Environ. Sci. Technol. 1993, 27, 749-756
Method for the Determination of Some Soluble Atmospheric Carbonyl Compounds Yln-Nan
Lee' and Xlanliang Zhou
Department of Applied Science, Environmental Chemistry Division, Brookhaven National Laboratory, Upton, New York 11973 A technique was developed for the measurement of soluble atmospheric carbonyl compounds, which uses a pyrex coil gas-liquid scrubber sampler in conjunction with a highperformance liquid chromatograph (HPLC) equipped with a UV-visible detector for separation and identification following derivatization with 2,4-dinitrophenylhydrazine (DNPH). Carbonyls exhibiting a Henry's law solubility similar to or greater than that of formaldehyde (FA) can be determined by this method; these include FA, glycolaldehyde (GA), glyoxal (GL), and methylglyoxal (MG). The typical sampling and analysis conditions are as follows: gas and liquid flow rates through the coil 2.3 L min-l and 0.30 mL min-l, respectively; derivatization time 4 h at [DNPH] = 0.10 mM and pH 2.4; HPLC sample injection volume 1.2 mL; 6-mm path-length flow cell (360 nm for FA and GA; 400 nm for GL and MG). Based on liquid standards and field-developed chromatographic characteristics, the limits of detection are -0.005 ppb (in the gas phase) for MG, -0.01 ppb for GL, and -0.02 ppb for FA and GA. Because of the short air-liquid contact time in the coil sampler (510 s), interferences from aqueous-phase reactions of 0 3 are insignificant. Also, at the low pH of the scrubbing solution, interference resulting from reactions of carbonyls with S(1V) is unimportant. A number of other potential interferences were examined and found to be unimportant. This technique was employed in a field measurement at a rural site in Georgia as part of the Southern Oxidants Study in summer 1991. The concentrations of FA, GA, GL, and MG observed ranged up to 8, 0.6, 0.1, and 0.2 ppb, respectively.
Introduction Atmospheric carbonyl compounds are either derived from direct emissions ( I ) or produced as reaction intermediates from oxidation of hydrocarbons initiated by OH radicals and 03 molecules (2-5). Because of their photolysis reactivities in the solar spectrum of the lower troposphere, carbonyls play critically important roles in atmospheric free-radical production and photooxidant formation (e.g., refs 6 and 7). The identification and characterization of atmospheric carbonyls therefore constitute important tasks in gaining insight into not only reaction mechanisms by which hydrocarbons are oxidized and degraded but also pathways and efficiencieswith which photooxidants are generated. Currently, a number of atmospheric carbonyls are being determined on a fairly regular basis (4),mostly with a trapping technique using solid adsorbents coated with derivatization reagents such as 2,4-dinitrophenylhydrazine (DNPH) followed by liquid chromatographic analysis of the derivatives (8-13). These methods, however, focus primarily on relatively nonpolar species such as formaldehyde (FA), acetaldehyde, and acetone and are not suitable for highly polar and highly water soluble species, such as a-oxygenated carbonyls including glycolaldehyde 0013-936X/93/0927-0749$04.00/0
0 1993 American Chemical Society
(GA), glyoxal (GL), and methylglyoxal (MG). Measurement of the latter compounds has been sparse; indication of the presence of some of these species,e.g., methylglyoxal, comes mainly from analysis of rainwater and fogwater ( 1 4 , 15). Since this class of carbonyls is believed to be produced as intermediates in the oxidation of atmospheric hydrocarbons such as isoprene and toluene (3, 16, 17), information on their concentrations is critical to our understanding of the atmospheric oxidation of these hydrocarbons. Furthermore, since these soluble species can be efficientlyremoved from the atmosphere by wet scavenging processes, knowledge of their distributions can lead to an improved assessment of atmospheric lifetimes and transport scales of hydrocarbons in general. In this paper, we describe amethod for the determination of gas-phase concentrations of the following carbonyls: glycolaldehyde,glyoxal, methylglyoxal, and formaldehyde (the Henry's law solubilities are listed in Table I). This method makes use of a coil gas-liquid scrubber sampling technique customarily applied to soluble species such as HzOz (20) and HzCO (21) in conjunction with the highly sensitive and selective analysis method of DNPH derivatization-HPLC separation (9). This technique was deployed in a field measurement at a rural site in southeastern Georgia in summer 1991,and the gas-phase concentrations of these a-oxygenated carbonyls along with formaldehyde were determined. Some results obtained from this field measurement are also briefly reported.
Experimental Section Sampling Technique. The sampling of water-soluble gaseous species by scrubbing with condensed water has been achieved in three different fashions: glass coil with concurrent gas and liquid flows (201,diffusion denuder tubing with countercurrent gas and liquid flows (22),and cryogenicfreezing of gaseous speciesalong with water vapor (23). In this work, we selected the glass coil sampling technique primarily because of the ease with which this system can be constructed in the laboratory and set up and maintained in the field. The glass coil sampler employed is essentially identical to that reported by Lee et al. (24). The sampling assembly consists of three parts (Figure 1):an all-Pyrex 10-turn coil (helix diameter -2 cm) made of 0.2-cm4.d. glass tubing for gas-liquid contact and scrubbing of soluble gases, an inlet section upstream of the coil for introducing sample air and scrubbing solution, and a widened glass section downstream of the coil for gas-liquid separation. Since gas inlets have been found to contribute to sample loss or retention for highly soluble gases, e.g., HzOz (25), we adopted two measures to minimize such surface effects: (1) the coil sampler is placed directly at the desired sampling spot to eliminate gas sample conduit tubings, and (2) the gas inlet to the coil is reduced to a pinhole of diameter -0.1 cm on a short piece of 'Is-in. Teflon tubing connected upstream of the coil, which also serves as the inlet of the scrubbing solution (Figure 1). Environ. Sci. Technol., Vol. 27, No. 4, 1993 749
Table I. Henry's Law Constants of Some Important Soluble Atmospheric Carbonyls species
HzCO HOCHoCHO CHOCHO CH3COCHO HOCH2COCH3
H, M atm-'
ref
3.0 x 103 4.6 x 103 4.1 x 104 1.0x 105 23 x 105 1.4 X lo6 3.7 x 103 3.7 x 104 1.8 x 103
Reference 18, 25 "C. Reference 19, 22 "C. 114" OD
4
-
sample time of 20 min, which was taken as the time over which the ambient sample is averaged. DNPH Scrubbing Solution. The DNPH derivatizing reagent, which also served as the scrubbing solution, was prepared largely as described by Kieber and Mopper (26). Briefly, reagent-grade DNPH (Kodak, 198%) was purified by recrystallizing twice from HPLC-grade CHaCN (Baker Chemical Co. or Baxter), dried in a vacuum desiccator for 2 days, and stored in the dark. A concentrated DNPH stock solution was prepared by dissolving 1.5 g of the purified DNPH crystal into a 300-mL solution of a 1:4:5 mixture of concentrated HCl (or 6 M HzS04), HzO, and CH3CN. This stock solution, stored in a dark glass container in a refrigerator, was stable for more than 2 months. A working DNPH scrubbing solution was prepared by diluting typically 1.6 mL of the concentrated stock with 450 mL of water (Millipore Milli-Q filtered, resistivity 18 MQcm at 25 "C) and extracting overnight with three portions of 10mL of CC14 to remove any residual hydrazones derived either from the concentrated DNPH stock solution or from the dilution water used. This purification step permitted consistently low and stable blank values and, therefore, low limits of detection. The DNPH concentration and pH of the scrubbing solution prepared this way were 100 f 2 pM and 2.40 f 0.05, respectively. Sample Collection Efficiency. The efficiency with which gases are scrubbed by a coil sampler is dependent on their Henry's law equilibrium constants (H) and on the rates with which such equilibria are established. Under Henry's law equilibrium, the ideal collection efficiency, Po, can be expressed as
-
13 mm OD+
4 rnm OD
2 rnrn ID
2mrnID+
y j.
Scrubbina Solution oui to Peristaltic Pump +------8ccm
___i
Flgure 1. Schematic diagram of the coil scrubber assembly (MFC, mass flow controller).
Po = [l + Fg/(HRTFl)l-l
The scrubbing solution is pumped up to the coil through a piece of thin Teflon tubing (Spaghetti PTFE, i.d. 0.05 cm, Berghof) by a peristaltic pump (4-channel RabbitPlus, Rainin), equipped with a 0.05-cm4.d. silicone pump tubing. The return flow from the gas-liquid separator is achieved by the same peristaltic pump on a different channel under the identical conditions. Even though the pumping rates of the two channels are nominally identical, the return flow is invariably slightly smaller than that pumped into the coil because of some evaporation during gas-liquid contact. This reduced flow helps to maintain a sharper time resolution of the samples by (1)avoiding accumulation, therefore mixing, of liquid at the gas-liquid separator and (2) allowing air bubbles to be regularly drawn into the return flow forming liquid segments which minimize axial mixing. The liquid samples were collected, measured, stored in Teflon vials, and analyzed after a specified derivatization time. The liquid flow rate, which permitted knowledge of the actual liquid volume into which the sample gases were incorporated, was calculated for each sample from the known volume and collection period. The sample gas was drawn through the coil assembly by a metal bellows pump connected to the gas-liquid separator of the glass coil assembly; the flow was regulated by a mass flow controller (0-5 L min-1 (std), Tylan) and recorded on a strip chart recorder. The liquid and gas flow rates were typically maintained at 0.30 f 0.02 mL min-l and 2.3 f 0.1 L min-l, respectively. The sample volume collected was typically 6 mL, corresponding to a 750
Environ. Sci. Technol.. Vol. 27, No. 4, 1993
(1) where R is the gas constant, Tis the absolute temperature, and Fgand Fl are the gas and liquid flow rates, respectively. For the typical experimental conditions we employed, Le., FJFl = 7.7 X l O 3 , P " I99% for H 1 3 X lo4M atm-l. The upper limit of Po for formaldehyde, the least soluble species in Table I, is -93 % . The actual time required for establishing gas-liquid equilibrium in a helical coil is not attainable from first principles. An estimate, however,may be calculated using the Gormley-Kennedy equation (27) for molecular diffusion in a laminar flow developed in a cylindrical tube for gases that are irreversibly adsorbed on the wall upon impinging. Using a gas diffusivity of 0.1 cm2 s-l, we calculated an efficiency of 90% (for irreversible uptake) in the linear length of the coil, -54 cm, at 2.3 L min-I. This efficiency is expected to increase if the gas-phase mass transfer is enhanced by turbulent mixing over that governed by molecular diffusion and would decrease if irreversible uptake on liquid surface is not achieved (28). The empirical collection efficiency, p, of the coil was determined using two identical coils connected in series and measuring the amount of material collected by each scrubber. In this experiment, the gas inlets of the coil scrubbers, i.e., the short Teflon tubing with the pinhole, were replaced by a piece of miniature T-connector (i.d. -0.1 cm) so that the two coils were connected together to a source of standard gas in such a way that the second coil was sampling the effluent gas of the first. Each coil had its own separately maintained flow of scrubbing solution. Since the collection efficiencies of the two coils are expected to be nearly identical, the concentrations C1
100
90
80
? CI
70 60
50 40
30' ' ' ' '
0
' ' ' ' 0.5
1 , / , , 1 , , , , 1 , , , , / , , , /
1
1.5 F , Llmin 9
2
3
2.5
Figure 2. Collection efflclency of the coil for formaldehyde: A, pH 2.10; 0,pH 2.44; 0 , pH 3.00.
Table 11. Scrubbing Efficiency of the Ten-Turn Glass Coil and the Limit of Detectioin of the Coil Method efficiency? % Fg= l.OOc
species
Fg= 2.3OC
HzCO HOCHzCHO CHOCHO CH3COCHO
41 100 84 88
68
100 98 98
LOD,bppb 0.02 0.02 0.01 0.005
The uncertainties are typically f 5 % , Conditions: liquid flow rate, 0.30 f 0.01 mL rnin-'; [DNPH] = 0.10 mM; pH 2.44 f 0.05; T = 25 f 2 OC. F = 2.3 L min-'. Liters per minute.
and Cz collected by coil 1 and coil 2, respectively, are related to the collection efficiency by
p = 1- C,/C,
(2) The standard carbonyl gases were generated by passing Nz (ultrahigh purity, 99.999%) through a bubbler gasliquid reactor (29) containing aqueous carbonyl solutions of known concentrations. The gas-phase concentrations of the carbonyls thus produced were maintained typically at 10 ppb. The collection efficiency for formaldehyde was determined as a function of gas flow rate and the pH of the scrubbing solution (Figure 2). The lowering of the efficiency with increasing pH at a fixed gas flow rate may indicate that the acid-catalyzed hydration of the dissolved HzCO (30),which is responsible for establishing the overall solubility of formaldehyde, is partially rate limiting. The converging of the efficiencies at -41 % at higher gas flow rate, i.e., 2.3 L min-l, presumably reflects the fact that the liquid-side mass transfer is enhanced by a thinner and faster flowing liquid film induced by the higher gas flow, at the expense of a shortened gas-liquid contact time. Under this condition, the uptake may be dominated by either aqueous or interfacial mass transfer (28). The collection efficiencies for glycolaldehyde, glyoxal, and methylglyoxal are significantly greater than that of formaldehyde (Table 11))but only GA exhibits a 100% efficiency. It should be pointed out that, despite the lower collection efficiency for FA, we chose the high sampling gas flow of 2.3 L min-l to maximize the sensitivity for the a-oxygenated compounds whose atmospheric concentrations are expected to be significantly lower than that of formaldehyde (17). The temperature effect of the collection efficiency for FA was briefly studied. Maintaining the coils at 0 "C using an ice bath, we increased the
Figure 3. HPLC chromatogramof the soluble carbonyl derivativesand the elution gradient profile: concentration of standards 1.0 pM. The three mobile phases that total 100% are, from the bottom, water, methanol, and acetonitrile.
-
-
efficiency by 20 9% at Fg= 2.0 L min-l and 10 % at Fg = 2.3 L min-1 over that at -25 "C (all at pH 2.44). We conclude that a temperature variation of f10 "C in the field leads only to a 5 3 % change in the collectionefficiency for the conditions we employed. HPLC Analysis and Calibration. A ternary gradient high-performance liquid chromatograph (SpectraPhysics, Model SP8800)equipped with a 25-cm Cureversephase column (Alltech; packing material 5-pm Hypersil ODs) was used in conjunction with a programmable UVvisible detector (Spectra-Physics, Model Spectra 200) for the separation and detection of the hydrazones. The signal was processed by an integrator (Spectra-Physics, Model Chrom-Jet). The solvents used were H20, CH3CN, and CH30H (both as HPLC-grade solvents from either Baxter or Baker). The gradient mixing was achieved by lowpressure mixing with the solvents degassed by He purging. The eluant flow rate was maintained constant at 1.6 mL min-l. The carbonyl compounds used for calibration and for establishing the HPLC separation conditions were all purchased from Aldrich in the highest purity available (FA,37 % solution; GL and MG, 40%solution; GA, dimer). Although pyruvic acid (PA) and hydroxyacetone were included in our HPLC analysis, their ambient concentrations were not determined (see below). Liquid standards were prepared by mixing small portions of the diluted solutions of these carbonyls (at millimolar levels) with the DNPH scrubbing solution and allowing for overnight reaction (see the kinetics section below). A chromatogram showing the separation of all the carbonyls along with the solvent gradient profile is given in Figure 3 (360 nm): baseline separation is achieved for all the species except GA and PA and the retention times (in min) are 8.2 (GA), 12.5 (FA),24.0 (GL), and 25.0 (MG). The peak between Environ. Sci. Technol., Vol. 27, No. 4, 1993
751
100
.-C
u) I
t; i.0
0.5
1 .o
1.5
i
2.0
Conc., pM
Figure 4. Callbration curves of the four soluble carbonyl compounds. The solid lines are the least-squares best flts of the data; the slopes and P are given in parentheses: 0 , formaldehyde (370, 1,000); 0, glycolaldehyde(253,0.996); A, glyoxal (454,0.994);0, methylglyoxal (481, 0.996).
GL and MG is derived from the MG stock, possibly as an impurity. In all subsequent analyses and calibrations, the UV-visible detector was programmed at 360 nm for FA and GA and 400 nm for GL and MG to maximize the sensitivities for the respective hydrazones (26). The calibration curves of FA, GA, GL, and MG were made using carbonyl standards covering typically 0.1-1.5 pM (Figure 4); a linear relationship between concentration and integrated peak area was observed for all the carbonyls within the concentration ranges of interest. The small positive and negative intercepts for GL and GA, respectively, are explained below. It may be pointed out that although a sample injection volume of 0.50 mL was used in establishing the calibration curves, avolume of 1.23mL was used in field analysis to increase the sensitivity of this method. We noted that the quality of the HPLC separations did not change with the increased injection volume. For the sampling conditions of Fg = 2.3 L min-l, FI= 0.30 mL min-’, an HPLC injection volume of 1.23 mL, and a scrubbing efficiency of loo%, a 1 pM aqueous concentration corresponds to 3.2 ppb gaseous concentration. It may be pointed out that hydroxyacetone exhibited a sensitivity significantly lower than the rest of the compounds tested, Le., by a factor of -50, too low to make this technique useful. This observation is consistent with the low sensitivity of ketones in general for a number of commonly employed derivatizing reagent (31). In addition, with the present gradient, PA is not completely resolved from FA, which is present at much greater concentration over PA, rendering the current method unsuitable for pyruvic acid. Blank Corrections. The background signals of the DNPH solution that must be corrected for were obtained by collectingthe DNPH scrubbing solution that had flowed through the entire liquid delivery system but not been exposed to the ambient air. This was achieved by two different approaches: One involved the sealing off the inlet pinhole of the glass coil with Teflon tape so that the air flow was cut off. The second arrangement shorted the liquid flow with the glass coil bypassed. Since the two approaches yielded nearly identical results, we settled on the second one for it was much less time consuming. The background signals were found to be quite small for formaldehyde and nearly nondetectable for the rest. The maximum background values observed during a 4-week 752
Environ. Sci. Technol., Vol. 27, No. 4, 1993
1 0
100
200
400
300
t, rnin
Figure 5. DNPH derivatization kinetlcs of some carbonyls. All are at 0.40 pM except for hydroxyacetone,2.0 pM. The lines are the flrstorder fits to the data, and the first-order rate constants (In mln-I) are given in the parentheses: A (- ---), glyoxal (0.034); 0 (- -), methylglyoxal (0.01 1); 0 (- -) glycolaldehyde (0.028); (-), formaldehyde (0.039); +, pyruvicacki(0.039); 0 (- -), hydroxyacetone (0.049). For clarity, a curve for PA Is not shown.
-
.
field measurement corresponded to -0.02 ppb for FA, 50.01 ppb and GA, and 10.002 ppb for GL and MG. DNPH Derivatization Kinetics. The reaction kinetics of DNPH with the soluble carbonyls were determined in order to evaluate the appropriate derivatization time. In this study, known amounts of carbonyls were allowed to react in the DNPH scrubbing solution, and the hydrazones produced were monitored as a function of time. The results obtained for the typical conditions employed, i.e., [DNPH] = 100pMandpH 2.45 (at room temperature), are shown in Figure 5. Since the concentrations of the carbonyls used (0.4 pM, except hydroxyacetone, 2 pM) were much smaller than that of DNPH, pseudo-first-order conditions were obtained. The characteristic reaction times (the inverse of the pseudo-first-order rate constants) are comparable for FA, GA, PA, and GL, Le., -30 min. Hydroxyacetone is the shortest, -20 min, and MG the longest, -90 min. The pH dependence of the derivatization kinetics was also evaluated for formaldehyde at two pH’s: 2.03 and 2.82. The results showed that the reaction was slowed by 20-40% compared to that at pH 2.45. Further, the equilibrium concentration of the hydrazone was also decreased by 30 % . Consequently, pH 2.4 appears to be optimal for DNPH derivatization and was used in all analyses. Because of the rather slow kinetics, the DNPH derivatization time needed is fairly long. In order to reach a >95% completion, a reaction time of - 4 h is necessary for the slowest reaction. In our 1991 Georgia field measurement, when this technique was first employed, the HPLC brought to the field was also needed for other analyses, making it difficult to carry out carbonyl analysis consistently on a fixed time interval of 4 h for all the samples. Consequently, we allowed overnight reaction and analyzed the samples typically 16-24 h after collection. A concern in applying this long derivatization time is the stability of the hydrazones. This aspect was examined in the laboratory by checking the hydrazones as a function
-
of time; they were found to be stable for a 24-h period. The exception is glyoxal, for which a -10% decrease was observed. A second concern stems from possible contamination of the samples by the air of the indoor environment of, for example, our research van. Consequently, we tested a few samples by splitting them into two portions, one stored with caps on and the other with caps off. The fact that no significant difference (*5% ) was observed between those two fractions of the same sample suggested that no significant contamination resulted from the indoor air. Finally, in view of the rather long derivatization, the aqueous stability of the carbonyl compounds themselves must also be examined. In this regard, we prepared diluted aqueous solutions of these carbonyl compounds (all at 2.5 pM, except hydroxyacetone, 12.5pM) and examined their concentrations as a function of time up to 100 h, using the DNPH-HPLC technique with a fixed 4-h derivatization time. The results showed that all of these carbonyls were stable under slightly acidic conditions, Le., at pH 2.82 and 4.25, up to 100 h. An exception is glyoxal which showed an -10% decay in 24-h period. The actual effect of this decay, however, may be unimportant as the DNPH derivatization of glyoxal is complete in -2 h.
Results and Discussion Ambient Measurement. The technique described in this work was deployed in the field for the first time in summer 1991 during a field expedition as part of the Southern Oxidants Study program aimed at gaining understanding of ozone formation in rural areas of the south. The sampling site was at a clearing inside George Smith State Park, -60 miles west by northwest of Savannah. In our setup, all the equipment, e.g., the peristaltic pump and the HPLC, was housed in a mobile research van. The coil assembly was placed at 1m above and -0.7 m away from the roof of the van, or -4 m above the ground, and the metal bellows pump pulling sample air was placed underneath the van with the mass flow controller located inside the van. The coil and the tubings delivering the scrubbing solution were shielded from the sunlight by A1 foil. The coil was also protected from rain by an 8-in. plastic funnel placed ,- 6 in. above in an inverted position. A typical chromatogram of the HPLC analysis of a sample (collected 1953-2015, July 24, local time) is shown in Figure 6A along with that of the corresponding blank. The blank shows negligible background for GA and MG, a fairly small background for FA, and a baseline shift near where GL is eluted. The peak at 16.5 minis an impurity of the scrubbing solution that increased in size with the age of the scrubbing solution, which was prepared fresh every other day. When the solution was fresh, this peak was almost nondetectable (Figure 6C). The sharp drop at 20 min reflects detector wavelength change from 360 to 400 nm and the accompanied auto zeroing. The skewed peak at -23 min, which is always observed, is probably derived partly from impurities in the scrubbing solution and partly from refractive index change associated with the solvent gradient. These peaks are unimportant as they are not near and therefore not affecting the target peaks. Inspection of the chromatogram of the sample (Figure 6A) shows that the correction of small background for FA and MG is straightforward. The background correction for GA and GL, on the other hand, may introduce somewhat larger uncertainties. In the case of GA,a straight
-
-
-
Flgure 6. Representativechromatogramsof field samples and blanks obtained during a field measurement in Georgla, 1991: (A) sample blank of July 24; (C) sample 0041-01 15, July 1953-2015, July 24; (6) 31. Wavelength switching took place at t = 20 min for (A) and (6) and at t = 15 min for (C).
valley to valley baseline was used in the integration routine. Use of a straight tangent skim rather than a concave baseline may lead to a slight underestimate of the concentrations of GA. The magnitude of this negative bias is estimated to be 110%. This integration protocol also gives rise to the slight negative intercept observed in GA's calibration curve. In the case of GL, we used peak height rather than peak area to minimize the uncertainties associated with the baseline shift which gave rise to a slight positive intercept in the GL's calibration curve. The peak heights were then converted back to areas using a linear correlation established between those two quantities at higher reagent concentrations, i.e., 10.2 pM. In making the background correction, we took the DNPH scrubbing solution that had been exposed to the liquid plumbing system but not to the ambient air as blanks. Ideally, a blank should be a "sample" which has been exposed to the ambient air stripped of the specific compounds of interest. However, methods for removing Envlron. Scl. Technol., Vol. 27, No. 4, iQQ3 769
the carbonyls, such as use of an activated charcoal filter, would also remove many other trace atmospheric constituents including ozone, making such an approach less than ideal. To lend support to the current practice, we examined a possible surrogate of such an ideal blank, a sample collected from 0041 to 0115 on July 31 (Figure 6C). Since it rained heavily on the preceding day (2.1 cm between 8 a.m. and 9 p.m. on July 30), and the sample was taken over the midnight hours, any highly soluble species, in the absence of important sources, should have all but disappeared. This expectation is consistent with the zero concentrations indicated for GA, GL, and MG in this sample, namely, the chromatogram of this sample is nearly identical to that of the blank (Figure 6B) so far as the highly soluble species are concerned. One apparent exception is that formaldehyde is still present in this night sample, consistent with its borderline solubility. From this surrogate blank, we conclude that the procedure used for establishing background corrections is indeed reasonable. It is interesting to note that the skewed peak at -23 min is somewhat larger in the blank than in the sample (Figure 6B and C). One possible explanation is that this peak may be due to a relatively volatile impurity of the scrubbing solution that got partially stripped during contact with ambient air. The impurity peak at -16.5 min is absent in Figure 6C because the scrubbing solution was only -4 h old. During our field measurement in Georgia between July 23 and August 16,1991, we collected typically 5 samples a day and made a total of 123 determinations of ambient concentrations of the soluble carbonyls. The variabilities of gas and liquid flows in 24-h period were comparable at -5%. In Figure 7, we display one day's record (August 4,1991) of the four carbonyls. The concentrations of GA, GL, and MG exhibited strong diurnal dependence, reaching maxima during early afternoon and reaching zero at around midnight. These nighttime minima are expected for highly soluble species that are subject to rapid loss by dry deposition. The concentrations of FA, GA, GL, and MG at this rural site for the period mentioned, ranged up to 8,0.6,0.1, and 0.2 ppb, respectively. An account of the measurement results will be reported in detail elsewhere. Measurement Uncertainties and Detection Limit. The limits of detection of this technique and its measurement uncertainties are governed not only by the method procedures and intrinsic instrument sensitivity and stability but also by chemical species present in the atmosphere that can affect the chromatographic separation and quantitation. They are considered below. Procedural and Instrumental Uncertainties. The uncertainties associated with the various processes involved in the methodology are listed in Table 111. While the flow rates of both the sample air and the scrubbing solution probably cannot be maintained better than k5%, the derivatization time can be improved to f 5 5%. Judging from standard calibration runs, the uncertainty in chromatographic integration is the smallest for formaldehyde and methylglyoxal, i.e., f 5 % , because of good baseline resolutions for these peaks. The uncertainties for glycolaldehyde and glyoxal are 10% ,because the former elutes as a shoulder peak on the DNPH reagent peak and the latter coincides with a minor baseline shift (Figure 6). Air Sample Matrix Effects. When this technique is applied to measuring the ambient air, unknown peaks may appear on the chromatogram, possibly affecting HPLC analysis. However, from the experience we accumulated
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754
Environ. Sci. Technol., Vol. 27, No. 4, 1993
0.6
i-
glycolaldehyde o 0
0
0
mothylglyoxal
0.06
0.04
t
0
O.OZI@,
, ,
8
, ,
I
, , ,
,;
0
0 0
,
,
0 ,
,
,:
0
0
4
8
12
16
20
24
time of day, hr Figure 7. Concentrations of the soluble carbonyls determined at a rural site in Georgia on August 4, 1991.
Table 111. Procedural Uncertainties of t h e Coil Methode sources scrubbing efficiency flow rates derivatization chromatographic integration
uncertainty, 7% -5 -5
-
-
10 1 5 for FA, 10 for MG, -15 for GAIGL
0 Conditions: gas flow rate, 2.30 L mind; liquid flow rate, 0.30 mL min-'; [DNPH] = 0.10 mM; pH 2.44.
during the 4-week measurement, we noted that, except for GA, no major interfering peaks were present (Figure 6A). The occasional detection of a peak at retention time 1min before the glycolaldehyde peak (consistent with that of glyoxylic acid) may affect the accuracy of its integration. Because of this, the uncertainty in GA is estimated to be *15 5%. Another possible source of interference is the indoor environment in which the DNPH solution is prepared and the samples are derivatized. If the air is contaminated with the carbonyls of interest, blank correction can be sizable, leading to large uncertainties. The blank values observed in both the laboratory and the instrumented van were negligible for GA, GL, and MG; the sizes of the blank peaks were near the detection limit of the system, corresponding to gas-phase concentrations of -0.002 ppb. The blank values of FA were somewhat greater, as may be expected, and they corresponded to up to 0.02 ppb. However,because formaldehyde concentration is in general quite substantial, these blanks did not lead to major
-
uncertainties. The limits of detection of this measurement technique, taking into account all of the uncertainties discussed above, the signal to noise ratio of the UV-visible detector, and the necessary blank corrections, are given in Table I1 for the specified conditions. The data collected also show that there is at least one unidentified peak that was fairly consistently present in the chromatograms at a retention time of 10.5min (Figure 6A). The identification of this peak is in progress. Possible Interferences. Although the coil technique exhibits high collection efficiencies for the soluble carbonyls of interest, it also simultaneously incorporates into the scrubbing solution a number of other soluble atmospheric species. Potential interferences from these concomitantly scrubbed materials are examined below. Negative Interferences. We first examine aqueous reactions of the carbonyls with other co-scrubbed species that would result in negative interferences. The major The species in this category include 03,SOZ,and "3. following analysis, however, shows that the contributions of these species are all quite minor. 03:With a low solubility, 1.3 X M atm-I (25 "C), ozone exhibits a small equilibrium concentration of 11.3 X M (for a gas-phase concentration of 1100 ppb). Its reaction kinetics with dissolved carbonyls is fairly slow, e.g., a second-order rate constant of -100 M-l s-1 with M, the effective formaldehyde (32). At lo31 1 X time constant for destroying the dissolved carbonyls is estimated to be 1100 days. For the short gas-liquid contact time (
