Further Studies of Dialysis as an Analytical Tool SIR: Analytical dialysis (2, 3 , b-8, 12-26) can be used for obtaining an estimate of molecular size of substances in the presence of others from which they cannot be separated, for detecting interaction with other solutes (8, IO), and for detecting heterogeneity (8, 22). I n the work reported here, the sensitivity of the dialysis technique for these purposes has been explored for the special situation in which scarcity of test material allowvs but a single loading of the dialysis cell, and in which the measurement of solute concentration results in chemical alteration so that monitoring without removal of diffusate is not possible. I n contrast to earlier work (11, 12) in which the retentate was analyzed, the present technique necessitated the analybiq of the diffusate. The method of analysis of two-solute systems differs essentially from that used by Craig, et al. ( 4 , 8 , 9 ) as it provides least squares estimates of the two escape rate coefficients even when they do not differ greatly. As will be shown, a computer program (1) written for compartmental analysis in studies of turnover rates can be used to analyze dialysis experiments in which relatively large portions of diffusate are removed for concentration determination, and to state in statistical terms the precision of escape rate coefficients and of a parameter indicating the composition. The membrane and model substances are chosen so as to make their separation by dialysis difficult in order to study the capabilities of the method on such systems. EXPERIMENTAL
The dialysis apparatus (Dialystat, Kational Instrument Laboratories, Rockville, hld.) was the same as used previously (10). Cellophane (Visking tubing, Nojax 36, supplied with the Dialystat) was tied with surgical linen thread (Ethicon, Code 009 C, size 0) to a glass tube of 21-mm. i.d., and excess cellophane was cut off, The membrane area exposed to the inside fluid was 3.45 cm*. For some experiments the cellophane was stretched in both directions by a technique similar to that of Craig, King, and Konigsberg (8, 9). X piece of cellophane tubing was tied off a t one end, and at the other it was tied to a graduated tube. Both bag and tube were filled with water, and air pressure was applied until either the volume inside the bag was doubled or until breakage of the bag. The membranes tied to the glass tube were stored in CH30H, or in 0.lM S a C l containing 100 mg. of KaXs per liter. The test material contained in 0.5 ml. of solvent was introduced within 10 seconds through a polyethylene tube (Clay-Xdams, Kew 658
ANALYTICAL CHEMISTRY
York, Intramedic PE90, 0.86-mm. id.), the tip of which touched the wall of the glass tube about 2 mm. above the inside surface of the membrane. A 50-ml. beaker with 10 ml. of solvent was placed under the membrane so as to leave 2 to 3 mm. of space between membrane and bottom, and its temperature was regulated by a water bath. The temperature of the diffusate was measured at the beginning and at the end of each run with a microprobe of a Telethermometer (Yellow Springs Instrument Co., Inc., Yellow Springs, Ohio). For concentration determination 1-ml. aliquots of diffusate were withdrawn at 2, 4, 6 minutes, a t about half time, and a t the end of dialysis (Method A ) , or a t the rate of 0.1 ml. per 12 seconds in the intervals 1-3, 3-5, and 5-7 minutes, and during 2 minutes a t about half time. At the end, two 1-ml. aliquots of diffusate were uqed (Method B ) . The final concentration in the retentate was determined on diluted aliquots. The concentration of the initial solution mas calculated from measurements on a dilution obtained by adding 0.5 ml. into 10 ml. of solvent through the same polyethylene tube as used in the dialysis experiment. The solutes were selected to be of relatively small molecular weight and to have absorption spectra that could be used for determining the sum of two solute concentrations a t one wavelength. Absorbance measurements (Beckman DB spectrophotometer) rather than chemical determinations were used for convenience, as the present work was concerned primarily with model systems that could be studied with precision. Since abqorbance was proportional to concentration for any one solute, absorbances were used in place of concentrations. For example, in a 1:1 mixture each component contributed 50% to the absorbance. The product of absorbance and volume will be referred to as L'amourit." The
0 Figure 1.
IO
20
0
IO
30 20
:
Homogeneity test by dialysis
0.004M recrystallized 2-naphthol-6,8-disulfanic acid in 0.01 N HCI. [a) 23.1 3OC., same experiment as listed in Table I; discontinuous withdrawals (Method A). (b) duplicate run at 23.1 O°C.
solutes, benzoic acid (Fisher), 2-naphthol-6,8-disulfonic acid dipotassium salt (Fisher), and adenosine-5'-monophosphate (Sigma, Grade Type IV) were shown by separate tests not to absorb on cellophane. .Ill data were analyzed first with the aid of an equation given earlier (10) and plotted as shown in Figure 1. The ordinate ZAAt is the sum up to time t of the values for hat =
for each dialysis period between two successive samplings. The slope of the straight line drawn through the points is the escape rate coefficient A. The sensitivity of fit to error in the nieasurement of the absorbance increases as the dialyzing solute approaches concentration equilibrium. I t is important to have the amount added initially agree with the sum of all portions measured in the experiment, the recovered amount. If the latter is larger, and the former is used for k in Equation 1, the expression in brackets may become negative. For this reaqon k mas always taken to be equal to the recovered amount. On the other hand, if the recovered amount is too small, the rate coefficient will be too large. By the use of the computer program as described below, a best fit can be obtained under all circumstances. However, as before, the calculated rate coefficient will be too low or too high, if the recovered amount is too high or too low, respectively. For the purpose of demonstrating the precision requirements for the dialysis of tiyo-solute systems, only the better experiments were used in the analysis by the computer program. To allow for the delay in establishing quasi-steady-state conditions in the membrane and for the noninstantaneous mixing of the diffusate, 0.2 minute was subtracted from all time readings, the value 0.2 having been obtained as the best estimate of the intercept from a large number of plots such as shown in Figure 1. This value and, for comparison, also 0.15 and 0.1 minute, were used for the experiments with mixtures of two solutes. The escape rate coefficients were corrected to 25' C. by multiplication with 298.16 q T / T q 2 g 8 1where 6, 7 is the viscosity of water and T is the absolute temperature. During any one experiment the temperature was constant to better than 10.1" C.; it ranged between 22.2' and 26.2' C. Least Square Fitting with the NIH SAAM-22 Computer Program (1). The equivalent model for the dialysis of single solutes with rapid withdraival of samples a t given time points has
been described previously (IO). The equivalent models for a single solute with continuous withdrawal of samples ( a ) , for a mixture of two solutes with rapid withdrawal of samples a t given time points ( b ) , and for two solutes with continuuus withdrawals (c) are shown below :
solute used was taken as being without error equal to the recovered amount, then the precision of X was high, the relative standard deviation ranging between 0.25 and 0.84yG (average 0.50yG) in 13 determinations, and having a value of 1.157, in one instance.
$53 *rc=1
*IC = 1
\O+3 - 4 - 9
wheref = L ~ ~ / L=! ~Z + ~ / V-~ t / l minute = 20 - t/l minute. The statistical weights for the observations, required in the program, were calculated under the assumption of equal absolute errors of absorbance readings in the range 0 to 0.5. Solutions with larger absorbances were diluted (definition of the symbols is available from the senior author on request). RESULTS
Single Solutes. The consistency of results with the assumption t h a t the transfer rate across the membrane is proportional to the concentration difference, is reflected in the standard deviation for one absorbance measurement and the standard deviation for the escape rate coefficient X as shown in Table I. If the value for amount of
Table I.
Absorbances a t 274 mp in the Dialysis of Single Solutes, and Rate Coefficients, A, Corrected to
Minutes 2 4 6 Diffusate 10 15 20
Retentate
The standard deviation for a single absorbance measurement, calculated from the fit of the dialysis curve, ranged between 0.0010 and 0.0086 (average 0.0035, or 4 to 6% of the lowest concentration measured). I n 10 out of 14 experiments this standard deviation was less than twice that of the spectrophotometer estimated to be about 10.002. The effect on X of an error in k is shown in an experiment with benzoic acid in 0.01N HC1 (24.5' C.) in which X was computed with k equal to the total added and to the recovered amount (98.84%). The values were X = 0.06566 (*0.00047), standard deviation of -4,= 0.0054, and X = 0.06591 (10.00017), standard deviation of A , = 0.0060, respectively. An error in k does not as much affect X as its standard deviation. The fit vias hardly affected, because the standard deviation of A , was high. The
agreement of replicate determinations of X with the same membrane seemed to depend chiefly on the precision of absorbance measurements, of the timing of sample withdrawals and of the temperature control. Examples for reproducibility are duplicate runs which gave the following values for X (1 standard deviation) : 0.02589( =kOo.OOOO9), 0.02637(*0.00014) on one day, and 0.02573(?~0.00009),0.02611 (10.00020) on another for the crude napthol disulfonic acid; and 0.0634(*0.00016), 0.0659(10.00017) for benzoic acid. Corollary Findings. The ratio of A's for benzoic acid (Sa) and 2-naphthol-6, 8-disulfonic acid (DS) was slightly greater (2.48, 2.33) for two unstretched membranes than for a membrane stretched t o the breaking point (1.91), indicating that the membrane was restrictive even though the niolecular size was far from the limit for passage. The rate of diffuqion of B d was increased by about 3070 by the stretching. Agreement of the values of X for B.1 in 0.01X HC1 (0.129) and in 0.1S S a C l (0.123) indicated that charge effects were negligible in the preqence of electrolyte. Charge effects hindered the escape of adenosine monophosphate when it was dissolved in HzO but not when it was dissolved in 0.1N XaC1, in which case a good fit of the experimental points (standard deviation of A. = 0.0020) was obtained. If computer facilities are not available, one can distinguish between small and large molecular weight contaminants from a plot such as Figure 1. The following scheme is suggested. Arrange samPlings at t l , t z , t 3 , t 4 , and a final one a t t ~ so, that a t tj 80 to 85% of the solute has escaped; choose t4 = t5/2, a t which time 60 to 65% will have escaped if the solute is 80% pure, or better, and if the contaminant does not escape more than twice as fast as the
40 Final
Discontinuous withdrawals, Vethod A; Membrane 1 - BA, 22.50' C.a DS (recryst.), 23.13" C.b Found Calcd. Found Calcd. 0.088 0.0881 0.068 0.0738 0.175 0.1751 0.155 0.1565 0.250 0.2523 0.240 0.2395 ... ... 0.377 0.3767 0.485 0.4826 ... ... 0 :674 0.6745 ... .,. 0.969 0.9624 0.346 0.3506 0.240 0.2472 (dil. 1 5) (dil. 1 11) 99.4 99.2
+
+
25" C. Continuous withdrawals, ;\lethod B; Membrane 2 BA, 26.20' L)S (crude), 22.30' C.b Found Calcd. Found Calcd. 0.098 0.1009 0.057 0.0614 0.200 0.1963 0.128 0,1293 0,277 0,2776 0.183 0.1982 0.407 0.4008 ... ... 0.495 0.4962 ... ... O:k66 0.5689 ... ... 0.833 0,8278 0.281 0.2807 0.431 0.4334 (dil. 1
+ 5)
Solute recovery, 76 98.9 A, ~ m minute-' . ~ ( + std. dev.) 0.0597(j30.00019) 0.0273(j30.00023) 0.0634 (*0,00016) Std. dev. of A. 0.0010 0.0028 0.0021 B.4 = Benzoic acid in 0.01N HC1; DS = 2-naphthol-6,8-disulfonic acid in 0.01N HC1. Best precision of A. * Worst precision of A.
(dil. 1
+ 5)
97.6 0.0261(~0.00020) 0.0086
VOL. 38, NO. 4, APRIL 1966
659
0.1 1
1
10.0 "I
s
0.1 I
I
I
I
0.6
0.7
0.8
IO.0I
..-0 4-
A"
-
>
0)
- - o
2
e
0.5
R
R
Figure 2. Analysis of two-solute system containing adenosine monophsophate (76.1% of total absorbance) and benzoic acid (23.9%)
Figure 3. Same as Figure 2 except that time scale correction is 0.1 5 minute
Method A. Case 3: XA is assumed to b e equal to 0.0232; AB as function of R; time scale correction 0.1 minute. ( X ) value aslevel of sumed to be correct. Solid lines cover range for F at significance
570
principal component. Choose tl = t4/10, 28 = 3t4/10. If all measurements are accurate to within i l % , then curvature of the plot can be detected if a contaminant is present to the extent of 10% of the total, and if the rate coefficient for the contaminant differs from that for the principal component by a factor of 2 or 2.5. If the contaminant diffuses more slowly than the principal component, the points of the plot from t = 0 to t 4 lie almost exactly on a straight line. If the contaminant diffuses more rapidly, the curve between t = 0 and ta is convex with respect to the abscissa. With more accurate data, a plot of h , calculated from Equation 1 for each individual interval, against time can be used t o distinguish between a more slowly- or a more rapidly-diffusing contaminant. In the first case the plot of A against time is convex, in the second case it is concave with respect to the abscissa. Mixtures of Two Solutes. For mixtures of t w o solutes, several types of treatment of the dialysis data are possible. Three parameters are involved: the escape rate coefficients of components A and B , h~ and AB, and a parameter describing the composition, R = k A / ( k ~ k B ) , where k~ and k B are the amounts of A and B used. One, two, or all three parameters may be unknown and require estimation. Case 1: ha and h B are known. The least squared estimate for R with its standard deviation is obtained with the aid of the program without iteration. Case 2: ha (or AB) and R are known. The least squares estimate for AB (or hA) with its standard deviation is obtained by the iterative procedure of the program. Case 3: hA is known. Diagrams of AB us. R indicate the probable range of values these parameters can assume within a chosen range for the ratio of variance to minimal variance ( F ) . Case 4: R is known. tz = t4/5, and
+
660
ANALYTICAL CHEMISTRY
0.9
h~ and A B are obtained by the iterative procedure of the program. This situation is unlikely t o be of interest in practice. Case 5: Sone of the three parameters is known. One may fit XA, AB, and R, or XA and R for different values of hn/X,+ The latter procedure was used. In all experiments the timing of withdrawal of samples was chosen so as to facilitate the detection of small proportions of a more rapidly diffusing component, the detection of more slowly diffusing components being less difficult ( 1 2 ) . These analyses were carried out for an experiment with a mixture of 76.1% adenosine monophosphate (Component A ) and 23.9Yc benzoic acid (Component B ) ( R = 0.761). Case 1. for XA = 0.0232 and A B = 0.0614, one obtains R = 0.799(+0.009) for 6 seconds, and R = 0.786(k0.008) for 12 seconds time scale correction, which is 5.0 and 3.3% (4.2 and 3.1 standard deviations) above the value for the assumed composition. The standard deviations for a single absorbance measurement, s, were 0.0028 and 0.0024, respectively. Case 2. for ha = 0.0232 and R = 0.761, one obtains XB = 0.0523(10.0013), s = 0.0019, for 6 seconds time scale correction, and AB = 0.0555 (i.0.0016), s = 0.0023, for 12 seconds time scale correction. The values for hB are 14.8 and 9.6Yc below the value listed under Case 1. Case 3. for ha = 0.0232, one obtains AB = 0.0501 i 0.0032, which is 18.4% below the value listed under Case I,and R = 0.746 + 0.022, which is 2.0% below the assumed value; s = 0.0020, 6 seconds time scale correction. In Figures 2 and 3 is shown hB as a function of assumed values for R for 6 seconds and 9 seconds time scale correction. The extent of the solid line indicates the interval of =k 5.05 for the variance ratio F , corresponding to the 5% level of significance. Case 5. in Figures 4 and 5 are s h o m ha and AB as functions of R for 6 seconds and 12
seconds time scale correction. Figure 5 is similar to Figure 4 except that the values for XB are higher and the best fit is shifted t o the far right. Within the 5YClevel of significance for the variance ratio, X A of Figure 4 ranges from 0.024 to 0.027, X B from 0.050 to about, four times this value, and R from 0.76 to about 0.97. Because the least squares fit in Figure 5 is esceptionally close (s < 0.0005) , only the range of F for the 0.5% level of significance includes the correct values for ha and A B . As was t o be expected, the smaller A B , the greater the proportion of Component B which is necessary for fitting the data, or the smaller R has to be. The range for ha was between 37, and 167, above the value listed under Case 1. I n Figures 6 and 7 are shomm the results of an esperiment with a mixture of 49.57, naphthol disulfonic acid and 50.5Yc ben~oicacid. The correct value for the rate coefficient AB falls within the range for the 57, level of the F test. The estimates for x A are between 12 and 34% too high. The composit,ion parameter R indicates about 63Yc naphthol disulfonic acid. Another experiment with 91.8% naphthol disulfonic acid and 8.2% benzoic acid $vas analyzed by the method for Case 5 to study the sensitivity for small molecular weight' cont,aminants. A membrane stretched t o the breaking point was used. The least squares fit gave X A = 0.0130 i 0.0012, R = 0.953 i 0.033, and standard deviation of A , = 0.0035; the actually used R of 0.918 falls within the confidence limits. The single-solute model gave XA = 0.0433 i 0.0003 and standard deviation of A , = 0.0040. The t,wo-solute model thus did not fit significnntly better than the singlesolute model ( F l , 5 = 0.75). From this analysis one may conclude that the threshold of sensitivity is above 8y0, if the standard deviation for a single absorbance measurement is i0.0035 and if the times for sampling are chosen as in the experiments presented. It was of interest, therefore, t o determine the permissible variability of measurements which allows to distinguish between a single-solute and a
the permissible standard deviation for an absorbance measurement as & 0.0028. DISCUSSION
0. I x" L
0
R 0 0.7
0.8
0.9
R
I .o
Figure 4. Analysis of two-solute system containing adenosine monophosof total absorbance) phate (76.1 and benzoic acid (23.970)
Yo
Method A. Case 5: ha and X g as functions of R; time scale correction 0.1 minute. ( X ) values assumed to b e correct. Solid lines cover range for F at the 5% level of significance
tno-.olute system for the case that the rate coefficients of the two solutes differ by a factor of two and that R = 0.9. If the times of sampling from the diffusate are at (O.O), 2, 4, 6, 20, and 40 minutes, and from the retentate a t (0.0) and 40 minutes, and if XA = 0.025 and Xg = 0.05, the reduction in the sum of squaied deviation. of A. due to reglession by fitting two X's instead of one X, is 5.292 X 10-j. This value \vas obtained by fitting the singlesolute inodel t o data generated by the two-solute model. I n order that the fit of the latter be significantly better than t h a t of the former, the ratio of 5.292 x to the mean squared deviation of A , for a tno-solute model fitted t o eyperimental data would have to be greater than Fi,S. For the 5% level of significance, F1,jis equal t o 6.61, and the mean squared deviation due to eiror must not be greater than 8.006 X l o p 6 . This gives the limit for I
The characterization of a mixture of solutes by its behavior in diffusion through a membrane in terms of two rate coefficients and one composition parameter appears to be useful whenever the range of rate coefficients is small. I n the method presented, a probable range of sets of values for these three parameters is given in the form of graphs, the range being limited on the basis of the significance level for the goodness of fit. The analysis of experimental data in nhich the ratio of two rate coefficients was about 2.5 showed that the rate coefficient of the slower component can be reliably estimated not only when this component is in excess but also in a 1: 1 mixture. The estimate for the rate coefficient of the fast component is coupled ivith that for the composition. The smaller the proportion of the fast component, the greater is the estimate for the rate coefficient. The timing of the withdrawal of samples is very critical in the initial stage. A shift in the time scale by =tO.O5 minute (or about 27, of the reciprocal rate coefficient of the faster component) affects the goodness of fit of the first sample value a t 1.8 or 1.9 minutes. If, in the given example for 23.9T0 fast component, one were to place a large weight on the first measurement, then results computed (for 0.2 minute time scale correction) with the known values for both rate coefficient would seem to show that either the absorbance measurement for the retentate a t the end was 2.57, too low, or that the actual volume of the retentate was 2% greater than the intended 500 J. The obvious indeterminacy in the estiniation of the larger rate coefficient is caused by the relative inaccuracy of the first
1
0.I
x" L
0
K 0
0.8
0.9 R
I
.o
Figure 5. Same as Figure 4 except that time scale correction is 0.2 minute
sample value of 0.070 absorbance and by the uncertainty in the length of the first tiine interval. Severtheless, the estimates for both rate coefficients a t the center of the range for R (though not a t the best fit) mere fair (with the exception of Figure 5 ) , while R was biased upwards in both the 23.9Y0 and benzoic acid experiment. For the 49.E17~ resolution of a mixture of two solutes, by the means described here, particularly for reasonably good determination of proportions, the two rate coefficients would have to differ by more than a factor of 2.5. Deviation from linearity of the plot of ZXAt us. t as in Figure 1 is about as sensitive a test for heterogeneity as is the F test for the improvement of the fit by using a two-solute instead of a single-solute model. However, if the rate coefficient for a pure substance is known, the presence of admixtures is more easily detected by a deviation from the true value of the apparent rate coefficient computed for the singlesolute model than by detection of significant curvature in the ZXAt plot. The technique described may find uses in quality control of industrial
0.1
x" L
0
R
0.1 4"
0 0.5
0.6
0.7 R
0.8
0.9
Figure 6. Analysis of two-solute system containing naphthol disulfonic acid (49.5% of total absorbance) and benzoic acid (50.570) Continuous withdrawals (Method 6). Case 5: ha and AB as functions o f R; time scale correction 0.15 minute. ( X ) values assumed to b e correct. Solid lines cover range for F a t 5% level of significance
: K 0 0.5
0.6
0.7
0.8
0.9
K Figure 7 . Same as Figure 6 except that time scale correction i s 0.2 minute VOL. 38, NO. 4, APRIL 1966
661
products whenever a change in particle size is the most prominent feature. ACKNOWLEDGMENT
The senior author is greatly indebted to blones B e m a n and to blarjorie F. Weiss for discussion and help with the computer computations. All these were carried out on the I B M 7094 of the Of Standards, VVashington, D. C . LITERATURE CITED
(1) Berman, hI., Shahn, E., Weiss, RI. F., Bzophys. J . 2, 275, 289 (1962).
(2) Carr, C. W.,“Dialysis,” W, G. Berl, ed., Yol. 4 , 1-43, Academic Press, h’ew York, 1961. ( 3 ) Collander, R., Soc. Scz. Fennica, Commentatzones Bzol. 2 , 1 (1926). (4) Craig, L. C., Ansevin, A,, Bzochem. 2 , 1268 (1963). (5) Craig, L. C., Hausmann, kT.2 .4hrens,
E. H., Jr., Harfenist, E. J., i l x a ~ . cHEM. 23, 1326 (1951). (6) Craig, L. C., King, T . P., “Dialysis,” D. Glick, ed., Yol. 10, 175-99, Acadernic Press, New York, 1962. ( 7 ) Craig, L. C., King, T. P., J. Am. Chem. SOC.77, 6620 (1958). (8) Craig, L. C., King, T. P., Stracher, A., Ibid., 79, 3729 (1957). (9) Craig, L. C., Konigsberg, Wm., J . Phys. Chem. 65, 166 (1961). (10) Hoch, H., Sinnett, S. L., Miller,
P. O., llahady, I. B., Biochem. 4, 1931 (1968). (11) Hoch, H., Turner, 31.E., Biochim. Bzophys. Acta 38, 410 (1960).
(12) Hoch, H., Rilliams, R. C., ANAL. CHEM.30, 1258 (19%). (13) Renkin, E. AI.) J . Gen. Physiol. 38, 225 (1954). (14) Spandau, H., Angew. Chem. 63, 41 (1951). (15) Spadau, H., Monographien zu“.4ngewandte Chemie” and “ Chemie-Ingenieur-Technik” Xr. 63 (Weinheim 1951). (16) Spandau, H., Brunneck, E., Angew. Chem. 65, 183 (1953). HANSHOCH P H Y L L0I.~hIILLER Physical Chemistry Laboratory \.eterans Administration Center hlartinsburg, R.Va.
Evaluation of Pyrolytic Boron Nitride Crucible Thimble in Carrier Gas Fusion Determinations of Gases in Metals SIR:Substantially high oxygen blanks are normally associated with the carrier gas fusion technique for the determination of oxygen in metals when operating temperatures are increased above 2000°C. After thorough degassing of the conventional crucible assembly, the principal source of this blank is the carbon monoxide formed by the high-temperature reaction of the graphite or carbon black insulation with the quartz crucible thimble, according t o the equation : SiOn
+ 2C
-+
2CO
+ Si
k33.5m-4.
4-
p m m
T6.5rnm
T
60mm
Figure 1.
1
Crucible and funnel as-
sembly for blank rate studies 662
ANALYTICAL CHEMISTRY
I n order to appreciably reduce this oxygen blank, it is necessary to eliminate one or both of the reactants participating in the reduction reaction. Smith and coworkers ( I , 2) apparently achieved a significant reduction in the oxygen blank by eliminating both the insulation and the quartz thimble. The output of a high energy induction generator (20 k ~or. larger) was efficiently coupled to the crucible by means of a special current concentrator so that the bare crucible could be heated to the high temperatures required. The design of this concentrator and the overall furnace assembly was rather elaborate (1). Another alternative would be to eliminate the Si02 reactant by utilizing some material other than quartz as the crucible thimble. Of necessity, this material would have to be an electrical nonconductor, thermally stable, mechanically strong, and should not itself contribute to the blank. Since boron nitride appeared to fulfill these requirements, an investigation was undertaken to evaluate its performance under helium carrier gas fusion conditions. Two types of thimbles were examined, namely, a hot-pressed form, containing a nominal 3 to 4 weight % boron oxide, and a pyrolytic form prepared by a vacuum deposition process. The latter thimble was essentially free of oxygen. Both thimbles were specially fabricated by the Carbon Products Division of Union Carbide Corp. For the blank rate studies, the Ultra Carbon Corp. C-625 graphite crucible and F-703 graphite funnel, reduced in size as shown in Figure 1, were floated within the respective thimbles on -200-
mesh UCP-2 graphite powder insulation. Heating of the crucible assembly was accomplished by means of a Xodel T5N-1 Lepel high frequency induction generator having a frequency rating of 250 to 600 kc. and a nominal power output of 5 kw. The crucible assembly degassing procedures were comparable for each of the thimbles. Temperature readings were made with a Pyro optical pyrometer calibrated according to instructions supplied by the Pyrometer Instrument Co. The helium carrier gas was purified by passing it through a 13X molecular sieve trap cooled to liquid nitrogen temperature. A small collection trap, also containing a 13X molecular sieve cooled to liquid nitrogen temperature, isolated the gases evolved
i
00116S0
1750
l8SO
1960 2OSO
2150
2250
2uO
CRUCIBLE TEMPERATURE (‘Cl
Figure 2. Oxygen blank rates with various crucible-thimbles
