Quantifying compounds of a sample can only be done with STD?
Posted: Thu Dec 22, 2011 5:23 am
by seamoro
Quantifying compounds of a sample can only be done with STD? is this statement correct?.... hello........i doing research...extracting compounds from a mushroom.........i just want to confirm this.......i can only Quantify a compound if i have a standard?.........i can just run a sample.....on GCMS or LCMS...... identify the compounds and based on their peak areas in comparison to total peak area....quantify the % of each component in the sample?....is this correct....?/
Re: Quantifying compounds of a sample can only be done with
Posted: Thu Dec 22, 2011 8:32 am
by Alexandre
You will have rough idea doing your way. The problem is that different compounds have different responses.
You can estimate other way by isolating g or mg of compound of interest from your mushroom and then calculate % by weight.
It is not always needed to have STD to know concentration, e.g. if compound is well known it's spectral properties are tabulated and one can, say, by measuring UV absorbance recalculate concentration from the coef of molar extinction (Beer law).
Re: Quantifying compounds of a sample can only be done with
Posted: Fri Dec 23, 2011 6:35 pm
by lmh
... enlarging on Alexandre's answer, there is also the happy situation where you are working on a family of chemicals that contain a single chromophore, but are modified distant to the chromaphore, with the result that they all have the same extinction coefficient. A special case is where you have derivatised a set of compounds that do not absorb, and added a light-absorbing group. In these cases, it's usually convenient to run just one "typical" example for which a standard is available, and apply it to all the others for which no standard is available.
I'd add that "quantify" means different things to different people. Many labs/disciplines have methods that fall short of what an unbiased observer would like to see, but which are founded on a pragmatic approach, and which have been used so long that no one wants to hear uncomfortable questions! The academic world is often only interested in whether the relative abundances of a range of chemicals change, not in what the absolute distribution is, so an inaccurate area % may be adequate. To another person, % must mean % without bias.
For some chemicals for which I can't get standards, I use UV and LC-MS together. MS identifies; UV is more quantitative (given that I can find a related chemical as standard); but I check that UV peak area correlates with MS peak area. If it doesn't, I have to decide whether it's misidentification of UV peaks, or misquantification of MS peaks.