Instrumental Parameters and Spectral Quality (Lab Report Sample)

INFRARED SPECTROSCOPY 2 LAB: Instrumental Parameters and Spectral Quality

PURPOSE:
The purpose of this Lab is to calibrate the wavenumber scale of the FTIR spectrometer, minimize background and saturation, as well as determine the effect of variations in range, resolution, and number of scans so as to determine quality of spectrum signal-to-noise ratios.
BACKGROUND:
Atoms of molecules when they attain sufficient vibrations that correspond in frequency to IR radiation do absorb the IR radiation. Those vibrations can then cause absorption bands in IR spectra which are unique for each compound. An IR spectrometer consists of basically a radiation source, a monochromator, and a detector. The source of radiation can be any inert solid heated electrically to about 1000-1800oC e.g. the Nerst glower, Globar or Nichrome coil. The monochromator is a dispersive device for a broad spectrum of radiations used to split a continuous calibrated series of electromagnetic bands to determine the wavelengths or frequency. Detectors can be either thermal or photon e.g. thermocouples, thermistors or pneumatic devices. They measure the heating effect produced by infrared radiation. Various physical properties changes can then be quantitatively determined.
In this Lab we shall utilize a Fourier Transform IR Spectrometer (FTIR) which is superior in terms of speed and sensitivity over ordinary IR spectrometers. FTIR has an extended capability of IR spectroscopy and can be applied in areas that are impossible for dispersive IR spectrometers. It also allows us to measure all frequencies simultaneously. The FT system is composed of a source of radiation, an interferometer, and a detector (see a simplified layout of FTIR in the appendix section: Appendix 1A). Although the same source of radiation is the same, the approach utilized in FTIR is different hence can measure absorption at different components. The interferometer consists of 3 active components; a moving mirror and a fixed mirror, and a beam splitter (Appendix 1A).
APPARATUS AND CHEMICALS:
Nicolet Nexus 6700 or Nicolet Nexus 670 FTIR spectrometer
Liquid nitrogen to cool the detector
Polystyrene film, 1.5 mil (thickness 0.0015 inches = 38.1 Ојm) and 3.0 mil
PROCEDURE:
In the lab
Instrument Start-up
After ensuring that the spectrometer is equipped with the ordinary sample compartment (and not the ATR accessory), the “OMNIC” icon is double clicked to start it up. The “File” menu is then pulled down and the “Options” clicked to check “Use selected file type” and ensure the file type is TCAMP-DX (*.JDX). This saves the spectra in an ASCII version which can be used in Excel.
After that, a click is made on the “Expt set” button (experimental setup) followed by another click on the “Bench” tab. Where it says “Window”, the appropriate choice of the substance window in use is selected. The scan range is also set by entering the recommended range values for a “Max. range limit” and “Min. Range limit”.
On the “collect” tab, the desired number of scans is typed and an appropriate resolution selected. “% transmittance” is then selected for the “Final format” while for “Correction” “None” is chosen. The files can then be renamed later. Under “Background handling”, “Collect background after 1000 times” is selected and then all that is saved by clicking “Ok”.
Spectrum of Air
Returning to the “Bench” tab, the interferogram (raw signal coming from the detector) is found. There is a small peak in the middle which is not easy to see. Liquid nitrogen is put on the detector to cool it so that the beak in the middle could glow and become visible. The amplitude must be above four before any spectra is collected. The “Automatic atmospheric suppression” is unchecked in the “Collect” tab and the “Expt set” window closed.
The purge gas valve is opened after which a “Col Bkg” clicked to collect background spectrum. Since the purge gas had been flushing the spectrometer for a while, one should not expect air in the system, hence there should be no bands in the background spectrum.
The purge gas valve is closed and the sample compartment opened for a few seconds to let air in before it is closed again, and the sample spectrum collected. The displayed spectrum show several bands due to air absorption. The purge gas is then opened to replace the air in the sample compartment with the gas. The collect tab in Expt set is then opened again and the “Automatic atmospheric suppression” selected to introduce small artefacts into the spectra. After this, the Expt set window is closed once more and a sample spectrum collected again. The bands due to air absorption are gone now, and the two spectra are saved.
Polystyrene Spectra
In the experimental set-up, a change is made to all the necessary parameters to collect the spectrum of a polystyrene film. The background-collected spectrum is then used to calibrate the wavenumber scale of the spectrometer. A 1.5-mil polystyrene film is inserted in the sample compartment and the compartment closed before a background spectrum is collected immediately with 2 scans and a resolution of 1 cm-1. The background is taken every five minutes until the air bands cease to change in size.
A spectrum of polystyrene is then collected using the following combinations of number of scans and resolution. (The “Data spacing” corresponding to each value is noted down. It is important to remember that each time the parameters are changed the background and sample spectrum are run. The five polystyrene spectra are then saved in .CSV format.
Number of scans 2 2 2 8 32 Resolution 1 4 16 4 4