Imaging the morphology of solvent-free prepared MALDI samples

Abstract

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. The development of solvent-free sample preparation methods has enabled MALDI to analyze insoluble materials and, interestingly, can provide higher-quality mass spectral data. Although the utility of solvent-free sample preparation for MALDI has been demonstrated, the reasons for its success are only now being discovered. In this study, we use microscopy tools to image samples prepared using solvent-free methods to examine the morphology of these samples. The samples are prepared using a simple vortex method. Our results show that the average particle size of typical MALDI matrices is reduced from their original tens to hundreds of micrometers to hundreds of nanometers. This size reduction of the matrix occurs in one minute using the vortex method. We also observe remarkably smooth and homogeneous sample morphologies for the laser to interrogate, especially considering the relatively crude methods used to prepare our samples. [5][6][7][8][9][10]. MALDI can generate important data on the telomer repeat units, end groups, and average molecular weights of these materials. MALDI methods have been developed to address a broad variety of different polymer materials containing different chemistries. One of the key issues in traditional MALDI sample preparation is making true solutions of the analyte and the matrix [9]. 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. While several groups investigated solventfree sample preparation methods at nearly the same time, the methods developed by Trimpin, Räder, and coworkers have gained widespread use [11][12][13][14][15]. To make the sample preparation step easier, less time consuming, and reduce the risk of cross-contamination, we developed a simple version of the solvent-free sample preparation method, now called the vortex method [16]. Although the utility of solvent-free sample preparation for MALDI has been clearly demonstrated, the investigations into why it works and what impact these data have on our overall understanding of the MALDI process have only recently begun [17,18].To develop increased understanding of the utility of the solvent-free MALDI sample preparation method, we have investigated the morphology of these samples using different imaging experiments-optical imaging [19], scanning electron microscopy (SEM) [20], atomic force microscopy (AFM) [21], and time-of-flight secondary ion mass spectrometry (ToF-SIMS) [22]. Previous work has shown the utility of investigating MALDI sample morphology using imaging methods. We have used SEM to investigate the morphology of electrospray deposited samples [23] ...

Quantitative matrix-assisted laser desorption/ionization mass spectrometry

“…Their acoustic droplet ejector provides better control of conditions affecting protein extraction and matrix crystallization, and offers the ability to deposit matrix onto small surface features accurately. Mass spectral quality and reproducibility was found to be better than that obtained with manual pipette spotting [26].…”

“…For example, solvent-free sample preparation methods enable analysis of insoluble materials and reportedly provide higher quality mass spectra. The surfaces of these samples have been examined and the authors report remarkably smooth and homogeneous sample morphologies [26]. There are numerous other refinements to the general MALDI process that have the potential to improve the precision and accuracy of quantification, but they are yet to be assessed rigorously in a quantitative setting.…”

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

“…X-ray diffraction was performed to reach atomic level information [17,18]. A similar approach, involving various imaging techniques, has recently been reported to investigate the morphology of solventfree MALDI samples [19].Solid-state nuclear magnetic resonance (NMR) has proven to be a valuable spectroscopic method to characterize solid-state structure at the atomic level. Currently, using state of the art techniques, such as magic angle spinning (MAS) [20] to increase resolution, and cross polarization (CP) [21] to improve sensitivity, it is possible to obtain high-resolution solid-state NMR spectra similar to those obtained in the liquid state.…”

“…X-ray diffraction was performed to reach atomic level information [17,18]. A similar approach, involving various imaging techniques, has recently been reported to investigate the morphology of solventfree MALDI samples [19].…”

“…The present study is focused on solvent-free MALDI samples consisting of 2,5-dihydroxybenzoic acid (2,5-DHB) and caesium fluoride ground together in the solid state at different molar ratios. This particular matrix/salt system was selected as it is currently used in our laboratory for MALDI of large poly(ethylene oxide) polymers, and because it allows complementary NMR data to be obtained from 13 C, 133 Cs, and 19 F spectra.…”