Anal. Chem. 1988, 60,2497-2500
2497
Determination of Yttrium in Complex Nickel-Base Alloys Using Microwave Dissolution and Inductively Coupled Plasma Optical Emission Spectrometry Paul A. Vozzella* a n d David A. Condit United Technologies Research Center, Silver Lane, M S 94, East Hartford, Connecticut 06108
A rapid, precise method for determining yttrlum in complex nickel-base superalloys uses microwave-assisted acid dissolution for sample preparation and inductively coupled plasma optical emission spectrometry for analysis. By careful minimization of the HF concentration required for dissolution, yttrium can be determined at levels down to 10 ppm f10% with a relative standard deviation less than 5 % when using the 360.07-nm emission line.
Yttrium additions to the Ni-Cr-A1 alloy system have been shown to improve high-temperature cyclic oxidation behavior ( I , 2). In support of similar investigations, a rapid and sensitive method for determinating yttrium in high-temperature nickel-base superalloys was developed. The yttrium content of Ni-Cr-A1-Y, Fe-Cr-AI-Y, and Cdr-Al-Y alloys are routinely determined in this laboratory using aqua regia for dissolution and inductively coupled plasma optical emission spectrometry (ICP-OES) for analysis. With microwave-assisted acid dissolution, sample preparation time of these alloys has been reduced substantially. However, difficulties arise when these alloys are modified with refractory elements such as tantalum and tungsten, as is the case with nickel-base superalloys. The refractory elements readily hydrolyze during dissolution with aqua regia unless hydrofluoric acid is used. The addition of hydrofluoric acid may cause losses due to the low solubility of yttrium fluoride. Previously, this problem has been addressed by evaporating solutions to dryness and removing the excess HF, followed by redissolving the residue in hydrochloric acid (3). For this method, sample preparation was the rate-determining step. The objectives of this work were to develop a rapid dissolution procedure for nickel-base superalloys containing yttrium, using a reagent matrix in which yttrium would be stable, and to determine yttrium by using ICP-OES with an emission line free from matrix element spectral interferences. In addition, the emission line selected was required to be sensitive enough to allow reliable measurements of yttrium to be made a t a lower limit of 10 ppm in the alloy. Yttrium has been determined in complex geological samples by using ICP-OES, but optimum results for low concentrations usually require a separation technique ( 4 ) . In this investigation, separation and/or concentration techniques were not considered because of the time restraints introduced by these methods. A spectrophotometric technique for determining yttrium in nickel-base alloys has been reported (5) but was not evaluated because of its low tolerance for fluoride, a vital component of the solvent required for high-temperature nickel-base alloys. In addition, no tolerance data were given for Cr, a major constituent of the alloys under study, and the tolerance limits for those elements that were listed would be exceeded when analyzing samples containing very low levels of yttrium. EXPERIMENTAL SECTION Two studies were undertaken to evaluate the effect of hydrofluoric acid concentration on the determination of yttrium by 0003-2700/88/0360-2497$01.50/0
ICP-OES in either nitric acid or aqua regia solutions. The first uses synthetic yttrium standards to study yttrium emission stability as a function of time to establish the maximum time yttrium stays in solution at a given HF concentration. The other study evaluates the effectiveness of boron in complexing free fluoride, thereby allowing yttrium to remain in solution longer. Standard solutions of yttrium containing varying amounts of HF were brought to volume with a boric acid solution. The emission intensity of the yttrium signal was then measured over a period of time. Following these yttrium stability studies, a third study was undertaken to determine the stability of yttrium in solutions containing HF in the presence of matrix elements indigenous to high-temperature nickel-basesuperalloys. Finally, the guidance gained from these studies aided in the analysis of a model high-temperaturenickel-base alloy. Thus, a methodology evolved which defined the dissolution conditions, sample size and analysis timing required for anticipated yttrium alloy concentrations of 10-2000 ppm. Chemicals and Instrumentation. All concentrated acids used were Reagent Grade quality. Water was distilled and deionized (DDIW). A boric acid solution was prepared by dissolving 15.0 g of dry H3B03in DDIW and diluting to 1 L. A commercial yttrium standard, lo00 wg/mL, was used (Spex Industries catalog no. AQY2). The National Bureau of Standards standard reference material employed was No. 899, “Tracealloy C”, nominal weight percent composition 0.12 C, 12.0 Cr, 8.5 Co, 1.75 W, 0.9 Nb, 2.0 Al, 2.0 Ti, 0.01 B, 0.1 Zr, 1.75 Ta, 1.2 Hf, and balance Ni. A Thermo Jarrell-Ash Plasma 200 inductively coupled argon plasma optical emission spectrometer (ICP-OES)was used in this investigation. The Plasma 200 is a sequential scanning instrument employing a peristaltic pump for sample uptake and an adjustable cross-flow nebulizer. It is outfitted with a dual monochromator consisting of a 1/6-mEbert-Fastie premonochromator and a 1/3-m Ebert-Fastie primary monochromator to provide spectral resolution of 0.02 nm. Typical operating parameters were 1.2 kW power, 18 L/min and 1.0 L/min argon respective coolant and auxiliary flows, and 30 psi and 0.5 L/min respective nebulizer pressure and argon flow. In addition, sample delivery rate to the nebulizer was 1.0 mL/min and the observation height was optimized for maximum emission intensity. A consumer microwave oven (Whirlpool, Model MW-8750), rated at a maximum power output of 700 W was used to assist sample dissolution. The oven, located in a fume hood, was outfitted with a wind-up mechanical carousel to ensure uniform thermal exposure of the samples. The oven was typically operated at 30% power. As an added safety precaution when operating the oven, a 115-V ac proximity switch was installed on the exterior of the oven door. This was done after learning that at least one burn injury has occurred when using a consumer microwave oven for a laboratory application (6). The added switch was in series with the power module supplying the magnetron and independent of, but not in place of, the manufacturer’s safety devices. Thus, whenever the oven door was open, the magnetron was disabled. Sample dissolution containers were 250 mL capacity polycarbonate, flat bottom centrifuge bottles with polypropylene screw caps (Cole-Palmercatalog no. 56106-10). To minimize exposure of the oven to acid fumes during microwave dissolution,the sample bottles were placed inside a larger polycarbonate container with a tight-fitting lid (Cole-palmercatalog no. 5-6760-05and 5-6760-09, respectively). This container, which readily holds 1 2 sample bottles, was unloaded in a fume hood after the microwave exposure cycle to prevent exposure to acid fumes and allow for evaporation of any acid condensate on bottle exteriors. It is noteworthy that 0 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER
a,
4
5 , 1988
Table 11. Effect of Individual Matrix Elements on Recovery of 1.00 pg/mL Yttrium in Aqua Regia Solutions
PPm y > h = 371.03 nm
element Ni Cr Time, hrs
Flgure 1. Stability of 1.00 pg/mL Y solutions containing 5 % HNO, t HF: (m) 1%, (A) 2 % , (0) 3%, (A)5 % , and (0) 10% HF.
X=
PPm y ,
360.07 nm
o'8 0.6
E
I
0,41
elemental composition range, w t % 59-70 9-20 5-10 2-12 2-10 2-9 1-5
W Ta Mo Hf
co
Ti A1 A1 + 2-570 HF
solution concn. range: mg/mL 3-7 0.25-2 0.1-1 0.1-1 0.05-0.5 0.01-0.12 0.25-1 0.05-0.2 0.1-1.2
1-2
0.1-1.2
1.2
measd Y concn, kg/mL 1.04 f 0.02 1.04 i 0.02 1.07 f 0.02 1.02 f 0.02 1.02 i 0.03 1.00 f 0.01 1.03 i 0.02 1.01 f 0.01 0.85 f 0.1 1.05 i 0.02
'Concentration ranges based on 0.5 g/100 mL, 1.0 g/100 mL, emission measurements taken at X = 360.07 nm. Table 111. Recovery of Yttrium Added to NBS SRM 899 Sample Solutions (0.5 g/100 mL), Containing Aqua Regia
HF
+
0.2
equivalent ppm Y in alloy 100 200 500 1000 2000
we found minimizing power and time required for dissolution prevented acid condensation on bottle exteriors. Yttrium Solution Stability. To determine the stability of yttrium in solutions containing hydrofluoric acid, 1gg/mL yttrium solutions containing 5% "OB and from 1to 10% HF were made up from a 1000 gg/mL commercial standard. The ICP was calibrated with a 1.00 pg/mL yttrium standard containing 5% HN03. The yttrium content of these solutions, measured over a 48-h period, starting with 1h, are given in Figure 1. The benefit of adding a fluoride complexing agent such as boric acid was evaluated for solutions containing 5% and 10% hydrofluoric acid. Boric acid was added to the yttrium-spiked solutions by diluting them to a final volume of 100 mL with 1.5 wt % boric acid (70 mL). The yttrium contents, given in Table I, were measured at time intervals the same as above up to 48 h. An abbreviated version of the above study was performed for solutions made up with aqua regia. The term "aqua regia" in this work refers to an aqueous solution containing 10% HCl + 5% "OB. These results are given graphically in Figure 2. Effect of Alloy Matrix Elements. Given in Table I1 are the elemental composition ranges considered typical of high-temperature nickel-base superalloys along with solution concentration ranges and yttrium recovery measurements. To evaluate the spectral and/or stability effect of the individual alloy elements on the determination of yttrium by ICP, aqua regia solutions containing these elements were spiked to contain 1.00 gg/mL Y. The ICP was calibrated against a pure yttrium standard containing aqua regia only. The concentration ranges were determined assuming sample sizes of 0.5 g/100 mL and 1.0 g/100 mL for the low and high limits, respectively. The last column represents the measured yttrium values in solutions containing these ranges of matrix element concentrations.
ppm Y measured (A = 360.07 nm) 5% HF 2% HF 99 193 393 772 1114
98 204 510 1022 2054
Determination of Yttrium in Ni-Base Superalloys. Nickel alloy samples were provided as machined turnings or chips. Machined samples were degreased with trichloroethylene (TCE), in a well-ventilated hood to minimize TCE exposure, rinsed with DDIW, and dried at 110 "C. The samples (0.2-1.0 g) were weighed directly into dry 250-mL polycarbonate bottles, the required acids pipetted into the containers, and the caps tightened by hand. The sample bottles were placed inside a larger polycarbonate container with a tight-fitting lid. Heating was accomplished in the microwave oven at the 30% power level. A typical dissolution time for a group of eight samples was 15 min. After dissolution was complete, the containers were allowed to cool for a few minutes, the caps carefully removed in a fume hood, and the samples made up to final volume (100 mL) with DDIW using an automatic liquid dispenser. Samples of a proprietary Ni-base superalloy containing Cr, Al, W, Ta, and Mo were dissolved in aqua regia containing 3-10% HF, spiked with yttrium and made up to a final volume of 100 mL. The yttrium concentration in each of the final solutions was 1.00 pg/mL. These solutions allowed the stability of yttrium in the presence of hydrofluoric acid and alloy matrix elements to be evaluated. Results, measured over a 48-h period, indicated the solutions were stable within 5%. With NBS SRM 899,0.5-g and 1.0-g samples were weighed into polycarbonate sample bottles and yttrium standard solution was added a t a level equivalent to 5-100 pg of Y/g of sample. Next, 10 mL of concentrated HCl, 5 mL of concentrated "OB, and 3 mL of concentrated H F were added by pipet, and the bottles were capped and heated in the microwave oven. After cooling, the sample volumes were made up to 100 mL with DDIW. The yttrium in solution was determined by ICP-OES using matrix matched standards to calibrate the ICP. The same solutions were then analyzed for yttrium using standards matched only in acid content (no HF) and containing no matrix elements. The results were essentially the same as above. In addition, the recovery of higher concentrations of yttrium was evaluated by adding an appropriate amount of yttrium standard solution to 0.5-g samples of NBS SRM 899 to give yttrium levels equivalent to 100-2000 ppm in the alloy. Sample dissolution was then achieved by adding aqua regia containing either 2% or 5% HF, using microwave-assisted heating, and
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
Table IV. Recovery of Yttrium Added to NBS SRM 899 Sample Solutions (0.2 g/100 mL) Containing Aqua Regia 2%
HF
equivalent ppm Y in alloy 100 200 500
+
ppm Y measured (X = 360.07 nm) after 1 h after 48 h 103 203 503
1000
1004
2000
1999
101 208 522 1029 2031
bringing up to a final volume of 100 mL with DDIW. The yttrium concentration of these solutions was measured within 1h against a pure yttrium standard in aqua regia; the results are given in Table 111. Finally, to determine the stability of the higher yttrium concentrations at 1 and 48 h, yttrium standard solutions were added to smaller 0.2-g samples of NBS SRM 899 to simulate 100-2000 ppm yttrium in the alloy followed by dissolution in 5 mL of H20 + 10 mL of HC1+ 5 mL of HNO, + 2 mL of HF and made up to 100 mL with DDIW. The yttrium concentrations were measured after l and 48 h, once again using a pure yttrium standard in aqua regia. These results are given in Table IV.
RESULTS AND DISCUSSION The ICP is particularly suited for determining yttrium, i.e. excellent sensitivity, wide linear dynamic range, and minimal spectral interferences. For the yttrium stability studies, any of the more sensitive emission lines were suitable. However, with the addition of matrix elements common to nickel-base superalloys, spectral interferences eliminate all but two emission lines, 371.03 and 360.07 nm, from serious consideration. NBS SRM 899 at 1.0 g/100 mL concentration level was used to evaluate spectral interferences at these two lines. A direct spectral interference from T i (200 ppm) was found at the 371.03-nm line. At the 360.07-nm emission line, no direct spectral interferences were observed, and the Zr peak just to the right of the yttrium peak could effectively be eliminated by using a narrow peak search window (0.033 nm wide). Therefore, the more sensitive 371.03-nm line was used for the yttrium stability studies in different acid media while the less sensitive, interference-free 360.07-nm line was used for more complex solutions containing alloy matrix elements. Yttrium Solution Stability. Nitric acid solutions ( 5 % ) containing 1.00l g of Y/mL were found to lose stability rapidly as the hydrofluoric acid concentration exceeded 3% (Figure 1). Addition of boric acid to complex excess fluoride was effective in maintaining stability at the 5% H F level but ineffective at 10% (Table I). However, in the presence of aqua regia, yttrium solutions at a 1.00 pg/mL level were stable for up to 48 h with H F concentrations up to 5% (Figure 2). Aqua regia solutions containing 10% H F had lost yttrium after 1 h. Boric acid preserved the yttrium stability in this matrix up to 6 h, but for less than 24 h. Effect of Alloy Matrix Elements. Individual matrix elements commonly encountered in nickel-base superalloys were found not to interfere with yttrium quantitation by ICP-OES (at X = 360.07 nm) with the exception of aluminum (Table 11). However, in the presence of 2-5% HF, the aluminum interference disappeared. This observation was found to be repeatable but could not be readily explained. It was also observed that in sample solutions containing many of the refractory elements, such as Cr, Al, Ta, and Mo, common to high-temperature nickel alloys, yttrium stability a t the 1.00 pg/mL level was insensitive to H F concentrations as high as 1070,even though no fluoride complexing agent was present. Presumably this is due to the increased competition for the available fluoride by the refractory elements, making
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the formation of insoluble yttrium fluoride less favorable. Determination of Yttrium in Ni-Base Superalloys. Microwave heating for sample dissolution was selected because it has the ability to accelerate sample dissolution, thus meeting the requirement for a rapid dissolution technique (7-9).The 250-mL polycarbonate sample bottles proved to be well-suited for the dissolution of the nickel alloys. Polycarbonate is essentially transparent to microwave radiation, allowing maximum coupling with the metallic sample and the aqueous dissolution medium. The clarity of the bottles also allows the analyst to determine when sample dissolution is complete. Some acid fumes leak from around the bottle cap threads during sample dissolution; this leakage prevents excessive pressure buildup in the container which could lead to bottle rupture. Liquid loss (by weight) was determined to be negligible under the experimental conditions employed. Because of concern about the corrosive nature of these acid fumes, the sample bottles were placed in a larger polycarbonate container with a tight-fitting lid during the heating process (IO). The larger container (described in the Experimental Section), although not 100% effective in containing the acid fumes which leak out of the sample bottles, does minimize the quantity of fumes coming in contact with the oven components. Fumes are vented into a hood after removing the container from the oven. Another advantage of the second container is its ability to contain any liquid leakage from a sample bottle in the event of a failure. After the sample dissolution cycle is complete, a measured amount of water is added to the sample bottle to achieve the desired final volume. Thus, a sample is made up to volume and analyzed in the same container into which it was weighed and dissolved,significantly reducing the amount of labware and time required for sample preparation. In the approximately 3 years that this technique has been used by this laboratory, no bottle has failed due to excessive pressure buildup. The NBS SRM 899 was considered a good representative composition of a “typical” high-temperature nickel-base superalloy and was readily available in a convenient, milled form. Samples of this alloy, spiked with yttrium to simulate 5-100 ppm yttrium in the alloy, demonstrated recoveries within 5% of the added amounts with relative standard deviation values typically less than 5% for n = 5 measurements. Furthermore, these results were generated with non-matrix-matched standards. Thus, even when nonmatrix material standards are used to calibrate the ICP, spiked yttrium concentrations are recovered. However, its should be noted that a different model ICP using a different type nebulizer might be more sensitive to viscosity changes that results from these relatively high (0.5 and 1.0 g/100 mL) sample concentrations. Thus, individual laboratories should evaluate their particular equipment before assuming that matrix-matched standards are not required. The bulk of the stability and recovery data generated in this investigation centered around solutions containing 1.00 bg of yttrium/mL of solution. This is equivalent to 100 ppm yttrium in a dissolved alloy sample of 1g, diluted to a final volume of 100 mL. The data discussed above showed that at this level of yttrium, HF concentrations as high as 10% had no detrimental effect on the measured yttrium concentration in the presence of the typical matrix elements evaluated. However, at yttrium concentrations in excess of 100 ppm, an H F concentration of 5% resulted in low yttrium recoveries while 2% HF had no deleterious effect (Table 111). To remedy this, going to a smaller sample size of 0.2 g/100 mL resulted in acceptable recoveries (Table IV).
CONCLUSION Yttrium can be readily determined in nickel-base superalloys over the concentration range 10 to at least 2000 ppm
Anal. Chem. 1988, 6 0 , 2500-2504
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with an overall accuracy within &5%. Weighing samples (0.2-1.0 g) directly into polycarbonate containers and dissolving the alloys in aqua regia using a minimum amount of concentrated HF, and employing microwave heating, is a fast, convenient method of sample preparation. Controlling sample size and acid concentration eliminates the need for fluoride complexing agents. Using the 360.07-nm emission line, matrix-matching of yttrium standards with alloy elements is not necessary. Total analysis time for a set of four samples in duplicate typically takes 2 h.
ACKNOWLEDGMENT The authors thank Gerald S. Golden for his helpful suggestions and comments during the course of this investigation and Robert W. Dean for his assistance in preparation of the manuscript. LITERATURE CITED (1) Whittle, D. P.; Stringer, J. Philos. Trans. 295, 309-329.
R . SOC. London, A
1980,
(2) Tien, J. K.; Pettit, F. S. Metall. Trans. 1972, 3 , 1587-1599. (3) Fornwalt, D. E., Pratt & Whitney Aircraft, East Hartford, CT, unpublished work, 1985. (4) Bolton, A.; Hwang, J.: Voet, A. V. Spectrochim. Acta, Part B 1983. 388, 165-174. (5) Hsu, C. G.; Pan, J. M. Analyst (London) 1985, 110, 1245-1248. (6) Kingston, H. M., National Bureau of Standards, Inorganic Analytical Research Division, Gaithersburg, MD, personal communication, March 1987. (7) Matthes, S. A.; Farrell. R . F.;Mcakie, A. J. Technical Progress Report 120, Bureau of Mines, Analytical Support Services Program, April 1980. (8) Nadkarni, R. A. Anal. Chem. 1984, 56, 2233. (9) Mahan, K. I.; Foderaro, T. A.; Garza, T. L.; Martinez, R. M.; Maroney, G. A.; Trivisonno, M. R.; Willging, E. M. Anal. Chem. 1987, 59, 938. (IO) Wieland, J. R., PCC Airfoils Inc., Minerva, OH, personal communication, March 1987.
RECEIVED for review February 9, 1988. Accepted August 22, 1988. This work was presented in part at the 38th Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, 1987 (paper no. 1016).
Evaluation of Inductively Coupled Plasma Mass Spectrometry for the Determination of Trace Elements in Foods R. Duane Satzger Elemental Analysis Research Center, Food and Drug Administration, 1141 Central Parkway, Cincinnati, Ohio 45202
Inductively coupled plasma mass spectrometry has been investigated for the determination of trace elements in foods. The effects of the malor elements in foods (Na, Mg, P, K, Ca) on the response of several trace elements found In foods (AI, Cr, Zn, Mo, Cd, Pb) were evaluated. Enhancement or suppresslon of response Is generally less than 10% in the presence of 1000 mg L-’ of each major element added. The effects of mlxed concomitants (Na, Mg, P, K, Ca) at concentratlons of 0.1 % and 0.2% total dlssolved solids on Zn, Mo, Cd, and Pb at 0.01 and 0.10 mg L-’ were investlgated. The greatest suppression observed with the mixed concomitant solutions was on the response of 0.01 mg L-’ Zn. Results are presented for dry ashed reference materials, which compare well with those reported by NBS.
Responsibilities of this laboratory include assessment of the nutritional and/or toxicological significance of trace elements in foods. Elements of interest include those that are naturally incorporated in the food matrix in addition to contributions from agricultural chemicals, processing, environmental contamination, and, in recent years, product tamperings. Several analytical instrumental techniques have traditionally been required to obtain information on major and ultratrace element concentrations. Most nutritional elements are present a t parts-per-million levels in foods and can therefore be determined by using inductively coupled plasma optical emission spectroscopy (ICP-OES) or flame atomic absorption spectrometry (I, 2 ) . However, many elements present a t ultratrace levels, such as Cd and Pb, require the use of more sensitive and time-consuming techniques such as graphite furnace atomic absorption spectrometry (GFAAS) or differential pulse anodic stripping voltammetry (DPASV).
Sub-part-per-billion determination of P b by GFAAS requires multiple injections and voltammetric techniques for P b at this level require an extended electrochemical preconcentration step (3). Inductively coupled plasma mass spectrometry (ICP/MS), on the other hand, offers comparable or better multielement sensitivity with direct solution nebulization ( 4 ) . This is a tremendous advantage when investigating an unknown, such as in a product-tampering case. In many situations, samples can be rapidly screened for the type of contaminant. If the adulterant can be isolated, an elemental “fingerprint” may be established in an effort to define its origin ( 5 ) . One drawback is that the sample matrix reportedly has an effect on the analyte response, requiring extensive dilution of the sample in order to reduce sample induced matrix effects. However, these reports have not been consistent in type or degree of matrix induced interference reported (6-10). Matrix effects reported by other investiators appear dependent on the instrument used in the study as well as source operating conditions such as sampling distance, power, and nebulizer flow rate. On the basis of these studies (7-IO), it is apparent that instruments that utilize greater sampling distances (15-20 mm) require higher nebulizer flow rates (0.8-1.2 L min-l) to obtain an optimum analyte count rate. At the above sampling distances and flow rates, matrix-induced effects appear more severe than those effects reported at shorter distances (110 mm) and lower nebulizer flow (0.5-0.7 L min-’) rates ( 4 ) . Source grounding configuration, interface design, and ion optical configuration, including the pressure in the ion lens region, may play a role in elucidating the origin of matrixinduced changes in analyte response. In recent work, Munro et al. (11) investigated the application of ICP-MS to the determination of V, Mo, Cd, and P b in biological standard materials. This work studied molecular
This article not subject to U.S.Copyright. Published 1988 by the American Chemical Society
