This question is based on the paper by Ogwu & Cohen, 1998.
Article A Simple Colorimetric Method for the Simultaneous Determinat...
It was mentioned that a 1.33 mM of cysteine stock solution was prepared and later an aliquots of 300 ul containing 100 nmol of cysteine was used as the standard.
I understand that in the stock solution, every 1 litre contains 1.33 mmol of Cysteine. Which in turn, equivalent to 1.33 nmol of cysteine in every 1 ul of solution.
I tried to work out the calculation many times, but I always ended with 399 nmol of cysteine in 300 ul of aliquots.
Can anyone tell me how the calculation in the paper actually works out and where did I go wrong? Thanks!
Yes, math is inflexible that way.
There is an error in the published paper: either the stock solution was actually 0.33 mM, or the standard aliquot had 399 nmol of cysteine.
It is less likely that the aliquot was 75 microliters (which would give the 100 nmol).
I just read the original method published in 1967, does it make any difference if the molecular weight of the chemical was cysteine hydrochloride instead of just cysteine?
It shouldn't - they have to use a larger mass of cysteine hydrochloride compared to cysteine, but 1.33 mM is 1.33 x10-6 moles/liter either way - whether it is moles/liter of cysteine or moles/liter of cysteine hydrochloride. Does the original method claim that 300 microliters of a 1.33 mM solution contains 100 nmoles?
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1270629/
Thank you for your reply.
The original research (as above) did not specified the concentration of the stock solution, but to quote directly from the method, here is what being mentioned:
"The reaction mixture, containing 0.5 ml of aq. cysteine hydrochloride solution (0.05 - 0.5 umole), 0.5 ml of acetic acid and 0.5 ml of acid ninhydrin reagent 1 or 2, was mixed thoroughly. "
You may also refer to figure 3. in the paper for the standard curve reported.
The original 1967 paper lays out the method for determining cysteine with ninhydrin. Their method 2 is the one you want to use: the reagent has excess concentrated HCl so it won't matter if you use cysteine or cysteine hydrochloride.
The 1998 paper has an error - it is a typographical error as described before. The methods of the 1998 paper are probably OK - but I would run standards and check everything - they are sloppy with math.
- Badges
- Science topic
- Similar topics
- Chemistry