A MALDI Sample Preparation Method Suitable for Insoluble Polymers

Solid-state Nuclear magnetic resonance (NMR) was used here to explore structural characteristics of samples to be subjected to matrix-assisted laser desorption/ionization (MALDI) and prepared without the use of any solvent. The analytical systems scrutinized in NMR were mixtures of a 2,5-dihydroxybenzoic acid (2,5-DHB) matrix and caesium fluoride (CsF), used as the cationization agent in synthetic polymer MALDI mass analysis, at different molar ratios (1:1, 5:1, and 10:1). Complementary information could be obtained from 13 C, 133 Cs, and 19 F NMR spectra. Grinding the matrix together with the salt in the solid state was shown to induce a strong modification in the molecular organization within the MALDI sample. The evidenced mechano-induced reactions allow strong interactions between the matrix and the cation, up to the formation of a salt, and only occur in the presence of some water molecules. Addition of a poly(ethylene oxide) polymer as the analyte did not further modify the observed molecular organizations. Although relative matrix and salt concentrations in the scrutinized samples were unusual for MALDI analysis, mass spectra of good quality could be obtained and revealed that cation attachment on polymers during the MALDI process is not a matrix-independent event since a lower ionization efficiency was obtained from highly organized solid samples, mostly consisting of 2,5-DHB caesium salt species. (J Am Soc Mass Spectrom 2009, 20, 1906 -1911) © 2009 American Society for Mass Spectrometry M atrix-assisted laser desorption/ionization (MALDI) is widely used as an efficient ionization method to characterize synthetic polymers by time-of-flight mass spectrometry (TOF MS) [1,2]. A key point in the success of a MALDI-MS analysis is sample preparation, that is, proper matrix and cation selection as well as solid-state organization. The matrix actually plays multiple roles in the MALDI process and should accordingly present specific properties. Some criteria are clearly defined, such as a strong absorptivity at the employed laser wavelength or good vacuum stability, while other requirements, such as a good miscibility of the matrix with the analyte in the solid state, are not easily related to physico-chemical parameters of the matrix. As a result, prediction for the optimal matrix-polymer system is still not possible and sample preparation methods are usually developed from published protocols that were shown to work for a given polymeric system [3]. In particular, solvent-free MALDI was recently proposed as an alternative to the conventional dried droplet method [4,5].Despite MALDI being a widely used technique, mechanisms underlying this desorption/ionization process are still not fully understood. To investigate the MALDI process, many studies have focused on sample morphology using different imaging experiments. Sample heterogeneity and distribution of molecules within the MALDI solid sample have been explored using microscopy techniques, such as optical microscopy [6,7], scanning electron microscopy [8 -...

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confidence: 99%

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confidence: 99%

Solid state nuclear magnetic resonance as a tool to explore solvent-free MALDI samples

Pizzala

Barrère

Mazarin

et al. 2009

“…Many interesting polymeric analytes are either completely insoluble or present significant challenges in making analytically useful solutions. To address these issues, solvent-free sample preparation methods have been developed [11][12][13][14][15]. Previous papers have demonstrated that solvent free sample preparations can produce analytically useful polymer MALDI data for insoluble analytes.…”

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confidence: 99%

Extending the solvent-free MALDI sample preparation method

Hanton

Parees

2005

112

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is an important technique to characterize many different materials, including synthetic polymers. MALDI mass spectral data can be used to determine the polymer average molecular weights, repeat units, and end groups. One of the key issues in traditional MALDI sample preparation is making good solutions of the analyte and the matrix. Solvent-free sample preparation methods have been developed to address these issues. Previous results of solvent-free or dry prepared samples show some advantages over traditional wet sample preparation methods. Although the results of the published solvent-free sample preparation methods produced excellent mass spectra, we found the method to be very time-consuming, with significant tool cleaning, which presents a significant possibility of cross contamination. To address these issues, we developed an extension of the solvent-free method that replaces the mortar and pestle grinding with ball milling the sample in a glass vial with two small steel balls. This new method generates mass spectra with equal quality of the previous methods, but has significant advantages in productivity, eliminates cross contamination, and is applicable to liquid and soft or waxy analytes. (J Am Soc Mass Spectrom 2005, 16, 90 -93)

“…These are grinding by mortar and pestle [4,5], ball-mill [1][2][3][4], and vortexing [6]. Analyte, matrix, and salt are added to a vessel and, in the case of the ball-mill or vortex methods, suitably sized balls are used to assist in the homogenization process.…”

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confidence: 99%

“…Two general methods exist for the dry transfer of the homogenized powder from the vessel to the MALDI plate. In one method a pressed pellet is affixed to the plate with double-sided adhesive tape [5] and in another a loose powder [1][2][3][4] is transferred by the use of a small spatula, gently pressing it to the plate and producing a thin film.…”

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confidence: 99%

Multisample preparation methods for the solvent-free MALDI-MS analysis of synthetic polymers

Trimpin

McEwen

2007

A limitation of any current approach using solvent-free MALDI mass spectrometry is that only one sample at a time can be prepared and transferred to the MALDI-plate. For this reason, multiple-sample preparation approaches for solvent-free MALDI MS analysis of synthetic polymers were developed that are simple and practical. One approach multiplexed sample preparation by simultaneously preparing multiple samples. With this approach, as many as 384 samples could be prepared by addition of analyte, matrix, salt, and 1-mm metal beads to each well of a 384-well disposable bacti plate, capping the plate with the lid and homogenizing all samples simultaneously using a common laboratory vortex device. Besides the time savings achieved by a single vortex step for multiple samples, an additional advantage of this method relative to previously reported solvent-free preparation methods is that the mixing volume per sample is reduced, which allows a reduction in the amount of analyte required. This method, however, still requires the transfer of each homogenized sample to the MALDI plate for subsequent analysis. Here we report a novel approach that combines multiple simultaneous solvent-free sample preparation with automatic sample transfer to the MALDI target plate. This approach reduces the possibility of cross-contamination, the amount of sample and matrix consumed for an analysis, and the time required for preparation of multiple samples. These methods were shown to provide high-quality mass spectra for various synthetic polymer standards with M n values to 10 kDa. The methods are efficient in that small sample amounts are required, the sample/salt/matrix ratio is not critical, and the time necessary to achieve sufficient homogenization of multiple samples is less than 5 min. . A recent study based on the ball-mill homogenization/loose powder transfer method revealed that solvent-free MALDI-MS simplifies the measurement and improves the result qualitatively and quantitatively [3]. In general, the solvent-free method gives more homogeneous analyte/matrix mixtures and higher shot-to-shot and sample-tosample reproducibility than does the solvent-based method [3,4]. Consequently, less laser power has to be applied [3,4], which yields milder MALDI conditions and thus less fragmentation [1,3]. This method, for the same reasons, also reduces background signals [3] and improves the resolution of the analyte signals [3,4].For synthetic polymer analyses, three general methods exist for homogenization. These are grinding by mortar and pestle [4,5], ball-mill [1][2][3][4], and vortexing [6]. Analyte, matrix, and salt are added to a vessel and, in the case of the ball-mill or vortex methods, suitably sized balls are used to assist in the homogenization process. For polymers having M n Յ 10 kDa ball-mill, vortex, and mortar-and-pestle approaches appear to work equally well [4,6]. Two general methods exist for the dry transfer of the homogenized powder from the vessel to the MALDI plate. In one method a pressed