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Food Analysis Lecture 11 (2/22/2005)

Food Analysis Lecture 11 (2/22/2005). Infrared Spectroscopy (2). Qingrong Huang Department of Food Science. Reading materials: chapter 24, IR Spectroscopy, Food Analysis. Principles IR Spectroscopy. Energy: E=h  where:  is the frequency in hertz

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Food Analysis Lecture 11 (2/22/2005)

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  1. Food Analysis Lecture 11 (2/22/2005) Infrared Spectroscopy (2) Qingrong Huang Department of Food Science Reading materials: chapter 24, IR Spectroscopy, Food Analysis

  2. Principles IR Spectroscopy Energy: E=h where:  is the frequency in hertz In IR, frequency is commonly expressed as wave numbers ( , in Reciprocal cm, or cm-1) Where Near-IR: 800-2,500 nm or 4,000 – 12,500 cm-1 Mid-IR: 2,500-15,000 nm or 667 – 4000 cm-1

  3. Principles IR Spectroscopy • Absorption of radiation in this region by a typical organic molecule • results in the excitation of vibrational, rotational, and bending modes, • while the molecule itself remains in its electronic ground state. • Molecular asymmetry is a requirement for excitation by infrared • radiation and fully symmetric molecules do not display absorbance in • This region unless asymmetric stretching or bending transitions are • possible. Symmetric stretch Assymmetric stretch Symmetric bending

  4. Principles IR Spectroscopy • For the purpose of routine organic structure determination, the most • important absorptions in the infrared region are the simple stretching • vibrations. For simple systems, these can be approximated by • considering the atoms as point masses, linked by a “spring” having a • spring constant k and following Hooke’s Law. Using this simple appro- • ximation, the equation shown in below can be utilized to approximate • the characteristic stretching frequency (in cm-1) of two atoms of mass • m1 and m2, linked by a bond with a spring constant k: Where =m1m2/(m1+m2) , also called “reduced mass”

  5. Useful Functional Group

  6. Mid-IR Spectrum

  7. FT Instrumentation • Two types of spectrometers: • Dispersive instruments: use a monochrometer, e.g. UV-Vis • Fourier transform (FT) instruments: all wavelengths arrive at • the detector simultaneously and a mathematical treatment called • FT is used to convert the results into a typical IR spectrum. An • Interferometer is used in IR. • Interferometer splits an IR beam and recombines it by reflecting • back the split beam with mirrors. As the pathlength of one beam is • varied by moving its mirror, the two beams will interfere either • constructively or destructively, depending on their phase difference. • The interferogram showing intensity versus pathlength is then • converted by Fourier transformation into an IR spectrum, giving • Absorbance vs. frequency.

  8. IR Instrumentation Light source: Nichrome wire that glows when an electrical current is passed through; Interferometer: no monochrometer Detector: thermocouple detector, whose output voltage varies with changes caused by varying levels of radiation striking the detector.

  9. IR Instrumentation

  10. Near-IR Spectroscopy NIR (800-2500 nm) is more widely used for quantitative analysis of foods than are mid-IR.

  11. Near-IR Spectroscopy

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