Time-of-flight mass spectrometry: an increasing role in the life sciences

Although available commercially since the mid-1950s, there has been a renewed and increasing interest in time-of-flight mass spectrometers during the last decade or more. Improvements have been made in mass resolution; and high-speed data acquisition systems have been developed which enable the recording of all ions in each time-of-flight cycle. Most importantly, these instruments have been coupled with several new ionization techniques, which are capable of desorbing the relatively large and intractable biopolymers whose structures are of interest to molecular biologists, biochemists and biophysicists. Primarily these are techniques which employ pulsed lasers, fission fragments and pulsed ion beams, for which a 'non-scanning' and/or high-transmission analyzer provides considerable analytical advantage. In this report we review some basic principles of the time-of-flight mass analyzer, highlighting efforts to improve dynamic focusing for instruments forming ions in the gas phase and static focusing for desorption instruments, and the progression from time-slice to time-array detection. We also review some of the accomplishments of instruments employing the time-of-flight analyzer, including: molecular weight determinations for peptides and small proteins; the analysis of tryptic digests, crude extracts and whole cells; the structural analysis of glycolipids, phospholipids and lipopolysaccharides; and the determination of covalent and metal-linked peptide dimers. We conclude with some recent developments in combining the time-of-flight analyzer with liquid chromatography using the continuous flow probe technique.