CA1064726A - Apparatus and method for uranium determination - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

This invention is a method that uses the previously undiscovered fact that the phosphorescent lifetime of the uranyl ion at very dilute concentrations in an aqueous solution is anomalously long in comparision to that of other commonly-occurring phosphorescent species, wherein the uranyl ions in the low concentration sample are excited by projecting electro-magnetic energy into the sample, terminating the projection of electromagnetic energy into the sample and measuring the characteristics of the decaying uranium phosphorescence after termination of the incident electromagnetic radiation.

Description

1C16~7~6 This lnvention relates to a method ~or detecting uranium compounds in a translucent or transparent salllple that possibly contains a uranlum compound that luminesces in response to ultraviolet light.
It i5 not broadly new to use the luminescent char-acteristics of uranium compounds for the purpose of detecting a uranium compound in a sample. Uranium has a characteristic green phosphorescence which can be isolated by optical ~ilters and its lntens~ty measured with a photo detector, whose elec-trical output is considered to be an indicatlon of the uraniumconcentration ln the smaple. The known luminescent methods, however, are very complex, slow and prone to contamination.
For example, in accordance with one method commonly used~ an aqueous sample thought to contain a uranlum compound ls flrst evaporated carefully to dryness and the re~idue then fused at a high temperature wlth a carbonate-~luoride flux ko produce a glass-like disc. The disc is then placed in an op~ical fluorimeter wherein it is illuminated by ultraviolet light to cause the uranium to luminesce. As lndicated above, uranium luminesces with a characteristic green colour which is isolated and measured as described.
With the improvements described in this invention, it is possible to make a direct measurement on translucent or trans-parent samples to detect the existence of uranium compounds in the samples. The measurements can be made "in the field" as opposed to a laboratory. In mineral exploration, it is o~ten important to know t-he uranium content in natural waters, i.e., those$`waters which occur naturally as lakes and rivers, or ground waters. Typically, natural waters away from uranium-bearing mineralization might contain 0.1 ppb uranium or less butwaters draining or in contact with such mineralization might -- 1 ~

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have values in the 1-100 ppb uranium range. The importance of the present invention to mineral exploration is readlly apparent.
The use of the invention, however, is not restricted to mineral exploratlon. It can be used to advantage in any case where one wants to detect uranium compounds in translucent or transparent samples.
With these and other ob~ects in view, ~his method relates to a method of detecting uranium compounds in a trans-lucent sample through utilizatlon of the anomalously long llfe time of phosphorescence of the uranyl lon in concentra~ion le~els of less than about one part per milllon, comprising the steps of excit~ng the uranyl ions in the sample by pro~ecting electromagnetic energy; terminating said pro~ection of electro-magnetic energy lnto the sample; and measuring the decay with time o~ the uranium phosphorescence after the termination of the incident electromagnetic radiat;ion.
The invention will be clearly understood after refer-ence to the following detailed specl~ication read in con~unction with the drawings.
In the drawings: ~
Figure 1 is a graph illustrating the decay of lntensity of luminescence of hexavalent uranium and luminescence due to naturally occurring compounds in natural waters; and - Figure 2 is a block diagram Or apparatus in accordance with this invention.
As indicated, this invention makes use of the fact that under excitation by ultraviolet light, uranium compounds fluoresce ~, ` ~6~'~Z6 with emisslon of a characteristlc green light. It is belleved that only hexavalent uranium, u6 and the uranyl ion, UO2 o~ some uranyl complexes, phosphoresce, the other quadrivalent uranium ion being non luminescent.
Thls inventlon makes use o~ the fact that uranium has an anomalously long luminescent decay tlme. Uranium compounds which are detected by this invention luminesce when irradiated with ultraviolet light. When the irradiating light is suddenly cut off, the luminescence lntensity decreases to zero in a finite amount of time. The lifetimes of most strongly luminescent compounds are much less than 1 micro second and may be more o~ten measured ln nanoseconds. Uranium compounds that lumlnesce in response to ultraviolet light are anomalous in that their life time of luminescence ls of the order o~ 100-500 micro seconds in suitable aqueous solutlons. It may even be of the order of ~everal mllliseconds lf the uranium is held in ~rystalllne form, or in a solid solution such as an aqueous solution that has been frozen to -20 - -30C.
Figure 1 is a graph lllustra~i~g the ll~etime o~
; 20 lumlnescence a~ter termination of ultraviole~ ~citation for an U uranium compound, compared to other compounds, that luminesce in natural waters. As an example of the method, a short ultraviolet light pulseS between 5-10 nanoseconds longg was generated by a laser and directed at a number of aqueous samples, some of which contalned a uranium compound that luminesces in response to ultravlolet llght. The resultant luminescence, in some cases, was an intense burst o~ luminescence that decayed very rapidly, typically in less than 1 microsecond.
In other cases, however, there was a "tall" of luminescence that decayed over a very much longer lifetime. Those samples where lumlnescence decayed very rapidly did not contain a uranium compound that phosphoresces in response to ultraviolet light. The anomalously long lifetime of the luminescence of uranium compounds constitutes a basis for the detection of the uranium compounds.
Measurement of ~he intensity of the "tail" of lumines-cence due to the uranium phosphorescence is a measure of the presence and amount of the uranium compound in the transparent sample. The intensity measurements are made by a suitable photo detec~or and electronic means. If the measurements are made after luminescence due to materials other than uranium compounds have died down, then the measurement of intensity is substantially due to the luminescence of the uranium compound. It will be noted that, in the case of the sample tested in Figure 1, luminescence due to naturally occurring com~ounds other than uranium compounds had suhstantially decayed after about 25 micxoseconds. Thus, any measurement of luminescent intensity after a period of 25 micro-seconds following termination of the ultraviolet light into the sample is a measurement of an intensity that is substantially due to luminescence of uranium compounds in the sample.
It is desirable to increase the intensity of lumines-cence due to uranium compounds relative to other possibly inter-fering species in the sample. This can be done in several ways including, increasing the viscosity of the sample; reducing the quenching effect of some commonly-occurring ions in the solution;
or removing the uranium compounds into an environment that is either more suitable for uranium luminescence or less contaminated with ions that can quench uranium luminescence. As an example of the effect of viscosity, it is known that uranyl compounds in crystalline form luminesce very strongly with lifetimes of the order of 1 millisecond or more. The "crystalline" properties of the sample can be enhanced by adding a small ~uantity of viscous solvent, such as phosphoric acid. Increasing viscosity in this ~L~64~ 6 way provides a more suitable matrix and increases the relative luminesc~nce of uranium component. Freezing the solution to -30 40C is another way in which the s~ple may be caused to yield considerably inhanced luminescence.
Some commonly occurring ions in the sample solution can quench uranium luminescence to varying degrees. It is accordingly highly desirable to eliminate such effects. It has been found that sodium pyrophosphate is very effective as a "masking agent" in that it not only masks the quenching effect of the interfering ions but leaves the uranyl ion in a suitable environment for effective fluorescence. In solution, it forms complexes with those interfering ions whose presence;~in their normal form gives rise to undesirable effects. The sodium pyrophosphate comple~es the transition metal ions which are then effectively "masked" by the much larger surrounding phosphate radicals. The efficiency of RU~ a masking process is pH
affected. Additionally, the luminescent yield itself is believed to be similarly affected and it is thns desirable to control the pH value with a suitable buffer. Pyrophosphate compounds other than sodium are within the scope of the invention because it is the pyrophosphate anionic complex that is active.
It has been found experimentally that for optimum luminescence the sample should be about neutral (pH-7j and, accordingly, a phosphate buffer is adjusted for neutrality.
The sodium pyrophosphate is then added to the buffer in the ratio 1:10. Prior to the uranium analysis the pyrophosphate-buffer solution is added to the sample in approximately a 1:10 ratio.
Due to the viscosity of the reagent, care must be taken to ensure that the sample is stirred or otherwsse agitated to obtain a homogenous mixture.
The ratio 1:10 is not critical but, at higher ratios, high concentrations of interfering ions in the sample may be 6~'726 insufficiently masked. At lower ratios the loss o~ sensitivlty due to dilution of the sample by the reagent becomes increasingly noticeable.
In Flgure 2, a block diagram o~ a suggested apparatus for practicing the invention is illustrated. A laser 11 generates a short but intense ultraviolet light pulse. In the embodiment ; illustrated, the laser ~s a nitrogen laser con~tructed a~ter the design o~ Small et al which delivers a peak power o~ about 20 kilowatts at a wave length of 3371Angstrom in a pulse that lasts about 10 nanoseconds. The pulse rate of the laser is 15 per second and it consumes about 5 watts o~ electri~-l~p~e~.
A lens 12 focuses the laser beam into a transparent cuvette 13 that contains an aqueous sample to be tested and a sodium pyroph~sphate masking agent, so that any uranium compound ln the sample will ~umi~e~ce in response to ultraviolet light.
The resultant green luminescence of a uranium compound that might be in the sample is isolated by a filter 14 before irradlating the photocathode sur~ace of the photodetector 15. The filter 14 is an optlcal thin-~ilm ~ilter.
, The output of the photodetector 15 is ampliried and fed to an electronic gating circuit 16. ~ second photodetector 17 monitors the ~ncident laser beam and is used to trigger a delay circuit 18 that allows only that luminescence attributable to uranium to pass through the gate circuit by operating to permi~; passage only aft~r a time delay within which lumlnescence due to materials other than uranium have dropped to a value at which they are insigni~icant. The delay period has been typically set at between 20-50 microseconds.
The resulting periodic signal is fed to an integrator 19 which is allowed to sum the intensity of the luminesence primarlly due to uranlum for 16 pulses and then give a readout o~ intensity.

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Following readout, it is reset by an electronic clock circu1t 20 driven by the trlgger circuit. Just prior to reset, a sample and hold circuit 21 samples the integrator and transfers the information to a meter 22 and recorder 23.
The time delay after the termination of the pro~ection of the ultraviolet pulse into the sample after which inten~ity measurement takes place is capable of variation and qulte ~easibly might be as small as 5 microseconds. It should be sufflciently long a~ter the termLnation of the pulse of light so that there will be no electrical noise lnterference, such as is commonly present from a pulsed laser. It also must be a duration longer than the lifetime Or luminescence of the other ~; compounds in the material that luminesce. Sources of ultraviolet light other than a laser beam can be used although~;the highly directional properties of such a beam are very desirable. For example, a pulsed Zenon arc lamp might be used, which has an out-put ranging from the ultraviolet to the infrared and would thus require considerable spectral filtering to avoid light scattering.
The laser is to be pre~erred because it has output at only one wave length (3371A) although it is not thought of as the only possible source of ultraviolet light that can irradiate ; the sample. The laser repetit~on rate in the example glven of 15 pulses per second was convenlently locked to a subharmonic o~
the frequency of the power supply, the frequency of this being 60 cycles per second. I~ the laser repetltion rate was increased, it would require more power to provide the greater number Or pulses per second. By increasing the pulse frequency, it would, of course, lmprove the d~tectivn limlt for a g1ven period of luminescent intensity integration.
In a typical uranium measurement, the nitrogen laser is set to deliver about 20-30 kilowatts peak power in a beam focussed onto a small cuvette into which some 5 ml of the sample, ~ i9L7;~6 including reagent, i5 placed. The luminescence of the sample is monitored by two photomultipliers. In front of one of these a filter with a spectral response optimized for uranium lumines-cence (4900-5~00A) is placed; in front of the other, the reference detector, a filter with peak transmission a~ wavelengths 4500-4700A is placed. With such a dual detector system and suitable electronics recorder responses of the order or 10V per ppb uranium are obtained. Detector limits of 0.03 ppb uranium have been achieved. The effect of interfering ions has been checked to verify the efficacy of the buffer-masking system. It has been found tha~ 10-100 ppm quantities of Cl- and C03 and 10-50 ppm level~ of Fe and Mn are masked effectively by the present system. Without the mas~ing reagent, such quantities totally quench the uranium luminescence signal.
SUPPLEMæNTARY DISCLOSURE
The terms fluorescence, phosphorescence and luminescence, as applied to the radiation emitted by some substances on being irradiated by a source of electromagnetic energy of suitable wave-lengths, are now defined. Fluorescence applies to radiation emitted during the time that the substance is illuminated but that decays very rapidly if the irradiation is interrupted.
Some subs~ances show a persistence of emission, after the exci-tation is terminated, that may last for a few microseconds to many seconds or even longer. This is known as phosphorescence.
Luminescence i9 used herein as a general term for radiation emitted where the presence or absence of persistence is not specified.
Contrary to indications of the available literature, we have found that at very low concentrations of uranium, U
less than abou~ one part per million, the half lifa of phos 1i[~64~Z6 phorescence of uranium compounds in solution increases sharply as the concentration decreases, with the result that the half life of phosphorescence of uranium in such a solution is anomalously long with respect to non-uranium compounds associat-ed therewith. Thus, uranium com~ounds can be resolved from other phosphorescent species by this anomalous half life of phosphorescence.
Prior to this invention, the half life of phosphores-cence had not been used as a diagnoxtic method to analyse for uranium in a sample.
The following drawings and specification supplement th~s invention. In the supplemental drawings:
Figure 3 is a graph illustrating the variation of the time constant of decay of intensity of the phosphorescence of hexavalent uranium, with the concentration of uranium, in an aqueous solution;
Figure 4 is a graph which shows the variation of the luminescent intensity of a uranyl solution containing pyrophos-phate and metaphosphate with tha pH of the solution;
Figure S shows the correspondence between some analyses for uranium using this invention and by other techniques of analysis on samples of natural watersi and Figure 6 shows a similar correspondence diagram for solid samples of geological origin.
As indicated above, this invention makes use of the known fact that under excitation by ultraviolet light, uranium compounds phosphoresce with emission of a characteristic green light. It is believed that only hexavalent uranium, u6 , present in the uranyl ion, UO2 , phosphoresces, uranium of _g_ ,, :. .

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other valences being essentially non-luminescent.
One of the important aspects of this invention is -the discovery that at very low concentrations (U less than one part per million) the half-life of phosphorescence of uranium in an aqueous solution increases very rapidly as the concentration decreases.
The half-life or lifetime of phosphorescence can be defined as follows. Under continuous excitation a fluorescent substance emits a continuous luminescence. I, however, the excitation is abruptly terminated, the luminescence does not decay immediately but over a finite length of time. This decay commonly follows an exponential form:

It o Equation 1.
where It is the intensity at time t;
Io is the intensity at the moment the excitation is interrupted;
and K is a constant. The lifetime is defined here as the time for which It = IOè 1 or 0.3710.
The published literature indicates that for simple uranyl forms (i.e., uranyl nitrate, perchlorate) in water solution the lifetime is of the order of 1 to 4 x 10 6 sec.
Moreover, Benson et al. (Chem. Phys. Letters, V. 35, p. 135, 1975) wrote that although the decay becomes longer at de-creasing UO2 concentrations, their data indicated that at infinitely dilute concentrations an extrapolated value of 6 x 10 secs would pertain. These measurements were made over a range of about 0.01 - 2.0 mil/litre (approximately 4 x 103 800 x 103 ppm). We have, however, found tha-t below about ten parts per million U there is instead a very marked progressive increase 30 of lifetime from about 8 x 10 6 sec at 70 ppmU, more or less ,., , --10--, :

i47~6 as shown in the literature, to more than 80 x 10 6 seC at 50 ppbU. This is shown in Figure 3 where the points marked o represent data taken from ~enson and the points marked x represent data obtained by the present inventors and which are used in ~he practice of this invention.
This finding is of practical significance because in many real samples, as opposed to laboratory standard solutions which are usually made up in deionised water, there arP fluorescent species present other than uranium~
In general, however, the more highly fluorescent the species, the shorter is the corresponding lifetime. For instance, many organic compounds, which might be of either synthetic or natural origin, fluoresce intensely; but their lifetime is measured as a few nanoseconds. For given intensities of luminescence, the greater the difference between the lumin-escent lifetime of desired species and that of the interference, the lower the level of the former that may be detected. For ;` the data given by Benson et al., a practical system for sub-ppb U levels would not be possible for most natural waters because the uranyl lifetimes are not sufficiently different from the luminescent lifetimes of ather compounds likely to be pre-sent.
Figure 1 is an illustration which compares the decay of the intensity of tAe phosphorescence of organic compounds found in natural waters after excitation by an ultraviolet light with the corresponding intensity of the phosphorescence of a uranium compound. An examination of the graph shows that after 15 microseconds, all phosphorescence had decayed except that of uranium. By making a measurement at a time after phosphorescence due to the other compounds in the solu~ion was i:

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essentially low but before the phosphorescence due -to uranium has decayed, one can detect the presence of a uranium compound and, by measuring its intensity, determine the uranium content quantitatively.
A further reason for working at very dilute levels of uranium is as follows: Uranium at relatively hiyh concen-trations in real samples (e.g. the aqueous solution obtained from the acid digestion of a geological sample) is almost invariably accompanied by other metal species that quench or reduce to a varying degree the intensity of fluorescence. The degree of quenching due to a given species depends on the con-centration of that species but is independent of the uranyl concentration. An important benefi-t of the practice of the present discovery is that by diluting the sample to the very low levels of uranium exhibiting the "anomalous" liEetime, the quenching effects of interfering species are significantly re-duced.
It is considered that the method of this invention is useful at uranium concentrations of less than about one ; 20 part per million. Typically, natural waters away from uranium bearing mineralization might contain 0.1 ppb uranium or less, but waters draining or in contact with such mineralization might contain 1 to 100 ppb uranium. These concentrations are very much below one ppm. From an examination of the curve of Figure 1, it will be apparent that the half-life of phosphor-escence of any uranium compound that might be pxesent is anomalously long and the detection of uranium compounds would be correspondingly easy. The method, therefore, is important in the exploration for uranium.
In cases where the uranium content is thought or shown to be more than the optimum range for detection by the ~, ~L~6~7Z6 method, one would dilute the sample progressively to bring it down to a sufficiently low uranium concen~ration where the half-life of phosphorescence would be anomalously long.
As another aspect of this invention, we have found, at the low uranium concentrations mentioned earlier, that the effect of adding certain anions, notably polyphosphates, to the sample to be tested is to increase markedly the initial quantum yield, that is the quantity, lo f equation 1. At the same time the lifetime either remains constant or is only slightly reduced. We have also found that there is no enhancement of organic luminescence by the addition of these anions and, th~s, such addition can be used to enhance the uranium luminescence selectively.
We have found the effect of the phosphorescence en-hancement is pH-dependent. The variation of phosphoxescence intensity with pH for pyrophosphate and metaphosphate additives is illustrated in Figure 4. For consistency of instrument calibration the pH of the solution is maintained at the opti-mum value for phosphorescent intensity.
~ The normalized relative yield is the ratio of intensity of phosphorescence of a dilute uranyl solution (2 ppbU) to which the polyphosphate reagent has been added com-pared to the emission of that same solution but without the addition of the reagent. Table 1 illustrates the normalized relative yield for various polyphosphate additives.
_BLE 1 ADDITIVE FORMULA NO~LIZED RELATIVE YEILD
_ _ _ _ Sodium phrophosphate 4 27 80 Sodium tripolyphosphate 5 3 10 44 30 Sodium metaphosphate ~ 3)i3 ~o Sodium trimetaphosphate 3 309 40 ,, 369~7~6 ~ lthough sodium salts of khs polyphosphate anions were used in the experiments described in this table, the nature of the anion is not importank and any convenient soluble salts of the polyphosphate anion would be effective.
This family of reagents is sensitive to acid and will decompose eventually to simple monophosphates in highly acidic solutions, more rapidly if the solutions are warmed much above 70F. The monophosphate solution so generated is considerably less effective in stimulating the luminescent efficiency of the uranyl ion. A buffer is, therefore, added to reduce the acidity of the solution.
While there is a difference in the effect of the polyphosphates listed in Table 1 they are all suitable for the invention.
In all cases listed, the polyphosphate was added to the buffer in ~he ratio of about 1 t:o 10. Prior to the uranium analysis the polyphospha~e-buffer solution is added to the sample in approximately a 1.0 to 10 ratio. Due to the vis-cosity of the reagent, care must be taken to ensure that the sample is stirred or otherwise agitated to obtain a homogeneous mixture.
Sources of ultraviolet light other than a laser beam of the device of Figure 2 can be used, although the highly di-rectional properties of such a beam are very desirable. For example, a pulsed Zenon arc lamp might be used, which has, how-evex, an output ranging from the ultraviolet to the infrared and would thus require considerable spectral filtering to avoid light scattering. The nitrogen laser is to be preferred because - it has significant output at only one wave length (337 nm) al-thought it is not thought of as khe only possible source of ultra-violet light that can irradiake khe sample.

647;~6 Ultraviolet light is not considered as the only possible excitation source. In fact, other electromagnetic energy sources providing wave lengths of less than about 450 nanometers, which is the maximum wave length that can be used to excite uranyl phosphorescence, can also be employed.
The laser repetition rate in the example given o~ 15 pulses per second was conveniently locked to a sub-harmonic of the frequency of the power supply, the frequency of this being 60 cycles per second. Increasing the pulse frequency would improve the detection limit for a given period of signal in-tegration, but at the expense of increasedlaser power con-sumption.

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In a typical uranium measurement, the nitrogen laser is set to deliver about 20 - 30 kilowatts peak power in a beam focussed into a small cuvette into whichsome 5 - lO ml of the ; liquid sample, including reagent, is placed. A filter with a spectral response optimized for uranium phosphorescence (480 -540 nm? was used in front of the photomultiplier. A small silicon photodiode monitored the intensity of fluorescence due to organic species in the sample. With such a detector system and suitable electronics, recorder responses of the order of lOV per ppb uranium are obtained. Detection limits of 0.03 ppb uranium have been achieved.
The effect of interfering ions has been checked to verify the èfficacy of the buffer-polyphosphate additive. It has been found that useful uranium analyses can be made even in the presence of as much as lO - lO0 ppm quantities of Cl and CO3 and 10 - 50 ppm levels of Fe and Mn + - levels that would otherwise have effectively made such measurements im-possible.

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2~;i The readout of the instrument is calibrated to yield concPntrations of uranyl ion of very much less than one part per million because, as noted above, it is in this range where the half-life of uranyl phosphorescence is optimi~ed and maximum rejection of fluorescent interferences can be ex-pected. If a sample tested is believed to have a higher con-centration of uranium the sample is progressively diluted by known ratios until a concentration within the normal range of the instrument is obtained. From the scale reading and the amount of dilution necessary to obtain it one can determine the concentration of the uranyl ion of the original solution.
Persons skilled in the art may determine other tech-niques for using the basic method which makPs use of the pre-sent discovery of the anomalously long half-life of decay of uranyl phosphorescence in low concentration solutions.
Figure S shows a comparison of uranium analyses made on samples of natural waters using the present invention (designated UA-3 in this embodiment), with those made on the same samples by either fused-disk fluorimetry or fission-traok analyses. The range of uranium concentrations was from0.05 to 200 ppb. The correspondence between these two sets of data is excellent and proves the potential accuracy of measurements made using this invention.
Figure 6 shows a similar comparison diagram of analyses made on solutions derived from solid samples (soils, silts and rocks~. The range o uranium concentrations herein was from about 300 ppb to 2000 ppm. For these measurements the uranium was first extracted from the sample by acid digestion and then appropriately diluted with deionised water ', 30 to less than 1 ppm before analysis with the UA-3. The straight "
~ -16-7Z~i line graph is a testimen-t to the accuracy of the method of this -type of analysis.
Whereas the foregoing discussion has largely re-ferred to uranium compounds in an aqueous solution, it is apparent that similar comments will apply to solutions of uran-ium compounds involving other solvents.

Claims (13)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of detecting uranium compounds in a trans-lucent sample through utilization of the anomalously long life time of phosphorescence of the uranyl ion in concentration levels of between one and one hundred parts per billion, comprising the steps of:
exciting the uranyl ions in the sample by projecting electromagnetic energy;
terminating said projection of electromagnetic energy into the sample; and measuring the decay with time of the uranium phos-phorescence after the termination of the incident electro-magnetic radiation.

CLAIMS SUPPORTED BY

SUPPLEMENTARY DISCLOSURE

2. A method of detecting uranium compounds in a trans-lucent sample through utilization of the anomalously long life time of phosphorescence of the uranyl ion in concentration levels of legs than about one part per million, comprising the steps of:
exciting the uranyl ions in the sample by projecting electromagnetic energy;
terminating said projection of electromagnetic energy into the sample; and measuring the decay with time of the uranium phos-phorescence after the termination of the incident electro-magnetic radiation.

3. A method as in Claim 2 including the step of diluting the concentration of uranium in the sample to below the level of about one part per million.

4. A method as in Claim 2 wherein said electro-magnetic energy is ultraviolet light.

5. A method as in Claim 2 or Claim 3 or Claim 4 wherein said translucent sample is an aqueous solution.

6. A method as claimed in Claim 2 or Claim 3 or Claim 4 including the steps of increasing selectively the phosphorescent characteristics of the uranium compounds by adding a polyphos-phate compound.

7. A method as claimed in Claim 2 or Claim 3 or Claim 4 including the steps of increasing selectively the phosphorescent characteristics of the uranium compounds by adding a polyphos-phate compound, said sample being buffered to maintain a pH be-tween 5 and 10.

8. A method as claimed in Claim 2 or Claim 3 or Claim 4 including the steps of increasing selectively the phosphorescent characteristics of the uranium compounds by adding a polyphos-phate compound, said sample being buffered to maintain a pH
between 6 and 8.

9. A method as claimed in Claim 2 or Claim 3 or Claim 4 including the steps of increasing selectively the phosphorescent characteristics of the uranium compounds by adding a polyphos-phate compound, said sample being buffered to maintain a pH of about 7.

10. A method as claimed in Claim 2 or Claim 3 or Claim 4 including the steps of increasing selectively the phosphorescent characteristics of the uranium compounds by adding a polyphos-phate compound, said polyphosphate compound being a polyphosphate compound taken from the group of pyrophosphate, tripolyphosphate, tetraphosphate, trimetaphosphate, tetrametaphosphate and hexa-metaphosphate.

11. A method as claimed in Claim 2 or Claim 3 or Claim 4 including the steps of increasing selectively the phosphorescent characteristics of the uranium compounds by adding a polyphos-phate compound, said polyphosphate compound being a polyphosphate compound taken from the group of pyrophosphate, tripolyphosphate, tetraphosphate, trimetaphosphate, tetrametaphosphate and hexa-metaphosphate, said sample being buffered to maintain a pH
between 5 and 10.

12. A method as claimed in Claim 2 or Claim 3 or Claim 4 including the steps of increasing selectively the phosphorescent characteristics of the uranium compounds by adding a polyphos-phate compound, said polyphosphate compound being a polyphosphate compound taken from the group of pyrophosphate, tripolyphosphate, tetraphosphate, trimetaphosphate, tetrametaphosphate and hexa-metaphosphate, said sample being buffered to maintain a pH
between 6 and 8.

13. A method as claimed in Claim 2 or Claim 3 or Claim 4 including the steps of increasing selectively the phosphorescent characteristics of the uranium compounds by adding a polyphos-phate compound, said polyphosphate compound being a polyphosphate compound taken from the group of pyrophosphate, tripolyphosphate, tetraphosphate, trimetaphosphate, tetrametaphosphate and hexa-metaphosphate, said sample being buffered to maintain a pH of about 7.