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Quantitative vibrational spectroscopy on liquid mixtures: concentration units matter
Henk-Jan van Manen1, Jan Gerretzen, Martijn Smout
1Nouryon Chemicals B.V., Expert Capability Group Measurement & Analytical Science, Zutphenseweg 10, 7418 AJ Deventer, The Netherlands. henkjan.vanmanen@ru.nl.
The Analyst
|May 17, 2021
Summary
Spectroscopic intensities correlate linearly with volume-based concentrations, not mass-based ones. Using volume fractions in quantitative spectroscopy improves accuracy and avoids errors in calibration models.
Area of Science:
- Analytical Chemistry
- Spectroscopy
- Physical Chemistry
Background:
- Quantitative vibrational absorption spectroscopies utilize Beer's law for concentration determination.
- Conflicting literature exists regarding the use of volume- versus mass-based concentration units in spectroscopy.
Purpose of the Study:
- To clarify the relationship between spectroscopic intensities and concentration units (volume vs. mass).
- To determine the optimal concentration units for accurate spectroscopic calibration.
Main Methods:
- Near-infrared, mid-infrared, and Raman spectroscopy were employed on binary solvent mixtures.
- Multivariate analysis methods, including classical least squares (CLS) and partial least squares (PLS), were utilized.
- Simulation studies were conducted to quantify errors.
Main Results:
- Spectroscopic intensities show a linear relationship with volume-based concentration units.
- Mass-based concentration models introduce non-linearity, especially when solvent densities differ.
- PLS models using mass fractions can compensate for non-linearity but may introduce errors up to 10-15%.
Conclusions:
- Volume-based concentration units are preferred for optimal spectroscopic calibration.
- Using volume fractions enhances accuracy in academic and industrial spectroscopic practices.
- Density differences between solvents explain the non-linearity observed in mass-based models.
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