Analytical method standardized

Sample type Key biomarkers Recommen- ded analytical methods Standards Advantages Disadvantages... 

Traceability is one major factor that can be achieved via CRMs as main means in the held of chemical metrology. In general, CRMs are applied for the validation of analytical methods. Standard solutions are then used for instrument calibration. Nonetheless, CRMs should not be understood as the solution for all problems in chemical measurement. It goes without saying that the matrix of a CRM should match the analytical problem as exactly as possible. It is clear that there are not CRMs available for all matrices and analytes. Thus, it is important to have the best matrix match. 

Standardization refers to the compounds (standard compounds) and to analytical methods (standard methods). The reference material is defined as "material or substance one or more of whose property values are sufficiently homogeneous and well established to be used for the calibration of an apparatus, the assessment of a measurement method, or for assigning values to materials."247... 

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All these methods begin with combustion of the sample resulting in the sulfur being oxidized to SO2 and SO3. Table 2.3 summarizes the different analytical methods with references to the corresponding standards. 

Normality is an older unit of concentration that, although once commonly used, is frequently ignored in today s laboratories. Normality is still used in some handbooks of analytical methods, and, for this reason, it is helpful to understand its meaning. For example, normality is the concentration unit used in Standard Methods for the Examination of Water and Wastewaterf a commonly used source of analytical methods for environmental laboratories. 

Examine a procedure from Standard Methods for the Analysis of Waters and Wastewaters (or another manual of standard analytical methods), and identify the steps taken to compensate for interferences, to calibrate equipment and instruments, to standardize the method, and to acquire a representative sample. 

Consider the situation when the accuracy of a new analytical method is evaluated by analyzing a standard reference material with a known )J,. A sample of the standard is analyzed, and the sample s mean is determined. The null hypothesis is that the sample s mean is equal to p. 

The most commonly employed standardization method uses one or more external standards containing known concentrations of analyte. These standards are identified as external standards because they are prepared and analyzed separately from the samples. 

An external standardization allows a related series of samples to be analyzed using a single calibration curve. This is an important advantage in laboratories where many samples are to be analyzed or when the need for a rapid throughput of samples is critical. Not surprisingly, many of the most commonly encountered quantitative analytical methods are based on an external standardization. 

An analytical method is standardized by determining its sensitivity. There are several approaches to standardization, including the use of external standards, the method of standard addition. 

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. 

The scale of operations, accuracy, precision, sensitivity, time, and cost of methods involving redox titrations are similar to those described earlier in the chapter for acid-base and complexometric titrimetric methods. As with acid-base titrations, redox titrations can be extended to the analysis of mixtures if there is a significant difference in the ease with which the analytes can be oxidized or reduced. Figure 9.40 shows an example of the titration curve for a mixture of Fe + and Sn +, using Ce + as the titrant. The titration of a mixture of analytes whose standard-state potentials or formal potentials differ by at least 200 mV will result in a separate equivalence point for each analyte. 

Precipitation titrimetry is rarely listed as a standard method of analysis, but may still be useful as a secondary analytical method for verifying results obtained by other methods. Most precipitation titrations involve Ag+ as either an analyte or... 

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. 

Quantitative analytical methods using FIA have been developed for cationic, anionic, and molecular pollutants in wastewater, fresh waters, groundwaters, and marine waters, several examples of which were described in the previous section. Table 13.2 provides a partial listing of other analytes that have been determined using FIA, many of which are modifications of conventional standard spectropho-tometric and potentiometric methods. An additional advantage of FIA for environmental analysis is its ability to provide for the continuous, in situ monitoring of pollutants in the field. ... 

An important step in developing a standard method is to determine which factors have a pronounced effect on the quality of the analytical method s result. The procedure can then be written to specify the degree to which these factors must be controlled. A procedure that, when carefully followed, produces high-quality results in different laboratories is considered rugged. The method by which the critical factors are discovered is called ruggedness testing. ... 

Before sample preparation, surrogate compounds must be added to the matrix. These are used to evaluate the efficiency of recovery of sample for any analytical method. Surrogate standards are often brominated, fluorinated, or isotopically labeled compounds that are not expected to be present in environmental media. If the surrogates are detected by GC/MS within the specified range, it is... 

Most specifications and analytical methods have been given (72). Most of the standards have remained unchanged for the past half-century. They were designed for acid recovered from wood tar condensates. Ah acid of commerce easily passes these tests. 

Analytical Procedures. Standard methods for analysis of food-grade adipic acid are described ia the Food Chemicals Codex (see Refs, ia Table 8). Classical methods are used for assay (titration), trace metals (As, heavy metals as Pb), and total ash. Water is determined by Kad-Fisher titration of a methanol solution of the acid. Determination of color ia methanol solution (APHA, Hazen equivalent, max. 10), as well as iron and other metals, are also described elsewhere (175). Other analyses frequendy are required for resia-grade acid. For example, hydrolyzable nitrogen (NH, amides, nitriles, etc) is determined by distillation of ammonia from an alkaline solution. Reducible nitrogen (nitrates and nitroorganics) may then be determined by adding DeVarda s alloy and continuing the distillation. Hydrocarbon oil contaminants may be determined by ir analysis of halocarbon extracts of alkaline solutions of the acid. 

Automated analyzers may be used for continuous monitoring of ambient poUutants and EPA has developed continuous procedures (23) as alternatives to the referenced methods. Eor source sampling, EPA has specified extractive sampling trains and analytical methods for poUutants such as SO2 and SO [7446-11-9] sulfuric acid [7664-93-9] mists, NO, mercury [7439-97-6], beryUium [7440-41-7], vinyl chloride, and VOCs (volatile organic compounds). Some EPA New Source Performance Standards requite continuous monitors on specified sources. 

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