Analytical method selection

A review is presented here of certification approaches, followed by several of the major agencies and individual developers of RMs for chemical composition, addressing some of the many associated scientific aspects that significantly impinge on the conduct and outcome of the analytical characterization exercises. These include definition of analytical methods selection of analytical methodologies, analysts and laboratories in-house characterization and cooperative inter-laboratory characterization. 

There are several factors involved in defining the limitations of an analytical method. Selecting the right method for defining these limitations can be as important as the actual definitions. Factors that must be taken into consideration in defining detection and quantification limits are ... 

Parr JL, Bollinger MJ, Carlberg KA. 1986. Monitoring requirements and analytical method selection for EPA programs (RCRA, CERCLA, SDWA, CWA). In Haztech 86 International Conference... 

Liquid-liquid extraction is used extensively and successfully (6). If the analytes are acidic or basic, as is often the case when HPLC is the analytical method selected, appropriate ionization suppression can be employed to affect the desired extraction. Back extraction of the analytes into an appropriately buffered aqueous volume can then serve to isolate and concentrate. Anionic and cationic surfactants, or so-called ion-pairing reagents, can be added prior to extraction to increase the partition coefficients of the trace organic ionic compounds. 

The selectivity of fluorescence is one of its most important advantages as an analytical method. Selectivity is obtained mainly through structural restrictions. Only certain types of molecules possess the ability to fluoresce and often only under specified conditions. However, this fact is also the main reason for the limited use of fluorescence for analysis. Nevertheless, the formation of fluorescent derivatives of compounds which do not fluoresce permits the method to be extended into most areas of concern to the analytical chemist. 

An overview of Phase II and III TIE procedures outlined by the U.S. EPA (1993a, b) are provided in Tables 3 and 4, and were previously presented in Novak et al. (2002). The treatments, procedures and analytical methods selected for a Phase n and III TIE are directly related to those treatments observed to effectively eliminate or reduce toxicity during Phase I. Consequently, the specific approach can only be... 

The analytical method selection is an intricate part of the DQO process because of the variety of existing analytical methods and techniques. A chemist experienced in environmental analysis should make the selection using the action level as a starting point and refining the choice based on other aspects of the project DQOs. 

Following are three general rules for the analytical method selection process ... 

Rule 1. Intended use of the data and the regulatory framework guide analytical method selection. 

Justification for all ingredients used Justification for all analytical methods selected... 

SMI Analytical Method Selection SAS Prolog Mainframe/Mini IBM-PC Natural Language Understanding Centralization 1 Easy Query 1... 

An example of the use of an analytical methods selection expert which was developed with the KDS3 system is shown in Table 1. In this example the user is attempting to select a method for the determination of total sediment aluminum concentration according to the requirements of the Water Quality Branch of Environment Canada in Ottawa. From the main menu, aluminum, Al, is selected as the analyte of interest. The presentation of a set of values in a menu is a useful technique in the KDS3 shell when mutually exclusive options are to be selected. The KDS3 shell then prompts the user with a series of questions that can be answered with Y(yes), N(no), or (don t know or don t care), based on the conditions set up by the developer. 

Following the movement of airborne pollutants requires a natural or artificial tracer (a species specific to the source of the airborne pollutants) that can be experimentally measured at sites distant from the source. Limitations placed on the tracer, therefore, governed the design of the experimental procedure. These limitations included cost, the need to detect small quantities of the tracer, and the absence of the tracer from other natural sources. In addition, aerosols are emitted from high-temperature combustion sources that produce an abundance of very reactive species. The tracer, therefore, had to be both thermally and chemically stable. On the basis of these criteria, rare earth isotopes, such as those of Nd, were selected as tracers. The choice of tracer, in turn, dictated the analytical method (thermal ionization mass spectrometry, or TIMS) for measuring the isotopic abundances of... 

An analytical method is selective if its signal is a function of only the amount of analyte present in the sample. In the presence of an interferent, equations 3.1 and 3.2 can be expanded to include a term corresponding to the interferent s contribution to the signal. Si,... 

An analytical method is selected on the basis of criteria such as accuracy, precision, sensitivity, selectivity, robustness, ruggedness, the amount of available sample, the amount of analyte in the sam-... 

When the analytical method s selectivity is insufficient, it may be necessary to separate the analyte from potential interferents. Such separations can take advantage of physical properties, such as size, mass or density, or chemical properties. Important examples of chemical separations include masking, distillation, and extractions. 

Although many quantitative applications of acid-base titrimetry have been replaced by other analytical methods, there are several important applications that continue to be listed as standard methods. In this section we review the general application of acid-base titrimetry to the analysis of inorganic and organic compounds, with an emphasis on selected applications in environmental and clinical analysis. First, however, we discuss the selection and standardization of acidic and basic titrants. 

Clinical Applications Perhaps the area in which ion-selective electrodes receive the widest use is in clinical analysis, where their selectivity for the analyte in a complex matrix provides a significant advantage over many other analytical methods. The most common analytes are electrolytes, such as Na+, K+, Ca +, H+, and Ch, and dissolved gases, such as CO2. For extracellular fluids, such as blood and urine, the analysis can be made in vitro with conventional electrodes, provided that sufficient sample is available. Some clinical analyzers place a series of ion-selective electrodes in a flow... 

Environmental Applications Although ion-selective electrodes find use in environmental analysis, their application is not as widespread as in clinical analysis. Standard methods have been developed for the analysis of CN , F , NH3, and in water and wastewater. Except for F , however, other analytical methods are considered superior. By incorporating the ion-selective electrode into a flow cell, the continuous monitoring of wastewater streams and other flow systems is possible. Such applications are limited, however, by the electrode s response to the analyte s activity, rather than its concentration. Considerable interest has been shown in the development of biosensors for the field screening and monitoring of environmental samples for a number of priority pollutants. 

The importance of between-laboratory variability on the results of an analytical method can be determined by having several laboratories analyze the same sample. In one such study seven laboratories analyzed a sample of homogenized milk for a selected alfatoxin. The results, in parts per billion, are summarized in the following table. 

More attention to selecting and obtaining a representative sample. The design of a statistically based sampling plan and its implementation are discussed earlier, and in more detail than in other textbooks. Topics that are covered include how to obtain a representative sample, how much sample to collect, how many samples to collect, how to minimize the overall variance for an analytical method, tools for collecting samples, and sample preservation. 

Analytical methods aie utilised by all branches of the chemical iadustry. Sometimes the goal is the quaUtative deterniiaation of elemental and molecular constituents of a selected specimen of matter othertimes the goal is the quantitative measurement of the fractional distribution of those constituents and sometimes it is to monitor a process stream or a static system. Information concerning the various iadividual analytical methods may be found ia separate articles dispersed alphabetically throughout the Eniyclopedia. The articles ate iatroductions to topics each of which is the subject of numerous books and other pubhcations. 

Catalyst testing and evaluation have been revolutionized by computers, automated test reactors, and analytical methods. With modem equipment, researchers can systematically prepare and screen many catalysts in a short time and efftciendy deterrnine, not only the initial catalytic activity and selectivity, but also the stabiUty and the appearance of trace products that may indicate some new catalytic properties worthy of further development. 

A number of analytical methods have been developed for the determination of chlorotoluene mixtures by gas chromatography. These are used for determinations in environments such as air near industry (62) and soil (63). Liquid crystal stationary columns are more effective in separating m- and chlorotoluene than conventional columns (64). Prepacked columns are commercially available. ZeoHtes have been examined extensively as a means to separate chlorotoluene mixtures (see Molecularsieves). For example, a Y-type 2eohte containing sodium and copper has been used to separate y -chlorotoluene from its isomers by selective absorption (65). The presence of ben2ylic impurities in chlorotoluenes is determined by standard methods for hydroly2able chlorine. Proton (66) and carbon-13 chemical shifts, characteristic in absorption bands, and principal mass spectral peaks are available along with sources of reference spectra (67). 

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