Developed for GC-MS users, this article describes how selected ion flow tube mass spectrometry (SIFT-MS) differs from gas chromatography mass spectrometry (GC-MS) both in its technical features and its operational capabilities. Both techniques analyze volatile organic compounds (VOCs) but have different fundamental strengths, which means the techniques complement rather than replace each other.

SIFT-MS is a form of direct mass spectrometry that analyses whole air in real time with typical detection limits at parts-per-trillion level (by volume). Real-time analysis is achieved by using precisely controlled soft chemical ionization that does not require chromatography. These factors also allow long-term calibration stability, reduced maintenance, and easy configuration of our instruments. Consequently, SIFT-MS is an ideal tool for rapid screening and real-time monitoring applications, especially where users may have non-technical backgrounds.

Alternatively, GC-MS provides very good specificity, but relies on electron impact ionization and chromatographic separation. Therefore it is relatively slow, has high maintenance requirements, and must be operated by skilled personnel, when compared to SIFT-MS. As a result, gas chromatography mass spectrometry complements the fast screening capabilities of selected ion flow tube mass spectrometry with slow but very selective analysis.

The following table summarizes key characteristics common to SIFT-MS and GC-MS.


CharacteristicSIFT-MS GC-MS
Compounds analyzed:VOCs and certain inorganic gasesVOCs and semivolatile VOCs (SVOCs), inorganic gases
Suitable matrices:Gas (including headspace)Gas (including headspace), liquid
Speed of analysis:A fraction of a second to minutes, depending on requirementsTypically 10 to 45 minutes (determined by elution time for analytes)
Detection limits:Real-time analysis at sub-ppbv concentrations / sub-ng L-1Routinely sub-nanogram (ng) level, but dependent on system inertness, sample matrix and ionization method
Sample preparation:Generally preparation free due to analysis of whole airUsually requires preparation and/or preconcentration (for example, solvent extraction, purge and trap, thermal desorption, SPME)
Analyte separation:Real-time analysis because there is no chromatographic separation of analyte, which also eliminates discrimination. Compounds resolved due to the relatively simple chemical ionization ‘fingerprints’Separation of analytes using appropriate column and temperature program. May require several runs through the GC to separate analytes of differing polarity (discriminatory)
Ionization mechanism(s):Three standard soft chemical ionization agents (H3O+, NO+, O2+)Typically 70 eV electron impact ionization; sometimes chemical ionization
Ion selection and detection:Quadrupole mass selection and particle multiplier detection (ion counting)Typically use quadrupole-based mass selection and measure ion current
Data collection modes:Full Scan Mode and Selected Ion Monitoring (SIM) – the latter allows real-time quantitative analysisFull Scan Mode and Selected Ion Monitoring (SIM)
Compound identification:High – multiple reagent ions, but no chromatographyVery high due to chromatography
Quantitation:Real-time quantitation from reagent and product ion intensities, reaction rate coefficients and sample flow rateFrom full calibration of system for particular analytical method
Calibration:Calibration is required infrequently; for some applications it is not required at allCalibrated regularly using a set of dilutions of known concentrations
Validation:Routine validation using automated on-line analysis of certified gas standardValidation involves use of spiked samples and blanks in the analytical sequence
Ease-of-Use:Technical and non-technical operationTechnical operators only
Maintenance requirements:Low – primarily vacuum pumpsHigh – frequent fouling of column and ion source

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An overview of SIFT-MS


Selected ion flow tube mass spectrometry uses soft chemical ionization reactions coupled with mass spectrometric detection to rapidly quantify VOCs in real time from whole-gas samples. Three standard chemical ionization agents (or reagent ions) are used in SIFT-MS: H3O+, NO+ and O2+. These reagent ions are mass selected (Figure 1) and react with trace VOCs in very well controlled ion-molecule reactions but do not react with the major components of air, allowing SIFT-MS to analyze whole air for trace VOCs to pptv levels.

Soft chemical ionization yields a smaller number of product ions per compound than electron impact mass spectrometry (as used in standard GC-MS, for example), so gas chromatographic separation is unnecessary. This speeds sample throughput and provides instantaneous quantification of VOCs. Use of multiple reagent ions also greatly reduces interferences, markedly increasing the specificity of SIFT-MS compared with most other direct mass spectrometry technologies.

Selected Ion Flow Tube Mass Spectrometry SIFT-MS Schamatic

Figure 1. Schematic representation of the SIFT-MS technique

How SIFT-MS complements GC-MS


This section describes the principles of the SIFT-MS technique that are essential to understanding how it complements gas chromatography mass spectrometry. In particular, it focuses on how soft chemical ionization is applied very precisely in SIFT-MS, allowing it to provide unparalleled selectivity among direct mass spectrometry techniques, and creating an ideal companion technique for GC-MS.

a. Chemical ionization in SIFT-MS

Chemical ionization (CI) uses a molecular ion to transfer charge on to the target compound (analyte). CI is “softer” than many other types of ionization, so it transfers less energy to the analyte, resulting in less fragmentation. SIFT-MS is a unique CI-MS technique because it precisely controls ion energies to allow repeatable, real-time quantitative analysis. Another benefit is long-term calibration stability.

SIFT-MS uses softer chemical ionization (CI) agents than GC-MS and terms them “reagent ions” (or “precursor ions”). The standard reagent ions used in SIFT-MS are H3O+, NO+, and O2+. By applying these ions in a soft ionization process, SIFT-MS encounters significantly reduced fragmentation compared to harsher CI and electron impact (EI) ionization.

Figure 2 compares ionization of ethylbenzene using 70-eV EI (as used in GC-MS) and 12.1-eV O2+ CI (as used in SIFT-MS). Reduced fragmentation means chromatography is unnecessary, which allows our technolog to be applied as a real-time technique.

GC-MS and SIFT-MS ionization comparison

Figure 2. Electron impact and chemical ionization of ethylbenzene illustrate the much simpler fragmentation observed for SIFT-MS than standard gas chromatography mass spectrometry.

 

Fragmentation and chromatography mean GC-MS can have higher selectivity than the somewhat cleaner mass spectra produced by SIFT-MS. Therefore, in certain applications involving complex mixtures, SIFT-MS is ideal as a rapid screening tool, while GC-MS is ideal for methodical identification and quantitation of every compound. The strength of SIFT-MS is its fast, broad analysis and hence it is complementary to rather than competitive with GC-MS.

b. Real-time resolution of isomeric and isobaric compounds

The triple reagent ion system of SIFT-MS (H3O+, NO+, and O2+) is able to resolve certain isobaric and isomeric compounds. A simple example is provided in Table 1 for the acetone and propanal isomers of C3H6O. The NO+ reagent ion provides the most effective differentiation because it reacts via a different mechanism for the two compounds and yields a single product ion for each.

Table 1.Product ions formed from reaction of the SIFT-MS H3O+, NO+ and O2+ reagent ions with isomeric compounds acetone and propanal.
Reagent ion Acetone product ion (m/z) Propanal product ion (m/z)
H3O+(CH3)2CO.H+ (59)CH3CH2CHO.H+ (59)
NO+(CH3)2CO.NO+ (88)CH3CH2CO+ (57)
O2+(CH3)CO+ (58); CH3CO+ (43)CH3CH2CHO+ (58); CH3CH2CO+ (57)

 

Figures 3 and 4 illustrate how a hypothetical multi-component sample is analyzed using electron impact mass spectrometry (using, GC, GC-MS, and SIFT-MS), respectively. In Figure 3, the high degree of fragmentation arising from EI ionization is shown. Without GC, EI-MS is complicated and allows few compounds to be targeted uniquely. However, the same mode of ionization applied in GC-MS allows compounds to be separated in time through the GC column, while the relatively unique mass spectral “fingerprints” of each compound can be used to identify and quantify the compound.

hypothetical multi-component sample is analyzed using electron impact mass spectrometry (using, GC, GC-MS and SIFT-MS)

Figure 3. 70-eV electron impact mass spectrometry (a) without and (b) with gas chromatography. The hypothetical 15-component sample is derived from the US EPA Compendium Method TO-15 and generated from mass spectra in the NIST library (https://webbook.nist.gov/chemistry/).

 

In Figure 4, the same mixture is analyzed using the three standard SIFT-MS reagent ions. All fifteen compounds can be resolved in real-time without using chromatography

In Figure 4, the same mixture is analyzed using the three standard SIFT-MS reagent ions. All fifteen compounds can be resolved in real-time without using chromatography.

Figure 4. Three standard SIFT-MS reagent ions (a) H3O+, (b) NO+ and (c) O2+ for real-time resolution of the 15-component sample shown in Figure 3. Data were taken from the Syft compound library. Red numbers identify some unique ions useful for quantitation.

 

Summary


SIFT-MS offers high sensitivity, real-time and non-discriminatory analysis of VOCs in whole air in a very easy to use package. The absence of chromatography columns and very clean, precise chemical ionization reduce maintenance requirements and increase stability, compared to gas chromatography mass spectrometry. These factors make selected ion flow tube mass spectrometry a very powerful, complementary technique for GC-MS, which is a good compound identification tool, but cannot analyze rapidly and requires highly trained operators.

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