Environ. Sci. Technol. 1996, 30, 859-863
Carbonyl Collection Efficiency of the DNPH-Coated C18 Cartridge in Dry Air and in Humid Air ERIC GROSJEAN AND DANIEL GROSJEAN* DGA, Inc., 4526 Telephone Road, Suite 205, Ventura, California 93003
The performance of the DNPH-coated C18 cartridge with respect to sampling of ppb-ppm levels of carbonyls has been tested in dry air (RH ) 3-7%) and in humid air (RH ) 55 ( 10%). In humid air, experiments with cartridge (upstream)-impinger (downstream) sampling trains indicated no cartridge breakthrough, and good agreement was found between measured and nominal concentrations. In dry air, there was good agreement between measured and nominal concentrations for ketones but not for aldehydes. For carbonyls produced in-situ as reaction products in ozone-alkene experiments, formation yields in dry air and humid air were the same for ketones but were lower in dry air than in humid air for aldehydes, especially for formaldehyde. For the ca. 20 carbonyls studied, the results indicate good performance of the DNPH-coated C18 cartridge when sampling carbonyls in humid air and poor performance when sampling aldehydes, especially formaldehyde, in very dry air. Implications of the results are briefly discussed.
Introduction Carbonyls in air are often measured by liquid chromatography as their 2,4-dinitrophenylhydrazine (DNPH) derivatives following collection on C18 cartridges coated with DNPH (1-9). This method has been employed to measure carbonyls in outdoor air, indoor air, and vehicle exhaust (1-9) as well as in laboratory studies of carbonyls as reaction products of the atmospheric oxidation of hydrocarbons (10-24). While the performance and limitations of the DNPHcoated C18 cartridge have been characterized in a number of studies (1-9), little information is available regarding applications in dry air. Using cartridge (upstream)impinger (downstream) sampling trains, Grosjean (5) has examined the C18 cartridge collection efficiency for formaldehyde and acetaldehyde in air at RH g 28%. Cartridge collection efficiencies were 96, 95, and 88% for formaldehyde and 99, 99, and 92% for acetaldehyde at RH ) 71, 44, and 28%, respectively. These results suggested, for formalde* Corresponding author telephone: 805-644-0125; fax: 805-6440142.
0013-936X/96/0930-0859$12.00/0
1996 American Chemical Society
hyde and to a lesser extent for acetaldehyde, a decrease in cartridge collection efficiency at the lowest humidity tested, RH ) 28%. Results regarding the sampling performance of the DNPH-coated C18 cartridge in dry air are lacking, not unexpectedly since most applications of the method involve sampling of carbonyls in humid air, i.e., ambient air, indoor air, and vehicle exhaust (1-9). The need to examine the influence of air humidity on the performance of the DNPH-coated C18 cartridge was suggested by a number of observations made in the course of recent laboratory studies of carbonyls as products of the ozone-olefin reaction (13, 15, 18, 23, 24). Formation yields of several aldehydes in dry air (RH ) 3-7%) were lower than those measured in humid air (RH ) 55 ( 10%), especially for formaldehyde. For example, yields of formaldehyde in dry air were 0.13 for 1-butene and 0.11 for 2-methyl-1-butene (24). Those measured in humid air were substantially higher and were closer to the value of 0.50 that is consistent with considerations regarding the mechanism of the ozone-alkene reaction (10-19, 23, 24), e.g., 0.41 from 2-ethyl-1-butene (24), 0.48 for 1-octene (18), 0.53 for 1-decene (18), 0.42 for β-pinene (15), and 0.34-0.50 for unsaturated aliphatic alcohols (13, 23). It is the objective of this article to show that, when compared to those measured in humid air (RH ) 55 ( 10%), carbonyl concentrations measured using the DNPHcoated C18 cartridge in dry air (RH ) 3-7%) are essentially the same for ketones but are lower for several aldehydes, especially for formaldehyde. We will verify, using cartridgeimpinger sampling trains [the performance of the impinger is not influenced by air humidity (5)], the absence of breakthrough when sampling carbonyls on DNPH-coated C18 cartridges in humid air. We will show that measured and nominal concentrations of carbonyls agree well in humid air and that this agreement holds in dry air for ketones but not for aldehydes. Using the ozone-alkene reaction to prepare carbonyls in-situ, we will compare carbonyl formation yields in ozone-alkene experiments carried out in humid air and in dry air, and we will show that the results of these comparisons are consistent with those of the comparisons between measured and nominal carbonyl concentrations. Implications of the results with respect to sampling carbonyls in air using DNPH-coated C18 cartridges will be briefly examined.
Experimental Methods Carbonyl Measurements. C18 cartridges (Sep-Pak, shortbody “Classic”, Waters Chromatography) were coated with acidic DNPH in our laboratory as described previously (25). Samples were collected for 30 min or 60 min at a flow rate of 0.7 L/min and were analyzed by liquid chromatography with UV-visible detection (8). The analytical and sampling aspects of the method have been the object of comprehensive studies (8, 25). Especially relevant to the present work are the results described in ref 25 regarding elution recovery tests, breakthrough studies, replicate analyses, and collocated cartridges. Experiments in Humid Air with Cartridge-Impinger Sampling Trains. The five carbonyls formaldehyde, acetaldehyde, 2-butanone, cyclohexanone, and biacetyl were prepared in-situ at ppb levels by reaction of ozone (0.1-0.2
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TABLE 1
Summary of Results for Cartridge-Impinger and Impinger-Impinger Sampling Trains in 2-Methyl-1-Butene-Ozone-Cyclohexane Experiments in Humid Air (RH ) 55 ( 10%) carbonyl concentration formaldehyde
acetaldehyde
2-butanone
cyclohexanone
biacetyl
upstream cartridge (ppb) 112.6 70.2 65.2 19.7 21.1 38.1 23.8 26.2 24.7 16.4 16.4 36.0 24.0 24.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 downstream impinger (ppb) 6.5 2.4 0.0a 100 100 100 100 100 100 100 100 100 100 100 % of total carbonyl collected g94.2b g96.7b 100 on upstream cartridge upstream impinger (ppb) 249.8 130.0 117.3 downstream impinger (ppb) 28.0 23.7 0.0 % of total carbonyl collected g89.9b g84.5b 100 in upstream impinger a
0.0 ) below detection.
b
20.3 34.1 45.8 35.0 35.0 26.8 23.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0 100 100 100 100 100 100 100
Lower limit, formaldehyde contamination in downstream impinger.
ppm) with 2-methyl-1-butene (1-2 ppm) in humid air (RH ) 55 ( 10%) in a 3.7-m3 all-Teflon chamber (10, 11). Ozone and 2-methyl-1-butene were allowed to react in the dark and at ambient temperature and p ) 1 atm of air in the presence of sufficient cyclohexane (400 ppm) added to scavenge the hydroxyl radical. After all of the ozone had reacted, samples were collected using cartridge (upstream)impinger (downstream) sampling trains that were collocated with impinger-impinger sampling trains. The sampling flow rate was 0.19 L/min. The impingers contained acidic DNPH in HPLC-grade acetonitrile (5). The protocol for experiments involving cartridge-impinger and impinger-impinger sampling trains has been described previously (5). Comparison of Nominal and Measured Carbonyl Concentrations. Experiments in humid air (RH ) 55 ( 10%) were carried out in 3.7-m3 all-Teflon chambers (1024) covered with black plastic. Nominal carbonyl concentrations were calculated from the amount of carbonyl injected and the known chamber volume. The experiments involved carbonyls studied singly (2-butanone, methacrolein, 2-ethylacrolein) or as mixtures (acetone + butanal + cyclohexanone; acetaldehyde + butanal + hexanal + cyclohexanone; acetaldehyde + acetone + butanal + hexanal; acetone + 2-butanone + 3-pentanone + 3,3dimethyl-2-butanone). Loss of carbonyls to the chamber wall was measured for ca. 15 carbonyls including those relevant to this study. When compared to the duration of the experiments (and also to the duration of the ozonalkene experiments described below), the measured carbonyl wall losses made a negligible contribution to the measured carbonyl concentrations. Experiments in dry air included two sets of experiments. Hexanal and nonanal were studied in our laboratory using the 3.7-m3 Teflon chambers described above but with dry cylinder air (Air Liquide, “instrument dry air” grade,
