Sampling in gas chromatography

Tipler, A. An introduction to headspace sampling in gas chromatography. Fundamentals and theory, www.perkinelmer.com... 

Condition (a) above is not exclusive to headspace analysis in fact, it is a pre-requisite for quantitative analysis of any sample in gas chromatography. Essentially the same is true for condition (b). Condition (d) is primarily a design problem. Finally, constancy of p is assured by proper automation of the system (i.e. by exact repetition of the operational parameters) and by the fact that the calibration standard is carried through the same MHS steps as the sample itself Therefore, the greatest problem is posed by the need to ensure equilibrium between the two phases in the vial. 

The automation of the introduction of liquid samples in gas chromatography does not differ substantially from that described above for liquid chromatography, although the nature of the mobile phase calls for a slightly different instrumental design. There are several commercially available models, the commonest of which have no sample turntable. 

The mobile phase is the vaporized sample in gas chromatography or the solvent used to elute the sample over the stationary phase in liquid chromatography. 

The methods for the collection and introduction of gas samples in gas chromatography (GC) are described. Containers for samphng gases, sorption pipes, two- and three-position multiport valves and chambers capable of changing pressure with a mobile piston are presented. The methanizer in which the catalytic reduction of carbon monoxide and carbon dioxide to methane occurs is also presented. 

Gas chromatography (GC) and mass spectrometry (MS) are the most common methods of chemical analysis. These tests can be done on urine and blood samples. In gas chromatography, the sample is vaporized in the presence of a gaseous solvent and introduced into the instrument. 

Analytical separations may be classified in three ways by the physical state of the mobile phase and stationary phase by the method of contact between the mobile phase and stationary phase or by the chemical or physical mechanism responsible for separating the sample s constituents. The mobile phase is usually a liquid or a gas, and the stationary phase, when present, is a solid or a liquid film coated on a solid surface. Chromatographic techniques are often named by listing the type of mobile phase, followed by the type of stationary phase. Thus, in gas-liquid chromatography the mobile phase is a gas and the stationary phase is a liquid. If only one phase is indicated, as in gas chromatography, it is assumed to be the mobile phase. 

In gas chromatography (GC) the sample, which may be a gas or liquid, is injected into a stream of an inert gaseous mobile phase (often called the carrier gas). The sample is carried through a packed or capillary column where the sample s components separate based on their ability to distribute themselves between the mobile and stationary phases. A schematic diagram of a typical gas chromatograph is shown in Figure 12.16. 

A chromatographic column provides a location for physically retaining the stationary phase. The column s construction also influences the amount of sample that can be handled, the efficiency of the separation, the number of analytes that can be easily separated, and the amount of time required for the separation. Both packed and capillary columns are used in gas chromatography. 

Despite their importance, gas chromatography and liquid chromatography cannot be used to separate and analyze all types of samples. Gas chromatography, particularly when using capillary columns, provides for rapid separations with excellent resolution. Its application, however, is limited to volatile analytes or those analytes that can be made volatile by a suitable derivatization. Liquid chromatography can be used to separate a wider array of solutes however, the most commonly used detectors (UV, fluorescence, and electrochemical) do not respond as universally as the flame ionization detector commonly used in gas chromatography. 

The quantitative determination of a component in gas chromatography using differential-type detectors of the type previously described is based upon meas urement of the recorded peak area or peak height the latter is more suitable in the case of small peaks, or peaks with narrow band width. In order that these quantities may be related to the amount of solute in the sample two conditions must prevail ... 

The injection device is also an important component in the LC system and has been discussed elsewhere (2,18). One type of injector is analogous to sample delivery in gas chromatography, namely syringe injection through a self-sealing septum. While this injection procedure can lead to good column efficiency, it generally is pressure limited, and the septum material can be attacked by the mobile phase solvent. 

The temperature program mode is the most widely used separation technique in gas chromatography [155]. As well as reducing separation times for samples with a wide tailing point... 

In gas chromatography samples are separated by distribution Ssetween a statloneu y phase and a mobile phase by adsorption,... 

PBECOLUMN SUBTRACTION REAGENTS USED FOR QUALITATIVE SAMPLE IDENTIFICATION IN GAS CHROMATOGRAPHY... 

M.P. Colombini, F. Modugno, M. Giacomelli, S. Francesconi, Characterisation of Proteinaceous Binders and Drying Oils in Wall Painting Samples by Gas Chromatography Mass Spectrometry, Journal of Chromatography, A, 846, 113 124 (1999). 

C. Mathe, J. Connan, P. Archier, M. Mouton, C. Vieillescazes, Analysis of frankincense in archaeological samples by gas chromatography mass spectrometry, Annal. Chim., 97, 433 445 (2007). 

Barcelo D, Porte C, Cid J, Albaiges J (1990) Determination of organophosphorus compounds in Mediterranean coastal waters and biota samples using gas-chromatography with nitrogen-phosphorus and chemical ionization mass-spectrometric detection. Int J Environ Anal Chem 38(2) 199-209... 

Carrizo D, Grimalt JO (2006) Rapid and simplified method for the analysis of polychlor-onaphthalene congener distributions in environmental and human samples by gas chromatography coupled to negative ion chemical ionization mass spectrometry. J Chromatogr A 1118(2) 271-277... 

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