Internal standard tracked dilution to overcome challenges in dried blood spots and robotic sample preparation for liquid chromatography/tandem mass spectrometry assays

Dried blood spot (DBS) technology is an emerging alternative for sample collection in bioanalysis. Dilution for DBS samples is a challenge due to its solid sample format. Currently, DBS samples requiring dilution were first extracted as regular samples and then diluted with extracted blank samples containing internal standard (IS). Since the dilution step is a volume-critical step, extra care has to be taken to achieve accurate dilution when dealing with limited volume extracted samples. Here, we introduce an alternative sample dilution for liquid chromatography/tandem mass spectrometry (LC/MS/MS) assays using IS to track the dilution step. Dilution factor-adjusted IS working solution was first added to the sample requiring dilution before sample processing; subsequently, the processed sample was approximately diluted into the assay linear response range before LC/MS/MS analysis. We define this approach as IS-tracked dilution. The advantage of this approach is that the diluting step is tracked by the IS and is no longer a volume-critical step. Another recognized challenge related to sample dilution is automatic sample dilution using a liquid handler. This IS-tracked dilution may also help address some of the challenges for automatic sample dilution of liquid samples. This new dilution approach was proven to be effective and convenient in both plasma assays and DBS assays using omeprazole as a probe compound. Copyright (C) 2011 John Wiley & Sons, Ltd.