Applied_Spectroscopy_Practica_01_01 - 28

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Applied Spectroscopy Practica
Spectral regions providing the highest variation for the different
components are listed in the Supplemental Material. Spectral
pretreatments are sometimes required to reduce systematic
noise, baseline variations, light scattering, pathlength
differences, etc.21-23 In our case, using the first derivative
of the spectra, as opposed to the normal spectra, provided
better RMSEC values due to reduced peak overlap and
baseline distortions. Mean centering was applied to the
spectra and a spectrum outlier diagnostic was applied to
identify and eliminate outliers (only in the case of the
Cy-only model).
Results and Discussion
Attenuated total reflection (ATR) was selected as the sampling
method of choice because it can be readily used by
operators at the plant level. Ideally, the PS samples would
be analyzed directly in their solid (for pellets) or viscoelastic
(for pastes) state to allow for a faster and more sensitive
quantification of the Cy and H content. However, this direct
sample presentation method makes the analytical outcome
prone to random or systematic errors if there is heterogeneity
in the bulk distribution of the solvents because ATR only
probes the surface of the sample, with a penetration depth
on the order of 0.6-2.3 µm over the spectral range of interest
when using a diamond ATR element. Notably, a partial
evaporation of the Cy or H at the surface of the samples
would result in a systematic underestimation of their true
content in the bulk sample. In view of this, we decided to dissolve
the samples in a common solvent for all three components
(PS, Cy, and H) to ensure a homogeneous distribution
and unbiased results. This solvent must be minimally volatile
during analysis, and it should interfere as little as possible
with the characteristic bands of the compounds of interest.
Several candidates were considered and DCE was selected
as a good compromise in terms of volatility and spectral
interference (see Fig. 2a). A concentration of 30 wt% of PS
in DCE was used throughout as a compromise between
the need for sensitivity, which improves at higher PS concentration,
and a sufficiently low viscosity to enable homogenization
of the sample solutions.
An investigation of the pure component spectra, shown in
Fig. 2a, was conducted to evaluate the possibility of a univariate
analysis of the Cy and H contents in PS using isolated
bands of sufficient intensity. For simple systems, the Beer-
Lambert law can be used directly to quantify the species of
interest. In practice, the spectral information is often
obscured, even for moderately complex systems, due to
the overlap of bands originating from different components.
This was particularly the case for H: although it presents
very intense C-H stretching bands between 2800 and
3000 cm−1, the overlap with the C-H stretching bands of
both PS and Cy led to strong nonlinearity for univariate calibrations.
On the other hand, good candidate bands can be
identified for Cy. A spectrally isolated band is also needed
for PS to serve as an internal standard because, while its concentration
is well controlled in the calibration standards, it is
unknown for samples collected at the pilot plant. The best
univariate calibration was obtained using the intensity ratio
of the 813 cm−1 Cy band (shifted to 818 cm−1 in DCE) and
1601 cm−1 PS bands but, as can be seen in Fig. S1
(Supplemental Material), a nonlinear trend was obtained
(see the nonrandom residuals). Univariate analysis of Cy in
PS thus provides a quick general guidance for the recycling
process optimization, but it cannot provide accurate results.
In particular, the presence of a large H content provoked systematic
errors due to the partial overlap of the baseline
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