Selectivity of an analytical methods

The selectivity of an analytical method is determined by comparing test results from the analysis of samples containing impurities (related compound), degradation products (originated from samples submitted to stress conditions), or placebo ingredients with those obtained from the analysis of samples without impurities, degradation product, or placebo ingredients. 

A literature search was usually the first step that resulted in the selection of an analytical method consistent with one of the common sampling methods. The objective of these methods is to... 

In the design of a study and the selection of an analytical method, it is very important that adequate attention be paid to the extent of validation and field applicability of a particular method. Not all of the methods have been validated to the same extent. It is the analyst s responsibility to determine the data quality needed before initiating the application of a particular method. 

Even using uncertainty factors, the problem of determining the reliability of qualitative methods has not be solved because the usual statistical approaches are often not applicable. In residue analysis, this problem is often amplified because concenftations frequently are in the low or even sub-ppb range. Most promising appears to be a model that helps in estimating, in arbitrary units, the overall selectivity of an analytical method on the basis of partial selectivity indices. Selectivity indices are nothing more than a combination of the above-mentioned tools with the experience obtained within the European Union from recognized laboratory experts (26). 

The selectivity a in linear HPLC does not express selectivity in the same sense as we have defined the selectivity of an analytical method above. It is still a reasonable measure of analytical selectivity, because the resolution / of two neighboring peaks (in this case the analyte and the interferent peak, respectively) is directly proportional... 

Selectivity of an analytical method is the extent to which, in a given procedure, other substances affect the determination of a substance according to a given procedure . When using an analytical method, consideration needs to be given to the effect that (a) impurities, (b) degradants and (c) the matrix might have on the ability of the method to discriminate between these components and the analyte. 

The terms selectivity and specificity are often interchangeably used. The term specificity generally refers to a method that produces a response for a single analyte only, while the term selective refers to a method that provides responses for a number of chemical entities that may or may not be distinguished from each other. Because there are very few methods that respond to only one analyte, the term selectivity is usually more appropriate. The USP defines selectivity of an analytical method as its ability to accurately measure an analyte in the presence of interferences, such as synthetic precursors, excipients, enantiomers, and known (or likely) degradation products that may be expected to be present in the sample matrix. 

The selection of an analytical method may also be influenced by external regulatory requirements or by special requests of the customer. The following terms often appear in the analytical jargon and were compiled by Garfield [8] the classification of methods relies on purpose of the method or the administrative background. 

The selectivity of an analytical method can be improved by modification of medium conditions for the measurement. A very important way to increase the selectivity is by masking the interfering ions. The most often utilized masking reagents are oxalate, sulfate, phosphate, and EDTA. The main characteristics of compounds resulting after the masking process are... 

Another way to improve the selectivity of an analytical method is by the selective extraction of the analyte from the matrix followed by its determination. 

It has been proved that the selectivity of an analytical method is directly connected to the complexity of the matrix from which the analyte must be determined. As a result, the same method can be more selective or less selective, depending on the qualitative and quantitative composition of the matrix from which the analyte must be determined. For example, two types of electrochemical sensors are described for the assay of thyroid hormones L-T3 and L-T4. The first is an amperometric biosensor based on L-amino acid oxidase (l-AAOD),270 whereas the other is an amperometric immunosensor based on anti-L-T3 and anti-L-T4.271 If T3 and T4 have to be determined from phramaceutical products, both types of sensors have the necessary sensitivity and selectivity. When it is required to determine both hormones in biological fluids or in thyroid tissue, the proposed biosensors are not selective enough because l-AAOD catalyzes the reactions of both thyroid hormones. The amperometric biosensors can only make the discrimination between the two thyroid hormones, namely, L-T3 and L-T4, since the specific antibody reacts only with the specific antigen. 

Selectivity of an analytical method refers to the degree 10 which the inelltod i.s free from interference by other species contained in the sample matrix. Unfortunately, no analytical meihtxl is totally free from interference from other species, and frequently steps must he taken to mininiize the effects of these imerrercnces. 

Although the steps in solving analytical problems usually follow the order listed above, knowledge of basic statistics is useful not just for handling the data and method validation but is required for proper sampling and selection of an analytical method. The statistics and definitions needed to understand what is meant by accuracy, precision, error, and so on are covered in Section 1.3. Students not familiar with these terms and concepts may want to read Section 1.3 at this point. Steps (1) and (2) are covered in this section, while steps (3) through (5) are discussed in the sections following Section 1.3. 

The selectivity of an analytical method is a measure of the degree to which the determination of an analyte is interfered with by accompanying analytes or matrix components. K fully selective analytical method enables the analytes to be selected for... 

As discussed previously in Section 6.2.1, the selectivity of an analytical method can be summarized as the ability of the method to measure and differentiate the analytes in the presence of components that may be expected to be present, e.g. metabohtes, impurities, degradants or matrix components. The terminology specificity is sometimes used in the hterature as a synonym for selectivity but more properly describes the property of an analytical method for which only the target analyte gives any response whatsoever, i.e. the extreme case of selectivity. (It seems unlikely that a truly specific method exists for any analyte.) One of... 

The selection of an analytical method is dependent on required response time, sensitivity, and whether a concentration measurement or an isotopic ratio of is required. When rapid determination... 

Selectivity is defined as the ability of the dosage method to render results that are independent from interfering compounds, no matter what the nature of such compounds (co-eluted products, matrix interferents, molecules resulting from bleeding of the stationary phase of the capillary column, and others). Note that the selectivity of an analytical method depends on both the parameters relative to GC-MS coupling and the sample preparation process. 

---