The word 'sample' is used differently in statistics as compared to among the chemists.

If you have 20 buckets of water from 20 lakes, one from each lake, then you would have a sample of 20. (We statisticians say that you have one sample.) Then the "unit of investigation" would be the bucket of water. Of course there would be a random variation between the buckets.

Maybe you have measured each bucket 3 times, so that you get 3 values for each bucket. But the sample size would still be 20 (n=20). Of course there there can be a random measurement error so that there is a variation among the measurement. There are two sources of variation, one between the buckets and one between the measurements. (By the way, it better to have 60 buckets of water (from 60 lakes) and one measurement per bucket than to have 20 buckets and 3 measurements per bucket.)

What is what among the "a, b and c" values and negative control. Please explain more.