Environ. Sci. Technol. 1996, 30, 2687-2703
Air Quality Model Evaluation Data for Organics. 2. C1-C14 Carbonyls in Los Angeles Air ERIC GROSJEAN,† D A N I E L G R O S J E A N , * ,† MATTHEW P. FRASER,‡ AND GLEN R. CASS‡ DGA, Inc., 4526 Telephone Road, Suite 205, Ventura, California 93003, and Department of Environmental Engineering Science, California Institute of Technology, Pasadena, California 91125
As part of a larger experiment that provides a comprehensive set of observations to be used for testing air quality models for organic air pollutant transport and reaction, ambient air samples have been collected using DNPH-coated C18 cartridges at four urban locations and one background location in the Los Angeles area and have been analyzed for carbonyls as their DNPH derivatives. Twenty-three carbonyls have been identified and their concentrations measured: 14 aliphatic aldehydes (from formaldehyde to tetradecanal), two aromatics (benzaldehyde and m-tolualdehyde), three ketones (acetone, 2-butanone, and cyclohexanone), one unsaturated carbonyl (crotonaldehyde), and three dicarbonyls (glyoxal, methylglyoxal, and biacetyl). Another 19 carbonyls have been tentatively identified including 11 low molecular weight (MW) and intermediate MW carbonyls (of which four may be due to reactions of ambient NO, NO2, and ozone with DNPH on the sampling cartridge), four C4-C6 dicarbonyls present at trace levels, and four high MW aliphatic carbonyls (C15-C18). Total carbonyl concentrations (4-h samples) averaged 22 ppb at the urban locations and 3.5 ppb at the background location and were highest (29 ppb) at the Azusa, CA, monitoring site that is downwind of downtown Los Angeles. Formaldehyde (urban average 5.3 ppb), acetaldehyde, and acetone accounted for 24%, 18%, and 7%, respectively, of the total carbonyls on a ppbv basis. The nine high MW carbonyls (C8-C14) accounted for 11-14% of the total carbonyls. The acetaldehyde/formaldehyde concentration ratio averaged 0.75 at the urban locations. Ranking of the measured carbonyls with respect to removal of the hydroxyl radical showed acetaldehyde to be the most important followed by formaldehyde and nonanal. Diurnal and spatial variations in ambient carbonyls levels are briefly examined and appear to be consistent with both direct emissions and insitu formation during eastward transport over the urban area.
S0013-936X(95)00758-9 CCC: $12.00
1996 American Chemical Society
Introduction Carbonyls are of critical importance in atmospheric chemistry as products of the photochemical oxidation of virtually all hydrocarbons and as precursors to free radicals, ozone, and peroxyacyl nitrates (1-5). Carbonyls are also important species against which the predictions of computer-based kinetic air quality models that attempt to describe ozone formation in the atmosphere can be tested (6-10). Several carbonyls including formaldehyde, acetaldehyde, and acrolein have received regulatory attention as eye irritants, toxic air contaminants, mutagens, and carcinogens (1114). In spite of their importance, carbonyls have not been measured historically as part of federal, state, or local air pollution monitoring networks. Three carbonyls (formaldehyde, acetaldehyde, and acetone) are currently monitored in the many urban areas of the United States that exceed the national ambient air quality standard for ozone (15). In this paper, we describe the methods and results of a study in which ambient levels of carbonyls have been measured at five southern California locations during the 1993 smog season. This study of ambient carbonyls has been carried out as part of a larger effort whose main objective is to acquire a comprehensive data base for organic air pollutants that can be used to test the performance of a photochemical airshed model designed to predict both particulate and gaseous organic concentrations directly from data on emissions from sources (16). The carbonyls measured in this study are in the range C1-C14, i.e., from formaldehyde to tetradecanal. These carbonyls have been identified using a combination of methods including three liquid chromatography (LC) methods with isocratic elution and single wavelength detection, one LC method with gradient elution and diode array detection, and one method involving chemical ionization mass spectrometry. This study was motivated by several considerations. Past studies of ambient carbonyls in urban air have been limited to formaldehyde and acetaldehyde and less frequently to several of the low molecular weight carbonyls (17-25). To our knowledge, higher molecular weight carbonyls have not been measured. The last two studies of ambient carbonyls in southern California have been carried out more than 5 years ago (22, 23). More recent information is needed to assess, for example, current population exposure (1114) and the contribution of carbonyls to photochemical smog formation (6, 8-10) as well as to examine the impact on ambient levels of carbonyls of new air pollution rules regarding exhaust reactivity (25) and recent changes in vehicle fuel composition (26). Of special interest with respect to current levels of carbonyls in urban air are oxygenated compounds such as alcohols and ethers whose use as vehicle fuel additives may result in higher levels of carbonyls in exhaust emissions (24-26).
Experimental Methods Field Operations. Ambient air samples were collected on Sept 8-9, 1993, at five southern California locations: a * Corresponding author telephone: (805) 644-0125; fax: (805) 6440142. † DGA, Inc. ‡ California Institute of Technology.
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source-dominated location (Long Beach, 3648 N. Long Beach Blvd.), a downtown location (Los Angeles Dept. of Water and Power Building 3, 1630 N. Main St.), a neardownwind location (Azusa, 803 N. Loren Ave.), a smog receptor site (Claremont, instrument trailer located near Foothill Blvd.), and a “background” location (San Nicolas Island, 130 km WSW of Long Beach and generally upwind of the urban Los Angeles area). This 2-day period was selected for detailed examination because it represented a severe photochemical episode in the Los Angeles area. Ozone concentrations peaked above 300 ppb in the eastern portion of the air basin near Claremont on Sept 9, while 1-h average nitrogen dioxide concentrations exceeded 200 ppb at Diamond Bar upwind of Claremont that day. At each location, air was drawn directly (without inlet sampling lines in order to minimize carbonyl loss) through C18 cartridges coated with DNPH. The cartridges were connected, via 6 mm diameter Teflon sampling lines, to portable, dual sampling units that housed calibrated flowmeters, vacuum gauges, timers, sampling pumps, and associated electrical and tubing connections (17, 22). The sampling duration was 4 h (0000-0400, 0600-1000, 12001600, and 1800-2200 h PDT), and the sampling flow rates were 0.55-0.77 L min-1. The volumes of air sampled were 0.14-0.21 m3. At the background location, samples of 5-, 8-, and 11-h duration were collected from Sept 7 to Sept 9, 1993. The volumes of air sampled were 0.16-0.37 m3. All flowmeters employed in the five dual sampling units were calibrated three times, i.e., before, during, and after the field measurements. The flowmeter calibrations were carried out using a flowmeter previously calibrated at EPA as the reference flowmeter. Prestudy and poststudy calibrations agreed within 2%. The EPA-calibrated flowmeter and one of the field flowmeters were subsequently audited using a certified digital bubble flowmeter. Linear regression of the audit data (certified bubble flowmeter/ other device) showed good agreement with slopes of 1.051 ( 0.005 for the EPA-calibrated flowmeter (R ) 1.0, n ) 7, range 38-3620 mL min-1) and 1.067 ( 0.026 for the field flowmeter (R ) 0.999, n ) 7, range 99-3610 mL min-1). Sampling Cartridges. Batches of C18 cartridges (SepPak short body “classic”, Waters Chromatography Division, Millipore Corp.) were coated with acidic solutions of twicerecrystallized 2,4-dinitrophenylhydrazine (DNPH) in HPLCgrade acetonitrile (27). After being washed with acetonitrile, coated with freshly prepared acidic DNPH, and allowed to dry in carbonyl-free air, each cartridge was sealed with Teflon tape, wrapped in acetonitrile-washed aluminum foil, and stored in a glass vial with a Teflon-lined cap. Batches of vials were sent to the field sites and back in zip-lock bags that contained DNPH-coated filters; these filters acted as passive samplers for carbonyl impurities. All cartridges were stored refrigerated in the dark at the field locations and upon return to the laboratory. Shipping controls and field controls were included at all field locations. Following sample collection, the cartridges were eluted with 2 mL of acetonitrile (27). The acetonitrile extracts were analyzed by liquid chromatography as described below. Carbonyl-DNPH Standards. All solvents employed were HPLC-grade (Baker, Aldrich, VWR Scientific). DNPH (Aldrich) was recrystallized twice from hot ethanol, rinsed three times with ethanol, and dried under slight vacuum in a dessicator. Carbonyls were from commercial sources (Aldrich, Sigma, Fluka, or PolyScience, highest purity available). Carbonyl-2,4-dinitrophenylhydrazones were
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prepared and recrystallized as described previously (28). Calibration standards were prepared by dissolving the solid hydrazones in acetonitrile. Calibration curves, i.e., absorbance at a given wavelength (peak height or peak area) vs concentration, were constructed as described previously (17-22, 24, 28). The slopes of these calibration curves, i.e., response factors, were used to calculate carbonyl concentrations in the ambient air samples. Calibration standards were included daily along with each batch of samples analyzed. Liquid Chromatography Analysis. Acetonitrile solutions of carbonyl-DNPH standards and of air samples were analyzed by liquid chromatography (LC) with UV detection (17-22, 24, 28). Two LC systems and four analytical methods were employed. The four separation methods included three isocratic elution methods with single wavelength detection at 360, 385, or 430 nm and one gradient elution method with diode array detection (200600 nm). The first LC system included a Valco injection valve with a 20-µL sample loop, a SSI Model 300 pump, a Perkin Elmer Model LC 75 UV-visible detector, and a Hitachi Model D-2000 integrator/recorder. The column used was a C18 Axxiom ODS (Cole Scientific Inc.) with a C18 guard cartridge (Brownlee Applied Biosystems). The detection wavelengths were 360, 385, and 430 nm. Three isocratic elution modes were employed: 53:47 by volume acetonitrile-water eluent, eluent flow rate 1.4 mL/min, column pressure 1700 psi (Method A); 80:20 by volume acetonitrile-water, eluent flow rate 2.0 mL/min, column pressure 1900 psi (method B); and 54:36:10 by volume acetonitrile-water-tetrahydrofuran, eluent flow rate 1.2 mL/min, column pressure 2200 psi (method C). Method A allowed the resolution of the DNPH derivatives of many carbonyls up to C9 (28). Method B yielded shorter retention times than method A for DNPH derivatives of higher molecular weight carbonyls, thus resulting in better limits of detection (by a factor of about 10 on a peak height basis) for these carbonyls. Figure 1 shows the chromatogram of a mixture of the DNPH derivatives of the n-C10, n-C11, n-C12, n-C13, and n-C14 aldehydes using method B. Method C enabled us to separate the following sets of DNPH derivatives that co-eluted or were partially resolved using method A: n-pentanal and glyoxal; n-hexanal and methylglyoxal; the three C3 carbonyls acetone, acrolein, and propanal; and the four C4 carbonyls crotonaldehyde, 2-butanone, methacrolein, and n-butanal. The second LC system included a Hewlett Packard (HP) Model 1090/Series II liquid chromatograph and a HP 1090 diode array detector. In order to obtain directly comparable results, all LC analyses were carried out using the analytical column that had been employed with the other LC system using methods A, B, and C. The column temperature was 40 °C, the eluent flow rate was 1.4 mL min-1, and the injection volume was 20 µL. Acetonitrile and water were sparged with helium for 10 min. To complement the three isocratic methods A, B, and C, a gradient elution method (method D) was employed with 49:51 by volume CH3CNH2O for the first 26 min, from 49:51 CH3CN-H2O to 100% CH3CN over the next 14 min, and 100% CH3CN afterwards. The 290-600 nm UV-visible spectra were recorded online using the diode array detector. We have previously reported on the 290-600-nm spectra of C1-C10 carbonylDNPH standards (28, 29). The spectra of the DNPH derivatives of decanal, undecanal, dodecanal, tridecanal,
FIGURE 1. (a) Chromatogram of a mixture of the DNPH derivatives of (from left to right) decanal (retention time ) 6.05 min), undecanal (8.26 min), dodecanal (11.39 min), tridecanal (15.88 min), and tetradecanal (22.23 min) using method B (detection wavelength ) 360 nm). (b) Absorption spectrum (290-600 nm) of tetradecanal-DNPH recorded using method D. (c) Chemical ionization mass spectrum of a mixture of the DNPH derivatives of decanal, undecanal, dodecanal, tridecanal, and tetradecanal. The corresponding protonated molecular ion peaks are m/e ) 337, 351, 365, 379, and 393, respectively.
and tetradecanal have not been reported before. The 290600-nm absorbance spectrum of tetradecanal-DNPH is shown in Figure 1. Spectra of the C10-C13 carbonyl-DNPH derivatives, not shown, were similar. Absorption maxima were 360 nm for n-C13-DNPH and 361 nm for n-C10, n-C11, n-C12, and n-C14-DNPH. These absorption maxima are consistent with those of the DNPH derivatives of lower molecular weight monofunctional aliphatic aldehydes (28, 29). Mass Spectrometry Analysis. Mass spectrometry analyses were carried out using a HP 1090/Series II liquid chromatograph, a HP Model 5989 A mass spectrometer, and a HP Model 59980-B particle beam interface. Mass spectra were recorded in the chemical ionization (CI) mode with methane as the reagent gas (30). The eluent was 100% CH3CN, the eluent flow rate was 0.4 mL min-1, and the instrument’s autosampler was used to inject 20-µL aliquots of the samples directly into the particle beam interface. No liquid chromatography column was used. This approach
reflected the following considerations: nanogram detection limits were needed for mass spectrometry detection of carbonyls in the ambient air samples; these low detection limits could be achieved with 100% CH3CN eluent but not with water-CH3CN eluent due to the strong negative effect of water on the sensitivity of the particle beam interface; with 100% CH3CN eluent, many low molecular weight carbonyls co-elute or are poorly resolved on the LC column, which is therefore of limited usefulness under these conditions; the LC column was not needed to obtain mass spectra of carbonyl-DNPH standards (28, 30). Methane CI mass spectra of the DNPH derivatives of C1-C9 carbonyls have been described in previous work (28, 30). Spectra of the C10-C14 n-alkanal-DNPH were recorded in this work. Consistent with data for lower molecular weight homologues (28, 30), these spectra contained the protonated molecular ion (MH, where M is the molecular weight of the carbonyl-DNPH) as the most abundant fragment. Thus, as is shown in Figure 1, the CI mass
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TABLE 1
Summary of Carbonyl-DNPH Retention Times and Response Factors for Isocratic Elution Methods A, B, and C method Aa
method Bc
retention time (min) response formaldehyde acetaldehyde acetone acrolein propanal glyoxylic acid crotonaldehyde 2-butanone methacrolein n-butanal 2-methylpropanal 3-penten-2-one pyruvic acid benzaldehyde cyclohexanone 3-methyl-2-butanone 3-methylbutanal 2,2-dimethylpropanal 2-methylbutanal 2-ethylacrolein n-pentanal glyoxal glutaraldehydef m-tolualdehyde n-hexanal methylglyoxal 2-methylcyclohexanone nopinone 2,3-butanedione n-heptanal 4 acetyl-1-methylcyclohexene n-octanal 2,3-pentanedione n-nonanal n-decanal n-undecanal n-dodecanal n-tridecanal n-tetradecanal
3.20 ( 0.16 4.17 ( 0.24 5.49 ( 0.37 5.92 ( 0.43 6.04 ( 0.36 7.69 ( 0.11 7.69 ( 0.25 8.88 ( 0.70 8.27 ( 0.83 9.14 ( 0.83 8.90 ( 0.55 10.4 ( 0.1 11.2 ( 1.2 12.1 ( 1.6 13.0 ( 1.3 12.5 ( 0.1 12.1 ( 0.6 12.4 ( 1.2 12.5 ( 1.2 13.0 ( 1.3 13.1 ( 0.9 13.5 ( 1.7 14.7 ( 1.5 17.9 ( 1.0 21.4 ( 3.0 23.0 ( 2.8 25.5 ( 2.6 29.4 ( 3.0 30.8 ( 3.1 34.0 ( 5.3 39.9 ( 4.0 61.9 ( 2.3 62.9 ( 6.4 99.1 ( 4.1
factord
238 ( 33 153 ( 18 101 ( 14 128 ( 6 89.3 ( 9.9 66.9 ( 8.6 45.0 ( 1.3 47.8 ( 4.8 46.5 ( 6.0 52.2 ( 1.6 37.2 ( 0.0 34.1 ( 4.6 34.6 ( 4.2
method Cb
retention time (min) response factor retention time (min) response factor
1.27
5.24 ( 0.00 7.08 ( 0.01 9.70 ( 0.00 10.58 ( 0.01 11.65 ( 0.01
152.0 ( 3.8 91.3 ( 2.8 63.2 ( 1.2 70.9 ( 0.9 48.7 ( 0.9
1.55
15.77 ( 0.04 16.59 ( 0.02 17.64 ( 0.02 18.61 ( 0.03
44.3 ( 1.4 22.7 ( 0.5 40.7 ( 0.9 25.0 ( 0.5
1.70
24.17 ( 0.06 24.58
18.7 ( 0.5 21.3 ( 2.2
30.5 ( 0.1 36.5 ( 0.3
13.3 ( 0.0 39.3 ( 4.0
37.5 ( 0.2 50.4 ( 0.1 53.2 ( 5.4
10.7 ( 0.0 6.7 ( 0.0 10.9 ( 1.1
1.01 1.12 1.25
387 295 212
28.1 ( 6.3 83.3 ( 5.9e
1.75
20.8 ( 2.9 16.1 ( 2.1 21.8 ( 1.9 4.2 ( 0.4 6.8 ( 0.4
2.15 1.65
83 137
2.62
74
2.72 2.96 3.53
66 56 45
9.6 ( 1.2 7.5 ( 0.8 4.5 ( 0.5 24.0 ( 3.6 3.0 ( 0.3
4.71 6.13 8.4 11.5 16.1 22.5
31.9 22.5 15.0 11.0 7.5 4.5
a 53:47 acetonitrile-water. b 54:36:10 acetonitrile-water-tetrahydrofuran. c 80:20 acetonitrile-water. attenuation setting ) 5. e Average value, nonlinear response, see text. f Mono-DNPH derivative.
spectrum of a mixture of the five C10-C14 carbonyl-DNPH includes major peaks at m/e ) 337 (MH of decanal-DNPH), 351 (C11-DNPH), 365 (C12-DNPH), 379 (C13-DNPH), and 393 (C14-DNPH). The smaller peaks at m/e ) 338, 352, 366, 380, and 394 result from the isotopic contribution of 13C to the MH peak (28, 30). The peak at m/e ) 199 is the protonated molecular ion of the DNPH reagent (28). Retention Times and Absorbance Ratios. Confirmation of the carbonyl structure involved the comparison of retention times, 200-600 nm UV-visible spectra, and chemical ionization mass spectra to those of a data library constructed in our laboratory as part of past studies (28) and augmented by the data for C10-C14 carbonyls obtained in this study. From the UV-visible spectra, 430 nm/360 nm and 385 nm/360 nm absorbance ratios were calculated using the four elution methods. These absorbance ratios have a diagnostic value to identify aliphatic carbonyls (which absorb with maxima near 360 nm), aromatic carbonyls (maxima near 385 nm), and dicarbonyls (maxima near 430 nm) in ambient air samples that contain unknown carbonyls (28). Retention times for the three isocratic elution methods A, B, and C are compiled in Table 1. Absorption maxima and 430/360 nm absorbance ratios measured using isocratic elution have been reported previously (28). Chemical ionization mass spectra have also been described previously (28, 30). Retention times, 385/360 nm absorbance ratios,
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d
Peak height (mm) for 1 µg mL-1 carbonyl,
and 430/360 nm absorbance ratios for the gradient elution method (method D) have not been reported before and are listed in Table 2. Gradient elution, isocratic elution with three eluents, 200-600 nm diode array absorption spectra, and chemical ionization mass spectra were also used to verify that peaks assigned to a single carbonyl-DNPH did not in fact contain co-eluting compounds. Carbonyl-DNPH Retention Times vs Carbonyl Carbon Number. Previous work has shown that, on C18 analytical columns and with acetonitrile-water eluent, carbonylDNPH derivatives of functional homologues (e.g., monofunctional alkyl-substituted aldehydes, unsaturated aldehydes, saturated ketones, unsaturated ketones, aromatic aldehydes, oxoacids, dicarbonyls) elute according to carbonyl carbon number (28). We observed the same trend for the isocratic elution and gradient elution methods employed in this study. This is illustrated in Figure 2, which shows plots of retention times vs carbon number for n-alkanals using methods A, B, and D (results for method C, not shown, are similar to those for method A). Polynomial fits to the log (retention time) vs carbonyl carbon number data (polynomial equations gave better fits than linear least squares regressions) yielded the following relations:
TABLE 2
Summary of Retention Times, Response Factors and Absorbance Ratios for Carbonyl-DNPH Derivatives Using Gradient Elution Method D with Diode Array Detector response factora
carbonyl DNPH reagent formaldehyde acetaldehyde 2,3-butanedionec acetone acrolein propanal crotonaldehyde 2-butanone methacrolein n-butanal benzaldehyde cyclohexanone glyoxald n-pentanal m-tolualdehyde glutaraldehydec methylglyoxald n-hexanal 2,3-butanedioned nopinone n-heptanal 4 acetyl-1-methylcyclohexene n-octanal n-nonanal n-decanal n-undecanal n-dodecanal n-tridecanal n-tetradecanal
retention time (min) 2.30 3.93 5.34 5.84 7.45 7.45 8.45 11.0 12.2 12.2 13.0 16.5 19.0 20.3 20.7 23.7 23.7 25.8 26.2 28.7 28.3 29.4 30.0 31.7 33.6 35.2 36.6 37.9 38.9 39.8
peak height
peak area
23.8 14.3
288.6 207.5
8.74 8.74 7.88 5.40 4.24 4.24 4.31 2.65
165.5 165.5 161.3 131.8 134.0 134.0 123.4 89.9
1.05 2.66 3.53
56.4 113.1 78.2
3.24 5.32
71.1 90.9
5.99 6.74 5.56
74.0 93.1 69.8
7.57
65.6
absorbance ratiosb
430/ 360
385/ 360
0.39 0.24 0.36 0.17 0.38 >0.60 0.37 0.77 0.40 >0.60 0.39 0.89 0.46 6.32 0.39 0.88 0.27 5.14 0.36 4.19 0.37 0.26 0.33
0.62 0.81 0.94 0.76 1.03 >1.15 0.96 1.30 >0.97 >1.15 0.97 1.38 0.99 4.50 0.99 1.39 0.70 4.02 0.95 3.65 1.04 0.78 0.90
0.25 0.25 0.25 0.24 0.24 0.24 0.24
0.77 0.75 0.74 0.72 0.72 0.71 0.70
a Peak height (milliabsorbance units ) mAu) and peak area (mAu s) for 1 µg mL-1 carbonyl. b 430 nm/360 and 385 nm/360 nm absorbance ratios from 200-600 nm (diode array) absorbance spectra. c MonoDNPH derivative. d Di-DNPH derivative.
log tR ) 0.14N + (2.8 × 10-3)N2 + (8.42 × 10-5)N3 + 0.342 (method A, n ) 9, R ) 1.00) (E1) log tR ) 0.022N + (7.1 × 10-3)N2 - (1.18 × 10-4)N3 0.022 (method B, n ) 14, R ) 1.00) (E2) log tR ) 0.048N + (4.06 × 10-2)N2 - (3.1 × 10-3)N3 + 0.63 (method C, n ) 6, R ) 1.00) (E3) where tR is the retention time of the carbonyl-DNPH derivative and N is the carbonyl carbon number. These relations are useful to identify carbonyls in complex mixtures including ambient air samples by comparison of the measured retention times of their DNPH derivatives to those calculated in eqs E1-E3. Quantitative Analysis. Concentrations of carbonyls in the air samples were calculated using the response factors measured using external carbonyl-DNPH standards synthesized in our laboratory. For all but one carbonyl (glyoxal), calibration curves were linear over a range of concentrations that bracketed those of the carbonyls in the ambient air samples (17-22, 28). For glyoxal, peak height vs concentration data exhibited upwards curvature, and concentrations were calculated using a polynomial fit to the calibration data:
FIGURE 2. Retention times (tR) of carbonyl-DNPH derivatives vs carbonyl carbon number for n-alkanals using methods A (isocratic elution, solid triangles), B (isocratic elution, solid squares), and D (gradient elution, open squares).
y ) 3.9 + 11.9H + 1.1H2
R ) 1.00, n ) 6 (E4)
where y is the glyoxal concentration (in ng mL-1), and H is the peak height, mm, on attenuation setting 5. Response factors for isocratic elution (peak height, methods A-C) are listed in Table 1. Those for gradient elution (method D, peak height, and peak area) are listed in Table 2. To verify consistency in retention times and response factors, single carbonyl-DNPH calibration standards and two calibration mixtures were analyzed along with every batch of ambient air samples. One calibration mixture was prepared in our laboratory from single carbonyl-DNPH standards and contained, in HPLC-grade acetonitrile, the DNPH derivatives of 13 carbonyls: the C1-C9 aliphatic aldehydes, benzaldehyde, acetone, 2-butanone, and cyclohexanone. The other calibration mixture was obtained from a commercial supplier and contained the DNPH derivatives of 13 C1-C8 carbonyls, each at a nominal concentration of 3 µg/mL carbonyl in acetonitrile (ERA013, Lot No. CAP 27275-55, Radian Corp.). Both calibration mixtures were employed for daily checks of consistency in retention times and overall system performance. The commercial mixture also provided an independent “audit” for the response factors measured using the carbonyl-DNPH standards synthesized in our laboratory. Response factors were measured in several comparisons of the laboratory standards and the commercial mixture for 10 carbonyls and for two analytical methods, methods A and C. Least squares linear regression of the data indicated reasonable agreement:
Rf (CM) ) (0.92 ( 0.02)Rf (LS) + (4.7 ( 2.9) R ) 0.996, n ) 25 (E5) where Rf is the carbonyl-DNPH derivative response factor, and CM and LS denote the commercial mixture and the mixture of laboratory standards, respectively. Liquid Chromatography Detection Limits. For formaldehyde, acetaldehyde, acetone, and propanal, detection in ambient air was limited by the amount of “background” carbonyl content of the DNPH-coated C18 cartridge. For all other carbonyls, which were not detected in cartridge
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TABLE 3
Analytical and Ambient Air Detection Limits ng of carbonyl/sample analytical detection limitb
carbonyla
method A
formaldehyde acetaldehyde acetone acrolein propanal crotonaldehyde methacrolein 2-butanone n-butanal benzaldehyde cyclohexanone glyoxal n-pentanal m-tolualdehyde methylglyoxal n-hexanal nopinone n-heptanal 4-acetyl-1-methylcyclohexene n-octanal n-nonanal n-decanal n-undecanal n-dodecanal n-tridecanal n-tetradecanal
7 11 17 14 19 26 35 38 36 50 50 44 61 82 79 106 252 178 228 380 570
method B
cartridge background contentc
13 21 23 26 31 38 54 76 115 157 230 380
93 (50-140) 200 (130-290) 118 (90-150) NDe 50 (30-80) ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND
equiv lowest quantifiable limit in ambient air (ppb)d
method A 0.48 0.71 0.32 0.04 0.13 0.06 0.08 0.08 0.08 0.07 0.08 0.12 0.11 0.11 0.17 0.17 0.29 0.24 0.26 0.47 0.63
method B
0.03 0.03 0.03 0.04 0.03 0.05 0.06 0.08 0.11 0.13 0.18 0.28
a Listed according to increasing retention times (and decreasing response factors). b From response factors listed in Table 1, detection wavelength ) 360 nm, injection aliquots ) 20 µL, cartridge elution volume ) 1.7 mL, and using a peak height of 1 mm as the detection threshold. c Mean value for two batches of cartridge blanks and two batches of field controls, n ) 12, range for each batch given in parentheses. d Volume of air samples ) 156 L (4 h at 0.65 L min-1). e ND, not detected.
blanks, the detection limit in ambient air could be calculated from the analytical detection limit and the volume of air sampled. Cartridge background content and analytical detection limits for methods A and B are listed in Table 3. Also listed in Table 3 are the corresponding ambient air detection limits. These limits of detection were calculated for a volume of air sampled ) 156 L, which corresponds to a 4-h sample collected at 0.65 L min-1 and therefore applies to the urban air samples (detection limits for San Nicolas Island, where longer samples of 5, 8, and 11 h duration were collected, are correspondingly lower). Ambient detection limits for formaldehyde, acetaldehyde, acetone, and propanal, while being limited by the cartridge background content, are suitable for the purpose of this study since the ambient concentrations of these carbonyls were many times higher than the limits of detection. For all other carbonyls, the low analytical detection limits translate into ambient air detection limits of ca. 0.1 ppb for the low molecular weight carbonyls (up to about C7) using method A and of ca. 0.1-0.28 ppb for the higher molecular weight carbonyls (up to C14) using method B. Tests of Cartridge Sampling Performance. The performance of the DNPH-coated C18 cartridge for sampling airborne carbonyls has been studied in detail previously (27). In this study, four cartridge sampling performance tests were carried out: (a) two consecutive cartridge elutions to verify sample recovery; (b) replicate LC analyses to assess analytical precision; (c) sampling with two DNPH-coated C18 cartridges in series to verify absence of breakthrough; and (d) collection and analysis of collocated samples to assess overall precision (sampling + analytical). Tests a and b were carried out using ambient air samples collected at the four urban locations on Sept 7-9. Tests c and d were
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carried out by collecting additional ambient air samples at one location, Azusa, on Sept 12, i.e., shortly after the completion of the field study. Results of the cartridge sampling performance tests are listed in Table A-1 in the supplementary microfilm material that accompanies this paper and can be summarized as follows. Second elution tests indicated complete recovery of all carbonyls in the first elution of the cartridge. Tests with two cartridges in series indicated no breakthrough for any of the carbonyls. Relative standard deviations (RSDs) for replicate analyses were often less than 5%; larger RSDs were observed when carbonyl concentrations were closer to the detection limit. Results for collocated samples showed consistently higher RSDs, for a given carbonyl, than those measured in replicate analyses. Thus, the overall measurement uncertainty reflects to a large extent uncertainties in the sampling step. The results obtained in this study are consistent with previous work, which could be consulted for a more detailed discussion of cartridge sampling performance tests (27). Previous work has shown that the collection efficiency of DNPH-coated C18 cartridges is not influenced by air humidity in the range of ambient air humidity relevant to this study (22). Control tests described in more detail elsewhere (31) have shown that unidentified compounds (possibly including carbonyls) form when sampling, using DNPH-coated C18 cartridges, the pollutants NO, NO2, and ozone in purified air at concentrations comparable to those observed in urban air during this study. These compounds may account for four of the 19 unidentified carbonyls observed in the ambient air samples (see Results and Discussion).
FIGURE 3. Liquid chromatography analysis of an ambient air sample collected in Azusa, CA, Sept 8, 1993, 0600-1000 h PDT, using method D with diode array detection. Chromatograms are shown for three detection wavelengths, 360 (top), 385 (middle), and 430 nm (bottom). Retention times for some of the peaks are 2.3 min for unreacted DNPH; 3.9, 16.7, and 20.2 min for the DNPH derivatives of formaldehyde, benzaldehyde, and glyoxal, respectively; 33.5 min for nonanal-DNPH; 35.2, 36.8, 37.8, 38.8, and 39.8 min for the DNPH derivatives of the C10-C14 n-alkanals; and 40.6, 41.3, 42.0, and 42.8 min for the DNPH derivatives of carbonyls tentatively identified as C15-C18 n-alkanals and/or their isomers.
Results and Discussion LC and MS Analysis of Ambient Air Samples. All ambient samples contained many carbonyls: a typical chromatogram included at least 30 peaks. This is illustrated in Figure 3 for a sample collected in Azusa and analyzed using gradient elution with diode array detection. The chromatograms in Figure 3 are shown for three detection
wavelengths: 360, 385, and 430 nm. The early part of the chromatogram (from left to right) contains unreacted DNPH and the DNPH derivatives of formaldehyde, acetaldehyde, acetone, and other low molecular weight carbonyls. The middle section of the chromatogram contains intermediate MW monofunctional carbonyls and low MW dicarbonyls, e.g., benzaldehyde (peak at tR ) 16.7 min, whose height
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FIGURE4. Chemicalionizationmassspectrumofanambientairsample collected in Azusa, CA, Sept 8, 1993, 0600-1000 h PDT. Peaks include MH of the DNPH derivatives of formaldehyde (m/e ) 211), acetaldehyde (225), acetone + propanal (239), butanal + 2-butanone (253), pentanal and C5 isomers (267), cyclohexanone (279), hexanal and isomers (281), benzaldehyde (287), heptanal and isomers (295), tolualdehyde and isomers (301), octanal and isomers (309), nonanal and isomers (323), decanal and isomers (337), glyoxal (419), methylglyoxal (433), and biacetyl (447). Peaks at m/e ) 297, 371, and 409 are system peaks, see text.
increases from 360 to 385 nm) and glyoxal (peak at tR ) 20.2 min, whose height increases from 360 to 430 nm). The last section of the chromatograms shown in Figure 3 includes DNPH derivatives of higher MW carbonyls. The peaks at tR ) 35.2, 36.8, 37.8, 38.8, and 39.8 min correspond to the C10-C14 aliphatic aldehydes decanal, undecanal, dodecanal, tridecanal, and tetradecanal, respectively, as was verified by comparison with authentic standards. Several peaks that correspond to higher MW carbonyls are also present. A comparison of the measured retention times to those calculated from extrapolation of relationships between retention times and carbonyl carbon number indicates that the peaks with retention times of 40.6, 41.3, 42.0, and 42.8 min in Figure 3 correspond to DNPH derivatives of C15-C18 carbonyls. The diode array spectra exhibited absorption maxima near 360 nm, thus indicating that these compounds are DNPH derivatives of aliphatic monofunctional carbonyls. Thus, the four carbonyls are likely to be the C15-C18 n-alkanals (pentadecanal, hexadecanal, heptadecanal, and octadecanal) and/or ketone or branched-chain aldehyde isomers. Although being less sensitive than liquid chromatography, the chemical ionization mass spectrometry method nevertheless provided important information, i.e., gave independant confirmation of carbonyl structure for the most abundant carbonyls present in the ambient air samples. This is shown in Figure 4 for an ambient air sample collected in Azusa (LC chromatograms of the same sample have been shown in Figure 3). The major peaks in the mass spectrum of the sample shown in Figure 4 are at m/e ) 211 (MH of formaldehyde-DNPH), 225 (acetaldehyde), 239 (propanal + acetone), 253 (butanal + 2-butanone), 265 (pentanal), 279 (cyclohexanone), 281 (hexanal and isomers), 287 (benzaldehyde), 295 (heptanal and isomers), and 301 (tolualdehyde isomers). Less abundant peaks are present at m/e ) 309 (octanal and isomers), 323 (nonanal and isomers), 337 (decanal and isomers), 419 (glyoxal), 433 (methylglyoxal), and 447 (biacetyl). The relative abundances of the CI-MS peaks were consistent with the carbonyl concentrations measured by liquid chromatography. Three peaks of variable abundance at m/e ) 297, 371, and 409 were present in the CI mass spectra of the ambient
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air samples. These peaks are absent from the CI mass spectra of single carbonyl-DNPH and from those of carbonyl-DNPH standard mixtures. We have observed these peaks in the mass spectra of all cartridge samples collected in ambient air (this study) and in laboratory studies of hydrocarbon-ozone and hydrocarbon-NOx reactions (32-34). We believe these three peaks to be “cartridge system peaks” that do not provide information on carbonyls in the matrix air sampled. Carbonyls Identified in Ambient Air. Twenty-three carbonyls have been identified in ambient air: the C1-C14 saturated aliphatic aldehydes from formaldehyde to tetradecanal; the saturated ketones acetone, 2-butanone, and cyclohexanone; the unsaturated aldehyde crotonaldehyde; the dicarbonyls glyoxal, methylglyoxal, and 2,3-butanedione (biacetyl); and the aromatics benzaldehyde and m-tolualdehyde. In addition to the 23 carbonyls listed above, up to 19 other carbonyls were present in some or all of the ambient air samples. These unidentified carbonyls included, in order of increasing retention times, 11 compounds recorded using methods A and C (U1-U11, of which four may form by reactions of NO, NO2, and O3 with DNPH on the sampling cartridge, see below), four dicarbonyls recorded in two of the samples collected in Azusa and analyzed using method D and CI-MS (U12-U15), and four high molecular weight carbonyls recorded using methods B and D (U16-U19). Retention times, absorbance parameters, and other relevant information for these carbonyls are compiled in Table 4. Also given in Table 4 are tentative structural assignments that are briefly discussed below. Of the 19 unidentified carbonyls listed, four (U1, U2, U3, and U5) may be contributed in part or entirely by reactions of NO, NO2, and/or ozone with the DNPH-coated C18 cartridge while sampling ambient air. Tentative Structural Assignments for Unidentified Carbonyls. As mentioned earlier in this section, the four high molecular weight carbonyls labeled U16-U19 are, with a reasonable degree of probability, the C15-C18 aldehydes pentadecanal, hexadecanal, heptadecanal, and octadecanal. Aldehydes up to C18 have been previously reported in marine air (35) but not in urban air. The four dicarbonyls observed using method D and labeled U12-U15 are higher molecular weight homologues of those identified, i.e., glyoxal, methylglyoxal, and biacetyl. They elute after biacetyl and before nonanal. Possible candidates include the C4, C5, and C6 dicarbonyl isomers. Several C4-C6 dicarbonyls have been identified before, for example, 2,3-pentanedione in cigarette smoke (36), glutaraldehyde and adipaldehyde as products of the atmospheric oxidation of cyclic olefins (37, 38), and 2-butene1,4-dial and 3-hexene-2,5-dione as oxidation products of aromatic hydrocarbons (4). Of the 11 unidentified carbonyls observed using method A (U1-U11), the two labeled U8 and U10 gave an excellent match with authentic standards of the C9 cyclic ketones nopinone and 4-acetyl-1-methylcyclohexene, respectively. These two ketones form in the atmospheric oxidation of terpenes, nopinone from β-pinene and 4-acetyl-1-methyl cyclohexene from D-limonene (32). They may also have direct emissions, as yet undocumented, from biogenic or anthropogenic sources. The unidentified carbonyls U1 and U3 elute before formaldehyde and between acetaldehyde and acetone, respectively. Their absorption spectra are consistent with
TABLE 4
Retention Times, Absorbance Parameters, and Tentative Structural Assignments for Unidentified Carbonyls absorbance ratios retention time (min)
method A
method Da
unidentified carbonyl
method A
method D
absorption max (nm)
430/ 360
385/ 360
430/ 360
385/ 360
U1
2.6
2.9
370
0.19
1.22
0.30
1.05
U2
3.0
3.6
310
0.15
0.21
0.09
0.31
356
