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Anal, Chem. 1981, 5 3 , 2057-2060
Piezoelectric Crystal Coating for Detection of Organophosphorus Compounds George G. Guilbault, Jean Affolter, and Yutaka Tomlta Department
of Chemistty, University of New Orleans, New Orleans, Louisiana 70 722
Edward S. Kolesar, Jr." Air Force Systems Command, Aerospace Medical Division, School of Aerospace Medicine, Crew Technology Divlsion, Crew Environments Branch, Brooks Air Force Base, Texas 78235
Copper complexes have been evaluated for the detection of organophosphorus compounds In alr. Dlisopropyi methylphosphonatr (DIMP) was used as a model for the phosphorus esters. Bldentate copper complexes possess the hlghest activity for DIMP, and thls effectiveness was evaluated by chemically binding the complex to XAD-4, an experimental resln polymer. The calibration curve Is linear between 0 and 20 ppb DIMP (the range investigated). No signlflcant interference from automobile exhaust gas, chosen as a posslble pollutant, was observed.
The purpose of this effort was to evaluate a series of copper complexes for the detection of organophosphorus compounds in air using ti coated piezoelectric quartz crystal. Piezoelectric crystals have been used as sorption detectors and microbalances, and several authors have discussed the theory and applications of this technology in the literature (1-8). In a previous effort concerned with detecting organophosphorus compounds, it was discovered that l-n-dodecyl-3-hydroxyiminomethylpyridinium iodide (3-PAD) was an adequate coating and that ita sensitivity could be increased by forming a ternary mixture with Triton X-100 and NaOH (8, 9). Several papers published on catalysts for hydrolysis of phosphorus esters cite the use of copper complexes; this fact suggesta that this class of compound is worthy of investigation. The most interesting study is from Wagner-Jauregg, et al. (IO). They found that Cu(I1) salts catalyze the hydrolysis of diisopropylphosphonate, and the catalytic effect is enhanced above pH 7. Copper has to be complexed to remain in solution, and lig,mds containing basic nitrogen were used for this purpose. The most effective catalyst found is the copper(I1) 2,2'-dipyridyl cation. Two monodentate or one bidentate ligand yield8 the optimal activity. With more ligand bonding, the activity (decreasesmarkedly. Copper was found to be the best metal among tKose tested: nickel, cobalt, iron, and manganese. Since then, other workers have made use of copper as a catalyst for the hydrolysis of phosphorus esters (11-13). The overall reaction occurs in two steps: first, the copper complex binds the phosphorus ester reversibly; second, the adduct-product is irreversibly broken down by hydrolysis. The property of the first reaction is most usefulsin this effort. Copper complexes may adsorb and desorb phosphorus esters in air. The ,second reaction is unlikely to occur'on a copper complex used as a coating for a crystal, because of the low content of water in air; and thus, humidity is not expected to interfere. A significant portion of organophosphorus compounds contain either the phosphoryl or thiophosphoryl groups. As has been the practice in many previous studies concerning phosphoryl-containing compounds, diisopropyl methyl0003-2700/81/0353-2057$01.25/0
phosphonate (DIMP) was used as a model compound in this effort (2, 8, 9, 14).
EXPERIMENTAL SECTION Apparatus. The experimental instrumentation used to evaluate the coated piezoelectricquartz crystal detector is shown in Figure 1. Nitrogen flows through a sintered glass moistened with DIMP and generates a saturated gas mixture. The DIMP content in the gas flow can be adjusted by varying either the temperature of the waterbath or the gas flow. In this effort, the temperature was maintained constant, and the gas flow was varied. A six-port valve permits the selection between pure nitrogen and a nitrogen-DIMP mixture. The flow rates are the same in both cases. A septum is used before the mixing coil to permit the controlled injection of the gas samples to be evaluated by the detector. All the tubing carrying DIMP is made of either copper or glass. The piezoelectric crystals (International Crystal Manufacturing Co.) were 9 MHz AT-cut quartz crystals with goldplated electrodes (14 mm diameter). After the electrode surfaces were coated with the copper complex, crystal placement and pbsition in the detector cell were accomplished by using an HC 25/U socket (International Crystal Manufacturing Co.). The instrumentation consisted of a low-frequency transistor oscillator (International Crystal Manufacturing Co.) powered with a regulated power supply (Heath Kit Model IP-28). The applied voltage was regulated to 6 V dc for all measurements. The oscillator output frequency (crystal vibrating frequency) was monitored by using a frequency counter (digital display) with a 0.1 Hz resolution capacity (Heath-SchlumbergerModel SM-4100). A permanent record of the detector's frequency changes was generated by connecting a digital-to-analog converter (DAC) circuit to the output of the frequency counter. The DAC provided a voltage output signal directly proportional to a change in crystal vibrating frequency. The output of the DAC was connected to a potentiometric recorder. Thus, a frequency change was recorded as a numerical quantity (frequency counter display) and as a voltage fluctuation (potentiometric recorder). The apparatus was tested in the monitoring and batch modes; that is, applying DIMP on the sintered glass or injecting gas containing DIMP through the septum. Higher values and smaller standard deviations have been obtained with the monitoring mode. The reason for this difference is attributed to the length of time required to establish an equilibrium for DIMP between the crystal's coating and the gas phase of the batch mode. Thus, assays in this work, unless otherwise specified, have been made with the monitoring mode. Chemicals. Cu(butyrate)z. One gram of CuC12.2Hz0and 0.52 g of butyric acid (1:l molar ratio, copper is in excess) are dissolved in 100 mL of water and the solution is made basic with concentrated NH, to a pH >9. Cu(butyrat& is extracted several times with 1-butanol until a significant color change is observed in the extracting solvent. The excess solvent is then evaporated under vacuum with a Rotavapor (Buchi Instruments). The finely ground residue is dried overnight in a glass tube flushed by nitrogen at 150 "C. The composition was determined by elemental analysis. The substance has no melting point and decomposes at 270 "C. 0 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981
&S y
t
wCRYSTAL
NITROGEN
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c1
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MIXING FLOW REGULATOR
s
OSCILLATOR
8
2.0
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-+-I-
c
0
I I
\
I
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COUNTER
6-PORT VALVE, z PosirioHs
ANALOG~TO~O1G1TAL CONVERTER
m*,,,,,,,,, RECOROER
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O
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Flgure 1. Experimental instrumentation used to evaluate the piezoe-
lectric crystal coatings.
Cu(butyrate)2.ethylenediamine. A 0.1-g sample of Cu(bumol) is dissolved in an excess of 1-butanol; tyrate)2 (4.2 X and, similarly, a 0.025-g sample of ethylenediamine (4.2 X mol) is dissolved in the same solvent. The two solutions are mixed and the solvent is evaporated with a Rotavapor (Buchi Instruments). The finely ground residue is dried overnight in a glass tube flushed by nitrogen at 150 "C. The composition was determined by elemental analysis. The remainder of the compound is used for coating the crystal. Cu(b~tyrate)~.diethylenediamine.The preparation of this compound is similar to that of Cu(butyrate)2.ethylenediamine. The molar ratio of Cu(butyrate)2and diethylenetriamine is 1:l. One part of the resulting substance is used for checking its composition by elemental analysis, and the remainder is used for coating the crystal. C~(butyrate)~.$ethylenediamine. The preparation of this compound is similar to that of Cu(butyrate)2.ethylenediamine; the ratio of Cu(butyrate)z to ethylenediamine is 1:2. One part of the resulting substance is used for checking its composition by elemental analysis, and the remainder is used for coating the crystal. Polymer Bonded Copper Complexes. The XAD-4-Cu2+ diamine, XAD-4-Cu2+ triamine, and XAD-4-Cu2+ tetraamine complexes were synthesized and given to us by Donald Owens (Tulane University, New Orleans, LA). The composition of the complexeswas determined by elemental analysis and UV and IR spectrometry. The coarse beads were finely ground in an agate mortar so that the crystal could be coated by spraying the polymer dust onto its electrodes. Crystal Coatings. In order to determine if the number of ligands bonded to copper influence the bonding to phosphorus esters in air, the same way as in water, we prepared several butyrate complexes with differing amounts of ligands. Butyrate as an anion was selected because it makes the corresponding copper salt soluble in 1-butanol and permits the preparation of the complexes in a water-free solvent. The anion is believed to have little influence on the affinity of the complexes for phosphorus esters, Copper diamine, triamine, and tetraamine bonded to a polymer have also been assayed. The crystals were coated on both electrode surfaces for all work. The assayed substances are listed in Table I. The copper butyrate complexes were dissolved in methanol; the solution was applied to crystal's electrodes and allowed to dry. The XAD-4-copper polymers were sprayed onto the crystal's electrodes that were humidified with methanol and dried and the excess polymer wiped off with a soft piece of paper. The polymer is believed to adhere to the crystal's electrodes by electrostatic forces. The constant frequency reading observed for these coated crystals during several days of actual use is an indication of good stability. RESULTS AND DISCUSSION Sensitivity Studies. Diisopropyl methylphosphonate (DIMP) was used as a model for phosphorus esters. The results are summarized in Figure 2.
NUMBER OF LIGANDS PER COPPER ATOM
3
4
Figure 2. Adsorption values of copper butyrate salts and copper complexes bound to a polymer for DIMP.
All reactions are reversible, that is, DIMP is quantitatively desorbed when the crystal is flushed with pure nitrogen. The bidentate complexes, which are the best catalysts for hydrolysis, proved to be the best coatings. The XAD-4-Cu2+ diamine coating has a higher affinity for DIMP than does the corresponding copper butyrate complex. The results shown in Figure 2 are the average of six measurements; the standard deviation is 3.5%. The higher adsorption surface of the polymer may be a possible explanation for this observation. How does XAD-4-Cu2+ diamine compare with 3-PAD, already shown to be a good selective substance for DIMP ( I ) , and with SE-30, an unselective silicon oil? The results are summarized in Table 11. The XAD-4-Cu2+ diamine coating has been found to have a higher affinity for DIMP than the 3-PAD coating. Calibration Curve. The calibration curve of the XAD4-Cu2+ diamine coating is linear from 0 to 20 ppb DIMP (the range investigated). Approximately 20 ppb is 40% of the vapor pressure of DIMP at 20 "C, and this value is within the range of what is to be expected in contaminated air. A calibration c w e of l pg of XAD-4-Cu2+diamine showed a linear response of AF vs. DIMP concentration over the range of 1-20 PPb. Effect of Base. The catalytic hydrolysis of phosphorus esters by copper complexes is increased in an alkaline medium (IO). An assumption was made that the adsorption of DIMP could be enhanced by making the coating basic. For verification of this assumption, 2% NaOH was dissolved in glycerol, and this solution was mixed with various amounts of Cu(butyrate)2.ethylenediamine. Sodium glycerolate increased the adsorption of DIMP; however, no maximum was observed. A plot of AF/(ppb DIMP/pg coating) vs. percent of sodium glycerolate added showed a linear increase from (0.5 AF/ppb DIMP)/Fg coating at 0% sodium glycerolate to (1.4 AF/ppb DIMP)/pg coating at 100% sodium glycerolate. The mixing of the substances produced no synergystic effect. The overall adsorption is due to the contribution of each substance. This finding suggests that the basicity has an effect only on the second step of the hydrolysis of phosphorus esters, that is, irreversible breakdown of the phosphorus ester (catalyst-adduct). Lifetime. The XAD-4-Cu2+ diamine substrate has been found to be the best coating; however, without a long-lasting adhesion to the crystal's electrodes, this coating is of little practical value. A successful procedure has been developed for fixing the polymer permanently. First, the crystal was coated with poly(hexadecylmethacrylate), a highly viscous liquid; second, finely ground XAD-4-Cu2+ diamine was sprayed onto the crystal. The excess was wiped off with a soft paper. The adsorption of DIMP on poly(hexadecy1meth-
ANALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981
Table I. Coatings Evaluated (Identification Symbols and Associated Subscripts Identify the Substrates Depicted in Figure 2) polymer backbone identification symbol coating (1) (2) '40
Cu(butyrate),
A2
Cu(butyrate),.H N NH,
A3
Cu(butyrate),.H H NH NH,
A4 0 2
0, 0,
I
u2
t i
Table 11. Adsorption Values of Two Selective Substances for DIMP and an Unselective Substance (SE-30)a substance XAD-4-Cu2+diamine 3-PAD SE-30
( A F (Hz)/ ppb DIMPI/ pg coating
NH2
u
(1) (2) (3)
Table 111. Response of Copper Chelate Coating to t o Interferencesa concn, AF, interference PPm Hz carbon monoxide ammonia
2.6 1.9
hydrogen sulfide
0.4
a 3-PAD was assayed in pure state, although its known value is higher if this substance is mixed with a basic solution and a surfactant. Data in this table were obtained by recording the frequency change (Hz) observed following exposure olf a crystal coated with 2 pg of each of the three substrates to 5 ppb of DIMP. Thus, the AFresponse of 3-PAD woiild be 29 Hz (Le., 19 Hz/5 ppb DIMPI2 pg coating is l.9).
sulfur dioxide hydrogen chloride benzene toluene chloroform
a
1250 1250 250 1250 2 50 1250 250 1250 2 50 1000
26
2 18 0 12
4
40
15b 9
1000 500
13
49
100 to 1 5 ppm of DIMP = 591 Hz. response. AF
acrylate) is only (0.05 Hz/ppb DIMP)/Fg coating, approximately 25 times less than that on XAD-4-Cu2+ diamine. The calibration curve is linear between 0 and 20 ppb DIMP. The addition of the glue or plasticizer does not affect the calibration curve obtained in the absence of binder. Interferences. How selective is the XAD-4-Cu2+ diamine coating for DIMP in a background of pollutants which are commonly encountered in ambient air? Automobile exhaust gas, an adequate model for air pollutants, was selected. Samples were taken with a 50-mL syringe from the exhaust pipe of an idling V-8 gasoline engine. (The pollutant concentration is the highest for an idling engine.) The car had no catalytic converter in its exhaust system. The samples were diluted 1 : l O with air and injected through the septum into the detectoir (Figure 1). The assays were repeated several times with exhaust gas and pure air. Despite the crude technique used, the relative standard deviation was approximately 4 % . The 1 : l O exhaust gas mixture, which featured highly polluted air far above the average level of that in cities, airports, or highways, yielded a response equivalent to approximately 7 ppb DIMP. This result must, however, be considered tentative. The samplings of DIMP and exhaust gas have been made by different methods: monitoring and by batch. The batch method is likely to give lower results than monitoring, due to the length of time required to reach equilibrium between the gas phase and the crystal coating. The batch method for exhaust gas was chosen for simplicity. Nevertheless, even if a 1-decade error allowance is made, the XAD-4-Cu2'+ diamine coating still appears to be a highly selective costing for DIMP. Finally, tests were made concerning the effect of common atmospheric HC1, interferences (CO, NH3,H2S, SOz,benzene, toluene, and chloroform) on the response of a crystal coated with the Cu tetraamine chelate immobilized on the XAD-4
IH2 [yq,
NH
Cu(butyrate),.2H N NH, XAD-4-Cu2+diamine XAD-4Cu2+triamine XAD-4-CuZ+ tetraamine
(3)
? * +
u L/
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11
Irreversible
resin. The results (Table 111) indicate no serious interference from any of the diverse substances tested. Similar results, would be expected for the diamine and triamine chelates. CONCLUSIONS The coated piezoelectric quartz crystal coated with an XAD-4-Cu2+ diamine substrate has demonstrated high sensitivity and selectivity toward DIMP. When compared with the 3-PAD coating substrate developed in a previous effort (8,9),the XAD-4-Cu2' diamine coating has demonstrated an approximate 1.5-fold improvement in affinity toward DIMP. Thus, the high sensitivity and selectivity of the XAD-4-Cu2+ diamine coating toward DIMP predict that it will be an excellent choice for the detection of structurally related organophosphorus compounds. LITERATURE CITED King, W. H., Jr. Anal. Chem. 1984, 36, 1735-1739. Scheide, E. P.;Guilbault, G. G. Anal. Chem. 1972, 44, 1964-1968. Hlavay, J.; Guilbault, G. G. Anal. Chem. 1977, 49, 1890-1898. Oiln, J. 0.; Sem, G. J. Afmos. fnvkon. 1971, 5 , 653-668. King, W. H., Jr. ResJDev. 1989, 20(4), 28-34. King, W. H., Jr. Res./Dev. 1989, 20(5), 28-33. Sauerbrey, 0. Z. fhys. 1959, 155, 206-222. Guilbault, G. G.; Tomita. Y.; Kolesar, E. S., Jr. Final Report for USAF (SAM-TR-80-21), Contract F-33615-78-D-0617-30, 1980, NTIS A091705 Tomita, Y.; Guilbault, G. G. Anal. Chem. 1980, 52, 1484-1489. Proper, Wagner-Jauregg, T.; Hackley, B. C.; Lies, T. A,; Owen, 0. 0.; R. J. Am. Chem. Sac. 1955, 77, 922-929. Epstein, J.; Rosenblatt, D. H. J. Am. Chem. SOC. 1858, 80, 3596-3596. Gustafson, R. L.: Chaberek, S., Jr.; Martell, A. E. J. Am. Chem. Soc. 1963, 85, 598-601. Murakami, Y.; Martell, A. E. J. Am. Chem. Soc. 1984, 86, 2119-2129. Shakelford, W. M.; Guilbault, G. G. Anal. Chim. Acta 1974, 73, 383-389.
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Anal. Chem. 1981, 53, 2060-2067
RECEIVED for review March 2,1981. Accepted August 20,1981. This work was supported by the Department of Defense, United States Air Force, Air Force Systems Command, Aerospace Medical Division, USAF School of Aerospace Med-
icine, Brooks AFB, TX, under Contract F33615-78-D-06170030 and Army Research Office Contract DAAG-29-80-K-002, This paper has been reviewed by the USAF Office of Public Affairs and is approved for publication.
Isotopic Determination of Uranium in Picomole and Subpicomole Quantities J. H. Chen and G. J. Wasserburg" Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 9 1 125
A procedure for the separation of U and measurement of U isotopes in meteorltic, lunar, and terrestrlal samples is described. Accurate 23'U/235Uratios and U concentration measurements can be done on a routine basis at the level of mol of U by isotope dilution using a double tracer composed of 233Uand n3eU. Precise determination of the number of U atoms in a sample can be done at the level of 108-10s atoms. The technique involves careful control of the chemical procedures to ellmlnate laboratory contamination and precise calibration of the mass spectrometer in order to obtain high reproducibliity. The 238U/23sU ratios were measured on a wide variety of samples which include (1) bulk meterorltes, (2) lunar samples, (3) coarse-grained Ca-AITI-rich inclusions and (4) fine-grained AI-Mg-Na-rich inclusions from the Aliende meteorlte, and (5) phosphates from three meteorites. The results show the same Isotopic values withln limits of errors as that measured on normal terrestrial U to within 4%0. Ail of the analyzed meteorite samples are very ancient and several of them show large fractionations of U relative to Th and Nd. These data imply that 247Cmwas
