Not tarred with the same brush: Study attempts to define the varied molecular makeup of coal tars

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  • Published: Oct 15, 2016
  • Author: Ryan De Vooght-Johnson
  • Channels: Gas Chromatography
thumbnail image: Not tarred with the same brush: Study attempts to define the varied molecular makeup of coal tars

Mother Nature

Coal tars can differ considerably from one another in their molecular profiles. It is these differences that troubles industry, for the phenolic makeup of coal tars dictates their stability.

Do not tar all coal tars with the same brush for under their apparently standard black-treacle viscosity and appearance, coal tars can differ considerably from one another in their molecular profiles. It is these differences that troubles industry, for the phenolic makeup of coal tars dictates their stability.

Highly abundant in petroleum, coal extracts, and bio-oils, the phenolic compounds we take advantage of for our fuels were first created by plants many years ago. Over time, these plants became submerged by earth—perhaps when a flood ravaged the area—becoming trapped deeper and deeper in the Earth’s underbelly as the plants above decomposed into soil. Under immense pressure and temperature, the buried vegetation, over many, many years, carbonised into coal.

Given that this creation of coal isn’t an exact science, it is highly likely for the composition to vary from coal to coal. There are no quality controls or standard operating procedures here. We do not have a say in coal production, but we can determine its molecular makeup as it is gathered deep from within the Earth’s core.

‘It is important to determine the phenolic compounds and their molecular compositions in the coal tars,’ writes Sutian Mar, first author and analytical chemist from China University of Petroleum in Beijing.

GC-APCI-MS

Writing in Rapid Communications in Mass Spectrometry, Ma, together with colleagues and Kejun Qian of Waters Corporation, set out to characterise the phenolic compounds in coal tar containing a myriad of hydrocarbons.

Following the trends of hydrocarbon analyses, which are meticulously chronicled in their paper, Ma opted for GC-based resolution owing to its ‘efficient separation ability’ combined with negative atmospheric pressure chemical ionization (APCI), which has been proven to have ‘high responses for phenolic compounds.’

For their study, Ma procured coal from the National Institute of Clean and Low-Carbon Energy of China and distilled the samples into narrow fractions under atmospheric conditions. The group then resolved these highly complex mixtures using a mid-polarity DB-35 column over a 38-minute gradient—ramped incrementally from 80 to 300°C—fed directly into a tandem quadrupole for mass analyses.

Phenolic makeup

Comparing their novel method with an identical GC-setup hooked up to a flame ionization detector, when aligned side-by-side the chromatograms showed that the former provided a more detailed panorama of the phenolic makeup of coal tars, particularly for the lowly volatile phenols. These compounds with high boiling points, Ma comments, are likely to be ‘hydroxy phenanthrenes/anthracenes, hydroxyl dibenzofurans, and hydroxyl fluoranthenes/pyrenes’ based on their masses.

‘The relative abundance of mass peaks,’ adds Ma, ‘exhibited a distinct composition and distribution of phenolic compounds in each fraction.’ The alkyl phenols containing fewer than seven carbons, for example, were largely within the fractions eluted at temperatures lower than 200°C. Indenols and benzenediols, by contrast, were found lurking among the 240–260°C fraction and the naphtols within the lowly volatile 280–280°C range.

Since 2010, scientists have attempted to ionize phenolic compounds through soft methods alone, reasoning that mass spectra are too complex already. However, given that compounds are not fragmented when ionized in this manner, the all-important structural information required to identify isomers is not given. Ma sees APCI as the solution, albeit one that requires some more work.

‘Regardless of ionization efficiency among the isomers, the relative abundances of mass peaks could be considered as the semi-quantitative concentrations of various phenolic compounds,’ writes Ma.

Related Links

Rapid Commun. Mass Spectrom., 2016, 30, 1806–1810. Ma et al. Characterization of phenolic compounds in coal tar by gas chromatography/negative-ion atmospheric pressure chemical ionization mass spectrometry.

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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