Table V.
Determination of 4M26B in Steam Turbine Oils
Comparison of Results by Colorimetric, GLC, and I R Methods % 4M26B w./v. Turbine ColoriSo. metric GLC IR I 0.04 0.04 0.04 2 0.13 0.13 0.13 3 0.09 0.06 0.07 4 0.10 0.09 0.09 5 0.07 0.04 0.06 6 0.14 0.14 0.14 7 0.06 0.07 0.06 8 0.12 0.13 0.12
resolution spectrometer; because of this it is only recommended in those cases where interference is suspected in the other methods or where the identity of the antioxidant is in doubt. Comparative results have been obtained on eight samples of steam turbine oils from an industrial power station. These oils had been in service for periods ranging from 50 to 160 months and had proved impossible to analyze satisfactorily by previously published methods. The excellent agreement between results from the three methods is illustrated in Table V. ACKNOWLEDGMENT
The repeatability of all three methods is better than i: 0.01% at the 0.2% level and the choice of method largely depends upon the instrumental facilities available and upon the frequency of analysis. However, the spectrometric method is probably the most difficult to carry out and requires an expensive high
We are grateful to Jim Whiston for his assistance in the radioactive tracer work, and to Mrs. L. F. C. Underwood and Mrs. L. A. Paxton who assisted with the experimental work. LITERATURE CITED
(1) Am. SOC.Testing Materials-ASTM Standards on Petroleum Products and
Lubricants, Designation D1473-61T, Part 11, p. 1514 (1961). (2) Anglin, C., Mahon, J. H., Chapman, R. A., J . Anr. Food Chem. 4, 1018 (1956): (3) Bain, G. H., A p p l . Spectry. 10, 193 (1956). --, (4) Braithwaite, B., Penketh, G. E., Analyst 88, 297 (1963). ( 5 ) Buttery, R. G., Stuckey, B. N., J . Agr. Food Chem. 9, 283 (1961). (6) Cook, C. D., J . Org. Chem. 18, 261 (1953). (7) Filipic, V., Ogg, C., J . Assoc. Ofic. Agr. Chemists 43, 795 (1960). (8) Jennings, E. C., Curran, J. T., Edyards, D. G., ANAL.CHEM.30, 1946 11958). (9) Mahon, J. H., Chapman, R. A., Ibid., 23, 1116 (1951). (IO) Phillips, M. A., Hinkel, T. D., J . Agr. Food Chem. 5 , 379 (1957). (11) Poti, E., Gent, L. L., Pomatti, R. C., Levin, E., ANAL. CHE?d. 2 5 , 1461 (1953). (12) Sloman, K. G., Romagnoli, R. M., Cavagnol, J. C., J . Assoc. Oflc. Agr. Chemists 45,76 (1962). (13) Szalkowski, C. R., Garber, J. B., J . Agr. Food Chem. 10, 490 (1962). RECEIVEDfor review April 29, 1963. Accepted August 5, 1963. \ -
Multiple Reflection Cells for internal Reflectio n S pect rometry N. J. HARRICK Philips Laboratories, Irvingfon on Hudson, N. Y .
b The geometry of multiple reflection cells determines the ease with which internal reflection spectrometry techniques can b e utilized. A number of cells which have certain advantages over ones previously used are described. The double-pass cell, for example, where the beam enters and emerges from the same end of the cell, after traversing the length of the cell twice, has its other end free so that it can b e dipped in a liquid to measure its spectra, requiring no additional seals or infrared windows.
T
OTAL internal reflection spectrometry has found numerous applications since the technique was first proposed (3, 4, 6, 8). In many of these applications distinct advantages are found over conventional techniques. For example, in measuring the spectra of liquids, troublesome interference patterns found in thin liquid cells are eliminated; no additional windows are required; the strength of the absorption can be adjusted by adjusting the angle of incidence or the height of the liquid in the cell; there is no defocusing of the beam by the windows and the liquid; and there is no change in reflectivity losses with change of liquid.
188
ANALYTICAL CHEMISTRY
I n frustrated multiple internal reflection (FMIR), the surface is sampled many times and weak absorptions are thus amplified. The advantages thus gained over a single reflection approach are that the spectrum of thin films, even monolayer films, can be measured and that measurements on bulk materials can be made far from the critical angle, and the resulting spectrum resembles more closely that obtained from transmission measurements. The type of internal reflection cell employed has a considerable bearing on the ease with which measurements can be made. Fahrenfort (3, 4 ) has used a cell in the form of a hemicylinder for one reflection and a modified hemicylinder for as many as five reflections where the angle of incidence could be adjusted over a certain range. Harrick (6,-8) and others (1, 11) have employed thin flat platee where multiple (up to 400) reflections occurred between planeparallel surfaces and the cells were operated a t a predetermined angle of incidence. In both types of cells the angle of incidence is well defined. Cylindrical rods have been used as photorefractometers (9, 10) and more recently bent rods (6, 13) have been proposed as cells for use in the internal reflection technique for obtaining the
optical absorption spectrum of liquids. This paper points out some possible disadvantages of cylindrical rods as light pipes and draws attention to other plane-parallel geometries we have been using, which have a number of advantages over those used previously. First, we recall that the electric field strength a t the interface and the depth of penetration in the rarer medium, hence the strength of coupling to the absorbing rarer medium, depend on both the angle of incidence and the relative index of refraction a t the interface (6). The dispersion in the index of refraction in the vicinity of absorption bands thus leads to apparent band shifts and distortions using these internal reflection techniques (4), especially when working near the critical angle. Thus it is important that the angle of incidence be known, so that allowance can be made for these effects. CYLINDRICAL ROD OPTICS
When cylindrical rods are used as light pipes for internal reflection spectrometry, the light beam should preferably be directed a t some angle toward the surface to enhance the number of reflections and thus the strength of absorption. In this cme, however.
Figure 1. Schemcltic representation of cylindrical rod optics
there is no longer a unique angle of incidence. This can be appreciated by considering a cylinder as shown in Figure 1 with plane-parallel ends, where for simplicity we consider no refraction a t the entrance and exit faces. If collimated light fills the entrance face and is directed a t an angle of incidence of 45" in the median plane, the angle of incidence will be 60" on the surface of the cylinder where the tangent plane makes an angle of 45' with respect to the horizontal-a substantial change. In fact, the whole spectrum of angles of incidence between 45" and grazing is found for the curved surface. The light not striking the median plane follows a screwlike path down .;he rod and all the exit light forms a conc: with a hole in the center as shown in Figure 1. This light pattern will obviously give rise to complications in conventional monochromators if the cylindrical rod is placed in the sampling space. If only the light representing: the desired angle of incidence is collected, there is a considerable loss of power. If more complicated optics are used and all of the light is collected, it will represent a wide spectrum of angles of incidence with inherent distortion and broadening. We have experimental evidence for this. Some improvements can be made by appropriately shaping the ends of the rod and by using very high index materials, although it is unlikely that a single mode with a unique angle of incidence can be exited in rods which are large compared to the wavelength of the light being used. Thus our conclusion is that although the use of cylindrice,l rods in internal reflection spectrom1:try may have advantages because of their simplicity in preparation, they should be used with caution. Because of the complicated optical paths, calculations cannot easily be made. Bends in the rod further complicate calculatio 3s; hence calibrations are necessary. MULTIPLE REFLECTION CELLS
Most of the multiple reflection cells used to date were cut from suitable material in the form of thin plates and the ends were bevelel to form either a trapezoid or a parallelogram with the angle of the bevel determining the interior angle of incidence. In these
single-pass cells the entrance and exit windows were located a t opposite ends of the plate. After the light was introduced into the cell a t the entrance window, it was propagated down the length of the cell by means of multiple internal reflection from opposing planeparallel surfaces and then emerged from the exit window a t the opposite end. Even though the plate thickness was large compared to the wavelength of the light employed, single-mode excitation was achieved because of the well defined angle of incidence. Descriptions are given here of a number of cells where the light is still reflected from plane-parallel surfaces but which have advantages over those previously used for certain applications. V-Shaped Cell. The first is that shown in Figure 2, which is similar to a sample prepared for one of the original tests of the internal reflection spectrometry technique (8). There is no defocusing of the light beam if the distance the light travels within the cell equals the distance between the ends of the cell multiplied by the index of refraction of the crystal-i.e., L = nd. (More correctly: L = nd cos r/cos i where i and T are, respectively, the half-angular spread in the incident and refracted beams a t the entrance face, Both i and T are near 0"; hence cos i cos r 1.) The condition L = nu' is satisfied for materials with refractive index, n,greater than or equal to 2, if the internal angle of incidence, e, is chosen as
-
N
1 2
0 = - cos-1
N(n - 2) nN-4n+4
(1)
and if the angle between the two arms is 28. 0 involves both the index of refraction and the number of reflections, N . The number of internal reflections is given by N
d
= - csce 1
Table 1.
cote
+ 2 (1 - Cotv)
(2)
n
_ _ _ d~
-4
~~~
Figure 2. Simple multiple reflection cell which can be inserted in sampling space of monochromator without additional optical elements Solid lines show course of I;ght b e a m in absence
of cell
and can be adjusted by adjusting the thickness, t, of the light pipe or the distance, d , between the ends of the arms. N must be a multiple of 4 and must be greater than 4 except for n = 2. The critical angles and the angles of incidence required for a specified number of reflections are given in Table I for a number of suitable materials. For materials with index of refraction less than or equal to 2, the appropriate angle of incidence is greater than 45" and is given by
e
=
sin-1
u'x
(3)
-
and the angle between the two arms is still equal to 20. The number of reflections is given by
t
A' = nd cos0
(4)
Appropriate parameters for materials which might be used to construct such cells are given in Table 11. Since there is no defocusing of the light beam by this
Typical Infrared Transparent Materials and Parameters for Cell of Type Shown in Figure 2 Where n 2 2 eb A' 1 ° C n ec= N 27" 15' 2-22 20 4 14' 30' 30 ' 1-9 3.5 16' 45' 20 39 0.6-40 12 2.4 24' 40' 45" Vis-25 2 30 ' Any number
-
Ge Si
KRS-5
AgCl Critical angle for material-air interface. Internal angle of incidence. c Ueeful wavelength range in microns. (1
Table II.
&Os MgO
NaCl
Typical Infrared Transparent Materials and Parameters for Cell of Type Shown in Figure 2 Where n 5 2 (Any number of reflections which is multiple of 4 can be used) n 0.5 e A' - A" 50" 25' UV-6 1.7 36' 30' 50" 25' Vis-9 1.7 36' 30' 1.5 47O 54" 35' Vis-15
VOL. 36, NO. 1, JANUARY 1964
189
41-
3t-
--a W
a
?aa W n
a
0
30
60
90
ANGLE OF INCIDENCE (e)
Figure 4. Number of reflections and aperture vs. angle of incidence for single-pass or double-pass multiple reFigure 3. Double-pass multiple reflection cells
cell and since the exit beam is coaxial with the entrance beam, such a cell can be placed directly in the sampling space of a monochromator without any additional optical components and without disturbing the optics in the spectrometer. This type of cell must, of course, be operated a t a fixed angle of incidence. Double-Pass Cells. Figure 3 shows some double-pass cells which we have found very convenient in our work. Here the light beam enters and emerges from the same end of the cell. The light is again propagated down the length of the cell by multiple reflection from opposing plane-parallel faces but totally reflected a t the far end and then returns and emerges from an exit window located near the entrance window. Advantages of this doublepass cell over the single-pass cells are the following: For a cell of given length and fixed angle of incidence, the light beam suffers twice as many reflections, hence the sensitivity is twice as great; the entrance and exit beams pass through a common pivot point, making optical alignment much easier; for measuring the spectra of liquids or monitoring reactions in liquids, since one end of the cell is completely free, it can be dipped directly in a beaker and no additional window or seals are required; for use in vacuum systems or applications to analysis of eauents from gas chromatographs where enclosure of the sample is required, only one infrared window is necessary and access is required to only one side of the apparatus. Except for an angle of incidence of 45", the angle of incidence for the reflection a t the far end will, of course, be different-via., the complement of the 190
ANALYTICAL CHEMISTRY
flection cells
angle of incidence on the side. If there is concern that this may lead to a broadening of the absorption band, the end of the cell can be metalized or masked so that the reflection here will not play a part in the absorption. Because the angle of incidence a t the end of the cell is the complement of the angle on the sides, the range of angles of incidence which can be employed is from the critical angle, ec, to (90 - ec), while in the single-pass cells the range is Bc to 90". This is no particular disadvantage for high index materials, where measurements a t grazing incidence are of no interest anyway. The range, for example, for a Ge - HsO interface is 18.5"to 71.5". Figure 3a shows such a cell designed for a predetermined angle of incidence and a definite number of reflections. The structure shown in Figure 3b, on the other hand, permits an adjustment of the angle of incidence and also the number of reflections but only for a few specific angles of incidence, because the exit and entrance beams must pass through a common pivot point. When the angle of incidence is changed, the path length in the cell changes, which will alter the location of the external focus. This can be compensated for either by separate manual adjustment or a direct coupling to one of the mirror settings. When thin plates are employed, the light beam suffers many internal reflections and the light fills the whole of the plate. The entrance and exit windows now act as focal points and since they have a common pivot point, alignment of the cell in the apparatus is considerably simplified. For a structure similar to that shown in Figure 3a, 50y0 of the emerging radiation will be
directed back toward the source. This loss can largely be eliminated by making the entrance window smaller than the exit window, as shown in Figure 3c. This is especially true for external optics, where one deals with the image of B slit which is generally narrow compared to the radiation source and the entrance window of the cell. This has worked successfully in our experiments. If it is desired to vary the angle of incidence continuously, the structure shown in Figure 3d can be used with, of course, a loss of 50% of the light intensity; however, the sensitivity of these cells is twice as great as that of the single-pass cells. CELL PREPARATION
There is a fairly large choice of infrared transparent materials from which cells may be constructed. High index materials have the advantage that there is a wide choice in angle of incidence and thus it is possible to employ many reflections when the cell length is limited. It is preferable to work with reflections from plane surfaces because the optics are then simpliied. When many reflections are employed, smooth surfaces are required, since the reflection must be specular rather than diffuse. Scattering not only leads to power loss but makes balancing in double-beam arrangements more of a problem. Smooth surfaces can be prepared by mechanical polishing. For certain experiments the damaged surfaces resulting from mechanical polishing are undesirable. Electropolishing techniques developed recently for semiconductors (11)are thus attrac-
tive. Other etching techniques-e.g., spin etch-are under investigation. The success of a thermal etch on silicon followed by annealing (1) is also encouraging. Surfaces "as grown," such as dendritic crystals m d semiconductor sheets ( 2 ) ,might be adopted for this use. With certain materials cleaved surfaces might also be employl2d. The opposing flat surfaces must be parallel to a high degree in samples designed for many refle:tions. If the surfaces are off parallel by 0.1 ", for example, the internal angle of incidence will change by 10" after LOO internal reflections. The light be:tm will not reach the exit window if it i s directed from the thicker toward the thinner end of the cell and the working angle is less than 10" from the critical mgle. In general, it is preferable to cut and prepare the cell in one piece. When the structure becomes complicated, such as that shown in Figure 2 and Figure 3 4 it may be assembled from a number of pieces by optical contact along the dotted lines. (Optical contact can readily be achieved between two surfaces polished flat to within onetwentieth wavelengtl- of sodium light.) The angle of incidence desired is chosen according t o the depth of penetration and electric field strength required in the rarer medium (6). The angle of inciden:e also determines the number of refleci ions and aperture of the cell. These can be calculated from geometrical considerations and are shown in Figure 4. The number of refllxtions is given by 1 A: = - cote
t
(5)
for single-pass cells and is twice this for double-pass cells. Here Z is the length of the plate and t is its thickness. For a maximum aperture the entrance and exit windows should be cut so that the light enters and leaves at normal incidence. This also eliminates polarization and dispersion in refraction by the windows, which only complicate matters. The aperture is defined as that portion of window which transmits light internally reflected at the desired angle of incidence. For a.igles of incidence less than 45", only part of the bevel contributes to the apzrture: A = 2tsinO
For
e
(6)
= 45Oto 90" A = t se,: e
(7)
For double-pass cell,$ A is the total aperture of the entrance plus exit windows, while for single-pass cells A represents the apertupe of the entrance or exit window. For double-pass cells the total aperture remains constant
MONOCHROMATOR
Figure 5. Experimental layout for double-beam operation
regardless how it is divided between the entrance and exit windows. The dimensions of the cell are determined from the following considerations. The width of the cell is chosen to be equal to or greater than the height of the spectrometer slits-e.g., 1 to 1.5 cm. The length to thickness ratio of the cell determines the number of reflections once the angle of incidence is selected. We have constructed cells of thicknesses ranging from 0.25 to 5 mm. and lengths from 1 to 10 cm. The performance of multiple reflection cells which were properly constructed has been very encouraging. Cells constructed from highly transparent material with surfaces adequately polished and plane-parallel transmitted close to theoretically expected power even for hundreds of reflections, the principal loss arising from reflection a t the entrance and exit windows. Even weak bulk absorptions can be troublesome, since the path length in the bulk may be large. In some of our double-pass cells, for example, the path length in the bulk is 25 cm. INSTRUMENTATION
The choice of cell type will depend on what instrumentation is available. The advantage of the one shown in Figure 2 is that it can be placed directly in the sampling space of a monochromator without any additional optics. The cells shown in Figure 3 can be inserted directly in commercially available ATR attachments or in modifications thereof. The sampling space provided in many spectrometers is often inadequate for many experiments-e.g., those requiring vacuum systems or furnaces. For this reason it is often preferable to work with a monochromatic beam taken out of the instrumentsLe., external optics. Space is then no limitation and, furthermore, heating of the cell by the light from the infrared source is minimized. By the introduction of plane mirrors, the slit image can be oriented so that the cells shown in Figure 3 can be operated in a vertical position. Double-beam operation gives en-
hanced sensitivity because with the resulting flat base line higher amplifications can be employed, and source fluctuations and atmospheric absorption bands are canceled to a high degree. For high degree of balancing, care must be taken that the same portion of the infrared source contributes to both beams. Surface roughness and dispersion in transmission and reflection of the cell will contribute to unbalance. Equal lengths of cell material should be traversed by both the reference and sample beams to compensate for any absorptions characteristic of the cell material-e.g., lattice vibrations. Various arrangements might be employed for double-beam operation. A double-beam system we have used both with single-pass and double-pass cells is shown in Figure 5. The latter cells make alignment very simple, since the entrance and exit beams have a common focal point especially when the sample detector is mounted on a goniometer with the pivot point directly below the focal point of the cell. Collecting the reflected component from the entrance window of the cell as the reference beams ensures that both detectors see the same portion of the infrared source. Effects of dispersion in the refractive index of the cell are not eliminated here. This can be done by transmitting the reference beam through a compensating crystal and reflecting the sample beam from a mirror made from the same crystal. Balancing can be accomplished using external beam attenuators or can be done electrically. We have done the latter. LITERATURE CITED
(1) Becker, G. E., Gobeli, G. W., J . Chem. Phys. 38,2942 (1963). (2) Dermatis, 6. N., Faust, J. W., Conference paper 62-514, Winter Meeting AIEE. 1962. (3)-Fahienfort, J., Spectrochim. Acta 1 7 , 698 (1961). (4) Fahrenfort, J., Ksser, W. M., Ibid., 18, 1103 (1962). ( 5 ) Hansen, W. N., ATAL. CHEM.35, 765 (1963). (6) Harrick, K. J., Ann. N . Y . Acad. Sci. 101, 928 (1963). (7) Harrick, N. J., Phys. Rev. 125, 1165 (1962). (8) Harrick, N. J., Phys. Rev. Letters 4, 224 (1960); J . Phys. Chem. 64, 1110 (1960). (9) Kapany, K. S., Pike, J. N., J . Opt. SOC.Am. 47, 1109 (1957). (10) Kapany, N. S., Pontarelli, D. A., Appl. Optics 2, 425 (1963): ., Ibid.. in press (1963). (11) Shame. L. H.. Proc. Chem. SOC. ( L o n d o i ) 1961, 461. (12) Sullivan, M. V., et al., J . Electrochem. SOC.110, 412 (1963). (13) Wilks, P. A., Jr., Wilks Scientific Corp., South Norwalk, Conn., private communication, 1963. '
RECEIVEDfor review July 12, 1963. Accepted September 23, 1963. Division of Analytical Chemistry, 145th Meeting, ACS, Kew York, S. Y., September 1963. VOL. 36, NO. 1, JANUARY 1964
191
