US3485565A - Combustion apparatus,particularly for preparation of samples for radioactive isotope tracer studies - Google Patents

Description

Dec. 23. 1959 N .4. KAARTINEN 3,435,555

COMBUSTION APPARA'I'US, PARTICULARLY FOR PREPARATION OF SAMPLES FOR RADIOACTIVE'ISOTOPE TRACER STUDIES Filed May 15, 1958 l I I l 11/11/1111 I 1 [Fill 0000004700 00 0 awn/me. Mad 6. z/r/rrzrmmg,

United States Patent 3,485,565 COMBUSTION APPARATUS, PARTICULARLY FOR PREPARATION OF SAMPLES FOR RADIOAC- TIVE ISOTOPE TRACER STUDIES Niilo H. Kaartinen, Turku, Finland, assignor to Packard Instrument Company, Inc., Downers Grove, Ill., a corporation of Delaware Filed May 13, 1968, Ser. No. 728,553 Claims priority, application Finland, May 16, 1967, 1,390/ 67 Int. Cl. F23d 11/38 U.S. Cl. 431-3 8 Claims ABSTRACT OF THE DISCLOSURE Method and apparatus for the processing of fluid materials, particularly in the preparation of samples for radioactive isotope tracer studies by combustion of starting materials containing such isotope tracers. The sample is burned in a combustion chamber which tapers upwardly and inwardly above the sample receptacle so as to approximate the shape of the flame of a burning sample, and the combustion products are continuously exhausted from the combustion chamber and passed through a heat exchanger which condenses the condensable vapors in the combustion products. Oxygen is fed into the combustion chamber at a controlled rate during combustion, and after combustion nitrogen gas is fed into the combustion chamber and exhausted therefrom through the heat exchanger so as to purge the system of any remaining gaseous production products.

The present invention relates generally to the processing of fluid materials. In its principal application, the invention relates to methods and apparatus for the preparation of samples for radioactive isotope tracer studies and, more particularly, to an improved method and apparatus for preparing such samples by combustion of the starting material containing the isotope tracer.

It is a primary object of the present invention to provide an improved combustion chamber for burning liquid or solid samples in an open system so that the combustion products are continuously removed from the combustion chamber, and including means for facilitating thorough cleaning of the chamber in a rapid and eflicient manner after each combustion. Thus, a related object of this aspect of the invention is to provide such an improved combustion chamber which has virtually no memory, even when used to burn radioactive samples.

Other objects and advantages of the invention will become apparent from the following detailed description and upon reference to the accompanying drawings, in which the single figure is an elevation view, partially in section, of combustion apparatus for use in the preparation of samples for radioactive isotope tracer studies and including a schematic diagram of a portion of the fluid and electrical systems associated therewith.

While the invention will be described in connection with certain preferred embodiments, it will be understood that it is not intended to limit the invention to these particular embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalent arrangements as may be included Within the spirit and scope of the invention as defined by the appended claims.

Turning now to the drawing, there is illustrated a portion of a sample preparation system for use in the preparation of samples for radioactive isotope tracer studies, such as studies involving tissue distribution and residue levels of drugs in plants and animals. In the preparation of such samples, a sample of the starting material containing the radioactive isotope tracer, such Patented Dec. 23, 1969 as a sample of the plant or animal tissue, is burned to convert the carbon in the starting material to carbon dioxide and the hydrogen to water, and the radioactive isotope tracer is then recovered from the resulting combustion products. For example, if the particular radioactive isotope tracer employed is C, it appears in the combustion products as CO gas; if the tracer is tritium H), it appears in the combustion products as H 0 in the form of a condensable vapor. Although C and H are the most commonly employed tracers, it will be understood that a number of other radioactive isotopes may be employed, such as S which is. converted to sulfate during combustion.

In order to provide samples which can be analyzed for radioactivity, the compounds containing the isotope tracers are'recovered from the combustion products, and separated from any materials therein which might interfere with the radioactivity determination. For example, the H 0 is recovered by cooling the combustion products to condense the vapors therein, including the H 0, after which the condensed vapors are separated from the remaining gases. The *CO may also be recovered by condensation or freezing at extremely low temperatures, such as by the use of liquid nitrogen for example, but it is more conventional to react the CO with a liquid trapping agent such as ethanolamine; the resulting reaction product is then recovered and mixed with a liquid scintillator to provide a sample suitable for use in making a radioactivity determination.

Referring now more specifically to the drawing, the sample to be burned is placed in a sample basket 10 which forms a part of the electrical ignition system, and also functions as a catalyst for eflicient combustion for the sample contained therein. The basket 10 is suitably made of platinum or a platinum-rhodium alloy, so that the basket can be used both as an electrical resistor in the ignition system and as a catalyst for the combustion of the sample. A pair of electrical conductors 11 and 12 extend upwardly from a mounting plate 13, to support the basket 10 at the upper and lower ends thereof, while also making electrical contact with the basket to connect it into the electrical ignition system. The conductors 11 and 12 extend vertically down through the plate 13 and terminate in depending connector pins beneath the plate 13.

In order to facilitate the loading of successive samples, the mounting plate 13 is supported on the top of a small platform 14 threaded on to the end of a pneumatic piston rod 15. To load a sample in the basket 10, the pneumatic cylinder and piston assembly 16 associated with the rod 15 is actuated to retract the piston rod 15, thereby lowering the basket 10 through an opening 17 in the bottom of a combustion chamber 18. The sample is then loaded in the basket, and the cylinder and piston assembly 16 is actuated to advance the rod 15 and thereby raise the basket 10 through the opening 17 into the combustion chamber 18. As the platform 14 enters the opening 17 a sealing ring 19 mounted in a groove in. the outer periphcry of the platform 14 engages the tapered walls of the opening 17 to form a gas-tight seal therewith.

For the purpose of igniting a sample contained in the basket 10 after it has been raised into the combustion chamber 18, the connector pins depending from the plate 13 fit into complementary electrical receptacles 20 in the top of the platform 14. The receptacles 20, in turn, are connected to an electrical igniter circuit including a power source such as battery 21 and an ignition switch 22 for applying an electrical voltage across the basket 10, which serves as a resistive type heating element in the igniter system. Thus, the sample is ignited by simply closing the switch 22, which is opened again as soon as combustion has been initiated.

In order to supply the oxygen required for combustion of the sample contained in the basket 10, pure oxygen is supplied to the combustion chamber 18 through a valve 23, a flow meter 24, and a pair of cooperating passageways 25 and 26 formed in the platform 14 and the plate 13. The gas discharge passageway 26 in the plate 13 is positioned directly beneath the center of the basket 10, so that the oxygen is fed directly into the combustion zone. The oxygen flow rate is adjusted, via the valve 23 and flow meter 24, to a level slightly above that required to support combustion of the sample in the basket 10, so that there is a slight excess of oxygen within the combustion 'chamber. Consequently, there is generally a relatively thin layer of an oxygen-rich atmosphere be- .tween the combustion flameand the inside walls of the combustion chamber 18, as indicated by the arrows in the drawing. This excess oxygen rises through the combustion chamber and is exhausted from the combustion chamber 18 along with the combustion products through a lateral exit 27 at the top of the chamber.

In accordance with one aspect of the present invention, the combustion chamber is open at the upper end thereof with the sidewalls extending upwardly and inwardly above the sample basket so as to approximate the shape of the flame of a burning sample, thereby minimizing the volume of oxygen-rich atmosphere around the flame, and the walls of the combustion chamber are preheated so as to maintain the wall temperature above the condensation temperature of the vapors contained in the combustion products. With this design, the combustion products tend to be swept directly into the exit 27, with the rising layer of oxygen-rich atmosphere along the chamber sidewalls tending to isolate the combustion products from the sidewalls. Moreover, any combustion products that do contact the chamber walls re main in the gas state, even during initiation of the combustion, because the walls are preheated and maintained at a temperature above the condensation temperature. Thus, in the illustrative embodiment of the combustion chamber the walls of the combustion chamber 18 extend vertically upwardly past the sample basket 10, and then slope inwardly above the basket so as to approximate the shape of the flame represented in broken lines. Surrounding the combustion chamber 18 is a cylindrical vessel 30 which defines an annular cavity around the outer surface of the chamber 18 for receiving a preheating fluid. To center the combustion chamber 18 within the vessel 30, the upper end thereof meshes with a complementary mounting element 31, while the lower end fits into a complementary hole in the bottom wall of the vessel 30.

Prior to ignition of the sample contained in the basket 10, the liquid contained in the annular cavity between the combustion chamber 18 and the vessel 30 is heated by means of a heating coil 32 at the lower end of the cavity. The liquid distributes this heat along the walls of the combustion chamber 18 so that the walls are uniformly heated to a temperature above the condensation temperature of the vapors contained in the combustion products to be produced. It has been found that the preheating of the combustion chamber walls to maintain the combustion products in gaseous form even during ignition, combined with the flame-shaped configuration of the chamber, permits the combustion products to be exhausted from the combustion chamber, on a continuous basis, so efliciently that there is virtually no residue of combustion products deposited on the chamber walls. The illustrative system also prevents condensation within the exit 27 of the combustion chamber 18, since the exit is also surrounded by the peheated liquid in the annular cavity between the combustion chamber 18 and the surrounding vessel 31.

As the exhausted gases leave the exit 27, they enter a transfer tube 34 which is insulated to maintain the fluids passing therethrough in a gaseous state. In the particular embodiment illustrated, the transfer tube 34 is double Walled with a metallic inner shell and an insulating outer shell to minimize the heat loss therethrough. From the transfer tube 34, the gaseous combustion products are passed through a suitable heat exchanger (not shown) for cooling the exhausted combustion products to condense the vapors therein, with the condensed vapors being collected in a counting or sample vial when the radioactive isotope tracer is contained therein. When the radioactive isotope tracer is still in gaseous form after being discharged from the heat exchanger, the gases are passed through a suitable recovery device such as a reaction column for reacting the isotope compound with a trapping agent, for example.

.When thecombustionof a given, sample has been com.

pleted, the valve 23 is closed to terminate the oxygen supply to the combustion chamber, and a valve 60 is opened to supply an inert gas such as nitrogen to the combustion chamber via the same flow meter 24 and passageways 25, 26 previously used to suply the oxygen. This inert gas, which is supplied under a slight pressure, sweeps upwardly through the combustion chamber 18 so as to purge the chamber of any remaining combustion products, and continues on through the chamber exit 27. Consequently, it can be seen that the entire system including the combustion chamber 18 is immediately purged of all gaseous combustion products following each sample combustion.

In one example of the invention, ten one-gram samples of tritium-labelled samples were combusted in sequence in the same equipment, with a blank sample, i.e., a sample containing no radioactive tracer, being combusted after each labelled sample. The combustion of each sample was initiated by the electrical igniter, heated to a temperature of about 1500 C., and the oxygen flow rate was set at about two liters per minute. The pressure inside the combustion chamber during combustion was less than 0.1 atmosphere above atmospheric pressure. The walls of the combustion chamber were pre-heated and thermostatically maintained at approximately 170 C. which was sufficient to prevent any noticeable condensation of the combustion products on the inside walls of the combustion chamber. During combustion, the combustion products were continuously exhausted through the upper end of the combustion chamber into a heat exchanger. From the heat exchanger, condensed vapors including condensed H O dripped into a counting vial connected to the heat exchanger, while the remaining gases passed on through the vial and were vented to the atmosphere.

The combustion of each sample was completed in about 45 seconds, after which the oxygen was turned OE and the nitrogen supply to the combustion chamber was turned on so that nitrogen was fed into the combustion chamber at a rate of seven liters per minute for about five to ten seconds.

The radioactivity level of the tracer in the starting material placed in the combustion chamber was 100,000 disintegrations per minute (d.p.m.). When the sample collected in the counting vial was analyzed for radioactivity, a count of 42,000 counts per minute (c.p.m.) was measured. The counting efliciency of the analytical method was determined to be 42% so that the measured count of 42,000 c.p.m. indicated that there was no loss whatever, i.e., there was recovery of the radioactive material. To check the accuracy of the radioactivity measurement made for the recovered material, the same amount and type of radioactive isotope tracer that was injected into the original starting material was placed in a second counting vial and analyzed for radioactivity in the same equipment used to analyze the recovered sample. The count measured for this second counting vial was identical to the measurement for the first sample, i.e., the count was 42,000 c.p.m. in each case, thereby confirming that the recovery was in fact 100%. Over the series of ten samples, the standard deviation of recovery was determined to be 0.7%, which is about the same degree of variability accounted for by statistical variati ns in the samples plus the accuracy of the analytical instrument without automatic standardization. Based on a comparison of the counts of the radioactive samples and the alternate blank samples, a memory of 1/ 10,000 or less was obtained consistently throughout the entire series of samples. The 42% counting efficiency compares with maximum efficiencies of 25% to 36% obtainable by comparable methods used previously, the improvement being due in large measure to the fact that there was little or no oxygen present in the sample so that quenching efiects were minimized or perhaps even eliminated. In addition to the increase in efficiency, there was a corresponding reduction in background, so that the resulting [figure of merit (efficiency squared divided by background) was significantly increased. For example, with the 42% efiiciency, the background was 27 so that the figure of merit was 650, which compares with a figure of merit of 370 obtainable by the conventional previous methods. The total time required to prepare the above samples was such that about 30 to 40 samples could be prepared per hour.

I claim as my invention:

1. An improved combustion apparatus comprising means forming a combustion chamber for burning material to produce gaseous combustion products, exit means at the top of said combustion chamber for continuously exhausting said gaseous combustion products from said combustion chamber, and a receptacle within the lower portion of said combustion chamber for holding the material sample to be burned, the side walls of said combustion chamber extending upwardly and inwardly above said receptacle so as to approximate the shape of the fiame of the burning material and thereby minimize the volume of atmosphere around the flame.

2. An improved combustion apparatus as set forth in claim 1 further characterized by means for supplying oxygen to said combustion chamber at a controlled rate during the burning of said material, and control means associated with said combustion chamber for terminating the oxygen supply and supplying an inert gas to the combustion chamber upon completion of the burning of said sample so as to sweep any residual combustion products out of said chamber.

3. An improved combustion apparatus as set forth in claim 1 further characterized by means for preheating the walls of said combustion chamber so as to maintain said walls above the condensation temperature of the vapors contained in the combustion products.

4. An improved combustion apparatus as set forth in claim 1 further characterized by means for moving said receptacle in and out of said combustion chamber for the loading of successive samples therein.

5. An improved combustion apparatus as set forth in claim 1 further characterized by means for supplying oxygen to said combustion chamber at a controlled rate during the burning of said material, control means associated with said combustion chamber for terminating the oxygen supply and supplying an inert gas to the combustion chamber upon completion of the burning of said material so as to sweep any residual combustion products out of said chamber, and means for preheating the walls of said combustion chamber so as to maintain said walls above the condensation temperature of the vapors contained in the combustion products.

6. A combustion method comprising the steps of providing a combustion chamber including a. receptacle within the lower portion thereof for holding .a material to be burned with the side walls of said combustion chamber extending upwardly and inwardly above said receptacle so as to approximate the shape of the flame of the burning material and thereby minimize the volume of atmosphere around the flame, burning a sample of material in said combustion chamber to produce gaseous combustion products, and continuously exhausting said gaseous combustion products from the top of said combustion chamber.

7. A combustion method as defined in claim 6 further characterized by the step of preheating the Walls of the combustion chamber so as to maintain said walls above the condensation temperature of the vapors contained in said gaseous combustion products.

8. A combustion method as set forth in claim 6 further characterized by the steps of supplying oxygen to said combustion chamber at a controlled rate during the burning of said material, and terminating the oxygen supply and supplying an inert gas to said combustion chamber upon completion of the burning of said material so as to sweep any residual combustion products out of said chamber.

References Cited UNITED STATES PATENTS 2,712,351 7/1955 Roth et al 43l158 XR 2,771,357 11/1956 Wroughton 84.1 2,938,577 5/1960 Hughey 4313 2,955,088 10/1960 Beerbower et al. 252-3011 3,015,127 1/1962 Stalego 431158 XR 3,192,985 7/1965 Livingston et al. 431-3 3,202,139 8/1965 Livingston et al. 431--3 XR FREDERICK L. MATTESON, JR., Primary Examiner R. A. DUA, Assistant Examiner US. Cl. X.R.

Images