is my sentences correct

The -ed ending "obtained" (i.e. the use of the past participle) does not affect whether you use "is" or "was". Depending on which of these you use, the sentence will have different meanings.

I simplify the sentence to make my point clear:

(1) "The yield obtained with deproteinization time of 30 minutes is 2.76 %".

The present "is" can be employed, as here, to discuss something that is habitual, normal, usual, commonly experienced, etc. [Every time the process is carried out,] the yield obtained is 2.76%.

If, however, you are talking about one single experiment, you would use 'was'.

(2) "The yield obtained with deproteinization time of 30 minutes was 2.76 %". [When I carried out the experiment,] the yield was 2.76%

Also, it would be better to say "with a deproteinization time of 30 minutes"

Will this work for you? " The yields obtained with deproteinization times of 30, 60, and 90 minutes without neutralization were 2.76%, 1.83%, and 0% respectively ." I base that on the fact that there were three different yields ( plural ) at three different times. ( Thus the plural past-tense "were" )

Yes. Absolutely.

thank you, you made it clear

yes, i did not realize it, thank you