Droplets MALDI

FIGURE 8.12 (a) Laserspray mass spectrum of bovine insulin obtained by laser ablation of a 2,5-dihydroxybenzoic acid/bovine insulin mixture prepared using the dried droplet MALDI sample preparation method, (b) The DriftScope presentation of the IMS/MS acquisition of the data. 

Figure 12.3. MALDI-MS signals from a droplet deposited in the rat brain tissue section containing a drug candidate are plotted against acquiring time which is directly related to the locations of each pixel.

Figure 12.4. An ion image of a droplet in the rat brain slice was obtained after reconstmction of MALDI signals from Figure 12.2.

In contrast to MALDI the principle of ESI is based on fine spraying of sample solution. The initially formed electrically charged droplets are rapidly desolvated, permitting the individual dendrimer species to be observed as isolated ions in the mass spectrum. 

J. B. Young and L. Liang, An impulse-driven liquid-droplet deposition interface for combining LC with MALDI MS and MS/MS, J. Am Soc. Mass Spectrom., 17 (2006) 325-334. 

One of the requirements in MALDI-MS analysis is the use of a liquid matrix. The electrowetting-on-dielectric (EWOD) method has been used to move and mix droplets containing proteins and peptides with the liquid matrix, all of which were situated at specific locations on an array of electrodes. With this method, insulin (1.75 pM), insulin chain B (2 pM), cytochrome c (1.85 pM), and myoglobin (1.45 pM) have been analyzed [518]. 

It is important to note that each of these ionization sources, either a laser in MALDI-TOF or the high voltage ionization of droplets in ESI-MS/MS or LC-MS/ MS, will each produce a different spectrum of detectable ions and intensities because the effectiveness and nature of peptide ionization is quite different for each source. In addition, the presence of multiple peptides that influence each other s ionization potential notably through ion suppression makes most peptide ion measurements only semiquantitative. 

A number of different sample preparation methods have been described in the literature [37,38], A collection of these protocols is accessible on the Internet [39,40], The original method that is always the most widely used has been called dried-droplet. This method consists of mixing some saturated matrix solution (5-10 pi) with a smaller volume (1-2 pi) of an analyte solution. Then, a droplet (0.5-2 pi) of the resulting mixture is placed on the MALDI probe, which usually consists of a metal plate with a regular array of sites for sample application. The droplet is dried at room temperature and when the liquid has completely evaporated to form crystals, the sample may be loaded into the mass spectrometer. 

On-probe purification using derivatized MALDI probe surfaces has been described to simplify the sample preparation process. Various developments in this field have allowed the introduction of new techniques such as the surface-enhanced laser desorption ionization (SELDI) [42], The surface of the probe plays an active role in binding the analyte by hydrophobic or electrostatic interactions, while contaminants are rinsed away. In the same way, this technique uses targets with covalently coupled antibodies directed against a protein, allowing its purification from biological samples as urine or plasma. Subsequent addition of a droplet of matrix solution allows MALDI analysis. 

Fig. 1. MALDI and electrospray processes are represented schematically. Electrospray ionization involves the formation of charged droplets, while MALDI occurs through the laser evaporation from a crystallized sample/matrix mixture. In both techniques, the sample ions are extracted and transmitted through electrostatic lenses to a mass analyzer...

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