A two-dimensional (2D) correlation method generally applicable to various types of spectroscopy, including IR and Raman spectroscopy, is introduced In the proposed 2D correlation scheme, an external perturbation is applied to a system while being monitored by an electromagnetic probe With the application of a correlation analysis to spectral intensity fluctuations induced by the perturbation, new types of spectra defined by two independent spectral variable axes are obtained Such two-dimensional correlation spectra emphasize spectral features not readily observable in conventional one-dimensional spectra While a similar 2D correlation formalism has already been developed in the past for analysis of simple sinusoidally varying IR signals, the newly proposed formalism is designed to handle signals fluctuating as an arbitrary function of time, or any other physical variable This development makes the 2D correlation approach a universal spectroscopic tool, generally applicable to a very wide range of applications The basic property of 2D correlation spectra obtained by the new method is described first, and several spectral data sets are analyzed by the proposed scheme to demonstrate the utility of generalized 2D correlation spectra Potential applications of this 2D correlation approach are then explored

Generalized Two-Dimensional Correlation Method Applicable to Infrared, Raman, and other Types of Spectroscopy:

A novel concept in vibrational spectroscopy called two-dimensional infrared (2D IR) spectroscopy is described. In 2D IR, a spectrum defined by two independent wavenumbers is generated by a cross-correlation analysis of dynamic fluctuations of IR signals induced by an external perturbation. 2D IR spectra are especially suited for elucidating various chemical interactions among functional groups. Notable features of the 2D IR approach are: simplification of complex spectra consisting of many overlapped peaks; enhancement of spectral resolution by spreading peaks over the second dimension; and establishment of unambiguous assignments through correlation analysis of bands selectively coupled by various interaction mechanisms. The procedure for generating 2D IR correlation spectra and the properties of the 2D spectra are discussed in detail. Examples of 2D IR spectra are presented for atactic polystyrene and the proteinacious component of human stratum corneum to demonstrate the utility of this technique.

Two-Dimensional Infrared (2D IR) Spectroscopy: Theory and Applications

Publisher Summary Generalized two-dimensional (2D) correlation spectroscopy is described to show how this technique can be applied to a very broad range of spectral analysis problems. In this spectroscopy, the underlying similarity or dissimilarity among systematic variations in spectroscopic signal intensities is examined. It develops parallel to multiple pulse-based 2D spectroscopy techniques and employs a somewhat different approach to constructing 2D spectra. The signal variations are induced by an external perturbation or physical stimulus applied to the sample, and the pattern of the intensity variation is systematically analyzed by a simple cross-correlation technique. The correlation intensities thus obtained are displayed in the form of a 2D map defined by two independent spectral axes for further analysis. Such a 2D map is referred to as a 2D correlation spectrum, even though many of today's 2D correlation studies include applications in the field outside of spectroscopy, such as chromatography and microscopy. An illustrative example is given for the analysis of attenuated total reflection (ATR) IR spectra collected during the crystallization of biodegradable polymer poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) to demonstrate the merits of this technique, such as the enhancement of spectral resolution, detection of coordinated changes among spectral signals, and determination of relative directions and sequential order of intensity variations.

https://journals.sagepub.com/doi/pdf/10.1366/0003702001950454

Generalized Two-Dimensional Correlation Spectroscopy

Developments in mid-infrared FT-IR spectroscopy of selected carbohydrates

Pharmaceutical applications of Mid-IR and Raman spectroscopy.

The application of step-scan interferometry to two-dimensional infrared (2D IR) spectroscopy is described. In this 2D FT-IR experiment, a step-scan interferometer is used to study a system undergoing dynamic changes induced by an external perturbation. Because step-scanning removes the spectral multiplexing from the temporal domain, the time dependence of the sample response to the perturbation can be retrieved more conveniently, in comparison to conventional rapid-scan techniques. Time-resolved IR data are then converted to 2D IR correlation spectra. Peaks located on a 2D spectral plane provide information about interactions among various functional groups associated with the IR bands. In the step-scan mode, the FT-IR multiplex advantage is retained; thus, spectral regions far removed from each other can be correlated with the use of 2D analysis from a single scan. 2D FT-IR spectra for a composite film of isotactic polypropylene and poly(γ-benzyl-L-glutamate) subjected to a small-amplitude sinusoidal strain are presented. The 2D FT-IR spectra clearly differentiate bands arising from the polyolefin and polypeptide. Overlapped bands are deconvoluted into individual components on the 2D spectral plane due to their different dynamic behavior. The applicability of step-scan 2D FT-IR to a variety of dynamic experiments is discussed.

Application of Step-Scan Interferometry to Two-Dimensional Fourier Transform Infrared (2D FT-IR) Correlation Spectroscopy

Ultra-rapid-scanning Fourier transform infrared spectrometry

This article describes the modification of a commercial Fourier‐transform infrared (FT‐IR) spectrometer for step‐scan operation. Step‐scan operation decouples the FT‐IR spectral multiplexing from time and is therefore applicable to a variety of time‐dependent spectroscopic experiments, including, particularly, photoacoustic and photothermal spectroscopy. The step‐scan instrument described controls the retardation (moving mirror position) with a feedback loop. The loop uses path difference, or phase, modulation of the reference laser intensity, together with lock‐in amplifiers to detect the mirror position. Since the interferogram can be sampled at intervals as small as 1/4 λHeNe, the maximum free‐spectral range is 31 600 cm−1. For initial positioning of the mirror, stepping can be as rapid as 100 Hz. The current software will allow data collection at ∼1.6 Hz, although the mirror settling time of ≤20 ms would allow data to be collected at 20–30 Hz stepping frequency with more efficient software. The mirror...

Step‐scan Fourier‐transform infrared spectrometer

Dynamic rheo-optical spectra of two isotactic polypropylene samples of different states of order were measured by step-scan FT-IR spectrometry. Experimental details and data collection considerations are discussed. The features in the dynamic spectra are found to be due to small wave-number shifts and absorption changes. The dynamic rheo-optical spectra depend strongly on the pretreatment of the samples, as well as on the polarization state of the infrared radiation used for analysis.

Step-Scan Fourier Transform Infrared Study on the Effect of Dynamic Strain on Isotactic Polypropylene

The capabilities of a step-scan Fourier transform spectrometer of obtaining time-resolved spectra are reported. As a demonstration of the method, time-resolved spectra from a pulsed fluorescent lamp are presented. The potential of step-scan interferometry for time-resolved infrared measurements of a variety of transient phenomena is discussed.

Christopher J. Manning

Papers

Application of Step-Scan Interferometry to Two-Dimensional Fourier Transform Infrared (2D FT-IR) Correlation Spectroscopy

Ultra-rapid-scanning Fourier transform infrared spectrometry

Step‐scan Fourier‐transform infrared spectrometer

Step-Scan Fourier Transform Infrared Study on the Effect of Dynamic Strain on Isotactic Polypropylene

Time-resolved spectroscopy using step-scan Fourier transform interferometry