Specifications detector response

Essential features of an automated method are the specificity, ie, the assay should be free from interference by other semm or urine constituents, and the sensitivity, ie, the detector response for typical sample concentration of the species measured should be large enough compared to the noise level to ensure assay precision. Also important are the speed, ie, the reaction should occur within a convenient time interval (for fast analysis rates), and adequate range, the result for most samples should fall within the allowable range of the assay. 

GC/ECD or a halogen-specific detector (HSD) (Method 8080) is the technique recommended by EPA s Office of Solid Waste and Emergency Response for determining a- and [3-endosulfan and endosulfan sulfate in water and waste water at low-ppb levels (EPA 1986a). At these low concentrations, identification of endosulfan residues can be hampered by the presence of a variety of other pesticides. Consequently, sample clean-up on a Florisil column is usually required prior to analysis (EPA 1986a). 

The effort required to establish identity of a nitrosamine in an environmental sample depends on the nature of the problem and the specificity of the primary detection system. TEA response is much stronger evidence of identity than response from a flame ionization or nitrogen-specific detector. If TEA response is supported by chemical (9) or ultraviolet photolysis (8) supporting data, identification is adequate for many... 

Solute property detectors, such as spectroscopic andj electrochemical detectors, respond to a physical or chemical] property characteristic of the solute which, ideally, is] independent of the mobile phase. Althou this criterion is rarely met in practice, the signal discrimination is usually sufficient to permit operation with solvent changes (e.g., flow programming, gradient elution, etc.) and to provide high sensitivity with aj wide linear response range. Table 5.4. Solute-specific detectors complement ulk property detectors as they provide high ... 

Detectors are composed of a sensor and associated electronics. Design and performance of any detector depends heavily on the column and chromatographic system with which it is associated. Because of the complexity of many mixtures analysed and the limitation in regard to resolution, despite the use of high-resolution capillary columns and multicolumn systems, specific detectors are frequently necessary to gain selectivity and simplify the separation system. Many detectors have been developed with sensitivities toward specific elements or certain functional groups in molecules. Those detectors that exhibit the highest sensitivity are often very specific in response, e.g. the electron capture detector in GC or the fluorescence detector in LC. Because... 

The first classification is based on the nature of the detector response. Table 4.7 ranks several chromatographic detectors as specific and nonspecific. A nonspecific or universal detector responds to all solutes present in the mobile phase and this performance makes it a... 

Amarnth and Amamth [15] described a specific method for the determination of penicillamine and cysteine. Treatment with 1,1-thiocarbonyl diimidazole converts penicillamine to 5,5-dimethyl-2-thioxothiazolidine-4-carboxylic add. The detection limit is 2 pmol of the drug per injection, and the detector response is linear up to 1 nmol. 

Interference occurs when compounds co-elute with the analytes and are not detected directly by a specific detector. The effect is to create negative peaks or an erratic response for the analyte. This problem can be identified by using a non-specific detector such as an ion trap MS detector, an MS in the electron impact ionization mode, or a flame ionization GC detector. 

The potential of modern chemical instrumentation to detect and measure the conposition of coirplex mixtures has made it necessary to consider the use of methods of multivariable data analysis in the overall evaluation of environmental measurements. In a number of instances, the category (chemical class) of the compound that has given rise to a series of signals may be known but the specific entity responsible for a given signal may not be. This is true, for example, for the polychlorinated biphenyls (PCB s) in which the clean-up procedure and use of specific detectors eliminates most possibilities except PCB s. Such hierarchical procedures simplify the problem somewhat but it is still advantageous to apply data reduction methods during the course of the interpretation process. 

Before the ruggedness test could be contemplated it was essential to fully validate the method with respect to other method characteristics. Hence the following tests were carried out, specificity, spectral purity of chromatographic peaks, linearity of detector response, and repeatability over 100 injections. Satisfactory results were achieved for all these experiments before we continued to the ruggedness test. 

System Suitability. Although method validation is performed once at the end of method development, system suitability tests are performed on a specific system periodically (usually daily) or prior to each batch during validation and sample analysis to determine the system performance (see Chapter 13). During method development or/and upon completion of the validation, system suitability data should be evaluated and used to define acceptance criteria to use before starting sample analysis. System suitability tests include (1) the reproducibility of retention time, (2) adequate sensitivity to quantify LLOQ (minimum detector response), (3) appropriate sensitivity to quantify ULOQ (within range of detector), and (4) chromatographic separation. 

Enzymes can be used in several ways in chromatographic applications to improve selectivity or to enhance the detector response. Applications may involve enzymes with either a broad specificity toward a group of related compounds or a high specificity toward a particular compound. In the field of drug residue analysis, most current applications concern enzymatic reactions taking place in separate reactors incorporated in LC systems before or after the analytical column. Reactors with immobilized enzymes have proven to be suitable in such continuous flow systems. 

When specific detectors are used to quantify a certain class of compounds in the presence of a background of many nonspecific compounds, it is not enough that the specificity be sufficient to get well-resolved peaks for the desired compounds. It is also important that the presence of the nonspecific background does no not change the response factors for the compounds of interest. 

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