( 3 ) Calvin, M. (to United States Atomic
Energy Commission), U. S. Patent 2,856,418 (Oct. 14, 1958). (4) Crandall, H., Thomas, J. R. (to United States Atomic Energy Commission), u. s. Patent 2,892,681 (June 30, 1959). ( 5 ) Crandall, H. W., Thomas, J. R., Reid,
J. C., United States Atomic Energy Commission CN 2657 (1945); Nucl. Sci. Abstr. 1 1 , 12369 (1957). ( 6 ) Diehl, H., Smith, G. F., “The Iro; Reagents: Bathophenanthroline, etc., G. Frederick Smith Chemical Co., Columbus, Ohio, 1960. ( 7 ) Flaschka, H., ter Ham, K., Bazen, J., Mikrochim. Acta 1953, p. 345. (8) Flaschka, H., Abdine, H., ChemistAnalyst 45, 58 (1956). (9) Hagemann, F. J. (to United States Atomic Energy Commission), U. S. Patent 2,632,783 (March 24, 1953). (10) Hellwege, H. E., Schweitzer, G. K., 4nal. Chim. Acta 29, 46 (1963). ( 1 1 ) Hill, R. D., Gesser, H., J . Gus Chromatog. 1 , 10 (1963). (12) Kinnunen, J., Merikanto. B.. ChemistAnalyst 43, 93 (1954).
(13) Korbl, J., Pribil, R., Zbid., 45, 102 (1956). (14) Larsen, E. M., Terry, G. A., J . Am. Chem. SOC.75, 1560 (1953). (15) Magnuson, L. (to United States Atomic Energy Commission), U. S. Patent 2,830,066 (April 8, 1958). (16) Martens, R. I., Githens, R. E., Jr., ANAL.CHEM.24,991 (1952). (17) Moshier, R. W., Schwarberg, J.. E.,
Morris, M. L., Sievers, R. E., Pittsburgh Conference, March 1963. (18) Omori, T., Wakahayashi, T., Oki, S., Suzuki, N., J . Znorg. Nucl. Chem. 46, 2265 (1964). (19) Reid, J. C., Calvin, M., J . Am. Chem. SOC.72, 2948 (1950). (20) Ross, W. D., ANAL. CHEM.35, 1596 (1963). (21) Ross, W. D., Wheeler, G., Zbid., 36,266 (1964). (22) Ross, W. D., Sievers, R. E., Wheeler, G., Zbid., 36, 598 (1965). (23) Schultz, B. G., Larsen, E. M., J . Am. Chem. SOC.72, 3610 (1950)., (24) Schwarberg, J., Moshier, R. W., Walsh, J. H., Talanta 1964, p. 1213. (25) Schwarzenbach, G., Flaschka, H.,
“Komplexone. Titration mit Hilfe von Komplexonen, . .,” Firma B. Siegfried, Zofingen, Switzerland, 1953. (26) Schweitzer, G. K., Anal. Chim. Acta 30, 68 (1964). (27) Sievers, R. E., Ponder,
B. W., Morris, M. L., Moshier, R. W., Znorgaa.
Chem. 2, 693 (1963). (28) Smith, G. F., Richter, F. P., “Phen-
anthroline and Substituted Phenanthroline Indicators,” G. Frederick Smith Chemical Co., Columbus, Ohio, 1944. (29) Van Winkle, 0. (to United States Atomic Energy Commission), U. S. Patent 2,895,791 (July 2.1, 1959). (30) Wakahayashi, T., Oki, S., Omori, T., Suzuki, N., J . Znorg. Nucl. Chem. 46, 2255 (1964). (31) Werner, L., Perlman, I., Calvin, M.
(to United States Atomic Energy Commission), u. s. Patent 2,894,805 (July 14, 1959).
RECEIVED for review February 19, 1965. Accepted June 1, 1965. Division of Analytical Chemistry, 148th Meeting, ACS, Chicago, Ill., September 1964.
Variable Angle Reflection Attachment tor the Ultraviolet, Visible, and Infrared WILFORD N. HANSEN North American Aviation Science Center, Thousand Oaks, Calif.
b A variable angle reflection attachment which operates in the ultraviolet, visible, and infrared regions has been developed for spectrometers. The design is simple, especially as an internal reflection unit, in which case it comprises a right angled mirror and a right angled prism. The sample is placed in contact with one short face of the prism at which various angles of incidence are obtained by rotating the mirror. The attachment is useful to obtain attenuated total reflection spectra, to obtain indices of refraction directly by a single reflectivity measurement at less than critical angle or by determining the critical angle itself, or to obtain reflectivity data at two polarizations or two angles from which both optical constants can be calculated.
I
NTERNAL REFLECTION SPECTROMETRY
has a number of unique advantages, and the methodology and apparatus exploiting these advantages have developed over the past few years ( 3 , 4).
.I simple reflection attachment is described here which can be used to convert almost any ultraviolet, visible, or infrared absorption spectrometer into an internal reflection spectrometer. This device can be used as an ATR unit to obtain transmission-like spectrh. 1142
e
ANALYTICAL CHEMISTRY
PRISM
9 I 360 MOTION
Figure 1 . Arrangement of prism and mirror of variable angle reflection unit for a 45” angle of incidence at sample
Indices of refraction of substances which are not too absorbing can be obtained as a function of wavelength by measurements of reflectivity at angles less than the critical angle or by the critical angle method itself at any wavelength. Both optical constants-Le., the refractive index and the absorption coefficient-can be calculated by using the device to obtain reflectivities at two angles or two polarizations (1, 2). APPARATUS
General Description. The device comprises a right angled prism and a right angled mirror arranged as shown in Figure 1. T h e mirror is rotatable
about a line through its 90” apex, and the prism can b e translated in a direction parallel to its long face. A simple geometrical analysis shows t h a t the direction of a light ray is not changed by having passed through the device, regardless of the original direction of the ray. There will in general be a parallel displacement, but this is readily eliminated by sliding the prism as indicated. Thus, any angle of incidence, 8, on the sample face can be obtained at will, within certain limits. (The angles obtainable are limited only by the net “window width” of the unitLe., as 8 approaches normal or grazing, the width of beam that can be accommodated approaches zero). Figure 2 shows the relative prism, mirror, and beam positions for 6 = 70’. Note that a t other than 0 = 45’ for an isosceles prism, there will be refraction. Despite the value of e and the refraction, the beam direction is still the same after leaving the device as it was before entering it. To see this it is helpful to note that the beam entering the prism face is parallel to the beam leaving the prism face, both inside the prism and outside the prism. Also note that as the unit as a whole is translated perpendicular to the light beam (see Figure 3) the direction of the beam is unchanged. The distance traveled by the beam in air and the distance traveled in the prism are also unchanged. I n fact the distance traveled in air by any ray is exactly the same as if the unit were not there. For prisms with low indices it is
PRISM
MOTION
-
~
PRISM
MOTION
Figure 2. Relative position of prism and mirror for an angle of incidence at the sample of 70" Refraction i s shown for a prism of index 1.5
necessary to silver the short face not contacting the sample to prevent loss of light when t h e reflection is nontotal a t this face. For prisms with high indices, like silicon and germanium, it is advantageous to use a nonisosceles shape in order to permit a wider range in 8. Special Features. d reflection unit of this simple design can be used in virtually a n y spectrophotometer with sample compartment large enough to accommodate it. (Of course, if polarized light is to be used a polarizer must also be provided.) From the general description given above several features can be seen: (1) Known angles of incidence are easily obtained] provided an appropriate angular scale is fastened to the mirror. (2) The beam collimation is usually nl times as good as that of the original beam in the plane of incidence] where n1 is the refractive index of the prism. This is the best collimation that it is possible to obtain in a practical way. More specifically, consider a light beam of rectangular cross section with its short dimension in the plane of incidence (the usual arrangement). .4n angular spread of 6 in the plane of incidence outside the prism will give rise to a spread of 6/nl in the angle of incidence, e, on the sample inside the prism. 25
3
Figure 3. Schematic illustrates how point of reflection scans across sample as the unit as a whole is moved perpendicular to the incoming light beam Here the angle of incidence is fixed a t 50'
The angular spread of the beam perpendicular t o the plane of incidence may be much larger than 6 without increasing the spread of 6/nl in e. A geometrical analysis shows that a perpendicular angular spread of nlp outside the prism gives a spread of p inside and a spread of e - sin-l(sin e cos p) in 0. For example, if 0 is ca. 45", a beam spread of 9" outside a prism of index 3.0 would give a spread in e of only 0.08". For small e (near normal incidence) this difference in the effect on e of beam spread parallel and perpendicular to the plane of incidence will tend to disappear. At small e, however, reflectivity is in general insensitive to angle, and e need not be so well defined. The practical way, therefore, to collimate the beam further, if needed, for reflectivity measurements is simply to mask it to reduce the angular spread in the plane of incidence. Well defined angles of incidence are therefore easily realized. (3) Because all working surfaces have their normals in a common plane, the plane of incidence, light polarized either parallel or perpendicular to this plane will remain so, provided the prism is homoWAVELENGTH (MICRONS) 5
4
geneous. (4)Since the path length of any ray in air is the same with and without the unit present there is absolutely no appearance of atmospheric absorption bands when the unit is used in an unpurged double beam spectromter. (5) The fact that the unit as a whole can be shifted back and forth perpendicular to the beam without changing any parameters permits the sample surface to be scanned by the beam. A constant reading with scan indicates a uniform sample. Or the sample may cover only part of the sample face, and the other part be bare or covered b y a standard substance like silver. By merely moving the unit slightly a direct comparison can be had between the sample reflectivity and that of the standard, independent of instrument drift, etc. Within obvious angular limits, the standard can be total reflection, the very best standard available. The unit can also serve as a convenient variable angle ezternal reflection unit. For this purpose the prism is removed, a mirror takes the place of one short prism face and a flat sample alone replaces the prism-sample inter7
8
9
IO
12
15
20
FREQUENCY (CM )
Figure 4.
Base lines (no sample present) for variable angle unit with KRS-5 prism at two different angles of incidence in a Perkin-Elmer 42 1 spectrophotometer VOL. 37, NO. 9, AUGUST 1 9 6 5
0
1143
n
-
70000
1
I
7~ 60000
0.71
U
v
f
06r
50000
2 W
-
40000
LL LL
$
30000
V
gt-
20000
E
10000
a
042 .44 .46 .48 . 5 0 52
54 56
58 .60 .62 .64 66
WAVELENGTH (microns) 40
42
44
46
48
50
52
54
56
58
60
62
64
66
Figure 6. Absorption coefficient, CY, of eosin-Y solutions calc. from reflectivity data
WAVELENGTH (microns1
Figure 5.
ATR spectra for aqueous eosin-Y solutions
Relative concentration of 1 .O corresponds to SO groms/liter ATR prism has n g = 1.787. 0 = 55'
face. All of the above mentioned features except (4) and the total reflection part of (5) still obtain. If desired, feature (4) can be restored by enclosing the space ordinarily occupied by the prism (using an appropriate window instead of the long face) and filling the space with a nonabsorbing gas. This space can be filled with a transparent liquid and all of the above listed features of the prism case will be restored. An advantage of the liquid prism is easy optical contact with hard solids. RESULTS AND DISCUSSION
Performance as an ATR Unit. One of the uses of t h e reflection unit is to obtain transmission-like spectra by means of A T R . We have used the unit for this purpose in the ultraviolet, visible, and infrared. Figure 4 shows the performance of the unit in the infrared with no sample present; a Perkin-Elmer 421 spectrometer was used. The prism was KRS-5. The mirrors were aluminum evaporated onto the faces of a right angled prism. The 100% trace (unit out) and traces for the unit set a t 0 = 45' and e = 60" are shown. Note that because the sample can be placed on either short face of the prism, the 60" trace is also a 30" trace. Several features are noteworthy: (1) The efficiency-Le., the transmittance of the unit without sample -is close to that theoretically possible considering the reflection losses a t two KRS-5 surfaces plus losses a t the two aluminum mirrors. This is true despite the fact that the Perkin-Elmer 421 has a short focal length and large angular spread of the beam (small j value), a fact which emphasizes any effects of defocusing caused by the unit. 1 144
0
ANALYTICAL CHEMISTRY
Of course, some defocusing does occur. The high efficiency throughout, however, proves that defocusing by the unit is unimportant. (2) The efficiency of the unit is maintained over a wide choice of e, traces for all angles between ca. 27' and 63" lying between the two shown. A wider angular range can be used for KRS-5. However, for e < 25" or e > 65' one short face of the prism must be silvered. (3) Atmospheric absorption causes no more noise than with the unit out, even though the instrument was unpurged. (4) The traces a t all angles are nearly as flat as the regular 100% line of the instrument permitting drift-free ATR spectra. The slight increase in the break a t 2000 cm.-l for the 60' trace is due to the slight polarization of the beam by the beam entering and leaving the KRS-5 prism a t non-normal incidence. The case cited above K-as chosen because it was about the most unfavorable encountered or expected. In the UV-VIS-NIR range, using optical glass prisms, the energy losses are generally found to be less than the case cit,ed. Because of the lower prism index there is less lost by reflection; however, this is partly offset since the aluminum mirrors are less efficient. Defocusing contributes to the loss, but only the divergent rays are lost in this fashion, improving the net collimation of the beam. With ordinary optical glass prisms the easily obtainable range in 0 is ca. 15' to 75'. As examples of ATR spectra in the visible region the results of measurements on a series of aqueous eosin-Y solutions of regularly varying concentration are presented. The observed ATR spectra are shown in Figure 5. The
ordinate here is [(reflection absorbance, " defined as .4 = log,, I o / I where I is the intensity of light reflected from the sample and I" is the intensity reflected when no sample is present (total reflection). This special absorbance is read directly on the absorbance scale of standard spectrometers if the scale is adjusted such that the absorbance given without the sample present (total reflection) is zero. Unit relative concentration represents 50 grams per liter. The prism index varied from 1.816 at 0.43 micron to 1.781 a t 0.66 micron. The angle of inridence was 55" with a total spread of 0.5". The polarization was parallel to the plane of incidence. T o the extent that ATR spectra are transmission-like, they are a direct measure of the variation of the absorption coefficient, CY, with wavelength. The spectra can be compared directly with plots of a shown later in Figure 6. When this comparision is made it will be seen that only a t the lowest concentrations do the relative heights of the two peaks compare closely in the two figures. This type of deviation of the ATR spectra from the transmission spectra has been called "distortion." I t can readily be understood by considering what is happening to the index of refraction as shown in Figure 7 . I n the 0.54-micron region, especially, n2,the real part of the refractive index of the sample is increasing appreciably, causing considerable increase in the intensity of the A T R spectrum in this region. The ATR spectra are, however, similar to the transmission spectra, especially a t low concentrations. Direct Measurement of Refractive Index. If 8 is chosen smaller than the critical angle, the reflection measurement gives a rather direct measure of n2. Reflection spectra for the above eosin-Y solutions but with e smaller than critical, are given in Figure 8. From these data, in regions where CY is
Z N C
x
W
n
w
.4
2
I-
2 a LL W
a
I .3
42 .44 46 48 50 52 54
56 58 60 62 64 66
WAVELENGTH (microns)
Figure 7. Real index of refraction, n2, of eosin-Y solutions calcd. from reflectivity data
not too large, n2can easily be calculated ( 2 ) . The equation is
where RI is the reflectivity measured a t less than critical angle. An analogous equation will be found for parallel polarization in reference ( 2 ) . The angle 6 should be enough less than the critical angle so that it is out of the region where reflectivity changes so rapidly with 8 that error is introduced by the uncertainty in e. The similarity between A and n2 as a function of wavelength can be seen by comparing Figures 7 and 8. The reflection unit also makes it easy to determine n2 by the critical angle method wherever a is not too large. The 90” mirror is rotated until the critical angle, e,, is reached as evidenced by a sudden shift in the indicated energy. It will be recalled that n = sin 8, where n = a2/nl. This method is convenient at any wavelength. When no sample is present-Le., when the second phase is air -this method can be used to determine the index of the prism itself. Thus, it is not necessary to know the dispersion curve of the prism before measurements can be made. As an example of the use of the above methods, the index of water was measured. The results agreed with the zero concentration (pure water) curve of Figure 7. Determination of Both Optical Constants. From complex Fresnel formulas for reflectivity at a plane boundary, R is a function of n, a, 8, and P
WAVELENGTH ( m i c r o n s )
Figure 8. Reflection spectra at less than critical angle for eosin-Y solutions Relative concn. of 1 .O corresponds to 50 grams/liter. = 1.787. e = 4 5 0
where P is a measure of the polarization. The variables 8 and P are subject to choice. I t is possible to invert the reflectivity equations and solve for n and a in terms of R1 and Rz, reflectivities measured at two different polarizations or two different angles. The reflection unit described above was designed to permit convenient and accurate measurement of RI and Rz using a standard automatic scanning spectrophotometer. As a n example of this usage of the reflection device the optical constants n~ and a have been calculated as a function of wavelength for eosin-Y solutions from the reflectivity data given in Figures 5 and 8. For these calculations a computer program is used which inverts the reflectivity equations for parallel polarization at two different angles-Le., solves for n and a from A at two e’s. The resultant
The prism no
optical constants are plotted in Figures 6 and 7 . The reflection unit has also been used to determine optical constants of metals, for which the method proves to be extremely useful ( I ) . LITERATURE CITED
(1) Hansen, Wilford ?IT.,
“Internal Reflection Spectroscopy and the Determination of Optical Constants,” I S A Transactions, in press. (2) Hansen, Wilford N., Spectrochim. Acta 21, 815 (1965). ( 3 ) Hansen, Wilford N., Horton, James A., A N A L .CHEM.36, 783 (1964). ( 4 ) Harrick, N . J., Ibid., p. 188.
RECEIVED for review March 12, 1965. Accepted June 7, 1965. Presented in part at the 16th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1965.
VOL. 37, NO. 9, AUGUST 1965
1 145
