at river mile 78. Both compounds represent common sub-structures in various pesticide and dye molecules, and several of the companies located along the river have patents using these compounds (30-32j. It is possible that these compounds are actually present in the river water as such, but it is also possible that they are formed in the GC injection port by pyrolytic degradation of larger pesticide or dye molecules (see above). All three binaphthyl-sulfone isomers (no. 92) were identified in the river water near Philadelphia. Product literature for one of the companies in the area indicates production of condensed sulfonated polymers derived from naphthalene sulfonic acid and maleic anhydride. It seems likely that the binaphthylsulfones are formed as by-products during preparation of this commercial product. 

Cortes, J. M. Sanchez, R. Diaz-Plaza, E. M. Villen, J. Vazquez, A. Large Volume GC Injection for the Analysis of Organophosphorus Pesticides in Vegetables using the Through Oven Transfer Adsorption Desorption (TOTAD) Interface. J. Agric. Food Chem. 2006,54, 1997-2002. 

Argon is analyzed by mass spectrometry (characteristic ion m/z 40) or by gas-solid chromatography. Its concentration can be increased by several times by selective adsorption over a suitable adsorbent followed by thermal desorption of the gas onto the GC injection port. 

After cooling the vials to room temperature, add 2 ml hexane, close the vial again and vortex for 10 s. Transfer the upper (hexane) layer to a glass tube with a cone bottom and carefully evaporate the hexane with a gentle stream of nitrogen at room temperature. Finally, dissolve the residue in 100 pi (plasma) or 80 pi (erythrocytes) of hexane and transfer the sample to a GC injection vial with a crimp cap or a screw cap. Inject 1 pi into the GC. 

The same statistical procedures were used here to evaluate the effects of humics on batch and continuous LLE. A base extraction procedure (19) was required for processing methylene chloride extracts prior to GC injection in order to protect the GC column from contamination by humics. This process led to losses of 2,4-dichlorophenol and the chlorinated biphenyls. Therefore, these compounds were not used in the evaluation of the CLLE in the presence of humics. All other compounds were not affected by the base extraction procedure. The ANOV procedure tested each compound for changes in concentration by comparing early batch extraction recoveries (from freshly prepared solution) to later ones (after the 12.5-L extraction). This process was done separately for Parts 1 and 2. It was therefore possible to test each compound for time-dependent decreasing concentration with and without the presence of humics. 

The TMS ether derivative should be kept in a desiccator until analyzed because it is very susceptible to moisture and hydrolyzes easily. Excess amounts of pyridine usually result in a large and tailing peak on a chromatogram and interfere with the baseline separation of the cholesterol peak. It is therefore recommended to evaporate pyridine under a nitrogen stream and to dissolve the residue in n-hexane. Excess silylating agents usually produce a white residue in the resulting n-hexane solution. Microfiltration prior to GC injection is recommended. 

Headspace SPME is a solventless extraction method where a silica fiber coated with adsorbant or absorbant polymer material is exposed to a gas phase to extract analytes. The food of interest is placed in a closed or open container (such as a mouth simulator). After extraction, the fiber is desorbed in a GC injection port for separation and detection of the extracted analytes. 

The fiber is always fully exposed during desorption in the GC injection port. Fractional exposure can be consistently achieved by either drilling a notch in the injection holder or by restricting the plunger with an O ring. The amount exposed must be measured from the... 

SPME is a sample-preparation technique based on absorption that is useful for extraction and concentration of analytes either by submersion in a liquid phase or exposure to a gaseous phase (Belardi and Pawliszyn, 1989 Arthur et al., 1992). Following exposure of the fiber to the sample, absorbed analytes can be thermally desorbed in a conventional GC injection port. The fiber behaves as a liquid solvent that selectively extracts analytes, with more polar fibers having a greater affinity for polar analytes. Headspace extraction from equilibrium is based on partition coefficients of individual compounds between the food and headspace and between the headspace and the fiber coat-... 

Desorption depth in the GC injection port only needs to be determined once for the specific GC. For optimum desorption, the center of the SPME fiber should be injected to the hottest spot in the injection port. This can be determined by using expensive accessories commercially available from GC manufacturers, or by extracting a simple standard sample and injecting the SPME fiber at different depths. The depth producing the sharpest, most intense peak is the ideal injection depth. By setting the O ring at the desired point (as shown in Figure Gl.6.1), the injection depth can be maintained consistently. 

Techniques of chromatographic analysis continue to develop and for up-to-date methods, the specialist literature should be consulted [62, 63]. In all cases, reaction samples have to be taken at known time intervals and quenched by an appropriate method (sudden cooling, change of pH, dilution, etc.) before chromatographic analysis. It is important to check the stability of the reaction component to the chromatographic and work-up conditions. For example, are the compounds to be analysed thermally stable to the GC conditions (Conditions inside a GC injection port and, indeed, within the column are not unlike those of a heterogeneous catalytic reactor ) Are they stable to the pH of the HPLC eluent An obvious restriction is that chromatographic component analysis does not lend itself to the study of fast reactions. 

Figure 18. Left Temperature-programmed preparative-scale separation of racemic enflurane by GC. Injected amount 30 /A, 8.4 cm/s helium. Bight Over-loading experiment at 40°C and 8.4 cm/s helium (Schurig and GTosenick, 1994).

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