2459
Anal. Chem. 1983, 55, 2459-2460
Absorption Cell wlth Fiber Optics for Concentration Measurements In a Flowing Gas Stream Kathy
A. Saturday
E. I. du Pont de Nemours & Co., Savannah River Laboratory, Aiken, South Carolina 29808 Continuous, real-time measurement of gas concentrations is required for process control, gas phase kinetics, and monitoring. In many situations, the detection of transient or rapid changes in concentration is essential to properly observe and control the experiment or process. Only an in-line monitor can produce the timely analyses required in such cases. However, installation of an on-line analyzer may not be desirable in remote or inaccessible locations or where the equipment will be subjected to a harsh or contaminated environment. Space or budget limitations might also prevent installation of dedicated instrumentation a t each sampling location. In these cases, the analyzer must be separated from the sampling point without significantly lengthening the analysis time. Constant monitoring of the concentration of a gas phase reaction product was accomplished during kinetics experiments in our laboratory by using a simple optical absorption cell equipped with fiber optics. The optical monitor cella were added to existing experimental equipment a t key sampling locations where space limitations were a definite consideration. With fiber optics, the light source and detector assemblies could be separated from the monitor cell and located where space was available, interference with system operations was minimized, and exposure of the instrumentation to radioactive contamination was low. To facilitate the connection of the fiber optics to the experimental apparatus, the optical system was kept simple. Delicate alignments were avoided since the apparatus was contained in a glovebox and access to the monitor cells was limited. Changing the analysis site from one monitor cell to another was a trivial operation, allowing a single light source and detector assembly to service all experimental configurations. The optical absorption monitor described in this article is mechanically and optically simple in design, yet sensitive, reliable, easy to operate and quickly adaptable to many analysis situations. EXPERIMENTAL SECTION Absorption Cell. The absorption cell was constructed from Varian Mini-Con Flat hardware and specially fabricated adapters that couple the fiber optics to the cell (see Figure 1). Standard Varian viewing ports with sapphire windows No. (954-5140) were attached to nonadjacent arms of the 0.75 in. i.d. cross (Varian No. 952-5070) that formed the body of the cell. The window flanges were sealed to the cell with copper O-rings that were secured by six 8-32 machine screws inserted through the flange on the cross, the window flange, and into the tapped fiber optics adapter. To attach the fiber optic cable to the cell, the ferruled cable end was inserted into the adapter and held in place by the set screw. The cell was connected to a in. line in the experimental apparatus by use of copper O-ring sealed flanges (Varian No. 954-5135) that were welded to the line. The sample optical path length was 7.6 cm. Light Source and Fiber Optics. A low-pressure mercury lamp (Model 6035, Oriel Corp., Stamford, CT) supported in an Oriel Model 6365 mount served as the light source for the optical monitor. Two-meter fused silica fiber optic cables (OC-1450,Math Ass~ciates,Inc., Port Washmgton, NY)transmitted the light from the source to the monitor cell and from the cell to the detector. The ferruled end of the fiber optic cable was held in close proximity to the lamp with the distance between them adjusted to maximize the light transmission through the fiber. Detection Equipment. An RCA 1P28 photomultiplier tube in a fiber optics adapted Oriel Model 7060 photomultiplier housing was used to detect the light transmitted through the cell. Either 0003-2700/83/0355-2459$01.50/0
Table I. Performance of In-Line Absorption Cell
gas UF, F2
ClF, PuF,
analysis wavelength, measdcross nm section, emz 253.7 365.0 253.7 365.0 253.7 365.0 253.7
1.59 X 2.33X 1.77 X '4.99 X 9.71 X 2.88 X 6.57 x
lo-'* lo-"
lo-" lo-**
max min measdetecturable able pressure, quantity, torr torr 15 115' >760 >760 300 >760 3
0.05 4 5 15 1
30 0.01
' Maximum pressure measurable is limited by vapor pressure of UF, . the 253.7-nm or the 365.0-nm line in the lamp spectrum could be used for analysis by inserting the appropriate interference filter (Oriel Model 5640 or 5643) in the filter assembly on the detector housing. The photocurrent was processed with an Oriel Model 7072 readout and a strip chart recorder, or alternately, with a programmable digital multimeter (Model 3478A, Hewlett-Packard, Loveiand, CO) which was interfaced to a Hewlett-Packard 9845B computer. Calibration. The system was calibrated by obtaining a plot of absorbance as a function of pressure for each of the gases utilized or produced in the system. The light intensity transmitted through the fibers and an empty cell, Io, was determined after the cell was evacuated to a pressure of 30 mtorr or less. The neat gas was then allowed into the cell and light transmission readings were taken at several pressures, which were measured by Validyne (Northridge, CA) APlO absolute pressure transducers. All calibration curve8 were linear, with correlation coefficients of 0.990 or higher. A cell constant (sample pathlength times absorptivity, ab) was determined for each gas and was identical for all cells, indicating uniformity in the path length. Cross sections calculated from experimental data assuming a 7.6 cm pathlength agreed with previously published values to within 10%.
RESULTS AND DISCUSSION The in-line absorption cell was utilized to study the preparation of uranium and plutonium hexafluorides in a flowing system. Continuous in-line monitoring was required to obtain the desired kinetic data, since drawing samples at frequent intervals would have significantly changed the gas flow and total system pressure during the course of the experiment. Several cells were placed in key monitoring locations in the system and withstood exposure to corrosive fluorine-containing atmospheres. Measurements of 0-2.5 absorbance units were routinely made despite high light loss in the cell and absorption by the fiber optic cables. The high light losses resulted from the simple optical system of the cell. Light exiting from the fiber optic cable diverges as it traverses the cell. Subsequently, only a small fraction of the light transmitted to the cell can be collected by the second fiber for transmission to the detector. However, the amount of light transmitted by the system was sufficient for the required analyses and could saturate the detector if the bias voltage was too high. The light transmission of the system is also limited by the absorption of the 253.7-nm mercury line in the fiber optic cables. In the output spectrum of a low-pressure mercury lamp, the intensity ratio 0 1983 American Chemlcal Society
2460
Flgure 1. Optical
Anal. Chem. 1983, 55. 2460-2461
detectable quantity denotes the partial pressure of the gas at which a signal-to-noise ratio of 5 is obtained. To measure smaller partial pressures, a straight nipple section could he added between the body of the cell and the window flange to increase the path length. The long path length design was not tried in this study, but a collimator might he necessary to obtain sufficient light throughput and the effects of the additional path length on dead volume in the flowing system need to he evaluated. In a kinetics experiment, the transmission of the cell is monitored as a function of time and subsequently processed to obtain the reaction rate. Besides the kinetics experiments, the in-line absorption cells were found to he generally useful in monitoring photolysis experiments, passivation of the experimental equipment, and decomposition rate experiments.
monitor conslruction.
of the 253.7- and 365.0-nm lines is much greater than unity, hut after transmission and absorption by the 2-m fiber optic, this ratio is reduced to near unity. For the current analysis situation, the fiber absorption simplified the operation of the optical monitoring cell since the monitoring wavelength could he changed without adjusting the detector gain. However, the fiber absorption limits the maximum cable length that can be used to transmit the 253.7-nm radiation. The performance of the in-line monitor cell is shown in Table I for several gases. The column labeled minimum
Neodymium Fluoride Mounting for and Americium
01
Registry No. UF,, 7783-81-5; F,, 7782-41-4;CIF,, 7790-91-2; PuF,, 13693-06-6.
RECEIVEDfor review February 16,1983. Accepted August 11, 1983. This paper waa prepared in connection with work done under Contract No. DE-AC09-76SR00001 with the US. Department of Energy.
Spectrometric Determination of Uranium, Plutonium,
Forest D. Hindman Radiological and Enuironmental Sciences Laboratory, Department of Energy, 550 Second Street, Idaho Falls, Idaho 83401 This procedure was developed to take the place of electrodeposition ( I ) and is an extension of the Lieberman and Moghissi procedure (2). When applied to 10 g of soil or 500-mL water samples, the nuclides to he determined were separated from the sample matrix by a barium sulfate precipitation technique (3). The nuclides were separated from each other hy solvent extraction (3,4). And then americium was separated from the rare earths by column chromatography (5). The pure nuclides were mounted on HT-100 filters by this procedure, omitting the separations descrihed herein. Some samples had to he scoped to get an estimation of the sample size needed for analysis. Others had to be analyzed qualitatively to determine what nuclides were present. Both objectives were achieved with aliquanta of the original sample small enough to keep the total mass of calcium, thorium, uranium, and rare earths below 200 rg. Under these conditions there was no need to make prior separations of the nuclides from the sample matrix, and tracers were not necessary, hut could he used if desired. Unlike electrodeposition or precipitation with cerium hydroxide (61,this method accommodates substantial quantities of common interferences such as iron, aluminum, titanium, and zirconium since they form strong fluoro complexes. In addition, uranium cannot he carried quantitatively on cerium hydroxide. Most problems are with poor resolution, not poor yields, and the samples can he recovered easily for further purification. With experience, visual examination of samples prior to filtration suffices to identify the occasional sample which requires further purification. Usable resolution has heen obtained with 50 g of neodymium plus 50 pg of uranium-236. This
anide not subject 10
U S
Copyright.
The decontamination of one nuclide from another is around lo3 and the yields are 95-98%.
EXPERIMENTAL SECTION Instrumentation. The a spectrometry system has heen described previously (3). Reagents. All solutions were stared in polypropylene bottles. Neodymium Nitrate. A solution containing 0.5 mg/mL of neodymium in 0.2 M nitric acid was prepared from neodymium nitrate. Carbon Suspension. A 47-mm GA-6 Metricel filter (Gelman Instrument Co., Ann Arbor, MI) was fumed in 5 mL of sulfuric acid. The suspension was cooled and diluted to 50 mL with water. Substrate Suspension. Ten milligrams of neodymium as a nitrate solution and 20 mL of hydrochloric acid were diluted to 400 mL with water. Ten milliliters of 48% hydrofluoric acid was added and the solution was diluted ta 500 mL. Then 2-3 mL of carhon suspension was added. Tracer Solutions. The uranium, plutonium, and americium tracer solutions were purified (7) and standardized (3)as descrihed previously. Procedure. The sample, such as water, a smear, or air dust, was wet ashed with hydrofluoric, nitric, and perchloric acids in a Teflon beaker and transferred to a 250-mL Erlenmeyer flask, The solution was fused in sulfuric acid and enough sodium and potassium sulfate to dissolve the sample in 200 mg each of sodium acid sulfate and potassium acid sulfate melt. The melt was cooled and dissolved with heat in 10 mL of 3 M hydrochloric acid, The solution was held at the boiling temperature for 5 min and then transferred to a clear, 50-mL, round-hottomed, polycarbonate centrifuge tube (Nalge Co., Rochester, NY) and then 0.1 mL of neodymium nitrate solution and 5 mL of 30 M hydrofluoric acid were added with swirling after each addition. The sample was allowed to stand for 30 min. A 25-mm DM-450, filter was mounted on the stainless steel support in a polysulfone twist-lock funnel
Published 1983 by the American Chemlcal Societ)
