Tardigrades, carrot carotenoids and the DIY Raman spectrometer (V)
The thumb-sized, solid state green laser (30mW or 50mW) for our Raman experiment was
already depicted in our recent May issue.
It is ideally suited, due to its intense green light, its well-defined
output wavelength (532 nm) in combination with a reasonable bandwith and
wavelength stability over time. No need for active cooling, control or
recalibration. It is simply doing its job, without boasting, reliably.
Exactly like some politicians, isn't it?
Fig. 2: The output of the same laser signal as in fig. 1, but this time with a much better resolution on the x-axis. In this zoom view it is becoming apparent that our green laser (similar as virtually all green 532 nm lasers, so-called DPSS solid state lasers) is delivering a very fine signal at a position close to 532,0 nm. The bandwidth FWHM ("Full Width at Half Maximum") appears to be in the range of ca. 1 nanometer - perfectly sufficient for our needs.
Fig. 3: In the case of Raman spectroscopy the position of the Raman bands has to be translated to so-called Raman shift values. Those shift values are indicating the distance from the laser signal, with the laser signal arbitrarily set to zero. Furthermore the wavelength shift is being converted to its reciprocal value, thus ending up with a [1/cm] unit. At first sight all this might sound terribly confusing - but it is just a math trick in order to get identical Raman shift values even when using different laser wavelengths. As a consequence the Raman shift e.g. of a diamond will be always about 1332 [1/cm], no matter whether a green or a red laser are used. Those among you who are getting headache from this can simly use one of those fine online wavelength to Raman shift converters. In any case the wavelength of the green 532 nm laser light will then appear as a Raman shift of zero.
One might think now about simply switching on the laser, turn its beam towards the sample and hope to receive a perfect clean Raman spectrum in return (like the one shown in fig. 5). Sadly, no! First of all there is an excess of stimulating laser radiation which has to be filtered away - otherwise it would overwhelm and spoil the spectrum:
Fig. 4: The Raman catastrophe - a hopeful spectrum of carotene, made without long pass filter, just for demonstration purposes. The spectrum is being overwhelmed by the laser radiation, drowning all the Raman signals in a terrible mess.
This is what we would like to reach instead: a perfectly usable Raman raw spectrum
of ß-carotene (this time we used no tardigrade carotene,
but intead carotene from carotene grocery pastilles for the hypochondriacs and
artificial tanning addicts among us).
Fig. 6: But there are some more pitfalls to consider. In the spectrum above only a long-pass filter was used. This filter is well able to filter out the primary laser but it doesn't influence anything on the further right side of the spectrum, beyond 350 cm-1. As a consequence some very weak satellite signals of the triggering laser will become visible as well. Even though the laser is concentrating most of its radiation at 532 nm (Raman shift zero) it is still delivering some spectral impurities (satellite signals) which are becoming visible due to the enormous amplification needed for Raman spectroscopy. As a consequence additional signals, like the one at Raman shift 1900 are showing up in the spectrum. This is no problem for the whole-hearted spectroscopist who might be able to deal with the situation. But the beginner will be irritated by those ghost signals and - worst case scenario - interpret them as bands resulting from the sample under investigation. As a consequence a so-called laser line filter in front of the laser can be recommended. It is simply cleaning the laser signal, preserving the 532 nm signal and removing everything else. Nevertheless one should keep in mind that any laser cleaning filter will weaken the Raman bands as well. So, at least in some tricky cases or for the first experiments, one might be better off to work without the laser cleaning filter in order to make the Raman signals better visible.
Fig. 7: Last but not least one should be aware of the individual measurement environment. As we are measuring in the wavelength region of visual light, fluorescent light, old-fashioned 'energy saving' lamps and also strong sunlight might interfere with our spectra. E.g. the band with a Raman shift of 2420 as shown above did appear only when an energy saving (fluorescent) lamp was switched on in the lab room during the time of the measurement.
Fig 8: A further typical source of mistakes: in this example the sensor background noise was forgotten. As a consequence the resulting spectrum is an overlay of the actual spectrum and the (different) susceptibilities of the CCD sensor channels (pixels). Modern spectrometers are able to perform this kind of background correction automatically, whereas in the case of older spectrometers it might be necessary to make a measurement without the sample first and use it as a blank frame for background substraction afterwards.
Even though all this might sound tough for some of you
it must be emphasized that experimental Raman spectroscopy can be tremendous fun and
intellectually rewarding, in particular for those who cannot afford more expensive
professional equipment at a price tag of, let's say 20,000 US$.
© Text, images and video clips by
Martin Mach (firstname.lastname@example.org).