this application, identical equipment to that utilized in the dynamic calibration of gas detection devices can be used (6). The limitations of a permeameter or permeameter tubes are those imposed by the critical point of the gas and the mechanical properties of the tube wall material. A gas whose critical point is below that a t which the measurements are t o be performed cannot be used because the emission rate is not described by Equations 1 and 3. I n this casz, the internal tube pressure is described by Equation 5 and the emission rate appears as if the tube is in the depletion stage. A reasonably low internal pressure (pi)level must be maintained to prevent large deviations from Henry’s law and rupturing of the tube wall. Accuracy limitations are imposed only by the dimensional, temperature, and gas detection accuracy of the system. A n experimental technique must be implemented t o ensure that thermal and the subsequent mass transport equilibrium are fully established during the measurements. The permeability coefficient is determined by measuring
y, with a detector and using Q c and Equation 1 to compute the permeability coefficient (P,) or the group Poe-*IRT. Figure 2 presents the Teflon sulfur dioxide permeability coefficient determined in this fashion as a function of temperature from data of references (1) and (6). A t 25 “C, it is 2.45 X lo-” cc/sec-cm2-Torr/cm. The permeability activation energy ( E ) is 1080 cal/mole as given by the slope (EIR)of the data. The diffusion coefficient (D,) is determined by Equation 4 and from the time required t o establish a steady emission rate after a n abrupt and small change in tube temperature. A plot of log (D)GS. l / T yields the diffusion activation energy (Eo). Once P, and D, are established, the gas solubility in the tube material is calculated from the ratio Pm/Ds at all temperatures.
RECEIVED for review September 30, 1970. Accepted August 9, 1971.
Construction and Calibration of an Apparatus for Absolute Measurement of Total Luminescence at Low Levels Richard Bezman and Larry R. Faulkner Coolidge Chemical Laboratory, Harrard Unicersity, Cambridge, Mass. 02138 An apparatus which features an integrating sphere for the reproducible collection of sample luminescence and a detection system of uniform response with wavelength has been constructed for the absolute measurement of total luminescence from sources of irregular geometry. The instrument was designed especially for use with low-level sources (less than l O I 3 photons/sec). A stepwise procedure permits calibration, repeatable to =t3%, for low luminescence intensities by means of the ferrioxalate actinometer exposed with a high intensity source. The stepwise nature of the calibration gives high confidence in its accuracy, and it is apparently independent of the geometry of the monitored source and the angular distribution, the decay time, and the spectrum (at wavelengths shorter than 600 nm) of its luminescence.
FROMTIME TO TIME,investigations of luminescent materials have demanded measurements of luminescence intensity that are both absolute, in that the results may be expressed in “absolute” photonic units, and total, in that they account for all light emitted a t any wavelength in any direction. This need has appeared most frequently when luminescence efficiencies have been of interest ( I , 2). Of course, methods involving fluorescence, phosphorescence, chemiluminescence, and scintillation measurements currently comprise a n important segment of analytical chemistry. Yet the development of these methods as viable analytical techniques continues to depend upon gradually improved understanding of fundamental processes. Measurements of luminescence yield have historically been a prominent means for advancing such knowledge, and, not surprisingly, the utility of the measure(1) J . N. Demas and G. A. Crosby. J . Phys. Chem., 75, 991 (1971). (2) C. A. Parker, “Photoluminescence of Solutions.” Eisevier. Amsterdam, 1968.
ments has been proportional to the ease and reliability with which they could be made. Only in determinations of fluorescence and phosphorescence quantum yield has measurement of total luminescence met unqualified success. This special case appears because the measurements are taken at steady-state; because a fairly intense, well-defined excitation beam can be used; and because the angular distribution of emission can be made completely random. Absolute measurements are not always needed in this application, because one seeks t o know only the ratio of total emission to the total quanta absorbed. However for luminescence that is not optically excited, emission intensities often change rapidly with time, sample geometries are often not constant or regular, the angular distribution of emission is usually not uniform, and absolute, rather than comparison, measurements must be used t o obtain emission yields of luminescence. It is plain that methods suitable for work in fluorescence spectrometry are not ordinarily useful for chemiluminescence, electroluminescence, or radiation-induced luminescence measurements. Furthermore, the luminescence generated in experimental work is usually a t such a low level that the usual means for determining absolute intensities, such as actinometry, cannot be applied to the sources themselves or to secondary steady-state sources of comparable intensity. Standard emitters, moreover, are generally far too intense for use in the direct calibration of photomultiplier tubes, and attempts at masking usually only aggravate geometric problems. To meet the requirements for our work in electrogenerated chemiluminescence, we have constructed and calibrated a photometric apparatus that yields absolute measurements of total emission regardless of the geometry, decay rate, or spectrum (at wavelengths shorter than 600 nm) of emission. Be-
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
* 1749
Figure 1. Schematic view of photometric apparatus on the vertical bisecting plane cause the calibration procedure has been tedious and timeconsuming, the instrument was designed for long-term stability, and it includes provision for periodic checks on the calibration. Although it was designed specifically for our work, the photometer is undoubtedly suitable, either as described or with only minor modification, for many other types of research. At the very least, the somewhat novel stepwise calibration procedure should be of value in the preparation of conceptually similar instruments. Apparatus Design and Construction. Our specifications were met by the instrument shown in Figure 1, in which the integrating sphere collects and samples the source output, the quantum counter-photomultiplier combination serves as a detection system of uniform wavelength response, and the secondary standard lamp provides the capability for periodic calibration confirmation. In our application, of course, the source is a n electrolytic cell, which enters the sphere and is supported by the 24/40 standard-taper joint. However, the joint is a convenient entry port for any source of suitable size, hence the utility of the apparatus is not restricted to electrochemically-generated luminescence. Integrating spheres, which are spherical enclosures whose inner surfaces are coated with diffusely-reflecting substances of very high reflectivity, are exceptionally convenient devices for collecting reproducible fractions of the output of sources contained within ( I , 3, 4 ) . This is because the nature of the interior surface results in a distribution of reflected light intensity that is uniform over the entire inner surface and is dependent only on the source intensity. It is a n important consequence that the intensity of reflected light on the surface does not depend upon the position of the source inside the sphere or upon the angular distribution of emission. Thus, if the detector receives only reflected light (Le., does not view the source directly), the geometric factor involved in the photometric calibration will also be independent of source location or angular distribution of emission. It is clearly from this property that devices based on integrating spheres derive their utility for measurements of intensity from sources of widely disparate geometry. They have been quite commonly employed, and, indeed, our design was prompted by the device used in the chemiluminescence work of Zweig, Hoffmann, Maricle, and Maurer (5). (3) R. P. Teele, “Applied Optics and Optical Engineering,” McGraw-Hill, New York, N.Y., 1965, pp 29-32. (4) L. S . Forster and R. Livingston. J . Chem. Plzys., 20, 1315 (1952). (5) A. Zweig, A. K. Hoffmann, D. L. Maricle, and A . H. Maurer, J . Amer. C/wm. SOC.,90, 261 (1968). 1750
The sphere employed in this work was made from a 3-1. round-bottom flask having a standard-taper 24/40 neck joint. The flask was modified, as shown in Figure 1, by the addition of a 5-cm diameter detector port. The sphere’s inner surface was sprayed, according to the manufacturer’s instructions, with Eastman 6080 White Reflectance Paint, which has a constant diffuse reflectance over at least the range from 360600 nm. Tests showed that the coating was completely opaque ; hence the requirement of a purely diffusely-reflective inner surface was satisfied. Perhaps the most substantial improvements to be offered here over the usual practice pertain to the calibration procedure, which is discussed in the following section, and in the detection system. In particular, the detector sensitivity has been rendered effectively wavelength-independent by interposing between the phototube and the sphere a fluorescent screen “quantum counter”, which is simply a 3.0-g/l. solution of rhodamine B in ethylene glycol. Both substances were Eastman Organic Chemicals White Label reagents. The operating characteristics of this screen were investigated extensively by Melhuish ( 6 ) , who found that, within his 2 % measurement precision, such solutions can eliminate the wavelength-dependence of photodetector sensitivity for incident light of wavelength between 280 and 600 nm. A general discussion of fluorescent screen quantum counters has been given by Parker ( 2 ) . The fluorescent solution in our apparatus is contained in a 50-mm diameter cell of 1-cm path length, which was fabricated of optical glass by Precision Cells, Inc. The cell is supported by a small shelf attached t o the front wall of the phototube enclosure and is held securely by a pair of spring clamps. Because optical glass does not transmit well in the ultraviolet, the presence of this cell in our apparatus effectively limits its applicability t o wavelengths longer than about 350 nm. Directly interchangeable quartz cells are available which can easily extend the useful range t o 280 nm and below. The photomultiplier itself is a n EM1 Type 9656R, a 2-inch, 10-stage, end-window tube with “Super S-11” response. The principal reason for its selection was that the Super S-11 quantum efficiency characteristic provides a reasonable overall gain to about 670 nm, yet its room temperature noise-insignal level at the voltage at which it is operated (1000 V) is quite low. A special dynode chain was designed in order to maximize the tube range of linear intensity response. A large chain current (1.7 mA) and relatively small resistances (51 K) are (6) W. H. Melhuish, J . Opt. SOC.Amer., 52, 1256 (1962).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
used t o establish the interdynode voltages of the first nine stages, whereas Zener diodes are employed to stabilize the photocathode-to-first-dynode voltage, the voltage between the ninth and tenth dynodes, and the tenth-dynode-to-anode voltage at 150, 86, and 86 V, respectively. Power to the dynode chain is furnished by a Kepco Model OPS 2000B operational power supply, which is programmed as a voltage regulator. The anode current is always monitored through a current-to-voltage transducer which maintains the anode a t virtual ground. This attention paid t o photomultiplier operation has resulted in a linear relationship between photon flux o n the detector and anode current which extends even to the highest practical anode current (100 FA). The essential effect of this circuitry is much the same as that of Santini and Pardue (7), but some simplification has been afforded by the present design. Although temperature compensation has been neither employed nor found necessary, it could easily be included, if it were needed for a particular application, by appropriate modification of the programmable power supply. A Keithley Model 610B electrometer was used to read out photocurrents and their integrals for the calibration procedure described below. The photomultiplier is housed in a 4 X 5 X 6-in. aluminum box whose inside was finished in flat black paint. A n iris diaphragm assembly was mounted o n the front of the box between the integrating sphere and the housing windows, and the joint between the iris and the sphere was sealed with a neoprene O-ring. The photomultiplier itself is supported by its base pins and is protected from stray magnetic fields by a Mu-metal shield. The dynode chain components are located in a separate compartment to avoid heat transfer into the tube either by radiation or through the base connections. A pair of small (No. 330) incandescent lamp cartridges, one red and one yellow, are mounted in Dialco holders inside the photomultiplier enclosure (only one shown in Figure 1) and serve as secondary intensity standards. Different colors were selected so that calibration checks could be made at two widely different intensities. The bulbs are powered by a constant 73-mA current obtained by appropriate programming of a Kepco C K 40.8 M power supply. To assure long-term stability, the operating current was chosen to be only 90% of the lamps’ design current. The assembled apparatus was mounted o n a sturdy wooden base. Provision was made for easy, reproducible removal and replacement of the integrating sphere, because part of the calibration procedure required direct access t o the phototube window. The entire instrument is located in a light-tight cabinet, which has been fitted with opaque electrical connectors. A n interlock switch protects the photomultiplier from room light levels by removing the high voltage whenever the cabinet door is opened. Calibration. The absolute calibration of the photometric apparatus rests upon a recognition that, in this particular system, the overall absolute sensitivity is separable into two factors, one of which is associated with the geometric collection efficiency, the other being related t o the detector sensitivity. Thus the conversion factor, F, for transforming photocurrent (or charge) into total rate of source emission (or overall total emission), can be written
F
=
Fd/Fcpiieie
(1)
Here Fd is the detector sensitivity and represents the ratio of light absorption by the quantum counter t o the resulting (7) R. E. Santini and H. L. Pardue, ANAL.CHEW,42, 706 (1970).
photocurrent. The quantity F a p h e r e is the fraction of total emission in the integrating sphere that is absorbed by the quantum counter. Recognizing the separability of these two factors is important because it enables one t o use high intensity sources and actinometric measurements in a stepwise calibration procedure for a device intended for low-level work. The absolute calibration of the photometric apparatus therefore consisted of the following series of determinations:
(1) The linearity of the relation between photocurrent and light intensity a t the photocathode was verified for the range of intensities needed in the calibration. (2) Fspheie was measured by determining via chemical actinometry the ratio of the number of quanta absorbed by the quantum counter t o the number emitted beforehand inside the sphere. (3) Fd was measured by determining the ratio of the integrated photon flux at the face of the fluorescent screen to the resulting photomultiplier anode current. (4) The intensities of the secondary standard lamps were compared with the source used in step 3. It is clear that the calibration procedure rests in essence upon the actinometric measurements of total photon flux. For all such measurements, the potassium ferrioxalate actinometer devised by Hatchard and Parker was utilized (2, 8). The ferrous iron produced by the photoreaction was determined spectrophotometrically as the 1,lo-phenanthroline complex. The ferrioxalate salt was prepared from reagent grade materials according to the procedure given by Hatchard and Parker, and spectrophotometric tests indicated that solutions prepared from it contained no measurable amounts of ferrous iron. A Bausch and Lomb Spectronic 20 spectrophotometer was used for the measurement of optical absorption by the ferrous1,lo-phenanthroline complex at 510 nrn. The instrument performance was tested by a procedure in which the absorbances of standard solutions were measured. A Beer-Lambert plot of the data was linear to a complex concentration of lO-4M and yielded a molar absorptivity of 1.08 X l o 4l./molecm. This figure compares favorably with the literature value of 1.10 x 1041./mole-cm. The procedure of Hatchard and Parker for exposing and developing the ferrioxalate solution was used without rnodification. I n all cases, a 1-cm depth of 0.15M solution was employed, and exposure times were adjusted to give a measured absorbance of 0.1-0.8. The light source used for these determinations was a “Penray” argon discharge lamp (Ultraviolet Products, San Gabriel, Calif.). Because one kind of measurement required that the source be immersed in the actinic solution, the lamp was rendered waterproof by application of a film of acid-proof insulating varnish to the envelope-handle junction. The light output was stabilized by operating the lamp a t a constant current of 8 rnA, which was obtained by appropriate programming of the Kepco OPS 2000B power supply. A question arises with regard to the quantum efficiency for ferrioxalate photoreduction that is properly used for calrulations involving actinic measurements obtained with the argon lamp. This source emits its output mainly in two groups of lines, one located between 395 and 433 nm and another a t wavelengths longer than 697 nm. Only the former group results in the photoreduction of ferrioxalate. However, the (8) C. G. Hatchard and C. A . Parker, Proc. Roy. Soc. (Lo/?do!f), 235A, 518 (1956).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
1751
quantum efficiency for the reaction (ferrous ions produced per photon absorbed) varies in 0.15M solutions from about 1.15 to 1.01 over the 395-433 nm wavelength range; hence a weighted quantum yield c $ ~which , takes account of the relative intensities of the absorbed lines, must be employed. Th,e value used in this work was calculated according to
where Z(X) is the relative intensity of the line at wavelength A, and +(A) is the quantum yield for photoreduction at that wavelength. The summations include all the argon lines from 395 t o 433 nm. The values of Z(X) were measured with a n Aminco-Bowman spectrophotofluorometer, and when they were used in conjunction with the +(A) data of Hatchard and Parker, a value of 1.03 was obtained for $ w . Chen has shown that the sensitivity of the spectrophotofluorometer is almost constant in the spectral region in question (9), so this procedure seems entirely reasonable. The initial step in the determination of the photomultiplier intensity response characteristic was the measurement of the attenuation factor of the iris diaphragm. Two metal posts were attached t o the front of the photomultiplier housing to limit the motion of the iris operating lever; hence, a constant, reproducible open-to-closed area ratio was maintained. With the iris fully opened, the current through a small lamp placed in the integrating sphere was adjusted to give a n anode current of about 1 pA. The iris opening was then reduced to its minimum, the anode current was remeasured, and, finally, the lamp was turned off so that the dark current could be determined. The attenuation factor is simply the ratio of the first current to the second, after a correction for the dark current (0.3 t o 2 nA) had been applied to both. I n this work 3. Because the largest anode current this factor was 355 allowed in the attenuation factor measurements was only 0.05 % of the dynode chain current, and because a current-tovoltage transducer was used to make the current measurements, the determination was free of errors due to detector nonlinearity. The detector response characteristic at higher anode currents was determined by increasing the intensity of the light source in the sphere in steps, and comparing the photocurrent observed with the iris open t o that expected from the product of the iris factor and the photocurrent observed with the iris closed. (Photocurrent is the anode current corrected for dark current.) The photomultiplier response as determined by this comparison method was completely linear to an anode current of 100 pA (10% of the maximum allowable value). Higher values were neither investigated nor allowed to flow during experiments, so that tube fatigue and gain changes would be avoided. The determination of Fsphere was begun by measuring the total output of the argon lamp. The lamp was immersed in enough ferrioxalate solution t o cover all parts of the envelope with at least 1.O cm of liquid. The solution was then exposed for about 20 min, after which the amount of ferrous iron produced was determined colorimetrically. Measurements at several solution temperatures indicated that the output of the lamp is not significantly dependent upon its envelope temperature. A series of measurements showed the lamp to produce 9.57 =t 0.10 X 1014 photons/sec in its blue-violet region. The lamp was then placed inside the sphere in a position such
*
(9) R. F. Chen, Anal. Biocliem., 20, 339 (1967). 1752
ANALYTICAL CHEMISTRY, VOL. 43,
that only reflected light appeared at the exit port. In order t o simulate the conditions of our application more closely, a borosilicate glass tube, having the same shape and size as the portion of our electrolytic generating cell that protrudes into the sphere, was placed in the position occupied by the cell, The vessel used for the fluorescent screen was filled with the actinic solution, and the argor. lamp was operated for a n appropriate time. The collection rate at the location of the fluorescent screen was determined t o be 6.63 + 0.11 X 1013 photons/sec; hence Fsphere was 6.93 i 0.14 X 10-2. The third step in the calibration sequence, the determination of the ratio of integrated photon flux absorbed by the fluorescent screen to the photomultiplier anode current, was complicated by two factors. First, the red and infrared emission of the argon lamp, while ignored by the actinometer, would have contributed measurably t o the photocurrents. Furthermore, the sensitivity of the photomultiplier is so great that with light levels suitable for actinometry, intolerably high anode currents would have resulted. Both problems were solved by appropriate filter usage-an Oriel Optics G774-4000 blue bandpass filter was placed ahead of the fluorescent screen, and an Oriel G-53-79 0.01 %-transmittance neutral density filter was inserted behind it. During this set of measurements, the integrating sphere was removed from the photomultiplier base board, and the argon lamp was mounted about 2 in. ahead of the iris diaphragm, which was in its open position. The cuvette used for the quantum counter was filled with actinic solution, and the integrated photon flux at the screen was measured. Several such determinations yielded a flux of 7.91 i 0.05 x 1013 photons/sec. The cuvette was subsequently cleaned, dried, and refilled with rhodamine B solution. The argon lamp was switched on, and the photomultiplier anode current was determined. A series of these measurements yielded 8.37 + 0.3 P A as the photocurrent. The argon lamp was then turned off, and each of the secondary standard lamps was connected, in turn, t o the constant current source. The photocurrent due to each was recorded, so that in subsequent work with the system the photomultiplier gain could be adjusted to duplicate these standard photocurrents. It is perhaps a tribute to the stability of the system calibration that, in nearly one year of operation, no variations in the calibration greater than 1 have been noted. Following these measurements, the actual transmittance of the neutral density filter was determined. The argon lamp current was reduced so that, with the filter in place, the photocurrent was about 10 nA. Then the tube dark current was measured, the anode current was recorded with the filter in place, and, finally, the anode current was measured with the filter removed. The ratio of the second measurement to the third, both corrected for dark current, is the filter transmittance. In this case it was determined to be 4.74 X 10-4. From this transmittance, the integrated photon flux reported just above, and the attenuated photocurrent resulting from that flux, Fd is calculated to be 4.48 =k 0.09 X 109 photons/secpA. Thus the overall calibration factor, F , is the ratio of Fd to Ftphore, 6.46 = 0.19 X 1O1O photonslsec-,uA (or photons/ PC). CONCLUSION
It should be clear from the foregoing that the calibration of this photometric apparatus can be carried out with excellent precision. This attractive aspect is directly attributable to the stepwise nature of the scheme employed. Whether or not the calibration is accurate is a matter not easily discerned.
NO. 13, NOVEMBER 1971
Of course, if there were a means by which one could check the calibration with very great confidence, that means could itself serve as a calibration. Certainly none has come t o our attention. F o r the most part, confidence in the reliability of the calibration must be placed almost entirely in the reasonableness of the procedure. We believe such confidence is wellplaced, and our belief is reinforced by the agreement between our measurements (10) and those of Maloy and Bard (ZI) for the emission efficiency of the 9,lO-diphenylanthracene anioncation reaction. There are a few pitfalls in the operation of the apparatus that should perhaps be pointed out. Demas and Crosby ( I ) have recently described a pair of subtle effects which can alter the operation of integrating spheres. First, variations in the reflectivity of the sphere coating with wavelength can produce a strongly wavelength-dependent value for FSpllere. However, the sources of interest in our work emit in spectral regions in which the coating reflectance has been shown t o be quite constant (12); hence this effect was considered unimportant and its magnitude was not determined. Nevertheless, an assessment of the effect is worthwhile if short-wavelength sources are to be used, and it is conveniently carried out by comparing the spectrum from a continuous source recorded indirectly (10) R. Beznian and L. R. Faulkner, submitted for publication. ( 1 1 ) J. T. Malo) and A. J. Bard, J . A m v . C h m . Soc.. in press. (12) Eastman Kodak Co.. Rochester, N.Y., publications 5532 and 5533.
via the integrating sphere t o that obtained either directly or following one reflection from the Eastman paint. Demas and Crosby further indicated that if anything inside the sphere absorbs the source emission, the observed intensity can be greatly reduced because the light within the sphere is reflected so many times. This effect has not been a problem to us, but obviously it may be troublesome in certain instances. A third pitfall pertains to emitters having appreciable output at wavelengths longer than 610 nm. The fluorescent screen abruptly becomes transparent at about this wavelength ; hence long-wavelength light emerging from the sphere will register directly at the phototube, rather than being subjected t o the fluorescent scattering of the screen. This effect will appear as a n Fd value different from that obtained in the calibration procedure. We have encountered this problem in our studies of rubrene-containing systems (10), but in that case, the emission beyond 610 nm is small and a correction c a n be applied. Even recognizing problems like these, we nevertheless believe the present apparatus and calibration procedure represents a considerable improvement over the measurement methods ordinarily used in fields where they apply. RECEIVED for review June 28, 1971. Accepted August 23, 1971. The support of this research by the E. I. duPont de Nemours Company and the William F. Milton Fund of Harvard University is gratefully acknowledged. We are also indebted t o the National Science Foundation for a Graduate Fellowship awarded t o one of us (R. B.).
Fluorometric Determination of Submicrogram Quantities of Tin T. D. Filer Health Sercices Luhoratorj’, US.Atomic Energ!, Commission, Idaho Fulls. Idaho
A fluorometric procedure for the determination of tin using 3,4’,7-trihydroxyflavone (THF) has been developed that is much more sensitive than other common methods. After decomposition of the sample by a pyrosulfate fusion, the tin is extracted as the iodide into toluene from a sulfuric acid solution. The fluorescence of the tin(1V)-THF complex is measured in a sulfate buffer solution. The method has a detection limit of 0.007 pg, a precision to about 2% on 1 p g , and excellent tolerance to most common elements, antimony and tantalum being the only serious interferences.
FEWSENSITIVE PROCEDURES exist for the determination of microgram quantities of tin. Fluorometric procedures have been developed using the ammonium salt of 6-nitro-2 naphthylamine-8-sulfonic acid ( I ) , flavanol (2), and oxine-5sulfonic acid (3). The reaction of tin(I1) with the ammonium salt of 6-nitro-2-naphthylamine-8-sulfonic acid permits 0.1 pg/ml of tin t o be determined ( I ) . Since tin(I1) is the reactant species, reduction of tin and development of fluorescence must be carried out in a n inert atmosphere, and ions which are also capable of reducing the reagent will cause serious positive interferences in the procedure. The reaction of tin(IV) with Anderson and S . L. Lowy, Aim/. Chim. Actri. 15,246-53 (1956). ( 2 ) C. F. Coyle and C. E. White, ANAL.CHEM., 29, 1486-88 (1957). (3) B. K . Pal and D. E. Ryan, Auul. Chin?.Actu, 48, 227-31 (1969). (1) 5. R . A.
flavanol permits 0.1 pg/ml of tin to be determined, and only phosphate, fluoride, hafnium, and zirconium interfere. Close control of dimethylformamide concentration is necessary and satisfactory results are not obtained in the presence of chloride ion (2). The reaction of oxine-5-sulfonic acid with tin(I1) o r tin(1V) permits 0.001 pg,iml of tin t o be determined if the fluorescence is developed in a closely controlled concentration of ethanol, methanol, or p-dioxane; or 0.1 pg/ml of tin may be determined if the fluorescence is developed in water (3). Magnesium, cadmium, thorium, cobalt, fluoride, nickel, zirconium, hafnium, zinc, and aluminum are serious interferences in this procedure. The present procedure uses 3,4’,7-trihydroxyflavone (THF) which is 3 t o 15 times more sensitive to tin than the reagents listed above. Antimony, zirconium, hafnium, aluminum, gallium, tungsten, molybdenum, niobium, and tantalum interfere in the direct determination of tin. However, tin can be satisfactorily separated from most interfering ions by extracting the iodide from a sulfuric acid solution into toluene. EXPERIMENTAL
Apparatus. The instrumentation used was a Beckman DU spectrophotometer with a fluorescence accessory modified as described (4). A combination of Corning Filters with (4) C. W. Sill and C . P. Willis, ANAL.CHEM., 31,598 (1959).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
1753
