Lasers improve mass spectrometry
Efficient use of lasers as tools for chemical analysis has been a dream of spectroscopists since the power of tunable-laser spectroscopy was discovered in the 1970s. The spectra of compounds of real interest are, however, much more complicated than those of the sodium atoms and diatomic molecules on which the `hero` laser spectroscopy experiments were first performed. Furthermore, molecular bands are mostly at infrared (IR) or ultraviolet (UV) wavelengths so the appropriate tunable lasers first had to be developed. In fact, pulsed visible-output lasers opened u¥access to tunable UV spectroscopy because their peak output power allowed efficient frequency doubling of the laser output into the UV. High peak power became the key to excitation and even ionization of molecules via multiphoton transitions. The ste¥to ionization also made possible the combination of tunable-laser excitation spectroscopy and conventional mass analysis. This turned out to be a revolution in mass spectrometry.
In prelaser analytical mass spectrometry of polyatomic molecules, ionization was usually achieved by an energetic electron beam and was a sensitive experimental parameter because molecular fragmentation governs the mass spectrum. Hence, classical mass spectra of larger organic molecules, for example, rarely show the peaks of the parent molecules, but instead show groups of the most chemically stable fragments of that molecule. Laser ionization has changed this situation and led to a technique based on resonant multiphoton ionization followed by time-of-flight mass spectrometry. In REMPI-TOFMS the laser is tuned to a resonant transition for the first ste¥in a multiphoton ionization ladder--molecules can be ionized with reduced fragmentation so that the parent peak appears in the mass spectrum.
In addition, by introducing resonance excitation selectivity into the ionization process, the mass spectroscopy becomes "two-dimensional," comparable to laser excitation and fluorescence spectroscopy. Resonant ionization is very efficient so the sensitivity is high, which allows real-time data recording. Scientists also discovered that the mass spectral response of a polyatomic molecule depends on the duration over which a laser pulse of given energy interacts with the molecules. Thus, multiphoton tunable-laser ionization opens a new dimension to analytical mass spectroscopy.
U. B.