With conventional continuous wave (CW) Raman spectroscopy, spectra often contain broad background interference or distinct interfering bands when measuring samples outside sample enclosures in ambient light. This may limit the use of flexible and convenient measurement configurations and may force the use of closed sampling systems. [1, 2]
During time-resolved measurements, the detector is only active for extremely short repeated periods of time. A pulsed laser which is used in time-resolved Raman has a very high instantaneous pulse energy and the instantaneous Raman scattering “burst” intensities can be quite high compared with conventional CW Raman scattering. These features lead to a collection of high amounts of Raman scattering in short repeated time periods i.e. the collected Raman scattering to background emission ratio is high. Because of this, time-resolved Raman is significantly less sensitive to continuous background interference like ambient light or thermal emission.
The rejection capabilities can be demonstrated by repeating time-resolved measurements in different levels of ambient light intensity. The following drill core sample measurements were carried out in three different lighting conditions:
1. In a closed sample enclosure
2. In an open sample enclosure
3. In front of a bright lamp
The measurements were carried out using a Timegate Instruments PicoRaman spectrometer and a 532nm pulsed laser. The measurement time was 2 minutes with each ambient light intensity level.
The drill core piece was placed inside a closed sample enclosure. This is a common measurement arrangement with CW Raman.
The sample closure was opened with room lighting (fluorescent lamps) switched on.
In addition to room lighting, a bright light was placed in front of the open sample enclosure and pointed directly at the sample.
The spectra measured in these three lighting conditions are presented below. As can be observed from the spectra, ambient lighting has a minimal effect on the measurements even when a bright light is pointing directly at the measurement spot.
Three spectra measured in different lighting conditions have minimal differences.
Time-resolved Raman provides a convenient way to carry out measurements as it is not sensitive to bright or changing lightning conditions. Furthermore, it does not require impermeable covering from ambient light. This enables the use of flexible and open sampling and measurement configurations.
Learn more about time-resolved Raman spectroscopy by downloading a PicoRaman brochure:
References
[1] Vandenabeele P., Practical Raman Spectroscopy - An introduction, John Wiley & Sons, 2013, pp 44
[2] Zhao J., Short M., Braun T., Lui H., McLean D., Zeng H., 2014, Journal of Biomedical Optics, 19(11)