An Easy, Sensitive and Automated Method to Measure VOCs in Drinking Water with ITEX

Editor’s note: Contributing to this blog post was Dr. Klaus Schrickel, Senior Application Scientist GC and GC-MS, Thermo Fisher Scientific, Dreieich, Germany.

Introduction

The detection limits for VOCs (volatile organic compounds) are changing continually. The latest change to the German drinking water regulation took place on the 20 June, 2023.

When we talk about VOCs, we are referring to anything in a boiling range from 50°C to 250°C. Typical compounds are halogenated substances and aromatic substances.

In DIN EN ISO methods, different methodologies on detecting VOCs are discussed.

The new norm is the DIN EN ISO 20595:2023-08. It states GC-MS and static headspace technology.

The following table states some of the required detection limits.

The next graphic shows a comparison of different headspace technologies.

This blog post will show what you can accomplish using ITEX (In-Tube-Extraction) in combination with a PTV and refocusing analytes on a packed glass liner.

Methodology

For the blog post, the following analytical components were used:

ITEX workflow:

1. Sample is applied to a vial that is crimped and shaken
2. The ITEX tool penetrates the septum, and the syringe above the trap brings the gas phase through the trap several times
3. Using a constant flow through the trap made from inert glass at low temperature, water can be removed; this feature was not used for the current application
4. Desorption into the injector is done at 280°C to desorb the analytes
5. The trap is then cleaned with gas flow
6. Due to active cooling, the ITEX tool is quickly ready for use again

The standard adsorbent for ITEX is Tenax TA 80/100 mesh, however, there are different adsorption materials available.

In Figure 3, static headspace (1 ml) is compared to ITEX (35 X 1 ml) technology. The graphic shows clearly that you can achieve a concentration factor of 20, using ITEX technology. ITEX is a fully automated tool on the RSH platform.

Beside the concentration factor, why is ITEX now of special interest? Figure 4 shows an injection using a loop headspace sampler. The later-eluting substance is focused well, however, earlier-eluting substances like Vinylchloride — which is very volatile — are not focused well. This results in bad peak shape, which is undesirable.

Combining the ITEX technology with a 2 mm deactivated packed glass liner in the PTV injector allows the refocus of even very volatile analytes. Those packed liners are commercially available.

It is important to note that, compared to other technologies, the Thermo Scientific PTV uses a fan to cool down the injector body very quickly using ambient air slightly above room temperature (cryo options down to -100°C are possible). No liquid cooling agents are necessary.

This has several advantages. Cooling agents increase costs, water can freeze out on the Tenax, and the Tenax can break down, which leads to non-reproducible benzene peaks in the chromatogram. Technologies such as Purge & Trap use cooling liquids or Peltier elements, which cool the trap to low minus temperatures. Using the special geometry of the 2mm liner in the Thermo Scientific PTV and the fan in the PTV to cool the injector, allows you to refocus without risking damage to the Tenax. Figure 5 showcases the effect of the refocusing on the glass liner.

Special options using the Thermo Scientific ISQ 7610

For this blog post a single quadrupole Thermo Scientific ISQ 7610 GC-MS was used, as well as a special column, the TG-624SilMS, 60m x 0,25mm x 1.4µm, which allows temperature programs up to 320°C. This makes the system flexible, in the event other applications need to be performed on the same system. Normally, headspace columns are the limiting factor, due to their maximum temperature.

Benefits of the ISQ 7610:

1. The ExtractaBrite Ion Source includes:
   1. 3 Lenses         
   1. RF lens
   1. Repeller
   1. Ion Volume
2. The Advanced EI Source (AEI) 
   1. Higher ion yield = better sensitivity
   1. Bundled ion beam = it is more robust against contamination
3. Timed Acquisition
   1. SIM with individual, overlapping windows
   1. Fast alignment between component table and instrument method via SIM bridge
   1. Easy method creation
   1. Easy addition of components
   1. Fast correction for RT shifts
4. Never Vent technology
   1. Vacuum Probe Interlock
   1. Easy changing of column and ion source without breaking the vacuum
   1. Easy changing from EI to CI or NCI

Results and conclusion

All regulatory limits could be reached with good S/N levels and respective calibration curves from 0.005 up to 10 ppb using 10 points. Blind amounts were measured and, if wanted, can be subtracted in the software. The precision (n=12) was measured with and without internal standards. The average rsd without internal standard was at 4.4% and with internal standard at 2.7 %, for 21 compounds listed (Table 2).

This blog post shows that ITEX is a very easy-to-use and cost-efficient tool to achieve results that are comparable to Purge & Trap results, but without the burden of low temperatures. This is accomplished using double focusing on the ITEX device and the packed inert glass liner. Using the Advanced-EI source, even lower detection limits can be reached. The complete workflow already works in several labs across Europe with sufficient detection limits as specified by the German drinking water directive.

To learn more about ITEX and see dedicated results, join us for our free webinar in German.

Webinar in German: 25 October, 2023, 11 a.m. CET

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