Sample matrix deposit

Basile, F. Kassalainen, G.E. Williams, S.K.R. Interface for direct and continuous sample-matrix deposition onto a MALDI probe for polymer analysis by thermal field flow fractionation and off-line MALDI-MS. Anal. Chem. 2005, 77, 3008-3012. 

Severe MS performance degradation has been correlated to exposure of porous Si to air. The DIOS substrates are therefore required to be stored in ethanol until needed. For the substrates coated with tissue sample, matrix deposition should occur immediately out of the same concern. 

Aerni H, Cornett D, Caprioli R. Automated acoustic matrix deposition for MALDI sample preparation. Anal. Chem. 2006 78 827-834. 

An analyte may be present in one material phase (either a solid or liquid sample) and, as part of the sample preparation scheme, be required to be separated from the sample matrix and placed in another phase (a liquid). Such a separation is known as an extraction—the analyte is extracted from the initial phase by the liquid and is deposited (dissolved) in the liquid, while other sample components are insoluble and remain in the initial phase. If the sample is a solid, the extraction is referred to as a solid-liquid extraction. In other words, a solid sample is placed in the same container as the liquid and the analyte is separated from the solid because it dissolves in the liquid while other sample components do not. 

For example, our preliminary MALDI MS studies employed a somewhat unconventional sample preparation approach in which the protein deposition step preceded the matrix deposition step. (In the conventional approach, the protein is co-deposited with the MALDI matrix solution.). .. Thus, in the first/next/colleclively group of experiments, we will explore different sample preparation methods to determine if other protocols also lead to an inverse relationship between surface-protein binding affinity and the MALDI ion signal. 

After collection, samples were divided into 5-cm sections or into sections determined by changes in sample matrix. Only sediment from the interior of the core (not in contact with the corer) was used to avoid possible contamination from the polycarbonate and from sediment fines forced along the wall of the tube. Sectioned samples were stored in solvent-rinsed bottles at 4 °C until analyzed. Some samples used in this study were collected from reaches of the submerged riverbed (where PCBs were deposited before the reservoir was filled). Sample locations (Figure 1) were determined in relation to buoys placed and maintained by the U.S. Army Corps of Engineers. Samples used in this study were limited to those found to be contaminated with at least 1 xg/g of total PCB to minimize quantification error during weight-percent calculations. 

Splitless injection involves keeping the injector split vent closed during the time the sample is deposited on the column, after which the vent is reopened and the inlet purged with carrier gas. In splitless injection, the inlet temperature is elevated with respect to the column temperature. The sample is focused at the head of the column with the aid of the solvent effect. The solvent effect is the vaporization of sample and solvent matrix in the injection port, followed by trapping of the analyte in the condensing solvent at the head of the column. This trapping of the analyte serves to refocus the sample bandwidth and is only achieved after proper selection of the solvent, column and injector temperatures. Splitless injection techniques have been reviewed in References 29 and 30. 

The second method is sandwich method which was developed from the two-layer method and used first for the analysis of the single mammalian cell lysates [67], The first thin layer is formed by the matrix-only solution. It is followed by deposition of the analyte solution and then deposition of second layer of matrix on the top of analyte layer. The sample is basically sandwiched between the two matrix layers and preparation results in a matrix-sample-matrix sandwich. This method is specifically used for detecting protein and peptides [67-69], The three-layer matrix-sample preparation method was also used as matrix-matrix-sample mode [70],... 

For a few tissue samples, the matrix deposition can be performed manually. A matrix solution is sprayed onto the tissue section with a hand-held thin layer chromatography (TLC) sprayer or an artist airbrush. The reproducibility of manual matrix deposition is an issue. When the manual sprayer is used, the MALDI target plate with the tissue section is held vertically about 15-25 cm from the sprayer nozzle. It is recommended to spray multiple coats of matrix across the tissue section and each coating cycle consists of passing the sprayer two to hve times across the tissue section and allowing the tissue to dry for about 1-5 min. This process is usually repeated between 10 and 20 cycles. 

There are three different types of GC/FTIR interfaces light-pipe, matrix isolation (MI), and cryo-deposition (also direct-deposition, cryotrapping). In the two latter techniques, the sample is deposited on a surface before measurement of spectra. All three techniques have been used for the analysis of CWC-related chemicals. Light-pipe interface has been the most popular, even though the usage of cryodeposi-tion in this type of analysis has been increasing over the years. 

This is the simplest technique but is prone to contamination or element loss by evaporation. Also the sample matrix may become too concentrated to pass through the nebuliser and burner without deposition. Since the matrix is also concentrated the final sample aspirated may suffer from matrix interference. This should be investigated and if necessary a method of standard additions used see section V. 

A wide variety of desorption ionization methods is available [7] desorption chemical ionization (DCI), secondary-ion mass spectrometry (SIMS), fast-atom bombardment (FAB), liquid-SIMS, plasma desorption (PD), matrix-assisted laser desorption ionization (MALDI), and field desorption (FD). Two processes are important in the ionization mechanism, i.e., the formation of ions in the sample matrix prior to desorption, and rapid evaporation prior to ionization, which can be affected by very rapid heating or by sputtering by high-energy photons or particles. In addition, it is assumed that the energy deposited on the sample surface can cause (gas-phase) ionization reactions to occur near the interface of the solid or liquid and the vacuum (the so-called selvedge) or provide preformed ions in the condensed phase with sufficient kinetic energy to leave their environment. 

In MALDI, the sample is deposited on a target and co-ciystallized with a solid matrix [14-15]. The target is transferred to vacuum and bombarded by photon pulses from a laser, in most cases a nitrogen laser (337 nm) nowadays. The ionization results from efficient electronic excitation of the matrix and subsequent transfer of the energy to the dissolved analyte molecules, which are desorbed and analysed as protonated or cationized molecules [7]. The ionization process is not fully understood. Extremely high molecular-mass compounds, e.g., in excess of 200 kDa, can be analysed using the MALDI, if performed on a time-of-flight mass spectrometer (Ch. 2.4.3). 

Flameless atomic absorption spectrophotometry is essentially very simple. A substrate upon which the sample matrix can be deposited is placed in or immediately adjacent to the spectrophotometer light beam, and a means of heating this substrate rapidly to 800°-3500°C is provided. Electrical resistance is usually the heating method used. The substrate... 

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. 

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