Explain briefly from the point of view of instrumental design, how the FTIR inst

Question

Explain briefly from the point of view of instrumental design, how the FTIR instruments generate an IR spectrum. How do FTIR instruments generate higher signal-to-noise ratios for spectral peaks compared to that of dispersive instruments? A part of a NMR output of a single scan is shown below. Expanded background signal is shown separately (same normalized scale). Estimate the signal to noise ratio of this spectrum. How would you to improve the signal-to-noise to at least 400. From the fringe pattern of an empty IR cell, the following wavenumbers were found for a series of consecutive fringes; 3010.11, 3058.97, 3107.89, 3156.75, 3205.64, 3254.54 and 3303.26. With a suitable graphical plot calculate the path length of the IR cell and its uncertainty.

Explanation / Answer

a)

FTIR spectrometers (Fourier Transform Infrared Spectrometer) Infrared spectroscopy exploits the fact that molecules absorb specific frequencies that are characteristic of theirstructure. These absorptions are resonant frequencies, i.e. the frequency of the absorbed radiation matches the transition energy of the bond or group that vibrates. The energies are determined by the shape of the molecularpotential energy surfaces, the masses of the atoms, and the associated vibronic coupling. Thus, the frequency of the vibrations are associated with a particular normal mode of motion and a particular bond type.

scientists have established various ways to utilize infrared light. Infrared absorption spectroscopy is the method which scientists use to determine the structures of molecules with the molecules’ characteristic absorption of infrared radiation. Infrared spectrum is molecular vibrational spectrum. When exposed to infrared radiation, sample molecules selectively absorb radiation of specific wavelengths which causes the change of dipole moment of sample molecules. Consequently, the vibrational energy levels of sample molecules transfer from ground state to excited state. The frequency of the absorption peak is determined by the vibrational energy gap. The number of absorption peaks is related to the number of vibrational freedom of the molecule. The intensity of absorption peaks is related to the change of dipole moment and the possibility of the transition of energy levels. Therefore, by analyzing the infrared spectrum, one can readily obtain abundant structure information of a molecule.

b)

FTIR spectrometers have several prominent advantages: (1) The signal-to-noise ratio of spectrum is significantly higher than the previous generation infrared spectrometers. (2) The accuracy of wavenumber is high. The error is within the range of ± 0.01 cm-1. (3) The scan time of all frequencies is short (approximately 1 s). (4) The resolution is extremely high (0.1 ~ 0.005 cm-1). (5) The scan range is wide (1000 ~ 10 cm-1). (6) The interference from stray light is reduced. Due to these advantages, FTIR Spectrometers have replaced dispersive IR spectrometers.

i)

RMS_S/N = 2.5 x S / Npp where S is the signal and Npp is the noise and 2.5 es a coverion factor

for a Npp = 0.005 and a S of 0.7 RMS_S/N = 2.5 x  0.7 / 0.005 = 350

ii)

using funtions like furrier transformate in the data

iii)

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