Amino Acid Clusters Formed by Sonic Spray Ionization

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108

The internal energy distributions of typical ions generated by desorption electrospray ionization (DESI) were measured using the "survival yield" method, and compared with corresponding data for electrospray ionization (ESI) and electrosonic spray ionization (ESSI). The results show that the three ionization methods produce populations of ions having internal energy distributions of similar shapes and mean values (1.7-1.9 eV) suggesting similar phenomena, at least in the later stages of the process leading from solvated droplets to gas-phase ions. These data on energetics are consistent with the view that DESI involves "droplet pick-up" (liquid-liquid extraction) followed by ESI-like desolvation and gas-phase ion formation. The effects of various experimental parameters on the degree of fragmentation of p-methoxy-benzylpyridinium ions were compared between DESI and ESSI. The results show similar trends in the survival yields as a function of the nebulizing gas pressure, solvent flow rate, and distance from the sprayer tip to the MS inlet. These observations are consistent with the mechanism noted above and they also enable the user to exercise control over the energetics of the DESI ionization process, through manipulation of external and internal ion source parameters. [1, 24, 29 -32], and explosives detection [4,8,[33][34][35][36][37]. Investigations into the fundamentals of the droplet dynamics and ionization mechanism have also been conducted through systematic measurements of the droplet size and velocity [38], computer simulations [39], surface charging effects [40], or more simply, by evaluating the mass spectra of various types of analytes in conjunction with variations in the experimental parameters [41]. These investigations suggested that the primary ionization mechanism in DESI is "droplet pick-up", i.e., extraction of the analyte into the droplet, followed by electrospray ionization (ESI)-like mechanism in which the progeny droplets are desolvated yielding ionized molecules. One of the arguments supporting this conclusion, the similarity between the ESI and DESI mass spectra, also implies that the two ion sources should generate populations of ions with similar internal energy distributions, P(E). It should be noted though, that the initial events in the two experiments are quite different, and recent simulations show that the process of formation of progeny droplets from the initial ϳ4 m droplets in DESI is not dependent on charge build-up as it is in ESI. These dissimilarities need not result in differences in the final internal energy (E) since the later processes occurring in the atmospheric pressure interface are probably the same for these two processes and, as we will show later, dominate the energetics of the process.Here we use the "survival yield" (SY) method [42] applied to a set of thermometer ions (benzylpyridinium cations) to judge internal energy deposition associated with soft ionization techniques [43] to measure and compare the internal energy distributions of the ions generated b...

Section: Methodsmentioning

confidence: 99%

Internal energy distributions in desorption electrospray ionization (DESI)

Nefliu

Smith

Venter

et al. 2008

Self Cite

108

“…Magic numbers for amino acid cationic clusters [11][12][13] and preference for homochirality [14,15] have been reported. Negatively charged clusters of amino acids have also been produced [13].The formation and energetics of gas-phase multiplycharged anions (MCA) is a subject that has attracted much attention [16 -18]. Even though several multiplycharged molecular anions have been measured, observations of multiply-charged cluster anions are few.…”

mentioning

confidence: 99%

mentioning

confidence: 99%

Multiply-charged negative clusters of adenosine-5′-monophosphate in the gas phase

Liu

Hvelplund

Nielsen

2005

Multiply-charged noncovalent cluster anions of adenosine-5=-monophosphate (AMP) were formed by electrospray ionization (ESI). Ions in higher charge states were observed when the ions were accumulated in an ion trap with helium buffer gas before detection. We determined the smallest size (n a ) or appearance size as a function of charge state (q), i.e., n a ϭ 4 for q ϭ 2, n a ϭ 8 for q ϭ 3, and n a ϭ 13 for q ϭ 4. The relation between n a and q can be described by a charged droplet model. When the size is larger than n a for a given q, the fragmentation pathway of an anion cluster is dominated by loss of neutral fragments. In contrast, when the size approaches the appearance size, only charged fragments are formed. Cluster cations such as metal clusters have been extensively studied during the last two decades [3,4]. Recently, the study of biomolecular cluster cations has become an active research field, and much work has dealt with the formation, dissociation, and charge partitioning among fragments of protein cluster cations [5][6][7][8][9][10]. Magic numbers for amino acid cationic clusters [11][12][13] and preference for homochirality [14,15] have been reported. Negatively charged clusters of amino acids have also been produced [13].The formation and energetics of gas-phase multiplycharged anions (MCA) is a subject that has attracted much attention [16 -18]. Even though several multiplycharged molecular anions have been measured, observations of multiply-charged cluster anions are few. Multiply-charged metal cluster anions were observed by electron attachment to monoanions in a Penning trap [19 -22] . Here we report, to our knowledge, the first observation of negatively, multiply-charged noncovalent clusters made of adenosine 5=-monophosphate (AMP). The dependence of cluster size on charge state was measured, and the results, are modeled by a charged droplet model. The fragmentation pathways of multiply-charged negative clusters are also presented. ExperimentalTwo setups were applied in the experiments: an electrostatic ion storage ring (ELISA) and an accelerator mass spectrometer (AMS). In both setups, negative clusters of AMP were produced by ESI. A saturated solution of AMP in methanol was sprayed.Detailed information about the ESI source (Figure 1a) is described in reference [32]. Briefly, the source is mounted on the high-voltage platform of the accelerator. A stainless steel hypodermic needle is, via a fused silica capillary, connected to a syringe containing a solution of the analyte. A syringe pump (Harvard apparatus) delivers a constant flow through the needle. A typical flow rate is 1ϳ2 l/min. A voltage of 3 to 4 kV relative to the heated capillary is put on the needle. The capillary is normally heated for desolvation of solvent molecules from analyte molecules. At the exit of the capillary, ions emerge into the first vacuum zone where a pressure of around 1 mbar is maintained. The ions focused by a tube lens go through a skimmer into a second region. The voltage on the tube lens varies from 50 to...

“…SSI is a spray ionization technique first introduced by Hirabayashi et al [64,65] in the mid1990s, and then adopted and developed further by others [66]. A unique feature associated with SSI is that both positive and negative ions are formed simultaneously.…”

Section: Ionization Approaches With Linear Ion Trapsmentioning

confidence: 99%

Evolution of instrumentation for the study of gas-phase ion/ion chemistry via mass spectrometry

Xia

McLuckey

2008

The scope of gas-phase ion/ion chemistry accessible to mass spectrometry is largely defined by the available tools. Due to the development of novel instrumentation, a wide range of reaction phenomenologies has been noted, many of which have been studied extensively and exploited for analytical applications. This perspective presents the development of mass spectrometry-based instrumentation for the study of the gas-phase ion/ion chemistry in which at least one of the reactants is multiply charged. The instrument evolution is presented within the context of three essential elements required for any ion/ion reaction study: the ionization source(s), the reaction vessel or environment, and the mass analyzer. Ionization source arrangements have included source combinations that allow for reactions between multiply charged ions of one polarity and singly charged ions of opposite polarity, arrangements that enable the study of reactions of multiply charged ions of opposite polarity and, most recently, arrangements that allow for ion formation from more than two ion sources. Gas-phase ion/ion reaction studies have been performed at near atmospheric pressure in flow reactor designs and within electrodynamic ion traps operated in the mTorr range. With ion trap as a reaction vessel, ionization and reaction processes can be independently optimized and ion/ion reactions can be implemented within the context of MS n experiments. Spatial separation of the reaction vessel from the mass analyzer allows for the use of any form of mass analysis in conjunction with ion/ion reactions. Time-of-flight mass analysis, for example, has provided significant improvements in mass analysis figures of merit relative to mass filters and ion traps. The development and evaluation of new tools has clearly been a central activity and contributor to the dramatic advances in the capabilities of mass spectrometry over the past several decades. In this perspective, we relate the evolution of instrumentation for research activities focused on the study and use of ion/ion reactions and offer it as one example of many where progress in an area of science is intimately related to and dependent on the development of instrumentation. It is provided in appreciation for the inspirational example of creativity in instrumentation development [1] provided by Cooks over the years. Electrospray ionization (ESI) [2], among its many contributions, enabled the development of macro-molecule ion/ ion chemistry as a research area. The multiple-charging phenomenon associated with electrospray makes possible the generation of multiply charged reactant ions. This capability leads to the possibility that at least one ion/ion reaction product can retain charge, which makes such a product directly amenable to study with mass spectrometry. In the past two decades, significant progress has been made in understanding and exploring the gas-phase ion/ion chemistry of high mass multiply charged ions. A variety of potential analytical applications have been developed accordingly. Co...