Background: Currently, manual pipetting of human sample material is still a key process in sample preparation for chromatographic and mass spectrometric analyses in the clinical laboratory. In most cases, however, pipettes used in clinical laboratories are only specified for handling water-like liquids. Actual pipetted liquid volumes can be verified by weighing within the sample preparation process, and the results can be corrected accordingly (gravimetric correction). The aim of our study was to evaluate and compare the effects of gravimetric correction in terms of accuracy and precision for an air cushion and direct replacement pipette.
Methods: Forty-fold serial determination of linezolid in a spiked serum sample by ID-LC-MS/MS was applied as an exemplary measurement procedure. Polypropylene tubes were weighed before the addition of 50 µL serum, after the addition, and after the addition of the internal standard solution. Coefficients of variation (CV) were calculated as an indicator of measurement precision.
Results: The use of a direct replacement pipette was associated with improved measurement imprecision than an air cushion pipette (CV 1.70% vs 2.49% for serum, uncorrected results). The results obtained after gravimetric correction showed improved precision with the use of an air cushion pipette compared to the conventional approach (CV 1.51% vs 1.61%). Using a direct replacement pipette, the impact of gravimetric correction on imprecision was negligible.
Conclusion: Using direct replacement pipettes in sample preparation enables more precise ID-LC-MS/MS analyses than using air cushion pipettes. Gravimetric correction can be a useful tool to improve the precision of LC-MS/MS measurement procedures when complex matrices such as human serum are handled with commonly used air cushion pipettes.
Keywords: Chromatography; Gravimetric correction; Mass spectrometry; Measurement imprecision; Pipetting.
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