also illustrate the influence of lit4 chloride (NaCl) ion on fluorescence. The addition of 1MNaCl causes a 20 % depression of histamine fluorescence and a 93% depression of spermidine fluorescence. The linearity of the fluorescence is not affected. The fluorescence of the OPT-complexes of histamine and spermidine is read at once, but both fluorophores are stable for at least 10 hours provided the solutions are protected from irradiation. The quenching effects of halogens on fluorescence in general should be of interest because of the increasing use of fluorescence spectrometry in analytical biochemistry. The role of halogen ions in the quenching of fluorescence as observed here can be explained on the basis of a heavy atom effect. The ability of the halogens to quench fluorescence is in the same order as their increasing atomic weights and relative position one to another in the periodic chart
of elements. An excited molecule fluoresces as a result of decaying from the lowest lying singlet state to the ground state. However, an excited molecule can also undergo inter-system-crossing (ISC) to a lower lying triplet state, in which case one does not observe fluorescence. In the case of the heavy atoms (e.g., halogens) there is an enhancement of singlet + triplet transition (ISC; intersystem-crossing) with a concomitant diminution in fluorescence. The fact that the OPT-spermidine fluorophore is so much more sensitive to quenching by halogen than is the OPT-histamine fluorophore might be related to the fact that on a mole to mole basis histamine is 25 to 30 times more fluorescent than spermidine. RECEIVED for review November 2, 1970. Accepted May 5, 1971. Work supported by USPHS Grant 5-R01-NB-06053.
Indirect Determination of Titanium by Atomic Absorption Spectrophotometry C. L. Chakrabarti’ and Mohan Katyal Department of Chemistry, Carleton University, Ottawa, Ontario, KlS5B6, Canada
IN ATOMIC ABSORPTION spectrophotometry, the principle of “indirect” determination is generally applied (1, 2 ) to nonmetals because the resonance lines for most of them lie in the ultraviolet region and their hollow cathode lamps are not readily available. Metals which do not atomize well are difficult to determine with low-temperature flames by atomic absorption spectrophotohetry because of the non-availability of free atoms in the electronic ground state to give significant absorption. The use of high-temperature flame-e.g., a nitrous oxideacetylene flame-however, makes it possible to determine such metals but the ionization (3, 4) due to high temperature must be suppressed by the addition of an ionization suppressor. However, a nitrous oxide-acetylene flame, especially, a fu$rich flame gives considerable flame emission (5) above 3000 A, which results in increased noise level. An indirect method for determining titanium involving a prior chemical amplification reaction has been reported by Kirkbright et al. (6). The present paper describes an indirect method for determining titanium, which is based on suppression of the absorbance of strontium caused by titanium in the presence of oxalate ion when the test solution containing strontium, titanium, and oxalate ions is nebulized in a 1
To whom all correspondence should be addressed.
(1) G. D. Christian and F. J. Feldman, Anal. Chim. Acta, 40, 173 (1968). (2) A. M. Bond and T. A. O’Donnell, ANAL.CHEM., 40,561 (1968). (3) D. C. Manning and L. Capacho-Delgado, Anal. Chim. Acta, 36, 312 (1966). (4) M. D. Amos and J. B. Willis, Spectrochim. Acta, 22, 1325 (1966). ( 5 ) R. N. Kniseley in “Flame Emission and Atomic Absorption
Spectrometry, Vol. 1, Theory,” Marcel Dekker, New York, N. Y., 1969, p 201. (6) G. F. Kirkbright, A. M. Smith, T. S. West, and R. Wood, Analyst, 94, 754 (1969). 1302
stoichiometric air-acetylene flame. This suppression of the absorbance of strontium caused by titanium in the presence of oxalate ion is due to compound formation (7) by Sr with Ti and oxalate ion. This method is very sensitive and makes use of a relatively emission-free stoichiometric flame. When the present work was completed, Ottaway el al. (8) reported another method for indirect determination of titanium, which was based on the enhancement of atomic-absorption signal of iron by titanium in a fuel-rich air-acetylene flame. EXPERIMENTAL
Apparatus and Reagents. A Techtron Model AA-3 atomic absorption spectrophotometer fitted with a R-136 photomultiplier tube; a Techtron (No. AB 51), 10-cm, air-acetylene, premix, laminar-flow, slot burner; a meter readout and a Sargent Recorder, Model SRL, and an ASL standard hollow cathode lamp for strontium were used. All reagents and chemicals used were of analytical reagent grade. Standard solutions were prepared with double-distilled water, and were made to contain 10% hydrochloric acid (v/v). This acid concentration was maintained in all test solutions in order to avoid any possible hydrolysis of titanium. Doubledistilled water was used throughout. The following experimental conditions were found to be optimum and were usedo in this study. Current, 10 mA; spectral band-pass, 3.3 A ; air flow, 8.5 l./min; acetylene flow, 1.8 l./min; (a nonluminous flame-just before the flame begins to show feathers). The height in the flame (from the burner top) at which the measurement: were made was 2 mm. The strontium line used was 4607.3 A. Procedure. In a 50-ml volumetric flask, the sample solution containing titanium is added to 25 ml of 35.0 m / m l (7) C. L. Chakrabarti and M. Katyal, Department of Chemistry,
Carleton University, Ottawa, Ontario, Canada, unpublished work, 1971. (8) J. M. Ottaway, D. T. Coker, and J. A. Davies, Anal. Letf., 3, 385 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
of SrZ+ solution (added as strontium chloride), 10 ml of 500 pg/ml of oxalate ion solution (added as potassium oxalate), and 5 ml of concentrated hydrochloric acid. The volume of the above is made up to 50 ml by adding distilled water. This is the test solution and contains 2.0 X 10-4M of Sr2+ (= 17.5 pg/ml of SrZ+), 100 pg/ml of oxalate ion, 10% (v/v) of hydrochloric acid, and the unknown concentration of Ti (as TiC14). As will be seen in the section on Results and Discussion, for best results, the concentration of Ti should be between 0.2 and 10 pg/ml of Ti. Using the optimized experimental conditions outlined above, the test solution is nebulized in the flame, and the absorbance of strontium is noted. This absorbance is related to the concentration of titanium by means of a calibration curve drawn with standards prepared and run under the same experimental conditions as the test solution. Since this method is based on compound formation, the effect of which is strongly dependent on the height in the flame at which the measurement is made and on the flame stoichiometry, it is of crucial importance that the height in the flame at which the measurement is made and the flame composition be carefully optimized. RESULTS AND DISCUSSION
Figure 1 (a linear plot) shows the effect of increasing concentration of titanium on the absorbance of strontium, the concentration of strontium being 2.0 X lO-4M. Figure 1 was obtained by using the optimized experimental conditions described. Higher ranges of titanium can be covered by different calibration curves if one starts with a higher concentration of strontium. However, this indirect method has advantage over the direct method only in the lower ranges of titanium (say, up to 10 pg/ml); the higher ranges of titanium ( > l o pg/ml) is best done by the direct method with a nitrous oxide-acetylene flame. Starting with a concentration of 2.0 X 10-4Mstrontium, 1 to 10 pg/ml of titanium has been successfully determined by this method. With 2.0 X lO-4M strontium, the sensitivity (defined as the concentration of titanium in pg/ml per 1 absorption decrease in the signal of strontium) of this method is 0.15. With strontium concentration of 2.0 X lO-4M and titanium concentration of 1 pg/ml, the absorbance is 0.073. The peak-to-peak noise of the final measurement is too small to be measured ((0.1 Z of the incident radiation). The standard deviation of a single measurement calculated from ten replicate measurements of 0.073 absorbance unit is 0.002 absorbance unit (E 0.075 pg/ml of titanium), and the coefficient of variation is 2.7x;. From these data, the detection limit (defined as twice the standard deviation) is 0.15 pg/ml of titanium. In the study of interferences described below, all cations were taken as chlorides, and the anions as ammonium salts. In the determination of 2 pg/ml of titanium by this method, the following ions with quantities (in pg/ml) shown in parenthesis did not interfere (interference was defined as a change in the absorbance value exceeding twice the standard deviation of the final measurement): Li+ (200), Na+ (200), K+ (lOOO), NH4+ (lOOO), CuZT(20), MgZf (20), Ba2+ (200), Zn2+ (200), Cd2+ (200), Sn4+ (20), Fe3+ (IOOO), Coz+ (200), NiZ+ (20), F- (20), I- (200), NOa- (200), CH8COO- (20). EDTA Na-salt (20,200) caused slightly negative interference (depression). Slightly positive interference (enhancement) was caused by the following ions when the quantities (in pgjml) shown in parenthesis were used: Mn2+(20,200), Cuz+ (200), Mgz+ (200), Ca2+ (200), Sn4+ (200), Ni*+ (200). The inter-
CONCENTRATION OF Ti, pg /ml
Figure 1. Effect of increasing concentration of titanium on absorbance of strontium Concentration of Sr2+ 2.0 X 10-4M ( = 17.5 fig/ml)
ference due to 200 pg/ml of Sn4+was eliminated when oxalic acid was used instead of potassium oxalate in the procedure. Negative interferences (depression) were observed with the following ions with quantities (in pg/ml) shown in parenthesis: A13+ (20), Zr4+ (20, 200), Hf4+ (20, 200), Cr3+ (20, 200), P 0 4 3 - (20, 200), F- (200). It is worth noting that the titanium of the burner used (the Techtron No. AB 51 burner was fabricated from titanium) did not interfere with the method. From the above, it can be seen that the method is of limited applications. However, the method should find applications in the determination of small quantities of titanium in the presence of alkali metals, iron, cobalt, zinc, and cadmium. Some of the possible applications of this method are in the determination of titanium in : “pure” chemicals, samples where the interfering ions are known to be absent, and when a nitrous oxide-acetylene flame is not available. The prime importance of this indirect method is the fact that sensitive determination of titanium may under certain conditions be made with an air-acetylene flame (this is especially important when a nitrous oxide-acetylene burner is not available). Also, compared to the indirect method by Kirkbright et al. (6), this method is simpler and less timeconsuming although the latter method (6) is about 10-fold more sensitive. Furthermore, the present method is applicable in the presence of high concentration of hydrochloric acid (which is frequently present in titanium solutions) while chloride must be absent in the method of Ottaway et al. (8) although the latter method (8) is also about 10-fold more sensitive. Titanium (7) causes similar decrease in the absorbance of calcium or barium (in the presence of appropriate quantities of oxalate ions in the solution); this decrease also gives linear calibration curves which can be used for the indirect determination of titanium. However, the curves given by strontium give the highest sensitivity (steepest slopes), and the best detection limit.
RECEIVED for review March 10, 1971. Accepted May 7, 1971. Dr. M. Katyal is a postdoctoral research fellow. The authors are indebted to the National Research Council of Canada and Ontario Department of University Affairs, for research grants.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
1303
