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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
Precise Determination of Stoichiometries for Heteropoly Complexes by Inductively Coupled Plasma Emission Spectrometry Manahen A. Fernandez and Glenn J. Bastiaans" Department of Chemistry, Georgetown University, Washington, D.C. 20057
Much of the poor precision and accuracy obtained from the analysis of complex samples with classical gravimetric, titrimetric, and colorimetric methods is due to the fact that the molecular form of the analyte and matrix components influence analytical results. Such effects have been observed for As (7, B), Mo (91,P (1&14), and W (11--13),all the elements which we are dealing with in our study. The extensive and time consuming sample handling procedures often necessary with classical methods also degrade piecision and accuracy. Representative relative standard deviations for such methods have been given as 3-14% for arsine based As analyses (15-19) and 3-7% for Mo (9). Such levels of precision are inadequate for our application. In contrast the molecular form of the analyte and specific sample matrix components have been found to have very little effect on the ICPES analyses of As (20,21),P (22, 23), Mo, and i T (24). Detection limits of less than 0.05 pg/mL were reported for all of these elements. Such high sensitivity derives from the fact that these elements are more efficiently atomized and excited within the plasma than in flames and furnaces. Sample handling procedures were quite minimal as well. In view of such advantages, significant improvements in the precision of elemental analysis should be achievable via ICPES. The analyses of the heteropoly complexes described below indicate the degree of precision we have been able t o achieve using ICPES with a special standardization procedure.
Inductively coupled plasma (ICP) emission spectrometry offers a precise method of analysis for determining the elemental stoichiometries of a class of complex inorganic ions, known as heteropolymolybdates and tungstates, containing Mo, W, As, and P. The precision of the new method compares very favorably with conventional colorimetric determinations of stoichiometry for these compounds. To attain the desired precision, a special two-step standardization procedure was employed. Several types of systematic errors of analysis were encountered. The probable causes of these interferences as well as methods for eliminating their effects are discussed. Results of this study indicate that precise as well as sensitive analyses are possible with ICP emission spectrometry.
I n t h e majority of applications of atomic spectrometry to t h e analysis of metals, optimization of the sensitivity of analysis is given greatest importance. There are many cases, however, where accuracy and precision of analysis must be given primary consideration. An example of the latter case is the analysis of complex inorganic compounds for two or more elements whose correct stoichiometry is unknown and must be determined. T h e problem to the analyst in this situation is to distinguish between a series of possible elemental ratios which conceivably may differ by as little as 6%. Superior accuracy and precision of analysis is obviously required. In this paper a procedure for accurately and precisely determining the As, P, Mo, and W stoichiometry for a series of heteropolyanions using inductively coupled plasma emission spectrometry (ICPES) will be described in order to demonstrate the precision and accuracy inherent in this method of analysis. Heteropolymolybdates and heteropolytungstates are interesting classes of inorganic ions which may commonly contain 6 t o 18 parent atoms (Mo or W)in a single acid or salt molecule in addition to one, two or more heteroatoms such as As, P, and many others ( 2 ) . The assignment of the correct parent atom t o heteroatom ratio of newly synthesized compounds of this type can be difficult in disadvantageous cases, because the collective confidence limit of the analyses for the individual elements in the heteropolyanions must be less than the relative differences between the possible stoichiometries. For example, a choice between stoichiometries of 17:2 or 18:2 involves an elemental ratio difference of only 5.6'70. Commercial analyses of these compounds for As and P have been inadequate because of results varying as much as 30% from expected values (2). In the past, structures and molecular formulas have been confirmed by X-ray crystallographic analysis (3+). Crystallographic procedures are, of course. time consuming, especially if molecular formulas are not known with certainty. In certain cases satisfactory crystals for this type of determination are not obtainable and other means of analysis are required. 0003-2700/79/0351-1402$01.00/0
EXPERIMENTAL Apparatus. The individual components of the equipment used in this study are listed in Table I. The gas flow control system originally obtained with the plasma torch was judged to be unsatisfactory. Two flow controllers were installed to improve control over the flow rates of the plasma and cooling gas lines. The nebulizer/aerosol gas line along with the plasma and cooling gas lines were equipped with more accurate flow meters. An additional modification consisted of cutting an extra observation port in the rear of the chassis of the matching port module. The overall plasma unit (torch assembly and matching network) was mounted on a milling table driven by a stepping motor in order to provide for accurate horizontal positioning of the torch. Vertical positioning was attained by separating the torch assembly (torch, coil, insulating blocks, and nebulizer chamber) from the unit box and mounting it on an aluminum plate which was in turn attached to a vertical translator mounted on the unit box. Emitted radiation was observed by focusing the radiation on the entrance slit of the monochromator with a single lens using 1:l magnification. The operating conditions employed in this work were: incident power. 1.4 kW; reflected power, < 2 1%'; Ar flow, coolant, 12.5 L/min. plasma, 0.64 L/min, nebulizer, 0.62 L/min; sample aspiration rate, 1.0 m l j m i n . Both monochromator slits were set at 50 pm. The observation height was optimized for each analyte. Reagents. Stock solutions containing 1000 pg/mL of As, P, LV. or Mo were prepared by dissolution of analytical reagent grade As20,+NH4H2P0,,H,WO,, and (NH4)6hfoi024.6H20, respectively, in deionized water. For water insoluble compounds, base was added as the minimum required amount of concentrated ",OH or as 0.01 N NaOH for samples involving Na salts. C
1979 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
Table I. Experimental Components Employed description
item R - F generator matching network plasma torch
Type HFP-1500 D, Plasma-Therm Inc., (Route 73, Kresson, N . J . 08052) Type MN-1500 E, Plasma-Therm Inc.
Model PT-1500, Plasma-Therm Inc. plus a second torch of similar dimensions, made in-house automatic power APCS-3, Plasma-Therm Inc. 76-mm F/3 S I Quartz Lens, ESCO external optics Optics monochromator 0.5-m Czerny-Turner, SPEX Industries (Box 798, Metuchen, N.J. 08840) Photomultiplier Tube, type 4840 detector (RCA) Model 82-375 Jarrell-Ash Co. power/supply (Waltham, Mass.) current pre410 Micro-Microammeter, Keithley Instruments Inc. (Cleveland, Ohio) amplifier digital voltmeter 5328A Universal Counter, HewlettPackard (Santa Clara, Calif. 95050) recorder Model MR, Sargent-Welch (Skokie, Ill.) flow meters 150-mm Series 7600, Matheson Gas Products flow controllers Series 8287, 8289, Matheson Gas Products torch translator Unislide A2504, Velmex Inc. (Holcomb, N.Y.) x-y, Milling Table, 023-700-c plasma unit (Ralmike’s, Plainfield, N.J. translator 07080) stepping motors Slow-Syn, M092-FC08 and MD63FC06, Superior Electric Co. (Bristol, Conn.)
Signal Measurement. Emission intensities were measured with an integrating digital voltmeter connected to the output of the photomultiplier preamplifier. Intensities were recorded as the mean of five 10-s integrations from the DVM. A blank using deionized water was made under the Same conditions before and after the aspiration of each set of sample and standard. RESULTS AND DISCUSSION Sample Analysis. Because of the interference effects discussed below, a two-step calibration procedure was employed for the analyses of the heteropolyanions. I n the first step, a sample bracketing technique (25) was used to assign estimated concentrations of analytes to t h e sample. Here, standard solutions containing a single element and no matrix were employed a t signal levels above and below t h a t of t h e sample. In t h e second step, a comparison standard was prepared containing t h e analytes of interest, base, and ap-
propriate matrix components a t approximately t h e same concentrations as the s a m d e . Final s a m d e concentrations were then obtained by direct comparison between the second standard and the sample signal levels. The use of the special comparison standards was found to compensate for the systematic errors created by the interference effects encountered in the ICP system. Results of the analyses of several heteropolymolybdates and -tungstates containing As are reported in Table 11. Precision levels, expressed as 95% confidence limits, are listed for t h e experimentally determined elemental ratios. T h e stoichiometries of the As/Mo and some of the As/W compounds have been confirmed by X-ray crystallographic analysis ( 3 3 ) . T h e W-P compound, phosphotungstic acid, was included t o show the feasibility of this type of analysis. It is a well characterized commercially available solid whose W / P ratio is exactly known. It should be noted that the 95% confidence limits listed in Table I1 are within the worst case uncertainty limits required for the unambiguous assignment of stoichiometry for all t h e listed compounds. Interferences and Wavelength Selection. I n order t o obtain the level of accuracy and precision of the results reported in Table 11, several interferences which cause systematic errors had to be detected and identified. T h e interferences encountered may be classified into t h e usual ionization, physical, and spectral types of effects observed in t h e use of flames, arcs, and other types of atom reservoirs. When samples containing Mo or W are injected into t h e ICP, the relative emission intensities from the atoms and ions formed from both elements were found to vary when an easily ionized element, such as Na, was present in the sample matrix. Presumably this effect occurs through a change in the electron population in the plasma which causes a shift in t h e Saha equilibrium governing t h e relative concentrations of atoms and ions. It is also possible that t h e atoms and ions may experience different excitation temperatures under plasma conditions where local thermodynamic equilibrium does not exist. Figure 1 illustrates t h e variation of the ratio of atom line to ion line intensities as a function of height for both Mo and W with and without the presence of Na in the sample matrix. It can be seen t h a t this ratio varies with height even in t h e absence of Na, very probably because of temperature variations with height. However, the presence of Na very clearly causes an increase in the atom/ion intensity ratio at all heights observed for both elements. A practical consequence of this ionization effect is the fact t h a t one must compensate for t h e variations in emission intensity when an alkali metal is present. One strategy might be to add a relatively large amount of an easily ionized element to all samples and standards to act as a spectroscopic buffer which would maintain a constant plasma electron population. This approach, however, would involve large total solute concentrations and would aggravate physical types of in___--___
Table 11. Results of Analyses of Heteropoly Compounds
formulas Na,[(CH,),Nl , M o , O , , ( C H , A S ) , ~ ~ H , O (CN,H,),[(CH,),AsMo,o, ,OH] .H,O (CN PH6) [ (C H As):Mo 0 H 1.4H: 0 (CN3H,),[(C,H,As),W,0:~H].2H:0 (CN ,H, ). (CH ,AsW 0 H). 3H 0 (CN 3H,)7(C,H; ASW 7 0 , ;H).3HI 0 (CN 3H6 [ (CH ,AS)>W,O HI , 9H20 ~
~
~
(CN,H,):[(C,H,AS),W,O,~H] ,7H20 P2Oi. 24 WO i. 25H 0
ParentiHeteroatom Ratio ___ absolute real found error
relative error, o/o
3.000
3.006
0.006
0.20
4.000
4.016
0.40
3.000 3.000
2.993 2.965 6.963 7.039 2.990 2.920 12.047
0.016 - 0.007
7.000 7.000
3.000 3.000
12.000
1403
-__-____~__
~
~
~
0,035 0.037 0.039 0.010 0.080 0.047
0.23 1.15 --0.53 0.54 ~
-0.33 2.66 0.39
95% Confidence Limits of the Mean absolute relative, %
-0.060 0.068 to.077 t0.030 0.068 :
+
0.058 0.044 - 0.048 10.135
+
2.0 11.7 r2.6 - 1.0 +
! 1.0
-0.8 Z l . 5
1.6 ‘ 1.1
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
700
-
t
Table 111. Effect of Addition of 1500 FglmL NH,Cl to Solutions Containing As, P, Mo, and W as a Function of Plasma Height matrix to element/ anawavelyte length. ratio. " , nm w/w Pi213.6 30 A ~ 1 2 2 8 . 8 7.5 M01386.4 30 M01202.0 30 W/407.4 7.5 W/200.8 7.5
12
I4
18
16
20
O B S E R V A T I O N H E I G H T (rnm a b o v e c o i l 1
Figure 1. Variation of atom/ion line emission intensity with the height of observation. (W) W, 250 pg/mL and Na, 600 pg/mL; (0)W, 250 pg/mL, ( 0 )Mo, 50 pg/mL and Na, 600 pg/mL; (0)Mo, 50 bg/mL. Wavelengths employed: W(1) = 407.4 nm, W(I1) = 207.9 nm; Mo(1) = 386.4 nm, Mo(I1) = 202.0 nm
IOOL
80
t
:"I v1
Z 6 O ,
f
501
W
)Oi l 20
I
'
l
l
IO 12 14 16 OBSERVATION H E I G H T
!
1
18
20
(mm)
Figure 2. Dependence of analyte signal on observation height. All the measurements were made in the center of the plasma and the results are normalized for each line with respect to the maximum signal obtained on that set. W(1) = 407.4 nm; W(I1) = 207.9 nm; W concentration = 250 pg/mL. P(1) = 213.6 nm; P concentration = 50 pg/mL; Mo(1) = 386.4 nm; Mo(I1) = 202.0 nm; Mo concentration = 250 Fg/mL; As(1) = 228.8 nm; As concentration = 100 pg/mL
terference effects. T h e method of compensation employed in this work was simply to add corresponding amounts of alkali metals to our second standard only when they were found to be present in a specific sample. T h e physical types of interferences observed in this work include both effects involving the nebulization process and those actually taking place in t h e plasma. In many cases, however, it is difficult to distinguish the ultimate origin of t h e interference. One significant effect which is primarily a function of plasma conditions is the variation of emission intensity with height of observation in the plasma. Figure 2 reveals the variation of emission intensity with height for atoms and/or ions of Mo, U',As, and P. Such variations can be attributed t o both an increase in vaporization efficiency with height for refractory elements such as M o and W as well as to the decrease in plasma temperature with increasing height seen by other workers (26, 27). At greater heights
% Effect relative to Signal of Pure Analyte Height, mm _____12 14 16 18 20
-7.3
-1.9 -7.5 -8.9 -2.4 -2.5
-7.5 -G.9 -8.2 -3.0 -2.9 -0.9 -7.9 -8.0 - 8.7 -9.7 -10.9 -10.3 -1.6 -2.8 -2.3 - 3 . 4 -1.1 -2.6
8.3 -3.0 8.2 - 10.2 -1.7 -1.0
(16-18 m m ) increased vaporization dominates the signal of refractory W and Mo while a decrease in temperature more strongly influences the emission of more volatile P and As. Other factors, of course, may influence the intensity-height variations found in this system. T h e lateral diffusion of vaporized sample has not been accounted for nor have possible changes in the existing degree of thermodynamic equilibrium. These factors, however, should yield smaller variations in emission intensity. Regardless of the ultimate causes of intensity-height variations, one must obviously be quite careful to optimize and control the height of observation when performing analytical work to assure good sensitivity, precision, and accuracy. A second type of physical interference which must be considered when one is attempting a precise analysis is the effects caused by the presence of a sample matrix. For all analyte species treated, it was found that emission intensity a t all heights invariably decreased when extra solute material was added to standard solutions of the analyte of interest. Table I11 lists the relative decrease in emission intensity observed for several atom and ion lines when NH4C1 was added as the extra solute. Since for a given element the relative decrease is fairly constant a t all heights, one may conclude that the decrease is primarily due to a change in the efficiency of the sample introduction (nebulization and aerosol transport) process. There is good evidence indicating that the efficiency of the type of pneumatic nebulizer and aerosol chamber employed does change with the total solute concentration of the solution being aspirated (28). Variations in the degree of suppression reflect differences in the matrixto-analyte ratio between elements observed. The presence of a sample matrix may affect conditions in the plasma as well. Changes may occur in the plasma temperature, spatial distribution of sample, plasma electron population, and the degree of thermodynamic equilibrium achieved. Such effects could not be distinguished from the predominant effect caused by the change of sample introduction efficiency with solute concentration. Compensation for the above physical interferences was again achieved by matching the total solute content of sample and secondary comparison standard. In the long run more effective solutions to the problems of physical type interferences must involve the development of a more reproducible sample introduction system. The final type of systematic error detected which affected analytical results and which influenced selection of analytical wavelengths was a spectral type of interference. In this work extraneous light from both line and continuum type sources was found to reach the detector. A line source of emission was attributed t o be t h e cause of extraneous light when both a large signal increase was observed and a close lying Mo or W line could be found in wavelength tables (29) or could be observed in the experimental spectrum of Ma or W. A
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
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___Table IV. Evaluation of Wavelengths ( 1 2 mm above coil, 50/50 pm slits) element As
(100 pg/mL)
Mo (500 ,ug/mL)
wavelength, nm
SIN ratio
element
wavelength, nm
193.7 197.2 199.0 200.3 228.8* 235.0 278.0
97.2 57.1 31.3 77.5 100.0 28.5 81.2
W (500 ugiml)
202.0* 317.0 379.8 386.4 390.3
58.8 58.7 42.8 100.0 44.2
200.8 203.0 207.9* 255.1 272.4 294.7 400.9 407.4 429.5 430.2
“continuum like” source of light was assumed when a general increase in emission intensity over a large spectral region (20-30 nm) could be observed upon the addition of Mo or W. T h e source of the “continuum like” radiation could be due to any combination of the following phenomena: (1) scattering of light off the spectrometer diffraction grating, (2) molecular band emission, (3) emission of incandescent solid particles not yet vaporized (30, 31) and (4) association continua such as those caused by the recombination of alkali metals with hydroxy radicals in flames (31, 32). T h e presence of 600 pg/mL of Mo was found to yield a continuum type interference of 15-77% upon the following lines of As: 193.7, 197.2, 199.0, 200.3, and 235.0 nm. The effect upon the As 228.0-nm line was only 1% , but a line interference of >500% was measured at the 278.0-nm line. A similar continuum spectral interference from 1500 pg/mL of W was observed upon the As lines a t 197.2, 200.3, and 228.8 nm and the P lines a t 213.6 and 214.9 nm. W atomic lines increased signals >200% for the As lines at 193.7, 199.0, 235.0, and 278.0 nm. Figure 3 illustrates how the observed degree of spectral interferences typically varied with plasma height. The increase with height of Mo and W spectral interferences upon As and P correlates well with the increase in the intensity of W and Mo lines shown in Figure 3. This correlation suggests that scattered light from the many lines of the Mo and W spectra may be primarily responsible for the observed interferences. Emission from incandescent particles would be expected to decrease a t greater heights because of more complete vaporization. In addition there are no known emission bands of simple W or Mo molecules. such as MooBor W03, a t the wavelengths employed. Association continua are also less likely in an inert Ar environment. Perhaps the best way of minimizing and compensating for such spectral interferences is to employ a high resolution spectrometer capable of better optically isolating a particular emission line and of automatically measuring and accounting for other sources of light from the plasma. However, spectral interferences can also be overcome using the less expensive 0.5-m monochromator employed in this work through the standard matching technique and proper wavelength selection. Analytical wavelengths in this study were selected on the basis of their immunity from interferences as well as their signal-to-noise ratio. Table IV contains the signal-to-noise ratios observed with the ICP system for several lines of As, P, Mo, and W. T h e wavelengths marked with an asterisk in Table IV indicate those used for analysis. In order to promote precision the lines selected were those having the highest ratios unless a large interfering effect was present. A significant source of uncertainty in these analyses was the instability of the pneumatic nebulizer. Careful control over such parameters as the pressure of the aspirating gas and the head pressure of the waste solution drain had to be maintained in order to obtain reproducible results. Frequent
P
-_
SIN ratio 18.5 49.1 100.0
41.3 21.8 36.4 40.4 52.8 23.7 46.2
213.6* 214.9
(50 ui4iml)
100.0
73.3
____.__
+ 401 z 0
0 W LL
b
20
I O
12
14
I6
I8
20
O B S E R V A T I O N HEIGHT (mmabove c o i l )
Figure 3. Variation of the spectral interferences with the observation height. As(W): effect of 600 pg/mL of W on 50 pg/mL As signal, = 228.8 nm. As(Mo): effect of 600 pg/mL Mo on 100 pglmL As signal, X = 228.8 nm. P(W): effect of 600 pg/mL W on 50 pg/mL P signal, X = 213.6 nm
plugging of the nebulizer also complicated the experimental procedure. In view of such difficulties it seems likely that improvements in sample introduction should further increase the precision attainable with this analytical technique.
CONCLUSION It has been shown that through careful calibration with standards that are closely matched with sample in terms of matrix components and analyte concentration, precise and accurate analyses can be accomplished with an inductively coupled plasma emission spectrometer. Because the concentrations of the analytes employed in these measurements were well above their detection limits, the signal-to-noise ratio of the signals observed were large. This factor contributed to the excellent precision of the reported results. I t also has been demonstrated that analysis via the ICP is a realistic alternative to colorimetric and atomic absorption procedures for samples containing greater than trace levels of analyte. The advantages of the ICP procedure include simplicity of sample preparation; relatively rapid analytical procedure, especially in cases of multielement analysis; high precision and accuracy with careful calibration procedures; and immunity from many physical interferences. It must be noted that the ICP procedure is at a disadvantage with respect to spectral interferences when compared to atomic absorption. However, since several analytical lines are available for each element, spectral interferences can often be minimized or avoided by the proper selection of the wavelength of observation.
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
It is anticipated t h a t high precision analyses via the ICP can be expanded to a wide range of elements and can be applied t o a large variety of samples which are currently analyzed by other methods.
Thompson, K. C.; Thompson, D. R. Analyst (London) 1974, 9 9 , 595. Fiorino, J. A.; Jones, J. W.; Capari, S. G. Anal. Chem. 1976, 48, 120. Terashima, S. Anal. Chim. Acta 1976, 86, 43. Dickinson, G. W.: Fassel. V. A. Anal. Chem. 1969, 4 7 , 1021. Kirkbright, G. F.: Ward, A. F.; West, T. S. Anal. Chlm. Acta 1973, 6 4 , 353. Kirkbright. G. F.; Ward, A. F.; West, T. S. Anal. Chim. Acta 1972, 62, 241. Fassel, V. A.; Kniseley, R. N. Anal. Chem. 1974, 46, 1110A. Boumans. P. W. J. M.; DeBoer. F. J., Spectrochim. Acta, Part B 1975, 30, 309. Schrenk, W. G. in "Flame Emission and Atomic Absorption Spectrometry-Vol. 2"; Dean and Rains, Ed.; Dekker: New York, 1971: p 314 Kalnicky, D. J.; Kniseley, R. M.; Fassel, V. Spectrochim. Acta, Part 6 1975, 30, 511. Kornblum, G. R.; DeGabn, L. Spectrochim. Acta, Part 6 1977, 32, 455. Skogerboe, R. J.; Olson, K . W., Appl. Spectrosc. 1978, 32, 181. Meggers, W. F.; Corliss, C. H.; Scribner, B. F. "Tables of Spectral-Line Intensities" Natl. Bur. Stand. ( U . S . ) .Monogr. 1975, No. 145. Schrenk, W. G., "Analytical Atomic Spectroscopy": Plenum Press: New York. 1975: p 223. Gaydon, A. G.; Wolfard, H. G. "Flames, Their Structure, Radiation and Temperature"; Chapman and Hall: London, 1970. Gaydon, A. G. "Dissociation Energies and Spectra of Diatomic Molecules", 3rd ed.: Chapman and Hall: London, 1968.
LITERATURE CITED Cotton, F. A,; Wilkinson, G. "Advanced Inorganic Chemistry", 2nd ed.. Wiley-Interscience: New York, 1966; pp 938-946. Rajkovic-Blazer, L. M. PhD. Thesis, Georgetown University. Washington, D.C., 1978; p 22. Kwak, W.; Rajkovic, L . M.; Stalick, J. M.; Pope, M. T.: Quicksall, C. 0. J . A m . Chem. SOC. 1976, 75,2778. Barkigia, K. M.; Rajkovic, L. M.: Pope, M. T.; Quicksall, C. 0. J . A m . Chem. SOC. 1975, 9 7 , 4146. Kwak, W.; Rajkovic, L. M.; Pope, M. T.; Quicksall, C. 0.: Matsumoto, K ; Sasaki, Y. J . A m . Chem. SOC.1977, 9 9 . 6463. Wasfi, S.; Kwak, W.; Pope, M. T.; Barkigia, K. M.; Butcher, R. J.; Quicksall, C. 0. J . A m . Chem. SOC. 1978. 100, 7786. Haywood, M. G.; Riley, J. P. Anal. C h m . Acta 1976, 85, 219. Fishman, M.: Spencer, R . Anal. Chem. 1977, 4 9 , 1599. Sutcliffe, P. Analyst(London) 1976, 101, 949. Halmann, M. "Analytical Chemistry of Phosphorus Compounds ' : Wiley-Interscience: New York. 1972. Kolthoff, I. M.; Elving, P. S. "Treatise on Analytical Chemistry". Part 11, Vol. 5; Interscience: New York. 1961: pp 341-354 Wilson, C. L.; Wilson, D. W. "Comprehensive Analytical Chemistry", Vol IC; Elsevier Publishing Co.: New York. 1962; p 220. Smith, D. P. FhD. Thesis, Georgetown University. Washington, D.C , 1975: p 40. Landis, A. M. Ph.D. Thesis. Georgetown University, Washington, D.C.. 1977; p 56. Lichte, F. E.; Skogerboe. R. K. Anal. Chem. 1972, 44. 1480. Skogerboe, R . K.; Bejmuk, A. P. Anal. Chim. Acta 1977. 94. 297.
RECEIIFDfor review February 14,1979. Accepted May 1,1979. This project was supported by National Science Foundation Grant No. CHE75-22848. Presented in part a t the Fifth Annual Federation of Analytical Chemistry and Spectroscopy Societies Conference.
Comparative Interference Study for Atomic Absorption Lead Determinations Using a Constant Temperature vs. a PuIsed-Type Atomizer Lynn Robert Hageman, John A. Nichols, Puligandla Viswanadham, and Ray Woodriff Department of Chemistry, Montana State University, Bozeman, Montana 59717
belong all commercial atomizers, such as the Massman and the mini-Massman designs, the sample is vaporized and atomized under conditions of rapidly changing temperature. Their interference tendencies are similar. In the second type, exemplified by the atomizers developed by L'vov ( 2 ) in Russia and by Woodriff and his associates (2, 3 ) a t Montana State Llniversity in the United States, the sample is vaporized into a tube which has already achieved a preset optimal atomization temperature (2-7). At least a few workers in both Russia and the United States have recognized from the early days of electrothermal atomizers that constant temperature was a vital feature for interference-free analyses. L'vov (8) in particular has justified this belief on both theoretical and experimental grounds. Lead was chosen as the element of interest in this study for several reasons: (1) the importance of lead analyses in many different matrices for environmental and health studies; ( 2 ) the availability of extensive literature on lead determinations and interferences in pulsed-type atomizers; and ( 3 ) the desire t o extend the type of comparative interference study, previously implemented with a constant temperature furnace for manganese (91, to a more volatile element-for which interferences are very common in pulsed type atomizers because of the limited ashing temperature. Many simple matrix interferences observed in pulsed-typed atomizers have been documented in the literature. Regan and iyarren (I0, I I ) reported about 40% enhancement of lead AAS signals from a 1% ascorbic acid solution, using a Perkin-Elmer
During the one to three seconds necessary to heat commercial electrothermal atomizers to the desired atomization temperature, many reactions take place, and analyte compounds may be lost from the rapidly heating furnace at varying temperatures with varying matrices-often at sub-optimal temperatures with inadequate atomization, since residence times are short. Thus, matrix interferences are common in these pulsed-type atomizers. However, the same solutions, when atomized in a constant temperature furnace (CTF), show no significant matrix interferences. Lack of ruggedness of analytical procedures using pulsed-type atomizers seems to be an inherent limitation, whereas equipment ruggedness limitations of the CTF are amenable to elimination by appropriate attention to engineering aspects of fabrication. Difficult samples representing common matrices reveal the ease of obtaining interference-free results directly with the CTF-and the difficulty, even with pretreatments,of correcting for interferences on a routine basis in pulsed-type atomizers.
T h e various electrothermal atomizers available commercially for atomic absorption spectrometry (AAS) provide a lei-!: sensitive, as well as relatively inexpensive, means of analyzing for lead. However, effects such as matrix interferences have disenchanted many users. Electrothermal atomizers current11 in use can be classified into two categories, pulsed-type and constant temperature atomizers. In the former type. to which 0003-2700/79/0351-1406$01 O O i O
C
1979 American Chemical Societ)
