Techniques for Monitoring the Quality of Ultrapure ReagentsNeutron Activation and X-Ray Fluorescence J. W . Mitchell, C. L. Luke, and W. R . Northover Bell Laboratories, Murray Hili, N . J , 07974
Reagents distributed by several commercial suppliers as electronic or ultrapure chemicals were analyzed quantitatively by instrumental neutron activation to determine total concentrations of sodium, chlorine, copper, and manganese impurities. Data obtained in this way for the analysis of ultrapure nitric, acetic, hydrofluoric acids, hydrogen peroxide, and ammonium hydroxide show reagents much purer than corresponding reagent grades. However, comparisons of these data with quotations of purity provided by suppliers showed several discrepancies. A sensitive X-ray fluorescence method is also described for the determination of trace transition elements in particulates separated from various commercially available, high purity, water soluble salts. Evidence of significant contamination of pure salts by iron in particulate form is reported.
Highly pure chemicals have become essential for many research and industrial applications. For example, these reagents have been sought by the electronics device industry ( I , 2 ) for trace analysis ( 3 ) , and for fabricating new products ( 4 ) . Commercial suppliers, stimulated by increasing demands for chemicals better than the usual reagent grade, have introduced special lines of ultrapure or electronic grade products ( 5 ) . Careful characterizations prior to commercial distribution have been reported for some of these chemicals. Kershner, Joy, and Barnard concentrated samples from lots of highly pure acetic, hydrochloric, hydrofluoric, nitric, and perchloric acids and determined trace impurities by emission spectroscopy (6). After comprehensively reviewing the literature, and considering cost, convenience, and other factors, Barnard and coworkers concluded that survey emission spectroscopy has proved the most valuable technique for the determination of trace impurities in pure chemicals (7). On the other hand, Hughes et al. analyzed ultrapure water by quiescent evaporation in Teflon, followed by emission spectroscopy and questioned the actual presence of the constituents detected, since the measured quantities may have been derived from the air, electrodes, or containers during their analytical procedure (8). Problems in preventing contamination during the determination of trace elements have been reported by other analysts (9, 1 0 ) . Contamination during the particularly susceptible processes of purification, analysis, and containment of pure chemicals has produced problems partially responsible for the availability of a number of chemicals in uncertain states of purity. Significant enhancement of the difficulty in selecting suitable chemicals for intended application has resulted as well. Previous investigators used polarographic methods to measure nanogram (ng) amounts of Fe, P b , Cd, and Cu in highly pure HC1, HC104, "03 and "4OH (11). Kuehner described detailed procedures for preparing, containing, and storing ultrapure acids (12). In the present investigation, commercially available ultrapure acids, ammonium hydroxide, and hydrogen peroxide
were analyzed by exposing samples to a flux of thermal neutrons and assaying the induced radioactivity by gamma ray spectroscopy. The sensitivity of the method eliminated the need for preconcentrating the trace elements, and greatly decreased the risk of contamination. The analysis of reagents usually includes procedures for determining the percentage of residue remaining after evaporation or filtration. These residues, resulting primarily from insoluble compounds, dust, soot, hair, and other airborne particulates, constitute only a very small fraction of the total weight of the sample. Airborne particulates, however, have been found to contain high concentrations of cations (13, 1 4 ) , thus representing a potential source of significant contamination of pure chemicals. The concentration of particulates in inorganic salt solutions and the efficiency of their removal by filtration have been determined by light scattering techniques (15). Although this method is convenient for measuring the concentration of particles, the determination of the elemental composition of the particulates is not possible. The present investigators have used an extremely sensitive X-ray fluorescence method to determine directly cations in particulates separated from commercially available ultrapure salts. After dissolving the sample and filtering through 0.8-p porosity Millipore filter disks, transition elements in the particulates were identified and quantitatively determined by X-ray fluorescence analysis of the filters.
EXPERIMENTAL Reagents. Acetate, chloride, hydroxide, a n d carbonate salts o f sodium, a m m o n i u m chloride, n i t r i c , acetic, a n d hydrofluoric acids, hydrogen peroxide, a n d a m m o n i u m hydroxide were purchased as u l t r a p u r e o r electronic grade reagents. Liquid c h e m i cals, described as highly p u r e reagents, were stored a t l o w t e m perature ( 5 5 "C) in containers p r o v i d e d by t h e supplier a n d equilibrated for t w o days a t r o o m temperature p r i o r t o analysis. To p r e v e n t c o n t a m i n a t i o n by airborne particulates, reagents were opened i m m e d i a t e l y before analysis, sampled, weighed, a n d (1) H. G. Griffin and T. D. George, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1972, paper No. 72. (2) Werne Kern, RCA Rev., 31,207 (1970). (3) M. Zief, Ind. Res., p 36, April 1971, (4) W. G. French, W. R. Northover, A. D. Pearson, and A. R . Tynes, American Ceramic Society Meeting, Philadelphia, Pa., 1970, paper 5-65-70. (5) M. Zief, Amer. Lab., pp 55-7, October 1969. (6) N. A. Kershner, E. F. Joy, and A. J. Barnard, Jr., Appl. Spectrosc.. 25, 542 (1971). (7) A. J. Barnard, E. F. Joy, K. Little, and J. D. Brooks, Talanta, 17, 785 (1970). (8) R . C. Hughes, P. C. Murau, and G. Gundersen, Anal. Chem. 43, 691 (1971). (9) David N. Hume, Advan. Chem. Series. 67, 30 (1967). (10) David E. Robertson, Anal. Chem.. 40, 1067 (1968). (11) John K. Taylor, Ed.. Nat. Bur. Stand. (U.S.)Tech. Note. 545, 49 (1970). (12). E. Kuehner, Nat. Bur. Stand. ( U . S . ) Tech. Note 549, 60 (1971). (13) E. June Maienthal and R . A. Pauison, Nat. Bur. Stand. ( U . S . ) Tech. Note, 545, 53 (1970). (14) J. Ruzicka and J. Stary, "Substoichiometry in Radiochemical Analysis," Pergamon Press, New York, N.Y., 1968, p 57. (15) D. H. Freeman and W. L. Zielinski, Jr., Ed.. Nat. Bur. Stand. (U.S.) Tech. Note. 549, 62 (1971). A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 8, J U L Y 1973
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4 rnl PLEXIGLAS
Table I. Decay Corrected Net Photopeak Areas of Isotopes in Standard Solutions 56Mna 38cib 24NaC 64CUd 601 77 601 21 61817 60966 61368 631 40 SUCTlOM FLASK
Figure 1. Coprex Microdot filtration apparatus
packaged in a laminar flow hood located in a room continuously supplied with a positive pressure of filtered air. Samples a n d Standards. Stock solutions, prepared by dissolving the pure salts, manganese nitrate, copper nitrate, and sodium chloride, were standardized using conventional analytical techniques. Aliquots of these standard solutions were then diluted with pure water to a known volume. The final concentration of the cation in solution was several orders of magnitude larger than any corresponding trace impurity in the deionized quartz-distilled water. Polypropylene or polyethylene vials and tubing and quartz ampoules were cleaned by successive rinses in methanol, deionized quartz-distilled water, hot nitric acid, and pure water. After rinsing these containers several times by pouring the reagent directly from its freshly opened bottle, the vials or ampoules were filled and the reagent was frozen with liquid nitrogen. Quartz ampoules were then sealed with an oxy-hydrogen flame. Polyethylene tubes were heated in a quartz-lined inverted crucible furnace and sealed by pressing the ends between Teflon rods or platinumtipped forceps. Containers not immediately used were dried under heat lamps in a laminar flow hood and stored. Sealed samples were enclosed in plastic bags, and stored under solid carbon dioxide before being removed from the clean room and transported to the reactor laboratory. Samples were then analyzed within 24 hours. Neutron Activation Analysis. In the pneumatic tube facility of the Industrial Reactor Laboratory at Plainsboro, N.J., samples and standards were simultaneously irradiated with thermal neutrons at a flux of 2-3 X 1013 n/cm2-sec. Solutions of Naf and Cu2+ were used to monitor the thermal flux a t different positions in the polyethylene rabbit (6 X l ? % - i n )Variations . of 3 to 6% were detected. After 10- to 30-minute irradiations, measured aliquots (1 to 10 ml) of the solutions were transferred to 30-ml glass vials or 50-ml plastic beakers and diluted to a known volume. Gamma ray activities were measured by counting under identical geometrical conditions for 10- to 100-minutes using a 1024 channel pulse height analyzer equipped with a lithium drifted germanium detector which was 1170efficient for capture of the 1.332 MeV photopeak of 6OCo. Net photopeak areas for the 0.847 MeV y of 56Mn and 1.347 or 2.75 MeV y of S a Z 4 were computed by Covell’s method and corrected for decay (16). Activity from 64Cu was measured after a 10-hour delay. Occasionally, samples were irradiated up to 100 hours in a well-thermalized graphite column at a flux of 1-2 X 1011 and immediately counted to determine activity from “Cu. 18F produced during the irradiation of HF in the pneumatic tube facility (neutron flux of energy 20.6 MeV = 2 X 1OI2 n/ cm2/sec) prevented the nondestructive determination of Cu in this reagent. Other possible interferences from fast neutron reactions were shown to be insignificant by counting several samples after irradiation in an 80-mil thermal shield of cadmium. (16) D. F.
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Covel1,Anai.Chem.. 31, 1785 (1959)
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1 9 7 3
1713 1519 1790 1663 1520 1556
1390 1138 1224 1293 1133 1190
5651 5303 5383 5772 20257e 19401e
aEnergy of y,0.847 MeV; Vol. of soln, 1.0-ml; concn. 1.06 X g/ml. ?2.75 MeV, 2.0-ml, 2.86 X glrnl. * 2 . 1 7 MeV, l . O - m l , 4.42 X g/ml. d0.511 MeV, l.O-ml, 1.08 X l o - ‘ g/ml. eMeasured with a sodium iodide detector.
X-Ray Fluorescence Analysis of Particulates. Five-tenths of a gram of an ultrapure salt was completely dissolved in 2.0 ml of deionized quartz-distilled water in a 10-ml Teflon (TFE) beaker. The solution was then filtered through a Millipore disk with 0.8-p pores in an apparatus previously described by C. L. Luke (17) and recently modified by J. E. Kessler and S. M. Vincent (18). In the new apparatus shown in Figure 1, insoluble residues were collected in a circular area with a diameter of 100 mils. After the sample was filtered and washed with a few drops of pure water, a gray or black spot, which consisted of contaminating dust and other particulates, was found on all disks. The disks were dried and the microdots were counted for 40 seconds, using a 50-mil aperture on a curved-crystal vacuum X-ray Milliprobe spectrograph equipped with a lithium fluoride crystal and a tungsten target. X-Ray intensities from the paper disks were measured at characteristic wavelengths for Cu, Ni, Co, Fe, Mn, Cr, and V. After blank corrections from the paper disk were applied (see Blank 2, Table Vj, the concentration of each metal present in the sample was then obtained by comparison with suitable calibration disks. It was previously demonstrated by one of us that no significant uncertainties arise from the comparison of X-ray intensities from trace metals in particulates on filters with corresponding trace metals precipitated and collected on calibration disks (19). Calibration disks were prepared as follows: One ml of pure water, a drop of 0.01% aqueous solution of meta-cresol purple, 0.2 ml of 10% HC1 containing 10 pg of Ti(1V) per milliliter and an aliquot of a solution containing 100 ng of each of the seven metals were added to a 10-ml Teflon beaker. The resulting mixture was neutralized to the distinct purple color of the indicator ( i e . , pH 9) by dropwise addition of pure isopiestically prepared ammonium hydroxide solution (20). Five drops (ca. 0.25 ml) of a 2% solution of sodium diethyldithiocarbamate was then added. The mixture was swirled, covered, allowed to stand five minutes, and filtered. The precipitate obtained was washed and then treated as described above. In order to provide for background and reagent blank correction, a second disk was prepared in which the addition of the 100 ng of the 7 metals was omitted (see Table Vj. The precipitation and collection of Fez-, Coz-, Cuzf, Cr3+. and Mn2+ on calibration disks were monitored by radioisotope techniques. Triplicate measurements showed 98.5 f 0.9, 83.6 f 2.4, 94.0 f 1.1, 98.6 f 0.9, and 99.5 f 0.4 percentage recoveries of 100 ng of each metal in the presence of the respective isotopes, 59Fe, 60C0, 64Cu, W r , and 54Mn. Data in Table V show X-ray counts for the remaining elements to be reproducible within f 7 % or better.
RESULTS AND DISCUSSION Neutron Activation Analysis. Since primary standards of comparison in the form of corresponding chemicals with certified trace elements are not available, synthetic standard solutions doped with trace amounts of Cu, C1, Mn, and Na were used. Net photopeak activities from a series of standard solutions that were periodically irradiated and analyzed during a ten-hour period are shown in Table I. (17) C. L. Luke,Anai. Chim. Acta. 41, 237 (1968). (18) J. E . Kessler and S. M . Vincent, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1972, paper No. 70. (19) C. L. Luke, T . Y . Kornetani, J. E . Kessler. and T. C. Loornis, Environ. Sci. Techno/., 6, 11 05 (1 972) (20) H . lrwing and J. J. Cox, A n a i y s t (London). 8 3 , 526 (1958).
Table II. Concentration of Impurities (ng/ml) Found in Nitric Acids Elernent
Mn
CI
Na
1
1 2 3 4a 1 2 3 4a 1
10.2 6.13 8.1 0.6 65 79 55 50 66 50 98 30 19 12 13 13
2
cu
a
Supplier
Sample
3 4a 1 2 3 4a
0.511
2
3
4
0.9 7.3 2.2
29.7 49.0 16.5
...
...
...
259 330
57
...
...
... 53 66 59 500 109 103 124
80 1465 1400
6.1
...
... ... 357
...
...
,..
.,. 8
...
100 355 . . .
...
, . .
500 13
5
0.7
8 8 100
Concentration reported by supplier.
Table I l l . Concentration of Trace Elements (ng/ml) Found in Hydrofluoric Acid Elernent Sample Mn
Na
CI
1 2 3 4a 1 2 3 4a 1 2
3 4a
Supplier
2
1
0.3 0.3 0.6 0.5 12 23 16 20 175 180 1El2
0.2 0.5 . , .
... 11 15 14
... 270 472
1 OClO
... 100
3
0.3 0.3 0.2 ... 12 27 17 . . . 800 805 79 5 ...
4 1 .o 1.1 0.9
... 129 140
... 100 28 1 287 242 80
a Concentration reported by supplier.
The average deviations of the mean of these data, 61265 f 843, 1627 f 97, 1228 k 76, and 5527 f 184, show the conditions for irradiation to be reproducible within k 3 to 6%. It is also apparent that no significant losses of trace elements from the standard solution to the walls of the irradiation container are occurring. Blank values were measured by irradiating clean, empty capsules, rinsing the inside of each with 1:l HKOs and analyzing the rinse solution by gamma ray spectroscopy. The trace elements being determined were not detected in solutions obtained by examining several capsules prior to analyzing reagents or during later periodic checks. Thus, blank values for Na, Cl, Mn, and Cu were less than 10, 10, 0.01, and 1.0 ng respectively. The gamma ray spectra, lB, 2A, 3A, and 4 in Figure 2 were obtained by simultaneously irradiating a series of nitric acids, counting equal volumes for the same length of time, and recording on equivalent ordinate scales. A rapid qualitative survey method, which was convenient for quickly rejecting the most contaminated sample or for selecting the product that was clearly the best, was based on a visual comparison of photopeak heights of isotopes undergoing negligible decay during the counting interval for the series of samples. Quantitative results for Cu, Na, Mn, and C1 were reported in Table 11. The reported values were obtained by separately analyzing two or three different samples of the reagent. During previous work in which
0.511
Figure 2.
1.37 2. I 7 2.75 E N E R G Y OF G A M M A R A Y , M e V
Gamma ray spectra of impurities in nitric acids
Table I V . Concentration of Trace Elements (ng/ml) Found in Acetic Acids Elernent
Mn
Supplier Sample
cu
1 2 3 4a 1 2 3 4' 1
CI
3 4a 1
Na
2
2 3 4' a
1 7.0 13.3
... 1 903 91 6 ... 400 66 ...
... 30 655 799 ... 300
2
3
4
5.0 90 24
2.4 2.7 3.0 ... 359 359
2.4 1.5 2.0 ...
0.5 0.5 ...
...
...
50 4.8
, . .
...
... ...
. , .
9 101 377
3
100 187 159
...
...
300
...
, . .
39 35
... ... ... ... 5 133 66 58 500
Concentration reported by supplier.
A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 8, JULY 1 9 7 3
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Table V. Calibration Data for X-Ray Analysis Metals
cu
Ni
co
Fe
cpsa Blank 1 Net cps Blank 2 c
1131 f 2 0 631 f 4 7 500 f 43 225 f 14
1087 f 45 233 f 2 854 f 43 63 f 4
634 f 50 84 f 3 550 f 47 32 f 1
901 f 8 340 f 14 555 f 20 83 f 2a
Total
a Average of values from two calibration disks obtained during a counting period of 40 sec. disks. Average of values from four blank sample disks.
Table VI. Determination of Impurities in Particulates Separated from Ultrapure Salts Conc. of Trace Elements Sample
Cu
Ni
Go
Fe
48
134 4 1 6
152 3
436 70 16 54 23 04
Na2C03b Na~C03 NaCl
N.D.C
KI NH4CI
N.D. N.D.
NaAc
N.D.
13
N.D.
2
31 2
N.D.
N.D.
a Average of three determinations. ment not detected.
gm/gm)a
Mn
Cr
V
89
23
83
N.D. N.D. N.D.
N.D. N.D. N.D.
N.D. N.D. N.D. N.D. N.D.
4 N.D.
5 N.D.
Reagent grade chemical.
Ele-
similar techniques were used, repetitive measurements of 1.000 ppm Cu and 0.1107 ppm of Mn in synthetic standards showed 0.8 and 5.3% deviation of the means, respectively, from the known doped levels (21). Replicate irradiations and determinations of Na a t 100 ppb showed *lo% relative error. Except in a few cases where widely disparate values resulted from particulate impurities, the concentration reported in Tables I1 to IV represent actual reagent impurities with uncertainties ranging from f 5 to =k30%. The improved quality of the electronic or ultrapure chemicals is evident by comparing data in Table I1 with results for product 3, a reagent grade chemical. The information reported by supplier 1 and the results obtained by these analyses suggest continued deterioration in the quality of the reagent after the supplier’s analysis. The nonagreement of the reported and determined data for trace elements in products 2 and 3 emphasizes the need for the potential user to characterize chemicals carefully before using them in special applications in which the concentration of impurities is critical. Results for the analysis of hydrofluoric and acetic acids are reported in Tables III and IV. Nitric acid, 3, and hydrofluoric acid, 3, were both described as reagent grade chemicals containing low concentrations of Na. An examination of data in Tables I1 and I11 show an order of magnitude greater sodium in the nitric acid supplied in a borosilicate glass vessel, than in hydrofluoric acid contained in a polyethylene bottle. Triplicate analysis of an electronic grade H2Oz indicated 117 f 24 ng/ml of sodium, but the product obtained by purification of this reagent in this laboratory contained 42 f 13 ng of sodium per milliliter. Analysis of reagent grade NH40H for Mn, Na, C1, and Cu, indicated 2.5, 297, 577, and 186 ng/ml, respectively, while an electronics grade product contained 6.6, 26, 407, and 4 ng/ml of these impurities. By appropriately modifying counting techniques and varying irradiation times, traces of other elements in pure (21) J . W. Mitchell, J. E. Riley, and W. R. Northover, J . Radioanai. Chem.. in press.
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, J U L Y 1973
Mn
393 = 25i 63 f 3 331 f 28 27 f 2
Cr
V
441 f 31 76 f 0 365 f 30 36 f 2
480 f 31 372 f 10 108 f 37 15 f 1
Average value obtained by counting two blank calibration
chemicals can be quantitatively measured by spectroscopy of characteristic gamma rays from the isotopes, 60mC0, 52V, IlJmIn, 122Sb, 76As, 72Ga, 27Mg, and 82Br. Attempts were made to determine Co, Fe, Sc, Cr, Ag, Cd, and Zn by irradiating samples for 100 hours in quartz ampoules. Direct analyses of the reagents were precluded by the development of excessive pressure, which exploded ampoules during irradiation. If adequate techniques to prevent contamination are used, the method could be extended to include the determination of these traces by evaporating samples to dryness prior to irradiation. However, the possibility of making unreliable assessments of purity is enhanced due to potential contamination or nonquantitative recoveries of the evaporated residues. Analysis of Particulates. In order to show the reproducibility of the X-ray method used in the present investigation, data obtained from two sets of calibration disks, both prepared within one hour, are shown in Table V. The background intensities for copper, iron, and vanadium are caused by line interference due to La radiation of tungsten, presence of iron in the reagents and in the X-ray spectrograph, and interference from KP radiation of titanium, respectively. In spite of this, the net counts obtained for 100 ng of the seven metals show that the sensitivity of the method is very high and reasonably good precision was obtained. The sensitivity (counts per second/ ng) calculated from the average of the values in Table V was as follows: Cu = 5, Ni = 8, Co = 5, Fe = 5, Mn = 3, Cr = 4, and V = 1. It was also established that a plot of the average counts us. the concentration of metal was linear over the range of 0 to 100 ng of metal. Replicate analyses of 0.5-gram samples of a typical ultrapure sodium carbonate for iron present in particulates showed 16, 41, 27, and 70 ppb of this metal to be present. The variability of the data obtained was undoubtedly due to the non-uniform distribution of particulates in the solid reagents. This variation would probably be significantly reduced by analyzing larger samples. Neutron activation analysis of a sample of sodium carbonate indicated 19 f 8 ppb Cu and 3.3 f 1.8 ppb Mn. X-Ray fluorescence data for impurities in particulates separated from a sample of the same sodium carbonate showed 17 ppb of Cu and no detectable Mn, which indicates significant contamination of the sample with copper from the particulate matter. Average results of analysis in triplicate of particulates in several commercially available pure salts are given in Table VI. The reagent grade NaZC03 is easily identified by inspecting these data. It appears that iron contamination, one of the most common problems in ultrapurification, is significantly induced through fall-out of airborne particulates. Received for review September 13, 1972. Accepted January 30, 1973.
