ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
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Plasma Emission Determination of Trace Heavy Metals in Salt Water Matrices Danton D. Nygaard’ Harold E. Edgerton Research Laboratory, New England Aquarium, Boston, Massachusetts 02 7 10
recorder. Detection limits and analytical precision were determined by integrating the photomultiplier output for specified periods of time. During the automated replicate standard addition analyses, the photomultiplier output was sampled 120 times during an 8-s period and averaged (13).
The Spectraspan dc plasma emission spectrometer is evaluated an an analysis tool for the determination of trace heavy metals (cadmium, chromium, copper, lead, nickel, and zinc) in seawater and acid digestates of ocean sediments. Sodium, calcium, and magneslum In seawater are shown to increase both the background and elemental line emission intensities. Optimum analytical emission lines and detection limits are reported. Analytical precision is evaluated by replicate standard addition analyses of known solutions.
RESULTS A N D DISCUSSION Effect of EDTA Incorporated into Sample Matrix. Preliminary investigations were troubled by memory effects, caused by adsorption of metal ions onto the Tygon peristaltic pump tubing and polyethylene atomizer housing of the spectrometer. In an attempt to alleviate this problem, disodium EDTA was added to the metal ion and blank solutions a t the 10 FM concentration level. This addition produced a fivefold enhancement of the metal emission line intensity for a 50-ppb Cu solution, without significantly affecting the blank background emission. At the same time, memory effects were eliminated. On the basis of these experiments, 10 W MEDTA was added to all samples and blanks in subsequent work. A similar use of EDTA to inhibit metal ion adsorption onto surfaces has been reported by Hoyle (16). Salt Effects. The literature suggests that the addition of easily ionizable metals to the sample matrix can dramatically alter the properties of the dc plasma ( 5 , 9). T h e emission spectrum in the vicinity of the 327.4-nm Cu line was scanned for a 50-ppb Cu solution using both distilled water and 0.5 M NaCl as solvents. The results, as shown in Figure 1,confirm the literature reports. T h e enhanced background continuum seen in the NaCl matrix may be due to grating scatter of the sodium emission lines. However, the Echelle monochromator system employs double monochromation, with a prism following the grating in the light path, which should eliminate this source of background, unless light is scattered around the prism. Continuum background emission has been reported for sodium solutions in flames (I 7, I 8 ) , resulting from recombination reactions in the flame. I t is also possible t h a t salt particles in the plasma scatter black-body emission, emanating from the tungsten electrodes, into the monochromator. The increased intensity of metal emission lines in matrices containing easily ionizable elements has been attributed t o a shift in the free atom-ion equilibrium due to an increase in the concentration of electrons (19). However, after correction for background emission, the 224.7-nm Cu ionic emission line showed the same increase in intensity as the atomic line when 0.5 M NaCl was added to the matrix. Keirs and Vickers (20) have described the same enhancement effect for KCl, and attribute the enhancement t o increased penetration of the sample into the plasma. Keirs and Vickers (20) also discuss the possibility that dc arcs struck in monatomic gases may not reach local thermal equilibrium, unless the arc is seeded with an easily ionizable element, thereby increasing the electron concentration of the plasma. Identical results were obtained when 0.25 M Na,SO, was substituted for NaC1, indicating the anion plays little part in the enhancement. The same enhancement of both background continuum and metal line emission intensities with the addition of 0.5 M NaCl to the sample was also observed for the other metals studied (Cd, Cr, P b , Ni, and Zn).
T h e dc plasma was introduced as an excitation source for atomic emission spectrometry by Margoshes and Scribner ( I ) and Korolev and Vainshtein ( 2 ) . Modified designs have been characterized by a number of other authors (3-9). Spectrametrics Inc. now produces an atomic emission spectrometer which employs a dc plasma arc as the excitation source. This report describes an attempt to evaluate the Spectrametrics instrument as an analysis tool for the determination of selected trace heavy metals (Cd, Cr, Cu, P b , Ni, and Zn) in seawater and acid digestates of ocean sediments. T h e high concentrations of sodium, calcium, and magnesium in seawater are shown t o increase both the background and elemental line emission intensities. Results are reported concerning the optimum analytical emission line and detection limit for each element. Analytical precision is evaluated by replicate standard addition analyses of known solutions, and discussed in terms of the various sources of noise. Finally, conclusions are drawn concerning the utility of the instrument for the analysis of trace heavy metals in salt water matrices.
EXPERIMENTAL Apparatus. All data was taken on the Spectraspan I11 plasma emission spectrometer, which uses an inverted V shaped dc plasma struck between tungsten electrodes as the excitation source ( I O , 1 I ) and high resolution Echelle optics in the monochromator (12). The replicate standard addition analyses were carried out with the instrument interfaced to a Technicon automatic sampler and a Wang 614 programmable calculator (13). Reagents. Standard metal ion solutions were prepared on the day of use by dilution of commercially available 1000-ppm standard solutions. Ten micromolar EDTA was added to each solution in order to minimize adsorption of the metal ions onto surfaces. Synthetic seawater was prepared according to Morel (14) from reagent grade chemicals. All concentrated salt solutions were passed through columns of Chelex-100resin to remove heavy metal impurities (15). Procedures. The entrance slit to the monochrometer was focused on the notch below the V-shaped, current-carrying portion of the plasma, and the focus optimized while aspirating a 500-ppb stock solution of the element of interest into the plasma. Metal emission lines were scanned by pressing a rubber wheel, driven by a constant speed motor, against the monochromator wavelength selection wheel. During wavelength scans, the photomultiplier output was interfaced, through a current t o voltage converter, t o an x-y Present address, Department of Chemistry, Bates College, Lewiston, Me. 04240. 0003-2700/79/0351-0881$01 OO/O
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1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979 ___1
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Figure 3. Copper emission intensity, corrected for background, as a function of NaCl concentration at the 327.4-nm copper line for a 5 0 p p b Cu solution
