Biomedical Engineering Reference
In-Depth Information
20
15
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0
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Sample Sequence No.
System Suitability
CS and QC
Incurred Sample
Fig. 9
Low and variable internal standard responses caused by autosampler problem occurred
during the middle of run injection. Extraction: evaporation-free protein precipitation; sample
volume: 50 mL; IS volume: 150 mL. Reproduced from ref. [
36
] with permission from Elsevier
Malfunction of Autosampler
In bioanalysis, extracted samples are usually stored in either autosampler vials or
wells in a plate (such as 96-well plate) sealed with pierceable caps or covers. During
injection, the autosampler needle has to pierce the caps or covers to load samples.
The debris may completely or partially block the autosampler needle, which would
result in no sample or variably low sample volumes injected. Accordingly, no or
randomly low IS responses are observed. As most autosamplers have a built-in nee-
dle flushing mechanism, the debris in the needle might be flushed out later partially
or completely. Therefore, the injected volume can be back to normal at a later time
without an operator's intervention. Apparently, when a needle will be blocked and
when the blocked needle will be cleared by flushing, as well as how it will be
blocked (completely or partially) are difficult to predict. Hence, there would be no
clear pattern for this type of IS variations. However, the affected injections normally
have lowered IS responses (Fig.
9
). Despite lowered IS responses, the accuracy of
quantitation can usually be maintained except for situations where no or very low
amount of samples are injected, resulting in responses outside the limit of linear
range or unacceptable S/N.
To solve the problem, the affected run can be reinjected in a different
LC-MS system or on the same system after the needle of the autosampler is
cleaned.
