nutts said:
I used the "bold last letter" method to emphasise a corrected spelling.

The spelling of the word is correct... the formatting of the letters is non-uniform...
Anyway, I believe your reasoning behind your incorrect spelling doesn't hold water... I understand why you have used the argument, but in this case it is flawed.
Isotope in this case is not similar to "metal"... rather it is more similar to "brick". For example, "bricks" can imply a number of a similar type of brick. Isotope would fall into this type of plural, i.e. isotopes can imply a number of similar types of isotope
And in order to keep this on-topic, we always used the flat metal screen across the fireplace and even if the hearth exists, then you can always hook it to the wall at each corner, but a few inches off the floor. If marking the hearth is an issue, then add a rubber (type) strip to the bottom of the fireguard
Bricks is a strange case, as it appears to be used randomly.
"Brick-wall" for instance. Not Bricks-wall. "Brick-built", not Bricks-built.
"What's this wall made of?" "Its made of brick"
Or to describe me... "built like a brick sh*t house"
Ergo the word "brick" is used as both a plural and singular.
Ditto "glass".
"What have you got there?" "Some glasses..." (this implies a pair of specs or some Champagne flutes)
"What have you got there?" "Some glass..." (this implies "some glass" in the literal sense)
And I believe the same goes for isotope.
"What have you got there?" "Some isotopes..." (this implies isotopes of more than 1 element)
"What have you got there?" "Some isotope..." (this implies isotopes of just 1 element)