Sample collection, pretreatment

The objective ia any analytical procedure is to determine the composition of the sample (speciation) and the amounts of different species present (quantification). Spectroscopic techniques can both identify and quantify ia a single measurement. A wide range of compounds can be detected with high specificity, even ia multicomponent mixtures. Many spectroscopic methods are noninvasive, involving no sample collection, pretreatment, or contamination (see Nondestructive evaluation). Because only optical access to the sample is needed, instmments can be remotely situated for environmental and process monitoring (see Analytical METHODS Process control). Spectroscopy provides rapid real-time results, and is easily adaptable to continuous long-term monitoring. Spectra also carry information on sample conditions such as temperature and pressure. 

Sample Collection, Pretreatment, and Analysis. Sediment-bound phosphorus in the Genesee River was studied by sampling bottom sediment, fine-material washed from bottom sediment, suspended sediment, and water column particulate material at six stations on the river. The sampling program was planned to be synoptic with complete chemical and hydrological parameters recorded at each site. One kilogram surficial sediment samples were collected in midstream at most sites during six field trips... 

Sample preparation is an extremely critical step in the course of a speciation analysis, as the original sample has to be transformed into a form which can be subjected to analysis, while the original distribution of an element over its various chemical forms may not be altered. In general, determination of Hg species involves the following steps (1) sample collection/pretreatment/ preservation/storage (2) extraction of Hg from the matrix/cleanup/preconcentration (3) separation of Hg species of interest (4) detection. 

Indeed AMS is a state-of-art ultrasensitive technique for trace analysis, but the success of AMS depends on many crucial steps before the actual measurement in the AMS system. In fact, sometimes expertise from several areas of science is required to make the AMS experiments reliable. Some of the crucial points are sample collection, pretreatment, sample preparation for ion source, development of separation methods for isobars, etc. Like all other analytical techniques, it is also necessary to measure standard, blank, and background, simultaneously with the sample. Below some of the crucial steps are described in nutshell. 

Off-line SFE is inherently simpler for the novice to perform, since only the SFE (and analyte collection) step needs to be understood. In off-line SFE further cleanup or a pretreatment step can be employed to eliminate interferences. With off-line SFE, sensitivities are limited by the fact that only about 1 p,L of the collection solvent is generally injected into the GC. The daily sample throughput can be higher using offline SFE, since SFE-GC requires that the GC be used for a sample collection device (rather than performing chromatographic separations) during the SFE extraction, whereas several off-line extracts can be loaded into an autosampler for unattended GC analysis. 

Surface-water samples are usually collected manually in precleaned polyethylene bottles (from a rubber or plastic boat) from the sea, lakes, and rivers. Sample collection is performed in the front of the bow of boats, against the wind. In the sea, or in larger inland lakes, sufficient distance (about 500 m) in an appropriate wind direction has to be kept between the boat and the research vessel to avoid contamination. The collection of surface water samples from the vessel itself is impossible, considering the heavy metal contamination plume surrounding each ship. Surface water samples are usually taken at 0.3-1 m depth, in order to be representive and to avoid interference by the air/water interfacial layer in which organics and consequently bound heavy metals accumulate. Usually, sample volumes between 0.5 and 21 are collected. Substantially larger volumes could not be handled in a sufficiently contamination-free manner in subsequent sample pretreatment steps. 

Urine collection Pretreatment, monthly during treatment, and during Week 4 of reversal Pharmacokinetic samples Blood collected at specified times after dosing on Day 1 and during Weeks six and 12... 

Stoeppler, M. Analytical aspects of sample collection, sample storage and sample pretreatment, in Trace Element Analytical Chemistry in Medicine and Biology (ed.) Bratter, P. and Schramel, P., Vol. 2, p. 909, Berlin—New York, Walter de Gruyter Co 1983... 

We have found that rigorous sorbent pretreatment procedures (e.g., Soxhlet extraction and thermal desorption) in concert with a well-established quality control program will successfully control potential contamination effects arising from the sample collection media. Furthermore, a well-executed quality control program will permit identification of spurious data points attributable to media contamination when and if they do occur. 

Another consideration about sample collection and preparation is the interest in knowing how the analyte is distributed in the sample. If only the total amount is important, the sample can be totally decomposed. On the other hand, if the analyte binding ability must be evaluated, pretreatment should keep analyte integrity in all parts of the sample. 

Figure 1 indicates an example of how pretreatment of the Incoloy 800 reactor had a very large effect on the acetylene conversion (or on the kinetics of acetylene decomposition). The coke-coated Incoloy 800 reactor (the coke had been deposited on this reactor when butadiene reacted at 500°-700°C.) used in Run 14 resulted in much lower acetylene conversions in the range of 450° to 550°C compared with the same Incoloy 800 reactor after the coke had been burned off with oxygen and after the reactor had been contacted with hydrogen until nearly all surface oxides were eliminated. Most conversion results for the 11 gas samples collected during Run 15 are shown in Figure 1. Gas Samples 1 through 3 at 350°, 400°, and 450°C, respectively, indicated almost no acetylene conversions. A small amount of carbon dioxide was produced at 450°C, indicating some metal oxides had still been present on the surface after the hydrogen pretreatment. The temperature was then increased to 500°C, and the conversions then increased from 66% to 99% during the first 23 min (Samples 4 and 5). Some carbon oxide production was noted in Sample 4 but none in Sample 5 or in any later samples of the run presumably...

A few of the many manuals on chemical analysis which include instructions on sample collection may be mentioned. For over 100 years, AOAC INTERNATIONAL, has published detailed experimental procedures for sampling, subsampling and pretreatment of natural materials with the latest volume (Horwitz 2000) containing procedures for materials such as agricultural liming materials, fertilizers, plants, animal feed and dairy products in various chapters. A chapter on sampling dairy and related products is in the book edited by Marshall (1993) on Standard Methods for the Examination of Dairy Products. Specific sampling procedures for various food commodities are summarized by Ihnat (1982), based mostly on the AOAC manual. 

In the following sections the methods and techniques used for sample collection and pretreatment, fractionation, and detection of metalloid- and metal-containing species will be considered. 

Microwave-assisted desorption coupled to in situ headspace solid-phase microextraction (HS-SPME) was first proposed as a possible alternative pretreatment of samples collected from workplace monitoring. Therefore, pretreatment that takes a short time and uses little or no organic solvents has led to the recent development of a new extraction technique. Solid-phase micro-extraction (SPME) coupled with GC analysis has been used successfully to analyze pollutants in environmental matrices. MHS has been developed to achieve one-step, in situ headspace sampling of semivolatile organic compounds in aqueous samples, vegetables, and soil [7, 55-58]. 

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