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Anal. Chem. 1985, 5 7 , 1754-1756
(9) Pella. P. A.; Lorber, K. E.; Heinrich, K. F. J. Anal. Chem. 1978, 50, 1268.
RECEIVED for review February 11, 1985. Accepted April 18, 1985. Certain commercial equipment, instruments, or ma-
terials are identified in this report to specify adequately the experimental procedure. Such identification does not imply recommendation of endorsement by the National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
Detection of Organophosphorus Compounds with a Coated Piezoelectric Crystal George G. Guilbault* and Janos Kristoff
Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148 Donald Owen
Department of Mechanical Engineering, University of New Orleans, New Orleans, Louisiana 70148
Pieroeiectrlc quartz crystals coated wlth PVP-TMEDA and PVBC-TMEDA were found to be sensltlve to DIMP in the parts-per-bllllon concentratlon range. These coatlngs have outstandlng advantages compared to those previously described: higher sensitlvlty, faster response, and longer llfetime. I n addition, the cailbratlon curves pass through the origin, and the reproducibility and base line stabllity are withln f2 Hr. Also, the response and recovery tlmes are on the order of mlnutes for the PVP-TMEDA coatlng and on the order of seconds for the PVBGTMEDA copper complex. No serious interferences were observed.
The oscillating quartz piezoelectric crystal was first described for use as an analytical device by King (1). In this technique a coating specific for detection of one compound (i.e., organophosphorus compounds) is placed on the electrode surface of the quartz crystal. If interaction occurs, the presence of the compound is indicated; the magnitude of the frequency change quantifies the amount of compound present, according to the Sauerbrey equation
AF=-kAm where A m is the amount of substance adsorbed onto the crystal. Research for new coatings sensitive to organophosphorus compounds is of great practical importance, especially for compounds developed for chemical warfare (CW) applications. The optimization of coating sensitivity and selectivity toward the highly toxic cholinesterase inhibitory organophosphorus pesticides and CW agents is of interest to many scientists. Previously, we devoted many efforts to the development of a piezoelectric cyrstal detector for organophosphorus compounds in air. The ferric chloride (FeC13)complex of pesticides and the cobalt (Co) complex of isocyanobenzoylacetate (IBA) were used as adsorptive coatings (2,3).The FeC1, complex showed a very slow response and lacked the desired sensitivity. The application of the IBA coating was limited by a very short lifetime. A ternary mixture coating of l-n-dodecyl-3-(hydroximinomethy1)pyridinium iodide (&PAD), Triton X-100, and sodium hydroxide (NaOH) was reported to have faster response, longer lifetime, and higher sensitivity for the parts-per-million (ppm) range (4-6). Diisopropyl methylphosphonate (DIMP) was used as a model compound for detection and assay of 0003-2700/85/0357-1754$01.50/0
organophosphorus compounds with the G-agent structure (pentavalent phosphorate with an active leaving group, X OR
I
RO--6-X
I1
0
Additionally, L-histidine hydrochloride was also reported as an excellent coating for assay of organophosphorus compounds of the malathion type (4-6). Guilbault and Kristoff used uncoated crystals for the detection of organophosphorus vapors (7). Several papers suggest the use of copper complexes for hydrolysis of phosphorus esters ( 8 , l l ) . In this application, copper is complexed, and ligands containing basic nitrogen are used. The reaction takes place in two steps: first, the copper complex binds the phosphorus ester reversibly; second, the adduct-product formed is irreversibly broken down by hydrolysis. This second reaction is unlikely to occur on a copper complex used as a coating for the crystal. From a practical viewpoint, the copper complexes of importance have a fast, reversible, and weak complexation toward organophosphorus compounds at ambient temperature. Indeed, copper complexes have already been evaluated for the detection of organophosphorus compounds in air (12, 13). In this study, two types of copper(I1) chelates were investigated. (Tetramethylethylenediamine)copper(II) chloride (TMEDA) was connected to hydrophilic poly(viny1pyrrolidone) (PVP), and to hydrophobic poly(vinylbenzy1) chloride (PVBC) polymers. Thus, PVP-TMEDA polymerbonded chelates were synthesized and evaluated.
EXPERIMENTAL SECTION Instrumentation. A schematic diagram of the experimental instrumentation is shown in ref 7. Nitrogen gas flowed through a drier reservoir filled with phosphorus pentoxide (PzO,) mixed with glass rings. The gas flow was divided into three parts: FI is the flow stream for desorption; F,,for dilution; and F3, for generation. At all times PI = F2 F3 The required concentration of DIMP in the gas flow was adjusted by varying the (F2/F8)ratio. In this work, the temperature was kept constant by a water bath, and the gas flow rates were varied. The flow rate of the gas entering the detector cell was 100 cm3/min. The cell design was essentially the same as that described earlier (14). The gas stream was focused on both faces of the coated crystal. A four-port valve was used to make a selection between either pure nitrogen or the carrier gas containing DIMP. An injection port was inserted into
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0 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
the pure nitrogen stream for the injection of different gas samples. All tubing carrying DIMP was made of glass, Teflon, or stainless steel. A 9-MHz, AT-cut quartz crystal with gold electrode (International Crystal Mfg. Co., Oklahoma City, OK) was used. The low-frequencyoscillator (InternationalCrystal Manufacturing Co.) was powered by a Heathkit IP-28 power supply (Heath-Schlumberger, Benton Harbor, MI), The applied voltage was regulated to 9 V dc. The frequency of the oscillator was measured with a Heath-Schlumberger Mode SM-4100 frequency counter. A digital-to-analogconverter was added to the frequency counter so that a permanent record of the frequency changes could be simultaneously recorded. In order to obtain a gas flow saturated with the organophosphorus vapor, the Fanitrogen stream was bubbled through the liquid DIMP by means of a small pore size sintered glass frit. A filter packed with glass wool was used after the generator bottle to trap all liquid DIMP aerosol. Since glass wool adsorbs DIMP on its surface, the gas flow was passed through the adsorber for half an hour prior to the measurements to ensure a stable and saturated DIMP atmosphere. Gas Chromatographic Analysis. Gas chromatography was used to c o n f i i vapor concentrations generated by the evaporation method for DIMP. The vapor samples were dispersed through a flask containing 25 mL of isooctane chilled in a water bath at -5 to 0 OC. After a specific collection time, the contents of the flask were transferred to a volumetric flask and aliquots were injected into a Perkin-Elmer Sigma 1B chromatographic system. A 50-m 0.25 pm 5% methyl phenyl SE52 fused silica WCOT capillary column from Quadrex Corp. was used with a nitrogen phosphorus detector. The concentration of the vapor was calculated from the equation (5) (wgAW.4) Cppm Ft(M.W.) where Cppmis the concentration of vapor (ppm), pgA is the total analyte mass determined by gas chromatographic analysis (g), F is the flow rate of vapor stream dispersed through the solvent trap (L/min), t is the time of bubbling vapor through trap (rnin), and M.W. is the molecular weight of the sample (g/mol). This was a very accurate analysis of parts-per-million and parts-perbillion concentration levels of vapor, provided that all the vapor analyte is successfully trapped in the solvent for subsequent gas chromatographic analysis. Results obtained by GC analysis were in good agreement with those estimated from vapor pressure data, indicating the validity of assumptions of the later technique. Chemicals. The TMEDA copper(I1) chloride and polymers (PVP, PVBC) were synthesized as described below; DIMP and organic chemicals were obtained from commercial sources and used without further purification. Inorganic gases were from lecture bottles (Matheson Co., Inc., East Rutherford, NJ). (Tetramethylethylenediamine)copper(II) Chloride. A 3.41-g portion (0.02 mol) of CuC12.2H20was dissolved in 100 mL of dimethylacetamide (DMAC), and then 2.32 g (0.02 mol) of tetramethylethylenediamine was added resulting in an azure precipitate. The precipitate was filtered, rinsed with 100 mL of DMAC, and dried in vacuo at 65 "C for 12 h (% Cu = 63.5/286.5). PVP-(Tetramethylethylenediamine)copper(II)Chloride. One gram of PVP was dissolved in 18.0 g of HzO and then 1.6 g of, (tetramethylethylenediamine)copper(II) chloride solution was added and mixed thoroughly. A dark blue film formed when cast. PVBC (Poly(vinylbenzy1)Chloride. Approximately 25.0 g of vinylbenzyl chloride was placed in a 150-mL test tube equipped with an N2 purge and magnetic stirrer. Then 0.25 g of 4% a,d-azoisobutyronitrile (AIBN) added to 75 mL of chloroform was added and the mixture was brought to 65 OC. It was maintained at this temperature with constant stirring for 10 h. The solution was cooled and polymer precipitated as feathery flakes by dripping slowly into hexane with high agitation. After drying at 60 "C in vacuo for 24 h 18.9 g of brittle, clear light green polymer was obtained. The polymer was redissolved in CHC1, to make a 10% (wt) solution with 0.656 mequiv of Cl- per gram solution. PVBC + (Tetramethylethy1enediamine)co per(II) Chloride. TO10.0 g of 10% PVBC solution, 0.31 g of CuB(TMEDA) chelate (63.5 g of Cu/286.5 g of chelate) was added. The chelate partially dissolved. This solution was filtered and cast to form an emerald green film 7% by weight in copper.
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Table I. Response and Recovery Times for the PVP-TMEDA Complex concn, ppb 5.1 8.9 14.0 20.9 25.9 30.0 36.5 41.1
47.0 a
'
meana AF,Hz
av dev from mean, Hz
response time, min
13 36 62 100 122 138 155 172 180
1 0.25 0.5 0.25 1 1.7 0.75 3.0 4.7
1 1 1.5 1.5 2 2 3 5 5
recovery time, min
1
1 2 2 3 3.5 4 4.5 6 7
Average of four measurements.
Table 11. Response and Recovery Times for the PVBC-TMEDA Complex concn, wb
meana
av dev from mean,
U,Hz
Hz
response time, s
recovery time, s
5.1 6.4 8.9 11.5 15.1 17.0 20.9
4 6 8 12 15 17 26
0.50 0.25 0.50 0.50 0.25 1.0 0.75
4 4 5 5 5 10 30
20 20 25 30 30 60 120
Average of four measurements. Crystal Coating. An aqueous solution of the PVP-TMEDA chelate was made and a small amount of the solution was applied, with a microsyringe, to each electrode face. After evaporation of the water, a uniform, thin layer was formed on the electrode surfaces. For a completely dry coating to be obtained, the crystal was flushed with dry nitrogen for a few hours. Due to the hydrophobic behavior of PVP, water vapor was excluded during the investigations. The PVBC-TMEDA coating was prepared from a chloroform solution, using a procedure similar to that in the preceding paragraph. After application to the electrodes, the chloroform evaporated, and the crystal was ready for evaluation. In addition, this coating can be exposed to the atmosphere without concern for water-vapor effects.
RESULTS AND DISCUSSION Experimental results obtained for the PVP-TMEDA and the PVBC-TMEDA copper complexes are summarized in Tables I and 11. The calibration curves are shown in Figure 1. The PVP-TMEDA complex has a linear response to DIMP between 0 and 30 ppb. Above the 30 ppb concentration level, a saturation effect is observed, the response and recovery times increase, and the reproducibility decreases. Within the 0 to 30 ppb concentration range, this coating can be characterized as having good base line stability, excellent reproducibility, and relatively low response and recovery times (times to r&acha steady-state and return back to base line, respectively). The PVBC-TMEDA copper chelate has a linear calibration curve in the 0-20 ppb range. No other coating sensitive to DIMP yields a similar degree of reproducibility or low response and recovery times. Above 20 ppb, the response to DIMP increased and no saturation was observed. Interferences. Also examined were various organic and inorganic materials which could be expected in air as pollutants. To study the effect of interfering materials, the injection method was used (5.0-mL gas sample was injected in all cases). The results obtained are summarized in Table 111. As shown in Table 111, no serious interferences were observed-except
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
2001
0 0
IO
20
30
1
1
1
1
40
50
60
70
ppb DIMP Figure 1. Calibration curves for the PVP-TMEDA and PVBC-TMEDA
complexes with
ACKNOWLEDGMENT
DIMP.
The authors wish to thank Mary Jane Van Sant who ran the GC analyses of DIMP and Malathion, reported herein. Registry No. DIMP, 1445-75-6;quartz, 14808-60-7.
Table 111. Response of Coatings to Interferencesa AF, Hz
interference none (DIMP, 10 ppb) hydrogen chloride (HCl) ammonia (NHJ nitrogen dioxide (NO,) sulfur dioxide (SO,) carbon monoxide (CO) hydrogen sulfide (H2S) benzene (C,H,) toluene (C,H,) chloroform (CHC13) automobile exhaust
Water was a serious interference in assays using the PVPTMEDA coating but was not in the PVBC-TMEDA studies. Thus, for practical use in a field assay of organophosphorus compounds, the PVBC-TMEDA substrate is recommended. Up to 80% relative humidity can be tolerated with little effect. Optimum Flow Rate. During the investigations, a flow rate of 100 cm3/min was found to be optimum. Below 100 cm3/min, the sensitivity increased; however, the response and recovery times also increased. Above 100 cm3/min, the sensitivity and base line stability decreased. Lifetime. The lifetimes of the coatings were measured under a continuous carrier gas-flow condition. After 25 days of use, the loss in sensitivity was 20% for the PVP-TMEDA chelate and only 8% for the PVBC-TMEDA complex. Optimum Coating Mass. The optimum coating mass is approximately 50 pg for the PVP-TMEDA substrate and 60 pg for the PVBC-TMEDA complex. In the case of the PVBC-TMEDA chelate, the sensitivity is practically the same in the 20-50 pg mass range. Above 50 pg, the base line stability decreases. The PVP-TMEDA copper chelate shows increasing sensitivity with an increase in the mass of coating used; however, the base line becomes unstable.
concn, ppm
PVPTMEDA
PVBCTMEDA
40
1000 1000 1000 1000 1000
41
10 11 14 14 10 10 11 10 10
1000
100 100 100 1:lOb
48 40 41 40 41
43 40 42 42
17 11
"All interferences were run in the presence of 10 ppb DIMP. *Dilution. for ammonia (NH,), with the PVP-TMEDA coating; and for chloroform, with the PVBC-TMEDA substrate. The effect of chloroform may be due to the dissolution of the coating. Since the concentrations examined are much higher than those typically found in air, no serious interference would be expected.
LITERATURE CITED King, W. H., Jr. Res. Dev. 1989, part 1, 20 (4), 28; part 2, 20 (5), 28. Schelde, E. P.; Gullbault, G. G. Anal. Chem. 1972, 4 4 , 1764. Shackelford, W. M.; Gullbault, G. G. Anal. Chim. Acta 1974, 73, 383. Gullbault, G. G.; Tomlta, Y.; Kolesar, E. S., Jr. SAM-TR-80-21, July 1960 (NTIS AD A091705). Guilbault, G. G.; Tomlta, Y.; Kolesar, E. S., Jr. Sens. Acfuafors 1981, 2, 43. Tomita. Y.; Gullbault, G. G. Anal. Chem. 1980, 52, 1484. Gullbault, G. G.; Krlstoff, J. Anal. Chlm. Acta 1983, 149, 337. WagnerJauregg, T.; Hackley, B. E.; Lies, T. A,; Owen, 0. 0.; Proper, R. J. Am. Chem. Soc. 1955, 7 7 , 922. Epsteln, J.; Rosenblatt, D. H. J. Am. Chem. SOC. 1958, 80, 3596. Gustofson, R. L.; Chaberek, S.; Martell, A. E. J. Am. Chem. SOC. 1983, 85, 596. Murakaml, Y.; Martell, A. E. J. Am. Chem. SOC. 1964, 86, 2119. Gullbault, G. G.; Affolter, J.; Tomita, Y.; Kolesar, E. S., Jr. SAM-TR-816,May 1981 (NTIS AD A100967). Gullbault, G. G.; Affolter, J.; Tomlta, Y.; Koiesar, E. S.. Jr. Anal. Chem. 1981, 53, 2057. Hlavay. J.; Gullbault, G. G. Anal. Chem. 1977, 49, 1890.
RECEIVED for review September 4,1984. Resubmitted March 1,1985. Accepted March 26,1985. The financial assistance of the U.S.Air Force (Aerospace Medicine Branch, Brooks Air Force Base, San Antonio, TX) is gratefully acknowledged.
