Labs have used polystyrene (PS) as a reference and calibration standard for infrared spectroscopy (IR) for decades. Although it is not always necessary, PS may also be used to calibrate Fourier transform infrared spectroscopy (FTIR) instruments. The advantage of polystyrene as a standard is that it exhibits strong and sharp absorption peaks at a number of wavelengths in the IR region. By comparing the apparent positions of absorption peaks in a PS standard spectrum to the expected positions of these peaks, you can determine how well calibrated an FTIR instrument is.
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Things you need
- Polystyrene film reference standard
- Reference standard certificate of calibration
Acquire an infrared spectrum of the polystyrene reference film standard on the FTIR that is to be calibrated. This is done by simply placing the film in the beam path of the instrument and using the "scan" command. Depending on the instrument and methodology used, a background or "blank" scan may be acquired first and subtracted from the standard scan.
Have the FTIR software locate the major absorption peaks on the resulting PS spectrum and list the peak positions in reciprocal centimetres overlaid on the spectrum or in a separate document.
Examine the position of the peaks in the PS standard spectra. The spectra should have well-defined absorption peaks at the following approximate wave numbers: 3060, 2850, 1944, 1601, 1583, 1154 and 1028cm^-1. The exact positions expected for these peaks will be detailed in the calibration certificate accompanying the PS standard.
Ensure that the absorption peaks in the PS standard spectrum are within the confidence intervals given for each peak in the standard calibration certificate. For example, if the certificate states that the 1944cm^-1 peak should appear at 1944 +/- 0.6 cm ^-1, ensure that the spectrum has a peak in this range. If all peaks in the spectrum appear in the appropriate range, the FTIR instrument may be considered properly calibrated with respect to wavelength accuracy.
Interpretation of Polystyrene Standard
Tips and warnings
- Most modern FTIR instruments have an internal calibration based on a HeNe laser, and so calibration by polystyrene standard may not be strictly necessary.
- The intensity of absorption for various peaks in the standard PS film may also be tracked over time to ensure that the instrument performance is not degrading.
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