Co-eluting impurities

Stmctural confirmation is essential for chemical development analyses to assure that the proposed compound is correct. For example if an impurity peak in a UV monitored trace increases it is essential to confirm that it is due to the proposed compound and not a co-eluting impurity. An important application is... 

An additional advantage inherent to the FSFDs is that the presence of co-eluting impurities can be observed. These compounds that would normally go undetected and affect quantitation should be readily seen, either as additional fluorescence if the compound does fluoresce or as reduced fluorescence or skewing of the spectrum if the compound only absorbs UV light When working at typical analytical levels, the fluorescence signals from two compounds are additive, the first problem can be both spotted and corrected for. 

Co-eluting impurity. Our prep HPLC method was of lower resolution than that of analytical. Impurities eluting closely to the target compound may get collected in the same fraction. 

A new method was developed for the separation of three known and ten new anthraquinone pigments. In addition, five new pigments were determined by fourier transformation mass spectrometric detection (FTMS) as co-eluting impurities (120). The analyses were performed on a Kinetex C18 column. 

FIGURE 8 Critical regions in a chromatogram to evaluate ( I) co-elution of impurities, (2) highly retained compounds (apolarity issue), (3) non-retained compounds (polarity issue), and (4) co-elution with the API (isomers). 

If reference materials of the suspected impurities are available, the drug substance or finished drug product should be spiked at an appropriate level to demonstrate that the result is unaffected by the addition of the impurity. Figure 2 shows examples of individual chromatograms of the API and three known process impurities. As shown here, none of the three process impurities interfere with the API peak, although peaks for impurities A and C appear to overlap and could co-elute if both were present in the sample. This specificity may be acceptable if the method was designated as an assay method for the quantitation of the API. For a method intended to quantitate process impurities, the overlap of these two components would in most cases be unacceptable due to the inability of the method to accurately measure the two individual components. 

Complete resolution of the LC peaks of interest is not always necessary as shown in Figure 6.36. In this case the identity of the minor co-eluting peak was known from LC-MS, which aided in the identification of the unknown impurity. 

Optional) Inject the internal standard(s) to determine if there are any degradation products or impurities that may co-elute and absorb at the wavelength of the analytes, and to determine the purity correction of the internal standard. 

MS triggering of stop-flow spectra even provides an additional feature - it is now possible to monitor the peak purity as a triggering criterion, instead of just the peak maximum in conventional UV-triggered experiments. Thus, in the case of partial co-elution, the stopping of the LC pump can be delayed until a peak elutes without major impurities. 

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