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PTR-TOF-MS with High Temperature Front End

Photo

This PTR-TOF-MS was built for a commercial UK company and is an extension of the PTR instrument designed for Nottingham University.

This new instrument has a larger reflectron (150mm diameter compared to 100mm), and a longer effective flight length of 2 metres (compared to 1.2m). It also has a higher temperature capacity at the front end to allow semi-volatile materials to be analysed more successfully. The oven operates at 100°C, but there is a separate 200°C heater on the capillary inlet line, and a 200°C heater on the PTR source to prevent low volatility molecules from condensing in the inlet lines leading to the reactor cell.

This increases the range of compounds that can be detected by the instrument. As well as the ability to monitor a continuous flow of gas drawn into the PTR reactor, it is possible to use thermal desorption methods for analysis of semi- volatiles, because the transport to the reactor is sufficiently hot.


Graph showing desorb peak

Shown here is an example of such a mode of operation: the plot shows the intensity of the protonated molecular ion (c/s for mass 221) for semi-volatile di-tert-butyl benzoquinone (DBQ) as a function of time, as 10ng of the compound was evolved from a thermal desorber unit.

A full mass spectrum is available at every point on the 'chromatogram', and in the case of a multi-component system, one of the key advantages of using a TOF analyser is not only the high speed of analysis, but also the ability to acquire all masses simultaneously. A mass spectrum from a one second slice at the peak of the chromatogram shows a relatively intense peak at mass 117.

Graph showing spectrum


A one second mass spectrum extracted retrospectively from the peak of the mass 221 chromatogram.

Note the more intense mass 117.1 peak, which is a good fit to the protonated dimer of acetone (C3H6O)2+H+ used for applying the DBQ onto the desorber.


Graph showing chromatogram


Since all data was recorded, it is possible to request the chromatogram for this peak retrospectively. This is shown here. Note the different time evolution of the acetone compared with the DBQ - its high volatility means that it was already evaporating as the heater was turned on.

Since there is no 'overhead' to acquiring all data recorded at the detector, the data set is extremely rich and the ability to interrogate the data set retrospectively is one of many advantages offered by the TOF-MS method.


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Last updated: 14:49 17/02/2014

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