Question

1.) Explain the advantages of a FT instrument over a constant wavelength diffuse instrument.

2.) Explain why Infrared spectroscopy uses interferometers rather than constant wavelength spectrometers.

Answer 1

• FT-IR Advantages There are three major advantages in the performance of an FT-IR spectrometer over a dispersive infrared spectrometer. These advantages have been the reason for the switch to the more modern FT-IR technique in the last decade by infrared spectroscopists. Multiplex Advantage An interferometer in an FT-IR instrument does not separate energy into individual frequencies for measurement of the infrared spectrum. Each point in the interferogram contains information from each wavelength of light being measured. Every stroke of the moving mirror in the interferometer equals one scan of the entire infrared spectrum, and individual scans can be combined to give better representation of the actual absorbance of the sample. In contrast, every wavelength across the spectrum must be measured individually in a dispersive spectrometer. This is a slow process, and typically only one measurement scan of the sample is made in a dispersive instrument. The FT-IR advantage is that many scans can be completed and combined on an FT-IR in a shorter time than one scan on a dispersive instrument. The multiplex advantage results in faster data collection of an FT-IR spectrum. Throughput Advantage An FT-IR instrument does not use a slit to limit the individual frequency reaching the sample and detector as a dispersive instrument does. There are also fewer mirror surfaces in an FT-IR spectrometer, so there are less reflection losses than in a dispersive spectrometer. Overall, more energy reaches the sample and hence the detector in an FT-IR spectrometer than in a dispersive spectrometer. This means that the signal-to-noise ratio of an infrared spectrum measured on an FT-IR is higher than the signal-to-noise ratio attained on a dispersive instrument. Higher signal-to-noise means that the sensitivity of small peaks will be greater, and details in a sample spectrum will be clearer and more distinguishable in the FT-IR spectrum than the dispersive spectrum of the same sample. In addition, high-resolution measurement of infrared spectra is of higher quality on an FT-IR system. The slit on a dispersive instrument must severely limit the amount of energy reaching the sample in order to measure data points spaced closely together on a high resolution spectrum, resulting in poor quality spectra. The process is also extremely slow due to the coordination of the grating and slit systems to collect the large number of data points required. Precision Advantage An FT-IR spectrometer requires the use of a laser to control the velocity of the moving mirror and to time the collection of data points throughout the mirror stroke length for each scan. This laser is also available as a source of wavelength calibration within the instrument. The laser wavelength is a constant value, and the x-axis data points of the FT-IR spectrum are automatically referenced to this known value to maintain internal precision and accuracy of the wavelength positions. Spectra collected with an FT-IR spectrometer can be compared with confidence whether they were collected five minutes or five years apart. This capability is not available on a dispersive infrared system. External calibration standards are required to control the accuracy of a dispersive instrument, making spectra less comparable due to instrumental unknowns during and between scans. Accuracy and precision in infrared spectra are much higher when collected on an FT-IR
• question 02.

1. The wavelength accuracy or Connes' advantage. The wavelength scale is calibrated by a laser beam of known wavelength that passes through the interferometer. This is much more stable and accurate than in dispersive instruments where the scale depends on the mechanical movement of diffraction gratings. In practice, the accuracy is limited by the divergence of the beam in the interferometer which depends on the resolution.