Anal. Chem. 2009, 81, 1595–1599
Real-Time Monitoring of Hazardous Air Pollutants Gregory J. Francis,†,‡ Vaughan S. Langford,† Daniel B. Milligan,† and Murray J. McEwan*,†,‡ Department of Chemistry, University of Canterbury, Christchurch, New Zealand, and Syft Technologies Limited, 3 Craft Place, Middleton, Christchurch, New Zealand Selected ion flow tube mass spectrometry (SIFT-MS) is a technique that offers real-time alternatives to existing methods for monitoring hazardous air pollutants (HAPs) in the environment using chemical ionization. The use of this technique requires knowledge of the kinetic parameters of the reagent ions H3O+, NO+, and O2+ that are most commonly used. We report here measurements with these reagent ions of kinetic parameters for 17 HAP molecules ranging from 1,1-dichloroethene to nitrobenzene. From these data, limits of quantitation are established for all 17 compounds on a commercial SIFT-MS instrument and are found to be well below the time-weighted averages required by legislating bodies for workplace conditions. Volatile organic compounds (VOCs) that are toxic and are dispersed in the local environment are primarily derived from anthropogenic origins. There are obvious advantages in being able to monitor the VOCs in the air we breathe to ensure standards of purity and that the VOC concentrations are below safe levels.1 The United States has implemented an amendment to the Clean Air Act in 1990 that lists 188 different compounds described as “hazardous air pollutants” or HAPs.2,3 Monitoring levels of HAPs is an important issue for occupational safety and health, and NIOSH (U.S. National Institute for Occupational Safety and Health) has developed guidelines to which workplaces in the United States must conform.4 The safety limits for exposure to an HAP can be defined as an 8 h time-weighted average (TWA), and this is the maximum concentration for a normal 8 h work day and a 40 h work week to which nearly all workers may be repeatedly exposed day after day without adverse effects.4 Having established the TWA limits for the 188 HAPs, it is necessary to implement convenient methods for monitoring their concentrations in the environment. The U.S. Environmental Protection Agency (EPA) has described a range of analytical methods using different sampling methodologies and analytical techniques that are approved to monitor the specified * Corresponding author. E-mail:
[email protected]. † University of Canterbury. ‡ Syft Technologies Limited. (1) Berry, K. D. Environ. Sci. Technol. 1986, 20, 647–651. (2) United States Environmental Protection Agency. The Plain English Guide to the Clean Air Act; Office of Air Quality Planning and Standards Pub EPA456/K-07-001. Research Triangle Park, NC, 2007. (3) United States Environmental Protection Air Act. 1990. (4) U.S. Department of Health and Human Service. Pocket Guide to Chemical Hazards, Number 90-117; National Institute for Occupational Safety and Health, 1990. 10.1021/ac802510h CCC: $40.75 2009 American Chemical Society Published on Web 01/21/2009
HAPs. These approved methods are described as TO-14A5 and TO-15.6 In both methods, samples may be collected into rigid stainless steel canisters that have a volume of ∼6 L. On arrival at the laboratory, GC or GC/MS analysis is the method of choice. Selected ion flow tube mass spectrometry (SIFT-MS) is a comparatively new analytical method that does not suffer from the sample preparation constraints that other analytical techniques require.6,7 In the SIFT-MS technique, mass-selected ions are admitted to a flow tube reactor where chemical ionization processes convert a small fraction of the mass-selected reagent ions into ion products of the analyte. The analyte concentration is found from the ratio of analyte product ions to reagent ions providing the kinetic parameters of the reagent ion chemistry with the analyte is known. The traditional reagent ions used for chemical ionization in SIFT-MS are H3O+, NO+, and O2+. The kinetics of these ions with most HAPs is not known. This present study examines the capability of SIFT-MS to quantify 17 HAP compounds using the reagent ions mentioned. As the kinetics of these 17 compounds have not been established we first establish these data and then use the known kinetic parameters to determine the limits of quantitation (LOQs) on a commercial SIFT-MS instrument. The sensitivity limits are then compared with the TWA limits required by the various health agencies. EXPERIMENTAL SECTION The instrument used in this investigation is a transportable commercial SIFT-MS instrument.8 The details of this instrument have been included in earlier papers9,10 and are similar to earlier SIFT instruments in principle but with some important differences. In the commercial instrument the flow tube is approximately 30 cm long and 5 cm in diameter and is bent through 180° so as to stack the downstream chamber above the upstream chamber, thereby reducing the footprint of the instrument. Two inert buffer gases are used to contain the ions. Helium is added through the (5) United States Environmental Protection Agency. Compendium Method TO14A. In Compendium Methods for the Determination of Toxic Organic Compounds in Ambient Air, 2nd ed.; Center for Environmental Research Information: Cincinnati, OH, 1999. (6) United States Environmental Protection Agency. Compendium Method TO15. In Compendium Methods for the Determination of Toxic Organic Compounds in Ambient Air, 2nd ed.; Center for Environmental Research Information: Cincinnati, OH, 1999. (7) Smith, D.; Spanel, P. Mass Spectrom. Rev. 2005, 24, 661–700. (8) Voice 100, Syft Technologies Ltd., Christchurch, New Zealand; http:// www.syft.com. (9) Francis, G. J.; Wilson, P. F.; Milligan, D. B.; Langford, V. S.; McEwan, M. J. Int. J. Mass Spectrom. 2007, 268, 38–46. (10) Milligan, D. B.; Francis, G. J.; Prince, B. J.; McEwan, M. J. Anal. Chem. 2007, 79, 2537–2540.
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Table 1. List of 17 Compounds Drawn from the TO-14A and TO-15 Methods Investigated in This Work uncertainty/%g a
b
compd
TO
mass
VP/Torr
1,1-dichloroethene (E/Z)-1,2-dichloroetheneh vinyl acetate 1,4-dioxane methyl methacrylate epichlorohydrin cumene N,N-dimethylformamide 2-chloroethyl ether chloroacetic acid urethane acrylamide N,N-dimethylaniline isophorone styrene oxide 1,2,4-trichlorobenzene nitrobenzene
15 14A 15 15 15 15 15 15 15 15 15 15 15 15 15 Both 15
96.94 96.94 86.09 88.11 100.1 92.52 120.2 73.09 143.0 94.50 89.09 71.08 121.2 138.2 120.2 181.5 123.1
500 335 83.0 37.0 28.1 12.0 3.2 2.7 0.71 0.69 0.54 0.53 0.50 0.38 0.30 0.18 0.15
-1 c
PA/kJ mol
813.9 797.4 831.4 791.6 887.5 765.4
d
800.3
f
IE/eV
method
TWA ppmv
kexp
BR
9.8 9.7 9.2 9.2 10.0 10.6 8.7 9.1
absolute absolute absolute absolute absolute absolute absolute absolute relative relative relative relative relative relative relative relative relative
100 200 8.5 25 100 5 50 10 5 0.3i 1.3j 0.1 5 25
±15 ±15 ±15 ±15 ±15 ±15 ±15 ±15 ±25 ±25 ±25 ±25 ±25 ±25 ±25 ±25 ±25
±20 ±20 ±20 ±20 ±20 ±20 ±20 ±20 ±30 ±30 ±30 ±30 ±30 ±30 ±30 ±30 ±30
10.8 10.2 9.8 7.1 9.1 9.1 9.0 9.9
870.7 941.1 893.5
e
5 1
a “TO” refers to the method of which the compound is a member: either the TO-14A, the TO-15, or both analytical methods. Each method is discussed in the text. b VP is the partial vapor pressure of the analyte in torr. The vapor pressures are sourced from the TO-14A and TO-15 method documents (refs 5 and 6). c Proton affinities sourced from the NIST Webbook (ref 11). d Ionization energies also sourced from the NIST Webbook (ref 11). e Definitions are discussed in the text. f The definition of TWA is discussed in the text; however, the specific value here is the U.S. NIOSH/OSHA workplace safety value (ref 12). g “kexp” refers to the uncertainty of the experimental rate coefficients of H3O+, NO+, and O2+ reacting with the neutral analyte. “BR” refers to the uncertainty on the measurement of the branching ratio. h The compound studied was a mixture of both the E and Z stereoisomers. The vapor pressure listed is a linear combination of both isomers in a 1:1 mixture. i The Great Britain TWA value has been used as the U.S. TWA value is not defined. j The Texas 30 min ambient air safety limit has been used as the U.S. TWA value is not defined.
Table 2. Rate Coefficients for the 17 HAP Compounds with H3O+ and Clusters, NO+, and O2+ Ions Measured at 298 K and 0.5 Torr kexp [kc]/10-9 cm3 molecule-1 s-1 c,d compd
µD/Debyea
R/Å3 a
H3Ob
H3O+ · H2O
H3O+ · (H2O)2
H3O+ · (H3O)3
NO+
1,1-dichloroethene (E/Z)-1,2-dichloroethenee vinyl acetate 1,4-dioxanef methyl methacrylate epichlorohydrin cumeneg N,N-dimethylformamide bis(2-chloroethyl) ether chloroacetic acid urethane acrylamide N,N-dimethylaniline isophorone styrene oxide 1,2,4-trichlorobenzene nitrobenzeneg
1.62 1.11 1.83 0.00 3.13 0.65 0.79 3.82 0.53 1.71 2.43 3.58 2.00 4.62 1.93 1.36 4.22
7.78 7.78 8.47 8.12 10.0 7.43 16.0 7.24 11.6 5.49 7.01 6.29 15.6 16.06 15.42 15.64 14.7
2.4 [2.6] 0.3 [2.2] 3.2 [2.9] 2.4 [1.7] 3.2 [3.0] 2.2 [1.8] 2.4 [2.6] 3.4 [5.1] [2.1] [2.5] [3.4] [4.8] [3.3] [5.8] [3.3] [2.8] [5.4]
