Radiofrequency-only daughter scan mode to provide more spectral information in liquid chromatography—thermospray tandem mass spectrometry

Abstract

A method is presented for increasing the number of specific ions in LC-thermospray mass spectra by means of a quadrupole tandem mass spectrometer (Finnigan TSQ-70) in the radiofrequency-only daughter (RFD) scan mode. The method can be used for screening a large number of compounds eluted from an HPLC system. MS-MS in the usual daughter, parent or neutral loss scan mode (on retention time altered for each eluted compound) in this particular instance is very laborious or even impossible. In the RFD scan mode the first quadrupole is operating as a high-pass mass filter. Only ions with masses equal to or above the arbitrary selected cut-off mass will enter the collision cell. With a low collision offset voltage mainly molecular ions will be present in the third quadrupole, which is acting as a mass analyser in the full-scan mode. With medium and high collision offset voltages, daughter ions are generated in the collision cell. By using two or three different alternating collision offset voltages during one analysis, both molecular and daughter ions can be acquired, increasing the specificity of the mass spectrum. First, data on optimization of the low collision offset voltage and the collision gas (argon) pressure with a mixture of alachlor, atrazine, aldicarb and barban are presented. Next, spectral information and data about the sensitivity of twenty compounds (alachlor, aldicarb, aniline, atrazine, benzothiazole, carbendazim, chloridazon, diazinon, dimethoate, diuron, ethylenethiourea, isocarbamide, isoproturon, metamitron, metolachlor, monolinuron, propachlor, sethoxydim, simazine and warfarin) in the RFD scan mode at three collision offset voltages (-6, -20 and -40 V) are presented and compared with the single-stage Q3MS scan mode. The sensitivity proved to be the same or better at collision offset voltages of -6 and -20 V, partly because adducts and eluent clusters were decreased significantly or even disappeared. At a collision offset of -40 V the sensitivity decreased for many compounds and the more intense ions mainly had low m/z values, which are less specific. The RFD scan mode, using a -6 and -20 V collision offset voltage alternating in each scan, is demonstrated by screening a surface water sample (river Rhine) spiked with ten compounds at a level of 1 μg/1. It resulted in chromatograms with increased spectral information, the same or better signal-to-noise ratios, less eluent clusters and no adducts.