Direct sub-atmospheric pressure ionization mass spectrometry: Evaporation/sublimation-driven ionization is amazing, fundamentally, and practically.

This paper covers direct sub-atmospheric pressure ionization mass spectrometry (MS). The discovery, applications, and mechanistic aspects of novel ionization processes for use in MS that are not based on the high-energy input from voltage, laser, and/or high temperature but on sublimation/evaporation within a region linking a higher to lower pressure and modulated by heat and collisions, are discussed, including how this new reality has guided a series of discoveries, instrument developments, and commercialization. A research focus, inter alia, is on how best to understand, improve, and use these novel ionization processes, which convert volatile and nonvolatile compounds from solids (sublimation) or liquids (evaporation) into gas-phase ions for analysis by MS providing reproducible, accurate, sensitive, and prompt results. Our perception on how these unprecedented versus traditional ionization processes/methods relate to each other, how they can be made to coexist on the same mass spectrometer, and an outlook on new and expanded applications (e.g., clinical, portable, fast, safe, and autonomous) is presented, and is based on ST's Opening lecture presentation at the Nordic Mass spectrometry Conference, Geilo, Norway, January 2023. Focus will be on matrix-assisted ionization (MAI) and solvent-assisted ionization (SAI) MS covering the period from 2010 to 2023; a potential paradigm shift in the making.

Resolving Isomers of Star-Branched Poly(Ethylene Glycols) by IMS-MS Using Multiply Charged Ions.

Ion mobility spectrometry (IMS) mass spectrometry (MS) centers on the ability to separate gaseous structures by size, charge, shape, and followed by mass-to-charge (m/z). For oligomeric structures, improved separation is hypothesized to be related to the ability to extend structures through repulsive forces between cations electrostatically bonded to the oligomers. Here we show the ability to separate differently branched multiply charged ions of star-branched poly(ethylene glycol) oligomers (up to 2000 Da) regardless of whether formed by electrospray ionization (ESI) charged solution droplets or from charged solid particles produced directly from a surface by matrix-assisted ionization. Detailed structural characterization of isomers of the star-branched compositions was first established using a home-built high-resolution ESI IMS-MS instrument. The doubly charged ions have well-resolved drift times, achieving separation of isomers and also allowing differentiation of star-branched versus linear oligomers. An IMS-MS "snapshot" approach allows visualization of architectural dispersity and (im)purity of samples in a straightforward manner. Analyses capabilities are shown for different cations and ionization methods using commercially available traveling wave IMS-MS instruments. Analyses directly from surfaces using the new ionization processes are, because of the multiply charging, not only associated with the benefits of improved gas-phase separations, relative to that of ions produced by matrix-assisted laser desorption/ionization, but also provide the potential for spatially resolved measurements relative to ESI and other ionization methods.

Laserspray ionization on a commercial atmospheric pressure-MALDI mass spectrometer ion source: selecting singly or multiply charged ions.

Multiply charged ions, similar to those obtained with electrospray ionization, are produced at atmospheric pressure (AP) using standard MALDI conditions of laser fluence and reflective geometry. Further, the charge state can be switched to singly charged ions nearly instantaneously by changing the voltage applied to the MALDI target plate. Under normal AP-MALDI operating conditions in which a voltage is applied to the target plate, primarily singly charged ions are observed, but at or near zero volts, highly charged ions are observed for peptides and proteins. Thus, switching between singly and multiply charged ions requires only manipulation of a single voltage. As in ESI, multiple charging, produced using the AP-MALDI source, allows compounds with molecular weights beyond the mass-to-charge limit of the mass spectrometer to be observed and improves the fragmentation relative to singly charged ions.

Transport of ammonium perfluorooctanoate in environmental media near a fluoropolymer manufacturing facility.

In order to understand better the pathways for transport of ammonium perfluorooctanoate (APFO) from a point source, a focused investigation of environmental media (water and soil) near a fluoropolymer manufacturing facility (Site) was undertaken. Methods were developed and validated at 2 microg kg(-1) [the limit of quantitation (LOQ)] in soil, and at 50 ng l(-1) in water. Environmental media were sampled from a public water supply well field located north of the Site, across a river. The data suggest that APFO air emissions from the Site are transported to the well field, deposited onto the soil, and then migrate downward with precipitation into the underlying aquifer.

Method development for the determination of residual fluorotelomer raw materials and perflurooctanoate in fluorotelomer-based products by gas chromatography and liquid chromatography mass spectrometry.

The methodology for the determination of perfluorooctanoate (C(7)F(15)COO-, PFO), fluorotelomer alcohols (FTOHs: 6-2, 8-2, and 10-2), perfluorooctyl iodide (PFOI), and 8-2-8 fluorotelomer alcohol ester in complex fluorotelomer-based commercial products has been demonstrated and validated. Sample preparation procedures allowing determination of residual levels of these compounds were developed. The analytes were detected either by LC/MS/MS (PFO), LC/MS (FTOHs), or GC/MS (PFOI, 8-2-8 ester). The methods were validated by investigating the recoveries of analytes spiked at multiple levels to authentic sample matrices. The recoveries generally were between 70 and 130%. The limits of detection were in sub-microg/g range and the limits of quantitation were in the mug/g range. The methods were applied to fluorotelomer-based raw materials and fluorotelomer-based surfactants and polymeric products and represent methods useful for the determination of higher carbon chain length homologs as well.

Efficient "total" extraction of perfluorooctanoate from polytetrafluoroethylene fluoropolymer.

To determine the optimum conditions for the complete extraction of perfluorooctanoate (PFO) from polytetrafluoroethylene fluoropolymers, sample preparation and pressurized solvent extraction (PSE) conditions were investigated. Solvent extraction temperature, solvent residence time, relaxation time between extractions, and the effects of heating before PSE showed that methanol at 150 degrees C extraction temperature and a 12 min solvent residence time were the most efficient conditions. Preheating the polymer before extraction at 150 degrees C for 24 h significantly enhanced the quantity of PFO removed. Heating above 150 degrees C resulted in loss of PFO. PFO was determined by liquid chromatography with tandem mass spectrometry.

Analysis of solids, liquids, and biological tissues using solids probe introduction at atmospheric pressure on commercial LC/MS instruments.

Direct analysis of samples using atmospheric pressure ionization (API) provides a more rapid method for analysis of volatile and semivolatile compounds than vacuum solids probe methods and can be accomplished on commercial API mass spectrometers. With only a simple modification to either an electrospray (ESI) or atmospheric pressure chemical ionization (APCI) source, solid as well as liquid samples can be analyzed in seconds. The method acts as a fast solids/liquid probe introduction as well as an alternative to the new direct analysis in real time (DART) and desorption electrospray ionization (DESI) methods for many compound types. Vaporization of materials occurs in the hot nitrogen gas stream flowing from an ESI or APCI probe. Ionization of the thermally induced vapors occurs by corona discharge under standard APCI conditions. Accurate mass and mass-selected fragmentation are demonstrated as is the ability to obtain ions from biological tissue, currency, and other objects placed in the path of the hot nitrogen stream.

Fluorescence-based peptide labeling and fractionation strategies for analysis of cysteine-containing peptides.

This study demonstrates that 1,5-I-AEDANS (5-({2-[(iodoacetyl)amino]ethyl}amino)naphthalene-1-sulfonic acid) can be used as a versatile fluorescence-based peptide quantification tool and provides readily interpretable tandem mass spectra for de novo peptide sequencing. Two AEDANS-cysteinyl-peptide fractionation strategies were evaluated. One AEDANS-cysteinyl-peptide fractionation strategy employs immobilized metal affinity chromatography (IMAC) to recover AEDANS-labeled peptides and reduce the complexity of peptide mixtures. In an alternate solid-phase approach, 1,5-I-AEDANS was coupled to an o-nitrobenzyl-based photocleavable resin to produce a resin that can label and isolate thiols and cysteine-containing peptides with a modified-AEDANS label (mAEDANS: 5-((4-amino-4-oxobutanoyl){2-[(iodoacetyl)amino]ethyl}amino)naphthalene-1-sulfonic acid). This fractionation protocol enriches cysteine-containing peptides more specifically than the IMAC strategy. Using micro-LC-ESI-MS with an on-line fluorescence detector and a Q-TOF mass spectrometer, we generated fluorescence-based elution profiles and corresponding positive ion mass spectra of AEDANS-labeled peptides. This study demonstrates that AEDANS-peptides produce positive ion ESI-MS mass spectra with detection limits comparable to those of the unlabeled peptide. Collision-induced dissociation (CID) of fluorescent AEDANS-peptides revealed readily interpretable product ion spectra with the label intact. Similar to the AEDANS-labeled peptide, an mAEDANS-labeled thiol is fluorescent and CID of a mAEDANS-labeled peptide also reveals an interpretable product ion spectrum with the label intact.

Comparison of pressurized solvent and reflux extraction methods for the determination of perfluorooctanoic acid in polytetrafluoroethylene polymers using LC-MS-MS.

Both pressurized solvent extraction (PSE) and reflux extraction in various solvents were used to select the most efficient system for the determination of the quantity of perfluorooctanoic acid (PFOA) present in polytetrafluoroethylene polymers. After evaporating the solvent, PFOA was determined using liquid chromatography tandem mass spectrometry. Ethanol, water and methanol gave comparable results and were shown to be good solvents for this extraction. Acetonitrile was a reasonable solvent using the reflux extraction method, but not with PSE. Chloroform resulted in poor recovery for both extraction methods. PSE proved to be the more efficient extraction method.

Method for the determination of perfluorooctanoic acid in air samples using liquid chromatography with mass spectrometry.

Perfluorooctanoic acid is a completely fluorinated carboxylic acid that is usually used in the ammonium salt form as a processing aid in the production of many fluoropolymers and fluoroelastomers. Ammonium perfluorooctanoate readily dissociates in water to give the ammonium and perfluorooctanoate ions. Perfluorooctanoate has been reported to be present in low levels in human serum in the United States and Europe. This study reports on the development and validation of a method for the determination of perfluorooctanoic acid in air samples. This method uses the Occupational Safety and Health Administration (OSHA) Versatile Sampler (OVS) with a nominal 0.3 micro m filter and polystyrene resin sorbent (XAD-2 or XAD-4) followed by determination of the perfluorooctanoate anion by liquid chromatography mass spectrometry. The method was validated in the range of 0.474 to 47.4 microg/m3 for a 480-L sample. Breakthrough studies showed samples could be collected at 1 L/min for 24 hours or at 15 L/min up to 8 hours without breakthrough. Extract storage stability tests showed that sample extracts in methanol remain stable in glass autosampler vials for up to 13 days following initial injection. Perfluorooctanoic acid stability on OVS tubes was unaffected at both refrigerated and ambient temperatures. The overall average retention efficiency was 92.1% with a pooled RSD95 of 5.8% at five concentration levels (0.474 microg/m3 to 47.4 microg/m3).

Carrier ampholyte-free solution isoelectric focusing as a prefractionation method for the proteomic analysis of complex protein mixtures.

The field of proteomics requires methods that offer high sensitivity and wide dynamic range. One of the strategies used to improve the dynamic range is sample prefractionation, such as microsolution isoelectric focusing (IEF). We have modified a commercial solution IEF instrument, the Rotofor, to prefractionate protein mixtures by carrier ampholyte-free solution IEF. The focusing chamber of the Rotofor was divided into several compartments by polyacrylamide membranes with imbedded Immobiline mixtures of specific pH values. When an electric field is applied, each protein migrates to the compartment confined by membranes with pH values flanking its isoelectric point. The approach was demonstrated for the focusing of myoglobin into a predicted compartment, as well as the separation of a complex soluble yeast protein mixture into several distinct fractions. The proteins were dissolved in water or 30% isopropanol. The method is applicable to both gel-based and solution-phase protein identification methods, without the need for further sample preparation.