METHODS TO IMPROVE THE FRAGMENT ION ABUNDANCE AND REPRODUCIBILITY OF DAUGHTER ION SPECTRA IN HYBRID TANDEM MASS-SPECTROMETRY

Hybrid tandem mass spectrometers contain an RF-only quadrupole collision cell. However, the transmission properties of such a device are known to be low. Also, owing to the large number of parameters involved in setting up collision cell conditions, the variability of daughter ion spectra is pronounced. This variability causes problems when attempting quantitative analysis using tandem mass spectrometry. It is shown that an increased daughter ion abundance of peptides can be obtained by altering the acidity of the collision gas. The pentapeptide leucine-enkephalin was subjected to collision (at maximum collision energy 400-430 eV) with hydrogen sulphide, argon and ammonia. Low-mass fragment ions, abundance, e.g., immonium ions, increased ca. 3.5-fold on going from the basic gas ammonia to the acidic gas hydrogen sulphide. Reproducibility of daughter ion spectra is possible, using the transmission properties of the collision cell. The characteristics of an RF-only quadrupole collision cell are such that the parent ion intensity at the second detector varies as a function of collision ion energy and displays oscillatory properties. Using the nodes and anti-nodes of the parent ion transmission, it is possible to achieve reproducible daughter ion spectra of a wide range of biopolymers. The tuning of the RF-only quadrupole collision cell is mass dependent and not compound dependent, hence it is possible to optimize and tune it using a standard such as perfluorotributylamine before subjecting valuable or scarce sample to tandem mass spectrometry.