Quantitation Gas Chromatography

[phuonghuynh]

Hi all,

Could anybody explain me quantitation in GC is carried out on the basic of peak area ratio or peak height ratio?

I think quantitation in GC is carried out on the basic of peak area ratio more accurate than the basic of peak height ratio but my boss didn't agree.

Thank-you very much for your advices!

[Chris Jones]

G'day!

Yep, peak area is proportional to the amount of material leaving the column. However the detection method will influence how reliable this is.

In very simplistic terms...

If 1 mg of compound X gave a peak area of 2000 units, 2 mg should give an area of 4000 units. This is definately true of flame ionisation detection.

Also, if compound Y has a molecular weight of 200 g/mol, and compound Y has a molecular weight of 400 g/mol, one mg of each will result in two very different peak sizes.

However if you are comparing the ratio of one compound to another and using mass spectrometry, you need to be careful of the ease of ionisation of the compound. For example, alcohols will ionise more easily than hydrocarbons, and hence give a greater area for the same amount of sample.

These problems are all overcome through the testing of standards.

Hope this helps! And welcome to the forum

[phuonghuynh]

Thank-you very much for answering my question!

I understand what you said. I am using Agilent 7890A with flame ionisation detection.

My Boss said that we didn't calculate the result in HPLC on the basic of peak height ratio because of overloading peaks.

You know, in GC when you increase the concentration of the sample, the height of this peak will be increase proportionally but this is not true in HPLC. In HPLC when you increase the concentration of the sample to the limit, the height of this peak can't be increase anymore and the peak broden. So, we calculate the result in HPLC on the basic of peak area ratio.

In GC, if the sample is not purity there are a lot of ghost peak. Some of them can be overlap the main peak and this makes the area of main peak increase. So, We have to calculate the result in GC on the basic of peak height ratio.

I didn't agree of his idea but i can't find explain to him. I didn't also find any articles which relate this problem.

Can you help me?

Anyways, thanks a lot!!!

[chromatographer1]

The same effect of peak overload (actually the problem is peak dissymmetry) can occur in GC. Sometimes (emphasis on sometimes) one can measure on the basis of peak height in GC but not always, and actually not often either. It is especially a problem in gas-solid chromatography, but ALWAYS try to avoid using peak height as a measurement criterion and use peak area instead.

best wishes,

Rod

[AICMM]

phuonghuynh,

The way it was explained to me long ago (when I was a lad!) was this. Capillary presents (typically) nice sharp peaks so peak height gave you an easy means of quantification (ruler.) Packed columns tended to broaden peaks so peak area would take that into account but harder to measure in the old days. The proper use of standards and modern data systems makes this moot, really, although I almost always use peak area now rather than peak height. Peak area on an FID is also nice for the use of a single response factor.

Best regards.

[HW Mueller]

This has been discussed extensively before, in short: Avoid using peak hight unless peaks overlap. You have to do the calibration in the method you use, one obviously can´t use an area calibration to run samples via peak hight, and vice versa.

[phuonghuynh]

Thank-you very much for your advices!

It's very helpful to me.

Best wishes!

[Ron]

Either peak height or peak area can be used in GC, however as a general rule peak area will give better linearity and precision than peak height. Most data systems allow either to be used, and ir is easy to try both and see which gives the best results.