Application of Diffusion Denuder Samplers to the Determination of Semi Volatile Organic Material

The application of diffusion denuder samplers to the determination of gas and particulate phase semi-volatile organic material is illustrated with results from three different studies, one each using the BIG BOSS, PC-BOSS and IOVPS samplers.

Semi-volatile organic compounds lost from particles during sampling and subsequently collected by an XAD-II trap and semi-volatile organic compounds retained by the quartz filters during sampling have been chemically characterized for <2.5 |im particles in BIG BOSS samples collected at Azusa in the Los Angeles Basin. The XAD-II sorbent beds included significant concentrations of aliphatic, acidic and aromatic organic compounds. Similar compounds were also detected in the GC-MS analysis of the filter extracts. However, the compounds retained by the filter were of higher molecular weight. The distribution of compounds lost from particles during sampling and remaining on the particles during sampling is illustrated by the GC/MS results for paraffinic compounds (Figure 5).

The pattern seen in Figure 5 is typical of results obtained for all classes of compounds and all samples studied to date. For those compounds which have been characterized, the envelopes of each class of compounds remaining in the particles and lost from the particles overlap. For each compound class, the more volatile compounds predominate in the material lost from the particles and collected in the XAD-II bed during sampling. In contrast, the higher molecular weight organic compounds are retained by the particles during sampling. For example, particulate n-tetradecane and n-pentadecane are found only in the XAD-II bed and not in the particles after sampling (Figure 5). Hydrocarbons lower in molecular weight than these two compounds are found in comparable concentrations in the XAD-II beds of both Samplers 1 and 2 of the BOSS (Figure 2) indicating they originate mainly from the breakthrough of some fraction of the gas phase component of these species. In contrast, n-tetracosane and higher molecular weight aliphatic hydrocarbons are retained by the particles during sampling and are not found in the XAD-II sorbent beds (Figure 5). Compounds of intermediate molecular weight, e.g. n-decosane, are partially lost and partially retained by the particles. Also illustrated

Figure 5 GC/MS data (m/z = 85) for paraffinic compounds; (A) retained by particles and (B) lost from particles during collection on a filter (from Tang, 1994).

by the GC-MS data is the increased tendency for lower molecular weight semi-volatile organic compounds to be retained by the particles during sample collection as the polarity of a given molecular weight compound increases. For example, n-heptadecane (MW 226) is largely lost from particles during sampling (Figure 5). However, lauric acid (MW 214) and fiuoranthene (MW 202) are largely retained by the particles during sampling.

Results for the determination of PAH compounds in indoor air obtained with the IOVPS and with a conventional filter-sorbent sampler are given in Figure 6. As indicated in Figure 6(A), about 90% of the phenanthrene, pyrene and chrysene are present in the gas phase. However, about 60% of the more volatile phenanthrene (MW 178) and pyrene (MW 202) are lost from the filter of the filter pack during sample collection. In contrast, the loss of the less volatile chrysene (MW 228) was negligible. These results are comparable to those given above for the Azusa study with the BIG BOSS.

Recent studies have indicated that the U.S. Environmental Protection Agency (EPA) PM10 air quality standard does not provide adequate human health protection because the fine particle (PM2.5) component of PM10 is related to observed health effects at concentrations substantially below the PM10 standard. As a result, EPA has promulgated a PM2.5 air quality standard. In order to implement the new PM2.5 standard, a Federal Reference Method (FRM) for fine particulate monitoring has been proposed (see Schaefer, 1997). The PM2.5 FRM is a single filter pack sampling method with gravimetric determination of the collected mass.

For the reasons outlined above, the FRM will tend to not measure semi-volatile fine particulate constituents. The amount of semi-volatile material is expected to be a substantial fraction of the total PM2.5

Figure 6 Retention and loss of particulate PAH compounds during sampling. (A) The lower concentrations determined by a filter pad, compared to IOVPS, is due to losses from particles during sampling. (B) Concentrations of both particle and gas phase PAH with the IOVPS.

NV ammonium NV organic nitrate

NV ammonium NV organic nitrate

Lost ammonium Lost SVOC

nitrate

Figure 7 Average composition of PM2.5 in Riverside CA, including semi-volatile ammonium nitrate and organic material lost during sampling from particles collected on a filter.

Lost ammonium Lost SVOC

nitrate

Figure 7 Average composition of PM2.5 in Riverside CA, including semi-volatile ammonium nitrate and organic material lost during sampling from particles collected on a filter.

mass observed in many urban areas. As a result, the proposed Federal Reference Method may under-determine Rne particulate mass. A comprehensive field study to evaluate the PC-BOSS and compare with results obtained by other PM2.5 sampling methods, including the FRM has been conducted in Riverside, California. Riverside was chosen for the study because high particulate pollution resulting from summer inversions is expected. Both annual and 24 hour maximum concentration of PM10 exceeded the federal standards in 1995 and high concentrations of particulate semi-volatile ammonium nitrate and organic materials are expected to be present in this area.

The average result for the determination of the composition of Rne particulate matter in Riverside during August and September 1997 are given in Figure 7. Substantial amounts of both ammonium nitrate and semi-volatile organic material were lost from the Rlters of both the PC-BOSS and the PM2.5 FRM. The average loss of ammonium nitrate (34%, 1.7 |gm~3) was smaller than that for the semi-volatile organic material (54% of total Rne particulate organic material, 9.5 |g m~3). As a result of the loss of these species, the PM2.5 FRM lost an average of 39% of the fine particulate material during the collection of the sample.

The results obtained in these three examples illustrate the importance of correctly sampling for semi-volatile particulate organic material.

See also: II/Extraction: Solid-Phase Extraction. Membrane Separations: Filtration. III/Atmospheric Analysis: Gas Chromatography: Supercritical Fluid Chromatography. Solid-Phase Extraction with Discs.

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