Abstract:
An instrument for the analysis of a sample material to detect the presence of specific substances that thermally decompose into electronegative species comprises a sensitized surface, means for heating the sensitized surface in a gaseous environment to generate a gaseous boundary layer adjacent the sensitized surface, means for causing the sample material to interact with the sensitized surface to form negative ions, a collector electrode maintainable at a different electrical potential from the sensitized surface to cause a current of these negative ions to the collector electrode, and means for measuring the ion current. The sensitized surface is formed on a ceramic body that is impregnated with an alkali metal. The composition and temperature of the sensitized surface, and the composition of the gaseous boundary layer are selected according to the specific substance to be detected.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This is a further development in thermionic detection techniques, and provides a method and apparatus for flameless thermionic detection of specific substances that thermally decompose into electronegative species. 
     2. Prior Art 
     In 1936, J. P. Blewett (Physical Review, Vol. 50, p. 464, 1936) described studies in which efficient filament sources of positive ions of the alkali metals were produced by heating synthetic alkali aluminum silicates. These alkali-glass sources were shown to be abundant emitters of positive ion current and poor emitters of negative ion current, especially when heated to temperatures close to the melting temperature of the alkali-glass. 
     In 1951, Rice (U.S. Pat. No. 2,550,498) described a method and apparatus for electrically detecting vapors of certain substances by sensitizing a hot surface with a material from the class of alkali metals and their compounds, bringing sample vapor into contact with the hot surface, and measuring the current of positive ions produced at the hot surface by the presence of the sample vapor. A preferred embodiment of Rice&#39;s apparatus consisted of two concentric platinum cylinders with appropriate diameters such that gas vapors of interest could flow through a gap between the inner and outer cylinders. The cylinders were electrically biased so as to cause the motion of positive ion current in the direction from the inner cylinder to the outer cylinder. The inner cylinder further surrounded a helical heater coil wound on an alumina cylinder. Natural alkali impurities within the alumina ceramic served to produce the required sensitizing action for a relatively short operating time. Rice taught that the active life of a sensitized alumina could be increased or restored by soaking the alumina in a water solution containing an alkali metal salt. For even longer life, Rice further taught that the alumina cylinder could be replaced by a cylinder of alkali glass such as one of those described by Blewett. The detector described by Rice was found to be especially effective in the detection of compounds containing halogen atoms. 
     In 1957, Roberts (U.S. Pat. No. 2,795,716) described an improved electrical vapor detector which used a positive ion source that provided a relatively long life compared to that source described in the Rice patent. The positive ion source described by Roberts consisted of a cylindrical alumina ceramic core upon which was wound a heater coil. The alumina core and heater coil were covered on their outer surfaces with a coating of positive ion emitting material. For the positive ion emitting material, Roberts used the alkali-glasses described by Blewett. The alkali-glass was powdered and mixed with a suitable ceramic cement in the desired proportion, and the mixture was coated on the alumina core and heater coil and allowed to set. 
     In 1975, Kolb and Bischoff (U.S. Pat. No. 3,852,037) described a selective ionization detector which used an electrically-heated alkali-galss bead maintained in a heated softened state during operation of the detector. Kolb and Bischoff theorized that the softened glass state acted to maintain an adequate supply of alkali to the surface of the glass by means of molecular motion within the body of the glass. Kolb and Bischoff described a detector in which the alkali glass bead was mounted above a burner nozzle to which a mixture of a combustible gas and a sample gas was supplied. A collecting electrode was located above the glass bead, with an electrical bias applied between bead and collector so as to direct negative ion current to move from the bead to the collector. Kolb and Bischoff further taught that specificity for particular individual substances could be obtained by suitable selection of gas flows and by selection of the appropriate alkali metal used in the alkali-glass bead. For example, rubidium-glass appeared best suited for detection of nitrogen compounds, whereas sodium-glass was especially good for phosphorus compounds. 
     In 1977, Burgett et al. (Journal of Chromatography, Vol. 134, p. 57, 1977), described a new nitrogen-phosphorus detector for gas chromatography. The active component in this detector was described as a ceramic cylinder coated with an alkali salt activator similar to that described by Rice. Actually, this alkali-ceramic cylinder has the physical appearance of a ceramic core covered by a glass-like outer shell, similar to the positive ion source described by Roberts. Like Rice and Roberts, the alkali-ceramic cylinder in Burgett&#39;s detector was suspended in the center of a collector cylinder and positive ion current arriving at the collector was measured. Electrically, one end of Burgett&#39;s alkali-cylinder was connected to the collector cylinder, and the other end was connected to a source of electrical heating power. The electrical potential difference between the alkali cylinder and the collector electrode was mostly provided by an electrical fringe field caused by biasing the collector at a high voltage with respect to a flame jet structure external to the collector. 
     The nature of the ionization mechanism operating in these prior art devices has not been well established. However, both Rice and Kolb and Bischoff theorized that the probable ionization mechanism involved release of neutral alkali atoms from the alkali source and subsequent gas phase ionization of the alkali vapors by reaction with sample compounds. Consequently, according to prior art teachings, the alkali source was considered as serving mainly to supply neutral alkali vapors to the gaseous environment of the source. 
     Those devices of the prior art which sensed positive ion current suffered from the fact that when heated, the alkali-sensitized sources were themselves abundant positive ion emitters even without the presence of a sample. Consequently, there always existed in these positive ion detectors a large background signal which acted to mask the responses obtained from samples. This high background level also was highly susceptible to noise variations caused by changes in such things as gas flows or contamination. 
     The devices described by Rice, Roberts, and Burgett et al. involved sensitized elements in which only the surface layers contained the activating alkali compound. Consequently, the lifetimes of these sensitized elements were limited by the depletion of active material from the surface layers. 
     In the alkali-glasses described by Blewett and Kolb and Bischoff, the alkali metal was present throughout the body of the glass; and Kolb and Bischoff theorized that active material in the glass surface layer was continually replenished by migration of alkali atoms from within the body of the glass. However, such synthetic alkali-glasses are difficult to manufacture because a glass melt must be made starting from dry ingredients. Furthermore, if a particular shape of the alkali-glass bead is desired, that shape must be formed while working with the glass in the molten state. A special complication in manufacturing, forming, and operating alkali-glass beads is the fact that the softening and melting points of the glass depend strongly on the type and density of the alkali metal used in the glass formulation. This property severely limits the freedom of manufacturing alkali-glass beads of widely varying formulations. In general, increasing the alkali atom content of a glass usually results in a decrease in melting point. Consequently, alkali-glass beads are often restricted in use at high temperatures due to the onset of glass melting. In fact, Kolb and Bischoff have taught that the alkali-glass beads in their device must be operated above the glass softening point for satisfactory operation. In the Kolb and Bischoff device, only a slight overheating of the bead is often sufficient to cause physical destruction of the bead by melting. 
     In the devices described by Kolb and Bischoff, and by Burgett et al., the electric field established between the alkali-bead and the collector electrode is highly non-uniform, and may appropriately be described as an electrical fringe field. As a consequence, the response characteristics of these two prior art devices are known to be highly dependent on the precise spatial location of the alkali-bead with respect to the collector or any other electrode that is at a different voltage from the bead. 
     SUMMARY OF THE INVENTION 
     This invention provides a thermionic ionization method and apparatus to analyze chemical substances by the process of emission of negatively charged particles from a heated and appropriately sensitized surface. The heated, sensitized surface is situated in a gaseous environment such that there exists around the surface a hot and possibly chemically reactive gaseous boundary layer. Sample compounds are directed to impinge into this gaseous boundary layer and onto the hot surface for the purpose of decomposing the sample compounds. Depending on the temperature of the sensitized surface and the chemical composition of the gaseous boundary layer, certain types of sample compounds form decomposition products which and highly electronegative. These electronegative species subsequently form gaseous negative ions by extracting electrons from the heated, sensitized surface. An electrical potential difference exists between the sensitized surface and a collector electrode so as to cause negative ions to move to the collector. This current of negative ions at the collector electrode is used to indicate and measure the presence of the sample compounds in question. The controllable parameters which are most critical in determining the type of samples detected are the composition of the sensitized surface, the temperature of the sensitized surface, and the chemical composition of the gaseous boundary layer. The surface temperature and boundary layer composition determine the chemical products formed in the decomposition of sample compounds. The surface temperature and the composition of the sensitized surface determine the negative charge emission characteristics of the surface. 
     It is a particular object of this invention to provide a method of forming a sensitized surface by mixing an alkali compound uniformly with a ceramic cement material, and using this mixture to form an alkali-ceramic bead in which there may be embedded an electrical heating coil. Being composed of a ceramic material, the resultant bead is capable of operating over a wide range of temperatures without danger of melting. In addition, the method of forming the bead is relatively simple, and allows a wide range of different ceramic-alkali compound formulations to be used. The purpose of the alkali compound is to lower the electronic work function of the ceramic so as to more easily allow the emission of negatively charged particles from the surface of the bead. Depending on the specific sample response desired, the composition of the alkali-ceramic bead may be chosen to provide the best response signal with regard to sensitivity as well as specificity. 
     It is also a particular object of this invention to provide a method of detecting substances containing nitrogen or phosphorus atoms with a high degree of specificity. According to this method, the alkali-ceramic bead is heated to surface temperatures in the approximate range of 600° C. to 1000° C. The bead is situated in a gaseous environment comprising an oxygen-containing gas such as air and a very dilute concentration of hydrogen (0.05≦H 2  /O 2  ≦0.20, approximately). For this mixture of gases, the boundary layer of the bead may be considered to contain radical chemical species such as H atoms, O atoms, and OH molecules similar to the chemical environment commonly found in H 2  -air flames. However, in the present method, the concentration of H 2  is too low to provide a self-sustaining H 2  -air flame, if the heating power to the bead were to be removed. In the presence of nitrogen or phosphorus compounds, this chemical environment in the boundary layer is favorable for the formation of nitrogen- or phosphorus-containing decomposition products which are highly electronegative. Although the exact identities of these electronegative species have not been firmly established, species such as CN, NO 2 , and PO 2  are known to have the required electronegativity property. 
     It is also a particular object of this invention to provide a thermionic ionization method of detecting substances which thermally decompose into electronegative fragments in an inert chemical environment. According to this method, the alkali-ceramic bead is situated in an inert gaseous environment such as pure nitrogen. Consequently, any electronegative species that are formed are the result of the thermal decomposition chemistry involving the constituent atoms of the sample compound itself. According to this method, highly specific responses are obtained for compounds which contain NO 2  molecular groups, halogen atoms, or oxygen atoms. Also, according to this method, the specificity of response can be further enhanced by selecting the proper bead surface temperature for the type of response desired. For example, specific response to NO 2  compounds is best achieved at relatively cool bead temperatures in the approximate range of 400° C. to 600° C. 
     It is also a particular object of this invention to provide a thermionic ionization method for non-specific detection of hydrocarbon compounds. According to this method, the alkaliceramic bead is situated in an oxygen-containing gaseous environment with either no hydrogen present (H 2  /O 2  =0) or with a relatively high concentration of hydrogen (H 2  /O 2  &gt;0.2, approximately). This chemical environment is favorable for the formation of decomposition products containing C and O atoms, and such species are known to be highly electronegative. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic view of a selective thermionic detector according to the present invention. 
     FIG. 2 is a plot of the logarithm of emission current versus the reciprocal of surface temperature for an alkaliceramic bead used in a thermionic detector according to the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     FIG. 1 shows a schematic, cross-sectional view of a thermionic ionization detector according to the present invention. An alkali-ceramic bead 1 of either cylindrical or spherical shape is molded about a resistive electrical coil 2 commonly formed from either nichrome or platinum wire. The resistive coil 2 is powered by a source of electric current 3 for the purpose of heating the alkali-ceramic bead. The heated bead 1 is disposed in a gaseous environment that is generated by controllable gas flows directed to the proximity of the bead 1 via a center passageway 4 through an inner cylindrical structure 5, and via an outer annular passageway 6 between the inner cylindrer structure 5 and an outer cylindrical structure 7. The flow of gas past the hot bead forms a gaseous boundary layer 8 of high temperature, and possibly of highly reactive chemical composition. 
     The bead is positioned within a cylindrical electrode structure 9, which serves as a collector electrode for negatively charged ions. The collector electrode 9 is disposed coaxially within the outer cylindrical structure 7, and is perforated to permit gas glow therethrough from the outer passageway 6 into the proximity of the bead 1. 
     The collector electrode 9 is electrically connected to an electrometer 10, which is used to measure the magnitude of ion current collected. An electrical voltage source 11 provides a negative bias voltage to the bead 1, thereby causing negative ions formed at the bead&#39;s surface to move in a direction toward the collector electrode 9. Sample compounds are introduced to this detector along with the gas flow carried in passageway 4. The proximity of the bead 1 to the adjacent end of the cylindrical structure 5 is such that sample compounds impinge directly onto the end of the bead 1 nearest to the cylindrical structure 5. 
     For the embodiment illustrated in FIG. 1, suitable dimensions are as follows: 
     
         ______________________________________Component        Dimensions______________________________________Bead cylinder 1  4-mm diameter and 4-mm heightCylindrical structure 5            3-mm outside diameterCylindrical structure 7            14-mm inside diameterCollector electrode 9            7-mm inside diameter______________________________________ 
    
     For components having the above dimensions, the distance from the top of the cylindrical structure 5 to the bottom of the bead 1 is suitably 1.5 mm. These dimensions are not to be considered as restrictive; larger or smaller dimensions can be used with corresponding scaling adjustments in the gas flows. 
     In FIG. 1, the alkali-ceramic bead 1 is shown connected to its heating current supply by electrical connections which emanate in a direction perpendicular to the axis of the collector electrode 9. For this configuration, the collector electrode structure immediately surrounding the bead is constructed of an open, screen-like material to allow a symmetrical gas flow field surrounding the bead. In alternative embodiments, the electrical leads to the bead can extend in a direction parallel to the axis and out the top of the collector cylinder 9; and the collector electrode 9 can be formed with a completely solid cylindrical wall. 
     As shown in FIG. 1, the alkali-ceramic bead 1 is preferably located entirely within the collector electrode 9 so as to produce a well-defined electrical field between the concentric bead and collector structures. This relative positioning of the bead with respect to the collector minimizes the importance of precise location of the bead, and minimizes the bead bias voltage required to achieve efficient negative ion collection. In the present invention, bias voltages of -4 volts to -12 volts are commonly used, in contrast to the hundreds of volts required with prior art devices where the bead was located in an electrical fringe field. 
     For the specific detection of nitrogen and phosphorus compounds, typical flow rates for gases supplied to the detector are as follows: 
     
         ______________________________________Gases             Flow Rates______________________________________Air through passageway 6             150 ml/min to 250 ml/minHydrogen through passageway 4             3 ml/min to 5 ml/minSample compounds contained in              10 ml/min to 100 ml/minan inert gas such as nitrogenor helium through passageway 4______________________________________ 
    
     To obtain non-specific response to hydrocarbon compounds, the hydrogen flow is either increased to a flow rate greater than 8 ml/min or turned off altogether. To obtain responses specific to compounds containing NO 2  groups, halogen atoms, or other electronegative constituents, an inert gas of low thermal conductivity such as nitrogen, instead of air or hydrogen, is supplied through passageways 6 and 4. These gases and flow rates are to be considered as representative, but not restrictive. It is, of course, possible to use other types of gases than those mentioned in order to extend the specific response characteristics of a detector according to this invention. 
     The alkali-ceramic bead 1 is formed by mixing together proportionate amounts of an alkali compound, a ceramic cement, and water. The resultant slurry is then coated over the electrical heating coil 2 and allowed to harden. The ceramic cement preferably contains 100% inorganic constituents such as Al 2  O 3  or AlSiO 2 . Preferred characteristics of the ceramic cement are that it withstand temperatures in excess of 1000° C.; that it be non-porous and make gas tight seals; that it form high-strength bonds; that it resist thermal shock; and that it exhibit low shrinkage. Cements of this type are commercially available in either dry powder form or in pre-mixed, ready-to-use form. An example of a suitable ceramic cement is Super Refractory Cement C-10 manufactured by Dylon Industries, Inc. of Cleveland, Ohio. 
     The type of alkali compound used in forming the alkali-ceramic bead 1 depends on the intended use of the alkali-ceramic bead. Generally, it is preferable that the alkali compound have a low volatility at the desired operating temperature of the bead. Alkali surfaces have been found to be especially suitable. Other kinds of alkali compounds that might be used include alkali carbonates and alkali chlorides. For specific detection of nitrogen and phosphorus compounds, alkali-ceramic bead formulations consisting of 6% by weight of Rb 2  SO 4  and 94% by weight of ceramic cement have exhibited excellent sample response characteristics with operating lifetimes exceeding 2000 hours. 
     In a thermionic detector according to this invention, the sample response currents and background currents obey the physical laws governing thermionic emission of charge from heated surfaces. According to the well-known Richardson-Dushman equation, the thermionic emission current from a hot surface is proportional to the mathematical factor e -w/t , where W is the electronic work function of the surface and T is the surface temperature. Consequently, a graph of the logarithm of emission current versus 1/T for the alkali-ceramic bead should yield a straight line with a negative slope that is proportional to the work function. Such a plot is shown in FIG. 2 for both the background current emitted with no sample present, and for the response current to nitrogen or phosphorus compounds. 
     FIG. 2 illustrates that both the background and response currents exhibit the dependence on bead surface temperature expected for thermionic emission. Furthermore, the difference in the slopes of the graphs for response current and for background current indicates that the response current can be viewed as being caused by a reduction of the effective work function of the bead when the bead&#39;s surface is surrounded by electronegative decomposition products of the sample. 
     In the present invention, the work function of the surface of the alkali-ceramic bead is dependent on both the type and density of the alkali compound used in the bead composition. Thus, for a series of beads, each of which contains the same density by volume of a different one of of the following compounds: Na 2  SO 4 , K 1  SO 4 , Rb 2  SO 4 , or Cs 2  SO 4 , the bead work function for the different beads varies in decreasing order according to Na&gt;K&gt;Rb&gt;Cs. This is the same order as the ionization potentials of the alkali metals. Similarly, for two beads composed of two different densitites of Rb 2  SO 4 , the bead with the higher alkali density will have the lower work function. The data illustrated in FIG. 2 was obtained for a bead containing 6% by weight of Rb 2  SO 4  and having a work function of approximately 3.4 electron volts when surrounded by the hydrogen-air gas mixture commonly used for specific nitrogen and phosphorous detection. 
     The emission of negative charge from the alkali-ceramic bead of this invention depends on the surface temperature of the bead as well as the work function of the bead. The surface temperaure is also important in determining the extent of decomposition of sample compounds. Consequently, enhanced sensitivity and specificity can be obtained by choosing the bead composition to give the best sample response at the desired operating temperature. For example, if the desired response requires a high surface temperature for the preferred decomposition chemistry, then a bead of relatively low work function may provide too large a background current in comparison to the sample current. In that case, better response is obtained by using a bead composition of higher work function. Similarly, specific responses requiring relatively low surface temperatures would be expected to be optimized by using bead compositions of relatively low work function. 
     An alkali-ceramic bead according to this invention can be an abundant source of positive ion current, if it is biased at a positive voltage with respect to the collector electrode. However, positive ion operation is generally characterized by very low sample response and very high background. Therefore, negative ion operation generally is superior in providing the desirable combination of high sample response and low background. 
     This invention has been described in terms of a preferred embodiment. It is to be recognized, however, that variations in the composition of the alkali-ceramic bead and modifications in the configuration of the thermionic detector apparatus might be appropriate for certain applications and yet be within the scope of the invention. Accordingly, this invention is to be broadly construed, and is limited only by the following claims.