by Mark J. Perkins, Vaughan S. Langford and Murray J. McEwan AL
High-Throughput VOC and Inorganic Gas Analysis: Automated SIFT-MS
Introduction Selected ion flow tube mass spectrometry (SIFT-MS) enables direct, real-
time monitoring of volatile organic compounds (VOCs) and inorganic gases. Unlike traditional chromatography-based techniques, there is no requirement to separate compounds prior to analysis. Not only can chal- lenging compounds, such as formaldehyde, ammonia and small sulfur compounds, be analyzed without derivatization or preconcentration, but these analyses can be completed in seconds. Automating these analyses realizes the full potential of the fast SIFT-MS-based measurements for diverse applications.
Automation of traditional chromatographic techniques allows instru- mentation to be productive 24/7, which is critical in many labs, since sample throughput is typically only 20–60 samples in a 24-hour period. Because the breadth of analysis is constrained by chemical reactivity, mul- tiple columns or several techniques may be needed to comprehensively analyze just one sample. In addition, very dilute samples may require a preconcentration step.
SIFT-MS is a direct mass spectrometry technique1 that delivers real-time,
comprehensive gas and headspace analysis to ultratrace levels with a wide linear range.2
It couples ultrasoft, precisely controlled chemical
ionization with mass spectrometric detection to rapidly quantify VOCs and permanent gases to low-ppt concentrations by volume (pptv) (see Figure 1). Eight chemical ionization agents (reagent ions) are applied in SIFT-MS instruments: H3
O+ , NO+ , O2 , O2 +, O– , O2 –, OH– , NO2 – and NO3 –. These
react with VOCs and inorganic gases in well-controlled ion–molecule reactions, but do not react—or react only very slowly—with the major components of air (N2
trace and ultratrace levels without preconcentration. and Ar). This enables SIFT-MS to analyze air at
Rapid switching of the eight reagent ions provides higher selectivity than other direct mass spectrometry techniques. A comparative study found good correlation between SIFT-MS and GC/MS.3
Automated SIFT-MS Autosampler integration is the simplest and most cost-effective way
to leverage high sample throughput from the rapid, direct gas analysis provided by SIFT-MS. An autosampler improves repeatability and repro- ducibility compared to manual analysis and eliminates operator error.
Because SIFT-MS is a direct mass spectrometry technique (that is, it has no preseparation using chromatography), its requirements for an auto- sampler differ from those commonly used with GC. GC-based techniques require rapid injection to achieve good chromatographic separation; chromatography leads to prolonged analysis as individual compounds elute from the column over time. However, SIFT-MS requires steady sample injection for the duration of the analysis, because analysis is car- ried out simultaneously with injection (Figure 2). The GERSTEL (Linthicum, Md.) Multipurpose Sampler (MPS) has proven to be the best-suited off- the-shelf autosampler system for SIFT-MS. Autosampler integration opens diverse applications of the technique for contract and R&D laboratories.
High-throughput static headspace analysis (SHA) SIFT-MS offers very rapid, highly sensitive quantitation of VOCs and inor-
ganic gases from headspace samples. Because chromatographic separation has been eliminated, SIFT-MS provides simple analysis of very volatile sol- vents as well as chromatographically challenging compounds, including ammonia, formaldehyde and hydrogen sulfide. Figure 3 shows an example of rapid screening of three different polymers for residual monomers.
Figure 1 – Schematic representation of the SIFT-MS as applied in the Voice200ultra (Syft Technologies, Christchurch, New Zealand).
AMERICAN LABORATORY 17
Figure 2 – Sample-injection and analysis requirements of GC-based tech- niques and SIFT-MS.
JANUARY/FEBRUARY 2017
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