1830
Anal. Chem. 1983, 55,1830-1832 0.6 r
The intercept on the absorbance axis is due to tetrachloropalladic(I1) acid and cacotheline. The tetrachloropalladic(I1) acid concentration remains constant but the concentration of cacotheline decreases as part of it is converted to dihydrocacotheline in proportion to the concentration of carbon monoxide. The calibration curve is a straight line, however, as demonstrated by the following:
‘t3.CIO ‘+O.CO?
AAtotal
=
AAH2[PdC141
+ (eCaco(0)2ACCaco(0)z1) 4(e for each substance at 520 nm)
(tCaco(OH)zACCaco(OH)z~)
-ACCaco(O)n
=
-
ACC~CO(OH)~
C C ~ ~ ~ (assuming ( ~ H ) ~stoichiometric reduction) AAtotal 3
0
200
400
I
500
I
I
BOO
CONCENTRATION,
I
I
I000
I
ppm
Figure 1. Calibration curve for absorbance vs. concentration of carbon monoxide. Average and range of values for five determinations are shown, with standard deviations listed beside each set of data.
Tetrachloropalladic(I1) acid has nearly the same yellow hue in solution as cacotheline. Cacotheline probably is protonated a t the amine groups in the reagent solution. The reactions for the reduction of tetrachloropalladate(I1) and subsequent reduction of cacotheline are as follows (Caco(0)z= cacotheline; Caco(OH)z = dihydrocacotheline):
-
+ H20 Pd + COz + 4H+ + 4C1Pd + Caco(0)2 + 2H+ Pd2++ Caco(OH)2 Pd2++ 4C1- + 2H+ H2[PdCl4] +
HZ[PdCl4] CO
CO
-
-
+ H20+ Cac0(0)~
HzLPdCI,I
C 0 2 + Caco(OH),
=
[(ECaco(OH)z - ~Caco(0)z)CCaco(OH)~~l = kCCaco(OH)z
The method described here is unique in using a stable one-solution reagent. In acetic acid solution substantially less than 90%, the violet dihydrocacotheline produced by reaction with carbon monoxide fades, probably due to oxidation of the dihydrocacotheline by atmospheric oxygen. In the 90% acetic acid media, the dihydrocacotheline color is stable indefinitely. Thus, both the reagent solution before use and the solution containing the reaction products are stable on storage. Registry No. I, 561-20-6;CO, 630-08-0; PdCld2-,14349-67-8.
LITERATURE CITED ( 1 ) Smith, R. G.; Bryan, R. J.; Feldstein, M.; Levadie, B.; Miller, F. A.; Stephens, E. R.; Whlte, N. G. (Subcommlttee 4 of the Intersoclety Committee) Health Lab. Sci. 1970, 7 , (January Supplement) 75. (2) Lambert, J. L.; Hamlln, P. A. Anal. Lett. 1971, 4 , 745. (3) Lambert, J. L.; Wiens, R. E. Anal. Chem. 1974, 4 6 , 929.
RECEIVED for review December 30,1982. Accepted May 26, 1983. This research was supported in part by National Science Foundation Grant CHE-7915217 and by NSF Grant SPI8013291 to Yuan C. Chiang.
Portable Piezoelectric Crystal Detector for Field Monitoring of Environmental Pollutants Mat H. Ho and George G. Guilbault* Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70122 Bernd Rietz National Institute of Working Environment, DK-2900 Hellerup, Denmark In general, there are two types of toxic chemicals in the atmosphere we breathe: acute toxins and cumulative toxins. Many acute toxins can be tolerated at sublethal concentrations. Cumulative toxins, on the other hand, may cause a harmful effect even upon exposure to minute amounts over long periods. Toxins in the atmosphere, either acute or cumulative, need to be monitored. For acute toxins, a continuous monitoring device is required to ensure rapid detection of the lethal concentrations. For cumulative toxins, a cumulative dosimeter is required to measure the time-weight-average (TWA) exposure. Most of present monitoring systems involve a sampling step, using activated charcoal, silica gel, Tenax GC, or passive sampling devices (1,2),and transport of the sample back to the laboratory for subsequent analysis. The results of an air sample taken one day are usually not known until the next day and a worker in the field may be poisoned before the harmful level is identified. Direct monitoring devices are preferred, but they are either bulky, high power consuming, complicated to operate, or expensive or lack sensitivity and selectivity. There is, therefore, a need for a direct monitor
that is portable for field use, lightweight, low power consuming, inexpensive, rugged, and simple to operate and yet is reliable, sensitive, and selective. It appears that the piezoelectric crystal can be developed as such a device. In an attempt to demonstrate this concept, a portable field monitor was constructed by using a coated piezoelectric crystal for direct monitoring of toluene in a Danish printing plant. The principle of the detector is that the frequency of vibration of an oscillating crystal is decreased by the adsorption of a foreign material on its coated surface (3). Toluene is adsorbed on the coating, thereby increasing the mass on the crystal and decreasing the frequency. This change in frequency is proportional to the concentration of toluene in the atmosphere.
EXPERIMENTAL SECTION Apparatus. The detector, which is 20 X 14 X 9 cm in dimensions and weighs 2.5 kg, was developed for field use. Figure 1shows the schematic diagram of the detector. The device consists of two crystals, one as a sensor and the other as a reference (9 MHz, AT-cut piezoelectric quartz crystals from International Crystal Mfg. Co., Oklahoma City, OK). Only the sensor crystal was coated with Pluronic F-68. Two oscillators were employed
0003-2700/83/0355-1830$01.50/00 lg83 American Chemical Society
ANALYTICAL CHEMISTRY, VOL.
YmB.l.i --Y.r..?2 reset
A
55, NO. 11, SEPTEMBER 1983
1831
sample update switches
converter
Flgure 1. Schematic diagram of the detector: (A) silica gel columri, (B)
activated carbon columln, (C) filter.
to drive these crystals as described elsewhere ( 4 ) . The frequency outputs of the two oscillators were combined in a mixer circuit and the difference of frequency between the sensor and reference crystals was obtained. Since the sample was adsorbed only on the surface of the coated sensor crystal, the frequency output was directly proportional to the concentration of' the sample assayed. Rechargeable nickel-cadmium batteries, which are capable of 8 h of continuous operation, provide all power for the electronics and for the sampling pumip (Anatole J. Sipiri Co.). The sampling valve was a four-port Vallco valve (Varian Instrument Division, Palo Alto, CA). The detector cell, which was described earlier (5),provides a small dead volume and permiai it to be incorporated into the detector box. Phuonic F-68 was disriolved in acetone and the crystal was coated with a microsyringe. About 60 pg of Pluronic F-68 was coated on both sides of the crystal. The detector is commercially available from Universal Sensors (PO Box 736, New Orleans, LA 70148). Reagents. Pluronic F-68 was obtained from Fluka Chemical Corp., Hauppauge, NY. Acetone and toluene were reagent grade from Matheson, Coleman and Bell, Cincinnati, OH. The standard vapor concentrations of toluene for calibration were prepared by the diffusion method. A Kin-Tek Precision Calibration System, Model 570 (Kin-Tek Laboratory, Texas City, TX), was used and a diffusion cell was constructed as describied previously (4). Procedure. Purified air was first passed over the coated sensor crystal until the base line became stable. The sampling valve was then rotated to introduce the standard vapor concentration of toluene or contaminated air into the detector cell. The frequency decreased due t o the adamption of toluene onto the coating. A steady reading of frequency was obtained when equilibrium was established. The sampling valve was then rotated again and purified air was allowed tx) pass through for desorption. Purified air was obtained by using activated charcoal imd silica gel columns. Water vapor interfered and was removed by using a Nafion permeation tube (6).
RESULTS AND DISCUBSION Toluene vapor was adsorbed on the PLuronic F-68 coating and a decrease in frequency was observed. Unlike the injection system where the frequency response was recorded as a peak maximum, the response in this system was taken from the frequency difference between the base line and the steady reading after equilibrium was established as shown in Figure 2. The response time arid recovery time were affected by the flow rate. The response time and the recovery time became quite slow, as the flow irate decreased below 40 mL/min. As the flow rate increased from 40 mL/min to 120 mL/min, the response time decreased from 60 s to 3b s. Faster response and recovery can be obtained by increasing the flow rate. However, a t higher flow rates the sensitivity decreased markedly due to incomplete adsorption of toluene vapor on the coated electrode. An optimum flow rate of 100 mL/min was observed. The sensitivity was affected not only by flow
0
1
2
3
4
5
6
7
8
9
10
t i m e , min
Flgure 2. Detector response to various concentrations of toluene.
rate but also by the amount of coating since the greater the amount of coating, the more the toluene vapor was adsorbed. This result indicates that the sorption involves multilayers rather than only a surface layer. The crystal became overloaded and the vibration ceased when the amount of coating was too large. [t is apparent that the optimum amount of coating is about 60 pg. The loss of coating, due to evaporation at the high flow rates, is one of the major difficulties when the detector is used for continuous monitoring. Since the amount of coating affects the sensitivity of the detector, poor reproducibility is obtained if the coating is lost. Also the drift of the base line makes the detector become less reliable. With Pluronic F-68 as coating, no loss of coating is observed and very stable base lines are obtained after 2 months of operation. A response time and recovery time were obtained in about 40 s and 30 s, respectively. Various inorganic and organic substances, which could be present at workplace sites and could interfere with the assay of toluene, were tested. The injection method was used for inorganic gases because of its simplicity. The injection method is likely to give lower results than the continuous flow method due to the limit of sample size and the time required to reach equilibrium. Nevertheless, even if 1 decade error allowance is made, no interferences from inorganic gases (CO, NH,, SOz, HC1) a t 100 ppm concentration are expected. Organic vapors, especially benzene and alkylbenzenes, give some interferences. At printing plants less than 5% (v/v) concentrations of such organics could be expected to be present. The frequency response of 100 ppm of toluene was almost identical with that obtained for 100 ppm of toluene 5 ppm of benzene, p-xylene, ethylbenzene, or mesitylene. Thus no interference is anticipated in this application. Water vapor interfered and was removed by using a Nafion permeation tube (6). Water vapor was selectively removed by diffusion through a permaselective membrane of Nafion tubing without loss of toluene. A linear calibration plot was obtained from 1 ppm to 200 ppm with an average relative standard deviation of 2%. After the portable piezoelectric crystal detector was developed and evaluated, a field test was performed in the real environment. The detector was tested in the operating room of the Danish printing plant incooperation with the Danish National Institute of Working Environment. This was a large printing plant located in the greater Copenhagen area, where concern for worker safety upon exposure to toluene vapor is great. The results of readings on the piezoelectric detector were compared with the two accepted procedures for monitoring toluene in the printing plant, the photoionization detector and the Drager tube. The photoionization detector (Model PI 101, HNU Inc., Newton, MA) is based on an ionization of the molecule to be assayed by absorption of UV radiation (7). The Drager tube (National Drager, Inc.,
+
1832
Anal. Chem. 1983, 55, 1832-1833
Table I. Comparison of Results Obtained by Piezoelectric Crystal Detector and Two Other Methods (HNU Photoionization Detector and Drager Tube) sampling site XI
x, x, x'l x, x,
X6
re1 std dev, %
P.Z.a
toluene concentration, ppm HNU Drager tube
34
35
45
60
35
40
34 195
30
> 200
30 40
2.0
a Piezoelectric crystal detector. tion detector.
25 40 5.0
35 50 35 30 >200 30 35
HNU photoioniza-
Pittsburgh, ?A), a semiquantitative (10-15% deviation) device good in the range of 5 to 400 ppm, is based on the oxidation of toluene by iodine pentaoxide in the presence of sulfuric acid (8). Seven sites were chosen at various locations in the plant: two a t the cutting and assembly (X, and X2),two near the mixing container stations which feed the color containers (X3 and X4), one at a location between the two color containers (X5),one by the control panel (X6),and one in the middle of the hall (X7). Four readings were taken with the piezoelectric crystal unit, one with the Drager tube, and several with the photoionization detector. The results obtained with the three units are shown in Table I, indicating that the piezoelectric unit gave data consistent with the photoionization detector
and Drager tube. Moreover, the results obtained with the piezoelectric unit were better in relative standard deviation than the other two. The portable coated piezoelectric crystal detector has been demonstrated to be a potential device for continuous monitoring of organic vapors in the industrial atmosphere. The only drawback in this type of detector is the selectivity. Stationary phases of chromatography such as Pluronic F-68, although sensitive, are not particularly selective. Further work should be devoted to the area of finding more specific coating materials. Registry No. Toluene, 108-88-3.
LITERATURE CITED (1) Amerlcan Society for Testlng and Materlals "Sampling and Analysis of Toxic Organics in the Atmosphere"; ASTM: Philadelphia, PA, 1979; ASTM Special Technical Publication 721. (2) West, P. W. Am. Lab. (Fairfield, Conn.) 1980, 7 , 36. (3) Hlavay, J.; Guilbault, 0. G. Anal. Chem. 1977, 4 9 , 1890. (4) Ho, M. H.; Guilbault, G. G.; Rietz, B. Anal. Chem. 1980, 52, 1489. (5) Ho, M. H.; Gullbault, G. G.; Scheide, E. P. Anal. Chem. 1982, 5 4 , 1998. (6) Simmonds, P. G.; Kerns, E. J. Chromatogr. 1979, 186, 785. (7) Manual, HNU P I 101 Unit: HNU Inc.: Newton, MA, 1978. (8) Leichnitz, K. "Detector Tube Handbook", 3rd ed.: Dragerwerk Ag Lubeck: Federal Republic of Germany, 1976.
RECEIVED for review May 4, 1981. Resubmitted June 3, 1983. Accepted June 20,1983. This work was conducted with the financial assistance of the Army Research Office (Grant No. DAAG29-77-G0226) and a travel grant from the North American Treaty Organization (Grant No. NATORG 1719) which permitted a joint research effort between United States and Danish Laboratories.
Comparison of Quartz and Pyrex Tubes for Combustion of Organic Samples for Stable Carbon Isotope Analysis Thomas W. Boutton,* William W. Wong, David L. Hachey, Lucinda S. Lee, Mercedes P. Cabrera, and Peter D. Klein Stable Isotope Program, Children's Nutrition Research Center, USDAIARS, Department of Pediatrics, Baylor College of Medicine, Texas Children's Hospital, Houston, Texas 77030 Most laboratories that perform routine stable carbon isotope determinations on organic samples now employ a sealed-tube combustion method similar to that described by Buchanan and Corcoran (1). According to this method, the sample is sealed into an evacuated length of quartz tubing containing copper oxide as an oxidant and baked at 800-900 "C. This method is more efficient in processing large numbers of samples when compared to dynamic methods, where gases are repeatedly cycled through a furnace by means of a Toepler pump ( 2 ) . Sofer (3)recently has demonstrated that petroleum samples could be combusted to COz in borosilicate (Pyrex) glass at 550 "C without loss of precision or accuracy of 613C values compared with combustion in quartz tubing 850 "C. Sofer (3)also suggested that other organic samples could be combusted at 550 O C in Pyrex as well. Because Pyrex tubing is approximately 14% of the cost of quartz tubing, we have compared 613C values and percent carbon obtained on a variety of organic substances using both methods, e.g., combustion in quartz tubing at 850 "C and in Pyrex tubing at 550 "C.
EXPERIMENTAL SECTION Combustion tubes (9 mm 0.d.) were cut to 30 cm lengths and sealed at one end. Approximately 2 g of copper oxide wire and
9 mm2of silver foil were placed in each of the quartz tubes, which were subsequently baked at 850 "C for 1 h to remove potential organic contaminants. Pyrex combustion tubes were baked at 550 "C for 1h, while the copper oxide and silver foil for the Pyrex tubes were purified separately at 850 "C for 1h. Sample handling cuvettes, approximately 5 cm long, were made from 6 mm 0.d. quartz or Pyrex tubing, sealed at one end, and baked for 1h at the appropriate temperature. Approximately 3-10 mg of dry sample was weighed into prebaked cuvettes which then were slid into the larger 9 mm 0.d. combustion tubes. Loaded combustion tubes were attached to a vacuum manifold with 3/8 in. Ultra-Torr unions, evacuated to torr, and sealed with a torch. Each sealed tube was 7 X inserted into a in i.d. ceramic pipe to minimize the effects of potential explosions and placed inside a muffle furnace. Quartz tubes were baked at 850 "C for 1 h, and Pyrex tubes were baked at 550 "C for 1 h. The temperature of the muffle furnace was increased slowly so that approximately 1.5 h were required to reach 550 "C, and 2 h t o reach 850 "C. After combustion, the tubes were allowed to cool to room temperature inside the furnace for 12 h. Gases from the combusted sealed-tubes were released into a vacuum line by use of a tube cracker ( 4 ) . Gases were passed through a methanol-dry ice trap to remove water vapor before being frozen into a liquid nitrogen trap. Noncondensable gases were pumped away. The volume of purified COz was measured
0003-2700/83/0355-1832$01.50/00 1983 American Chemical Society
