236
Anal. Chem. 1988, 60,236-240
Surface Acoustic Wave Gas Sensor for Nitrogen Dioxide Using Phthalocyanines as Chemical Interfaces. Effects of Nitric Oxide, Halogen Gases, and Prolonged Heat Treatment M a a r t e n S. Nieuwenhuizen* a n d Arnold J. Nederlof
Prins Maurits Laboratory TNO, P.O. Box 45, 2280 A A Rijswijk, The Netherlands
The effect of CO, NO, and 0, on the response of a SAW (surface acoustlc wave) chemosensor for NO, has been studled. A descrlptlon Is glven of the measurlng equlpment exlstlng of a mass flow controlled automatlc gas dllutlon system. Copper and Iron phthalocyanine were used as the chemkal Interface. Slmultaneously, the Influence of amblent atmospheres (N, and 0,) was Investigated. Predlctlons from ultraviolet-vldble experiments In solutlon do not hold for gaseous envhonments. Also the effect of electronegative gases like the halogens was studled. Response up to 40 tlmes the NO, response was measured. Prolonged heat treatment affects the senSnlvlty for NO2 negatlvely as well as the response tlme. Thls asks for a more stable chemlcal Interface. All results are discussed In terms of general pers such as selectlvlty, senslformance crlterla for gas m tlvlty, response tlme, reverslblllty, and stablllty.
Surface acoustic wave (SAW) devices are attractive for chemical microsensor applications because of their small size, low cost, sensitivity, and reliability. In previous papers (1-5) we already reported on our SAW chemical sensor research concentrating on the development of a SAW gas sensor for NOZ. In order to obtain a selective sensor system, metallophthalocyanines were investigated as the chemical sensor interface because these p-type organic semiconductors strongly interact with electronegative compounds like NOz. Recently, the effect of the central atom of the phthalocyanine on sensitivity, selectivity, and response time was studied as well as the effects of the layer thickness of the chemical interface and the SAW operating frequency ( 4 ) . It was concluded that copper phthalocyanine (CuPC) applied to one delay line of a dual delay-line SAW device via physical vapor deposition showed the best compromise with respect to the above-mentioned sensor requirements. At the time selectivity of the NOz sensor was tested by using a number of gases (CO, COz, NH3, SOz, HzO, methane, and toluene) that may act as potential interferences. In this paper we describe the measuring system and report on additional work focusing on some questions which arose as to whether NO is also interacting with the CuPC as NOz because NO and NOz often occur simultaneously. When the effect of NO was studied, the effects of CO and Oz also were tested both in air and in nitrogen. Iron phthalocyanine (FePC) predicted stronger interactions with NO, CO, and O2 (UV experiments in solution). Therefore besides CuPC also FePC was tested as a chemical interface. The response of the sensor to chlorine, bromine, and iodine was tested as these are very strongly electronegative gases and were reported to show large effects on the semiconducting properties of phthalocyanines (6). Finally, the effect of prolonged heat treatment on the response of the sensor as well as the morphology of the chemical 0003-2700/88/0360-0236$01.50/0
Table I. Characteristics of the Various SAW Devices Used in This Study: Layer Thickness ( h ) ,Frequency of the Measuring (f,)and the Reference Delay Line (fo)as well as Drift ( D ) and Noise (A') in the Frequency Difference Signal at 150 "C
sensor code h , prn CuPC65 CuPC65O CuPC67 CuPC68 FePC73 a
0.43
b
0.28 0.18 0.29
fl, MHz
fo, MHz
52.46 52.43 78.68 78.82 78.71
52.56 52.53 78.83 78.75 78.46
D ,Hz/rnin 0.8 0.3 4.3
N , Hz " 7 6
0.7
8
-13.1
30
After 6 weeks at 150 "C. *Morphology changed (see SEM pic-
tures).
interface was tested. As PCs are able to sublimate at elevated temperatures, their long-term stability was suspected to be poor. In the Experimental Section a description is given of the measurement system developed in our laboratory including an automated gas or vapor generation system using mass flow controllers. EXPERIMENTAL SECTION Copper and iron phthalocyanine, NOz, NO, CO, Clz, Br,, and I2 as well as dimethyl sulfoxide were obtained commercially. W spectroscopic data of solutions of PCs in dimethyl sulfoxide were obtained with a Beckmann DU-7W-Vis spectrophotometer. A stock solution was made and nitrogen was bubbled through for 24 h, followed by the appropriate gases for 1 h. The configuration of the SAW devices and electronics is described elsewhere (3). In Table I the characteristicsof the devices and the chemical interfaces are given, being the layer thickness, the frequencies of both delay lines, and the maximum drift and peak-to-peak noise during NOz measurements at 150 "C. The phthalocyanine layers were evaporated onto the quartz substrates by physical vapor depition using a stainlesssteel mask at mbar and a source temperature suffkient for 0.02 pm/min growth at a source distance of 8.5 cm. The layer thickness and morphology were determined by scanning electron microscopy with a Philips SEM 515 system at 30 keV. An experimental system has been built to generate vapors from bubblers, evaporators, or permeation tubes or to dilute pure gases from gas bottle sources. In Figure 1 the measuring system is depicted schematically. Pressurized air or another gas (a) is used as a diluting gas which is cleaned first by a coal filter (b). From a vapor generator (c) or a gas bottle (d) a primary stream controlled by a Hi-Tec F 201-EA mass flow controller (MFC) (gl) is diluted in a mixer (kl). In a second mixer (k2) part of the above-mentioned stream is diluted again using three MFCs: one containing the vapor or gas (g3), one containing dry gas (g4), and one (g5) containing water vapor from a bubbler (1) brought together in a mixer (k3). In this way the relative humidity is controlled. Relative humidity was measured with a Novasina MIK 3OOO-C capacitive RH-meter (n). From the final mixture a flow (10 L/h) is blown into the measuring cell via a Mace solenoid three-way valve (0)upon the start of a measurement. This flow is controlled by a sixth MFC (g6) in 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 3, FEBRUARY 1, 1988 237 Table 11. Sensitivities (S)and 80% Response Times ( t ) of CuPC67 and FePC73 for NOz, NO,CO, and O2 at 150 OC in Air and Nitrogen at 150 and 30 O C CuPC67
FePC73 S,
S,
atmosphere air air NZ N2
Flgure 1. Schematic representation of the measuring system. Characters refer to the text below.
combination with a central vacuum system (f). The measuring cell (m) consists of a milled brass cylindrical box containing the SAW device bonded to a ceramic plate. A silicone adhesive is used for proper heat conduction and to prevent the reflection of unwanted bulk acoustic waves. The back of the ceramic plate is provided with a thick film resistor in order to heat the sensor. Waste gas streams are blown into the fume hood (el, e2). NO2 and NO are monitored (n) with a Foxboro Miran 1A CVF infrared analyzer at the appropriate wavelengths. Chlorine and bromine concentrations are calculated from dilution ratim and weight l a w s of the source, while iodine is trapped in tetrachloromethane and measured by UV spectroscopy. The SAW device is fed by a 12-V power supply. A HewlettPackard HP 86 B computer (9) controls the temperature (&0.01 "C) inside the cell by using a temperature sensor and a thick film resistor as heat supply. The frequency signals are measured with an HP 5335A frequency counter (I) Monitor . signals as well as temperature signals are fed into the computer via analog-to-digital (A/D) converters (p). The MFCs are operated by a central unit (h) with settings by hand or remote setpoints from the computer using digital-to-analog(D/A) converters (9). Frequencies,monitor signals, temperature, and time are collected on an HP 9121 floppy-disk drive (u) in blocks of 30 measurements in order to present them later with a screen (t)or an HP 7470A plotter (v). From the frequency plots the response and sensitivity are calculated. Subtraction by the frequency of the reference delay l i e yields a response which is largely independent of temperature and pressure effects. The response of the sensor is related to the quantity of chemical interface material present. In practice the response (R) is normalized by dividing by the layer thickness. The sensitivity (S) is defined as the response divided by concentration (Hz/(ppm.pm)) in the initial linear part of the response curves.
RESULTS AND DISCUSSION NO, CO, and O2 Measurements. During all previous experiments air was used as the reference gas. In air NO is easily oxidized into NO2, especially in catalytic environments such as metal surfaces or even PCs. As a result the NO response of the sensor is very much disturbed by the presence of NOz. In order to study the response of the sensors especially for NO and 02,nitrogen was used as a reference gas. The measurement system was purged with nitrogen for 24 h. In air PCs act as p-type organic semiconductors but in nitrogen Heiland and Kohl (7) observed PCs to be n-type semiconductors. As SAW responses are controlled both by conductivity effects and mass effects, changing from air to nitrogen might have an influence on the SAW responses. In Table I1 the sensitivities and response times of two sensors (CuPC67 and FePC73) a t 150 OC for a number of gases in air and nitrogen are listed. The sensitivity of CuPC for NOz is smaller in nitrogen than in air with comparable response times. Also the effect of treatment of the PC layer with NO was observed. In air as well as in nitrogen this results in an increasing sensitivity, but this is accompanied by a strongly increased response time, especially in nitrogen. FePC shows no difference in sensi-
N2 N2 N2 NZ (30 "C) NZ (3OOC)
gas NO2 NOZ" NO2
NOza NO
co 02
CO 0 2
Hz/
(ppm-pm)
t,
min
Hz/ (ppmepm)
t,
min
42 69
2 8
72
6
31 39
3 35
73
6 6
4 0 0
14
-5 -0.lb
1 1
-10-5
2
-10-4
1
-10-2 -10-4
6
-0.3b
6 1 1
"After measurements with NO at 150 "C. bInitial responses. tivities in air compared to nitrogen, but after the NO experiments, sensitivity for NOz is much smaller, while response times are unaffected. Possibly, NO is reacting with the PC or changing its morphology in some way. With respect to NO, Uchida et al. (8) mention 1:l axial NO/PC complexes. Using X-ray photoelectron spectroscopy, they observed a very much central metal atom dependent electron transfer between the central metal atom and NO. This phenomenon differs from the interaction between NOz and PC. Although both gases are electronegative, NOz tends to extract an electron from the electron clouds of the aromatic rings of PC, while NO interacts with the metal atom. Catalytic properties of CuPC and FePC are mentioned by Steinbach and Joswig (9). They report on the reduction of NO into both nitrogen and ammonia. To study interactions between PCs and CO, NO, and O2 especially FePC has been used as a hemoglobin model compound. Collamati (10) reports on the formation of reaction products of FePC and O2in solution as studied with UV-vis spectroscopy. Stynes et al. (11)mention the synthesis of a number of FePC derivatives containing CO, while Calderazzo et al(12) found that FePC shows irreversible Oz binding and reversible CO binding. From the general effects of the toxic action of CO in blood a strong interaction between the porphyrin system and CO is known. In order to predict the behavior of CuPC and FePC with respect to the gases studied, it was tried to obtain some information from UV-vis spectra of these compounds in DMSO saturated with NOz, NO, CO, or O2with N2 as a reference gas, although it was realized that correlations might be poor due to the fact that the solution environment is different from the PC layers. In Figure 2 these spectra are given for CuPC and FePC, respectively. In the case of CuPC no clear effects can be observed, except for the appearance of NO signals in spectrum c. In the case of FePC in all cases spectral shifts were obtained in the 300-400 nm and 600-700 nm region indicating interactions between FePC and all gases. With Ozaccording to Calderazzo (12) shifts to higher wavelengths indicate the formation of oxygenated compounds. With NO, shifts to higher wavelengths indicate the formation of 1:l NO/FePC complexes according to Ascenzi et al. (13). With some reserve one might expect the same kind of interactions when using FePC layers in the gas phase. The sensitivities for NO as listed in Table I1 for CuPC and FePC are much smaller than for NOz, while in the case of FePC even a negative response was observed. Negative responses have been observed before ( 4 ) when studying the effects of NH3 and HzO, which are also complex forming gases. With CuPC the response time for NO is much longer than
238
ANALYTICAL CHEMISTRY, VOL. 60, NO. 3, FEBRUARY 1, 1988
absorption
300
A
i
S,
I
I
400
500
,
I
600
I
700 h inm 1
800
li
concn,
Hz/
gas
ppm
(ppmsfim)
t , min
EA,^ eV
M
r, nm
NO2 N02' C12 Br2 I?
100 100 50 50 13
16 7 678 78 67
1 48 23 23 15
3.91 3.91 3.61 3.36 3.06
46 46 71 160 254
0.36 0.36 0.36 0.40 0.46
" After halogen gas experiments. *From ref B
i
absorption
Table 111. Sensitivities (S)and 80% Response Times ( t ) of CuPC68 for NOz, Cl,, Br2, and I2 at 150 "C as well as the Electron Affinities (EA) from the Literature, Molecular Masses ( M ) ,and Critical Diameters ( r )
15.
Table IV. Results of Prolonged Heat Treatment" NO2
3"
time, weeks
S , Hz/(ppm.fim)
min
S, Hz/(ppmym)
min
0 3 6
46 92 34
3 27 42
-3.7 -4.4 -4.2
10 12 10
t,
t,
"Sensitivities (SIand 80% response times of the response of CuPC65 for NO2 and NH3 at 150 O C after heating at 150 "C for 0, 3, and 6 weeks.
A inml
Flgure 2. (A) UV-vis spectra of solutions of CuPC in DMSO saturated with N, (a), NO, (b), NO (c), O2(d) and CO (e). (B) UV-vis spectra of solutions of FePC in DMSO saturated with N, (a), NO, (b), NO (c), 0, (d) and CO (e).
for NOz, while with FePC no difference is found. The sensitivities for CO and Ozdo not reflect the predictions from the UV-vis experiments at all. No correlation is found between the shift of the 307-nm signal in Nz, being +11,+55, +13 and +16 for NOz, NO, 02, and CO, respectively, and the sensitivities as given in Table 11. CuPC is not sensitive to CO or O2 at 150 "C, while FePC does only show initial effects indicating irreversible interactions. At lower temperatures (down to 30 "C) very low sensitivities were observed. Response times indicate that probably only surface effects will occur. Chlorine, Bromine, and Iodine Measurements. In the literature some papers describe the interaction between PCs and electronegative gases like the halogens. Gentry and Walsh (14) measured conductivity changes of PbPC caused by chlorine at several temperatures. A rapid initial response was observed, followed by a slower one. They concluded the effect to be surface adsorption followed by reaction or absorption or diffusion in the bulk, like we (4) also postulated for the NOz response, although with SAW, apart from conductivity effects, mass changes also play a role. Heiland and Kohl (7) also measured conductivity changes of PCs due to chlorine, but we feel they only measured the initial response as mentioned by Gentry and Walsh because very different response times are reported by both groups. Snow et al. (15) report on the simultaneous conductivity and SAW measurements of the interaction of Langmuir-Blodgett films of PC derivatives with iodine. We seriously doubt whether their explanation of SAW responses as only mass changes is right as conductivity changes play a role too. Nevertheless, they observed interesting phenomena. Response times of the SAW and conductivity measurements differed substantially when measured at the same time, which possibly indicates a strong surface effect on conductivity changes which were fastest. In Table I11 the results for CuPC a t 150 "C with chlorine, bromine, and iodine are summarized. They all show a much
L
0
60
I
I
120
180
t imin)
Flgure 3. Response curves of CuPC65 for 100 ppm NO, at 150 " C after prolonged heat treatment at 0 (a), 3 (b), and 6 (c) weeks.
higher response than NOz, especially chlorine. This is accompanied by much larger response times. Four effects can be mentioned: SAW responses result from changes in conductivity and in mass which both may result from surface adsorption or from bulk effects. The strength of the interactions as well as the conductivity effects might be reflected by the electron affinities of the gases. Mass changes are controlled by molecular mass and size. Size also controls whether or not bulk effects play a role. Therefore in Table I11 also the electron affinities, molecular masses, and critical diameters of the gas molecules are listed. From this table it is clear that neither these values nor a combination thereof do correlate with the observed sensitivities or response times, indicating a combination of effects. Furthermore, a destructive effect of the halogens on the NOz sensitivity and response times was observed (Table 111). Prolonged Heating Experiments. In ref 4 it is mentioned that the stability of evaporated PC layers at elevated temperatures and in gas streams is poor above 150 "C. It is also noted that CuPC layers show the highest relative stability, especially when compared with FePC or PbPC. In order to obtain some more information on the stability of CuPC, CuPC65 was heated at 150 "C for 6 weeks. In Table
ANALMICAL CMMISTRY. VOL.
!.
BO. NO. 3. FEBRUARY 1.
1988
239
*
npUr 4. Scanning electron rnlcr~aphs01 the C U E layer of
CuPCB5 before (lelt) and alter (rlght) the heating exprlments.
lV the sensitivitiea for NOz are listed measured after 0.3,and 6 weeks. Since previously CuPC showed substantial response to ammonia (4). the sensitivity for this compound is given as well. With respect to the NO2 response it was found that after 3 weeks a higher sensitivity is observed followed by a lower sensitivity after 6 weeks. Response times increase continuously. Both effects are also clearly observed in the response curves depicted in Figure 3. A two-mechanism response curve is visualized consisting of a rapid initial effect (surface) followed by a slower effect (bulk). Upon aging,the initial response does not seem to slow down but its magnitude is smaller. The relative contribution of the second effect to the response shows the game maximum as the total response. Its contribution in the response time is strongly increasing with time. Finally, recovery times are hardly affected. The response to NHS is hardly affected because the response mechanism differs from that for NO2 as is reflected by its opposite sign ( 4 ) . The abovementioned effects are caused by a change in the morphology of the CuPC layer, as c h e m i d changes are not believed to take place under these conditions. Figure 4 shows scanning electron micrographs of the CuPC layer before and after the heating experiments. Obviously, sintering has taken place by some kind of e v a p o r a t i o n / r e c r y s t n mechaniim. A8 a result the outer surface becomes less porous (initial response) causing larger diffusion paths into the layer (response times). In future this problem will he solved by chemical immobilization of tetracarboxy PCs via a spacer molecule (a silane reagent) (16,17). The expected smaller responeea of these monomolecular layers will partly he overcome by operating a t higher frequencies as the SAW sensitivity will be higher then. Another advantage of chemical immobilization will be shorter response times due to less influence of diffusion phenomena. CONCLUSIONS With CuPC and FePC, differencea in NO2 sensitivitiea and response times between air and nitrogen as background were observed. Possibly, NO is reacting with the PC or changing its morphology in some way, because after exposure to NO changes in the behavior of the sensor were observed. The results for NO in nitrogen indicate the formation of coordination compounds. Its effect in air will mainly result in the formation of NO2 in oxidizing atmospheres and nitrogen or ammonia in reducting atmospheres, possibly enhanced by the catalytic effect of PCs as well as high temperatures and metal surfaces (measuring cell). The sensitivities to CO and O2did not reflect the predictions from the UV-vis experiments.
CuPC is not sensitive to CO or O2a t 150 "C. while FePC doea only show initial effects. At lower temperatures (down to 30 OC) very small sensitivities were observed. The halogen gases showed a strong interference with sensitivity and stability, confirming the strong interaction of electronegative compounds with PCs. Sensitivity to NO2 decreases after 6 weeks a t 150 "C and response times increase dramatidy. NH3response is hardly affected. These effects seem to be caused by morphology changes of the PC layer. A possible solution to the stability problem is chemical immobilization of tetracarboxy PCs via a spacer molecule (a silane reagent), which is currently being studied. ACKNOWLEDGMENT The contributions to the work described in this paper of Mr.Barendsz, Mr.Oudmayer and Mr.Bosman of our l a b ratory are gratefully acknowledged. Mr. Duvalois is acknowledged for measuring the scanning electron micrographs. We also thank Dr. Venema and Mr. Vellekoop of the Department of Electrical Engineering of Delft University of Technology, The Netherlands, and Mr. Nieuwkoop of the Institute of Applied Physica TNO-TH, Delft, The Netherlands, for their collaboration within our multidisciplinary project team. Registry No. CUPC, 147-14-8: FePC, 132.161; CO. 630-08-0; NO, 10102-43-9; 02,7782-44-7; NO2, 10102-44-0;Nz,7727-37-9; Br2. 7726-95-6; CI,, 1782-505; I*, 7553-56-2.
LITERATURE C I T E D (1) Sarsn6s2. A. W.: Vk. J. C.: NLnrrrsnhulrsn. M. S.;NbmIImp. E.: VsLhmp. M. J.; Wjwn. W. J.: Venmna. A. Rocsedhpaof !iw IEEE ulbaoonk8 Symposknn. San FnnCLOM. CA. 1985. 586-590. (2) NIOUWOIIMZ(HI. M. S.: Barsnbrr. A. W.: NIWW*OOP.E.: VelWmp. M. J.; V~MM. A. E k t r W I . Len. 1088. 22. 184-185. (3) V-m. A,: Niau*kmp. E.: VslWmp. M. J.: Bsrsndsr. A. W.; Nbu. wenhuizen. M. S. Ssns. Achamr 1986. 10. 47-84. 141 NiaUwenhUbesl. M. S.:-1. A. J.: Barendsz. A. W. AMI. m.. .. weceding I" mls b l m . 151 E.: Vsllak-. M. J.: (hilasn. W. J.: h r , ~Venema. , ~. A.: Nbu*k-. sndsr. A. W.': N&&I&en,' M. S.I& Tram: I&&.. Fm&bcbier end Frequency CMnd 1987. 34. 148-155. (8) Oirrchol. Th. G. J. van; LBBuwen. D. van: Msdama. J. J . E ~ c ~ L w M I .
ppsr
~
~
Chem. 1972. 37,373-365. 171 Helland. 0.: K h l . H. Rmeedlna(l0ImS Imanatbral C a n t a m on &iaStale'Sen&s and Armatm. T r a w '85.280-263. PI L k h & K ' Soma. M.: Onirhi. 1.:Tamam. ~~.K. J . cham. Soc.. Ferad3v
..
~
iwe.
T ~ S ~ IS .
~
~
75.2839-2856
Jaw,
(9) Sleinbach. F.; H.J. J . &Id. 1078. 55. 272-280. (10) CaLmaIl. I I m g . Chim. A m 1980, 124, 61-66. (11) stynes. D. v.; JBWS B. R. J . ~ m m. . soc. i n n . 96.
2733-2738. (12) CBlderauo. F.; Frediani. S.: Jams. B. R.; PamPaM. G.: Rsmn. K. J.: S a m . J. R.: Serra. A. M.: VbU. D. IMQ. Unn. 1982. 21. 2302-2306.
Anal. Chem. 1888, 60, 240-244
240
(13) AscenZi. P.; Brunorl. M.; Pennesl. 0.; Ercolanl, C.; Monacelll, F. J. Chem. Soc,balron Trans. 1987, 369-371. (14) Gentry, S. J.; Wakh, P. T. Proceedings of the 2nd Internatlonal Meeting on Chernlcal Sensors, Bordeaux, 1986, 209-212. (15) Snow, A. W.; Barger, W. R.; Klusty, M.; WohltJen, H.; Jarvis. N. L. Langmuk 1988, 2, 513-519. (16) Barendar, A. W.; Nieuwenhuizen,M. S. Dutch Patent 85-02705. (17) Nieuwenhuizen, M. S.; Nederlof, A. J.; Coomans, A. Frezenius' 2.
Anal. Chem ., submitted for publication.
RECEIVED for review June 8,1987. Accepted September 14, 1987. Financial support was given by the TNO Centre for Microelectronics (Delft, The Netherlands).
Direct Determination of Dissolved Cobalt and Nickel in Seawater by Differential Pulse Cathodic Stripping Voltammetry Preceded by Adsorptive Collection of Cyclohexane-1,2=dione Dioxime Complexes John R. Donat* and Kenneth W. Bruland Chemistry Board of Studies and Institute of Marine Sciences, University of California, Santa Cruz, Santa Cruz, California 95064
A hlahly rendthe vottammetric tedmlque was developed for the dlrect detennlnatlon of cobalt and nlckel In seawater at plcomolar and nanomder concentratlons, respectively. Cyck~hexane-l,2dlOlw,dlaxlme (nkxlme) complexes of Co( I I ) and Nl( I I ) were concentrated from 10 mL of sample onto a hanging mercury drop electrode by controlled adsorptlon and the current rerultlng from r.ductlon of Co(I I ) and Nl( I I ) was meawed by dmerential pldse cathodlc stripping vottammetry. Detakd experlments were conducted to determine the optlmal ligand type and concentretlon, buffer type and concent r a m pH, and adoorptkn potential. Maxhnum SenellMty was obtaltwd by uskrg a nloxlme concentratlon of 1 X I O 4 m , a HEPES buffer concentratlon of 0.03 m , a solutlon pH of 7.6, and an adsorptlon potentlal of -0.6 V. Replkate analyses of seawater reference materlals yielded excellent agreement wlth CertMled values. Analytlcal preclrdon for Co and NI at coastal and open ocean concentrations was approximately 45% relative standard devlatbn. Detectlon Umlts for Co and NI depend upon reagent Manks and are 6 pM and 0.45 nM, respectlvely, for 15-mln adsorption perlods.
The most common methods for determining Co and Ni concentrations in seawater have used graphite furnace atomic absorption spectrometry (GFAAS) preceded by a concentration step involving either coprecipitation, solvent extraction, or a chelating resin (1-4). While these methods are sensitive, they also require relatively expensive instrumentation and relatively large sample volumes, are generally time-consuming, and are not conveniently performed at sea. In contrast, voltammetric techniques allow direct anaiyte determinations without separate preconcentration steps and have the potential to be used on board ship, thereby permitting nearly real-time analyses. Recently, Sensitive voltammetric techniques that rely on adsorption of surface-active metal complexes onto a hanging mercury drop electrode (HMDE) followed by a reductive stripping step have been developed. Trace metals determined in natural waters by this approach include Ni and Co ( 5 , 6 ) , Cu Zn (8),Fe (9),La, Ce, and P r (IO), Mn ( I I ) , and Cu, Cd, and P b (12).
(a,
0003-2700/88/0360-0240$01.50/0
This class of voltammetric techniques has been called adsorptive voltammetry by some authors and cathodic stripping voltammetry by others. The technique we report here for the direct, simultaneous determination of Co and Ni in seawater belongs to this class and will be called differential pulse cathodic stripping voltammetry (DPCSV) because of the pulse modulation applied during the stripping step and the direction of resulting current. The reported concentrations of dissolved Co and Ni in oceanic waters are 10-100 pm and 2-12 nm, respectively (1, 4,13,14). Although DPCSV techniques have been developed for Co and Ni by using the ligand butane-2,3-dione dioxime (dimethylglyoxime, DMG) (5,6),they did not appear to have the sensitivity required for the determination of Co in seawater. The DPCSV technique presented here utilizes the ligand cyclohexane-1,2-dione dioxime (nioxime), which has a high selectivity for Co and Ni similar to DMG and forms complexes with Co(I1) and Ni(I1) that are cyclic and square planar, thereby possibly enhancing their adsorption onto the HMDE. We investigated the use of this ligand in order to develop the most sensitive technique possible, particularly for Co with its low concentration in seawater. This technique is more sensitive than the DPCSV techniques employed previously and is faster and uses less expensive instrumentation and smaller sample volumes than GFAAS methods which require separate preconcentration steps. Results of detailed studies aimed at optimizing the analytical parameters for maximum sensitivity in the determination of Co and Ni in seawater are presented.
EXPERIMENTAL SECTION Electrochemical Equipment. In all experiments, an IBM EC 220-1A stripping voltammeter was used interfaced with an EG&G Princeton Applied Research (PAR) Model 303A static mercury drop electrode (SMDE) in the hanging mercury drop electrode (HMDE) mode. The stripping voltammeter was modified to increase ita sensitivity for seawater analysis as follows (15): The pulse width was shortened to 33 ms, and the pulse amplitude and frequency were made operator-selective between 0 and 100 mV and 2 or 5 s-l, respectively. A "large" mercury drop (2.8 mm2 surface area) was employed as the working electrode. The Ag/AgCl reference electrode was modified by depositing AgCl onto the silver wire and filling the glass electrode sheath with a saturated solution of ultrapure KCl (Merck). The auxiliary 0 1988 American Chemical Society
