What is the maximum size that can be measured with an infrared microscope?
It is said that analysis is generally possible up to 10 µm, although this depends on the measurement method and the type and state of the sample. This is related to instrument sensitivity (light level) and wavelength (IR range is 2.5 to 25 µm). For example, measurements with the aperture set to 10 µm may result in ghost peaks or peak broadening at 10 µm or above (up to 1000 cm-1).
Fig. 1 shows measurements of novolac resin on a metal sheet using various aperture sizes.
To avoid noise effects, a different number of measurements was used for each aperture size.
|30µm: Resolution: 8 cm-1, Accumulation: 20 (approx. 8 s)
10µm: Resolution: 8 cm-1, Accumulation: 100 (approx. 40 s)
5µm: Resolution: 8 cm-1, Accumulation: 400 (approx. 160 s)
Fig. 2 shows a magnified image of Fig. 1 in the range from 1300 to 700 cm-1. It shows that while novolac resin can be identified at 5 µm, changes in the spectrum shape are clearly apparent at the low-wavenumber side. (See Fig. 2)
As shown above, care is required when using an aperture up to 10 µm due to deformation of the spectrum shape.