Direct Determination of Trace Quantities of Lead, Bismuth, Selenium, Tellurium, and Thallium in High Temperature Alloys by Non-Flame Atomic Absorption Spectrophotometry G. G. Welcher, 0. H. Kriege, and J. Y. Marks Materials Engineering & Research Laboratory, Pratf & Whitney Aircraft, East Hartford, Conn. 06 108
The determination of ppm concentrations of lead, bismuth, selenium, thallium, and tellurium in high temperature alloys is difficult because of the sensitivity required, the complex matrices, and the lack of standard materials. A new analytical method has been developed which offers significant advantages over other procedures for the determination of these critical trace elements in high temperature alloys. The new method utilizes highly sensitive non-flame atomic absorption spectrophotometry with simultaneous background correction for the determination of trace elements without prior separation. This technique is reliable and the most rapid available for the analysis of high temperature alloys for all five trace elements.
Systematic laboratory investigations as well as service experience have demonstrated that several low melting elements are harmful to the strength of high temperature alloys even when present at very low concentrations. Much metallurgical interest has been centered on the effects of 0.3 to 10 ppm of lead, bismuth, tellurium, selenium, and thallium on nickel-base alloys ( 1 ) .In a similar manner, various impurities can have a significant influence on the properties of other high strength metallic systems. The analysis for these trace elements in high temperature alloys is difficult because of the complex matrix which limits the choice of dissolution medium and gives rise to many chemical and spectral interference problems; the low concentration at which the trace elements of interest are present; and the lack of proper standard materials containing these trace elements. Present analytical methods are limited in applicability and are often tedious. A sensitive emission spectrographic procedure has been described for the determination of as little as 0.3 ppm tellurium in nickel alloys ( 2 ) .The tellurium is separated from the matrix by precipitation as the metal and removal on a Millipore filter. The paper and precipitate are transferred to a graphite electrode, ashed, and burned in a dc arc. This method is reliable and sensitive but quite time-consuming. A carrier distillation emission spectrographic procedure (.3) has been suggested for the determination of lead, bismuth, and tin. The sample is dissolved in acid, brought to dryness, and ignited to the oxide. The oxides are mixed with a silver chloride-lithium fluoride carrier and burned in a dc arc. The technique has the required sensitivity for lead and bismuth, gives precise results, and has the advantage of determining both lead and bismuth concurrently. However, the procedure is slow, is subject to some spectral interferences, and requires a skilled spectrographer to obtain consistently good results. Another emission spectrographic procedure has been described for the rapid deter(1) R . J. Henricks and M. L. Gell, "Trace Element Effects in Cast NickeCBase Superalloys," PSM Materials Engineering Congress, Chicago, Ili., October 3, 1973. (2) J. Y Marks, R. Cone, and E. Leao, Appl. Spectrosc., 25, 493 (1971). (3) M. G. Atwell and G. S.Golden, Appl. Spectrosc., 24, 362 (1970).
mination of lead and bismuth ( 4 ) .A small amount of metal chips is covered with lithium carbonate and excited in a dc arc. The technique is quite rapid and reliable but requires carefully matched solid standards for best results. With proper standards, the method may b e used to determine thallium but is not applicable to the determination of selenium or tellurium. Extraction with tri-n-octylphosphine oxide has been used to concentrate bismuth, lead, and thallium prior to measurement by flame atomic absorption ( 5 ) or emission spectrography (6). Both procedures are highly sensitive but quite tedious and are not applicable to selenium or tellurium. The X-ray fluorescence of selenium precipitated as the metal has been used to measure 3 to 100 ppm of selenium in copper- nickel- and iron-base alloys (7); however, this method is very time-consuming. Spark sources mass spectroscopy has seen increasing use as a method for simultaneous detection of many trace elements in high temperature alloys. The sample is machined into two pins which serve as electrodes. The samples are sparked in high vacuum and the mass spectrum is recorded on a photographic plate in a series of graded exposures. The plate is then visually analyzed by a disappearing line method to determine element concentration. Comparison of results obtained by this method with results by more precise techniques has been disappointing with only qualitative agreement. The lack of proper solid standards and the segregation of trace elements in nickel alloys (accentuated by the small amount of sample actually analyzed) are serious problems for the mass spectrographic method. Because of the many disadvantages of present methods for the determination of lead, bismuth, tellurium, selenium, and thallium in complex high temperature alloys, work was initiated to develop a procedure which would be accurate, applicable to all the elements of interest, rapid, and not dependent upon solid reference materials. Recent reports of work on atomic absorption spectroscopy utilizing non-flame atomization suggested that this technique might satisfactorily meet these requirements.
EXPERIMENTAL Equipment. A Perkin-Elmer Model 403 atomic absorption spectrophotometer with deuterium background corrector, the HGA 2000 non-flame atomizer, and Model 056 recorder were utilized for all absorbance measurements. All temperatures reported here were measured using the calibrated current meter provided with the instrument. Standard hollow cathode lamps were used for lead, bismuth, tellurium, and thallium while a Perkin-Elmer electrodeless discharge lamp served as the selenium light source. The atomizer tube was purged with argon. Solutions were introduced into the graphite furnace by Eppendorf automatic micropipets. Reagents and Stock Solutions. Stock solutions of lead, bis(4) M. G. Atwell and G. S. Golden, Appl. Spectrosc., 27, 464 (1973). (5) K. E. Burke, Analyst, (London).97, 19 (1972). (6) L. E . Harper, "Solvent Extraction Spectrochemical Method for Trace Amounts of Pb, Bi and Sn in High Temperature Alloys and Raw Materials Used in Their Production", TRW Metals Division Report T10009A (1972). (7) C. M. Albright, K. E . Burke, and M. M. Yanak, Talanta, 16, 309 (1969).
A N A L Y T I C A L CHEMISTRY, VOL. 46, N O . 9 , AUGUST 1974
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T a b l e I. Operating P a r a m e t e r s and D e t e c t i o n L i m i t s Element
Pb
Wavelength, nm Spectral band width, nm Solution volume, MI Sample concentration, mg/ml Drying temperature, "C Drying time, sec Char temperature, "C Char time, sec Atomization temperature, "C Atomization time, sec Detection limit,b ppm in sample
283.3 0.7 50 20 150 20 400 60a 2000 5 0.1
Interrupt purge gas flow during initial 30 seconds of char cycle. a peak signal-to-background ratio of two.
Bi
223.1 0.2 20 20
150 20 800 45 2200 5 0.1
214.3 0.7 50 20 150 20 600 60a
2200 5 0.2
se
196 . O 0.7 50 20 600 20 1000 45 2400 5 0.1
TI
276.9 0.7 50 4 150 20 500 60a 2000
5 0.1
Detection limit is defined as the concentration of analyte in the sample that produces
muth, tellurium, selenium, and thallium were prepared by dissolving high purity metals in dilute nitric acid. Solutions containing less than 0.1 mg/ml were prepared daily. Analytical reagent grade acids were used for sample dissolution and stock solution preparation, and deionized water was used for all dilutions. Solutions containing a total metal concentration of 1 gram per 50 ml were used to optimize variables for lead, bismuth, selenium, and tellurium. A metal concentration of 0.2 gram per 50 ml was used for thallium for best repeatability. Preparation of Reference Alloys. Standard materials were not available which contained known amounts of the five elements of interest; consequently, trace elements were added in successively higher concentrations to three 16-lb ingots of a nickel-base alloy containing 10% Co, 8%Cr, 1%Ti, 6%AI, 6%Mo, 4%Ta, and 2% Hf. Another ingot was cast under the same conditions to which no trace metal additions were made. Chips were prepared from these materials and analyzed by existing analytical methods. Homogeneity of these materials was confirmed by analysis of samples from the top, middle, and bottom of each ingot. These alloys were used in establishing optimum conditions for maximum reliability and accuracy of the method. Recommended Procedure for Sample Analysis. Dissolve 1 gram of metal chips in 30 ml of a 1:l:l mixture of nitric acid, hydrofluoric acid, and water by warming in a Teflon beaker. When dissolution is complete, reduce the volume to approximately 5 ml by evaporation. Cool, add about 20 ml of water, and heat to dissolve all salts. Cool, transfer to a 50-ml plastic volumetric flask, and dilute t o the mark with water. Prepare standards by doping either well-characterized samples of base alloy or by combining proper proportions of stock solutions of the matrix elements to approximate the composition and concentration of the samples. Select the instrumental conditions as indicated in Table I. Carefully adjust the HCL or EDL beam position to bring it into best coincidence with the deuterium arc beam. The temperature programmer is operated in the automatic mode such that the purge gas flow is interrupted eight seconds prior to atomization and is not resumed until the atomization cycle is completed. Atomize samples and standards and prepare a graph of absorbance us. trace element concentration.
RESULTS AND DISCUSSION Background Studies. Initial experiments with dilute
acid stock solutions of the elements indicated adequate sensitivity for the non-flame technique; however, in the presence of a nickel alloy matrix, very poor detection limits for the analyte elements were obtained because of the high background absorbance. This background signal was still apparent, although somewhat reduced, when the automatic deuterium background correction system was used. Before further studies were made, mirrors in the optical system were adjusted t o bring the deuterium arc beam into optimum geometric alignment with the hollow cathode beam. Adequate background correction could then be achieved with the two beams properly aligned. Non-specific absorbance and emission background signals varied differently as a function of wavelength. This difference was illustrated by measurement of both absorp1228
Te
Table 11. Efficiency of D e u t e r i u m Arc in B a c k g r o u n d Absorbance Correction Signal-to-background ratio Without background correction
With background correction
5 .O 1.1 1.4 40 1.2
12 9 .o 4.0 190 3 .O
tion and emission background signals from a nickel-base alloy containing less than 1 ppm lead a t 217.0 and 283.3 nm. The nickel alloy concentration was 1 gram per 50 ml of solution and absorbance measurements were made without deuterium arc correction. The emission signal was recorded by chopping the light emitted from the furnace at the frequency of the detector. Although the emission signal was greater a t 283.3 nm in agreement with recently published data for the graphite tube alone (8), the absorbance signal was greater a t 217.0 nm. Therefore, two opposing factors should be considered when choosing the proper analyte wavelength in samples of high solute concentration. First, the more intense continuum emission a t higher wavelengths results in poorer detection limits due t o shot noise. Second, at lower wavelengths, detection limits suffer because of the higher nonspecific absorbance from the matrix. This attenuation of the analyte beam is not adequately compensated for by the deuterium arc correction system because of the inherent mismatch between the two beams. The second factor is predominant in determining detection limits in samples containing large solute concentrations. Thus, optimum detection limits for elements measured in a high matrix concentration may be obtained a t wavelengths higher than those used for more dilute solutions. Table I1 shows the effectiveness of the deuterium arc in correcting for matrix background when measuring lead, bismuth, selenium, thallium, and tellurium. Optimum heating parameters were used to give the best possible signal-to-background ratios. The most significant improvement was for bismuth where a ninefold increase in signalto-background ratio was noted. Finally, the background signals due t o the graphite furnace alone and due t o the nickel alloy matrix were compared at the bismuth 223.1-nm line. The signal for the nickel alloy matrix was two orders of magnitude higher than that for the graphite furnace alone. These studies (8) J. D. Kerber, A. J. Russo, G. E. Peterson, and R. D. Ediger, At. Absorption Newsleff., 12, 106 (1973).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 9 , AUGUST 1974
lor
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0NICKEL
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1600
1700
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1900
1800
L
2000
_L 2100
L
1200
1
1
2300
2400
1 2500 2600
ATOMIZATION TEMPERATURE ("2)
m
CHAR TEMPERATURE ('C)
Figure 3. Atomization temperature vs. sensitivity for lead
Figure 1. Char temperature vs. sensitivity for lead, bismuth, and selenium io r
Y
2 V
s
30
6
C U l C K l l ALLOY + 4PPM BISMUTH
v)
-uIcllL
0
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2200
2iiO
ATOMIZATION TEMPERATURE ('C)
0 bo0
l
100
I
I
I
1
800
1000
I200
1400
Figure 4. Atomization temperature vs. sensitivity for bismuth
CHAR TEMPERATURE ('C)
Figure 2. Char temperature vs. sensitivity for thallium and tellurium
demonstrate the magnitude of background effects and suggest the criticality of proper beam alignment and background correction. Heating Programs. Studies were made to optimize the heating parameters for each element. Drying temperatures and times had little effect on absorbance signals between 150 and 400 O C , except in the case of selenium, where higher dry temperatures resulted in better reproducibility. The char cycle was necessary, even though no organic matter was present, to minimize background absorbance. This is probably accomplished in two ways. First, volatile matrix components are removed during this cycle, thus reducing background. Second, thermally unstable fluorides and nitrates may be converted to oxides and carbides which result in lower background absorbance upon atomization. Figures 1 and 2 show the effects of char temperature on lead, bismuth, selenium, thallium, and tellurium in a nickel alloy. Data are reported as relative absorbance which is the arbitrary signal peak height in chart divisions. All of the curves are similar in two respects: first, there is a range of temperatures where a relatively small change in sensitivity is noted; second, sensitivity decreases rapidly at higher char temperatures. In general, low char temperatures are recommended for the best combinations of detection limit and reproducibility. The changes in sensitivity with changing char temperatures are interesting in that they may be correlated with the volatility of compounds which may be formed in the graphite tube. The rapid decreases in sensitivity for selenium and tellurium at relatively low char temperatures are
P
9
E
;
/
20
ATOMIZATION TEMPERATURE I'CI
Figure 5. Atomization temperature vs. sensitivity for selenium
indicative of the high volatilities of the fluorides and oxyfluorides of these elements. Loss of these elements a t higher temperatures may correspond to formation of more thermally stable oxides or nickel bimetallic compounds. Decreases in sensitivity for lead, bismuth, and thallium at higher temperatures correspond roughly to the boiling points of their respective fluorides (1290 "C, 1027 "C, and 655 "C). There is no apparent explanation for the rapid increase in sensitivity for thallium a t low char temperatures. Proper atomization temperature was important in obtaining maximum sensitivity, minimum background, and best precision. Figures 3-7 show the effects of atomization temperature on sensitivity and background in a nickel-base alloy for lead, bismuth, selenium, thallium, and tellurium, respectively. The curves for lead, bismuth, thallium, and tellurium are similar in that sensitivity reaches a maximum and then decreases a t higher temperatures. The reason for this is not clear but may involve a decrease in analyte con-
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 9, AUGUST 1974
* 1229
5or
0 NlCKfl A l L O I
~
I
I PPM THALLIUM
3
1200
1400
NICKEL ALLOV
iaoo
1600
2000
2200
2400
i
2600
ATOMIZATION TEMPERATURE ('C)
Figure 6. Atomization temperature vs. sensitivity for thallium
Pb
1.00 1.02 0.88 1.17 0.74
Nickel only Nickelalloy* Iron alloyC Cobaltalloyd
Bi
Se
1.00 1.03 0.92 0.77 0.92
1.00 1.88 1.88 1.33 2.18
TI
1.00 0.74 0.82 0.92 0.92
i
zoo
2000
LLW
2300
14w
ATOMIZATION TEMPERATURE ('C)
i
2100
complete recovery of thallium was achieved; however, the bismuth signal was still negligible. Based on bismuth recoveries by other techniques from solutions containing hydrochloric acid, it must be concluded that both bismuth and thallium, when present as chlorides, are lost during preatomization heating. The bismuth absorbance signal is also suppressed in the presence of sulfuric acid. Hydrochloric acid suppressed the signal for lead. The decrease in signal was directly proportional to the char temperature. A t a 600 "C char temperature, the lead absorption signal was completely suppressed.
Relative sensitivity Matrix
iaoo
Figure 7. Atomization temperature vs. sensitivity for tellurium
Table 111. Effects of Various Matrix Materials on Relative Sensitivities for Pb, Bi, Se, T1, and Ten
No matrix
1800
Te
1.00 1.10 1.04 0.81 0.97
a Analyte concentrations are 3 ppm in the matrix (0.06 pg/ml in solution). Co-S';'c Cr-17; Ti-GL/, A1-Cf$ Mo-4'; Ta-2C/, Hf. Fe-185% Cr-9cyi Ni. Co--4:; Mo-37i Nb-15; Ta-4'; W-1.5';, Mn.
* Ni-107;
Table IV. Comparison of Results by G r a p h i t e Atomizer a n d O t h e r Techniques Pb, ppm
ppni _ _Bi,_ ~ _ _
Alloy standard
Graphite atomizer
Em. spec.
Graphite atomizer
1 2 3 4
1.9 3.1 4.7 0.5
2 4 6 0.6
0.3 0.4 0.4
