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Applied Spectroscopy Practica
6
Figure 3. Performance of the separate PLS models for the
quantification of either Cy-only or H-only in PS.
Table I. Determination of the Cy or H content in PS samples
collected at specific steps of the recycling process as determined
using the individual cymene or H PLS models.
Sample
Precipitated paste
Cy (wt%) H (wt%) Spectrum fit (%)
24.9 ± 0.6 N/A
54
Washed paste (W1) N/A
Washed paste (W2) N/A
Pellet (W1)
Pellet (W2)
1.8 ± 0.2 N/A
1.1 ± 0.2 N/A
11.8 ± 0.4 66
12.9 ± 0.1 70
99
97
fit values typically <70%, due to the unaccounted presence of
Cy. Such single-solvent models are therefore not reliable
except for the pellets, leading to the need for a model combining
both Cy and H.
A PLS calibration model was thus built to describe the four
components (PS, Cy, H, and DCE) over the concentration
range relevant to a PS dissolution/precipitation recycling
Figure 4. Performance of the combined (Cy + H) PLS model for
the quantification of Cy and H in PS.
process. A very good correlation (R2 >0.998) was found
between the predicted and real concentration values for all
components. Figure 4 shows the results for the two solvents
of interest and Fig. S2 (Supplemental Material) for PS (after
eliminating the DCE contribution). Compared to the previous
single-solvent models, larger values of RMSEC (0.0297 and
0.156) and RMSEP (0.0865 and 0.147) were obtained for Cy
and H, respectively. Regarding the robustness of the model,
the RMSECV values obtained for Cy and H are 0.0781 and
0.189, respectively. A small difference between RMSEP and
RMSECV values is normally an indication of the predictive ability
of a model.
The model was applied to the same series of recycled PS
samples recovered at the pilot plant (Table II). The spectrum
fit percentages are systematically improved with values
≥98.8%. This is particularly the case for the pastes for
which the single-solvent models failed badly. This is easily
understood when noting that the Cy and H contents were
substantial in all types of pastes, leading to the breakdown
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