It is possible to grow large, protein doped matrix crystals under near equilibrium conditions, rather than in a rapidly drying droplet [11,17]. Supersaturated matrix solutions containing protein will form crystals that can be used directly in an ion source. Supersaturation can be achieved by heating, cooling or slow evaporation. The protein-doped crystals can be cleaved to expose well defined faces to the laser beam.
In general the slow crystallization approach favors the detection of high mass components over low mass peptides, regardless of pH and solution
Producing large protein-doped crystals has several disadvantages compared to the fast drying (non-equilibrium) crystallization techniques described above.
1. It is slower. Crystals take hours to grow, Definitely not practical for large-scale, high-throughput applications.
2. Peak broadening is often observed.
3. High mass accuracy is out of the question due to the irregular geometry of the sample bed.
4. Growing crystals requires more analyte (10-100x) than traditional methods.
However, even with those difficulties some advantages are also realized:
1. Crystals can be grown from solutions with involatile solvents at concentrations that suppress ion signals from dried droplet experiments.
2. High concentrations of non-protenaceous solutes do not affect crystal doping. Detergents are an exception.
3. Mixtures of polypeptides can be incorporated into crystals and analyzed.
4. Crystals can be easily manipulated. Common operations are washing, cleaving, etching and mounting.
5. The crystals are very rugged.
6. The crystals provide more defined starting conditions for fundamental MALDI Ionization studies.