RRF value in unknown sample concentrations

[R&D-Chemist]

Recently I developed a method for GC solvents based on RRF consideration.

Now I am stuck at point, (how to calculate the concentration of the analyte), when we use method for calculating PPM of residual solvent in the given sample.

How to find CA? (Formula worked fine when I had known concentration of sample).


Example:

RRF=(A-a * C-is)/(A-is*C-a)

A relative response factor (RRF) is determined for each analyte using the following equation:

Where:
AA = the area of the analyte to be measured
AIS = the area of the specific internal standard
CA = the known concentration of the analyte in the solution (μg/mL)
CIS = the known concentration of the internal standard in the Solution (μg/mL)

Thanks

[chromatographer1]

Which did you do? or not do ?

You did not make up a std having the analyte of interest and the internal std analyte at known concentrations to give you a RRF?

Or

You made a sample with an internal std at an unknown concentration?

If you have a relative response factor for the internal std and you know the concentration of the IStd and the analyte response then it is simple algebra.

What pieces of the puzzle are you missing? I am afraid I am misunderstanding something here.

CA = AA * CIS / AIS / RRF

Rod

[R&D-Chemist]

This is the way i have to do the calculations as per my method.
1. Using the following equation, calculate relative response factor (RRF) for each residual solvent:
same formula from previous post.

2. Using the RRF above, calculate the amount of each residual solvent in ppm:


PPM=(RRF sample/RRF) *(std*uL/100.0mL)*2.0/(sample wt in mg)*1000

Where,

RRFsample = Individual peak RRF value for each sample injection
RRFstd = Average RRF of Working Standard
µL, std = Volume of Stock Standard
100.0 mL =Stock Standard final volume
2.0 mL = Sample Dilution
Sample, wt, mg = Sample weight (mg)
1000 = ppm conversion factor

Now i have to calcluate amount of the residual solvent in given sample.

since i don't know the concentration of the sample. how should i calcultae
the RRF(based on my formula).

Thanks you for input.

[chromatographer1]

You calculated RRF, let's call them response factors, the response of the analyte per response of the internal std based on an equal wt/wt ratio.

Let say for example, hexane's response is twice the response of methanol based on a weight per weight sample. RRF = 0.5 for MeOH/C6

In other words is a response for methanol at 1mg/mL is 100,000 counts then the response for hexane at 1mg/mL is 200,000 counts.

Once you know the response factor, then say in a sample of unknown sample concentration, the hexane peak area is 50,000 counts. The methanol peak (ISTD) in your dissolution solvent still gives you 100,000 counts.

That means the concentration of the hexane in the sample is 0.5mg/mL.

But you don't know the concentration of the sample that gives you a hexane concentration of 0.5mg/mL.

But if you know the concentration of the hexane in the sample on a wt/wt basis then you can calculate the wt/vol concentration of the sample solution.

Let's say you know the sample contains 2 % hexane, then the unknown sample concentration is 50x that of the hexane peak of 0.5mg/mL, and the sample solution concentration is 25 mg/mL (50 times 0.5 mg/mL)

To calculate the concentration of the sample you must know the concentration of the solvent analyte based on a wt/wt basis and you must know the response relative to a known concentration internal standard.

Otherwise, you cannot calculate the unknown concentration of the sample in solution.

best wishes,

Rod

[Peter Apps]

Have you ever run any standards with known concentrations of the substances that you have to measure in the samples ?

You cannot determine an RRF by injecting samples of unknown composition.

Peter

[R&D-Chemist]

Yes, I did run the standards with known concentrations of the substances that I have to measure in the samples.

thanks

[Peter Apps]

OK, so from the results of the standards (with internal standards included) you can calcualte the RRF for each analyte using the equation in the first post.

The confusion may come from the equation in your second post. When you run samples (with internal standards) you do not measure relative response factors, you measure relative response (as a peak area ratio) and then use the RRF that you calculated from the standards to calculate the quantity of analyte from that relative response.

So instead of:

PPM=(RRF sample/RRF) *(std*uL/100.0mL)*2.0/(sample wt in mg)*1000

The equation should be:

PPM=(RR sample/RRF) *(std*uL/100.0mL)*2.0/(sample wt in mg)*1000

assuming that the numbers and units fit your sample preparation.

Peter

[R&D-Chemist]

Thanks Peter - got the solution

Thanks to all for their input