cross sections of the (p,n) reactions are all of the same order of magnitude while for the (n,y) reactions they encompass several orders of magnitude. Consequently, and generally speaking, an impurity must be present a t a much higher level than the others to prevent their detection; this situation is very different from the situation of neutron activation where a trace element can sometimes prevent the determination of most other impurities. LITERATURE CITED (1)R. Gijbels and J. Hosta, Anal. Chim. Acta, 39, 89-93,(1967).
(2)D. F. C. Morris, D. N. Slater, and R. A. Killick, Talanfa, 8, 373-376 (1961). (3)J. N. Barrandon, P. Benaben, J. L. Debrun, and M. Valladon, Anal. Chim. Acta, 73, 39-48 (1974). (4)J. T. Routti and S. G. Poussin Nucl. lnstrurn Method 72, 125-144 (1969). (5)Enzo Ricci and R. L. Hahn, Anal. Chem., 39, 794-797 (1967). (6)C. S. Williamson, J. P. Bougeot, and J. Picard, Rep. C. E. A. R. 3042 (1966). (7)J. N. Barrandon, P. Benaben, and J. L. Debrun, unpublished results. (8)J. L. Debrun, N. J. Barrandon, and P. Benaben, unpublished results. (9)L. A. Currie, Anal. Chern., 40,586-593 (1968). (IO) K. A. Keller, J. Lange, H. Munzel, and G. Pfennin "Landolt-Bornstein lables," Group I, Volume 5,pari b, p 170,Springer-Verlag.(1973).
RECEIVEDfor review May 8, 1974. Accepted October 7, 1974.
Substoichiometric Neutron Activation Determination of Sodium: Extraction of Sodium Dicyclohexyl- 18-Crown-6 Tetraphenylborate J. W. Mitchell Bell Laboratories, Murray Hill, N.J. 07974
D.
L. Shanks
Duke University, Durham, N.C. 27706
A high accuracy method for the determination of traces of sodium by neutron activation was developed by selectively extracting sodium into chloroform as the dlcyclohexyl-18crown-6 ion-association complex with tetraphenylborate. Procedures for the substoichiometric extraction of sodium are described and results for the determination of sodium in a synthetic standard solution and in silicon dioxide samples are reported. These results, 0.240 f 0.004 and 3.81 f 0.18 pg/g, respectlvely, indicate an absolute error of 4.6% for the determination of sodium in the standard.
Many crown reagents, macrocyclic polyethers that exhibit the unique property of selectively and strongly complexing alkali and alkaline earth cations, have been synthesized (I). Although general complexation properties of the compounds have heen studied (2-4), the use of these reagents for practical separations in analysis has been lacking. We report here the first substoichiometric method for the selective radiochemical separation of sodium following thermal neutron irradiation. A substoichiometric quantity of dicyclohexyl-18-crown-6 (Crown) in the presence of excess tetraphenylborate (TPhB-) is used to reproducibly isolate sodium by solvent extraction into CHC13. EXPERIMENTAL Reagents. Dicyclohexyl-18-crown-6 was obtained from Aldrich Chemical Company, Inc. Twenty grams of the crude reagent were dissolved in excess n-hexane in a Teflon beaker. The brown viscous oil that settled to the bottom of the beaker contained most of the unreacted starting materials. The supernatant n-hexane phase was transferred to a second beaker and evaporated with gentle heating until a viscous, oily residue remained. This product was cooled in a Dry Ice-acetone bath and stirred vigorously until converted into a milky white paste that did not crystallize after storage for several weeks. The purified product was stored in a cleaned 642
ANALYTICAL CHEMISTRY, VOL. 47, NO. 4 , APRIL 1975
polyethylene bottle and protected from light by completely wrapping the bottle with opaque tape. A gas chromatogram of the white product showed the presence of one major peak accompanied by a nonresolved companion peak, which contributed approximately 25% to the total peak area. These compounds were presumed to be the A and B isomers and no attempts were made t o separate them. Sodium-22 was purchased from New England Nuclear, while the isotope, 24Na,was made by neutron irradiation a t the Industrial Reactor Laboratory at Plainsboro, N.J. Sodium tetraphenylborate was supplied by Fisher Scientific, Inc. Other chemicals and reagents were reagent grade. Substoichiometric Extraction of Sodium. T o measure the substoichiometric extraction of Na+ as a function of pH, aqueous phases were prepared by mixing appropriate amounts of sodium tetraphenylborate and 22Na,and diluting to near volume in 10-ml volumetric flasks with demineralized water. After small amounts of HC1 were added to adjust the final p H of the solution in the range 1-5, each was diluted to volume. Water previously equilibrated with CHC13 was used for all dilutions. For the range of pH from 7 to 11.5 appropriate amounts of (0.01-1.0 ml) of 0.1M NaOH were used. The organic phase was prepared by dissolving weighed amounts of dicyclohexyl-18-crown-6 and diluting to volume with CHC13 pre-equilibrated with water. Stock solutions 0.1M in polyether were diluted suitably to prepare organic phases. Equal aliquots of each phase were equilibrated in 15-ml centrifuge tubes, centrifuged for 60 seconds, and suitable aliquots were then counted on a well-type sodium iodide detector. The extraction of sodium was also investigated by equilibrating different volumes (3 to 7 . 5 ml) of a 0.OlMaqueous solution of sodium tetraphenylborate tagged with '?Na tracer. During these experiments, the equilibrium pH was maintained in the range 5 to 8, the concentration of polyether initially present in the organic phases was held constant at O.OIM, and 5-ml organic phases were used for the series of extractions. The effect of [TPhB-] on the extraction of sodium was followed by extracting a series of 5-m1 aqueous phases (0.1M in total sodium, but 0.001 to 0.08M in tetraphenylborate) with an equal volume of 0.01M polyether in CHC13. Extraction Rates and Reagent Stability. The time required for equilibrium to be attained during substoichiometric extractions of sodium was determined by starting a set of phases to equi-
librate on a wrist-action shaker a t the same time and then removing samples after 5, 15, 20, 30, 45, and 130 min. The activity extracted into the organic phase reached a constant value after 15 min and decreased slowly after equilibrating longer than 45 min. In most experiments, phases were equilibrated for 30 min. The stability of the organic and aqueous phases was monitored by mixing 5-ml aliquots of each phase taken from freshly prepared stock solutions, equilibrating for 30 min, and measuring the 22Na activity in the organic phase. The procedure was repeated periodically over a period of 24 hours. Interference by Anions and Cations. T o measure the influence of various anions on the extraction of sodium, aqueous phases were made 0.005M in Br-, C1-, I-, Nos-, s04'-, C~H302-,OH-, F-, C104-, or SCN-, 0.01M in NaTPhB, and tagged with "Na tracer. T h e total concentration of sodium in each aqueous phase was held constant at 0.015M by using the sodium salt of the anion to be tested. Thus, the concentration of the interfering anion was 50% as large as the concentration of TPhB-. The '*Na activity extracted into the organic phase in the presence of the test anion was compared with the y-ray activity isolated when the aqueous phase contained only 0.015M NaTPhB. Although the control solution contained the same [Na+] as the test solution, the [TPhB-] in the former was 50% larger than the amount in the test solution. Attempts to prepare NaTPhB solutions by neutralization of H T P h B were abandoned. In some experiments, the aqueous phases were prepared to contain different percentages by volume of methanol. The appropriate aliquot of methanol along with other components of the aqueous phase were added to the volumetric flask and then diluted to volume with demineralized water. Although the volumes of methanol and water are not strictly additive, these solutions are designated as containing the volume percent of methanol initially added. The effects of cations were determined by extracting sodium in the presence of 80 pg of K, Cs, Ca, Ba, Co, and Mn. A chloride solution of the test cation was used to dope aqueous phases containing 0.01M Na+ and 0.01M TPhB- to the 20-ppm level. After the p H of each was adjusted between 6 to 8,4- or 5-ml aqueous phases were extracted with an equal volume of O.OO5M polyether in CHC 1:1. Irradiations. A standard solution containing 502.6 wg/g of sodium was prepared by dissolving primary standard sodium oxalate. After a 0.5-ml aliquot of this solution was diluted by a factor of IO3 with demineralized, quartz-distilled water, 4-ml samples were sealed in polyethylene tubes. Carbonate-free solutions of sodium hydroxide were diluted immediately before use t o a final sodium concentration of 3.54 >: IO-' g/ml and used as comparison standards. A 1,0042-g sample of silicon dioxide containing traces of Mn, Na, Cu. Co, Cr, Fe, P t , and C1 was dissolved in a Teflon bomb in a mixture of 1 ml of 1:1 "03 and 10 ml of isopiestically prepared HF. The resulting solution was fumed with 0.5 ml of Ultrex HC104, and diluted to 50 ml with demineralized, quartz-distilled H20. The sample was diluted further by a factor of two before packaging for irradiation. Samples and standards were irradiated for 20 minutes in a flux of thermal neutrons of IOLz3 n cm-> sec-' in the pneumatic tube facility a t the Industrial Reactor Laboratory in Plainsboro, N.J. Following neutron irradiation, aqueous phases to be extracted were prepared by mixing :LC5ml of the NaOH comparison standard, 1.9 ml of 0.1M NaTPhB, and 2.5 ml of 0.01M NaOH carrier in a 25-ml volumetric flask and diluting to volume. The irradiated reference standard solution, (2.5 ml of Na*C204), was treated in the same way. A 2.5-ml aliquot of the solution resulting from the dissolution of the Si02 sample was treated similarly. The comparison standard of NaOH, simultaneously irradiated with dissolved silicon dioxide samples. was then prepared in the same manner. In this case, 2.5-ml of nonirradiated sample (dissolved Si02 sample) was also added to make the aqueous phase prepared from the comparison standard identical in composition to the irradiated sample. After measuring the p H of the aqueous phase and adjusting if necessary to ensure a value in the range 5.0 to 8.5, five-ml portions of the aqueous phases were equilibrated with equal volumes of 0.005M polyether for 30 min in 15-ml centrifuge tubes.
RESULTS A N D DISCUSSION Theory of Substoichiometric Extraction of Sodium with Crown-Tetraphenylborate. By using excess dicyclohexyl-18-crown-6 in an organic phase, an alkali cation can be extracted from an aqueous phase that contains a t least an equivalent amount of a suitable associating anion.
In the case of sodium and tetraphenylborate, the overall reaction for this process is given by Na*(,)
+
+
TPhB',,,
Crown(,, e
N a * Crown TPhB(,,
(1)
where the subscripts (a) and (0)refer to reagents present predominantly at equilibrium in the aqueous and organic phases, respectively. The extraction constant for this reaction can be written as [Nao Crown TPhB], Kex = ]Na*] [T PhB'][ C rownIo
(2)
where { [Na Crown TPhB],)/([Na+]) is the distribution , [Na+] refers to all aqueous phase species ratio, ( K D ) and containing sodium. K D is dependent on several equilibria: 1)the concentration of TPhB- in the aqueous phase which is governed by the reaction HTPhB(,)
+
H*(,)
+
TPhB'(,)
(3)
2) the chelation reaction
Na+(,)
+
Crown(,,
=
N a * Crown+(,)
(4)
3) the association reaction Nan Crown*(,)
+
TPhB-,,,
=---L
Na4 Crown TPhB( o)
(5)
and also on 4) the dissociation of the ion-association complex in the organic phase via the reaction Naa Crown TPhB(,,
+=
N a - Crown(,,+
+
TPhB(,,-
(6)
At sufficiently low concentrations of hydrogen ion in the aqueous phase and with an organic solvent of low or medium dielectric constant, reaction 3 proceeds and reaction 6 is suppressed. Measurements of the equilibrium constant for reaction 4 document the great tendency for this reaction to proceed in the forward direction ( 5 , 6 ) . To exploit reaction 1 for substoichiometric extractions, several conditions must be met. The crown reagent must be consumed via reaction 4 to the extent of 99% or better. Sodium must not be dissolved in the organic phase by the process Na+(,)
+
TPhB',,,
NaTPhB,,
(7) The sodium-Crown ion-association complex with TPhBmust have a high preference for distribution into the organic phase and cannot dissociate appreciably. T o achieve reproducible extractions of sodium with substoichiometric amounts of Crown, conditions must be selected such that the distribution ratio is governed only by the concentration of Crown in the organic phase. Under substoichiometric conditions, the extraction of sodium is independent of the equilibrium pH in the range 5.0 to 7.5 as shown in Figure 1. With aqueous phase concentrations of the anion, TPhB-, greater than or equal to the molar concentration of Crown in the organic phase, a constant amount of sodium is extracted into the organic phase as shown in Figure 2. Thus, reaction 4 is sufficiently strong that a substoichiometric quantity of Crown produces reproducible isolation of sodium from solutions initially containing excess amounts of the cation and anion. In the presence of methanol, the solubility of salts in organic solvents containing Crown is increased and aqueous phase complexation of sodium by reaction 4 is enhanced ( I , 5 ) ; however, data in Table I show that concentrations of methanol up to 10% in the aqueous phase have no effect on the substoichiometric extraction of sodium. 4
A N A L Y T I C A L CHEMISTRY, VOL. 47,
NO. 4 , A P R I L 1975
643
z 0
IV
a
Table I. Dependence of Extraction of Sodium Tetraphenylborate on Concentration of Methanol in Aqueous Phase
t
50
[MeOHI, ? V i V
E IX W
22Na ~n CHC13, mm-1°
2%a,
extracted
0 0.1 0.5
W
0
2z W 0 K
w
a
I
2
3
6
5
4
7
8
9
IO
II a
21039 39.1 21405 40.4 21351 38.4 1.o 21407 39.3 3 .O 21440 39.4 5 .O 21112 38.4 10.0 21225 38.1 25.0 20232 36.2 [Crown] = 0.0046M, VuLP= V,, = 5.0 ml, [ N a T P h B ] = 0.01M
PH OF AQUEOUS PHASE AT EQUILIBRIUM
Figure 1. Substoichiometric extraction curve for Na+ [Nat] = O.OlM, [NaTPhB] = O.OIM, [Crown] = -0.005M
Table 11. Extraction of Sodium-Crown Complex as a Function of Time 22Na in C H C I ~ ,min-l
Extr. time, min.
5 15 20 20 20 30 45 130
z
0 L
V
a a
IY ..
w
W
W
I-
z 0 w
20643 21780 21828 21784 21758 20780 21595 18907
22 Na,
OO
extracted
36.3 38.2 38.5 38.6 38.4 36.6 38.5 34.9
W
1 0
IO
1 20
1 30
1 40
1
50
I 60
I
I
70
80
MOLAR CONCENTRATION OF ( T P h B ) - IN AQUEOUS PHASE
I
x102
Figure 2. Dependence of substoichiometric extraction of sodium on
[TPhB-]
Reproducible Extractions. The data in Table I1 show that sodium tetraphenylborate solutions shaken with CHC13-Crown organic phases for a t least 15 and not more than 45 minutes are a t equilibrium. After phases were shaken longer than 45 minutes, the 22Naactivity in the organic phase decreased considerably from the mean obtained during 15 to 45 min. This phenomenon must likely result from the dissociation of the sodium-Crown complex in the organic phase, reaction 6. The rate of loss of 22Na activity from the organic phase was also dependent on the purity of the Crown reagent. When crude reagent as received from the supplier was used, precise extractions could be obtained by equilibrating phases for 15 to 30 minutes. However, after 1.5 hours no activity remained in the organic phase. Once organic phases were freshly prepared from purified reagent, aqueous phases could be shaken for 15 to 45 min with aliquots from the stock organic phase up to 24 hours later without affecting the precision of the substoichiometric extraction. Phases equilibrated after aging for 1, 3, 5, 23.5, and 47 hours showed 22065, 22397, 22675, 22645, and 16754 counts min-’, respectively, of 22Nain the organic phase. The precision of substoichiometric extractions from solutions initially containing different amounts of sodium tetraphenylborate is shown in Table 111. Under the extreme conditions, different aqueous phase volumes and reagent concentrations, the mean of the data a t molar ratios of Crown to NaTPhB 1 1.0 (solutions 5 to 9) has a relative standard deviation of 1.3%.Since, in practical applications, the volume of the aqueous phases resulting from treating 644
A N A L Y T I C A L CHEMISTRY, V O L . 47, NO. 4, APRIL 1975
Table 111. Substoichiometric Extraction of Crown Complex From Aqueous Phases Containing Different Amounts of Sodium Vol. aq. S o h No.
phase, c m - 3 0
NaTPhB, mmol
1
3 .O 4 .O 4.5 5 .O 5.5 6 .O 6.5 7 .O 7.5
0.030 0.040 0.045 0.050 0.055 0.060 0.065
2 2 h a in CHC13, min‘l
15541 15753 15863 16041 16377 16303 6 16789 7 0.070 1640 1 8 0.075 16714 9 16517b a [Crown] = 0.010M, V,,, = 5 ml, [NaTPhB] = 0.01M. * Mean, u = 219. Only solutions 5 to 9 were used in computing the mean. These are the only phases in which substoichiometric conditions exist, i t . , [Naja, > [Crownlor,. 2 3 4 5
aliquots of a sample solution and the concentration of sodium and tetraphenylborate in each solution are held constant, reproducibility better than 0.5% should be possible. Interferences. The influence of various anions on the extraction of sodium is shown in Table IV. In comparing the 22Na activity isolated under interference-free conditions (control) with the activity extracted in the presence of an anion (concentration equal to 50% of the amount of NaTPhB in the aqueous phase), it was assumed that the overall equilibrium was not affected by the foreign anion and that the rate of extraction of Na Crown T P h B was not altered by the anion being tested. The repeatability of duplicate extractions in the presence of various anions was f 0 . 2 to 0.7%with one value a t 1.1%.The interference ratio, I.R. = (22Naorganic phase activity obtained in the presence of anion)/(22Naorganic phase activity obtained when
~~
Table IV. Effect of Anions on Substoichiometric Extraction of Sodium Crown Complex Anion‘
NaTPhB Br‘ c1-
IN03’
soa2NaTPhB Ac-
OH‘ F‘ NaTPhB
%a
activity in CHC13
4648 4421 4530 4257 4220 4411 4268 4176 4227 4115 4648 4460 4865
*
35
i
33
i
* * *
17 14 49 35
Table VI. Determination of Sodium in Standard Oxalate Solutions %a
I.Rab
... 0.95 0.97 0.92 O .93 0.95
...
0.98 0.99 0.96
...
C104’ 22 0.96 SCN’ i 11 1.05 [Anion] = O.O5M, [Na-] = 0.015M, [TPhB-] = 0.01M. *See text.
activity isolated
“Na Cation
NaTPhB
K
cs Ca Ba co
In
activity
CHC13, m i n - l
11611 10400 11111 10890 10829 11433 17013 17231
*
+ i
i
*
53 85 23 4 191 49 122 30
I. R.C
... 0.89 0.96 0.94 0.93 0.98
NaTPhB ... Mn i 1.01 a [ M n + ] = 20 ppm, V, = V, = 4 ml, [Crown] = O.O5M, [Na] = 0.010M, [TPhB-] = 0.01M. * Average of two measurements. c See text.
test anion was absent), deviated -3 to -7% from 1.0. A deviation of the same magnitude resulted for the ratio of the activity of sodium extracted from two aqueous phases, each containing 0.015M Na+, but 0.010M and 0.015M TPhB-, respectively. Thus, no significant interference was observed for 0.005M concentrations of any of the anions tested. Inasmuch as the alkali and alkaline earth salts of tetraphenylborate are insoluble, serious interferences from major amounts of K, Cs, Ba, and Ca might be expected. Except for K, the interference from 80 pg of these cations was unimportant as shown in Table V. No interference from the transition cations, Co and Mn, was detected. The ratio of the concentration of sodium carrier to that of the impurity elements is -10 to 1. Maintaining this ratio in a practical sample is easily accomplished, regardless of the initial concentration of sodium. The appropriate amount of sodium carrier is added after the sample is irradiated. It can also be demonstrated that the effects of most major constituents in the sample on the quantitative result are either eliminated completely or rendered negligible by substoichiometric separations. To accomplish this, the composition of the aqueous phases of the sample and of the comparison standard are made identical by adding nonactivated sample to the comparison standard during post-irradiation processing. Results on such interference-free determinations by substoichiometric neutron activation will be reported later (7). The high selectivity of the extraction of sodium can be attributed to favorable chelation due to a more optimum match of the ionic radius of sodium than other cations to the size of the Crown ring (1).
activity
isolated from
Samplea
cpm m l - l
std,C cpm m 1 - l
1 2 3 4
480 502 491 499
385 375 331
[Nalfound, Ugd
0.233 0.244 0.239 0.243 0.240 i 0.004
...
364 i 21d Prepared by dissolving and diluting primary standard sodium oxalate. Vorg (4 ml) counted for 5 min. CStandard solution of NaOH, [Na] = 3.54 X 10-7 g/ml, Vc,rg (3 ml) counted for 10 min. Mean of Activity from standards used to calculate [Xa] found.
Table VII. Determination of Sodium in Silicon Dioxide ‘%a
Table V. Effect of Cations on the Extraction of Sodium-Crown-Tetraphenylborate ComplexQ
‘%a
from sample,b
activity
2 4 ~ aactivity‘
Sample
from sample
from std
[ N a l found, n g / g
1 2 3
22910 20510 22990
7222 7138 7093
3.905 3.537 3.990 3.81 * 0.18
Na in standard = 1.231 X 10-6 g.
Quantitative Determinations of Sodium. Standard reference samples certified for traces of sodium are not readily available. Instead, a solution containing 502.6 pg/ml of Na was prepared by dissolving the appropriate amount of primary standard sodium oxalate. A 0.5-ml aliquot of the prepared solution was then carefully diluted by a factor of lo3 immediately before the determination of sodium by neutron activation. A carbonate-free stock solution of sodium hydroxide was diluted suitably and used as a comparison standard. The results in Table VI computed from the equation Iliample= WStdw A - l e -
Astd
(8)
where Wsample and Wstd are the amounts of sodium in the oxalate and hydroxide solutions, Asample and Astd are the corresponding activities of 24Na substoichiometrically extracted into the respective organic phases, V is the volume of the standard solution, and F is the fraction of the prepared standard aqueous phase that was extracted. The mean of the data is within 4.6% of the theoretical amount of sodium in the solutions analyzed (sodium oxalate). Sodium was also determined in a matrix containing pg/g amounts of each of the cations Pt, Ir, Co, Fe, Mn, Cr, Cu, and C1. Results for the analysis of this sample for sodium are shown in Table VII. The high precision of the method and the selectivity for sodium, demonstrated by the absence of isotopes of GomCo, 56Mn, 64Cu, and 38Cl in the Ge(Li) gamma ray spectrum of the organic phase, suggests that this procedure is well suited for the separation and determination of traces of sodium in samples that cannot be counted directly. Although interferences from other trace impurities in a high-purity sample due to overlap of photopeaks with both of those of 24Namight be expected to occur only rarely during gamma-ray spectrometry with high resolution lithium drifted germanium detectors, the precision of measurements and the detection limits for sodium can be improved considerably by selective separation prior to counting in a well-type sodium iodide detector. ANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 4 , APRIL 1975
645
LITERATURE CITED (1) C. J. Pederson and H. K. Frensdorff, Angew. Chem., lnf. Ed. Engl.. 11, 16 (1972). (2) H. K. Frensdorff, J. Am. Chem. SOC.,93,4684 (1971). (3) R. M. Izatt. J. H. Ryfting. D. P. Nelson. B. L. Hayrnore, and J. J. Christensen, Science, 164, 443 (1969). (4) K. H. Wong. G. Konizer, and J. Srnid, J. Am. Chem. SOC.,92,666 (1970).
(5) H. K. Frensdorff, J. Am. Chem. SOC.,93,600 (1971). (6) R. M. Izatt, D. P. Nelson, J. H. Ryfting. B. L. Hayrnore, and J. J. Christensen, J. Am. Chem. SOC.,93, 1619(1971). (7) J. W. Mitchell, unpublished work.
RECEIVEDfor review August 23, 1974. Accepted December 18, 1974.
Determination of Oxygen- 18 in Water Contained in Biological Samples by Charged Particle Activation R. A. Wood, K. A. Nagy, N. S. MacDonald, S. T. Wakakuwa, R. J. Beckman, and H. Kaaz Laboratory of Nuclear Medicine and Radiation Biology and Department of Radiological Sciences, University of California,
Los Angeles, CA 90024
A comparator activation method is described for the determination of oxygen-18 in water, using the nuclear reaction I80(p,n)l8F ( Tjn, 110 min). A measure of the 0.511-MeV annihilation radiation from 18F yields a measure of the l80. The method is simple, requiring no chemical processing of the water other than distillation. The only competing reactions in the distilled Hzi80 are the I60(p,a)l3N and I3C(p,n)l3N ( T i n , 10 min). These interferences are eliminated by storing the samples for 2 hr before counting. This method of I8O analysis in the Hzi80 yields precise results and is comparable in accuracy to mass spectrometry. Water samples as small as 1.5 1.11 can be analyzed for l80 content using this activation technique. The primary advantage of this method of I8O analysis in water over mass analysis and other activation techniques is the simplicity of sample preparation and radioassay, permitting the use of large numbers of samples in l80tracer studies.
In studies of oxygen metabolism in various biological systems, the use of stable l8O as a tracer has become increasingly important and has created a need for rapid analytical techniques. In many of these studies the final product for lS0analysis is HzisO. The method most commonly employed in the analysis of l8O in water is mass spectrometry (1-3). However, the need to isolate the oxygen of water as a simple gas (e.g., CO or COZ) prior to mass analysis, requires complex vacuum trains as well as a complicated gas collecting apparatus. Other methods that have been used for lS0analysis in water are density measurement techniques ( 4 , 5 ) . The difficulties of sample preparation and the need for relatively large samples limit the use of this procedure in most tracer experiments. Activation analysis methods have frequently been used as a practical alternative to mass spectrometry or density measurements. In activation methods, the desired stable isotope is made to undergo a suitable nuclear reaction. The resulting nuclide is then measured and yields a measure of the parent isotope. The principal objective of this study was to develop a technique for measuring the '*O content of small samples of water ( 1 2 0 pl). Because activation is particularly suited to the analysis of small samples, we investigated the use of this method. Radioactivation methods used to determine 1 8 0 in H2180 include: the measurement of neutron emission following CY646
ANALYTICAL CHEMISTRY. VOL. 47, N O . 4 , APRIL 1975
particle bombardment (6, 7); the counting of delayed neutrons of 4.14-sec "N produced from triton bombardment of lRO(8);the measurement of charged particles emitted from the bombardment of l80with protons or deuterons (9);and by thermal neutron activation of ' 8 0 to form 29-sec (10). These methods are excellent in that they show high specificity and can measure I S 0 in very small samples. The principal limitation of many of these methods is the need for rapid sample transfer systems and specialized counting equipment. A more practical nuclear reaction for determining I8O in Hz180 is the ls0(p,n) 110-min 18F.The principal advantages of this conversion are that the relatively high capture cross section (500 mb a t 5 MeV) permits high conversion fractions in short bombardment periods, and that the long half-life of the product nuclide permits multisample analysis, resulting in a significant reduction in the cost per determination. Methods using the (p,n) reaction have been utilized to detect enriched l8O in the course of photosynthesis (II), and to determine oxide film thickness on various metal surfaces (12).In this paper the (p,n) reaction for IaO analysis is applied to the determination of lSO in HzI8O using standard comparator activation techniques (13). EXPERIMENTAL Apparatus. The proton accelerator used was a compact 91-cm multiparticle AVF medical cyclotron obtained from Cyclotron Corporation, Berkeley, CA. It is housed a t the University of California a t Los Angeles Medical Center. The 2- and 20-pl disposable glass capillary pipets were obtained from Drummond Scientific Company and are accurate to f0.25% of stated volume. The spinning wheel target assembly used in this study was modified from an assembly used by Watson e t al. ( 1 4 ) for the measurement of cross-sections in charged particle activations. Our wheel was designed to rotate samples through the cyclotron beam a t 800 RPM in order to reduce errors due to the nonhomogeneity and drift of the beam. The wheel contained three arc slots 1 inch across (Figure 1).Radial grooves were made across the arc slots 3.75' apart to accommodate the glass capillary pipets. A second aluminum disc was made to fit over the first to hold the pipets in place during hombardment, The wheel assembly was attached to an aluminum plate that was fitted with an electric motor, pulley, and belt to rotate the target wheel. An electrically insulated Faraday cup was also attached to the aluminum plate, positioned so as to collect the cyclotron beam after it passed through the target wheel. Two target wheels were constructed: one to hold eighty-one 20-pl capillary pipets and one to hold one hundred twenty-nine 2-pl pipets. The entire assembly was mounted on the end of the cyclotron beam tube.
'
