Determination of Soluble Cadmium, Lead, Silver, and Indium in Rainwater and Stream Water with the Use of Flameless Atomic Absorption Anthony Rattonetti lllinois State Water Survey, Box 232. Urbana. I l l . 61801
Flameless atomic absorption (AA), because of its small sample volume requirements and low absolute detection capability, should be a propitious analytical method for any study where there is a large number of samples, or when the metals of concern are in the ng ml-1 range and below. Because the technique is new, there is a paucity of information regarding interferences during the direct determination of a given element as it occurs in a genuine sample matrix. Using stream water and rainwater, this study found interferences in the direct determination of 1) the environmentally hazardous metals, cadmium and lead, and 2) silver and indium, metals which are used in meteorological research. This paper describes how the interferences were eliminated. EXPERIMENTAL Apparatus. All samples were filtered through a 47-mm diameter, 0.5-pm pore Celotate (Millipore) filter. Prior to filtering the samples, 100 ml of de-ionized water was passed through the filter to leach out the water soluble blank that would be contributed to the sample filtrate. Ten-ml glass stoppered erlenmeyer flasks were used in performing the extractions. A modified Burrell Wrist Action shaker was used to mix the organic and aqueous layers. As designed, this shaker simulates a snap of the wrist to create a swirling motion to the contents of the flasks which are mounted directly on the shaker side arm shafts. This motion could not effectively mix the organic and aqueous layers; however, complete dispersion of the layers was accomplished by fastening the flasks to a 20-cm laboratory clamp which was perpendicularly mounted to the shaker side arm shaft and agitated for 2 min a t high speed. Pipets with disposable polypropylene tips were used to remove part or all of the organic phase and dispense it on the tantalum ribbon. Care was taken to ensure that none of the aqueous layer was introduced into the pipet. Polypropylene tipped pipets were also used when dispensing aqueous samples and standards on the ribbon, as it was found that a loss of In occurred in the pipet when using a stainless steel syringe needle. Reeves et al. ( 1 ) reported a similar loss when dispensing silver with a stainless steel needle. The flameless unit used was an Instrumentation Laboratory (IL) 355. It consists of a tantalum ribbon enclosed in an argon purged cell. Quartz windows at each end of the cell allow the hollow cathode lamp (HCL) beam to pass through and be aligned directly over the ribbon. The beam is focused as close to the ribbon as possible without causing interference from the continuum emission of the tantalum during atomization. A separate module supplies variable argon flow to the cell and current to the ribbon during the two cycles: dry-pyrolyze and atomize. The ribbon is narrowed in the center to increase resistance and thereby raise the temperature at the sample deposition site. It can hold a sample volume of 0.1 ml. A more complete description is given by Hwang et al. (2). An IL model 353 atomic absorption spectrophotometer capable of automatic background correction was used. A minimum amount of instrumental damping was used. Instrumental response was recorded on a Honeywell 194 recorder. Reagents. All standard solutions of the metals were made from either a reagent grade soluble salt or metal dissolved in the minimum amount of acid and diluted to the desired concentration with distilled de-ionized water. All other chemicals and solvents used were of reagent grade. Technical grade argon was used. ( 1 ) R. D . Reeves, 6. M . Patel, C. J. Molnar. and J. D. Winefordner, Anal. Chem.. 45, 246 (1972). (2) J. Y . Hwang, C. J. Mokeler. and P. A. Ullucc~,Ana/. Chern., 44, 2018 (1972).
Procedure. The effect of argon flow and temperature was examined using aqueous standard solutions of the metals. The argon flow rate was variable to 5 1. min-I. The temperature was estimated by placing a minimum amount of a solid or liquid on the ribbon and increasing the current until the known melting or boiling point was attained. This method was also used by Hwang et al. ( 2 ) . When analyzing nanogram amounts of dried sample, the ribbon may heat beyond the temperature estimated by this method, as there is then considerably less material on the ribbon, resulting in a smaller heat capacity. Known amounts of cadmium and lead were added to genuine samples to check for naturally occurring matrix interferences, as would be indicated by a capricious recovery upon analysis. The known amounts were added to 0.05 ml of sample after it was dispensed on the ribbon. If recovery was favorable, a direct determination using simple aqueous standards for calibration was compared with the standard additions technique on the sample. Solvent extraction was used in the event of major and variable matrix effects. Matrix suppression occurred when samples were spiked with silver or indium; however, this was not a practical disadvantage, as the work by Gatz et al. (3),Warburton and Young ( 4 ) , and Woodriff et al. ( 5 ) portended that the amounts of silver or indium occurring in rain either naturally or through meteorological experimentation would be too low to be conveniently determined by a direct flameless AA method. Therefore, solvent extraction was used to concentrate the sample and the flameless AA parameters needed to analyze the metal in the organic matrix determined. Organic phases heavier than water were used because: the small volumes used (0.1-0.3 ml) would tend to undergo evaporation losses if the solvent was above water, and because the heavier solvents coalesced into a sphere which made removal easy with a pipet.
RESULTS AND DISCUSSION Argon Flow Rate. The main effect of the argon purge is the removal of ambient oxygen from the cell to prevent oxidation of the tantalum ribbon and the formation of analyte oxides which would thereby reduce the number of free atoms in the cell. This effect is illustrated by the dotted portion of the lead plot in Figure 1. In this instance, the sample was dispensed and the dry cycle started immediately. Sixty seconds were allowed to elapse before initiating the dry cycle for the solid lines. At values below 4 1. min-l for lead, the purge cannot efficiently remove the oxygen. Beyond 4 1. min-l, the 60-sec delay period before starting the dry cycle is not needed. Cadmium, silver, and indium have a similar behavior. Since a purge of 5 1. min-l had no adverse analytical effects, it was used to ensure the rapid removal of vapors produced during the dry-pyrolyze cycle which could form refractory compounds with the analyte atoms. Temperature. Figure 2 shows that cadmium, silver, and lead all had a temperature beyond which the absorbance decreased. Some of the reasons Hwang et al. (2) ascribed for this are the following: increasing absorption values up to the temperature favoring maximum free atom formation; decreasing absorption beyond it due to (3) D. F. Gatz. N . A . Dingle. and J, W . Winchester, J. Appl. Meteorol.. 8, 229 (1969). (4) J. A . Warburton and L. G . Young, Anal. Chem.. 44, 2043 (1972). (5) R. Woodriff, B. R. Culver. D. Shrader, and A . B Super, Anal. Chem., 45, 230 (1973).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 6. MAY 1974
9
739
Table I. Lead and Cadmium Extraction and Direct Determination Parameters 400
300
t
___
Pb DRY CYCLE STARTED IMMEDIATELY
1
Lamp current, mA P.M. (R213) voltage Slit width, r m Dry-pyrolyze, "C Atomize, "C Sensitivity, grams D.L.,c absolute, grams relative, ng ml --1. Need for background correction Extraction ratio
g each
lo-'
A~-A--A
A
A,
- DRY CYCLE DELAYED
Pb
Cd
2170 7 800 160
2288 10 620 320 loo;= 350, 60 set.* 100 1700 1700 3 x 10-11 4 x 10-12 1 X 10-11 1 . 5 X 1O-I2 0.002d 0 , 03e
Yesj
No 25/1
...
*
~n
I-L-,~/LA-A-A
2
1
Figure 1.
Used in Pb recovery study with aqueous matrix. Used to pyrolyze organic matrix of extracted Pb. Defined as S/N = 2. 5-ml sample ex-
4 Ar PURGE, liters per m i n
tracted, aU organic phase dispensed. ' 0.05-ml dispeneed sample. Hydrogen continuum (Varian catalog No. 56-100024-00) operated at 30 mA.
6
5
3
Effect of argon flow on absorbance
600
I
1
I
)
( / O - O - O
'
I
I
1
1
-
Pb
-
\
\
500:
1
Table 11. Lead and Cadmium Extraction and Direct Determination Results
Sourcea
Stream Stream Stream Stream Stream Stream Rain Rain Rain Rain a
loot
400
1
A /*'
800
1200
1600
2000
2400
TEMPERATURE, " C
Figure 2.
Effect of temperature on absorbance
possible loss of sample with an abrupt spurt a t high temperature prior to a proper atomization; and a decrease in the number of atoms in the volume above the ribbon because of thermal expansion. Indium behavior was subject to another factor. A study in our laboratory showed that indium occurring in a de-ionized water matrix was prone to volatilize off the ribbon between the completion of drying and the reaching of the peak temperature for atomization. A low drying temperature and high atomization temperature minimized this effect. Volatilization prior to the initiation of the atomize cycle is prevented with a lowered drying temperature, whereas with the high atomize temperature, the ribbon heats rapidly allowing only a short interval for loss to occur. Fuller (6) has reported another pre-atomization loss in the determination of copper and nickel using a graphite furnace atomic absorption atomizer. Cadmium and Lead Direct Determination. The results of these studies are shown in Tables I and 11. A direct determination of cadmium with calibration using (6) C. W. Fuller, A n a l . Chem. A c t a . , 62, 4 4 2 ( 1 9 7 2 ) .
740
ANALYTICAL CHEMISTRY, VOL. 46, NO. 6, MAY 1974
Original Pb Added in sample deconcentermined by tration, Yo Re- extraction, ng mg-1 covery ng ml-1
10 10 10 10 10 10 10 100 100 100
0 0 0 0 0 0 90 67 93 60
1.70 2.95 1.15 2.25 2.20 2.00 2.00 55.00 33.00 44.00
Cd Direct, ng ml-1
Standard additions, ng ml-1
2.00 1.85 1.90 1.95 0.08 0.03 1.30 0.44 0.58 0.34
2.20 1.70 1.80 1.95 0.07 0.03 1.25 0.48 0.64 0.32
Identical samples used for Pb and Cd.
aqueous standards is in good agreement with the standard additions technique when background correction is used. Lead could not be done by standard additions in stream water because the spiked lead was completely suppressed. This lack of recovery does not result from the loss of volatile lead compounds during drying, as the drying temperature could be varied from 50 to 300 "C with no effect on the recovery of spiked lead in stream water or rainwater. The suppression of the lead is evidently due to molecular reformation of the lead compounds in the dense atom cloud directly over the ribbon. Increasing the atomization temperature to 2300 "C offered no improvement. The work by Reeves et al. ( I ) further suggests that molecular recombination is the cause. They were able to decrease the attenuation of lead atoms with increasing height above their graphite rod atomizer with the addition of a hydrogen diffusion flame. This flame eliminates the cool zone over the rod and the unburned hydrogen acts as a releasing agent against chemical interferers. Purging the cell used in this current study with hydrogen might offer some relief from the suppression, although Hwang et al. ( 2 ) have evidenced that the use of hydrogen as a purge gas, in this cell, gives a reduced sensitivity for lead in a de-ionized water matrix as compared to argon. There was some lead recovery with the rainwater samples, but it was variable from sample to sample. Hwang e t al. (7) were able to overcome matrix effects in the direct ( 7 ) J Y Hwanq, P A Ullucci, and C J Mokeler. A n a / Chem 45, 795 (1973).
~~
Table 111. Silver Extraction Parameters and Results of an Analysis f o r Naturally Occurring Silver
Dry-pyrolyze, 350 "C, 60 sec 3280.7 A Atomize, 1250 "C Lamp current, 10 mA P.M. (R213) voltage 700 V Sensitivity, 2 X lo-" gram Slit width, 160 ,urn D.L.,a absolute, 5 X gram Background correction, not relative, 0.001 ng ml-l, needed 5-ml sample, all organic phase dispensed Extraction ratio, 50/1 A,
Source
Rain Rain Rain Rain Rain Rain Rain Stream Stream Stream a
Defined as S/N
=
~
~~~~
Table IV. Indium Extraction Parameters A, 3039.4 A Lamp current, 10 mA P.M. (R213) voltage, 620 V Slit width, 320 pm Extraction ratio, 33/1
a
Dry-pyrolyze, 350 "C, 120 sec Atomize, 2700 "C Sensitivity, 4 X lo-'" gram D.L.,o absolute 1 X gram relative 0 . 0 1 ng ml -l, 10-ml sample, all organic phase dispensed with dry-pyrolyze time of 210 sec
Defined-as S/N = 2.
Ag, ng ml-1
0.008
0.124 0.002 0.031 0.015
0,049 0.216 0.062 0,042 0,006
2.
determination of lead in blood by employing the method of standard additions to one sample and using the values obtained as a standard for the rest of their samples. The matrix of rainwater is not as uniform as that of blood; therefore, such a method is precluded here. A dithizonechloroform (500 mg/liter) system which extracts lead from a sample solution to which citrate has been added and adjusted to a pH of 9-11 with dilute ammonia recovered the lead from the stream water and rainwater. Matrix effects are not unique to lead or to tantalum flameless AA. Takeuchi et al. (8) reported interferences in the determination of magnesium and copper with another variety of tantalum AA device. They advised solvent extraction as a possible solution to overcome the matrix interference. Clark et al. (9) disclosed how zinc is affected by a variety of ions when a graphite furnace was used. The effects were largely negative but a few positive results were shown. Solvent extraction solved all but a few of their problems. Only in several cases where the interferer was in excess concentration of 10,000 times that of zinc did the solvent extraction system fail to correct the effects. The broad temperature range over which cadmium exhibits high sensitivity indicates the strong propensity of cadmium to maintain free atoms. Such a quality is an asset in tantalum flameless AA where there is a cooling of the atomic vapor as it leaves the ribbon surface, favoring their recombination back into a molecular species. Silver and Indium. The results of these determinations are shown in Tables I11 and IV. Silver was extracted with a dithizone-carbon tetrachloride system (250 mg/liter) from a sample solution 0.1-0.2N in nitric acid. Silver ion is vulnerable to adsorption on container walls (10) and photochemical reduction in the presence of halide ions. To maintain the silver in solution, 0.1 gram of sodium thiosulfate was added to each rainwater and stream water collector prior to sample collection. The sodium thiosulfate dissolved in the rain or stream water and formed the wellknown silver-thiosulfate ligand. West et al. (10) also used this ligand to prevent adsorption losses. Our studies have (8) T. Takeuchi, M. Yanagisawa, and M. S u z u k i , Taianta. 19, 465
(1972). (9) D. Clark, R. M . Dagnall, and T. S . West, Anal. Chirn. Acta, 63, 1 1 (1973). (lo) F. K. West, P. W. West. and F. A . Iddings, Anal. Chem.. 38, 1566 (1966).
concluded that less thiosulfate was needed to maintain silver in the sub ng ml-1 range than was needed in the previous study which dealt with higher concentrations. Sodium carbonate (0.1 gram) was also added to each collector to ensure an alkaline solution; otherwise the natural acidity of rain would cause a decomposition of the sodium thiosulfate. Prior to making the sample 0.1-0.2N in nitric acid, the thiosulfate must be destroyed; otherwise it would precipitate free sulfur. Under such conditions, the amount of silver recovered in the extraction is nil. It was found that an addition of hydrogen peroxide (1 ml of 30% Hz02 for a 5-ml aliquot of the sample) would effect the decomposition of the thiosulfate and the solution could be adjusted to the proper acid normality with complete recovery of the silver in the subsequent extraction. These procedures assume an original rainwater sample of 200 ml or less and a stream water sample of 50 ml. Indium was extracted, using an 8-hydroxyquinolinechloroform system (2.5 grams/liter), from a solution buffered at a pH of 3.5. The buffer was made by adjusting the pH of a saturated solution of potassium hydrogen phthalate to 3.5 with nitric acid. Two ml of the buffer was then added per 10 ml of sample to be extracted. The pyrolysis product of the indium 8-hydroxyquinolinate is resistant to pre-atomization volatilization and yields less sensitivty than indium in a simple aqueous matrix. The highest sensitivity of the extracted indium was obtained by further narrowing the center of the ribbon. This reduced the capacity to 0.05 ml of-sample and raised the temperature to about 2700 "C; however, the sensitivity was approximately a factor of four less than the same amount of indium in a distilled de-ionized water matrix. The organic phase of some samples exhibited background absorption during atomization. Such samples also gave a reduced recovery of spiked In when extracted. This is probably because the source of the background absorption disposed indium to molecular recombination. Samples which did not cause background absorption from their organic phases gave complete recovery of extracted In. It is therefore imperative that such background absorption be exposed in samples by first checking for background absorption at 3039.4 A with a hydrogen continuous source before analyzing with an In HCL. This reduced recovery of In would not be evidenced by the use of automatic background correction and would lead to an underestimation of the amount of In in a sample. CONCLUSIONS The precision of triplicate determinations is f5% when the amount of analyte dispensed on the ribbon is ten times the absolute detection limit (D.L.) and the same ribbon is used for all measurements. About 100 determinations can be made before a ribbon must be replaced. The rainwater and stream water samples used in the studies on cadmium, lead, and indium were chosen from a pool of about 4000 samples from the 1972 summer of projANALYTICAL CHEMISTRY, VOL. 46, NO. 6, MAY 1974
741
ect METROMEX (11, 12). METROMEX operations are based in metropolitan St. Louis, Mo., and vicinity being aimed, in part, a t studying the effect of atmospheric pollution as it relates to inadvertent weather modification. The wide range of metal concentrations in rainwater from such an area and the capricious nature of rainfall volume proved these techniques to be well suited to such conditions.
( 1 1 ) S.S.Miller, Enwron. Sci. Techno/., 6,508 (1972). (12) S. A . Changnon, Jr., F . A . Huff. and R. G . Sernonin, Buli. Amer. Meteor. Soc.. 52,958 (1971).
ACKNOWLEDGMENT This research has been under the general direction of Richard G. Semonin and Donald Gatz. Received for review August 20, 1973. Accepted December 7, 1973. The work has been supported under contract AEC-1199 of the US. Atomic Energy Commission and contract 14-06-D-7197 with the Division of Atmospheric Water Resources Management, Bureau of Reclamation, U S . Department of the Interior. Part of this paper was presented as Paper No. 205 a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1973.
Analysis of 1,2,3,4,10,1O-Hexachloro1,4,4a,5,8,8a-hexahydro-1,4-endo-exo-5,8dimethanonaphthalene by Nuclear Quadrupole Resonance Spectrometry Dina Gegiou Experimental Research Department, General State's Laboratories, 16 A. Tsocha St., Athens, Greece
Nuclear quadrupole resonance (NQR) spectrometry has been used for elucidating the structure of several organochlorine compounds commonly used as pesticides and of a number of cyclodiene chlorinated insecticides including the title compound. commercially known as aldrin (1, 2 ) . In all the examined compounds, assignment of signals to specific chlorine atoms was made utilizing spectra-structure correlation charts and other data, except in the case of aldrin in which one resonance frequency only was observed (2). The absence of other resonances in the spectrum of aldrin was considered as due to lack of instrument sensitivity or to a considerable disorder in the crystal lattice. In order to check the latter possibility, the aging technique was used in which a sample of aldrin was maintained for several days at a temperature just below the melting point. Aging did not produce any detectable effect on the NQR spectrum. In addition, samples of aldrin were crystallized from hexane, acetone, and benzene with no apparent change in the NQR spectrum (2). In the present note, we report the complete NQR spectrum of aldrin at various temperatures, since sometimes more than one temperature is needed in order to avoid missing one or more resonances, because of accidental superposition in frequency at a single temperature (3). The observed signals were tentatively assigned to the specific chlorine atoms following the reasoning outlined by Roll and Biros (2). EXPERIMENTAL The sample of aldrin used in this work was analytical grade of 99% purity, purchased from Shell Chemical Co., New York, N.Y., without further purification. (1) E. G. Brame, Jr.,Ana/.Chern., 39,918 (1967) (2) D.B. Roll and F. J . Biros, Anal. Chern., 41,407 (1969). (3) P. J. Bray, R. G. Barnes, and R . Bersohn, J. Chern. Phys., 25, 813 (1956). 742
ANALYTICAL CHEMISTRY, VOL. 46, NO. 6, MAY 1974
The spectra, shown in Figure 1, were obtained at four different temperatures, namely at 77 "K (liquid nitrogen), 193 "K (dry iceacetone), 273 "K (ice-water), and 300 "K (room temperature), because of the stability and ready accessibility of these baths, using the Decca-Radar nuclear quadrupole resonance spectrometer. A half-gram sample of aldrin was placed in a glass tube of 1-cm diameter and positioned in the spectrometer coil. Frequency measurements were accurate to fO.001 MHz and temperature measurements to f l "C.
RESULTS AND DISCUSSION Figure 1 shows that the NQR spectrum of aldrin exhibits at room temperature three resonance signals. while at the lower temperatures. the expected six signals. Previously ( 2 ) ,only one signal was observed a t room temperature, probably because of instrument insensitivity. The 35Cl pure quadrupole resonance frequencies as a function of temperature are given in Figure 2. No evidence of a phase transition or discontinuity in the plot is observed; the resonance frequency decreases at higher temperatures in qualitative agreement with Bayer's theory ( 4 ) . Therefore, the changes in the spectral pattern of Figure 1 are attributed to lattice or torsional oscillations of the molecule. The high frequency signals ( e . g . , at 77 "K) at 38.485 and 38.382 MHz may be assigned to the two vinylic chlorine atoms, since they are known to occur at higher frequencies than aliphatic chlorine atoms (2). Among the four remaining signals, the two higher lying signals at 37.225 and 37.140 MHz are assigned to the two chlorine atoms of the dichloromethylene group. since spectra-structure correlations seem to indicate (1) that the resonances of dichlorosubstituted carbon atoms occur at higher frequencies than monochloro-substituted carbon atoms; thus, the signals of (41 H
Bayer. Z Physik. 130,227 (1951)
