Sample treatment prior to analysis

Each of these operations can affect adversely the final result of the analysis. 

Some procedures are given for various types of compounds in Chapter 5. Several examples of a complete sample treatment prior to analysis will be presented here for illustration. 

Maruyama and Takemori [6] isolated norepinephrine and dopamine from brain tissue prior to GC analysis by the procedure outlined in Fig. 2.1. A 1-ml volume of 0.05 N oxalic acid (saturated with NaCl) and 3.5 ml of 25% n-butanol in isopropanol are added to a small homogenization test-tube containing one (380—480 mg) or two entire mouse brains and the mixture is homogenized. The homogenate is centrifuged at 2500 g for 5 min in a clinical centrifuge adapted for this purpose. Then 3 ml of the pure phase of 

A number of other interesting examples of multi-step separations of complex mixtures can be found in the book by Karger et al. [10]. 

From the complexity of the above procedures, it is obvious that the result of the analysis depends on a large number of factors. The most general approach to the solution of the problem consists in elaboration of a standard procedure, its strict observation and its testing by means of an independent method, e.g., with the use of the sample compounds labelled with 14C. The determination of the total recovery by means of an internal standard, i.e., a chemically very closely related compound, a defined amount of which is added to the sample before the start of the procedure, is not exact as it is difficult to ensure that all of the properties of the standard that contribute to the final result are completely identical with the properties of the compounds being determined. With simpler procedures, when only a few different steps are applied, it is usually sufficient if a 

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A procedure for sample extraction or sample treatment prior to analysis... 

Sample preparation procedures for GC are generially more involved. For example, for methadone hydrochloride, 0.5jV sodium hydroxide is added to give the free base, followed by extraction with methylene chloride. An internal standard is added after the extract is dried with anhydrous sodium sulfate [3, p. 970]. The assay of interleukin-la formulated with human serum albumin does not require any sample treatment prior to analysis by capillary electrophoresis [44]. 

Whatever the chemistry, MALDI mass spectrometry has proved to be very effective in validating results of synthesis performed on PEG. Nevertheless some drawbacks were spotted. First, a chemical reactivity directed by the acidic properties of the matrix co-crystallized with the sample under investigation was observed with acid-sensitive Schiff bases. The side reaction was avoided when a nonacidic matrix (2,4,6-trihydroxyacetophenone) was used. Second, mixture analysis was quite difficult to conduct due to the presence of large overlapping distributions. For these two reasons ESI mass spectrometry was also investigated to monitor PEG-mediated syntheses as it requires no specific sample treatment prior to analysis and is easily coupled to a liquid chromatography to profile mixtures. 

The sample preparation plays a very important role for the analysis of a drug and its metabolites in biological tissue sections using mass spectrometric imaging. Several factors in IMS sample preparation must be considered, from sample collection to surface treatment prior to analysis in order to produce high quality, reliable, and reproducible results. 

The introduction of GC as an analytical technique has had a profound impact on both qualitative and quantitative analysis of organic compounds. Identification of compounds by GC can be accomplished by their retention times on the column as compared to known reference standards, by inference from sample treatment prior to chromatography, " or by the concept of retention index. " The latter method and tables of retention indices " with associated conditions have been reported. " Although qualitative data and analytical techniques for identification of compounds are well-established " and relative retention data for over 600 substances also have been published, " the main utility of GC undoubtedly lies in its powerful combination of separation and quantitative capabilities. Use in quantitative analysis involves the implementation of two techniques being performed concurrently, i.e., separation of components and subsequent quantitative measurement. 

Microwave-assisted extraction has also been used as a solid sample treatment prior to speciation analysis [264-266], leaving the organometallic compound moiety intact. This is a prerequisite for a successful extraction procedure to be applied prior to speciation analysis and can be met by careful optimization of the conditions of the microwave attack. Open-vessel treatment is preferred to pressurized bomb systems commonly used in the analysis for total metals because it offers milder reaction conditions — the increase in temperature is governed to a great extent by the boiling point of the solvent — and easier control of process variables [266]. 

Solid samples and especially if the matrix is not known, need transformation prior to the measurement. In fact, the measured fluorescence of these materials only concerns a thickness of several micrometres beneath the surface. This thickness depends upon its composition and of the angle of incidence of the primary X-rays this can be from between several angstroms (if the incident angle is acute), up to half a millimetre. All superficial heterogeneity can have important consequences and cause variations in the result. This is the principal reason for surface treatment prior to analysis. 

Naturally, the overall successful quantitative analysis is a multifaceted problem of sample choice, quantitative recovery during extractions and purification steps, choice of internal standards, signal recording technology, etc. Most of these topics are beyond the scope of this chapter, as they are not deemed to be particularly characteristic of biochemical GC. However, sample treatment prior to its introduction into a gas chromatograph will be the subject of a later discussion. 

For the last twenty years much work has been done in the study of the thermal stability of lignocellulosic materials by thermal analytical methods. Since these materials are complex mixtures of organic polymers, thermogravimetric (TG) analysis causes a variety of chemical and physical changes depending on the nature of the sample and its treatment prior to analysis. These problems have been reviewed recently. ... 

From an environmental point of view, analysis of waste waters is of great importance for characterizing their potential danger, and, in the case of emission to the environment, for detecting their spread and/or their origin. As for seawater, the matrix of waste water is complex and usually requires a treatment prior to analysis. Numerous examples of CE applications to different typologies of waste waters are reported in the current literature. CE was relevant to determine inorganics in samples of many different industrial processes (leather industry, pulp... 

While MALDI is a typical off-line ionization technique/interface, one can also implement classical interfaces to carry out off-line (or at-line) analyses of reaction mixtures. Aliquots can be obtained from a reaction mixture (a dynamic sample/matrix) at specific time points, and injected to the ion source of a mass spectrometer for analysis (e.g., [49-54]). In some cases, quenching is conducted [49, 50]. If the transient intermediates (e.g., radicals) are to be detected, it is important to assure that the reaction has not finished at the time of ionization [55]. Short-lived intermediates can be reacted with auxiliary compounds in order to enable subsequent measurements in the methodology referred to as spin-trapping (see, e.g., [56, 57]). The off-line analyses based on aliquoting of dynamic matrices, and subsequent separation, provide temporal resolutions in the order of a few minutes (see, e.g., [58]). Nonetheless, they are uncomplicated, and - in some cases - they may enable offline sample treatment prior to detection by MS. For instance, inorganic ions, present in the reaction mixture, can readily be removed on an exchange resin to render the collected samples compatible with MS [59]. 

Gas chromatography (GC) is an outstanding tool for the analysis of volatile, semivolatile and nonpolar compounds. Automation of sample treatment prior GC analysis is the bottleneck in GC analysis. It is of utmost interest to avoid laborious and time-consuming operations as well as sample contamination. 

When interference effects are large in magnitude, external sample treatment is usually the only means of alleviating the problem. This is often accomplished by chemical separation of the analyte species from the sample matrix prior to analysis. If this approach is not possible, then the interference probably precludes the determination of the desired analyte element under the prevailing conditions. 

It is important to remember that decisions on the treatment of samples prior to analysis should always be based on sound knowledge of what the results are going to be used for. It is therefore important to establish a good dialogue with the customer prior to carrying out any tests. 

The concentration obtained from the standard curve is rarely the final answer in a real-world instrumental analysis. In most procedures, the sample has undergone some form of preanalysis treatment prior to the actual measurement. In some cases, the sample must be diluted prior to the measurement, as mentioned in Workplace Scene 6.3. In other cases, a chemical must be added prior to the measurement, possibly changing the analyte s concentration. In still other cases, the sample is a solid and must be dissolved or extracted prior to the measurement. 

When a more specific detection system is used instead, a rigorous sample cleanup may not be necessary. This is actually the case with most of the microbiological and immunochemical detection systems applied in residue analysis. Owing to the selectivity and sensitivity of their detection principle, homogenization with an aqueous buffer is often tire only treatment required prior to analysis. Moreover, these detection systems are usually independent of the sample size as, in many cases, a single drop of milk or tissue fluid is sufficient to carry out a successful analysis. 

Application of some kind of sample treatment may have the potential to improve substantially the detection of certain antibacterials in milk by microbial routine methods (59). Treatment, for example, of milk samples with ammonium oxalate solution prior to analysis can lead to lower limits of detection of tetracyclines by both microbial inhibition and microbial receptor assays. This is due to the fact that tetracycline residues tend to form chelates with divalent cations and bind to proteins, which reduce their antibacterial efficacy. However, the oxalate treatment causes splitting of complex and/or protein bonds without increasing the detection limits of other antibacterials commonly used in dairy cows. 

In this assay, milk samples could be analyzed without dilution, but tissue and egg white samples should be homogenized in 5% trichloroacetic acid, centrifuged, and brought to pH 7 prior to analysis. Egg yolks required a separate treatment involving mixing with a pH 7.4 phosphate-EDTA buffer, incubation for 3 h at 25 C with cephalexin antiserum and enzyme-label cephalexin, and centrifugation. The assay could detect cephalexin down to 30 ppb in milk, 60 ppb in egg yolk, and 400 ppb in hen tissue. 

Garcia Gutierrez [19] has described an azo coupling spectrophotometric method for the determination of nitrite and nitrate in soils. Nitrite is determined spectrophotometrically at 550 nm after treatment with sulfuric acid and N-1 -naphlhylclhylcnediamine to form an azo dye. In another portion of the sample, nitrate is reduced to nitrite by passing a pH 9.6 buffered solution through a cadmium reductor and proceeding as above. Soils were boiled with water and calcium carbonate, treated with freshly precipitated aluminium hydroxide and active carbon, and filtered prior to analysis by the above procedure. 

All procedures applied to a sample prior to analysis may include pre-treatment (e.g., filtration, homogenization). Volume 1(10). 

The preparation of a derivative of a sample compound prior to GC is a significant potential source of both qualitative and, in particular, quantitative errors. Almost all reactions that are used for derivatization are organic syntheses adapted to the micro-scale. This approach makes full use of an advantageous property of GC, namely the need to take only very small amounts of the sample for the analysis, but on the other hand, it makes heavy demands on the quality of the materials used and the precision of the operating procedures. As GC has especially been used in analyses of complex mixtures with large contents of various components, such as biological samples, the operations necessary for the preliminary separation of the compounds of interest from the sample, e.g., extraction or TLC, are often involved in the entire procedure, and make it even more complicated. With some reactions, the necessity for an anhydrous medium requires the application of drying (lyophilization) in the treatment of the sample. 

In a number of cases, simple dissolution of a solid sample in an appropriate solvent is possible and some laboratory reagents may even be analysed without further treatment. Prior to flame analysis, the best solvent is dilute hydrochloric acid, provided of course that the major matrix elements are not silver, lead or another element which forms a sparingly soluble chloride. If additional oxidising ability is required, concentrated nitric acid may be added to the solvent. This acid is the preferred solvent when the analysis is to be completed by electrothermal atomisation. If the material contains large amounts of silica it may be necessary to add hydrofluoric acid after preliminary digestion with hydrochloric acid (see Chapter 4g). Care should of... 

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