GC Injection systems

1 GC Injection systems. Many specialised injection systems are used with the GC in the modern laboratory to deliver the complete sample to the column without alteration. The conventional injection method is simply to inject a small volume (about 1 pL) of a dilution of the sample using a syringe with a fine needle, which pierces a silicon rubber septum and delivers the sample into a heated chamber where it is vaporised and carried on to the column in the stream of the carrier gas. Some of the carrier gas-containing vaporised samples may be split from the main column flow and vented to reduce the amount of sample 

The high temperature of the injector, typically 250 °C, means that this method is not suitable for analytes that are subject to thermal degradation. For these materials an on-column method is preferable, in which the solution of the sample is injected directly into the narrow capillary column with a fine needle. On-column injection techniques are also more suitable for extremely dilute samples, as more material is delivered to the column, but are less suitable for dirty samples containing non-volatile contaminants, which accumulate on the column. 

The use of a headspace-injection technique may be preferable if the analytes are contained within a non-volatile or corrosive matrix that cannot be injected directly (e.g. perfume in a washing powder). The basic principle of headspace injection is the delivery of a volume of vapour from the space above the sample material to the GC column. This can be achieved in several ways 

Only materials volatile at the sampling temperature are transferred to the GC. Therefore, most headspace injection systems include some means of either gently heating the vial containing the sample or bubbling a gas through the non-volatile liquid to purge the volatiles, which can then be 

If the target analytes do not readily evaporate from the matrix they are in, then another recently improved technique - direct thermal desorption - is more effective. This is often used for dry, non-volatile matrices like wood, soil, spices or resins. The sample is placed directly in the liner or desorption chamber, which is then flushed with inert carrier gas and heated rapidly to transfer the volatiles to the analytical column. 

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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. 

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]. 

Gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) analyses GC analyses are performed on a WCOT (wall-coated open tubular) polydimethylsiloxane fused silica capillary column (1 fim film thickness, 50m x 0 32mm ID ) Shorter columns for example 25m, or columns with thinner films, for example 0 2/im, may also be used successfully Injection of the silylated sample (2/d) is performed with a moving needle-type injector, other types of capillary GC injection systems may be used after optimization of the concentration and amount of sample to be injected With a 50-m capillary... 

Vacuum systems are occasionally used for sample concentration [50,51]. While vacuum systems may be used routinely, they are particularly suited to aroma isolates containing relatively higher boiling and heat sensitive components. More recently, large volume GC injection systems have minimized the need for concentration [52]. 

Important to environmental analysis is the ability to automate the injection, as weU as the identification and quantitation of large numbers of samples. Gc/ms systems having automatic injectors and computerized controllers have this capabiUty, even producing a final report in an unattended manner. Confirmation and quantitation are accompHshed by extracting a specific ion for each of the target compounds. Further confirmation can be obtained by examining the full scan mass spectmm. 

SPME has been utilized for deterrnination of pollutants in aqueous solution by the adsorption of analyte onto stationary-phase coated fused-siUca fibers, followed by thermal desorption in the injection system of a capillary gas chromatograph (34). EuU automation can be achieved using an autosampler. Eiber coated with 7- and 100-p.m film thickness and a nitrogen—phosphoms flame thermionic detector were used to evaluate the adsorption and desorption of four j -triazines. The gc peaks resulting from desorption of fibers were shown to be comparable to those obtained using manual 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... 

There are several types of sample introduction systems available for GC analysis. These include gas sampling valves, split and splitless injectors, on-column injection systems, programmed-temperature injectors, and concentrating devices. The sample introduction device used depends on the application. 

Cool the reaction mixture and inject 1-2 fA into the GC/MS system. TBDMS Procedure... 

To aid in determining the number of carboxyl groups, prepare a derivative using trideuterated Methyl-8 (Pierce cat. no. 49200), using the same procedure previously given. Inject 1-2 pd of the trideuterated methyl ester separately or mix equal portions of the nondeuterated methyl ester with the trideuterated methyl ester, and inject 2 pd immediately into the GC/MS system. From the mass difference, it is easy to determine the number of carboxyl groups present. 

Inject 1-2 fA chloroform or methylene chloride (bottom layer) into the GC/MS system. 

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). 

Flumetralin was extracted from tobacco using Soxhlet extraction. A 5-g amount of Florisil (5% deactivated) was transferred directly on to the filter disk of a Soxhlet extractor followed by another 5 g of Florisil mixed with 5 g of ground tobacco sample as an upper layer. A 60-mL volume of hexane and 3mL of a 4 agmL internal standard solution were placed in a 250-mL round-bottom flask prior to attaching the Soxhlet extractor. The unit was placed on a heating mantle and the hexane was refluxed through the extractor at the rate of about 250 mLh for 4.5 h. After cooling, 0.5 pL of the extract was injected directly into a GC/FCD or GC/MS system. 

Inject an aliquot of the GC-ready sample solution into the GC/MSD or GC/FTD system. 

Inject the cleaned-up sample into the GC/MSD or GC/FTD system operated under the same conditions as employed for standardization. Compare the peak areas of the analytical samples with the calibration curve. Determine the concentrations of pyraflufen-ethyl, E-15 (E-1), E-16 (E-2) and E-3 present in the sample. 

Inject the cleaned-up sample into the GC/NPD system operated under the same conditions as used for standardization. 

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