Analytical Procedure of Chemical Imaging Technique

Description

Chemical imaging also known as quantitative-chemical mapping is the analytical capability of generating a visual image of component distribution from simultaneous spectral and spatial, time information measurements. Hyper spectral imaging, as opposed to multispectral imaging, measures contiguous spectral bands rather than spaced spectral bands. The main idea is that for chemical imaging, the analyst can take many data spectrum measurements at a specific chemical component in a specific spatial location at the same time, this is useful for chemical identification and quantification. Alternatively, choosing an image plane at a specific data spectrum (PCA-multivariable data of wavelength, spatial location at time) can map the spatial distribution of sample components, assuming that their spectral signatures differ at the chosen data spectrum.

Chemical imaging uses the same fundamentals as vibrational spectroscopic techniques, but adds information by simultaneously acquiring spatially resolved spectra. It combines the benefits of digital imaging with the characteristics of spectroscopy. In a nutshell, vibrational spectroscopy studies the interaction of light with matter. When photons interact with a sample, they are either absorbed or scattered. Specific energy photons are absorbed, and the pattern of absorption provides information, or a fingerprint, on the molecules present in the sample. Chemical imaging, on the other hand, can be performed in one of the following observation modes: Optical absorption, emission (fluorescence), transmission, or scattering (Raman). The fluorescence (emission) and Raman scattering modes are currently thought to be the most sensitive and powerful, but also the most expensive.

Transmission measurements involve radiation passing through a sample and being measured by a detector placed on the opposite side of the sample. The amount of energy transferred from incoming radiation to the molecules can be calculated as the difference between the number of photons emitted by the source and the number measured by the detector. The same energy difference measurement is performed in a diffuse reflectance measurement, but the source and detector are located on the same side of the sample, and the photons measured have re-emerged from the illuminated side of the sample rather than passing through it. The energy can be measured at one or more wavelengths, after a series of measurements, the response was calculated.

A critical aspect of acquiring spectra is that the radiation must be energy selected in some way, either before or after interacting with the sample. A fixed filter, tunable filter, spectrograph, interferometer, or other device can be used to select a wavelength. It is inefficient to collect a significant number of wavelengths using a fixed filter approach, so multispectral data is typically collected. Interferometer-based chemical imaging necessary the collection of entire spectral ranges, resulting in hyper spectral data. Tunable filters can provide multi or hyper spectral data, depending on the analytical requirements.

Hyper spectral imaging is most often applied to either solid or gel samples, and has applications in chemistry, biology, medicine, pharmacy, food science, biotechnology, agriculture and industry. Chemical imaging using NIR, IR, and Raman is also known as hyper spectral, spectroscopic, spectral, or multispectral imaging. Other ultra-sensitive and selective imaging techniques, such as UV-visible or fluorescence micro spectroscopy are also in use. In biology and medicine, many imaging techniques can be used to analyse samples of all sizes, from the single molecule to the cellular level.

Any material whose functionality is dependent on chemical gradients may be amenable to investigation using an analytical technique that combines spatial and chemical characterization. As manufactured materials become more complex, the demand for this type of analysis increases. Chemical imaging techniques are critical for understanding modern manufactured products and, in some cases, are non-destructive, preserving samples for further testing.

The spatial distribution of sample components determines the functionality of many materials, both manufactured and naturally occurring. Extended release pharmaceutical formulations, for example, can be achieved by using a barrier layer coating. The presence of this barrier controls the release of active ingredient, and imperfections in the coating, such as discontinuities, may result in altered performance. Imperfections or contaminants in silicon wafers or printed microcircuits can cause these components to fail in the semi-conductor industry. Chemical gradients also play a role in the functionality of biological systems, a single cell, tissue, or even whole organs function due to the precise arrangement of components. It has been demonstrated that even minor changes in chemical composition and distribution can serve as an early warning sign of disease.