Gas chromatography interfaces

Fig. 11.15. Gas chromatography interfaces (jet separator, top membrane separator, bottom). In the jet separator, momentum of the heavier analyte molecules causes them to be sampled preferentially by the sampling orifice with respect to the helium carrier gas molecules (which diffuse away at a much higher rate). In the membrane separator, the analyte molecules are more soluble in the silicone membrane material leading to preferential permeability. Helium does not permeate the membrane with the same efficiency and is vented away.

J. Staniewski, H.G. Janssen, C. A. Cramers and J. A. Rijks, Programmed-temperature injector for large-volume sample introduction in capillary gas chromatography and for liquid chromatography-gas chromatography interfacing , J. Microcolumn Sep. 4 331-338(1993). 

Jiang GB, Ni ZM, Wang SR, et al. 1989. Organic mercury speciation in fish by capillary gas chromatography interfaced with atomic absorption spectrometry. Fresenius Z Anal Chem 334(l) 27-30. 

Mass spectrometry is an extremely versatile detection system for gas chromatography. Interfacing an HPLC system to a mass spectrometer is a much more difficult task, however. Describe the major reasons why it is more difficult to combine HPLC with mass spectrometry than it is to combine GC with mass spectrometry. 

Some sampling accessories [e.g., infrared microscopes (see Chapter 14) and gas chromatography interfaces (see Chapter 23)] are too big to fit in even the largest sample compartment. In these cases, the accessories are mounted outside the instrument and the modulated beam is directed to them by means of a switch-able mirror (often known as di flip mirror). It is very rare that the beam is returned to the detector mounted inside the instrument, so a second detector is usually a component of accessories that are mounted outside the spectrometer in the external beam. Several contemporary instruments are also equipped with flip mirrors to direct the beam to different possible internal beam paths, in one or more of which a different accessory may be mounted permanently. In this way, the user can switch rapidly from one type of measurement to another, making optimal use of a single spectrometer. 

The possibiHties for multidimensional iastmmental techniques are endless, and many other candidate components for iaclusion as hyphenated methods are expected to surface as the technology of interfacing is resolved. In addition, ternary systems, such as gas chromatography-mass spectrometry-iafrared spectrometry (gc/ms/ir), are also commercially available. 

Coupled liquid chromatography-gas chromatography is an excellent on-line method for sample enrichment and sample clean-up. Recently, many authors have reviewed in some detail the various LC-GC transfer methods that are now available (1, 43-52). For the analysis of normal phase eluents, the main transfer technique used is, without doubt, concurrent eluent evaporation employing a loop-type interface. The main disadvantage of this technique is co-evaporation of the solute with the solvent. 

Figure 12.19 Schematic diagram of the interface system used for supercritical fluid cliromatography-gas chromatography. Reprinted from Journal of High Resolution Chromatography, 10, J. M. Levy et al., On-line multidimensional supercritical fluid clrromatogi a-phy/capillary gas chromatography , pp. 337-341, 1987, with permission from Wiley-VCH.

On-line coupling of normal-phase liquid chromatography (NPLC) and gas chromatography is today a well developed and robust procedure and has been regularly applied to environmental analysis. When a fraction of the NPLC sample is introduced in to the GC unit, a large-volume interface (LVI) is needed but, due to the volatility of the organic solvent used in NPLC, this does not present such a great problem. 

One example of normal-phase liquid chromatography coupled to gas chromatography is the determination of alkylated, oxygenated and nitrated polycyclic aromatic compounds (PACs) in urban air particulate extracts (97). Since such extracts are very complex, LC-GC is the best possible separation technique. A quartz microfibre filter retains the particulate material and supercritical fluid extraction (SPE) with CO2 and a toluene modifier extracts the organic components from the dust particles. The final extract is then dissolved in -hexane and analysed by NPLC. The transfer at 100 p.1 min of different fractions to the GC system by an on-column interface enabled many PACs to be detected by an ion-trap detector. A flame ionization detector (PID) and a 350 p.1 loop interface was used to quantify the identified compounds. The experimental conditions employed are shown in Table 13.2. 

One of the first examples of the application of reverse-phase liquid chromatography-gas chromatography for this type of analysis was applied to atrazine (98). This method used a loop-type interface. The mobile phase was the most important parameter because retention in the LC column must be sufficient (there must be a high percentage of water), although a low percentage of water is only possible when the loop-type interface is used to transfer the LC fraction. The authors solved this problem by using methanol/water (60 40) with 5% 1-propanol and a precolumn. The experimental conditions employed are shown in Table 13.2. 

Gas chromatography/mass spectrometry (GC/MS) is the synergistic combination of two powerful analytic techniques. The gas chromatograph separates the components of a mixture in time, and the mass spectrometer provides information that aids in the structural identification of each component. The gas chromatograph, the mass spectrometer, and the interface linking these two instruments are described in this chapter. 

The combination of chromatography and mass spectrometry (MS) is a subject that has attracted much interest over the last forty years or so. The combination of gas chromatography (GC) with mass spectrometry (GC-MS) was first reported in 1958 and made available commercially in 1967. Since then, it has become increasingly utilized and is probably the most widely used hyphenated or tandem technique, as such combinations are often known. The acceptance of GC-MS as a routine technique has in no small part been due to the fact that interfaces have been available for both packed and capillary columns which allow the vast majority of compounds amenable to separation by gas chromatography to be transferred efficiently to the mass spectrometer. Compounds amenable to analysis by GC need to be both volatile, at the temperatures used to achieve separation, and thermally stable, i.e. the same requirements needed to produce mass spectra from an analyte using either electron (El) or chemical ionization (Cl) (see Chapter 3). In simple terms, therefore, virtually all compounds that pass through a GC column can be ionized and the full analytical capabilities of the mass spectrometer utilized. 

Merritt DA, KH Freeman, MP Ricci, SA Studley, JM Hayes (1995) Performance and optimization of a combustion interface for isotope ratio monitoring gas chromatography/mass spectrometry. Anal Chem 67 2461-2473. 

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