Anal. Chem. 1981, 5 3 , 245-248
(5),1.104456 g/cm3 at 25 "C = Dd = density of deuterium oxide (7), and 0.9970429 g/cm3 at 25 "C = D, = density of pure natural water (7).From the above equation, the density of protium oxide is 0.997 027 4 g/cm3. The instrument constant "k" is calculated from (3) where TI = the period reading for pure natural water, Tz= the period reading for air, D, = the density of water at 25 "C (g/cm3), and D, = the density of air at 25 "C and ambient pressure (g/cm3). If the density of air is not known, it can be calculated from 0.0012930 P D, = (4) 1 (0.00367 X t)%
+
where D, = the density of air (g/cm3), t = temperature ("C) and P = pressure (torr). The density of the unknown sample (d,) is calculated as
+
d, = k(T2 - TW2) D,
(5)
where k = the instrument constant (eq 3), T, = the instrument reading for the unknown sample, T, = the instrument reading for pure natural water, and D, = the density of water at 25 "C. The atom percent deuterium oxide (N) is calculated from (8)
N =
D, - D, Dp((Md/Mp) - 1) +'Ds(1 - (MdDp/Mpd))
x 100 (6)
where at 25 "C D, = density of sample, D, = density of protium oxide, Dd = density of deuterium oxide, Mp = molecular weight of protium oxide, and h f d = molecular weight of deuterium oxide. Constants as of April, 1980, are D, = 0.997 027 4 g/cm3, Dd = 1.104456 g/cm3 (5),Mp = 18.01505 g/mol (6), and Md = 20.027604 g/mol (6, 7). The determination of deuterium in lithium hydride-lithium deuteride mixtures using the vibrating probe density meter is a useful analytical
245
technique. Comparing results with other methods of analyses has shown that the electronic density determination is more precise and less time consuming than mass spectrometry and nuclear magnetic resonance. The vibrating probe is more accurate than the other methods of analyses on a broad range of concentrations because there is no dependency upon empirical calibration with a "known" deuterium standard. High-purity natural water was used to calibrate the electronic density meter, but any precisely known density standard could have been used.
ACKNOWLEDGMENT The authors are grateful to R.L. Jamison, Jr., and J. W. Charles, Jr., for their help in locating and calculating the most recent values for constanta in the equations and to the Nuclear Magnetic Resonance Laboratory and the Isotopic Laboratory for their analyses of the samples. Thanks is given to the Materials Testing Laboratory for modification of the Leco induction furnace.
LITERATURE CITED (I) Mettler Instrument Corp. "Operating Instructions for Mettler Density Instrument DMA 60/601";Mettler Instrument Corp: Hightstown, NJ, 1977. (2) Johnson, E. E., Union Carbide Corp.-Nuclear Division, Oak Ridge Y12 Plant, Oak Ridge, TN; personal communication to L. A. Stephens, Sr., March 20, 1978. (3) Kohlraush, "Praktische Physiks"; G. G. Toebner; Stuttgart, Germany, 1968: Val. 3. Sectlon ~22. ........... (4) Furman. N. Horwell In "Scott's Standard Methods of Chemical Analyses", 5th ed.; Van Nostrand: New York, 1948;p 389. (5) Table of Isotopes "CRC Handbook of Chemistry and Physics", 58th ed.; Chemical Rubber Publishing Co.: Cleveland, OH, 1978; p 8271. (6) Pure Appl. Cbern. 1974,37,600. (7) Jones, Frank, National Bureau of Standards, Washington, DC, personal communication, Feb 1978. (8) Kirschenbaum, Isidor "Physical Propertles and Analysis of Heavy Water", National Nuclear Energy Series; McGraw-Hill: New York, 1951;p 15.
RECEIVED for review May 27,1980. Accepted November 12, 1980. The Oak Ridge Y-12 Plant is operated by the Union Carbide Corporation's Nuclear Divison for the Department of Energy under U.S.Government Contract W-7405-eng-26.
Radiotracer Techniques for Evaluation of Selenium Hydride Generation Systems D. C. Reamer,* Claude Veillon, and P. T. Tokousballdes' Human Nutrition Research Cenfer, USDA, Building 307, Room 2 15, Beltsville, Maryland 20705
Several SeH, systems and materials of constructlon are evaluated by use of '%e as a radiotracer. Polypropylene, two types of Teflon, and both silanired and unsllanlzed glass are evaluated. Glass and polypropylene exhibit the greatest absorption of selenium and silanired glass the least. A quartz furnace atomic absorption system Is described havlng a detection llmlt of 3 ng.
The utilization of hydride generation/atomic absorption spectrometry (AAS)for the analysis of selenium is continually Present address: 22 Narcissou Street, Kiffissia, Athens, Greece.
increasing in popularity (1-12). Siemer and Koteel (8)compared different hydride generation/AAS techniques and mentioned methods of optimizing systems to obtain maximum sensitivity. However, little information is available on the adsorptive effects of various construction materials for hydride generators. McDaniel et al. (5) used radiotracers to evaluate a variety of existing procedures for SeHz generation. With their system, it was reported that the SeHz was being transferred from the solution to the atomization source with an efficiency approaching 90%. The present paper describes radiotracer studies designed to investigate the performance of different types of materials used in the construction of SeHz generators. The evolution and transport of SeHz from the acid medium can be affected by the type of material used in
Thls article not subject to US. Copyright. Published 1981 by the American Chemical Society
246
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2. FEBRUARY 1981 m
LIC.
HARCOAL
&-Ty
SYRINGE-
t
l
FURNACEHEATEDTO 900.C
m e 1. oiagrarn of Temn (TFE) and pciypropylene reaction vessels. the construction of the hydride generator. EXPERIMENTAL SECTION Instrumentation. The selenium analyses by AAS were performed with a Perkin-Elmer 503 atomic absorption spectrometer equipped with a selenium electrodeless discharge lamp (EDL). Modifications of the atomic absorption unit involved replacement of the burner with an electrically heated quartz absorption tube connected to the SeH, generation system. The absorption tube was constructed from a 10 mm X 18 cm quartz. tube left open at both ends. A 6 mm X 10 cm quartz gas inlet was connected a t the center and the 12-cm central portion of the absorption tube was wrapped with asbestos tape, then wrapped with Chrome1A wire, and then with more asbestos tape. A variable transformer was used to apply a voltage across the nichrome wire. The cell, mounted in a holder, was fitted into the receptacle of the adjustable burner mount of the instrument. The absorption tuhe was optically aligned in the light path of the atomic absorption unit so as to maximize the intensity of source radiation passing through its opening. An air inlet was lncated in the glass transfer line between the generator and the quartz.furnace. The gas flows were monitored with calibrated rotameters (air 150 mL min, hydrogen 300 mL/min). For the radiotracer studies, Se '.m samples was counted in a Searle Analytic 1185 y spectrometer. Adsorbed 75Seon apparatus was counted in a Nuclear Chicago 825 small animal whole body counter. Reagents. All reagents were a t least reagent grade, and high-purity deionized water was used throughout. The NaBH, was obtained as a powder (No. 87658, Ventron Corp., Danvers, MA) and was of the highest purity obtainable (99%). Solutions of 5% NaBH, were made daily in 0.1 M NaOH. The radioactive '5Se tracer (sodium selenite) was obtained from New England Nuclear, Boston, MA. A 1000-ppmselenium stock solution was prepared hy dissolving elemental selenium in a minimum amount of nitric acid and diluting with 6 M HCI. Further dilutions were made when necessary. Reagent grade HCI and HISOI were also used. The SeH, generating solutions used were 3.5 N in HCI and 0.9 N in H,SO,. Hydrogen Selenide Generators. Several different generating systems were developed and various construction materials were tested to ascertain their operating efficiency. The materials used were polypropylene, two types of Teflon, and both silanized and unsilanized glass, The design of the Teflon tetrailuoroethylene (TFE) and polypropylene generators evaluated is shown in Figure 1. The generators were machined from solid blocks of material and fitted with Teflon fittings. The generator was connected with a Teflon or glass transfer tube either to a polystyrene tube containing cocoanut charcoal to adsorb the generated 75SeH2or to the quartz furnace, respectively. A syringe fitted valve was positioned in the nitrogen line to allow for purging of air from the system prior to the injection of the NaBH,. The injected NaBHl was forced into the sample solution by the purge gas and the generated %eH, was quantitatively trapped in the charcoal for subsequent counting, or conducted into the fumace. The glass generator (Figure 2) differed in design in that the NaBH, was injected into the purge gas from the top to be mixed with the sample. When the furnace was attached to the generator,air was
d
F m e 2. W g a m of glass reactkm ve&
and heated quartz furnace.
introduced into the system just before the furnace entrance U) mix with the hydrogen and produce an air/hydrogen flame at the furnace "1"'junction. Tracer Procedure. Radioactive selenium was added to s a m plea and standards to determine the efficiency of the hydride generators and the distrihution of the SeH, during the generating process. The generated gnseous SeH, was adsorbed on rnmanut charcoal to obtnin mass balnnces. AU pans of the reaction vessel were separately counted fur -%e in the whole body 1 counter and included in the mass balance calculations. A11 solutions and rharroal adsorbent were counted for '%e in the well-type 'I counter. Analytical Procedure. At the start of the analytical cycle when wing a charcoal trap. a 10mL sample containing a %e spike was placed in the reaction vessel and closed to the environment. A polystyrene tube containing approximately 0.5 g of charroal was attnrhed to the exit port of the generator. and the system was purged with nitrogen. By use of a 5ml. polypropylene syringe. 4 ml. oi5% NaRH, was injected into the nitrogen stream over an inter\,al of 5-7 8. The system continued to be purged with nitrwen for 1 min after the NaBH. injection. After each reaction, the generation vessel was drained and rinsed with water. The NaBH, injection port was also rinsed to prevent a premature reactiun of the next sample with residual NaRH,. The apparatus was disassembled. and all the D ~ were S counted for "Se in the whole body counter. When the qumz furnace was used, the procedure followed was slightly different. After the sample was placed in the reacton vessel and closed, the system was first purged for approximately 1 min with nitrogen, and then with hydrogen. Air was then introduced into the furnace. pmducinp;a small air/hydrogen flame at the entrance oi the furnace. Next. the NaRH, was injected as prewously described and the SeH, passed into the furnare and atomized by the flame and heatpd flame. After the injection, the absorption4gnalreturned tothebanelinein 12-15s. Thesystem was rinsed as previously desrrihed and was ready for the next sample. Kach sample required 4 - 3 min to analyze.
RESULTS AND DISCUSSION Conditions in t h e Sample Mix. It was necessary to initially optimize the production of SeH, by determining the quantity of NaBH, to be injected and by evaluating the types of acids and their required concentrations. The design of the hydride generator was investigated with several types of material used in the construction of the generator. By use of radiotracer '%e, the mncentrations of HCI and H2S0, were evaluated and optimized for maximum selenium signal. F w 3 illustrates the effect of the HCI roncentration on the SeH2 production. Similar results were obtained for H,SO,, and the optimum acid concentrations were found lo he :1 N HCI and 0.9 N HISO,. Figure 4 shows that for the glass reaction vessel a 7% NaBH, concentration was optimum, for the glass reaction vessel. A 5 % solution was used in this work because less hydrogen was evolved at the lower concentration with only a slight decrease in sensitivity. The NaBH, was prepared in 0.1 N NaOH to decrease its rate of decomposition.
ANALYTICAL CHEMISTRY, VOL. 53. NO. 2, FEBRUARY 1981
I
I
2
I
I
3
4
I
247
I
5
6
HCI MOLARITY Flgure 3. Effect of HCi concentration on SeH, formation. rl.m,mn
NO
Flgure 5. Effect ol Um nmbw of reactions on fhs quantity of &im
retained by the Teflon (TFE) reaction veszas.
CWARCOAL-
120rnl
VOIY".
N O ~ H +
Flgure 4. Effect of NaBH, wncentration on peak Msht and quantity of selenium adsorbed by the walls of the glass reaction vessel. Table I. Summary of 'ISe Adsorption onto Reaction Vessels % "'e retained
material nolvnronvlene +efion (~;FE) Teflon (PFA) glass silanized glass resilanized glass
32 t 8 13.7 + 16.8 11 f 3 23.1 f 13 7.6 t 5.6 2.0 * 1.1 ~~
~
Flgure e. Diagram of Teflon (PFA) reaction vessel. no. of trials 5
45 9 59 18 40
Evaluation of Material in theConstruction of Hydride Generators. The retention of SeH, by materials used in the hydride generators was evaluated by spiking samples and standards with "Se and following it throughout the analytical procedure. Table I summarizes the results from five different reaction vessel surfaces. 'Ihe data demowtrate the imwrtance of knowing what is happening on the walls of the reaction vessel hefore the SeH? reaches the atomization source. The polypropylene generator (Figure 1 J retained an average of 32% of a l M p p h solution of selenium. Once retained, the selenium could not be removed with water hut was removed with 2 N nitric acid. 'I'he'retlon T F E generator (Figure 11 reacted in an unusual way. Figure 5 is a summary of 15 data pointa from three trials. It initially retained greater than 30% of the generated SeH,, but as subsequent reactions were performed the quantity retained decreased. It is possihle that the available adsorption sites were being filled and therefore deactivating the Teflon surface toward further SeH, adsorption. When the Teflon generator was washed with concentrated nitric acid, the selenium was removed and with further hydride generations the surface was shown t n have reverted
'
!
::L 30
20
10'1
2
3
4
5
6
7
8
9
1
0
Reaction No
Effectof Um m b e r of reacticns on Um quantity of seleniun retained by ihe Teflon (PFA) reaction vessel. Figue 7.
to ita original high selenium retention characteristics. A second type of Teflon generator using a Teflon perfluoroalkoxy (PFA) resin was also evaluated (Figure 6). The generator was constructed from a commercially available 1WmL screw top veasel. Figure 7 illustrates what occur8 when a poorly designed generator is used. Because of the vesael size, it was necessary to stir the solution to achieve good mixing
248
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
A
CHARCOAL SOLUTION VESSEL
IOOC
CHARCOAL T R A P
ASOLUTION
v) 0
40 20 -
4- I
REACTION No
Flgure 9. Effect of the nmber of reacths on the quantity of selenium retained by the silanized glass reaction vessel. REACTION No.
Figwe 8. Effect of the number of reactions on the quantity of selenium retained by the glass reaction vessel. during the introduction of the NaE%H4.Even with the stirring, only 70% of the generated SeH, reached the charcoal trap and an additional 10% was retained by the Teflon PFA walls. The remaining selenium was unreacted in the solution. The large surface area of the generator prevented adequate mixing of the NaBH4 with the selenium in solution. With a welldesigned generator, a Teflon PFA hydride generator could be acceptable, if the analyst was willing to accept a consistent 10% loss of the generated SeH, to the container walls. Since glass is the most popular material used in the construction of hydride generators (Figure 2), it was the most exhaustively evaluated. As shown in Figure 8, the quantity of SeH, that reached the charcoal trap was inversely related to the quantity that was retained on the generator walls. The majority of the radioactivity was found on the upper portion of the reaction vessel, indicating that the selenium had been converted to the hydride, and then was attached to the glass walls and probably decomposed to its elemental form. After many reactions there was an orange coloration on the upper portion of the reaction vessel indicative of elemental selenium. The retained selenium was partially removed with water and was totally removed with concentrated nitric acid. The wide variations in the quantity of selenium retained from one reaction to another make it impossible to obtain good analytical precision, in spite of the near completeness of the conversion to SeH,. For minimization of this problem, the surface of the glass was deactivated by coating it with a silanizing agent (dimethyldichlorosilane). A 10% solution of dimethyldichlorosilane was prepared in methanol, and the glass parts of the generator were heated in the solution and oven dried a t 90 "C. Repeat treatments ensured a complete coating of the glass surfaces and lasted for at least 50-100 reactions. The silanization greatly reduced the SeH, retention of the glass walls (Table I). Figure 9 illustrates the consistent recovery in the charcoal trap of the radioactive selenium spikes. After resilanization of the glass generator, the quantity of SeH, retained was approximately 2% and an average of 5% of the selenium was not converted to the hydride. Therefore, with this hydride generation system, 93% of the selenium can reach the atomization source to be analyzed. Quartz Furnace, The quartz furnace was modified to allow for air to be introduced into the hydrogen flow just before it reached the heated tube. This produced an air/ hydrogen flame at the entrance to the heated portion of the furnace. When the airlhydrogen mixture reached the furnace heated at 900 "C, a flame spontaneously occurred. The air and hydrogen flow rates were adjusted to position the flame
just at the furnace entrance. When the N a H 4 was introduced into the acidic sample, the generated hydrogen forced the flame into the furnace increasing the flame size. The end 3 cm of the quartz furnace were not heated so that excess hydrogen leaving the furnace would have cooled sufficiently so that it would not spontaneously burn when coming in contact with the air. If flames did occur a t the ends of the furnace, an absorption signal resulted due to absorption by flame gas species. The flame eliminated the need to remove water and HCl vapors prior to their introduction into the furnace, and it increased the analytical sensitivity by aiding in the atomization of the SeH,. absorbance signals during the analysis of blanks due to gas perturbations in the furnace were minimal because of the furnace flame, as well as the deuterium arc background corrector. Calibration curves are linear from the detection limit up to about 100 ng/mL when measuring peak height and can be extended to at least 500 ng/mL by measuring peak area. The detection limit of any analytical technique depends on both the size of the sample and the absolute detection limit. The absolute detection limit achievable by this procedure is limited by the gas perturbations in the furnace when the SeHz is generated and atomized. The detection limit (twice the standard deviation of the blank) is 0.3 ng of Se/mL on a concentration basis, and with a standard 10-mL sample size, 3 ng of selenium in a sample can be detected.
LITERATURE CITED (1) (2) (3) (4) (5)
(6) (7) (8) (9) (10) (11) (12)
Siemer, D.; Hagemann, L. Anal. Left. 1975, 8 , 323. Walker, H.; Runnels, J.; Menyfleld, R. Anal. Chem. 1978, 48, 2056. Florlno, J.; Jones, J.; Caper, S. Anal. Chem. 1978, 48, 120. Vljan, P.; Wood, G. Talenta 1978, 23, 89. McDanlel, M.; Shendrlkar, A,; Relszner, K.; West, P. Anal. Chem. 1978, 48, 2240. Ihnat, M. J. Assoc. Off. Am/. Chem. 1978, 59. 911. Clinton, 0. Analyst(London) 1977, 702, 187. Siemer, D.; Kotwl, P. Anal. Chem. 1977, 49, 1096. Flanjak, J. J . Assoc. Off. Anal. Chem. 1978, 67, 1299. Egaas, E.;Julshamn, K. At. Absorpf. News/. 1978, 77, 135. Robblns, W.; Caruso, J. Anal. Chem. 1979, 57, 891a. Jackwerk, P.; Wilier. P.; Hohn, R.; Berndt, H. At. Absorpf. News/. 1979. 78, 66.
Received for review September 8,1980. Accepted November 6,1980. D.C.R. is a Research Associate, Children's Hospital, Boston, MA, and is supported in part by General Cooperative Agreement No. 58-32U4-0-127. P.T.T. was a Research Associate, Chemistry Department, University of Maryland, College Park, MD, and supported in part by Specific Cooperative Agreement No, 1090-20912-013A. Presented in part a t the 31st Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 1980. Specific manufacturer's products are mentioned herein solely to reflect the personal experiences of the authors and do not constitute their endorsement nor that of the Department of Agriculture.
