11 Extending the solvent range
Following the initial evaluation, three solvents were selected to assess whether higher proportions of solvent were usable. Based on there physiochemical properties, DMSO, DMAC and DMI were assessed up to 100% solvent as matrix. Figure 4 shows the responses to fi ve analytes added at generic level 250 (see Table 2), although all analytes were evaluated [5]. It is quite clear the partitioning of non-polar compounds is particularly diminished as the solvent proportion increases. Additionally, for DMI, partitioning of methanol is signifi cantly enhanced.
Figure 2: Example headspace injection of ‘level’ 250 from water. The shaded section shows the region of the injection over which concentration measurements were averaged.
Initial evaluation to 10% solvent in water
The fi rst step in evaluating the compatibility of non-aqueous solvents with SIFT-MS is to determine the dilution level in water that can be tolerated without signifi cant affects on signal responses. For this, level 250 standards were prepared in water- solvent mixtures up to 10% solvent concentration. Figure 3 shows the affects on the response to toluene and butanol only (for clarity), although all analytes were evaluated. The response is shown relative to signal response in 100% water. Other than for methanol, the more non-polar analytes are more adversely affected as the solvent concentration increases. Also, there is a marked decrease in response above 6% triacetin and this is due to it being immiscible in water above 6.1% and selectively removing non-polar compounds from the aqueous fraction. The increasing response for butanol with DMF is due to an interference product ion from the DMF artifi cially infl ating the apparent butanol signal.
Summary/Conclusion
The data presented demonstrates that by selecting suitable solvents, it is possible to expand the range of matrix solvents beyond just water, and for 5% solvent in water, the sensitivity of the analysis remains essentially the same as for wholly aqueous systems. There are limitations, including a reduction in sensitivity for non-polar analytes as the proportion of solvent increases, and the presence of interfering ions from the solvent itself which can limit the range of analytes for particular solvents choices. Table 4 summarises the solvent limits, and limitations that apply to each solvent evaluated. However, with suitable method development it is now possible to extend automated SIFT-MS headspace analysis to non-aqueous solvent systems.
Table 4: Summary of solvent limits and analytical limitations for the solvents evaluated in this study.
DMAC DMF
DMSO DMI
MeOH Triacetin
Solvent Limit Comments ≤50% ≤10% ≤25%
Impacts analysis of acetone.
Impacts analysis of butanol, butylamines etc. Impacts analysis of benzene and isooctane.
≤100% Watch for impurities in solvent, including adsorption from air.
≤10% ≤6%
Impacts analysis of isooctane. Analysis limited to NO+
Limited by miscibility of water.
References 1. MJ Perkins, VS Langford. Rev Sep Sci. 3, e21003.
https://doi.org/10.17145/rss.21.003
Figure 3: Toluene and butanol signal responses to increasing solvent levels in water, from 0 to 10%.
Table 3 shows the data generated for repeatability and linearity for all analytes at 5% solvent concentration.
Table 3: Summary of linearity and repeatability for all 14 analytes at 5% solvent concentration. (* excludes butanol due to interferences, ** excludes benzene due to interferences, *** excludes TCE due to low sensitivity with NO+
reagent ion). Matrix Water
5% DMF* 5% DMI
5% DMSO** 5% DMAC
5% MeOH*** Triacetin
R2
0.998 - 0.999 0.994 - 0.999 0.996 - 0.999 0.998 - 0.999 0.990 – 0.998 0.999 – 1.000 0.996 - 0.999
Repeatability “Level” 50 1.2 - 2.6% 1.5 - 6.5% 1.3 - 8.0% 0.8 - 7.0% 2.1 – 7.6% 1.2 – 5.6% 1.3 - 9.6%
“Level” 250 2.1 - 6.9% 0.7 - 7.0% 0.8% - 4.8% 1.1 - 4.4% 1.3 – 6.3% 1.6 – 2.8% 0.9 - 5.1%
“Level” 500 0.9 - 2.7% 0.6 - 4.6% 1.4 - 4.8% 0.7 - 3.8% 1.2 – 4.5% 0.7 – 4.1% 0.6 - 5.4%
2. D Smith, P Španěl, N Demarais, VS Langford, MJ McEwan. Mass Spec. Rev. e21835.
https://doi.org/10.1002/mas.21835.
3. D Hera, VS Langford, MJ McEwan, TI McKellar, DB Milligan. Environments 4, 16.
https://doi.org/10.3390/environments4010016.
4. VS Langford. Chemosensors 11, 111.
https://doi.org/10.3390/chemosensors11020111.
5. MJ Perkins, VS Langford, LP Silva. Analytica 4, 313-335.
https://doi.org/10.3390/analytica4030024
reagent ion products.
Figure 4: Response of a range of analytes as solvent proportion in the matrix is increased to 100%.
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