A method and the necessary equations were developed for quantitative spectral analysis of surface-active analytes adsorbed onto attenuated total reflectance (ATR) internal reflection elements (IRE) in the presence of analyte solution. The method allows, for the first time, (1) quantitative determination of the Gibbs surface excess ( ) of analyte adsorbed onto the ATR IRE by use of the correctly derived equations, in the presence of a solution of the analyte, and (2) quantitative ATR determination of bulk solution analyte concentrations when the spectra are dominated by analyte adsorbed to the IRE. Either internal or external reference standards may be used. The results for adsorption of cetylpyridinium chloride (CPC, 0.010 M) onto a ZnSe IRE were validated by comparison with (a) values determined by adsorption of CPC onto ZnSe powder and with (b) literature values determined by Langmuir-Blodgett / IR techniques. Integration of the ATR absorption equation over a step (or ramp) concentration profile at the interface gives an absorbance per reflection equation with a term in de (Harrick's "effective thickness"), representing the bulk solution contribution, and a term in de/dp (dp is Harrick's "penetration depth"), representing the contribution of surface-adsorbed analyte:A/N = ecde + e(2000de/dp)r where c is the bulk analyte concentration, e is the molar analyte absorptivity, and N is the number of ATR reflections. The results demonstrate that spectrophotometers have become sufficiently advanced to allow analysis of surfactants in dilute aqueous solution (0.01 M) using methylene C-H stretching bands, which are nearly obscured by the O-H stretching band of H20.

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A general method for spectroscopic determination of the incidence angle and number of reflections in ATR devices is described. This technique extends the inherent advantages of ATR by simplifying general quantitative spectroscopic measurements. Effects of polarization, beam focusing, errors of alignment, indices of refraction, handling of the optical elements, and surface activity of internal standard are discussed. Measured ATR absorbances are fitted mathematically with the use of incidence angle as a parameter. A standardizing solution, molar absorptivities for that solution, and knowledge of the dimensions of the internal reflection optical element are required. Application of the method to a cylindrical internal reflection (CIR) device gives the averaged incidence angle in the CIR with greater accuracy than is otherwise obtainable. With the use of a side-focusing Fourier transform spectrophotometer, the average incidence angle in the Spectra-Tech, Inc. “Micro-cell” “CIRCLE” CIR was 49.68 degrees ± 0.78%, and the number of solution-sensing internal reflections was 7.52 ± 1.95%.

The separation of tervalent lanthanides (M 3+ ) by centrifugal partition chromatography (CPC) with the extractants 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (HPMBP, HL) and 1-phenyl-3-methyl-4-capryloyl-5-pyrazolone (HPM-CP, HL) in the toluene-water phase pair and the factors influencing the separation efficiencies have been investigated. The CPC efficiencies are mainly limited by the slow dissociation of the M 3+ -acylpyrazolone complexes (ML 3 ) occurring exclusively at the toluene-water interface, as indicated by a direct linear correlation between the reduced plate heights (CETP ck ) and the half-lives (t 1/2 ) of the dissociation of the ML 3 complexes determined by independent kinetic studies. The lanthanide-acylpyrazolone system represents the first example of a separation by multistage countercurrent distribution wherein the efficiencies are mainly limited by interfacial processes analogous to conventional liquid chromatographic systems. Dramatic improvements in the efficiencies of these separations could be obtained by the addition of the neutral surfactant Triton X-100 to the toluene phase and the metallochromic indicator Arsenazo III (AZ) to the aqueous phase. The improvement in the efficiencies with Triton X-100 was due to the increased interfacial areas resulting from the adsorption of the surfactant, and that in the case of AZ was due to the increased interfacial area and interfacial catalysis of the formation and dissociation of the ML 3 complexes. As a result, the addition of AZ provided much higher efficiencies than did the addition of Triton X-100. Further, the addition Triton X-100 did not significantly alter the selectivities of the ligands for M 3+ , but the addition of AZ resulted in much poorer selectivities. Significant differences were observed in the efficiencies of separations with HPMBP and HPMCP under various experimental conditions, which stem from differences in the interfacial dissociation rate constants of ML 3 complexes and the interfacial areas generated, indicating that CPC separation of tervalent lanthanides with acylpyrazolones in the toluene-water phase pair is driven mainly by interfacial processes.