TuO-06


ACCURATE MASS MEASUREMENT AT LOW RESOLUTION BY USING ELECTROSPRAY MASS SPECTROMETRY ON A QUADRUPOLE INSTRUMENT

Andrew N. Tyler, Brian N. Green*, Eddie Clayton+ and Michael Morris*

Department of Chemistry, Harvard University, Cambridge, MA 02138, USA
+Zeneca Pharmaceuticals, Alderley Park, Macclesfield, Cheshire SK10 4TG
*Micromass UK Ltd, Tudor Road, Altrincham, Cheshire WA14 5RZ


Hitherto, the accurate mass measurement of the molecular weight of organic compounds has generally been made at high-resolution by using double-focusing or Fourier transform instruments. In principle these measurements can also be made at low resolution providing certain criteria are met. One major criterion, common to all mass measurements by mass spectrometry, is that the sample and reference peaks have homogeneous composition, i.e. they are composed of ions having a single elemental composition. Operating at relatively low resolution increases the probability of peak overlap i.e. heterogeneous peaks occurring, in which the peaks to be mass measured contain more than one elemental composition. However, since electrospray ionisation (ESI) generates relatively background-free spectra, one can often avoid such interferences by judicious choice of reference material.

Other important criteria are:

  1. Data are collected with sufficient channels on the m/z scale to allow the positions of the peaks to be defined with sufficient accuracy.
  2. Peaks contain sufficient ions to ensure that statistically significant data are obtained.
  3. Both sample and reference peaks on the m/z scale are measured in the same way.
We demonstrate the use of a single quadrupole equipped with ESI to make accurate mass measurements on organic samples with molecular weights in the range 190-750 Da. Our technique gives accuracies comparable with those achieved with high-resolution instruments, with measured masses within 5 ppm of the expected masses from a single experiment of 1 minute duration. Results will be presented showing the accuracy and precision from repeat measurements on samples producing positive- negative-, singly- and doubly-charged ions.

The practical application of the technique will be described and the theoretical and practical limitations will be discussed.