Anal. Chem. 1984, 56,569-573
sought in the first place, and thus is impossible. Often though, the-analyst has qualitative knowledge of whether or not N,' > N:. Any knowledge of this kind can and should be put to good use. That row of AN which corresponds to the smallest value of N,' should have the largest values of the incremental molar additions. This requires abandoning the symmetric design of additions, causing the condition of (ANTAN)-lto worsen. Nevertheless, statistical precision is gained a t the expense of amplification of deterministic error. In the twocomponent example chosen, redesigning the experiTent such that Win,= ('/&Minc reduces the variance in No by approximately 50%. The above considerations suggest a procedure analogous to the Stein two-stage sampling procedure in statistics. Out of a total or rt standard additions, the first (r - .$t data points could be used to calculate a rough estimate of No. With this, AN can be rechosen so that &/&= No1/N22. In summation, the GSAM algorithm provides the obvious advantage of simultaneously estimating initial concentrations of analytes when matrix effects and mutual interference effects are present. To obtain full advantage of this technique, care
569
must be applied in the analysis.
ACKNOWLEDGMENT The authors thank Maynarhs de Koven for helpful discussions. LITERATURE CITED (1) Saxberg, 6. E. H.; Kowalski, B. R. Anal. Chem. 1979, 57, 1031.
(2) Grayblll, F. W. "Theory and Applicatlons of the Linear Model"; Duxbury Press: North Scituate, MA, 1976. (3) Moran, J. M., personal communlcatlon. (4) Rathman, L. D.; Crouch, S. R.; Ingle, J. D. Anal. Chem. 1972, 44,
1375.
(5) Mendenhaii, W.; Scheaffer, R. L. "Mathematical Statistics with Applications"; Duxbury Press: North Scltuate, MA, 1973. (6) Franke, J. P.; de Zeeuw, R. A.; Hakkert, 6. R. Anal. Chem. 1978, 50,
1374. (7) Jochum, C.; Jochum, P.; Kowalskl, B. R. Anal. Chem. 1981, 53, 85. (8) Skoog, D. A.; West, D. M. "Fundamentals of Analytlcal Chemistry"; Holt Rlnehart and Winston: New York, 1976. (9) Harwlt, M.; Sloane, N. J. A. "Handamard Transform Optics"; Academ ic Press: New York, 1979.
RECEIVED for review July 15, 1983. Accepted December 7, 1983. This material is based upon work supported by the National Science Foundation under Grant CHE-8004220.
Portable Generator for On-Site Calibration of Peroxyacetyl Nitrate Analyzers Daniel Grosjean,*' Kochy Fung, John Collins, Jeffrey Harrison, and Edmund Breitung
Environmental Research & Technology, Znc., 2625 Townsgate Road, Westlake Village, California 91361
Parts per bllllon (ppb) levels of the atmospherlc pollutant peroxyacetyl nltrate (PAN, CH,C( 0)00N02) are synthesized In a portable photochemical flow reactor from lrradlated mixtures of acetaldehyde, chlorlne, and nitrogen dloxlde In alr. Direct, on-site calibration of PAN analyzers (e.g., electron capture gas chromatographs) can thus be performed In the range 2-400 ppb. The constant PAN output of the generator Is quantitated by Ion chromatography followlng alkaline hydrolysis of PAN to acetate In Implngers, whose collection efficiency for PAN and potential lnterferents has been established. Generator output stablllty, PAN output as a lunctlon of temperature, and effect of matrix air humldlty have also been characterized.
Peroxyacetyl nitrate [PAN, CH3C(0)OON02, systematic nomenclature name ethane peroxoic nitric anhydride ( I ) ] is a major product of photochemical transformations involving oxides of nitrogen in the atmosphere (2). Of the several methods available for trace levels measurements of PAN in laboratory, smog chamber, and ambient atmospheres, electron capture gas chromatography (EC-GC) is the most widely employed (2). Instrument calibrations are generally performed by dilution of high concentrations of PAN (- 1000 ppm in N,) prepared by standard methods (2) and stored in compressed gas cylinders. However, the stability of PAN in these conditions is highly erratic (2) and there has been, for a number of years, a critical need for developing a method suitable for calibration of EC-GC instruments directly in the Present address: Daniel Gros'ean and Associates, Inc., Suite 645,
350 N. Lantana St., Camarillo;&A 93010.
0003-2700/84/0356-0569$01.50/0
parts per billion (ppb) range relevant to atmospheric levels. The objective of this paper is to describe such a method, which has been developed in our laboratory to calibrate automated EC-GC instruments employed for PAN measurements as part of smog chamber studies of hydrocarbon photochemistry (3) and of field experiments conducted in urban atmospheres (4). A portable PAN generator has been constructured, in which known, stable, ppb amounts of PAN are generated by chlorine-initiated photooxidation of acetaldehyde in the presence of NO2 in air, a method first described by Gay et al. (5). The portable generator can be used for direct, on-site calibration of EC-GC PAN analyzers in the range of -2-400 ppb. The PAN output of the generator is measured by ion chromatography following alkaline hydrolysis of PAN to acetate ion.
EXPERIMENTAL SECTION PAN Analyzer. Portable PAN analyzers constructed in our laboratory include a Shimazdu electron capture detector (63Ni) and electrometer, a 0.3 X 50 cm Teflon column filled with 10% Carbowax 400 on 60/80 mesh Chromosorb G, an 8-port sampling valve with a %cm3sampling loop actuated by a timer-controlled solenoid value with a cycle time of 15 min, and an insulated, compact frame housing all components (dimensions 43 x 30 x 15 cm). Typical operating conditions are as follows: oven temperature, 30 O C ; detector temperature, 30 O C ; and nitrogen carrier gas flow rate, 40 cm3min-'. PAN is eluted in -5 min under these conditions. PAN Generator. The portable PAN generator includes a temperature-controlled permeation oven (dimensions 46 X 35 X 20 cm, weight -2 kg) and a photochemical flow reactor (dimensions 76 X 46 X 28 cm, weight - 2 kg). The oven houses chlorine, acetaldehyde, and nitrogen dioxide permeation tubes (for example, 3 cm tube for CH3CH0and wafer F653 for C1, and NOz, Metronics). Constant temperature in the range 30.0 to 45.0 0 1984 American Chemlcai Soclety
570
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
"C is achieved with a 100-W cartridge heater controlled by a circuit using a Motorola CA3059 chip, a YSI thermistor, and a box fan. Flow rates are controlled with calibrated flowmeters. The NO2 permeation rate is measured with a ThermoElectron Corp. Model 14B/E chemiluminescent analyzer calibrated according to US. Environmental Protection Agency procedures including gas-phase titration with National Bureau of Standards traceable NO and NO2 standards. The measured efficiency of the instrument's converter for reduction of NO2to NO was 198% at 375-400 "C. Nitric acid, a trace contaminant (0.4-2.9%) commonly present in commerical NO2 permeation tubes, is an interferent in the NO2 and NO, (NO + NO2) modes of chemiluminescent analyzers (6) and is removed from the air stream by collection on a nylon filter (7) upstream of the chemiluminescent analyzer. The NO2permeation rate thus measured is free of interference from nitric acid. The acetaldehyde permeation rate is measured by using the liquid chromatography method of Fung and Grosjean (8). Carbonyl impurities in the acetaldehyde permeation tube (e.g., acetic acid, formaldehyde, acetone) do not interfere with acetaldehyde measurements under these conditions. The chlorine permeation rate is not measured. While a method involving titration with NOCl and chemiluminescent detection of the evolved NO has been described (9),NOCl is a highly toxic compound. Instead of titration with NOCl, we measure the chlorine atoms output as chloride ion (see reactions 1and 2 below) by ion chromatography following effluent trapping in impingers containing 10 mL of 5 X N KOH in water. The photochemical reactor unit includes a 115 cm long, 4.5 cm diameter U-shaped glass tube and four 20-W General Electric F20T12BL black lights mounted on either side of the tube on an angled aluminum frame. The distance from the lamps to the outside wall of the tube is -5 cm. The inlet of the glass tube receives the effluent from the permeation tubes, and the outlet is connected to a 200-cm3mixing bulb where hydrocarbon-free dilution air may be added with a Brooks dual channel mass flow controller. The reactor volume is 1800 cm3,which gives residence times of -15-60 min at typical generator flow rates of -30-120 cm3m i d . Carbon dioxide, which may complicate, as carbonate, the ion chromatographic analysis of acetate after alkaline hydrolysis as is described below, is removed from the matrix air with a 15 cm long X 5 cm diameter soda-lime scrubber. PAN Synthesis. Upon irradiation of mixtures of chlorine, acetaldehyde, and nitrogen dioxide in air, PAN is formed according to the following reaction sequence involving chlorine photolysis, reaction of chlorine atoms with acetaldehyde, reaction of the acetyl radical with oxygen, and reaction of the peroxyacetyl radical with nitrogen dioxide (5) Clz + hv 2C1 (1)
--
4
C1+ CH3CHO CH3CO + 0 2
CH,C(O)OO + NO2
HCl + CHaCO
(2)
CHaC(0)OO
(3)
CH3C(O)OON02
(4)
That PAN is indeed formed in these conditions has been verified by Fourier transform infrared spectrometry analysis of the generator output in two independent laboratories. Recorded absorption bands gave an excellent match with literature data (2, 5,101. Preparation of PAN in the Static Mode. In order to carry out additional characterization and interference studies under a variety of conditions and copollutant concentrations (e.g., presence or absence of chlorine, different levels of NO2 and ozone, etc.), PAN was also synthesized in the static mode by sunlight irradiation of appropriate organic compound-NO, mixtures in purified air, using 4-m3containers constructed from Teflon FEP film. PAN was thus prepared by irradiation of mixtures of acetaldehyde (0.5-1.0 ppm), NO2 (0.5-1.0 ppm), and chlorine (0.25-0.5 ppm) as is described by reactions 1-4, as well as by irradiation of NO2(0.2 ppm) with propene or cis-2-butene (1ppm). Concentrations of PAN (electron capture gas chromatography), NO, NO2 (chemiluminescence), ozone (ultraviolet photometry), and other reactants and products were monitored during these experiments. Alkaline Hydrolysis Method. The amount of PAN synthesized in the photochemical flow reactor is determined by
sampling the reactor effluent with a microimpinger containing N KOH 10 mL of aqueous dilute alkaline solution (e.g., 5 X or NaOH). PAN decomposes in alkaline solutions to yield nitrite and acetate (2) CH&(O)OON02
+ 20H--
CH3COO- + NO2-
+ 02 + H2O (5)
Quantitation of PAN as acetate is performed with a Dionex Model 10 ion chromatograph. For typical operating conditions (8 X IO4 M sodium borate eluent, 10 pmho FS, pump rate = 30%), the detection limit for acetate ion is -0.02 g / m L . The corresponding detection limit for PAN is, for example, -2.8 ppb for a 10-mL impinger sample collected for 30 min at a flow rate of 1.0 L/min. Calibrations are performed by using standard acetate solutions in the range 0.05-10.0 gg/mL. An example of generator PAN output determination as acetate is as follows: In a typical experiment (generator temperature, 25 "C; permeation tube oven temperature, 45 "C; generator flow rate, 119 cm3 min-'; dilution air flow rate, 640 cm3 mi&; total flow rate, 759 cm3min-'), a 22-h hydrolyzate sample was found to contain 150 2.2 ( l u ) pg of acetate, which corresponds to a PAN output rate of 0.233 f 0.0034 gg min-l in the diluted air stream. This in turn corresponds to a PAN concentration of 63.6 0.9 ppb in the diluted air stream, or a generator output concentration of 406 6 ppb. Removal of PAN in the alkaline solution is verified throughout the experiment by monitoring the generator effluent downstream of the impinger with an electron capture gas chromatograph.
*
*
*
RESULTS AND DISCUSSION Generator Output Stability. For on-site calibration of field instruments, output stability is an important feature of the generator. Variations in PAN output may be due to a combination of several factors including variations in permeation oven temperature (this will affect the permeation rates and hence the PAN yield), reactor temperature, and drift of the PAN measurement device, e.g., the electron capture gas chromatograph (EC-GC). Permeation rates a t an oven temperature of 45.0 f 0.2 OC were found to vary by less than 0.2% over a 64-h period as measured by monitoring the NO2 output with a chemiluminescent analyzer. At a PAN level of 60 ppb, the stability of the PAN output was *1.2% over a 24-h period, as measured by EC-GC, and was f3.5% over a 7-day period, as measured with a chemiluminescent NO, analyzer. On-site calibrations of PAN analyzers repeated after 3 months of continuous operation in the field matched well the initial calibrations (e.g., 11/14/1980 and 01/30/1981, PAN ppb = 0.35 peak height, em, +1.05, R = 0.999, n = 8, range 2-22 ppb; 11/02/1982 and 02/23/1983, PAN ppb = 0.21 peak height, cm, - 0.003, R = 0.999, n = 10, range 10-40 ppb). PAN Output as a Function of GeneratorTemperature. During on-site calibrations, the permeation oven is held constant at 45 "C, while the generator temperature is close to, and may vary with, that of the surrounding environment. The thermal stability of PAN (equilibrium 4) is known to decrease rapidly with increasing temperature (11 )
k4 =
exp(-104 kJ/RT) s-'
(6)
e.g., the stability of PAN decreases by a factor of -20 between 15 "C and 35 "C. To investigate the temperature dependence of the PAN output, the photochemical reactor was wrapped with insulation material and allowed to stabilize as indicated by a stable PAN peak on the EC-GC chromatogram. The corresponding temperature was measured with a type K chromel-alumel thermocouple with an ice/water bath reference junction strapped onto the outside surface of the reactor tube. More insulation material was added to obtain a higher temperature, and the process was repeated to obtain the PAN output temperature profile shown in Figure 1. A significant temperature effect is observed, with, for example, a -20% de-
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
571
Table I. Summary of Matrix Air Humidity Effects and PAN Removal in Aqueous Solutionsa PAN peak height relative to dry airC PAN mode concn, ppb over water watere acidic solutionf dynamic (generator) static (Teflon chamber)
64 60 55 210 310 42
0.984 (4)g
0.982 (3) 0.943 ( 6 ) 0.907 (12) 0.972 (2) 0.957 (3) 0.982 (3)
0.963 (3) 0.986 (3)
0.903 (3) 0.918 (4)
PAN from irradiated a Complete removal of PAN in alkaline solutions was also verified in all experiments. Dew point of -15 to -30 "C at temperatures of 20 to 30 "C. acetaldehyde-chlorine-nitrogen dioxide mixtures in air. Matrix air humidified by passing over 1 0 mL of water in microimpinger. e Through 10 mL of water in impinger, flow Number of experiments in parentheses. N HCl in impinger. rate 1 L/min. f 1 0 mL of aqueous 5 X
-
- ~-
~~
-~
~~
~~
Table 11. Removal of NO, in KOH and H,O Impingers y = downstream x = upstream NO, concn, ppb NO,, PPb impinger 70-160 0.792 KOH 70-620 H2 0 0.817 crease in PAN putput when raising the generator temperature from 23 "C to 35 "C. PAN Removal by Alkaline Impingers. Removal of PAN by alkaline aqueous solutions was found to be essentially complete. With an EC-GC downstream of the impinger, measurements were performed with impingers containing 0.5 N KOH, 0.5 N NaOH, 0.1 N NaOH, and 0.005 N KOH, at sampling flow rates of 0.12,0.14,0.76, and 0.97 L min-', and with sampling times ranging from 15 min to 22 h. Complete disappearance of the PAN peak from the chromatogram was observed in all cases, for PAN generated in both dynamic (flow reactor) and static (Teflon chamber) modes, and at PAN concentrations of up to 400 ppb. Under the operating conditions described earlier and with a 2 cm3 air sample, the EC-GC detection limit for PAN is 1ppb. The trapping efficiency of the alkaline solutions is thus 199.75%. A value of 296% was independently obtained by IC determination of the acetate ion with two impingers in series. Influence of Matrix Air Humidity on EC-GCResponse to PAN. Since the water vapor content of the effluent air increases downstream of the impinger, it is important to verify the influence of matrix air humidity on the EC-GC response to PAN. Conflicting results have appeared in the literature concerning the effect of humidity on PAN measurements using electron capture detectors (12-16). In experiments conducted in both static and dynamic modes (Table I), we found a measurable, but small (13%) decrease in PAN peak height when passing a dry air stream over an impinger containing water. When the dry air stream was directed into the water impinger, a similar small decrease in PAN peak height was observed. In contrast, we observed a PAN loss of up to 10% in impingers containing aqueous acidic solutions (e.g., 5 x N HCl), or in impingers containing water but exposed long enought to the matrix air so that substantial acidification had occurred by retention of HC1, nitric acid, and other acidic compounds formed, along with PAN upon irradiation of CH3CHO-Clz-NO2 mixtures. Rationale for Determination of PAN as Acetate Rather Than as Nitrite. Alkaline hydrolysis of PAN yields equimolar amounts of acetate and nitrite ions (reaction 5), and determination of PAN as nitrite, as well as acetate, is possible (2,16-18). However, under our conditions, the unreacted NOz is expected to interfere as nitrite in aqueous alkaline impingers (19, 20)
+ HzO 2 N 0 2 + 20H2N02
--
2H+ + NO2-
+ NO3-
HzO + NOz- + NO3-
(7)
(8)
intercept
R
8.7 12.9
0.994 0.996
n 10 13
'OI
I
020
25
30
35
40
45
PHOTOLYSIS TUBE TEMPERATURE,
C'
Figure 1. Change of PAN output as a function of photochemical reactor temperature (reactor flow rate, 116 mL min-'; total flow rate, 759 mL min-l; permeation oven temperature, 45 "C).
To verify this, experiments were conducted with 50-600 ppb of NOz in purified air in both static and dynamic modes using 10 mL of water and 10 mL of 0.05 N-0.5 N KOH or NaOH in impingers, with sampling flow rates of 0.7-1.0 L min-l. The results, summarized in Table 11, confirm that -20% of the NOz is removed in water and alkaline traps, in agreement with earlier work (21),and that the amount remoed may result in a substantial interference in the determination of PAN as nitrite. In addition to direct removal of NOz, other reactions may introduce errors in the quantitation of PAN as nitrite under our conditions. NO2 may react with chlorine atoms (22)
NO2 + C1- ClONO NOz + C1- CINOz
-
or with C10 radicals (9, 22, 23) C1+ O3 C10
+
+ Oz
(9) (10) (11)
C10 NO2 CION02 (12) to form CINO, products which may decompose to nitrite in alkaline solutions. Indeed, sampling of CIONOz in the stratosphere has been achieved by use of alkaline solution-impregnated filters (24). Experiments we conducted with irradiated mixtures of 0.1-0.5 ppm chlorine and nitrogen dioxide
572
ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1984
h
8
v
h
s
4
h
0'
z + 0 0 z z
un
0
z 2 0
E
u, 6
8
Z
8+ 8+ P
i
*
G G
in air in both static and dynamic modes showed a substantial decrease in the apparent NOz concentration measured by chemiluminescence (e.g., the actual NO2 concentration plus the instrument response to ClNO, species) when inserting alkaline impingers upstream of the chemiluminescent NO, analyzer. In addition, a number of oxidants may convert nitrite to nitrate while sampling the generator effluent. For example, ozone is produced in our system upon photolysis of NOz. Molecular chlorine is readily collected on alkaline filters (25) or in aqueous alkaline solutions, where the predominant forms of free chlorine are HOC1 and C10- (26) which have oxidizing properties (27). Photochlorination and oxidation initiated by chlorine atoms have also been documented (28). The above discussion and results indicate that quantitation of PAN as nitrite could not be considered for a flow reactor whose effluent unavoidability contains unreacted materials and reaction products that are known or potential interferents. In contrast, no detectable amounts of acetate could be found in impinger samples collected during control experiments conducted with CH3CH0 in pure air (up to 1.0 ppm), mixtures of CH3CHO (0.1-1.0 ppm) and NOz (0.1-0.5 ppm), and mixtures of CHBCHO (0.1 ppm) and ozone (0.14 ppm) under our conditions. Thus, no significant oxidation of acetaldehyde to acetic acid took place in the air stream and/or the impinger during sampling. Other potential sources of interferences were investigated, including trace levels of acetic acid in the matrix air, acetate ion content of the water employed to prep- the alkaline solution, and acetic acid as an impurity in the acetaldehyde reagent. Of these, only the latter was measurable, varying from one permeation tube (or reagent bottle) to the next, and required reagent "blank" correction factors of up to a few percent. Addition Design Considerations. In its present configuration, the PAN generator provides a constant output of PAN at the ppb levels directly relevant to ambient air and smog chamber measurements. As a portable unit requiring only power and a source of matrix air, the generator is particularly suitable for on-site calibration of EC-GC PAN analyzers and has been employed in this laboratory for instrument calibrations not only at various locations in Southern California but also in the western and eastern United States. Other peroxy nitrates besides PAN can be readily obtained by replacing the acetaldehyde permeation tube by a permeation tube containing the appropriate aldehyde. For example, peroxybenzoyl nitrate can be prepared from benzaldehyde (29). Future design considerations will focus on features aimed at optimizing the generator output with respect to PAN yield. For example, it is difficult to obtain commercial permeation tubes with reproducible and optimum (with respect to PAN yield) permeation rates. The outputs of PAN and other products thus vary when one or more permeation tubes have to be replaced. This requires a complete recalibration involving the steps listed in Table 111. Constant and optimal PAN yields could be obtained by independent temperature and flow control of each permeation tube, combined with adjustments of other parameters including the reactor flow rate (i.e., residence time), the light intensity (e.g., number of lamps and distance from the reactor), and the reactor temperature.
ACKNOWLEDGMENT Fourier transform infrared spectra of our PAN generator output were kindly provided by P. L. Hanst, U.S. Environmental Protection Agency, and M. Molina, University of California, Irvine. R. Lewis and D. Nies of Environmental Research & Technology, Inc., participated in a number of PAN generator calibrations and acetate analyses, respectively. Registry No. PAN, 2278-22-0.
Anal. Chem. 1984, 56,573-575
LITERATURE CITED Martinez, R. I. Int. J. Chem. Kinet. 1980, 12, 771-775. Stephens, E. R. A&. Environ. Sci. Techno/. 1989, 1 , 119-146. Grosjean, D.; McMurry, P. H. “Secondary Organic Aerosol Formation: Homogeneous and Heterogeneous Chemical Pathways”; ERT Document No. P-A098-04, National Technical Information Service PB-82262262, Springfleid, VA, 1962. Grosjean, D. Environ. Sci. Techno/. 1983, 17, 13-19. Gay, B. W., Jr.; Noonan, R. C.; Bufalini, J. J.; Hanst, P. L. Environ. Sci. Technoi. 1978, 10, 82-85. Joseph, D. W.; Spicer, C. W. Anal. Chem. 1978, 50, 1400-1403. Groslean, D. Anal. Lett. 1982, 15, 785-796. Fung, K.; Grosjean, D. Anal. Chem. 1981, 53, 168-171. Vanderzanden, J. W.; Birks, J. W. Chem. Phys. Len. 1982, 88, 109-1 13. Hanst, P. L. Adv. Envlron. Sci. Techno/. 1971, 2 , 91-213. Cox, R. A,; Roffey, M. J. Environ. Sci. Techno/. 1977, 1 7 , 901-906. Hoidren, M. W.; Rasmussen, R. A. Envlron. Sci. Technoi. 1978, 70, 195-187. Nieboer, H.; VanHam, J. Atmos. Environ. 1978, 70, 115-120. Lonneman, W. A. Envlron. Sci. Techno/. 1977, 1 1 , 194-196. Watanabe, I.; Stephens, E. R. Environ. Sci. Techno/. 1978, 12, 222-223. Nieisen, T.; Hansen, A. M.; Thomsen, E. L. Atmos. Environ. 1982, 16, 2447-2450. Penkett, S.A.; Sandalls, F. J.; Loveiock. J. E. Atmos. Environ. 1975, 9, 139-140. Sandalls, F. J.; Penkett, S.A.; Jones, B. M. R. “Preparation of Peroxy-
573
acetylnitrate and Its Determination in the Atmosphere”; HMSO: London, 1974; AERE Report R-7807. Lee, Y.-N.; Schwartz, S.E. J. Phys. Chem. 1981, 85, 840-848. U S . Environmental Protection Agency. “Comparability of nine methods for monitoring NOp in the ambient air”, Report EPA-650/4-74-012, Washington, DC, March 1974. Cox, R. A. J. Photochem. 1974, 3 , 175-188. Uselman, W. M.;Levine, S.2.; Chan, W. H.; Calvert, J. G.; Shaw, J. H. I n “Nitrogenous Air Pollutants: Chemical and Biological Implications”; GrosJean, D., Ed.; Ann Arbor Science Publishers: Ann Arbor, MI, 1979; pp 17-54. Lee, Y. P.; Stimpfie, R. M.; Perry, R. A.; Mucha, J. A.; Evanson, K. M.; Jenning, D. A.; Howard, C. J. Int. J. Chem. Kinet. 1982, 74, 71 1-732. Boneiii, J. E.; Greenberg, J. P.; Lazrus, A. L.; Spencer, J. E.; Sedlacek, W. A. Atmos. Environ. 1978, 12, 1591-1594. Berg, W. W.; Winchester, J. W. J. Geophys. Res. 1917, 82, 5945-5953. Aoki, T.; Munemori, M. Anal. Chem. 1983, 55, 209-212. Joiiey, R. L. Prepr. Pap. Nati. Meet., Div. Envlron. Chem., Am. Chem. SOC.1982, 68-71. Oliver, B. G.; Carey, J. H. Environ. Sci. Techno/. 1977, 1 1 , 893-895. Fung, K.; Grosjean, D. Presented at the Pacific Conference on Chemistry and Spectroscopy, Pasadena, CA, Oct 26-28, 1983.
R~~~~~~~for review July 26, 1982, R~~~~~~~~~~September 2, 1983. Accepted Novmeber 7, 1983.
CORRESPONDENCE In Vivo Monitoring of Dopamine Release in the Rat Brain with Differential Normal Pulse Voltammetry Sir: Chemical neurotransmission is achieved by release of neurotransmitter substances from nerve terminals into the synaptic cleft where they partidy diffuse into the extracellular fluid. Therefore in vivo monitoring of extracellular neurotransmitter concentrations inside the mammalian brain is of pride interest. Several neurotransmitters such as dopamine (DA) are easily oxidizable and many studies (for review see ref 1and 2) have focused on the following question: Is in vivo electrochemistry capable of monitoring spontaneous DA release in the rat striatum (the striatum is a brain region densely innervated by dopaminergic terminals)? Up to now the answer to this question was no; it has been reported that extracellular DA concentration was below the detection limit (50 nM) (1-5). Moreover, such monitoring was complicated by the fact that ascorbic acid (AA) and 3,4-dihydroxyphenylaceticacid (DOPAC) which are easily oxidizable, are present in the extracellular space in much higher concentrations (1-6). Separation of AA from catechols such as DA and DOPAC was achieved by means of treated carbon fiber electrodes ( 4 , 5 ) . Unfortunately, with these electrodes, DOPAC and DA oxidize at almost the same potential. Therefore, in the present study, in an attempt to monitor DA alone, DOPAC was eliminated from the brain by inhibiting its synthesis. This was achieved by pargyline pretreatment of the rats (5). In order to measure mammalian extracellular DA concentration, we employed, in conjunction with our treated carbon fiber electrodes (7),a promising electrochemical technique developed by Osteryoung’s group (for review on electrochemical techniques see ref 8). This technique combines the advantages of differential pulse voltammetry (DPV) and of normal pulse voltammetry (NPV). Due to its sensitivity and selectivity, DPV has been widely used. With DNPV, as well as with DPV, the oxidation current is differentiated by means of a measuring pulse (see Figure 1). The advantages of NPV 0003-2700/84/0356-0573$0 1.5010
for in vivo electrochemistry have been recently pointed out (3): Since the electrode is a t a resting potential during the greater part of the scan, it minimizes the filming of the electrode surface by electrogenerated products and, therefore, it improves the stability of the response. As with NPV, but unlike DPV, the DNPV scan is entirely pulsed (Figure 1). With this technique we were able to detect very low DA concentrations (detection limit 5 nM) and to monitor from the striatum of pargyline-treated rats a catechol peak due to oxidation of the DA released in the extracellular fluid by dopaminergic nerve endings.
EXPERIMENTAL SECTION Chemicals. Dopamine (DA),3,4-dihydroxyphenylaceticacid (DOPAC), and ascorbic acid (AA) were dissolved in phosphate buffered saline (PBS) solution (KCl, 0.2 g/L; NaCl, 0.8 g/L; NazHP04.2Hz0,1.44 g/L; KHZPO4,0.2 g/L; pH 7.4). In order to prevent spontaneous oxidation of DA, AA was always present in DA solutions. However, the DA and DOPAC response did not depend on the AA concentration (between 50 and 500 wM). Treated Carbon Fiber Electrodes. Their preparation has been described elsewhere (9). Some of them have been furnished by Solea Tacussel Co. (72, rue d’hlsace, 69100 Villeurbanne, France). They were electrochemically treated according to a procedure slightly modified from that previously described (7): An anodic potential of a triangular wave form (initial potential, 0 V; amplitude 2.9 V; frequency 70 Hz) was applied for 20 s. Then, a continuous potential (-0.8 V) was applied for 5 s. Finally a continuous potential (+1.5 V) was applied for 5 s. During this treatment electrodes were immersed in the PBS solution. Apparatus. Electrochemical treatments as well as automatic measurements with DPV or with DNPV were performed by means of a new apparatus (“Biopulse”,Solea Tacussel) specially designed for this purpose. Reference and auxiliary electrodes were as previously described (7). DNPV parameters are defined in Figure 1 and their values were T = 0.5 s, t l = 70 ms, t z = 30 ms, A V = 0 1984 American Chemical Society
