Simple and reliable quantitation of ppt-level PAHs in air using TD–GC–MS

Summary

This Application Note describes the performance of a sorbent tube and optimised analytical protocol dedicated to the detection of ppt-level polycyclic aromatic hydrocarbons (PAHs) in ambient air. Using active (pumped) sampling with analysis by thermal desorption–gas chromatography (TD–GC), this innovative approach completely avoids the labour-intensive solvent-extraction protocol employed in many standardised methods. The optimised analytical method also offers increased sensitivity due to increased extraction efficiency, absence of a dilution stage, and efficient transfer of compounds to the GC.

Introduction

Polycyclic aromatic hydrocarbons (PAHs) are widely recognised as a particularly harmful group of organic compounds, with documented carcinogenic, mutagenic and teratogenic properties. PAHs are formed as a result of the incomplete combustion of organic materials such as coal, gasoline and wood, and as a result they are particularly prevalent in urban and industrial environments. This, combined with their toxicity, has resulted in the implementation of very low limit levels for urban and workplace air. Although PAHs can be analysed by GC–MS, they cover a relatively wide volatility range, from naphthalene (b.p. 218°C) to benzo[ghi]perylene (b.p. 500°C). This means that the lower-volatility PAHs tend to occur bound to particulate matter, as well as being present in the air. This is reflected in many global standardised methods for the analysis of PAHs (including US EPA Method TO-13A,¹ ISO 16000-13,² ISO 16232,³ ISO 12884⁴ and Chinese EPA Method HJ 584⁵), some of which require the use of a quartz filter to trap the particulates, backed-up with a sorbent cartridge to collect the vapour-phase fraction (and indeed any analytes that are released from the particulate matter during sampling). This filter-cartridge system is rigorously conditioned before being used for air sampling. It is then solvent-extracted, concentrated, and subjected to silica gel cleanup followed by directly injection into the GC–MS system. This lengthy, multi-step process is labour-intensive, making it difficult to automate, and prone to errors in sample handling and data logging (‘chain of custody’). In addition, the use of solvent risks the more volatile analytes being lost during the evaporation stage, raises the possibility of crosscontamination between samples, and means that the protocol is not environmentally responsible. Consequently there is demand for more efficient techniques for the sampling of PAHs and their introduction to the GC system. Thermal desorption offers a number of well-known advantages over solvent extraction methods for a wide range of VOCs and SVOCs,⁶ the major one being the greatly improved sensitivity due to the avoidance of dilution, the high extraction efficiency, and efficient transfer/injection into the GC. This sensitivity advantage means that the 1000+ litre volumes (and very large sampling flows) required for traditional methods can be substantially reduced, simplifying the sampling process. TD also interfaces with the same type of GC–MS analytical equipment widely used for monitoring VOCs such as benzene in ambient air. In this Application Note, we describe the outstanding performance offered by a new sorbent tube developed by Markes that is dedicated to the analysis of PAHs. This ‘PAH tube’ is employed in conjunction with an optimised TD–GC– MS method employing Markes’ UNITY™ or TD-100™ thermal desorbers. This approach, building on earlier work with PAHs,⁷ complements typical VOC monitoring methods for analysis of lower-boiling atmospheric pollutants (such as US EPA Method TO-17,⁸ ISO 16017,⁹ ISO 16000-6,¹⁰ and Chinese EPA Methods HJ 644¹¹ and HJ 732¹²) by offering excellent reproducibilities and linearities for these challenging analytes. In addition, the high linear velocities in the flow path and the temperature uniformity in the TD instrument provide the well-resolved chromatography that is needed to confidently distinguish between the various PAHs.

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