Soft ionisation mass spectrometric techniques and the environmental sciences (2015)
On 10th December 2014, the Environmental Chemistry Group and the Molecular Physics Group of the Institute of Physics co-organised a half-day meeting to discuss recent applications and developments of soft chemical ionisation mass spectrometric techniques in the field of environmental sciences, with a focus on the areas of atmospheric chemistry and physics. This followed on from two successful meetings on the same topic in 2012 and 2013. The meeting comprised four talks from invited speakers; each forty minute presentation was followed by ten to fifteen minutes of questions and discussion.
The first talk, given by Professor Thomas Karl (University of Innsbruck, Austria), was entitled Eddy Covariance Measurements of NMVOC using PTR-(Q)MS, PTR-TOF-MS and PTR-QI-TOF-MS. The speaker began with an overview of the role of non-methane volatile organic compounds in determining tropospheric ozone levels, the oxidative capacity of the atmosphere, and the formation of organic aerosols. He highlighted the need for reducing uncertainties associated with NMVOC (non-methane volatile organic compounds).
The first talk, given by Professor Thomas Karl (University of Innsbruck, Austria), was entitled Eddy Covariance Measurements of NMVOC using PTR-(Q)MS, PTR-TOF-MS and PTR-QI-TOF-MS. The speaker began with an overview of the role of non-methane volatile organic compounds in determining tropospheric ozone levels, the oxidative capacity of the atmosphere, and the formation of organic aerosols. He highlighted the need for reducing uncertainties associated with NMVOC (non-methane volatile organic compounds).
Thomas Karl discussed the range of spatial and temporal resolutions covered by different approaches to the study of NMVOCs, from leaf level through to earth observation, and then focussed on tower-based and airborne flux measurements. These techniques can offer temporal resolutions of less than one minute; periods of measurement from some tower sites extend over several years. The techniques used to quantify (NMVOC) fluxes have advanced from more indirect gradient methods through virtual disjunctive eddy covariance (pioneered by Professor Karl) to direct eddy covariance of a large number of compounds with the recently developed PTR-ToF-MS (Proton transfer time-of-flight mass spectrometry) method.
Thomas Karl stressed the particular importance of flux-based techniques in the tropics, where large species diversity hampers calculation of emissions by scaling up leaf-level measurements. |
Recent advances including the use of an Ionicon PTR-ToF-MS for measurement of methanol fluxes by eddy covariance, with the high mass resolution enabling methanol to be distinguished from isobaric ions. PTR-MS flux data across the USA and South America have been used to derive upper and lower bound estimates of dry deposition of the isoprene oxidation products methyl vinyl ketone and methacrolein to vegetation. Professor Karl also showed airborne measurements of anthropogenic VOCs in Mexico City and discussed source distributions. In the final part of the talk, he introduced a newly developed technique, proton transfer reaction quadrupole interface time of flight mass spectrometry, and its current application to measuring NMVOC fluxes, including benzene, over the city of Innsbruck. The speaker concluded with a summary of progress in CIMS (chemical ionisation mass spectrometry) over the last fifteen years and the consequent advances in flux quantification.
The second speaker, Dr Racheal Beale (Plymouth Marine Laboratory), described her work in quantifying concentrations and air-sea exchange of acetone in situ. She discussed the importance of acetone in the troposphere and the uncertainty in the extent and drivers of net oceanic exchange resulting from the sparse number and limited duration of measurements. The speaker described a novel membrane inlet PTR-MS technique for measuring acetone in seawater and demonstrated its application during several field campaigns. These included: ICON, where concentrations were measured from the Mauritanian upwelling during April and May 2009; the Atlantic Meridional Transect (AMT) 19 and AMT 22 research cruises, which travelled from the UK to Chile during October and November in 2009 and 2011 respectively; and long term (January 2011 to March 2012) weekly measurements at a research station 10 km off the Plymouth coast.
Seawater acetone concentrations measured during AMT 19 and 22 ranged from 2 to 24 nM and 5 to 36 nM respectively. Surface seawater concentrations during the cruises were comparable to the highest concentrations observed in the North Atlantic Gyre, where biological activity is low, and were highest in the high temperate latitudes. High concentrations were also observed in the eutrophic Mauritanian upwelling. During AMT 22, eddy covariance air-sea flux measurements were also undertaken using PTR-MS and compared with air–sea flux predicted from a two-layer model using the measured water and air concentrations. Observed and predicted acetone fluxes were in good agreement. During AMT 22, the Atlantic Ocean was a net sink for acetone at high northern latitudes and a net source in the subtropics, whereas the South Atlantic represented a zero net flux. Dr Beale concluded with planned future work, including the use of a switchable reagent ion source to aid separation of glyoxal and propanal contributions to the acetone signal.
The second speaker, Dr Racheal Beale (Plymouth Marine Laboratory), described her work in quantifying concentrations and air-sea exchange of acetone in situ. She discussed the importance of acetone in the troposphere and the uncertainty in the extent and drivers of net oceanic exchange resulting from the sparse number and limited duration of measurements. The speaker described a novel membrane inlet PTR-MS technique for measuring acetone in seawater and demonstrated its application during several field campaigns. These included: ICON, where concentrations were measured from the Mauritanian upwelling during April and May 2009; the Atlantic Meridional Transect (AMT) 19 and AMT 22 research cruises, which travelled from the UK to Chile during October and November in 2009 and 2011 respectively; and long term (January 2011 to March 2012) weekly measurements at a research station 10 km off the Plymouth coast.
Seawater acetone concentrations measured during AMT 19 and 22 ranged from 2 to 24 nM and 5 to 36 nM respectively. Surface seawater concentrations during the cruises were comparable to the highest concentrations observed in the North Atlantic Gyre, where biological activity is low, and were highest in the high temperate latitudes. High concentrations were also observed in the eutrophic Mauritanian upwelling. During AMT 22, eddy covariance air-sea flux measurements were also undertaken using PTR-MS and compared with air–sea flux predicted from a two-layer model using the measured water and air concentrations. Observed and predicted acetone fluxes were in good agreement. During AMT 22, the Atlantic Ocean was a net sink for acetone at high northern latitudes and a net source in the subtropics, whereas the South Atlantic represented a zero net flux. Dr Beale concluded with planned future work, including the use of a switchable reagent ion source to aid separation of glyoxal and propanal contributions to the acetone signal.
In the third talk, Dr Markus Kalberer (University of Cambridge) gave a comprehensive overview of the use of soft ionisation ultra-high resolution mass spectrometry (UHRMS) for characterising the organic fraction of atmospheric particles. The average chemical composition of tropospheric aerosol includes a large fraction of organic material. The chemical composition of this material is important, for example in determining health impacts, but largely unknown due to challenges in determining temporal and spatial variability, measuring trace quantities, and characterising complex mixtures. Dr Kalberer explained how the high mass resolution and accuracy of UHRMS can help to meet these challenges because they enable us to determine elemental composition.
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Though less able to measure the entire chemical space (polarity, solubilities, functional groups), the reduced fragmentation produced by soft atmospheric pressure ionisation is useful for analysing complex mixtures in laboratory-based smog chamber experiments and also for characterising samples collected in the field. Dr Kalberer described the use of techniques such as Nano-Desorption Electrospray ionization (DESI), Liquid Extraction Surface Analysis (LESA) and Extractive Electrospray ionization (EESI) (“online ESI”) to minimize sample workup and associated artefacts and to provide temporal information of increasing resolution.
In the final talk, Dr Gavin Phillips (University of Chester) described the use of iodide chemical ionization mass spectrometry (ICIMS) for the measurement of peroxyacetyl nitrate (PAN) and peroxyacetic acid (PAA) using alternatively heated and ambient temperature inlet lines. He contrasted measurements made at a boreal forest site in Finland (HUMPPA-COPEC campaign) with those from a mountaintop site in Germany (PARADE). PAA was found to be a large proportion of the sum of PAA and PAN during the HUMPPA campaign, which was characterised by above average temperatures, high BVOC (biogenic volatile organic compounds), and low NOx mixing ratios. In contrast, during the PARADE campaign PAN dominated, reflecting lower temperatures and NOx levels. Dr Phillips moved on to the use of ICIMS alongside aerosol mass spectrometry to investigate production of ClNO2 through uptake of N2O5 on chloride containing aerosols in the field.
The meeting was attended by ca. 40 delegates, and it was agreed to hold a fourth meeting in the autumn of 2015 – details to follow.
In the final talk, Dr Gavin Phillips (University of Chester) described the use of iodide chemical ionization mass spectrometry (ICIMS) for the measurement of peroxyacetyl nitrate (PAN) and peroxyacetic acid (PAA) using alternatively heated and ambient temperature inlet lines. He contrasted measurements made at a boreal forest site in Finland (HUMPPA-COPEC campaign) with those from a mountaintop site in Germany (PARADE). PAA was found to be a large proportion of the sum of PAA and PAN during the HUMPPA campaign, which was characterised by above average temperatures, high BVOC (biogenic volatile organic compounds), and low NOx mixing ratios. In contrast, during the PARADE campaign PAN dominated, reflecting lower temperatures and NOx levels. Dr Phillips moved on to the use of ICIMS alongside aerosol mass spectrometry to investigate production of ClNO2 through uptake of N2O5 on chloride containing aerosols in the field.
The meeting was attended by ca. 40 delegates, and it was agreed to hold a fourth meeting in the autumn of 2015 – details to follow.