Chromatography Forum

Since you are a student I will pose the answer in the form of a question: The calibration plots relative detector response against quantity (mass or concentration for example) of analyte. If the calibration is linear, then relative response per unit quantity is constant over the range of the calibration. By relative response I mean the response to the analyte divided by the response to theinternal standard.

Do you expect the relative detector response per unit quantity to change if you make up new solutions ?

Peter

If I were to assume that the solutions are perfectly the same AND the GC is good, would I expect them to change? No. But since I've been seeing variability in my previous injections when injecting the same standard 3 times I suppose this means I have to use the same solution for calibration and experimentation.

You are confusing the poosr repeatability of your method with a change in detector response. Let me pose another question - will the repeatability change if you make up a different set of solutions ?

Peter

When you say repeatability doesn't that imply the change in the peak areas, retention times, peak heights (and other factors) for the same solution injected under the same conditions over several times?

Peter, please have a look at this http://i.imgur.com/GndBQ.jpg It's a print screen shot of the injections I used for the calibration.
I hope you can explain things based on this, as you can see my understanding isn't great.

Hi Fang

Yes, repeatability comprises the things that you list. Do you think that any of these will change if you make up a new set of solutions ? Hint; repeatability is (or should be) measured by repeated injections of the same solution, and from your earlier posts it depends on the way that the instrument is set up and operated. Will making new solutions change the way that the instrument is set up and operated ?

I must decline your request to analyse your raw data - I have plenty of data of my own that I need to analyse.

Peter

Will making new solutions change the way that the instrument is set up and operated ?

No, it will not. The GC instrument is the same as is the temperature and pressure settings since it's set and loaded when I use the instrument.

But I still observe different readings for the same solution over different injections for a given concentration and for the same concentration a different batch of solution gives different readings. I'm afraid I don't quite follow you.

Will making new solutions change the way that the instrument is set up and operated ?

No, it will not. The GC instrument is the same as is the temperature and pressure settings since it's set and loaded when I use the instrument. So this answers your question "Is the calibration curve only valid for the solutions prepared? I mean, if I use up the standard solutions or even if I use up the internal standard solution or if I prepare a new standard solution is the calibration curve still usable?" (24 Sept). If you make new solutions the instrument and method will not change, therfore the repeatability will not change. I would add that with a method like yours that is not very repeatable it owuld be a good idea to run a calibration each time that you run a batch of samples.


But I still observe different readings for the same solution over different injections for a given concentration this is the repeatability of the instrumentand for the same concentration a different batch of solution gives different readings this is due to the repeatability and to the difference between the solutions. If you calculate the mean peak area for each set of replicate injections of a given solution at a given concentration the effects of poor repeatability will be reduced, and you should find that the mean of (peak area per unit mass of analyte) stays the same for different concentrations and different solutions . I'm afraid I don't quite follow you.

Peter