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

Abstract

AbstractA coal tar pitch has been fractionated by preparative and analytical size exclusion chromatography (SEC) to provide narrow 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, Mp, and by calculating number (Mn) and weight (Mw) 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 Mp 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 Mp values below m/z 3,000 and the calibration of SEC by polymers. Working with Mp gave better results than the Mn and Mw 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.