Gas chromatography sampling

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

Gas chromatography Sample is dissolved and TFA is derivatized and analyzed by capillary GC with flame ionization detection. 

Average of five composite samples analyzed by electron capture gas chromatography. Samples collected from drip zone of tree. 

The individual fatty acid composition of fats and oils are routinely assayed by gas chromatography. Samples are transesterified with methanol to convert the... 

Figure 20-10. Gas chromatography sample addition syringes (A) For liquids. (B) Micro syringe. (C) For gases. (D) Gas tight valves. (Courtesy - Alltech Associates Inc., Deerfield, IL)...

Reactions are driven to completion, with no apparent decomposition, in less then 10 minutes by fusing the sample with potassium hydroxide in an inert atmosphere. After concentration of the volatile products, they are separated and determined by gas chromatography. Sample losses are minimised by performing the total analysis in a single piece of apparatus. Fluids, gum rubbers, and resins are handled with equal... 

Table 1. Alternative methods of sample injection are summarized below and in Table 1, with schematic diagrams in Figures 2 and 3. Gas chromatography sample injection systems ...

Although gas chromatography can give the concentration of each component in a petroleum gas or gasoline sample, the same cannot be said for heavier cuts and one has to be satisfied with analyses by chemical family, by carbon atom distribution, or by representing the sample as a whole by an average molecule. 

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. 

Preparing a Volatile Sample Gas chromatography can be used to separate analytes in complex matrices. Not every sample that can potentially be analyzed by GG, however, can be injected directly into the instrument. To move through the column, the sample s constituents must be volatile. Solutes of low volatility may be retained by the column and continue to elute during the analysis of subsequent samples. Nonvolatile solutes condense on the column, degrading the column s performance. 

Gas chromatography is widely used for the analysis of a diverse array of samples in environmental, clinical, pharmaceutical, biochemical, forensic, food science, and petrochemical laboratories. Examples of these applications are discussed in the following sections. 

Clinical Analysis Clinical, pharmaceutical, and forensic labs make frequent use of gas chromatography for the analysis of drugs. Because the sample s matrix is often incompatible with the GC column, analytes generally must be isolated by extraction. Figure 12.25b shows how gas chromatography can be used in monitoring blood alcohol levels. 

Time, Cost, and Equipment Analysis time can vary from several minutes for samples containing only a few constituents to more than an hour for more complex samples. Preliminary sample preparation may substantially increase the analysis time. Instrumentation for gas chromatography ranges in price from inexpensive (a few thousand dollars) to expensive (more than 50,000). The more expensive models are equipped for capillary columns and include a variety of injection options and more sophisticated detectors, such as a mass spectrometer. Packed columns typically cost 50- 200, and the cost of a capillary column is typically 200- 1000. 

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 first set of experiments describes the application of gas chromatography. These experiments encompass a variety of different types of samples, columns, and detectors. Most experiments maybe easily modified to use available equipment and detectors. 

Determination of DMAC in Air. DMAC can be measured in air by passing a known amount of sample through water in a gas-scmbbing vessel and then analyzing the solution either chemically or by gas chromatography. 

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