Electrospray as a sample deposition for MALDI-MS was suggested by Owens and co-workers , and independently, by Axelsson et al. [References, 19]. In this technique, a small amount of matrix-analyte mixture is electrosprayed from a HV-biased (3-5 KV) stainless steel capillary onto a grounded metal sample plate, mounted 0.5-3 cm away from the tip of the capillary.
Electrospray sample deposition creates a homogenous layer of equally sized microcrystals and the guest molecules are evenly distributed in the sample. The method has been proposed to achieve fast-evaporation and to effectively minimize sample segregation effects. The presence of cation adducts in the MALDI spectra from electrodeposited samples demonstrates that solution components are less segregated than in equivalent dried-droplet deposits.
Electrospray matrix deposition was used by Richard Caprioli and collaborators in 1997 [References, 21] to coat tissue samples during the MALDI –based molecular imaging of peptides and proteins in biological samples. Matrix-only solution was electrosprayed on TLC plates by Clench and co-workers for the direct MALDI analysis of the impurity spots of tetracycline samples [References, 28].
Electrospray deposited samples have been shown to give several advantages over traditional droplet methods:
1. The reproducibility of MALDI results from spot-to-spot within one sample desposit, and from sample-to-sample for multiple depositions, is much improved. Typical sample-to-sample variations are in the 10 to 20% range.
2. The correlation btween analyte concentration and matrix signal is also improved. Quantitation with internal standards has been reported by Owens [References, 18].
3. The sample deposits are much more resistant to laser irradiation. More shots can be collected from any single laser spot location.
4. The method offers a possible path for interfacing MALDI sample preparation to Capillary electrophoresis and liquid chromatography.
1. Slower. It takes 1 to 5 minutes to create a useful deposit. It also takes time to switch to a new analyte since the capillary must be thoroughly cleared of any leftover sample from the last measurement before spraying can start.
2. Salt adducts are a problem and desalting of the matrix and the sample is usually needed to eleminate cationization signals.
3. Extra equipment is required, along with training.
4. It involves the use of dangerous high voltages.
Aerospray (pneumatic spraying) has been suggested as an alternative sample spraying method. Recent results by Wilkins et. al. have demonstrated high degree of reproducibility for this sample preparation technique [References, 32]. Homogeneous thin films can be easily made, with good spot-to-spot and sample- to-sample reproducibility.
The potential exists to combine both techniques, using aerospray for the nebulization and an electric field to control solvent evaporation and droplet size.