MoP-21


QUANTITATIVE CLECTROSPRAY MASS SPECTROMETRY FOR DIRECT ANALYSIS OF ILLICIT ALKALOID MIXTURES - A NEW SEMIEMPIRICAL MODEL FOR ESI WITH MULTICOMPONENT MIXTURES

D. Selby*, R. WellsÝ, J. MurbyÝ and M. Guilhaus*

*School of Chemistry, The University of New South Wales, Sydney 2052 Australia
ÝAustralian Government Analytical Laboratories, 1 Suakin Place, Pymble, NSW 2173


Electrospray ionisation mass spectrometry (ESI MS) is a sensitive method for the detection and identification of alkaloids and many other drugs. The mechanism of ESI is thought to involve several solution-phase, surface and gas-phase processes. The resulting competition for charge in electrosprayed mixtures of analytes leads to non-linearities in response that make direct quantitative analysis of mixtures by ESI impossible unless the non-linear relationships can be predicted.

We have developed a non-linear model for the direct analysis of an n-component mixture in which the competition for charge is predicted. The model requires an internal standard to be present in each sample and the prior determination of a constant for each of the components to be analysed. Responses for each of the components in the mixture in a particular analysis are entered into a spreadsheet wherein an iterative algorithm determines the concentration of each of the components relative to that of the internal standard. The model suggests that interfacial and gas-phase processes as well as solution processes (diffusion) must be taken into account to model the competition for ionisation in ESI of mixtures.

For a blind study involving a large number of illicit alkaloid analyses the model gave errors of less than 5% total alkaloids. The errors were distributed normally only when the solution effects were incorporated in the model. The work has resulted in a method which can screen illicit heroin samples in a fraction of the time currently taken by classical methods based on gas-chromatography/mass-spectrometry.