Specificity of analytical methods

SPECIFICITY (of analytical method) The ability of the method to measure the analyte in a mannerthat isfree from interference from other components that may normally be excepted to be present such as ingredients, impurities and degradation products. 

The focus of this chapter is photon spectroscopy, using ultraviolet, visible, and infrared radiation. Because these techniques use a common set of optical devices for dispersing and focusing the radiation, they often are identified as optical spectroscopies. For convenience we will usually use the simpler term spectroscopy in place of photon spectroscopy or optical spectroscopy however, it should be understood that we are considering only a limited part of a much broader area of analytical methods. Before we examine specific spectroscopic methods, however, we first review the properties of electromagnetic radiation. 

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. 

Specifications and Analytical Methods. Butanediol is specified as 99.5% minimum pure, determined by gas chromatography (gc), sohdifying at 19.6°C minimum. Moisture is 0.04% maximum, determined by Kad-Fischer analysis (dkecdy or of a toluene a2eotrope). The color is APHA 5 maximum, and the Hardy color (polyester test) is APHA 200 maximum. The carbonyl number is 0.5 mg KOH/g maximum the acetal content can also be measured dkecdy by gc. 

Specifications and Analytical Methods. Purity is specified as 99.5% minimum, by gc area percentage, with a maximum of 0.1% moisture by Kad-Fischer titration. Color, as deUvered, is 40 APHA maximum samples may darken on long storage. 

Specifications and Analytical Methods. Sulfur hexafluoride is made to rigid specifications. Per ASTM D2472-81 (reapproved 1985) (50), the only permissible impurities are traces of air, carbon tetrafluoride (0.05 wt % max), and water (9 ppm by wt max dew point —45° Cmax). 

The fermentation-derived food-grade product is sold in 50, 80, and 88% concentrations the other grades are available in 50 and 88% concentrations. The food-grade product meets the Vood Chemicals Codex III and the pharmaceutical grade meets the FCC and the United States Pharmacopoeia XK specifications (7). Other lactic acid derivatives such as salts and esters are also available in weU-estabhshed product specifications. Standard analytical methods such as titration and Hquid chromatography can be used to determine lactic acid, and other gravimetric and specific tests are used to detect impurities for the product specifications. A standard titration method neutralizes the acid with sodium hydroxide and then back-titrates the acid. An older standard quantitative method for determination of lactic acid was based on oxidation by potassium permanganate to acetaldehyde, which is absorbed in sodium bisulfite and titrated iodometricaHy. 

Specifications and Analytical Methods. The purity of 2-pyrrohdinone is determined by gas chromatography and is specified as 98.5 wt % minimum. Maximum moisture content is specified as 0.5 wt %. Typical purities are much higher than specification. 

There are several comprehensive reviews of analytical methods for vitamin K (19,20). Owiag to the preseace of a aaphthoquiaoae aucleus, the majority of analytical methods use this stmctural feature as a basis for analysis. Several identity tests such as its reaction with sodium bisulfite or its uv spectmm exploit this characteristic. Although not specific, titrimetric, polarographic, and potentiometric methods have also been used (20). 

In addition to this drive to look beyond manufacturing to specifications, new analytical methods such as molecular weight distribution, Mooney relaxation, and other measures of polymer processibiHty are being explored. 

Ethylene oxide is sold as a high purity chemical, with typical specifications shown ia Table 14. This purity is so high that only impurities are specified. There is normally no assay specification. Proper sampling techniques are critical to avoid personal exposure and prevent contamination of the sample with trace levels of water. A complete review and description of analytical methods for pure ethylene oxide is given ia Reference 228. 

Some internationally harmonized guidelines regarding specifications and tests, impurities and validation of analytical methods have particular relevance to the development of chiral drugs and are discussed below. In addition, the impact of work on the common technical document is considered. 

Radioisotope-labeled nitrosamines have proven valuable in development of analytical methods and for demonstrating efficiency of recovery of nitrosamines from tobacco products and smoke (37-39). The very high specific activity required for low part-per-billion determinations has discouraged most analysts from using this approach. Unless a radiochromatographic detector with adequate sensitivity is available, samples must be counted independently of the final chromatographic determination, and one of the advantages of internal standardization, correction for variation in volume injected, is lost. 

Low resolution MS yields specificity comparable to that of high resolution MS, if a relatively pure sample is delivered to the ion source. Either high resolution GC or additional sample purification is required. To obtain sufficient specificity, it is necessary to demonstrate that the intensities of the major peaks in the mass spectrum are in the correct proportions. Usually 10 to 50 ng of sample is required to establish identity unambiguously. Use of preparative GC for purification of nitrosamines detected by the TEA ( ) is readily adaptable to any nitrosamine present in a complex mixture and requires a minimum of analytical method development when new types of samples are examined. 

If analytical methods are validated in inter-laboratory validation studies, documentation should follow the requirements of the harmonized protocol of lUPAC. " However, multi-matrix/multi-residue methods are applicable to hundreds of pesticides in dozens of commodities and have to be validated at several concentration levels. Any complete documentation of validation results is impossible in that case. Some performance characteristics, e.g., the specificity of analyte detection, an appropriate calibration range and sufficient detection sensitivity, are prerequisites for the determination of acceptable trueness and precision and their publication is less important. The LOD and LOQ depend on special instmmentation, analysts involved, time, batches of chemicals, etc., and cannot easily be reproduced. Therefore, these characteristics are less important. A practical, frequently applied alternative is the publication only of trueness (most often in terms of recovery) and precision for each analyte at each level. No consensus seems to exist as to whether these analyte-parameter sets should be documented, e.g., separately for each commodity or accumulated for all experiments done with the same analyte. In the latter case, the applicability of methods with regard to commodities can be documented in separate tables without performance characteristics. 

Analytical procedures are classified as being compendial or non-compendial in character. Compendial methods are considered to be valid, but their suitability should be verified under actual conditions of use. To do so, one verifies several analytical performance parameters, such as the selectivity/specificity of the method, the stability of the sample solutions, and evaluations of intermediate precision. 

Any analytical method inherently carries with it limitations in terms of speed, allowable uncertainty (as MDL), and specificity. These characteristics of a method (or analytical technique) determine where and how the method can be used. Table 71-1 shows a method to relate purpose of analytical method to the speed of analysis and error types permitted. 

The implications of the analysis have to be considered before taking the sample or devising a sampling scheme. It is the responsibility of the analytical chemist, through discussion with the customer, to establish the real nature of the problem. How much cadmium is there in this sample is not sufficiently specific. You must always ask why the information is required. The answer affects both the sampling plan and the choice of analytical method. These will depend on the acceptable level of uncertainty in the final result. 

An important group of analytical methods is based on measurements of the change in isotopic ratio when active and non-active isotopes are mixed. In the simplest case, a known amount w1 of labelled analyte of known specific activity at is added to the sample. After isotopic mixing has been established sufficient of the analyte is separated (nor normally 100%) to allow the new specific activity a2 to be measured. Measurements of activity and the amount of the analyte separated are thus required. Subsequently the amount w2 of analyte in the sample may be calculated from equation (10.17). 

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