Dried droplet preparation

Note The conventional co-crystallization is usually termed dried droplet preparation. The original thin layer technique involves preparation of a thin HCHA layer from solution in acetone on top of which the analyte is placed in a second step without re-dissolving the matrix. [95,97]... 

There are two common matrix preparation methods, thin-layer and dried droplet preparation. For thin-layer preparations the matrix is tqiplied to the MALDl target plate in a volatile solvent, such as acetone. The solvent spreads and evaporates... 

For analytes of very low proton affinity, such as neutral carbohydrates and many synthetic polymers, cationization by Na, K or other metal cations is usually observed in MALDI (see Chapters 6-8). The cationization in all likelihood takes place in the expanding plume, and requires a codesorption of the analyte and the cations. Hence, the best results are obtained from sample locations where both species exist in close neighborhood, such as in the center of DHB-dried droplet preparations. Specific protocols have been developed for the MALDI of such analytes [66, 67]. 

Among the many modifications and variations of the simple dried-droplet preparation, two alternatives stand out as particularly useful and widespread, namely surface preparation and anchor sample plates. ... 

Anchor plates for the preparation of multiple samples have small hydrophilic islands, typically of 100-500 pm diameter, placed on a hydrophobic surface [147]. The hydrophobic surface prevents spreading of the sample solution over a larger area, as otherwise observed for dried-droplet preparations. Instead, the hydrophilic solution contracts onto these islands, thereby concentrating the matrix and analyte onto a small defined area upon solvent evaporation. This confinement to a smaller volume is particularly useful for analytes of low concentration in combination with proportionally lowered matrix concentrations, and also facilitates automated analyses of the fixed-location samples. Anchor sample plates are also commercially available as disposable targets prespotted with matrix and calibration spots. 

Comparison between vacuum subhmed matrices and conventional dried droplet preparation in MALDI-TOF mass spectrometry. J. Am. Soc. 

Figure 4.6 High-resolution MALDI image showing distribution of the peptide substance P (panel b) in a regular dried-droplet preparation using the matrix 2,5-dihydroxybenzoic acid (panel a). The image corresponding to the distribution of...

Figure 4.7 High-resolution MALDI image of a dried-droplet preparation of a homogeneous solution of three peptides substance P, meiittin and insulin using...

This does not mean that dried-droplet preparations cannot be imaged. Indeed, clearly dependent on the chemical structure, certain compounds do remain in the tissue (e.g., the (prenol) lipid ion at 610.444). For specific analyte molecules such as these latter compounds, the dried-droplet method can thus be used as a specific sample preparation method to wash away unwanted compounds. In this respect, it is interesting to note that the corpora cardiaca are the main synthesis sites of the insect juvenile hormone (JH), also a prenol lipid of the class of terpenoids. JH as such is not secreted in the adult stage of the insect imaged here, but the presence of other terpenoids in this tissue is expected. 

The crystallization process is a critical parameter in LDI and MALDI sanple preparation [50,105,106]. The conventional co-crystallization is usually termed dried droplet preparation. Dried droplet preparation yields comparatively large crystals, especially when slow evaporation, e.g., from aqueous solutions, is involved. Unfortunately, large crystals are detrimental for good shot-to-shot reproducibility and mass accuracy. 

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. 

The solid-sample preparation is usually achieved by the deposition on a metallic surface of the solution of matrix and analyte with a concentration suitable to obtain the desired analyte/matrix ratio. The solution is left to dry under different conditions (simply at atmospheric pressure, reduced pressure, or under a nitrogen stream). This method is usually called the Dried Droplet Method. In all cases, what is observed is the formation of an inhomogeneous solid sample, due to the different crystallization rate of the matrix and analyte. Consequently, the 10 molar ratio is only a theoretical datum In the solid sample, different ratios will be found in different positions and the only way to overcome this is to average a high number of spectra corresponding to laser irradiation of different points. 

The dried droplet method is the method originally introduced by Hillenkamp and Karas (Fig. 4.7). A saturated matrix solution, 5-10 g depending on the solubility of the matrix, is prepared in water, water-acetonitrile, or water-alcohol mixtures. In a second vessel, the sample is diluted to about 100 mg L Mn a solvent that is miscible with the matrix solution. The matrix and sample solutions are then mixed such that the final molar ratio is 10,000 1 with a final volume of a few tiL. 

The samples were prepared by mixing the lignin water soluhons with acetone (4 mg/ml, 50/50 water/acetone by volume). The sample soluhons so prepared were mixed with an acetone soluhon of the matrix (10 mg matrix soluhon per ml acetone). As the matrix 2,5-dihydroxy benzoic acid was used. For enhancement of ion formahon, NaCl was added to the matrix (10 mg/ml in water). The soluhons of the sample and the matrix were mixed in proportions 3 parts matrix soluhon 4- 3 parts lignin soluhon -I- 1 part NaCl soluhon, and 0.5 to 1 pi of the resulhng soluhon mix were placed on the MALDI target. After evaporation of the solvent the MALDI target was introduced into the spectrometer. The dry droplet sample preparahon method was used. 

Dried-droplet technique. The dried-droplet technique (also referred to as the one-layer spot) is the most widely practiced mode of sample preparation [30,50], In this technique, a few microliters of the sample solution (e.g., in 0.1% aqueous TFA) is mixed with an equal volume of the saturated matrix solution (prepared in the same medium) in a molar ratio of 1 1000 to 10,000. A drop of that mixture is applied onto the MALDI target and is dried slowly in the ambient air or by a gentle stream of cold air. Drying the sample spot under vacuum or in a refrigerator can improve the homogeneity of the sample preparation. 

The most common method for preparing a MALDI target, known as the dried droplet technique, is to mix the sample and matrix, usually in a ratio of about 1 5000 in lpl of solvent, and allow the mixture to dry on the target. Mixing can occur either before addition to the target or, more commonly, solutions of the sample and matrix are added to the target independently and allowed to mix. Most... 

The dried droplet standard MALDI sample preparation is very simple. Here, the sample and matrix are dissolved in solvents or solvent systems that are miscible with each other, and mixed either before deposition onto or directly on the MALDI sample support. The matrix-analyte droplet of typically 1 pi volume is then slowly dried in air, or under a forced flow of cold air. This results in a deposit of crystals which, depending on the matrix and preparation conditions (e.g., solvent evaporation rate), vary between submicrometer and several hundred micrometers in size. In cases of solvent systems with a high polarity, surface tension leads to a nonhomogeneous distribution of the individual crystals near the rim of the... 

For the MALDI-MS analysis of intact proteins, FI CCA (alpha-cyano-4-hydroxycinnamic acid), SA (sinapinic acid) or DHB (2,5-dihydroxybenzoic acid) matrices and the dried-droplet deposition method for sample preparation are typically used [13, 14] (Table 3.1). Depending on the properties of the protein, it is often necessary to test a series of solvents and matrices to optimize the outcome of the MALDI-MS experiment. Peptides and small proteins below molecular weight 20000 Da are often amenable to analysis using HCCA matrix and reflector TOF-MS mode, whereas larger proteins may produce better results with SA or DHB matrix in the linear TOF-MS mode. Hydrophobic proteins can be analyzed using the HCCA matrix dissolved in high concentrations of formic acid (up to 30%) [15]. When using cirmamic acid matrices, SA and HCCA, and the... 

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