Recent developments and applications of selected ion flow tube mass spectrometry (SIFT-MS).

Selected ion flow tube mass spectrometry (SIFT-MS) is now recognized as the most versatile analytical technique for the identification and quantification of trace gases down to the parts-per-trillion by volume, pptv, range. This statement is supported by the wide reach of its applications, from real-time analysis, obviating sample collection of very humid exhaled breath, to its adoption in industrial scenarios for air quality monitoring. This review touches on the recent extensions to the underpinning ion chemistry kinetics library and the alternative challenge of using nitrogen carrier gas instead of helium. The addition of reagent anions in the Voice200 series of SIFT-MS instruments has enhanced the analytical capability, thus allowing analyses of volatile trace compounds in humid air that cannot be analyzed using reagent cations alone, as clarified by outlining the anion chemistry involved. Case studies are reviewed of breath analysis and bacterial culture volatile organic compound (VOC), emissions, environmental applications such as air, water, and soil analysis, workplace safety such as transport container fumigants, airborne contamination in semiconductor fabrication, food flavor and spoilage, drugs contamination and VOC emissions from packaging to demonstrate the stated qualities and uniqueness of the new generation SIFT-MS instrumentation. Finally, some advancements that can be made to improve the analytical capability and reach of SIFT-MS are mentioned.

Understanding Gas Phase Ion Chemistry Is the Key to Reliable Selected Ion Flow Tube-Mass Spectrometry Analyses.

Ion-molecule reactions (IMR) are at the very core of trace gas analyses in modern chemical ionization (CI) mass spectrometer instruments, which are increasingly being used in diverse areas of research and industry. The focus of this Perspective is on the ion chemistry that underpins gas-phase analytical CI methods. Special attention is given to the soft chemical ionization method known as selected ion flow tube-mass spectrometry (SIFT-MS). The processes involved in the ion chemistry of the reagent cations, H3O+, NO+, and O2+•, and the anions, O-•, O2-•, OH-, and NO2-, are discussed in some detail. Stressed throughout is that an understanding of these processes is mandatory to obtain reliable analyses of humid gaseous media such as ambient air and exhaled breath. It is indicated that further research is needed to understand the consequences of replacing helium in some situations by the more readily available nitrogen as the carrier gas in SIFT-MS.

Rapid monitoring of volatile organic compounds: a comparison between gas chromatography/mass spectrometry and selected ion flow tube mass spectrometry.

RATIONALE: The gold standard for monitoring volatile organic compounds (VOCs) is gas chromatography/mass spectrometry (GC/MS). However, in many situations, when VOC concentrations are at the ppmv level, there are complicating factors for GC/MS. Selected ion flow tube mass spectrometry (SIFT-MS) is an emerging technique for monitoring VOCs in air. It is simpler to use and provides results in real time. METHODS: Three different experiments were used for the comparison. First SIFT-MS was applied to monitor the concentrations of 25 VOCs in a mixture at concentrations up to 1 ppmv using only a generic database for known kinetic data of three reagent ions (H3O(+), NO(+) and O2(+)) with each VOC. In experiment 2, a side-by-side comparison was made of 17 VOCs at concentrations between 1 ppmv and 5 ppbv after small corrections had been made to the SIFT-MS kinetic data. In a third experiment, a side-by-side comparison examined two groups of samples received for commercial analysis. RESULTS: In experiment 1, 85% of the VOC concentrations were within 35% of their stated values without any calibration of the SIFT-MS instrument. In experiment 2, the two techniques yielded good correspondence between the measured VOC concentrations. In experiment 3, good correlation was found for VOCs from three of the samples. However, interferences from some product ions gave over-reported values in one sample from the SIFT-MS instrument. CONCLUSIONS: These three experiments showed that GC/MS was better suited to monitoring samples containing large numbers of VOCs at high concentrations. In all other applications, SIFT-MS proved simpler to use, was linear with concentration over a much wider concentration range than GC/MS and provided faster results.

Selected ion flow tube studies of several siloxanes.

RATIONALE: People are using increasing amounts of siloxanes that ultimately end up in landfills and then in landfill gas and biogas digesters. Their presence poses difficulties for industries seeking to utilize the energy content of landfill and biogas, as the combustion process oxidises silicon to silicon dioxide that in turn damages engine parts. Rapid, efficient and accurate methods are needed to quantify their presence. METHODS: Selected ion flow tube mass spectrometry (SIFTMS) is an emerging real-time technique that has found application for monitoring trace volatiles in air. Samples containing the trace volatiles are simply drawn into the flow tube and convected in a stream of helium. Chemical ionization reactions from mass-selected reagent ions with the volatiles ensue. To quantify the volatiles in the sample, the ion chemistry of the reagent ion with each volatile must be known. RESULTS: Rate coefficients and product ion branching ratios were found for the compounds dodecamethylpentasiloxane, decamethylcyclopentasiloxane, decamethyltetrasiloxane, octamethylcyclotetrasiloxane, triethylsilanol, tetramethylsilane and hexamethyldisilazane. CONCLUSIONS: The ion-molecule reactions of the seven silicon-containing compounds examined here were fast, occurring at or near the collision rate, thus allowing for detection at low levels. The very simple reaction chemistry found of proton transfer, electron transfer and methyl loss will enable easy quantitation of the siloxanes in landfill gas and biogas using the SIFTMS technique.

Application of selected ion flow tube mass spectrometry to real-time atmospheric monitoring.

Data are presented for real-time atmospheric monitoring of volatile organic chemicals (VOCs) in air using selected ion flow tube mass spectrometry (SIFT-MS) technology. These measurements were made by one of the new generation of SIFT-MS instruments. Results are shown for five VOCs that were continually monitored from a stationary sampling point over a 4-day period: ethene, ethanol, 1,3-butadiene, benzene and toluene. All analytes except ethene in the study have at least two simultaneous and independent measures of concentration. These results demonstrate the great advances in SIFT-MS that have been made in recent years. 1,3-Butadiene is measured at a concentration of 9 pptv with a precision of 44%. For a 1-s integration time, a detection limit of 50 pptv is achieved. Instrument sensitivities are reported for all five analytes.

Real-time measurement of peroxyacetyl nitrate using selected ion flow tube mass spectrometry.

The on-line detection of gaseous peroxyacetyl nitrate (PAN) using selected ion flow tube mass spectrometry (SIFT-MS) has been investigated using a synthetic sample of PAN in air at a humidity of approximately 30%. Using the H(3)O(+) reagent ion, signals due to PAN at m/z 122, 77 and 95 have been identified. These correspond to protonated PAN, protonated peractetic acid and its water cluster, respectively. These products and their energetics have been probed through quantum mechanical calculations. The rate coefficient of H(3)O(+) has been estimated to be 4.5 x 10(-9) cm(3) s(-1), leading to a PAN sensitivity of 138 cps/ppbv. This gives a limit of detection of 20 pptv in 10 s using the [M+H](+) ion of PAN at m/z 122.

Monitoring chloramines and bromamines in a humid environment using selected ion flow tube mass spectrometry.

The selectivity and sensitivity of selected ion flow tube mass spectrometry (SIFT-MS) for individual breath analysis of haloamines has been improved by heating the flow tube in a commercial instrument to around 106 degrees C. Data is presented showing the marked reduction in the number density of water clusters of product ions of common breath metabolites that are isobaric with the product ions from monochloramine and monobromamine that are used to monitor the haloamine concentrations. These results have direct relevance to the real-time monitoring of chloramines in drinking water, swimming pools and food processing plants. However, once the isobaric overlaps from water cluster ions are reduced at the higher temperatures, there is no conclusive evidence showing the presence of haloamines on single breath exhalations in the mid parts per trillion range from examination of the breaths of volunteers.

Detection and quantification of chemical warfare agent precursors and surrogates by selected ion flow tube mass spectrometry.

The rate coefficients and branching ratios of 15 chemical warfare agent precursor and surrogate compounds reacting with H(3)O(+), NO(+), and O(2)(+) have been measured in the laboratory using the selected ion flow tube (SIFT) technique. Measurement of the relevant kinetic parameters for these agents has enabled quantitative monitoring using the SIFT-MS analytical technique. Thirteen of the 15 compounds studied were found to have real-time detection limits in the parts-per-trillion-by-volume concentration range when measured on a standard commercial Voice100 instrument, with specific compounds having detection limits below 100 parts-per-trillion-by-volume.

Real-time monitoring of hazardous air pollutants.

Selected ion flow tube mass spectrometry (SIFT-MS) is a technique that offers real-time alternatives to existing methods for monitoring hazardous air pollutants (HAPs) in the environment using chemical ionization. The use of this technique requires knowledge of the kinetic parameters of the reagent ions H(3)O(+), NO(+), and O(2)(+) that are most commonly used. We report here measurements with these reagent ions of kinetic parameters for 17 HAP molecules ranging from 1,1-dichloroethene to nitrobenzene. From these data, limits of quantitation are established for all 17 compounds on a commercial SIFT-MS instrument and are found to be well below the time-weighted averages required by legislating bodies for workplace conditions.

Determination of olive oil oxidative status by selected ion flow tube mass spectrometry.

The emergence of primary and secondary oxidation products in New Zealand extra virgin olive oil during accelerated thermal oxidation was measured and correlated with the concentrations of 13 headspace volatile compounds measured by selected ion flow tube mass spectrometry (SIFT-MS). SIFT-MS is a mass spectrometric technique that permits qualitative and absolute quantitative measurements to be made from whole air, headspace, or breath samples in real-time down to several parts per billion (ppb). It is well-suited to high-throughput analysis of headspace samples. Propanal, hexanal, and acetone were found at high concentrations in a rancid standard oil, while propanal, acetone, and acetic acid showed marked increases with oxidation time for the oils used in this study. A partial least-squares (PLS) regression model was constructed, which allowed the prediction of peroxide values (PV) for three separate oxidized oils. Sensory rancidity was also measured, although the correlations of headspace volatile compounds with sensory rancidity score were less satisfactory, and too few results were available for the construction of a PLS regression model. A fast (approximately 1 min), reliable method for prediction of olive oil PVs by SIFT-MS was developed.

An ICR study of ion-molecules reactions relevant to Titan's atmosphere: an investigation of binary hydrocarbon mixtures up to 1 micron.

Results are reported for studies of binary mixtures of hydrocarbons exposed to low-energy electron impact ionization. A variety of experimental methods are used: conventional ICR mass spectrometry, the standard double resonance in an ICR for determination of the precursor ions, and the modulated double resonance ejection in an ICR for the determination of the daughter ions. A flowing afterglow-selected ion flow tube experiment (FA-SIFT) was used for validation and examination of termolecular reactions. An extensive database of reaction kinetics already exists for many of these reactions. The main point of this study was the determination of the accuracy of this database and the identification of missing reactions and reaction channels. An effort was made to extend the study to the highest pressures possible to determine if any important termolecular reaction channels were present that were not recognized in earlier investigations. A new approach was used here. In the binary mixtures of hydrocarbon gases, mass spectra were obtained as a function of independent pressure changes of both gases. All the mass peaks in the spectra were fitted using existing kinetic data as a starting point. A model of the ion abundances was then produced from the solution of the partial differential equations derived from the kinetics in terms of reaction rate coefficients and initial abundances. The model was fitted to the data for all of the pressures by a least-squares fit to the reaction rate coefficients and initial abundances. The kinetic parameters were then adjusted if required.

Real-time detection of common microbial volatile organic compounds from medically important fungi by Selected Ion Flow Tube-Mass Spectrometry (SIFT-MS).

We describe a new method, Selected Ion Flow Tube-Mass Spectrometry (SIFT-MS) for the rapid and sensitive real-time detection and quantification of volatile organic compounds from medically important fungi, grown on a range of laboratory media. SIFT-MS utilises the chemical ionisation reactions of mass-selected ions to characterise volatile organic compounds (VOCs) that are produced as metabolites from fungi. This technique has the distinct advantage over others in that it readily detects low molecular weight, reactive volatiles, and allows for real-time, quantitative monitoring. The fungi examined in this study were Aspergillus flavus, Aspergillus fumigatus, Candida albicans, Mucor racemosus, Fusarium solani, and Cryptococcus neoformans grown on or in malt extract agar, Columbia agar, Sabouraud's dextrose agar, blood agar, and brain-heart infusion broth. Common metabolites (ethanol, methanol, acetone, acetaldehyde, methanethiol, and crotonaldehyde) were detected and quantified. We found the fingerprint of volatiles, in terms of presence and quantity of volatiles to be strongly dependent on the culture medium, both in terms of variety and quantity of volatiles produced, but may form the basis for species specific identification of medically important fungi.

A SIFT ion-molecule study of some reactions in Titan's atmosphere. reactions of N(+), N(2)(+), and HCN(+) with CH(4), C(2)H(2), and C(2)H(4).

The results of a study of the ion-molecule reactions of N(+), N(2)(+), and HCN(+) with methane, acetylene, and ethylene are reported. These studies were performed using the FA-SIFT at the University of Canterbury. The reactions studied here are important to understanding the ion chemistry in Titan's atmosphere. N(+) and N(2)(+) are the primary ions formed by photo-ionization and electron impact in Titan's ionosphere and drive Titan's ion chemistry. It is therefore very important to know how these ions react with the principal trace neutral species in Titan's atmosphere: Methane, acetylene, and ethylene. While these reactions have been studied before the product channels have been difficult to define as several potential isobaric products make a definitive answer difficult. Mass overlap causes difficulties in making unambiguous species assignments in these systems. Two discriminators have been used in this study to resolve the mass overlap problem. They are deuterium labeling and also the differences in reactivities of each isobar with various neutral reactants. Several differences have been found from the products in previous work. The HCN(+) ion is important in both Titan's atmosphere and in the laboratory.

Termolecular ion-molecule reactions in Titan's atmosphere. IV. A search made at up to 1 micron in pure hydrocarbons.

The results of a study of ion-molecule reactions occurring in pure methane, acetylene, ethylene, ethane, propyne, propene, propane, and diacetylene at pressures up to 40 microns of pressure are reported. A variety of experimental methods are used: The standard double resonance in an ICR, for determination of the precursor ions and the modulated double resonance ejection in an ICR, for the determination of the daughter ions. The FA-SIFT technique was used for validation and examination of termolecular reactions with rate coefficients that are less than 10(-26) cm(6) s(-1). An extensive database of reaction kinetics already exists for many of these reactions. The main point of this study was the determination of the accuracy of this database and to search for any missing reactions and reaction channels that may have been omitted from earlier investigations. A specific objective of this work was to extend the study to the highest pressures possible to find out if there were any important termolecular reaction channels occurring. A new approach was used here. In the pure hydrocarbon gases the mass spectra were followed as a function of the pressure changes of the gas. An initial guess was first made using the current literature as a source of the reaction kinetics that were expected. A model of the ion abundances was produced from the solution of the partial differential equations in terms of reaction rate coefficients and initial abundances. The experimental data was fitted to the model for all of the pressures by a least squares minimization to the reaction rate coefficients and initial abundances. The reaction rate coefficients obtained from the model were then compared to the literature values. Several new channels and reactions were discovered when the modeled fits were compared to the actual data. This is all explained in the text and the implications of these results are discussed for the Titan atmosphere.

Reactions of CH3OCH2+ with hydrocarbons and O, N, and S compounds: applications for chemical ionization in selected ion flow tube studies.

We report the results of a flowing afterglow ion source-selected ion flow tube study (FA-SIFT) of the reactions of the methoxymethyl cation, CH3OCH2+. Rate coefficients and product branching ratios are reported for twenty nine reagent molecules including those that constitute the major ingredients of air, the hydrocarbons CH4, C2H6, C3H8, n-C4H10, C2H2, C2H4, C3H4 (allene and propyne), C6H6, and the S-containing molecules H2S, CH3SH, C2H5SH, (CH3)2SH, and (C2H5)2SH. In addition, we examined the reactions with the N-containing molecules NH3, CH3NH2, (CH3)2NH, (CH3)3N, pyrrole, pyridine as well as CH3COCH3. The results can be summarized under three general reaction types: Reaction at the CH3 carbon, reaction at the CH2 carbon, and association. The results also indicate that the methoxymethyl cation can be used as a chemical ionization source for the detection of trace levels of S-containing compounds in saturated hydrocarbons.

Application of selected ion flow tube-mass spectrometry to the characterization of monofloral New Zealand honeys.

Honeys have a range of physicochemical and organoleptic properties, depending on the nectar source. Selected Ion Flow Tube-Mass Spectrometry (SIFT-MS) is an emerging technology that quantifies volatile organic compounds (VOCs) to low concentrations (usually parts-per-trillion (ppt) levels) and is here applied to monitor the aromas in the headspace of different New Zealand monofloral honeys. Honey aromas arise from VOCs in the honeys that differ according to the flower type from which they were derived. In this exploratory study, the headspaces of nine monofloral New Zealand honeys (beech honeydew, clover, kamahi, manuka, rata, rewarewa, tawari, thyme, and vipers bugloss) were analyzed using SIFT-MS without sample preparation. The purpose of the investigation was to identify the major volatiles in each of the honeys and to test the feasibility of using SIFT-MS to distinguish between New Zealand monofloral honeys. In the nine monofloral honeys sampled, a clear distinction was observed between them based on their aroma signatures.

Headspace analysis of Italian and New Zealand parmesan cheeses.

UNLABELLED: New Zealand is a leader in the global dairy industry. Milk powder is the principal export product, but there is also a prominent cheese manufacturing industry, catering more for the domestic market. The Selected Ion Flow Tube-Mass Spectrometric (SIFT-MS) technique was used to compare 4 New Zealand cheeses marketed as "parmesan" with 4 Italian Parmigiano Reggiano and Grana Padano cheeses. The cheese headspace was analyzed in real time without any sample preconcentration. Total of 38 volatile compounds in the cheese headspace were monitored with headspace concentrations varying between single digit parts per billion (ppb) to tens of parts per million (ppm). When the results were subjected to multivariate statistical analysis, a clear discrimination was found between the New Zealand "parmesan" and Italian cheeses based solely on the measured concentrations of these volatile compounds. If the volatile compounds used in the analyses were restricted to known odor-active compounds in Parmigiano Reggiano cheese, the ability to discriminate between the cheeses was maintained. The analyses also showed that it was possible to clearly differentiate between the different processing plants in individual countries. Important discriminatory volatiles in the samples tested were butanoic acid and phenylacetaldehyde for discriminating between Italian cheeses and ethyl butyrate, acetaldehyde and methylbutanals between New Zealand cheeses. We conclude that the New Zealand "parmesans" do not provide a good representation of the aroma of Italian "parmesans." PRACTICAL APPLICATION: SIFT-MS has been shown to clearly differentiate both country of origin and the manufacturer of "parmesan" cheeses made in Italy and New Zealand based on differences in volatile organic compounds. Thus this method will have benefit for use in the quality control of "parmesan" and other cheese varieties.

Improved peroxyl radical scavenging TOSC assay to quantify antioxidant capacity using SIFT-MS.

We report a new, fast, sensitive variation of the total oxyradical scavenging capacity (TOSC) assay for measuring the antioxidant capacity of pure compounds, plant extracts and biological fluids using selected ion flow tube mass spectrometry (SIFT-MS). The TOSC assay examines the partial inhibition of ethene formation in the presence of antioxidants that compete with alpha-keto-gamma-methiolbutyric acid (KMBA) for reactive oxygen species. The SIFT-MS-TOSC assay takes 15 s for each ethene analysis and the time interval between consecutive analyses is 20 s. We demonstrate the method by monitoring the antioxidant capacity of several standard radical scavengers of peroxyl radicals. For peroxyl radicals the measured SIFT-MS-TOSC concentrations necessary to produce 50% inhibition of radical reaction with KMBA are 6.1 +/- 0.3 microM for Trolox, 5.7 +/- 0.3 microM for ascorbic acid, 8.4 +/- 0.4 microM for uric acid and 38 +/- 2 microM for reduced glutathione.

Detection of monobromamine, monochloramine and dichloramine using selected ion flow tube mass spectrometry and their relevance as breath markers.

We report a fast, sensitive, real-time method to measure monobromamine, monochloramine and dichloramine using selected ion flow tube mass spectrometry (SIFT-MS). Relative rate coefficients and product distributions are reported for the reagent ions H3O+ and O2 +. Rapid reactions with the haloamines were observed with H3O+ and O2 + but no fast reaction was found with NO+. A slow reaction between NO+ and dichloramine was observed. We demonstrate the feasibility of determining these compounds in a single human breath for which the limit of detection is approaching 10 parts per billion (ppb). We also report preliminary measurements of these compounds in the breath of individuals where the concentrations of bromamine and chloramine ranged from 10 to 150 ppb.

Titan's ion chemistry: a laboratory perspective.

Some of the most interesting objects in the solar system are those bodies that have significant atmospheres. The discovery that Titan, Saturn's largest satellite, has a substantial nitrogen-based atmosphere makes it a prime extraterrestrial object of interest. The advent of the Cassini orbiter spacecraft program that is able to observe and sample Titan's ionosphere adds greatly to this interest. We report here a summary of some of the efforts that have been made in the laboratory to understand the processes responsible for chemical processing of the primary ions formed in Titan's ionosphere, into the ions observed by in situ sampling. The presence of significant hydrocarbons and the colder temperatures of Titan's ionosphere lead to a much greater complexity in the ion chemistry of Titan than is apparent in the ion chemistry of Earth. A review of all the ion-molecule chemistry investigated in laboratory studies relevant to Titan is included as a table. The complexity of some of the hydrocarbon ion structures formed in just three reactive ion-molecule sequences from the primary ions has required a new experimental methodology which is discussed.