US3451778A - Method of labeling - Google Patents

Description

June-24,1969 R. E. FEARON 3,451,778 I f METHOD OF LABELING Filed July 6, 1965 Sheet of 2 R E FEARON INVENTOR.

June 24, 1969 R. E. FEARON 3,451,778

' METHOD OF LABELING Filed July 6, 1965 Sheet 2 0:2

R E FEA/PON INVENTOR.

" BY war United States Patent 01 ice 3,451,778 Patented June 24, 1969 3,451,778 METHOD OF LABELING Robert E. Fearon, 530 S. Lewis Ave., Tulsa, Okla. 74104 Filed July 6, 1965, Ser. No. 469,678 Int. Cl. G01n 31/00 US. Cl. 23230 6 Claims ABSTRACT OF THE DISCLOSURE The chemical concept of labeling as it appears in the art has been greatly broadened beyond the ordinary meaning of the term as it is used in the English language. It has come to include not only those techniques that impart visible and outward evidences that are immediately and directly observable by the senses, but labeling, as the art has developed, now includes also a variety of procedures for imparting more or less obscure properties, ones requiring observation by means of refined instrumental techniques. As an example of the modern development of the art I refer to my US. Patent No. 3,013,958, entitled Isotopic Labeling. In this, the difference, that is imparted, and used for the identification of material, is the substitution of an atom of the same element in a molecular structure, but with a special isotopic choice of the substituted atom. Since in this previous art I do not limit myself to radioactive isotopes, but include labeling with stable isotopes as well, it is clear that the property of matter to which my prior labeling invention applies is indeed an obscure one. The only requirement which I impose is thatthe isotope by which labeling is accomplished must be rare in nature and easily recognizable.

I have now discovered a new form of labeling, a form essentially related to a characteristic other than isotopy. A requirement for labeling, by analogy with the teaching of my previous invention, is that the characteristic by which the labeling is accomplished must not be a common property of matter, but must be something rare in nature. I have succeeded in finding a quality suitable for identification, one usable and easily recognizable which not only is rare in nature, but in fact, does not exist at all, so far as is known. I label materials by admixing with them very small amounts of chemical compounds of rare gases, and thereafter observe a property of the labeled material due to the amount of such rare gas compound. Included in the rare gas compounds which I employ are the chemical combinations of the element xenon, including especially the perxenates, the xenic acid derivatives, the oxyfluorides of xenon, and the complex fiuo-xenates, and any other stoichiometric chemical compounds of xenon which may prove convenient.

I also employ clathrate compounds of any of the rare gas elements susceptible of this status of chemical combination. Moreover there have been predicted theoretically (and band spectra have been observed) diatomic electrically charged ions including such elements as argon, neon, and krypton. Theoretical chemists have pointed out the possibility that these ions may indeed have a chemical existence in the same sense that the onium" radicals exist. Thus, it is likely that chemical compounds of rare gases patterned along the onium plan will be found to exist and will include stoichiometric compounds of rare gases other than xenon and radon. Such chemical compounds, if any be found to exist, also fall within the purview of my invention, and their uses are contemplated herein, for reasons which will shortly appear. Clathrates of carbon tetrafiuoride, methane, dichloro-difluoro methane, and other gases are usefully employed in my method also.

The basic requirement of labeling in the modern state of the art, and set out in its broadest terms, is that a property or a peculiarity of some kind be imparted in a form that is useful for labeling purposes. To be useful for labeling purposes such property must be one exceedingly rare in nature, and one which is capable of being readily and definitely characterized to the exclusion of other properties of matter, so that accurate labeling identification is accomplished. In keeping with the modern concept of labeling, I have found that the addition of compounds of rare gases imparts properties that are absolutely unique, properties that have a maximum labe1 ing desirability. The reasons are easy to understand. First of all, chemical compounds of rare gases, whatever variety of compound, do not exist at all in nature. To the extent of chemical knowledge at this time, no such compounds have ever been observed in nature by any chemist, or discovered in any chemical analysis of natural raw materials anywhere in the world, from the beginning of time down to this moment. a

It is worth noting that up to about three years ago it was not seriously considered that there were, or ideally ever could be, any chemical compounds at all derived from the rare gases. The chemistry books of ten years ago characterize this group of elements as having zero valence, and being entirely incapable of chemical combination. The reason for the above situation is a curious one, and it is important at this point to understand it. Xenon, for example, has a hexafluoride which can be crystallized. Xenon hexafluoride is not successfully made by heating xenon and fluorine in an autoclave and cooling the products. The equilibrium amount of xenon hexafluoride recoverable in this manner is negligible, if indeed, not absolutely zero. Xenon hexafluoride, so it appears, exists in equilibrium at an elevated temperature. At the elevated temperature the equilibrium is reversible, and the reaction is very incomplete. In being slowly cooled to room temperature, the system of xenon plus fluorine passes through a series of states corresponding with less and less xenon hexafluoride in equilibirum. The technique which has been found effective to produce xenon hexafluoride from the equilibrium mixture at high temperature involves rapidly chilling portions of the high temperature mixture, producing a temperature change that is too rapid to allow chemical equilibrium to be maintained during the cooling process. In this way xenon hexafluoride derived from the high temperature mixture is saved at the low temperature to an extent sufficient that crystals of it can be recovered by condensation from the vapor.

Xenon hexafluoride is important in my labelling art since it is the starting point for making many chemical compounds of xenon. The hexafluoride obtained generally in the manner described is a truly distinct chemical combination of the element, having characteristics that would be expected of a hexafluoride of an element in its general vicinity in the periodic table. The hexafluoride of xenon does not attack glass or silicates at room temperature. It sublimates, as do many of the fluorides of elements which are nearby in the periodic table. Reacting with water and with other highly polar or ionic Solvents it produces a variety of derivatives in which the xenon is combined with oxygen and hydrogen, with oxygenhydrogen and metals, or with oxygen, hydrogen, and

complex radicals of one kind or another. The chemistry of xenon has become so extensive and complex at this time that the University of Chicago Press has published a book on this (Malm, J. G., Holt, B. D., Bane, R. W., Noble-Gas Compounds, H. H. Hyman, ed., p. 167, University of Chicago Press, Chicago, 1963). The purpose of this preceding description is to clearly set out the fact that there exists the means of practicing my method.

Particular advantages attach to the employment of rare gas compounds for tracing and labeling purposes. Among the important factors that favor the use of these compounds is their nontoxic character. The non-radioactive rare gases themselves (which are promptly produced from the compounds in many chemical environments) are entirely lacking in poisonous characteristics. Other chemical degradation products which come from rare gas compounds are ordinarily derived in such small quantities that they are not important. Very small amounts of hydrogen peroxide, ozone, water, and the like may be produced. Also, in compounds wherein rare gas radicals have been united to other chemical entities of a complicated nature, free radicals are produced. These shortly dimerize to make predictable products, or degrate in ways that are entirely foreseeable. Since it is the rare gas constituent alone that identifies a compound of the kind I employ in my tracing, I am at liberty to choose compounds in such a way that all the degradation products are harmless to man in the concentrations which I produce.

Another advantage of the rare gas tracers, as has been pointed out, is the uniqueness of molecules containing rare gases, in view of the fact that these molecules do not occur in nature. This advantage permits the immediate employment of any method of distinctively recognizing the molecules of rare gases. I apply new methods to the recognition of rare gas tracers whenever such new methods become available. Thus, my method of labeling by using compounds of the rare gases continually becomes more valuable as a consequence of the extensive efforts being devoted at all times to the recognition and analysis of molecules.

A further advantage of the use of rare gas compounds is in the fact that some of the rare gases are indeed very scarce in nature, making the distinctive recognition of gaseous products of decomposition quite feasible. Because of the uniqueness of the molecules containing rare gases and because of the rarity of some of the gases in nature, extremely low concentrations of these materials can serve for labeling purposes. Very frequently concentrations less than one part per million are useful.

It is an object of my invention to provide convenient and elfective chemical labeling means for identifying and distinguishing products that may appear in commerce; such as, textiles, pharmaceuticals, dyes, chemicals, etc.

It is an object of my invention to provide means as recited above in which the added labeling material is harmless, compatible lWith the material to which it is added, and low in concentration to such a degree that the cost of the labeling is slight.

It is an object of my invention to provide means and methods for recognizing the labeling constituents when these are present in material which I have labeled.

It is an object of my invention to provide a method of distinctively recognizing a particular material which has been labeled, distinguishing it from other material which has been similarly labeled and to provide a method for determining when the labeling was done.

It is an object of my invention to provide chemical labeling of dyes, drugs, plastics, textiles, etc., in a form such that neither the labeling additive nor its degradation products is appreciably toxic to man or animals.

It is an object of my invention to provide chemical labeling methods, and materials, such that less than one part per million of labeling additive is required to produce eifective identification of the labeled products.

It is an object of this invention to provide means, involving the use of xenon compounds, whereby the operator can determine the average temperature at which a preparation has been stored, and for how long it was kept at such temperature.

Since the rare gas compounds other than the fluorides are even more impossible to synthesize by any direct chemical combination, it is not surprising that these compounds are not found in nature. Because of its entire absence in nature, any compound of a rare gas, when added to something else, imparts entirely new and distinct properties to the material to which it has been added. The new properties thus imparted are attributable exclusively to the rare gas compound. They include the characteristic by which many such compounds continuously evolve small amounts of the rare gases which they contain. Such is the case with the stoichiometric crystallized compounds of xenon. The clathrates also evolve rare gases entrapped in their molecular structures, doing this according to an easily predictable law of nature, one which can be arrived at very readily from the standpoint of statistical mechanics. Thus, it is a property of any material labeled by a content of rare gas labeling substances that, when stored in a closed bottle, it will, after a time, exhibit a concentration of the rare gas substantially higher than the ambient level at which such gas is found in air. If a beam of resonant radiation derived from a sharp spectral line of the same gas be directed through the space in the top of the bottle containing such a labeled material, opacity to the resonant radiation depends critically upon the concentration of the rare gas that has been evolved into the space by slow decomposition of the rare gas tracer.

FIGURE 1 shows a system adapted to produce resonant radiation, and observe the absorption of the resonant radiation by rare gas atmospheres occurring in the vicinity of material labeled by a rare gas compound.

In FIGURE 1 I illustrate the production of pure resonant radiation and the procedure for directing it through an atmosphere which may contain the rare gas from which the resonant radiation is derived. In this figure I produce the resonant radiation by an electrical discharge through the gaseous atmosphere 4 contained in the glass envelope 5. The electrical discharge occurs between electrodes 3 and extends through capillary space 6. The atmosphere 4 may contain xenon at a pressure of approximately one micron Hg absolute. The electrons 3 are energized by the battery 1 and the current is limited by the resistance 2. Radiation 7 from the capillary space 6 passes through converging lens 8 and is brought to focus at 11 after passing through the transparent wall of the bulb 9. The focus 11 is in the interior space 10 of the bulb 9 which contains a low pressure gaseous atmosphere which, for example, may be pure xenon at a very low pressure. It is necessary that the xenon contained in the bulb be at a very low pressure for otherwise the pure monochromatic resonant radiation 12 re-emitted from the xenon atmosphere in the vicinity of the focus 11 is absorbed in the atmosphere before it can escape from the bulb. The pure monochromatic resonant radiation 12 passes through the converging lens 13 and is converged in the vicinity of the neck 15 of the bottle 14 after which it impinges on the light sensitive region 17 of the light detection apparatus 16. The electrical indicating signal from the light detecting apparatus 16 is transmitted over the wires 18 to the indicating electrical meter 19. The reading on the indicating electrical meter 19 is responsive to the intensity of the resonant radiation arriving at 17 and, therefore, indicates, among other things, the resonant absorption due to the atmosphere present in the neck of the bottle 14. If the bottle 14 contains a substance comprising a mixture that includes among other things a chemical compound of xenon, there is an extra amount of absorption observed in the passage of the resonant radiation 12 through the atmosphere in the neck 15 of the bottle 14.

The light source arrangement just described for generating resonant radiation may be replaced by any laser device adapted to produce the same spectral wave length and frequency, such laser device being the full equivalent of my light source up to and including the bulb 9 from which the resonant radiation 12 is emitted.

Another useful labeling property of rare gas compounds is the ability of such compounds to evolve heat when they decompose. To identify the presence of a rare gas compound in a mixture, all that is necessary is to heat the mixture in a thermally insulated region, supplying the heat from a constant source of energy such as an electric resistance element, and to concurrently record the temperature rise in the space. A temperature rise which is essentially linear can be produced and measured for all materials except those which contain something that decomposes.

To make the curve of temperature rise as exactly linear- 1y as possible, the thermally insulated space may be protected on the outside with a thermal guard. The slightest deviation from a linear temperature rise can be observed when two corresponding experiments of this kind are conducted, one with a rare gas labeling substance present and one without. To observe the difference, I establish means (such as a thermopile) extending between the two insulated spaces. FIGURE 2 shows a technique for performing this type of thermal analysis.

In FIGURE 2 I illustrate a form of heat detection apparatus for observing the presence of xenon compounds or clathrate compounds of other substances. In the illustration of FIGURE 2 as a matter of choice I have not provided the thermal guard arrangement surrounding the heated spaces. I have, however, provided a comparative means of measuring the warmup of two samples, A and 20B, which are alike in all particulars except that one may contain a compound of a rare gas element. In FIG- URE 2 the battery 27 provides electrical energy to the heating elements 27A and 27B which are connected in series. The battery 27 is connected to the heating elements 27A and 27B by the wires 28 and by the return circuit wire which completes the series connection of these heating elements. The heating element 27A is in the lefthand of two spaces wherein the warmup is to be compared, whereas the heating element 27B is in the righthand space. The two spaces are both insulated from their environment by being enclosed in a Dewar vacuum 21 contained in the toroidal glass device 20. For the purpose of decreasing the gain or loss of thermal radiation, the surface 22 is silvered. The glass apparatus is understood as a section through a three dimensional structure having approximate cylindrical symmetry about the axis AA'. Insulation material 22A is present in three places, as is indicated by the stippling. Thermopile junctions adapted to compare the heat evolvement in the two spaces 22B and 22C are indicated at 23. As long as the temperature during the warmup remains equal in the spaces 22B and 22C, the thermopile junctions are isothermal and no electrornotive force appears on the wires 24, or is indicated by the electrical recording unit 26. Should it be that the temperature on one side or the other deviates due to the evolution of heat by an exothermic chemical reaction such as the decomposition of a xenon compound, an electrornotive force will be indicated by the thermopile junctions 23 and will result in an electrornotive force on the wires 24 and will be indicated by the recorder 26.

A useful labeling property of some rare gas compounds is emission of light when the compounds undergo decomposition. Decomposition of a rare gas compound differs essentially from the decomposition of any other known compounds in one particular. The rare gas compounds are the only ones which decompose exothermically while yielding atoms, not molecules, of one of the constituents. The constituent so yielded is the rare gas itself, Quite frequently, because of the energy of the decomposition, atoms of the rare gas thus yielded appear in an excited state from which they immediately radiate electromagnetic quanta and return to the ground state. Accordingly, therefore, a technique for observance of the presence of rare gas compounds comprises heating a portion of any substance suspected to contain these materials in darkness and in the presence of sensitive light observing equipment such equipment may include, but is not limited to, apparatus employing photomultiplier tubes. Also suitable are photo tubes in which collected electric charge is increased by gas multiplication or some other convenient means of amplification. Infrared and ultraviolet radiation is included in this method of detection, to the extent that ligh sensitive equipment can detect such radiation.

FIGURE 3 shows an arrangement for practicing the recognition of rare gas compounds by heating substances in the dark. In FIGURE 3 I provide an oven 29 equipped with thermal insulation 30 between its inner and outer walls. The oven 29 is also provided with suitable glass or heat resistant plastic windows 33 adapted to transmit visible and near visible electromagnetic radiation. Inside the oven I afford a mirror in the shape of a portion of an ellipsoid of revolution. The ellipsoid of revolution illustrated at 31, and shown in section, is to be thought of in three dimensions as a surface resulting from the rotation of this portion of the ellipse about the axis B-B, a line passing through the foci of the ellipse. A support 32 is situated within the oven and contains sample material 32A. The sample material 32A is so placed that it lies in the close vicinity of one focus of the ellipsoid of revolution corresponding with the mirror 31. Because of the well-known properties of light, a mirror of such shape converges the light emitted from one focus, directing it to the other focus. In the vicinity of the second focus of the ellipsoidal mirror 31, I place a light detecting element 35 which communicates to a recording or indicating meter by means of electrical wires 36. The front of the oven and the light sensing device is protected from stray light by a blackened light shield 34 which is arranged to fit the front of the oven in such a way as to totally exclude room light. When an electrical signal appears on the wires 36 indicating the presence of visible or near visible radiation generated at the sample 32A and focused by the mirror 31, passed through the windows 33 and impinging on the light sensitive device 35, I conclude that there is present in the sample 32A a material of an unusual nature, with a very strong probability that the luminosity is caused by a chemical compound of xenon in the process of decomposition due to the heat within the oven 29. In use I continuously raise the temperature of the oven 29 and observe the emission of light as measured by the signal on the wires 36 using a recorder (not shown), which records the said electrical signals as a function of time.

Since molecules of rare gas compounds are entirely absent in nature, their addition to anything labels it to whatever extent these molecules have properties which are uniquely their own. It is, accordingly, within the purview of my invention to consider all properties of molecules which are uniquely related to rare gas compounds. Thus, selective absorption spectra can be identified, such spectra not being common to other molecules. Fluorescence phenomena peculiar to molecules containing rare gases may be used to identify them, to the exclusion of all other molecular substances. Raman spectra of the rare gas compounds are distinct and absolutely identifying. Nuclear magnetic resonance exists for any case in which there is a nuclear magnetic moment making such phenomea observable for a particular rare gas compound. Xenon 129 and 131, being nuclei of odd atomic weight, have nuclear magnetic moments, and recognition of them by nuclear magnetic resonance is, therefore, possible in all xenon compounds. These isotopes can. be distinguished because the gyromagnetic ratios of their nuclei is not equal. Moreover, nuclear magnetic resonance data for compounds of these isotopes shows the status of their molecular combination as well as the fact that they are xenon compounds.

The thermal neutron capture cross section of xenon 131 (which constitutes over 21% of the naturally occurring isotope) is 120 barns for thermal neutrons. For xenon 129, it is 45 barns. The captive cross section is negligibly small for the other naturally occurring isotopes of xenon. The processes are n-gamma capture, corresponding with the distinct and unambiguous capture spectra of these isotopes of xenon. No activation occurs, for the reason that the slow neutron capture leads to stable isotopes, xenon 132 and xenon 130. Plainly, therefore, every chemical compound of xenon has the above described property with respect to neutrons, for the reason that it contains Xenon 129 or xenon 131, or both.

Xenon 131 may be isotopically enriched if desired, starting with any sample of naturally occurring xenon, and likewise for xenon 129. Such isotopic enrichment is easily practiced by using a thermal diffusion column, if the operator desires. (Cyclotron resonance or other isotope-separation technique may be practiced.) I may then label pharmaceuticals and textiles distinctly, using, for the labeling, traces of chemical compounds derived from the xenon having a disturbed isotopic ratio.

Neutron gamma analysis (bombardment with thermal neutrons and production of the distinctive gamma spectrum of xenon 131) shows the presence of xenon 131, whereas other measurements such as the light emission measurement provided in my FIGURE 3 react to the total amount of xenon, as does the thermodynamic measurement corresponding with the apparatus of FIGURE 2.

The same propositions apply to xenon 129. A specific technique for performing neutron capture measurements to sense the presence of isotopes 129 and 131 of xenon by their distinctive cascades of capture radiation is set out in S. ,A. Scherbatskoys US. Patent No. 3,080,482. Scherbatskoys technique represents a convenient and useful procedure for performing this measurement in a quantitative manner, and enables the determination of the concentration of xenon 129, of xenon 131, and of each of these in a sample. Scherbatskoys apparatus also may be used to determine the ratio of concentrations of these isotopes by the inclusion of suitable computing machinery to calculate the ratio of the output from two of Scherbatskoys systems. One such system is set to sense xenon 129 only, whereas the other is adjusted to sense xenon 131 only. The output of each of the said systems feed separately into the inputs of the appropriate computational machinery for the determination of the ratio of these isotopes.

Thus, I can identify and distinctly determine materials labeled with xenon, even though they may be labeled with the same chemical compound of xenon, by detecting the different isotopic ratios characteristic of the respective labeling products. Nuclear magnetic resonance techniques, like the neutron test, are a measure of xenon 131 and xenon 129 only.

Because the frequency spectrum measured in a nuclear magnetic resonance machine is distinctly different for xenon 129 and for xenon 131, I can measure the relative amounts of these isotopes present in any sample containing them. To do so I place a sample containing both these odd isotopes of xenon in a constant magnetic field and excite nuclear magnetic resonance by means of auxiliary coils. The input is arranged at two frequencies, one being so chosen that xenon 129 responds to it to the exclusion of xenon 131. The second frequency of excitation is so chosen that xenon 131 responds, but not xenon 129. By suitable frequency discrimination, the outputs at the two frequencies are separately received and measured to indicate separately the concentrations of xenon 129 and 131 respectively, as present in the sample.

It is of particular interest to know the interval of time that has passed since a drug substance was made, for the reason that some drug materials undergo deterioration with time, and, therefore, should not be used after they become too old. Also, there are problems in plastics, textiles, and the like, in which it would be advantageous to know the time that passed since the manufacture of a material in commerce. Not only materials in commerce (and chemical intermediates) are of interest, but, also, on occasion, it is desirable to be able to determine how long a consumer product has been kept around since it was manufactured. I have discovered two techniques for performing this determination of time using properties of xenon isotopes as evidenced in their chemical compounds or otherwise.

Many perishable drugs have a safe keeping time of the order of a year. Therefore, it can be said that if the drug is less than one-half a year from the time of its manufacture, it is definitely all right. A way of assuring that any substance is not older than approximately half a year involves preparing a labeling additive consisting of a xenon compound which contains xenon 127 (of the type that descends from the isomeric transition). This sub-species of the isotope xenon 127 is radioactive, having a half-life of 32 days. The passage of six halflives corresponds with 192 days, which is slightly more than half a year. In this interval of time the radioactivity of a sample which initially contained xenon 127 decreases from its original value to an intensity 1 /2% of its original value. At the end of a year the ratio to the original intensity is roughly 1 to 5,000. Thus, it is apparent that the sensitive measurement of the radioactivity of a sample of material originally labeled with a xenon compound containing xenon 127 enables the determination of age and, particularly, permits the user to determine that the sample is or is not in its range of usefulness. Samples older than a year have such a negligible activity that they can be recognized without error.

A technique for enabling age determination over a greater range takes advantage of the fact that the escape of xenon from clathrate compounds of this element is isotope sensitive. There are different rates of escape, for example, if xenon 129 and xenon 131 are included. Thus, if a pharmaceutical is labeled with a clathrate compound of xenon containing xenon 129 and xenon 131, I can use the ratio determining procedures previously set out as an indication of the age of the preparation. The escape of xenon 129 from a clathrate is more rapid. Therefore, the ratio of the concentrations of xenon 131 to xenon 129 constantly increases with the passage of time. A factor which also enters in, as it happens, is the temperature. A larger weighting factor must be given to time spent at a high temperature. Accordingly, taken together with other measurements which determine the age of a preparation, the 131 to 129 ratio of xenon isotopes may be employed as a means of determining the temperature at which a preparation has actually been kept. If the temperature history is the main object of a labeling determination, I must then include some conventional age determining technique which is not temperature sensitive. For example, I can include labeling with tritium (as described in my US. Patent No. 3,013,958) and employ conventional tritium age determination techniques to obtain time in a manner that does not require knowledge of the temperature. Having this time as determined from the tritium age, I then measure the xenon 131 to 129 ratio in the same sample, which gives me another age figure that is not temperature independent. The ratio of the tritium to the xenon age is then made, using the arbitrary assumption of 20 centigrade temperature. (At 20 I take the xenon age factor of temperature dependency equal to one.) I compute the quotient of the apparent xenon age to the tritium age determination. This quotient gives the temperature weighting factor for the xenon age. From a table of temperature weighting factors plotted versus temperature, prepared for the given clathrate compound, I ascertain the average keeping temperature at which the preparation has been stored.

Another technique which I have found desirable involves the use of clathrate compounds to determine time of storage and temperature during storage but employs the characteristics of clathrate compounds in a different way. Particularly, I call attention to the fact that strongly bound inert molecules such as carbon tetrafluoride are capable of being contained in clathrate compounds. As those familiar with the art are aware, the pyrolysis temperature of carbon tetrafluoride is extremely high. It is, therefore, entirely feasible to assay clathrate compounds in which carbon tetrafluoride is contained in a cage molecule by using the heating process in the same way that it is used for inert gases. A very satisfactory temperature range exists in which the carbon tetrafluoride can be evolved as a gas, but is not destroyed by the heat.

Other extremely stable molecular structures are of interest besides carbon tetrafluoride. Methane, the pyrolysis of which occurs in the vicinity of 900 C., is sufliciently stable. Morever, dichlorodifluoromethane (known in commerce as Freon) is amply stable for my requirements. Just as is the case with the inert gases contained in a clathrate, so likewise the clathrate compounds of these inert molecules evolve the imprisoned gaseous constituent in a manner that is a determinable function of temperature and time. Accordingly, therefore, the assay of a solid which contained clathrate imprisoned Freon, carbon tetrafluoride, methane or the like, can carry information concerning the time and temperature of storage which it has endured. The manner of recovery of the information comprises heating the [solid clathrate to evolve the imprisoned gaseous constituents and determining the ratio of the gaseous constituents and correlating the ratio found with the ratio which existed when the material was manufactured. If 'I employ three clathrate compounds which have different algebraic formulas for their rate of loss at various temperatures, two independent ratios can be determined. From these data it is possible to uniquely determine both the time of storage and the temperature at which the storage occurred.

My more refined and more completely general method of employing clathrates, as I have outlined it, escapes entirely the requirement for the inclusion of a radioactive material for dating the time of manufacture. The carbon tetrafluoride, Freon, and related materials are entirely valid and useful for tracer purposes for the same reason that Xeon is. These substances, like xenon, are very rare in nature, possibly rarer than xenon. Moreover, pharmaceuticals, textiles, chemical intermediates of most types, plastics, and other substances of common experience which it is desired to label do not evolve these materials at all except when I have added a clathrate tracer compound deliberately. Sulphur hexafluoride is another example of a useful material serving in the same manner as does carbon tetrafluoride.

Considering my above description, together with the earlier examples of the use of xenon isotopes in clathrate compounds of xenon for tracing, I have been able to discover a general rule which is of help in guiding the operator who desires to explore the scope of my invention. The general rule is this, that I seek to use a chemical compound, clathrate, or otherwise, derived from a material which is either gaseous or has a very high vapor pressure at room temperature, and which under ordinary circumstances is very inert and indestructable chemically. I choose the gaseous or vaporizable material from among substances which are rare in nature and easily recognizable. I prepare chemical compounds, clathrate, or otherwise, derived from the inert vaporizable tracer substances. I then mix the clathrate compounds so prepared with the substances which it is desired to identify by tracing. Included in the scope of my discovery I may at times go further than merely mixing the compounds of my inert vaporizable tracer substances, by attaching these chemical compounds to the material being labeled, attaching by a chemical bond. Such attachment of the tracer as a prosthetic group is particularly feasible in connection with high polymers, plastics, elastomers, and the like, but this technique may be extended to other fields as desired. The attachment of the chemical grouping which carries the tracer substance through chemical bonding conveys the advantage that miscibility is no longer an issue, and that the chance of separation of the tracer material from the major portion of the batch being marked is prevented.

Methods of detection of evolved gases, such as carbon tetrafluoride, Freon, sulphur hexafiuoride, and the like are used in forms which also can be applied to xenon; and their use for xenon and the other noble gases is envisioned. Among useful methods of detection which I employ are mass spectrograph procedures, particularly including those which are presently in commercial use for leak detection. Equipment exactly analogous with that currently employed for helium leak detection, in fact, is very valuable. In addition, it separate and condense the gases or compress them and test by nuclear magnetic resonance. This procedure can be applied to all the gases which are composed of or contain nuclei with a magnetic moment. Sulphur hexafluoride meets the requirement for the reason that fluorine has a magnetic moment. Chlorine similarly has a rather large magnetic moment. Besides mass spectrography and nuclear magnetic resonance, I employ the other spectroscopic methods which have been referred to in connection with xenon (with the exception of the resonant radiation technique which does not apply very well to molecules). Tracing with carbon tetrafluoride clathrates and the like comprises also detection of the gaseous products (or the substances in the clathrate compound itself) by nuclear methods, including the neutron gamma process using Scherbatskoys technique, as has been previously mentioned.

Following the specific principles set out above, the operator will discern many useful ways to practice my tracer art, the larger number of which cannot be specifically included herein. All such methods and combinations of matter, falling within the spirit and scope of my invention, are included within it whether or not specifically recited above.

I claim:

1. The method of labeling a material which comprises adding to such material a chemical compound of a noble gas, and thereafter sensing with respect to the material having the thus added chemical compound a distinctive property of said chemical compound.

2. The method of labeling a material which comprises adding to such material a chemical compound of xenon, and thereafter sensing with respect to the material having the thus added chemical compound a distinctive property of said chemical compound.

3. The method of determining the apparent age of a material after its manufacture which comprises introducing therein at the time of manufacture in a trace amount a noble gas" chemical compound that decomposes in a fashion operative to alter the isotopic composition of the material, and thereafter measuring said isotopic composition, said measurement being directly related to the apparent age.

4. The method of determining the average temperature at which a material has been kept since its manufacture which comprises introducing therein at the time of manufacture in a trace amount a chemical compound which decomposes at a rate which is temperature dependent in a fashion operative to alter the isotopic composition of the material, measuring said isotopic composition to obtain the apparent age, measuring the true age of the same material by use of a radio-activity method, and comparing the apparent and true ages as determined by the two methods.

5. The method of determining the average temperature of a material during an interval of time comprising the steps of adding to such material immediately before the beginning of such time interval a tracer additive having 1 1 1 2 one property which changes in'a known manner as a funcv References Cited tion of temperatureand time, and a second property which UNiTED STATES PATENTS changes as a function of time only, measuring such said first-mentioned property at the beginning of said time in- 2,365,553 12/1944 H111 23230 X t'erval and at the end of said time interval, measuring 5 OTHER REFERENCES said second property at the beginning and at the end of said time interval, and determiningthe ratio of the respeC- Pauling General chemlstryv 1958 680*682' tive degrees of change in said two properties during said MORRIS Q WOLK Primary Examiner time interval. E I

6. The method of labeling a material which comprises 10 KATZ, l Examiner.

adding a clathrate compound in a trace amount to the said material and thereafter sensing a distinctive property US of the said clathrate compound in themixture, '250 83

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