Should Chemical names be used at the start of sentences?

Which of the following sentences is more grammatically correct:

'Calcium was reported in the sample, but not Na.'

or

'Ca was reported in the sample, but not Na.'?

Or are they both fine?

If you use the word for the first element, it reads much better if you also use it for the second one.

Calcium was reported in the sample, but not sodium.

If you clearly indicate the symbols of the elements in the previous text, like calcium (Ca), sodium (Na), then you can use only symbols in the following text.

Ca was reported in the sample, but not Na.

Even in scientific texts, they don't usually use just symbols when elements are mentioned for the first time.