33
Table 3. Results obtained for two vehicle cabins using TD-SIFT-MS and targeting the Chinese VIAQ compound suite.
Compound
Acetaldehyde Acrolein Benzene
Formaldehyde Styrene Toluene
Xylenes + ethylbenzene
Tube 1 / ug m-3 1.6 1.9
0.043 0.20 4.8 2.0 9.5
were expected to give a range of VOC concentrations, were collected at several sites around Christchurch, New Zealand: a park, a petrol station, and a busy intersection at 9:30 am (peak) and 7:30 pm (off-peak).
Figure 3 compares the concentrations of various environmental pollutants determined following one-hour active sampling on Carbotrap® 300 thermal desorption tubes at various sites (a local park, peak and off-peak intersection, and a gas station). Concentration measurements to sub-µg m-3 are readily achievable using TD-SIFT-MS.
3. Vehicle interior air quality (VIAQ)
Thermal desorption tubes are the standard sampling approach for determination of VOC emissions in motor vehicle cabins - to assess the so-called vehicle interior air quality (VIAQ).
Although VIAQ assessments have been made for many years, the need to analyse automobile interiors on a much wider scale was prompted by a Chinese government guideline standard implemented on 1 March 2012, which regulates the VIAQ of new vehicles [13]. This standard outlines stricter limits on eight harmful substances in the vehicle cabin: formaldehyde, acetaldehyde, acrolein, benzene, ethylbenzene, xylene, styrene, and toluene.
Larger testing volumes arising from legislative changes result in significant cost increases for manufacturers when conventional laboratory-based technologies (high-performance liquid chromatography (HPLC) and GC/MS) are used. Additionally, because analysis is offline and turnaround time is slow, failed product is identified too late.
In contrast, SIFT-MS can greatly reduce turnaround through comprehensive analysis and reduced testing time. SIFT-MS can also be integrated into production processes. There are several advantages resulting from
Tube 2 / ug m-3 1.7 1.4
0.37 0.63 3.5 1.4 8.7
coupling TD with SIFT-MS for VIAQ testing: (i) TDTs are the standard sampling approach to whole cabin testing, together with sub- assemblies and components, (ii) distributed sampling is facilitated, and (3) with suitable sorbent beds, a wider range of compounds can be analysed from one tube.
Formaldehyde, in particular, is a challenge for retention on most sorbents, so recoveries were investigated early in this work. Recovery for all compounds was determined from the ratio of the TD-SIFT-MS results to that of VIAQ standard measured with direct whole-air analysis of the gas standard. Table 2 shows that all compounds, except formaldehyde, have acceptable and reproducible recoveries.
Breakthrough was evaluated by connecting two TDTs in series and loading at an 80-sccm flow rate for 60 s. The results calculated based on the ratio between the second and first tubes is shown in Table 2. The aromatic compounds have breakthrough values < 1%; this small breakthrough is likely due to differing backgrounds between individual tubes. Acetaldehyde and acrolein breakthrough tubes do not show typical desorption profiles but an apparent step in concentration during the 220°C hold time. Formaldehyde showed very high breakthrough and even the second tube likely had further breakthrough, as the combined recovery from the two tubes is still only ca. 40%. Future work will focus on utilising a more suitable sorbent for formaldehyde if one can be identified.
Linearity was investigated by varying the VIAQ standard concentration loaded at a consistent flow rate (80 sccm for 60 s) from dynamically diluted standard (using nitrogen as the make-up gas) to give each concentration level. Linearity (i.e. R2
the gas stream, despite the adsorption sites being readily saturated (based on the insensitivity to loading time). Future work will identify and evaluate other sorbents for improved formaldehyde recovery.
Table 3 summarises data obtained for two samples taken from passenger car cabins (3 L of cabin air was sampled volume).
TD-SIFT-MS provides rapid, highly sensitive analysis for VIAQ applications, with potential to cover the full eight compounds required by Chinese regulations in one analysis (sorbent permitting).
Conclusions
By analysing thermal desorption tubes as volatiles are desorbed, TD-SIFT-MS eliminates the throughput-limiting step for conventional TD-GC/MS analysis: the chromatographic separation. Since, for example, key performance criteria for the US EPA Compendium Method TO-17R [1] are achieved by TD-SIFT-MS, it can be applied with confidence to routine TDT analysis, but with results delivered at significantly higher sample throughputs than the conventional method.
Furthermore, TD-SIFT-MS can potentially further reduce costs through its ability to analyse a wider range of compounds in a single analysis. As illustrated by the VIAQ case study, detection of low molecular weight aldehydes, in addition to more routine VOCs (e.g. BTX), means that one analysis is required, rather than separate analyses for VOCs from TDTs by TD-GC/MS and aldehydes from DNPH tubes by HPLC- ultra violet analysis.
Acknowledgements
We thank Dirk Bremer, Eike Kleine- Benne, and Kurt Thaxton from Gerstel for cooperation on the thermal desorption unit integration.
References )
results ranged from 0.95 for formaldehyde to 0.995 for acrolein. Interestingly, all compounds show linear response, including formaldehyde with its poor recovery/ high breakthrough. This indicates that the formaldehyde is still in equilibrium with
1. United States Environmental Protection Agency. Compendium method TO- 17 - Determination of volatile organic compounds in ambient air using active sampling onto sorbent tubes. Retrieved from
https://www3.epa.gov/ttn/amtic/files/ ambient/airtox/to-17r.pdf (1999).
2. United States Environmental Protection Agency. Method 325B - volatile organic compounds from fugitive and area sources: sampler preparation and analysis. Retrieved
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