GC, injection techniques

Table 4 GC injection techniques applied in pesticide residue analysis...

It is my impression that GC injection techniques are still far from being optimized to... 

Capillary GC injection technique for volatile compounds in headspace, purge and trap or thermodesorption systems. Instead of an... 

GC injection technique, also known as sandwich technique, whereby a liquid (solvent, derivatization agent) is first drawn up into the syringe, followed by some air to act as a barrier and finally the sample, Sandwich technique. 

GC injection technique using the PTV injector so that larger quantities of diluted extracts can be apphed from more than 2 pL up to a LC-GC coupling. The excess of solvent is evaporated through the spht line while analytes are concentrated in the inlet liner. A large difference between the boiling points (volatility) of the solvent and analytes is required. 

GC injection technique whereby the entire injection volume reaches the column with or without evaporation, splitless injection. 

Special focus has been set to the widely used and popular sample preparation methods as there are the pressurized liquid extraction (RLE), the thermal extraction of materials and food (outgassing), and in detail on the QuEChERS pesticide sample preparation used for GC-MS and LC-MS. As a consequence of the higher matrix load of these extracts and the recommended GC injection techniques, concurrent backflush, preventive maintenance and inlet deactivation became current topics of discussion. Olfactometry solutions have been added for apphcations in flavour analysis. 

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... 

Numerous types of GC injectors have been manufactured over the past four decades. The most commonly used injection techniques have been reviewed and described by Grob, who correctly states that analysts must fully understand the techniques before they can make the most appropriate choice for their particular application(s). For most GC capillary column applications, the split/splitless, programmed-temperature vaporization (PTV) and on-column injectors remain the most popular. However, over the last few years, technology has progressed rapidly to provide injectors that allow more of the sample extract on to the GC column without overloading it. 

Many of the classical techniques used in the preparation of samples for chromatography are labour-intensive, cumbersome, and prone to sample loss caused by multistep manual manipulations. During the past few years, miniaturisation has become a dominant trend in analytical chemistry. At the same time, work in GC and UPLC has focused on improved injection techniques and on increasing speed, sensitivity and efficiency. Separation times for both techniques are now measured in minutes. Miniaturised sample preparation techniques in combination with state-of-the-art analytical instrumentation result in faster analysis, higher sample throughput, lower solvent consumption, less manpower in sample preparation, while maintaining or even improving limits. 

Hinshaw [91] has addressed the problem of selection of the appropriate injection technique. Hundreds of liners are available per GC brand and application range. The most important injection techniques are listed in Table 4.16. 

As GC is not only used as a separation medium but also as an analytical technique detection has an important function. Even if the column tolerates high-solute levels, detector requirements may determine the best injection technique or they may dictate adding a sample dilution step before injection to bring injected quantities within the optimal operating range. GC instruments accommodate an extremely wide range of solute concentrations. Minimum and maximum solute... 

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... 

Table 7.31 Overview of direct injection techniques used in GC-MS analysis...

Various authors have described on-line LC-SFC coupling [947,948]. Coupling of LC to SFC with conventional-size LC columns, where only a small fraction of the peak of interest is transferred to the SFC, allows only for qualitative results, and does not address the need for improved sensitivity in cSFC. Cortes et al. [948] have described relatively large-volume sample introductions (>10 xL) into cSFC, using microcolumn LC in the first dimension. LVI-LC-cSFC provides enhanced sensitivity compared with conventional cSFC injection techniques. LC-cSFC is expected to be of utility in the characterisation of complex samples, and in the determination of components which are thermally labile do not contain significant chromophores or do not have sufficient volatility to be analysed by GC. 

There are three injection techniques for introducing a sample into a GC equipped with a capillary column split injection, splitless injection, and on-column injection. Split injection is the most often used injection technique. When a certain amount of FAME sample (1 to 3 ll) is introduced into the GC injector that is normally set at a temperature much higher than the boiling point of the solvent, the solvent vaporizes instantly in the carrier gas and creates a large volume of gas that contains all of the injected FAME in it. The carrier gas that contains the FAME is then divided into two streams from the injector one is directed onto the column, and the second is vented to the atmosphere, clearing the sample out of the injection chamber momentarily. This way, only a limited amount of sample is introduced into the column, to avoid column overloading, and injection time is short, to avoid peak broadening. 

Dilute samples can also be introduced into the GC system by on-column injection however, this technique requires special syringes or other devices and therefore will not be disci xp I in detail. Split or splitless injection, on the other hand, can be performed in a GC system equipped with a split/splitless injector without the need for additional devices. Investigators can simply change the control of the split/splitless valve and the oven temperature for the column to choose between the two injection techniques. 

Infant formula, tocopherol/tocotrienol analysis, 479, 487, 489 (table) Infrared spectroscopy, see also Fourier transform infrared spectroscopy fat measurement, 572 trans fatty acids, 505 Infrared/ultrasonic scanner for emulsion creaming, 597-598 Injection techniques for GC, 449 Insoluble recombinant proteins, purification of, 276... 

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-... 

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. 

Calibration standards can be of two types external standards and internal standards. With external standards, multiple concentrations of the standards are injected, areas are measured, and a calibration curve is platted. Unknown samples are then injected, chromatograms run, and areas are calculated and compared with the calibration curves to determine amounts of each compound present. With internal standards, known amounts of an internal standard are added to each known concentration of standard compound and areas or peak height response factors relative to those of the internal standard are calculated. When unknowns are run, a known amount of internal standard is added to the unknown sample, response factors are calculated relative to the internal standards, and amounts of each unknown present are calculated from the standards calibration factors. Internal standards are usually used to correct for variations in injection size due to different operators and injection techniques. Internal standards can also be used to correct for extraction variation in GC/MS target compound quantitation, this standard is referred to as a surrogate standard. Generally, an internal standard is used for one purpose or the other, not both at the same time. 

Chlorinated dioxins and dibenzofurans are best analyzed by GC/MS techniques, using both low- and high-resolution mass spectrometry. A measured amount of sample is extracted with a suitable solvent. The solvent extract containing the analytes is concentrated down to a small volume and then subjected to cleanup for the removal of interferences. The extract is injected onto the GC... 

Classical liquid-liquid and liquid-solid extractions are recently receiving additional examination, as new injection techniques for GC have made very simple, low-volume extractions feasible. Recently, several commercial systems for large-volume liquid injections (up to 150 pL all at once, or up to 1 to 2 mL over a short period of time) have become available. When combined with robotic sampling systems, these have become powerful tools in the trace analysis of a variety of sample types. Due to its simplicity, classical liquid-liquid extraction is often the method of choice for sample preparation. Some of the robotic samplers available for this type of analysis, such as the LEAP Technologies Combi-PAL robotic sampler, which has been licensed by several instrument vendors, are also capable of performing automated SPME and SHE. 

Interfacing the TEA to both a gas and a HPLC has been shown to be selective to nitro-based explosives (NG, PETN, EGDN, 2,4-DNT, TNT, RDX and HMX) determined in real world samples, such as pieces of explosives, post-blast debris, post-blast air samples, hand swabs and human blood, at picogram level sensitivity [14], The minimum detectable amount for most explosives reported was 4-5 pg injected into column. A pyrolyser temperature of 550°C for HPLC-TEA and 900°C for GC/TEA was selected. As the authors pointed out, GC uses differences in vapour pressure and solubility in the liquid phase of the column to separate compounds, whereas in HPLC polarity, physical size and shape characteristics determine the chromatographic selectivity. So, the authors reported that the use of parallel HPLC-TEA and GC-TEA techniques provides a novel self-confirmatory capability, and because of the selectivity of the technique, there was no need for sample clean-up before analysis. The detector proved to be linear over six orders of magnitude. In the determination of explosives dissolved in acetone and diluted in methanol to obtain a 10-ppm (weight/volume) solution, the authors reported that no extraneous peaks were observed even when the samples were not previously cleaned up. Neither were they observed in the analysis of post-blast debris. Controlled experiments with handswabs spiked with known amounts of explosives indicated a lower detection limit of about 10 pg injected into column. 

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