Analysis. The total cyanide content of the sample was determined in the alkaline solution by AgN03 titration. For low cyanide concentration, Le., 0.1-1 mg/l, the colorimetric method using chloramin T and pyridine-pyrazolone was employed (3). Regeneration. The resin was regenerated with -8% NHlOH solution and washed with 15 ml of distilled water.
RESULTS AND DISCUSSION The cyanide absorption capacity of the strongly basic resin was first checked with KCN solutions in the concentration range of 5-40 gfl. The resin in the burette was found to absorb about 200 mg of cyanide per sample and completely absorbed the cyanide contained in 2-ml samples of concentrated solutions. The complex cyanides were treated in the same way as the simple cyanides and there, too, a cyanide-free filtrate was obtained. The efficiency of the complex cyanide breakdown and the recovery of cyanide was checked by material balance.
Some typical results are summarized in Table 11,where it can be seen that the recovery of cyanide is complete. A third acid wash may be used to ensure complete elution of cyanides, but care must be taken that the solution in the beaker where the acid effluents are collected is sufficiently alkaline. The complex cyanide breakdown and the recovery of free cyanide take no more than 20 min. The resin is regenerated by NH40H instead ef NaOH to prevent the formation of insoluble metal hydroxides.
LITERATURE CITED (1) "Standard Methods for the Examination of Water, Sewage and Industrial Wastes", Tenth ed., Amer. Publ. Health Assoc. Inc., 1955. (2) F. J. Ludzack et at., Anal. Chem., 26, 1784 (1954). (3) J. Epstein, Anal. Chem., 19, 272 (1947).
RECEIVEDfor review June 14,1976. Accepted November 10, 1976.
Autoradiography of Californium-252 Wire Neutron Sources Burton Tiffany," K. W. MacMurdo, and R. H. Gaddy Savannah River Laboratory, E. 1. du Pont de Nemours and Company, Aiken, S.C. 2980 1
Californium-252 is an intense neutron source and gamma emitter having many medical and industrial applications ( I 1. The sources are often fabricated in the form of a wire, and a piece of the wire is encapsulated in a metal sheath. For some applications, such as the neutron irradiation of tumors, the uniformity of distribution of 252Cfin the wire must be known. This paper describes a technique for measuring the distribution of 252Cfby placing a source on a photographic plate sensitive to gamma radiation to produce an autoradiograph and then reading the radiograph with an automated densitometer.
EXPERIMENTAL Apparatus. A Digital Equipment Corporation PDP-8/1 computer with two Dectape transports and 8K words of core memory was interfaced to a Grant microphotometer, Series 800. The photomultiplier tube was interfaced to a 10-bit analog-to-digital converter. The microphotometer was equipped with computer-controlled SLO-SYN driving motors to move the densitometer stage in the 3c and y directions. The autoradiographs were recorded on 2 X 10 inch Kodak Spectrum Analysis No. 1photoplates. Plate Preparation. The glass photoplate was encased in an opaque plastic sleeve that was backed with a 2 X 10 inch wood board for strength. Gamma-radiation absorbers such as 10- to 30-mil-thick sheet stainless steel or 10-mil-thick sheet tantalum were added in front of the emulsion on the photoplate to slow the exposure rate for strong sources. The photoplate was packaged in a darkroom. Procedure. The photoplate package was placed on top of the zizCf wire with the emulsion facing the wire for a period of time determined by the estimated concentration of 252Cfper unit length of wire, the thickness and type of encapsulation, and the thickness and type of radiation absorber in the photoplate package. After exposure, the photoplate was removed from the package without radioactive contamination and developed in Kodak D-19 Developer. After fixing and drying, the photoplate was positioned to give a lengthwise traverse on the densitometer glass stage. The autoradiographic image was generally about 3 mm wide and often not straight. Figure 1A shows an autoradiograph of a wire which is 140 mm long and contains 50 pg z5zCfper 25 mm. The densitometer optics were set to give a resolution a t the emulsion of 1pm in the width and 1 mm in the height. The length of the image was measured in millimeters and used as input information to the computer. The autoradiogaph was manually centered on the minimum transmittance point a t one end of the image and thereafter was automatically centered by the computer-controlled y-stepping motor. The percent
difference in the transmittance value used in the y -centering search routine was set a t the start of the evaluation to give accurate centering with minimal y-centering movement of the stage. The photomultiplier output was set to give 100%transmittance for a background region of the photoplate. Readings were generally taken a t 0.1-mm intervals and averaged for 5 to 10 of these values to give a regional value. The length of the image is limited to 204 mm with the present computer program.
RESULTS AND DISCUSSION Autoradiographs were initially interpreted manually on a densitometer with minimum transmittance values recorded every 3 mm (the estimated resolution of the technique). Kodak Spectrum Analysis No. 1 photoplates produced a satisfactory image. The transmittances were averaged and the percent differences were calculated. Because this was a very time-consuming task, photoplate image interpretation appeared to be an ideal application of computerized densitometry. A computer program was written to automate the readings of transmittance values and to convert transmittance to absorbance, which is directly proportional to the concentration
A
al
L a , '
a
P o s i t i o n Along Autoradiograph
Figure 1. 252Cf autoradiograph and distribution profile (A) Autoradiograph of a 252Cfwire. (B) Plot of the percent difference of the absorbance from the average absorbance along the autoradiograph ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
*
517
of 252Cfin that region of the wire. Readings were taken at 0.1-mm intervals and averaged over 5 to 10 readings. The values are stored in the upper 4K words of core memory and, on completion of scanning the image, are averaged, and the percent differences are calculated. The average absorbance and its standard deviation are calculated and printed. The percent difference values are plotted by the teletype giving a visual profile of the relative californium per unit length along the wire. An example of this plot is given in Figure 1B for the autoradiographic image in Figure 1A. The quality of the image depends on the distance between the photoplate and the wire, the accuracy of positioning the plate parallel to the wire, and the radiation received during positioning of the plate. For sources containing greater than 50 pg per 25 mm, blurring of the image was excessive due to radiation received by the photoplate during positioning of the plate. Insertion of radiation absorbers increased the exposure times and minimized the blurring effect. The accuracy of the technique, estimated to be f 5 % , depends upon the reproducibility of positioning the wire relative to the photoplage (f3%) and variations in the emulsion re-
sponse at low exposure levels (62%).The electronic instability of the photomultiplier tube and the analog-to-digital converter have only a minor effect (f0.1%). The technique of computerized densitometry can be applied to interpreting any photographic image. A computer program can be written to generate a two-dimensional array of points in any desired configuration. Calculations and curve smoothing and fitting on a dedicated minicomputer provide a rapid, flexible technique that can be applied to interpreting a variety of photographic images.
LITERATURE CITED (1) Californium-252 Progress, No. 1 through 20 (1969-1976); issued serniannually by Savannah River Operations Office, ERDA. Aiken, S.C.
RECEIVEDfor review August 27,1976. Accepted November 15, 1976. The information contained in this article was developed during the course of work under Contract No. AT(072)-1 with the U.S. Energy Research and Development Administration.
Gas Chromatography/Mass Spectrometry Interface Simplified and Quantified James P. Lehman Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 4522 1
The value of the combined gas chromatography/mass spectrometry technique (GC/MS) needs no elaboration here. A perusal of the table of contents of this journal demonstrates the great utility of the technique and the wide use that is being made of it. There are, however, three problems associated with the technique as it is generally used which are apparent to workers in this field. These problems are: 1)the determination of the sample enrichment in that portion of the carrier gas stream which enters the MS ion source from the separator, 2) optimization of the GC/MS system for sensitivity while retaining MS resolution, and 3) the transfer of GC methodology from the GC laboratory to the G U M S system. A technique is described which readily and reliably determines the sample enrichment seen in that portion of the carrier gas stream which enters the MS ion source from the separator. This capability permits one to evaluate separator performance and to compare separators with each other under the various conditions in which they are used. Another technique is described which evaluates MS performance as a function of carrier gas flow into the MS ion source. One can then select the optimum operating conditions for best GC/MS sensitivity. In the GC/MS interface system shown in Figure 1,the full functionality of a two-column GC is maintained. This greatly facilitates the transfer of GC methodology to the GC/MS system and reduces considerably the amount of time that the expensive and sophisticated MS is used simply as a GC detector.
EXPERIMENTAL Gas Flow Determinations. The variable effluent splitters were adjusted to the desired split ratio by measuring the He flows a t the flame ionization detectors (FID'S) and the vent a t 3 in Figure 1. The conductance of the vent a t 2 was then adjusted to match that a t 3 by adding glass wool through the tube fitting a t the Valco High Temperature 4-port switching valve (Valco Instruments, Houston, Texas 518
ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
77055), 1in Figure 1, until the gas flows a t 2 and 3 were equal when the valve was turned to switch the flow from 2 to 3. The He flow (HesEp) into the separator was controlled by R s ~ p (Figure 1)which consisted of '/I6 inch 0.d. X 0.010 inch i.d. stainless steel (SS) capillary or, more conveniently, the length of 0.016-inch SS wire inserted into a 30-cm length of %6 inch 0.d. X 0.020 inch i.d. SS capillary. HesEp was determined by shutting off the make-up He supply with the valve a t 5 in Figure 1,adjusting the He flow from the column (HecoL) that was connected to the separator (column B in Figure 1)such that a positive flow was measured and recorded a t 3 (HecoL > HesEp), and then switching the column flow to 2 and measuring HecoL. HesEp is the difference of the two measurements. The He flow from the separator into the MS ion source (HeMs) was inch 0.d. X 0.020 inch or controlled by RMS(Figure 1) which was 0.030-inch i.d. SS capillary. The pressure measured a t any point in the MS analyzer tube is a measure of the mass flow through the MS. The pressure registered a t the ionization vacuum gauge (IG), 8 in Figure 1,was used in these experiments. The difference between the pressure registered with the GC/MS shut off valve, 7 in Figure 1, closed (BACKGROUND P I G )and that registered with the valve open was directly a measure of HeMs. (GC/MS PIG) Molecular Leak Calibration. The volume of the inlet system was determined by expanding a known volume of Ar into the inlet system, the initial and final pressures being determined with a McLeod gauge at the gas inlet. The conductance of the molecular leak for He was determined by introducing He into the inlet system and measuring the decreasing gas pressure as a function of time with the McLeod gauge. The IG at 8 in Figure 1 was calibrated in terms of He flow by recording the BACKGROUND PIG,introducing He to the inlet system and recording both P H and ~ the total PIGas the P H was ~ incremented. GC/MS Operating Procedure. T o prevent contamination of the interface and MS with air, a positive flow of He is maintained a t all times a t 3 in Figure 1 by adjustment of the metering valve a t 4. Differences between HecoL and HesEp are accommodated by the vent and metering valve a t 3 and 4. When HecoL > HesEp, the excess column effluent is vented a t 3 and if HesEp > HecoL, the deficit gas flow is provided by the make-up He supply controlled a t 4. The GC/MS shut off valve is opened fully and the sample is injected, the analytical column effluent being vented a t 2. When it is seen that the solvent peak has passed a t the FID, the switching valve is rotated and the column effluent is now being sent to the separator and MS.
