Diagnostic:
"Pseudo internal standard"
Internal standardization can improve precision
when the dominant sources of error are related to sample preparation or
injection; because such errors affect both the internal standard and
the analyte peak in the same way, they will tend to cancel out when the
ratio of areas is calculated. Internal standardization can degrade
precision when the dominant sources of error are related to separation,
peak shape, or integration; because such errors affect the internal
standard peak and the analyte peak differently, they will tend to
combine when the ratio of areas is calculated. Even if your method does
not use an internal standard, you can use this to help identify the
source of precision problems, so long as your chromatograms have at
least two peaks of comparable size.
Calculate the ratio
of peak areas (or peak heights) for two peaks on the chromatogram.
Compare the precision of this area ratio to the precision of the areas.
If the area
ratio is more precise than the areas, the dominant source of error
is probably "upstream" of the separation: injection, dilution, sample
preparation, etc.
If the areas
are more precise than the area ratio, the dominant source of error
is probably "downstream" of the injector: separation, baseline noise,
peak shape, integrator settings, etc.
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