PDA and UV detector

[Kaoula]

Hi All:
we have a problem, this problem is as follow:
we have tow HPLC apparatus the first equipped with UV detector and the second equipped with PDA detector.

When we analyze any material using the two apparatus with the same wave length and the same conditions , the apparatus with PDA detector give area more than the area with UV detector

Example: (1762543 and 432158)

This problem is normal or no please help me??!!

Thank you
Kaoula

[Dan]

Hello Kaoula,

There is not a lot of detail in your message, but I can assume you do not have a problem. It is not unusual for UV to give more signal than PDA under the same operating conditions.

In general, a PDA is not as sensitive as UV and you will get a smaller signal (less area, less peak height) with PDA than UV. This is not always true as modern PDA instruments can be as sensitive as UV instruments.

Many things will effect the signal from a detector; lamp energy, temperature, signal electronics, chromatography data systems, etc. You can even see differences with different UV detector models, either from the same or from different vendors.

All these reasons, are why standards are injected in the same analysis run as samples.

Regards,
Dan

[Kaoula]

Hi Dan:
Thank you for your advice but I have the following notes:
1.the area given from PDA detector is more than the area given from UV detector.
2.I have used the same standard and sample and analyze them using the two detector the percent of sample versus standard is 95 % with PDA detector and 89 % with UV detector

Please help me to explanation this phenomena ,

and its correct to use PDA detector in the development of new analytical method (R&D Lab) which will to be applied in QC Lab using UV detector ???

thank you very match
with best regard
Kaoula

[Uwe Neue]

I am not a detector expert. However, it appears to me that this must have something to do with the wavelength accuracy and the shape of the absorbance of the compound. I would read the manual of the PDA, and then play with the bandwidth. If this is not a solution, I would read the manual of the UV detector and look how the single wavelength is defined.

Actually, coming to think of it, check if the wavelength filter in the UV detector is the right one for the wavelength that you are using.

[tom jupille]

Kaoula, let me get this straight:

1. You are running a set of calibration standards on both detectors, and determining calibration plots on both detectors.

2. When you then run samples, you get 95% recovery on the system with the PDA and 89% on the system with the UV detector.

Am I reading your problem correctly?

[Dan]

Kaoula,

Sorry, I made a mistake in what you meant about the peak areas with the detectors. Now, that I understand it , then yes there is a problem.

To help troubleshoot your problem, some more information would be useful. Uwe makes a good point about checking the operation of the detectors. Are the PDA and UV operating okay? Are the parameters for bandwidth, time constant, etc. set properly?

Tom also makes a good point that the way you are doing the calibration and quantitation need to be checked. The results you gave in your second message may indicate that it is not a detector problem. Can you give more information on the calibration/quantitation procedures?

Lastly, you asked about methods development using the PDA and routine (QC) analysis with UV. Yes, you can do this. Using the PDA to do the methods development is very common. There are many benefits to this. In the pharmaceutical labs were I have worked, we usually use the PDA for the development stage. However, we also run development analyses with the UV. We want to test the method as best as possible before we transfer methods to the QC group. The PDA greatly helps the methods development, but we usually use UV for the routine work.

Regards,
Dan

[HW Mueller]

Kaoula, does your standard and sample have the same spectrum? Do you run both at the max wavelength? Are your relative values in relation to the standard on only one detector or do they refer to sample/ST of PDA and sample/ST of UV detector? What are the bandwidth of the two detectors?

[Mark Tracy]

What are the detector cell dimensions? You would get more area with a longer pathlength. You may or may not get better signal/noise.

Dan's comment about PDAs being less 'sensitive' is misleading. Most PDAs have worse S/N that monochromator-type UV detectors, and often also worse linear ranges. Assuming all other factors are equal (they rarely are), the detection limits are inferior for the PDA. Sensitivity is simply the slope of the calibration line.

[vofelyp]

Kaoula, I think this phenomena is generated two different failure.
1. Have You checked the wavelenght accuracy of the two detectors? Generally the specifications of the detectors is +/- 2 nm. According to this spec. the measuring wavelenght diff. may be 4 nm between your two detectors then they fit its spec.
2. The materials spectra similar or not? What is the slope of the spectra near by the measuring wavelenght? If the spectra slopes are very different a little wavelenght diff. is caused big AU difference