Electrospray sample deposition

Numerous developments in MALDl sample preparation methods have focused on improving sample homogeneity. Among these methods, electrospray sample deposition (BSD) is the most effective technique to achieve sample homogeneity. BSD, when coupled with MALDl-MS, has been reported to markedly improve sample homogeneity and to result in enhanced sensitivity and signal reproducibility [53, 

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Wei, H. Nolkrantz, K. Powell, D.H. Woods, J.H. Ko, M.-C. Kennedy, R.T. Electrospray Sample Deposition for Matrix-Assisted Laser Desorption/lonization (MALDI) and Atmospheric Pressure MALDI Mass Spectrometry With Attomole Detection Limits, Rapid Commun. Mass Spectrom. 18, 1193-1200 (2004). 

Different methods to improve the reproducibility of MALDI spectra were suggested and the majority of them are based on improving the homogeneity of the matrix analyte cocrystals by optimizing the sample preparation method. Using electrospray sample deposition seems extraordinarily helpful in this respect (Wetzel et al. 2004). 

Electrospray sample deposition (ESDEP) is a sample preparation method where matrix and analyte solutions are sprayed on the target surface under the influence of a high-voltage electric field [44,45]. ESDEP is reported to yield much better shot-to-shot and spot-to-spot reproducibility than the dried-droplet method. The improved results are ascribed to the small and evenly sized crystals that are formed, and as a consequence, improved homogeneity of the MALDI sample surface. Hanton et al. [45] analyzed PEG1450, a narrowly distributed poly(ethylene oxide) sample, using an uncommon... 

The signal from electrospray sample deposition shows little shot-to-shot variability as long as the sample depth is thicker than the amoimt of sample that can be ablated by a few laser shots at the same location of the target (42). This methodology also shows good shot-to-shot repeatability for biopolymers (41). 

Figure 7.4. Electrospray sample deposition setup employed for preparation of MALDI sample slides. (Reproduced with permission ftom Axelsson, J. Hoberg, A.-M. Waterson, C. Myatt, P. Chield, G. Varney, J. Haddleton, D. Derrick, J. Improved reproducibility and increased signal intensity in MALDI as a result of electrospray sample preparation. Rapid Commun. Mass Spectrom. 1997, 11, 209-213.)...

Desorption electrospray ionization (DESI) uses an aqueous spray directed at an analyte deposited on an insulating surface (Fig. 8) [14,15], The sample is in the solid form at atmospheric pressure. The spray of charged droplets is produced, as in electrospray ionization (ESI), by passing an aqueous solution (i.e., methanol-water, containing some... 

Once the section is mounted to the sample plate with the desired orientation, matrix solution is deposited on the tissue surface by electrospray deposition, aero spray, or using robotics to deposit small matrix droplets across the tissue surface before MALDI analysis [13]. The most common and least expensive devices available for applying matrix are hand-held aerosol sprayers or air brushes. The main advantage of these devices is that, with careful application, a dispersion of very small droplets... 

Mass spectrometry tests were carried out using a LCQ deca XP + ion trap mass spectrometer (Thermo Finnigan, San Jose, CA, USA). The electrospray emitter tip nib structure was placed on a dedicated holder that is introduced in the inlet of the mass spectrometer. A platinum wire was subsequently inserted in the reservoir feature of the nib, and was connected at its other end on a metallic part on the holder where HV is normally applied for ionization purposes. Solutions to be analyzed were dropped in the reservoir feature using a gel-loader tip typically 5-10 pL of sample were deposited in the reservoir. Liquid was subsequently seen to fuse at the tip of the nib due to capillary action in the slot. HV was then applied, and a spray could be observed with HV values in a... 

Wetzel, S.J., Guttman, C.M. and Flynn, K.M., The influence of electrospray deposition in matrix-assisted laser desorption/ionization mass spectrometry sample preparation for synthetic polymers. Rapid Commun Mass Spectrom, 18 (2004) 1139-1146. 

FI- and FD-MS are truly "soft" ionization methods, with little excess energy being deposited into the ions that are formed. In a recent study some labile, low molecular weight polyesteramides were analyzed by FD-MS, electrospray (ESI-MS), and MALDI-MS—all of which are techniques that produce high abundances of molecular or quasimolecular ions. In this study ESI-MS was found to be the "softest" method (i.e., the one showing the least ion fragmentation and sample decomposition) FD-MS finished second, and MALDI-MS was third. 

Electrospray deposition. In electrospray deposition, the analyte-matrix solution is passed through a capillary held at high potential and is sprayed over the MALDI plate to provide a much-improved homogeneous sample preparation [52]. 

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