Metrohm Vision – Theory - Manual (Page 14)

12 ▪▪▪▪▪▪▪
Specified
Segment Size Points per
segment, s Specified
Gap Size Points per
gap, g
1 – 2 1 1 – 2 1
3 – 6 3 3 – 6 3
7 – 10 5 7 – 10 5
11 – 14 7 11 – 14 7
15 – 18 9 15 – 18 9
19 – 22 11 19 – 22 11
2.4 Third and Fourth Derivative
The third and fourth derivatives are calculated as higher order finite-difference derivatives, analogous
to the methods described above for first and second derivatives.
2.5 Standard Normal Variate
Standard Normal Variate is a scatter correction method used commonly to normalize spectra when
the effective pathlength varies among samples in a data set. Such pathlength variation can occur
when measuring the spectra of granular or powdery samples because a) sample presentation in a cell
is not perfectly reproducible, or b) particle size varies between samples.
The spectrum is mean centered and then divided by its standard deviation:
s
s s
s s
n
i
SNV i
i
i
n
=
−
−
−
=
∑
( ) 2
1
1
2.6 Baseline Correction
Baseline correction is used to change spectral offset by subtracting either a spectral value at a
specified wavelength, or a constant value entered manually.
2.7 Detrend
Detrend is a method that can be used to remove baseline offset, slope, or curvature from a spectrum.
This is accomplished by calculating a baseline function as the least squares fit of a polynomial to the
sample spectrum, and then subtracting that function from the spectrum.