Environ. Sci. Technol. 1993, 27, 2814-2820
Evaluation of Ozone and Water Vapor Interferences in the Derivatization of Atmospheric Aldehydes with Dansylhydrazine Daniel R. Rodler, Lubos Nondek,?and John W. Blrks’ Department of Chemistry and Biochemistry and Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado 80309-02 16
The dansylhydrazine (DNSH) and 2,4-dinitrophenylhydrazine (DNPH) techniques for the determination of carbonyl compounds are both susceptible to interferences from ozone oxidation and water vapor hydrolysis. Here we report the results of an investigation of these potential interferences for a new DNSH microcatridge technique and describe means of minimizing or completely eliminating them. Studies were carried out using both silanized, controlled-pore, glass beads and octadecylsilane (CIS) particles as the reagent support. All intereferences were minimized by the use of CISparticles. Ozone was found to cause partial oxidation of the DNSH reagent but had no significant effect on the hydrazone products for mixing ratios up to 300 ppbv 03,with or without the presence of up to 100% relative humidity. Thus, ozone is not a significant interference so long as DNSH is in substantial excess over the carbonyl compounds being derivatized. Water vapor provides an interference by shifting the equilibrium between the hydrazones and an alcohol intermediate. Addition of trichloroacetic acid to the microcartridge was found to reduce this interference at 100% RH to "
:;,
C2
Zero Air lOO%RH 3oOppbvO3 lOO4RH.3Wppbv03
C:
Interferences for dansylhydrazine and the hydrazones of formaldehyde. acetaldehyde. and propanal (a)on Silanlzed glass bead Sorbent In the absence of trichloroacetic acld (n = 5);(b) on silanized glass bead sorbent in the presence of trichloroacetic acid. (n = 5); (c)on octadecyl sillca sorbent in the absence of trichloroacetic acM (n = 5): (d) on octadecyl silica sorbent in the presence of trlchloroacetkacM (n = 4). The error bars represent one standard deviation. (e)Normalized peak areas showing water vapor interferences for dansylhydrazine and hydrazones of straight chain aldehydes through hexanal and benzaldehyde on silanized glass bead sorbent in the presence of hydrochloric acM (n = 3). Flpure 6. Normallzed peak areas showing water vapor and ozone
interference illustrated in this figure is consistent with observations made using both sorbents, regardless of the presence or absence of an acid; i.e., the propanal hydrazone is the most affected by water vapor. The water vapor interference is dependent on the relative humidity of the sample, the acidity of the reagent solution used to load the cartridge, and the type of sorbent. This is believed to be primarily, if not entirely, due to the changing pH of the microenvironment in which the reaction occurs. The initial level of acidity is determined by the conditions used when the microcartridge is loaded. During sampling, water vapor is adsorbed onto the sorbent and presumably reaches equilibrium with the airstream that flows through the cartridge. The amount of water adsorbed is related to the extent to which the airstream is humidified, with the hydrophilic glass beads adsorbing much more water than the hydrophobic octadecyl silica. The amount of water adsorbed determines the volume in which the acid is solvated, and thus the acidity and pH. The water vapor interference may be attributed to hydration of thecarbon-nitrogen double bond. Hydrazine derivatization of carbonyl compounds occurs in two stages, addition and dehydration (6): 2818
Environ. Sd. Technol.. VoI. 27. No. 13. 1993
S
+
%N H/
H,O
'N-DNS
I
H 3
Under neutral or mildly acidic conditions, the equilibrium between 2 and 3 is located more toward 2 due to hydration. However, under more strongly acidic conditions, dehydration (2 3) is enhanced, and the equilibrium between 2 and 3 is shifted toward 3. The addition of water to the reaction microenvironment shifts the equilibrium back toward 2 by diluting the acid and possibly by increasing the activity of water. Cis and Trans Isomers. In the chromatograms obtained, it is apparent that the chromatographic process is capable of separating the cis and trans (with respect to the C=N bond) isomers of the dansylhydrazones (Figure
-
5 and refs 1 and 2), e.g., for the propanal hydrazone: I4
The exception of this is formaldehyde, the only symmetrical aldehyde. The trans isomers are the preferred products of the derivatization reaction. The trans:& ratio is greater than 3 for all the aldehydes we have analyzed. This is to be expected since the cis derivatization product is more sterically hindered than the trans product. The trans isomer is more elongated and permits greater interaction between the alkyl chain of the hydrazone and the octadecyl silica of the column, translating into a greater retention time for the trans isomer versus the cis isomer. It is interesting to consider the relative impact of hydrolysis on the two isomers. By comparing chromatograms in which the hydrazones were exposed to 1 L of dry air to chromatograms in which the hydrazones were exposed to 1L of air containing 100% RH, it is readily apparent that the cis isomers were hydrolyzed to a very small extent, while the trans isomers were extensively hydrolyzed. In fact, the transcis ratio decreases to roughly unity. This finding permits detection of any water vapor interference by following the transxis ratio. The dependences of extent of hydrolysis on length of the alkyl chain (C3 most strongly affected) and on the isomeric structure (trans more susceptible to hydrolysis than cis) are not readily explained in terms of mechanistic organic chemistry. Differences in reaction rates of isomers are often due to differences in steric hindrance in formation of the transition state. The large difference in reactivity of cis and trans isomers found for the dansyl hydrazones could be explained by steric hindrance if the lone electron pair of the S-N nitrogen catalyzes the nucleophilic attack of water on the carbon of the C=N bond. However, both the isomeric and alkyl chain length effects may involve surface interactions and have complex explanations. Ozone as an Interference. A negative interference for ozone has been reported for the DNPH method (7,8) and identified in a preliminary study for DNSH (2). The effects of exposing the hydrazine and hydrazones to an air sample containing high levels of ozone, with and without water vapor, are shown in Figure 6a-d. It is apparent that regardless of the humidity and presence or absence of the catalyst, there is no net oxidation of the hydrazones. However, at 300 ppbv ozone, there was -40% reduction in the hydrazine reagent when the silanized glass bead sorbent was used. This is improved to - 2 5 % by using the octadecyl silica sorbent. Interferences of this magnitude are not severe enought to have an impact on sample
derivatization, because we typically work with at least a 10-fold excess of hydrazine. Isoprene Oxidation within the Cartridge. We carried out a preliminary investigation of the effects of sampling an airstream containing 10ppbv isoprene, which is comparable to levels observed in a forest atmosphere during the warmer seasons (9,IO),along with various levels of ozone and relative humidity. At high concentrations of O3 and HzO, 300 ppbv ozone, and =loo% RH, the oxidation of isoprene produced the expected formaldehyde, methylvinyl ketone, and methacrolein for both glass beads and CIS. In addition, a mixture of Crj-C~oaldehydes and several unknown peaks were produced. This likely occurred as a result of free-radical polymerization of isoprene initiated by the Criegee biradical intermediate. I t was also observed that at high levels of ozone and relative humidity (300 ppbv, ~ ~ 1 0 0RH) % the hydrazine and hydrazones disappeared almost completely from the chromatogram when TCA was not used. Initially, it was thought that the radicals might have reacted at the dimethylamino group, thereby quenching the fluorescence. However, it was determined that under these conditions all of the hydrazine and hydrazone peaks were destroyed in UV absorbance chromatograms as well. This leads us to believe that either a polymer product, which incorporated the DNSH into its structure, was formed and did not elute from the cartridge or, less likely, the radicals produced in the cartridge destroyed the aromatic ring system. Additional work to further characterize the potential in-cartridge reactions of isoprene and other reactive species of interest, including acrolein, methacrolein, and methyl vinyl ketone, with ozone is in progress. A more thorough understanding of this potential interferences is important to any cartridge sampling method, including the DNPH method, for atmospheric carbonyl compounds.
Conclusions Water vapor is a serious interference in the DNSH derivatization technique that can be almost entirely eliminated through the use of octadecyl silica as the sorbent in combination with trichloroacetic acid. Ozone is not a significant interference, for mixing ratios up to 300 ppbv, provided that an excess of hydrazine is present. The destruction of the hydrazine reagent by ozone is less for octadecyl silica than for silanized glass beads. Artifact peaks may be created when the air sample contains 10 ppbv isoprene and high levels of ozone and relative humidity, and this effect requires further investigation for the DNPH technique as well.
Acknowledgments We thank Paul Goldan and Bill Kuster for their contributions to the designs of the ozonizer and humidifier, David Parrish and John Holloway for assistance in calibration of the ozonizer, Shelley Copley for helpful discussions of the hydrolysis mechanism, and Fred Fehsenfeld and Jim Roberts for a number of helpful suggestions. This work was supported by a grant from the Global Change Program of the National Oceanic and Atmospheric Administration (Grant GC91135). D.R.R. thanks CIRES for a Graduate Student Research Assistantship during part of this work. Environ. Sci. Technol., Vol. 27, No. 13, 1993
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Literature Cited Nondek, L.; Milofsky, R. E.; Birks, J. W. Chromatographia ~. 1993,32, 33. Nondek, L.; Rodier, D.; Birks, J. W. Environ. Sci. Technol. lY92,26,1174. Rodier, D.;Birks, J. W., University of Colorado, Boulder, CO, submitted for publication 1993. Goldan, P. D.; Kuster, W. @. NOAA Aeronomy Laboratory, Boulder, CO, personal communication, 1993. Grosjean, D.; Fung, K. Anal. Chem. 1982,54,1221. Smith, p. A. S. Derivatives of Hydrazine and other Hydronitrogens Having N-N bonds; Benjamin/Cummings: Reading, MA, 1983; p 15.
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(7) Smith, D. F.; Kleindeinst, T. E.; Hudgens, E. E. J . Chromatogr. 1989,483,431. (8) Arnts, R.R.; Tejada, S. B. Enuiron. Sci. Technol. 1989,23, 1428. (9) D.;Wofsy, S. C.; Jacob, D. J. Geophys. Res. 0 2 Pierotti, . , 1990,95, 1871. (10) Martin, R. S.; Westberg, H.;Allwine, E.;Ashman, L.; Farmer, J. C.; Lamp, B. J. Atmos. Chem. 1991,13, 1.
Received for review March 4, 1993. Revised manuscript received August 31, 1993. Accepted September 13, 19933.' a Abstract published in Advance ACS Abstracts, October 15,1993.
