Separation techniques sample preparation

Because preparation involves specialized procedures and instruments, most of analytical laboratories have Sample Preparation Sections, such as Organic Extraction and Metal Digestion shown in Figure 4.2. (The General Chemistry Section does not have a separate preparation group, as sample preparation is usually part of the analytical procedure). Because laboratory accuracy and precision strongly depend on the individual s technique, sample preparation personnel must be trained in each procedure, and their proficiency be documented. The laboratory must have a set of SOPs for preparation methods performed and must ensure that the Sample Preparation Group personnel are trained to follow them to the letter. 

Benzoic acid and its salts may be determined by titration with sodium hydroxide after extraction of the benzoic acid from an aqueous food suspension into chloroform, and evaporation of the chloroform and any acetic acid present (AOAC method 963.19). Vanillin interferes with this determination and a more selective method involves the determination of benzoic acid in an ether extract by UV absorption at 272 nm, as described in AOAC method 960.38. An alternative method of isolating benzoic acid from food involves the use of steam distillation and TLC separation. These sample preparation techniques are used in AOAC method 967.15 prior to the determination of benzoic acid by UV absorption. 

This separation technique has been employed primarily for preparative types of separations because detailed knowledge of the properties of the sample is required. Also, because this separation results in discrete zones of sample ions which are virtually pure, it makes sense to use this technique when the sample size is large. This technique is ineffective when the levels of impurities are small with respect to the target compound small amounts of sample ions do not form zones well and tend to mix with the target compound. Information on this technique is available (30). 

Theoretical and applied aspects of microwave heating, as well as the advantages of its application are discussed for the individual analytical processes and also for the sample preparation procedures. Special attention is paid to the various preconcentration techniques, in part, sorption and extraction. Improvement of microwave-assisted solution preconcentration is shown on the example of separation of noble metals from matrix components by complexing sorbents. Advantages of microwave-assisted extraction and principles of choice of appropriate solvent are considered for the extraction of organic contaminants from solutions and solid samples by alcohols and room-temperature ionic liquids (RTILs). 

Although on-line sample preparation cannot be regarded as being traditional multidimensional chromatography, the principles of the latter have been employed in the development of many on-line sample preparation techniques, including supercritical fluid extraction (SFE)-GC, SPME, thermal desorption and other on-line extraction methods. As with multidimensional chromatography, the principle is to obtain a portion of the required selectivity by using an additional separation device prior to the main analytical column. 

All separation techniques which allow the isolation of a certain amount of product can be qualified as being preparative . In contrast, analytical techniques are devoted to detect the presence of substances in a sample and/or quantify them. How-... 

Further techniques which may be applied directly to the solvent extract are flame spectrophotometry and atomic absorption spectrophotometry (AAS).13 The direct use of the solvent extract in AAS may be advantageous since the presence of the organic solvent generally enhances the sensitivity of the method. However, the two main reasons for including a chemical separation in the preparation of a sample for AAS are ... 

The sample preparation in LC analysis is as important as the chromatographic separation itself. The procedure will often require considerable skill copied with a basic understanding of chromatographic methodology. The analyst will need to have some familiarity with micro techniques including general micro-manipulation, microfiltration, centrifugation and derivatization. 

The two examples of sample preparation for the analysis of trace material in liquid matrixes are typical of those met in the analytical laboratory. They are dealt with in two quite different ways one uses the now well established cartridge extraction technique which is the most common the other uses a unique type of stationary phase which separates simultaneously on two different principles. Firstly, due to its design it can exclude large molecules from the interacting surface secondly, small molecules that can penetrate to the retentive surface can be separated by dispersive interactions. The two examples given will be the determination of trimethoprim in blood serum and the determination of herbicides in pond water. 

Analysis of methyl parathion in sediments, soils, foods, and plant and animal tissues poses problems with extraction from the sample matrix, cleanup of samples, and selective detection. Sediments and soils have been analyzed primarily by GC/ECD or GC/FPD. Food, plant, and animal tissues have been analyzed primarily by GC/thermionic detector or GC/FPD, the recommended methods of the Association of Official Analytical Chemists (AOAC). Various extraction and cleanup methods (AOAC 1984 Belisle and Swineford 1988 Capriel et al. 1986 Kadoum 1968) and separation and detection techniques (Alak and Vo-Dinh 1987 Betowski and Jones 1988 Clark et al. 1985 Gillespie and Walters 1986 Koen and Huber 1970 Stan 1989 Stan and Mrowetz 1983 Udaya and Nanda 1981) have been used in an attempt to simplify sample preparation and improve sensitivity, reliability, and selectivity. A detection limit in the low-ppb range and recoveries of 100% were achieved in soil and plant and animal tissue by Kadoum (1968). GC/ECD analysis following extraction, cleanup, and partitioning with a hexane-acetonitrile system was used. 

As an alternative approach towards the above requirement, Somorjai introduced the method of electron lithography [119] which represents an advanced HIGHTECH sample preparation technique. The method ensures uniform particle size and spacing e.g. Pt particles of 25 nm size could be placed with 50 nm separation. This array showed a uniform activity similar to those measured on single crystal in ethylene hydrogenation. The only difficulty with the method is that the particle size is so far not small enough. Comprehensive reviews have been lined up for the effect of dispersion and its role in heterogeneous catalysis [23,124,125]. 

Separation and detection methods A survey on determination of tin species in environmental samples has been published by Leroy et al. (1998). A more detailed overview of GS-MS methodology has been published by Morabito et al. 1995) and on sample preparation using supercritical fluid extraction has been described by Bayona (1995)- The techniques are now under control, so that routine procedures are available at a relatively low cost (Leroy et al. 1998). 

Until this point, the sample preparation techniques under discussion have relied upon differences in polarity to separate the analyte and the sample matrix in contrast, ultraflltration and on-line dialysis rely upon differences in molecular size between the analyte and matrix components to effect a separation. In ultrafiltration, a centrifugal force is applied across a membrane filter which has a molecular weight cut-off intended to isolate the analyte from larger matrix components. Furusawa incorporated an ultrafiltration step into his separation of sulfadimethoxine from chicken tissue extracts. Some cleanup of the sample extract may be necessary prior to ultrafiltration, or the ultrafiltration membranes can become clogged and ineffective. Also, one must ensure that the choice of membrane filter for ultrafiltration is appropriate in terms of both the molecular weight cut-off and compatibility with the extraction solvent used. 

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