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Fast thermal desorber coupled to a PTR-TOF-MS

Kore has recently designed, built and delivered a compact system for sample desorption, concentration and GC separation (PREFICS) prior to analysis by a mass spectrometer. As a spin-off from this work we have adapted our design of a rapid thermal desorber so that it can couple directly to the inlet of a PTR-TOF-MS. The first one has been tested successfully and has now been delivered to the University of Birmingham's Molecular Physics group under the direction of Dr. Chris Mayhew.

A conventional swab is inserted into the unit and then the jaws of the swab desorber are closed. The mechanism makes a high quality seal in such a way that pressurised gas can be forced through the swab with no leak of carrier gas or desorbed analyte. Swabs may even be reused multiple times.

Photo of swab desorber

In its simplest embodiment, air is drawn through the swab at atmospheric pressure at a rate determined by the flow into the PTR reactor (a few cm3/sec). The gas, in this case air, is pre-heated by the unit so that when it passes through the swab material the air flowing through is at the same temperature as the jaws, i.e. the desired desorption temperature (50°C to 200°C). At the exit of the swab desorber a short heated line of approximately 50mm in length allows the swab desorber to be close- coupled to the inlet of the PTR instrument. The internal components are all treated with a Silcosteel 2000 coating to guarantee that analytes are not lost to internal stainless steel surfaces.

Initial tests at Kore have been very encouraging. The Kore PTR-TOF-MS can operate in a pseudo "GC-MS" mode, in which the evolution of any species can be monitored as a function of time. Any mass species can be investigated by retrospective analysis of the data set, and concentration variations with time down to 100ms (the clear out time of the PTR reactor) can be plotted.


The photograph above shows the swab desorber close-coupled to a Kore PTR- TOF-MS

Results

Solutions of TNT in acetonitrile / methanol were deposited on a suitable swab. The solvent was allowed to evaporate for 30 seconds, the data acquisition started, and then the swab was inserted into the unit. The first data plot shows two masses: the solvent peak for acetonitrile and the molecular ion for TNT (both protonated). The desorption and detection is rapid, and the analyte has been completely desorbed within ~20 seconds of starting the desorption process.

Transient plots

In the second plot are three 'stacked' data sets for three different loadings of the swabs: 100ng, 10ng and finally 1ng. At 1ng, the detected signal is, of course, lower than at higher loadings, but is still clearly generating molecular ions for TNT at above the background. Swab temperature 140°C.

Transient plots

Having established that it was possible to detect a 1ng loading of TNT, we then investigated the optimum swab desoption temperature, in an attempt to detect even lower amounts of TNT deposited on a swab.

Transient plots

In this plot we see three overlaid traces of the protonated molecular ion for TNT at mass 228 for three different loadings. The swab was operated at 150°C for these acquisitions. We were able to detect TNT signals from a 250pg loading of a swab. Note also how quickly the compound is desorbed from the swab, and how the data acquisition is finished in less than 10 seconds for these low amounts of material.

(Data acquired by Kore and Ramón Gonzalez-Mendez of Birmingham University)

Experiments at Birmingham are now underway to explore various parameters such as the optimum desorption and PTR analysis conditions for various analytes, as well as detection limits and memory effects etc. There is no compound separation using the desorber with the PTR because there is no gas chromatograph. Nevertheless, the technique may still be very useful when looking for a known set of analytes owing to the rapid data acquisition time and the ability to monitor for all mass peaks in parallel.

Kore Technology Ltd would like to acknowledge the fact that the swab desorber technology was largely developed with funding from the UK Ministry of Defence.


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09:26 11/10/2013

© Kore Technology Limited 2013