Comparison of a preliminary procedure for the general unknown screening of drugs and toxic compounds using a quadrupole-linear ion-trap mass spectrometer with a liquid chromatography–mass spectrometry reference technique

Abstract

Liquid chromatography–tandem mass spectrometry (LC–MS–MS) might be a complement to GC–MS and HPLC–diode array detection for the general unknown screening (GUS) of drugs and toxic compounds, particularly when using information- or data-dependent acquisition (IDA or DDA), an auto-adaptive MS–MS product-ion scan mode where, at each unit time, the m/ z ratios above a given intensity threshold are selected for fragmentation. A new quadrupole-linear ion-trap mass spectrometer (LC-QqQlinear ion-trap) was evaluated for GUS using IDA. For the first detection step (so-called “survey scan”) the single quadrupole “enhanced” MS mode (EMS), where ions are accumulated then filtered in the Q3-linear ion-trap, was used. The so-called “enhanced” parent ion scan mode (EPI) used at two alternated fragmentation energies gave the best signal intensity and the best mass spectral information when adding mass spectra obtained in low and high fragmentation conditions, respectively, both in the positive (+20 and +50 eV) and negative (−15 and −40 eV) modes. Solid-phase extracts of serum spiked with eight test compounds (chosen for their retention times distributed along the 30-min long chromatogram and for ionising in both the positive and negative modes) were analysed in parallel with this LC–MS–MS technique and with a reference LC–MS method run on a single-quadrupole instrument where low and high in-source fragmentation conditions in the positive and the negative ion modes are alternated. A C 18, 5 μm (150×1 mm I.D.) column and a gradient elution of acetonitrile in pH 3, 2 m M ammonium formate, were used for both. Higher signal-to-noise ratios were obtained with the LC-QqQlinear ion-trap instrument than with the reference technique, resulting in mass spectra devoid of contaminant ions and at least as informative as the reconstructed single-MS spectra. After optimisation of the IDA intensity threshold for the detection of tiny chromatographic peaks in noise, five out of the eight compounds (milrinone, lorazepam, fluometuron, piretanide and warfarin) could be unambiguously identified at the concentration of 0.1 mg/l in serum, in the positive or negative modes, or in both, versus only two by LC–MS. All of them could be identified at 1 mg/l by both techniques. These preliminary results show that the sensitivity and mass structural information brought by this new LC-QqQlinear ion-trap instrument may help design an efficient toxicological GUS procedure.