Environ. Sci. Technol. 1994, 28, 221 1-2215
Evaluation of Isoprene Oxidation as an Interference in the Cartridge Sampling and Derivatization of Atmospheric Carbonyl Compounds Danlel R. Rodler and John W. Birks’ Department of Chemistry and Biochemistry and Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado 80309-02 16
The heterogeneous oxidation of isoprene by ozone and radicals formed in the reaction of ozone with the hydrazine derivatization reagent, either 2,4-dinitrophenylhydrazine (DNPH) or dansylhydrazine (DNSH), is shown to be a serious interference in the cartridge sampling and derivatization of atmospheric aldehydes and ketones. Positive artifact peaks of formaldehyde, methacrolein, methyl vinyl ketone, and several larger carbonyl compounds are formed, presumably through the oxidation of isoprene and its polymerization products. Formation of artifact peaks is nonlinear with respect to sampling time, with longer sampling times resulting in a greater proportion of ozone/ isoprene reaction products. These previously unreported interferences are potentially significant for other atmospheric alkene species, which also may react with ozone on sorbent surfaces to generate carbonyl compounds in situ. Artifact peaks for methyl vinyl ketone and methacrolein are minimized by acidifying octadecyl silica (C18) cartridges prior to sampling, but in this case high molecular weight aldehydes are produced by the reaction of ozone with the C-18 stationary phase. Data obtained through cartridge sampling of air masses containing isoprene or other alkenes and moderate levels of ozone (250 ppbv) should be carefully evaluated with respect to this interference. Introduction Sampling of atmospheric aldehydes and ketones with solid sorbent cartridges followed by derivatization with 2,4-dinitrophenylhydrazine(DNPH) has been frequently employed during the past decade (1-8). Cartridge sampling generally offers greater sensitivity than impingers and is more convenient for field measurements. Ozone has been found to interfere with the DNPH derivatization of atmospheric carbonyls using cartridges (9) and impingers (10). At ozone mixing ratios in the hundreds of ppbv, it has been reported to reduce hydrazone yields (9), generate several products from the ozone/DNPH reaction (IO),and create artifacts from C-18 sampling cartridges (9). Potential interferences from SO2 ( 3 ) ,NO, (3), and photochemical reactions during sampling (6) also have been investigated. The effect, however, of heterogeneous oxidation of atmospheric olefins within sampling cartridges has not been reported. We have found that oxidation of atmospheric alkenes and dienes may occur in the presence of ozone or other oxidants derived from ozone and, like the oxidation of the C-18 bonded phase, appears to be related to the presence of hydrazine, acidity of the cartridge loading solution, and ozone mixing ratio of the sample. Both oxidation processes yield a wide range of positive interferences. In the case of atmospheric olefins, the oxidation products are dependent on the structure of the olefin and may be as varied as the parent compounds. However, 0013-936X/94/0928-2211$04.50/0
0 1994 American Chemical Society
oxidation of the C-18bonded-phase primarily yields several late eluting peaks that are thought to be long-chain n-alkanals (11). Both processes are variable, depending on olefin and ozone mixing ratios, the acidity of the sampling cartridge microenvironment (controlled by the acidity of the reagent loading solution, the nature of the solid sorbent surface-percent carbon loading and extent of endcapping, factors which determine the hydrophobicity of the surfaces-and the level of water vapor in the air sample), concentration of the hydrazine reagent, and the sampling duration and volume. This work presents results demonstrating the formation of artifacts from the heterogeneous oxidation of isoprene by ozone. Isoprene was selected to illustrate this effect because of its high ambient mixing ratio in forested areas and its high reactivity toward ozone. Experimental Section
Chemicals. The aldehydes, ketones, isoprene, N,iV,”,N’-tetramethyl-1,4-phenylenediamine,dansylhydrazine (DNSH),and 2,4-dinitrophenylhydrazine(DNPH) were obtained from Aldrich, Milwaukee, WI. Trichloroacetic acid (TCA) and phosphoric acid were purchased from Mallinckrodt, Paris, KY. The hydrazone standards were prepared by introducing the carbonyl compounds to an acetonitrile solution containing the hydrazine (approximately M DNPH and 10-3 M TCA) and allowing at least 1h to ensure complete derivatization. The HPLC mobile phase consisted of acetonitrile and water, both from Burdick and Jackson, Muskegon, MI. The hydrocarbonfree air used as zero air was obtained from Denver Welding Supply, Denver, CO. Controlled-pore glass beads (PG1000-120, 80-120 mesh, 22.7 m2/g) were obtained from Sigma, St. Louis, MO. The glass beads were silanized with a mixture of trimethylchlorosilane and hexamethyldisilazane, also obtained from Sigma. Apparatus. The HPLC apparatus was comprised of a Spectra-Physics SP8800 ternary gradient pump, Rheodyne 7335 column inlet filter, Rheodyne 7125 six-port injection valve, Alltech Hypersil ODS (C-18) column (15 cm, 4.6 mm i.d.), Kratos Spectroflow 773 UV absorbance detector, Kratos FS970 fluorometer, and a Hewlett Packard HP3396A integrator. Supelco SP-13P and DuPont P4000 sampling pumps were employed in sample collection. Sampling microcartridges and cartridge shells (30 mm X 3.0 mm i.d.) were obtained from Tessek (Melcor, Sunnyvale, CA) and have been described previously (12, 13). Sep-Pak C-18cartridges were purchased from Waters (Milford, MA). The 5-mL gas-tight syringes (PressureLok Series A-2) used to construct the syringe impingers were purchased from Dynatech Precision Sampling Corporation, Baton Rouge, LA Two modifications were made to the syringe. First, the plunger guide was adapted to connect to 1/8-in. stainless steel tubing, permitting the impinger to be connected to the sampling pump. The Envlron. Sci. Technol., Vol. 28, No. 12, 1994
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other modification was a fitting that connects the conventional syringe outlet to 1/16-in. Teflon tubing, thereby allowing a preconcentration column to be connected to the syringe. Preconcentration columns for analysis of the impinger samples were manufactured from stainless steel tubing (2.1 mm i.d.1 obtained from a discarded microbore HPLC column which was cut into 2.5-cm segments packed with fully endcapped Bio-Si1 C-18 HL silica (15-35 pm, 18% carbon loading) from Bio-Rad Laboratories, Richmond, CA. Ozone concentrations in the range 0-300 ppbv were obtained from a UV ozonizer of our own construction. Relative humidities from 0 to nearly 100% were generated by passing dry zero air through a humidifier tube of our own construction and were diluted as necessary with dry zero air. Gas-phase isoprene standards were generated with an exponential dilution apparatus of our own construction; it is contained within the temperaturecontrolled oven of a Hewlett Packard 5730A gas chromatograph. Tylan mass flow controllers (FC 260) were used to control and measure the gas flow rates. The UV ozone generator and exponential dilution apparatus were described previously (14, 15). Sampling Procedure. Both Sep-Pak and high-pressure cartridges were cleaned with 5 mL of purified acetonitrile prior to sampling. The solvent remaining in the cartridges was removed with UHP helium at approximately 100 mL/min for 2 min. Unless otherwise noted, the cartridges were loaded with DNPH or DNSH by pulling 2 mL of a 0.1 mg/mL DNPH or DNSH solution in acetonitrile (acidified to lo3M with TCA) through the cartridges and subsequently dried by using a flow of UHP helium. The impingers were rinsed with 3 mL of purified acetonitrile followed by 3 mL of purified water. They M were then filled with 4.5 mL of purified water TCA) to which 100 pL of chromatographically purified DNPH in 5545, water:acetonitrile was added. The purified DNPH was prepared by injecting 20 pL of DNPH in acetonitrile (8.0 mg/mL) and collecting the column effluent post-detector for the portion of the reagent peak that exceeded 2 AU. Sampling cartridges and impingers were prepared within 5 min of sampling and analyzed within 1h after the sample was taken. To collect the air sample, the cartridge or impinger was connected to a battery-powered pump and the inlet@)placed within the sampling tube of the exponential dilution apparatus. Cartridges were discarded (Sep-Pak) or repacked (highpressure microcartridges) after sampling isoprene/ozone mixtures to eliminate cross-contamination of the samples. Derivatization Procedure. The DNSH derivatization reaction for the high-pressure microcartridges was performed in-cartridge by wrapping the cartridge in aluminum foil to minimize both contamination and loss of carbonyl compounds, sealing it in a vial, heating in an oven at 60 "C for 20 min, and allowing the vial and cartridge to return to room temperature. After sampling, the C-18 Sep-Pak cartridges containing DNPH were eluted into a vial with 1mL of purified acetonitrile, and 20 pL of the vial contents was analyzed after 40 min at room temperature. The impinger solutions were allowed to react a t room temperature for 40 min prior to preconcentration and analysis. Procedure for On-Line Microcartridge and Preconcentration Column Injections. High-pressure microcartridges were used to determine the extent of heterogeneous oxidation on silica gel sorbent. Analysis of the microcartridges was performed by placing the cartridge 2212
Envlron. Sci. Technol., Vol. 28, No. 12, 1994
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T i (min) Flgure 1. Chromatogram of a 15-L air sample containing 10 ppbv isoprene and no ozone collected with a preacidified C-18 Sep-Pak cartridge.
in a holder connected to the injection valve in place of the conventional injection loop, filling the holder loop with water, and turning the valve to the inject position. The water transfers the analytes to the head of the analytical column where they are focused upon interaction with the hydrophobic sorbent. The preconcentration columns, used in the analysis of the impinger samples, were analyzed by connecting them to the injection valve in place of the injection loop, turning the valve to the inject position, and backflushing the analytes onto the column with the mobile phase. The injection valve remained in the inject position through the duration of the chromatogram. Chromatographic separation of DNPH derivatives was achieved with 5545 (water:acetonitrile) for the first 5 rnin followed by a linear ramp to 595 (water:acetonitrile) at 45 min and was maintained a t 595 for the remainder of the chromatogram. For DNSH derivatives, the separation was performed with 63:37 (water:acetonitrile) for the first 8 min followed by a linear ramp to 2575 (water:acetonitrile) a t 45 min and maintained a t 25:75 for the remainder of the chromatogram. Detection of DNPH derivatives was performed by UV absorbance at 360 nm. DNSH derivatives were detected by fluorescence with excitation at 240 nm using a >470 nm emission cutoff filter.
Results and Discussion Formation of Artifact Peaks in Heterogeneous Oxidation of Isoprene. The formation of several peaks due to the heterogeneous oxidation of isoprene is apparent from comparing the chromatogram obtained from a 15-L sample of zero air containing 10 ppbv isoprene (Figure 1) with the chromatogram obtained from a 15-L sample of zero air containing 10 ppbv isoprene and 270 ppbv ozone (Figure 2). Both chromatograms were obtained with the DNPH/Sep-Pak method. The apparent mixing ratios of the expected oxidation products of isoprene are given in Table 1. The data were obtained using Sep-Pak cartridges loaded with DNPH and TCA, except where noted, and are corrected by subtraction of the system blank (the signal measured for an air sample containing appropriate levels of isoprene and water vapor, but no ozone). The air samples (15 L)
ISOPRENU03 ARTIFACTS
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o.OOo!. 0
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Flgure 2. Chromatogram of a 15-L air sample containing 10 ppbv isoprene and 270 ppbv ozone collected with a preacidified C-16 SepPak cartridge.
Table 1. Products Formed in In-Cartridge Oxidation of Isoprene in Presence of DNPH
1 1 10 10 10 10
90 270 90 270 270 90
C-18 C-18 C-18 C-18 C-18 silica
yes yes yes yes
no yes
C1 MVK MACR (ppbv) (ppbv) (ppbv)
0.30 0.68 2.8 4.1 4.0 2.8
8
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T i (min)
T i(min)
isoprene ozone preacid(ppbv) (ppbv) sorbent ified
0
0.8 0.058
2.5 0.18
were collected over a period of 30 min for all but the silica gel samples, which were 1L in 15 min. All samples contain approximately 90 % relative humidity. Significant levels of formaldehyde (Cl) were produced for all conditions in Table 1. Observed concentrations far exceed what would be predicted for gas-phase reactions. There is an approximately 30-9residence time in the generation system prior to sample collection, and in-cartridgeresidence times for the silica gel and Sep-Pak cartridges are approximately 100 and 30 ms, respectively. Interferences from C-18 Sorbent. Several lateeluting peaks occur in the chromatogram when an airstream containing ozone is sampled with an acidified C-18 sorbent (e.g., the last six large peaks of Figure 2). The unknown peaks were absent from the cartridge blank, appeared at much lower levels in samples containing ozone that were acidified after flushing from the cartridge, and were absent in samples taken using silica gel sorbent with the same reagent loading solution (Figure 3). Reagent loading solutions containing M TCA or phosphoric acid were employed with negligible differences in the interference. The source of these peaks is likely the oxidation of octadecyl silica, which has been reported previously (9, 11). The appearance of several peaks indicates that the process is more complicated than a simple oxidation of the siloxane linkage to yield an aldehyde. Effect of Acidity on Artifact Formation. An interesting feature of the data in Table I is the effect of acidity on the apparent mixing ratios. For most acidified Sep-Pak samples, the apparent mixing ratios of methyl vinyl ketone (MVK) and methacrolein (MACR) were
Flgure 3. Chromatogram of a 1-L sample of 10 ppbv isoprene and 270 ppbv ozone collected with a preacidified silica gel microcartridge.
