Biomedical Engineering Reference
In-Depth Information
Once an internal standard is selected, its concentration must be properly deter-
mined based on the particular situation, such as impurities of reference standards,
concentration range, and detection sensitivity towards the analyte and its internal
standard, and finally experimentally verified to ensure adequate linearity, accuracy,
and precision.
Due to the differences between spiked samples (calibration standards and quality
controls) and incurred samples as well as the potential inter sample differences,
variations in internal standard response during incurred sample analysis are some-
what expected and they should be monitored with predetermined criteria to identify
any potential bioanalysis abnormality. Many different factors (not necessarily only
matrix effect) could cause variations in internal standard responses during incurred
sample analysis. The same phenomenon, e.g., consistently higher IS responses for
all the samples of a subject, can be caused by different reasons. Accordingly, each
case should be dealt with individually with an open mind.
Stable isotope labeled internal standards may be the best, but they cannot always
follow an analyte to compensate the variations of experimental conditions, particu-
larly deuterated internal standards. In addition, low variation in internal standard
responses may not be interpreted as good results, though it is favored. Stable inter-
nal standard response is good only when it is sure that the internal standard behaves
the same way as the analyte does.
Acknowledgments
We would like to thank Dr. Wen Jin in the University of Guelph for her
reviewing of the draft manuscript and valuable comments. In addition, Tan would like to thank his
family (Cailin and Joyce) for their support during the preparation of this topic chapter.
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