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32 May / June 2021


Table 2. Recoveries and breakthroughs determined using direct analysis- and TD-SIFT-MS for the Chinese- regulated VIAQ compounds using TDTs packed with Carbotrap® 300 sorbent.


Compound


Acetaldehyde Acrolein Benzene


Ethylbenzene + Xylene Formaldehyde Styrene Toluene


Recovery


1.05 ¬± 0.07 1.03 ± 0.01 0.96 ± 0.03


0.916 ± 0.001 0.21 ± 0.02


0.880 ± 0.009 0.837 ± 0.004


Carbopack™ X (Method 325). The temperature was then ramped from 60 to 280°C at 120°C min-1 120 seconds.


% Breakthrough 11% ± 3%


3.0% ± 0.6%


0.07% ± 0.01% 0.37% ± 0.06% 71% ± 9%


0.21% ± 0.07% 0.29% ± 0.01%


, where it was held for


In the environmental monitoring case study, the nitrogen desorption gas was utilised at a fl ow rate of 41 mL min-1


, a constant pressure


of 54 kPa. The thermal desorption split was 1:1. The initial TD temperature was 60°C for 60 s, followed by a ramp rate of 240°C min-1


and a hold temperature of 300°C for 30 s. This more optimised desorption program enabled a total runtime of less than 3 min.


For the VIAQ study, the custom gas standard was loaded onto TDTs packed with Carbotrap® 300 at a fl ow rate of 80 sccm for 60 seconds. The program used with the thermal desorber had an initial temperature of 20°C with a 30-s hold time, followed by a ramp to 220°C at 60°C min-1


, and a 2-min


hold time at 220°C. The TD operating pressure was 4 kPa which gave a sample fl ow rate of 30.7 sccm.


Results and Discussion


This section fi rst presents a brief summary of performance of TD-SIFT-MS in the context of a regulatory environmental method, before presenting data that demonstrate application to measurement of volatiles in outdoor and indoor environments.


1. Validation of TD-SIFT-MS


In this section, benchmarking of the analytical performance of TD-SIFT-MS is conducted by comparison with acceptance criteria for two GC-based US EPA thermal desorption methods: (i) Compendium Method TO-17R (active sampling of the more volatile organic components of the hazardous air pollutant (HAP) list [1]), and (ii) Method 325 (passive sampling for benzene over a two-week period at the fenceline of oil refi neries [2]).


Figure 1 shows an example desorption profi le for TD-SIFT-MS corresponding to a


,


loading of 52 ng of each target compound. The SIFT-MS instrument measures the target compounds in real-time as they are desorbed from the TDT - there is no cryofocusing step. This means that concentration is calculated and plotted initially in volume-to-volume units as a function of time. This desorption ‘envelope’ is integrated by software to determine the mass of target compound on the sorbent tube. Note that desorption is completed in less than three minutes. With rapid cooling of the thermal desorption unit, throughputs of up to 20 tubes per hour can be realised.


Measurement linearity is summarised in Table 1 for TD-SIFT-MS measurements made over the concentration range required by the US EPA TO-17R method (0.5 to 100 ppbV). These results easily meet acceptance criteria, with correlation coeffi cients (R2 the high 0.999 range.


) in


Table 1 also gives the method detection limits (MDLs) in terms of minimum detectable mass on the Tenax® TA sorbent tube (in nanograms). Assuming that one litre of air is sampled, the data correspond to MDLs of 70.8, 73.1, and 83.8 pptV for benzene, toluene, and xylene, respectively. This is well below the TO-17R requirement of <500 pptV.


MDLs for Method 325, which involves two weeks of passive sampling of benzene onto specifi ed TDTs at oil refi nery boundaries yields MDLs shown in Table 1 (these are based on Carbopack™ X uptake volumes for passive sampling over two weeks). In addition to benzene, results for toluene and xylene performance are included. Again, for Method 325 TD-SIFT-MS easily meets the MDL requirement of <50 pptV. On the tube itself, these concentrations correspond to benzene, toluene, and xylene MDLs of 0.09, 0.21, and 0.33 ng, respectively.


Finally, Table 1 summarises the precision and preliminary recovery data determined as part of the study at about double the MDL for the TO-17 method. A question for further work is why some recoveries are poor (lower than 80%).


Overall, the performance of TD-SIFT-MS meets the criteria in Section 14 of US EPA Method TO-17R [1].


2. Environmental monitoring


In this section, results are presented that apply the TD-SIFT-MS technique to environmental monitoring - in this case, using the active sampling approach given in US EPA Compendium Method TO-17. In addition to BTX, styrene and 1,3-butadiene were targeted in this campaign.


Prior to undertaking the sampling campaign, the repeatability of TD-SIFT-MS for analysis of environmental pollutants was assessed by desorption of replicate tubes loaded from the gas standard containing benzene, toluene, ethylbenzene, xylene (BTEX) and styrene in nitrogen. The results are summarised in Figure 2. It is evident that TD- SIFT-MS analysis is highly repeatable, with RSD values less than 5% for all compounds.


Environmental air samples which


Figure 3. Environmental pollutant concentrations at four sampling sites determined using TD- SIFT-MS.


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