Abstract
Here we design, construct, and characterize a compact Raman-spectroscopy-based sensor that measures the concentration of a methanol–water mixture with 0.5% accuracy. The sensor measures the concentration with a precision of 0.2% with a 1 second measuring time, and with longer measurement times, the precision reaches as low as 0.006%. We characterize the long-term stability of the instrument over an 11-day period of constant measurement, and confirm that systematic drifts are on the level of 0.02%. We describe methods to improve the sensor performance, providing a path towards accurate, precise, and reliable concentration measurements in harsh environments. This sensor should be adaptable to other water–alcohol mixtures, or other small-molecule liquid mixtures.
Highlights
Raman spectroscopy is a powerful analytical technique that provides detailed information on molecular vibrations
While Raman spectroscopy has been demonstrated for sensing the concentrations of solutions [4–8], liquid mixtures [9–12], and gas mixtures [13], the accuracy is typically observed to be on the level of a few percent, the precision is often not quantified, and long-term systematic uncertainties are not investigated
This accuracy level is similar to the “detection limit” for the concentration of a water-toluene mixture reported by Hashimoto et al [20] using a sophisticated femtosecond laser system
Summary
Raman spectroscopy is a powerful analytical technique that provides detailed information on molecular vibrations. Since a Raman measurement only requires optical access to the sample, and can be completed using relatively inexpensive lasers and spectrometers, it is ideally suited for probing the precise concentrations of mixtures in a laboratory, field, or industrial setting. Demonstrations are typically completed in a laboratory setting and with bulky, expensive equipment It is not clear if a simple, inexpensive Raman spectrometer can be used to measure concentrations with high levels of accuracy, precision, repeatability, reliability, and immunity from environmental perturbations. Estimate that is largely immune to variations in laser power, optical alignment, and fluctuations in the absorption of the sample. This simple approach provides an estimate of the methanol concentration to within 3% accuracy, but with high repeatability. We find that the sensor provides a precision of 0.2% with a one-second averaging time, and 0.006% with a 1000-second averaging time
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