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Glass backed TLC plates are a godsend for those working with TLC staining solutions. Spots are a lot clearer when viewed backwards through the glass, and the surface will hold up under harsh conditions and heavy heating better than aluminum. As an added bonus, because the plate can’t bend the spots always rise straight.
The Tools
There are several different types of glass cutters available, primarily differentiated by their cutting edge. We use a diamond tipped cutter for the repetitive, close work and a hardened carbide wheel for the long initial cuts. The carbide wheel was around $6 at a local hardware store, and will be dull after a dozen cuts or so. But while it’s sharp there’s little better.
The Cutting Process
Begin by measuring out the length of your final plates. By default I divide our 20x20cm sheets into quarters, for ease of measurement. Mark deep on the plate with a pencil, so the markings are visible on both sides. Measure from both sides.
Line up a metal straight edge on the two markings, and use either a carbide wheel or fresh diamond tip to make a single cut across the sheet. You should here a soft scratching sound, and the cut should be a light green line in the glass, visible from all angles. Keep the cutter almost completely vertical with firm pressure, and don’t make more than one cut.
Breaking off the narrow sheets is the most challenging part of the process, which is why I favour a new cutting wheel. Once your cut has been made angle the sheet slightly, cut down. Array your fingers evenly across the sheet and with medium pressure push down. If your cut is sound and your pressure even you should get a nice, straight break. 
Switching to the diamond tipped cutter, score the sheets into segments of a comfortable width. Again, you want clean, single cuts that are a faint green colour when the light catches them. The narrower the plate the harder the break. If you work close 1.5cm is plenty of room for five spots, but keep to your own comfort level.
To break off the plates find a sharp surface and put the cut about a millimetre over the edge. I’m using a glass plate for my surface in this photo, but I’ve heard good things about marble tiles as well. Ask your local glassblower if there’s nothing handy; most counters don’t have a sharp enough edge.
Place the straight-edge flush with the cut and push directly down [1]. The plate should snap off and both it and the straightedge will drop to the counter. Slide the next cut into position and repeat as needed.
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[1] If you’re plates are breaking diagonally check your positioning. For the longest time I was pushing from about half a centimetre to the side, which broke a good 10% of the plates.
Chemtips 
I made a template on a piece of computer paper and laminated it. It had the 4 rows marked off and each row was segmented into the width of the TLC plate. Placing the large glass TLC sheet on top allowed the scoring lines to show through – measure once cut ad nauseum. 🙂
I had three widths of TLC plates in stock at all times. The largest TLC plate was as wide as my TLC chamber would hold. I could get 10-12 lanes on there for monitoring column chromatography. The middle size comfortably held three lanes for basic reaction monitoring: SM, cospot, rxn. The smallest size barely held one lane and was used if I needed to sample different eluent ratios without wasting a lot of TLC glass.
I do almost the exact procedure, but to break my plates once scored, I take my ruler and place it underneath the plate, lined up with my score. I place my fingers across the line as evenly as possible and push gently but firmly. I almost always get a perfect break. Holding up the plate and trying to break it the way you’ve shown always makes me nervous of cut fingers 🙂
The first break is definitely the hardest to do properly. The way my fingers are splayed there’s little chance of injury (my hands move away from the lower half of the sheet after it breaks).
I’m always interested in improving my technique though, so the next time I make plates I’ll give your method a try.