determinations were made on standard samples of this material to verify or to re-establish the analytical calibration. The nature of the determination was such that manual operation required approximately 30 minutes of operator time to scan each sample and calculate the result of each determination. This made the year’s operation equivalent to approximately 65 man-weeks. During this period, setup and processing time for the automatic apparatus was estimated a t 15 man-weeks. Maintenance and repair time was estimated a t 5 man-weeks. Thus the automatic analyzer saved approximately 45 manweeks of routine analytical time on one determination during one year. Delays because of backlog were minimized and many samples were analyzed that could not have been handled otherwise. Even more important, capable operators were freed for more challenging work and instrument time was made available during normal working hours. The system has been used on problems involving well-resolved analytical bands, overlapping bands, double-beam compensation, and atmospheric background interference. In nearly all cases the over-all analytical results have been as good as or better than with manual operation. In a check of instrumental repro-
ducibility] a single sample was analyzed 65 times during a one-day period. The standard deviation for a single determination was 0.045% of the mean determined value. Over approximately 3 months, the relative standard deviation of periodically repeated analyses of a series of standard samples was 0.1% of the mean value. A similar but smaller series of checks performed manually over the same period on similar samples gave a relative standard deviation that was not significantly different. In nearly all cases, use of the automatic infrared analyzer permitted the analysis of replicate samples to improve precision at little increase in cost. The analyzer also permitted the inclusion of standard samples a t more frequent intervals than would have been possible with manual analysis. In this way, accuracy of the analyses was improved a t very low cost. Finally, the automatic analyzer freed valuable personnel and equipment for nonroutine operations during the regular working day, since the routine, automated analyses were performed primarily during otherwise nonproductive off-shift hours. ACKNOWLEDGMENT
The authors gratefully acknon-ledge the helpful assistance of Carl G. Brod-
hun and Harry Weyl in the assembly and maintenance of the electronic equipment. LITERATURE CITED
(1) American Cancer Society Conference on Application of Automation t o
Urinary Steroid Assays, New York, N. Y., Sept. 18-19, 1961; Science 136,
160 11962’1. (2) Brackett, F. S., J . Opt. SOC.Am. 50, 1193 (1960). (3) Camels, J. W., Brame, E. G., Day, C. E., ANAL.CHEM.33,813 (1961). ~~
~
(4)Johnson, D. R., Eastern Analytical Symposium, New York, N. Y., Nov. 2-4, 1960, Paper 3. (5) New York Academy of Sciences Conference on Automatic Chemical Analysis, Ann. N . Y . Acad. Sci. 87, 609 (1959). (6) Re‘inke,’ R. C., Herscher, L. W., Perkin-Elmer Instrument News 1, No. 4 (1950). ( 7 ) Savitzky, A., Ax-4~.CHEY. 33, KO. 13, 25 A (1961). (8) Siggia, S., “Continuous Analysis of
Chemical Process Systems,” Wiley, NPWYnrk. 1 R A R (9, W e s t G t , D. F., A N A L . CHEY., 33, I
812 (1961). (10) White, J. U., Liston, 11. D., Simard, R. G., Ibid., 21, 1156 (1949).
RECEIVEDfor review June 14, 1962. Accepted August 30, 1962. Fifteenth Annual Sunimer Symposium, Division of Analytical Chemistry and ANALYTICAL CHEMISTRY, College Park. Md.. June 1962
Ra pid, No ndest ructive Determina t io n of Beryllium Using Van de Graaff X-Rays CHARLES A. LEVINE and JOSEPH
P. SURLS, Jr.
The Dow Chemical Co., Western Division, Piftsburg, Calif.
b A method is described for the rapid nondestructive analysis for beryllium utilizing the well known Be9 (7,n) 2a reaction. The y-rays were produced b y conversion in a tungsten target of electrons accelerated b y a Van de Graaff machine. Some of the neutrons resulting from the reaction of the y-rays with beryllium were captured in a silver disk to produce radioactive silver isotopes. The induced radioactivity was counted with a Geiger-Miiller tube apparatus and was proportional to the beryllium content of the irradiated sample. The average time required for a determination was 4 minutes. With a 20-gram sample, the limit of detection in a single determination was 0.00270.
W
for beryllium generally must be handled cautiously because of the toxicity of its compounds. ~ V. Gorshov and I n the late 1 9 3 0 ’ ~G. 1614
ET ANALYSIS
ANALYTICAL CHEMISTRY
coworkers (1) determined beryllium by photoneutron emission, irradiating the ores with 7-rays. Using 48 mc. of radium as the y-ray source and irradiating a 560-gram ore sample for 1.5 hours, the authors noted that beryllium contents of about 0.45%, amounting to 2.5 grams of beryllium, could be determined with an error of about 13% of the beryllium present. I n more recent years Rfilner and Edwards (S), Iredale ( 2 ) , and others have perfected the technique] using up to curie quantities of strong y-emitters such as Sblz4 as the radiation source and counting the neutrons emitted with BF3 tubes or scintillation counters. With sophisticated techniques-e.g., using 12 BF3 neutron counters and 1.5 curies of S b l 2 L a s little as 4 mg. of beryllium can be detected in a 0.5-cc. sample during a 1-hour irradiation. The need for rapid, sensitive beryllium determinations in low grade ores and the increasing availability of high
energy particle accelerators and their attendant high x-ray fluxes led to the development of the method here. While 1 curie of SblZ4has an x-ray intensity of about 2 X lo3 roentgens per hour a t 2 cm., it is easy to obtain 2 X lo6 roentgens per hour from a medium size accelerator. iin analysis, taking about 4 minutes, can detect less than 1 mg. of beryllium in a 20gram sealed sample. The use of an x-ray flux from a particle accelerator instead of from Sblz4 brings the additional advantages of relative freedom from radiation hazards and from frequent source replacement. EXPERIMENTAL
Apparatus and Procedure. X-rays, obtained by converting 2.1 m.e.v. electrons from a Van de Graaff accelerator, impinge on the beryllium containing sample. Fast neutrons are given off by the well known Be9 (y, n ) 2a reaction. The number of
I
Figure 1.
\
Experimental arrongement
neutrons is a direct function of the number of beryllium atoms present. The neutrons are moderated in surroundine polyethylene and a certain proportiog are captured in silver disk. Those captured by the silver form radioactive silver isotopes, mainly 24second Ag"0 and 2.3-minute Ag'oB. The irradiation is stopped and the silver disk is rapidly transferred to a Geiger counter where the amount of induced radioactivity is determined. The radioactivity is directly proportional to the amount of beryllium present. The sequence of events can be summarized as follows:
Figure
ing other than the standard shield to ""il..".. L""1 .I_^.._ . I lrUUrr
"arn&I"ulIu.
Ag'm (2.3 mh.) Agno, (24 see.)
3.
(G.-M. detector) In practice, because of the short half-life of the Ag"O, the sample is irradiated for 80 seconds, a standard TO seconds areused to transfer the silver d1sk to the Geiger counter, and the induced activity is measured for 30 seconds. With the conditions used, the induced activity is about 40 to 80 c.p.m. per milligram of beryllium, dependent upon the geometry of the sample holder and the moderator. Figure 1 is a drawing of the apparatus. The turntable is 20 inches in diameter, and, when activated, switches the target and silver disk assembly t o the Geiger tube in somewhat. !,,s than the ten seconas. i n e nign voltage to thl ' e Geiger tube is left off during the Van d,e Graaff irradiation to mevent a drastisC shortening of the life df the Geiger tube Concurrent with activating the turn table, the Geiger high voltage is tnrne,J on, and counting is started a t the em1 of the ten seconds allowed for transfe r time. The Geiger tube needs no shield rnl
.
Table I. Effect of Moderator in Sample on Sensitivity
Be, g.
Sample
0.046
0.046
Sugar, g. 0.000 1.530
Counts per minute per mg. of RI 56.7 f 1.0 56.9 f 1.80
1.
>..LT
l l l C SaLIqJ," 'IUILL-
ers shown in Figure 2 are of two types. One is a 2.5-cc. polystyrene vial and the other, for larger samples, . . is a 16-cc. metal pill box, Materials Used. I n the development of the method, C.P. grade B e 0 and BeS04.4H,0 were used as primary standards. Other chemicals were -~ 3 . .,. L.Y. giaue, noc Iurmer pnrineo ~~~1 , ~
thermal neutrons &(Ad
rnL.
~
~
,
1
DISCUSSIONS
Effect of Moderator. The amount and configuration of t h e neutron moderator have a large effect because they govern the percentage of fast neutrons t h a t are slowed down and captured by t h e silver. The moderator can be any hydrogenous material-polyethylene or paraffin is convenient t o use. When standardizing on a sample container size and shape and moderator configuration, the only possible change in moderator becomes that amount of moderating material which may be in the sample itself. This becomes insignificant if the sample can be kept small in relation t o the amount of moderator close to the silver disk. Table I shows the magnitude of this effect when using the small 2.5-cc. sample holder shown in Figure 2. In the case of the larger 16-cc. sample holder, a significant error can be m a d e e.g., 1.2 grams of water in a 6-gram sample causing a 10% increase in counts per minute per milligram of beryllium. Effect of %Ray Energy. The energy of the x-rays impinging on the beryllium sample has a large effect. The energy must be greater than 1.66 m.e.v. as this is the threshold for the Be ( y , n) reaction. Further, however, the cross section of the reaction changes very rapidly between 1.66 and 3.0 m.e.v. so that x-ray energy must be closely controlled. An added factor when making x-rays by converting energetic
electrons from an electron accelerator is that the efficiency of conversion increases rapidly with energy. Figure 3 shows the sum of these two effects or the net sensitivity of analysis us. the accelerating voltage of the accelerator. By controlling the voltage t o within 5 kv., the variation in sensitivity during a series of analyses can be kept very small. This control is readily accomplished by installing a hair line on the Van de Graaff voltmeter to correct parallax reading errors. Effect of Sample Size. Size of the sample within the sample holder has a noticeable effect only with the large sample holder. A smaller sample size places a greater percentage of any beryllium present closer to the silver disk, and, in the case of the 16-cc. pill box, substantially further away from the moderator. This, of course, affects the sensitivity. The amount of this effect is shown in Figure 4. It must be corrected for, either by using standard samples of approximately the same size or by reference t o the graph.
Co"ntr/min-mgBa
Figure 3.
Effect of accelerator volta g e on sensitivity
VOL 34, NO. 12, NOVEMBER 1962
1615
When the 2.5-cc. sample capsule is used, this effect is smaller than the statistical variation in the analyses and can be ignored. No variation in counts per minute per milligram of beryllium, greater than standard deviation, was seen when successively larger samples of beryl (4.6% Be) ranging from 0.719 gram to 7.455 grams were run in the small containers. Similarly, dilution of 0.862 gram of the beryl n.ith up to 8.467 grams of S a C l had no effect. Interfering Substances. Possible interfering substances fall into three categories. First are moderators, which may change the ratio of neutrons emitted by the beryllium t o neutrons caught by the silver. The magnitude of this error has been discussed above. Second are neutron absorbers, such as boron and cadmium, which might decrease the number of neutrons caught by the silver. Third are elements which would capture neutrons to form radioactive species other than L4g110-10s, and which would be counted by the Geiger counter as additional radioactivity. In the case of relatively large amounts of high neutron absorbers, here again no effect is seen using the m a l l sample containers while a distinct error can be made using the large containers. This, of course, is because larger amounts of the neutron absorbers can be placed in the larger containers. Table I1 shows, for the small containers, the effect of Cd and I3 in amounts equal to 35 times and 7 times the beryllium present, respectively. For the large containers, the effect of Cd and B up to 200 times and 50 times the beryllium present is shown. Elements which might be activated by the neutrons to form radioactive isotopes will, in general, not appreciably contribute to the counting rate because,
CC.
2.5 2.5 2.5 16 16 16
2
0
6
4
Sornple
Figure 4.
Be, g. 0.046 0.046 0.046 0.046 0.046 0 046
SO?, g.
if @-emitters,the thickne*s of thc silver disk will absorb the emission, and if y-emitters, the insensitivity of the Geiger counter toward ?-rays will discriminate against them. If a detector more sensitive to y-radiation, such as a scintillation crystal. were used, this problem would have to be considered. L-sing an x-ray sensitive scintillation crystal, one would also have the activities due to photoexcitation of certain elements by the Van de Graaff 2.1m.e.1'. x-rays. These elements nould include Se, Cd, Sr, In, Dy, Hi, Ir, and h u (i). Of these, only Hf and Ir might make enough activity to be troublesome xhen a Geiger counter is used. The entire problem of evtraneous induced activities in the sample can be eliminated by arranging the apparatus such that only the silver disk is transferred to the Geiger tube for counting. Any sill er in the sample, as il-ell as the silver disk itself, becomes radioactive from the x-ray beam by photoactivation to 44-second -4g1°7m and 40-second AglOgm. However, the n-eak emissions from these isotopes can be completely screened from the counter
Sample CdSO,, g.
_
_
_
HIROI g
...
...
... ...
1 5836
...
3.021
24.0 ...
... 11.56
...
14.72
.
Counts, 30 sec
1 2
1454 14T8 1453 1484 1439 1508
3
4 5 6
1616
ANALYTICAL CHEMISTRY
Yet rounts/min.
-d/C=s
38 2 38 5 38 2 38 6 38 0 38.9
2897 2942 2891 2954 2863 3002
"01,
IO
2
, iL
cc.
by using a thin 2-mil polyethylene film over the Geiger tube windo\\. This film has almost no absorption for the stronger AglOE-l10 @-particles. Duration of Irradiation Time. The accuracy of the analysis is governed by counting statistics. The more activity that is counted, the lower will be the percentage error. Table I11 shows the variation and deviation in six consecutive runs on a sample of beryl and the sensitivity calculated from the results. Standard deviation in the number of counts, N , from the Geiger counter is dX,and the ratio of observed to theoretical standard deviation errors for the set, from data in column 2 , is 0.67. This indicates the apparatus was functioning properly n-ithin expected statistical limits (4). hmount of activity induced in the silver and counted can be increased by increasing the sample size, increasing the irradiation time to take adrantage of 2.3-minute Ag'O8, counting for a longer time, or optimizing the moderator arrangement. The 80-second irradiation time and 30-second counting time, as described, do not take full advantage of 2.3minute Agio*. For maximum accuracy, these times should be increased to about 10 minutes each. The shorter times are a compromise to decreaqe the time involved per analysiq.
Counts ~ per min. per mg. of Be 5 6 . 7 11 . 0 56.6 f 1 . 0 55.0 Zk 1 . 0 1 7 . 5 11 . 0 43.6 f 1.0 415*10
Table Ill. Variation and Deviations on Consecutive Analyses
Run
8
Effect of sample size on sensitivity
Effect of Neutron Absorbers Mixed with Samples
Table 11.
Sample
holder,
140
Counts per min. per mg. of Be 6 3 0 i ~ 1 6 64Ozk16 6 2 8 1 1 6 6 4 1 1 1 6 6 2 3 f 1 6 6 5 2 f l 6
APPLICATIONS
This method is now used routinely to analyze beryllium ore., leach residues from the ores, and flotation products from ber\*lliuni t ontaining minerals. The Be content. of most of these samples range from 0.055 to 4 q . -111 anal)-sis takes about 4 minutes. The accuracy depends on the total number of counts detected, and therefore is a function of both sample sizr and Be content. At about O.lCc Iie, 3. 20-gram sample could be analyzed to a standard deviation error of 3c0 of the beryllium present. At the O.O1yc Be level. the same size sample could be analyzed (at the wme lirril~:il>i~itvlrvel) to
10.5% of the beryllium present. The statistical error could be reduced using larger samples or repeating the analysis several times. The method has been adapted to solution analysis using the large 16-cc. sample container by making the appropriate correction for the neutron moderating effect of the \vater.
Ibid., Report S o . P.G.-171, pp. 55772. (4) U. 6. Dept. of Health, Education, and Welfare, “Radiological Health Handbook,” PB 121784R, p. 133 (Office Tech. Services, September 1960). (5) V‘agner, C. D., Lukens, H., Otvoa, J. IFT., Intern. J . Appl. Radiation Isotopes 11, 30-7, (August 1961).
LITERATURE CITED
(1) Aidarkin, B. C., Gorshov, G. V.,
Grammakov, A. G., Kolchina, A. G., Tr. Radievogo Inst., Akad. Nauk SSSR, 8993 (1957); C.A. 54, 18175 (1960). ( 2 ) Iredale, P., United Kingdom Atomic Energy Authority Research Group, Report No. AERE-EL/M-108, Atomic Energy Research Establishment, Harwell, Berks, England (1960). (3) Milner, G. W. C., Edwards, J. W.,
RECEIVEDfor review June 18, 1962. Accepted August 21, 1962.
Condensation Nuclei, a New Technique for Gas Analysis FRANK W. VAN LUIK, Jr., and RALPH E. RIPPERE’ General Engineering laborafory, General Electric Co
b A technique for measuring low concentrations of gas involves conversion of gas molecules to liquid or solid submicroscopic airborne particles which act as condensation nuclei. Water vapor is caused to condense on the particles. The opacity of the vapor is then measured and related b y electrical signal to gas concentration. This procedure has many promising applications in air pollution control, toxic vapor detection, and continuous monitoring of chemical processes. technique for measuring low gas concentrations by conversion of gas molecules to condensation nuclei has been developed. An understanding of the technique is dependent upon a knowledge of condensation nuclei and the capability of the condensation nuclei detecting instrument. A general description of the technique is given here, with a few of the possible instrument configurations that have been developed in the authors’ laboratory and some performance test data on stdected gases. NET\
., Schenecfady,
N. Y THE DETECTOR
Figure 1 indicates the amount of saturation required to cause a “particle” of water to act as its own condensation nucleus ( 1 ) . While the major proportions of condensation nuclei are not water particles, once the growth process is triggered by a nucleus and water molecules start to attach themselves to it, the growing particle acts as if it \\ere completely composed of water. Therefore, Figure 1 can serve as a guide in suggesting the amount of supersaturation required to condense water on particles of a given size. The time required for the growth process-from nucleation a t about 0.001micron diameter to visible droplet about 1.0 micron in diameter-is about 7 milliseconds (’7). This speed of growth is an important factor in the instrumentation, and the growth in size of the particle provides an amplification factor which can be utilized to give extreme sensitivity t o this method of measurement.
The condensation nuclei detector
(CND) developed by General Electric Co. is shown schematically in Figure 2 (6). The power supplies, rotor driving mechanisms, and auxiliary equipment are omitted froni the figure.
The air iainple containing the particles to be detected and measuredj is drawn through the inctrument by the vacuum pump. ‘l’lie siimple first enters the humidifier, n-iic I e its relative humidity is raised io 100% d h Ratt‘r. The sample then passes through a rotating valve into the cloud chamber, where it is expanded adiabaticallj- by the valving and transport system, causing the saniple to cool and the relative humidity to rise to any desired supersaturation up t o 400% ( 5 ) . This unstable condition can be lelieved by three prosesses: Condensation in nhich the nucleus of the droplet is a single water molecule;
’OOr
CONDENSATION NUCLEI
Condensation nuclei are liquid or solid submicroscopic airborne particles, each of nhich can act as the nucleus for the formation of a water droplet. Their size may vary from 0.001 to 0.1 micron in diameter. Such nuclei are found in nature in concentrations ranging from a few hundred to hundreds of thousands per cubic centimeter of air. The nucleus derive3 its name from its triggering role in causing supersaturated water vapor t o condense into droplets. Water vapor in air that contains no particles will not start t o condense into droplets until the air is SO070 supersaturated. Prcsent address, Computcr Depart, Sunnlvale, Calif.
ment, Gcneral Electrir Co
2
300
I
w
Figure 1. Relative humidity necessary for condensation
E
200
01 10-6
.om1
I
I
IO-’ .OOl
10-6
I
10-6
I
10-4
SIZE CM RADIUS .I 1.0 S I Z E MICRONS
.01
GAS MOLECULE CONDENSATION NUCLEI ARC NUCLEI TOBACCO SMOKE
I
10-3
10.0
J
IO-*
100.0
RANGE OF PARTICLE SIZES
FOG
t?zzzzz
VOL. 34, NO. 12, NOVEMBER 1962
1617
