Proton transfer reaction mass spectrometry (PTR-MS) provides online, high sensitivity (low pmol/mol) determination of volatile compounds in air with high time resolution. This is a mature technology that has been commercially available for more than 15 years and is in use in a wide variety of scientific fields. Currently, there are several hundred PTR-MS instruments in operation but thus far there has been very little attention paid to ensuring measurement comparability and currently no specific SI traceable infrastructure exists. This is of particular importance for example in environmental monitoring where emission data from many different PTR-MS datasets are used as inputs to models, the outputs of which are compared to observations at monitoring sites to evaluate their predictive capabilities. Improved comparability between model inputs and the monitoring sites will improve both the reliability and accuracy of atmospheric models. There is an increasing interest in PTR-MS measurement, especially for disease diagnosis from exhaled VOCs in breath. Traceable calibrations will become very important as diagnostic VOC breath testing translates into clinical practice. Here we will present an evaluation of the utility of the PTR-MS technique to a wide variety of trace gases, assess the performance characteristics of the instrument, shown the sensitivity dependence to the matrix gas composition including humidity, show how an improved reference gas standard can more accurately determine the mass dependent transmission efficiency of protonated ions and provide a full uncertainty budget for PTR-MS measurements.
Dave Worton is a Senior Research Scientist leading the Environmental Gases team of NPL’s Gas and Particle Metrology Group, providing the traceability that underpins global atmospheric and urban air quality monitoring of greenhouse gases, emission and reactive gases and volatile organic compounds. Dave is a Chartered Chemist and member of the Royal Society of Chemistry and has over 15 years’ experience of developing and applying novel instrumentation to analyse and better characterize organic compounds using gas chromatography and mass spectrometry based techniques and has published over 50 peer-reviewed papers.