The use of multi-channel silicone rubber traps as denuders for polycyclic aromatic hydrocarbons

Department of Chemistry, Faculty of Natural and Agricultural Sciences, University of Pretoria, South Africa.
Analytica chimica acta (Impact Factor: 4.51). 06/2012; 730:71-9. DOI: 10.1016/j.aca.2011.11.013
Source: PubMed

ABSTRACT Atmospheric polycyclic aromatic hydrocarbons are ubiquitous environmental pollutants, which may be present both in the gaseous phase and adsorbed onto the surface of particles. Denuders are sampling devices which have been effectively employed in such partitioning applications. Here we describe and characterise a novel miniature denuder consisting of two multi-channel silicone rubber traps (each 178 mm long, 6 mm o.d. containing 22 silicone tubes), separated by a quartz fibre filter for particle phase collection. The denuder only requires a small portable personal sampling pump to provide sampling flow rates of ∼0.5 L min(-1). Theoretical considerations indicated that the air flow through the denuder was expected to be laminar, and the linear velocity arising from longitudinal diffusion was found to be negligible. The calculated particle transmission efficiency through the denuder was found to be essentially 100% for particles>50 nm, whilst the experimental overall efficiency, as determined by CPC and SMPS measurements, was 92 ± 4%. The size resolved transmission efficiency was <60% for particles below 20 nm and 100% for particles larger than 200 nm. Losses could have been due to diffusion and electrostatic effects. Semi-volatile gaseous analytes are pre-concentrated in the silicone of the trap and may be thermally desorbed using a commercially available desorber, allowing for total transfer and detection of the collected analytes by GC-MS. This enhances detection limits and allows for lower sampling flow rates and shorter sampling times, which are advantageous for studies requiring high temporal resolution.

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Available from: Patricia B C Forbes, Jul 21, 2014
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    • "Multi-channel PDMS traps and thermal desorption in a commercial system were successfully used for monitoring of atmospheric polycyclic aromatic hydrocarbons (PAHs) [18] [22] [23] [24]. Advantages of the multi-channel PDMS trap over commercial packed or coated sorptive devices are its open tubular structure and low pressure drop associated with laminar multi-channel flow [21] [22] [24]. Compared to multi-channel traps consisting of a bundle of GC capillary columns which contain nonsorptive outer coatings, both the inside and the outside of the multi-channel silicone rubber trap provide sorptive surfaces. "
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    ABSTRACT: The organochlorine insecticide DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane) is still used for malaria vector control in certain areas of South Africa. The strict Stockholm Convention on Persistent Organic Pollutants (POPs) allows spraying on the inside of traditional dwellings with DDT. In rural villages contaminated dust presents an additional pathway for exposure to DDT. We present a new method for the determination of DDT in indoor air where separate vapour and particulate samples are collected in a single step with a denuder configuration of a multi-channel open tubular silicone rubber (polydimethylsiloxane (PDMS)) trap combined with a micro quartz fibre filter. The multi-channel PDMS trap section of the denuder concentrates vapour phase insecticide whereas particle associated insecticide is transferred downstream where it is collected on a micro-fibre filter followed by a second multi-channel PDMS trap to capture the blow-off from the filter. The multi-channel PDMS trap and filter combination are designed to fit a commercial thermal desorber for direct introduction of samples into a GC-MS. The technique is solvent-free. Analyte extraction and sample clean-up is not required. Two fractions, vapour phase and particulate phase p,p'-DDT, o,p'-DDT; p,p'-DDD, o,p'-DDD; p,p'-DDE and o,p'-DDE in 4 L contaminated indoor air, were each quantitatively analysed by GC-MS using isotopically labelled ring substituted (13)C(12) -p,p'-DDT as an internal standard. Limits of detection were 0.07-0.35 ng m(-3) for p,p'-DDT, o,p'-DDT, p,p'-DDD, o,p'-DDD, p,p'-DDE and o,p'-DDE. Ratios of airborne p,p'-DDD/p,p'-DDT and of o,p'-DDT/p,p'-DDT are unusual and do not match the ideal certified ingredient composition required of commercial DDT. Results suggest that the DDT products used for indoor residual spraying (IRS) prior to, and during 2007, may have been compromised with regards to insecticidal efficacy, demonstrating the power of this new environmental forensics tool.
    Analytica chimica acta 06/2012; 730:112-9. DOI:10.1016/j.aca.2012.02.054 · 4.51 Impact Factor
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    ABSTRACT: In rural parts of South Africa the organochlorine insecticide DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane) is still used for malaria vector control where traditional dwellings are sprayed on the inside with small quantities of technical DDT. Since o,p'-DDT may show enantioselective oestrogenicity and biodegradability, it is important to analyse enantiomers of o,p'-DDT and its chiral degradation product, o,p'-DDD, for both health and environmental-forensic considerations. Generally, chiral analysis is performed using heart-cut multidimensional gas chromatography (MDGC) and, more recently, comprehensive two-dimensional gas chromatography (GC×GC). We developed an off-line gas chromatographic fraction collection (heart-cut) procedure for the selective capturing of the appropriate isomers from a first apolar column, followed by reinjection and separation on a second chiral column. Only the o,p'-isomers of DDT and DDD fractions from the first dimension complex chromatogram (achiral apolar GC column separation) were selectively collected onto a polydimethylsiloxane (PDMS) multichannel open tubular silicone rubber trap by simply placing the latter device on the flame tip of an inactivated flame ionisation detector (FID). The multichannel trap containing the o,p'-heart-cuts was then thermally desorbed into a GC with time-of-flight mass spectrometry detection (GC-TOFMS) for second dimension enantioselective separation on a chiral column (β-cyclodextrin-based). By selectively capturing only the o,p'-isomers from the complex sample chromatogram, (1)D separation of ultra-trace level enantiomers could be achieved on the second chiral column without matrix interference. Here, we present solventless concentration techniques for extraction of DDT from contaminated soil and air, and report enantiomeric fraction (EF) values of o,p'-DDT and o,p'-DDD obtained by a new multidimensional approach for heart-cut gas chromatographic fraction collection for off-line second dimension enantiomeric separation by (1)D GC-TOFMS of selected isomers. This multidimensional method is compared to the complementary technique of comprehensive GC×GC-TOFMS using the same enantioselective column, this time as the first dimension of separation.
    Analytica chimica acta 06/2012; 730:120-6. DOI:10.1016/j.aca.2012.03.028 · 4.51 Impact Factor
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    ABSTRACT: A study carried out at the University of Pretoria characterised aerosol particle morphology of residential coal combustion smoke. The general approach in this study was on individual particle conglomerations because the radiative, environmental, and health effects of particles may depend on specific properties of individual particles rather than on the averaged bulk composition properties. A novel, miniature denuder system, developed and tested at the University of Pretoria, was used to capture particle emissions from the coal fires. The denuder consists of two silicone rubber traps (for gas phase semi-volatile organic compound monitoring) in series separated by a quartz fibre filter (for particle collection). The denuders were positioned 1 m away from the fire and were connected to pumps that sampled ~5 litres of air over a 10 min sampling interval. A JSM 5800LV Scanning Electron Microscope with a Thermo Scientific EDS was used to analyse the structure and morphology of different aerosol samples from the quartz fibre filters. Eight samples from the different fire lighting methods were selected for SEM analysis. The punched samples were sputter coated with gold for ~15 minutes using a K550 Emitech Sputter Coater. Results show that apart from the fine and ultra-fine particles, coal smoke from domestic burning also contains aerosols greater than 5 µm in diameter. Consequently, we describe the potential for generation of 'giant' carbonaceous soot conglomerates with outer diameters of 5 to 100 µm. However, the exact mechanism for formation of such large soot conglomerates remains to be determined. We also describe the presence of spherules and solid 'melted toffee' irregular surfaces. Circumstantial evidence is used to postulate and discuss the possible modes of formation in terms of condensation, and partial melting. This work provides a description of the modes of formation and transformation of conglomerates originating from low temperature (<8000C) coal combustion.
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