Mass Spectrometry & Spectroscopy
Volatile organic compounds analysis in drinking water with Headspace GC/MSD using hydrogen carrier gas and hydroInert source
Bruce Quimby PhD, Anastasia A. Andrianova PhD, Agilent Technologies
Recent concerns with the price and availability of helium have led laboratories to look for alternative carrier gases for their gas chromatography mass spectrometry (GC/MS) methods. For GC/MS, hydrogen is the best alternative to helium, and offers potential advantages in terms of chromatographic speed and resolution. However, hydrogen is not an inert gas, and may cause chemical reactions in the mass spectrometer electron ionisation (EI) source. This can lead to disturbed ion ratios in the mass spectrum, spectral infi delity, peak tailing, and nonlinear calibration for some analytes. Therefore, a new EI source for GC/MS and GC/MS/MS was developed and optimised for use with hydrogen carrier gas. The new source, named HydroInert, was used in the system evaluated here to test volatile organic compounds (VOCs) in drinking water. In addition to the new source, the chromatographic conditions were optimised to provide separation of 80 volatile compounds in 7 minutes. Standards and samples were analysed in both scan and SIM data acquisition modes. For the scan data, spectra were deconvoluted with MassHunter Unknowns Analysis software and searched against NIST 20 to assess the spectral fi delity. In both modes, quantitative calibration was performed for the 80 compounds over the range of 0.05 to 25 µg/L. As demonstrated in this note, the system gives excellent results for the analysis of VOCs in drinking water.
Figure 1. Instrument configuration. Introduction
One of the analyses commonly used to ensure that the quality of drinking water is the measurement of volatile organic compounds (VOCs). These compounds can appear in drinking water by contamination from numerous sources, including industrial and commercial operations. Another common source is when VOCs are formed by the addition of chlorine (used to disinfect the water) and react with natural organic matter in the source water.
Regulations governing the allowable concentration of VOCs in drinking water vary by country and region but are typically in the low µg/L (ppb) range. Due to the large number of potential contaminants, and the need to measure them at such low levels, GC/MS systems are commonly used. GC/MS offers both the sensitivity and selectivity required to identify and quantify VOCs. Purge and trap [1] and static headspace [2, 3] are two commonly used automated sampling techniques that extract the VOC analytes from water samples and inject them into the GC/ MS. This method uses a system confi gured to perform static headspace/GC/MS analysis of VOCs in drinking water, optimised for using hydrogen as the carrier gas.
Both scan and SIM modes of data acquisition were evaluated. Scan is useful for confi rming the identity of found targets, and for identifying nontarget compounds. It can also be used retrospectively to search for compounds that may become of interest in the future. SIM has a substantial advantage in the signal-to-noise ratio and is preferred where quantitation to low levels is required.
Experimental
The Agilent 5977C Inert Plus MSD was coupled to the Agilent 8890 GC equipped with a multimode inlet (MMI) and an Agilent 8697 headspace sampler. A HydroInert source (G7078-60930 for the fully assembled source with 9 mm lens) was used in the MSD, and autotuned using the etune tuning algorithm. The analytical method used an Agilent Ultra Inert straight-through 1.0 mm GC inlet liner and a DB 624 UI column, 20 m × 0.18 mm, 1 µm. The Headspace Sampler was connected to the GC carrier gas inlet line between the GC control pneumatics and the GC injection port. A pulsed split injection was used with the split ratio set to 21:1.
Eight calibration levels ranging from 0.05 to 25 µg/L were prepared in water by spiking 5 µL of a corresponding stock solution (which also included the ISTD) into 10.0 mL of water in a 20 mL headspace vial. Five grams of anhydrous sodium sulphate were weighed into each vial before the addition of water and spiking solution. After capping, each vial was vortexed vigorously for 20 seconds, before placement in the headspace sampler. The spiking stock solutions were prepared in methanol using an Agilent 73-compound standard (DWM-525-1), an Agilent six-compound gas standard (DWM-544- 1), and an Agilent three-compound ISTD mix (STM- 320N-1), containing fl uorobenzene (internal standard), 1,2-dichlorobenzene-d4 (surrogate), and BFB (surrogate). The ISTD/surrogate mix was added to each calibration stock solution at a level to give 5 µg/mL of each compound in the water. Agilent MassHunter Workstation software was used for data acquisition and processing.
Heated transfer line
9 mm Extractor Lens
MMI Inlet
(Hydrogen) 5 6 20 m DB-624 UI
HydroInert Source
5977C MSD 8890 GC 8697 HS 1 4 3 2
Figure 1. System confi guration.
Figure 1 shows the system confi guration used here. The operating parameters are listed in Table 1.
Table 1. Gas chromatograph, mass spectrometer, and headspace sampler parameters for VOCs analysis.
INTERNATIONAL LABMATE - NOVEMBER 2022
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