More inclusive or selective ionization for mass spectrometry using obstructive sonic spray ionization and voltage polarity switching

Abstract

SSI with an obstruction improves the sensitivity of various samples compared with SSI. An applied voltage onto the obstruction further enhances the analyte ion abundances to approximately equal that of ESI but has the added advantage that switching the voltage polarity from positive to negative on the obstruction emphasizes different compounds present in the sample.

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

“…Briefly, in inlet ionization (Movie S1, supporting information), an inlet tube connects atmospheric pressure with sub‐atmospheric pressure and, depending on the matrix and inlet geometry used, optimizes at different inlet temperatures, similar to different voltages used in ESI and laser fluence in MALDI. An illustration is shown in Figure for several matrices used in MS (Scheme ) in which a few tenths of a μL of a co‐crystallized sample of the respective matrix solution is mixed with a 1 ppm solution of gramicidin S on a microscope cover slip slide, allowed to dry, and a small particle is introduced into the inlet.…”

“…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.…”

“…Consequently, using mass spectrometers optimized for ESI‐ and MALDI‐MS, the ease of adaptation of the methods based on the new ionization processes varies. It may only require turning off the high voltages and/or laser, or it may necessitate more significant physical modification(s) to obtain analytical useful results . As an example, the currently best performing MAI matrix, 3‐NBN, on Waters SYNAPT G2 and G2S instruments works exceptionally well for analyte ionization on the intermediate pressure MALDI sources without the use of the laser, but performs best at room temperature on the ESI Z‐Spray source blocks, and above the melting point of the matrix, 114–117 °C, the analyte ion abundance essentially ceases on this instrument .…”

“…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.…”

Alleviation of ion suppression effect in sonic spray ionization with induced alternating current voltage

“…Some studies resorted to the usage of high voltage and Venturi effect to improve the sensitivity and availability of SSI, resulting in the advent of electrosonic spray ionization (ESSI) and easy ambient sonic‐spray ionization . Recently, an obstruction was placed directly in the ion pathway of SSI to make a variation that could obtain enhanced ion abundance and improved selectivity . But this series of variations needed either complex assisting instrument or relatively high spray voltage, and the study on ion suppression effect of SSI is still lacking and therefore is worthwhile to be focused upon.…”