PHYSICAL CHEMISTRY OF MILK

Lesson 31. SPECTRUM OF ELECTROMAGNETIC RADIATION

Module 12. Nuclear chemistry

Lesson 31
SPECTRUM OF ELECTROMAGNETIC RADIATION

31.1 Introduction

Human eye perceives the difference in the colour of various compounds and things around them e.g. quinone is yellow; chlorophyll is green; 2,4-dinitrophenylhydrazone derivatives of aldehydes and ketones range in color from bright yellow to deep red, asprin is colourless. So human eye is functioning like a spectrophotometer analysing the light reflected from the surface of solid or passing through a liquid. Sunlight seen as single white colour is in fact composed of a wide range of radiation with different wavelengths in the ultraviolet, visible and infrared portions of the spectrum. The electromagnetic spectrum of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object. In principle this spectrum is infinite and is continuous.

31.2 Classification of Electromagnetic Radiation

When sunrays are passed through a prism, the rays bend in accordance to their wave lengths. Electromagnetic radiation such as visible light is commonly treated as a wave phenomenon, characterized by a wavelength or frequency. Wavelength is defined as distance between adjacent peaks (or troughs), and may be designated in meters, centimeters or nanometers (10-9 meters). Frequency is the number of wave cycles that travel past a fixed point per unit of time, and is usually given in cycles per second, or hertz (Hz).

Generally electromagnetic radiation is classified on basis of its wave length. Visible wavelengths cover a range from approximately 400 to 800 nm. The spectrum along with the wave length for the visible light is shown in Fig. 31.1.

Fig. 31.1 Wavelength in nm-visible spectrum

It could be observed that red colour has the longest wavelength while violet has the shortest wavelength, while the remaining colours are in decreasing order of wavelengths. People perceive this radiation as the visible light, ultraviolet (UV) rays, x-rays and gamma rays. The behaviour of electromagnetic radiation depends on its wavelength and on the amount of energy it carries with when it interacts with single atoms and molecules. This energy is measured per quantum (photon).

31.3 Electromagnetic Specturm

Spectrum of electromagnetic radiation is shown in Fig. 31.2 and presented in Table 31.1

Fig. 31.2 Spectrum of electromagnetic radiation

Electromagnetic spectrum can broadly be grouped into non-ionizing and ionizing radiation. The non ionizing radiation when obtained by low induced current is non thermal while high induced current generates heat which is effectively used in the microwave ovens. Photo electric excitation is used for producing photochemical effect.

Table 31.1 Regions of the Electromagnetic Spectrum

(Source: http://csep10.phys.utk.edu/astr162/lect/light/spectrum.html0

The ionizing radiation is more useful in medical field. Rays generated during ionization produce several effects such as damage to the DNA and also breaking of the bonds which are of immense help in treatment of cancer, correcting birth defects and also causing mutation.

In non-ionizing radiation, the extremely low frequency rays and very low frequency are included. These are generally referred as radio waves. The next range of frequency is categorized as microwave, which is followed by infra red rays. The visible range is forming the next group of rays.The ultra violet rays, X-rays and gama rays are considered under the category of ionizing radiation.

31.4 Lambert-Beer Law

This law relates to the absorption of light and to the properties of material through which the light travels. Transmittance is the term used to the amount of light penetrating a solution. It is expressed as the ratio between the intensity of transmitted light (It) to the intensity of initial light of light beam (lo).

T= It / Io

Where;

T – Transmittance
It – Intensity of the transmitted light
I0 – Intensity of the initial light beam

The law states that there is a logarithmic dependence between transmission (or transmissivity) of light through a substance, T, and the product of absorption coefficient of substance, α, and the distance the light travels through the material (i.e. the path length), ℓ. The absorption coefficient can, in turn, be written as a product of either a molar absorptivity of the absorber, ε, and the concentration c of absorbing species in the material, or an absorption cross section, σ, and the (number) density N' of absorbers.

For liquids, these relations are usually written as:

The transmission (or transmissivity) is expressed in terms of an absorbance which, for liquids, is defined as

Thus, if path length and molar absorptivity (or the absorption cross section) is known and if the absorbance is measured, then the concentration of the substance (or the number density of absorbers) can be deduced. If the concentration is expressed as a mole fraction i.e. a dimensionless fraction, the molar absorptivity (ε) takes the same dimension as the absorption coefficient, i.e. reciprocal length (e.g. m⁻¹). However, if the concentration is expressed in moles per unit volume, the molar absorptivity (ε) is used in L·mol⁻¹·cm⁻¹, or sometimes in converted SI units of m²·mol⁻¹. The absorption coefficient α' is one of many ways to describe the absorption of electromagnetic waves.

31.5 Mass Spectrometry

The mass spectrometer is an instrument which can measure the masses and relative concentrations of atoms and molecules. It makes use of the basic magnetic force on a moving charged particle. This is an analytical technique that measures the mass-to-charge ratio of charged particles. This technique is useful for the determination of mass of particles, elemental composition of a sample or a molecule, in finding out the chemical structure of molecules such as peptides and other chemical compounds. Main principle of this technique consists of ionizing chemical compounds to generate charged molecules or molecule fragments and measuring their mass-to-charge ratios. Mass spectrometer is the instrument used for this analytical technique. Mass spectrophotometer has three associated components. They are;

· The ion source: This component of the instrument will help in ionization of the sample in which an electron is removed from cations.
· The Mass analyser: This component of the instrument will sort out ions and separate them according to their mass and charge
· The detector: The sorted and separated ions are then measured and the results are recorded

The mass spectroscopy is useful in

(1) measuring the size of nano-particles in the absence of sophiscticated transmission electron microscopy or x-ray diffraction;
(2) detecting toxins in pharmaceuticals, biotechnology, and other foods
(3) can identify multiple compounds of pesticides at once that enter the food supply.

31.6 Nuclear Magnetic Resonance (NMR)

Nuclear magnetic resonance (NMR) is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation. Nuclear magnetic resonance spectroscopy is a research technique which is used for determining physical and chemical properties of atoms or the molecules of a compound. It also gives information about the structure, dynamics, reaction state, and chemical environment of the molecules. The technique is based on the nuclear magnetic resonance of the atomic nuclei in the molecules. In the scientific field this technique is being used to investigate properties of the organic molecules. Samples for which this technique is used range from small compounds with one dimensional protons or carbon–13 to large protein or nucleic acids using 3 or 4 dimensional techniques. This technique provides a wide range of information for a large number of samples including solutions and solids.

NMR spectroscopy is employed in a wide range of food safety areas broadly aimed at averting significant chemical, biological or microbiological threats to the food chain. This technique is more useful about new or immerging threats. Under such circumstances the analysis of specific compounds using traditional methods is time-consuming, expensive and often unsuccessful in reaching the goal of controlling risk. NMR is focussed on the development of non-targeted methods that are able to rapidly determine the presence of unspecified hazards.

NMR spectroscopy has also been used to study the effect of microbes on food composition. Applications include monitoring maturation processes, shelf-life determination and the measurement of freshness like storage history of a food product may be determined by considering the biochemical composition.

NMR methodologies can be used for food authentication i.e for the verification of geographical and botanical origin, and for the verification of labelling claims such as “organic” and those relating to more direct health implications including the characterisation of pronutrients.

NMR spectroscopy increasingly is playing a role in understanding a range of activities, for example, assessing ingredient purity, characterising functional ingredients and investigating the effect of novel processes on final product composition.