A Mechanism for Ionization of Nonvolatile Compounds in Mass Spectrometry: Considerations from MALDI and Inlet Ionization

Abstract

Mechanistic arguments relative to matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) address observations that predominately singly charged ions are detected. However, recently a matrix assisted laser ablation method, laserspray ionization (LSI), was introduced that can use the same sample preparation and laser as MALDI, but produce highly charged ions from proteins. In MALDI, ions are generated from neutral molecules by the photon energy provided to a matrix, while in LSI ions are produced inside a heated inlet tube linking atmospheric pressure and the first vacuum region of the mass spectrometer. Some LSI matrices also produce highly charged ions with MALDI ion sources operated at intermediate pressure or high vacuum. The operational similarity of LSI to MALDI, and the large difference in charge states observed by these methods, provides information of fundamental importance to proposed ionization mechanisms for LSI and MALDI. Here, we present data suggesting that the prompt and delayed ionization reported for vacuum MALDI are both fast processes relative to producing highly charged ions by LSI. The energy supplied to produce these charged clusters/droplets as well as their size and time available for desolvation are determining factors in the charge states of the ions observed. Further, charged droplets/clusters may be a common link for ionization of nonvolatile compounds by a variety of MS ionization methods, including MALDI and LSI.

Matrix-Assisted Ionization-Ion Mobility Spectrometry-Mass Spectrometry: Selective Analysis of a Europium–PEG Complex in a Crude Mixture

“…Specificity for certain matrices was reported in the development of MALDI (e.g., sinapinic acid for high mass proteins [64], DCTB for fullerenes and polymers [65], and tetracyanaquinodimethane for polycyclic aromatic hydrocarbons [66]), and ESI (e.g., metal salts [67] and peptides [4]). This is similar to questions raised for ESI and MALDI here and in other work [4,56,[68][69][70][71]. An explanation of how MAI can selectively ionize europium-containing species is of interest and may be determined with future work.…”

Localization and imaging of gangliosides in mouse brain tissue sections by laserspray ionization inlet

“…I-B), which operates independent of a voltage or a laser ( 56 ). MAII and LSII depend on the appropriate matrix (57)(58)(59), similar to MALDI ( 60-63 ), but do not require absorption at the laser wavelength employed. In LSII, the laser ablation only acts as the method to transfer a matrix/analyte sample into the inlet of the mass spectrometer, permitting the use of highly focused lasers to improve spatial resolution.…”

“…Approximately 20 laser shots were summed for each spectrum. LSII-MS was performed on the SYNAPT G2 by converting the nano-ESI source to a LSII source according to published procedure (54)(55)(56)(57)(58)(59)68 ) using the 337 nm nitrogen laser employed for LSII-MS on the LTQ Velos.…”

“…The absence of fragmentation with 2,5-DHAP matrix may be the result of lower thermal requirements in LSII ( 53,72 ). We are currently investigating mechanistic arguments that may assist in providing an explanation for these observations ( 58,59 ). The mass spectrum of purchased GD1 using 2,5-DHAP as matrix is shown in Fig.…”

“…To demonstrate that the observed fragmentation was a consequence of MALDI and not simply caused by instrumentation differences relative to the LTQ Velos, supplementary Fig. VII-B shows an LSII acquisition of GD1 with 2,5-DHAP on the SYNAPT G2 using the nano-ESI source converted to a LSII source ( 54,55,(57)(58)(59)68 ). Abundant [M-2H] 2 Ϫ ions are observed without adduction or fragmentation.…”