High-throughput analysis of hydrophilic VOCs in water is simplified through the application of direct mass spectrometric analysis and automated sample delivery. This article demonstrates the linearity and repeatability achievable for low molecular weight alcohols using automated SIFT-MS.


Headspace analysis of hydrophilic species in water is more challenging than that of hydrophobic species, such as aromatic hydrocarbons, due to their higher affinity for the aqueous phase. Direct analysis using SIFT-MS typically enhances throughput more than three-fold compared to purge and trap-gas chromatography (GC) methods. This application note describes a feasibility study that applied direct headspace analysis of aqueous ethanol and pentanol using SIFT-MS.

Aqueous solutions of ethanol and pentanol were analyzed using a Syft Technologies Voice200ultra SIFT-MS instrument integrated with a GERSTEL Multipurpose Sampler (MPS) (GERSTEL, Mülheim an der Ruhr, Germany) equipped with a GERSTEL agitator/incubator and headspace vial racks. The solutions were incubated for 15 minutes at both 40 °C and 60 °C. Headspace was sampled using a 2.5-mL headspace syringe (heated to 150 °C; fill speed of 200 μL s-1; viscosity delay of 5 s) and injected into the SIFT-MS instrument’s inlet at a flow rate of 50 μL s-1 (giving a total flow rate of ca. 435 μL s-1 with the nitrogen make-up gas flow).

Figure 1 shows the results obtained for ethanol analysis from standard solutions incubated at 60 °C, illustrating very stable injection by the autosampler and rapid measurement by the SIFT-MS instrument.

Figure 1 shows the results obtained for ethanol analysis from standard solutions incubated at 60 °C, illustrating very stable injection by the autosampler and rapid measurement by the SIFT-MS instrument

Figure 1. SIFT-MS selected ion mode analysis of ethanol as headspace from the standard solutions is slowly injected into the instrument following incubation at 60 °C.

 

Figure 2 shows the results from the analysis of aqueous solutions of ethanol and pentanol, incubated at both 40 °C and 60 °C and demonstrates very good linearity across the range. The measured headspace concentration for pentanol is approximately 40% higher than for the equivalent ethanol solutions due to the ethanol preferentially remaining in the aqueous phase. Whilst this preliminary investigation used a linear range from 20 to 100 nL mL-1, the upper and lower limits of this analysis could be extended by at least two orders of magnitude. The error bars correspond to 3 standard deviations of the repeated measurements made during a single 50-second sample injection (Figure 1).

Figure 2 shows the results from the analysis of aqueous solutions of ethanol and pentanol, incubated at both 40 °C and 60 °C and demonstrates very good linearity across the range.

Figure 2. Linearity of water headspace analyses for ethanol and pentanol using SIFT-MS at incubation temperatures of 40 °C and 60 °C.

This study demonstrates that SIFT-MS can rapidly analyze trace hydrophilic compounds in water, such as low molecular weight alcohols. Direct analysis using automated SIFT-MS is sensitive, linear, and repeatable. SIFT-MS provides substantial throughput increases over traditional purge and trap-GC methods, since no preconcentration or drying is required. Further, static headspace-SIFT-MS may offer a simplified approach to the determination of Henry’s Law coefficients.

Written by  Dr. Mark J. Perkins and Dr. Vaughan S. Langford

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