Peak height vs peak area

[hibu]

Hi all!

I am measuring ethanol with gc and my sample is strong liquor. It is prepared by pipetting 2 ml of strong liquor into a 20 ml volumetric flask and adding 0,4 ml of n-propanol. The flask is filled up to the mark with water. The n-propanol is added as internal standard and its final volume is 2 %.

The concentrations of my calibration solutions are 1 %, 2 %, 3 %, 4 % and 5 % (the solute is ethanol). In each of these solutions 2 % of n-propanol is added as internal standard. As a result, I get peak areas of ethanol and n-propanol. To calculate concentration of the sample i divide area of ethanol with area of propanol of each calibration solution and plot them on a curve. The ratio between area of ethanol and area of propanol of the sample is placed on the graph and the concentration can be calculated.

My question is, my instruction tells me to calculate the concentration using also the peak heights. I only know the height of the sample (peaks of ethanol and n-propanol). Do I need to calculate the peak heights of the calibration solutions using peak areas of calibration solutions as well to get to the concentration?

Any help will be appreciated!

[Consumer Products Guy]

I am measuring ethanol with gc and my sample is strong liquor. It is prepared by pipetting 2 ml of strong liquor into a 20 ml volumetric flask and adding 0,4 ml of n-propanol. The flask is filled up to the mark with water. The n-propanol is added as internal standard and its final volume is 2 %.

Any help will be appreciated!

You asked for help.....

Wow - your solutions are VERY concentrated. Once the ratio of ethanol and n-propanol are "set" in your aqueous solutions, you can dilute much more. Most times aquoues injections are held to 0.5 microliters or less.

I'd only use peak areas; we only used peak height when estimating limits of detection/quantitation.

And why even bother with a volumetric flask? Especially when adding your internal standard AFTER going to the mark, because you'd have to positively mix well. And the actual volume of the mix or the water doesn't matter because you're doing internal standard.

What I'd do is is use a disposable vial like 20 - 40ml or so with a cap that makes a good seal. I'd add a constant volume water (like about 80% of the vial capacity), THEN I'd pipet in your sample (because of the water already there, you'd lose less ethanol to volatilization), then add the internal standard, then cap and mix well. Then I'd dilute this further with water.

I think your written sample procedure is sloppy.

If I was designing the sample preparation for robustness, I'd likely prepare my internal standard solution in water, dispense accurate volume of that into my 20 - 40ml vial, then add my 2ml sample, then cap and mix, then dilute further with water. I think adding 0.400 ml of internal standard solution is not that quick, easy, or accurate, unless you're using an accurate dispenser such as Hamilton.

[hibu]

Thank you for your comments!

[James_Ball]

Hi all!

I am measuring ethanol with gc and my sample is strong liquor. It is prepared by pipetting 2 ml of strong liquor into a 20 ml volumetric flask and adding 0,4 ml of n-propanol. The flask is filled up to the mark with water. The n-propanol is added as internal standard and its final volume is 2 %.

The concentrations of my calibration solutions are 1 %, 2 %, 3 %, 4 % and 5 % (the solute is ethanol). In each of these solutions 2 % of n-propanol is added as internal standard. As a result, I get peak areas of ethanol and n-propanol. To calculate concentration of the sample i divide area of ethanol with area of propanol of each calibration solution and plot them on a curve. The ratio between area of ethanol and area of propanol of the sample is placed on the graph and the concentration can be calculated.

My question is, my instruction tells me to calculate the concentration using also the peak heights. I only know the height of the sample (peaks of ethanol and n-propanol). Do I need to calculate the peak heights of the calibration solutions using peak areas of calibration solutions as well to get to the concentration?

Any help will be appreciated!

The peak height would be read from the chromatogram not calculated from the area. The relationship between height and area depends on the shape of the peak and would be very difficult to calculate. If you print the chromatogram it should show the peak height and area, or if on a chart recorder you can measure the peak height from the baseline to the top of peak, assuming it doesn't exceed the top of the chart recorder.

[James_Ball]

I am measuring ethanol with gc and my sample is strong liquor. It is prepared by pipetting 2 ml of strong liquor into a 20 ml volumetric flask and adding 0,4 ml of n-propanol. The flask is filled up to the mark with water. The n-propanol is added as internal standard and its final volume is 2 %.

Any help will be appreciated!

You asked for help.....

Wow - your solutions are VERY concentrated. Once the ratio of ethanol and n-propanol are "set" in your aqueous solutions, you can dilute much more. Most times aquoues injections are held to 0.5 microliters or less.

I'd only use peak areas; we only used peak height when estimating limits of detection/quantitation.

And why even bother with a volumetric flask? Especially when adding your internal standard AFTER going to the mark, because you'd have to positively mix well. And the actual volume of the mix or the water doesn't matter because you're doing internal standard.

What I'd do is is use a disposable vial like 20 - 40ml or so with a cap that makes a good seal. I'd add a constant volume water (like about 80% of the vial capacity), THEN I'd pipet in your sample (because of the water already there, you'd lose less ethanol to volatilization), then add the internal standard, then cap and mix well. Then I'd dilute this further with water.

I think your written sample procedure is sloppy.

If I was designing the sample preparation for robustness, I'd likely prepare my internal standard solution in water, dispense accurate volume of that into my 20 - 40ml vial, then add my 2ml sample, then cap and mix, then dilute further with water. I think adding 0.400 ml of internal standard solution is not that quick, easy, or accurate, unless you're using an accurate dispenser such as Hamilton.

It looks like he is adding the internal before filling to the mark on the flask which would make it a constant concentration in the final 20ml volume.

A 500ul syringe would allow adding 0.4ml of internal quite accurately I would think.

[hibu]

Thank you for your helpful comments!

[DR]

To answer your question as I understood it,
Use either peak heights or areas as read from your data system or integrator for all injections.

If using heights, it's EtOH height/IPA height
If using areas, it's EtOH area/IPA area
and you plot the ratios...

[hibu]

Thank you for your comment!