Internal energy distribution in electrospray ionization

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

108

“…3 ) and p-methoxybenzylpyridinium tetrafluoroborate (para-OCH 3 ) were purchased from the Florida Center for Heterocyclic Compounds, University of Florida, Gainesville, while p-cyanobenzylpyridinium chloride (para-CN) was synthesized in-house following the method described elsewhere [36]. Micromolar (10 Ϫ6 M) solutions of the para-CH 3 , para-OCH 3 , para-NO 2 , and para-CN salts were prepared in MeOH/H 2 O (1:1) solution.…”

Section: Chemicals and Reagentsmentioning

confidence: 99%

Non-aqueous spray solvents and solubility effects in desorption electrospray ionization

Badu‐Tawiah

Bland

Campbell

et al. 2010

The use of non-aqueous solvents in desorption electrospray ionization mass spectrometry (DESI-MS) is explored by analyzing a set of 43 compounds using binary mixtures of chloroform, tetrahydrofuran, and acetonitrile as the spray solvent. Comparisons of data obtained from chloroform/tetrahydrofuran (1:1) and chloroform/acetonitrile (1:1) spray solvents with the standard aqueous-based spray solvent (methanol/water, 1:1) shows that the non-aqueous systems have practical value for DESI, especially in the analysis of hydrophobic compounds. Non-aqueous spray solvents were used to ionize thermometer molecules (benzyl pyridinium salts) and showed lower internal energies (softer DESI ionization compared with methanol/water, 1:1), a result that has parallels in known solvent effects in electrospray ionization and is explained by solvent effects on surface tension. Consideration of octanol/ water partition coefficients (K ow ) of the 43 analytes in the light of their DESI results reveals the importance of the solubility of analyte in the spray solvent in producing high quality mass spectra. This finding provides additional support for the droplet pick-up description of the DESI mechanism, which is based on analyte dissolution in the spray solvent, followed by splashing of subsequently arriving droplets in the liquid film to form microdroplets of dissolved analyte. DESI solvent optimization can be improved by the use of K ow of the analyte as an indication of the polarity of the most appropriate solvent system. [4 -6], one member of the growing family of ambient ionization methods [7][8][9][10][11][12][13][14][15][16][17][18], is due in part to the ability to optimize the spray solvent to the analyte(s) of interest, both by choice of the solvent and, in some cases, by addition to it reagents that chemically modify the analyte for enhanced ionization [19 -21]. Surface effects in DESI have also been studied [22], and it has been concluded that surface properties such as wettability play a role in charge-transfer. Solvent effects in DESI have not been studied systematically, although there are a number of instances in which alternatives to the common methanol/water system have been reported [23][24][25][26][27]. In the current work, we explore the use of non-aqueous spray solvents and their effects in DESI to extend the range of application of DESI and seek to more fully understand their effects and to interpret this data in light of the DESI mechanism.A number of proposals have been made about the mechanism of DESI, the most important of which is summarized under the rubric of the "droplet pick-up" mechanism [4,28,29]. As is the case in other desorption ionization techniques, both those performed under vacuum and those in the ambient environment, the mechanism of DESI can be divided into two steps: desorption/ ablation of analyte from the condensed phase and ionization of the analyte molecules. There is evidence from experiments [29] and simulations [28] that DESI ionization follows the formation of a thin liquid film of spr...

“…Simply setting the relevant software control to the same temperature setting does not ensure that the same degree of heating is achieved by the different heating configurations. The work of Collette et al and others [24,26,45] to characterize the temperature of the thermometer ions therefore becomes of practical utility in estimating the impact of ion source temperature with respect to its influence on the reproducibility of spectra. However, a simpler strategy than fitting the data from multiple thermometer ion species to a Boltzmann distribution might facilitate its use in routine analysis of analytical samples.…”

Section: Source Temperaturementioning

confidence: 99%

A method for monitoring and controlling reproducibility of intensity data in complex electrospray mass spectra: A thermometer ion-based strategy

Lecchi¹,

Zhao²,

Wiggins³

et al. 2009

Reproducibility in mass spectral data is important in both biomarker discovery and spectral database searching. We report a strategy, employing a series of substituted benzylpyridinium thermometer ions that can be used to monitor changes in performance of multiple aspects of an electrospray ionization source that impact the intensity axis of a spectrum. Performance attributes, which could confound even isotope-based quantification strategies, are readily assessed using a mixture of thermometer ions. Based on the observed behavior of the ions, a procedure is proposed for monitoring instrument performance and compensating for factors that affect reproducibility across both time and instruments. (J Am Soc Mass Spectrom 2009, 20, 398 -410)