This isn’t the first post about focus. In the previous post focus it became very clear – to me at least – how important proper focus is for the throughput of the optical system. The previous post was only about the planoconvex lens and the optical fiber. This will be about the microscope objective and the sample.
In the very beginning of my Raman shopping spree I aquired something that looks a lot like a Nikon CFI Plan Apo VC 20X. And you know what they say: If it quacks like a duck..
The microscope objective is very suitable for (this) Raman application because of the very large numerical aperture¹ (0.75) and the large diameter of the exit pupil (app. 14mm):
The catch is that the working distance is 1 mm. It’s on the low side of what you’d expect for 20x microscope objective, but I guess it’s the trade-off for the high NA. So what’s the problem with that then?
It means that the sample must not be further away than 1 mm from the front lens, and this where the trouble starts. No cuvettes that I could find anywhere have a wall-thickness different from 1.25 mm. The obvious answer is then to use a different sample holder.
So what thin-walled glass ware is out there? I couldn’t find anything but 10 mm NMR-tubes. They’re not cheap ..but they’re not terribly expensive either. However I don’t see any way of fitting one inside the Raman spectrometer.
Eventually I ended up with this:
It’s a 3M 6220J M125 diamond whetstone. The particle size is 125 micron, which roughly translates to grit 120. It cost me around 30€, which was 3x the price of a pair of used 5.00 mm quartz cuvettes from Hellma.
The second picture is not for the faint of heart, now you’re warned, and it did take some will-power to put the diamond abrasive to the quartz:
It took several hours of work to remove 0.55 mm of quartz glass on one side of one cuvette. The wall thickness is now around 0.70 mm, thin enough to put the sample in the focal point of the microscope objective.
Obviously, the wall of the spectroscopic cell was no longer crystal clear, but polishing with cerium(IV)oxide took care of that.
Cerium(IV) oxide comes in a variety of particle sizes, and I have no idea what the stuff I have is, but the most common values are in the 1-3 µm range. This is equivalent to grit 8000 or more, but polishing with cerium(IV) oxide is actually chemical-mechanical planarization – and that’s all I know about that.
During the polishing of the cuvette a small amount of CeO₂ snug inside the glass ..and stayed there. Cerium(IV) oxide is very hard to dissolve, and 50% sulfuric acid wouldn’t do it and neither would conc. sulfuric acid. 50% H₂SO₄ and 35% H₂O₂ was also ineffective, and neither conc. HNO₃ nor conc. NH₃ did anything.
It turns out CeO₂’s so difficult to get rid of, that there are publications about the kinetics of it. And after consulting the literature² I ended up being able to remove the CeO₂-traces with 110°C hot concentrated sulfuric acid.
And here’s the fruit of my (unexpectly hard) work:
It’s hard to make product pictures of glass-ware, so I filled them with an indigo carmine solution. The cuvette on the right is the slim version, with one wall 0.55 mm thinner than the other.
Here’s another photograph, just because:
I have a pair of cheap glass cuvettes from china, that I will file down once I forget how much work it took. The upside is that they are made from BK7 or something similar, so they should be a bit softer than quartz.
¹ There are other microscope objectives out there with higher NA, but anything with a NA > 1 requires oil-immersion and most of them have a working distance of a cover glass ie. just 0.17 mm. Large WD and large NA makes the price explode..
² Dissolution of Cerium Oxide in Sulfuric Acid
Um N., Miyake M. and Hirato T. Green Energy and Technology pp 165-170