Langmuir 1992,8, 351-352
351
A Simple, Effective, and General Method of Enhancing Peak Intensities in Attenuated Total Reflectance Fourier Transform Infrared Spectra of Powdered Solids Brian M. Lynch* and Susanne L. Chisholm Spectroscopy Laboratory, Department of Chemistry, Saint Francis Xavier University, Antigonish, Nova Scotia B2G ICO, Canada Received No'vember 18, 1991 Attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectral features of inorganic and organic powders are markedly enhanced when overlaid by nondissolving liquids as compared with measurementsfor dry materials. The extent of enhancementis controlled by the ratio of refractive indices of the liquid as compared with that of the ATR reflective element and quantitatively follows the predicted depth of penetration of the evanescent wave. This simple, nondestructive modification of sampling for ATR spectroscopy should find general use in analytical vibrational spectroscopy for powdered materials; it is of particular value for less intense peaks.
Introduction Recently' we obtained excellent attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectra for particulate silicas deposited on a horizontal internal reflectance element from aqueous media. Although we did not realize it at the time, the ATR FT-IR spectra of silica powders were markedly enhanced by our experimental procedure and are more informative than spectra obtained from directly deposited dry powders. This communication proposes and confirms a simple physical rationale, defines the conditions for enhancement, and illustrates applications to inorganic and organic species. Theory It does not seem to have been recognized previously that spectral enhancement is a predictable phenomenon where a liquid overlays an insoluble material on a horizontal IRE. In an attenuated total reflection experiment, penetration depth, D,, of the evanescent wave at any specific wavelength, A, is a function of (a) the angle of incidence, 6, of the internally reflected beam and (b) the ratio of the refractive index n2 of the rarer medium divided by that of the denser nl. The function is of the form shown in eq 1 (after Harrick2) D, = A/(2?m,[sin2 6 - [n2/nl1210~5)
(1)
As the ratio term n2/nl increases, the evanescent wave penetrates further; consequently, a greater quantity of the deposited insoluble material will be sampled, with enhanced analytical sensitivity. This letter reports examples of the predicted enhancements for inorganic and organic materials.
Experimental Section Spectroscopy. ATR FT-IR spectra were obtained using a Contact Sampler (Spectra-Tech, Inc.) accessory [an out-ofcompartment horizontal attenuated reflectance cell with a zinc selenide element ( 4 5 O angle of incidence)]. The spectrometer was a Bruker IFS 25 FTIR spectrometer, operated under the following conditions: 50 scans at 2 cm-l resolution; triangular apodization; zero-filling factor 2; spectra range 4000-600 cm-l. Spectra-FileIR Plus software,version 1.2 (Heydenand Son,K0ln, Germany), was used for spectral acquisition, storage, manipu(1) Smith-Palmer, T.; Lynch, B. M.; Roberts, C.; La,Y. Appl. Spectrosc. 1991,45, 1022-1025. (2) Harrick, N.J.ZnternalReflectionSpectroscopy;Harrick Scientific Corp.: Ossining, NY. 1979; p 30 (second printing).
lation, and plotting, controlled by a Laser 28612 MS-DOS microcomputer. Reference spectra were obtained at the beginning of each measurement for the accessory cell, either empty or filled with an appropriate liquid. Materials. The overlaying liquids were commercial samples of at least 99.5% purity. Barium carbonate was J. T. Baker's Analyzed Reagent,Lot 025111. Pentafluorobenzaldehydesemicarbazone waa prepared as reported by Kolb and co-workers.3 Poly(4-methoxystyrene)was Aldrich ChemicalCompanyCatalog No. 18,220-6. Mineral samples (albite, anhydrite, barite, bytownite, calcite, celestite, labradorite, leucite, min-u-si1(particulate silica), nephelite, oligoclase, strontianite, willemite) were provided through the courtesy of Professor R. F. Cormier, Department of Geology, St. Francis Xavier University.
Results and Discussion Figure 1shows partial ATR FT-IR spectra comparing powdered barium carbonate measured in air, and when covered by a series of seven liquids of increasing refractive indices (water, acetone, deuterium oxide, 2-propanol, methanol, methyl sulfoxide, 1,1,2,3-tetrachloro-2-propene), illustrating the out-of-plane bending feature around 855 cm-'; similar results were obtained for the more intense stretching feature around 1420 cm-l. Corresponding enhancements were found for 13 common minerals in powder form covered by water as compared with air. Figure 2 illustrates correlation of the appropriate peak areas for the out-of-plane bending mode with the partial depth-of-penetration expression 1/(2a[sin26- [ndn11210.5, showingthat the signal enhancement is indeed dominated by expression 1. Perfect agreement is not expected: decreased interparticle friction [through lubrication with the superior liquid phases1 also increases coverage of the IRE. Also, our approach neglects the dependence of the refractive index of a liquid upon the radiation frequency. We also examined representative powdered organic species (pentafluorobenzaldehydesemicarbazoneand poly(4-methoxystyrene)) in air or covered by n-hexane. Figure 3 shows two ATR FT-IR spectra of pentafluorobenzaldehyde semicarbazone from 1000-1800 cm-l. The areas under the principal peaks in the two media between 800 and 1800 cm-l are precisely, linearly related to each other (the coefficient of determination is 0.988). Similarly, for poly(4-metho~ystyrene)~ areas under the principal (3) Kolb, V.M.; Stupar,J. W.; Janota, T. E.;Dum, W. L.J. Org. Chem. 1989,54, 2341-2346.
0743-7463/92/2408-0351$Q3.00/0 0 1992 American Chemical Society
352 Langmuir, Vol. 8,No. 2, 1992
Letters
1.50 h
080-
h
30 Wovenumber cm-'
Wavenumber cm-'
Figure 1. Partial ATR FT-IR spectra for barium carbonate under eight different media: a, air; b, water; c, acetone; d, deuterium oxide;e, 2-propanol;f, methanol; g, methyl sulfoxide; h, 1,1,2,3-tetrachloro-2-propene.The absorbances for curve a have been multiplied by 10.
_]
12.0
8.0
845865 cm.-1 4.0
Ob Peak Area = - 13.7 + 14.2 Dp
"." .
Coefficient of Determination = 0.917
1.0 1.2 1.4 1.6 1.8 Depth ofPenelration[Dpl Relativeto&
Figure 2. Peak areas for carbonate bending absorptions versus relative calculated depth of penetration of evanescent wave: a, air; b, water; c, acetone; d, deuterium oxide; e, 2-propanol; f, methanol; g, methyl sulfoxide; h, 1,1,2,3-tetrachloro-2-propene. peaks at 801,825,1174,1244,and 1509 cm-l are precisely interrelated. This simple, nondestructive modification of sampling for ATR spectroscopy should find general use in analytical
Figure 3. ATR FT-IR spectra for pentafluorobenzaldehyde semicarbazone,from lo00 to 1800 cm-l (superior media, n-hexane and air).
vibrational spectroscopy for both inorganic and organic powdered materials. Obviously, it is of particular value for less intense peaks (for example, the carbonate outof-plane bending mode).
Acknowledgment. S.L.C. was supported by a 1991 Undergraduate Summer Research Assistantship from the Natural Sciences & Engineering Research Council of Canada (NSERC). The purchase cost of the Contact Sampler was supported in part through two awards to B.M.L.: 1991-1992Research Partnership Grant No. CRD 101598from NSERC; 1991-1992 Research Agreement No. 122 2 91 from Energy, Mines, and Resources Canada. We acknowledge reassuring discussions with John Reffner of Spectra-Tech, Inc., and with Jim Harrick of Harrick Scientific Corporation. Registry No. Albite, 12244-10-9;anhydrite, 1479804-0;barite, 13462-86-7; bytoronite, 82601-67-0; calcite,13397-26-7; celestite, 14291-02-2;labradorite, 12173-78-3;leucite,9011-14-7; silica,763186-9;nephelite, 1302-72-3;oligoclase, 12174-07-1;strontainite, 14941-40-3; willemite,14374-77-7;poly(4methoxystyrene),2493644-5; barium carbonate, 513-77-9; water, 7732-18-5;acetone, 672-propanol,67-63-0; methanol, 64-1;deuterium oxide,7789-20-0; 67-56-1; methyl sulfoxide,67-68-5; 1,1,2,3-tetrachloro-2-propene, 20589-85-9;n-hexane, 110-54-3.
