Formation of Non-Specific Protein Cluster Ions in Matrix-Assisted Laser Desorption/Ionization: Abundances and Dynamical Aspects

Intact non-covalent biological complexes resulting from interactions in solutions have been observed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF-MS). However, their analysis is hampered by the presence of non-specific, non-covalent complexes (clusters) produced under MALDI conditions. A study of the influence of laser energy and ion extraction conditions allows information to be obtained on (i) the collision regime in the plume and (ii) ion stability, through the abundances and time-of-flight (t_tof) deviations of bovine insulin clusters using a sinapinic acid matrix. In the linear mode, it is shown that (i) the neutral contribution to quasi-molecular peaks is below 10% and (ii) for related cluster species, the t_tof difference between neutrals and ions is less than 10 ns. Although the abundance of cluster ions increases relatively to the abundance of the monomeric ions with laser energy, heavier cluster ions are unfavored at higher laser energy. On the other hand, the abundance of clusters is, to a first approximation, independent of the electric field in the first acceleration region (750–32,500 V cm⁻¹) and of the extraction delay (up to 4000 ns). Gaussian-like distribution profiles are observed, whatever the acceleration field, for all cluster ion signals. This is in good agreement with the natural isotopic distribution. There is no significant difference between monomeric and cluster ion initial velocities (420 m s⁻¹ ± 18 m s⁻¹). Our results suggest a single origin for bovine insulin cluster ions that are observed by using a sinapinic acid matrix. The clusters should be formed promptly after the laser pulse (< 10 ns), in the selvedge of the surface. Smaller cluster ions could be formed by consecutive fragmentations of higher mass molecular clusters occurring after plume formation and before a significant ion acceleration.