Ch20A-TCF

Ch20A-TCF - Quantitative Chemical Analysis Quantitative...

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Quantitative Chemical Analysis Quantitative Chemical Analysis Chapter 20A: Fourier Transform Spectroscopy Fourier Transform Spectroscopy Part 1: Infrared Spectroscopy
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2 Fourier transform spectroscopy - especially useful in infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy Use of IR and NMR to determine the functional groups and structures of organic molecules is covered in Chem 322 at USC. We will not discuss this. In Chem 300 we will consider the basis for these useful forms of spectroscopy.
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3 Infrared Spectroscopy Infrared selection rule: photon absorption must result in a change in the dipole moment of the molecule. 3n – 6 modes; 3n – 5 modes if linear O O OO stretch no µ still no µ No IR absorption no OCO 2 no 3 modes absorb IR, one does not Three types of IR spectrometers Dispersive instruments – Prism (historic) (refraction of light) – Grating (diffraction of light) Interferometer instruments – FT IR
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Interferometry – the Michelson Interferometer δ = retardation NOTE : Distance mirror moves = δ /2 4 •This interferogram is from a single wavelength source • Real source is continuous over entire IR frequency range of interest • Detected signal is missing all IR absorptions of the sample -- How do we get the spectrum out of this?
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5 The spectrometer IR: 4,000 to ~400 cm 1 Red laser at, say, 650 nm (~15,300 cm 1 )
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6 The Fourier Transform •How do we get the frequency spectrum out of the interferogram? •Interferogram is an analog intensity vs distance signal. (continuous) (voltage) * (mirror position) Mirror is moved at constant rate, so distance time So, interferogram is an analog intensity vs time (I,t) signal •First, analog signal is converted to a digital signal – a series of (I,t) data points – by an analog-to-digital converter ( ADC ) •Second, multiple spectra are acquired and the digital signals added together. •Third, a Fourier transform is carried out to convert the data from a set of (I,t) data points to (I, ν ), (intensity,frequency), points. FT is a coordinate transformation from the time domain to the frequency domain * IR detectors: thermocouple (voltage); pyroelectric (voltage); photoconductor (current); photovoltaic (voltage)
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7 Let’s get a feeling for what this means Consider the closely related Fourier series.
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This note was uploaded on 03/28/2011 for the course CHEM 300L at USC.

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Ch20A-TCF - Quantitative Chemical Analysis Quantitative...

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