Biochemistry- How to increase the sensitivity of the assay?

Question

Conditions of assay:

3mL = final volume

1L Biruet reagent made of the following. Each Test tube has 1.5mL reagent!

- 1.5g CuSO4.5H2O

- 6g Sodium potassium tartate

- 300 mL of 10% w/v, sodium hydroxide

- Water to make up 1L

Which of the following is the best way to increase the sensitivity of an assay (ie assay lower concentrations of protein)?

A- Make up a more concentration Biuret solution so you can use a smaller volume of it in the assay

B- Double the volume of everything (making the final volume 6mls)

C- Increase the volume of protien solution added (keeping everything else the same)

D- Miss out the water from each tube

E- Half the volume of everything (making the final volume 1.5mLs)

thanks

Answer 1

In general, reducing the volume of an assay will increase the sensitivity, so for the question posed, it would most likely be (E), to halve the volume of everything and make the final volume 1.5 mls.  This doesn't always work, but for most biochemical assays, it does.  None of the other choices will work.  

Answer 2

C - increase the volume of protein solution added. Adding twice as much sample means there's twice as much protein compared to the amount of biuret reagent. In practice, you'd want to put more sample in but keep the volume the same, so you would miss out the water/diluent from each tube.

Option A is wrong - using more concentrated Biuret solution but less of it means you are still adding the same absolute amount of reagent.

B and D and E are wrong for the same reason - all these things still keep the same ratio between the sample and the reagent. If the given  amount of reagent will only reliably allow you to detect a certain amount of protein, the only option is to add more protein.