Solution 4.12 - Chemometrics: Data Analysis for the Laboratory and Chemical Plant

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

1. A graph of total ion current is given below.

2. A plot of scores of PC2 v PC1 is as follows.

The lower limb corresponds to the faster eluting compound, the farther from the origin the greater the intensity. The graph then changes direction, the region in the bottom right hand corner corresponding to a region of coelution, finally the graph moves back to the origin in the direction of the slower eluting compound.

3. The plot of scores versus time for the first two PCs is below.

The first PC represents mainly the overall intensity. The scores of the second PC are negative for the fastest elutiing component and positive for the slower eluting component. Because there is some noise and chromatographic peak tailing the trend of the second component at high elution time is not entirely clear.

Notice that there is no real significance in the order of the mass spectral peaks so successive points should not be joined up, unlike for the scores plot where successive points are related sequentially in time.

The loadings are presented below.

Ions at m/z 41, 68 and 67 are probably most associated with the fastest eluting compound, where m/z 78 is associated with the slowest eluting compound. Masses m/z 95 and 96 are probably associated with both compounds. These deductions can be made by comparing the loadings and scores plots or could be clarified using a biplot.

The resultant scores plot is given below.

The first few points are of very low intensity and dominated by noise. When scaling each row to a constant total, these become equally important to the later points and because they do not show the same trends they have a disproportionate influence on the graph.

4. The scores plot changes in appearance.

The scores are now approximately on a straight line, the points with negative scores corresponding to the beginning of the peak, and those with positive scores to the end of the peak. The position along the straight line relates approximately to the composition.

There is very little difference in the loadings plot as the main aim was to scale the points in time rather than masses.


5. The standardised scores and loadings are as follows.

 

 

 

The standardised loadings are now approximately on the arc of a circle, reflecting the fact that there are two significant components. The masses associated with each pure compound are at the ends. The scores plot is fairly similar to the raw scores plot except that it is shifted in origin.

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