Funnelling ions boost sensitivity of mass spectrometer

Posted by mjs76 at Jun 27, 2012 05:02 PM |
How ionised water is used to detect trace compounds in the air.

Proton Transfer Reaction Mass Spectrometry (PTR-MS) is an important area of research in our Department of Chemistry, and a new paper describes how this technique can be made even more sensitive and useful than it already is.

Developed in the 1990s, PTR-MS can detect extremely low concentrations of volatile organic compounds (VOCs) with a ‘limit of detection’ measured in parts-per-billion or sometimes even parts-per-trillion. As such, it is a vital component of both our Real-time Air Fingerprinting Technology (RAFT) project and our Diagnostic Development Unit (DDU – the so-called ‘Star Trek bed’.)

Proton Transfer Reaction MS works by using hydronium, H3O+, which is essentially water with an extra proton. as an ion source, created by simply passing a DC current through water. Using hydronium, a PTR mass spectrometer can separate out molecules which are more attractive to protons than water, such as VOCs, from the ‘background noise’ of the main constituents of air (oxygen, nitrogen, CO2 etc) which have a proton affinity below that of H2O.

This type of mass spectrometry incorporates a ‘drift tube’, a cylindrical electrode to which a radio frequency voltage is applied, creating a electrical field that keeps the ions moving at a constant rate, thereby allowing quantitative (rather than just comparative) measurements of the trace chemicals being studied. Ions exit the drift tube into the mass spectrometer itself, of which several different types are available.

One stumbling block is that the pressure in the drift tube must be higher than that inside the mass spectrometer, which allows only a very small aperture between the two. This severely limits the number of ions actually making it through with obvious consequent limitations in the sensitivity of the device.

A few years ago, researchers in the USA working on a different type of mass spectrometry, electrospray ionisation MS, improved on the drift tube by creating a ‘radio frequency (rf) ion funnel’. This is a series of ring electrodes of progressively smaller internal diameter which effectively channel the ions into a more concentrated beam before they enter the mass spectrometer. It can be thought of as a wall-less funnel.

Mass spectrometer being used to analyse breath. The ion funnel is in the blue box.

The Leicester team of Professor Andrew Ellis, Professor Paul Monks, Dr Robert Blake, Dr Iain White and PhD student Shane Barber, with colleagues from Kore Technology in Cambridgeshire, experimented with the effect of applying a combined drift tube/rf ion funnel to a PTR mass spectrometer. The funnel they used was made from 29 stainless steel plates, each 0.2mm thick and spaced 3.2mm apart using ceramic rods, with the internal diameter of 40mm in the first part (drift tube) gradually reducing to 6mm in the funnel section.

When nitrogen containing traces of methanol, acetone and other gases at known, minute concentrations was injected into the apparatus with the tube/funnel fitted, the number of ions reaching the ion detector increased significantly over the base level. In fact, the experiment was so successful that it threatened to wear out the ion detector. The limit of detection was reduced to less than 200 parts per trillion and the team believe that, with a bit of work, that can be got down to single figures.

This simple technique will greatly enhance the sensitivity of our RAFT and DDU equipment and all future applications of PTR-MS. The paper ‘Increased sensitivity in proton transfer reaction mass spectrometry by incorporation of a radio frequency ion funnel’ (doi: 10.1021/ac300894t) has been published in the journal Analytical Chemistry.

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