General Applicability of the Channels Ratio Method of Measuring Liquid Scintillation Counting Efficiencies ELIZABETH T. BUSH Nuclear-Chicago Corp., 333 Easf Howard Ave., Des Plaines, 111.
b Determination of the counting efficiencies of liquid scintillation samples as a function of the ratio of the count rates in two channels of the pulse height spectrum i s examined for a much wider variety of quenching agents, solvent systems, and fluor concentrations than previously reported. The behavior of the curves of efficiency vs. channels ratio i s examined as a function of small changes in multiplier phototube and discriminator voltages and other instrumental variables, and the over-all accuracy of the method i s assessed. It i s concluded that the method i s much more widely applicable than i s generally realized and that extremely useful results are obtainable when instrumental parameters are properly chosen.
0
of the outstanding advantages of liquid scintillation counting has been its ability to accept a wide variety of rsdioactive sample materials without the necessity of converting them to uniform chemical composition. This has resulted in great saving of sample preparation time, but has presented the alternative problem of determining counting efficiency individually for each sample composition. Recently, a method was described by Baillie ( I ) , and subsequently by Herberg (3) and Bruno and Christian (2): by which efficiency could be quantitatively related to the ratio of the counting rates in two channels of the pulse height spectrum of a liquid scintillation beta spectrometer. If the instrument provides for counting in two channels simultaneously, each sample need be counted only once, and no further operations on the samples are necessary to determine the counting efficiency. This provides significant advantages over the traditional methods of internal standardization or dilution. A calibration curve can be established by counting a series of samples of varying efficiencies (degrees of quenching), but containing known amounts of radioactivity. The observed counting efficiency, E , is plotted against the ratio, R, of the counting rates in the two channels. When unknowns are counted under identical instrumental conditions, the channels ratios may be calculated NE
1024
ANALYTICAL CHEMISTRY
and the corresponding efficiencies read from the curve. Baillie, Bruno, and Christian (1, 2 ) reported on quenching correction ( E us. R) curves for nine quenching agents, some of which were colored. Bruno and Christian found that a single curve sufficed for CY4 samples quenched with any of five different substances, both colored and colorless. They studied only one solvent-fluor system, but suggested that different curves would be found for other solvents. Baillie, on the other hand, found diverging curves for colored and chemically quenched (colorless) samples of C1', but reported that dilution of his (unspecified) solvent with secondary solvents gave the ssine curve as other chemical quenching agents. In each case, the investigators were able to determine counting efficiency from the measured ratio with an accuracy comparable to or surpassing that of other methods. But while the calibration curves remained unchanged for months in one laboratory, in the other it was necessary to recalibrate every few days. I t appears that relatively few invest,igators have taken advantage of this potentially simple and rapid method of
Table 1.
systems
+ POPOP* ++ POPOP POPOP +
Hyamine hydroxide Ethanolamine Water Acetone Benzoic acid Oleic acid Potassium hydroxide Tributyl phosphate Alanine Adenine Albumin Insulin n
EXPERIMENTAL
The isotopes studies were CY4 and HI. An attempt was made to include the solvent and fluor systems and the reagents most commonly used to prepare such samples, as well as a wide variety of quenching agents, including bio!ogical material. The effects of fluor concentration and of sample volume were also studied. Table I lists the fluors, solvents, and sample materials used. The study included about 50 sample compositions containing Ha and about 70 with C14. Samples were counted in 20-ml. vials of No-Sol-Vit glass supplied by the Wheaton Glass Co. H3 or 0 4 was added to all samples in the form of labeled toluene. Since toluene was not a component of all the sample compositions to be studied, it was necessary to introduce it in amounts which could not significantly dilute
Components of Samples Used in Quenching Correction Studies
Solvent-fluor Toluene PPO" Toluene PPO Toluene PRDc Dioxane PPO Dioxane PPO naphthalene
++ + + +
determining counting efficiency, because they are not sure either as to how many calibration curves are required for the different solvent and sample types they are using, or how the precision of the method is affected by instrumental variables such as voltage drifts. The present study is addressed to these problems.
2,5-Diphenyloxazole.
Secondary solvents Methanol Ethanol Xylene Anieole 1,2-Dimethoxyethane Ethylene glycol 2-Methoxyethanol Other Components Urine Blood precipitate from acetone Blood precipitate from ZnS04 NaOH NaOH Blood supernatant from ZnSO, Liver homogenate supernatant from ZnS04 Liver homogenate precipitate from ZnS04 Iodine Yellow wax Indigo Methyl violet Crystal violet,
+ +
Rlalachite green
Oxvuen ----4
* 2-p-Phenylenebis(5-phenyloxazole).
* 2-( 4-Biphenylyl)-5-phenyl-1,3,4-ox~diazole.
++ NaOH NaOH
from unity occurs for unquenched samples. Since these may, in general, be expected to have the highest count rates, less time will be necessary to achieve the desired statistical precision in the channel with the lower rate. RESULTS
ENERGY (PULSE HEIGHT)
Figure 1.
Spectrometer channels in relation to beta spectra
these samples, or frtil to dissolve in them. I n the largest group of samples, each received 100 J. of active toluene. These samples had volumes of 10 ml., except for some which were prepared to study the effect of sample volume, and which ranged from 3 to 18 ml. A significant source of error in the internal standardization method, the difficulty in measuring a small volume of active solution, was therefore present in this part of the experiment. In order to assess its contributicn to the over-all error, two further sets of samples were studied, containing 3 ml. of C1*-toluene and 1 ml. of H3-toluene, respectively, in 10- or 11-ml. total volume. The activity in each of the foregoing samples was 2.68 x IO4 or 1.71 X lo4 d.p.m. of 0 4 or 3.28 x lo6 d.p.m. of Ha. A series of low-activity samples of each isotope was also prepared in order to evaluate the channels ratio method at counting rates close 1,o the background level. Counting times wwe generally long enough to accumu1a':e a t least 20,000 counts in each chaniiel, except in the case of the low activity samples. The liquid scintillation counters, manufactured by Nuclear-Chicago Corp., included both two- and threechannel models. The criteria for choosing the counting charnels were that the statistical errors of cwnting should be minimized, and that the plot of efficiency us. channels ratio should have a useful slope over the desired range of efficiencies and be as newly linear as possible. The more nearly linear the calibration curves, the fewer points are needed to establish them accurately. To minimize statistical errors, the total portion of the f;pectrum which is included within the counting channels should be used in calculating the efficiency. This may be done either by dividing the spectrum into two adjacent channels and summtng their counts,
or by using one wide channel (from which the efficiency is calculated) and one narrow channel which is inculded within it. With reference to Figure 1, where the output pulse height spectrum of the (beta) counter is displayed in relation to three variable discriminator levels, these alternatives correspond to counting in channels LrL2 and &La, or channel &La plus either channel LI-La or &La. For a given total number of counts, statistical uncertainty in the value of the ratio is smallest when the ratio is unity. It is probably desirable that the greatest deviation
The shapes of the E us. R curves are determined by the positioning of the spectra in the counting windows and, therefore, by the gain and discriminator settings. Samples with varying degrees of quenching cannot all be counted with their best efficiencies a t a single setting. It is sometimes necessary to compromise between better efficiency and linearity of the curve. Since a principal objective of the present work is to present data for a wide range of quenching agents and efficiencies, the curves displayed here represent what were felt to be the best operating conditions for this large spread of samples. For a group of samples showing a smaller range in quenching, higher efficiency and better linearity may be obtained by optimizing the conditions for that group. It. was found that adjacent channels were most suitable for counting H3, with the L1-L efficiency plotted against the ratio of the counts in the upper window to those in the lower [R = (IJTLo)/(LI-L1)]. R is a much more slowly varying function of E for H3 than for C14. Use of adjacent channels gives the maximum change in R. When this ratio is plotted for C1' over a wide range of efficiencies, a very nonlinear curve results. Overlapping channels were therefore used for C1', with Ll-L3 efficiency plotted us.
R = (Li-Lt)/(LrLs).
CHANNELS RATIO
Figure 2. Quenching curve for labeled toluene
C14 samples
containing 100 pl. of
Wide-channel (11-1,) efficiency vs. ratio of counts in channels ( L ~ - l s ) / ( l ~ - l J
VOL 35, NO. 8, JULY 1963
1025
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
CHANNELS RATIO
Figure
3.
Quenching curve for H3 samples containing 100 pl. of labeled toluene Total efficiency in channels Ll-Lz plus
LZ-L3
The behavior of the E vs. R curves will be discussed as a function of the several variables which were studied. Sample Composition. CARBON-14 SAMPLES. A single quenching correction curve sufficed for all solvents, fluors, and quenching agents studied, so long as samples were not highly colored. Lightly colored samples containing biological materials such as urine, Hyamine solutions of blood, liver homogenate, etc., in which the quenching was chiefly chemical in nature, fell on this same curve. Highly colored samples diverged from the curve of chemical quenching as color became more intense. This agrees with Baillie's findings. Figure 2 shows the data for about 70 samples containing 100 pl. each of toluene-CI4. All the colored samples falling below the main curve were rather intense solutions of dyes, colored wax, or iodine, with the exception of the three yellow samples a t the bottom. These were blood samples of medium color intensity whose quenching was primarily chemical. Blue samples exhibited little quenching, as might be expected from the fact that the fluorescent emission spectrum is peaked a t 3500 to 4500 A. Samples which separated into two liquid phases showed anomalous behavior, as did partially frozen samples. Solidly frozen dioxane samples usually fell on the curve. Counting was done a t about -2" C. unless otherwise indicated. Results for colorless samples containing 3 ml. of toluene-Cl4 showed a reduced dispersion of points about the curve, indicating a reduced error due to pipetting. Eight quenching agents or 1026
ANALYTICAL CHEMISTRY
vs. ratio of channels ( L ~ - L ~ J / ( L ~ - L ~ l
secondary solvents were represented in this set. Since these samples were made up before the larger set, their compositions were unselected with respect t o the data of Figure 2. Samples containing the same quenching agents show a random distribution around the curve in Figure 2. TRITIUM SAMPLES. Data for IT3 samples containing 100 pl. of labeled toluene are shown in Figure 3. All samples fall very nearly on a single curve, although highly colored samples have slightly higher values of R for given values of E . This latter effect
was also shown by samples containing PPO without POPOP. On the basis of these data samples were selected whose points fell below, above, and on the curve, and samples having these compositions were made up with 1 ml. of H3-labeled toluene. The improved precision in measuring activity is indicated in Figure 4. (The average deviation from the curve of points for colorless samples is 0.098% in efficiency in the latter case, compared with 0.21% in the former.) This last group of samples was counted in four different instruments. Intensely colored samples were found t o deviate from the curve of chemical quenching by an average -0.5% in efficiency, with a maximum observed Samples condeviation of -0.75%. taining PPO as the only fluor showed a maximum observed deviation of -0.9%. The results for PBD and PPO POPOP in various fluor concentrations and with different solvents and quenching agents were consistent with a single curve, with one exception: In one of the four instruments a small negative deviation in efficiency (less than 1%) mas also shown by samples containing dioxane plus naphthalene. The effect of phase separation was similar to that in C14 samples. Some samples which were apparently solidly frozen did not fall on the curve, while others did. Duplicate Samples. A number of duplicate samples were made in the course of this work. Although pipetting was done with all reasonable care, the efficiencies of duplicate samples might differ by 1 or 2% in absolute value. This amounted to fractional differences of up to 10%
+
CHANNELS RATIO
Figure 4. Quenching curves for H3 samples containing at -1.5" and 6.7" C.
+
3 ml. of
labeled toluene
a t lower efficiencies, which was more t h a n could reasonably be accounted for by error6 in pipettiiig of reagents. Nevertheless, these apFeared to be differences in the degree of quenching, since tlie samples showed the expected relationships betw-een efficiency and channels ratio: their points fell on the expected curves. These differences might be attributed to aging processes during the time intervals of days or weeks between preparation of the "duplicate" samples. Prepared samples were stored in the counter at -1.5" C., while scintillation solutions and other ingredients were stored at room teniperature. Some difference may be due to the vials themselves. Although the labeled niaterial was volatile, there was usudly no evidence of evaporation from the vials for several weeks. Adsorption of labeled compounds on the glass might present a problem in the aging of :some other types of samples. This would be expected to shift t'lie pulse height spectra toward lower energy. \Ve h a r e observed this phenomenon in liquid scintillation counting of high specific activity salts of p32.
Temperature. H3 curves were sensitive t o temperature, as shown in Figure 4. The dottell lines connect points for t'he same samples, showing t h a t with a n increase in teniperature E decreased slightly, while R decreased considerably. C14 curves were not very sensitive to temperature. Although the same effects were operative on the pulse height spectra of the two isotopes, the CI4 ''tmiperature-comcurves were pensated." A decreass.: in E was accompanied by an increase in R which moved C14 point,s along; the curve. An average error of less than 1% in efficiency was found in u i n g a calibration curve for - 1.5" C. to determine the efficiency of samples at $-7 O C. Certain Optical Effects. Samples containing light-colored precipita.tes or filter paper on the bottoms of the vials did not deviate from the curves. d sample containing carbon black on the bottom fell far from t h e curve. (The activity was in solution i n all cases.) X gel sample of toluene plus Cab-OSi1 fell on the curve when first prepared, and showed very little quenching. Subsequently its efficiency decreased greatly without a correjponding change in R. Upon examination, the gel was then found to be distributed unevenly over the sides of the bottle. It is possible that with the right viscosity and great care in handlmg, reproducible results might be obtained with gel samples; but this study was not pursued. Sample volume could be varied from 3 to 18 ml. in the casr: of C14 without causing deviations front the curve, ancl
over a somewhat narrower range for H3. Efficiency was slightly higher a t 10 to 15 ml. Severe "fogging" of the vials, siinulat'ed by a light coating of soap, caused deviations in efficiency of 1 to 2y0, usually positive, for C14. Tormal fogging or fingerprints are not expected to cause errors. Instrument Parameters. The effects on t h e curves of small cliariges in multiplier phototube high voltage and discriniinator va,lue were studied t o assess t h e errors t o be expected from instrumental drifting. It was determined t h a t no measurable error could be expected from possible drifts in discriminator values. The expected error from high voltage drift.; may be expressed as follows for a tritium curve of the type shown: for the masiinuin change in high volt'age permitted by the regulator for a 30-volt line volta,ge change, the absolute value of E as determined fr(Jn1 R vould vary by 2 x at 1% efficiency, or by 5 X at 15% efficiency. That is, the error in the calculated activity of a sample (given by b E / E ) would range from 2% at E = 0.01 to 0.33y0 a t E=0.15. This is the maximum allowable error. In actual practice, the curves m-ere not observed to drift outside the limits predicted by counting statistics over periods of many weeks. The curves were reproducible aft'er changing the high voltage and discriminator values if the controls were accurately reset. This latter is an important requirement. In routine v-ork it would seem to be good practice to count a set of sealed quenched standards in every run, just as background and unquenched standard samples are usually counted. Level of Radioactivity. A set oi samples was made up t o have net count rates near tlie background level. C14 samples contained 1Oi d.p.m., and their count rates varied from about 0.75 to 2 times background in each cha,nnel, depending on the amount of quenching. Tritium samples contained 820 d.p.m., and varied in count rate between 0.35 and 7 times background in each channel. The quenching agent used was acetone. S e t count rates were calculated for each sample, using the measured background rates for each sample type. The efficiencies and ratios based on the net count rates showed the same relationships as those of higher activity samples. Background measurements were made on many sample compositions, but no general relationship between background rate and efficiency was found for either H 3 or CY4. Some factors in the variation of that part of the background due to the sample system are: (1) chemical quenching, ( 2 ) color quenching, (3) fluor type and concentrat'ion,
(4) fluorescence 01 components other than the scintillation solute, (5) efficiency for 7-ray interaction, (6) efficiency for Cherenkov radiation, (7) natural content of contemporary carbon, (8) natural activity of the vial, and (9) phosphorescence of some sample materials. I n so far as the effects of these could be separat'ed, a,n estimate was made of the amount by which the background rate changed when each factor mas varied independently. (Sample phosphorescence was not studied.) The observed limits of these changes may be of some general interest, since it is necessary to estimate the possible uncertainty in the background and the magnitude of the resulting error in order to choose appropriate counting procedures for samples of lorn activity. The following are the (approximate) maximum observed variations in background in the summed H3 channels with changes in each of those factors which could be distinguished, expressed as percentage changes in the background rate of a sample of pure toluene PI'O POPOP: chemical quenching, color quenching, 55%; fluor (presence or absence of secondary solute), 11%; solvent system (including effects of 4, 5, 6, and 7 above), 15y0;and vial, 15% (all vials were from a single shipment by the \Theaton Glass Co.). The magnitude of the variation in the C14 window was generally less, while in the lower energy € I 3 channel alone it was higher. Chemical quenching is distinguished from dilution in this context by letting it refer to the change in efficiency produced by adding to a given solvent system not more than 30y0 by volume of colorless quenching a,gents. No correlation was observed between efficiency and background rate in samples containing different chemical quenching agent.s. In a series of samples having increasing amounts of the same quenching agent, the background rate sometimes, but not always, varied monot-onically with efficiency. The background of colored samples always decreased with increasing Concentration of quencher. It is not always feasible, or even possible, to control all of the abovementioned factors in preparing blanks for low actirity samples. It is therefore of interest to compare the errors caused by using incorrect background rates in the channels ratio and other methods of efficiency determination. In the channels ratio method the values of both the ratio and the net count rate used in calculating the activity may be in error if the true background rates in each channel are not known. These errors may be additive or they may partially cancel each other, depending upon the part'icular counting conditions
+
+
VOL. 35, NO. 8, JULY 1963
1027
Table
II.
Error in Calculated Value of Disintegration Rate Due to Background Variation of -20% from Its Assumed Value
(Calculations for tritium samples having net count rates of 150 c.p.m. at various efficiencies) Channels ratio mcthod R
E
E’
1.50 1.25 1.00
0.2064 0.1532 0.1000 0.0681 0.0468
0.2027 0.1481 0.0941 0.0623
0.85
0.75
0.0405
employed. When internal standardization is used, the error introduced by uncertainty in the value of the background count rate appears only in the calculation of the net count rate. The relative magnitudes of the errors of these two methods were compared for an actual set of counting conditions. The error introduced by using the wrong value for the background count rate will be called the “background error,” defined as the fractional difference, (D’-D)/D, between the true disintegration rate, D , of the sample and the apparent value, D’, calculated as follows: D = C/E
(1)
D’ = C’/E’
(2)
where C is the true net count rate, E the true counting efficiency,C’ = C S A B , AB being the difference between the assumed and actual values of the background rate, and E‘ is the apparent efficiency. It is assumed that E = E’ for the internal standardization methodi.e., that the background rate cancels out of the expression for the efficiency. Using the channels ratio method, E’ is determined in the following way: A standard calibration curve is established by counting high activity samples, so that no background error is involved. The background rates of an unquenched sample are measured in each channel. The ratio of the true net counting rates of a low activity sample in the two channels is adjusted for a given background change in each channel: R‘ = (C, AhBI)/(C2 A B d , where the subscripts identify appropriate channels for the isotope in question. The new ratio R’ is then used t o find E’ from the calibration curve. I t is obvious that if E’ = E, as in the internal standardization method, the tractional error in the disintegration rate equals the fractional error made in determining the net count rate, since from Equations 1 and 2
+
+
D’-D = C‘-c __
E
1028
ANALYTICAL CHEMISTRY
-
D’-D D
-0.022 -0.0069
+0,020 + O . 047
+o. 109
D’-D - = - = -C’-C D ED
Internal standardization method D’-D D
0.040
0.040
0.040 0,040 0.040
C’-C
C
If, however, (C‘-C)/E = (E‘-E)/E, it is easily shown that D’-D = 0: and there is no background error. By using the channels ratio method, this situation may be achieved or approached in many cases by proper choice of the counting conditions. This depends on being able t o obtain, simultaneously, certain values of the slope of the curve of E us. R and of the ratios of the sample and background rates in each channel. For a given set of counting conditions and a given background change the background error will vary from positive to negative over a range of efficiencies, passing through zero a t some value. Samples having a restricted spread of efficiencies about this value will be counted with a very small background error. Table I1 illustrates this for Ha samples having net count rates of 150 c.p.m., or five times background, a t all efficiencies. The assumed value of the background was that observed for an unquenched sample: 10 and 20 c.p.m. in the lower and upper energy channels, respectively. The true value of the background was taken as 20% lower than the assumed value. An experimental curve of E us. R was used. This was a “typical” curve similar to that of Figure 3, with counting conditions chosen according to the general criteria set forth earlier, rather than with any reference to background error. For this particular curve, the background error became zero a t about 15% efficiency. Other values of the gain and discriminator settings were investigated, and it was found that for a change of 20% in the background rate the background error for H3 samples could be made less than the fractional error in the count rate alone-Le., the background error of the internal standardization method-for all but the most highly quenched samples. Zero background error was made to occur a t
any efficiency down to 7% (not necessarily the lower limit), For C14samples, zero error could be obtained for medium and highly quenched samples. In addition to the two methods already discussed, efficiency may be determined by either of two dilution techniques (4). One involves successive additions of scintillation solution to a sample, with repeated countings. Since the background is a function of sample volume as well as degree of quenching, it appears that it would be very tedious to determine an appropriate background rate for each count. The other method requires a series of samples having the same total volume but decreasing amounts of the radioactive (quenching) material. It might be hoped that one could avoid the necessity of having a blank for each dilution by using a single background value, that of the unquenched sample, and that the background error would disappear in an extrapolation to infinite dilution. In the one experiment of this kind which was performed in the course of this work, the fractional error in net count rate increased with the decreasing activity and yielded a background error of 5% a t infinite dilution. DISCUSSION A N D CONCLUSIONS
Baillie has shown that by the proper choice of instrumental parameters the C14 efficiency in channel L1-L3 can be made numerically equal to the rat,io (L2-LJ/(Ll-L3) over a limited range of efficiencies. This ratio thus provides a very convenient measure of efficiency, but it suffers in general from greater statistical uncertainty than the ratio (L1-L2)/(L,-4). The latter approaches the value of 1 as quenching increases, while the former approaches 0. Use of a special calibration curve is necessary for highly colored C I 4 samples. It appears that all colors follow one curve, a t least until quenching becomes very severe. The error which will be made in using a single curve for H3 samples is small. Samples prepared and sealed in a nitrogen atmosphere are ordinarily employed as standards in liquid scintillation counting. These have higher counting efficiencies than samples in equilibrium with the oxygen in air and cannot be used directly t o standardize solutions which are counted in ordinary vials, unless a correction is made for quenching in the latter. The use of a quenching correction curve offers a very convenient method of standardizing a laboratory solution. A curve may be established by counting a set of variously quenched samples of the unknown solution, using a tentative value for the activity. The sealed standard is then counted and the curve is normalized to this point. Obviously, such an extrap-
olation to higher efficiency should be carried out on a cwve which is fairly linear in this region. The use of a “duplicate” sample to determine counting efficiency may result in an unexpectedly large error, particularly a t lower efficiencies. The channels ratio of the individual sample gives a more reliatile measure of its efficiency. The channels ratio method offers the following advantages over internal standardization or dilution techniques for determining efficiency: only a single counting is necessary in a twochannel instrument; the measurement
can be repeated a t any time, because the sample remains unaltered; and certain sources of error are eliminated, such m the assumption that efficiency is the same before and after adding an internal standard and measuring errors in pipetting small volumes. On the other hand, when activity is very low a longer counting time will be necessary to achieve a given statistical precision in the value of the ratio than to achieve the same accuracy in the counting rate. This extra counting time must be weighed against the time required of personnel to perform the sample manipulations involved in other methods.
ACKNOWLEDGMENT
Thanks are expressed to Ariel G. Schrodt and Richard B. Frank for helpful discussions in the course of this work. LITERATURE CITED
(1) Baillie, L. A., Intern. J. A p p l . Radiation Isotopes 8, l (1960). (2) Bruno, G. A., Christian, J. E., ANAL. CHEM.33, 650 (1962).
(3) Herberg, R. J., Ibid., 32, 1468 (1960). (4) Peng, C. T., Ibid., 32, 1292 (1960).
RECEIVEDfor review August 8, 1962. Accepted April 15, 1963. Division of Analytical Chemistry, 141st Meeting, ACS, Washington, D. C., March 1962.
Neutron Activation and Radiochemical Determination of the Molybdenum, Chromium, and Iron Content of Individual Stainless Steel Microspheres PHILIP A. BENSON and CHESTER
E. GLEIT
TRACERLAB, Divisiori o f laboratory for Electronics, Richmond, Calif.
b Stainless steel microspheres, with weights between 4.7 X 10-9 and 3.9 X gram, are isolated on methyl methacrylate films. After measurement of particle diameters by optical microscopy, the specimens are encapsulated in lead foil and irradiated with an intense flux of neutrons. Radiochemical procedures are used to isolate the M099, Cr61, Fe66859, and NpZ39 fractions. The abundance of molybdenum, chroniium, and iron in the individual spheres is determined from the measured activities, The experimental error is attributed to three principal solJrces. Uncertainty in particle diameters as measured by optical microscope produced a relaRadiotive error of 6 to 11%. chemical variations led to a relative error of 5%. Coiitamination of the sample was succes:;fully controlled in the Mo and Cr analyses. Impurities in the encapsulatirg lead foil were the largest source of error in the iron determination.
A
chemical analysis of samples as small as 10-9 gram is required in research programs related to the characterization. of air-borne particulate matter and interplanetary dust. Neutron activation aaalysis is a valuable analytical method for such small samples. Through tho use of high intensity neutron sources and subsequent radiochemical analysis over 30 elements can be quantitatitely measured in CCURATE
amounts smaller than 10-’2 gram. In practice, sensitivity is limited by contamination. The experiment described in this paper was undertaken to develop suitable handling techniques and to determine the practical limits of sample size, accuracy, and reproducibility in performing such analyses. Activation analysis, based on mea.+ urements of induced radioactivity, is essentially free of errors produced by contamination of the specimen after irradiation. Carrier solutions composed of stable isotopes, which are added in the first step of the radiochemical separations, and standard blank corrections minimize errors due to accidental additions of stable species. However, special techniques must be employed in isolating the samples to minimize contamination prior to the irradiation. In the present work, the individual stainless steel microspheres are carried on 10-7-gram films of methyl methacrylate. To prevent loss of activated species during irradiation by nuclear recoil the particles are encapsulated in high purity lead foil and are then irradiated in high purity quartz ampoules. Iron, molybdenum, and chromium have been determined in a variety of samples by activation analysis. Several papers discuss the determination of chromium in meteorites (3, IO), alloys ( I I ) , and high purity metals ( 2 ) . Chromium is determined by quantitatively measuring the activity of 28-day Cr51. This species is produced by
neutron capture of stable CrW. After radiochemical purification, the intensity of the 0.32-m.e.v. -pray of CrS1 is measured. Molybdenum is determined in a similar manner employing the M098 (n,y) Mogg reaction. However, 66-hour Mog9 is usually assayed by measuring its beta radiation with a Geiger-Muller or proportional counter (1, 5, 7 ) . To correct for Mog9produced by the fission of uranium, a sample of natural uranium is irradiated simultaneously with the samples and the ratio of Mo99 arising from (n,f) M099 and Np239arising from the UZ3* (n,?) UZJg-.c Np23greaction is determined. The samples are then analyzed for Np239,and an equivalent amount of Mog9 is subtracted from the Mog9 in the samples to correct for uranium impurity. Iron yields two radioactive isotopes on activation, arising from the nuclear reactions: FeS4 (n,y)Fe65 and Fe5*( ~ 2 , y ) F e ~A~method . of counting these two radioisotopes simultaneously to increase sensitivity has recently been described (4). EXPERIMENTAL
Standards and Reagents. Appropriate amounts of iron, molybdenum. and chromium salts were dissolved and diluted t o 1 liter with conductivity water t o yield a solution containing approximately 20 mg. of the element per ml. Five milliliters of redistilled HXO3 mere added to the iron carrier to prevent hydrolyzation. Dilute solutions of these standards VOL 35, NO. 8, JULY 1963
1029
