I’m a big advocate of proper sample preparation – after all, it’s an essential step in the analytical process. Typically, this important process enriches target analytes and enables sample clean-up, which eliminates matrix effects. You cannot correct errors that occur in this step even with the most sophisticated analysis. However, over the last few years there has been tremendous progress in mass spectrometry (MS) instrumentation combined with liquid chromatography (LC) that has, to some extent, rendered sample enrichment obsolete, when no significant signal suppression by matrix components occurs. In fact, this is often the case with very clean matrices, such as aqueous samples.
Successful determination of organic compounds down to the low ng/L range has been described, often in combination with injecting large water volumes (above 10 percent of the void volume of the analytical column), which focuses the target analytes on the stationary phase, allowing them to elute only after initiating the elution gradient. The approach clearly requires some retardation of target analytes under the initial eluent conditions, otherwise, we face two problems:
- Insufficient focusing will lead to broad peaks. For instance, in the past decade, the focus of water analysis beyond regular monitoring of priority pollutants has shifted to more polar contaminants and transformation products, so achieving sufficient retardation may be critical.
- Co-elution of most of the interfering polar matrix may suppress or unpredictably enhance the signal. Even in clean water samples, organic and inorganic matrix components are present in concentrations typically orders of magnitude higher than the target analytes.
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