Determination of Isotope Ratios of Known DeuteriumHydrogen Samples Using a Mass Spectrometer ROSLYN B. ALFIN-SLATER, S . 31. ROCK, AND MARIE SWISLOCKI Uniuersi t y of Southern California, Los Angeles, Consolidated Engineering Corporation, Pasadena, and Los Angeles C o u n t y Hospital, Los Angeles, Cali$. Gas samples of various known deuterium enrichments were prepared by reducing deuterium oxide-w-ater mixtures over zinc. The mass 3-mass 2 ratio f o r each of these standards was obtained on the Consolidated-Nier nlodel 21-201 mass spectrometer. The measured ratio w-as then plotted against known enrichment. The resulting calibration curve can be used to determine absolute enrichments of unknowns from their measured ratios. This method of calibration permits setting all instrumental factors for maximum stability and precision, regardless of whether or not this setting yields the absolute ratio. The curve obtained can be used f o r one instrument Over a considerable period of time under various instrumental conditions.
I
deuterium in the range of 0.01 to about 1%) it was necessary to evacuate the tube by pumping for 25 minutes in order t o eliminate the “memory cffect.” [This phe2900 nomenon has been discussed elsewhere 2.300 (9,1 3 ) . ] This time 220 0 could he somewhat .\o shortened by flush$ 2.1 0 0 N ing with normal 0 X hydrogen s e v e r a l times during the pump-out period. After a sample of 25 atom Toenrichment .I 0 0 had been passed .095 through t h e ins t r u m e n t f o r 20 .09 0 minutes, 4 hours of alternate pumping .08 5 6 8 10 I2 14 16 18 and flushinrr with MASS 2 PEAK IN VOLTS hormal hydrogrn (PROPORTIONAL TO PRESSURE) were required b:Figure 1. Fractionation during fore the ratio of Reduction of Water to HydrogenDeuterium Gas standard hydrogen returned to normal. Flushing with hi,liuni did not accelerate eliriiinatiori of the memory effect. .I11 samples were tested for t,he presence of air and water I)) checking the 32 and 18 peaks, respect,ively (5). The presence 01’ air increases the mass 3-mass 2 ratio; the plot of ratio against pressure has a greater slope when air is in the sample. The presence of water possibly Irnds to exchange reactions with deuterium present. Therefow, in this work, samples showing 32 or 18 peaks greater than 0.002 volt above the instrumental background voltage were discarded.
ATTERIPTIKG the determination of tieuteriuiii-hydrogen ratios by the mass spectrometer several difficulties were encountered. Various in\ estigators (6) also had reported difficulties in the attempt to obtain deuterium-hydrogen latios of maximum precision and accuracy. This paper describes a method of preparing samples of kno\T 11 deuterium enrichment, and a satisfactory procedure for using them to determine the isotope ratio of unknown samples by mass spectrometric methods. ?;
EXPERIMENTAL
Eight standard solutions were prepared by diluting 99.8’; deuteriuni oxide with doubly distilled water to give the following (mole per cent of deuterium oxide in excess of that ordinarily found in distilled water): 0.01, 0.03, 0.04, 0.3, 0.5, 1.0, 1.1, and 1.25. These samples, and a sample of doubly distilled water, ITere then reduced over zinc a t 400” C. according to the method of Rittenberg (11). The best reduction was obtained using commercial 10-mesh zinc previously treated with 25y0 hydrochlorir acid, thoroughly washed with water, alcohol, and ether, and d r i d in an oven a t about 80” C. Using a quartz tube 50 mm. long with an inside diameter of 11 mm., containing about 55 grams of zinc, approximately eight 20-mg. (0.02-nil.) samples could be reduceJ before the zinc needed replacement. The mixture of hydrogendeuterium formed was collected in a sample container over mercury in a previously evacuated system (1). The flask was then att,ached to the mass spectrometer, and mass 3-mass 2 ratios (HD/H,) were read for various magnitudes (in volts) of the mass 2 peak. The instrument used was the Consolidated-Xier RIodel 21-201 mass spectrometer (8, 1 3 ) . The double exit slit system of this inst,rument, designed to handle isotopes of carbon, nitrogen, and oxygen, cannot be used for hydrogen isotopes because the mass 2 ( H H +) and mass 3 ( H D ion beams are too far apart to be simultaneously collecte 1. This necessitates focusing first the m a s 2 ion and then the mas3 3 ion on one collector (collector 1 for the data that follow) and measuring the ratio of the voltages developed ( 5 ) . The magnetic s r m met,hod was used to avoid the voltage effect, ( 7 ) .
-
The necessity of measuring the two peaks a t different times reduces the precision of hydrogen measurements in comparison with the heavier materials such as carbon dioxide and nitrogen. Results obtained in this investigation were reproducible to about +3% of ratio over a period of 2 weeks, and to *lye of ratio or better over short periods of several hours. Both the value of the HD/H2 ratio and its reproducibility are affected by instrumental factors, particularly the setting of the source magnet ( 2 ) ., Therefore all instrumental variables were set so as t o obtain optimum precision rather than absolute accuracy. It was found that after admission of samples enriched x-it,h
DISCLSslOl A l l ) K E S L L l ’ s
The data obtained for tank hydrogen, hydrogel1 from unenriched doubly distilled water, and the eight synthetic samples are shown in Table I. The 0.03 and 1.2 atom % enrichment samples show the effect of fractionation during the reduction process (Figure 1). I n these cases sufficient sample (0.04 ml.) was reduced t o fill two sample containers completely. In each case the second sample collected has a higher mass 3-mass 2 ratio than the first; hence, to ensure good results, the water sample used must 42 1
ANALYTICAL CHEMISTRY
422 be completely reduced. With all the other samples this precau' enrichment samples, tion was taken. I n the 0.03 and 1.2 atom % the average atom per cent was used. Distilled water was taken as the base in computing enrichments, so that the deuterium oxide added was the actual excess. The ratios obtained on the mass spectrometer should be multiplied by the factor (2/3)1'2 because of the difference in pumping speed of materials of different masses from the source. The leak is so designed that gases of all masses enter a t the same rate. However, hydrogen pumps out more rapidly than HD, so that all ratios are high by the factor (3/2)'12. In the calibration method described below, it is not necessary to apply this correction. The formula used in computing atom per cent from mcasured mass 3-mass 2 ratios, r , was ( I O ) :
A,,%/lOO
= .in?
=
Table 11.
Ratio Extrapolated to Zero Pressure to Eliminate HJ+ rm %
Sample T a n k H1 Distilled Hz0 0.01 A Y E 0.03 .&$E 0.04 A g E
0.0158 0.0311 0.0432 0.0841 0.1005
0.3 AYoE 0 . 5 .\%E
0.520 0.804
rm
0.0821
A% 0.00644 0.0127 0.0176 0.0343 0.0410
0,425 0,656
0.212 0.327
%(2/3)'/2 0.0129 0.0254 0.0353 0.0687
A%E
...,
....
0.0049
0.0216 0.0283 0.199 0.314
2 + r
The measured ratio rises i t ith pressure, presumably because of the presence of H a Tions (3,9, I S ) . The computed enrichment also increases with pressure; therefore, subtracting the standard does not completely compensate for the effect of If3+. T o nullify this interference of H 3 + with the mass 3 ion HD+, the mass 3mass 2 ratio is plotted against pressure. I n this case the ratio is plotted against the nisgnitude of the mass 2 peak in volts, inasMass 3-Mass 2 Ratios for Synthetic Samples at Various Source Pressures Source rm % hfeasAtom % Computed ured, on Computed Atom % Atom 7 Pressure Mass from EnrichEnrichment %,own (Peak 2, rm Ro ment from Synthesis Volts) Spectrometer
Table I.
.
(tank Hz)
0.00 (hydrogen from doubly distilled water, normal) 0.01
6
1 2 ~ n- . 0259 -.0 ..-. .-_. n
0.0280 0.0343 0.0383 0.0418
0.0140 0.0171 0.0191 0.020'3
6
0.0393 0.0440 0.0469 0.0527 0.0588
0.0196 0.0220 0.0234 0.0263 0.0279
....
0.0266 0.0291 0.0331 0.0341 0.0366
0.0070 0.0071 0,0097 0.0078 0.0087
0.0454 (mean) 0.0495 0.0519 0.0546
0.0258 0.0270 0.0285 0.0283 0.0287
9
12 15 18 6
9 12
15 18
0.03
.. .
9 12 15 18
6 . 9 12 15
18
6
0.0861 0.0980 0.1024 0.1063 0.1125
0.0965 0,1000 0.1054 0.1123 0.1140
0.0566
. ..
, . . .
....
.... ....
0,1108 0.1185 0.1215 0.1295 0.1331
0.0554 0.0592 0.0607 0.0647 0.270 0.290 0.297 0.300 0.310
0,250
15 18
0.540 0.580 0.595 0.600 0.620
0.50
6 9 12 15 18
0.856 0.890 0.934 0.945 0.9G1
0.428 0.445 0.467 0.472 0,480
0.408 0.423 0.444 0.446
1.0
6 9 12 13 18
1.745 1.811 1.860 1.872 1.901
0.865 0.897 0.921 0.927 0.942
0,846 0.875 0.898 0.901 0.914
1.1
6 9 12
1.896 1,962 1.999
15
2.019 2.023
0.939 0.971 0.990 0,999 1.002
0.919 0.949 0.966 0.973 0.97-1
1.070 1.096 1.116 1.136 1.148
1.051 1.074 1,093
0.04
9 12 15 18 0.30
6 9 12
18 1.2
G 9 12 15 18
2.124 2.181 2.217 2.252 2.286
2.204 2.251 2.298 2.344 2.360
0.0665
0.0358 0,0372 0.0373 0,0384 0.0386 0.263 0.274 0.274 0.282
0.462
1.109 1.120
ATOM
Figure 2.
PER
CENT
rm us. Atom
ENRICHMENT
Enrichment a t 18 Volts of Mass 2
much as the volt'age, or peak magnitude, is directly proportional to pressure. The plot., for small enrichments, is linear, and can be extrapolated to the zero-pressure ordinate. The ratio a t this point, corrected by the pumping factor of (2/3)''2, should be very nearly the absolute ratio, barring instrumental bias in collection of the ions of the two masses. Table I1 shows a comparison betlveen known enrichment,s and those calculated by extrapolating in this manner from the data of Table I. The deviations are considerably greater than can be esplained by the expected variations in either the preparation of the samples or the measurenient of ratios. They are only partially explained by the fact that the intercepts are somewhat in doubt, particularly in larger enrichments, because of deviations from linearity-curves for 1.0, 1.1, and 1.2 atom % enrichment being concave downward. It was concluded therefore, that instrunicntal factors, such as source magnct placement, alter relative collection efficiencies a t masses 2 and 3; hence the synthetic samples were used to construct a calibration curve for the instrument (Figure 2). Ratios read at an arbitrary sample pressure in the mass spectrometer are plotted as the ordinate, the enrichment or excess over doubly distilled water as the abscissa. The standard pressure chosen ~ Y S 18 volts of the mass 2 peak, although any value above 12 volts ~ o u l dbe equally useful. Below 12 volts, however, the precision begins to drop because of the very small size of the mass 3 peak. The data are plotted to two scales, to permit reading to sufficient accuracy in the region below 0.05 atom % enrichment. The curve is linear over the range from 0 to 0.04 atom yo enrichnient, and linear within limits of exnerimental error over the entire range of 1.2 atom %enrichment. When an unknown sample is run, the standard samplc closest to it in ratio is also run. The measured ratio from this standard is plotted ag:iiiist its k n o ~ nenrichment on tlic calibration shect.
423
V O L U M E 2 2 , NO. 3, M A R C H 1 9 5 0 If this point falls on the established curve, the atom per cent enrichment corresponding to the ratio obtained for the unknown sample is read directly from the curve. If the ratJioof the standard falls slightly t o one side of the curve, a line is drawn through the new point parallel to the calibration curve. The enrichment corresponding to the ratio obtained for t,he unknown is then read from this corrected segment of curve. Thus, the calibration curve can be used for a considerable period of time, over somewhat changing operating conditions. Csing the method of calibration permits adjusting the instrument itself for maximum shbility, regardless of its effect on the measured value of tlic ratio. Deviation of measured from absolute ratios will thni be determined by the accuracy of synthesis of the standards and by the reproducibility of the instrumental measurements. 13y using adequate precautions in preparing standards, therefor(,, this method of calibration results in :tcuuracy approaching instrumental precision. Of particular interest in future comparisons of data obtained by various laboratories is the large difierence between the HD/H? ratio of unenriched tank hydrogen and that obtained from reduct ion of unenriched tiouhly distilled water. Tank hydrogen contains only one hdf to two t,hirds as much deuterium as does the hydrogcn from water. Furthermore, variations in deuterium content of hydrogen and water samples from various sources have been reported (4, 1 4 ) . It is true that any arbitrary standard can be chosen for one laboratory, but obviously the bme isotope concentration must be known and specified if data from different laboratories are to be coinpared. SUMMARY AYD COYCLUSIONS
A satisfactory procedure for determination of deukrium-hydrogen mixtures has bcen developed using the Consolidated-Sier 21-201 mass spectrometer. The minimum detectable difference from normal distilled water when frequent standards are run is 0.0004 atom %, or less under favorable conditions. Over the entire range, reproducibilit>- is * 1% of the ratio for periods of several hours and *3y0 of the ratio for longer periods. It was found that instrumental factors, including the set,ting of the source magnet, affect materially t.he value of the ratio of HD/H2 obtained. Hence, t,he instrument was calibrated with synthetic samples of kno\m deuterium enrichments, hydrogen obtaincd from doubly distilled water being used as zero enrichment,. I n preparing thc cdibrnting samples, it was found neces-
sary to collect hydrogen from the entire water sainple placed in the reduction apparatus, as a different ratio was obtained from the first and last half of the reduction. The calibration curve showing known enrichment plotted against measured H D / H H ratio can be used for a range of operating conditions and hence for an estended period of time and has the advantage of yielding results in nearly absolute enrichment. ACKNOWLEDGMEST
This ivork \Y:LS (tone under a grant from tlie Life Insurance Medical Research Foundation to H. J. Deuel, J r . The authors also rvish to thank Stella Alogdelis for her aid in preparing several samples, C. E. Berry of the Consolidated Engineering Corporation and R. J. Kinzler of the University of Southern California for thrir advice and cooperation, the Hancock Foundation for the use of its facilities, H. E. Pearson for his assistance in establishing the l l a s s Spectrometer L:thoratory at the Los Angeles County Hospital, and the Cancer Teaching Fund of the U. S. Public Health Scrviw for its partial support. LITERATURE CITED
(1) Alexander, R. W,, and Marx, W., unpublished research. (2) Berry, C. E., private communication. (3) Bleakney, W., Phys. Rev., 41, 32 (1932). (4) Bleakney, W., and Gould, A . , Ibid., 44, 265 (1933). (5) Consolidated Engineering Gorp., “Mass Spectrometer, Model 21-201, Operation and Maintenance Manual,” 1945. (F) Kirshenbaurn, A. D., Hindin, S.G., and Grosse. -4. V.,Nature, 160, 187 (1947). (7) Kier, A. 0.. “Preparation and Measurement of Isotopic Tracers,” p. 11, Ann Arbor, hfich., Edwards Bros.. 1946. (8) Nier, -4. O., Rev. Sci. Instruments, 18, 398 (1947). and Iluutad, B., (9) Nier, A . O., Inghram, M. G., Stevens, C. .4., Manhattan District Declassified Document, M.D.D.C. Rept. 197 (1947). (10) Rittenberg, David, “Preparation and Measurement of Isotopic Tracers,” p. 31, Ann Arbor, Mich., Edwards Bros., 1946. (11) Sprinson, D. B., and Rittenberg, David, U. S.Naval Med. BUZZ.,Supplement, p. 82 (March-April 1948). (12) Swartout, J. A , , and Dole, XI.. J . A m . C h e n . SOC.,61, 2026 (1939). (13) Washburn, H. W.,0’. S.Saval .Ired. Bull., Supplement, p. 60, (March-April 1948).
RECEIVED August 1. 1949. Contribution 226 f r o m t h e Department of Riocheinistry and Xutrition, University of Southern California. The deuterium oxide used i n this investigation was purchased from tho Stuart Oxygen Company on oliocation froin the Isotopes Division. U. 8. Atomic Energy Co:niiiiasion.
Determination of Certain Ortho-Substituted Phenols IIOB.iRT H. WILLARD
AND
A. L. WOOTEN’, L-nicersily of .VZichigun, / I n n ,Irbor, Mich.
A colorimetric method for the determination of o-phenglphenol or o-tertbutylphenol in the presence of their para isomers is based on the formation of their aristols and subsequent extraction with toluene. The optimum amoiin t of o-phenylphenol is about 2.5 mg. and of o-tert-brltylphenol is about 0.5 mg. An accuracy of about 1% is practical.
S
Y N T H E T I C coating rebins are prepared in large volumes from p-te? t-butylphenol and from p-phenylphenol. One of the iiiost objectionable impurities to be found in these p-phenols is the ortho isomer. The o-phenol tends t o cause a yellow color i n the final resin; poor drying characteristics and other undesirable features are also present. The present analytical methods were developed as controls for certain experimental processes, but it, is believed, that they may find application in the routine assay of commercial phenols. In t>hepresence of alkali and iodine, an o-alkyl or aryl phenol undergoes iodination and subsequent polymerization. Bordeianu
’
Present address. Reichhold Chemic& Inc., Ferndale. Jfich.
( 1 , 2 ) showed that polymerization occurs through the positions para to the hydroxyl groups. He showed that the action of oxidizing agents on 2,4diiodothymol and 2-iodo--i-bromothyrnol yielded a red polymer through the same diphenoquinone iodide. The results of Poplawski ( 4 ) C31I; CaH7 are in agreement with these. Wollett ( 5 ) found polymers with o = ~ = b = o molecular weights up t o 1400. This necessity for a free para I , LEI3 C H , I position indicates that a di-ophenol would interfere with the present procedure, but that an o,p-dialkvlphenol would not ~
!
