A useful binary matrix for visible-MALDI of low molecular weight analytes

Abstract

In this work, a new absorbing candidate, rhodamine (R) 575, is described, which forms the basis of a binary matrix operating at 532 nm. Analyte ionization is found to be much more efficient when the dye is combined with a proton donor such as hydrochloric acid or ␣-cyano-4-hydroxycinnamic acid, or a proton acceptor such as sodium hydroxide. This makes the matrix more generic than many others that have been tried. Furthermore, under visible illumination R575 produces very few chemical fragments, making it useful for small molecular weight analyte detection. Spectra for a variety of analytes are shown. Insight into the MALDI mechanism was obtained by comparing the similarities and differences of visible-MALDI with the more common UV and IR-MALDI strategies. (J Am Soc Mass Spectrom 2010, 21, 294 -299) © 2010 American Society for Mass Spectrometry O ver the last two decades, matrix assisted laser/ desorption ionization (MALDI) mass spectrometry (MS), using either ultraviolet (UV) [1,2] or infrared (IR) lasers [3], has become an important analytical tool for the analysis of molecules of chemical and biological interest. The success of MALDI-MS is largely due to the relative "softness" of this technique, which allows for ionization and detection of intact analytes with minimal fragmentation. IR-MALDI has been shown to be superior to UV-MALDI in this regard for the analysis of biological macromolecules including DNA and RNA [4], although the former technique typically operates at higher pulse energies and features higher sample consumption per laser pulse. Today, UV lasers and, in particular, the N 2 gas lasers with an output wavelength of 337 nm and Nd:YAG lasers with a wavelength of 355 nm dominate in MALDI-MS instruments; and they are usually less expensive than their IR counterparts.One could expect that MALDI mass spectrometry using visible wavelength lasers will have several advantages over UV-MALDI and IR-MALDI. Many macromolecules of interest absorb UV-light but are transparent at visible wavelengths. Evidence suggests that visible-MALDI is softer than UV-MALDI [5]. Similar to UV-MALDI, visible-MALDI is expected to require lower pulse energies than IR-MALDI. Furthermore, readily available pulsed visible laser sources such as the solid-state frequency-doubled Nd:YAG laser (532 nm wavelength) are becoming relatively inexpensive and have longer operational lifetimes compared with standard UV devices. Delivery of light via fiber optics is also more routine at visible wavelengths.Despite its potential, visible MALDI has not been extensively explored. Studies have dealt with rhodamine (R) B, R6G [5,6], neutral red [7], and 2-amino-3-nitrophenol [8] as possible matrices for visible-MALDI using the 532 nm output of a doubled Nd:YAG laser. Recently, Au-assisted visible laser MALDI has emerged [9] as a new analysis technique as well as visible surface-assisted laser desorption/ionization from a graphite substrate [10]. Our group recently studied visible-MALDI coumarin laser dyes as potential MALDI matrixes using the 480 ...

Laser microdissection and atmospheric pressure chemical ionization mass spectrometry coupled for multimodal imaging

“…In addition, there is a growing interest in determining the spatially resolved chemical distribution of lipids in tissue using different MSI approaches including matrix‐assisted laser desorption/ionization (MALDI) as well as desorption electrospray ionization (DESI) . In contrast to imaging the inked pattern discussed above, where the dye itself acted as a matrix for absorption of the laser light and ablation of material from the surface, unstained biological tissue is much less absorbent of UV laser light and therefore less amenable to direct LA from glass . To circumvent this problem, glass microscope slides covered with a membrane of PEN that has a high UV absorption and is a common surface substrate for laser microdissection were used for tissue imaging.…”

Microstructured Tungsten Oxide: A Generic Desorption/Ionization Substrate for Mass Spectrometry

“…The mass spectra exhibit a comparable sensitivity to pSi23–25 (quantities ranging from 10 picomoles to 10 femtomoles). Recently our MALDI instrument was calibrated against a commercial time‐of‐flight mass spectrometer and found to be ∼ one order of magnitude less sensitive 26. Thus, our reported detection limits are upper bounds.…”

Application of dyes as doping agents in MALDI-MS matrices for the signal enhancement of proteins

“…Previous attempts have been made to use many dye materials as MALDI-MS matrix both for conventional analysis and MALDI-MS imaging. [26][27][28][29] However, the use remains limited for either specic class of compounds, such as lipids, or these methods in most cases were not able to outperform, and thus replace, the ability of commonly used UV-MALDI-MS matrices such as as a-cyano-4hydroxycinnamic acid (HCCA), sinapic acid (SA) and 2,5-dihydroxybenzoic acid (DHB). Detailed studies on the interaction of dyes with protein when added along with matrices were previously reported by Salih et al which suggest the electrostatic interaction can occur between protein and dye molecules leading to the dye-protein complex formation and can induce other effects such as in-source and post source decay of the analyte ions.…”