Optimised methylene-blue detection and quantification using conventional Raman spectroscopy

Raman spectroscopy is a powerful technique based on specific molecular vibrations producing a characteristic "fingerprint spectrum" used for sample analysis. So far, the use of Raman quantitative analysis has not kept pace with its use for qualitative analysis due to some challenges, yet it has a great potential to be developed for measuring the intended property within the sample. Moreover, bulky laboratory Raman spectrometers are very pricey, complex and are designed to be as versatile as possible. The main aim of this work is thus to perform a preliminary study that will enable the future advanced quantitative analysis and design of a much simpler, yet field-transportable system, that will allow the detection and quantfication of minuscule amounts of toxins found in the environment and specific pesticides on the different plants. For that purpose, an existing Raman spectrometer is used to investigate the optimum conditions for the detection and quantification of a specific model-molecule with a well-known Raman spectrum, Methylene Blue (MB). In a systematic study, the influence of instrumental and sample-related parameters on the ability to detect very low concentrations of MB are therefore investigated. In particular, optimum excitation wavelength and power, laser spot size, and sample phase and configuration are found. Moreover, suitable methods for the calculation and minimisation of the limit of detection and quantification (LOD and LOQ) are applied under various experimental conditions. Finally, analytical models are established and the error of prediction are calculated and discussed. The presented results offer clear guidelines for the quantification study, design and development of a field-transportable Raman analysis system, which will be the subject of a future PhD work.