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Anal. Chem. 1985, 57, 2634-2638
(CO) analysis, as a relationship between total non-methane hydrocarbon had been established with this emission product previously. Figure 3 is a chromatogram for a dilute vaporized gasoline sample. Most of the major hydrocarbons are labeled using the Table IV GC/MS identification scheme. The quality assurance experiments for repeatability, stability, detection and quantitation limits, and linearity indicate the analytical system to be useful for the routine quantitation of hydrocarbons in the C2 to C,, range.
LITERATURE CITED (1) Dietzmann, H. E.; Blank, F. M. SA€ Tech. Pap. Ser. 1980, No. 790816, 19. (2) Smith, L. R.; Black, F. M. SA€ Tech. Pap. Ser. 1980,No. 800822, ‘3.1,. Urban, C. M.; Garbe, R. J. SA€ Tech. Pap. Ser. 1979,No. 790696,
20. Cadle, S. H.; Nebel, G. J.; Willlams, R. L. SA€ Tech. Pap. Ser. 1979, No. 790694, 20. Levlns, P. L.; Kendall, D. A,; Caragay, A, B,; Leonavdis, G,; Oberholtzer, J. E. SA€ Tech. Pap. Ser. 1974,No. 740216, 11. Hollis, D. L. Anal. Chem. l9S8,38, 390-416. Grob, K.; Grob, G. J. Chromatogr. 1971,8 2 , 1-13. Bertsch, W.; Chang, R. C.; Zlatkis, 2. J. Chromatogr. Sci. 1974, 1 1 , 175-182. Williams, F. W.; Umstead, M. E. Anal. Chem. 1968,40, 2234-2244. Ave, W. A.; Teli, P. M. J. Chromatogr. 1971,6 2 , 15-27. Stump, F.; Bradow, R.; Ray, W.; Dropkin, D.; Zweidinger, R.; Sigsby, J. SA€ Tech. Pap. Ser. 1982,No. 820776, 19. Grimmer, G.; Hildebrandt, A,; Bohnke, H. ZBL. Bakt. Hyg., I . Abt. Or@. 1973, 158, 22-34. Stenberg, U.; Westerholm, R.; Alsberg, T,;R ~ u,; Sundvall, ~ ~A, in ~ “Proceedings of the 6th International Symposium on Polynuclear Aromatic Hydrocarbons”; Cooke, M., Dennis, A. J., Fisher, G. L., Eds.; Battelle Press: Columbus, OH, 1982; pp 765-771. Gross, G. P. Exxon Research and Engineering Co., personal communication, Sept 1980.
(15) Kelly, N. A. J Air. Pollut. Control Assoc 1983, 33 ( Z ) , 120-125. (16) Russell, J. W. Envlron. Sci. Technol. 1975,9 , 1175-1178. (17) cox, R. D.; Devitt, M. A,; Lee, K. W ; Tannzhill, 0. K. Envlron. Scr. Technol. 1982, 16, 57-61. (18) Nelson, P. F.; Qulgley, S. M. Envlron Sci Technol. 1982, 16, 650-655. (19) Lonneman. W. A.; Kopczynski, S. L ; Darely, P. E.; Sutterfield, F. D. Environ. Sci. Technol. 1974,8, 229-236. (20) Lonneman, W. A.; Seila. R. L.; Bufalini, J. J. Environ. Scl. Technol. 1978, 12, 459-463. (21) Eaton, H. G.; Wllllams, F. W.; Smlth, D. E. J Chromatogr. Scl. 1983, 21, 77-84. (22) Black, F. M., High, L. E.; Fontijr?, A. Envlron. Scl. Techno/. 1977, 17, 597-60 1. (23) Altshuller, A. P.; Bufalini, J. J. Environ. Sci. Technol. 1971,5,39-64. (24) Cox, R. D.; Earp, R. F. Anal. Ghem. 1982,5 4 , 2265-2270. (25) Loffe. B. V.; Isidorov, V. A.; Zenkevich, I. G. Envlron. Scl. Technol. 1979, 13,864-868. (26) McEwen, D. J. Anal. Chem. 1966, 38, 1047-1053. (27) Spindt, R. S.; Barnes, G. J.; Somers, J. H. SA€ Tech. Pap. Ser. 1971, No. 710605, 9. (28) Seiztnger, D. E.; Dimitraidas, 8. J. J. Air. Pol/ut. ControlAssoc. 1972, 2 2 , 47-51. (29) Black, F. M.; Hlgh, L. E. SA€ Tech. Pap. Ser. 1977,No. 770144, 22. (30) Pellizzari, E. D.; Bunch, J. E.; Berkley, R. E.; McRae, J. Anal. Chem. 1976,48, 803-807. (31) Hampton, C. V.; Pierson, W. R.; Harvey, T. M.; Updegrove, W. S.; Marano, R. S. Envlron. Sci. Technol. 1982, 16, 287-298. (32) Lonneman, W. A.; Bellar, T. A.; Altshuller, A. P. Environ. Sci Techno/. 1988,2 , 1017-1020. (33) Seila, R. L.; Lonneman, W. A.; Meeks, S. A. J. Environ. Sci. Health, Part A 1976,A 1I , 121-130. (34) COrnmlttee on Environmental Improvement Anal. Chern. 1983, 55, 2210-221s. (35) Stenhagen, E.; Abrahamsson, S.; McLafferty, F. W. “Registry of Mass Spectral Data”; Wiley: New York, 1974. (36) Mass SPectrometV Data Centre “Eight Peak Index of Mass Spectra”; ~ , AWRE, Aldermaston: Reading, UK, 1974; RG74PR.
for review January 7, 19B5* Resubmitted July 2, 1985. Accepted July 11, 1985.
Piezoelectric Crystal Detector for the Monitoring of Ozone in Working Environments Henrik M. Fog* Bruel & Kjaer Industri AIS, DK-2850 Naerum, Denmark
Bernd Rietz Danish National Institute of Occupational Health, DK-2900 Hellerup, Denmark
A piezoelectric crystal monitor for the detection of ozone In ambient air of working environments has been developed. The frequency decrease caused by ozone reactlng wlth the 1,Cpolybutadlene crystal coating depends on the ozone concentration in a simple way. The detection limit is below 10 ppb ozone. Interferences from nitrogen oxides, formaldehyde, carbon monoxide, and phenol are Insignlflcant. Simultaneous sampling has been performed at the Danish Welding Instltute, using the piezoelectric crystal detector, the portable A I D ozone analyzer, and Drager detector tubes. The results of these comparlson measurements show an acceptable agreement.
Ozone-a bi-free-radical of triatomic oxygen-is generated from biatomic oxygen on exposure to radiation with wavelength between 185 and 210 nm. Because these wavelengths are encountered in solar radiation, ozone can be found in the upper atmosphere. As a pollutant in the lower atmosphere ozone occurs around sources of ultraviolet radiation and 0003-2700/85/0357-2634$01.50/0
X-rays, electric arcs (welding equipment), and mercury vapor lamps and in the vicinity of electrical sources, accompanying electrical discharges. Ozone is used as an oxidizing agent in the chemical industry, for purification and sterilization of public water supplies and for bleaching of oils, paper, textiles, waxes, starch, and sugar. Health complaints associated with ozone exposure may include headache, nose and throat irritation, cough, chest pain, pulmonary oedema and inflammation of the lung tissue (I). Primarily during arc welding, high concentrations of ozone may arise, if no efficient ventilation systems and/or other ycupational health and safety protecting precautions are used. Chronic exposure to low levels of ozone has been implicated in the occurrence of chromosomal aberrations ( 2 ) . A large number of methods for the determination of ozone have been in use, involving several different analytical techniques, such as chemical oxidation (3),absorption of ultraviolet light ( 4 ) ,catalytic decomposition (5),chemiluminescence (6) or fluorescence (7), and cleavage of an olefinic bond (8, 9). Several direct-reading ozone monitors are available utilizing the chemiluminescent reaction of ozone and ethylene. These 0 i985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
instruments have been demonstrated to be specific to ozone and subject to few interferences (10). In recent years coated piezoelectric crystal detectors have become of increasing interest for monitoring traces of toxic pollutants in the air of working environments as well as in the atmosphere, because these monitors occasionally show excellent sensitivity and selectivity. Piezoelectric crystal detectors for the assay of environmental pollutants, e.g., mercury, sulfur dioxide, and toluene, have been developed and are described in the literature (11-13). The principle of such a detector is that the resonance frequency of a coated crystal is decreased by adsorption of gaseous pollutants on the crystal surface. Sauerbrey (14) has shown that the change in frequency is proportional to the amount of material on the oscillating crystal, Detection of Ozone. Two types of reactions may be responsible for the monitoring of air pollutants using piezoelectric detectors. In reversible reactions the frequency of the detector depends upon the chemical Bquilibrium between the concentration of gas in the ambient air and gas bound to the substrate. As long as the substrate is far from saturated with gas, the relationship between the concentration of gas in the air (C in parts per million or parts per billion) and the frequency change of the detector (AF in hertz) will be
AF = -K1C In nonreversible reactions the frequency of the detector changes continuously as long as the crystal is in contact with gas A F = -K,CAt or dF/dt = -K2C where At is the time of exposure and K1and K2 are constants expressing the sensitivity of the methods. By use of the reversible detector, the sensitivity of the method is believed to be independent of the air flow rate, because the chemical equilibrium should be independent of the flow rate. However, by use of the nonreversible detector the sensitivity of the method will increase with the number of ozone molecules reacting with the substrate and therefore with the flow rate but decrease with the load (CCAt) since the active sites of the substrate are gradually blocked with ozone. Accurate and sensitive monitoring of ozone in working environments necessitates the development of an instrument which is capable of performing two types of measurements, thus allowing (a) stationary measurements at selected local positions of working environments and (b) the performance of measurements as personal monitoring. The detection principle of the equipment is based on a nonreversible reaction between an unsaturated hydrocarbon polymer (1,4-polybutadiene) as the coating material and ozone, probably with the formation of ozonides as reaction product.
EXPERIMENTAL SECTION The experimental apparatus is shown schematically in Figure 1.
For calibration purpose an ozone generating system-indicated by dotted lines- was used. The system consists of the following parts: A flowmeter (Brooks Instrument B. V. Emerson Electric Co., Veenendal, The Netherlands) was used for measuring the air flow through the ozone generating system. The air flow (1 L/min.) was supplied by a pump (Instrument Pump Model 10019, Gilian Instrument Corp., Wayne, NJ). The ozone generator (SOG-1, Ultra Violet Products, Inc., San Gabriel, CA) was used as the source of different ozone concentrations. The SOG-1 is designed as a source of stable ozone production providing up to approximately 0.8 ppm ozone. The device utilizes the photochemical reaction of the 185-nm emission line of mercury to produce ozone from oxygen. It consists of a stable radiation sowce,
2635
Flgure 1. Schematic representatlon of the experimental plezoelectrlc gas detectlon apparatus: OGS, ozone generatlng system; FM, flowmeter; P, pump; G, ozone generator FH, filter holder: MC, monitorlng crystal; MO, oscillator for monitoring crystal; RC, reference crystal; RO, oscillator for reference crystal; FDU, frequency difference unit; F N C , frequencyholtage converter; V/DC, voRage/digital converter; C, computer; D N C , digitallvoltage converter; R, recorder.
a reaction tube, and a radiation housing. The source consists of a Pen-Ray mercury discharge lamp inserted into the radiation housing and an external ballast power supply. The amount of ozone produced at any flow rate can be adjusted with a movable masking tube. The tube has a number of index marks down its length which are provided for the convenience of the user in establishing other operating conditions. Thus, the rate of ozone produced can be controlled by varying the flow rate of air through the reaction tube and/or by varying the amount of radiation by adjusting the discharge tube mask. Silicone rubber tubes were used for leading the air flow from outside through the calibration equipment to the ozone generator. Ozone-free air was obtained by passing air through cotton-wool (4). Ozone monitoring in working environments was performed as follows: An air flow of 500 mL/min, supplied by a pump (same type as used in the ozone generating system but fast running with reduced length of stroke thus eliminatingpulsation) and measured by a flowmeter (Fischer & Porter, GmbH, Gottingen, FRG), passes a Teflon filter (FS, Millipore, pore size 3.0 pm, 37 mm diameter, Millipore Corp., Bedford, MA) before entering the detector cell. The filter prevents solid particles, which may be present in the ambient air, from reaching the coated crystal, and thus erroneously indicating response. The filter is placed in a filter holder of polypropylene, which is connected to the detector cell by a tube made of Teflon (3 mm id.). These materials were chosen because of their resistance to ozone. The detector cell (probe) consists of a housing made of Teflon with drilled holes, through which the air flow is directed toward the coated monitoring crystal and passes the noncoated reference crystal. The Teflon housing was protected by a metallic casing. The crystals used in these studies are 9-MHz AT cut quartz crystals with gold-plated metal electrodes on both sides (Kristallverarbeitung, Neckarbischofsheim GmbH, FRG). The matching of monitoring and reference crystals presented no problems, since the equipment could compensate for a difference in resonance frequency of up to 20 kHz. For the same reason the exchange of monitoring crystal was easy. The frequency signals coming from both crystals are combined in a frequency mixer, resulting in a frequency difference. This difference is converted into an analog signal (voltage) using a frequency/voltage converter and can thus be recorded continuously. A voltage/digital converter prepairs the analog signal for differentiation by a computer (Rockwell AIM 65). After convertion to analog form the differentiated signal is recorded. Analog signals enter the recorder (Briiel& Kjaer, type 2306, portable level recorder or Servogor 120, BBC Goerz Metrawatt, FRG) as shown in Figure 1. The computer may be used for digital presentation of the measurements. According to the supplier specifications of the electronic components none of the transformation steps introduce significant errors. A tape recorder was used for recording the program for monitoring ‘and calculation of the signals (AutomaticCassette Recorder N 2206, Philips). During coating the crystals are connected to a special oscillator circuit. This allows control of the thickness of the coating. For this purpose a frequency counter (Digicount 302, HEB Digital-
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
l l . _ l l l _ _ _ l _ l l _ l l _ . l _ _ l _ l _ _ l l l l _ l l l l _
Table I. Reproducibility of Detector Response O3 concn, ppb
rate of frequency change A F / A t , Hz/min
__
~
IO
3.5
3.0
150 225 280 425
5.0
500
5.5 11.7 12.0 16.5 26.5
550
30.0
11.0 11.5 17.0 25.0 24.0
_
I
3.2 5.5 10.8 12.0 16.0 23.5 30.0
_
X,Hz/min 3.2 5.3 11.2
11.8 16.5 25.0 28.0
Conditions: flow through ozone generator, 1 L/min; flow through detector cell, 0.5 L/min. Coating material: 1,4-p01ybutadiene. _ _ _ I I _ I _ _
Table 11. Detector Response a t Different Air Flows and Constant Ozone Concentration air tlow, i d . 1 min 170 230
390 530
rate of frequency change X = * / A t (Hz!min) 3.2 7.0 9.0 12.9
5
10
15
20
25
Time (mini
Flgure 2. Detector responses F(t) (1) and df(t)ldt(2)of intermittent exposure with ozone (0 and 94 ppb).
Table 111. Determination of Interferences X , Hz/min
2.6 6.0 8.5
4.0
3.3
6.5
11.9
12.0
6.5 8.7 12.3
8.5
L
technik, FRG) was used. For supplementary calibration purposes and comparative tests in the field a portable ozone analyzer, Model 560 (Analytical Instrument Development, Inc., Avondale, PA), was used. Reagents. The coating material used was 1,4-polybutadiene (Buna CB 10, Bayer Kemi A/S, Copenhagen) with a molecular weight of approximately 4000 and 1,4- double bond content about 96% of the theoretical value. This material was dissolved in toluene (p.a., Merck, Darmstadt, FRG). Methods of Application of Coatings. The method of coating the crystal is very important because of its influence on reproducibility and sensitivity of the detection. Polybutadiene (0.2-0.5 g) was dissolved in toluene (150 mL), and this solution was applied over the entire surface of the electrode on both sides with a tiny brush. Care must be taken to apply the coating as uniformly as possible. The uniformity of the coating was obtained by repeated application of the crystals with substrate and measuring the decrease in frequency, using the frequency counter. An amount of coating was placed on each crystal until a decrease of 2ooO-10000 Hz in the basic frequency of the crystal was observed. Fresh-coated crystals were kept over activated carbon in order to eliminate traces of toluene released from the coating and ozone from the surrounding air. Calibration Measurements. Table I shows the reproducibility of the detector response a t different ozone concentrations, using a fresh-coated crystal for every single measurement. The results indicate (a) the relative standard deviation is less than 10% and (b) hornogenity of variance in the concentration range 70-425 ppb O3 (Bartletts test, x2(4),,005 = 9.488 > 1.32). The estimated standard deviation = 0.31 (degrees of freedom, 15). The ozone generator was calibrated using the alkaline potassium iodide method described by Cohen e t al. (25). However, for ozone concentrations below 100 ppb, this method gives rather varying results, With the portable ozone analyzer, AID Model 560, more accurate and reproducible results could be obtained during calibration. The influence of the air flow on the detector response at constant ozone concentration is shown in Table 11. The dependence is almost directly proportional. The detector responses F ( t ) (1)and dF(t)/dt (2) by alternating between 0 and 94 ppb ozone are seen in Figure 2. The delay of the dF(t)/dt signal is due to an averaging time of 120 s in computing the differentiated signal leading to a more stable response. A fast response of the dF(t)/dt signal is obtainable a t shorter averaging time if a corresponding reduction in stability can be accepted. A fast response
interfering compound
concn, ppm
NO2 NO CHzO phenol
15 50 10 3-5 1000
co
frequency change, Hz/min
interference corresp to O3 concn in ppb
