Robert...there are 100's of reports on methods
a circle of cotton twine soaked in meths....light then light, bend/snap
a nick with a 3 square [triangular] file ......then bend/snap
The mini grinder was from JayCar.......don't be tempted in using their reinforced cutting discs .....but get a 6 pack of 20 mm diameter diamond dusted cutting discs....[cheap as chips] $10.00 for 6 discs + 2 arbors including postage to OZ]
The diamond dust coated disc will slice through the toughened glass capiliary tube like butter ...cut off to length.....do not attempt to polish.....wave each end of the capiliary tube over a gas flame until the end of the glass tube is red.....[then air cool]...this actually fluidises the glass and the sharp edge with cutting marks are gone!
The finished end and face surface will be absolutely suitable for an O-ring to be compressed against
I also use the same diamond dusted cutting discs for cutting K&S brass telescopic tubing
Derek