Trace organic analysis, steps

The identification and quantitative determination of specific organic compounds in very complex samples is an area of intense current research activity in analytical chemistry Optical spectroscopy (particularly UV-visible and infrared absorption and molecular fluorescence and phosphorescence techniques) has been used widely in organic analysis. Any optical spectroscopic technique to be used for characterization of a very complex sample, such as a coal-derived material, should exhibit very high sensitivity (so that trace constituents can be determined) and extremely great selectivity (so that fractionation and separation steps prior to the actual analysis can be held to the minimum number and complexity). To achieve high analytical selectivity, an analytical spectroscopic technique should produce highly structured and specific spectra useful for "fingerprinting purposes," as well as to minimize the extent of overlap of spectral bands due to different constituents of complex samples. 

It is possible to produce artificial matrix materials [12]. Such materials can be prepared on a mass basis by weighing all components both to mimic the matrix composition and the content of trace elements or trace organic substances. The materials could help to have matrix materials available for which the exact contents and composition are known. As a consequence it would be, in theory, possible to certify them on a mass basis and validate methods with highly traceable materials. In organic trace analysis this would circumvent the unknown extraction step. In reality, this is much more difficult to achieve than can be expected. The real matrix composition of many materials is unknown — in particular for environment samples. The physico-chemical status of the various substances depends on the history of the material. Therefore, various natural samples of expected similar composition are different in behaviour. In addition, when preparing mixtures of solid components, losses cannot be excluded and unfortunately are not quantifiable. Attempts have been made where losses were demonstrated but not quantified [12]. Therefore, materials certified for matrix composition and analyte content on a mass basis do not yet exist or are not of real use for method validation by routine laboratories. They may be of interest for laboratories active in the field of fundamental research in chemical metrology where smaller quantities of material are handled. 

Figure 4.1 Contributions from different steps to total effort and time, and thus potential for error, in a trace organic quantitative analysis. Reproduced from Majors, LC/GC North America 9 (1991), 16, with permission from Dr. Ron Majors and the journal.

Prior to the GC analysis, the air sample is first concentrated by passing it through a small adsorbent column. Such a preconcentration step is common if trace organics are to be analyzed in dilute media (air, water, soil, etc.). 

The method detection limit is, in reality, a statistical concept that is applicable only in trace analysis of certain types of substances, such as organic pollutants by gas chromatographic methods. The method detection limit measures the minimum detection limit of the method and involves all analytical steps, including sample extraction, concentration, and determination by an analytical instrument. Unlike the instrument detection limit, the method detection limit is not confined only to the detection limit of the instrument. 

Thus, to conduct successful analyses for many organic and inorganic compounds at trace concentrations, it is necessary to extract these compounds and use a concentration step prior to analysis. Many of the techniques developed for preconcentration are described in specialized books [10]. Proper choice of the extracting solvent can often be the critical step in the procedure. 

Flame atomic absorption spectrometry can be used to determine trace levels of analyte in a wide range of sample types, with the proviso that the sample is first brought into solution. The methods described in Section 1.6 are all applicable to FAAS. Chemical interferences and ionization suppression cause the greatest problems, and steps must be taken to reduce these (e.g. the analysis of sea-water, refractory geological samples or metals). The analysis of oils and organic solvents is relatively easy since these samples actually provide fuel for the flame however, build-up of carbon in the burner slot must be avoided. Most biological samples can be analysed with ease provided that an appropriate digestion method is used which avoids analyte losses. 

If analysis is to be attempted with a detection system of only moderate selectivity, a substantial cleanup procedure may be required in order to enhance the concentration of the extracted trace residue while decreasing die concentration of possible interfering substances in the sample matrix. This is die case with most of the relatively nonspecific physicochemical detection systems used in residue analysis. Occasionally a sample may be suitable for direct physicochemical analysis after an extraction and concentration step. However, the majority of edible animal products need extensive cleanup to separate the compounds of interest from animal lipids and other natural organic substances prior to detection. For such detection systems, there has been a general rule dictating diat the cleaner sample, the better the result obtained. 

The aqueous or organic extract obtained at this step of analysis may be a very dilute solution of the analyte(s) of interest. It may also contain coextractives, which if allowed in the final extract will increase the background noise of the detector, making it impossible to determine trace level concentrations of the analyte(s). To reduce interferences and concentrate the analyte(s), the primary sample extract must be subjected to cleanup procedures such as liquid-liquid partitioning, solid-phase extraction, matrix solid-phase dispersion, ultrafiltration, immunoaffinity chromatography, and online trace enrichment. In many instances. 

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