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Abstract

Blister beetles (Coleoptera: Meloidae) are the main natural source of
cantharidin, but the compound titre is depended on several factors
including, age, sex and mating status of the insects. In order to
eliminate such uncertainty factors in physiological and chemical studies
deuterium labelled cantharidin (D(2)C) with no natural abundance is
normally introduced into the beetles' body to use it as a model for
studying the cantharidin behaviour in vivo. Experiments were achieved on
Mylabris quadripunctata (Col.: Meloidae) from Southern France and the
beetles were exposed to an artificial diet containing a defined amount
of D(2)C. On the other hand, because of the high similarity between the
two compounds they cannot be well quantified by gas chromatography. In
order to remove the burden, MRM technique was used for the first time
which could successfully create well-defined cantharidin and D(2)C peaks
and hence a precise measurement. MRM technique was examined using a
GC-MS Varian Saturn which collected MS/MS data of more than one compound
in the same time window of the chromatogram. It is especially useful
when coeluting compounds have different parent ions, i.e. m/z 84 for
D(2)C (coeluting isotopically-labelled compound) and m/z 82 for
cantharidin (beetle-originated compound). Using the routine GC-MS runs,
measurement accuracy may be significantly reduced because the D(2)C peak
is covered by the cantharidin huge peak while MRM could reveal the two
coincided peaks of cantharidin and D(2)C. Therefore MRM is hereby
introduced as the method of choice to separate cantharidin from D(2)C
with high sensitivity and thus provide a precise base of quantitation.