Confocal Raman Microspectroscopy: Theory and Application to Thin Polymer Samples

Abstract

Raman microspectroscopy can be used effectively to study very small samples or to study small areas within a transparent sample. With the application of the technique of confocal microscopy to a Raman microscope, the depth resolution of the instrument can be enhanced considerably. Confocal microscopy uses a pinhole, placed in the back image plane of the microscope objective, to block light from outside the focal plane. In this way the signal from the small volume element one wants to study can be better separated from the signals arising from the surrounding material. In this paper we show that the performance of the confocal Raman microscope can be described satisfactorily by geometrical optics. Furthermore, we have performed measurements to determine the depth resolution of our system for different combinations of objectives and pinholes. Finally, we report on the applications of this technique to different polymer systems, such as multilayer foils, fibers, and fiber composites.

Confocal Raman Microscopy: Performance, Pitfalls, and Best Practice

“…A rigorous description requires a rather complex mathematical treatment, 12 but the PSF reduces to a simple Lorentzian function under common conditions that are discussed later in this article. In an alternative approach, Tabaksblat et al 13 outlined a geometrical optics calculation of the PSF as a function of aperture size and verified the predictions by making measurements on multilayer polymer systems. For example, they focused a 1003, 0.9NA objective onto a 2 lm poly(ethylene) (PE) film placed on a thick poly(propylene) (PP) substrate.…”

The chemical analysis and replication of polydimethylsiloxane by-products located within electrical trees

“…This kit contains the base compound and a curing agent. In order to make the samples the 2 compounds were mixed at a ratio of 10:1, degassed and cured in a mould at 100 o C. Samples were then cut to size (6x10x15 mm 3 ) and a needle was slowly inserted in order to grow electrical trees.…”

“…Due to the small quantities of the by-products produced in electrical treeing however, this can be challenging. Previously published papers [2] have suggested the application of Raman microprobe spectroscopy (RMS) for this purpose as it has a potential lateral resolution of ~1μm [3] and is therefore able to characterise the exact chemical composition of a material. This method however is not without its flaws and is often used in conjunction with Fourier Transform Infra Red (FTIR) which although does not have the same resolution can help to identify the chemical makeup of the sample in question.…”

Design and first applications of a flexible Raman micro-spectroscopic system for biological imaging

“…the collected light is focused and transmitted through a pinhole located in front of the detector. The pinhole is optically conjugated with the interrogation volume, from which the Raman spectrum is acquired, such as it accepts only the light originating from the location of the laser spot and rejects the rest [49,55]. It is convenient to guide the signal to the spectrom- eter using a multimode optical fiber.…”

“…The confocality of the system is a function of several factors, such as objective lens, tube lens, and the diameter of the detection fiber core. A rigorous assessment of the confocal Raman microscopy can be found elsewhere [19,30,49].…”