MALDI sample preparation, applications

In the second part of the chapter, the application of MALDI mass spectrometry for probing noncovalent interactions will be discussed. While special precautions have to be taken to avoid dissociation of complexes during MALDI sample preparation and during the desorption/ionization step, there are some extremely useful features of MALDI that make it suitable for fast readout in solvent accessibility studies (via HDX) and even for the determination of noncovalent binding constants via the so-called SUPREX method. 

As noted above, whole-cell MALDI-TOF MS was intended for rapid taxonomic identification of bacteria. Neither the analysis of specific targeted bacterial proteins, nor the discovery of new proteins, was envisioned as a routine application for which whole cells would be used. An unknown or target protein might not have the abundance or proton affinity to facilitate its detection from such a complex mixture containing literally thousands of other proteins. Thus, for many applications, the analysis of proteins from chromatographically separated fractions remains a more productive approach. From a historical perspective, whole-cell MALDI is a logical extension of MALDI analysis of isolated cellular proteins. After all, purified proteins can be obtained from bacteria after different levels of purification. Differences in method often reflect how much purification is done prior to analysis. With whole-cell MALDI the answer is literally none. Some methods attempt to combine the benefits of the rapid whole cell approach with a minimal level of sample preparation, often based on the analysis of crude fractions rather... 

The second method of sample preparation for IMS is a matrix-coating method for MALDI imaging. In this chapter, we review the choices of matrix compound and solvent composition appropriate for IMS of tissue sections. Three kinds of matrix-application methods and examples of their use are illustrated. 

MALDI, which is LDI utilizing a particular sample preparation). Although the performance of MALDI is superior to LDI in the analysis of many groups of compounds, LDI is still the perferred choice in some important applications, including cmde oil analysis [155], fullerene detection in rocks [156], atmospheric aerosol analysis [157], semiconductors, and surface analysis [158]. Reference 21 is a comprehensive review of the use of LDI (and several other ion sources) in analysis of inorganics. 

MALDI tutorial with useful information concerning sample preparation, calibration, and applications. 

This chapter describes and discusses the mostly used IMS technology—MALDI-TOF-IMS, including sample preparation, data interpretation/handling, and its applications. 

The wide application of newly developed advanced Surface MALDI MS methods, such as laser desorption ionization (LDI), surface-enhanced laser desorption ionization (SELDI), direct ionization on silicon surface (DIOS), etc. have two shortcomings. First, problems with MALDI-TOF MS sample preparation for nano-sized systems have not been completely worked out. Second, the reliability and reproducibility of experimental data concerning similar bio-active systems is lacking. Sometimes molecular, associative and fragment ions cannot, be observed in the MALDI mass spectra. We are currently working on overcoming these difficulties with some encouraging results.13,14... 

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. 

Currently developed for many applications (Stoeckli et al. 1999 Stoeckli et al. 2001, 2002,2003 Chaurand et al. 2004), MALDI MSI is achieved by rastering sequentially the surface of a defined area while acquiring a mass spectrum from every location (see Figure 2). Atypical sample preparation for MSI involves the fixation of the sample, for example, tissue section, on a MALDI plate and the application of the matrix solution over the latter, either as a thin layer or as a spot pattern, to get co-crystallization of analytes with matrix while solvents evaporate. Once dried, the sample is introduced in the mass spectrometer, where, for each defined image position, short UV laser pulses are fired onto the surface to generate ions. Those are analyzed by the TOF instrument and a mass spectmm is acquired. 

Apart from automating the matrix application process, it is critical to evaluate the resulting matrix crystals and coating for analyte extraction, localization, and effect on tissue architecture. Several reports have shown that even for standard analytes mixed with matrices, there is an uneven distribution of the analyte within the MALDI crystals and that sample preparation influences the resulting distribution [18-20], Thus, other active areas of research are focused on optimizing matrix application and sample preparation protocols. 

In addition to some early applications in bioanalysis, ambient ionization mass spectrometry has been used as an imaging tool to study drug distribution in tissue sections. Most of the work reported so far involved the use of DESI as the ambient ionization method. Compared to other mass spectrometry-based tissue imaging techniques such as MALDI and SIMS, DESI allows tissue samples to be analyzed under ambient conditions without sample preparation, which simplifies the procedure and prevents the redistribution of analytes during matrix deposition. A major drawback of DESI as an imaging tool is its relatively low spatial resolution (typically 250 pm) and therefore cannot be used for cellular or subcellular imaging. 

The book covers several subtopics of miniaturization and mass spectrometry. It combines (i) technological developments in the quest for miniaturization of sample preparation and how to connect micrometer-sized devices to a mass spectrometer, (ii) various illustrations of fields that benefit from such a hyphenated technique and (iii) technological developments for the miniaturization of the mass spectrometer. Additionally, the book is not restricted to one ionization technique as is often the case for many reviews, but it reports on efforts for both the ESI and MALDI ionization techniques. After an introduction to miniaturization and mass spectrometry, the book is divided in three sections that respectively concern (i) ESI-MS applications, (ii) MALDI-MS applications and... 

MALDI MS is an analytical approach suitable for obtaining molecular weights of peptides and proteins from complex samples. MALDI MS can profile the peptides and proteins from single-cell and small tissue samples without the need for extensive sample preparation, except for cell isolation and matrix preparation. Strategies for peptide identification and characterization of post-translational modifications are versatile and broadly applicable. " ... 

Application of Solid-Phase Extraction Sample Preparation Before MALDI-MS 1355... 

The optimal matrix-sample preparation is ultimately dependent on the MALDI instrument used and the nature of the sample. Thus, the optimal sample preparation for a certain application has to... 

Application of Microdispensing and Nanovials for MALDI-MS Sample Preparation... 

---