Raman spectroscopy provides chemical-rich information about the composition of analytes and is a powerful tool for biological studies. With the ability to investigate specific cellular components or image whole cells, compatible methods of sample preservation must be implemented for accurate spectra to be collected. Unfortunately, the effects of many commonly used sample preservation methods have not been explored with cultured cells. In this study, two human cell lineages of varying phenotypes were used to investigate the effects of sample preservation methods. Cells were cultured directly onto quartz substrates and either formalin-fixed, desiccated or air dried. The results indicate that the methodology applied to cell cultures for Raman analysis significantly influences the quality and reproducibility of the resulting spectral data. Formalin fixation was not found to be as universally efficient as anticipated for a commonly used fixative. This was due largely to the inconsistency in sample preservation between cell lines and loss of signal intensity. Sample air-drying was found to be largely inconsistent in terms of spectral reproducibility. Our study shows that sample desiccation displayed good spectral reproducibility and resulted in a good signal-to-noise ratio. Lipid and protein content in both activated and inactivated cells were maintained and provided a more controlled method compared with air-drying, revealing that the speed of drying is important for sample preservation.
Raman spectroscopy assesses the chemical composition of a sample by exploiting the inherent and unique vibrational characteristics of chemical bonds. Initial applications of Raman were identified in the industrial and chemical sectors, providing a rapid non-invasive method to identify sample components or perform quality control assessments. Applications have since increased and sample sizes decreased, leading to the onset of micro-Raman spectroscopy. Coupling with microscopy enabled label-free sample analysis and the unveiling of total chemical composition. Latter adaptations of Raman have advanced into biomedical diagnostics and research. Alongside technical developments in filter systems and detectors, spectral peak intensities and improved signal-to-noise ratios have facilitated target molecule measurement within a variety of samples. Quantitative sample analysis applications of Raman have contributed to its increasing popularity. Through these exceptional capabilities, potential Raman spectroscopy utility in biomedical research applications has expanded, exemplifying why there is continued interest in this highly sensitive and often under-used technique.
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