(5) K . Faiigatter, V. Svobcda, and J. D. Winefordner, Appl. Spectrosc., 25, 347 (1971). (6) H. Kawaguchi, M. Hasegawa, and A. Mizuike, Spectrochim. Acta, Part 8,27, 205 (1972). (7) H. Kawaguchi, M. Hasegawa. and A. Mizuike, J. Jpn Spectrosc. Soc., 21, 36 (1972). (8) H. Kawaguchi, T. Sakamoto, and A. Mizuike. Ta4nta. 20, 321 (1973). (9) F. E. Lichte and R. K . Skogerboe, Anal. Chem., 45, 399 (1973). (10) C. A. Bache and D. J. Lisk, Anal. Chem., 30, 786 (1967). (11) D. N. Hingie, G. F. Kirkbright, and R. M. Bailey, Talanta, 16, 1223 (1969). (12) H. E. Taylor, J. H. Gibson, and R. K. Skogerboe, Anal. Chem., 42, 1569 (1970). (13) G. W. Wooten, E. L. Brown. and J. F. Moon, private communication (1973). (14) K . W. Busch and T. J. Vickers, Spectrochim. Acta, Part 8, 28, 85 (1973). (15) D. E. Nixon. V. A. Fassei, and R. N. Kniseiy, Anal. Chem.. 46, 210 (1974).
(16) S. Murayama, Spectrochim. Acta, Part8, 25, 191 (1970). (17) S.Lans, W. Lochte-Hokgreven,and G. Traving, 2.Phys., 176, 1 (1963). (18) D. S. Auld, H. Kawaguchi, D. M. Livingston, and 8.L. Vaiiee, 8iochem. Blophys. Res. Commun.,57, 967 (1974). (19) D. S.Auld, H. Kawaguchi, D. M. Livingston, and B. L. Vailee, Proc. Nat. Acad. Sci. USA, 71, 2091 (1974). (20) D. S. Auld, H. Kawaguchi. D. M. Livingston. and B. L. Vaiiee, Blochem Blophys. Res. Commun., 62, 296 (1975). (21) A. Poiicard, Harvey Lect., 27, 204 (1932). (22) G.Thanheiser and J. Heyes. Arch. Eisenhutfenwes., 11, 543 (1941). (23) D. Giick, Ann. N.Y. Acad. Sci., 157(1), 265 (1969).
RECEIVEDfor review November 18, 1974. Accepted February 7 , 1975. This work was supported by Grant-in-Aid LH-94 from the International Lead Zinc Research Organization, Inc.
Simultaneous Determination of Seven Trace Metals in Potable Water Using a Vidicon Atomic Absorption Spectrometer Kenneth M. Aldous, Douglas G. Mitchell, and Kenneth W. Jackson Division of laboratories and Research, New York State Department of Health, New Scotland Avenue, Albany, NY 12201
A multichannel atomic absorption spectrometer Is used for the determination of Zn, Cd, NI, Co, Fe, Mn, and Cu in potable waters. Metals are chelated with ammonium pyrroiidine dithiocarbamate at pH 4, and a 10-fold concentration is achieved by extracting into methyl isobutyl ketone. The organic phase is aspirated into an air-acetylene flame, and atomic absorption is measured simultaneously at the resonance lines of these elements by disperslng a 168-nm region of the lamp and flame spectrum across a vidicon array detector. Detection limits from 0.004 to 0.02 pg/mi have been obtained, with dynamic ranges up to 100 and relative standard deviations of 3 % at optimum concentrations. This performance, though poorer than by conventional singlechannel atomic absorption spectrometry, is adequate for routine monitoring of public water supplies and most waste waters.
Potable water contains metals a t both moderate and trace concentration levels. The major constituents, Ca, Mg, Na, and K, are present a t from 1 to 250 Kg/ml; other metals are below 1 kg/ml. The former group can be directly determined by conventional flame techniques using direct nebulization without pretreatment, while the latter group requires preconcentration. Atomic absorption (AA) spectrometry is the preferred analytical technique for most metals, with trace metals preconcentrated by ion exchange ( I ) , partial evaporation (Z), or chelation and solvent extraction (3, 4 ) . Atomic absorption methods are rapid, simple, precise, and accurate, and interferences seldom occur. However, with a conventional single-channel spectrometer, elements must be analyzed in sequence, an increasingly inefficient procedure as the number of metals per sample increases. Arc or spark source emission spectrometry is capable of simultaneously determining large numbers of elements per sample, but this technique requires a substantial capital investment and skilled spectroscopists and is not suitable for moderately sized laboratories. Several multielement spectrometers using AA, atomic emission (AE), and atomic fluorescence 1034
* ANALYTICAL CHEMISTRY, VOL.
47, NO. 7, JUNE 1975
spectrometry have been described in the literature ( 5 ) ,but these are not readily available, and their construction generally requires engineering skills not found in the average analytical laboratory. Vidicon television camera tubes enable the simultaneous observation of all wavelengths over a wide spectral range, and these detectors have recently been shown to be suited for both flame AA (6-8) and AE (9-11) spectrometry. Basically, part of the lamp and flame spectrum exiting from a monochromator is dispersed across a light-sensitive target to produce a charge pattern. This charge is read by a scanning electron beam, and the charge density a t each point on the target is obtained. The charge density is a function of radiative intensity, and the position is a function of wavelength. We have developed a multichannel AA spectrometer with a silicon-target vidicon detector responding in the ultraviolet region (7) and have used this instrument for the simultaneous determination of trace wear metals in used lubricating oils (8).Here we describe its further application to the simultaneous determination of seven trace metals in potable water samples. With this system, the major metals (Ca, Mg, Na, K) are not readily determined simultaneously because their principal resonance lines are spread between 285.2 nm (Mg) and 766.5 nm (K). All radiation over a wavelength range of 481.3 nm would have to be dispersed across the detector, and this would require a low-dispersion grating with impractically low resolution (7). However, the most important trace components, including Zn, Cd, Ni, Co, Fe, Mn, and Cu, have principal resonance lines between 213.9 nm (Zn) and 324.7 nm (Cu), and this 110.8-nm range can be readily dispersed onto a vidicon detector using a moderate-resolution monochromator.
EXPERIMENTAL Apparatus. The multichannel AA spectrometer, consisting of two multielement hollow cathode lamps (Jarrell-Ash, Waltham, MA), air-acetylene burner/nebulizer, 0.25-meter Ebert monochromator, and silicon-target vidicon detector, has been described previously (7). The vidicon tube with associated electronic console was the SSRI Model 1205 Optical Multichannel Analyzer (Prince-
RESULTS AND DISCUSSION Table I. Atomic Absorption Operating Conditions Primary light sources: Multielement hollow cathode lamps Lamp No. 1 (20 mA): Ni, Co, Fe, Mn, Cu, C r Lamp No. 2 (10 mA): Zn, Cd, Pb, Cu Flame: Air-acetylene. Flow rates adjusted to give a stoichiometric flame when nebulizing methyl isobutyl ketone Entrance slit: 50 pm Monochromator grating: 295 grooves/mm, blazed at 190 nm. Linear dispersion, 13.2 nm/mm Vidicon detector: Ultraviolet-sensitive silicon target, detecting radiation from 200 to 368 nm. Glass filter of 2.4-mm path length, attenuating radiation >300 nm.
ton Applied Research Corp., Princeton, NJ). The operating conditions for this application are given in Table I. Radiation from the lamps was combined by means of a half-silvered quartz mirror (beam splitter). The monochromator grating, which presented a spectral range of 168 nm to the vidicon target, was rotated until the resonance lines of all the elements of interest were simultaneously observed. The wavelengths are listed in Table 11. Reagents. Ammonium pyrrolidine dithiocarbamate (APDC) solution (5%) was prepared by dissolving 5 g of Baker grade reagent in 100 ml of deionized water and filtering through a 0.45-pm membrane filter. The solution was stored a t 4 O C in a dark bottle. The 10% phthalate buffer was prepared by dissolving 25 g of potassium hydrogen phthalate (NBS pH standard) and 3.5 ml of 1M HC1 in 250 ml of deionized water. All other chemicals were reagent grade. A stock solution was prepared containing Zn2+, Cd2+, Ni2+, Co2+, Fe3+, MnZ+,and Cu2+, all as their nitrates, a t 100 pg/ml. A series of standards a t concentrations from 0.002 to 0.5 pg/ml was obtained by appropriate dilution with deionized water. Potable water samples were collected from various municipal water supplies in New York State. Procedure. To 200 ml of a standard or a potable water sample were added 4 ml of phthalate buffer and 10 ml of 5% APDC. The solution was adjusted to pH 4 and transferred quantitatively to a 1000-ml separating funnel; 20 ml of methyl isobutyl ketone (MIBK) were added; and the funnel was shaken mechanically for 5 minutes. The organic phase was then removed and centrifuged. Standards and samples were nebulized into the stoichiometric air-acetylene flame. As described previously (7), the AA signals for the metals were obtained simultaneously by electronically subtracting the analyte signal of each from a prerecorded blank (pure MIBK) signal. The resulting spectra were automatically plotted onto a stripchart recorder. Calibration curves were constructed for each of the seven metals, and concentrations were calculated. Comparative single-element results were obtained by nebulizing the extracted standards and samples into an air-acetylene flame using a Techtron AA-120 single-channel spectrometer (Techtron, Palo Alto, CA), Le., by an accepted method for potable water analysis (12).
Optimization of Experimental Parameters. Selection of Flame Conditions. In a simultaneous multielement analysis, any single set of flame conditions is not optimum for all elements. Both flame type and flame stoichiometry are important. Certain metals, including Al, Be, Sc, Ti, and V, require a nitrous oxide-acetylene flame, and we have previously noted that emission interferences would be encountered using this flame with the vidicon AA spectrometer (8).Most other elements can be determined in an air-acetylene flame, though metals such as Cr, Sn, and Sr require a fuel-rich (reducing) flame. Most of the elements of interest in potable waters, however-and all of those determined during this study-are best determined in a stoichiometric air-acetylene flame. Primary Radiation Sources. Primary sources emitting resonance radiation of all elements of interest must be combined and passed along the same optical path through the flame. These resonance lines should be of approximately equal intensity because of the limited dynamic range of the silicon vidicon detector (ca. 1000). If a line of high intensity is set beside a line of low intensity, the latter may have a poorer SIN. For potable water analyses we combined two lamps (Table I) having a significant inherent variation in spectral intensity between some metals. The Mn 279.5-nm line is one of the most intense in a multielement hollow cathode lamp, and Mn is three or four times more sensitive than Fe, Co, and Ni. The result is that the Mn AA signal is about 20 times greater than the signals for Fe, Co, and Ni a t any given concentration. Experiments showed, however, that the latter three lines were not sufficiently weak to affect the SIN adversely. Recorder Readout. Since the Mn and Cu signals are more intense than those of the other metals of interest, the Zn, Cd, Ni, Co, and Fe peaks were plotted directly, and the recorder gain was reduced 10-fold to record the Mn and Cu peaks. This gave more easily readable spectra. The Cu 324.7-nm line is very intense in a hollow cathode lamp, and both lamps contained this metal, yet in Figure 1, the Cu signal appears similar in magnitude to that of Mn. To achieve this, the Cu radiation was attenuated by mounting a double thickness (about 2.4-mm path length) of borosilicate glass microscope slide over the appropriate section of the vidicon tube entrance window. If the Cu radiation were not attenuated, it would have been necessary to reduce the lamp currents to prevent saturation of the vidicon target at the Cu wavelength. This would have weakened the resonance lines of the remaining elements, with a further possible decrease in the SIN. The effect of the glass filter is seen from the spectra in Figure 2A, which were obtained by focusing the combined
Table 11. Vidicon Dynamic Ranges, Comparison of Detection Limits, a n d Maximum Permissible Limits for Trace Metals i n Water AA-120
MPL"
Vidicon dynamic range
Wavelength,
detection l i m i t ,
Detection l i m i t ,
Upper limit,
USPHS,
FWPCA,
Analyte
nm
W m l
ugiml
Wml
uglmlb
Ug/mlc
Zn
213.9 228.8 232.0 240.7 248.3 279.5 324.7
0.0005 0.0005 0.001 0.004 0.001 0.001 0.0006
0.004
0.20 0.20
5 .O
5.0
0.01
0.01
2 .o 1.o
0.005 0.5
0.3
0.009
1.o
Cd Ni
co Fe
Mn
0.005 0.01
0.02
1.o
... ...
0.2
...
0.05 2.0 cu 0.006 0.6 1.o 0.2 a Maximum permissible limits. United States Public Health Service (for potable water). CFederal Water Pollution Control Assn. (for agricultural irrigation).
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
1035
Mn
Cu.324.7 nm
I
cu
Cd
,327.4 nm
It
273.5
324.7
252.3 Mn, 279.5 nm
WAVELENGTH (nm) Figure 1. Vidicon AA spectra for seven metals extracted from a multielement aqueous standard (0.2 pg/ml) into MIBK
radiation from the two multielement hollow cathode lamps into the entrance slit of a 0.5-meter scanning Ebert monochromator (Jarrell-Ash). When the Mn 279.5-nm line and the two Cu lines were scanned without the filter, the Cu 324.7-nm line was about 8 times more intense than the Mn line. With the filter, the intensities of the Cu lines were reduced to 13% (324.7 nm) and 19% (327.4 nm) of the unattenuated values. This is to be expected from the transmission characteristics of the glass filter (Figure 2B). By thus attenuating the Cu 324.7-nm line, the intensities of the remaining metals could be maintained a t a high enough level to prevent any loss in S/N. Calibration Curves a n d Detection Limits. We have previously stressed the possibility of spectral interferences using this monochromator grating in the vidicon AA spectrometer due to the rather poor resolution (8).Although all principal resonance lines in this application are apparently resolved (Figure l), overlap with nonresonance lines and flame background radiation can occur, with consequent loss of linearity in the calibration curves and, hence, lower analytical dynamic range. A knowledge of the dynamic range for each element during the simultaneous analysis is therefore necessary. For the seven metals reported here (Table II), the lower limit is the detection limit, the concentration of metal a t which S/N = 2. This was calculated from the relative standard deviation of 10 replicates a t a concentration within a factor of 3 of this value. At higher concentrations the relationship between absorbance and concentration becomes nonlinear and, as the slope decreases, so does precision. The upper limit was therefore arbitrarily defined as the concentration a t which a change in signal from X + s (sig- s would imply a nal plus one standard deviation) to concentration change of f15%. This rather generous criterion was chosen because the largest possible dynamic range is desirable for simultaneous multielement analysis. The precision a t a concentration in the middle of the linear part of the calibration curve was calculated from 10 replicate determinations for each metal. In every case, the relative standard deviation was about 3%. 'Table I1 also gives the detection limits obtained with the single-channel instrument. Theoretical calculations have shown that a silicon vidicon detector should have a poorer
x
1036
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
v
w
UNATTENUATED
I
0 300
320
ATTENUATED
,/
i,tl4
340
360
WAVELENGTH (nm) Figure 2. Attenuation by use of a glass filter ( A ) Effect on Cu resonance lines. (B)Optical transmission of the glass filter
S/N than a photomultiplier tube a t radiative intensities typical of AA measurements (7). Our results have always shown a 7- to 10-fold decrease in S/N for the vidicon (6, 8 ) , and this is reflected in the comparative detection limits. Maximum permissible concentrations of these metals in potable waters and in waters used for irrigation purposes are also listed in Table 11. The vidicon detection limits, although poorer than by single-channel AA, are in every case low enough to determine significant concentrations of these metals. Analysis of Potable Water Samples. Comparative results by the vidicon spectrometer and single-channel AA for 20 potable water samples are shown in Table 111. Only in 3 samples did the concentration of the metal exceed the vidicon upper working limit, necessitating diluticn and a repeat analysis. For Zn, Fe, Mn, and Cu, the correlation coefficients were 0.941, 0.962, 0.984, and 0.999, respectively. In many cases, however, concentrations were below the vidicon detection limits, and consequently Table I11 gives no indication of the accuracy for Cd, Ni, and Co. For recovery tests, a potable water sample was chelated, extracted into MIBK, and analyzed for the seven metals on the vidicon spectrometer. An aqueous standard containing equal concentrations by weight of the analyte metals was then used to spike the sample a t three concentration levels.
Table 111. Comparative Results for Potable Water Samples, pg/ml: Vidicon vs. AA-120 Zn
Sample No.
1 2 3 4 5 6
7 8 9 10 11 12 13 14 15 16 17 18 19 20 .
Vid.
Ni
Cd AA-120
Vid.
...
0.13 0.09 0.12 >0.2 0.04 0.11 0.02 0.01 0.03
0.14 0.07 0.10 0.15 0.06 0.10 0.03 0.03 0.04 , 0.01 0.03 0.04 0.02 0.02 0.20 0.15 0.02 0.02 0.03 0.02 0.004 0.003 0.005 0.005 0.004 0.004 0.002 0.002 0.20 0.14 = not detected.
AA-I20
0.003 0.001 0.001 0.001 0.003 0.001 0.003 0.001 0.003 0.003
0
...
...
... ... ... ...
... ... ... ... ... ... ... ... ... ... ... ... ...
..
...
... ... ... ... 0.001 0.003 0.003 0.003
...
Vid.
co
AA-I20
... .. . .. . 0.01
.. . ... ,.. ... ...
... ... 0.01 ... ... 0.02 ... ... .. . . .. ..,
0.02 0.01 0.01 0.01
...
0.01 0.01 0.01 0.01 0.01
.. . ... ...
0.01
0.01 0.01 0.01 0.02 0.01
...
Vid.
AA-120
Vid.
AA-120
... ... ... ... ... . .. . ..
...
...
0.005 0.004 0.005
0.21
0.005
...
0.02 0.18 0.01 0.10 0.09 0.02 0.02 0.01 0.02 0.02 0.55 0.19 0.04 0.06 0.08 0.08 0.14 0.05 0.15 0.25
. .. * .. * ..
... . .. ... ... ... ... ... ... * .. ...
Table IV. Recovery from Potable Water Spiked with Multielement Aqueous Standards Recovered, Uglml
Added,
uglml
Zn
Cd
Ni
co
Fe
Mn
cu
0.00 0.10 0.15 0.20
0.01 0.12 0.15 0.18
0.00 0.10 0.15 0.19
0.00 0.11 0.15 0.23
0.00 g.09 0.14 0.23
0.00 0.08 0.14 0.19
0.01 0.14 0.18 0.25
0.01 0.12 0.17 0.25
Mn
Fe
These spiked solutions were analyzed on the vidicon spectram-eter after preconcentration. In every case recovery was complete within the limits of accuracy of the method (Table IV).
CONCLUSION A vidicon multichannel spectrometer can significantly improve the efficiency of a moderately large water analysis laboratory. Although not sufficiently versatile to handle all metals of interest, it can be used to analyze simultaneously seven important metals. The others can be analyzed separately by single-channel AA. Vidicon detection limits are somewhat higher but still satisfactory for routine potable water analysis. The system cannot be readily used to determine lead, because of inadequate sensitivity, nor chromium, because of spectral interference (8).I t is not useful for applications where high sensitivity is required, and with some highly polluted waters, the limited dynamic range will make dilution and repeat analysis necessary.
... ...
0.005
... ...
...
...
... ... ...
0.01 0.009 0.01 0.01
...
... 0.11 0.07 0.04
... ... ...
0.50 0.20 0.02 0.10 0.12 0.10 0.14 0.07 0.15 0.29
Vid.
... 0.01 0.009 0.05
...
... ... ...
0.14
...
0.02 0.38 >1.00 0.09 >1.00 0.04 0.20 0.18 0.01 0.47
cu AA-120
0.02 0.01 0.005 0.03 0.01 0.006 0.005 0.001 0.08 0.004 0.02 0.43 0.85 0.05 1.oo 0.06 0.24 0.22 0.03 0.49
Vid.
...
0.02
...
0.01
...
0.02 0.01 0.05
... ...
0.15
...
0.06 0.05 0.05
AA-I20
0.004 0.02 0.01 0.01 0.008 0.01 0.01 0.04 0.008 0.005 0.15 0.007 0.07 0.05 0.06
*..
...
0.15 0.002
0.17 0.003
0.45
0.50
...
...
Operation of the vidicon is only a little more complicated than in single-channel AA, and sample preparation is the same. Wavelengths are selected electronically, and it is simple to change the elements being determined. This versatility provides a distinct advantage over the direct-reader emission spectrometer, where alignment requires the precise placing of slits and where it is difficult to set up the instrument to determine several elements simultaneously. The most time-consuming step in the vidicon analysis is the measurement of peak heights from the stripchart recorder. Ideally the instrument should be interfaced with a minicomputer (7).
LITERATURE CITED D. G. Biechler, Anal. Chem., 37, 1054 (1965). R. C. P. Sinha, K. C. Singhal, and A. C. Banerji, Technology, 5, 121 (1968). J. Nix and T. Goodwin, At. Absorption News/., 9, 119 (1970). R. D. Ediger, At. Absorption News/., 12, 151 (1973). K. W. Busch and G. H. Morrison, Anal. Chem., 45, 713A (1973). K. W. Jackson, K. M. Aldous, and D. G. Mitchell, Spectrosc. Lett.. 6, 315 (1973). D. G. Mitchell, K. W. Jackson, and K. M. Aldous, Anal. Chem., 45, 1215A (1973). K. W. Jackson, K. M. Aldous, and D. G. Mitchell, Appl. Spectrosc., 28, 569 (1974). K. W. Busch, N. G. Howell, and G. H. Morrison, Anal. Chem., 46, 575 (1974). D. 0. Knapp, N. Omenetto, L. P. Hart, F. W. Plankey, and J. D. Winefordner, Anal. Chim. Acta, 69, 455 (1974). M. J. Milano, H. L. Pardue, T. E. Cook, R. E. Santini, D. W. Margerum, and J. M. T. Raycheba, Anal. Chem., 46,374 (1974). M. J. Taras, A. E. Greenberg, R. D. Hoak. and M. C. Rand, "Standard Methods for the Examination of Water and Wastewater," 13th ed., 1971, American Public Health Association, New York, p 21 1.
RECEIVEDfor review December 2, 1974. Accepted February 3,1975.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
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