Using gas-phase chloride attachment for selective detection of morphine in a morphine/codeine mixture by ion mobility...

Morphine and codeine are two important compounds of the opiate family that have vast applications in medicine. Several techniques have been reported for the determination of these opiates. Although ion mobility spectrometry (IMS) in positive ion mode can be applied for detection of both morphine and codeine, this technique on its own cannot detect a mixture of these two compounds because of the overlapping of their peaks.

An IMS instrument equipped with a corona discharge ion source operating in negative ion mode was used for the detection of anionic clusters of morphine and codeine. In normal negative ion mode, NO_x ^- , CO₃ ^- , and O_n ^- act as the main reactant ions (RIs) which can deprotonate the analytes. We also used chloroform as a dopant to produce Cl^- as an alternative RI.

Morphine has a phenolic and an alcoholic OH group, while codeine bears only an alcoholic OH group. Because the phenolic OH group is more acidic, only morphine is deprotonated in negative ion mode in a morphine/codeine mixture. Furthermore, since morphine has two OH groups that can act as hydrogen-bond donors, it acts as an anion receptor. Hence, in the presence of chloroform where Cl^- acts as the RI, morphine traps the Cl^- anion to form a morphine-Cl^- (Mor.Cl^- ) adduct ion, while because of its structure codeine does not have this capability.

Using the difference in the structures of morphine and codeine, two ionization methods were proposed for selective detection of morphine. Morphine is more acidic than codeine and has greater anion-receiving capability than codeine. Hence, it can both be deprotonated and form a adduct anion with Cl^- . The Cl^- attachment method is recommended for measurements at ambient temperature.