M. L. Rogers and R. L. White, Appl. Spectrosc. 41, 1052 (1987). 4. ... to George C. Pimentel," L. Andrews and W. J. Orville-Thomas, Eds.,. J. Mol. Structure 157, 1 ..... Raman Spectroscopy, R. J. H. Clark and R. E. Hester, Eds. (Hey- den & Son ...
alcohols, and alkanes. The results of measurements made on compounds deposited on the gold disk were compared and contrasted to the findings on the same compounds deposited on the gold disk in xenon and argon matrices. For all compounds, a measurable, significant difference was found. In most cases, the values of absorptions for compounds on the bare disk were lower than the values found in the argon and xenon matrices. Differences on the order of 20 cm -1 generally were found for the carbonyl absorptions of esters, while smaller differences were found for alkanes (i.e., 5 cm-1). The directions of the energy shifts induced by the sample phase were not found to follow the same trend for all alkane absorptions. Full width at half-height (FWHH) values were found to be much narrower in the matrix-isolated cases for the compounds, as compared to the bare disk at the same temperature (10 K). An absence of aggregation due to the presence of matrix gas atoms was postulated to be the controlling factor in F W H H value determination. These results substantiate that significant influences on the FTIR spectra of organic compounds are due to the use of a
noble gas as an inert sample phase. Additionally, the selection of a noble gas can affect the characteristics of the IR spectrum. 1. W. M. Coleman III and B. M. Gordon, Appl. Spectrosc. 43, 1004 (1989) and references therein. 2. R. A. Nyquist, Appl. Spectrosc. 49, 336 (1986). 3. M. L. Rogers and R. L. White, Appl. Spectrosc. 41, 1052 (1987). 4. "Matrix, Isolation Spectroscopy and Molecular Structure: A Tribute ~ to George C. Pimentel," L. Andrews and W. J. Orville-Thomas, Eds., J. Mol. Structure 157, 1 (1987) and references therein. 5. W. M. Coleman III and B. M. Gordon, Appl. Spectrosc. 41, 1431 (1987). 6. R.A. Nyquist, The Interpretation of Vapor Phase Infrared Spectra, Group Frequency Data (Sadtler Research Laboratories, Philadelphia, Pennsylvania, 1984), Vol. I. 7. C. J. Pouchert, The Aldrich Library of FT-IR Spectra (Aldrich Chemical Co., Milwaukee, Wisconsin, 1985), 1st ed., Vols. 1 and 2. 8. Sadtler Standard Infrared Grating Spectra Library (Sadtler Research Laboratories, Philadelphia, Pennsylvania, 1985). 9. Sadtler Standard Infrared Vapor Phase Spectra Library (Sadtler Research Laboratories, Philadelphia, Pennsylvania, 1985).
Diffuse Transmittance Spectroscopy of Polymeric Fibers A. T A B O U D O U C H T * and H. I S H I D A t Department of Macromoleeular Science, Case Western Reserve University, Cleveland, Ohio 44106
In this work, a new Fourier transform infrared spectroscopic technique (diffuse transmittance) is introduced. Its principle is based on the collection and the analysis of the diffusely transmitted infrared radiation through fibrous samples. The qualitative as well as the quantitative aspects of the method have been investigated. A comparative analysis of diffuse reflectance (DRIFT) and diffuse transmittance (DT) spectra shows that orientation of the sample has little effect on DT spectra, as compared with DRIFT spectra. With the use of ultra-high modulus polyethylene fabrics as a substrate coated with 12-nitrododecanoic acid, the concentration dependence of F(T), a function of the diffuse transmittance T, defined as F(T) = (1 - T)2/2T, was investigated within the coating thickness range of 0 to 50 nm. Index Headings: Infrared; Analytical methods; Surface analysis; Spectroscopic techniques.
INTRODUCTION Infrared spectroscopy :is one of the oldest analytical methods for molecular level characterization. The diversity of valuable information which can be extracted from the infrared spectrum (i.e., molecular structure, conformation, crystallinity, orientation, hydrogen bonding, etc.) has obviously contributed to the success of this method. Until recently, the transmission technique was the most popular, because of its simplicity. Its early application Received 30 January 1989. * Present address: Institut Algerien du Petrole Boumerdes, Algerie. t Author to whom correspondence should be sent.
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for surface analysis of powdery materials to investigate adsorption and catalysis provided one of the most direct means of observing interactions and perturbations occurring on the surface. 1,2 However, it was noticed that, as the radiation impinged upon samples, a large portion of the incident radiation was scattered, reducing the energy reaching the detector--which leads to poor-quality spectra. Hence, this scattering phenomenon was perceived as a major problem by the spectroscopists at that time. The poor quality of the infrared instruments--as well as the lack of adequate signal collection devices, which limited the quality of the spectra recorded--probably contributed to this perception. In order to minimize the losses due to scattering and maximize the direct transmitted energy, researchers used several techniquesl~: a decrease of particle size below the wavelength of the radiation, compression of the powder, and use of materials which scatter very little. Diffuse reflectance spectroscopy was the first technique which took advantage of scattered light--specifically, diffusely reflected radiation. However, its application in the early days was limited in the measurement of the ultraviolet-visible and the near-infrared regions of powdery samples using higher-sensitivity detectors. The application of the diffuse reflectance technique in the mid-infrared region was ignored, since the low-energy throughput of the grating instruments and the low sensitivity of the detectors did not permit spectra with an acceptable signal-to-noise (S/N) ratio to be recorded.
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APPLIED SPECTROSCOPY
Later, the advantages offered by the Fourier transform infrared instruments, namely, a high S/N ratio combined with the design of more sensitive detectors (e.g., MCT detector) and better reflectance optics, permitted the recording of quality spectra and initiated the success and popularity of diffuse reflectance spectroscopy in the midinfrared range. Another reason for this popularity is due to the fact that this characterization technique is considered to be nondestructive, because no optical element ' is in contact with the sample. Many diffuse reflectance theories have been proposed and many related review articles have already appeared2 -~ These theories have been broadly classified into two categories: the continuum and the statistical models2 Continuum theories involve the use of phenomenological constants, while statistical theories use fundamental optical quantities such as absorptivity, refractive index, and particle size. The most popular theory describing the diffuse reflectance process from powders has been developed by Kubelka and Munk. 7,s They derived a relationship which, in the case of an infinitely thick sample, takes the following familiar form: F ( R ~ ) = (1 - R ~ ) 2 / ( 2 R ~ ) ~- k / s
(1)
where R~ is the reflectivity of an infinitely thick sample. This theory relates F(Roo), a function of Ro~, to the absorption coefficient, k, and the scattering coefficient, s. This relationship, identified as the Kubelka-Munk function, is very important for spectroscopists because it predicts a linear relationship between the molar absorption coefficient and the maximum value of F(Ro~) for every peak, assuming that the scattering coefficient, s, is constant. If the assumption is true, then the relationship takes the following form: F ( R ~ ) = (1 - R~)2/(2Ro~) = 2.303ac/s = K * c
(2)
where a and c are the absorptivity and the concentration, respectively. K* is a constant. However, the application of this relationship for quantitative analysis is valid only for a very limited concentration range2 The primary reason for the breakdown is that the basic assumptions involved in this theory are not strictly valid. It has been derived for weakly absorbing samples, irradiated with diffuse light, having a constant value of the scattering coefficient over all the frequency range and ignoring the presence of specular reflectance. As Ishida 5 has pointed out, the application of D R I F T for quantitative analysis must be carried out with caution, as many factors--such as particle size of the sample and particle size of the diluentl°--influence the scattering properties and hence the spectral shape. Furthermore, the presence of specular reflectance accentuates the difficulties, especially when one is dealing with anisotropic (oriented) samples such as fibers. Initially designed for powdery samples, D R I F T has extended its field to surface analysis of polymer film, H polymer fibers, 12and glass fiber mats. ~3,~4Although qualitative analysis has been successful, the quantitative aspect has presented some difficulties, due to the contribution of specular reflectance to the spectrum. This specular component substantially influences the spectrum, because the spectral features of diffuse and specular reflectances ?undamentally differ in the vicinity of absorption bands. 15
Specular reflectance which is caused by reflection at the surface is predominantly a function of the refractive index, while diffusely reflected radiation, which partially penetrates the sample prior to being scattered back towards the surface, is primarily influenced by the absorption. In order to decrease the contribution of specular reflection to the D R I F T spectrum, several means have been used: a blocker, TMa diluent overlayer, 17and a method of angular dependent detection. TM We believe that the use of a blocker will not completely eliminate the difficulties associated with specular reflectance. The reason is that the curved exposed fiber surface leads to a distribution of angles of incidence, and, therefore, specular reflection will be generated at all angles. Because the blocker will only eliminate specular reflectance at certain angles, its performance for fibrous samples will be questionable. McKenzie et al. TM reported the lack of reproducibility of the D R I F T spectra of glass mats. These difficulties have been related to anomalous dispersive effects associated with the strong intensity bands of the glass, as well as to the glass fiber (anisotropy) orientation effect, which causes a spectral change as fiber orientation changes. These problems make any quantitative analysis difficult. However, these difficulties have been minimized by the addition of an overlayer of a nonabsorbing, scattering materiaW which makes the incident and the scattered radiation more isotropic. However, this overlayer does alter the amount of radiation reaching the sample. Another approach towards resolving the complications associated with the specular reflectance and anisotropy of particles is to consider the collection and the analysis of the diffusely transmitted radiation, the transmittance of the layer being expressed by the following equation: 4 T =
(3)
(1 + ~)2e~d - (1 - ~)2e-~ where T is the diffuse transmittance, ~ = [ k / ( k + 2s)]'/2, K = [k(k + 2s)] '~2, k is the absorption coefficient, and s is the scattering coefficient. The reason for this approach is that the forward scattering is primarily the result of multiple scattering, which is known to have no preferred direction of scattering; the scattering distribution thus should be isotropic. 4 The objective of this study is to explore the qualitative as well as the quantitative aspects of this technique, the diffuse-transmittance method. It is hoped that the collection of a signal free of any specular component will allow the characterization of anisotropic materials such as fibers--qualitatively as well as quantitatively. EXPERIMENTAL
Fiber Cleaning. Ultra-high-modulus polyethylene fabrics, Spectra 900, were kindly supplied by Dr. H. Nguyen of Allied-Signal Co. In order that surface contamination on the fibers could be eliminated, all the fabric samples were extracted in methylene chloride for a minimum time of 48 h, with the use of a Soxhlet apparatus. The cleaned samples were then dried for 48 h at 60°C in an air oven. The dried samples were stored in stoppered vials. FT-IR Diffuse Transmittance. All the spectra were recorded on an FT-IR spectrometer (Bomem DA3) with a APPLIED SPECTROSCOPY
1017
o
,°oils°,
< j
o
[
420
~,
SRPeef/ur/~r~cDiffuse {~/
40,
_
2-/
~
_
0
Fibers
1
NUMBEROF"FABRIC LAYERS
Diffuse-transmitted
FIG. 1. Optical layout for diffuse transmittance.
common transmission attachment. Because there was no access to an optical setup adequate for the collection of forward-scattered light and because it was desirable to maximize the energy throughput reaching the detector, the sample holder was positioned as close to the detector as possible. The spectrometer is equipped with an evacuation system capable of reaching a vacuum level down to 0.02 Torr in the sample chamber. The spectral range of the liquid-nitrogen-cooled mercury-cadmium-telluride detector is 5000-800 cm -1, with a specific detectivity D* = 3 × 10 l° cm Hz'/2/W. All the spectra were recorded at a resolution of 4 cm -1, and 400 scans were coadded at a vacuum level around (],.30 Torr. FT-IR Diffuse Reflectance. The spectra of the fiber mat samples were recorded on a Bomem DA3 FT-IR spectrometer with a diffuse reflectance cell (SpectraTech). All spectra were collected at a resolution of 4 cm -1, and 400 scans were coadded. Effect of Fiber Mat Orientation. The effect of fabric sample rotation has been investigated by both FT-IR techniques (DRIFT, DT). In both cases, a rotation of the sample around an axis perpendicular to its plane has been performed, and a spectrum has been recorded for each position. Preparation of Calibration Solutions. A chloroform solution of 10.0 g/L of 12-nitrododecanoic acid (Aldrich, used as received) was prepared. This solution was then used for the preparation of several solutions of lower concentration by dilution. The concentrations of the prepared solutions were determined on the basis of the estimated specific surface area of the polyethylene fiber fabric in order to get art estimated equivalent coverage thickness of the order of 2, 4, 6, 10, 20, 30, and 50 nanometers (nm) of the solute when 200 #L of the respective solution were uniformly deposited on the preweighted samples. The fabric sample weights were around 0.17 gram. The coating was achieved by depositing the solution, as uniformly as possible, with the use of a microsyringe. The solvent evaporation was performed in air at room temperature overnight. RESULTS AND DISCUSSION The optical phenomenon responsible for diffuse transmittance in the fiber mat is multiple scattering. The basic principle of diffuse transmittance is shown in Fig. 1. This technique is based on the collection and the analysis 1018
B
Volume 43, Number 6, 1989
3
FIG. 2. Effect of the number of fabric layers on (A) the signal intensity enhancement; (B) the signal-to-noise ratio.
of the diffusely transmitted radiation. This view is consistent with Fig. 2A, which shows the plot of the intensity enhancement of the overtone band of polyethylene (2018 cm -1) as a function of the number of polyethylene fabric layers. The intensity of this band increases nonlinearly as the number of fabric layers is increased. This situation is not typical of the traditional transmission mode, where, for very weak intensity bands, the Beer-Lambert law is expected to hold. Here, because the light scattering is the main contributing factor, a multiplication of exposed fibers leads to a nonlinear gain in the absorption intensity. The signal-to-noise ratio plotted against the number of polyethylene fabrics is presented in Fig. 2B. From this plot, it is evident that, in our case, a maximum of two fabric layers will give infrared spectra of optimal sensitivity, although the additional gain from the first layer is small. The validity of any technique used for infrared surface analysis must be demonstrated in terms of (1) its qualitative aspect (i.e., its sensitivity limits for surface species detection); and (2) its quantitative aspect: the spectra must be sensible to the change in concentration of the surface species and must exhibit a linear change as a function of concentration. However, in the diffuse transmittance technique, the radiation reaching the detector is believed to consist mainly of diffusely transmitted radiation, probably combined with a very small portion of the incident light which has been directly transmitted through some fibers. This makes the Beer-Lambert law not applicable for diffuse transmittance. An alternative solution is to consider the similarity in the intensity distribution of radiation for the forward and the backwarc scattering and postulate that a possible linear relationship could exist between the absorption bands and concentration if the DT spectra were generated with the use of the following equation by analogy to the KubelkaMunk relationship: F(T)
=
(1 -
T)2/2T
= f(c) = me
(4~
where T is diffuse transmittance, c is the concentration and m is a constant. In this work, all the spectra were generated by this equation (unless otherwise specified) The restriction imposed for the use of this relationshil: is as follows: 0 < T _< 1. The reason for this restrictioi. is related to the domain of the definition of this functioi. F(T), which is undefined for T = 0. A typical spectrurl
t
A/ /
A t800
L ~A) DT spectrumof coatedfibers ]
/
B) SubtractedDT spectrum
II 1600
of neatcoating 1400
1200
-3.8
800.
2100 2070 2040 2010 1980 1950 1920 t890 1860
FIG. 3. (A) Diffuse transmittance spectrum of polyethylene fabrics coated with 12-nitrododecanoic acid. (B) Subtracted diffuse transmittance spectrum. (C) Transmission spectrum of neat 12-nitrododoecanoic acid.
FIG. 4. Effect of fiber orientation on DRIFT spectra. (A) Spectrum at zero position. (B) Spectrum after 270° rotation. (C) Their difference.
wavenumbers
I000
generated by this function is shown in Fig. 3A. In Fig. 3, spectrum A represents polyethylene fibers coated with a 20-nm-thick layer of 12-nitrododecanoic acid. Spectrum B is the the spectrum after subtraction of the contribution of the neat polyethylene fibers. Spectrum C is that of neat 12-nitrododecanoic acid recorded with the use of the direct transmission technique. The analysis of spectra 3A and 3B shows the following: 1. Although having a deeper radiation penetration into the sample, in comparison to ATR (a typical surface spectroscopic technique), diffuse transmittance may be considered as a surface analytical technique, as verified by its ability to detect the presence of nitrododecanoic acid on the surface of polyethylene fibers. 2. This technique is very sensitive to surface species, in this case to the compound deposited on the surface of the fibers at an equivalent monomolecular layer thickness. 3. A comparison of the diffuse transmittance spectrum to that of the transmittance spectrum of neat nitrododecanoic acid in Fig. 3C does not show any difference in peak positions. In order for us to evaluate the potential of the diffuse transmittance technique, its comparison with the wellestablished diffuse reflectance will be useful mainly with respect to the application of these methods to fibrous samples, where orientation effects have been shown to significantly influence the quality of the spectra generated by the D R I F T technique, thus not allowing any quantitative analysis. The application of D R I F T to the surface study of glass fibers 14 and of polyester (PET) fibers 12 has shown that a change in the fiber orientation causes a change in the relative intensity of absorption bands in the same spectrum. In their comparative analysis of P E T fibers and P E T film, Graf e t al. 12 have shown that the reflectance spectrum of fibers is dominated by the specular reflectance in the region of strong absorption, while the contribution of diffuse scattering is significant in the region of weak absorption. This observation is in accord with the expectation based on the Fresnel relationship expressing the specular reflectance (R) as a function of n (index of refraction) and k (ex-
wavenumbers
tinction coefficient), which in the case of a normal incidence takes the following form: (n - 1) 2 + n2k 2 R = (5) ( n + 1) 2 + n2k 2" In order to compare the effect of orientation on D R I F T and DT spectra, we have recorded the spectra of polyethylene fabric samples at different orientations using both FT-IR techniques. Figure 4 shows two D R I F T spectra, in the frequency range of 2100-1860 cm -I, taken at different relative orientations (spectra A and B) and their difference (spectrum C). The simultaneous presence of positive and negative bands in the difference spectrum clearly illustrates the spectral changes induced by the sample rotation. Similar observations can be made when one is analyzing the diffuse transmittance spectra of the fiber samples recorded as a function of their orientation shown in Fig. 5: the presence of negative and positive bands in the difference spectrum also suggests the influence of the fiber orientation on the diffuse transmittance spectra. However, a comparison of the bandwidths in both difference spectra (Figs. 4C and 5C) shows a wider band in the D R I F T spectrum, compared with that in the
2100 2070 2040 2010wavenumbers i980 1950 1920 1890 1860 FIG. 5. Effect of fiber rotation on DT spectra. (A) Spectrum at zero position. (B) Spectrum aver 270 ° rotation. (C) Their difference.
APPLIED SPECTROSCOPY
1019
T A B L E I.
DRIFT DT
Influence of fiber rotation on the percent scatter (Sa).
A) 0 nm
Rotation angle (~)
45
90
135
270
% Scatter
4.9
2.7
13.6
4.5
(S°)
% Scatter
0.2
2.1
2.6
~
A
S . = [(XIY)o -
(xIY)°]I(X/Y)o
where X and Y are the integrated band intensity of the 2017-cm -~ and 1897-cm -~ bands, respectively, ( X / Y ) o is their ratio for the zero or reference position, and ( X / Y ) . is their ratio for orientation angle a relative to the zero position. The results for both infrared techniques are shown in Table I. The comparative analysis of the data relative to each technique shows that diffuse transmittance leads 1020
Volume 43, Number 6, 1989
"
=
6
4
~
8 ~A=758
C) 6 nm
1.8
(6)
A
B) 2 nm
~
(so)
DT spectrum, resulting probably from a distribution of band shift as the sample orientation changes. Accordingly, the phenomena leading to the orientation effect in diffuse reflectance and diffuse transmittance must be fundamentally different. ]In the case of DRIFT, the orientation effect has been shown to be related to the presence of the specular reflectance component. However, in the case of diffuse transmittance we believe that this phenomenon is due to a weak polarization of the incident light interacting with the oriented sample. It is well known that a weak polarization is always possible in the midIR GE/KBr beamsplitters, and its effect cannot be neglected, 19especially in the situation where one is dealing with oriented samples. Accordingly, we do believe that, in the diffuse transmittance experiment, the radiation reaching the detector would consist mainly of diffusely transmitted light. However, it is possible that a small portion of light may be directly transmitted through some loose fibers. Although it would be negligible in comparison to the portion of the light diffusely transmitted, its interaction with the sample dichroism is believed to induce the spectral change as the sample is rotated. Therefore, in essence, the diffuse transmittance spectrum can be considered as a compo,dte spectrum consisting of two superimposed spectra: namely, a pure diffuse transmittance spectrum and a direct transmittance spectrum. In the same way, the diffuse reflectance spectrum has been considered to consist of two superimposed spectra: one due to specular reflectance, the second due to diffuse reflectance. In order to determine and compare the effective contribution of each of these two factors to the quantitative aspect of the two techniques, we attempted the following approach: two overtone bands were arbitrarily selected-the 2017-cm -~ band and t:he 1897-cm -~ band, attributed, respectively, to polyet:hylene overtone crystalline bands2°--and their relative intensity change as a function of the sample orientation was calculated, with the assumption that the variation of this ratio is a measure of the orientation effects. The percent scatter (S.i) as a function of rotation angle a relative to an arbitrarily selected sample position (in our case zero angle position) was then determined according to the following relationship:
~
E) 20 n
~
~\A=741
~
~ 1 5 6 5 G) 50
]698 (COOI-I) J t645 wavenumbers
176D
i530
FIG. 6. Influence of the coating layer thickness on the DT spectra: (A) uncoated polyethylene fibers; (B) 2 nm; (C) 6 nm; (D) 10 nm; (E) 20 rim; (F) 30 nm; (G) 50 rim.
to less difference in the data and, accordingly, seems to be less affected by the variation of the fiber orientation. A relatively higher orientational effect, however, was expected in the case of DRIFT. We do believe, however, that this orientation effect would be stronger if we were dealing with a material higher in absorptivity than is polyethylene. In order to assess the sensitivity as well as the potential of this technique for quantitative analysis, we recorded the spectra of polyethylene fabrics coated with 12-nitrododecanoic acid, for different average layer thicknesses of the coating. The deposited layer thicknesses ranged from 2 to 50 nm. Figure 6 illustrates the spectra of the fibers coated with different thicknesses. The thickness of the coating is indicated next to each spectrum. The sensitivity of DT is demonstrated by the presence of the 1698-cm -1 band associated with the carboxylic acid group (-COOH) of the surface coating, which is notable even for a layer thickness as low as 2 nm. Figure 7 shows the resulting calibration curve generated by plotting, for the different layer thicknesses, the ratio of the peak area of the band at 1698 cm -1 (-COOH) to that at 1897 cm -1 (crystalline overtone band of the polyethylene substrate). Three examples of experimental data were used for each thickness in the curve, and the curve fitting
~) 1.5
o
1.0-
0.5z 01
lo
4o
3'o
4'o
Coating thicknes~ (nm)
FIG. 7.
Calibration curve.
~o
analysis using the regression analysis lead to a straight line passing t h r o u g h the origin of the axes and having a slope m = 0.0256 _+ 0.0006 and a correlation factor R = 0.97. This confirms the existence, within the studied concentration range, of a linear relationship between the F ( T ) area and the concentration of the surface species. CONCLUSIONS In this study, a new F T - I R spectroscopic technique, diffuse transmittance, has been introduced. Despite the lack of adequate signal collecting optics, which obviously limits the major part of the diffusely t r a n s m i t t e d signal from reaching the detector, this technique has been shown to present several advantages: 1. It is a sensitive surface analysis technique. 2. It is a very easy and simple technique. 3. It is totally nondestructive, as no optical contact with the sample occurs. It overcomes the disadvantage of using diluents like D R I F T , thus avoiding all the problems associated with the use of K B r - - s u c h as moisture, contamination, etc. It allows the s t u d y of the fiber sample in its natural environment. 4. It is practically unaffected by the sample anisotropy. This factor makes this technique very promising for quantitative fiber surface study. T h e effect of anisot r o p y can be further reduced by the use of parabolic collection optics, coupled with a blocker, which is placed at the pass of the directly t r a n s m i t t e d radiation.
weakly absorbing spectral region (or weakly absorbing material) of highly scattering materials. 1. L. H. Little, Infrared Spectra of Adsorbed Species (Academic Press, London/New York, 1966). 2. Experimental Methods in Catalytic Research, R. B. Anderson, Ed. (AcademicPress, New York/London, 1968). 3. G. Kortfim, Reflectance Spectroscopy (Springer-Verlag, New York, 1969). 4. W. W. Wendlandt and H. G. Hecht, Reflectance Spectroscopy (Interscience, New York, 1966). 5. H. Ishida, Rubber Chem. and Technol. 60, 497 (1987). 6. H. G. Hecht, J. Res. Nat. Bur. Stand., Sect. A, 80, 567 (1976). 7. P. Kubelka and F. Munk, Z. Tech. Phys. 12, 593 (1931). 8. P. Kubelka, J. Opt. Soc. Am. 38, 448 (1948). 9. H. G. Hecht, Anal. Chem. 48, 1775 (1976). 10. P. R. Griffiths and M. P. Fuller, in Advances in Infrared and Raman Spectroscopy, R. J. H. Clark and R. E. Hester, Eds. (Heyden & Son, London/Philadelphia/Rheine, 1982), Vol. 9, p. 63. 11. S. R. Culler, M. T. McKenzie, L. J. Fina, H. Ishida, and J. L. Koenig, Appl. Spectrosc. 38, 791 (1984). 12. R. T. Graf, J. L. Koenig, and H. Ishida, in Fourier Transform Characterization of Polymers, H. Ishida, Ed. (Plenum, New York, 1987), p. 397. 13. R. T. Graf, J. L. Koenig, and H. Ishida, Anal. Chem. 56, 773 (1984). 14. M. T. McKenzie, S. R. Culler, and J. L. Koenig, Appl. Spectrosc. 38, 786 (1984). 15. R. K. Vincent and G. R. Hunt, Appl. Opt. 7, 53 (1968). 16. R. G. Messerschmidt, Appl. Spectrosc. 39, 737 (1985). 17. M. T. McKenzie and J. L. Koenig, Appl. Spectrosc. 39, 408 (1985). 18. P. J. Brimmer, P. R. Griffiths, and N. J. Harrick, Appl, Spectrosc. 40, 258 (1986). 19. T. Hirschfeld, in Fourier Transform Infrared Spectroscopy, J. R. Ferraro and L. J. Basile, Eds. (Academic Press, New York, 1979), Vol. 2, p. 193. 20. S. Krimm, Fortschr. Hochp01ym.-Forsch.2, 51 (1960).
This technique can be advantageously applied to
Multiple Harmonic Electron Paramagnetic Resonance Spectroscopy by Simultaneous Detection YUHEI SHIMOYAMA* and H I R O S H I W A T A R I Department of Physics, Hokkaido University of Education, Hakodate 040, Japan (Y,S.); and Department of Molecular Physiology, National Institute for Physiological Sciences, Okazaki 444, Japan (H. W.)
A simultaneous measurement method was developed to permit the multiple detection of the first- and second-order harmonic displays of electron paramagnetic resonance (EPR) spectra. Application of integration and differentiation of amplitude and phase simultaneously, i.e., vector operations on the various displays, enables identification of signals expressed by the various harmonics. By using simultaneous detection, we found for the first time that signals indicated by the different displays are not always identical. The transformation from the first to the second harmonic displays (or vice versa) by vector operations indicated that the displays were not interchangeable when nonlinear response was involved. Simultaneous detection proved to be useful for measurements of the electron spin relaxation where multiple signals coexist. The present detection system provides a means for multivector EPR spectroscopy. Index Headings: Simultaneous detection; First harmonics; Second har-
Received 8 July 1988; revision received 10 September 1988. * Author to whom correspondence should be sent.
Volume 43, Number 6, 1989
monics; Vector integration; Vector differentiation; Saturation transfer EPR; Spin label.
INTRODUCTION In conventional electron paramagnetic resonance (EPR), spectra are generally recorded in the first harmonic display, V (1), in which the same frequencies are used for m o d u l a t i o n / d e m o d u l a t i o n , e.g., 50 kHz/50 kHz. A signal at the first h a r m o n i c shows a first derivative line shape, which is inherent to the principle of phasesensitive detection. T h e second harmonic display, V (2), is detectable with the use of m o d u l a t i o n / d e m o d u l a t i o n frequencies differing by a factor of two, such as 50 k H z / 100 kHz. In this case, the second harmonic signal essentially shows a second derivative, which has been used
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