A Robust Solution for the VOC Analysis of Air Toxics

As evidence grows indicating the harmful effects of low-level exposure to hazardous air pollutants such as air toxics, there is an increasing need for analytical labs employing canisters to detect ever-lower levels of analytes in air samples while maintaining high standards of analysis and application flexibility. This article describes the benefits of the CIA Advantage™ from Markes International (Llantrisant, U.K.) for the analysis of air toxics in canister air samples. Excellent results were obtained for VOC analysis of 62 analytes over a wide concentration range, which, together with cryogen-free automated operation, offers substantial benefits to analytical chemists measuring air toxics using canisters.

The CIA Advantage is designed to allow a wide range of sample concentrations to be analyzed without the need for dilution. Using a combination of loop sampling, large-volume sampling, and the ability to split sample flows, component concentrations ranging from ppt to low-percent levels can be handled on one instrument. CIA Advantage systems comply with U.S. EPA Method TO-15 for air toxics, and up to 27 canisters can be accommodated, allowing round-the-clock, automated operation without the need for cryogen.

Experimental: TD-GC/MS

The experimental setup employed a CIA Advantage instrument in conjunction with GC/MS. For full details, see Markes’ TDTS 99 (available at http://www.markes.com/Downloads/Application-notes.aspx).

Results and discussion: Splitless analysis of air toxics

Figure 1 - Total ion chromatogram for a 1-L standard containing 1 ppb air toxics, analyzed splitless, cryogen-free, and in full-scan mode using TD-GC/MS (quad), according to U.S. EPA Method TO-15.

Air toxics comprise various compounds with a range of volatilities from C3 hydrocarbons to polysubstituted benzenes. They often have low concentrations, which frequently require that the whole sample be transferred to the GC (splitless analysis) to ensure maximum sensitivity. Figure 1 shows the total ion chromatogram for a splitless analysis of a 1-L standard containing 1 ppb air toxics, according to U.S. EPA Method TO-15. Clearly, very good peak shape can be seen for the more volatile compounds, which is important when carrying out splitless analyses.

VOC analysis of a wide range of samples

A key benefit of the CIA Advantage for the analysis of these challenging samples is the backflush focusing trap (i.e., the analytes are injected into the GC in the opposite direction in which they are absorbed). This makes it possible to use more than one sorbent, allowing analysis of a wide range of compounds in a single run, as shown in Figure 1 (where a three-bed sorbent trap was employed).

Handling trace- and high-concentration analytes in air samples

Figure 2 - Responses for loop sampling (at 0.5 and 2 mL) and mass-flow-controlled sampling (at 25, 50, 100, and 200 mL) for selected compounds from the TO-15 standard, showing that a single curve can be fitted to both sets of points. Vertical scaling has been applied to some of the plots to allow them all to be shown on the same axes.

Ambient air samples can contain air toxics at a wide range of concentrations. The CIA Advantage-HL model can easily handle such situations because it can sample at both low volumes (using a sample loop) and high volumes (using a mass flow controller). Figure 2 shows that both low-volume and high-volume samples can be compared using the same calibration curve, for seven example compounds from the TO-15 standard.

Negligible carryover for toxic air pollutants

Figure 3 - Chromatogram of a 40-mL sample of a 1-ppm 62-component TO-15 standard (i.e., a heavily overloaded sample). Inset: Expansion of the peaks for 1,2,4-trichlorobenzene and hexachlorobuta-1,3-diene, with the chromatogram of a subsequent 500-mL nitrogen blank added in black, showing very low carryover, even for these high-boiling components.

For a system to be suitable for analyzing both trace- and high-concentration samples, it is critical that sample carryover is minimal. Otherwise, significant time can be spent cleaning the system, or (more importantly) the concentration of target compounds in low-concentration samples can be overestimated. Figure 3 demonstrates that the CIA Advantage recovers immediately from being subjected to an overloaded sample of TO-15 air toxics. Taking the stickiest and highest-boiling compound in the list—1,2,4-trichlorobenzene—only 185 pg was present in the subsequent blank run, compared to 303 ng in the original. Carryovers for the 62 compounds were in all cases <0.07%, with 82% of the compounds not being detected at all in the blank run.

Excellent peak shape

Figure 4 - A low-level rural air sample, highlighting the excellent peak shape achieved in the extracted-ion chromatograms of isopropanol and toluene following splitless analysis on the CIA Advantage.

It is also important for the system to be able to analyze samples without a split, to guarantee maximum sensitivity for low-concentration samples. The CIA Advantage can do this without compromising the quality of the peak shape, as shown in Figure 4.

Conclusion: Air toxic analysis results

The CIA Advantage provides excellent results over the full range of air toxics sampled from canister air. Cryogen-free operation reduces running costs, while the heated internal lines and efficient purge steps combine to avoid the problem of carryover, even with less volatile analytes.

This negligible carryover means that canister analysis can be confidently undertaken on samples of unknown concentration, facilitating automation and therefore increasing productivity. Furthermore, the CIA Advantage is fully compliant with U.S. EPA Method TO-15, making it especially well-suited to the analysis of air toxics.

Dr. Nicola Watson is Environmental Specialist, Markes International, Gwaun Elai Medi Science Campus, Llantrisant, RCT, CF72 8XL, U.K.; tel: +44 (0)1443 230935; fax: +44 (0)1443 231531; e-mail: enquiries@markes.com.