200
Anal. Chem. 1991, 63,208-212
Potentiometric Stripping Analysis Using Copper(I) and Determination of Chlorine Species Youqin Xie and Calvin 0. Huber* Department of Chemistry, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, Wisconsin 53201
Copper is potentiostaticallydeposited on a gold-film electrode. Foilowing the deposition, the copper is chemically oxidized by chlorine species in appropriate buffer medium. Copper( I ) was shown to be superior to copper(0) as the reduced form. Competltive copper oxldation by dissolved oxygen is eiimlnated. Copper( I ) deposition adheres to a Langmuir adsorption model. The stripping time equation for adsorbed copper( I ) and the potential-time function are derived theoretically and verified by experiments. The stripping time is proportional to the reciprocal of the concentration of chlorine species. SeiectivHy between hypochlorous acid and monochloramine can be obtalned by comparing the results at pH 6.0 with those at pH 2.5. The detection limit of 0.005 mg of CVL for both hypochlorous acid and monochioramine at a relative standard deviation of about 3 % .
INTRODUCTION Chlorine is commonly added to water for disinfection or biofouling control. The chlorine added is in equilibrium with hypochlorous acid (HOCl), the product of its hydrolysis. The equilibrium quantitatively favors the hypochlorous acid a t ordinary pH. The hypochlorous acid reacts with any ammonia present very quickly to form monochloramine (NH,Cl). Hypochlorous acid and monochloramine, which are distinctively toxic to different forms of aquatic life, are the two dominant species of chlorine in water. Spectrophotometric molar absorptivities of hypochlorous acid and monochloramine are too low to allow submilligram/liter concentration measurements. Various titrimetric ( I ) and amperometric (2-6) techniques are used to monitor these species. Rapid and accurate techniques suitable well below milligram/liter levels that are selective for hypochlorous acid and monochloramine are often needed. Chemical stripping analysis, the predecessor of potentiometric stripping analysis (PSA) (7-9), was used for the determination of some oxidizing agents such as Mn04- and Ce(1V) at parts per million (ppm) levels using a silver deposit (IO)and for the evaluation of the equilibrium constant for heterogeneous copper disproportionation (I1). Fayyad et al. reported the application of PSA for the determination of dissolved oxygen based on its reduction by Cd-Hg formed at a glassy carbon electrode (12). Potentiometric stripping analysis presents some unique features in comparison with anodic stripping voltammetry (ASV) for trace metal determinations. Like ASV, it consists of a potentiostatic deposition step and a subsequent stripping step. The deposition step in PSA is the same as that in ASV, but the stripping step in PSA is fundamentally different from that used in ASV in that stripping is achieved chemically by oxidant in the solution instead of by an applied potential function. During the stripping step, the working electrode potential at zero current versus time is recorded. PSA is an example of heterogeneous redox chemistry involving a layer of a metallic or other deposit, which may be the analyte or may be a reagent. The performances of PSA in flow systems have been investigated (13-15). A significant advantage for flow systems
is that the chemical stripping need not be performed in the same solution used for deposition, as in the batch mode. By suitable choice of the composition of the stripping solution, the analytical procedure can be optimized. In the present work, the mechanism of heterogeneous copper oxidation by chlorine is proposed. A technique using deposited copper species as a reagent for selective determination of parts per billion (ppb) levels of hypochlorous acid or monochloramine by potentiometric stripping analysis in flow systems is presented. The deposition of Cu(1) is found to adhere to a Langmuir adsorption model. The theoretical equations for stripping time and the potential-time relationship for adsorptive PSA in flow systems are derived. EXPERIMENTAL SECTION Reagents. Standard hypochlorite solutions were prepared from 5% sodium hypochlorite solution (Aldrich Chemical Company) after standardization with potassium iodide and sodium thiosulfate. Dilute solutions were prepared just prior to use by rapid, successive dilutions with nitric acid (pH 4.0) in doubly distilled water. Monochloramine was prepared by mixing appropriate solutions of sodium hypochlorite with at least a 2-fold molar exces of NH3 solution. The stock solution of gold(II1) (0.01 M) was prepared by use of hydrogen tetrachloraurate trihydrate (HAuCl4+3H2O) (Aldrich) and acidified to pH 1 to prevent hydrolysis when hydrochloric acid is used. Copper acetate (CuOAc2.H20)(Baker Analytical) was used to prepare the Cu(I1) stock solution (0.010 M). Buffer solutions (Clark-Lubs, KHP) were prepared by a standard procedure. All other solutions were prepared from ACS reagents and doubly distilled water. Apparatus. The flow system consisted of an eight-channel inlet valve (FIAtron FIA-Valve 2500, Milwaukee), an electrochemical flow cell, and a peristaltic pump (Masterflex). The dual thin-layer cell with a cell spacer thickness of 0.005 in. (Bioanalytical Systems Inc.) consisted of a glassy carbon working electrode (3-mm diameter) and a gold counter electrode (3-mm diameter). The Ag/AgCl(sat) reference electrode was connected to the cell through a reference compartment. All the potentials given in this paper are expressed versus this electrode. All connections were made with 1-mm-i.d. Teflon tubing. Flow rates were determined volumetrically and could be adjusted between 0.1 and 6.0 mL/min. The potentiostat used was the Pine Instrument Model RDE3. Preparation of the Gold-Film Electrode. The glassy carbon electrode was first polished with alumina powder (1 pm), then cleaned with 95% ethanol, and finally rinsed by distilled water before use. The gold-film electrode was prepared by depositing M Au(II1) solution in 1.2 M hydrochloric acid gold from 2.0 X at Ed = -0.2 V for 30 s, -0.3 V for 30 s, -0.4 V for 1.0 min, and -0.5 V for 2.0 min (Le., with a total plating time of 4.0 min), flowing at 1.2 mL/min. A well-plated gold-film electrode could be used for at least 1 week if stored in distilled water and handled with care. Procedure. After adjustment of the appropriate deposition potential and time, the rotating valve was operated to a position where the deposition solution (0.1 mM Cu2+in pH 4.0 acetate buffer) was allowed to enter the cell and deposition was started simultaneously. Deposition was discontinued, and the open-circuit potentiometric mode was initiated simutaneously with arrival of the stripping (i.e., sample) solution at the cell. A 5-s lag time for movement of the sample solution from the valve to the detector cell needed to be accommodated. After the E-t curve was recorded, a solution of 5 mg/L hypochlorous acid was passed through the cell for 10 s to remove residual copper, and finally
0003-2700/91/0363-0208$02.50/0 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL.
-200
pH = 4.3
pH = 5.0
pH = 5.6
03, NO. 3, FEBRUARY 1, 1991
209
-200
pH = 7.4
500 mV -100
-100
g
-
0
-150mV
0
B 5
s 8
1
P 100
Ui
bi
100
>
-
9
E
E
W
W
200
200
300
300
-
H
400
60 sec
H
60 sec
Figure 1. Stripping curves for copper in 0.1 M acetate buffer.
Table I. Stripping Potentials of Copper (mV) with Various Media at pH 4 step
concn, M
nitrate
acetate
phthalate
chloride
1
0.1
-46 -49
-70
-100
-65 -87
-157 -267
152
160
150
160
197 240
1.0
3
0.1
1.o
193 194
the valve was operated to allow pH 4.0 acetate buffer to enter the cell for 10 s before beginning the next cycle.
RESULTS AND DISCUSSION General Features of the Stripping Curves and Stripping Mechanism. When copper(I1) was reduced to copper(0) a t -500 mV and the stripping was done in sodium nitrate, phthalate, and acetate buffer electrolytes in the pH range 2-7, two or three stripping steps were observed for each stripping curve corresponding to different interfacial chemical redox reactions. The general shapes were independent of buffer system anions. Figure 1 shows those with acetate buffer. At low pH (2-4), step 1 was the dominant reaction, while step 2 was not observable. With an increase of pH, step 2 became the dominant one and step 1 vanished. Cupric Ion and Anion Effects. To investigate the copper stripping mechanism, 1.0 X 1048.0 x lo4 M Cu2+were added to the stripping solutions. As a result of the added Cu2+ion, the stripping potentials of the three steps moved anodic in every electrolyte. Plots of Ell2 against log [Cuz+] for all stripping steps were linear, suggesting that the products of all three steps are copper(I1) species. The slopes of step 1, step 2, and step 3 were 26.0 f 1.0, 28.0 f 1.0 and 55.0 f 1.0 mV, respectively. The intercept for step 1 was 79.0 mV vs Ag/AgCl(sat), or +0.300 V vs NHE, which is close to +0.340 V, the standard potential of the Cu/Cu2+ couple. The anion ligands in stripping solutions affect the stripping potentials of copper. Some experimental results are listed in Table I. In nitrate solution, all stripping potentials were virtually independent of the nitrate concentration, indicating that the cupric ion controls the stripping potentials. The negative shifts of the stripping potential of step 1 with anion (acetate, phthalate, and chloride) concentrations suggest complexation between copper(I1) and these anions and that the product of step 1 is copper(I1). The big negative shift of step 1 in a chloride medium indicates that copper(1) chloride is produced through disproportionation. The stripping potential for step 3 was independent of either acetate or phthalate concentration but moved anodic with an increase
U
-
400 30 sec 15sec 15 sec Figure 2. Deposition potential effects on stripping curves for copper in pH 4.5 acetate buffer.
of chloride concentration. Apparently copper(1) was oxidized to copper(I1) during step 3. Chloride stabilizes Cu(1) more strongly, while smaller formation constants for acetate and phthalate with Cu(1) and Cu(I1) are about the same. Examination of the E-t Curve. Xie and Chau (16) have derived the theoretical E-t equation for potentiometric stripping analysis using a rotating disk gold-film electrode and showed that E is linearly proportional to log [t112/(. - t)] with a slope of 6 0 / n mV. The experimental results here can be compared to that theoretical treatment. The slope found for step 1 was 27.0 f 1.0 mV with nitrate, acetate, and phthalate, and 45.0 f 1.0 mV with chloride. The slopes for step 2 and step 3 were 28.0 f 1.0 and 63.0 f 1.0 mV, respectively, with all the anions mentioned above. These data suggested that both step I and step 2 involved two-electron transfer, whereas step 3 was a one-electron transfer. The deviation of the experimental slope for step 1 with chloride from the theoretical value was attributed to the disproportionation of copper, in agreement with the anion effect results. Dependence of the Deposition Potentials. Figure 2 shows that, for deposition potentials more negative than -150 mV, there were two stripping steps and that the relative magnitudes were affected by the deposition potential. At deposition potentials more positive than -100 mV, only a single step a t about +200 mV was present. Copper Stripping Mechanism. The results outlined above provide a basis for ascertaining the stripping redox chemistry of copper (see Table I, Figures 1 and 2). The step at about -50 mV is apparently the formation of cupric ion in nitrate solution and complexed copper(I1) species in acetate and phthalate solutions. The magnitude of step 2 increases with pH, indicating the formation of hydrolyzed species. Using the hydrolysis constant data reviewed by Baes and Mesmer (In,it can be shown that the ratio of [CU~(OH)~~+]/[CUOH+] is about IO6;thus, the principal hydrolysis product of copper(I1) at moderate pH is Cu2(OH),Z+.The stripping potential of step 2 in nitrate solution is +0.130 V vs Ag/AgCl (+0.340 V vs NHE). The calculated standard potential for the Cu/ C U ~ ( O H )couple ~ ~ + is +0.360 V vs NHE a t pH 5. The independence of the stripping potential for step 2 on pH corresponds to no net proton transfer in the interfacial redox reaction. The E-t data showed that step 3 was a one-electron transfer, yet the potential was sensitive to both cupric and acetate species. In summary, the mechanism proposed is step 1 Cu
+ HOC1 + H+
-
Cu2+ + C1-
+ H20
210
ANALYTICAL CHEMISTRY, VOL. 63, NO. 3, FEBRUARY 1, 1991
and the surface excess of R just after the deposition is
step 2 2Cu
+ 2HOC1-
step 3 2Cu(I),d
+ 2C1-
Cu2(OH):+
-
+ HOC1 + H+
2Cu2+
+ C1- + HzO
Adsorption of Cu(1). In order to model the Cu(1) deposition, we consider a reversible deposition reaction followed by a rapid adsorption process:
0 + ne
-
R
where td is the deposition time. Stripping of cu(I),d. During the stripping period, the change of the surface excess in time must be equal to the flux of the oxidant from the solution to the electrode surface:
Rad
(1) The steady-state mass-transfer differential equations for a thin-layer cell are
where Jox is given by Jox = 0 . 8 3 D ~ ~ ~ 3 b 1 1 ~ 2 ~ - 1 ~ 6 ~ 0 ~ (15) Iip1Jz~~x
(3) where u, and uy are the magnitudes of the solution velocities in the x, y direction, respectively, with the y axis perpendicular to the surface of the electrode and the x axis along the surface in the direction of flow. If it is assumed that the adsorption process is so rapid that there is diffusion control, the surface excess is related to the volume concentration at the electrode surface, CR(y = O), by the adsorption isotherm. In the case where the surface concentration of R is so low that the adsorption isotherm can be linearized, i.e. rR(t) = bRCk(t)
with
bR =
rR,$R
(4)
where cg(t)is the surface concentration of R at any time, r R a is the saturation coverage of R, and PR = eXp(-AGoR/Rr), A G O R is the adsorption free energy. The other boundary conditions are cO(y,o) =
cR(Y,O) = 0
CO;
cO(o,t) = 0;
CR(O,t) =
lim CoCy,t) = Cb;
cb(t)
lim CR(y,t) = O
"Y
"Y
(5) (6)
(7)
The solutions of eqs 2 and 3 are the same as what were given by Levich (18) and Hanekamp (19):
where petripis the linear flow rate during the stripping step (centimeters/second) and CLx is the bulk concentration of the oxidant. If Cb, is in relatively large excess, then Jox can be considered as a constant during the entire stripping step. This differential equation must be solved by using the following conditions: t =
=
y=o
-DR(
y=o
= kCb(t)
(18)
rR,d/JOx
Shape of the Transient Potential-Time Curve. T o describe the concentration of Cu2+at the electrode surface during the stripping process in stripping solutions in which Cu2+is initially absent, the diffusion equation
must be solved with the following initial and boundary conditions:
t > 0;
-
y = 0;
y
m;
Co(y,O) = 0
lim C&,t)
=0
(21) (22)
Y-m
y=o; Do(dCo/dy) = -Dox(dCox/dy) = -Jox (23)
The solution of the diffusion equation a t y = 0 is Co(0,t) =
where b is the width of the cell spacer (centimeters), 1 is the length of the cell (centimeters), v is the kinematic viscosity of the solution (squared centimeters/second), and C(dep denotes the linear flow rate during the deposition step (centimeter/ second). By combining eqs 4,9, 10, and 8 and solving eq 8, we have
(17)
Substituting eqs 13 and 15 into eq 18 gives
kq
2)
rR(t) = 0
7,
=
t>0;
and
(16)
rR,d
The initial condition states that the surface excess is r & d after deposition. The boundary condition states that, at the end of the oxidation, rR(t) = 0 and t = T . The solution for stripping time is obtained by solving eq 14:
t = 0; Do($)
rR(t) =
t = 0,
2Joxt' I 2
(TDO) 'I2
Combination of eqs 14 and 18 gives
Substituting eqs 24 and 25 into the Nernst equation, we have
ANALYTICAL CHEMISTRY, VOL. 63,NO. 3,FEBRUARY 1, 1991
.16
9
211
i
-
-
/3
6
v)
.12-
F
.-F a .-a L
.08
5
-
-
.04
l01 0 0.4
1 0.6
0.8
1.0
1.2
1.4
Flow Rate (p-’l2) (ml/min)”/2 = 3.1 mL/min, Stripping time and flow rate: (A) pLdep changing potrip; (B) pship= 3.1 mL/min, changing pdep;stripping, 1.0 mg/L HOCI; other conditions, same as in Figure 3.
Figure 5.
50 -
Table 11. Analytical Characteristics
deposit 40-
linear range,
LOD,b
form
pH
compd
s,” %
mgCl/L
mg Cl/L
Cu(1)
2.5
HOCl NH&l HOCl HOCl NHPCl HOCl
3.0 3.0 3.5
12.00 52.00 52.00 51.00 51.00 51.00
0.005
h
v,
6.0
Y
F 0 .-c
a a ‘C
5
30-
Cu(0)
2.5 5.0
20-
5.6 6.0
0.025
a Estimated relative standard deviation at HOCl concentration of 1.4 X lo4 M based on 8 repetitive measurements. bLOD (limit of detection) = 3s‘/sensitivity.
10A’
0-
5.6
0.007 0.007 0.031 0.035
I
I
I
I
60
120
180
240
Deposition Tme (s) Figure 4. Stripping vs deposition times: Cu(I1) concentrations, (1) 5.0 X M, (3)5.0X M; other conditions, same M, (2)1.0X
lo-’
as in Figure 3.
Copper(I1) Concentration and Deposition Time Dependence. The expected linearity was obtained a t a Cu(I1) concentration below 10 pM with standard deviations of regression smaller than 0.7 s. Deviations from linearity were observed a t higher concentrations as expected. When approaching full surface coverage, the stripping time approaches its maximum value (T~,,). The concentration a t which the electrode surface becomes saturated with Cu(I1) is related to the deposition time. Extension of the linear range can be achieved by using shorter deposition times. The Langmuir model used above would predict a linear dependence of 1 / ~ on l/Cb:
This relationship was found experimentally as shown in Figure 3.
Figure 4 illustrates the exponential relationship between stripping time and deposition time at different levels of Cu(I1) as predicted by eq 19. Limiting deposition times are reached at about 3 min, independent of copper(I1) bulk concentration. Flow Rate Influence on cu(I),d Stripping. The effect of the flow rate from 0.2 to 5.0 mL/min on stripping time was examined for deposition from lo4 M Cu2+and with 1.4 X M HOCl in the stripping solution. The results are shown in Figure 5. The stripping time was independent of the flow rate during deposition. According to eqs 11, 13, and 19, an increase of flow rates would lengthen the stripping time. The function is such that for reasonable flow rates the increase is insignificant; i.e., the thickness of the diffusion layer is small enough so that the flow rate will not increase the stripping time significantly. In contrast, the stripping time was found to be linearly proportional to pstri;1/2, in agreement with eq 19, indicating that the oxidation of Cu(I),d was mass transfer controlled. Determination of Chlorine Species. The reciprocal of stripping time is proportional to the concentration of the oxidant in the solution. The slope of such reciprocal calibration plots is related to the mass-transfer coefficient and/or kinetic rate constant. Both Cu(0) (Ed = -0.50 V) and Cu(1) (Ed = +0.10 V) can be used for chlorine determinations. Potentiometric stripping determinations were carried out for chlorine concentrations of 0.025-2.00 mg of Cl/L. Analytical results are summarized in Table 11. The lower limit of detection was calculated as 3 times the standard error of the
212
Anal. Chem. 1991, 63, 212-216
estimate divided by the slope of the regression line. Complete selectivity for hypochlorous acid over monochloramine can be obtained by controlling the pH of the stripping solution. The stripping time with hypochlorous acid is invariant over the pH range 2 4 . The rate constant for the monochloramine/copper reaction is smaller than that for the hypochlorous acid/copper reaction at high pH. At pH greater than 5 , the reaction rates for oxygen/copper and monochloramine/copper reactions were approximately the same; Le., monochloramine is oxidatively inactive. Below pH 2.5, the stripping time with monochloramine was equivalent to the stripping time with hypochlorous acid. Thus, hypochlorous acid can be determined first a t pH 6.0 and then monochloramine by difference at pH 2.5. It was found that copper(1) was superior to copper(0) as the reduced form in almost every respect, as shown in Table 11. The stripping potential for copper(1) is ca. +250 mV vs Ag/AgCl, much more positive than that for copper(0) (ca. -50 mV). Thus, the competitive oxidation of copper by dissolved oxygen is eliminated when using copper(1). Many other oxidants, such as iodine, also do not interfere. Elimination of these interferences contributes to a lower limit of detection. Strong oxidizing agents, such as permanganate and hexavalent chromium, will interfere with the determination of chlorine species. A t pH 2.5, the rate constant of the Mn0,-/Cu(I) reaction is about the same as that of the HOCl/Cu(I) reaction, while the Cr/(VI)/Cu(I) reaction rate is about 1 order of magnitude lower. A t pH 6, about a 100-fold stoichiometric excess of Cr(V1) was tolerated. In addition, many metals, e.g., cadmium and zinc, cannot be reduced at the deposition potential of copper(1) and therefore do not interfere. A bulk gold substrate for copper(1) deposition was found to yield precision as good as that for a gold-film electrode. This should
enhance the convenience of the technique. Further applications of potentiometric stripping of copper(1) deposits are being persued. Registry No. HOC1, 7790-92-3; NH,Cl, 10599-90-3;water, 7732-18-5.
LITERATURE CITED (1) Standard Methods for The Examination of Water and Wastewater, 14th ed.; American Pubiic Health Association: Washington, DC, 1975; p 318. (2) Tsaousis, A. N.; Huber, C. 0. Anal. Chim. Acta 1985, 179, 319-323. (3) Morrison, T. N.; Huber, C. 0. Water Chlorination, Environmental Impact and Healfh Effects; Ann Abor Science: Ann Arbor, MI, 1983; VOl. 4, pp 751-759. (4) Coburn, J. T.; Kafil, J. B.; Huber, C. 0. Ibid. 1983;Vol. 4,pp 743-750. (5) Raab, D. H.; Huber, C. 0. Ibid. 1985;VoI. 5, pp 1073-1074. (6) Davies, D. A.; Huber, C. 0. Ibid. 1985: Vol. 5, pp 1091-1098. (7) Jagner, D.; Aren, K. Anal. Chim. Acta, 1978, 700, 375-388. (8) Jagner, D. Anal. Chem. 1978, 50. 1924-1928. (9) Jagner, D. Analyst 1982, 107, 593-599. (10) Bruckenstein, S.;Bixler, J. W. Anal. Chem. 1965, 3 7 , 786-790. (11) Bruckenstein, S.;Tindall. G. W. Anal. Chem. 1988, 40, 1402-1404. (12) Fayyad, M.; Tutunji, M.; Ramakrishnat, R . S.; Taha, 2 . A. Analyst 1986, 1 1 1 , 471-473. (13)Anderson, L.; Jagner, D.; Josefson, M. Anal. Chem. 1982, 5 4 , 1371-1376. (14)Frenzel, W.; Bratter, P. Anal. Chim. Acta 1986, 179, 389-398. (15) Jagner, D.; Josefson, M.; Aren, K. Anal. Chim. Acta 1982, 747, 147- 156. (16) Xie, Y.; Chau, T. JinanLiyiXuebao 1987, 2(1),65-71; Chem. Abstr. 1988, 108, 48245. (17) Baes, F., Jr.; Mesmer. R. The Hydrolysis of Cations; John Wiley 8 Sons: New York, 1976. (18) Levich, V. Physico-chemical Hydrcdynamics ; Prentice-Hall: Engle-
wood Cliffs, NJ, 1962. (19) Hanekamp, H. B.; Van Niewkerk, H. J. Anal. Chim. Acta 1980, 127, 13-22.
RECEIVED for review July 31, 1990. Accepted November 2, 1990.
Supercritical Fluid Chromatography with a Helium Microwave- Induced Plasma for Chlorine-Selective Detection Liming Zhang and Jon W. Carnahan* Department of Chemistry, Northern Illinois University, DeKalb, Illinois 60115
Randall E. Winans* and Paul H. Neil1 Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439
A helium microwave-induced plasma was coupled directly to a capillary supercritical fluid chromatograph. Carbon dioxide was utilized as the mobile phase. The chromatographic system, interface, and plasma source are described. For the determination of chlorine, the near-infrared atomic emission line at 837.6 nm was chosen to avoid molecular emission band interferences in the visible spectral region. Signal intensity reduction was observed with increased supercritical fiuld chromatography mobliephase pressure. Chromatograms for mixtures of chlorine-containingcompounds are illustrated. The calibration range was 3 orders of magnitude, and the chlorine detection limit was 40 pg/s.
* Authors t o whom correspondence should be addressed.
INTRODUCTION Capillary supercritical fluid chromatography (SFC) has gained popularity for the separation of high molecular weight compounds (1-5). Since the mobile phase of SFC is gaseous at atmospheric pressure, the column effluent is compatible with many of the conventional gas chromatography (GC) detectors. Examples include the flame ionization detector (FID) (6-8), thermionic ionization detector ( 9 ) ,flame photometric detector ( I O ) , electron capture detector ( l l ) ,mass spectrometry (12),helium microwave-induced plasma (MIP) (13, 14), and radio frequency plasma (15). Among the various detection methods, those based on atomic emission offer many potential advantages. These advantages include inherent selectivity, relative freedom from interferences and the capability of multielement detection.
0003-2700/91/0363-0212$02.50/00 1991 American Chemical Society
