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Applications of d.c. argon plasma emission spectroscopy to saline waters: a study of enhancement effects
Spectmchimica Acta,Vol. 35B.p~.421fo430 Pergamon Press Ltd., 1980. printed inGreat Britain
Applications of d.c. argon plasma emission spectroscopy saline waters: a study of enhancement effects* DELYLEEASTWOOD,MARTHASCHULZHENDRICK and GENES• U.S.Coast Guard Research and Development Center Groton, CT 06340, U.S.A.
to
GLIERO
(Received 16 January 1980) Abstract-A commercially available d.c. argon plasma emission spectrometer was used to determine transition metals (3d and 4d) and also Be in salt and brackish water. The effects of salinity on the enhancement of emission intensities of the analyte lines were studied using an empirical approach combined with statistical analysis. One set of experiments deals with the effects of trace metal concentration and salinity on the relative emission intensities of 14 elements using a completely randomized experimental design, i.e. the sequence in which the 48 treatment combinations (12 levels of salinity and 4 levels of metal concentration) were measured was determined by randomization, with the results evaluated by an analysis of variance program. The second set deals with the effect of relative enhancement to salinity on selected ion and atom emission line pairs for various elements at 100 and 50% salinity relative to fresh water (0% salinity). These results are analyzed by a stepwise linear multiple regression analysis program using selected parameters of theoretical interest. It was discovered that the differences in relative enhancement for ion-atom emission line pairs for 100% salinity could be predicted basically with two variables. A coefficient of determination of 95% was achieved by employing the energy of transition for the atomic line and the number of unpaired d-electrons in the lower atomic state.
1. I~R~DU~I~N DIRECTCURRENT (d.c.) argon plasma emission spectroscopy, (DCP), like other plasma emission techniques, has gained considerable attention [l-5] in recent years as an alternate method of analysis to atomic absorption. A commercially available d.c. argon plasma emission spectrometer with tunable echelle monochromator which can be used either sequentially or simultaneously for up to 19 elements at a time has proven useful for a variety of sample types including sea water. The U.S. Coast Guard has a need for a reliable method to detect, identify and quantify hazardous chemicals and oils resulting from transportation-related spills. Since many of these spills involve sea or brackish water samples of variable salinity, the effect of salinity on the analytical methodology for trace element analysis becomes important. Enhancement effects due to alkali metals in sea water samples have been observed in d.c. plasma emission spectroscopy. Such effects, while enhancing the sensitivity of the method, may require matching matrices of samples, blanks and standards, using standard addition techniques or internal standards or using excitation buffers to compensate for alkali metal ions present in samples with unknown matrices. An understanding of the magnitude of the enhancement factor which will vary with element and emission line selected and also of the basic mechanism resulting in the enhancement will help in correcting for this effect. Earlier studies [6, 71 on a few elements using a 2-electrode d.c. argon plasma jet seemed to indicate a plateauing effect which allowed a lithium buffer to be used to *Part of this paper was presented Cambridge (1979). [l] [2] [3] [4] [S] [6]
at the 21st Coll. Spectr. Int. and 8th Int. Conf. Atomic Spectr.,
R. A. BURDO and M. L. SNYDER,Anal. Cbem. 51, 1.502 (1979). G. W. JOHNSON,H. E. TAYLORand R. K. SKOGERBOE,Appl. Specfrosc. 33, 451 (1979). H. L. FELKEL, JR. and H. L. PARDUE, Anal. Chem. 50, 602 (1978). R. K. SKOGERBOE, I.T. URASA and G. N. COLEMAN,Appl. Specnosc. 30, 500 (1976). D. W. GOLIGHTLY and J. L. HARRIS, Appl. Specrrosc. 29, 233 (1975). W. G. Cox, Proc. Int. Conf. Enu. Sensing and Assessment, Las Vegas 1975, Vol. 2, Session 4, p. 6. Las Vegas, Nevada, U.S.A. (1975). [7] M. D. SANDSand W. G. Cox, Preptints 3rd Annual Meeting FACSS, joint with 19th Coil. Spectrosc. 1~. and 6th Int. Cont. Atomic Spectrosc., Philadelphia 1976. Abstr. 100. 421
422
D. EASTWOOD,M. S. HENDRICK and G. SOGLIERO
compensate for unknown salt concentrations. More recently, a spectrometer employing a 3-electrode jet, which seemed to produce a “hotter” plasma, was used for measurements on a larger number of elements [8, 91. These experiments indicated that the enhancement effect due to alkali metals was not a simple phenomenon so that compensation with a lithium or similar alkali metal buffer to reach a plateau region was not always practical. Minor constituents of seawater such as Ca may cause spectral interference either directly or indirectly (through stray light or ghosts) but this is a relatively straightforward phenomenon and will not be further discussed here. Various authors [lO-131 indicated that the inductively coupled plasma (ICP) results indicate lack of local thermal equilibrium (LTE). Some scientists [14] have suggested that different methods of measuring temperature in the DCP give different results and that these conflicting observations indicate that the DCP also shows lack of LTE. Therefore, in describing a mechanism for the enhancement we cannot think in terms of temperature and electron density; consequently, the theory as given in BOUMAN’S book [15] does not apply. In this paper, we take an empirical approach to the problem. By a stepwise process, we hope eventually to elucidate the underlying mechanism of enhancement by alkali metals or at least to be able to predict the results well enough to correct to desired precision. Experiments to be described in this paper fall into two sets. The first set deals with the effect of trace metal concentration and salinity on the relative emission intensities of 14 elements using a completely randomized experimental design i.e. the sequence in which the 48 treatment combinations (12 levels of salinity and 4 levels of metal concentration) were measured was determined by randomization, with the results evaluated by an analysis of variance program (ANOVA). The second set deals with the effect of relative enhancement due to salinity on selected ion and atom emission line pairs for various elements at 100 and 50% salinity relative to 0% salinity. These results are analyzed by a stepwise linear multiple regression analysis program using appropriate selected parameters of theoretical interest. The results of the regression analysis are particularly interesting since it was established that the differences in relative enhancement for ion-atom emission line pairs could be predicted basically from only two parameters. 2.
APPARATUS AND TECHNIQUES
2.1 Instrumentation Instrumentation was the SpectraMetrics SpectraSpanR III d.c. argon plasma emission echelle spectrometer [16-181 with 3-electrode plasma jet. This spectrometer can be used to quantitate either simultaneously, with a multi-element capability of up to 19 elements with a cassette of preselected lines (see Table l), or sequentially by peaking on the emission line of interest (see Table 2). Argon was welder’s grade from Linde. [8] D. EASTWOOD,M. S. HENDRICK,S. CALLAWAY and W. G. Cox, Proc. 1978 New England Conf. Current Deuei. Spectrochem. Excitation and Analysis, Woods Hole, MA 1978 Enu. Quart. (U.S.D.O.E. Envtl. Lab), July 1979, I 463. [9] T. R. GILBERT,Proc. 1978 New England Conf. Current Deuei. Spectrochem. Excitation and Analysis, Wood’s Hole, MA 1978 Eno. Quart. (U.S.D.O.E. Envt. Lab), July 1979, I, 429. [lo] J. M. MERMET,Spectrochim. Acra 30B, 383 (1975). [ll] G. R. KORNBLUMand L. DE GALAN, Spectrochim. Acta 32B, 71 (1977). [12] D. J. KALNICKY, V. A. FASSEL and R. N. KNISELEY,Appi. Spectrosc. 31, 137 (1977). [13] P. W. J. M. BOUMANS and F. J. DE BOER, Spectrochim. Acta 32B, 365 (1977). [14] J. LEEMAN and W. ELLIOT, SpectraMetrics, Inc. Andover, MA., private communication. [15] P. W. J. M. BOUMANS, Theory of Spectrochemical Excitation. Hilger and Watts, London/Plenum, New York (1966). [16] P. N. KELIHER and C. C. WOHLERS, Anal. Chem. 48, 333A (1976). [17] P. N. KELIHER, Res. Dev. 27, (6), 26 (1976). [18] A. T. ZANDER and P. N. KELIHER, Appl. Spectrosc. 33, 499 (1979).
A study of enhancement
Table
1. Relative
emission
response
Emission Element*
Wavelength
Be1 AI1 VI Crl Mn II Fe II co1 Nl I GUI &I$ MoI Cd II TlI
(A)
2348.61 3961.52 4379.24 4254.35 2576.10 2599.40 3453.50 3414.73 3273.96 2138.56 3798.25 2144.38 5350.46 4057.83
Pb III
30%
effect
to salinity
response salinity
1.24 2.25 1.30 1.46 2.17 1.76 1.41 1.29 1.25 0.64 1.33 1.33 1.90 2.04
and concentration
at highest 50%
423
measured
salinity
variables
concentrationt 100%
Experimental errorf
salinity 1.44 2.84 1.22 1.37 2.82 2.18 1.49 1.35 1.24 0.77 1.35 1.64 2.18 3.64
1.35 2.42 1.20 1.36 2.39 1.90 1.42 1.30 1.24 0.70 1.26 1.47 1.95 2.41
* Concentrations: 0 ppb, 40 ppb, 200 ppb and 1 ppm unless noted otherwise. t Concentrations: 1 ppm except for Zn, normalized against 1 ppm at 0% salinity. $ Estimate of the intrinsic variability in measurement data obtained from ANOVA, § Zn concentrations: 0 ppb, 56 ppb, 167 ppb and 500 ppb. 11Pb concentrations: 0 ppb, 62.5 ppb, 250 ppb and 1 ppm.
0.0283 0.1374 0.0408 0.0406 0.0657 0.0534 0.0312 0.0374 0.0265 0.0183 0.0585 0.0337 0.0631 0.1828
&.
Instrumental parameters such as argon pressure (nebulizer, 1.4 bars; electrode sleeves, 3.45 bars) were adjusted to maximize the emission intensity of the line of interest with reasonable stability and then held constant. 2.2 Reagents” Artificial seawater was prepared using ultrapure chemicals from the Alfa Division of Ventron with concentrations of the major cations (Na, Mg, Ca, K, Sr and B) occurring in ocean water equivalent to the American Society of Testing and Materials (ASTM) Table
2. Enhancement
Element*t Be SC V Cr Mn Fe Fe co Ni CU Zn Mo$ Rh PdS Cd
differences
Wavelengths (A) Atom line Ion line 3130.42 3642.79 3093.115 2843.259 2576.10 2599.40 2599.40 2286.169 2316.049 2192.269: 2061.91 2816.158 2511.030 2488.928 2144.38
2348.61 3907.49 4379.240 4254.35 4030.76 2522.85 3440.61 3453.501 3414.738 3273.96 2138.56 3170.35 3692.36 2763.09 2288.02
* All transitions are s+p unless t Lines are resonance (to ground $ d +p transition for atom line.
(observed
and calculated) parameters
with
ion
and
atom
emission
lines
and
Unpaired d electrons (Lower atomic state)
Energy of atom Transition (cm-‘)
Y1 (Experimental)
Y1 (Calculated)
0 1 4 5 5 4 4 2 1 0 0 5 2 0 0
42,565 25,585 22,829 23,499 24,802 39,626 29,056 28,948 29,276 30,535 46,745 31,533 27,075 36,181 43,692
-0.01
-0.0138 0.8538 -1.3261 1.4133 1.3557 0.5834 1.0508 0.8219 0.6747 0.5181 -0.1986 1.0581 0.9047 0.2685 -0.0636
noted otherwise. state) unless noted
0.73 1.42 1.44 1.38 0.52 1.11 0.83 0.66 0.61 -0.04 1.04 0.70 0.48 -0.32
with 5.
* Editor’s nore: We discourage the use of the concentration units ppm and ppb if they refer to @g ml-’ and ng ml-’ in solutions rather than pg g-i or ng g-i m solids. In favour of speed of publication we sometimes connive at the misuse of ppm or ppb in articles appearing in this journal, if no confusion may arise.
D. EASTWOOD, M. S. HENDFUCK and G.
424
SGLIERO
procedure [19], using Na as NaCl, Mg as MgS0;7Hz0, Ca as CaC1;4H,O, K as KCl, Sr as Sr(NO,), and B as H,BO,. These major components were dissolved in deionized water (ASTM Type I) [20] and acidified to
[19]1978 Annual Book [20] [21] [22] [23] [24]
A study of enhancement
effect
425
Zn I 2138.56%
4.0r-
.g 3.0t I .G co :g 2.05 a9 .' z z ly i.o-
Metal concentration, Fig. 1. Relative emission at three levels of salinity. O-0-0
intensities of Zn 0.. . . 0.. . . . 0 100% salinity.
ppm
I 2138.56A vs Zn concentration 0% salinity, + --+ --+ 30% Standard prepared at 2 ppm.
in ppm salinity,
effects for the treatment combinations of 30, 50 and 100% salinity at the highest metal concentration. The last column shows the estimate of the experimental error, 6, for each element. This represents the amount of residual variation or “random variability” that remains after the variability due to salinity and metal concentration have been removed. The least squares estimates of the main effects and interactions for all treatment combinations, obtained from ANOVA, are used to calculate the points shown in the following examples. Figure 1 shows the effect of Zn concentration on relative emission intensities for three salinities of 0,30 and 100% for the 2138.56 w atomic emission line at the highest Zn concentration measured. Figure 2, for the same Zn line shows the effect of varying salinity at 4 different concentrations. Figures 3 and 4 represent plots of similar data for the 2576.10 A Mn ionic emission line employing the same variables. In this case, a much greater enhancement is observed. The reproducibility and accuracy of these experiments is reflected in the value of the experimental error, &, (Table 1, last column). Some elements, i.e. MO, Tl, Al, Pb, showed background and spectral interference at the selected emission lines. (For Al, as previously reported [25], by scanning over the spectral line using a small scanning Zn I 2138.568
I
40.0
I
I
fB.0 Salinity,
80.0
I
1
100.0
120.0
%
Fig. 2. Relative emission intensities of Zn I 2138.56A vs % of salinity of Zn concentration. 0.. . . 0.. . . . Cl Oppb, +-+-+ 56ppb, 0---O---O d - - - B - - - B 500 ppb. Standard prepared at 2 ppm. [25] M. S. HENDRICK, D. EASTWOOD and W. G. COX, Abstracts Spectrosc., Cleveland 1979. Abstract 053.
Pittsburgh
at four levels 167ppb,
Conf. on Anal.
Chem. and A@.
426
D. EASTWOOD, M. S. HENDRICK and G. SOGLIERO
Mn II 2576.108
0.4 0.6 Metal concentration, ppm
0.2
0.0
1.0
0.8
Fig. 3. Relative emission intensities of Mn II 2576.10 8, vs Mn concentration in ppm at three levels of salinity. 0 . . . . 0 . . . 0 0% salinity, + - - - + - - - + 30% salinity, and O-0-0 100% salinity. Standard prepared at 10 ppm.
motor and a Bascom-Turner recorder with a microprocessor to record the line shapes, the spectral interference had been found to be due to Ca). Even in cases of spectral interferences, results were consistent and it was generally possible to subtract out the background. Examining the data for all the preselected emission lines on the cassette, it was observed that the largest enhancement seemed to result for ionic emission lines for transition elements with unfilled d-subshells. This observation led to further experimentation which is described in Section 3.2. 3.2 Emission enhancemenr due to salinity for ion-atom
pairs
3.2.1 Experiments. Table 2 shows 15 carefully selected ion-atom emission line pairs for 14 elements that were studied at 100 and 50% salinity. These elements include open and closed shell transition metals; i.e. with 3d and 4d electrons, and also Be. The lines selected were strong lines, not complex or broad, with sufficient intensity for measurement at the l-5 ppm level. Where possible, the ion and atom lines for each element were selected to have similar electronic transitions, electronic states and energies. These choices explain why s+p transitions and resonance lines dominate; however, the study was not restricted to these lines, nor were all ion-atom pairs comparable. Mn II 2576.108
I 20.0
I
40.0
I
60.0 Salinty, %
I
I
80.0
100.0
Fig. 4. Relative emission intensities of Mn II 2576.10 8, vs % of salinity of Mn concentration. 0 . . . 0. . . 0 0 ppm + - + - + 40 ppb, O-O-O A - - - A - - - A 1 ppm. Standard prepared at 10 ppm.
120.0
at four levels 200 ppb, and
A study of enhancement
effect
427
3.2.2 Stepwide multiple regression analysis. The experimental values for YI are also listed in Table 2. Y, is defined as the difference in the relative emission enhancement at 100% salinity for ion-atom emission line pairs, normalized against the measured intensities for each line at 0% salinity. Thus Y 1=
I
--L
I
r7100%A 100% I I,O%
I A,O%
(1)
and is normalized so that I1,ooh= I,,o, = 1. Change in background and interferences were subtracted by means of blanks for both 0 and 100% salinity. The largest positive experimental values of Yr (for 100% salinity measured at 1 ppm concentration) occur for elements such as V, Cr and Mn, which have several unpaired d-electrons, while relatively small negative values are obtained for elements with filled d subshells such as Zn. Experimentally observed enhancements due to 100 and 50% salinities for ionic and atomic lines for elements with different numbers of unpaired d-electrons, are shown in Figs. 5 and 6. The atomic line shows greater enhancement than the ionic line for filled d-shell systems, such as Zn and Cd, and the opposite is true in unfilled d-shell systems. Y, represents the splitting (or difference) in these enhancements at 100% salinity. A rough plot (not shown) of Y, vs atomic number for the 3d and 4d elements, indicated an apparent parabolic relationship in position along a row of the periodic table corresponding to filling up of the d-subshell. This suggested the hypothesis that YI could be predicted by establishing a functional relationship between Yr and several variables, one of which might relate to the number of unpaired d-electrons. A number of variables, which for theoretical reasons could possibly contribute to the enhancement phenomenon, were then selected. These variables, using values from MOORE [26], MEGGERSet al. [27] and BOUMANS [15], were the following: XI X, X, X, X, X, X,
= ionization potential (ion), = ionization potential (atom), = reciprocal of square root of atomic weight, = excitation potential of upper ionic state, = energy of transition (ionic line), = total number of unpaired electrons (lower atomic state), = gAi transition probability (ionic line), &6000K x*= z 5ooo K (Z, = partition function of ion), x,=
Z:, 6000 K z 5ooo K (Z, = partition function of atom), A
XI0 = atomic number, XI1 = total number of unpaired d electrons in lower atomic state, X,, = total number of d electrons in unfilled subshell for lower atomic state, XI3 = energy of transition (atomic line). Three additional variables were initially considered: excitation potential of upper atomic state, total number of unpaired electrons (lower ionic state), and gA,, the transition probability (atomic line). These variables were found to be highly correlated with certain variables already in the set being considered and were dropped from further consideration at this time in order to reduce the amount of input data. Modeling the predicted value, Yr, as a linear combination of these variables; i.e.
P~=co+clx~+c*x*+... P61
+c13x13,
(2)
C. E. MOORE, Atomic Energy Levels, Vol. l-3. NSRDS-NBS 35. U.S. Government Printing Office (1971). [271W. F. MEGGERS, C. H. CORLISS and B. F. SCRIBNER, Tables of Spechal Lines Intensities, Part 1, Nat. Bur. Stand. (U.S.) Monogr. 145, U.S. Government Printing Office (1975).
428
D. EASTWOOD.M. S. HENDRICKand G. SOGLIERO
0
I
L
I
I
50
100
0
I
J
50
100
Salinity,
0
I
I
50
loo
%
Fig. 5. Relative emission intensities of ion and atom line pairs for Mn, Ni, and Zn at 50 and 100% salinity relative to 0% salinity. Mn I 4030.76 A, Mn II 2576.10 A, Ni I 3414.73 A, Ni II 2316.04& Zn I 2138.56A and Zn II 2061.91 A. O------O----0 Atom line A-A-h Ion line.
a systematic consideration of the effect of these variables upon the response variable was undertaken by using a stepwise multiple regression analysis.* As the name implies, the stepwise multiple regression brings in the independent variables, one at a time. The selection criteria for entering a variable is based upon the correlation between the independent variable and the response variable. As each new independent variable is brought into consideration, a new “least-squares” regression equation is formed, and the coefficient of determination, R*, is calculated, (Note: R* is a measure of the “goodness-of-fit.” It quantifies the proportion of variability in the original response variable that can be explained by the independent variables that are used in the regression equation.) As more independent variables are entered into the regression equation, the value of R* increases. Ideally, one would like R* to be high with as few independent variables as possible.
4.0 -
1 0
Rh
I
50
I
100 Salinity,
Fig. 6. Relative emission intensities of ion and atom line pairs for Rh and Cd at 50 and 100% salinity relative to 0% salinity. Rh I 3692.36 A, Rh II 2511.03 A, Cd I 2288.02 A and Cd II 2144.38 .&. 0 - - - 0 - - - 0 Atom line A-8-A Ion line. * This technique was implemented on the Data General NOVA 840 computer using an algorithm developed by EFROYMSON[28] and a modified version of a UNIVAC STAT PACK program. [28] M. A. EFROYMSON,Marhematical Methods for Digital Computers. (Edited by A. Ralston and H. S. Wilf), Volume 1. Chapter 17, p. 191. Wiley, New York.
A study of enhancement
Predicted
effect
Y,
Fig. 7. Plot of measured relative emission intensity, Yr, (100% salinity) vs the predicted relative emission intensity, 3,. Predicted values were obtained from a multiple regression equation. The independent variables used were: (1) the energy of transition (cm-‘), and (2) the number of unpaired d-electrons (lower atomic state).
Using the 15 experimental points of Y1, listed in Table 2, a coefficient of determination, R2, of 94.56% was obtained from the regression analysis with only two independent variables. The first one entering was X13, the energy of transition for the atomic line. It accounted for 80.71% of the variability in the data. With the introduction of the second variable, X1i, the total number of unpaired d-electrons in the lower atomic state, an R2 of 94.56% was achieved. The “least-squares” regression equation obtained was Y1 = 1.8682 + 0.1168 X1, - 0.4422 x 1O-4 X1,.
(3)
The predicted values, Y1,, calculated from (3) and plotted against the experimental values, Y1, are shown in Fig. 7. It should be noted that the total number of unpaired electrons, X,, was highly correlated with the total number of unpaired d-electrons, Xil, (both for the lower atomic state) for the transitions studied. For these variables, for the data used in this paper, r = 0.975, where r is the estimated correlation coefficient. Using the 15 experimental values at 50% salinity, a coefficient of determination of 92.61% was obtained with variables X, and Xi,. In this case, X, entered rather than Xii, since the total number of unpaired electrons was slightly more correlated to the experimental response variable at 50% salinity than was the total number of unpaired d-electrons. Therefore, these alternate highly correlated independent variables should not be excluded from further consideration. Attempts to obtain good predictive equations for response variables of the form I I,l~Oo~lII,Oo~ and I~,~O&Ia,Oo~ with the same experimental points and the same set of independent variables were not satisfactory. A combination of ten variables was required before R* reached values of 94 and 97% for these cases, respectively. This indicates that perhaps some key variables are missing, or the form of the variables should be altered. One additional form of the response variable, Y2, was considered; namely the ratio of
430
D. EASTWOOD,M. S. HENDRICKand G. SOGLIERO
The coefficient of determination in this case, with two variables was 88.73%, which represents a slower convergence than for the difference of these variables. The prediction equation obtained is: pZ = 2.1936 + 0.0943 X, - 0.2855 x 1O-4 X,, .
(4)
4. DISCUSSION The main result is that in normal seawater using a d.c. plasma emission jet, the difference in relative enhancement of ion-atom line pairs of 14 elements, can be reasonably predicted by using a regression equation with only two variables, one of which-the total number of unpaired d-electrons-had apparently not been previously considered. BOUMANS [29] has suggested that our results might substantiate that the d.c. plasma considered in this work is a non-LTE source. Our results do not permit, however, to draw conclusions as to the mechanisms involved, e.g. the Penning ionization mechanism as suggested by MERMET [30] or the auto-ionization mechanism as suggested by LOVER [31] both for argon ICPs. Although the results presented here do not require a transport mechanism and do not rule out a Penning ionization mechanism involving argon metastable atoms as intermediates, further explanation of the theoretical mechanism must wait for later work. Also, an understanding of the enhancement of the ion and atom lines separately in seawater or other alkali metal matrices will probably involve additional variables not considered thus far . 5. CONCLUSIONS
Enhancement due to salinity and concentration has been systematically investigated for 14 elements using d.c. argon plasma emission spectrometry. Progress has been made in understanding and predicting the difference in the relative enhancement due to salinity of ionic and atomic emission lines. This should permit the development of improved analytic procedures by appropriate matrix corrections and choices of internal standards. Further experimental and theoretical studies are underway to investigate the general mechanism for enhancement by alkali metals in d.c. argon plasmas. If predictable, this enhancement could become an advantage by improving sensitivity and lowering detection limits. Acknowledgements-The participation of JOHN LEEMANof Rob Roy Associates, WALTERCox of the Naval Underwater System Center, Newport, Rhode Island and ALEX SCHEELINEof the National Bureau of Standards and University of Iowa in stimulating technical discussions relating to experimental and theoretical aspects of this work is gratefully acknowledged by the authors. Portions of this work were presented at the XXIst Colloquium Spectroscopicum International and 8th International Conference on Atomic Spectroscopy, Cambridge, U.K. July 1979. [29] P. W. J. M. BOUMANS,Philips Research Laboratories, 5600 MD Eindhoven, The Netherlands, private communication. [30] J. M. MERMET,C. R. Acad. Sci. Ser. B 281,273 (1975). [31] R. J. LOVES, Autoionizing Lines in ICP Emission Spectromerry. Abstracts 6th Annual Meeting FACSS, Philadelphia 1979, Abstract No. 052.
Applications of d.c. argon plasma emission spectroscopy saline waters: a study of enhancement effects* DELYLEEASTWOOD,MARTHASCHULZHENDRICK and GENES• U.S.Coast Guard Research and Development Center Groton, CT 06340, U.S.A.
to
GLIERO
(Received 16 January 1980) Abstract-A commercially available d.c. argon plasma emission spectrometer was used to determine transition metals (3d and 4d) and also Be in salt and brackish water. The effects of salinity on the enhancement of emission intensities of the analyte lines were studied using an empirical approach combined with statistical analysis. One set of experiments deals with the effects of trace metal concentration and salinity on the relative emission intensities of 14 elements using a completely randomized experimental design, i.e. the sequence in which the 48 treatment combinations (12 levels of salinity and 4 levels of metal concentration) were measured was determined by randomization, with the results evaluated by an analysis of variance program. The second set deals with the effect of relative enhancement to salinity on selected ion and atom emission line pairs for various elements at 100 and 50% salinity relative to fresh water (0% salinity). These results are analyzed by a stepwise linear multiple regression analysis program using selected parameters of theoretical interest. It was discovered that the differences in relative enhancement for ion-atom emission line pairs for 100% salinity could be predicted basically with two variables. A coefficient of determination of 95% was achieved by employing the energy of transition for the atomic line and the number of unpaired d-electrons in the lower atomic state.
1. I~R~DU~I~N DIRECTCURRENT (d.c.) argon plasma emission spectroscopy, (DCP), like other plasma emission techniques, has gained considerable attention [l-5] in recent years as an alternate method of analysis to atomic absorption. A commercially available d.c. argon plasma emission spectrometer with tunable echelle monochromator which can be used either sequentially or simultaneously for up to 19 elements at a time has proven useful for a variety of sample types including sea water. The U.S. Coast Guard has a need for a reliable method to detect, identify and quantify hazardous chemicals and oils resulting from transportation-related spills. Since many of these spills involve sea or brackish water samples of variable salinity, the effect of salinity on the analytical methodology for trace element analysis becomes important. Enhancement effects due to alkali metals in sea water samples have been observed in d.c. plasma emission spectroscopy. Such effects, while enhancing the sensitivity of the method, may require matching matrices of samples, blanks and standards, using standard addition techniques or internal standards or using excitation buffers to compensate for alkali metal ions present in samples with unknown matrices. An understanding of the magnitude of the enhancement factor which will vary with element and emission line selected and also of the basic mechanism resulting in the enhancement will help in correcting for this effect. Earlier studies [6, 71 on a few elements using a 2-electrode d.c. argon plasma jet seemed to indicate a plateauing effect which allowed a lithium buffer to be used to *Part of this paper was presented Cambridge (1979). [l] [2] [3] [4] [S] [6]
at the 21st Coll. Spectr. Int. and 8th Int. Conf. Atomic Spectr.,
R. A. BURDO and M. L. SNYDER,Anal. Cbem. 51, 1.502 (1979). G. W. JOHNSON,H. E. TAYLORand R. K. SKOGERBOE,Appl. Specfrosc. 33, 451 (1979). H. L. FELKEL, JR. and H. L. PARDUE, Anal. Chem. 50, 602 (1978). R. K. SKOGERBOE, I.T. URASA and G. N. COLEMAN,Appl. Specnosc. 30, 500 (1976). D. W. GOLIGHTLY and J. L. HARRIS, Appl. Specrrosc. 29, 233 (1975). W. G. Cox, Proc. Int. Conf. Enu. Sensing and Assessment, Las Vegas 1975, Vol. 2, Session 4, p. 6. Las Vegas, Nevada, U.S.A. (1975). [7] M. D. SANDSand W. G. Cox, Preptints 3rd Annual Meeting FACSS, joint with 19th Coil. Spectrosc. 1~. and 6th Int. Cont. Atomic Spectrosc., Philadelphia 1976. Abstr. 100. 421
422
D. EASTWOOD,M. S. HENDRICK and G. SOGLIERO
compensate for unknown salt concentrations. More recently, a spectrometer employing a 3-electrode jet, which seemed to produce a “hotter” plasma, was used for measurements on a larger number of elements [8, 91. These experiments indicated that the enhancement effect due to alkali metals was not a simple phenomenon so that compensation with a lithium or similar alkali metal buffer to reach a plateau region was not always practical. Minor constituents of seawater such as Ca may cause spectral interference either directly or indirectly (through stray light or ghosts) but this is a relatively straightforward phenomenon and will not be further discussed here. Various authors [lO-131 indicated that the inductively coupled plasma (ICP) results indicate lack of local thermal equilibrium (LTE). Some scientists [14] have suggested that different methods of measuring temperature in the DCP give different results and that these conflicting observations indicate that the DCP also shows lack of LTE. Therefore, in describing a mechanism for the enhancement we cannot think in terms of temperature and electron density; consequently, the theory as given in BOUMAN’S book [15] does not apply. In this paper, we take an empirical approach to the problem. By a stepwise process, we hope eventually to elucidate the underlying mechanism of enhancement by alkali metals or at least to be able to predict the results well enough to correct to desired precision. Experiments to be described in this paper fall into two sets. The first set deals with the effect of trace metal concentration and salinity on the relative emission intensities of 14 elements using a completely randomized experimental design i.e. the sequence in which the 48 treatment combinations (12 levels of salinity and 4 levels of metal concentration) were measured was determined by randomization, with the results evaluated by an analysis of variance program (ANOVA). The second set deals with the effect of relative enhancement due to salinity on selected ion and atom emission line pairs for various elements at 100 and 50% salinity relative to 0% salinity. These results are analyzed by a stepwise linear multiple regression analysis program using appropriate selected parameters of theoretical interest. The results of the regression analysis are particularly interesting since it was established that the differences in relative enhancement for ion-atom emission line pairs could be predicted basically from only two parameters. 2.
APPARATUS AND TECHNIQUES
2.1 Instrumentation Instrumentation was the SpectraMetrics SpectraSpanR III d.c. argon plasma emission echelle spectrometer [16-181 with 3-electrode plasma jet. This spectrometer can be used to quantitate either simultaneously, with a multi-element capability of up to 19 elements with a cassette of preselected lines (see Table l), or sequentially by peaking on the emission line of interest (see Table 2). Argon was welder’s grade from Linde. [8] D. EASTWOOD,M. S. HENDRICK,S. CALLAWAY and W. G. Cox, Proc. 1978 New England Conf. Current Deuei. Spectrochem. Excitation and Analysis, Woods Hole, MA 1978 Enu. Quart. (U.S.D.O.E. Envtl. Lab), July 1979, I 463. [9] T. R. GILBERT,Proc. 1978 New England Conf. Current Deuei. Spectrochem. Excitation and Analysis, Wood’s Hole, MA 1978 Eno. Quart. (U.S.D.O.E. Envt. Lab), July 1979, I, 429. [lo] J. M. MERMET,Spectrochim. Acra 30B, 383 (1975). [ll] G. R. KORNBLUMand L. DE GALAN, Spectrochim. Acta 32B, 71 (1977). [12] D. J. KALNICKY, V. A. FASSEL and R. N. KNISELEY,Appi. Spectrosc. 31, 137 (1977). [13] P. W. J. M. BOUMANS and F. J. DE BOER, Spectrochim. Acta 32B, 365 (1977). [14] J. LEEMAN and W. ELLIOT, SpectraMetrics, Inc. Andover, MA., private communication. [15] P. W. J. M. BOUMANS, Theory of Spectrochemical Excitation. Hilger and Watts, London/Plenum, New York (1966). [16] P. N. KELIHER and C. C. WOHLERS, Anal. Chem. 48, 333A (1976). [17] P. N. KELIHER, Res. Dev. 27, (6), 26 (1976). [18] A. T. ZANDER and P. N. KELIHER, Appl. Spectrosc. 33, 499 (1979).
A study of enhancement
Table
1. Relative
emission
response
Emission Element*
Wavelength
Be1 AI1 VI Crl Mn II Fe II co1 Nl I GUI &I$ MoI Cd II TlI
(A)
2348.61 3961.52 4379.24 4254.35 2576.10 2599.40 3453.50 3414.73 3273.96 2138.56 3798.25 2144.38 5350.46 4057.83
Pb III
30%
effect
to salinity
response salinity
1.24 2.25 1.30 1.46 2.17 1.76 1.41 1.29 1.25 0.64 1.33 1.33 1.90 2.04
and concentration
at highest 50%
423
measured
salinity
variables
concentrationt 100%
Experimental errorf
salinity 1.44 2.84 1.22 1.37 2.82 2.18 1.49 1.35 1.24 0.77 1.35 1.64 2.18 3.64
1.35 2.42 1.20 1.36 2.39 1.90 1.42 1.30 1.24 0.70 1.26 1.47 1.95 2.41
* Concentrations: 0 ppb, 40 ppb, 200 ppb and 1 ppm unless noted otherwise. t Concentrations: 1 ppm except for Zn, normalized against 1 ppm at 0% salinity. $ Estimate of the intrinsic variability in measurement data obtained from ANOVA, § Zn concentrations: 0 ppb, 56 ppb, 167 ppb and 500 ppb. 11Pb concentrations: 0 ppb, 62.5 ppb, 250 ppb and 1 ppm.
0.0283 0.1374 0.0408 0.0406 0.0657 0.0534 0.0312 0.0374 0.0265 0.0183 0.0585 0.0337 0.0631 0.1828
&.
Instrumental parameters such as argon pressure (nebulizer, 1.4 bars; electrode sleeves, 3.45 bars) were adjusted to maximize the emission intensity of the line of interest with reasonable stability and then held constant. 2.2 Reagents” Artificial seawater was prepared using ultrapure chemicals from the Alfa Division of Ventron with concentrations of the major cations (Na, Mg, Ca, K, Sr and B) occurring in ocean water equivalent to the American Society of Testing and Materials (ASTM) Table
2. Enhancement
Element*t Be SC V Cr Mn Fe Fe co Ni CU Zn Mo$ Rh PdS Cd
differences
Wavelengths (A) Atom line Ion line 3130.42 3642.79 3093.115 2843.259 2576.10 2599.40 2599.40 2286.169 2316.049 2192.269: 2061.91 2816.158 2511.030 2488.928 2144.38
2348.61 3907.49 4379.240 4254.35 4030.76 2522.85 3440.61 3453.501 3414.738 3273.96 2138.56 3170.35 3692.36 2763.09 2288.02
* All transitions are s+p unless t Lines are resonance (to ground $ d +p transition for atom line.
(observed
and calculated) parameters
with
ion
and
atom
emission
lines
and
Unpaired d electrons (Lower atomic state)
Energy of atom Transition (cm-‘)
Y1 (Experimental)
Y1 (Calculated)
0 1 4 5 5 4 4 2 1 0 0 5 2 0 0
42,565 25,585 22,829 23,499 24,802 39,626 29,056 28,948 29,276 30,535 46,745 31,533 27,075 36,181 43,692
-0.01
-0.0138 0.8538 -1.3261 1.4133 1.3557 0.5834 1.0508 0.8219 0.6747 0.5181 -0.1986 1.0581 0.9047 0.2685 -0.0636
noted otherwise. state) unless noted
0.73 1.42 1.44 1.38 0.52 1.11 0.83 0.66 0.61 -0.04 1.04 0.70 0.48 -0.32
with 5.
* Editor’s nore: We discourage the use of the concentration units ppm and ppb if they refer to @g ml-’ and ng ml-’ in solutions rather than pg g-i or ng g-i m solids. In favour of speed of publication we sometimes connive at the misuse of ppm or ppb in articles appearing in this journal, if no confusion may arise.
D. EASTWOOD, M. S. HENDFUCK and G.
424
SGLIERO
procedure [19], using Na as NaCl, Mg as MgS0;7Hz0, Ca as CaC1;4H,O, K as KCl, Sr as Sr(NO,), and B as H,BO,. These major components were dissolved in deionized water (ASTM Type I) [20] and acidified to
[19]1978 Annual Book [20] [21] [22] [23] [24]
A study of enhancement
effect
425
Zn I 2138.56%
4.0r-
.g 3.0t I .G co :g 2.05 a9 .' z z ly i.o-
Metal concentration, Fig. 1. Relative emission at three levels of salinity. O-0-0
intensities of Zn 0.. . . 0.. . . . 0 100% salinity.
ppm
I 2138.56A vs Zn concentration 0% salinity, + --+ --+ 30% Standard prepared at 2 ppm.
in ppm salinity,
effects for the treatment combinations of 30, 50 and 100% salinity at the highest metal concentration. The last column shows the estimate of the experimental error, 6, for each element. This represents the amount of residual variation or “random variability” that remains after the variability due to salinity and metal concentration have been removed. The least squares estimates of the main effects and interactions for all treatment combinations, obtained from ANOVA, are used to calculate the points shown in the following examples. Figure 1 shows the effect of Zn concentration on relative emission intensities for three salinities of 0,30 and 100% for the 2138.56 w atomic emission line at the highest Zn concentration measured. Figure 2, for the same Zn line shows the effect of varying salinity at 4 different concentrations. Figures 3 and 4 represent plots of similar data for the 2576.10 A Mn ionic emission line employing the same variables. In this case, a much greater enhancement is observed. The reproducibility and accuracy of these experiments is reflected in the value of the experimental error, &, (Table 1, last column). Some elements, i.e. MO, Tl, Al, Pb, showed background and spectral interference at the selected emission lines. (For Al, as previously reported [25], by scanning over the spectral line using a small scanning Zn I 2138.568
I
40.0
I
I
fB.0 Salinity,
80.0
I
1
100.0
120.0
%
Fig. 2. Relative emission intensities of Zn I 2138.56A vs % of salinity of Zn concentration. 0.. . . 0.. . . . Cl Oppb, +-+-+ 56ppb, 0---O---O d - - - B - - - B 500 ppb. Standard prepared at 2 ppm. [25] M. S. HENDRICK, D. EASTWOOD and W. G. COX, Abstracts Spectrosc., Cleveland 1979. Abstract 053.
Pittsburgh
at four levels 167ppb,
Conf. on Anal.
Chem. and A@.
426
D. EASTWOOD, M. S. HENDRICK and G. SOGLIERO
Mn II 2576.108
0.4 0.6 Metal concentration, ppm
0.2
0.0
1.0
0.8
Fig. 3. Relative emission intensities of Mn II 2576.10 8, vs Mn concentration in ppm at three levels of salinity. 0 . . . . 0 . . . 0 0% salinity, + - - - + - - - + 30% salinity, and O-0-0 100% salinity. Standard prepared at 10 ppm.
motor and a Bascom-Turner recorder with a microprocessor to record the line shapes, the spectral interference had been found to be due to Ca). Even in cases of spectral interferences, results were consistent and it was generally possible to subtract out the background. Examining the data for all the preselected emission lines on the cassette, it was observed that the largest enhancement seemed to result for ionic emission lines for transition elements with unfilled d-subshells. This observation led to further experimentation which is described in Section 3.2. 3.2 Emission enhancemenr due to salinity for ion-atom
pairs
3.2.1 Experiments. Table 2 shows 15 carefully selected ion-atom emission line pairs for 14 elements that were studied at 100 and 50% salinity. These elements include open and closed shell transition metals; i.e. with 3d and 4d electrons, and also Be. The lines selected were strong lines, not complex or broad, with sufficient intensity for measurement at the l-5 ppm level. Where possible, the ion and atom lines for each element were selected to have similar electronic transitions, electronic states and energies. These choices explain why s+p transitions and resonance lines dominate; however, the study was not restricted to these lines, nor were all ion-atom pairs comparable. Mn II 2576.108
I 20.0
I
40.0
I
60.0 Salinty, %
I
I
80.0
100.0
Fig. 4. Relative emission intensities of Mn II 2576.10 8, vs % of salinity of Mn concentration. 0 . . . 0. . . 0 0 ppm + - + - + 40 ppb, O-O-O A - - - A - - - A 1 ppm. Standard prepared at 10 ppm.
120.0
at four levels 200 ppb, and
A study of enhancement
effect
427
3.2.2 Stepwide multiple regression analysis. The experimental values for YI are also listed in Table 2. Y, is defined as the difference in the relative emission enhancement at 100% salinity for ion-atom emission line pairs, normalized against the measured intensities for each line at 0% salinity. Thus Y 1=
I
--L
I
r7100%A 100% I I,O%
I A,O%
(1)
and is normalized so that I1,ooh= I,,o, = 1. Change in background and interferences were subtracted by means of blanks for both 0 and 100% salinity. The largest positive experimental values of Yr (for 100% salinity measured at 1 ppm concentration) occur for elements such as V, Cr and Mn, which have several unpaired d-electrons, while relatively small negative values are obtained for elements with filled d subshells such as Zn. Experimentally observed enhancements due to 100 and 50% salinities for ionic and atomic lines for elements with different numbers of unpaired d-electrons, are shown in Figs. 5 and 6. The atomic line shows greater enhancement than the ionic line for filled d-shell systems, such as Zn and Cd, and the opposite is true in unfilled d-shell systems. Y, represents the splitting (or difference) in these enhancements at 100% salinity. A rough plot (not shown) of Y, vs atomic number for the 3d and 4d elements, indicated an apparent parabolic relationship in position along a row of the periodic table corresponding to filling up of the d-subshell. This suggested the hypothesis that YI could be predicted by establishing a functional relationship between Yr and several variables, one of which might relate to the number of unpaired d-electrons. A number of variables, which for theoretical reasons could possibly contribute to the enhancement phenomenon, were then selected. These variables, using values from MOORE [26], MEGGERSet al. [27] and BOUMANS [15], were the following: XI X, X, X, X, X, X,
= ionization potential (ion), = ionization potential (atom), = reciprocal of square root of atomic weight, = excitation potential of upper ionic state, = energy of transition (ionic line), = total number of unpaired electrons (lower atomic state), = gAi transition probability (ionic line), &6000K x*= z 5ooo K (Z, = partition function of ion), x,=
Z:, 6000 K z 5ooo K (Z, = partition function of atom), A
XI0 = atomic number, XI1 = total number of unpaired d electrons in lower atomic state, X,, = total number of d electrons in unfilled subshell for lower atomic state, XI3 = energy of transition (atomic line). Three additional variables were initially considered: excitation potential of upper atomic state, total number of unpaired electrons (lower ionic state), and gA,, the transition probability (atomic line). These variables were found to be highly correlated with certain variables already in the set being considered and were dropped from further consideration at this time in order to reduce the amount of input data. Modeling the predicted value, Yr, as a linear combination of these variables; i.e.
P~=co+clx~+c*x*+... P61
+c13x13,
(2)
C. E. MOORE, Atomic Energy Levels, Vol. l-3. NSRDS-NBS 35. U.S. Government Printing Office (1971). [271W. F. MEGGERS, C. H. CORLISS and B. F. SCRIBNER, Tables of Spechal Lines Intensities, Part 1, Nat. Bur. Stand. (U.S.) Monogr. 145, U.S. Government Printing Office (1975).
428
D. EASTWOOD.M. S. HENDRICKand G. SOGLIERO
0
I
L
I
I
50
100
0
I
J
50
100
Salinity,
0
I
I
50
loo
%
Fig. 5. Relative emission intensities of ion and atom line pairs for Mn, Ni, and Zn at 50 and 100% salinity relative to 0% salinity. Mn I 4030.76 A, Mn II 2576.10 A, Ni I 3414.73 A, Ni II 2316.04& Zn I 2138.56A and Zn II 2061.91 A. O------O----0 Atom line A-A-h Ion line.
a systematic consideration of the effect of these variables upon the response variable was undertaken by using a stepwise multiple regression analysis.* As the name implies, the stepwise multiple regression brings in the independent variables, one at a time. The selection criteria for entering a variable is based upon the correlation between the independent variable and the response variable. As each new independent variable is brought into consideration, a new “least-squares” regression equation is formed, and the coefficient of determination, R*, is calculated, (Note: R* is a measure of the “goodness-of-fit.” It quantifies the proportion of variability in the original response variable that can be explained by the independent variables that are used in the regression equation.) As more independent variables are entered into the regression equation, the value of R* increases. Ideally, one would like R* to be high with as few independent variables as possible.
4.0 -
1 0
Rh
I
50
I
100 Salinity,
Fig. 6. Relative emission intensities of ion and atom line pairs for Rh and Cd at 50 and 100% salinity relative to 0% salinity. Rh I 3692.36 A, Rh II 2511.03 A, Cd I 2288.02 A and Cd II 2144.38 .&. 0 - - - 0 - - - 0 Atom line A-8-A Ion line. * This technique was implemented on the Data General NOVA 840 computer using an algorithm developed by EFROYMSON[28] and a modified version of a UNIVAC STAT PACK program. [28] M. A. EFROYMSON,Marhematical Methods for Digital Computers. (Edited by A. Ralston and H. S. Wilf), Volume 1. Chapter 17, p. 191. Wiley, New York.
A study of enhancement
Predicted
effect
Y,
Fig. 7. Plot of measured relative emission intensity, Yr, (100% salinity) vs the predicted relative emission intensity, 3,. Predicted values were obtained from a multiple regression equation. The independent variables used were: (1) the energy of transition (cm-‘), and (2) the number of unpaired d-electrons (lower atomic state).
Using the 15 experimental points of Y1, listed in Table 2, a coefficient of determination, R2, of 94.56% was obtained from the regression analysis with only two independent variables. The first one entering was X13, the energy of transition for the atomic line. It accounted for 80.71% of the variability in the data. With the introduction of the second variable, X1i, the total number of unpaired d-electrons in the lower atomic state, an R2 of 94.56% was achieved. The “least-squares” regression equation obtained was Y1 = 1.8682 + 0.1168 X1, - 0.4422 x 1O-4 X1,.
(3)
The predicted values, Y1,, calculated from (3) and plotted against the experimental values, Y1, are shown in Fig. 7. It should be noted that the total number of unpaired electrons, X,, was highly correlated with the total number of unpaired d-electrons, Xil, (both for the lower atomic state) for the transitions studied. For these variables, for the data used in this paper, r = 0.975, where r is the estimated correlation coefficient. Using the 15 experimental values at 50% salinity, a coefficient of determination of 92.61% was obtained with variables X, and Xi,. In this case, X, entered rather than Xii, since the total number of unpaired electrons was slightly more correlated to the experimental response variable at 50% salinity than was the total number of unpaired d-electrons. Therefore, these alternate highly correlated independent variables should not be excluded from further consideration. Attempts to obtain good predictive equations for response variables of the form I I,l~Oo~lII,Oo~ and I~,~O&Ia,Oo~ with the same experimental points and the same set of independent variables were not satisfactory. A combination of ten variables was required before R* reached values of 94 and 97% for these cases, respectively. This indicates that perhaps some key variables are missing, or the form of the variables should be altered. One additional form of the response variable, Y2, was considered; namely the ratio of
430
D. EASTWOOD,M. S. HENDRICKand G. SOGLIERO
The coefficient of determination in this case, with two variables was 88.73%, which represents a slower convergence than for the difference of these variables. The prediction equation obtained is: pZ = 2.1936 + 0.0943 X, - 0.2855 x 1O-4 X,, .
(4)
4. DISCUSSION The main result is that in normal seawater using a d.c. plasma emission jet, the difference in relative enhancement of ion-atom line pairs of 14 elements, can be reasonably predicted by using a regression equation with only two variables, one of which-the total number of unpaired d-electrons-had apparently not been previously considered. BOUMANS [29] has suggested that our results might substantiate that the d.c. plasma considered in this work is a non-LTE source. Our results do not permit, however, to draw conclusions as to the mechanisms involved, e.g. the Penning ionization mechanism as suggested by MERMET [30] or the auto-ionization mechanism as suggested by LOVER [31] both for argon ICPs. Although the results presented here do not require a transport mechanism and do not rule out a Penning ionization mechanism involving argon metastable atoms as intermediates, further explanation of the theoretical mechanism must wait for later work. Also, an understanding of the enhancement of the ion and atom lines separately in seawater or other alkali metal matrices will probably involve additional variables not considered thus far . 5. CONCLUSIONS
Enhancement due to salinity and concentration has been systematically investigated for 14 elements using d.c. argon plasma emission spectrometry. Progress has been made in understanding and predicting the difference in the relative enhancement due to salinity of ionic and atomic emission lines. This should permit the development of improved analytic procedures by appropriate matrix corrections and choices of internal standards. Further experimental and theoretical studies are underway to investigate the general mechanism for enhancement by alkali metals in d.c. argon plasmas. If predictable, this enhancement could become an advantage by improving sensitivity and lowering detection limits. Acknowledgements-The participation of JOHN LEEMANof Rob Roy Associates, WALTERCox of the Naval Underwater System Center, Newport, Rhode Island and ALEX SCHEELINEof the National Bureau of Standards and University of Iowa in stimulating technical discussions relating to experimental and theoretical aspects of this work is gratefully acknowledged by the authors. Portions of this work were presented at the XXIst Colloquium Spectroscopicum International and 8th International Conference on Atomic Spectroscopy, Cambridge, U.K. July 1979. [29] P. W. J. M. BOUMANS,Philips Research Laboratories, 5600 MD Eindhoven, The Netherlands, private communication. [30] J. M. MERMET,C. R. Acad. Sci. Ser. B 281,273 (1975). [31] R. J. LOVES, Autoionizing Lines in ICP Emission Spectromerry. Abstracts 6th Annual Meeting FACSS, Philadelphia 1979, Abstract No. 052.