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The deviation of the measured concentration of the antibody in the model solutions from the nominal value should not exceed 20% for the concentration of 750 ng/mL.

The meaning is: when the nominal concentration specified for the model solution is 750 ng/mL, the values obtained through measurement should not deviate by more than 20% from 750 ng/mL.

I'm not sure whether to use the definite or the indefinite article here.

On the one hand, it is the concentration, it is the level specified for some model solutions.

On the other hand, I remember that we write "keep the solution at a temperature of 10 °C". We use the indefinite article here, and I drilled myself into using it, because it is hard to "feel" why there should be that a there. And "for a/the concentration of ZZZ ng/mL" is a construction that looks similar.

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We need more context to be absolutely certain, but generally you would use the indefinite article in a sentence like that. I agree that your example of "a temperature" is exactly analogous. In fact, I would say that any unit of measure is subject to exactly the same rules (a length of five centimeters, a weight of 10 grams, a resistance of 5 ohms, etc.)

However, (and this is why I say more context is needed) if the text you are quoting has already specified the 750 ng/mL concentration, then the definite article is correct. (I'm not sure if your third sentence is referring to this specification or a previous specification.) If, for instance, there were an earlier sentence stating, "The antibody solutions should be prepared at a nominal concentration of 750 ng/mL", then the would be appropriate here, because it is referring to a specific, previously defined, concentration. In this case, the "750 ng/mL" is actually redundant, since it has already been specified, but would be included as a reminder to the reader.

Another possibility is that the text calls for a range of antibody concentrations: "The antibody solutions should be prepared at concentrations of 250, 500, 750, and 1000 ng/mL." The definite article would also be appropriate in this case, although I would consider that wording awkward if so. I would refer to "the 750 ng/mL concentration". In this case, the reference is not redundant, but it is referring to a specific, previously defined concentration.

I don't know that there is a hard and fast rule that I can articulate, but as a native speaker I definitely get a "feel" for which is correct. The way to think about it may be that the article in phrases like this indicates "concentration" alone, and not the entire phrase. So, if a concentration is being specified for the first time, we call it "a concentration", because it is one of the infinite number of possible concentrations, which we use the of phrase to define. If we refer to the concentration later, we use the definite article, because we are referring to a specific, previously defined concentration, which we use the of phrase to identify.

A couple of examples might help:

I picked an apple out of the basket and placed it on the table. I picked the apple up later and ate it.

It's clear that the second sentence refers to the same apple as the first, even though (because, actually) we have switched from the indefinite to the definite.

Bring the solution to a temperature of 10°C.... Keeping the solution at the temperature of 10°C, ...

The definite article signals to the reader that the temperature has previously been defined (keeping in mind that there might be many steps between those two sentences). I would not say that using the indefinite in the second sentence would be incorrect, but using the definite implies a continuity between the two. On the other hand, using the definite article in the first sentence would be incorrect, assuming that is the first time the temperature of the solution has been specified.

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