Peak purity confirmation

Caution needs to be exercised in confirmation of method specificity, as peak purity cannot be inferred from the absence of any mass spectrometric signals, as an absence could be because of ion suppression, or the response may too low to be detected, or the unknown may have different ionisation properties. Peak purity should be tested by using an orthogonal technique. 

A more popular approach for detemining peak purity has been the use of diode-array detectors which were first introduced in 1982 [50]. Peak homogeneity of similiar benzodiazepines [51] and theophylline have been determined by this technique [52]. Rapid-scan detectors are also useful in confirming the presence of known components such as colorants, which are commonly used in drug products [53,54] and identification of related substances [55,56]. 

Dual Channel Plot from a Diode Array Detector Confirming Peak Purity Courtesy of the Perkin Elmer Corporation... 

Before the quantitative information contained in a chromatographic peak can be used, the purity of the peak should be confirmed. Only after we are sure that no coeluting impurity was present, which could have contributed to the peak response, can we convert the peak s area or height into quantitative information based on the equivalent response of a pure standard. This peak purity analysis can be based on the comparison of the various spectra recorded during the elution of the peak. If the peak is pure, then, apart from concentration differences, the spectra taken at several points during a peak s elution should all be identical and the match scores obtained should be very close to the perfect match scores. If significant deviations are encountered, this can be seen as an indication of impurity. 

One of the most important aspects of peak purity analysis that is often overlooked is the fact that any peak purity algorithm can only confirm the presence of impurities, but can never unambiguously prove that a peak is pure. 

Peak purity analysis is very useful in chromatographic method development, to confirm that all components have been chromatographically separated, and in quality control, to warn the analyst that an unexpected coeluting impurity has appeared. 

Intended use User requirement specification Analysis of drug products and substances Automated analysis of up to 100 samples/day Limit of quantitation <0.05% Automated confirmation of peak purity and identity with diode-array detection Compatible with 2-mm to 4.6-mm i.d. columns... 

The identities of individual mustard glucosinolates were confirmed by negative ion eJectrospray mass spectrometry as was LC peak purity. This LOMS method may be used to identify species which are not commercially available as pure standards since mass spectrometry can be used lo check for all known glucosinolate anions. 

Main band peak purity should be confirmed. 

Specificity is one of the most important characteristics of a stability-indicating method and should be determined as one of the first validation parameters. A specific method can accurately measure the analyte of interest even in the presence of potential sample components (placebo ingredients, impurities, degradation products, etc.). When criteria for specificity are not met, this often indicates that the method is not sufficiently developed furthermore, it is likely that criteria for accuracy, precision, and linearity may also not be fulfilled. A major objective of determining specificity is to ensure peak purity of the main compound to be determined in other words, confirm that no related compoimd or product ingredient co elutes and interferes with the measurement of the assayed compound. Specificity is required from the sample matrix as well as impurities and potential degradation products. 

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