Matching average masses of pitch fractions of narrow polydispersity, derived from matrix-assisted laser desorption ionisation time-of-flight mass spectrometry, with the polystyrene calibration of SEC

A coal tar pitch has been fractionated by preparative and analytical size exclusion chromatography (SEC) to provide harrow time-elution fractions, expected to have relatively low molecular dispersities. The fractions were characterised by analytical SEC, by UV-fluorescence spectroscopy, and by laser-desorption MS and by MALDI-MS using sinapinic acid as matrix. The MALDI spectra were evaluated by measuring the peak-intensity mass, M-p, and by calculating number (M-n) and weight (M-w) average masses. The high mass limits of the spectra were determined by three methods based on (i) truncating the spectrum when the signal fell below 5 times the standard deviation of signal at the instrument limit, (ii) taking the upper mass as defined by a minimum slope of the spectrum from baseline, and (iii) taking all of the high mass signal as from sample. The laser desorption spectra were evaluated by M-p values only; the results below m/z 1,000 were in excellent agreement with the polymer calibration. The MALDI mass spectra gave a good fit between M-p values below m/z 3,000 and the calibration of SEC by polymers. Working with M-p gave better results than the M-n and M-w values from Method (i). Results from Methods (ii) and (iii) were significantly worse. Reasons for the lack of agreement between results above m/z 3,000 by MALDI and above m/z 1,000 by LD-MS have been discussed.