Is the incorporation of analytes into matrix crystals a prerequisite for matrix-assisted laser desorption/ionization mass spectrometry? A study of five positional isomers of dihydroxybenzoic acid

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A new instrument and method is described for laterally resolved mass spectrometric surface analysis. Fields of application are in both the life sciences and the material sciences. The instrument provides for imaging of the distribution of selected sample components from natural and artificial surfaces. Samples are either analyzed by laser desorption ionization (LDI) time-of-flight mass spectrometry or, after preparation with a suitable matrix, by matrixassisted laser desorption ionization (MALDI) mass spectrometry. Areas of 100 ϫ 100 m are scanned with minimal increments of 0.25 m, and between 10,000 and 160,000 mass spectra are acquired per image within 3 to 50 min (scan rate up to 50 pixels per s). The effective lateral resolution is in the range of 0.6 to 1.5 m depending on sample properties, preparation methods and laser wavelength. Optical investigation of the same sample area by UV confocal scanning laser microscopy was found to be very attractive in combination with scanning MALDI mass analysis because pixel-identical images can be created with both techniques providing for a strong increase in analytical information. This article describes the method and instrumentation, including first applicational examples in elemental analysis, imaging of pine tree roots, and investigation of MALDI sample morphology in biomolecular analysis. (J Am Soc Mass Spectrom 2002, 13, 735-748)

Section: Smaldi Mass Analysis Of Peptide Samplesmentioning

confidence: 98%

Scanning microprobe matrix-assisted laser desorption ionization (SMALDI) mass spectrometry: Instrumentation for sub-micrometer resolved LDI and MALDI surface analysis

Spengler

Hubert

2002

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256

“…The impacts of changes in laser fluence and crystal structure on the initial ion distribution of 2,5-dihydroxybenzoic acid (DHB) were examined because they are considered to be the major determinants of MS signal quality. DHB was selected as the target molecule because it is the best-studied matrix compound, both experimentally and theoretically [13,[35][36][37][38]. In the low fluence region, the appearance thresholds of positive and negative matrix ions should provide an unambiguous examination of the hypothesis of ion-pair production: the positive and negative ions should have identical thresholds if they are ion-pairs, whereas the presence of different thresholds would imply that ion-pairing is likely only a minor pathway.…”

Section: Atrix-assisted Laser Desorption/ionizationmentioning

confidence: 99%

Incoherent production reactions of positive and negative ions in matrix-assisted laser desorption/ionization

Liu

Lee

Wang

2009

Utilizing synchronized dual-polarity matrix-assisted laser desorption/ionization (MALDI) mass spectrometry, we found good evidence of the incoherent production of positive and negative matrix ions. Using thin, homogeneous 2,5-dehydroxybenzoic acid (DHB) matrix films, positive and negative matrix ions were found to appear at different threshold laser fluences. The presence of molecular matrix ions of single charge polarity suggests that the existence of DHB ion-pairs may not be a prerequisite in MALDI. Photoelectrons induced by the laser excitation may assist the production of negative DHB ions, as shown in experiments conducted with stainless steel and glass substrates. At high laser fluences, the relative yield of positive and negative matrix ions remained constant when homogeneous matrix films were used, but it fluctuated significantly with inhomogeneous crystal morphology. This result is also inconsistent with the hypothesis that matrix ion-pairs are essential primary ions. (MALDI) provides a convenient means to produce protein and peptide ions for mass analysis [1, 2], but its application to the studies of biopolymers such as carbohydrates is still limited by insufficient ion yields [3,4]. It is generally accepted that the protonated matrix molecules are the essential initial ions responsible for the production of positively-charged analytes [5], normally via proton transfer reactions. But such a qualitative description is insufficient to provide guidelines for the optimization of experimental conditions for the wide variety seen in biological samples. The paucity of our knowledge of detailed ionization mechanisms makes the optimization of MALDI performance a highly empirical process and thus the primary bottleneck of this method. In the last decade, Karas and coworkers and Knochenmuss and coworkers have respectively established a cluster model [6,7] and a two-step model [8 -10] to explain MALDI. The two models are divergent in several aspects, including the origin of the initial ions that trigger the subsequent reactions, the role of ion-neutral reactions during material desorption, the contribution of photoelectrons in the reactions, etc. The two reaction models seem not converge over time mainly due to the insufficient information provided by conventional mass spectrometric methods. It is thus desirable to develop our understanding of MALDI from novel perspectives, such as the correlation between primary positive and negative ions at the time of ionization.A recent report based on a synchronized dual-polarity MALDI-TOF method stated that the spectral pattern of MALDI-generated positive ions were approximately equal to those of the negative ions for some moderate-size proteins [11]. There was thus some speculation over whether the positive and negative ions are produced in a pair-wise fashion in MALDI [12,13], or whether they are the result of preformed ion-pairs in the crystal before laser irradiation [6]. More recently, Dashtiev and coworkers [14] systemically studied the correlation of positive and ne...

“…Because a large number of experimental parameters influence the ionization processes that occur in MALDI, the ionization mechanisms and kinetics are not fully understood despite its widespread usage and popularity. In an attempt to better understand and control MALDI, several groups have focused on studies directed at elucidating the mechanisms that lead to MALDI [18][19][20][21][22][23][24]. The proposed mechanisms are based on empirical observations, but thus far no single mechanism has been proposed that is able to explain all of the experimental observations.…”

Section: Introductionmentioning

confidence: 99%

“…Incorporation of the analyte into the matrix crystal lattice is believed to be a prerequisite for a successful MALDI analysis, especially when the dried droplet sample preparation technique is used [22]. Thereafter, laser ablation brings the analyte and matrix into the gas phase.…”

Section: Introductionmentioning

confidence: 99%

Sodium Cation Affinities of Commonly Used MALDI Matrices Determined by Guided Ion Beam Tandem Mass Spectrometry

Chinthaka¹,

Rodgers²

2012

The sodium cation affinities of six commonly used MALDI matrices are determined here using guided ion beam tandem mass spectrometry techniques. The collision-induced dissociation behavior of six sodium cationized MALDI matrices, Na + (MALDI), with Xe is studied as a function of kinetic energy. The MALDI matrices examined here include: nicotinic acid, quinoline, 3-aminoquinoline, 4-nitroaniline, picolinic acid, and 3-hydroxypicolinic acid. In all cases, the primary dissociation pathway corresponds to endothermic loss of the intact MALDI matrix. The cross section thresholds are interpreted to yield zero and 298 K Na + −MALDI bond dissociation energies (BDEs), or sodium cation affinities, after accounting for the effects of multiple ionneutral collisions, the kinetic and internal energy distributions of the reactants, and dissociation lifetimes. Density functional theory calculations at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G* and MP2(full)/6-311+G(2d,2p)//B3LYP/6-31G* levels of theory are used to characterized the structures and energetics for these systems. The calculated BDEs exhibit very good agreement with the measured values for most systems. The experimental and theoretical Na + −MALDI BDEs determined here are compared with those previously measured by cation transfer equilibrium methods.