19 nov. 2013

Monochromator: Grating / Encoder

One of the most common patterns in the noise test if a monochromatic instrument is when an encoder fails ( see the post: Diagnostics: Lamp Bias / Encoder Peak / Water Vapour Bands) .
It affect to the noise level scale (out of tolerance limits, for the RMS and Peak to Peak noise), but it can affect also to the wavelength scale (.
The encoder is the spare that measure with a high accuracy and precision the position of every wavelength which come out from the grating, at the same time it has a motor which moves the grating, as you can see in the video).
The video shows how the polichromatic light from a lamp comes out from a slit and hits the grating which will generate monochromatic light to another slit and to the sample and detectors. This type of monochromator is known as pre-dispersive. (See the post: NIR: Post-dispersive concept to see the difference) 
One complete scan is a movement of the grating from left to right. In order to decrease the noise, normally, 32 scans are acquired to average (32 for the reference and 32 for the sample).
When the axis of the encoder motor ( which moves the grating ) does not move correctly ( due to high friction ), we can see a peak at 1500 nm aproximately.
If this persist, it can be the time to replace it.
The noise spectra of an instrument with encoder problems can be similar to this:
Here we can see crearly the peak at 1500 nm, and also other noise due to the water vapour bands.
In this picture we can see the grating and the motor-encoder
Once mounted, it is necessary adjust in order to match the peaks of the polyestirene internal filter with the correct standards wavelengths . This point is important because we don´t want any change in the performance of the calibrations, so the spectrum pof the polyestirene after and before the encoder change should match as better as possible.

We can scan (after the encoder change) a sealed check sample, and  compare the spectrum and results with the previous ones (dates when the instrument was performing fine, or when it was performing bad (due to noise by the encoder) and to get our own conclusions.
The picture shows a Check Cell (a sealed cup) with a product inside (soya meal ). Seal the cup prevent that there are few changes in the composition from one day to other.



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