Let us start with some basics. A UV cell, aka a UV cuvette, can be made of a few different materials. The most popular materials are disposable plastic, optical glass and, of course, UV quartz. Out of the three possible options only one of them will not break when dropped. Can you guess which one? That’s right- the disposable plastic cells are pretty robust and can handle drops with ease. On the other hand we have Optical Glass and UV Quartz for which there is a good chance they will break if mishandled.
A client once asked, “Is there anything you can do to make these cuvettes stronger?” Well, you can make the bonding of the joints stronger, which could result in a stronger cell. However, glass is still a fragile material (does anyone remember Mr. Glass from the movie “Unbreakable”) and until someone invents unbreakable glass, then you’ll just have to be careful when working with a UV cell.
Assembly of a UV Cell
To understand why it is tough or even impossible to repair a UV cuvette, we have to understand how a cell is assembled. A standard Type 1 spectrophotometer cuvette is made with five different pieces of glass. They are assembled as follows:
Preparing the UV Cuvette Plates
Before even getting to the assembly phase, we need to slice a smaller block from an ingot. Once we have the smaller block, we slice the block into smaller plates. The plates are then put onto grinding machines to get them close to the 1.25 mm thickness that we use for our standard wall thickness.
Frosted vs Polished Plates
In the first polishing phase, we use a coarse polishing powder for this process since it takes off the most material in the shortest amount of time. If we are making the walls with a frosted finished, then the process stops here and the plates are sent for cutting to the correct width. Once they are cut we then use a fine polish on the other side of the walls to make them smooth so no particles get stuck on the inside walls.
To make the polished walls a special piece of equipment called a lapping machine is used. To get an optically transparent finish we use a fine polishing powder and we need to apply a fine polish to both sides of the plates. The plates are then cut to 45 mm in height and 12.5 mm in width.
Many UV cell manufactures still make cells by hand. This technique uses a set of square metal jigs (see photo below) to assemble a cuvette. The base of the cuvette is put into the jig first and then the side walls are fused onto the base of the cuvette. Next we attached the front and back windows. Finally the cuvettes are annealed to remove any stress that was caused during the fusing process.
Fire Fusing Vs. Fritting a UV Cuvette
When we spoke about cuvette strength earlier in this article this is what we were referring to. Let’s go each of these processes so we can fully understand them.
To frit a UV cell, you apply a glass powder to the edges of the plates where the face plate will be attached. The assemblies are then placed in a furnace and the glass frit melts, gluing the plates together. The fritting process is less labor intensive and much cheaper to do. This method is a popular choice for Chinese cell manufactures. Fritting a cell is a great technique if you are on a tight budget, but it can be a problem is you use solutions that are not frit friendly and they can eat away at the frit and cause the cell to break.
Also using a fritted cuvette at high heat or with corrosive materials is a big no-no. Any temperatures over 400ºF will cause the frit to loosen and the UV cuvette will fall apart.
The technique we use for most of our cuvettes is fire fusing. Using this technique, the plates are fused with a hydrogen-oxygen torch. Quartz melts at about 1,100 C and when the quartz softens and another plate is attached they meld together into one piece of glass. It is like taking two sticks of butter and softening the ends and then pressing them together and freezing them. What you get is one nice seamless cuvette that is amazingly strong and can resist the most volatile chemicals since it doesn’t use any glues or frits.
Of course, the downside to this process is that it takes some time and is more expensive than fritting.
The key to having good fire fusing is the cleanliness of the plates. If the plates have a speck of dust or dirt, this can create bubbles and other imperfections in the fusing boundaries making a weak joint. Therefore, if you are working in a dirty factory, even if you are using fire fusing, the fusing can still stink. Just because a company says they use fire fusing does not necessarily make it a better choice. Overall, fire fusing is the better option but more skill is needed than fritting.
It’s all about quality versus quantity. With frit you can make a ton of UV cuvettes at a low price with ok quality. Or you can make a few robust fire fused cells with a higher price but amazing quality.
So now that we have the knowledge of how a UV cell is assembled we can see that there is a ton of work that goes into it. When a cell is dropped and breaks at the seam, even something that is a “clean break,” is it not worth investing the time and effort in trying to re-polish and re-bond the cuvette. It is much cheaper just to buy a new one and move on with your experiments.
Hope this was helpful!
Please contact us with any questions.
Here’s to your success!!
The FireflySci Team