1064 nm Dispersive Raman Micro-Spectroscopy and Optical Trapping of Pharmaceutical Aerosols

Peter J Gallimore, Nick M Davidson, Markus Kalberer, Francis D Pope, Andrew D Ward

Research output: Contribution to journalArticlepeer-review

7 Citations (Scopus)
298 Downloads (Pure)


Raman spectroscopy is a powerful tool for investigating chemical composition. Coupling Raman spectroscopy with optical microscopy (Raman micro-spectroscopy) and optical trapping (Raman Tweezers) allows microscopic length scales and hence femtolitre volumes to be probed. Raman micro-spectroscopy typically uses UV/visible excitation lasers, but many samples including organic molecules and complex tissue samples fluoresce strongly at these wavelengths. Here we report the development and application of dispersive Raman micro-spectroscopy designed around a near-infrared continuous wave 1064 nm excitation light source. We analyse micro-particles (1-5 μm diameter) composed of polystyrene latex and from three real-world pressurised metered dose inhalers (pMDIs) used in the treatment of asthma: salmeterol xinafoate (Serevent®), salbutamol sulfate (Salamol®), and ciclesonide (Alvesco®). For the first time, single particles are captured, optically levitated and analysed using the same 1064 nm laser, which permits a convenient non-destructive chemical analysis of the true aerosol phase. We show that particles exhibiting overwhelming fluorescence using a visible laser (514.5 nm) can be successfully analysed with 1064 nm excitation irrespective of sample composition and irradiation time. Spectra are acquired rapidly (1-5 min) with a wavelength resolution of 2 nm over a wide wavenumber range (500-3100 cm-1). This is despite the microscopic sample size and low Raman scattering efficiency at 1064 nm. Spectra of individual pMDI particles compare well to bulk samples and the Serevent pMDI delivers the thermodynamically preferred crystal form of salmeterol xinafoate. 1064 nm dispersive Raman micro-spectroscopy is a promising technique which could see diverse applications for samples where fluorescence-free characterisation is required with high spatial resolution.

Original languageEnglish
Pages (from-to)8838–8844
Number of pages7
JournalAnalytical Chemistry
Issue number15
Early online date29 Jun 2018
Publication statusPublished - 7 Aug 2018


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