Mass Spectrometry & Spectroscopy


Extension of residual solvents analysis by SIFT-MS to non-aqueous solutions Mark Perkins, Element Materials Technology


Automated selected ion fl ow tube mass spectrometry (SIFT-MS), by avoiding the need for chromatographic separation of analytes, signifi cantly increases sample throughput for headspace sample analysis. However, the technique is susceptible to saturation by non-aqueous solvents matrices, leading to erroneous results [1], and water has traditionally been used as the universal solvent for headspace analysis. By selecting solvents with suitable physiochemical properties this limitation can be avoided and this article describes the evaluation of six solvents via linearity and repeatability measurements on 14 commonly used residual solvents.


The SIFT-MS technique NO+


and ultra-trace levels without pre-concentration. Rapid switching between reagent ions provides high selectivity because the multiple reaction mechanisms give independent measurements of each analyte [4]. The multiple reagent ions frequently remove uncertainty from isobaric overlaps in mixtures containing multiple analytes.


O2


[3]. These reagent ions react with VOCs and other trace analytes in well-controlled ion-molecule reactions, but they do not react with the major components of air (N2


, O2 , O2 - and NO3


SIFT-MS (Figure 1) uses soft chemical ionisation (CI) to generate mass-selected reagent ions [2] that can rapidly quantify volatile organic compounds (VOCs) down to part-per-trillion concentrations (by volume, pptV). Up to eight reagent ions (H3 -, NO2


O+ +, O- , OH- -) are obtained from a microwave discharge in air , and Ar). This enables direct, real-time analysis of samples to be achieved at trace ,


Dimethylacetamide (DMAC)


Dimethylformamide (DMF)


Dimethyl sulfoxide (DMSO)


1,3-Dimethyl-2- imidazoldinone (DMI)


Methanol (MeOH) Triacetin


265 153 189 225 65 259


Table 1: Solvents evaluated in this study. Solvent


Boiling Point / °C


Log Kow


-0.77 -1.10 -1.98 -0.3 -0.77


Product ions from solvent H3


O+ 88 29, 74 79 115 33 NO+ 87, 117


72, 73, 103


78, 108 114 62


O2


+ 43, 87 43, 73 79 113, 114 31, 32


0.25 45, 59, 159 159, 248 103, 115, 116, 145


Table 2 shows the 14 analytes used to assess the non-aqueous solvents mixtures and these cover a range of chemical classes and polarities. As there was signifi cant variability in headspace partitioning, fi ve different concentration ranges were used. For the sake of clarity ‘level’ is used throughout and Table 2 correlates ‘level’ with actual solution concentration for each group of solvents and Figure 2 shows a typical headspace injection.


Figure 1: Schematic diagram of the SIFT-MS technique, which utilises soft chemical-ionisation for direct analysis of samples.


Syft Technologies’ TracerTM SIFT-MS instrument was coupled with a multipurpose autosampler (MPS Robotic Pro, Gerstel) controlled using Gerstel’s Maestro software. Analysis of samples was carried by transferring 2.5 mL of headspace from an incubated vial, via a heated syringe, followed by injection into the SIFT- MS instrument at 50 µL s-1


. High-purity nitrogen was used as a make-up gas as a


balance for the required instrument sample flow. It should be noted that, unlike conventional GC-based headspace analysis, sample is transferred to the instrument over 50 seconds as slow introduction of sample is required to obtain suitable signal responses. A typical injection plot is shown in Figure 2.


Experimental


. This ensures that, at suitable concentrations in water, the partitioning into the headspace is minimal, thus ensuring that the analyser response is not saturated. Despite its low boiling point, methanol can be used due to its very slow reactions with the NO+


Table 1 summarises the six solvents used in this study. A characteristic of all solvents, with the exception of methanol, is their high boiling point and low Log Kow


reagent ion, however, this does limit methods to those that only use NO+ the reagent ion. as


20 50


100 200 250 300 400 500


Table 2: Generic ‘level’ used and the relationship to actual solution concentration. Solution Concentration / ppm


‘Level’


Benzene Toluene


Trichloroethylene (TCE)


0.01


0.025 0.05 0.1


0.125 0.15 0.2


0.25


Chloroform Isooctane Propanal


Tetrahydrofuran (THF)


Butanone (MEK)


Acetone Acetonitrile


2-Propanol (IPA) Nitromethane


0.04 0.1 0.2 0.4 0.5 0.6 0.8 1


0.4 1 2 4 5 6 8


10 1


2.5 5


10


12.5 15 20 25


2 5


10 20 25 30 40 50


1-Butanol Methanol


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