grateful to Wilson D. Langley, State University of S e w York a t Buffalo, for his helpful discussions during the preliminary phases of this study. LITERATURE CITED
(1) Hamilton, P. B., ANAL. CHEM.35, 2055 (1963). (2) Hamoir, G., “Biochemist’s Handbook,” Cyril Long, ed., p. 666, Van Nostrand, Princeton, N. J., 1961.
(3) Koessler, K. K., Hanke, M. T., J . Biol. Chem. 39, 497 (1919). (4) McIntire, F. C., Clements, L. N.,
Sproull, M., ANAL.CHEM. 2 5 , 1757 (1953). (5) MchIanus, I. R., J. Biol. Chem. 225, 325 (1957). (6) MacPherson, H. T., Biochem. J . 36,59 (1942). (7) Pauli, H., 2. Physiol. Chem. 42, 508 (1904). (8) Severin, S. E., Yudaev, I. A., Biokhimiya 16, 386 (1951).
(9) Tallan, H. H., Proc. SOC.Ezptl. Biol. Med. 89, 553 (1955). (10) Tallan, H. H., &loore, S., Stein, W. H., J. Biol. Chem. 211, 927 (1954).
(11) du S’igneaud, I’., Behrens, O., Ergeb. Physiol. Biol. Chem. Ezptl. Pharmakol. 41, 917 (1939). RECEIVEDfor review April 29, 1966. Accepted June 23, 1966. Work supported by grants from the Muscular Dystrophy Associations of America and the National Institute of Arthritis and Metabolic Diseases, U.SP.H.S. (Grant A M 05311)
Spectrofluorometric Analysis of Rare Earth Chelates of Thenoyltrifluoroacetone, Benzoylacetone, and Dibenzoylmethane by Computer Spectrum Stripping Techniques ELIZABETH C. STANLEY,’ BRUCE 1. KINNEBERGI2 and LOUIS P. VARGA Department of Chemistry, Oklahoma State University, Stillwafer, Okla.
b Fluorescence studies of rare earth tetrakis chelates with the /3-diketone ligands thenolyltrifluoroacetone, benzoylacetone, and dibenzoylmethane in acetonitrile solution indicated the feasibility of a general method for rare earth analysis. By choosing a ligand having the proper chelate triplet state energy level, selective excitation of the resonance levels of the rare earth ions was observed allowing controlled elimination of some overlapping spectral features. Emission data from the single beam spectrofluorometer were corrected, normalized, machine plotted and a spectrum stripping program was found capable of identifying the components of a rare earth mixture b y systematic search and identification of the maximum peak wavelengths. The long wavelength cut-off of our spectrofluorometer limited observation of intense fluorescence to samarium, europium, and terbium chelates, but the wavelengths of possible intense line fluorescence are presented for all rare earths studied. Phosphorescence was observed for gadolinium and lutetium thenayltrinuoroacetone chelates a t
oo c.
B
of a favorable combination of energy level spacings, resonance energy level lifetimes, and shielded orbitals, sharp line fluorescence is observed for many rare earth chelate solutions. For several systems an electronic energy level population inversion is possible such that stimulated emission of light, laser action, has been observed, and ECAUSE
1362
ANALYTICAL CHEMISTRY
considerable work has gone into a search for potential liquid laser materials (5, IS, 17, If). Energy transfer processes involved in the fluorescence and phosphorescence of interesting systems such as chelates of the rare earths with benzoylacetone (l-phenyl-lJ3-butanedione) and dibenzoylmethane (1,sdiphenyl-l,3-propanedione) , were first studied extensively by Crosby, Whan, and coworkers (6, 7 , 1 2 , 2 0 )and later by others (S, 4 , 11). Ligands such as thenoyltrifluoroacetone [4,4,4-trifluoro1-(2-thienyl)-l,3-butanedione] have been shown to be especially promising energetically from the formation of strongly fluorescing tetrakis rare earth chelates (5,17) and a number of these have been prepared (I). Excitation of the 4f shell resonance levels of the lanthanide ion in a chelate “cage” proceeds by the mechanism of intramolecular energy transfer from the triplet state energy level associated with the complex as a whole (6,7,20). Since chelate triplet state levels vary according to the particular ligand used, selective excitation of the strong resonance levels of the lanthanide ions in any mixture may be observed for a series of ligands properly chosen for their triplet state levels as shown in Figure 1. If the lanthanide ion’s lowest resonance energy level is above the chelate triplet state energy level fluorescence line emission is not observed, but phosphorescence band emission due to transitions from the triplet state direct to the ground state may be observed (20). Thus the sharp fluorescence lines observed for many of the rare earths, and which are a unique property of the
particular metal ion, may be turned off or on allowing a systematic search and identification procedure compatible with computer spectrum stripping. This suggested, further, that the fluorescence method would fulfill the conditions of specificity and sensitivity necessary for the trace analysis of the rare earths. Previous analyses of the rare earths by a number of techniques have been reviewed (19,26). The determination of trace quantities has always presented difficulties and the fluorescence method of analysis for simple aquated rare earth ions lacked sensitivity (10). Fluorescence analyses in aqueous tungstate, oxalate, or tetraborate-containing solutions a t pH 9 were shown by .ilberti and llassucci ( 2 ) to be sufficiently sensitive and selective, but the nature of the absorbing-fluorescing species, likely dimers or higher aggregates] was not determined. I n the present paper the feasibility of a sensitive qualitative and quantitative determination for the rare earths by spectrofluorometric methods ivas studied using tetrakis chelate systems in acetonitrile. In addition, digital data processing methods were tested on a general program for the identification of the rare earths present in a complex mixture. EXPERIMENTAL
Reagents and Chemicals. Rare earth oxides and metals (99%) were obtained from the Lunex Co. and rare earth chlorides (99.9%) from 1 Present address, University of Illinios, Urbana, Ill. 2 Present address, University of Oklahoma, Norman, Okla.
4c
' IO'cm"
3(
34 34 32 3c
2f 2€ 24 T T A Tridef Level B Triplet Level
3
22 4
x
3
'F.
TL
IGq. D Triplet Level
I
I€
I€ Id
li IC
a a
6
E
cm
SI?
4
2
r
E
-
a
b ? - - I-- l I 1
'F% Ce
"4
Pr
51'I
I
I
Ji
s77
C
atsIr,
a
39
4174 Nd
(Prn)
Figure 1. Experimental triplet energy levels of thenoyltrifluoroacetone (TTA), benzoylacetone (B) and dibenzuylmethane (D) tetrakis complexes of trivalent rare earths compared to the rare earth ion levels. 511 + 6 1 for ~ Dy refers to Dy+* (23). Most nonresonance levels have been omitted. -, rore earth ion resonance levels (8, 121; -+known fluorescence transitions (12, 15, 23); - - + probable transitions of strongest lines observed [See Ref. (12) for DyBa]. Shown also is the excitation wavelength range used in this study. Tris chelate triplet levels for B and D agree with the levels given here (12). The intense fluorescence transitions shown for Sm, Eu, and Tb were observed in this study
VOL. 38, NO. 10, SEPTEMBER 1966
1363
Table I. Fluorescence Properties of Rare Earth Chelates
Excitation wavelength,
Peak positions, mfia Sm Eu Gd Tb Dy Ho Er Tm 580 405 nd nd nd nd 53NW) 563(s) 592 600 613(s) 604(w) 640(w) 648 B 385 592 nd 532(w) 579 nd 490 nd nd 608(s) 564(s) 592 545b) 600 614(s) 580(w) 605(w) 640(w) 648 TTA 390 594 nd 533(w) 580(w) phos. 490 nd 450 520(w) nd 606(s) 562(s) 593 peak 545(s) 525(w) 598 613(s) ~ 5 1 0 58O(w) 535(w) 604(w) 605(s) 645 B = strongest peak; w = weak peak, broad band or shoulder, wavelength approximate; nd = not detected
Ligand D
mfi
Pr 594 608(s)
Nd nd
Yb
Lu-
nd
phos. peak -510
Yb
Lu
0
Table II. Fluorescence Properties of Rare Earth Chelates Relative intensities for strongest peaks"
Ligand Pr Nd Sm 0.3 D 0.02 1.0 B 0.04 120. TTA 0.2 Intensity of SmB assigned as unity.
Eu 30. 225. 2000.
Gd
Tb
0.75
0.5 1.2
Dy
H O
Er
0.004
0.05
Tm
0.02
0
K and K Laboratories. Dibenzoylmethane and benzoylacetone (Eastman White Label) were recrystallized from methanol. Thenoyltrifluoroacetone (Eastman White Label) was used without further purification. Piperidine (Matheson Coleman and Bell practical grade) was purified by distillation initially and later used successfully without distillation. Absolute ethanol (U.S.I. reagent quality) was used directly. The solvent for the fluorescence studies was Mallinckrodt Nanograde T M acetonitrile (Lot PGXF) used without further treatment. Preparation of Rare Earth Chelates. Rare earth chlorides (MClZ.6HzO) were prepared from the metals or oxides and hydrochloric acid. Following the procedure of Bhaumik, et al. (6) a warm solution of 3 mmoles of the rare earth chloride in 15 ml. absolute ethanol was added to a warm solution of 12 mmoles of the chelating agent and 12 mmoles of piperidine in 25 ml. absolute ethanol. After initial stirring, the reaction mixture was allowed to cool to room temperature. Following slow crystallization (2-4 days), the product was filtered, washed with cold absolute ethanol, and dried for several days in a vacuum desiccator. Analyses for C, H, N , S, and gravimetric rare earth analyses supported evidence that the rare earth chelates with dibenzoylmethane (D), benzoylacetone (B), and thenoyltrifluoroacetone (TTA) were piperidinium salts corresponding to the empirical formula ML4.HP, where M = rare earth ion, L = ligand, and, H P = piperidinium. 1364
ANALYTICAL CHEMISTRY
Impure products of uncertain composition were obtained when attempts were made to hasten the preparation by heating the reaction mixture or by ovendrying (18). The colors of the solid chelate salts were the same as the chlorides with some shift toward canary yellow for the less intense shades. Lutetium chelates, however, remained colorless. A useful indication of purity was found in the melting point data (I!) since, for any chelate series, melting points changed monotonically with atomic number. Apparatus. A Farrand single-beam dual monochromator spectrofluorometer (104244B-Ser. 190) was used for all fluorescence measurements. The wavelength range of this instrument was 220 to 700 mp for both the excitation and emission monochromators. The light source was a Hanovia 150-watt xenon arc lamp. Manually interchangeable slits of 20-mp bandwidth were used in the excitation moncchromator and of 5-mp bandwidth in the emission monochromator. The sample cells were 1-cm. fused silica, placed in a circulating ice water constant temperature sample compartment purged with dry nitrogen. Current from a RCA 1P28 photomultiplier detector was amplified by a modified RCA microammeter (WV-84C) which drove a variable load, and the signal was recorded by a TI recorder (model PWS). The sensitivity range of the spectrofluorometer was varied by the microammeter range selector and the load resistance setting. The remaining features of the instrument correspond to recent descriptions (14, 16). In-
strumental sensitivity was high, with 10-6 pg./ml. quinine sulfate (in 0.1N H2S04)detectable. Calibration of the spectrofluorometer to correct the variations in intensity of the lamp and response of the photomultiplier tube as functions of frequency followed the method of Drushel, Sommers, and Cox (9, 18). These data are unique for each instrument. Procedure. Prior to the analysis of each chelate solution the peak intensity [&SIof 1 pg./ml. quinine sulfate in 0.1N H2SO4excited at 350 mp was recorded to establish the operating response of the spectrofluorometer. To determine which chelates fluoresced with sufficient intensity to be useful in the study of mixtures, 1.0 X 10+M solutions in acetonitrile of all of the available chelates were prepared. The excitation wavelength for maximum fluorescence intensity at 0" C. was determined for each, and the relative fluorescence intensities compared. The peak positions and excitation wavelengths are shown in Table I and the approximate relative intensities for the chelates are listed in Table 11. From the data, solution concentrations were prepared which would provide nearly equal maximum intensities (at the same attenuation) for each member of a set with a given ligand. Between three and six spectra were determined for each of the rare earth chelates. The observed spectra were manually converted to digital form using a cross-hatched template. Intensities above an interpolated baseline were read a t 2.5 mp intervals from 475 to 675 mp and the averaged values
from the several spectra were punched on II3M cards together with range and load resistance data, the standard quinine sulfate peak intensity, and the xenon lamp calibration factor. Xine such sets of standard spectra were used in the analysis. The major source of error in the procedure-Le., the difficulty in determining accurate numerical intensities at intervals of 2.5 msled to an average deviation a t each point of nearly 77c. The measured peak wavelengths, however, were reproducible within 1 mg. For each ligand, mixtures containing all possible combinations of fluorescing chelates were prepared with concentrations designed to give well-defined peaks for all components. Duplicate spectra were run for most of the mixtures and the averaged spectral data were punched on cards to await computer interpretation. RESULTS
Chelate Triplet Levels. The triplet level of the dibenzoylmethide chelates shown in Figure 1 was determined by the observed fluorescence emission of P r D , SmD, and E u D , but not T b D as indicated in Table I. The benzoylacetone triplet level was placed from the observed emission of the T b B chelate but not ErB. The thenoyltrifluoroacetone triplet level was placed considering the observed excitation of ErTTA and the requirement that the triplet level be lower than absorbing singlet levels in the excitation energy range. The placement of these levels in Figure 1 appears to fit the observations but the exact positions are not implied. Emission Intensities. Although fluorescence was observed for 1.0 x 10-3M PrD, PrB, PrTTA, HoTT.4, and ErTTA solutions, and weak phosphorescence for LuTTA, Table I1 shows the emission intensities for these chelates were too low for practical determinations with the present instrument. .kt higher concentrations, intensities for these chelates were still small, due to selfquenching effects. Since the presence of these rare earths in a mixture would have little effect on the spectra they were not considered further in the analytical scheme. Other rare earths were excluded because no fluorescence was observed. Inspection of Figure 1 reveals that an emission monochromator and detector sensitive to the near-infrared region would be required to observe the intense resonance fluorescence lines of Pr, Nd, Dy, Ho, Er, Tm, and Yb. The emission observed for GdTTA was phosphorescence rather than fluorescence. The exceptionally strong phosphorescence band is characteristic of chelates of Gd, La, and Lu (with filled or half-filled orbitals) and %'as most intense for gadolinium. To our knowledge no ligands have been found which have chelate triplet state levels above the first resonance level of gadolinium. As
where CFACT, the instrumental correction factor, is a function of emission frequency; QS is the measured quinine sulfate peak intensity; QSS is the standard quinine sulfate peak intensity; RANGE is the microammeter setting; RES is the load resistance; and X E is the xenon lamp correction factor, a function of the excitation wavelength. The corrected curve was normalized by dividing all points by the largest value of E',,,,. In the normalization procedure the last four factors in the above expression cancel out, but they were retained in the program to allow later intensity studies. After correction and normalization, data for the nine standards were stored on IBM cards. Analysis of Mixtures. I n Table I11 are given the identification numbers for each mixture analyzed, components and concentrations of rare earth chelates in each mixture, and the components found by the machine program.
inspection of Figure 1 indicates, such a system would exhibit line fluorescence in the ultraviolet. Thus the following study was concerned only with the chelates: SmD and EuD; SmB, EuB, and TbB; SmTTA, EuTTA, GdTTA, and TbTTA. Except for relative intensities it may be noted from Table I that the spectra were characteristic of the rare earth ion present, and showed only minor dependence upon the ligand. Computer Correction, Normalization, and Plotting of the Emission Data. The expression converting the observed experimental emission intensities, Ye,, at 2.5-mp intervals from 475 to 675 mg, to corrected intensities, E',,,,, is
Y,,,,
=
Ye=,. CFACT
9
_QS_ . _RANGE __. QSS
RES
X E , (1)
Table 111.
Run number
10, 11, 12 13, 14, 15 16, 17 18 19, 20 21,22 23 24 25 26 27 28, 29, 30 31, 32
Results of Mixture Analyses
Components added, M 1 x 10-3, 2 x 10-3 SmD EUD 1 x i0-6,5 x 10-5 SmB 1 X 10-3 EUB 1 x 10-6, 5 x 10-5 SmB 4 X 10-4 TbB 1.6 x EUB 4 x 10-5 TbB 2x SmB 2 x 10-4 EuB 2 x 10-8 TbB 2 x 10-3 SmTTA 1 x EuTTA 4 X 10-7, 6 X lo-' SmTTA 1 . 2 x GdTTA 5 x 10-3 SmTTA 2 X 10-6 TbTTA 1 x 10-3 EUTTA 3 x 10-7 GdTTA 5 x 10-3 EuTTA 8 X 10-7 TbTTA 5 x 10-3 GdTTA 5 x 10-3 TbTTA 1 x 10-3 SmTTA GdTTA TbTTA SmTTA EUTTA
33, 34 35, 36 37, 38
GdTTA EUTTA GdTTA TbTTA SmTTA EUTTA TbTTA SmTTA EUTTA GdTTA TbTTA
2 x 10-6 6 x 10-3 1 x 10-3 2 x 10-6 5 x 10-7 5 x 10-3 5 x 10-7 6 x 10-3 1 x 10-3 1.5 x 10-6 5 x 10-7 1x
1 . 5 X 10-6 5 x 10-7 6 x 10-3 1 x 10-3
Components found Sm Eu Sm Eu Sm Tb Eu Tb Sm Eu Tb Sm Eu Sm Gd Sm Tb Eu Gd Eu Tb Gd Tb Eu Sm Gd Tb Sm Eu Gd Eu Gd Tb Sm Eu Tb Sm EU Gd Tb
VOL. 38, NO. 10, SEPTEMBER 1966
1365
-90
+ + + + + + +
i
.80 +
+
0
. .
X
I
i
= 8
SPECTRUM SPECTRUM SPECTRGM SPECTRUU SPECTRUM
OF MIXTURE
_ _ __
A F T E R REMOVAL OF _. EU
AFTER REMOVAL OF
EU AND SH AFTER REMOVAL OF E U i S H AN0 TE AFTER REMOVAL OF E U I S M ~ T B AN0 GO
+ +
i
.70 +
+
+ +, +
R E
L A
.bo
T I V
+
E -50
0 U A N T A
+
+ +
+
i
+ +
.
i
I \
.40 +
P I
p,
I
\*
1. I I
I
.oo
. .-..-..-..-..-..-..-. +
-4750
.SO00
2.105
2.000
1.904
1.739
FREQUENCY-RECIPROCAL
Figure 2.
0
-6210
-6100
1.600
1.538
-6 7 5 0
1.481
MICRONS
Successive stages in the spectrum stripping analysis of a 4-component TTA mixture
To illustrate the analysis procedure let us take the emission data for the 4component mixture of Run 37. The emission spectrum after correction and normalization is shown as the upper curve in Figure 2. Information was then given the computer that this was a TTA mixture and that at least 4 components were possible. The peak wavelength ( X M A X ) for the mixture was then compared to the peak wavelengths of the standards, and the first component identified (europium, standard $17). Then the curve for the europium standard was subtracted from that of the mixture. The remaining curve was normalized again, and a new peak wavelength was again compared with the standards, identifying a second component (samarium, standard #6). This procedure was repeated as many times as there were possible components. After two subtractions, experimental error made it necessary to allow a deviation in X X A X of *0.005 micron for line fluorescence, and *0.0075 micron for the GdTTA phosphorescence band. For some dibenzoylmethane and thenoyltrifluoroacetone mixtures it was noted that the first S m peak (at 0.5625 micron) was more intense than the 1366
1.666
-..
-.*
-..J*.1.818
t
ANALYTICAL CHEMISTRY
second (at 0.5975 micron) so the program was modified to accept identification of Sm by either peak wavelength. The results of the spectrum stripping procedure are seen in the lower curves Figure 2. For purposes of the plot, of course, the lower curves were not normalized at each step as were the data in the analysis program. hfter subtraction of the four components, no significant peaks remained and the lowest curve was essentially zero. An "extra" component xas identified in only one c w . From visual observation of the spectrum, it was suspected that europium was present as a trace impurity in the gadolinium used, and the machine program confirmed this in Run 27. A listing of the computer program written in Fortran IV is available on request. Included is the set of correction factors, CFACT and X E , and a complete data set from a sample calculation. DISCUSSION
Although only optimized concentration ratios were studied here, the results found allow more specific analysis procedures to be devised. For instance, in
the analysis of gadolinium for trace impurities, preparation of the benzoylacetonate chelate would allow identification of terbium, samarium, and europium without interference from gadolinium. (Runs 19, 20). For the analysis of terbium, the dibeneoylmethide chelates would give fluorescence peaks for samarium and europium only (Runs 10, 11, 12). Quantum efficiencies for fluorescence of the europium chelates indicate that europium could be detected at trace levels in all other rare earths. It was found that quenching affected fluorescence intensity, but the results of Figure 2 show that a stored standard spectrum subtracted from a mixture spectra leaves a qualitatively undisturbed spectra of the remaining elements. The TTA chelates offer the greatest sensitivity for rare earth analysis, although the other chelate systems, as pointed out, provide selectivity when that quality is needed. Limits of detection for the TTA chelates were a p proximately 0.02 p.p.m. for Eu, 2 p.p.m. for Sm and 10-100 p.p.m. for the remaining fluorescing rare earths. Inspection of Figure 1 reveals that she preparation of several chelate sys-
tems with triplet state energies between 5ooo and 12,000 cm.+ would help distinguish several rare earths with resonance fluorescence transitions in the near-infrared region. Using the present chelate systems, fluorescence studies in this wavelength region are in preparation and it is anticipated that the spectrum strippiug program discussed here will be a powerful tool in the analysis of the closely spaced and overlapping emission lines of this rare earth group.
LITERATURE CITED (1)Alberti, G., Massucci, M. A., ANAL. CEEM.38, 214 (1966).
(2) Bauer, H., Blanc, J., Ram, D. L., J . Am. Chem. Soe. 86, 5125 (1964). (31 Bhaumik. M. L.. J . Chem. Phw. ‘ b, 3711 (lk). (4) Bhaumik. M. L., El-Sayed, . M. A., . Ibid., 42, 787 (1965’). (5)Bhaumik, M. L., Fletcher, P. C., Nugent, L. J., Lee, S. M., Higa, S., Telk, C. L., Weinberg, M., J . Phys. Chem.68,1490 (1964). (6) Crosby, G . A., Whan, R. E., Alire, R. M., J . Chem. Phys. 34,743 (1961). (7) Crosby, G.A., Whan, R. E., Freeman, .T. J .. .r. 66. 2493 (19621. --, - . -Phvs. Chem. (8j-Dieke, G. H., Cross‘white,’ H. M., Appl. Opt. 2,675 (1963). (9) Drushel, H. V., Sommers, A. L., Cox, R. C., ANAL. CEEM. 35, 2167 ( 1963). (10) Fassel, V. A., Heidel, R. H., ANAL. CHEM. 26, 11% (1954). (11) Filipescu, N., Sager, W. F., Serafin, F. A., J . Phys. Chem.68,3324 (1964). (12) Freeman, J. J., Crosby, G . A., J . Phys. Chem. 67,2717 (1963). (13)Lem icki, A., SameLson, H., Brecher, C., J . &em. Phys. 41, 1214 (1964) (14) Lott, P. F., J . Chem. Educ. 41,A327 (1964). ’
-0.-
ACKNOWLEDGMENT
The authors express appreciation to Dale D. Grosvenor and his s t d of the Oklahoma State University Computer Center for their advice and assistance. The plotting subroutine was written by E. J. Kobetich, Kansas State University. Acetonitrile purified in a GLC prep column was kindly furnished us by Continental Oil Co. during the early stages of this project.
(15) Melby, L.R., Rose,N. J., Abramson, E., Caris. J. C., J . Am. Chem. Soc. 86, 5ii7 (1964). (16) Muller, R. H.,ANAL. CHEM. 37, (13),93A (1965). (17) Schimitschek, E. J., Nehrich, R. B., Trias, J. A., J . Chem. Phys. 42, 788 (1965). (18)Stanley, Elizabeth C., MS Thesis, Oklahoma State University, May 1966. (19) Vickery, R. C.,“Analytical Chemistry of the Rare Earths,” Pergamon Press New York, 1961. (20) Whan, R. E., Crosby, G. A., J . Mol. Spectr. 8, 315 (1962). (21) Winston, H.,Marsh, 0. J., Suzuki, C. K.. Telk. C. L.. J . Chem. Phus. 39. 267 (i963).‘ (22) Woyski, M. M., Harris, R. E., Kolthoff, I. M., Elving, P. J. (eds.), “Treatise on Analytical Chemistry,” Part 11, Vol. 8, pp. 1-146, Interscience, New York, 1963. (23) Yariv, A., Gordon, J.P.,Proc. I E E E , pp. 4-29, January 1963. “
I
RECEIVEDfor review April 1, 1966. Accepted July 5, 1966. This work waa supported, in part, by the Oklahoma State University Research Foundation and by an NSF Summer Fellowship for Teaching Assitants, 1964.
Occurrence of Bias in the Spectropolarimeter AUGUSTE L. ROUY
and BENJAMlN CARROLL
Chemishy Department, Rutgers-The State University, Newark, N. 1.
A spectropolarimeter may exhibit a bias when the phase shift in an amplifier is coupled with a synchronously rectified signal that is not strictly of a simple harmonic form. Highly absorbing samples and especially samples that scatter appreciable radiation may overburden an amplifier and cause the phase shift. The bias i s usually levo although under certain conditions it may become dextro. Bias effects which have been reported as large as a few hundred millidegrees may be fully accounted for on the basis of the present treatment. The general case of bias is considered and some specific examples are examined.
S
INVESTIGAMRS have observed a bias in the readout of the spectropolarimeter (4, 6, 8). The reported optical activity data of other investigators have been challenged on this account (4, 6, 7, 10,12). The bias is exhibited frequently with solutions of high optical density, say above 2 or 3 and at much lower optical density for turbid solutions. Although the bias may be reproducible, the bias effects of absorption and scattering are not entirely additive so that controls for such systems may
EVEBAL
not be reliable (4, 6). Thus results in the literature for some systems having rotations in the range of 10-l to lo-* degree may be in serious error. A n explanation of this effect is attempted here. The bias appears to be frequently of a levo character. For example Resnik and Yamaoka (8) found that a clear solution of pot,assium dichromate which is neither optically active nor fluorescent yielded an apparent levo rotation of more than a hundred millidegrees. Hayatsu showed that spurious optical rotations up to 180 millidegrees could be obtained on a number of commercial spectropolarimeters when appreciable scattered light was present. In previous publications (2, 9) we have considered the limitations of the spectropolarimeter, and have shown that they may attain a sensitivity of about a millidegree in the absence of absorption or scattering and a lower sensitivity as the transmittance of the sample is decressed. The special effects due to scattering have also been discussed (3, 11). In all this work only the optics of the polarimeters were treated; the combined effects of the optics and the electronics were not considered. The possible existence of a bias in the spectropolarimeters was brought to our attention by E. Anders (1). This
phenomenon appears to be of a general character and may be explained as a coupled effect of the optics and electronics of the instrument. The order of magnitude of the bias is deduced for a few cases and is compared with experimental values. ORIGIN OF THE BIAS
Assuming that the optics of the polarimeter is aligned properly, the origin of the bias appears to be in the phase &it introduced by the amplifier when it is overloaded (6). It has been shown that the phaw shift alone will not produce the bias (10)unless it is coupled with the signal resulting from the process of synchronous rectification. Again if the signal is strictly of simple harmonic form, no bias will result. The point in absorption and scattering at which the bias will appear in an instrument will depend then upon its amplifier and the character of the oscillating signal intercepted by the phototransducer. TREATMENT OF THE GENERAL CASE
The oscillating signal arises from the oscillation of one of the polarizing el+ ments; either a magneto-optic (Faraday) effect or some kind of mechanical linkage is used for this purpose (8). Figure 1 is a diagram indicating the VOL 38, NO. 10, SEPTEMBER 1966
0
1367
