ENTRAINING VAPORIZED SAMPLE IN METASTABLE GAS

Abstract: Method and apparatus for analyzing a sample by metastable transfer emission spectroscopy. The sample is vaporized within a chamber (20) and the sample vapor is entrained in a stream or metastable gas flowing through the chamber. A carrier gas separate from the metastable gas used to excite the sample atoms is not required. In addition to performing its primary function of exciting the sample atoms or molecules, the metastable gas removes any traces of the sample that might otherwise be left behind on the sample vaporizer (28).

Description: ENTRAINING VAPORIZED SAMPLE IN METASTABLE GAS Background of the Invention 1. Field of the Invention The present invention broadly relates to methods and apparatus for exciting the atoms of a vap¬ orized sample to higher atomic energy levels by trans¬ ferring energy to the sample from a metastable gas. Such methods and apparatus are commonly used in meta¬ stable transfer emission spectroscopy. More particu¬ larly, the present invention is directed to means for mixing the vaporized sample with the metastable gas. 2. Description of the Prior Art Metastable transfer ' emission spectroscopy (MTES) is a recently developed technique for analyzing the elemental composition of a solid, liquid or gaseous sample. The MTES technique is described in U.S. Patent 4,148,612 to Taylor et al.; U.S. Patent 4,150,951 to Capelle et al.; and copending U.S. patent application Serial No. 165,529 entitled "Gas Mixing Apparatus for Metstable Transfer Emission Spectroscopy" filed jointly by the present applicant and Robert L. Schmidt on July 3, 1980 and assigned to the same assignee as the pres¬ ent application. MTES involves the use of metastable gas, which is a gas having a substantial number of its atoms or molecules excited to atomic or molecular energy levels above the ground state, wherein the atoms or molecules remain in their excited states for a rela- tively long time, generally for a time ranging from a • microsecond to a few seconds. Typically, the meta- stable gas is created by using an electromagnetic field ■ . ; to directly or indirectly excite a stable gas such as ■ nitrogen or one of the noble gases. In MTES, if the sample to be analyzed is not <V. in gaseous form, it must be vaporized by some means. Various sample vaporization means are known in the art, one example being an electrical resistance heater. In the operation of an MTES system, the -sam¬ ple vapor and the metastable gas are directed to flow into a common mixing region. ' In most applications involving solid or liquid samples, the amount of sample vapor is so small that it must be entrained in a car¬ rier gas in order to direct its flow. Known MTES sys¬ tems employ one of the inert gases as a carrier. In 1 such systems, the carrier gas flows past the sample vaporizer, entrains the sample vapor, and conveys the sample vapor to the mixing region. Upon mixture with the metastable gas, the atoms or molecules of the sample vapor become excited via nergy transfer to energy levels above the ground state. After being excited, the sample atoms or mole- cules almost immediately give up their excess atomic energy by emitting a photon of light and returning to fϋ*RE _ OMPI " the unexcited ground state. A spectrometer placed out¬ side a transparent window near the mixing region ana¬ lyzes the light emitted by the excited sample atoms. f The wavelength and intensity of the emitted light re¬ spectively identify and quantify the constituents of the sample. As just described, the known MTES systems require plumbing for two separate gas flow paths lead¬ ing into the mixing region. One plumbing line carries the metastable gas, and the other plumbing line carries the entrained sample vapor and the carrier gas. Known MTES systems also require apparatus for mixing the gases from the two paths thoroughly enough to ensure that all of the sample atoms will be excited by the metastable gas. For example, such mixing ap¬ paratus is the subject matter of the copending patent application cited earlier. Summary of the Invention The present invention is a method and appara¬ tus for analyzing a sample material by vaporizing the sample within a chamber, entraining the sample in a stream of metastable gas flowing through the chamber to permit a transfer of energy from the metastable gas to c * the sample vapor, and measuring radiation emitted by the sample vapor in response to the energy transfer. An advantage of the present invention over known MTES systems is that, instead of using a separate - .- carrier gas, it uses the metastable gas to entrain the sample vapor. This eliminates the need for two separ¬ ate plumbing lines or conduits to convey the carrier and the metastable gases, and eliminates any need for special mixing apparatus to ensure thorough mixing of the sample vapor with the metastable gas. An additional advantage of the present inven¬ tion is that it can be implemented with the sample vaporizer positioned within the flow of metastable gas, whereby a "scrubbing" action of the metastable gas en¬ sures that the sample is completely vaporized, leaving no residue on the vaporizer- Brief Description of the Drawings Figure 1 is a schematic diagram of an MTES system according to the present invention. Figures 2 and 3 are sectional views of an embodiment of a cylindrical chamber and a vaporizer comprising a flat pan for supporting the sample. Figure 4 is a sectional view of an embodiment of a cylindrical sample chamber and a vaporizer com¬ prising a cylindrical member for supporting the sample- Figures 5 and 6 are sectional views of an embodiment of a vertically oriented cylindrical sample chamber and a vaporizer comprising a cup for supporting the sample. Figures 7 and 8 are sectional views of an embodiment of a cylindrical sample chamber and a vap- OMPI orizer comprising a perforated disc on which the sample is deposited. Description of the Preferred Embodiment Figure 1 shows in schematic form the pre¬ ferred embodiment of an MTES system according to the present invention. In the preferred embodiment, metastable ac¬ tive nitrogen is the metastable gas used to excite the sample atoms or molecules. In operation, vacuum pump 10 causes nitrogen gas to flow out of container 12, through conduit 14, and into microwave cavity 16. In¬ side the cavity, the nitrogen gas is. subjected to microwave radiation which excites the gas to metastable higher energy levels. The """* resulting metastable nitro¬ gen gas flows out of microwave cavity 16, through con¬ duit 18, and into sample chamber 20. Sample vaporizer 22 is centrally located within sample chamber 20. The solid or liquid sample to be analyzed is initially deposited on vaporizer 22. The vaporizer preferably comprises an electrical heat¬ ing element through which current is conducted to heat the sample to a temperature typically between two and three thousand degrees Celsius, well above the vapori¬ zation temperature of the chemical constituents of the sample. . This heat causes the sample to boil off the surfaces of the vaporizer and form a vapor within sample chamber 20. The metastable nitrogen gas continuously flows through sample chamber 20 while the sample is being vaporized. The flowing metastable gas entrains the sample vapor and carries it out of sample chamber 20 into viewing tube 24. To minimize turbulence, sample chamber 20 and viewing tube 24 are preferably contiguous, coaxial cylinders having equal inner dia¬ meters. The metastable nitrogen gas transfers energy to the entrained sample vapor, exciting the sample atoms or molecules to higher energy levels. Almost immediately after becoming excited, each sample atom or molecule returns to its ground state by emitting a photon of light. To enable spectrophotometer 26 to observe the light emissions, viewing tube 24 may have a transparent window facing spectrophotometer 26, or else may be transparent all around. Spectrophotometer 26 measures the wavelength and intensity of the emitted light to identify and quantify the chemical consti¬ tuents of the sample. Figures 2-8 illustrate various alternative embodiments of sample chamber 20 and vaporizer 22. Figures 2 and 3 show an embodiment of sample vaporizer 22 comprising a flat pan 28 on which a solid or liquid sample may be deposited. An electrical heating element 30 is affixed to the bottom of pan 28 so as to conduct heat through the pan to vaporize the OMPI sample. Pan 28 is supported near the center of cylin¬ drical sample chamber 20, preferably by the electrodes 32 and 34 which supply power to heater 30. As indi¬ cated by arrows in Figure 2, the metastable gas sup¬ plied through conduit 18 flows in direct contact with the sample deposited on pan 28, entrains it, and car¬ ries it through sample chamber 20 into viewing tube 24. To maximize the amount of metastable gas that contacts the sample and pan 28, conduit 18 is preferably angled as shown so that the metastable gas flows toward the pan from above. Figure 4 shows an embodiment of vaporizer 22 comprising a cylindrical tube 36 inside of which the sample is deposited. As indicated by arrows in the figure, the metastable gas is directed through the center of the tube 36 to entrain the sample. Figures 5 and 6 show an embodiment of the MTES system in which conduit 18, cylindrical sample chamber 20, and viewing tube 24 are essentially rotated by 90° relative to the preceding embodiments so that their cylindrical axes are vertical. Vaporizer 22 comprises a cup 38 in which the sample is deposited. Electrodes 32 and 34 suspend cup 38 near the center of sample chamber 2-0. In operation, sample vapor leaves the surface of cup 38 and occupies a region near the cup. As indicated by arrows in Figure 5, the meta¬ stable gas flows around the perimeter of cup 38 and entrains the samDle vaDor. Figures 7 and 8*.show an embodiment of vapori¬ zer 22 comprising a perforated disc 40 suspended co- axially within sample chamber 20 by electrodes 32 and 34. The sample is deposited on the surface of disc 40 facing viewing tube 24. In operation, the metastable gas flows through the numerous perforations in disc 40, as indicated by arrows in Figure 7, and entrains the sample vapor. Unlike prior MTES systems .which included separate plumbing lines or conduits for carrying the metastable gas and a carrier gas, the invention just described has only a single gas flow path. This simp¬ lified structure beneficially reduces both the size and cost of an MTES system- * An MTES system according to the present in-, vention has the further advantage of eliminating any need for special mixing apparatus such as that used in prior systems to mix the sample vapor flowing through one length of tubing with the metastable gas flowing through a separate tube. A further advantage of the described embodi¬ ment of the invention is that the metastable gas flow¬ ing around sample vaporizer 22 has a "scrubbing" ability whereby it removes all traces of the sample from the surfaces of the vaporizer more effectively than would a stable carrier gas. This increases the accuracy of any quantitative measurements by ensuring OMPI that all of the sample vaporizes and is excited by the metastable gas.

Claims: I claim: 1. A method of analyzing one or more con- stituents of a sample material, comprising the steps of: supporting the sample within a chamber; vaporizing the sample constituents to be analyzed; conveying metastable gas through the chamber so as to entrain the vaporized sample constituents; and measuring radiation, emitted by the entrained _ vaporized sample constituents. 2. Apparatus for analyzing one or more con- stituents of a sample material, comprising: a chamber; "* -. . means for supporting the sample constituents to be analyzed; means for conveying metastable gas through the chamber so as to entrain the vaporized sample con- stituents; and means for measuring radiation emitted by the entrained vaporized sample constituents. —--— 3. Apparatus according to claim 2, wherein: the vaporizing means releases the vaporized sample constituents into a region within the chamber; and • - the conveying means directs the^metastable gas through the region occupied by the vaporized sample constituents . 4. Apparatus according to claim 2 or 3, wherein the vaporizing means comprises means for heat¬ ing the sample to a temperature greater than the vapor¬ ization temperature of the sample constituents to be analyzed.

Inventors: PEACOCK JON
Applicants: BECKMAN INSTRUMENTS INC
Application year: 1981
Main classification: G01J3
IPC codes: G01N21