Detector Linearity

[Warren]

Does anyone have any suggestions about a problem I have with detector linearity?

I am qualifying a new Agilent VWD with solutions of hexadecanophenone at 240nm. Direct injection into the detector with a syringe.

It is not passing our specs (%RSD is above 6%). I have flushed the sytem, put in a new lamp and prepared new solutions. Result is the same.

Using the same solutions and the same syringe on another detector at exactly the same time gives me a %RSD <1.0.

Other symptoms: Absorbance values are lower than expected for all my solutions but get worse as concentration increases.

Agilent assures me that the detector pases all their OQ/PV tests.

Any assistance would be greatle appreciated.

[Rob Burgess]

I'm not sure of an exact solution, but what I would do is ask Agilent what they do for IQ/PQ/PV linearity test.

I'm assuming they do something similar to what you are doing at the moment i.e. direct injection of "calibration solutions".

I suppose we're in a lucky position in a way because we get the vendor to get do all our Q's onsite when the product is delivered!

[MK]

are you looking at RSD% of responses normalized to concentrations (=response factor)?

This is carried out in a different way in Agilent OQ/PV: Caffeine standards are injected into detector through restriction capillary. The test has no criteria for response factor RSD%. They only look at correlation coefficient (R2>0.999 if I remember right). I've seen an agilent OQ/PV test with a perfect R2 of 0.999 but with a poor RSD% of ~6 for resp.factors. I wont start to criticize agilent's spec here, but I think their definition for "detector linearity" may simply differ from yours.

few things you could look at:

-recalibrate your detector and make sure holmium and intensity tests passes
-make sure that all your samples are below 2 AU
-carefully clean the cell with diluent between samples
-try if blanking out the diluents from standards makes any difference

[Uwe Neue]

What is your solvent? Maybe it provides a large background...

[HW Mueller]

When injecting anything directly into the UV detector with a syringe I usually have enormous problems due to air, dissolved, or bubbles, or whatever. That´s probably why Agilent uses the restrictor, most likely using degassed solutions delivered via the pump? I am having a considerable reservation on Warren´s lower than expected absorbance.... are you sure on that?

[JM]

Hi,
We had a similar problem of lower absorbance in one of our PDA ( agilent) when compared with others of same make during one of our analysis. It turn out to be problem with optics due to fogging. this can happen due to staning in flow cell as well.

best to check with agilent .

JM

[Warren]

Hi all

Thanks for the suggestions.
The detector is a new VWD detector. It passes all other tests e.g. wavelength accuracy, dark current, has been calibrated etc.

Though I am unsure what is meant by intensity test?

We are looking at the %RSD of sensitivity (=Absorbance/concentration).

All standards are made up in methanol and are below 2AU. Using the same standards in the same syringe on another detector at the same time gives me a %RSD <1.0%. I have done this test on many detectors any times so I know it works.

Using a restrictor and pumping the standards in the way Agilent does has given me exactly the same results. The same appliess when using a different flow cell.

Anyone know any more about potential problems cause by the optics? e.g. fogging?

Warren

[tom jupille]

I don't have "hands-on" experience with that detector, but does the software let you look at the actual energy (intensity) values either at a particular wavelength or over a range? That might give an indication of low light throughput problems.

[HW Mueller]

One would think fogging would produce drifting?
Warren,
what do you do to zero the detector? Is there a drift?

[MK]

Intensity test (if you run chemstation):

-fill the cell with water
-go to diagnosis
-select lamp/tests
-select intensity test and press start

the test gives you the lamp intensities at several wavelength ranges. In addition to poor lamp energy, intensity test may fail due to dirty cell or poor water quality. I'm not sure about VWD but atleast in 1100 DAD you can easily remove the cell and repeat the test. This gives you the effect of cell and water inside the cell.

[Warren]

The problem turned out to be with the optics bench - which Agilent replaced for us. Then the linearity test passed very well. Seems that there may have been a problem with the grating at 240nm.

thanks for all your suggestions.

Warren