Determining a 'safe' high-mass limit in matrix-assisted laser desorption/ionisation time-of-flight mass spectra of coal derived materials with reference to instrument noise

Three methods for determining a 'safe' estimate for high-mass limits of MAL DI spectra of coal derived liquids were explored, using a sample of coal-ta r pitch and its pyridine-insoluble fraction. Go-addition of increasing numb ers of single-shot spectra (10, 30, 50 and 100 pulses) showed visually obse rvable reductions in noise levels, consistent with robust and statistically meaningful signals, Three separate types of postacquisition calculation we re used to identify high-mass limits of the spectra, (i) A literature metho d indicated high-mass limits similar to those observed visually-as a shift from baseline at the highest masses, nearly 350 000 u for the coal tar pitc h and about 390 000 u for its pyridine insoluble fraction. (ii) Comparing i nstrument signal with pre-selected multiples of the standard deviation, upp er mass estimates of between 40-60 000 u for the coal-tar pitch and about 9 5 000 u for its pyridine-insoluble fraction were found. (iii) Calculation o f the slope was used to identify 'lift-off' of the spectrum from baseline. The angle between the smoothed spectrum and the baseline was matched to a p re-selected value (e.g. 0.5 degrees and 1 degrees). However, the arbitrary specification of the key parameter did not establish this last method on a firm basis. The choice of a criterion for estimating high-mass limits of MA LDI spectra remains a semi-quantitative procedure; a reasonably conservativ e high-mass limit may be estimated by comparison of signal with five times the standard deviation, However, evaluation of size exclusion chromatograms of the present samples using polystyrene standards suggests that molecular mass distributions of pitch samples arrived at by MALDI mass spectrometry are, at least partly, determined by the limitations of available instrument s. Copyright (C) 1999 John Wiley & Sons, Ltd.