Adsorption, Neg Intercepts, Calibration [August 12, 2004]

[admin]

By Donna Sirenski on Thursday, August 12, 2004 - 03:07 pm:

I have been challenged with the analysis of low concentrations of Triethanolamine in aqueous samples. I have observed a large negative intercept for my calibration curve. In fact, the negative intercept is larger than the response for the TEA in the samples. Surprisingly, my final result (TEA content) for several samples compares well with an analysis performed using LC/MS. My peer suggests that the method is still valid. I should point out that the curve is actually quite linear (r2>0.997) over the calibration range (100-500ppm) and I prepared my standards using the sample matrix, which is loaded with non-volatile material.

I can't find any information regarding large negative intercepts and I wonder if anyone can share their opinions regarding the validity of a calibration curve with large negative intercepts. I am thinking that the intercept is negative due to adsorption in the inlet which I can't seem to overcome and I do not want to have to derivitize the samples.

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By anonymous on Friday, August 13, 2004 - 10:10 am:

Could you post your line equation so we can see how large is "large"?

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By carla on Sunday, August 15, 2004 - 03:18 pm:

What do you mean with "negative intercepts"?

Do you mean the calibration curve crosses the amount/concentration axis or negative peaks?

[chromatographer1]

I suspect loss of the TEA in your system is causing the large negative intercepts of the regression line at the y-axis of your calibration graph at x=0.

Your R squared value is not great but not the worst possible, especially for a bad actor like TEA.

I assume you are getting a peak at 100 ppm spike, so at least you know your limit of detection is not above 100 ppm.

As long as the active sites in your system stay consistent in their retention of TEA samples you will be able to measure accurately in the future, BUT if your system changes your numbers will be inaccurate.

You are required to run multiple standards and calculate a recovery of each standard IMMEDIATELY before you run your sample and confirm the measurement with another std IMMEDIATELY after your sample injection to show your measurement is accurate and consistent with the standards.

It has been done before and will be done for problem analyses in the future, I am sure. I wish these problems did not exist, but at least they keep some analytical chemists employed.

Chromatographer1