1400
ANALYTICAL CHEMISTRY Table V. Accuracy of Results Element Iron Manganese Jlagnesium
No. of
Detns. 17 20 22
Conen. Range 0.080-0.13 0.020-0.040 0.27 -0.50
%.A?.
Deviation 7 8
8.2 5.6
The average deviations of approximately 8%’for iron and manganese and 5.6% for magnesium are considered adequate for routine testing of sample lots. Because there is considerable heating of the sample electrode nheii the Multisource is used, exposures must be of relatively short duration to avoid spattering of the solution. A spark stand equipped with water-cooled electrode holders would minimize heating of the sample electrode. The precision could probably be improved by using a conventional high voltage spark source, because this would allow one to use longer exposure periods, a slower optical system, and a slower emulsion. Use of a simple electrode with a truncated cone bottom similar t o that described by Scribner and Ballinger for the analysis of bronzes (11)would also probably improve the precision of the method. ACKNOWLEDGMENT
This investigation was conducted under the general direction of Paul M. Ambrose, chief, College Park Branch, Metallurgical
Division, and under the imniediate supervisioii of Howard F. Carl, chipf, Physical and Chemical Section. The author wishes to thank A. M. Sherwood and A. H. Macmillail for chemical analysis performed, Shirley Hodgson for preparation of many of the samples, and Charles E. White, of the rniversity of Maryland, for his many helpful suggestions LITER.4TURE CITED
(1) Baird, W. S.. “Proceedings of 6th Suniniei, Conference on Spectroscopy and Its Applications,” pp. 80--7. New York, John Wiley & Sons, 1939. (2) Colin, R. H., and Gardiier, D. .I..A S ~ L (’HEM.. 21, 7 0 1 4 (1949). (3) Feldman, C., I M . . 21, 1041-5 (1949). (4) Ihid., pp. 1211-15. (5) Gassman, A. G., and O’Neill, R. H.. Ihid., 21, 417-18 (1949). ( 6 ) Hasler, M. F., and Dietert, H. LV., J . Opticnl SOC.A m . , 33. 218-32 (1943). (7) Keirs, R. J.. and Englis, D. T.. IN). ENG.CHEM.,ANAL.ED.. 12, 275 (1940). (8) Office of Naval Research, Washington, D. C., “Titanium,” Report of Symposium on Titanium, 1949. (9) Russell, H. N., Astrophys. J., 66, 283-328 (1927). (10) Russell, H. N., Zbid., 66, 347-438 (1927). (11) Scribner, B. F., and Ballinger, J. C., J . Resmrch .Y,itl. H U T .
Standards (to be published).
RECEI\ E D June 7 ,
1950
Spectrochemical Analysis of Radioactive Solutions CYHUS FELDMAN, Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn., MYHON B. HAWKINS, Rudioisotope Applications Company, Berkeley, Calif., MARVIN MURRAY, Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn., AND DONALD R . WARD, Isotopes Division, U . S . Atomic Energy Commission, Oak Ridge, Tenn. A description is given of a chamber designed to permit electrodes for spectrochemical analysis to be loaded outside the spectrographic laboratory, excited with spark or other intermittent excitation, and disposed of with a minimum risk to personnel. If strippable films are used to protect the most heavily e;posed surfaces, residual radiation levels of -2 mr. per hour at contact on the chamber parts can easily be attained for samples customarily encountered. 7
I
S ASALYZING any type of material speckrochemically, one naturally attempts to attain the maximum accuracy, precision, sensitivity, and speed. In analyzing radioactive materials, however, the main consideration is safety, and the folloKiiig factors must be considered first:
Protection of operators from a, 0,and y radiation (shielding). Protection of operators from contact with radioactive materials, hlinimization of number of handling and transfer operations prior to excitation of sample. Prevention of air contamination, especially during excitation of sample. Safe and rapid disposal of used electrodes. Safe, rapid, and complete decontamination of auxiliary apparatus. The general interests of safety and efficiency are usually served best by standardizing techniques as much as possible. When the samples are tadioactive, safety and efficiency also affect the choice of excitation technique and sample form. CHOICE OF EXCITATION TECHNIQUE
Because the dischargm used in spectrochemical analysis are disruptive, they must be conducted within an enclosed chamber in order to provide shielding and prevent contamination of the air. It must be possible to decontaminate such a chamber quickly and safely, in order both to safeguard personnel and to have the chamber available for re-use BS soon as possible. The effectiveness and dispatch with which a surface can be
decontaminated tend to vary inversely with its area. The minimum permissible internal area of a chamber is dictated largely by the size and power rating of the discharge taking place within it. If the discharge continuously liberates large amounts of heat and vapor, as is the case 4 ith the direct current arc, the chamber must have a volume large enough to prevent overheating. This is a matter both of convenience of handling and of possiblr spectral line interferences from chamber materials which might be volatilized into the discharge if the chamber walls were too close to the arc. The use of spark techniques, however, lowers the temperature within the chamber, and makes it possible to adopt two measures which greatly simplify the decontamination problem: The size of the chamber caavity may be greatly reduced, thus decieasing the contaminated area. The area exposed to contamination may be coated before exposure with a strippable film. -4fter exposure, this film may be removed and the contarnination eliminated. This technique can be used only with spark excitation, because the film materials available cannot withstand the temperatures prevailing in an arc chamber. Almost all of the radioactive materials analyzed in the authors’ laboratory occur in solution form. Because this laboratory’s nonradioactive solutions are usually analyzed by either the copper spark method (1, 3 ) , or the porous cup method (e),
V O L U M E 22, NO. 11, N O V E M B E R 1 9 5 0
1401 sli ing out of the tube. A copper electrode in its tube is shown i n g g u r e I, D. The loaded tube is then inserted into a. stainless steel-lined hole in the Fluorothene (plast,ic polymer of chloratrifluoroethylene) rotor, E , and pushed through until the end of the copper electrode emerges into the hemicylindrical Cavity hollowed out in the end surface of the rotor. This entire assembly is then inserted into the stainless steel cassette, F,and secured by inserting the threaded turning pin, A , through the slot in the cassette and screwing it into t'le tapped hole shown on the outside curved surface of the rotor. The camplet.ed assembly is s h o r n in Figure 2.
Figure 1. Components of Cassette Assembly it appeared that the most serviceable protection chamber would be one built expressly for spark excitation of solutions by either of these methods. A chamber has been developed which permits the sample to he carried t o the laboratory, excited, and disposed of without, exposure of operators to dangerous levels of radiation or airborne contsminatian. I t ' c a n be decontaminated in 2 to 3 minutes, with negligible risk to the operator. The chamber consists essentially of two cassette assern1,lier and a chamber block. The cassette assemblies contain the electrodes, and can be .closed in such a way as to protect the operator against radiation or contaminated dust coming from th8 sample.
Figure 4.
Spark Chamher Assembled
ASSEMBLY AND OPERATION O F CHAMBER
When the copper spsrk method is to be used, the order 01 operations is as follows (see Figure 1): A copper rod 0.25 ineh'in diamet,er. machined itrs described in II. 3. is inserted mrt-wa\ into a elbsely fitting stainless steel tibe. "A long U-sh'aped cu"i has been'made in the wall of the tupe, and the metal tongue thus formed is bent into the form of a wave (-) of very law amplitude. The resulting spring action prevent.s the electrode from ~~~~~
~~~
The cassette is then closed by turning the rotor until the pin reaches t,he endbf its slot. Figurr 3 shows the cassette assembly when closed. Two such assemblies are prepared and sent to the laboratory submitting the sample. They are then opened as shown in Figure 2 and placed on a suitable support. The radioactive solution is deposited on the end of the electrodes by IL remotecontrol DiDet. and dried to % residue bv means 01 i n infrared'lsmp. If Cocoon or;tective raatings are uskd ( m a n u f a c t k d by the R. If. Hollingshead Company, Camden. S . J.). care should be taken to Id o&heat&g. A sheet of metal or
ena of the verticd'hale passing through the
n telining screws. The block is provided with indows consisting of glass or fused quartz isks 'held in suitable-recesses by spring Ips. It is supported on the optical bench 9 shown in Fimre 5. Once the r6sottes are in place, the cham:r cavity is created by rotating the rotors ithin the cassettes by means of the turnin g pins. Figure 4 shows the appearance ' the chamber after this has been done. ~
~
~
~~~
~
=
Figure 2. Cassette Assembly Open
~~~~
~~
~
ANALYTICAL CHEMISTRY
1402
ing the e l e c t , d e gap. The surface of this csvlty, most of which .is on t,he rotors, is the only surface directly exposed to contamination. The steel sleeve^ lining the hole in the rotor through which the rlect.rode tube is thrust project 5 to 10 mm. from the outside end surface of t.he rotors; excitation is provided by clamping the rlectriral leads t.o this projecting portion (see Figure 5). The insulation furnished by the body of the rotor has proved sufficient to prrmit the use of an A.R.L. spark source (pesk voltage -35 kv.j.' The spark gap is given its final adjustment with the aid of a projected shadow image, and the exposure is made. The electrical leads are then disconnerted, and the rotors are turned to the closed position, thus mvefing the greatest part of the contaminated area. The entire chamher, still containing the cassettes, is then removed from the supporting yoke and carried to t,he laborittory for disposal bf electiodes and decontamination. m e of t,he electrodes, each cassette assembly is re ill closed, from the chamber block. It is then inverted over a c isposable cont,ainer;such as a widemouthed bottle, and opencd, ;md the contaminated electrode is pushed out of its tube from behind, into the bottle. The cassette assembly is then closed, :md the bottle is capped and discarded. After both elect.rodt.s have bem disposed of, the cassette assemblies and the ch;mbw block are ready for decontamination. If the porous graphite cup terhnique Ysto be used, the porous cup electrode is first dipped open side down into a dilute solution Zapon until most of the side surface, hut none of the end, or apxrking surfare, has been coated (8, Figure 1, B ) . This prevents corrosion of t.he electrode tube by any acid solutions which may he inserted into the porous cup electrode. (Alternatively, parts coming in contact, with corrbsive solutions may he made of t.ant:tlum or platinum.) The POTOUS cup electrode is then ins w t d into an elwtrode tube open end first, unt,il the sparking rurfwe projects about 0.25 inch from the end of the tube. This is used as thc upper electrode. A graphite rod 0.25 in diameter is used as the lower electrode. The chamher is assembled 8 s nhove, and the sample solution is inserted into the porous cup ivom outside the chamber (8, Figure 1, A ) . Subsequent diap n d and decontamination are nrromdished as with the C O D D ~ ~ XI
-
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1g"
Il"..,",,"..
,.,
....... ". "_
....."..,
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,."
....... _."
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P. E. Urown and MI-workers). Before entering the vacuunl line, the air'emef&ing from the outlet passes through 10 inches of cotton wadding and B gas washer whose gas inlet tube in fitted with B coar8e fritted glass disk. The scrubbing liquid is chosen to correspond to the chemical nature of the radioactive spccics. Tests of the gas strewn before and after scrubbing show that, this step removes 90 to 99.8% of the activity remaining in the air stream after pasage through the cot,ton. Tests have shown no radioactivity to be present in the condensate or oil from thc vacuum pump. DECONTAMINATION OF CHAMBER
Technique. It has drays proved possible to reduce surface u h v i t y of the chamber parts to a negligible level 1-2 nr. pcr hour for ( 8 plus 7 ) activity or -30 decompositions per minute for a act,ivity a t a distance of 1 inch] by some camhinxt.ion of chemical treatment and abrasion. The. chemical niethods of decontamination. have involved disassembly of the chnmbrr below the surface of a chemical hnth whose ingredients were chosen, if possible, to form B complex a i t h the radioactive chemical species. The baths were renewed until activit.y was down to the level mentioned. Chemical decontamination methods have several disadv:uitages, however. Their effect on 8- and r-act,ive substances, especially on purous surfaces, tends to follow a law of diminishing returns. Surf w u s cant,aminated with radioactive isotopes of elements amenable to chemical treatment, such 8s tho alkalies, alkalinr earths, common metals, etc., may require several hours for decontamination; thc Isst traces of the more stubborn elements, such ils tungsten, columbium, etc., may yield to nothing short of smdpaper. Their effect on some a-active substances, such as plutonium and americium, is both ~ l o mand incomplete. Pralongcd shritsion of the surface is &IW&IS ~ ~ C C S S B.I (. Ythus . inereasina the daneer to bho operator. The use of drastic chemical agents is limit,ed by the nature of the material of the chitmber. Although lhc use of Fluoroin constructing thcne (oolvmerized monochlorotriRuoroet,h?l~~e) the ro&s makes the]m proof against m y enmbinat.ion of chemic.:d rengcnt,s (41,t,he sta:inless st,eel parts are particularly suxrcptihlc t,oattaak by hydroch 1 0 1.ir acid and alkalies.
111view of thc diffi< ties ronneetod with eimnicrl dwoiitiiininiition methods, it appi .ed that the problem niight bc solved niow r,ffectively hy nvoidi : cont:rniinalion of the surface in t h r tirst Chamhm n o 6 in USP provide at l e u t 2300 mg; per sq. em. oi shielding in all directions, so thxt once the casset.te amemhlirrs have heen closed, after evaporation of t.he residue, the operatop is completely protected from a and 8 radiation. Protection against y r3diation,.'ho~,evrl.,is comparatively slight, even aft,er the cassrtbes have been inserted into the chamher hloek (SW Tahle I).
{,l*U!.
.ittem,,t,s were firs t:tnrination a i t h a I
AVOIDANCE OF AIR CONTAMINATION
The breathing in of rsdioaetive solid particles may be particularly dangerous because of the extremely high levo1 of radii)tion in the immediate neighhorhood of such a prticle ( 6 ) . I t is therefore urgent that the vapors creat.ed by the escit.at,ion of radioactive samples he kept under control. In order to prevent the escape of radioactive vapors into the room, a partial vacuum is mltiitained in the .chamber. KO attempt is m d e to have t,he chitmhei vacuum-tight; clearunces of 0.2 to 0.3 mm. are purposely left between chamber block and cawette, cassette and rotor, etc. The air whioh thus leaks into the ehmnhcr is drawn off t.hrougli a smnll outlet drilled through the body of the block near the rear (glass) h d o w (see Figure 4). The current of the air seeping into the chamber makes the escape of radioactive vapors into the room virtually impossible. Air samples taken during the sparking of residues which read -5 roentgen per hour a t a distance of 3 inches when unshielded have shown no activity shove background (measurements made by
Figure 5.
Spar,kC h a m b e r and Filters in Operating
Position
1403
V O L U M E 22, N O . 11, N O V E M B E R 1 9 5 0 (nater-soluble polyethylene glycols). After exposure, this coating was removed by immcrsing the parts in a bath of warm complexing agent and flushing this away with warm water. This procedure considerably decreased the amount of activity deposited on the chamber parts themselves, but some chemical and abrasive treatment was still necessary for surfaces contami‘nated with plutonium, americium, or large amounts of @ and yactive substances. The most successful coating agent found so far has been Cocoon, a liquid which, when spread on a surface, dries rapidly t o give an elastic, strippable film. The coating may be applied either by dipping or by cementing suitably shaped pieces of the dried film to the surface to be protected.
Table 11.
Sample No.
Effectiveness of Decontamination with Strippable Coating”
-4ctivity on Coating Part nfter Sprctrogrnphic lhposnre, Activity on ?art Residual Activity on hafore Removal after Removal of I’nit before Coatinr of Coating Coating 400 4.0
