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ANALYTICAL CHEMISTRY, VOL. 50, NO 3 , MARCH 1978
counted for 10 h with and without a magnetic field. Figure 4 shows the results obtained. In this case also, without a magnetic field t h e 4.8-keV V x rays, obtained from Cr after irradiation are completely obscured by t h e background produced by t h e 8 particles emitted from the irradiated phosphate. By applying a magnetic field, the V K a x rays may be accurately integrated and measured. T h i s experiment is of special importance for the analysis of biological materials by nondestructive activation analysis. T h e other emitters contained in these matrices (Na, K, C1) produce isotopes with much shorter half-lives than 32P(14 d) and Cr (27 d ) after neutron activation. Thus, after an appropriate waiting period, the 32Pwill remain the only serious source of interference with the nondestructive determination of Cr in these matrices. Comparison of Experimental and Calculated Results. A magnet of 3.4 kG with circular poles of about 2.5-cm diameter was used in t h e experiments. Because of the shape of the magnet, the source-detector distance was about 5 cm. T h u s one of t h e conditions on which the calculations were based, t h a t the magnet should fill the entire source-detector distance, was not fulfilled. As a result, the fraction of d particles removed from the surface of the detector cannot be obtained exactly from Table I. However, this table indicated t h a t for our setup, at least 99% of 32Pp particles (1.71 meY) will be removed by t h e magnet compared with t h e experimental value of 95-9770 (calculated from Figure 2). There are several possible factors which could contribute to t h e slight discrepancy between the experimental and calculated results. (i) In the calculations, it was assumed that we have a radioactive point source and a collimator which prevents all t h e @ particles which d o not hit the detector in t h e absence of a magnet from hitting it after deflection. On t h e other hand, in the experiments a 6-mm diameter source was used without a collimator to get enough activity. (ii) T h e deflected @ particles could collide with different materials which as a result will emit bremsstrahlung. T h e
latter are detected by t h e detector but have not been considered in the calculation. (iii) Equation 1 is valid only in vacuum. It does not consider the possible collision of $ particles with air molecules and the resulting change in their momentum. However, experiments were also carried out at lower pressures down to 1Torr without any further reduction in the background, indicating that the presence of air probably does not contribute to the discrepancy. (iv) The calculations yield the fraction of 3 particles removed by the magnetic field, but do not take into account that, as a result, the spectra of the p particles which reach the detector are changed as compared to the initial spectra. This change of spectra has to be calculated in order to evaluate the changes in the detector background. T o this purpose, the response functions of the detector t o monoenergetic electrons of various energies, will have to he studied. iv) The whole area of the detector was considered to have the same efficiency whereas it was found (7) that for x-ray measurement, there are radial gradients in the efficiency of Si(Li) detectors.
ACKNOWLEDGMENT The authors thank Raia Nothman for her help and initiative in carrying out the experiments.
LITERATURE CITED (1) M. Mantel and S. Amiel, Anal. Chem.. 44, 548 (1972). (2) M. Mantel and S. Amiel, J . Radioanal. Chem., 16, 127 (1973).
(3) M. Mantel and S. Amiel. Anal. Chem., 45, 2393 (1973). (41 S Arniel. M. Mantel. and 2. 8. Alfassi. J . Radioanal. Chem.. 37. 189
(1977). (5) E. Segre, "Experimental Nuclear Physics", Vol3, Pergamon Press, London, 1952, p 428. (6) I . Kaplan, "Nuclear Physics", Addison-Wesley, New York, N.Y., 1962, D 363. (7) 2 B Alfassi and R Nothman Nucl Instrum Methods, 143, 57 (1977)
R K E I \ E D for review June 8. 1957. Accepted November 11, 1977. This work was supported by the US-Israel Binational Foundation.
Preservation of Some Trace Meta s in Samples of Natural Waters K. S. Subramanian, C.
L. Chakrabarti," J. E. Sueiras,'
and 1. S. Maines
Metal Ions Group, Department of Chemistry, Carleton University, Ottawa, Ontario, Canada K1S 586
The loss of 11 trace metals on storage of both synthetic water samples and natural water samples in Pyrex glass, Nalgene linear polyethylene, and Teflon containers has been studied using graphite furnace atomic absorption spectrometry. The amount of each trace metal lost has been studied as a function of time in the pH range from 1.5 to 8.0. Acidification to pH < 1.5 with nitric acid and storage in Nalgene containers are found to be the most effective ways of preventing the loss of trace metals from natural waters.
Several workers have reported significant loss of trace metals from aqueous solutions upon storage-the extent ' O n leave f r o m the Department of Chemistry, Santiago de Compostela University, Santiago de Compostela, Spain. 0003-2700/78/0350-0444$01.00/0
varied, among other things, with the type of container, contact time. pH. and the initial concentration of the metal. Eichholz et al. ( I ) reported loss of trace metals from hard water stored in polyethylene and borosilicate glass containers. Robertson (2) reported significant losses of Ag, Co, Fe, In, Sc, and U on storage of seawater in Pyrex and polyethylene containers. Durst and Duhart ( 3 ) could find no suitable containers for storage of dilute aqueous solutions of .4g. Posselt and Weber ( 4 ) and King et a!. ( 5 ) observed that a t p H > 7 , cadmium in distilled water was adsorbed more by glass than plastic containers; however, at pH < 2, adsorption of cadmium by either of these containers was negligible. Other workers (6-9) reported the need of acidifying the samples with " 0 3 to p H < 1 in order to prevent precipitation and adsorption of trace metals by container walls. I t was reported t h a t the amount of trace metal lost was directly related to the length of storage time and inversely related to t h e concentration of the trace metal (20-22). C 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
Most of the published literature t o date is on the loss of trace metals from synthetic samples of dilute aqueous solutions. Little has been reported about the magnitude a n d t h e rate of loss of trace metals on storage of natural water samples in glass or plastic containers. However, t h e above studies indicate that there is a problem of severe loss of trace metals from natural waters. I n view of t h e above, it was thought advisable t o study the effect of p H a n d container material on t h e stability of natural water samples under routine conditions of sample collection and storage, a n d t o find out the most suitable container that prevents or a t least decreases the extent of the loss. T h e metals studied were Al, Cd, Co, Cr, Cu, Fe, M n , Ni, Pb, Sr, a n d Zn.
EXPERIMENTAL Apparatus. The amounts of trace metals were determined using a Perkin-Elmer atomic absorption spectrophotometer,model 503, equipped with a Heated Graphite Atomizer 2ooO ( H G A - 2 0 ) , single-element hollow cathode lamps as narrow line sources, and a Perkin-Elmer Deuterium Background Corrector. Container for Water Samples. Pyrex glass (borosilicate glass--100-mL volumetric flasks with ground-glass stoppers) and Nalgene (linear polyethylene, 250-mL and 1000-mL screw-cap bottles) were used as containers for water samples. Teflon beakers (fitted with tight-fitting lids) were used for zinc because of the severe contamination found with Pyrex and Nalgene containers. Reagents. (a) Ultrapure water of resistivity 18.3 megohm-cm was obtained from a Milli-Q2 System (Millipore Corporation). (b) Stock solutions of Al, Cu, Fe, Ni, and Zn were prepared by dissolving pure metals in a minimum amount of ultrapure "OB, evaporating off the excess acid and making it up with ultrapure water-the final solution contained 1% (v/v) "OB. Stock solutions of Cd, Co, Cr, Mn: Pb, and Sr were prepared by dissolving a suitable oven-dried AnalaR salt of each metal separately in ultrapure water and acidifying the solutions to pH 2.0. All stock solutions were stored in clean polyethylene bottles (with the exception of zinc, which was stored in a Teflon bottle) and contained 1000 pgjmL of metal. Appropriate standard solutions were prepared by serial dilution of the stock solutions with ultrapure water immediately prior to analysis. (c) Humic acid (Technical Grade, Aldrich Chemical Company, Milwaukee, Wis.) was purified by leaching it for 3 days with 0.1 M HNOB(stirring continuously with a magnetic stirrer) to remove heavy metals. The leached acid was washed with ultrapure water and dried a t 180 "C. A stock solution containing 100 pg/mL of humic acid was prepared by dissolving 5.0 mg of the purified acid in 5.0 mL of 2 M Na2C03 (which had been purified by electrolysis), and then diluting the solution to 500 mL with ultrapure water. (d) Synthetic water samples and Rideau River (Carleton University site, Ottawa, Ontario) water samples studied are described below. The bulk composition of the synthetic water samples was as follows (13): (i) Inorganic bulk matrix (in pg/mL): CaZt, 40; Na+, 12; HC03-, 115; Cl-, 25; and 25; Organic bulk matrix: humic acid, 5 pg/mL; (ii) Trace metals: the values selected were based on the results of analysis of a Rideau River water sample by graphite furnace atomic absorption spectrometry (GFAAS). The values selected are presented in Table I. Procedure. Immediately prior to use, all containers were cleaned sequentially as follows: a detergent wash, tap water rinse, soaking in 2% "OB for 24 h, distilled water rinse (6 times), and ultrapure water rinse (6 times). After the cleaning operation, any containers found to give blanks (with ultrapure water acidified to pH 1.0 with nitric acid) having detectable concentrations of the trace metals were rejected. Stability of water samples was studied using both synthetic water samples and Rideau River water samples at pH's: 1.5, 2 . 5 , 4.0, 6.0, and 8.0. The p H values were adjusted with ultrapure "OB. Unacidified natural water samples were found to have a pH of about 8.0 and were used as the water samples at pH 8.0. The samples were stored in two containers of each material (Pyrex glass and Nalgene linear polyethylene for all metals except zinc for which Teflon was used) at each of the above pH values at 23-24 "C. (Before adjustment of pH and storage, the river water samples were filtered through a 0.45-pm membrane filter). The amount of each trace metal lost was determined by GFAAS on days: 1,
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Table I. Analytical Lines and Operating Conditions for Graphite Furnace Atomic Absorption Spectrometry Atomization parameters
Element
line, nm
Charring tempa, "C
Ag A1 Cd Co Cr Cu Fe Mn Mo Ni Pb Sr V Zn
328.1 309.2 228.8 240.7 357.9 324.8 248.3 279.5 313.3 232.0 283.3 460.7 318.4 213.9
300 1200 250 800 900 700 900 900 1200 800 500 600 1300 400
Sensitivity,b
lx
Tempn, Time "C S 2400 2400 1500 2500 2500 2500 2500 2500 2500 2500 2100 2500 2500 2000
5 6 4 6 6 6 6 6 8 8 4 8 8 5
lo-" g
10 35 3 80 40 IO 50 25 120 220 30 80 600 1.3
a Temperatures represent the meter settings o n the control panel of the power supply. Mass for 0.0044 absorbance with the purge gas flow in the normal mode excepting for Al, Cd, Pb, and Zn for which the interrupt mode was used. Except for A1 when argon was used as the purge gas, for all other elements nitrogen was used as the purge gas.
2. 3, 4, 5: 10, 20, and 30. The GFAAS technique was chosen because it has the extremely high sensitivity required for determining, without pre-concentration, the low concentration levels (0-50 ng/mL) at which trace metals are present in natural waters, and also because it has the high selectivity, precision, and day-to-day reproducibility required for reliable analytical results. For the determination of a particular metal, 5-, 50-, or 100-pL volumes (depending on the sensitivity of the metal) of samples were introduced into the graphite furnace with an Eppendorf syringe fitted with disposable plastic tips. Prior to use, the tips were decontaminated from traces of metals by soaking them for 24 h in 570 nitric acid (Baker Ultrex), followed by four rinses with Ultrapure water. The sequential "dry", "char", and "atomize" program of the HGA-2000 was followed, and the peak absorbance noted. The results of five replicate analyses of each test solution per container were averaged. The cumulative averages from the two containers for each metal at each pH were then used to draw the plots of YC loss of metal vs. time. These plots obtained for each metal in the natural water sample were then compared with those in the synthetic water samples to determine the change in loss, if any, due to difference in the composition between the natural water and the synthetic water samples. The amount of each trace metal in the river water sample immediately after collection, and at time intervals of 1, 2, 3. 4, 5,10, 20, and 30 days were obtained by reference to linear working curves prepared using a series of standard aqueous solutions of each metal, acidified to pH 1.0 using "OB (Baker Ultrex). A t this pH, the standard solutions were found to be stable a t least for one day. n'evertheless, compensation for small changes in the working curve due to conditions beyond the control of the analyst was made by running the standards at pH 1.0 a t about 30-min intervals during the analytical run. Also, on the days when the water samples were tested, a fresh calibration curve was prepared using a series of fresh (aqueous)standard solutions at pH 1.0. The relative standard deviation of each point in the calibration curve was 2-3 Yc. The concentrations of the trace metals in the synthetic water samples immediately after spiking and at the above time intervals were determined. The calibration curve obtained with aqueous standards was used for determining the concentration of the trace metals in synthetic water samples because it was found that the aqueous samples and synthetic water samples with the same analyte concentration gave the same instrumental response within the limits of experimental error, Le., no interference was observed
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
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a
n
I
I
10
1
20
CONTACT
0
30
TIME, DAYS
h
10
20
CONTACT
Figure 1. Loss of cadmium from Pyrex glass and Nalgene containers. Rideau River water sample (0.12 pg/L Cd); Pyrex glass: ( 0 )pH 1.6, 2.5, 4.0; ( 0 )pH 6.0; (A) pH 8.0. Nalgene: (0)pH 1.6, 2.5, 4.0, 6.0, 8.0. Synthetic water sample (0.10 pg/L Cd); Pyrex glass: ( 0 )pH 1.6, 2.5, 4.0; (V)pH 6.0; (0)pH 8.0. Nalgene: (0)pH 1.6, 2.5, 4.0, 6.0,
* 0 1 30
TIME,DAYS
Figure 4. Loss of iron from Nalgene container. Rideau River water sample (170 pg/L Fe): (0)pH 1.6; ( 0 )pH 2.5; (V)pH 4.0; (A)pH 6.0; (U)pH 8.0. Synthetic water sample (150 pg/L Fe): (0)pH 1.6; ( 0 ) pH 2.5; (V)pH 4.0; ( A ) pH 6.0; (0)pH 8.0
8.0 BO
-
60
LL
0 (0
40
J
O
1.5, for M n a t p H > 2.5, and for Cu (no Figure) a t p H > 4.0, when both synthetic and river water samples were stored in Nalgene or Pyrex glass containers. The following features of Figures 2-7 are worth noting; explanation of some of these features is offered below. (i) Most of the losses for the above metals (except copper) occurred within 5 days of storage. Virtually no further loss occurred u p to 30 days of storage. In the case of copper, most of t h e loss (35% at p H 6.0, a n d 50% a t p H 8.0) occurred in one day, and no further loss was observed up to 30 days. (ii) T h e amount lost increased with increasing p H irrespective of t h e composition of the water samples, and the nature of the container. Thus, in Pyrex glass containers, the percent loss in 30 days from river water samples a t p H 4.0, 6.0, and 8.0 was: for aluminum-53, 70, and 81, respectively (Figure 2); for iron-54. 78, and 85 (Figure 3); for manganese-43, 72, and 90 (Figure 5 ) : and for lead-48, 62, a n d 76 (Figure 7). T h e corresponding loss from synthetic samples was: for aluminum-49, 60, 65 (Figure 2); for iron-37, 70, and 70 (Figure 3); for manganese-35,62, and 77 (Figure 5 ) ; and for lead-same as for river water samples (Figure 7). In Nalgene containers, the percent loss in 30 days
* 447
at p H 4.0, 6.0, and 8.0 was: for iron-32, 44,and 58 (river water samples), and 22, 39, and 55 (synthetic water samples), respectively (Figure 4);for manganese-32, 60, and 78 (river water samples), and 32, 53, and 70 (synthetic water samples), respectively (Figure 6). Since the amount of adsorption generally increases with increasing pH (17), it is probable that the increasing loss of the metals with t.he increasing p H is due to adsorption by the container surface of hydroxcl or carbonato complexes of metals which predominate with increasing pH. Based on stability constant values ( I S ) , the various species may be identified as: colloidal hydrated aluminum oxide in the case of aluminum; CuHC03+ and C u C 0 3 a t p H 6.0, and C U ( C O ~ ) ~at' - p H 8.0 in the case of copper; and CdHC03+ at p H 6.0, and CdC03 at p H 8.0 in the case of cadmium. Benes and Smetana (18),who observed significant loss of iron to 10-j M) above p H 3.0 (e.g., -98% ;at p H > 6.6) attributed the loss to chemisorption of FeOH2+and Fe(OH)*+in the p H range from 2 to 5, and of colloidal hydrous ferric oxide above p H 5. T h e loss of iron observed in the present s8tudymay be explained similarly. Jenne (19) has reported t.hat hydrous manganese oxides are to remain coated on silicate surfaces. I t is probable that the loss of manganese in the present study may be due to adsorption of the hydrous oxide on the container surface, especially the glass surface. (iii) At a given pH, loss of metals from river water samples was always higher than from synthetic water samples (Figures 1-6). This is probably due to the greater biological activity of the river water samples. Additionally, in the case of iron, the organic acids (e.g., humic acids) in river waters have been reported (20) to hold t h e iron as peptized sols above p H 5; the rate of adsorption of these iron-organic colloids by t h e container walls may be greater than that of the inorganic iron sols. Similarly, the smaller concentration of cadmium in the river water samples (0.12 kg/L) t h a n in the synthetic water samples (1.00 pg/L) may be responsible for the greater loss of cadmium from the river water samples (26% ai, p H 6.0, and 31% a t p H 8.0 in 30 days) than from t h e synthetic water samples (17% a t p H 6.0, and 24% a t p H 8.0 in 30 days) (21). (iv) Loss of metals from the Pyrex glass Containers was higher than from the Nalgene linear polyethylene containers (Figures 1-6). For example, cadmium showed no loss on storage of synthetic water samples, and river water samples in Nalgene linear polyethylene containers at p H
