... Dynamic Headspace may be the solution.
A technique that meets the requirements of BOTH low detection limits AND good linearity.
Static Headspace (SHS) is an amazing technique for several reasons. As the two probably most significant can be mentioned: minimal sample processing and that it is gentle on the GC system, as heavier, non-volatile compounds are kept out of the system.
BUT. Then there is the sensitivity, which is not always sufficient, – especially when it comes to very polar components in aqueous matrix.
And what about SPME?
In combination with static headspace, Solid Phase Micro Extraction (SPME) can be used, which can increase the sensitivity a lot.
When SPME does not turn on either, one can investigate the option “Dynamic Headspace (DHS)” in combination with thermal desorption (TDU) and a cooled injection inlet (CIS). All fully automated with Gerstel solutions implemented on existing Agilent GC-MS or comparable system.
At SHS, the equilibrium between the liquid / solid phase and the gas phase is set. A very small amount of the gas phase (typically 1 ml) is injected into the GC system. For components that are far displaced towards, for example, an aqueous phase, the concentration in the gas phase will be quite small.
The principles behind DHS
Dynamic Headspace does not operate with a stable equilibrium ratio. The principle, on the other hand, is to push the equilibrium continuously during sampling.
By purging the sample with a continuous stream of inert gas, all components that have a vapor pressure on a subsequent trap can be collected. This trap consists of a thermal desorption tube with an active material (tenax, carbotrap, etc.) that binds the molecules.
In principle, it is possible to completely empty the sample of volatile compounds and thereby obtain a significant increase in sensitivity.
MSCi right now
At MSCi, we are currently challenging the DHS technique in practice, with a special focus on some very polar components in aqueous matrix, and we will continuously report on exciting possibilities.
To begin with, here is DHS “in action”
Overview
Schematic overview of the different steps in the DHS method
DHS "in action"
Watch our VIDEO with GERSTEL’s automated DHS setup.
Here combined with an Agilent GCMS system.
Some earlier data
Calibrating curve - sulfor
- Dimethyl disulfid
- 1 ml water sample.
- Conc: 0,05 to 0,50 µg/l
- GCMS – SIM mode
HS versus DHS
Comparison of standard mix run at HS and DHS, respectively. It should be noted that various instruments and columns have been used.
Top HS – SIM analysis
Middle DHS – Scan TIC
Bottom DHS – Scan EIC
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