Rapid-scan Spectra Research Articles

Anti-oxidative reactivity and mechanism of formic hydrazide, acethydrazide and butyrohydrazide towards reduction of single electron oxidant iridium(IV)

Reduction of a well-known single electron oxidant [IrCl6]2– by aliphatic hydrazides, viz. formic hydrazide, acethydrazide and butyrohydrazide, was investigated by use of several techniques including 1H NMR and rapid scan spectroscopies. It was demonstrated that the redox reactions exclusively follow second-order kinetics over a wide pH range; the observed second-order rate constants k' versus pH profiles have been established. When the features revealed by the rapid scan spectra, the reaction stoichiometry and the oxidation products were considered together, a reaction mechanism was proposed involving three parallel rate-determining steps in which [IrCl6]2– reacted with all the three protolytic forms of the hydrazides. Rate constants of the rate-determining steps and the dissociation constants of hydrazides at 25.0 °C and 1.0 M ionic strength were derived from the kinetic data. Activation parameters were determined for the reactions of [IrCl6]2– with the neutral forms of the hydrazides, and an outer-sphere electron transfer mode was implied. The reactivity of each protolytic form of the hydrazides was elucidated and its distribution versus pH was calculated. The reactivity of the protolytic forms was compared with that of the corresponding forms of aryl hydrazides, revealing that both the neutral and enolate forms of aliphatic hydrazides are less reactive than those of the aryl hydrazides towards reduction of [IrCl6]2–. It was concluded that aryl hydrazides are more important than aliphatic ones in the development of new hydrazide-based antioxidants in the future. Additionally, the protolysis equilibria of these aliphatic hydrazides in aqueous solution and their redox properties have been characterized and the dissociation constants in the case of butyrohydrazide are reported for the first time.

Background removal from rapid-scan EPR spectra of nitroxide-based spin labels by minimizing non-quadratic cost functions

Rapid-scan electron paramagnetic resonance spectroscopy is an emerging technique which substantially improves the signal-to-noise ratio and time resolution compared to conventional continuous-wave experiments. This allows the investigation of spin-labeled biomolecules and their structural dynamics on much shorter time scales than usually accessible. The EPR spectrum however is superimposed by a strong background that is caused by microphonic effects of the alternating magnetic field. This article discusses the use of non-quadratic cost functions for background removal of rapid-scan spectra. The method is validated for the most prominent type of spin-probes in the field of biochemistry: the nitroxide spin-label.

Photoacoustic Detection of Weak Absorption Bands in Infrared Spectra of Calcite.

Photoacoustic spectroscopic detection of infrared absorption often produces spectra with enhanced intensities for weaker peaks, enabling the detection of features due to overtones and combinations, as well as less-abundant isotopic species. To illustrate this phenomenon, we present and discuss photoacoustic infrared spectra of calcite. We use linearization of rapid-scan spectra, as well as comparing step-scan and rapid-scan spectra, to demonstrate that saturation is not the driving force behind these enhanced intensities. Our results point to a significant knowledge gap, since a theoretical basis for the enhancement of these weak bands has not yet been developed.

Intracellular Protein–Lipid Interactions Studied by Rapid-Scan Electron Paramagnetic Resonance Spectroscopy

Protein–membrane interactions play key roles in essential cellular processes; studying these interactions in the cell is a challenging task of modern biophysical chemistry. A prominent example is the interaction of human α-synuclein (αS) with negatively charged membranes. It has been well-studied in vitro, but in spite of the huge amount of lipid membranes in the crowded environment of biological cells, to date, no interactions have been detected in cells. Here, we use rapid-scan (RS) electron paramagnetic resonance (EPR) spectroscopy to study αS interactions with negatively charged vesicles in vitro and upon transfection of the protein and lipid vesicles into model cells, i.e., oocytes of Xenopus laevis. We show that protein–vesicle interactions are reflected in RS spectra in vitro and in cells, which enables time-resolved monitoring of protein–membrane interaction upon transfection into cells. Our data suggest binding of a small fraction of αS to endogenous membranes.

Oxidations of Benzhydrazide and Phenylacetic Hydrazide by Hexachloroiridate(IV): Reaction Mechanism and Structure-Reactivity Relationship.

Benz(o)hydrazide (BH) is the basic aryl hydrazide; aryl hydrazides have been pursued in the course of drug discovery. Oxidations of BH and phenylacetic hydrazide (PAH) by hexachloroiridate(IV) ([IrCl6]2−) were investigated by use of stopped-flow spectral, rapid spectral scan, RP-HPLC and NMR spectroscopic techniques. The oxidation reactions followed well-defined second-order kinetics and the observed second-order rate constant k′ versus pH profiles were established over a wide pH range. Product analysis revealed that BH and PAH were cleanly oxidized to benzoic acid and phenylacetic acid, respectively. A reaction mechanism was proposed, resembling those suggested previously for the oxidations of isoniazid (INH) and nicotinic hydrazide (NH) by [IrCl6]2−. Rate constants of the rate-determining steps were evaluated, confirming a huge reactivity span of the protolysis species observed previously. The enolate species of BH is extremely reactive towards reduction of [IrCl6]2−. The determined middle-ranged negative values of activation entropies together with rapid scan spectra manifest that an outer-sphere electron transfer is probably taking place in the rate-determining steps. The reactivity of neutral species of hydrazides is clearly not correlated to the corresponding pKa values of the hydrazides. On the other hand, a linear correlation, logkenolate = (0.16 ± 0.07)pKenol + (6.1 ± 0.8), is found for the aryl hydrazides studied so far. The big intercept and the small slope of this correlation may pave a way for a rational design of new antioxidants based on aryl hydrazides. The present work also provides the pKa values for BH and PAH at 25.0 °C and 1.0 M ionic strength which were not reported before.

Kinetic and mechanistic analysis of oxidation of 2-furoic hydrazide by hexachloroirradate(IV) in a wide pH range

Oxidation of 2-furoic hydrazide (FH) by hexachloroiridate(IV) ([IrCl6]2−) was studied kinetically in a wide pH range in aqueous solution of 1.0 M ionic strength. The oxidation reaction followed well-defined second-order kinetics: − d[IrCl62−]/dt = k′[FH]tot[IrCl62−], where [FH]tot denotes the total concentration of FH and k′ stands for the observed second-order rate constants. The established k′–pH profile displays that k′ increases drastically with pH and a plateau region exists between pH 4 and 6. A stoichiometric ratio of Δ[FH]tot/Δ[IrCl62−] = 1/4 was revealed by spectrophotometric titrations. 1H NMR spectroscopic studies indicated that FH was cleanly oxidized to 2-furoic acid. The kinetic data suggest a reaction mechanism in which all the three protolysis species of FH react with [IrCl6]2− in parallel, forming the rate-determining steps. Two stabilized hydrazyl radicals are generated in the rate-determining steps, in which a single electron is transferred to [IrCl6]2−. The two hydrazyl radicals react rapidly in consecutive steps requiring 3 mol of Ir(IV) to form 2-furoic acid as the final product. Rate constants of the rate-determining steps were deduced through a simulation of the rate expression to the k′–pH dependency data. Values of these rate constants demonstrate that the three protolysis species of FH have a huge reactivity span, changing by about 109 times toward reduction in [IrCl6]2− and that FH can be readily oxidized in neutral and basic media. Rapid scan spectra and the measured activation parameters suggest that an outer-sphere electron transfer is probably taking place in each of the rate-determining steps. This is the first kinetic study on the oxidation reactions of FH and provides concurrently the protolysis constants of FH (pKa1 = 3.04 ± 0.08 and pKa2 = 11.6 ± 0.1) at 25.0 °C and 1.0 M ionic strength.

Application of modulation excitation-phase sensitive detection-DRIFTS forin situ/operando characterization of heterogeneous catalysts

A new more general method and guidelines for the implementation of modulation excitation-phase sensitive detection-diffuse reflectance Fourier transform spectroscopy (ME-PSD-DRIFTS).

Design, modelling, and application of a low void-volumein situdiffuse reflectance spectroscopic reaction cell for transient catalytic studies

A new low void-volumein situreaction cell enables application of modulation excitation-phase sensitive detection-diffuse reflectance Fourier transform spectroscopy (ME-PSD-DRIFTS).

New kinetic and mechanistic findings in the oxidation of hydroxylamine by Cerium(IV) in perchloric acid media

AbstractThe oxidation of hydroxylamine (HONH2) by Ce(IV) in perchloric acid media took place fast and was followed by a stopped‐flow spectrometer. A rate law of −d[Ce(IV)]tot/dt = k′[HONH2]tot[Ce(IV)]tot was proved, where k′ stands for the observed second‐order rate constant and [Ce(IV)]tot and [HONH2]tot represent the total concentrations of Ce(IV) and HONH2, respectively. The dependencies of k′ on [H+] and on ionic strength were investigated. Rapid scan spectra recorded for a typical reaction course and a stoichiometric measurement unraveled several additional characters for the redox reaction. Based on experimental results, a reaction mechanism is proposed, which involves Ce4+, Ce(OH)3+, and Ce(OH)22+ attacking on HONH3+ in parallel. Moreover, these parallel attacking reactions are the rate‐determining steps, generating free radicals •ONH2, which are rapidly oxidized to nitrate by Ce(IV). Rate constants of these rate‐determining steps have been evaluated; a reactivity trend of Ce4+ > Ce(OH)3+ > Ce(OH)22+ is disclosed, but the reactivity differences among them are very small. The rate‐determining steps are rationalized in terms of an outer‐sphere electron transfer, and possible transition states are proposed in the cases of Ce(OH)3+ and Ce(OH)22+ to account for the small reactivity differences. The obtained kinetic and mechanistic results in the present work are compared with those acquired previously for the same reaction but in sulfuric acid and nitric media. It is concluded that the reaction media can play a dominant role in determining the reaction kinetics, mechanisms, and the product(s) in the oxidation of hydroxylamine by Ce(IV).

Kinetics and mechanism for reduction of Pt(IV) anticancer model compounds by Se-methyl L-selenocysteine. Comparison with L-selenomethionine

Se-methyl L-selenocysteine (MeSeCys) is one of the major organic selenium compounds acquired from the diet by human beings. It has been shown to have anticancer activity and cancer prevention functions. However, its antioxidant activity, largely related to its biological function, has not been well characterized so far. We here report a stopped-flow kinetic study of the reduction of the Pt(IV) anticancer model compounds trans-[PtX2(CN)4]2− (X = Cl; Br) by MeSeCys in a wide pH range. Overall second-order kinetics is established for the redox reactions, and spectrophotometric titrations indicate a 1:1 reaction stoichiometry. The MeSeCys is oxidized to its selenoxide form, as identified by high-resolution mass spectra. The proposed reaction mechanism involves parallel attack on one of the trans-coordinated halides of the Pt(IV) complexes by the selenium atom of the various MeSeCys protolytic species. Rate constants for the rate determining steps as well as the pKa values of the various protolytic species of MeSeCys have been determined at 25.0 °C and 1.0 M ionic strength. A bridged two-electron transfer mechanism for the rate-determining steps is supported by rapid-scan spectra, activation parameters, and by the much larger reaction rate of [PtBr2(CN)4]2− compared to [PtCl2(CN)4]2−. The experiments indicate that the reduction of [PtX2(CN)4]2− by MeSeCys proceeds via a similar reaction mechanism as L-selenomethionine (SeMet) studied previously. However, there is a large reactivity difference between these two selenium compounds, as a matter of fact the largest one observed so far when compared to other redox systems. It differs between the various protolytic species of MeSeCys and SeMet. The different reactivity of MeSeCys and SeMet in the reduction of various biologically relevant oxidants might account for their disparate efficacies as anticancer agents.

Oxidations of hydrazine and substituted hydrazines by hexachloroiridate(IV) in aqueous solution: Kinetic and mechanistic analyses

Oxidations of hydrazine (H2NNH2) and substituted hydrazines (t-C4H9HNNH2 and c-C6H11HNNH2) by [IrCl6]2− are found to obey overall second-order kinetics: −d[IrCl62−]/dt=k′[RHNNH2]tot[IrCl62−], where k′ pertains to the observed second-order rate constants and [RHNNH2]tot stands for the total concentration of hydrazines. The k′ versus pH profiles have been established over a pH range from 3 to 10. Rapid scan spectra recorded for the reaction courses indicate that there are no fast substitutions on [IrCl6]2− and/or strong associations between [IrCl6]2− and hydrazines during the reactions. The determined ratios of Δ[Ir(IV)]/Δ[RHNNH2]tot clearly endow a 4:1 stoichiometry for all the hydrazines studied. The oxidation product has been identified by NMR spectroscopy in the case of t-C4H9HNNH2. The proposed reaction mechanisms involve parallel attack of [IrCl6]2− on RHNNH3+ and RHNNH2, forming the rate-determining steps and producing hydrazyl free radicals; these are followed by several fast reactions. Rate equations have been derived, and the rate constants of the rate-determining steps at 25.0°C and 1.0M ionic strength have been calculated. It is demonstrated that our kinetic analysis is a good approach for determination of the pKa values of hydrazines. Monoalkyl substitutions on hydrazine enhance the reactivity of substituted hydrazines significantly, but the reactivity has no correlation with the pKa values. All the kinetic and spectral attributes bestow that an outer-sphere electron transfer takes place in the rate-determining steps. Moreover, all the kinetic and mechanistic discrepancies for the hydrazine-[IrCl6]2− reaction remained in literature have been clarified in the present work.

Oxidation of the drug tiopronin by Cerium(IV) in perchloric acid media: Kinetic and mechanistic analyses

Although several analytical methods based on the Ce(IV) oxidations were developed for determinations of the drug tiopronin (MPG) and some biothiols, the kinetic and mechanistic aspects of oxidations of the thiol-containing molecules by Ce(IV) were poorly deciphered if not all. The oxidation of MPG by Ce(IV) has thus been characterized kinetically in perchloric acid media in this work. The oxidation reaction follows overall second-order kinetics: -d[Ce(IV)]tot/dt=k′[MPG]tot[Ce(IV)]tot, where k′ stands for the observed second-order rate constant and [Ce(IV)]tot and [MPG]tot pertain to the total concentrations of Ce(IV) and MPG, respectively. The relation between k′ and [H+] has been established in the region of 0.030M≤[H+]≤2.00M. Several additional reaction features are revealed by the rapid scan spectra, product identification and stoichiometric study. All these features are rationalized by a proposed reaction mechanism which involves Ce4+, Ce(OH)3+, and Ce(OH)22+ attacking on the sulfur atom of MPG in parallel and forming the rate-determining steps. These steps are followed by up to seven fast reactions. Rate constants of the rate-determining steps have been derived showing a reactivity trend: Ce4+>Ce(OH)3+>Ce(OH)22+. This trend, being observed for the first time in this work, suggests an outer-sphere electron transfer taking place in the rate-determining steps, which is strongly bolstered by the dependence of the Ce(IV)/Ce(III) redox potential on [H+] measured previously. Our kinetic and mechanistic analyses indicate that a rather complex reaction mechanism is operating behind the simple second-order kinetics.

Stopped-flow kinetic analysis of the oxidation of semicarbazide by hexachloroiridate(IV)

A kinetic analysis of the oxidation of semicarbazide (SEM) by the single-electron oxidant [IrCl6]2− has been carried out by stopped-flow spectrometric techniques. The reaction proved to be first order each in [IrCl6 2−] and [SEM]tot, leading to overall second-order kinetics. The variation in the observed second-order rate constant k′ with pH was explored over the pH range of 0–7.11. Spectrophotometric titration revealed a stoichiometry of Δ[IrCl6 2−]/Δ[SEM]tot = 4:1 for the redox reaction. On the basis of the rate law, the redox stoichiometry, and the rapid scan spectra, a reaction mechanism is proposed which involves parallel attacks of [IrCl6]2− on both H2NCONHNH3 + and H2NCONHNH2 as rate-determining steps, followed by several rapid reactions. The rate expression, derived from the reaction mechanism, was utilized to simulate the k′–pH profile yielding a virtually perfect fit and indicating that the reaction path involving H2NCONHNH3 + does not make a significant contribution to the overall rate. The reaction between [IrCl6]2− and H2NCONHNH2 was further studied as a function of both temperature and ionic strength. From the temperature dependence, activation parameters were obtained as: ∆H 2 ‡ = 34.9 ± 1.5 kJ mol−1 and ∆S 2 ‡ = −78 ± 5 J K−1 mol−1. The observed ionic strength dependence suggests that the rate-determining step is between [IrCl6]2− and a neutral species of SEM. Hence, both the temperature and ionic strength dependency studies are in good agreement with the proposed reaction mechanism, in which the rate-determining step involves an outer sphere electron transfer.

L-selenomethionine reduces platinum(IV) anticancer model compounds at strikingly faster rates than L-methionine.

L-Selenomethionine (SeMet), the predominant form of selenium acquired from the diet by humans, has been used as a supplement, and exhibit some important functions like cancer prevention and antioxidative defense. Its interactions with Pt(II) anticancer drugs have been characterized, but its redox reactions with platinum(IV) anticancer prodrugs have not been exploited. In this work, the oxidation of SeMet by Pt(IV) anticancer model compounds trans-[PtX2(CN)4](2-) (X = Cl, Br) was characterized. A stopped-flow spectrometer was used to record the rapid scan spectra and to follow the reaction kinetics over a wide pH range. An overall second-order rate law was derived: -d[Pt(IV)]/dt = k'[Pt(IV)][SeMet], where k' pertains to the observed second-order rate constants. The k'-pH profiles showed that k' increased only about 6 times even though the solution pH was varied from 0.25 to 10.5. The redox stoichiometry was determined as Δ[Pt(IV)]/Δ[SeMet] = 1 : (1.07 ± 0.07), suggesting that SeMet was oxidized to selenomethionine selenoxide. The selenoxide together with its hydrated form was identified explicitly by high resolution mass spectral analysis. A reaction mechanism was proposed which encompassed three parallel rate-determining steps relying on the protolytic species of SeMet. Rate constants of the rate-determining steps were obtained from the simulations of the k'-pH profiles. Activation parameters were determined for the reactions of the zwitterionic form of SeMet with the Pt(IV) complexes. A bridged electron transfer process is delineated in the rate-determining steps and several lines of evidence support the bridged electron transfer mode. Strikingly, reduction of [PtX2(CN)4](2-) by SeMet is 3.7 × 10(3)-5.7 × 10(4) times faster than that by L-methionine. Some potential biological consequences resulting from the strikingly fast reduction are discussed.

X-band rapid-scan EPR of samples with long electron spin relaxation times: a comparison of continuous wave, pulse and rapid-scan EPR

X-band room temperature spectra obtained by rapid-scan, continuous wave, field-swept echo-detected and Fourier transform electron paramagnetic resonance (FTEPR) were compared for three samples with long electron spin relaxation times: amorphous hydrogenated silicon (T1 = 11 μs, T2 = 3.3 μs), 0.2% N@C60 solid (T1 = 120–160 μs, T2 = 2.8 μs) and neutral single substitutional nitrogen centres (NS0) in diamonds (T1 = 2300 μs, T2 = 230 μs). For each technique, experimental parameters were selected to give less than 2% broadening of the lineshape. For the same data acquisition times, the signal-to-noise for the rapid-scan spectra was one-to-two orders of magnitude better than for continuous wave or field-swept echo-detected spectra. For amorphous hydrogenated silicon, T2* (∼ 10 ns) is too short to perform FTEPR. For 0.2% N@C60, the signal-to-noise ratio for rapid scan is about five times better than for FTEPR. For NS0 the signal-to-noise ratio is similar for rapid scan and FTEPR.

X-band rapid-scan EPR of nitroxyl radicals

X-band rapid-scan EPR spectra were obtained for dilute aqueous solutions of nitroxyl radicals 15N-mHCTPO (4-hydro-3-carbamoyl-2,2,5,5-tetra-perdeuteromethyl-pyrrolin-1- 15N-oxyl-d 12) and 15N-PDT (4-oxo-2,2,6,6-tetra-perdeuteromethyl-piperidinyl- 15N-oxyl-d 16). Simulations of spectra for 15N-mHCTPO and 15N-PDT agreed well with the experimental spectra. As the scan rate is increased in the rapid scan regime, the region in which signal amplitude increases linearly with B 1 extends to higher power and the maximum signal amplitude increases. In the rapid scan regime, the signal-to-noise for rapid-scan spectra was about a factor of 2 higher than for unbroadened CW EPR, even when the rapid scan spectra were obtained in a mode that had only 4% duty cycle for data acquisition. Further improvement in signal-to-noise per unit time is expected for higher duty cycles. Rapid scan spectra have higher bandwidth than CW spectra and therefore require higher detection bandwidths at faster scan rates. However, when the scan rate is increased by increasing the scan frequency, the increase in noise from the detection bandwidth is compensated by the decrease in noise due to increased number of averages per unit time. Because of the higher signal bandwidth, lower resonator Q is needed for rapid scan than for CW, so the rapid scan method is advantageous for lossy samples that inherently lower resonator Q.

Electron Spin Relaxation and Heterogeneity of the 1:1 α,γ-Bisdiphenylene-β-phenylallyl (BDPA)/Benzene Complex

The electron spin-spin relaxation time (T(2)) for the 1:1 crystalline complex of α,γ-bisdiphenylene-β-phenylallyl (BDPA) with benzene was determined by continuous wave (CW) and rapid scan electron paramagnetic resonance (EPR). T(2) for individual BDPA particles found by simulation of rapid scan spectra or by simulation of the Lorentzian line shapes of CW spectra were in good agreement. The T(2) for small BDPA particles in air ranged from 80 to 160 ns, which corresponds to peak-to-peak Lorentzian linewidths of 0.82-0.41 G. The removal of oxygen from the samples had a greater impact on the line width for particles that had shorter T(2) in air. Heterogeneity in the g-value was not observed at X-band. Scanning electron microscope (SEM) images showed that the BDPA particles had varying morphology.

Comparison of continuous wave, spin echo, and rapid scan EPR of irradiated fused quartz

The E' defect in irradiated fused quartz has spin lattice relaxation times (T(1)) about 100 to 300 μs and spin-spin relaxation times (T(2)) up to about 200 μs, depending on the concentration of defects and other species in the sample. These long relaxation times make it difficult to record an unsaturated continuous wave (CW) electron paramagnetic resonance (EPR) signal that is free of passage effects. Signals measured at X-band (~9.5 GHz) by three EPR methods: conventional slow-scan field modulated EPR, rapid scan EPR, and pulsed EPR, were compared. To acquire spectra with comparable signal-to-noise, both pulsed and rapid scan EPR require less time than conventional CW EPR. Rapid scan spectroscopy does not require the high power amplifiers that are needed for pulsed EPR. The pulsed spectra, and rapid scan spectra obtained by deconvolution of the experimental data, are free of passage effects.

The S1YZ• Metalloradical EPR Signal of Photosystem II Contains Two Distinct Components That Advance Respectively to the Multiline and g = 4.1 Conformations of S2

The S2 state of the oxygen-evolving complex (OEC) of photosystem II is heterogeneous, exhibiting two main EPR spectral forms, the multiline and the g = 4.1 signal. It is not clearly established whether this heterogeneity develops during the S1 to S2 transition or is already present in the precursor states. We have compared the spectra of the S1YZ* intermediate, obtained by visible light excitation (induction of charge separation) of the S1 state at liquid He temperatures, (S1YZ*)vis, or by near-infrared (NIR) light excitation of the S2 state (utilization of the unusual property of the Mn cluster to act as an oxidant of Yz when excited by NIR), (S1YZ*)NIR. The decay kinetics of the (S1YZ*)vis spectrum at 11 K was also studied by the application of rapid-scan EPR. The two spectra share in common a signal with a characteristic feature at g = 2.035, but the (S1YZ*)vis spectrum contains in addition a fast decaying component 26 G wide. The analysis of the surface of the rapid-scan spectra yielded 270 +/- 35 and 90 +/- 15 s for the respective half-times of the two components of the (S1YZ*)vis spectrum at 11 K. (S1YZ*)vis advances efficiently to S2 when annealed at 200 K; notably the g = 2.035 signal advances to the multiline while the 26 G component advances to the g = 4.1 conformation. The "26 G" component is absent or very small, respectively, in thermophilic cyanobacteria or glycerol-containing spinach samples, in correlation to vanishing or very small amounts of the g = 4.1 component in the S2 spectrum. The results validate the assignment of S1YZ* to a true S1 to S2 intermediate and imply that the heterogeneity observed in S2 is already present in S1. Tentative valences are assigned to the individual Mn ions of the OEC in the two heterogeneous conformations of S1.

Photoacoustic infrared spectroscopy of Syncrude post-extraction oil sand

Rapid- and step-scan photoacoustic (PA) infrared spectra of three fractions of a Syncrude post-extraction oil sand were analyzed in detail in this work. The rapid-scan spectra showed that the samples were comprised primarily of kaolinite, quartz, silica, siderite, and residual hydrocarbons, and that the proportions of these constituents were different for each fraction. Depth profiling of the three post-extraction oil sands was accomplished using both rapid- and step-scan PA infrared spectroscopy. The results confirmed that kaolinite is more abundant in the near-surface region, whereas quartz and hydrocarbons are concentrated at greater depths. The modulation frequency dependence of the PA intensities for all three fractions was consistent with a model in which the samples are thermally thick; in other words, the thermal diffusion length (roughly equal to the sampling depth) was less than the particle sizes of all three samples. The results of this study are consistent with published reports on the PA infrared spectra of fine tailings generated during bitumen extraction and the spectroscopic and thermophysical characterization of clay soils and an appropriate model clay.