Do single matrix molecules generate primary ions in ultraviolet matrix-assisted laser desorption/ionization

Self Cite

The mechanisms of the reduction of Cu(II) in matrix-assisted laser desorption/ionization mass spectrometry (MALDI) are studied. In MALDI mass spectra, ions cationized by copper mostly contain Cu(I) even if Cu(II) salts are added to the sample. It was found that Cu(II) was reduced to Cu(I) by gas-phase charge exchange with matrix molecules, which is a thermodynamically favorable process. Under some conditions, large amounts of free electrons are present in the plume. Cu(II) can be even more efficiently reduced to Cu(I) by free electron capture in the gas phase. The matrices studied in this work are nicotinic acid, dithranol, and 2,5-dihydroxybenzoic acid. ϩ ) were observed in the analysis of proteins [11,17,19,20] and polar synthetic polymers, e.g., polyethylene glycols (PEG) [12], whereas in the analysis of apolar synthetic polymers, e.g., polystyrene (PS), the reduction of divalent metal ions was dominant [13][14][15][16]18].In this work, we present a detailed study of the reduction of divalent copper used as a cationization agent in MALDI. Different explanations of this reduction in the gas phase have been given, namely electron capture [21] and charge exchange with matrix [22]. The origin and the amount of free electrons in MALDI has not been comprehensively investigated before and many questions were still open [22]. Recently, studies of the origin of the free electrons in MALDI from our laboratory [23,24] have shown that free electrons are largely formed by photoelectric emission from a metal/ dielectric-substance interface, not by photoionization of matrix [21]. Our studies also showed that the yield of electrons strongly depends on the thickness of the matrix layer. In the case of nonmetallic surfaces, few electrons are produced.According to these results, we are able to investigate the roles of free electrons and matrices as possible reducing agents in MALDI separately. Blocking the electron source by using a nonmetallic sample carrier or a metallic target covered with an insulating layer provides the possibility to examine the effect of the gasphase charge transfer between divalent metal ions and the matrices. For investigation of the effect of the free electron capture, experiments have to be done in the absence of matrix by using direct laser desorption/ ionization (LDI) of the sample.Under continuous extraction conditions, for example on a time-of-flight (TOF) mass spectrometer, free electrons as the most mobile species can either be extracted very quickly (in the negative ion mode) or pushed back towards the target (in the positive ion mode) and thus have less overlap with the plume, i.e., electrons will be less available for capture by positively charged species. On the other hand, delayed extraction allows time for secondary ion-molecule reactions to occur, e.g., the charge transfer reaction between neutral matrix molecules and copper ions. In order to investigate the influence of the high extraction field on the free electron capture and the charge exchange reaction, we have compared the results ...

Section: With Matrix Present/no Free Electronsmentioning

confidence: 64%

Reduction of Cu(II) in matrix-assisted laser desorption/ionization mass spectrometry

Zhang¹,

Frankevich²,

Knochenmuss³

et al. 2003

Self Cite

“…Knochenmuss and coworkers proposed that energy pooling is a major reaction pathway responsible for the positive ion production [9,28], and the source of negative charges may be the photoelectrons ejected from the matrix-metal hybrid molecular orbitals [22]. On the other hand, negative matrix ions may be produced by the attachment of low-energy electrons during material desorption processes, as reported previously by Pshenichnyuk and coworkers [29 -31].…”

Section: Atrix-assisted Laser Desorption/ionizationmentioning

confidence: 65%

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...

“…A highly dense matrix plume must include very large clusters and aggregates. The local order in these clusters and aggregates is likely close to that in solid state, and this would promote the thermal-photoionization and energy pooling mechanisms [21,22]. Matrix ion production would occur in the first stage of ablation and analyte ion production would occur in the expanding plume after desolvation of aggregates [4].…”

Section: Ablated Volume and Ionizationmentioning

confidence: 99%

Irradiation effects in MALDI, ablation, ion production, and surface modifications. Part II: 2,5-dihydroxybenzoic acid monocrystals

Fournier

Marinach

Tabet

et al. 2003

Irradiation effects at low and high laser fluence on 2,5-dihydroxybenzoic acid large crystals were investigated. Contrary to what was observed for matrices as cinnamic acid derivatives, no chemical degradation of matrix is evidenced and continuous ablation as well as ion production resulted of extended irradiation in all the fluence range corresponding to classical matrix-assisted laser desorption /ionization. Ripples are formed on the base of the crater for a limited number of laser shots under moderate fluence. For extended irradiation, conical shape craters are formed with the axis of the crater oriented along the incident direction of the laser beam. A study of the craters showed that ablation through the ablated volume slowly varied with the laser fluence when a strong increase of ion production (matrix and analyte) was recorded. Ablation volume was found to vary non-linearly with the number of laser shots. On a same spot, the ablated volume and the ion production were measured as a function of the laser energy. With an increasing laser energy (or fluence), the ablated volume slowly increases when the ion production strongly increases. This gives evidence of a decoupling between ablation and ionization. is now a powerful method to produce ions of large molecules and its coupling with mass spectrometry (MS) provides a major analytical tool, particularly in proteomic studies and synthetic polymers characterization. To date, many of the methodologies developed for MALDI have been empirically determined. Indeed, the mechanisms involved in the ion production are very complex and cannot be experimentally studied in detail in all the time range of elementary processes. Briefly, these mechanisms involve ablation of molecules and aggregates from a solid state, the expansion of a plume from a very dense state to a very dilute gas. Ionization processes are likely involved in all the steps of the ablation and expansion. In addition, it is still difficult to predict for a small molecule absorbing at the laser wavelength whether it will be an effective or an inefficient matrix for the analysis of a given analyte. Some recent reviews gave an overview of the common ideas on these ionization mechanisms [2, 3] involving specific chemical reaction, ion-molecule reaction [2] or proton transfer in aggregates [3,4]. Generally, the mechanisms in MALDI are studied in two different ways. One deals with the ablation phenomenon mainly by molecular dynamics simulation [5,6] at very short scale (Ͻ Ͻ1 ns) and experimentally on the plume expansion [7] at medium and long-time scale (a few s to a few tens of s). The second deals with the ions produced by MALDI at the time scale of the ion time-of-flight, the role of the laser parameters [8], the target preparations and the nature of analyte ions. This subject occupies most publications in this field.In this article, we will focus on the relationship between the ablated volume and the analyte ion production and on the irradiation effects using large 2,5-dihydroxybenzoic acid single crystals...