Chromatography samples

Figure 3. Matrix Representation of Sample Analysis for P, Peaks and N Samples (Chromatography Data Matrix).

Analysis is the separation of a whole into parts. It is the qualitative and quantitative investigation and evaluation of samples. Chromatography and electrophoresis are the most powerful techniques of separation. Thousands of papers are published, as can be seen from the biannual reviews from Analytical Chemistry. Efficiency, selectivity, and sensitivity are ever-increasing, and a simple comparison between chromatograms from the 1980s to those from the 90s is impressive. Manufacturers have made tremendous efforts to give analysts the opportunity to achieve difficult separations with reliable, easy-to-handle instruments. 

Giabbai M, Roland L, Chian ESK. 1983. Trace analysis of organic priority pollutants by high-resolution gas chromatography and selective detectors (FID, ECD, NPD, and MS-DS) Application to municipal wastewater and sludge samples. Chromatography in Biochemistry Medicine and Environmental Research 13 41-52. 

An important field of application of analytical chemistry involves the isolation, identification, and quantification of components in complex samples. Chromatography is one of the most used techniques, because modern chromatographic methods have an excellent separation power, are versatile, and can be used with several detection techniques. 

See also Air Analysis Sampling. Chromatography Overview Principles. Clinical Analysis Sample Handling. Drug Metabolism Metabolite Isolation and Identification. Extraction Solid-Phase Extraction. Food and Nutritional Analysis Sample Preparation. Forensic Sciences Volatile Substances. Headspace Analysis Purge and Trap. Perfumes. Sample Handling Sample Preservation Automated Sample Preparation. Sampling Theory. 

Figure 3.4 Headspace analysis of volatile fatty acids from aqueous solution. Chromatographic conditions 15 m x 0.23 mm fused silica column coated with free fatty acid phase, 130°C (1 min), then increased by 6°Cmin to 200°C. Sample thermostatted at 120°C. Trace B is a blank. Peaks 1 = acetic acid 2 = propanoic acid 3 = 2-methyl propanoic acid 4 = butanoic acid 5 = 3-methyl butanoic acid 6 = pentanoic acid 7 = 4-methyl pentanoic acid 8 = hexanoic acid 9 = heptanoic acid 10 = octanoic acid 11 = nonanoic acid 12 = decanoic acid. Reproduced from Closta, W., Klemm, H., Pospisil, P. et al.. The HS-lOO, an innovative concept for automatic headspace sampling, Chromatography Newsletter, 11, 13-17, 1983.

Closta, W., Klemm, H., Pospisil, P. et al. (1983) The HS-lOO, an innovative concept for automatic headspace sampling. Chromatography Newsletter, 11, 13-17. 

Finally, other methods are used to obtain simulated distillation by gas phase chromatography for atmospheric or vacuum residues. For these cases, some of the sample components can not elute and an internal standard is added to the sample in order to obtain this quantity with precision. 

Liquid chromatography, having a resolving power generally less than that of gas phase chromatography, is often employed when the latter cannot be used, as in the case of samples containing heat-sensitive or low vapor-pressure compounds. 

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. 

The goal of chromatography is to separate a sample into a series of chromatographic peaks, each representing a single component of the sample. Resolution is a quantitative measure of the degree of separation between two chromatographic peaks, A and B, and is defined as... 

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. 

For most samples liquid-solid chromatography does not offer any special advantages over liquid-liquid chromatography (LLC). One exception is for the analysis of isomers, where LLC excels. Figure 12.32 shows a typical LSC separation of two amphetamines on a silica column using an 80 20 mixture of methylene chloride and methanol containing 1% NH4OH as a mobile phase. Nonpolar stationary phases, such as charcoal-based absorbents, also may be used. 

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