27 ene. 2015

Comparing spectra for standardization

These are spectra of a check sample acquired with and without an option called "compatibility mode", the sample is the same on both spectra, but they are different because in one of them an algorithm is applied, in order that the sample seems to be acquired in another instrument (different bandwidth, and some differences in the wavelength scale). If we compare the spectra without math treatments, we can see small differences (an offset), but we don´t realize if there is a shift in the wavelength scale:
Now we apply a first derivative to these spectra:
Here we can see some shifts in the wavelengths from one instrument to other, but we make a zoom to the areas in yellow to see in more detail.
 
 
Recently I read some advices from Mark Whesterhaus, and one way to decide which type of standardization to apply is to subtract one spectra from the other and to check if it looks to a raw spectrum, to a first derivative or to a second derivative and depending of this you can decide the better standardization algorithm.
This is the spectrum of one raw spectrum (from Master Instrument) substracted from the same sample raw spectrum scanned in Instrument 2 (Host):

The spectrum is very similar to the first derivative spectrum of the sample confirming that there is a instrument shift that must be considered during the standardization.
 
This option has been done exporting the same sample scanned in the same instrument with and without "compatibility mode", which is similar to scan the same sample in 2 different instruments.

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