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Proton Transfer Reactor / Atmospheric Pressure Chemical Ionisation MS (PTR/APCI TOF-MS)

Photo
The Nottingham PTRMS with Caroline Lamont Smith, part of the team that built it

This instrument has been designed for the University of Nottingham. In one mode it operates using chemical ionisation in a low pressure (~1mbar) drift tube, e.g. by using PTR (proton transfer reaction). The second mode employs APCI (atmospheric pressure chemical ionisation). To switch between the two modes requires a mechanical swap of the "front end" source. The open architecture of the instrument allows this to be done easily. Each mode can be used to both create both positive and negative ion mass spectra.

Example PTR data

A TOF-MS linked to a PTR system is highly attractive because it offers real-time, high time resolution, high sensitivity detection of all analyte species in parallel. In a quadrupole PTR-MS, as each new species (m/z) is added to the list of molecules to be monitored, less time can be spent monitoring the other species of interest, thus the sensitivity per molecule drops in proportion. Also, if 10 species are monitored for 100ms, then data for each individual species is only acquired every second, and thus time resolution suffers when multiple species are monitored.

These limitations do not apply to TOF-MS. All ions are detected; there is no concept of 'having to choose which mass to monitor'.


Photo

The PTR front end is a Kore design, and comprises a hollow cathode glow discharge source (HCGD) on the right side, a source drift region (to allow other chemistries to be achieved), and a PTR drift section into which the analyte molecules are introduced for collision with the primary ion beam. Just visible in the photograph are the parallel drift electrodes.


Prof. Andy Taylor and Dr. Rob Linforth have pioneered the use of atmospheric pressure chemical ionisation methods for the detection of flavour molecules using conventional quadrupole mass spectrometers. They are now looking to extend the speed and sensitivity in their research by using TOF-MS, as well as extending their ionisation methods to include proton transfer reaction chemistry (PTR).

An intriguing aspect of their research methodology is to monitor the appearance of flavour molecules with elapsed time on the breath of a person who is sampling a food product. Naturally, the basic experiment is quite short, and as much data as possible needs to be gathered in that time. The parallel detection capabilities of TOF-MS are very attractive from this point of view. One of the aims of this instrument is to be able to acquire sufficient data in 0.1 seconds to allow investigation of concentration variations with this time resolution.


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Last updated: 18:13 07/02/2014

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