Randy W. Purves

Separation of leucine and isoleucine by electrospray ionization-high field asymmetric waveform ion mobility spectrometry-mass spectrometry

An electrospray ionization (ESI) source was used to generate gas-phase molecular anions of the amino acids leucine and isoleucine ((M-H)−; m/z −130), which were separated by high-field asymmetric waveform ion mobility spectrometry (FAIMS) and detected by quadrupole mass spectrometry (MS). This combination of ESI-FAIMS-MS enabled selective determination of either amino acid in mixtures that contained at least a 625-fold excess of the other. Comparisons with conventional ESI-MS showed a 50-fold improvement in the signal to background ratio for a 1 µM solution of leucine.

Application of ESI-FAIMS-MS to the analysis of tryptic peptides

High-field asymmetric waveform ion mobility spectrometry (FAIMS) separates gas-phase analyte ions from chemical background, offering substantial improvements in the detection of peptides from complex protein digests. For a digest of enolase 1 (baker’s yeast), the focusing and separation offered by FAIMS produced an average intensity gain of 3.5 for the tryptic ions and reductions in background intensity of 5- to 10-fold when compared with ESI-MS. The increased signal-to-background in the ESI-FAIMS-MS experiment resulted in a greater number of identifiable peptides and therefore greater sequence coverage. Compensation voltage (CV) maps for a total of 282 tryptic peptides from thirteen proteins, generated according to charge-state, mass-to-charge ratios, and chain length, show that a majority of tryptic peptides can be detected by operating FAIMS at a few discrete values of CV rather than scanning CV across a wide range. The ability to reduce scanning requirements has potential benefits for coupling FAIMS with LC-MS. In select cases, FAIMS can be used to eliminate isobaric MS overlap between tryptic peptides; however, the primary advantage of FAIMS in an LC-FAIMS-MS analysis is foreseen to be the attenuation of chemical background noise rather than the separation of individual peptides. Using FAIMS to reduce mass spectral noise will offer improved detection of peptides from low abundance proteins in complex biological samples.

Evaluation of carrier gases for use in high-field asymmetric waveform ion mobility spectrometry.

Effects of carrier gas type (N2, O2, CO2, N2O, and SF6) on changes in the ratio of high-to low-field ion mobility, Kh/K, of cesium, gramicidin S, tetrahexylammonium, heptadecanoic acid, and aspartic acid in fields of up to 67 Td are presented. The theory of the mobility of ions at high E/N in different gases is discussed. Plots of Kh/K as a function of the ionic energy parameter, E/N, for the five ions in each of the gases were derived from experimental data collected using a high-field asymmetric waveform ion mobility spectrometer. The change in the ratio of high-to low-field ion mobility of cesium in carrier gases of O2 and N2 showed excellent agreement with literature values. The behavior of cesium in O2 and N2 is used to illustrate that the ratio Kh/K as a function of effective temperature is invariant with gas type as long as the well depth of the interaction potential significantly exceeds thermal energy. From these results, it appears that the well depth of the interaction potential of the heavier ions studied here, including gramicidin S, tetrahexylammonium, and heptadecanoic acid, with bath gases such as N2 and O2, is shallow relative to thermal energy.

Investigation of bovine ubiquitin conformers separated by high-field asymmetric waveform ion mobility spectrometry: Cross section measurements using energy-loss experiments with a triple quadrupole mass spectrometer

High-field asymmetric waveform ion mobility spectrometry (FAIMS) was used to separate gas-phase conformers of bovine ubiquitin produced by electrospray ionization. These conformers were sampled by a triple quadrupole mass spectrometer where energy-loss experiments, following the work of Douglas and co-workers, were used to determine their cross sections. The measured cross sections for some conformers were readily altered by the voltages applied to the interface ion optics, therefore very gentle mass spectrometer interface conditions were required to preserve gas-phase conformers separated by FAIMS. Cross sections for 19 conformers (charge states +5 through +13) were measured. Two conformers for the +12 charge state, which were readily separated in FAIMS, were found to have similar cross sections. Based on a method to calibrate the collision gas thickness, the cross sections measured using the FAIMS/energy-loss method were compared with literature values determined using drift tube ion mobility spectrometry. The comparison illustrated that the conformers of bovine ubiquitin that were identified using drift tube ion mobility spectrometry were also observed using the FAIMS device.

Separation of protein conformers using electrospray-high field asymmetric waveform ion mobility spectrometry-mass spectrometry

Abstract High-field asymmetric waveform ion mobility spectrometry (FAIMS) is a technique for continuous gas-phase ion separation at atmospheric pressure (760 Torr) and room temperature. FAIMS separates ions based on changes in mobility at high electric fields. Ion focusing in a cylindrical geometry FAIMS provides high ion transmission efficiency, thereby making FAIMS an ideal interface for transferring ions from an electrospray ionization (ESI) source to a mass spectrometer (MS). In this study, an ESI-FAIMS-MS instrument was used to study the conformers of the protein bovine ubiquitin. Multiple conformers for some charge states of bovine ubiquitin were resolved by FAIMS, including at least three for the +8 charge state. The number and abundance of the conformers of several charge states of bovine ubiquitin were dependent on solution pH and solvent composition. Mass spectra of individual conformers showed conformer-specific distributions of sodium and phosphate adduct ions. ESI-FAIMS-MS conformational data are compared with literature results that were collected using ESI drift tube mobility spectrometry/mass spectrometry.

Isotope separation using high–field asymmetric waveform ion mobility spectrometry

Abstract A new apparatus for gas-phase separation of stable elemental isotopes at atmospheric pressure is described. A gaseous mixture of chloride isotopes was generated using electrospray ionization and introduced into the analyzer region of a high-field asymmetric waveform ion mobility spectrometer (FAIMS). The ion current exiting the FAIMS was sampled into a quadrupole mass spectrometer for isotope identification.

Comparison of high-field ion mobility obtained from drift tubes and a FAIMS apparatus

Abstract High-field asymmetric waveform ion mobility spectrometry (FAIMS) separates ions by application of a high-voltage asymmetric waveform to closely spaced electrodes such that ions experience (transient) electric fields that are sufficiently high that the mobility deviates from its low field limit. We describe the use of FAIMS to determine the mobility of the chloride ion, m/z −35, at ∼760 Torr and ∼300 K in fields up to ∼65 Td. The high-field mobility of chloride was determined from FAIMS data using linear and nonlinear least squares techniques, and was found to agree well with published values of mobility determined by drift tube systems up to 50 Td.

Determination of Parts-per-Trillion Levels of Chlorate, Bromate, and Iodate by Electrospray Ionization/High-Field Asymmetric Waveform Ion Mobility Spectrometry/Mass Spectrometry

An electrospray ionization (ESI) source was used to transfer oxoanions (XO3−) of nitrogen, chlorine, bromine, and iodine to the gas phase for analysis by a combination of high-field asymmetric waveform ion mobility spectrometry (FAIMS) and mass spectrometry. The combination of ESI with FAIMS and mass spectrometry (ESI-FAIMS-MS) yields mass spectra that are virtually free of common spectral interferences. The unique separation and focusing properties of the FAIMS analyzer improved signal-to-background ratios (S/B) for all these analytes by 3–4 orders of magnitude compared with conventional ESI-MS and produced parts-per-trillion (pptr) detection limits for ClO3−, BrO3−, and IO3− in methanolic solution.

Detection of microcystins using electrospray ionization high-field asymmetric waveform ion mobility mass spectrometry/mass spectrometry

A combination of electrospray ionization, high-field asymmetric waveform ion mobility spectrometry, and mass spectrometry (ESI-FAIMS/MS) was used to analyze standard solutions of microcystins-LR, -RR, and -YR. The ability of FAIMS to separate ions in the gas phase reduced the amount of background in the mass spectrum without compromising the absolute signal for these microcystins. This reduction in background resulted in a ten-fold improvement in the signal-to-background ratio over conventional ESI-MS. Detection limits, using direct infusion, were determined to be 4, 2, and 1 nM for microcystins-LR, -RR, and -YR, respectively.

INVESTIGATION OF THE QUANTITATIVE CAPABILITIES OF AN ELECTROSPRAY IONIZATION ION TRAP/LINEAR TIME-OF-FLIGHT MASS SPECTROMETER

The quantitative analysis of pharmaceutical or environmental compounds represents an important application area of electrospray ionization (ESI) mass spectrometry (MS). Typically, these analyses are carried out using quadrupole mass analyzers. Recently, several other mass spectrometric configurations have been developed for ESI, including time-of-flight (TOF), which offers high sensitivity and fast data acquisition capabilities. This work investigates the capabilities and limitations of using ESI ion trap/TOFMS for quantitation. The analysis of sulfonamides, an important class of compounds used in the food industry, was selected for this study. It is demonstrated that a calibration curve exhibits a linear response for almost 3 orders of magnitude. The lower limit of the linear range is restricted by the chemical background noise while the upper limit is governed by the ESI source. The short term and long term reproducibility of the instrument are both demonstrated to be better than 8% relative standard deviation. The detection limits and linear dynamic range of this instrument are comparable with those obtained using a single quadrupole mass spectrometer for a single compound and superior for multicomponent analyses. For the analysis of five components, the detection limits using ESI ion trap/TOFMS are about three times lower. It is argued that improving the detection limits, through the use of tandem mass spectrometry, should further extend the linear dynamic range of the instrument for quantitation.