17 ene. 2012

Working in transflectance

Sample presentation in Near Infrared can be done in reflectance or in transmission. There are modules to work in each of them or in both. In reflectance the light goes through the sample and interacts with it and part of this light returns to the detectors (we don´t know the pathlength).
We cannot measure certain liquids this way (like water, oil,…) because light will goes through and did not return to the detectors, so we need to put behind the sample a reflector, so the light cross the sample, is reflected on the reflector´s surface and cross the sample again to the detectors. This way to analyze is called “transflectance”.
There are reflectors of different pathlengths (0.1mm, 0.2mm, 0.5mm,….), beeing this measurement  the gap between the glass of the cuvette and the surface of the mirror. So the real pathlength the light goes through is two times this value.
The pathlength must be choosing depending of the absortibity of the sample. If the water content is high we have to choose probably 0.1mm. For other samples (without or with few water) 0,5mm can be fine.
If we have a transmission module, this option is not necessary, and we use othe types of cuvettes which we will talk in future posts.


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