Food Analysis Lecture Notes PDF

Food analysis NUTR 210 Lect. 3  Basic principles of spectroscopy Basic Principles of Spectroscopy: Is the study of interaction of matter (molecular) with light or Study of interaction of electromagnetic radiation with the matter What is electromagnetic ra...

Food analysis NUTR 210 Lect. 3  Basic principles of spectroscopy Basic Principles of Spectroscopy: Is the study of interaction of matter (molecular) with light or Study of interaction of electromagnetic radiation with the matter What is electromagnetic radiation? Is a form of energy that propagates as both electrical (E) and magnetic (M) waves traveling in packets of energy called photons.  Basic principles of spectroscopy Electromagnetic radiation Frequency (v): It is defined as the number of time that electrical field radiation is oscillates/repeating in one second. - The unit for frequency is Hertz(Hz) - 1 Hz= 1 cycle per second Wavelength (λ) It is distance between nearest part of the wave in the same phase.  Basic principles of spectroscopy Important Definitions: Spectroscopy:  Is the study of molecular or atomic structure of a substance by measurement of its spectrum. Spectrum:  A range of electromagnetic energy (absorbed or emitted) separated by wavelength.  Spectroscopic methods are very informative and widely used for both quantitative and qualitative analyses.  Spectroscopic methods based on the absorption or emission of radiation in the ultraviolet (UV), visible (Vis), infrared (IR), and radio (nuclear magnetic resonance,NMR) frequency ranges are most commonly encountered in traditional food analysis laboratories.  Absorption/Emission spectroscopy Absorption spectroscopy :  An analytical technique which concerns with the measurement of absorption of electromagnetic radiation. e.g. UV(184-400) Visible (380 -780) Emission spectroscopy :  An analytical technique in which emission of Absorption: A transition from a lower level to a higher level particle or radiation is disperse according to with transfer of energy from the radiation field to an some property of emission and the amount of absorber, atom, molecule, or solid. dispersion is measures. e.g. Mass spectroscopy Emission: A transition from a higher level to a lower level with transfer of energy from the emitter to the radiation field. If no radiation is emitted, the transition from higher to lower energy levels is called nonradiative decay.  Basics of Light Light can take on many forms such as follows: 1 2 3 4 5 6 7 gamma microw Radio X-ray ultraviolet visible infrared radiation aves waves Wavelength increasing Energy/Frequency increasing The energy of the photon tells what kind of light it is. Radio waves are composed of low energy photons. Optical photons: (the only photons perceived by the human eye) = are a million times more energetic than the typical radio photon. The energies of X-ray photons range from hundreds to thousands of times higher than that of optical photons.  EFFECT OF ENERGY ON A MOLECULE The strength of the radiation energy will interact with the molecules in different ways: High energy sources produce breaking of bonds X-Ray, γ Rays, … Medium energy sources excite electrons UV / VISIBLE Spectroscopy Low energy sources produce vibrations in chemical bonds Infrared Energy Very low energy sources produce rotation of the chemical bonds Microwaves and Radio waves Each of these methods is distinct the different types of molecular or atomic transitions. QUANTITATIVELY: - For determining the amount of material in a sample QUALITATIVELY: – For identifying the chemical structure of a sample  UV / VISIBLE SPECTROSCOPY UV Radiation – Wavelength range 220 - 380nm VISIBLE Radiation – Wavelength range 380 - 780nm UV-Vis Spectroscopy : - Is one of the most commonly encountered laboratory techniques in food analysis. - Used in the determination the concentration of many different substances or concentration of different molecules in the solution.  Substances can be liquids or solids and measurements are made with instruments called SPECTROPHOTOMETERS or SPECTROMETERS  Modern instruments can be coupled to microscopes which allow solid samples and very small samples of solids and liquids to be analyzed both qualitatively and quantitatively  UV / VISIBLE SPECTROSCOPY UV Radiation – Wavelength range 220 - 380nm VISIBLE Radiation – Wavelength range 380 - 780nm Spectroscopy utilizing radiation in the UV-Vis divided into two generals: Absorbance and Fluorescence/emission spectroscopies,  UV / VISIBLE SPECTROSCOPY UV Radiation – Wavelength range 220 - 380nm VISIBLE Radiation – Wavelength range 380 - 780nm  Substances can absorb varying amounts of UV and/or Visible radiation at particular wave lengths – Coloured compounds absorb energy in both UV and visible region of the electromagnetic spectrum  The laws of Spectrophotometry There are two very important basic laws and a third one which is a combination of the two. LAMBERTS LAW – ABSORBANCE (A) proportional to the PATHLENGTH (b) of the absorbing medium. BEERS LAW - ABSORBANCE (A) proportional to the CONCENTRATION (c) of the sample. BEER- LAMBERT LAW - ABSORBANCE (A) proportional to c b A  cb A = Ecb (A is a ratio and therefore has no units) The constant E is called the MOLAR EXTINCTION COEFFICIENT  UV / Visible Spectroscopy Importance of the beer lambert law A = Ecb but if E and b are constant ABSORBANCE  CONCENTRATION and should be linear relationship Standards and calibration curve: Calibration curve are common in food analyses. is used to establish the relationship between analyte concentration and absorbance. The standard solutions are best prepared with the same reagents and at the same time as the unknown. Characteristics of UV-Vis Absorbing Species  UV-Vis spectroscopy do not use UV radiation below 200 nm.  The UV-Vis absorbs several functional groups such as. 1. Acetaldehyde (R-CHO) 2. Acetone (R2-CO) 3. Acetic acid (R-COOH) 4. β-Carotene 5. Oxalic acid 6. Phenol …..etc.  Infrared (IR) spectroscopy IR spectroscopy is used widely in the food industry for both qualitative and quantitative analyses of ingredients. It used to understand the structural of different chemical compound groups specially the structure of chemical groups (mostly the functional groups) Region of infrared that is most useful lies between 2.5-16 mm (625 - 4000 cm-1) depends on transitions between vibrational energy states