GC Instrumentation

The sample must be stable at the temperature required to cause vaporization, and the sample must have sufficient vapor pressure to be completely soluble in the carrier gas at the column operating temperatures. 

When injecting a sample, always position your thumb or finger over the syringe plunger. This prevents a blow-back of the sample by the carrier gas pressure in the injection port. 

The mixture is separated as the carrier gas sweeps the sample through the column. Columns are usually made from stainless steel, glass, or fused silica. The diameter and length of the column are critical factors in separating the sample mixture. 

Packed Columns. In packed columns the liquid (stationary) phase in contact with the sample contained in the mobile gas phase is maximized by coating a finely divided inert support with the nonvolatile liquid. The coated support is carefully packed into the column so as not to develop empty spaces. Packed columns are usually i or inch in diameter and range from 4 to 12 feet in length. These columns are particularly useful in the microscale laboratory, since they can be used for both analytical and preparative GC. Simple mixtures of 20-80 jlL of material can often be separated into their pure components and collected at the exit port of the detector. Smaller samples (0.2-2.0 (jlL range) will exhibit better separation. 

Capillary Columns. Capillary columns have no packing the liquid phase is simply applied directly to the walls of the column. These columns are referred to as waU-coated, open-tubular (WCOT) columns. The reduction in surface area (compared to packed columns) is compensated for by tiny column diameters (perhaps 0.1 mm) and impressive lengths (100 m is not uncommon). Capillary columns are the most powerful columns used for analytical separations. Mixtures of several hundred compounds can be completely resolved on a capillary GC column. These columns require a more sophisticated and expensive chromatography instrument. Capillary columns, because of their tiny diameters, can accommodate only very small samples, perhaps 0.1 jxL or less of a dilute solution. Capillary columns cannot be used for preparative separations. 

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Gas chromatography/ma.ss spectrometry (GC/MS) is an analytical technique combining the advantages of a GC instrument with those of a mass spectrometer. 

By combining a GC instrument with MS, the powerful combination of GC/MS can be used to analyze, both qualitatively and quantitatively, complex mixtures arising from a wide variety of sources. 

In 1994, Nam and King (68) developed a SFE/SFC/GC instrumentation system for the quantitative analysis of organochlorine and organophosphorus pesticide residues in fatty food samples (chicken fat, ground beef and lard). In this way, SFC was used as an on-line clean-up step to remove extracted material. The fraction containing pesticide residues is then diverted and analysed by GC. 

Gianesello et al. (120) described the determination of the bronchodilator brox-aterol in plasma by on-line LC-GC. After deproteination and extraction, the LC separation was carried out by using a mixture of -pentane and diethyl ether (55 45 (vol/vol) as mobile phase. A small cut of the LC chromatogram (shown in Figure 11.9(a)) was introduced at 85 °C into the GC via so-called concurrent solvent evaporation. Figure 11.9(b) demonstrates that a detection limit of about 0.03 ng/ml was obtained. A fully automated LC-GC instrument was described by Munari and Grob (121) and its applicability was demonstrated by the determination of heroin metabo-... 

Both multi-residue methods are presented in several parts, which separate general considerations from procedures for extraction, cleanup and determination/ confirmation. Whereas in EN 12393 several extraction and cleanup steps cannot be combined arbitrarily, the modular concept is utilized to a greater extent in EN 1528. In the latter standard, there is no limitation to the combination of several extraction procedures, mostly designed for different commodities, e.g., milk, butter, cheese, meat or fish, with different cleanup steps. Both standards, EN 1528 and EN 12393, do not specify fixed GC conditions for the determination and confirmation. All types of GC instruments and columns, temperature programs and detectors can be used, if suitable. 

Calibration data (e.g., linearity or sensitivity) are not discussed in detail between laboratories, but a typical calibration starts with 50% of the lowest fortification level and requires at least three additional calibration levels. Another point of calibration is the use of appropriate standards. In 1999 a collaborative study tested the effect of matrix residues in final extracts on the GC response of several pesticides.Five sample extracts (prepared for all participants in one laboratory using the German multi-residue procedure) and pure ethyl acetate were fortified with several pesticides. The GC response of all pesticides in all extracts was determined and compared with the response in the pure solvent. In total, 20 laboratories using 47 GC instruments... 

Table 12 Response of 19 pesticides in matrix extracts compared with the response in solvent (response in solvent = 100% mean of about 40 GC instruments)...

Optimizing the GC instrument is crucial for the quantitation of sulfentrazone and its metabolites. Before actual analysis, the temperatures, gas flow rates, and the glass insert liner should be optimized. The injection standards must have a low relative standard deviation (<15%) and the calibration standards must have a correlation coefficient of at least 0.99. Before injection of the analysis set, the column should be conditioned with a sample matrix. This can be done by injecting a matrix sample extract several times before the standard, repeating this conditioning until the injection standard gives a reproducible response and provides adequate sensitivity. 

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... 

FIGURE 5.14 Software to read individual files from an offline GC instrument into the 2D software. Figure courtesy of Kroungold Analytical (2007). 

GC instruments are designed so that columns can be replaced easily by disconnecting a pair of brass fittings inside the oven. This not only facilitates changing to a different stationary phase altogether, but... 

Experiment 44 A Study of the Effect of the Changing of GC Instrument Parameters on Resolution... 

There are three heated zones in a GC instrument. Which zones are these and why does each need to be heated ... 

Although in principle it is possible to simply use several GC instruments each equipped with a sample manager and a separate PC, this is really not efficient because it is expensive, and at the same time data handling becomes tedious. The first successful construction consisted of two GC instruments (e.g., GC instruments and data bus HP-IB) are commercially available from the firm Hewlett-Packard, Waldbronn, Germany), one prep-and-load sample manager PAL) (commercially available from CTC, Schlieren, Switzerland) and one PC 102). 

Recently, this system has been extended to include three GC instruments operated by one PC (Fig. 12) 103,104). Experience has shown that a single sample manager for all three GC instruments may cause problems because it is difficult to construct identical columns 104). Therefore, three sample managers were employed... 

Fig. 12. Picture of optimized unit for ee determination based on three GC instruments driven by one PC (104).

Catalytic tests were performed in an isothermal flow quartz reactor apparatus under atmospheric pressure, provided with on-line gas chromatographic (GC) analysis of the reagent and products by two GC instrument equipped with flame ionization and thermoconducibility detectors. The activity data reported refers to the behavior after at least two hours of time on stream, but generally the catalytic behavior was found to be rather constant in a time scale of around 20 hours. 

Modern GC instruments represent high resolution systems that are fully automated from sample injection to final data reduction. Utilization of new injection devices has provided the means to enhance the performance level significantly. Studies have shown, for example, that injection of the tranquilizer propio-nylpromazine and its sulfoxide into a hot injection port gave much poorer results than on-column injection at low temperature (44). In the latter case, however, nonvolatile sample components could enter the column. This disadvantage of classic sample injection can be eliminated through use of a programmed temperature vaporization (PTV) injector. 

A 20% polyethylene glycol succinate on Kieselguhr column (1.2 m, 443 K, 50 ml min helium carrier gas) was used for GC analyses. Deuterium-labelled compounds were analysed by NMR spectroscopy (JEOL C 60-HL equipment) after separation with a Carlo Erba Mod P preparative GC. Mass spectrometric analyses of the reaction mixtures were carried out with a Hewlett Packard 5890A GC instrument (25 m HP-20M column, 353-473 K) coupled with a 5970 MSD quadrupole mass spectrometer (El source, 70 eV, 1-s scans, HP 59970 MS ChemStation data system). 

Inject into GC instrument for specific period of time (see Basic Protocol, steps 5 through 8). 

Figure 6.3 Schematic diagram of an on-line SFE-GC instrument 1, carbon dioxide 2, high-pressure syringe pump 3, three-port valve 4, extraction cell 5, oven 6, gas chromatograph.

At cold spots the analytes can adsorb to the surface material and only a fraction will reach the detector. Subsequently, the adsorbed compounds can result in ghost peaks in later analyses. Transfer lines from for example, a TD instrument to the GC must be insulated and heated to avoid cold spots . The optimal temperature of the surfaces depends on the stability of the component, pressure, vacuum, etc. Some GC instruments have ceramic insulators at the inlet of the transfer line from for example, TD instruments and this material may act as a cold spot if the line is not heated at the inlet... 

Similar to GC instruments, HPLC instruments consist of an injection port, a separation column, a detector, and an instrument control/data acquisition computer. The use of liquid as a mobile phase influenced the design and construction materials of HPLC instrumentation elements. A sample extract or an aqueous sample is introduced into the separation column through an injection loop that can be programmed to receive various volumes of liquid (5 pi to 5 ml). 

Evaporate the organic solvent under a nitrogen stream and redissolve the residue in 1-3 ml hexane. Transfer 100 pi to a GC vial and cap for analysis. The exact volumes for this step have to be determined empirically and are dictated by the sensitivity and injection settings of individual GC instruments. 

FIGURE 4.7 The on-line ESy-GC instrument with extraction card shown in the figure inset. 

Figure 4.2. Schematic diagram of headspace extraction autosampler and GC instrument.

Capillary Gas Chromatography (HRGC). Analytical separations were performed on a Varian 3700 GC instrument as well as on a Carlo Erba type 5360 Mega Series gas chromatograph. The Varian 3700 GC system was modified with a hot split/splitless injector and additionally equipped with a commercially available inlet splitter (Gerstel, Mulheim/Ruhr) in order to install two capillary columns... 

It is clear from table 7.2 that in terms of extra-column dispersion a wide-bore capillary column requires instrumentation similar to that used for the packed column. However, the capillary column provides eight times as many plates (in a fifteen-fold analysis time). Conventional capillary columns require a reduction in the dispersion by about an order of magnitude, whereas narrow-bore columns require a further reduction by a factor of about 100. This, combined with the high pressures required, puts narrow-bore columns out of reach for current GC instruments. 

SFC instruments are hybrids of LC and GC instruments, and early commercial models were modifications of one or the other. If packed columns are used, the instrument is more like a liquid chromatograph with a reciprocating pump and a pressurized UV detector. If OT columns are used, the instrument is more like a gas chromatograph with a syringe pump and an FID. 

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