High Pressure MALDI
Matrix Assisted Laser Desorption/Ionization (MALDI) generates ions by shooting a laser at a small crystalline spot containing the analyte mixed with a “matrix” molecule which absorbs light strongly at the wavelength of the laser. The resulting plume of matrix and analyte molecules (and a small number of ions) is generally very hot because of the absorption of the laser light and is usually accelerated into the mass spectrometer which tends to vibrationally excite or “heat” the analyte molecules. Depending on the inherent stability of the analyte molecules, they can and do fragment spontaneously after ionization, a property that is often taken advantage of for structural determination and is called metastable decay or post-source decay. Metastable decay occurs to a greater extent in systems where the delay time between ionization and detection is long.
Is metastable decay a problem? Well, it depends. Like nozzle-skimmer dissociation in a electrospray ion source, it can be both useful and problematic. For relatively stable homogeneous samples, metastable decay gives you fragmentation that can be used for structural analysis – essentially giving you a free stage of MSn. However, for extremely fragile analytes or for mixtures of somewhat fragile analytes, fragmentation in the source is a very big problem.
For extremely fragile analytes, such as gangliosides, the spectra tend to show masses of the core molecule after loss of all the fragile portions, which makes determination of number or position of the fragile groups difficult. For mixtures of even moderately stable species, fragmentation in the source generates extremely complex spectra in which the fragment ions cannot be correlated with a particular parent ion, and determining the mass of the various original mixture components or the structure of any of them becomes very difficult.
High pressure MALDI is a technique for controlling the fragmentation in the MALDI source and simply involves raising the background pressure in the source to >100 mbar during the laser desorption event. This method cools the analyte and can substantially decrease metastable fragmentation. We are currently implementing this on the MALDI-FTMS instrument and plan to implement it also on the MALDI-qTOF instrument that should be operational in the spring of 2002.
We have currently published one paper on this topic, and have a second submitted. In addition, the results from this work have been presented at four conferences (ASMS 2001, ACS NERM 2001, 6th European FTMS meeting, Glycobiology 2001) and two local chemistry seminars (Cornell University, Northeastern University).
References:
1. O’Connor, P. B.; Costello, C. E., A High Pressure Matrix-Assisted Laser Desorption/Ionization Fourier Transform Mass Spectrometry Ion Source for Thermal Stabilization of Labile Biomolecules Rapid Commun. Mass Spectrom. 2001, 15, 1862-1868.
2. O’Connor, P. B.; Mirgorodskaya, E.; Costello, C. E., High Pressure Matrix-Assisted Laser Desorption/Ionization Fourier Transform Mass Spectrometry for Minimization of Ganglioside Fragmentation J. Am. Soc. Mass Spectrom. 2001, Submitted.
Presentations:
ASMS 2001 powerpoint talk
Glycobiology 2001 poster
