Introduction to GC-MS and SIFT-MS
Gas chromatography mass spectrometry (GC-MS) is a well-recognized technique for analysis of VOCs, offering high specificity as well as reliable quantitation (when calibrated). However, GC-MS requires a highly-skilled operator, and is not well suited to rapid analysis, due to the rather slow chromatographic process.
SIFT-MS complements GC-MS by providing real-time analysis of VOCs from whole air to part-per-trillion (by volume) concentrations. The real-time analysis combined with straightforward instrument operation procedures, make SIFT-MS an ideal option for screening applications where rapid analysis is advantageous, or where highly-trained personnel may not always be available to operate the instrumentation.
|Compounds analyzed||VOCs and certain inorganic gases||VOCs 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 requirements||Typically 10 to 45 minutes (determined by elution time for analytes)|
|Detection limits||Real-time analysis at sub-ppbv concentrations / sub-ng L-1||Routinely sub-nanogram (ng) level, but dependent on system inertness, sample matrix and ionization method|
|Sample preparation||Generally preparation free due to analysis of whole air||Usually 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 analysis||Full Scan Mode and Selected Ion Monitoring (SIM)|
|Compound identification||High – multiple reagent ions, but no chromatography||Very high due to chromatography|
|Quantitation||Real-time quantitation from reagent and product ion intensities, reaction rate coefficients and sample flow rate||From full calibration of system for particular analytical method|
|Calibration||Calibration is required infrequently; for some applications it is not required at all||Calibrated regularly using a set of dilutions of known concentrations|
|Validation||Routine validation using automated on-line analysis of certified gas standard||Validation involves use of spiked samples and blanks in the analytical sequence|
|Ease-of-Use||Technical and non-technical operation||Technical operators only|
|Maintenance requirements||Low – primarily vacuum pumps||High – frequent fouling of column and ion source|
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SIFT-MS 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.
This section describes the principles of the SIFT-MS technique that are essential to understanding how it complements GC-MS. 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 SIFT-MS to be applied as a real-time technique.
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.
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.
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 GC-MS. These factors make SIFT-MS 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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