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Anal. Chem. 1999, 71, 1851-1861

Liquid Chromatography Analysis of Carbonyl (2,4-Dinitrophenyl)hydrazones with Detection by Diode Array Ultraviolet Spectroscopy and by Atmospheric Pressure Negative Chemical Ionization Mass Spectrometry Eric Grosjean,*,† Peter G. Green,‡ and Daniel Grosjean†

DGA, Inc., Suite 205, 4526 Telephone Road, Ventura, California 93003, and Department of Environmental Engineering Science, Environmental Analysis Center, California Institute of Technology, Pasadena, California 91125

The (2,4-dinitrophenyl)hydrazones of carbonyls are separated by liquid chromatography and detected by ultraviolet spectroscopy (diode array detector) and by atmospheric pressure negative chemical ionization mass spectrometry. Results are presented for 78 carbonyls including 18 1-alkanals (from formaldehyde to octadecanal), 16 other saturated aliphatic carbonyls (5 C4-C7 aldehydes and 11 C3-C9 ketones), 16 unsaturated aliphatic carbonyls (9 C3-C11 aldehydes and 7 C4-C9 ketones), 13 aromatic carbonyls (including hydroxy- and/or methoxy-substituted compounds), 10 C2-C10 aliphatic dicarbonyls, 3 aliphatic carbonyl esters, and 2 other carbonyls. Isomers were observed for r,β-unsaturated ketones and saturated carbonyls that bear other oxygen-containing substituents, e.g. methoxyacetone, 2-furaldehyde, and the 3 carbonyl esters. For all but two of the carbonyls studied, the base peak in the negative APCI mass spectrum was the M - 1 ion (NO2)2C6H3NNdCR1R2 (R1 ) H for aldehydes), where M is the molecular mass of the carbonyl (2,4-dinitrophenyl)hydrazone derivative. The dicarbonyls 2,4-pentanedione and succinic dialdehyde reacted with DNPH to yield predominantly other products. Concentrations measured by ultraviolet spectroscopy (peak area) and by mass spectrometry (abundance of M - 1 ion) were in good agreement. Applications described include the measurement of 34 C1-C18 carbonyls at levels of 0.015-14 parts per billion (ppb) in urban air and the identification of carbonyls at ppb concentrations as reaction products in laboratory studies of the atmospheric oxidation of unsaturated organic compounds. The ability to identify carbonyls and to measure their concentrations at levels of parts per billion (ppb) or lower in complex mixtures is important in many areas, including environmental chemistry and biomedical research. In the field of air quality alone, areas of important research and regulatory applications include * Corresponding author. Fax: 805-644-0142. E-mail: [email protected]. † DGA, Inc. ‡ California Institute of Technology. 10.1021/ac981022v CCC: $18.00 Published on Web 03/19/1999

© 1999 American Chemical Society

urban air pollution, monitoring of human exposure to toxic contaminants present in indoor, outdoor, and workplace air, characterization of air emissions from combustion processes (e.g., wood burning, cigarette smoke, and biomass burning) and from industrial sources (e.g., oil refineries and chemical plants), policy decisions regarding the impact of vehicle fuelssincluding oxygenated fuelsson air quality, studies of in situ formation (photochemistry) and scavenging processes (e.g., cloud and rainwater chemistry and dry deposition) in the atmosphere, and laboratory studies of the complex reaction mechanisms that are involved in the atmospheric oxidation of volatile organic compounds emitted by anthropogenic and biogenic sources.1-10 While several direct spectroscopic methods are now available for the detection of the simplest carbonyl, formaldehyde, at ppb levels in air,11-14 measurements of all other carbonyls still rely on traditional methods, of which the most widely used involves liquid chromatography analysis of carbonyls as their (2,4-dinitrophenyl)hydrazones with ultraviolet detection. This method, first applied (1) National Research Council, Board on Toxicology and Environmental Health Hazards. Formaldehyde and other aldehydes; National Academy Press: Washington, DC, 1981. (2) Weschler, C. J.; Hodgson, A. T.; Wooley, J. D. Environ. Sci. Technol. 1992, 26, 2371-2377. (3) Seinfeld, J. H. Atmospheric chemistry and physics of air pollution; Wiley: New York, 1986. (4) Grosjean, D. J. Air Waste Manage. Assoc. 1990, 40, 1522-1531. (5) Tuazon, E.; Aschmann, S. M.; Arey, J.; Atkinson, R. Environ. Sci. Technol. 1998, 32, 2106-2112. (6) Rogge, W. F.; Hildemann, L. M.; Mazurek, M. A.; Cass, G. R.; Simoneit, B. R. T. Environ. Sci. Technol. 1998, 32, 13-22. (7) Grosjean, E.; Grosjean, D.; Gunawardena, R.; Rasmussen, R. A. Environ. Sci. Technol. 1998, 32, 736-742. (8) Carter, W. P. L. Atmos. Environ. 1990, 24A, 481-518. (9) Anderson, L. G.; Lanning, J. A.; Barrell, R.; Miyagishima, J.; Jones, R. H.; Wolfe, P. Atmos. Environ. 1996, 30, 2113-2123. (10) Zhou, X.; Mopper, K. Environ. Sci. Technol. 1990, 24, 1482-1485. (11) Tuazon, E. C.; Winer, A. M.; Graham, R. A.; Pitts, J. N., Jr. Atmos. Environ. 1978, 12, 865-875. (12) Platt, U.; Perner, D.; Pa¨tz, H. W. J. Geophys. Res. 1979, 84, 6329-6335. (13) Mackay, G. I.; Schiff, H. I.; Wiebe, A.; Anlauf, K. Atmos. Environ. 1988, 22, 1555-1564. (14) Lawson, D. R.; Biermann, H. W.; Tuazon, E. C.; Winer, A. M.; Mackay, G. I.; Schift, H. I.; Kok, G. L.; Dasgupta, P. K.; Fung, K. Aerosol Sci. Technol. 1990, 12, 49-55.

Analytical Chemistry, Vol. 71, No. 9, May 1, 1999 1851

to measuring carbonyls in ambient air ca. 20 years ago,15-16 is the method currently recommended by the Intersociety Committee17 and prescribed by air pollution control agencies including the U.S. Environmental Protection Agency.18 We have employed this method to measure carbonyls in ambient air, indoor air, and laboratory studies of atmospheric oxidation mechanisms.19-24 We have reported the use of a diode array detector (DAD) to improve analytical selectivity25,26 and the use of off-line chemical ionization mass spectrometry (CIMS) for identification and structure confirmation of carbonyls as their DNPH derivatives.27,28 In more recent work, we described the application of LC/UV (DAD) and LC/particle beam (PB) CIMS to the identification of carbonyls in laboratory studies of the reaction of ozone with alkenes and with unsaturated oxygenates29 and to the measurements of carbonyls at ppb levels in urban air.30 This combination of analytical methods, i.e., LC/UV (DAD) and LC/PBMS, resulted in a significant improvement over previous work with respect to the positive identification of a number of carbonyls; e.g., ca. 40 carbonyls could thus be identified and measured in urban air.30 However, the presence of water in the LC eluent has a strong negative effect on the sensitivity of the particle beam interface, and as a result, the nanogram detection limits that are needed for mass spectrometry detection of carbonyls in ambient air could only be achieved with ca. 100% CH3CN, i.e., at the detriment of chromatographic separation.30 To overcome this limitation, we have developed a new LC/ UV-MS method for analysis of part per billion levels of carbonyls as their DNPH derivatives. The method involves LC separation of carbonyls as their DNPH derivatives and simultaneous, independent detection by ultraviolet spectroscopy (DAD) and by atmospheric pressure negative CIMS. The results obtained using this method are described for 78 environmentally important carbonyls including aliphatic aldehydes and ketones (saturated and unsaturated), aromatic carbonyls, dicarbonyls, and polyfunctional carbonyls that bear a variety of oxygen-containing substituents. To illustrate the potential of the method, we describe two applications, one involving the determination of carbonyls at subppb to ppb concentrations in ambient air and the other involving (15) Kuwata, K.; Uerobi, M.; Yamasaki, H.; Kuga, Y. Anal. Chem. 1983, 55, 2013-2016. (16) Fung, K.; Grosjean, D. Anal. Chem. 1981, 53, 168-171. (17) Intersociety Committee. Methods of Air Sampling and Analysis, 3rd ed.; Lodge, J. P., Ed.; Lewis Publishers: Chelsea, MI, 1989; pp 293-295. (18) Zweidinger, R. Sampling and analysis for carbonyls (report, National PAMS Workshop); AREA Laboratory, U.S. Environmental Protection Agency: Research Triangle Park, NC, 1993. (19) Grosjean, D.; Fung, K. J. Air Pollut. Control Assoc. 1984, 34, 537-543. (20) Grosjean, D.; Miguel, A. H.; Tavares, T. M. Atmos. Environ. 1990, 24B, 101-106. (21) Grosjean, D. Environ. Sci. Technol. 1991, 25, 710-715. (22) Grosjean, E.; Williams, E. L., II; Grosjean, D. J. Air Waste Manage. Assoc. 1993, 43, 469-474. (23) Grosjean, D.; Grosjean, E.; Williams, E. L., II. Environ. Sci. Technol. 1994, 28, 186-196. (24) Grosjean, E.; Grosjean, D. Environ. Sci. Technol. 1997, 31, 2421-2427. (25) Druzik, C. M.; Grosjean, D.; Van Neste, A.; Parmar, S. S. Int. J. Environ. Anal. Chem. 1990, 38, 495-512. (26) Grosjean, E.; Grosjean, D. Int. J. Environ. Anal. Chem. 1995, 61, 47-64. (27) Grosjean, D. Anal. Chem. 1983, 55, 2436-2439. (28) Grosjean, E.; Grosjean, D.; Seinfeld, J. H. Int. J. Chem. Kinet. 1996, 28, 373-382. (29) Grosjean, E.; Grosjean, D. J. Atmos. Chem. 1997, 27, 271-289. (30) Grosjean, E.; Grosjean, D.; Fraser, M. P.; Cass, G. R. Environ. Sci. Technol. 1996, 30, 2687-2703.

1852 Analytical Chemistry, Vol. 71, No. 9, May 1, 1999

the identification of carbonyls (at ppb concentrations) as reaction products in laboratory studies of the atmospheric oxidation of unsaturated organic compounds. EXPERIMENTAL METHODS Carbonyl-DNPH standards were synthesized in our laboratory as described previously.25,26 Reagents and solvents were of high purity as reported in previous work.25,26 Carbonyls were from commercial suppliers (Aldrich Chemical Co., Lancaster Synthesis, Wiley Organics, Fluka Chemical Corp.) or were prepared as described previously by reaction of ozone with the appropriate unsaturated compounds. Thus, the dicarbonyl pinonaldehyde was prepared from R-pinene,26 the dicarbonyl 2-oxobutanal (CH3CH2C(O)CHO) was prepared from 1-penten-3-one,28 from 2-ethylacrolein,31 and from 4-hexen-3-one,29 the carbonyl ester methyl glyoxylate (HC(O)C(O)OCH3) was prepared from methyl acrylate29,32 and from methyl trans-3-methoxyacrylate (MTMA),32 and the carbonyl ester 2-oxoethyl acetate (CH3C(O)OCH2CHO) was prepared from trans-2-hexenyl acetate28 and from MTMA.32 We also employed commercially available mixtures (ERA-030, ERA037, and ERA-013K, Radian Corp.) that contained the DNPH derivatives of 13 and 15 simple carbonyls at certified concentrations of 0.01, 0.02, 0.05, 0.10, 0.50, 1.0, and 3.0 µg/mL (as carbonyl) in CH3CN. The instrument used for analysis of carbonyl-DNPH derivatives was a Hewlett-Packard model 1100 benchtop liquid chromatograph/mass selective detector which allows for independent acquisition of both ultraviolet (diode array detector, 200-600 nm) and mass spectral data (mass selective detector) for analytes present in a given LC run. The complete instrument consisted of the following HP 1100 components: binary pump, diaphragm solvent degasser, autosampler, diode array UV-vis detector (DAD), and the LC/MSD. The LC/MSD was connected directly downstream of the DAD with a short section of 0.25 mm inside diameter PEEK tubing. The LC column was a 5 µm, 150 × 4.6 mm C18 Axxiom ODS (Cole Scientific, Inc.) with a C18 guard cartridge (Brownlee Applied Biosystems). The eluent was CH3CN-H2O, starting with isocratic elution for 26 min (49 vol % CH3CN), continuing with gradient elution by increasing the CH3CN content from 49% at t ) 26 min to 100% at t ) 40 min, and ending with isocratic elution with 100% CH3CN. The eluent flow rate was 1.4 mL/min, the column temperature was 38 °C, and the volume of sample injected was 2-20 µL. The HP 1100 LC/MSD instrument includes two separate spray chambers to provide different ionization modes, an atmospheric pressure ionization electrospray (API-ES) and an atmospheric pressure chemical ionization (APCI) spray chamber.33 For the purpose of this study, we operated the MSD in the APCI mode and we recorded negative chemical ionization mass spectra (scan range m/z 50-600). In the APCI mode, the eluent is ionized by a corona discharge creating a reagent plasma which is then capable of chemically ionizing the analytes (primarily through proton transfer). This soft ionization mode is well-suited for the analysis of polar, labile compounds such as the DNPH derivatives of carbonyls. (31) Grosjean, D.; Grosjean, E.; Williams, E. L., II. Isr. J. Chem. 1994, 34, 365373. (32) Grosjean, E.; Grosjean, D. The reaction of unsaturated aliphatic oxygenates with ozone. J. Atmos. Chem. 1999, 32, 205-232. (33) Imitani, K.; Smith, C. Am. Lab. 1996, (Nov), 29-33.

Preliminary tests were carried out using the API electrospray method and resulted, consistent with previous tests we carried out using another LC/electrospray MS instrument, in detection limits that were approximately 2 orders of magnitude higher than those obtained using the negative-mode APCI method. Tests carried out with the LC/MSD using APCI in the positive mode gave mixed results, with variable degrees of fragmentation being observed for certain carbonyls and with detection limits that were 1 order of magnitude higher than those obtained using the negative-mode APCI method. The positive-mode APCI method, which may be useful to acquire structural information for different types of carbonyls, will be tested more thoroughly in future work. Parameters for the acquisition of mass spectral data were optimized by carrying out multiple injections of a standard mixture of 13 carbonyl-DNPH derivatives (using the instrument’s flow injection analysis option) while each parameter was varied five times. The fragmentor setting was optimized to obtain maximum M - H ions (where M is the molecular mass of the carbonylDNPH derivative) for all 13 compounds present in the test mixture (optimization with respect to fragment ions that may have diagnostic value will be examined in future work). Due to the increasing CH3CN content of the eluent, compounds that eluted after ca. 33 min gave small signals with a capillary current of 4 µA, and for these compounds, the response could be increased by factors of 50-100 by increasing the capillary current to 10 µA. The results reported in the next section were obtained with the following MS parameters: nebulizer pressure ) 60 psi, drying gas flow ) 4.0 L/min, drying gas T ) 350 °C, vaporizer T ) 500 °C, gain ) 5, fragmentor setting ) 50 V capillary voltage setting ) 1500 V, and capillary current ) 4-10 µA (10 µA for compounds that elute after ca. 33 min; see above). The response for the early eluting compounds (derivatives of C1-C4 carbonyls) was increased by 15-25% when the flow rate was decreased from 1.4 to 1.0 mL/ min. For the purposes of this study, we elected to use a flow rate of 1.4 mL/min in order to reduce the elution time of high molecular weight compounds. The use of the mass detector in the selected ion monitoring (SIM) mode instead of the full scan mode resulted in a 50-200% increase in the response for the BP (M - H) ion of all compounds in the standard mixture of 13 carbonyl-DNPH derivatives. Maintenance of the APCI spray chamber involved cleaning of the corona needle and spray shield with mild abrasive paper and CH3CN whenever the signal’s noise was observed to increase.

10 aliphatic C2-C10 dicarbonyls, and in Table 6 for 5 other carbonyls (methoxyacetone, 2-furaldehyde, and the 3 aliphatic ester carbonyls methyl glyoxylate, ethyl glyoxylate, and 2-oxoethyl acetate). Tables 1-6 include, for each carbonyl-DNPH derivative standard, the ratio of the retention time to that of formaldehydeDNPH, the wavelength of maximum absorption, the molecular weight of the carbonyl-DNPH derivative (the molecular weights of the mono-DNPH and di-DNPH derivatives for dicarbonyls), the base peak (most abundant ion) in the atmospheric pressure negative chemical ionization mass spectrum, and ions other than the base peak when observed. For many of the compounds listed in Tables 1-6, ultraviolet and mass spectral data were obtained from several independently prepared carbonyl-DNPH standards; e.g., 10 samples containing formaldehyde-DNPH were analyzed, of which three were different mixtures of formaldehyde-DNPH and acetaldehyde-DNPH in CH3CN and seven included formaldehyde-DNPH in different mixtures of 13 and 15 carbonyl-DNPH derivatives in CH3CN. The 10 samples yielded identical ultraviolet and mass spectral data, which are therefore listed as a single entry in Table 1. Omitted from Tables 1-6 for clarity of presentation are the 13C isotopic contributions to the base peak, e.g. the ion of mass/charge (m/ z) ratio ) 210 for formaldehyde-DNPH (abundance ) 10% of base peak). These ions, which are of diagnostic value, were observed and their abundances relative to those of the base peaks were recorded for all compounds. Also not included in Tables 1-6 is the ion of m/z ) 182, which was present in the spectra of all carbonyl-DNPH derivatives studied. Isomers of Carbonyl-DNPH Derivatives. Retention times and absorption maxima for many of the carbonyl-DNPH compounds studied here have been discussed in detail previously,25,26,30 including their relationships with the carbonyl’s number of carbon atoms and with the carbonyl structure (e.g., saturated, unsaturated, aromatic, etc.), and these relationships will not be discussed here. Since many of the carbonyl-DNPH derivatives listed in Tables 1-6 have not been studied previously, a brief discussion of isomer elution is given in this section. For all unsymmetrical carbonyl-DNPH derivatives, two isomers may exist, i.e., syn and anti34

RESULTS AND DISCUSSION Retention Times, Absorption Spectra, and Mass Spectra of Carbonyl-DNPH Derivatives. Absorption spectra (diode array detector, 200-600 nm) and atmospheric pressure negative chemical ionization mass spectra were recorded for 78 carbonyl-DNPH standards. Individual spectra are not shown due to space limitations. Examples of chromatograms and mass spectra are shown in Figures 1-3 and will be discussed later in this section. The results are summarized in Table 1 for 18 1-alkanals (from formaldehyde to octadecanal), in Table 2 for 16 other saturated aliphatic carbonyls (5 C4-C7 aldehydes and 11 C3-C9 ketones), in Table 3 for 16 unsaturated aliphatic carbonyls (9 C3-C11 aldehydes and 7 C4-C9 ketones), in Table 4 for 13 aromatic carbonyls (including several hydroxy- and/or methoxy-substituted compounds, of which one was also unsaturated), in Table 5 for

where R1 and R2 are the carbonyl substituents (R1 ) H for aldehydes) and X is the (2,4-dinitrophenyl)amino substituent -NHC6H3(NO2)2. Depending on the nature of the substituents R1 and R2, i.e., on substituent electronic and steric effects (and, for the syn isomer, on the possibility of hydrogen bonding between the substituents X and R1 and/or X and R2), one or both isomers may be stable, and if both isomers are stable, they may isomerize (generally to yield the more stable isomer) or they may simply coelute by virtue of having very similar chromatographic properties. Saturated carbonyls that bear alkyl substituents (Tables 1 and 2) yielded only one peak under the LC conditions of this study (although we have observed both isomers of acetaldehyde-DNPH (34) Binding, N.; Mu ¨ ller, W.; Witting, U. Fresenius’ J. Anal. Chem. 1996, 356, 315-319.

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Figure 1. (a) Liquid chromatography analysis of a mixture of the DNPH derivatives of 13 carbonyls by ultraviolet absorption at 360 nm (diode array detector, top) and by atmospheric pressure negative chemical ionization mass spectrometry (total ion current, bottom): C1, formaldehyde; C2, acetaldehyde; C3K, acetone; ACR, acrolein; C3, propanal; CR, crotonaldehyde; MEK, 2-butanone; MTH, methacrolein; C4, butanal; BZ, benzaldehyde; C5, pentanal; TOL, m-tolualdehyde; C6, hexanal. The corresponding m/z ions are indicated on the total ion current chromatogram (from single-ion-monitoring data; see examples in Figure 1b) and are the base peaks (BP ) M - 1) for the DNPH derivatives of C1 (m/z ) 209), C2 (223), ACR (235), C3K (237), C3 (237), CR (249), MTH (249), MEK (251), C4 (251), BZ (285), C5 (265), TOL (299), and C6 (279). (b) Examples of extracted ions for the mixture of carbonyl-DNPH derivatives shown in (a). From top to bottom, total ion current: m/z ) 235 (acrolein), m/z ) 237 (acetone, propanal), m/z ) 249 (crotonaldehyde, methacrolein), m/z ) 251 (2-butanone, butanal). 1854 Analytical Chemistry, Vol. 71, No. 9, May 1, 1999

Figure 2. Atmospheric pressure negative chemical ionization chromatogram (total ion current) of the DNPH derivatives of the carbonyl ester methyl glyoxylate, HC(O)COOCH3 (see data in Table 6). One of the two isomers elutes before formaldehyde-DNPH. In this example, methyl glyoxylate was prepared in situ by reaction of methyl acrylate with ppb levels of ozone.29,32 The chromatogram also includes unreacted DNPH (from the DNPH-coated C18 sampling cartridge) and the DNPH derivative of formaldehyde, which is the other major product of the methyl acrylate-ozone reaction.

by gas chromatography with detection by electron impact mass spectrometry (unpublished results from this laboratory)). Only one peak was also observed for the DNPH derivatives of aromatic carbonyls; see Table 4. Two peaks were recorded for saturated carbonyls that bear polar substituents (Table 6), i.e., methoxyacetone, 2-furaldehyde, and the three carbonyl esters methyl glyoxylate (Figure 2), ethyl glyoxylate, and 2-oxoethyl acetate, with both peaks yielding the same absorption spectrum and the same mass spectrum (mass spectra are discussed below). Unsaturated carbonyls may lead to four carbonyl-DNPH peaks, i.e., the syn-cis, syn-trans, anti-cis, and anti-trans isomers

The data in Table 3 indicate that unsaturated aldehydes and nonconjugated unsaturated ketones yielded only one peak (with the possible exception of cis-4-heptenal, although the small second peak observed for this compound, peak area ) 2% of that of the main peak, may be contributed by an isomer impurity). From one to three peaks with identical absorption spectra and mass spectra were observed for the five R,β-unsaturated ketones studied (of which the smallest peak, peak area ) 3-4% of that of the main peak, may also be contributed by isomer impurities). Dicarbonyls may yield two or four mono-DNPH derivatives, i.e., the syn and anti isomers of the possible mono derivatives

(one for symmetrical dicarbonyls and two for unsymmetrical dicarbonyls)

as well as up to four di-DNPH derivatives, i.e., the syn-syn, synanti, anti-syn, and anti-anti isomers

The data in Table 5 include results for six R-dicarbonyls and for four other dicarbonyls. For five of the six R-dicarbonyls, glyoxal, methylglyoxal, 2-oxobutanal, 2,3-pentanedione, and 3,4-hexanedione, only one peak was observed, that of the di-DNPH derivative, and the absorption maximum was 400-432 nm, consistent with previous work.25,26,28-31 The chromatogram of 2,3-butanedione (biacetyl) contained three peaks of which the main (97%) one was the di-DNPH derivative and the two minor ones (1% and 2%) were isomers of the mono-DNPH derivative (absorption maximum ) 362-369 nm for both peaks). Also included in Table 5 are data for the four dicarbonyls studied that are not R-dicarbonyls. The dialdehyde glutaraldehyde Analytical Chemistry, Vol. 71, No. 9, May 1, 1999

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Figure 3. Atmospheric pressure negative chemical ionization mass spectrometry analysis of the carbonyl products of the reaction of ppb levels of ozone with 4-hexen-3-one in the presence of cyclohexane: (a) total ion current chromatogram with DNPH derivatives of unreacted 4-hexen-3-one (three peaks; see discussion of isomers in text) and of the reaction products formaldehyde (C1), acetaldehyde (C2), cyclohexanone, glyoxal, and 2-oxobutanal; (b) mass spectrum of 2-oxobutanal (see data in Table 5).

yielded one peak, i.e., the di-DNPH derivative, and the keto aldehyde pinonaldehyde yielded two peaks, i.e., one mono-DNPH 1856 Analytical Chemistry, Vol. 71, No. 9, May 1, 1999

and one di-DNPH. The di-DNPH derivatives of glutaraldehyde and pinonaldehyde had absorption maxima at λ ) 368 nm, as expected

Table 1. Summary of Data for the DNPH Derivatives of 1-Alkanals

Table 3. Summary of Data for the DNPH Derivatives of Unsaturated Aliphatic Carbonylsa

carbonyl-DNPH carbonyl formaldehyde acetaldehyde propanal butanal pentanal hexanal heptanal octanal nonanal decanal undecanal dodecanal tridecanal tetradecanal pentadecanal hexadecanal heptadecanal octadecanal

RRTa

UV

1.00 1.40 2.30 3.65 5.98 7.92 8.54 8.96 9.28 9.52 9.74 9.92 10.09 10.24 10.43 10.62 10.85 11.12

maxb

MWc

355 364 365 366 367 366 366 365 363 362 362 361 361 361 361 361 360 360

210 224 238 252 266 280 294 308 322 336 350 364 378 392 406 420 434 448

carbonyl-DNPH BPd,e 209 223 237 251 265 279 293 307 321 335 349 363 377 391 405 419 433 447

a RRT ) retention time of carbonyl-DNPH relative to that of formaldehyde-DNPH (4.08 ( 0.02 min). b UV max ) wavelength of maximum absorption, nm, from 200-600 nm absorption spectrum recorded with diode array detector. c MW ) molecular weight of carbonyl-DNPH. d BP ) base peak (most abundant ion), m/z, in atmospheric pressure negative chemical ionization mass spectrum.e No ions other than BP and 13C contribution to BP (see text) were present in the spectra of the DNPH derivatives of 1-alkanals.

Table 2. Summary of Data for the DNPH Derivatives of Other Saturated Aliphatic Carbonylsa carbonyl-DNPH carbonyl aldehydes 2-methylpropanal 3-methylbutanal 2-methylbutanal 2,2-dimethylpropanal cyclohexylmethanal ketones acetone acetone-d6 2-butanone 2-pentanone 3-pentanone 3-methyl-2-butanone 3,3-dimethyl-2-butanone 2,4-dimethyl-3-pentanone cyclohexanone 2-methylcyclohexanone nopinoned

carbonyl aldehydes acrolein crotonaldehydec methacrolein 2-ethylacrolein trans-2-hexenal 2-methyl-2-pentenal cis-4-heptenal trans-2-decenal trans-2-undecenal ketones methyl vinyl ketone

RRT 2.01 3.08 3.44 5.55 7.60 7.67 7.78d 7.95e (2%) 9.41 9.64

2.68e (13%) 2.87e (3%) 3.06d 1-penten-3-one 4.76d 4.99e (12%) 3-penten-2-one 4.43e (3%) 4.83d 4-methyl-3-penten-2-one 6.94 4-hexen-3-onec 7.12e (4%) 7.38e (35%) 7.72d 6-methyl-5-hepten-2-onef 8.27 4-acetyl-1-methylcyclo- 8.64 hexene

UV max MW BP

other ionsb

380 382 381 379 382 384 365 366 381 380

236 250 250 264 278 278 292 292 334 348

235 249 249 263 277 277 291 291 333 347

none none none none 275 (7) 275 (3) 289 (7) 289 (7) 331 (6) 345 (5)

368 372 379 378 376 382 384 386 385 357 357 368 369

250 250 250 264 264 264 264 278 278 278 278 306 318

249 249 249 263 263 263 263 277 277 277 277 305 317

none none none 247 (8) 247 (9) none none 263 (3) none none none 289 (4) 301 (4)

a RRT, UV max, MW, and BP are defined in footnotes a-d of Table 1. b m/z; not including 13C contribution to base peak; see text. The percent abundance of the ion relative to that of BP is given in parentheses. c Predominantly the trans isomer. d Largest peak. e Smaller peak; percent of largest peak (peak height basis at 360 nm) given in parentheses. f Two coeluting peaks.

RRT UV max MW BP other ionsb 3.69 5.51 5.70 5.66 8.09

363 363 363 364 366

252 266 266 266 292

251 265 265 265 291

none none 263 (1) none none

2.01 1.98 3.44 5.51 5.51 5.52 7.78 8.27 5.36 7.94 8.30

368 367 369 371 370 370 370 370 373 371 372

238 244 252 266 266 266 280 294 278 292 318

237 243 251 265 265 265 279 293 277 291 317

none 237-242c none none 263 (2) 263 (2) 263 (1) 277 (8) 275 (23) 289 (25) 315 (5)

a RRT, UV max, MW, and BP are defined in footnotes a-d of Table 1. b m/z; Not including 13C contribution to BP; see text. The percent abundance of the ion relative to that of BP is given in parentheses. c Abundances relative to that of BP ) 3% (m/z ) 237), 4% (238), 6% (239), 13% (240), 24% (241), and 44% (242). d 6,6-Dimethylbicyclo[3.1.1]heptan-2-one.

since the two carbonyl groups are not conjugated. The dialdehyde succinic dialdehyde yielded three peaks, of which the two small peaks corresponded to one mono-DNPH derivative and one diDNPH derivative, respectively, and the largest peak (maximum absorption ) 338 nm, BP ) 247) was not a (2,4-dinitrophenyl)hydrazone. The dione 2,4-pentanedione yielded one peak (maximum absorption ) 310 nm, BP ) 262) which was not a

(2,4-dinitrophenyl)hydrazone. Dicarbonyls that are 1,3-dicarbonyls may react with substituted hydrazines to form the corresponding pyrazoles, e.g. for 2,4-pentanedione

where X is the 2,4-dinitrophenyl substituent. A more systematic study of the products of the reaction of DNPH with dicarbonyls that are not R-dicarbonyls will be carried out in future work. Atmospheric Pressure Chemical Ionization Mass Spectra. Atmospheric pressure negative chemical ionization mass spectra of carbonyl-DNPH derivatives have not been described prior to this work, and a summary of the results is given below. For all but two of the carbonyls studied (the two exceptions, the dicarbonyls 2,4-pentanedione and succinic dialdehyde, have been discussed in the preceding section), the base peak was the M 1 ion, i.e., (NO2)2C6H3NNdCR1R2,35 where M is the molecular mass of the carbonyl (2,4-dinitrophenyl)hydrazone derivatives. For 1-alkanals (Table 1), no ions other than the M - 1 base peak were observed. For other saturated aliphatic aldehydes and ketones (Table 2), the M - 1 base peak (BP) was accompanied (35) Kolliker, S.; Oehme, M.; Dye, C. Anal. Chem. 1998, 70, 1979-1985.

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Table 4. Summary of Data for the DNPH Derivatives of Aromatic Carbonylsa carbonyl-DNPH carbonyl

RRT

UV max

MW

BP

other ionsb

benzaldehyde o-tolualdehyde m-tolualdehyde p-tolualdehyde acetophenone 2,5-dimethylbenzaldehyde 2-hydroxybenzaldehyde (salicylaldehyde) 4-methoxybenzaldehyde (p-anisaldehyde) 3,4-dimethoxybenzaldehyde 4-hydroxy-3-methoxybenzaldehyde (vanillin) 4-hydroxy-3-methoxyacetophenone (acetovanillone) 3,5-dimethoxy-4-hydroxybenzaldehyde (syringaldehyde) 4-hydroxy-3-methoxycinnamaldehyde (coniferyl aldehyde)

4.75 7.13 7.29 7.35 6.77 8.15 2.97 4.98 3.07 1.75 2.37 1.54 2.67

384 386 385 388 382 389 391 398 398 402 393 436 415

286 300 300 300 300 314 302 316 346 332 346 362 358

285 299 299 299 299 313 301 315 345 331 345 361 357

none none none none none none none none none 329 (2), 315 (1) 343 (5), 329 (45), 313 (4), 298 (2) 360 (40), 359 (1), 345 (1) 356 (12), 355 (22), 325 (5), 310 (10)

a RRT, UV max, MW, and BP are defined in footnotes a-d of Table 1. b m/z; not including 13C contribution to BP; see text. The percent abundance of the ion relative to that of BP is given in parentheses.

Table 5. Summary of Data for the DNPH Derivatives of Dicarbonylsa carbonyl-DNPH carbonyl glyoxal methylglyoxal 2-oxobutanalc 2,3-butanedione succinic dialdehyde glutaraldehyde 2,3-pentanedione 2,4-pentanedione 3,4-hexanedione pinonaldehydeg

RRT

UV max

MW (mono)

MW (di)

BP

6.09 7.90 8.31 1.50e (1%) 1.79e (2%) 8.31d 0.81e (5%) 1.55d,f 6.42e (12%) 7.34 8.72 1.03 8.89 3.73e (9%) 9.07d

415 432 410 362 369 403 360 338f 368 368 402 310 400 368 368

238 252 266 266 266

418 432 446

417 431 445 265 265 445 265 247f 445 459 459 262 473 347 527

446 266 280 280 280 294 348

446 460 460 460 474 528

other ionsb 237 (14), 238 (16) 251 (14), 249 (17) 263 (12) none none 265 (7), 263 (48) 263 (80) 279 (10) 443 (8), 279 (15) 302 (14), 232 (6), 360 (7), 279 (0.1), 288 (5) 293 (5), 291 (12) none 345 (16)

a RRT, UV max, and BP are defined in footnotes a-d of Table 1. MW (mono) and MW (di) are the molecular weights of the mono-DNPH derivative and di-DNPH derivative, respectively. b m/z; not including 13C contribution to BP; see text. The percent abundance of the ion relative to that of BP is given in parentheses. c Prepared by reaction of ozone with 1-penten-3-one, 2-ethylacrolein, and 4-hexen-3-one. d Largest peak. e Smaller peak; percent of largest peak (peak height basis at 360 nm) is given in parentheses. f This compound is not the mono-DNPH derivative; see text. g (2,2-Dimethyl-3-acetylcyclobutyl)ethanal, prepared by reaction of ozone with R-pinene.

for several (but not all) compounds by a small M - 3 ion (BP-2) whose abundance was 1-5% of that of the base peak for alkylsubstituted ketones (e.g., 2% for 2-pentanone) and was higher for cyclic ketones, e.g., 23% of BP for cyclohexanone and 25% of BP for 2-methylcyclohexanone. The M - 3 ion (BP-2) was not observed in the spectrum of the isopropyl-substituted ketone 2,4dimethyl-3-pentanone, whose mass spectrum included a small M - 17 ion (BP-16, abundance ) 8% of that of BP). The spectrum of acetone-d6-DNPH (MW ) 244) contained, in addition to the BP at m/z ) 243, the six ions with m/z ) 237-242. The data in Table 2 also indicate that the three pentanone isomers studied, i.e., 2-pentanone, 3-pentanone, and 3-methyl-2-butanone, have identical retention times, absorption maxima, and mass spectra and thus could not be resolved if present as a mixture in a given sample, without first seeking chromatographic conditions suitable for their separation. As noted previously, the optimization of experimental conditions to generate distinct ion fragments for coeluting compounds that are structurally similar will be examined in future work. 1858 Analytical Chemistry, Vol. 71, No. 9, May 1, 1999

Table 6. Summary of Data for the DNPH Derivatives of Other Carbonylsa carbonyl-DNPH carbonyl

RRT

UV max

MW

BP

other ionsb

methyl glyoxylatec

0.89f (60%) 1.72e 1.24f (65%) 2.71e 1.14e 1.22f (13%) 1.59e 2.25f (30%) 2.14e 3.00f (25%)

355 357 356 359 360 356 363 370 392 383

268 268 282 282 282 282 268 268 276 276

267 267 281 281 281 281 267 267 275 275

none none none none 249 (18) 249 (16) none none none none

ethyl glyoxylate 2-oxoethyl acetated methoxyacetone 2-furaldehyde

a RRT, UV max, MW, and BP are defined in footnotes a-d of Table 1. b m/z; not including 13C contribution to base peak; see text. The percent abundance of the ion relative to that of BP is given in parentheses. c Prepared by reaction of ozone with methyl acrylate and with methyl trans-3-methoxyacrylate (MTMA). d Prepared by reaction of ozone with MTMA and with trans-2-hexenyl acetate. e Largest peak. f Smaller peak; percent of largest peak (peak height basis at 360 nm) is given in parentheses.

Mass spectra of the unsaturated aldehydes (Table 3) included BP ) M - 1 for all compounds. No other ions were present in the spectra of the C3-C5 compounds acrolein, crotonaldehyde, methacrolein, and 2-ethylacrolein, and small M - 3 ions (BP-2, abundance ) 3-7% of that of BP) were present in the spectra of the higher MW carbonyls. A second minor peak (2% of the major peak) was observed for cis-4-heptenal with UV and mass spectra identical to those of the major peak. As we have not observed more than one peak for the other unsaturated aldehydes studied, it is possible that this small peak is an impurity, e.g., the DNPH derivative(s) of trans-4-heptenal and/or other heptenal isomers (the stated purity of cis-4-heptenal is > 99%). The mass spectra of unsaturated ketones (Table 3) also included BP ) M - 1 for all compounds. The spectra of the R,βunsaturated ketones methyl vinyl ketone, 3-penten-2-one, and 4-hexen-3-one included no peak other than BP, that of 1-penten3-one included a small BP-16 ion (abundance ) 8-9% of that of BP), and that of 4-methyl-3-penten-2-one included a small ion at m/z ) 263 (abundance ) 3% of that of BP) that is most likely from a C5 unsaturated ketone impurity (e.g., 1-penten-3-one and/ or isomer, MW of carbonyl-DNPH derivative ) 264, BP ) M 1 ) m/z 263; see Table 3). The spectra of 6-methyl-5-hepten-2one and 4-acetyl-1-methylcyclohexene, which are not R,β-unsaturated, included small BP-16 ions (abundance ) 4% of that of BP). Mass spectra of the DNPH derivatives of aromatic carbonyls (Table 4) all included BP ) M - 1, and no other ions were observed except for the four polysubstituted compounds vanillin, acetovanillone, syringaldehyde, and coniferyl. The spectrum of vanillin-DNPH also included two small ions, BP-2 (abundance ) 2% of BP) and BP-16 (abundance ) 1% of BP). The spectra of the other three compounds included BP-2, BP-1, BP-16, BP-32, and BP-47 ions whose abundance relative to that of BP ranged from 1 to 45%. The spectrum of syringaldehyde-DNPH (MW ) 362) also contained ions at m/z ) 386 and 387, perhaps due to higher MW structural homologues, ion adducts, and/or other impurities. Mass spectra of the mono-DNPH and di-DNPH derivatives of dicarbonyls all included BP ) M - 1 (Table 5). The spectra of the mono-DNPH derivatives (which were observed for 2,3butanedione, succinic dialdehyde, and pinonaldehyde) included no ions other than the base peak. The spectra of the di-DNPH derivatives (which were observed for 9 of the 10 dicarbonyls listed in Table 5) included two other ions, namely the M - 3 ion (BP-2) of the mono-DNPH derivative for 6 of the dicarbonyls and the M - 1 ion (BP) of the mono-DNPH derivative for 7 of the dicarbonyls. The abundance of the BP-2 ion ranged from 12 to 80% of that of BP, and the abundance of the BP-1 ion ranged from 5 to 15% of that of BP. The spectrum of 2,3-pentanedione also included a second ion at m/z ) 443 (BP-16, abundance ) 8% of that of BP). As discussed earlier, 2,4-pentanedione reacted with DNPH to form one product, tentatively a (2,4-dinitrophenyl)pyrazole, and succinic dialdehyde reacted with DNPH to form an unidentified compound (BP at m/z ) 247, λmax ) 338 nm) as the major product and the mono-DNPH and di-DNPH derivatives as minor products. Mass spectra of the DNPH derivatives of other carbonyls all included BP ) M - 1 (Table 6). No other ions were present in the mass spectra of methoxyacetone, 2-furaldehyde, methyl glyoxylate, and ethyl glyoxylate, and the mass spectrum of the

carbonyl ester 2-oxoethyl acetate included a second ion at m/z ) 249 (possibly BP-32) whose abundance was 16-18% of that of the BP. Analysis of Sampling Cartridge Reagent. One major application of the analytical method described here is the identification of carbonyls at low levels in air. The sampling device employed in our laboratory consists of a C18 Sep-Pak cartridge which we impregnate with DNPH and phosphoric acid.36 While the sampling performance of this DNPH-coated cartridge has been thoroughly tested and documented,36 it is necessary to ascertain the compatibility of the sampling device with the analytical method described here. Samples collected using a DNPH/H3PO4-coated cartridge will, of necessity, contain unreacted DNPH and H3PO4, and it is therefore important to record the atmospheric pressure negative chemical ionization mass spectra of standards of DNPH, H3PO4, and both under the LC conditions described in this study. For DNPH, consistent with previous work,30,36 the retention time was 2.3 min and the absorption maximum was 353 nm. The mass spectrum included the base peak (m/z ) 197, i.e., M - 1) and smaller ions (listed in order of decreasing abundance) at m/z ) 163, 182, 168, and 179. Thus, the BP’s of DNPH and the corresponding fragment ions do not interfere with analysis of carbonyl-DNPH derivatives by atmospheric pressure negative chemical ionization mass spectrometry. The absorption maximum of H3PO4 is only slightly higher than that of CH3CN (215 nm vs 210 nm), and as a result, H3PO4 is essentially absent from UVvisible chromatograms recorded using a diode array detector. However, H3PO4 is readily detected by negative APCI mass spectrometry as a broad peak that elutes almost immediately upon injection of the sample and then slowly decreases in intensity over the next 3-3.5 min. The negative APCI mass spectrum of H3PO4 included the following ions (tentative assignments are given in parentheses): m/z ) 97 (monomer), 195 (dimer), 293 (trimer), 177 (pyrophosphate, i.e., dimer - H2O), and 275 (trimer - H2O). Quantitative Analysis: Comparison of UV Absorption and Atmospheric Pressure Negative Chemical Ionization. Shown in Figure 1 are data for a mixture of the DNPH derivatives of the 13 carbonyls formaldehyde, acetaldehyde, acetone, acrolein, propanal, crotonaldehyde, 2-butanone, methacrolein, butanal, benzaldehyde, pentanal, m-tolualdehyde, and hexanal. The amount injected was 60 ng (as carbonyl) for each compound. The upper portion of Figure 1 shows the peak abundance (absorbance units) measured at 360 nm using the diode array detector. The lower portion of Figure 1 shows the corresponding total ion current (counts) measured by atmospheric pressure negative chemical ionization mass spectrometry. Acetone + acrolein and 2-butanone + methacrolein, which coelute, are readily resolved by mass spectrometry with BP ) M - 1 ) m/z 235 (acrolein), 237 (acetone), 249 (methacrolein) and 251 (2-butanone). The absorption (diode array detector) and total ion current (TIC, atmospheric pressure negative chemical ionization) chromatograms are very similar, indicating that concentrations measured using the two methods are in agreement. For example, data for formaldehydeDNPH (10 independently prepared standards, amount injected ranging from 0.20 to 103 ng as carbonyl) and for acetaldehydeDNPH (10 independently prepared standards, amount injected ranging from 0.20 to 121 ng as carbonyl) yielded the relations (36) Grosjean, E.; Grosjean, D. Int. J. Environ. Anal. Chem. 1995, 61, 343-360.

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Y1 ) (4.35 ( 0.02)X1 + (0.10 ( 1.60), R2 ) 0.999 92 Y2 ) (4.62 ( 0.02)X2 + (0.13 ( 1.25), R2 ) 0.999 92 where Y is the abundance (counts) of the M - 1 base peak measured by atmospheric pressure chemical ionization mass spectrometry, X is the peak height (mAU) measured using the diode array detector (at 360 nm), the subscripts 1 and 2 are used for formaldehyde and acetaldehyde, respectively, R is the correlation coefficient (unit-weighted linear least-squares regression; plots not shown), and the stated uncertainties on slopes and intercepts are one standard deviation. Detection limits for mass spectrometry analysis range from 20 picograms (pg) for formaldehyde to 60 pg for tetradecanal at signal/noise ratios of 2/1 to 6/1. At a signal/noise ratio of g20/1, we estimate the lowest quantifiable amounts to be 200 pg for formaldehyde and 600 pg for tetradecanal. The noise in the mass spectrum is due in part to unreacted H3PO4 as discussed earlier, and limited experiments indicate that this noise can be reduced (and the detection limits correspondingly improved) by buffering the sample prior to mass spectrometric analysis. Examples of Application. Applications shown here to illustrate the potential of the method include two examples, one involving a laboratory study of the carbonyl products of the reaction of ozone with an unsaturated oxygenated compound, the ketone 4-hexen-3-one, and the other involving measurements of ambient concentrations of carbonyls in urban air. In the first example, mixtures of 4-hexen-3-one (ca. 1.0 ppm) and ozone (ca. 50-100 ppb) were allowed to react in the dark in purified air. The reaction vessel was a ca. 3.5 m3 smog chamber constructed from FEP Teflon film. Cyclohexane (400 ppm) was added to the reaction mixture to scavenge the OH radical, which may form as a product of the ozone-4-hexen-3-one reaction. After ozone had reacted, samples were collected on DNPH-impregnated C18 Sep-Pak cartridges whose preparation and sampling performance have been described in detail previously.30,36 The cartridges were eluted with CH3CN, and aliquots were analyzed by LC/UV and by LC/UV-APCI-MS as described in the Experimental Section. Data recorded for each analyte included absorption spectra and atmospheric pressure negative chemical ionization mass spectra. Comparison of these data to those given in Tables 1-6 for carbonyl-DNPH standards resulted in the positive identification of unreacted DNPH (from the sampling cartridge), unreacted 4-hexen-3-one, and the carbonyl reaction products acetaldehyde, 2-oxobutanal, formaldehyde, glyoxal, and cyclohexanone (the corresponding product yields and the relevant reaction mechanisms are reported elsewhere29,32). An example of a total ion current chromatogram for this sample is given in Figure 3, which also includes the atmospheric pressure negative chemical ionization spectrum of the reaction product 2-oxobutanal. Samples for control experiments, i.e., mixtures of 4-hexen-3-one and cyclohexane in air with no ozone present, were analyzed in the same way (data not shown) and indicated the presence of only two analytes, i.e., DNPH (MW ) 198, BP ) M - 1 ) m/z 197) and 4-hexen-3-one (MW ) 278, BP ) M - 1 ) m/z 277 for all three peaks; see discussion of isomers earlier in this section). In the second example, we collected ambient air in downtown Porto Alegre, Brazil, where the mixture of vehicle fuels is unique 1860 Analytical Chemistry, Vol. 71, No. 9, May 1, 1999

Table 7. Ambient Concentrations of Carbonyls in Downtown Porto Alegre, RS, Brazil, Sept 19, 1996a carbonyl formaldehyde acetaldehyde acetone acrolein propanal crotonaldehyde 2-butanone methacrolein butanal benzaldehyde 3-methylbutanal and/or isomers pentanal glyoxal acetophenone o-tolualdehyde m-tolualdehyde p-tolualdehyde

concn, ppb

carbonyl

concn, ppb

13.9 5.7 0.87

0.54 0.22 0.26

0.09 0.64 0.05 0.37 0.06 0.38 0.071 0.065

methylglyoxal hexanal 2,5-dimethylbenzaldehyde 2-oxobutanal 2,3-butanedione heptanal octanal nonanal decanal undecanal dodecanal

0.20 0.043 0.033 0.145 0.152 0.115 0.030 0.032

0.32 0.30 0.09 0.18 0.35 0.46

tridecanal tetradecanal pentadecanal hexadecanal heptadecanal octadecanal

0.024 0.028 0.030 0.041 0.035 0.015

a Sample collected from 7 to 10 a.m. at a sampling flow rate of 0.790 L/min (volume of air sampled ) 158 L).

in the world.7 The samples were collected on DNPH-coated C18 Sep-Pak cartridges and were analyzed by LC with UV detection30,36 and by LC/UV (DAD)-APCI-MS as described in the Experimental Section. These collection and LC analysis procedures are essentially identical to those recommended by the U.S. Environmental Protection Agency for ambient monitoring of carbonyl compounds using Compendium Method TO-11.37 Thirty-four carbonyls were positively identified. These carbonyls are listed in Table 7 along with their ambient concentrations, which ranged from 0.015 ppb for octadecanal to 13.9 ppb for formaldehyde. A more complete discussion of carbonyls and other air pollutants in the Porto Alegre urban area can be found elsewhere.38,39 CONCLUDING COMMENTS AND DIRECTIONS FOR FUTURE WORK At its present stage of development, the method described in this article offers several advantages, of which the most important is the positive identification, and this at sub-ppb to ppb levels in air, of many of the carbonyls that may be present in source samples and in ambient air (indoor and outdoor) and that may form as reaction products in laboratory studies that focus on elucidating reaction mechanisms for the atmospheric oxidation of hydrocarbons and other volatile organic compounds. Examination of the data in Tables 1-6 clearly indicates that the many carbonyls that can be present in complex environmental mixtures (including source samples, laboratory samples, and ambient air) may coelute and that, as a result, it is necessary to use mass spectrometry for positive identification of carbonyl-DNPH derivatives in environmental applications. This requires the construction of a library of mass spectra for standards of DNPH derivatives of (37) Determination of formaldehyde in ambient air using adsorbant cartridge followed by high performance liquid chromatography; U.S. Environmental Protection Agency, Office of Research and Development: Research Triangle Park, NC, 1988; Compendium Method TO-11. (38) Grosjean, E.; Grosjean, D. J. Braz. Chem. Soc. 1998, 9, 131-143. (39) Grosjean, E.; Grosjean, D.; Rasmussen, R. A. Environ. Sci. Technol. 1998, 32, 2061-2069.

many carbonyls. Data for 78 carbonyl-DNPH standards are described in this study. The method described here has not been fully exploited, and there are several directions for future work. Liquid chromatography conditions may be optimized (the LC conditions employed in this study are fairly “standard” for air quality applications), but it is unlikely that the numerous relevant carbonyls can be resolved by LC alone soon. Thus, work in this laboratory will continue to focus on the chemical ionization mass spectrometry component of the method. More specifically, we are gathering atmospheric pressure negative chemical ionization mass spectra for other classes of carbonyls relevant to air quality (e.g., hydroxy carbonyls, dicarbonyls, carbonyl esters, and keto acids). We also plan to explore the potential of optimizing conditions for fragment ions (as contrasted to optimizing conditions for M - H base peak ions in this work) as an additional tool to elucidate the structure of

DNPH derivatives of environmentally important carbonyls. ACKNOWLEDGMENT This work was been supported by R & D funds, DGA, Inc., and by the Department of Environmental Engineering Science, Environmental Analysis Center, California Institute of Technology. We thank the Hewlett-Packard Co. for the loan of the LC-MSD instrument, Dr. John Hughes (Hewlett-Packard Co.) for technical advice, and Dr. Jamie Schauer (Department of Environmental Engineering Science, California Institute of Technology) for making available several carbonyls.

Received for review September 14, 1998. Accepted February 6, 1999. AC981022V

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