IL30004A - Combustion methods and apparatus - Google Patents

Description

nana? *3p/im nwrf? jmaa w w IMPROVEMENTS IN OR RELATING TO COMBUSTION METHODS AND APPARATUS The present invention relates to a method of producing gaseous combustion- products and to comV stlon ap¾aratu3. 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 efficient 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..

According to one aspect of the invention, a combustion apparatus comprises 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 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 flame of the burning material and thereby minimize the volume of atmosphere around the flame.

According to another aspect of the invention, a method of producing gaseous combustion products comprises the stops 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.

The invention will be hereinafter described with reference to the accompanying drawing, 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 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 as a sample cf 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 14-C, it appears in the combustion products as gas; if the tracer is tritium (¾), it appears in the combustion products as H^O in the form of a condensable vapor.

Although 14-C and -5 are the most commonly employed tracers, it will be understood that a number of other radioactive isotopes may be employed, such as •55 which is converted to sulfate during combustion.

In crder 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^O is recovered by cooling the combustion products to condense the vapors therein, including the ^O, after which the condensed vapors are separated from the remaining gases. The AL.

CC>2 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 14 with a liquid trapping agent such as ethanolamin ; 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 efficient combustion for the sample contained therein. The basket 10 is suitably made of platinum or a platinum- hodium 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 and 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 ς£ 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 1 enters the opening 17, a sealing ring 19 mounted in a groove in the outer periphery 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 15 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 25, 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 2 , 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 between the combustion flame and 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 remain in the gas state, even during initiation of the combustion, because the walls are pre-heated 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 upwardl past .the sample basket 10, and then slope inwardly above the basket s. as to approximate the shape of the flame represented in broken lines. Surrounding the combustion chamber. 8 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 , 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 : essel 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 preheating that the 4e*^-ati»e °i "khe combustion chamber .walls- 'to maintain the combustion products in gaseous form- even during ignition, combined with the flame-shaped configuration of. tho chamber, permits the combustion products to bo exhausted from the combustion' chamber, on a continuous basis, so efficiently 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 preheated liquid in the annular cavity between the combustion chamber 18 and the surrounding vessel As the exhausted gases leave the exit 27, they enter a transfer tube 4- which is insulated to maintain the fluids passing therethrough in a. gaseous state. In the particular embodiment illustrated, ..the transfer tube 3 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 -th 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 tho isotope compound with; a trapping agent , for example . . ' · ■ ' ' ( When the combustion of a given .sampl . has ·■ been completed, 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 supply 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 ^Η?0· 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 off 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 (dpm). When the sample collected in the counting vial was analyzed for radioactivity, a count of 4-2,000 counts per minute (cpm) was measured. The counting efficiency of the analytical method was determined to be 4-2% so that the measured count of 42,000 cpm indicated that there was no loss whatever, i.e., there was 100% 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 t.hat 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 cpm 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 variations in the samples plus the accuracy of the analytical instrument without automatic st ndardization. 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 effects 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 2 efficiency, the background was 2 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.

Claims (9)

1. A. A 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 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 flame of the burning material and thereby minimize the volume of atmosphere around the flame.

2. A 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 eaid 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.

3. A combustion apparatus as set forth in claims 1 or 2 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. 3000^2

4. A combustion apparatus as sot forth in claim , 2 or 3 further characterized by means for moving said receptacle in and out of said combustion- chamber for the loading of successive samples therein.

5. A method of producing gaseous combustion products comprising the steps of providing. 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.

6. A combustion method as defined in claim 5 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.

7. · A combustion method as set forth in claims 5 or 6 further characterized by the steps of supplying oxygen to said combustion chamber at a controlled rate during the burning of said material, and termina ing gaej the oxygen supply and supplying an, inert(to said combustion chamber upon completion of the burning of said material so as to sweep any residual combustion products out of said chamber.

8. A combustion apparatus substantially as herein described with reference to the accompanying drawing.

9. A method of producing gaseous combustion products substantially as herein described.