Chromatography Forum

Hai every body,

I would like to know how to determine peak purity using PDA detector? Would any body please explain this .

Dear it is very easy with modern software available. If you have waters PDA then, creat a method for peak purity and follow instructions. You have to select the peak, the nosie level etc. The system automatically gives peak purity. Ur peak will be pure if purity angel is less than purity thresshold. In Agilent/ Dionex etc You will get a value. If it is close to 1000 peak is pure

Thanks

but how to determine peak purity with vwd uv detector

rc_12321, what type of software do you use? maybe someone here will be able to help you with the specifics.

yaoguocan, you can't perform peak purity tests with a vwd uv detector, you can perform a ratio test using different chromatograms from different WL, which could indicate but cannot prove conclusive peak purity. i wouldn't do it personally.

thanks for your kind reply .but are there any other detector can perform peak purity test . there is no pda in our lab.is ms ok

yes, you can do it with an MS also. make sure that your software can do the calulations. if yo uhave a modern software it shouldn't be a problem.

First of all, there is no such thing as "peak purity" in general. Various instrument manufacturers provide different ways of evaluating the possibility of coeluting compounds. In essence a "peak purity" parameter on a Waters system will have no relationship to a "peak purity" parameter on an Agilent, which will have no relationship to . . .

In order to do any kind of peak purity estimation, you need to obtain additional, non-chromatographic information.

If your UV detector will allow simultaneous dual-wavelength monitoring, then you can look at the absorbance ratio (if a peak is pure, the absorbance ratio will be constant across the entire peak; if you have multiple peaks and they have different absorbance spectra, then the ratio will change).

If you have MS, then you usually do not need to worry about peak purity. Assuming no fragmentation in the inlet (and no isomers), you simply monitor at the right m/z for your compound.

First of all, there is no such thing as "peak purity" in general. Various instrument manufacturers provide different ways of evaluating the possibility of coeluting compounds. In essence a "peak purity" parameter on a Waters system will have no relationship to a "peak purity" parameter on an Agilent, which will have no relationship to . . .

In order to do any kind of peak purity estimation, you need to obtain additional, non-chromatographic information.

If your UV detector will allow simultaneous dual-wavelength monitoring, then you can look at the absorbance ratio (if a peak is pure, the absorbance ratio will be constant across the entire peak; if you have multiple peaks and they have different absorbance spectra, then the ratio will change).

If you have MS, then you usually do not need to worry about peak purity. Assuming no fragmentation in the inlet (and no isomers), you simply monitor at the right m/z for your compound.

Dear
what do u mean by absorbance ratio (ratio of what,if i do have only one peak ) pl. clear
thank

You have to compare the responses at each of 2 wavelengths over time. If you open up the raw data files and extract the numbers corresponding to the time of your peak's elution, divide the slice area at wavelength 1 by the slice area of wavelength 2. If your peak is pure, that ratio should be pretty constant throughout all corresponding slices for the peak.

Thank you one and all
Dear DR Would you eloborte your statement?

You can perform a peak purity test with a PDA detector. but you have to be aware of the different terminology of that statement.

modern software coupled with a PDA detector permit you to see if the peak in your chromatoram is "pure". the pesk purity test will give you an indication of how pure the spectral profile is over the range of the peak in the chromatogram. this is done by comparing the different spectras of the peak at different retention times. a close to 1 or 100% or 1000 depend on the software will indicate how pure the spectras are. for most case save one this also means that your peak is made of one compound but and this is a big but, that is not enough to say that your peak is pure.
if you have 2 or more co eluting peaks, at exactely the same RT, one under the other you can still get a clean spectral profile. to make sure that your peak in your chromatogram is made of only one compound you must also perform a library matching with a standard as to ake sure that no co elution of compounds is taking place.
both test together can tell you if your peak in your chromatogram is made of one compound.
but you must have a PDA for that.
if you have no standard available then you must use other means of detection like MS to show that your peak is "pure" (made of 1 compound)


be aware that a ratio test will not tell you if you have 2 co-eluting peaks exactely one under the other.