Determination of Deuterium in Water LEWIS FRIEDMAN AND ADOLPH P. IKS.4’ Brookhaven National Laboratory, Upton, Long Island, N . Y . Difficulties encountered in the mass spectrometer analysis of hydrogen-deuterium mixtures produced from water samples stimulated an investigation of alternative procedures for the determination of the deuterium content of water. The reaction of zinc diethyl with water has been studied and conditions have been found under which quantitative yields of ethane and deuteroethane were obtained. Approximately 0.5 hour is required for the preparation of a gas sample from 0.01 ml. of liquid water. Memory effects are minimized in the sample preparation
T
H E conventional analytical procedure for the rapid determination of deuterium in water samples of limited size, of the order of 10 mg., involves its conversion to hydrogen by reaction with a suitable metal a t elevated temperature (3) followed by mass spectrometer analysis (I). R a t e r mag be run directly, if some sacrifice in accuracy can be tolerated ( 5 ) . This method suffers from “memory effects” resulting from heterogeneous exchange of hydrogen with water in the mass spectrometer and the conversion apparatus. Fractionation of the isotopes and voltage discrimination in the mass spectrometer make frequent empirical calibration of the instrument necessary for accurate results. A more efficient procedure would be the quantitative conversion of water to molecules which did not exchange hydrogen with water and of sufficiently high molecular weight so that fractionstion and voltage discrimination effects in the mass spectrometer introduced negligible errors when the isotopic species were determined. A preliminary investigation of the reaction of water with aluminum carbide
6H10
+ AlrCa +3CH4 + 2A13O3
did not produce quantitative yields of methane. They assumed no fractionation of isotopes in this process and this assumption Beems to be borne out by their results. The conversion of water to ethane by reaction with zinc diethyl
+ Zn(C2H& +ZnO + 2C2He D20 + Zn(CzH5) +ZnO + 2C2HjD H20
(5)
EXPERIMENTAL
Standard samples of water are converted to ethane in the a p paratm shown in Figure 1.
,
VACUUM LINE
k
Figure 1. Apparatus for Preparation of Ethane
Flask A is carefully dried and cooled to room temperature, and a 10-pi. sample of water is introduced into it, using a previously carefully dried micropipet which has been flushed several times with samples of the water to be analyzed. The flask, a 1-ml. bulb on the end of a standard-taper 12/30 joint, is then attached t o the apparatus, cooled with liquid nitrogen, and evacuated. After all the air is removed the droplet of water should be in the bottom of the bulb; if not, the flask is warmed and the water is condensed on the bottom of the bulb using liquid nitrogen. Zinc diethyl is then distilled from the reservoir flask, B, into A . Approximately 0.5 to 1 ml. of liquid is transferred for a run, using 10 pl. of water. The excess can be almost quantitatively recovered when the reaction is completed. The liquid nitrogen
7” + H10 +CHI hlg \ I OH CH,
(4)
showed some promise of virtually eliminating many of the difficulties associated with both the hydrogen procedure and the Zerewitinoff reaction. This paper describes a procedure for the quantitative conversion of water to ethane and the mass spectrometer determination of ethane-ethyl deuteride mixtures.
(1)
revealed that a side reaction producing hydrogen was very difficult to eliminate. In addition, i t was found that the methane-methyl deuteride system was unsatisfactory for spectrometer analysis. I n spite of the fact that the conditions of molecular weight and nonexchange with adsorbed water are fulfilled, the analysis is interfered with by the very same adsorbed water which introduces the memory effects nientioned above. A n y h y d r o x yl-con t a i n i n g molecule which ionizes on electron impact to produce masses of M l e = 17 and M / e = 16 must be kept out of the spectrometer if the instrument is to be used for deuterium analysis via the methane-methyl deuteride system. Orchin, U‘ender, and Friedel (9)have adapted the Zerewitinoff ( 7 ) active hydrogen deterniination for the analyses of deuterium in water. They found that the reactions
CH3MgI
system but serious errors result from fractionation of isotopes if the reaction is not carried to completion. Data are presented indicating the magnitude of this effect. The ethane-deuteroethane system was analyzed with a Consolidated-Nier mass spectrometer with a precision of 0.3%. The chief limitation of this method of analysis is interference by “natural” C13 in ethane at low deuterium concentrations. Over-all accuracy of about 2% can be obtained on routine analysis of water samples containing more than 0.5 atom % deuterium.
+ MgO.MgIz
I 876
a77
V O L U M E 2 4 , NO. 5, M A Y 1 9 5 2 Table I.
Mass Spectra and Sensitivities for Ethane and Ethyl Deuteride
CiHs Mass F U Sb 31 30 1.00 1.00 29 0.74 0.765 Sensitivity at parent mass 1.06 1 . 0 0 Friedman and Irsa. b Stevenson and Wagner (4). C Turkevich e l ol. (6).
_
_
CnHsD
Sb
TC
0.676
F* 1.00 0.701 3.06
1.00 0.608 2.47
1.00 0.731 3.304
-
1.00
1.00
TC
-
1.00
-
Q
bath surrounding it is removed and, as soon as the zinc diethyl starts t o melt, bubbles of ethane may be observed evolving from the liquid. A large excess of zinc diethyl is desirable to avoid local overheating which may induce a violent decomposition reaction. Although samples of water as large as 0.1 ml. have been decomposed, the hazard of a reaction in which the zinc ethyl itself decomposes increases with sample size. The reaction appears to proceed in two steps. Conversion proceeds smoothly to 50 to 75% of completion as A gradually warms t o room temperature. At this point bubbles may cease to evolve and the liquid assumes a viscous gelatinous appearance. Gentle warming with the hand or warm water will initiate further reaction, which proceeds vigorously for several minutes. The zinc diethyl becomes more fluid and some zinc oxide may precipitate from the solution. In order to ensure complete reaction, the zinc ethyl is refluxed four or five times for 5- to 10-second intervals by heating cautiously with the luminous flame of a microburner after the second phase of the reaction has stopped. The ethane and any water vapor entrained by i t is then condensed into A using a liquid nitrogen bath. This is replaced with a dry ice-acetone bath and the ethane is condensed in the first trap using liquid nitrogen. The stopcock between A and the rest of the apparatus is closed and A is gently heated with a luminous flame, refluxing the zinc ethyl again. The three traps in series are cooled with acetone-dry ice, secbutyl chloride-liquid nitrogen slush, and liquid nitrogen. The ethane is taken through these traps and any entrained zinc ethyl is separated by condensation. Traces of ethane in A produced by the last heating are also collected in the third trap. Finally the liquid nitrogen bath on the third trap is replaced by the secbutyl chloride bath and the product ethane is condensed in the evacuated sample bulb, C. The time required for the conversion of 10 GI. of water following the procedure outlined above is approximately 0.5 hour. The product ethane was analyzed in a Consolidated-Sier Model 21-201 mass spectrometer. Patterns for ethane and ethyl deuteride were determined a t 1500 volts ion-accelerating potential using magnetic scanning. A ratio of the sensitivity coefficients of the two gases is required for the super-position method of analysis. The Model 21-201 may be adapted for the determination of sensitivity coefficients of these isotopic molecules in the following manner. A 200-ml. bulb is attached to one of the two sample inlets to serve as a gas reservoir. Ethane is introduced through the other inlet and compressed to a pressure of 100 to 110 mm. by raising the Toepler pump to the point a t which the check valve is seated by the mercury. Gas pressure is then read and recorded. The gas is then expanded into the reservoir bulb and a final pressure compatible with the requirements of the spectrometer tube and leak is obtained. The sensitivity coefficient is then determined with a precision limited by the precision in reading a pressure of between 100 and 110 mm. on the sample system manometer, approximately 0.5%. The pump-out time of ethane in the spectrometer using acetone-dry ice refrigerant on the spectrometer trap was 2 minutes. Samples of light ethane run directly after 99.8% ethyl deuteride on this pumping schedule showed less than 0.1% contamination due to memory effect or holdover of the ethyl deuteride. Memory effects in the conversion apparatus were not observed, provided the reaction was carefully carried to completion. After a large number of runs a grayish white deposit of zinc oxide is observable on the glass, and a t this point it is profitable to clean the apparatus. DISCUSSION AND RESULTS
Typical calibration data, required for analysis of the ethaneethyl deuteride system, are presented in Table I. Data obtained
by Turkevich et al. ( 6 ) and Stevenson and Wagner ( 4 ) from samples prepared by the Grignard reaction are included for comparison. The differences in the sets of data are considered to be for the most part instrumental. It is important to note that the ratios of the sensitivity coefficients for deuterium substituted hydrocarbon and their protium analogs are not necessarily unity ( 2 , 4, 6 ) and that these ratios may vary instrumentally. The results obtained from analysis of a series of water standards prepared by weight dilution from distilled water and 99.8% deuterium oxide are presented in Table 11. Each value reported in the table represents a separate conversion of a 0.01-ml. water sample. Mass spectrometer precision in analysis of any particular gas sample was found to be better than 0.3%. The atom deuterium was calculated from the relative ion intensities of the 30 and 31 peaks in the observed spectra using the calibration data presented in Table I. This method can be used over a wide range of deuterium concentration. The lower limit is set a t between 0.5 and 1.0 atom % because of natural CIS interference. The upper limit depends essentially on the pattern stability in the mass spectrometer and the quality of heavy water available for calibration purposes. Samples containing 95% deuterium require a spectrometer precision of 0.1% to obtain an accuracy of 2%.
Table 11. Analyses of Deuterium in Water Sample 1
9 10
Experimental Atom % D 0.560 0.567 1.90 3.99 4.01 4.91 10.4 10.9 10.8 30.6 31.2 48.8 49.4 68.0 68.6 78.6 84.6 84.5
Calculated Atom % D 0,563
1.89 4.025 4.94 10.95 31.2 49.0 68.8 79.0 84.45
Table 111. Fractionation of Deuterium in Ethane Synthesis % Reaction 6.3 23.8 46.0 54.2 95.0
yo D Determined
yo Error
1.68 1.76 2.02 2.15 3.57
58 56 49 46 11
I t is evident that most errors in the method arise in the chemical conversion of water to ethane. If reasonable care is taken, errors in sampling and handling of 10 pl. of water can be readily eliminated. The problem of decontamination of the ethane from unreacted zinc diethyl is easily disposed of if the procedure is carefully followed. The most difficult error to eliminate is the problem of fractionation of the isotopes in the reaction-i.e., driving the reaction to completion. An investigation of the latter type error was made by running the reaction with separate samples of water containing 4 mole % HDO. At appropriate degrees of completion the reaction was quenched by cooling with liquid nitrogen and the extent of reaction in each case was determined by measuring the pressure of gas produced. As conditions of temperature were poorly defined, and the re-
am
A N A L Y T I C A L CHEMISTRY
action is kinetically complex (in part heterogeneous), no kinetic treatment of the data is made. The results of this fractionation study, presented in Table 111, show conclusively that serious errors are made if the reaction is not driven to completion.
(3) Rittenberg, D., ”Preparation and Measurement of Isotopic Tracers,” p. 31, 4nn Arbor, Mich., Edwards Bros., 1946. (4)
Stevenson, D. P., and Wagner, C. D., J . Chem. Ph~js.,19, 12 (1981).
(5) Thomas, B. W.,
i l . v . 4 ~ . CHEY.,22,
1476 (1950).
Turkevich, J., Friedman, L., Solomon, E., and Wrightson, F. AI , J . Am. Chem. SOC.,70, 2638 (1945). (7) Zeremitinoff,Bet-., 40, 2023 (1907).
(6) LITERATURE CITED
Alfin-Slater, R. B., Rock, S. M., and Swislocki, hl., ANAL. CHEM.,22, 421 (1950).
Orchin, M., ]Tender, J., and Friedel, R. A., Ibid.,21, 1072 (1949).
RECEIVED for review August 8, 1951. Accepted February 2 5 , 1952. Research carried out under the auspices of the U. s. Atomic Energy Commission.
Microdetermination of Deuterium in Organic Compounds JACK GRAFF’ .4ND DAVID RITTENBERG Department of Biochemistry, College of Physicians and Surgeons, Columbia University, Y e w York, IV. Y .
The usual procedures for determination of the deuterium concentration in the hydrogen of organic compounds require samples of about 100 mg. A study of various procedures in which the final analytical determination would depend on a mass spectrometric determination of hydrogen gas derived from the hydrogen of the organic sample has resulted in a micromethod requiring samples of from
T
HE common procedures for determining the deuterium
concentration of the hydrogen of an organic compound require the oxidation of the compound to yield water. The water is then purified and some physical quantity which is a function of the concentration of deuterium is precisely determined. The most widely used methods determine the density of the purified water. The densities of deuterium oxide and water differ by more than 105 p.p.m. and the density can be determined conveniently t o 2 p,p.m. by the falling drop procedure ( 4 ) . A sample sufficiently large to yield about 100 mg. of water is required for routine determinations. The procedure is long and requires careful attention t o details in order to obtain reliable results. The mass spectrometer can easily determine the deuterium concentration in as little as 0.1 cc. of hydrogen gas (4 micromoles). The analysis of water directly in a mass spectrometer is not feasible. If pure deuterium oxide vapor is admitted to a mass spectrometer, constructed of glass and Nichrome V, which has been baked until the water peak a t mass 18has a low intensity and no peaks are observable a t either mass 19 (HDO) or 20 (D20),a spectrum shown in Table I results. It is clear that the sample of deuterium oxide admitted into the mass spectrometer has become diluted with normal hydrogen atoms of compounds present in the instrument. Hydrogen atoms are present either as adsorbed water or as OH groups of the silicates of the glass vacuum chamber. Experience has shown that molecular hydrogen is the most suitable compound for mass spectrometric determination of deuterium. It is easily introduced and pumped out of the mass spectrometer, and impurities present either in the sample or in the mass spectrometer produce ion peaks far removed from those formed by hydrogen itself. Were it possible to liberate all the hydrogen from the water formed in the complete oxidation of an organic compound, it vould be possible to carry out an isotope determination on as little as 0.1 or 0.2 mg. of compound. While the oxidation of such quantities of organic compounds involves no technical difficulties, the conversion of this quantity of water 1 Present address. Department of Physiological Chemistry, Yale University. New Haven, Conn.
3 to 5 mg. The sample is burned in a stream of dry oxygen and the water formed is reduced by hot zinc to yield hydrogen. The accuracy is equal to that of previous macromethods and the time required for an analysis is considerably shortened. This method is especially useful in the study of the metabolism of natural compounds, where often i t is not feasible to obtain 100-mg. samples for analysis.
to hydrogen is not simple. The obvious methods such as reaction of water with sodium are not suitable, because these reactions do not completely convert all the water to hydrogen. Because of kinetic factors the isotope concentration of the deuterium in the hydrogen is from one third to one fourth that of the starting water ( 3 ) . The same fractionation exists in all reactions which do not go to completion.
Table I. Relative Intensities of Ion Beams at Masses 16, 17, 18, 19, and 20 after Introduction of Deuterium Oxide Vapor
Mass
(Intensity of mass 20 taken as 1000) Ion Intensities Immediately after ‘ admission of DzO Background into mass spectrometer
16
0.3 0.8 2.8 0.0 0.0
17 18 19
20
94
43 702 777 1000
ANALYSIS OF WATER BY EQUILIBRATION WITH HYDROGEN GAS
In an attempt to circumvent this difficulty, the authors studied the feasibility of equilibrating the water sample with a known amount of normal hydrogen. The reaction
HDO
+ Hz
HzO
+ HD
(1)
can be catalyzed in several ways ( 2 ) . As the equilibrium constant is known ( 5 ) , knowledge of the quantities of water and hydrogen employed and the isotope Concentration of the equilibrated hydrogen permits the calculation of the isotope concentration of the ifiitial water sample. Water formed by the combustion of a known amount of organic compound (about 5 mg.) was transferred to an evacuated vessel (about 300-ml. volume) and mixed with 1 ml. of normal hydrogen. The equilibration of hydrogen and water (Reaction 1) was carried out by a platinum
