Solution 3.8 - Chemometrics: Data Analysis for the Laboratory and Chemical Plant

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  • Published: Jan 1, 2000
  • Channels: Chemometrics & Informatics

1. The graphs are presented below.

It can be seen that B is intermediate in nature to A and C, partly by peak shifts and partly by the change in features.

Compound 2 appears to have a more complex spectrum to compound A.

2. The graphs are presented below.

A appears to consist of two peaks. The first, at low wavelength is at slightly too low wavelength to cross 0 in the first derivative, because using a 5 point filter, the lowest wavelength represented is 228 nm. This could be improved either by extending the wavelength range or increasing the digital resolution. The second (low wavelength) peak appears broader with a maximum at around 296 nm. Notice how the first derivative crosses the origin at approximately the same position as the second derivative exhibits a minimum.

3. The graphs are presented below.

They appear much more complex because there are three peaks within the region of spectrum presented. As judged by the minima in the second derivative, these are centred on 248, 234 and 312 nm. Notice that the first derivatives do not always cross the lines exactly at the same spot as the second derivative minima, this is because of overlap between neighbouring peaks and also low digital resolution. The higher the derivative, the greater the accuracy because the resultant peaks become sharper, providing noise does not interfere.

4. The graphs are presented below.

5-point derivative

7-point derivative

The 7-point derivative appears clearer than the 5-point derivative. There are no general guidelines for this, which depend on the spectral features and the digital resolution. Sometimes quite large windows are required.

The middle line (spectrum B) contains features of both the other pure spectra, including the maxima at around 240 and 260 nm and the minimum at around 250 nm. The region between 270 and 310 nm should consist of peaks due to both compounds. In favourable cases it is possible to resolve the peaks, but the spectra are slightly too broad in this situation. However, the second derivative of B does appear to consist of two partially resolved components, with a noticeable shoulder. This is not evident from the normal (unprocessed) spectrum.

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