Biomedical Engineering Reference
In-Depth Information
120
100
80
60
40
Methanol
20
Acetonitrile
0
0
10
20
30
40
50
Time (h)
Fig. 15
Response ratio of raloxifene-D
4
-6-glucuronide to raloxifene-6-glucuronide vs. the delay
of usage after the preparation of raloxifene-D
4
-6-glucuronide in methanol or acetonitrile
standards, it was because they were not in acetonitrile. They were in plasma matrix
from the very beginning and there was enough water for their solubilization.
Furthermore, sometimes a change of solvent supplier or lot could cause issues of
stability for an internal standard and therefore IS response variations. For example,
Napoli reported unusual IS response variation and stability problem when a differ-
ent lot of acetonitrile was used for the preparation of ascomycin working solution
(as the internal standard for tacrolimus, [
43
] ).
Lengthy Sample Treatment Procedure
When multiple sample treatment steps are used, especially derivatization is involved,
larger IS response variations are usually expected. For instance, in a method based
on LLE followed by derivatization and SPE, as large as fourfold difference in IS
responses was observed for CS and QC samples as well as incurred samples
(Fig.
16
). Despite this large IS response variation, the accuracy of the concentration-
known CS and QC samples was not impacted, it can be therefore concluded that the
concentrations of unknown samples are also reliable.
Others
In addition to the above, there might be other less common causes for IS response
variations. For example, due to mixed usage of rubber-lined caps and PTFE
(polytetrafluoroethylene)-lined caps during the liquid-liquid extraction shaking
(mixing) step, randomly low IS responses were observed for extracted samples, i.e.,
