Air Monitoring 41
Figure 1. EtO MDL in the presence of 2.5 ppbV acetaldehyde as a function of method length. The HON action level is marked by the horizontal line.
DISCUSSION
The MDLs presented in Table 2 and Table 3 refl ect the capabilities of single-compound SIFT-MS methods conducted over one minute (30 minutes for EtO). All the HON compounds have MDLs below the EPA action level for fenceline monitoring.
The new EPA regulations for fenceline monitoring of the HON compounds stipulate that alternative monitoring methods must achieve MDLs at least one-third of the action level for the monitored compounds. For any SIFT-MS method, lower MDLs are easily achieved by increasing the duration of the monitoring method. The improvement in MDL is approximately proportional to the inverse square root of the method length.
EtO provides a great example of how adjusting method length can signifi cantly enhance an MDL. A substantial dataset of real EtO one-minute scan data was aggregated over time, allowing for the calculation of MDLs for these aggregated datasets. For EtO, the MDL with a 1-minute method exceeds the HON action level. Figure 1 demonstrates the enhancement in the EtO MDL achievable by increasing the method duration.
The MDL for EtO drops below the HON action level when the method length exceeds 30 minutes. To achieve an MDL of one-third of the action level, the method length would need to be extended to about 3 hours. This duration is signifi cantly shorter than the 24- hour average concentration measured by the standard canister method for EtO, allowing for multiple high- precision samples to be collected within the required 24- hour period.
For all the HON compounds, SIFT-MS has dramatically better temporal resolution than the EPA specifi ed methods – passive and canister sampling – facilitating better source identifi cation in the event of the concentration exceeding the action level.
The EPA procedure states that MDLs can be estimated by examining the region of the calibration where there is a signifi cant change in sensitivity, i.e., a break in the slope of the calibration. To support the MDLs in Table 2 and Table 3 which were measured at a single concentration, low concentration calibrations were also performed for each compound down to low pptV levels. An example is shown in Figure 2 for 1,3-butadiene.
Figure 3. Low concentration calibrations using simple one- minute methods exhibit excellent linearity down to low pptV concentrations
of 220 pptV. There is a lower action limit (83 pptV) for facilities collocated with neoprene productions sources. Similar to the EtO approach, the chloroprene MDL can be reduced to below this action level by extending method length to fi ve minutes.
Syft Tracer’s fl exibility and ability to swiftly monitor diverse chemical species at low pptV concentrations makes it the ideal choice for the monitoring of trace levels of hazardous air pollutants. SIFT-MS is easy to automate, provides a simple data stream, and can operate unmonitored. It is the optimal solution for a facility aiming to go beyond basic compliance and achieve a higher standard of operational excellence. With a substantial database of ion-molecule reactions covering environmentally signifi cant compounds, SIFT-MS is well- positioned to adapt to future regulatory challenges.
CONCLUSIONS
SIFT-MS measurements offer signifi cant advantages over traditional techniques that require long sample collection times and off-site laboratory analysis. Unlike these methods, which can miss short-term fl uctuations in pollutant concentrations, SIFT- MS provides immediate and continuous data, allowing for timely detection of hazardous compounds. This capability enhances the precision of source identifi cation and enables quicker responses to potential environmental threats, ensuring more effective monitoring and protection of public health.
REFERENCES
Figure 2. 1,3-Butadiene low concentration calibration showing excellent linearity down to low pptV concentrations.
Concentrations measured using O2 ⁺ and NO⁺ exhibit excellent
linearity down to less than 20 pptV. This strong correlation shows that the 1,3-butadiene MDL presented in Table 2 is both realistic and attainable.
Additional examples of low concentration calibrations showcasing the linearity of SIFT-MS down to low pptV concentrations are presented in Figure 3.
The measurement of Chloroprene’s MDL was recently completed and recorded as 145 pptv which is below the HON action limit
Langford VS, Cha M, Milligan DB, Lee J (2023a). Adoption of SIFT-MS for VOC pollution monitoring in South Korea. Environments 10, 201.
https://doi.org/10.3390/environments10120201
US EPA (2016). EPA 821-R-16-006 - Defi nition and Procedure for the Determination of the Method Detection Limit, Revision
2. Swift SJ, Dryahina K, Lehnert A-S, Demarais N, Langford VS, Perkins MJ, Silva LP, Omezzine Gnioua M and Španěl P (2023). Accurate selected ion fl ow tube mass spectrometry quantifi cation of ethylene oxide contamination in the presence of acetaldehyde. Anal. Methods 15, 6435-6443.
https://doi.org/10.1039/D3AY01036H
Langford VS, Dryahina K, Španěl, P (2023b). Robust automated SIFT-MS quantitation of volatile compounds in air using a multicomponent gas standard. J. Am. Soc. Mass Spectrom. 34, 2630−2645.
https://doi.org/10.1021/jasms.3c00312
APPENDIX
) obtained from a microwave discharge in air are now applied in commercial SIFT-MS instruments. These reagent ions react with volatile organic compounds (VOCs) and other trace analytes in well-controlled ion-molecule reactions, but they do not react with the major components of air (N2
HOW SIFT-MS WORKS SIFT-MS uses soft chemical ionization (CI) to generate mass- selected reagent ions that can rapidly react with and quantify VOCs down to part-per-trillion concentrations (by volume, pptV). Up to eight reagent ions (H3 NO3
O⁺, NO⁺, O2⁺, O- - , OH- , O2 -, NO2 - and , O2 and Ar). This
enables direct, real-time analysis of air samples to be achieved at trace and ultra-trace levels without pre-concentration. Rapid switching between reagent ions provides high selectivity because the multiple reaction mechanisms give independent measurements of each analyte. The multiple reagent ions frequently remove uncertainty from isobaric overlaps in mixtures containing multiple analytes. To calculate the concentration of the compounds, the reaction rate constant (k) from the library entry is used (Langford et al. (2023b)).
Author Contact Details Samuel Edwards, Leslie Silva, Nathan Hoppens, William Pelet, Finlay Aitcheson • Syft Technologies Ltd • Address: 68 St Asaph Street, Christchurch Central, Christchurch - New Zealand • Tel: +64 3 338 6701 • Email:
info@syft.com • Web:
www.syft.com
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