Spontaneous Charge Separation and Sublimation Processes are Ubiquitous in Nature and in Ionization Processes in Mass Spectrometry.

Ionization processes have been discovered by which small and large as well as volatile and nonvolatile compounds are converted to gas-phase ions when associated with a matrix and exposed to sub-atmospheric pressure. Here, we discuss experiments further defining these simple and unexpected processes. Charge separation is found to be a common process for small molecule chemicals, solids and liquids, passed through an inlet tube from a higher to a lower pressure region, with and without heat applied. This charge separation process produces positively- and negatively-charged particles with widely different efficiencies depending on the compound and its physical state. Circumstantial evidence is presented suggesting that in the new ionization process, charged particles carry analyte into the gas phase, and desolvation of these particles produce the bare ions similar to electrospray ionization, except that solid particles appear likely to be involved. This mechanistic proposition is in agreement with previous theoretical work related to ion emission from ice. Graphical Abstract ᅟ.

Improving the Sensitivity of Matrix-Assisted Ionization (MAI) Mass Spectrometry Using Ammonium Salts.

In matrix-assisted ionization (MAI), analyte incorporated in a small molecule matrix is introduced into an aperture linking atmospheric pressure with the vacuum of a mass spectrometer. Gas-phase analyte ions are spontaneously produced without use of a laser or high voltage. Here we investigate analyte and background ion abundances upon addition of ammonium salts to various MAI matrix/analyte solutions. Regardless of the ammonium salt or matrix used, chemical background ions are suppressed and/or analyte ion abundance improved for basic small molecules, peptides, and proteins. Background ion abundances increase with increasing inlet temperature, but are suppressed with addition of any of a variety of ammonium salts without much effect on the total ion abundances of the analyte ions. However, at lower inlet temperature using the matrices 2-bromo-2-nitropropane-1,3-diol, 1,2-dicyanobenzene, and 3-nitrobenzonitrile (3-NBN), analyte ion abundance increases and any chemical background decreases upon addition of ammonium salt. The improvement in sensitivity using 3-NBN with ammonium salt allows full acquisition mass spectra consuming as little as 1 fmol of ubiquitin. More complete peptide coverage for 100 fmol of a BSA tryptic digest, and increased sensitivity of drugs spiked in urine and saliva were observed after ammonium salt addition to the 3-NBN matrix.

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

RATIONALE: A new variation of sonicspray ionization (SSI) significantly enhances its sensitivity with an advantage of producing selective or more inclusive ionization by changing the polarity of the potential applied to an obstruction in the path of the SSI spray. This technique also provides high sensitivity with water solutions, which is difficult for electrospray ionization (ESI) without added organic solvent. METHODS: An Orbitrap Exactive mass spectrometer with an IonMax source heated electrospray ionization (ESI) probe operating at its maximum sheath gas setting was used for SSI. Both positive and negative polarities varying from 0.1 to 4 kV were applied to a metal obstruction positioned in the path of the spray using an atmospheric solid analysis probe. All mass spectra for this study were acquired in the positive ion mode. RESULTS: SSI using a variety of obstructions improved the observed analyte ion abundance of small molecules, peptides, and proteins by approximately two orders of magnitude. The addition of a DC or AC voltage to a metal obstruction further enhanced the abundance of analyte ions by an additional two orders of magnitude. The relative abundances of the detected positive ions were considerably altered by switching the voltage polarity on the obstruction. CONCLUSIONS: 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.

High sensitivity steroid analysis using liquid chromatography/solvent-assisted inlet ionization mass spectrometry.

RATIONALE: Steroids can be injected to behave as therapeutic agents to promote muscle growth and strength. Areas of concern include synthetic steroids in consumer meat and milk products and the presence of anabolic steroids in athletes. Here we demonstrate a new ionization method for high sensitivity steroid analysis using liquid chromatography/mass spectrometry (LC/MS). METHODS: Solvent-assisted inlet ionization (SAII) mass spectrometry was coupled directly to an infusion pump or to a liquid chromatograph to determine the limits of detection and quantitation for selected steroids. LC/MS/MS data was acquired on a quadrupole time-of-flight (QTOF) mass spectrometer and high resolution-accurate mass LC/MS data was obtained on an Orbitrap mass spectrometer. RESULTS: The SAII limit of detection for infusion into the Orbitrap using high mass resolution and accurate mass was shown, for the steroids studied, to be low ppqt and the limit of quantitation using LC/MS was low ppt. Low ppb levels were detected with high signal-to-noise from spiked urine using a simple Ziptip procedure without sample concentration. CONCLUSIONS: LC/SAII-MS is more sensitive than electrospray ionization (ESI) at similar mobile phase flow rates for the analysis of steroids. Previous studies have shown LC/SAII-MS to have high sensitivity for analysis of peptides. The combined results suggests this easy to implement ionization method may advantageously replace ESI for a wide range of analyses.

Solvent assisted inlet ionization: an ultrasensitive new liquid introduction ionization method for mass spectrometry.

A new inlet ionization method requiring no voltage or laser, and using water, methanol, or water/organic solvent mixtures, is shown to produce mass spectra similar to those obtained with electrospray ionization (ESI) for small molecules, peptides, and proteins, at least as large as carbonic anhydrase, with sensitivity that surpasses ESI. With the use of wide mass range acquisitions at 100,000 mass resolution on an Orbitrap Exactive, detection limits below parts per trillion are obtained for small molecules such as arginine, ciprofloxacin, and acetaminophen. Low attomoles of bovine insulin consumed produced a multiply charged mass spectrum. Ions are generated, even using pure water as solvent, within the heated inlet tube linking atmospheric pressure with the first vacuum stage of the Orbitrap Exactive. The extremely high sensitivity observed at this early stage of solvent assisted inlet ionization (SAII) development suggests that inlet ionization may surpass nanoelectrospray in sensitivity but without the need for extremely low solvent flows.