α-pinene + O2 = ?: Study fills in blanks of α-pinene chemistry

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  • Published: Aug 17, 2016
  • Author: Ryan De Vooght-Johnson
  • Channels: Gas Chromatography
thumbnail image: α-pinene + O2 = ?: Study fills in blanks of α-pinene chemistry

Sink or swim

α-pinene is wafted into the atmosphere by coniferous forests, most notably those formed of huddled pines. There it reacts with atmospheric O2, creating low-volatile carboxylic acids and organic nitrates that coalesce into secondary organic aerosols.

The oxidation of α-pinene is not something we fully understand. At best just shy of three quarters of the products that form are accounted for, and at worst just over a half.

It's not as if scientists have turned a blind eye; many studies have been meticulously devised in pursuit of this particularly complex reaction. This uncertainty, Dr Rindelaub and collaborators write in Rapid Communications in Mass Spectrometry, stems from the ‘large number of reaction pathways’ for which ‘simple rules deduced from well-known reactions of smaller molecules do not necessarily apply.’

α-pinene is wafted into the atmosphere by coniferous forests, most notably those formed of huddled pines. There it reacts with atmospheric O2, creating low-volatile carboxylic acids and organic nitrates that coalesce into secondary organic aerosols (SOAs). Congregation of these atmospheric particles is particularly troublesome. For one, inhalation of this chemical cocktail may cause cancers of the lung and cardiovascular diseases. Secondly, SOAs adds to our global radiation budget.

In pine-needled forests, α-pinene, when prompted by OH radicals, governs the fate of mono-nitrogen oxides: does it sink or swim? A by-product of humankind’s fires, these ozone precursors, be it NO or NO2, can be sunk during the oxidation of biogenic volatile organic compounds (BVOC), forming non-volatile organic nitrates—RONO2, for example—that shelter oxidised nitrogen from succumbing to ozone (O3). In this respect, α-pinene can be said to reduce ozone formation, as well as contributing to its formation.

Defining x

The accounted for carboxylic acids and organic nitrates do not complete the equation, however. In addition to these, oxidation of α-pinene may spew out a multitude of other entities, including carbonyls, alcohols, and some still-to-be-identified multifunctional compounds. This ‘wide array of products,’ the authors reason,’ implies that a single analytical technique may not be adequate for complete characterisation of all α-pinene-derived species.’

In pursuit of the undefined Xs, the US scientists set out to detect the organic nitrates and organosulfates, plus any chance entities, that emerge from the oxidation of α-pinene by OH radicals. First, they used GC-ToF-MS, and later supplemented this with the HPLC-ToF-MS and LC-LTQ-MS/MS-based techniques more suited for detecting low-volatile and highly oxidised compounds. Striving for comprehensiveness, an array of ionisation methods was investigated, including the novel paper spray ionisation used to profile the gas and particle-phase products that get caught within strategically placed filters.

All these experiments were performed within the confines of a UV-bathed photochemical chamber, meticulously designed to mimic the atmosphere’s generation of nitrogen oxides and ozones.

A series of firsts

First and foremost, 32 entities caught up on filter traps were all-but identified with their toolbox, of which GC/MS contributed to the discovery of 18. Four of these were the ‘low-hanging fruits’ – pinocamphone, campholenic aldehyde, pinoaldehyde, and pinonic acid—identified as major products elsewhere. Clambering further up the metaphorical fruit-laden tree, and through picking at the treetops, were never-before detected products: pinocamphenol, fencholenic aldehyde, and α-pinene-derived nitrate isomers.

Adding to the list of firsts was the discovery of standalone, first-generation organic nitrates by accessible chromatographic techniques coupled to mass spectrometry. This ability to first separate and then analyse these entities is particularly exciting and, as the authors note, ‘extremely valuable’ to the field of atmospheric sciences.

Finally, the atmospheric chemists fashioned a paper spray ionisation source out of a filter sample, from which particle-phase oxidation products could be observed. ‘This ambient ionization method,’ Dr Rindelaub and colleagues concluded, ‘is a quick, effective method for filter sample analysis and relatively inexpensive, giving it great potential for use in ambient atmospheric sampling.’

Related Links

Rapid Commun. Mass Spectrom. 2016, 30, 1627–1638. Rindelaub et al. Chemical characterization of α-pinene secondary organic aerosol constituents using gas chromatography, liquid chromatography, and paper spray-based mass spectrometry techniques.

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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