DRAMIŃSKI - Spectroscopy

DRAMIŃSKI NIR-DRAM 100 - Near Infra Red Analyser of grain and flour content
The analyser is an advanced hi-tech device for measuring grain & flour composition by spectral analysis in the near-infrared spectral range.

NIR spectroscopy is the measurement of absorbed light directed on a sample in the wavelength region of 780 to 2500 nm. A non-destructive method of molecular analysis, NIR spectroscopy provides excellent quantitative data and requires little-to-no sample preparation. Some organic compounds, which tend to have complex band structures in this region, are quite identifiable in NIR spectroscopy. NIR spectroscopy can provide information on moisture, protein, fat and starch content. This spectroscopic technique is quite attractive because of its ability to produce consistent results, even as the concentration levels in a sample changes. NIR spectroscopy is commonly used in applications characterizing solid samples in the quality control of process flow and materials transport environments.

NIR spectroscopy can also be used where molecular vibrational analysis is required in the presence of interfering substances, such as glass or plastic containers, which make it ideal to perform final inspections in production lines and the laboratory. Dispersive NIR is the most common technique, which uses a spectrometer and a multi-channel detector to measure and record the NIR spectra.

NIR Spectroscopy has proven to be excellent for food and agricultural industries due to its ability to identify specific chemical bonds, such as N-H, C-H, and O-H. This provides a real quantitative tool for measuring fats, carbohydrates, fiber, proteins and moisture, which are all important measurements in food items.

NIR Spectroscopy also continues to grow as a QA/QC tool in the pharmaceutical and biotechnology markets. The technique can be applied to measure the mixing efficacy or the internal structure of a solid sample, such as a pill. It is suitable for chemists developing new compounds and as an online QA/QC tool for production. Other industries that have also accepted NIR as a satisfactory method include polymers, petrochemical and environmental industries.

The NIR instrument is not “calibrated” like a balance where the readings are merely adjusted up or down to a standard value.

The instrument has to be trained to recognise different products and constituents. This process of “training” is called the calibration procedure and herein lies the secret of success of this revolutionary technology.

For the training, a number of samples are analysed by traditional chemical analytical methods to determine the actual composition of the samples. Each of these samples is further placed in the NIR instrument and the reflectance values from the different wavelengths are obtained. With the aid of a microcomputer and powerful chemometric software the combination of analytical results and reflectance values are transformed to the calibration constants. This software is so powerful that great care must be taken that it does not merely present a statistical solution, but actually supplies a scientific solution that can be verified.

To develop any new calibration or even for maintaining existing calibrations, it is important to first physically source an ideal set of samples. For each product the sourced sample set must include samples that represent as much of the variation of the analytical and nutrient components that can be expected. This set should ideally also contain samples representing the natural variation that can occur. This includes the variation in cultivars, growing areas, growing conditions and growing seasons. Dersjant-Li and Peisker (2005) recently emphasised the large variation in nutritional composition between soya samples collected from different countries or even from different areas within the same country. Once a set of samples that covers most of the variation has been sourced, the majority of calibration software programs have a tool, which then aids in selecting a further sub sample set to prepare the calibration.

Furthermore, universal calibrations often supplied with the purchase of NIR instruments would rarely represent a true reflection of samples from local areas and usually need quite a bit of adjustment. This can be done by adding a number of carefully selected samples from a specific local product to the existing calibration data.

The scattered reflected and/or transmitted rays of each wavelength are concentrated onto a measuring cell. A number of reflections at different wavelengths are measured and then converted to analytical results by a microprocessor.

There is often a misunderstanding of the term NIR Reflectance. The rays are not merely reflected from the outside surface, but actually penetrate the sample. Each time a chemical bond is encountered that does not absorb the particular wavelength, the rays are scattered and reflected in all directions. These scattered beams may then be absorbed or reflected by other chemical bonds until a portion of the rays eventually exits the sample in all directions (Figure 5). The depth of penetration of the beam into the sample is not determined by the position of the detector, but rather by the strength of the light source.

The word “spectroscopy” is derived from the Latin root spectrum (appearance, image) and the Greek word skopia (to view). This definition is rather descriptive of the spectroscopic measurement itself i.e. to view a light image coming from a specimen (Miller, 2001).

In essence, NIR technology involves light interacting with matter where electromagnetic radiation occurs in the form of waves. The wavelength of a wave is the distance between the two peaks or high points and is indicated by the symbol λ (Shadow, 2000; Figure 1). A wavelength in the NIR spectrum is normally measured in nanometer (nm) where

1 nm = 10-9 m or 1000 nm = .001 mm.

That part of the spectrum visible to the human eye extends from about 400 nm to 800 nm, while the infrared spectrum extends from about 2 500 nm to 25 000 nm. Near infrared is considered as that part of the spectrum lying between the visible region and the infrared region. The range of wavelengths NIR covers are from 750 nm to 2 600 nm (Figure 2).

On the other hand, what would happen if we tried to reverse this process? That is, what would happen if we fired this special photon back into a ground state atom? That’s right, the atom could absorb that `specially-energetic’ photon and would become excited, jumping from the ground state to a higher energy level. If a star with a `continuous’ spectrum is shining upon an atom, the wavelengths corresponding to possible energy transitions within that atom will be absorbed and therefore an observer will not see them. In this way, a dark-line absorption spectrum is born

Unlike a continuous spectrum source, which can have any energy it wants (all you have to do is change the temperature), the electron clouds surrounding the nuclei of atoms can have only very specific energies dictated by quantum mechanics. Each element on the periodic table has its own set of possible energy levels, and with few exceptions the levels are distinct and identifiable.
Atoms will also tend to settle to the lowest energy level (in spectroscopist’s lingo, this is called the ground state). This means that an excited atom in a higher energy level must `dump’ some energy. The way an atom `dumps’ that energy is by emitting a wave of light with that exact energy.

In the diagram below, a hydrogen atom drops from the 2nd energy level to the 1st, giving off a wave of light with an energy equal to the difference of energy between levels 2 and 1. This energy corresponds to a specific color, or wavelength of light — and thus we see a bright line at that exact wavelength!

Discrete spectra are the observable result of the physics of atoms. There are two types of discrete spectra, emission (bright line spectra) and absorption (dark line spectra). Let’s try to understand where these two types of discrete spectra.

Continuous spectra arise from dense gases or solid objects which radiate their heat away through the production of light. Such objects emit light over a broad range of wavelengths, thus the apparent spectrum seems smooth and continuous. Stars emit light in a predominantly (but not completely!) continuous spectrum. Other examples of such objects are incandescent light bulbs, electric cooking stove burners, flames, cooling fire embers and… you. Yes, you, right this minute, are emitting a continuous spectrum — but the light waves you’re emitting are not visible — they lie at infrared wavelengths (i.e. lower energies, and longer wavelengths than even red light). If you had infrared-sensitive eyes, you could see people by the continuous radiation they emit!

Typically one can observe two distinctive classes of spectra: continous and discrete. For a continuous spectrum, the light is composed of a wide, continuous range of colors (energies). With discrete spectra, one sees only bright or dark lines at very distinct and sharply-defined colors (energies). As we’ll discover shortly, discrete spectra with bright lines are called emission spectra, those with dark lines are termed absorption spectra.