1788
Anal. Chem. 1985, 57, 1788-1790
50
-
40
-
constant at 2.7 V. The nonlinear relationship makes it more difficult to utilize rise time in this calibration technique. In conclusion, the fiber-optic acoustic calibration source is useful for the determination of the sensitivities of photoacoustic cells. With an ordinary pulse generator, the source provides sufficient acoustic intensity to calibrate less sensitive photoacoustic cells. The unit (pulse generator fiber-optic source) is portable and generates reproducible acoustic wave intensities (&0.3%). The fiber-optic probe is small enough to be used in almost any photoacoustic cell and is chemically inert to most common fluid media. Although it is not proven here, the source can be operated in the sine wave mode using a sine funtion generator (3) to calibrate gas-microphone photoacoustic cells. Finally, the sensitivity determined by the present method is not affected by the optical properties of the cell, but reflects only the acoustomechanical characteristics.
h
3
d 1
+
d
z
c?
0
30t
/
2ol1 10
0;
0.;
0.:
0.; VOLTAGE
0;s GRADIENT
110
112
1!4
(kV/rns)
Flgure 6. Effect of rise time (or voltage gradient) of electrical pulses on aCOUStlC wave emission, wlth quartz cuvette cell as the detector.
continuously. This can be achieved by changing either the amplitude or the rise time of the pulse generator output. Within the range of pulse amplitude from 1.6 V to 3.8 V, the acoustic wave intensity can be varied linearly to suit photoacoustic cells with different detection sensitivities. The limit of detection of a photoacoustic cell can be determined by decreasing the acoustic wave intensity continuously until a limiting signal to noise ratio of 3 is attained. The limit of detection is governed by the noise of the transducer and the signal processing electronics. The relationship between acoustic wave intensity and voltage gradient is not linear, as shown in Figure 6. The voltage gradient can be used to represent the rise time because the pulse amplitude was kept
ACKNOWLEDGMENT We are grateful to Donald R. Wiles for reading this manuscript. LITERATURE CITED (1) Patel, C. K. N.; Tam, A. C. Rev. Mod. Phys. 1981, 53, 517-550. (2) McClelland, J. F. Anal. Chem. 1983, 55, 89A-105A. (3) Lai, E. P. C.; Chan, B. L.; Chan, L. L. Anal. Chem. 1983, 55, 244 1-2444. (4) Voigtman, E.; Jurgensen, A.; Winefordner, J. Anal. Chem. 1981, 53,
.
. . .-.
1449. .- IAAR
(5) Lai, E. P. C.; Voigtman, E.; Winefordner, J. D. Appl. Opt. 1982, 27,
3126-3128. (8) Lloyd, L. B. Ph.D. Dissertation, University 1981; p 48.
of Utah, Salt Lake City, UT,
RECEIVED for review January 28,1985. Accepted March 18, 1985. This research was supported by a GR-5 grant from the Faculty of Graduate Studies and Research, Carleton University.
Isotopic Variatlons in Commercial High-Purity Galllum John W. Gramlich* and Lawrence A. Machlan Inorganic Analytical Research Division, Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, Maryland 20899 Since 1962 the Inorganic Analytical Research Division of the National Bureau of Standards has been conducting a program of absolute isotopic abundance and atomic weight determinations using high-precision isotope ratio mass spectrometry. Although this program has yielded extremely accurate atomic weights for reference samples, the uncertainty associated with a generally useful atomic weight is limited by the isotopic variations among readily available materials or by the lack of knowledge regarding such variations. Thus, an atomic weight determination should include a survey of possible natural or man-made isotopic variations. Research directed toward a redetermination of the atomic weight of gallium is in progress, partially spurred by a discrepancy between recently reported values for this element (1, 2 ) . Further impetus to survey the isotopic composition of gallium was supplied by research in the NBS Temperature and Pressure Measurements and Standards Division to accurately determine the triple-point temperature of gallium (3, 4 ) . Variations in the isotopic composition of gallium could affect the triple-point temperature if the samples studied showed
sufficiently large isotopic differences ( 4 ) . Isotopic fractionation of gallium by more than 10% has been reported when a continuous electrical current is passed through a capillary column containing the metal (5,6). Recent work in this laboratory has demonstrated gallium isotopic fractionation approaching 1% with ion exchange chromatography (7). Since commercially available high-purity gallium is generally purified by multiple recrystallization steps (3), the possibility exists for isotopic fractionation during the recrystallization, and thus variations among lots and manufacturers depending upon the exact treatment of the material. To determine if isotopic variations exist in commercially available gallium, 16 samples of high-purity gallium metal were analyzed using thermal ionization mass spectrometry. EXPERIMENTAL SECTION All samples were handled in a Class-100 clean air environment to minimize environmental contamination. A few milligrams of gallium metal from each lot was transferred from the original manufacturer’s container to a clean and previously weighed Teflon
This article not subject to U.S. Copyright. Published 1985 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
beaker. The beaker was then reweighed to determine the total amount of gallium transferred. The metal was dissolved in ultrahigh purity HN03 (8) and diluted to a volume sufficient to give a concentration of 100 pg of Ga/g in 1.6 mol/L "OB. botope ratio measurements were made on a NBS designed thermal ionization mass spectrometer with a 30-cm radius of curvature, 90" magnetic sector (9). The instrument was equipped with a thin lens "Z" focusing ion source and multicomponent deep-bucket Faraday cage collector. The remainder of the measurement circuitry consisted of a vibrating reed electrometer, voltage-tofrequency converter, scaler, and programable calculator. Timing, magnetic field switching, and data acquisition were controlled by the programmable calculator. Gallium was thermally ionized from a tungsten filament fabricated from 0.001 in. x 0.030 in. high-purity tungsten ribbon. After fabrication, the filaments were degassed in a vacuum and under a potential field for 45 min at a current through the filaments of 4.5 A. Filaments cleaned in this manner generally exhibited no detectable gallium signal or isobaric interferences in the gallium spectral region. Occasionally samples have shown small interfering peaks (presumably hydrocarbons) which can be resolved from the 69Gaand 'lGa mass positions. Even if not resolved, their contribution to an error in the ratio measurement of natural gallium would be less than one part in lo5. Pipets made from quartz tubing were used to transfer the samples from their containers to the filaments. The quartz tubing was cleaned by heating in 8 mol/L "OB for 48 h, followed by several rinsings with ultrahigh purity water. Approximately 5 pL of the sample solution (500 ng of Ga) was placed on the tungsten filament and dried with a current of 1 A through the f i i e n t for 10 min followed by a current of 3 A for 5 min. Because the rate and degree of isotopic fractionation during the mass spectrometric analysis may be affected by variations in the sample loading procedure, all samples were loaded on a programable sample dryer (10) which automatically and reproducibly controls the timing and currents during sample loading. After the initial drying,the filaments were transferred to a separate Class-100 clean air hood and heated for 15 s at 900"C, using an optical pyrometer for temperature adjustment (temperature not corrected for emissivity). This final step was to ensure conversion to the most stable crystalline form of gallium oxide, p-Ga203(11). During this high-temperature drying step, the air flow through the hood was turned off to allow more reproducible temperature settings. Gallium ion currents were measured with a constant accelerating potential of 9.8 kV. The ions of each isotope were brought into alignment with the collector by programable calculator controlled stepping of the magnetic field. The filament current was initially set at 2.15 A, corresponding to an optical pyrometer temperature reading of approximately 700 "C. Initially no gallium signal was observed; however, after 2-3 min, the Ga+ signal would rapidly grow in, reaching a total Ga+ ion current at the collector of approximately 2 X lo-" A at 5 min into the analysis. The f i i e n t current was adjusted at 5,10, and 15 min into the analysis to produce gallium ion currents of 2 X A, 4 X lo-" A, and 6 X 10-l' A, respectively. A t 20 min the signal intensity was adjusted to produce a total gallium ion current of 6 X A, and base line measurements were taken on each side of both isotopes. Data were collected between 20 min and 50 min into the analysis. Ten 1-s integrations of the ion current were made for each isotope before magnetic field switching, with an 8-5 time delay between isotopes to allow for magnetic field stabilization and settling of RC time constants of the measurement circuitry. The rate of isotopic fractionation during the measurement period was reproducible at between 0.02% and 0.03% per hour for the 69Ga/11Ga ratio. Variations in the chemical species on the filament and in the vapor phase in the ionization region can produce different isotopic fractionationpatterns and thus slightly different observed isotopic ratios (12). To verify that the standard sample loading and analysis procedure was not sitting on an "edge" which could cause variations between analyses, an alternative procedure was developed which we feel was sufficiently different to detect such a phenomenon. The principal modifications for the alternate procedure are as follows: tungsten ribbon from a different manufacturer was used, the final high-temperature sample preparation was at 860 "C for 1min, and the analysis was per-
1789
Table I. Specifications of Gallium Samples %
sample no. Ga-1 Ga-2 Ga-3 Ga-4
Ga-5 Ga-6
Ga-7 Ga-8 (SRM 994) Ga-9 Ga-10 Ga- 11
Ga-12 Ga-13
Ga-14 Ga-15 Ga-16
souroe
lot no.
Alcoa Eagle-Picher Alusuisse Eagle-Picher Indium Corp. of Am. Indium Corp. of Am. Ventron, Alfa Materials Research Corp.
3809B (2-59-77 F 17/220
Alcoa
3860 3854 8005 8002 3855 3856 F 17/252 5-57-76
Alcoa
Alcoa Alcoa Alcoa Alcoa
Alusuisse Eagle-Picher
nominal purity
99.9999+ 99.99999 99.9999+ A-24-76 99.99999 99.9 3792 99.999 081474 99.99999 31-AR16681 99.9999 99.9999+ 99.9999+ 99.9999+ 99.9999+ 99.9999+ 99.9999+ 99.99999+ 99.99999
formed at a hotter filament temperature, producing approximately three times the normal signal intensity. The alternate procedure produced an expected increase in isotopic fractionation during the analysis; however, the difference in the isotopic ratio between samples remained constant within experimental error. In addition, two of the samples, Ga-2 and Ga-3, were compared by using the silica gel technique reported by DeLaeter and Rosman (1)for gallium analyses.
RESULTS AND DISCUSSION The relative isotopic compositions of 16 commercial gallium samples were measured during this investigation. The sources, lot numbers, and nominal purities as specified by the manufacturers are reported in Table I. Several of these samples have been used to determine the triple-point temperature of gallium; thus additional information on the properties of these lots is available in the literature (3, 4). Since no standards certified for absolute isotopic composition of gallium currently exist, all isotopic ratio measurements reported in this paper are observed measurements not corrected for isotopic fractionation effects which occur during the thermal ionization process. To allow for future correction of these data to absolute values, all measurements were compared to sample Ga-8 which is scheduled for future issue as NBS Standard Reference Material 994, to be certified for absolute isotopic composition. The observed 6gGa/11Garatio for Ga-8 obtained during this work was 1.52841 f 0.000 11 (1s). All measurements on Ga-8 between 1976 and 1983 (51 determinations) yield an average "Ga/I1Ga ratio of 1.528 31 f 0.000 14 (1s). The long-term precision for isotopic ratio measurements on Ga-8 (ca. 0.01% standard deviation) is applicable to the ratio measurements on the other samples since parameters such as sample purity, analysis technique, etc., which can potentially introduce systematic errors and increased uncertainty in the measurement are comparable for all samples. Many of the gallium samples were reanalyzed several months after the initial measurements. In all cases, reanalyzed samples were within 0.01% of the initially obtained isotopic analyses. Table I1 gives the observed 6gGa/11Garatios obtained for the samples using both the standard analytical procedure as well as the alternate higher temperature procedure, as discussed in the experimental portion of this paper. Also listed are the ratios of the data for the standard (Ga-8) to the data for the other samples. All known parameters which could cause variations in the systematic error components of the measurements have been carefully controlled. Thus the ratio of ratios (Ga-8/sample) should provide an absolute difference
1790
Anal. Chem. 1985, 57, 1790-1792
Table 11. Observed Isotopic Ratios of Gallium Samples and Comparison with Ga-8 (SRM 994)
sample no. 1
6gGa/71Ga Ga-8 ratio/sample ratio Standard Procedure
Ga-1 Ga-2 Ga-3 Ga-4 Ga-5 Ga-6 Ga-7 Ga-8 Ga-9 Ga-10 Ga-11 Ga-12 Ga-13 Ga-14 Ga-15 Ga-16
1.52729 1.530 17 1.52646 1.527 75 1.52954 1.528 57 1.527 29 1.52841' 1.52696 1.52580 1.52559 1.52549 1.52569 1.52620 1.52612 1.52796
1.00073 0.998 85 1.00127 1.00043 0.999 26 0.999 90 1.00073 1.00095 1.00171 1.00185 1.00191 1.00178 1.00145 1.00150 1.000 29
Ga-3 value is 1.002 32. The corresponding values obtained by using the standard and alternate procedures are 1.002 43 and 1.002 50, respectively. The agreement between data observed for the silica gel technique and the methods of sample analysis reported in this paper gives support to the contention that the observed isotopic variations are real. Little information is available in the literature regarding isotopic variations of gallium in nature. Values reported during the past 35 years for the isotopic composition of gallium range over 3%; however this spread can possibly be explained by measurement imprecision. DeLaeter (13) reports a maximum deviation of 0.11% in six meteorites relative to the isotopic composition of his terrestrial standard. This work supports other reported observations of isotopic fractionation in gallium resulting from physical processes and may partially explain relatively large differences in recently reported values for the atomic weight of gallium (1,2). When high accuracy stoichiometry is required for gallium or its compounds, the isotopic composition of the particular sample should be given consideration.
Alternate Procedure Ga-1 Ga-2 Ga-3 Ga-4 Ga-5 Ga-6 Ga-7 Ga-8
1.526 60 1.529 74 1.52592 1.527 31 1.52894 1.52804 1.52685 1.52793b
ACKNOWLEDGMENT 1.000 87 0.998 82 1.00132 1.00041 0.999 34 0.999 93 1.00071
'13 determinations, s = 10.00011. b 7 determinations, s = 10.00007.
in the composition of the samples. On the basis of the review of the analytical procedures and data by the NBS Statistical Engineering Division, t h e uncertainty in the precision of the @Ga/'lGa ratios reported in Table I1 is estimated at 10.01 % (1s).
Two of the more extreme compositions, samples Ga-2 and Ga-3, were also compared by using the silica gel technique reported by DeLaeter and Rosman ( I ) . Although this technique does not appear to be as precise, the use of silica gelphosphoric acid for ionization enhancement provides a considerably different mechanism for ionization; by use of the 69Ga/71Garatios measured for these two samples the Ga-2/
The authors thank B. W. Mangum and D. D. Thornton for the gallium samples and for helpful discussions during this work. Registry No. Ga, 7440-55-3;@Ga,14391-02-7;'lGa, 14391-03-8. LITERATURE CITED DeLaeter, J. R.; Rosman, K. J. R. Int. J . Mass Spechom. Ion Phys. 1976, 27, 403-409.
Marlnenko, G. J . Res. Natl. Bur. Stand., Sect. A 1977. 87A, 1-4. Mangum, B. W.: Thornton, D. D. MetrolOga 1979, 75, 201-215. Mangum, B. W. Temperature 1982, 5. 299-309. Nlef, G.; Roth, E. C . R . Hebd. Seances Acad. Sci. 1954, 239, 162-184.
Goldman, M.; Nlef, G.; Roth,
E. C . R . Hebd. Seances Acad. Sci. 1956, 243, 1414-1416. Machlan, L. A.; Gramlich, J. W., in preparation. Kuehner, E. C.; Alvarez, R.; Paulsen, P. J.; Murphy, T. J. Anal. Chem. 1972, 44, 2050-2056. Shields, W. R., Ed. NBS Tech. Note ( U S . ) 1967, No. 426. Gramllch, J. W.; Shldeler, R. W. NBS Tech. Note ( U S . ) 1982, No. 1754. Roy, R.; Hill, V. G.; Osborn, E. F. J . Am. Chem. SOC. 1952, 7 4 , 719. Moore, L. J.; Heald, E. F.; Fllllben, J. J. Adv. Mass Spectrom. 1978, 7 A I 448-474. DeLaeter, J. R. Geochlm. Cosmochlm. Acta 1972. 3 6 , 735-743.
RECEIVED for review December 5, 1983. Resubmitted February 13, 1985. Accepted February 25, 1985.
Determlnatlon of Urea in Fertilizers Uslng Ion Chromatography Coupled with Conductometric Detection Dorita E. Menconi
Sierra Chemical Company, 1001 Yosemite Drive, Milpitas, California 95035 Government regulation of the fertilizer industry makes reliable measurement of the nutrient content of chemical fertilizers extremely important. One of the major nutrients found in fertilizer is nitrogen, which may be present as an inorganic salt of nitrate or ammonium or as urea. All of these species can be determined using ion chromatography (IC); however the first two species are detected by conductivity while the third requires spectrophotometric detection. In many laboratories the small number of urea determinations performed may not justify the additional cost of this special 0003-2700/85/0357-1790$01.50/0
detector and the separate columns required. The objective of this report is to describe of urea determination using IC coupled with conductometric detection. EXPERIMENTAL SECTION Approach. These are many different methods for the determination of urea (1-13). The approach used here is to make an aqueous solution of the fertilizer and hydrolyze any urea present by adding a 1% solution of urease. The urease enzyme catalyzes the hydrolysis of urea to ammonium ion. The ammonium ion, a conductive species, can be easily determined using IC with a 0 1985 American Chemical Society
