Transmission Geometry Laserspray Ionization Vacuum Using an Atmospheric Pressure Inlet

Abstract

This represents the first report of laserspray ionization vacuum (LSIV) with operation directly from atmospheric pressure for use in mass spectrometry. Two different types of electrospray ionization source inlets were converted to LSIV sources by equipping the entrance of the atmospheric pressure inlet aperture with a customized cone that is sealed with a removable glass plate holding the matrix/analyte sample. A laser aligned in transmission geometry (at 180° relative to the inlet) ablates the matrix/analyte sample deposited on the vacuum side of the glass slide. Laser ablation from vacuum requires lower inlet temperature relative to laser ablation at atmospheric pressure. However, higher inlet temperature is required for high-mass analytes, for example, α-chymotrypsinogen (25.6 kDa). Labile compounds such as gangliosides and cardiolipins are detected in the negative ion mode directly from mouse brain tissue as intact doubly deprotonated ions. Multiple charging enhances the ion mobility spectrometry separation of ions derived from complex tissue samples.

Novel ionization processes for use in mass spectrometry: ‘Squeezing’ nonvolatile analyte ions from crystals and droplets

“…Using a temperature range from 70 to 400 °C on an Orbitrap Q‐Exactive Focus mass spectrometer produces for 3‐NBN matrix exceptionally high analyte ion abundance without the need for high voltages or a laser , while other matrices, especially typical MALDI matrices, produce less abundant analyte ions, multiply and singly charged, and only at high temperatures. The temperature can be applied directly or indirectly to the tube, typically used as an ESI inlet . Charge separation occurs after the matrix is introduced to sub‐atmospheric pressure.…”

“…In vacuum ionization the matrix:analyte is introduced directly into the sub‐atmospheric pressure region of the mass spectrometer on a substrate where ionization commences . Intermediate pressure works well without, and sometimes with, the use of a laser, while ionization using high‐vacuum MALDI time‐of‐flight (TOF) mass spectrometers, with nearly collision‐free conditions, is more difficult, typically producing charge states between those observed with ESI and MALDI, if ions are produced at all .…”

“…A number of different mass spectrometers, in terms of analyzer types and vendors, have been used with the novel ionization processes demonstrating the importance of pressure, temperature, and collisions in optimizing ion formation . An example is shown in Figure for bovine serum albumin (BSA), 66 kDa, using mass spectrometers to ionize this 66 kDa protein intact and multiply charged using the new ionization processes.…”

“…Therefore, the charging and desolvation are associated with the matrix:analyte traversing through the cold or hot inlet tube, reducing the ion source to a feature intrinsically available with any atmospheric pressure ionization mass spectrometer without having to apply additional energy. A variety of different commercial MALDI and ESI sources have been employed for LSI and imaging applications including those from Thermo, Waters, and Bruker mass spectrometers . Recently, a miniature portable mass spectrometer was fitted with an LSI source …”

Ghost peaks observed after atmospheric pressure matrix-assisted laser desorption/ionization experiments may disclose new ionization mechanism of matrix-assisted hypersonic velocity impact ionization

“…Heating the capillary to 350°C provides effective desolvation of MALDI ions and reduces chemical noise compared with similar inlets kept at room temperature. [63] In the present funnel design, the highest temperature of the capillary was limited to 165° C; in addition, the capillary was only 85 mm long and had a much larger i.d. The latter two factors prevented the heat from capillary walls spreading inward to the center of the capillary, [64] thus limiting heating of the particulate matter travelling through the center.…”