ANALYTICAL CHEMISTRY, VOL. 50, NO,
9. AUGUST 1978
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Determination of Silver in Precipitation by Furnace Atomic Absorption Spectrometry J. D. Sheaffer' and Gerald Mulvey2 Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado 80523
R. K. Skogerboe" Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
A procedure for the determlnatlon of sllver In preclpltatlon at levels down to 1 X lod Fg/mL uslng furnace atomlc absorptlon and evaporatlve preconcentratlon Is descrlbed. The method Is shown to produce results whlch are preclse to better than f20% for trlpllcate analyses. Collaboratlve test and splke-recovery results also lndlcate that the accuracy Is In thls range, Results are presented whlch lndlcate the valldlty of the sample storage procedures used and problems whlch may be essoclated wlth such procedures. The method has been used for the analysls of preclpltatlon samples to demonstrate the occurrence of slgnlflcant Increases In the sllver content of snowfall 100 km or more downwlnd of sllver lodlde seedlng locatlons.
The effectiveness of silver iodide as a weather modification agent has been extensively studied. I t has been demonstrated that seeding with relatively small amounts of silver iodide, under proper dispersion conditions, can measurably increase precipitation from certain orographic and cumulus formations ( I ) . Higher concentrations of silver in seeded vs. unseeded precipitation is considered partial evidence of a n effective seeding operation. Similarly, the silver content of precipitation in downwind locations is a n important indication of inadvertent (extended area) seeding effects. T h e concentration of silver in natural, unseeded, precipitation is usually on the order of 104-10-5 pg/mL (2,3). Silver levels reported in seeded precipitation range upward to, but rarely exceed, pg/mL (4). Warburton (3) measured silver in snow collected 80 km downwind of a seeding program in the Sierra Mountains and noted that seeding approximately doubled the silver background level of 4 X lo4 pg/mL. The determination of these ultratrace concentration levels requires a n analytical method of unusually high sensitivity. Methods used t o successfully determine concentrations of silver in precipitation include neutron activation analyses (NAA) and furnace atomic absorption spectroscopy (FAAS) (4,5). Although NAA and FAAS are very sensitive methods, a preconcentration step is often required to bring silver concentrations to measurable levels. Preconcentration has been done by solvent extraction and cation exchange methods ( 4 , 5 ) . These techniques are time consuming and require great care, elaborate cleaning, very pure reagents, and frequent monitoring of efficiency. One analyst has avoided many of these problems by using evaporative preconcentration with FAAS (6). Relatively large volumes of precipitation are injected directly into a graphite furnace, evaporated to dryness, and atomized. T h e furnace configurations of many FAAS systems are too small for ef'Present address, Environmental Research and Technology, Inc. Fort Collins, Colo. 80521. 'Present address, Meteorology Research, Inc., Altadena, Calif. 0003-2700/78/0350-1239801 .OO/O
Table I. Summary of Instrumentation and Operating Conditions spectrophotometer atomization unit furnace conditions dry ash atomize background correction furnace sheath gas
analytical wavelength recorder
Varian-Techtron, Model AA5 Varian-Techtron, Model 6 3 5 s a t - 1 2 0 'C 5sat-300"C 5 s at 2000 'C Varian-Techtron, Model BC6, with H, continuum lamp special, long tube, fabricated from Poco Graphite, No, FX9 (see Figure 1) argon, welding grade at 5 L/min plus 10% methane in argon at 0.1 L/min during ash and atomize steps 328.1 nm Coleman, Model 65, strip chart recorder
ficient application of this preconcentration technique. T h e present report describes the use of an oversize graphite tube furnace in a flameless atomization system and includes procedural innovations for rapid evaporative preconcentration and analysis of silver in precipitat,ion. EXPERIMENTAL Apparatus. The atomic absorption system and operating conditions used are summarized in Table I. The standard graphite tube furnace was replaced by the oversize unit shown in Figure 1. The internal volume of this furnace is nominally 0.2 mL. A small indentation was drilled opposite the injection port to facilitate control of the localization of samples injected. Reagents. All nitric acid used was doubly redistilled in quartz and stored in Teflon bottles prior to use. Dilutions of this acid were made with distilled-deionized water previously stored in polyethylene bottles. The silver blank of 3 M acid prepared from these reagents was consistently less than 2 X 10" wg/mL. Analytical standards were prepared from AR grade AgN03 dissolved in 3 M HN03 and stored in Teflon bottles. A 0.1 O/o (w/v) solution of paraffin in AR grade cyclohexane was used as a pretreatment dip for the furnaces. Procedures. Falling snow samples were collected in plastic bags held 1 m above the ground by support frames. Samples were retrieved as soon as snowfall stopped and were held frozen until analyzed. Rainfall samples were collected in acid washed, 1-L polyethylene bottles fitted to the stems of 30-cm diameter polyethylene funnels set 1 m above the ground. Each rain water collection bottle was spiked with 10 mL of 3 M HN03 prior to collection t o inhibit losses of silver through wall adsorption. Rainfall samples were frozen as soon as possible after collection and retained in that form for storage. Sample collections were made by cooperative observers previously made aware of procedures necessary to preserve the samples and avoid contamination. In preparation for analysis, snow samples were thoroughly mixed in the bags and a fraction of each was transferred to smaller bags for melting at room temperature. A volume of 3 M HN03 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AUGUST 1978
sufficient to provide a 1% (v/v) acid concentration in the anticipated melt water volume was added to each sample on initiation of melting. Rainfall samples were allowed to completely melt in the collection bottles to avoid silver fractionation associated with freezing. Aliquots of each sample type were subsequently transferred to smaller plastic bags (Baggies) retained in beakers. All samples were kept covered except during the final analysis preparation step. Sample furnaces were dipped in cyclohexane solutions of paraffin and allowed to air dry. The resulting paraffin film prevented seepage of the water into the graphite lattice and was instrumental in confining sample aliquots to the central region of the furnace during evaporation. Samples were transferred to the furnaces by sequential injections of 50 p L each with an Eppendorf pipet. T o evaporate each injection, the furnaces were placed in an aluminum vee-notch holder device designed to hold up to 50 furnaces and situated on a hot plate. Evaporation of successive injections at -80 "C was typically continued until 1.0 mL of sample had been delivered. Preparation of approximately 40-50 sample furnaces in this manner required about 2 h. Following such preparation, sample furnaces were stored sealed in individual plastic vials until analyzed. For analysis, the furnaces were individually inserted into the work head of the atomizer and run through the atomization cycle under conditions stipulated in Table I. The individual response characteristic of each furnace was subsequently determined by injection and analysis of a series of standard solutions. This approach compensated for slight differences between furnaces and/or furnace alignment in the optical path.
RESULTS AND DISCUSSION S a m p l e S t o r a g e . T h e very low silver content of natural precipitation samples requires the use of containers that neither contribute a silver blank nor adsorb silver from kg/mL level. T h e silver aqueous solution a t the typical concentrations of small quantities of 3 M H N 0 3 , 0.03 M "OB, and distilled-deionized water were monitored for ten days during storage a t room temperature in new acid washed polyethylene bottles. No increases were observed in the 108
8 4 261 3 3 39i 7 only visible impurities. These impurities were easily separated from the snow by manual means. The comparatively better sample quality was the result of several factors determined by the snow cover and temperature regimes, Le., minimal dust Contamination, no insects, and closer attention by cooperative observers.
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ANALYTICAL CHEMISTRY, VOL 50, NO. 9, AUGUST 1978
T h e analysis results for each storm group were averaged to obtain a single datum per storm (11). Samples which contained snowfall from two or more storms were rejected from the data set. The resulting data set was subdivided in seeded and nonseeded subsets of 14 and 43 averaged measurements, respectively. Seeded samples were those collected when substantial seeding occurred in conjunction with favorable winds for transport of seeding material to the sampling area. Evaluation of each data subset via the Half-Normal Method described by Daniel (12) indicated that both fit a log-normal (Geometric) rather than a normal distribution. Evaluation for the presence of outliers resulted in the rejection of 1 and 5 inordinately high values from the seeded and nonseeded subsets, respectively, a t the 95% confidence level. The remaining data in these subsets of 13 and 38 average measurements were also shown to fit a log-normal rather than a normal distribution. This is consistent with the stochastic behavior of trace substances in natural systems as pointed out by Rustagi (13) and Ahrens (14). The geometric mean and standard deviation of the seeded subset were determined to be 1.7 X 10 f 0.3 X 10 pg Ag/mL; the values fur the nonseeded subset were 7.8 X & 2.4 X l o 4 Ag/mL. The difference between these means was determined t o be significant a t the 95Y~confidence level. This effectively indicates that seedling resulted in a mean increase in the silver content of the snowfall by approximately a factor of 2 as previously noted by Warburton (3). In this context, the results suggest the occurrence of extended area seeding effects at distances approximating 100 km or more when meteorological conditions are favorable. This is supported by the analysis of the silver data and the associated meteorology presented by Mulveq and Sheaffer (11). T h e accurate determination of Ag a t these concentration levels is more than a trivial problem. Results obtained in the
present study have indicated that precautions must be taken to avoid contamination. The presence of appreciable amounts of Ag in many reagents and the difficulty of purifying these further argues for the use of a minimal reagent approach such as that adopted herein. The accumulation of the majority of the Ag and the acid preservative in the portion of an aqueous sample which does not freeze at -20 "C emphasizes the necessity of completely thawing such samples in order to obtain a representative sample of the original for analysis. Finally, the analysis of rainfall for the present purpose must be based on the utilization of a sample collection system which remains closed until precipitation starts and closes soon after it ends. Otherwise, contamination by fugitive dust and insects will render the data highly suspect.
LITERATURE CITED (1) L. 0 . Grant and A. M Kahan, "Weather Modification for Augmenting Orographic Precipitation": J. Sinrpson and A. S. Dennis, "Cumulus Clouds and their Modification", in "Weather and Climate Modification", W. N. Hess, Ed., John Wiley and Sons, New York, N.Y., 1974. (2) E. Bollay Associates, Final Report, Bureau of Reclamation Contract No. 14-06-D-5573, 20 pp, 1965. (3) J. A. Warburton, Proceedings of International Conference on Weather Modification, Canberra, Australia, 1971, p 185. (4) R. Woodriff, R. B. Culver, D. Schrader, and A . B. Super, Anal. Chem., 45, 230 (1973). (5) J. A. Warburton, and L. G. Young, Anal. Chem., 44, 2043 (1972). (6) D. A. Segar, Department of Commerce, NOAA Laboratories, Miami, Fh., personal communication, 1976. (7) A. W. Strumpler, Anal. Chem., 45, 2251 (1973). (8) A. W. Strumpler, Armos. Environ., 10, 33 (1975). (9) J. Wisnuski, Proceedings of the Fourth Conference on Weather Wlication, 516 (1974). (10) J. E. Dye, C. Knight, and T. W. Cannon, Proceedings of the Fourth Conference on Weather Modification, Fort Lauderdale, Fla., 1974. (1 1) G. Mulvey and J. D. Sheaffer, submitted to J . Appl. Mefeorol. (12) Cuthbert Daniel, Technometrics, 1, 311 (1959). (13) J. S.Rustagi, Arcb. Environ. Hea/fh, 8, 68 (1964). (14) L. A. Ahrens, Geochim. Cosmochim. Acta, I , 49 (1954).
RECETCTD for review March 30,1978. Accepted May 12,1978.
Chemiluminescence Fiber Optic Probe for Hydrogen Peroxide Based on the Luminol Reaction Thompson M. Freeman and W. Rudolf Seitz" Department of Chemistry, University of New Hamsphire, Durham, New Hampshire 03824
A chemliuminescence (CL) fiber optic probe for hydrogen peroxide based on the iumlnol reaction has been constructed by immobilizing peroxidase in a polyacrylamide gel on the end of a fiber optlc. When the probe is immersed in a solution of peroxide in the presence of excess lumlnol, CL is generated as peroxide diffuses into the peroxidase phase. The fiber optic transmks CL to a detector. The theory of CL fiber optic probes has been developed assuming a first-order reaction In the enzyme phase. However, the peroxide probe shows response which is second order in peroxide. The detection limit is close to lo-' M peroxide. The time required to reach steady state varies with peroxide concentration. At M H,02, steady state is reached in about 4 s. Under the conditions usually employed, the CL intensity is limited by the rate of mass transfer from solution to the surface of the enzyme phase rather than by the activity of the peroxidase.
In this paper we report a new approach to using immobilized enzymes for analytical purposes, the chemilumines0003-2700/78/0350-1242$01 OO/O
cence (CL) fiber optic probe. This approach is applicable to any enzyme-catalyzed process that leads to the production of light. The enzyme catalyst is immobilized on the surface of a fiber optic. When the fiber optic is immersed in a solution of substrate, light will be generated as substrate diffuses into the immobilized enzyme phase. This light is transmitted through the fiber optic to a photomultiplier tube. The light level will reach a steady state when the rate at which substrate diffuses into the enzyme phase equals the rate at which substrate reacts in the enzyme phase. If the light-producing reaction is first order in substrate, then steady-state CL will be proportional to substrate concentration. CL fiber optic probes offer several potential advantages for chemical analysis. By immobilizing the enzyme catalyst, it is possible to reuse the enzyme many times. Often immobilization leads to an increase in enzyme stability. The advantages of immobilization have been well documented and have led to considerable work to develop analytical methods utilizing immobilized enzymes (1-3). The main advantages of CL methods are low detection limits and wide linear dynamic ranges with simple instrumentation ( 4 , 5 ) . It should C 1978 American Chemical Society
