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 technology to be applied as a real-time technique.
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.
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
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.