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Calibration
To calibrate LC systems, follow acceptable calibration methods using at least
five standards to generate a standard curve. The concentration range for
standards should cover the entire range of quality-control samples, typical
specimens, and atypical specimens.
To calibrate mass spectrometers, consult the calibration section of the
operator’s guide of the instrument you are calibrating.
Quality control
Routinely run three quality-control samples that represent subnormal,
normal, and above-normal levels of a compound. Ensure that quality-control
sample results fall within an acceptable range, and evaluate precision from
day to day and run to run. Data collected when quality control samples are out
of range might not be valid. Do not report these data until you are certain that
the instrument performs satisfactorily.
When analyzing samples from a complex matrix such as soil, tissue,
serum/plasma, whole blood, etc., note that the matrix components can
adversely affect LC/MS results, enhancing or suppressing ionization. To
minimize these matrix effects, Waters recommends you adopt the following
measures:
•
Prior to the instrumental analysis, use appropriate sample
pretreatment such as protein precipitation, liquid/liquid extraction
(LLE), or solid phase extraction (SPE) to remove matrix interferences.
•
Whenever possible, verify method accuracy and precision using
matrix-matched calibrators and QC samples.
•
Use one or more internal standard compounds, preferably
isotopically-labeled analytes.