Biomedical Engineering Reference
In-Depth Information
With the exception of the LC-MS/MS methods recently published, previous
assays were using either no IS [
241
] , structurally related IS [
194,
195,
218,
226,
227,
236,
237,
243,
244
], or a single SIL-IS [
193,
235,
238
] as a surrogate IS for the
quanti fi cation of tamoxifen/metabolites.
Since SIL-ISs are not always available and their use rather expensive, especially in
the case of multiple analytes analysis, the use of structurally related compounds or
analogue IS with different mass and with close or similar chromatographic behavior to
that of the analytes can represent an acceptable alternative. Nevertheless, in these latter
instances, ME variability between different sources of plasma (relative matrix effect
variability) must be investigated and quantified. From the assays operating with either
no IS or a unique analogue IS, only three methods quantitatively assessed for ME vari-
ability. Zweigenbaum J and Henion J [
235
] reported a signi fi cant ion suppression
which approximately halved 4-hydroxytamoxifen signal. This ion suppression was not
corrected by the IS and affected the precision and accuracy of the method that failed to
meet the acceptance criteria for 4-hydroxy-tamoxifen quantification. Furlanut et al.
[
241
] monitored Tam,
N
-D-Tam, and 4-OH-Tam in serum and tissue of BC patients,
employing external standard calibration and reported no ion suppression problem after
quantitative evaluation of ME. Unfortunately, no detailed information was available
regarding the extent of matrix effects variability and the number of plasma lots tested.
Only the recent method described by Beer et al. [
218
] thoroughly examined ME
variability using the quantitative approach proposed by Matuszewski et al. [
256,
257
] .
It is noteworthy that ME variability should be investigated even when using SIL-
ISs. In fact, SIL-IS may not fully correct for matrix effects, obviously when they do
not completely co-elute with their corresponding analyte. This phenomenon has
been particularly observed with deuterated SIL-IS that were found to be less lipo-
philic than their corresponding non-deuterated analogues, causing a slightly earlier
elution on a reversed-phase column [
258
] .
Although most recent developed assays used SIL-IS, only few methods quantita-
tively investigated potential ME variability on tamoxifen and its metabolites
quanti fi cation [
145,
245
] .
In our proposed assay [
145
], we thoroughly investigated ME both qualitatively
using the post-column infusion system proposed by Bonfiglio et al. [
259
] and quan-
titatively using the recommendations of Matuszewski et al. [
256,
257
] and the 2007
Washington workshop/conference report [
260
]. Although the qualitative examina-
tion of ME did not show any signal alteration, probably due to the infusion of high
concentration of analytes, quantitative ME examination showed an ion suppression
of approximately 40 % for the signal of
N
-D-Tam. We observed a similar extent of
ion suppression with the deuterated
N
-D-Tam (
N
-D-Tam-d5) and ascertained that
SIL-IS effectively corrected for the absolute and relative ME (or ME effect vari-
ability among six different lots of plasma). Therefore, this was a good illustration of
the value of SIL-IS use for an efficient control of residual matrix effects.
Besides the use of SIL-IS, another upstream and primordial approach that allows
to anticipate and drastically reduce matrix effects is the optimization of sample
preparation procedure.
