5 points calibration curve

Discussions about GC and other "gas phase" separation techniques.

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Hi,

My name is tinie and im new in using headspace GCFID. My project is now to test residual AN in NBR latex using headspace gas chromatography analysis.

As this is my first time developing new method, i would like to ask how do i plot 5 calibration curve? how to decide from which concentration i need to take for my 5 points calibration curve?

Hope to get a help from you guys.
Plotting the curve is easy. You just plot concentration (x-axis) vs. instrument response (y-axis) and get the regression constants. The tricky part is figuring out how much to add.

What is the concentration of AN you expect in the rubber?

I do a lot of headspace SPME and I pretty much always calibrate in the matrix and I use the method of standard addition for calibration. There are lots of things you have to worry about when you're dealing with a matrix like rubber gloves.

If you know the concentration you expect, you might try adding 5X, 10X, 15X, etc. that amount directly to the glove material and see where you are. Simply adding it to the vial doesn't mean you're good to go. You have to equilibrate it into the matrix.

If adding 5X doesn't increase the response much (or at all) after equilibration, then for sure you need to add more. If it gets you out of the linear range of your detector, then you've added too much.

If you're a total newbie to headspace analysis, I would get a good textbook on it and get yourself a good understanding of how headspace analysis works and also the matrix dependence of headspace analysis.

I highly recommend:

"Static Headspace Gas Chromatography, Theory and Practice" by Ettre and Kolb.

I don't see it on Amazon.com but I do see it on Ebay. My copy was printed in 1997 but the information in it is all still valid. 1997 is the green cover. The older one is dark blue/purple. Again, all of the information in it will still be valid. Headspace analysis is a pretty mature field.

There are specific examples of how to determine styrene in pellets, vinyl chloride in pvc polymer, etc. You can use those mindsets to decide how to approach your problem.

Good luck.
I saw we had some NBR gloves in our lab. I took one and cut it up. Analyzed 0.50 g sample and discovered it contains some residual xylene isomers. I tried the method of standard addition to calibrate for them (combined xylenes).

Standard Preparation

4.00 g and 2.00 g of glove material in separate 22 mL HS vials. I added 0.2 microliters of a neat mixture of xylene isomers (Fisher Scientific, I've had them for years) and sealed the vials. Heated overnight at 100 degrees C in a dri-bath heater I have in my lab. This step basically cooks the xylenes into the NBR matrix. Overnight could be overkill, the timing just worked out well for me.

The density of my xylenes standard is 0.868 g/mL.

0.868 g/mL x 0.0002 mL x 10^6 ug/g/2.00 g = 87 ug/g (ppm)
0.868 g/mL x 0.0002 mL x 10^6 ug/g/4.00 g = 43 ug/g (ppm)

Solid-Phase Microextraction

Fiber = Carboxen/PDMS (black) from Sigma-Aldrich
0.50 g sample size in 22 mL headspace vial
Preheat to 60 degrees C for 20 min.
Sample headspace in the vial for 30 minutes at 60 degrees C
Inject into the GC with FID detection.
Analyze the unfortified sample in duplicate to get some statistics on precision.

Detector response was linear over this range. Using method of standard addition (summed the areas of the xylene-isomer peaks), I determined the glove material to have 1.9 +/- 0.2 ppm total xylenes in it.

I took a guess at the concentration of the xylenes in the glove material and made my additions accordingly so that it'd likely still keep me in the linear range of the detector. My guess was pretty good but it would probably be better if I had calibrated more like adding 20 and 40 ppm instead of 40 and 80 ppm.

In any case, this simple experiment gave me an idea of what I needed to target. With this knowledge, I can refine my method development to arrive at the final conditions.
Hi,

Thank you for your sharing. I used ASTM D4322-96 method to analyze the residual of AN in latex using 1,2 dichlorobenzene as solvent and standard AN to plot my calibration curve.


This method use external calibration curve by adding 0.500 g of AN standard into 5ml of dichlorobenzene.

Below is the steps that i do.

1) dilute AN standard from pure to 1000 ppm.
2) Prepare 10ppm, 20ppm, 30ppm, 40ppm and 50ppm of AN using stock solution prepared.
3) Take 0.500 ml of each concentration and add 5 ml pure dicholorbenzene.
4) i run headspace and gc using FID detector.

is my method is correct?
I don't have access to the ASTM standards so I can't tell you if your method is correct.

Does the method call for dissolving the latex material in the dichlorobenzene?

My understanding is that those ASTM methods are developed and then validated across many labs. If you are tied to that method from a regulatory standpoint, you should follow it to the letter.

What I presented was what I would do if someone posed your problem to me. I'm not currently tied to that ASTM standard.
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