Simulation of high‐ and low‐resolution mass spectra for assessment of calibration methods

Calibrating mixtures of residual gases in quadrupole mass spectrometry (QMS) can be difficult since low m/z ratios of molecular ions and their fragments result in overlap of signals especially in the lower mass regions. This causes problems in univariate calibration methods and encourages use of full spectral multivariate methods. Experimental assessment of regression methods has limitations since experimental sources of error can only be minimised and not entirely eliminated. A method of simulating full spectra at low and high resolution to accurate masses is described and these are then used for a calibration study of some popular linear regression methods [classical least squares regression (CLS), partial least squares (PLS), principal component regression (PCR)]. Copyright # 2007 John Wiley & Sons, Ltd. Quadrupole mass spectrometry (QMS) is a low-cost mass spectrometric method with diverse applications. QMS is traditionally used with a front-end chromatographic separation technique (e.g. GC/MS, LC/MS). In direct sampling mass spectrometry, no separation method is applied and all components reach the analyser simultaneously. The quadrupole mass filter is commonly used as a residual gas analyser. It consists of four electrode rods set parallel to each other in a quadupolar arrangement, to which radiofrequency (rf) and direct current (dc) voltages are applied. The resulting electromagnetic fields cause ions injected into the mass filter with a particular charge-to-mass ratio to have stable (or unstable) ion trajectories as the dc and rf voltages are scanned. Ideally, the quadrupole electrodes should exhibit a hyperbolic profile in which case the ion motion will begoverned bythe Mathieu equation. 1 Thereare anumberof regions in which the ion motion is stable and these stable regions (zones) may be represented on the Mathieu stability diagram. The stability diagrams define parameters, a and q, which correspond to regions of stable trajectories. The a and q parameters are defined by: