I noticed that the % of an impurity peak sometimes diffrent from one HPLC instrument to another.
Using my Waters HPLC, I got almost the same % of impurity (3.5%) even using differnt mobile phases, while when other people ran the same sample on diffrent HPLC, they saw only <2% using the very same mobile phase and column!
Thanks for any suggestions

I am assuming you are getting your impurity concentration by area % or from relative response factors and not from a standard containing a known concentration of the impurity. I will further assume you are using UV detectors. If these assumptions are true, the difference could be due to variation in the actual wavelengths of the two detectors. If the wavelength you are using in your method is on a part of the spectrum of the unknown which has a significant "slope", slight variations in the actual detector wavelengths could result in significant variations in areas, though what you are seeing is greater than I would expect. Have you checked the wavelenth accuracy of the detectors?

The discrepancy may also come from differences in optical path and/or detector saturation limit.
Making the same assumptions as Russ, if you go from a 6mm to a 10mm detector and the main peak is saturating with 10mm, your impurities will grow while the main peak will grow less.

Expanding a bit on Russ's comment, even if the nominal wavelengths of the two detectors are the same, they may have a different bandpass. If you are on sloping part of the absorbance spectrum, this too can give you different results.

Another thought just struck me: was the same data system used with both HPLCs? If the same system (or same type) were the integration parameters the same (I know, you've checked this already; I'm just trying to cover all the bases). If a different system, try exporting the chromatogram from one data system to the other (if different brands, see if you can use export/import as AIA files) and reprocessing.

-- Tom Jupille / LC Resources Inc.

Many thanks for Russ, Beppe and Tom for taking the time to answer my question to which I believe it was answered thoroughly.

Since all the above sugestions seem reasonable, it would be quite interesting to know the source of the problem in your particular case - maybe more than one, particularly if more than two LC systems were involved.

Hi Bill,

First, Russ assumptions were correct in which impurity concentration by area % was considered and UV detectors were used at fixed wavelength (280 nm) for both LC systems.
I used Waters 996 photodiode array detector. The other LC system is an old Merck-Hitachi-655A variable wavelength UV monitor.
Apparently the problem originates from the difference of detectors characteristics. For example, I looked at the specifications, wavelength accuracy = +/-1nm for 996PDA and +/-2nm for Merck-Hitachi, spectral resolution (bandwidth) = 1.2 nm for 996PDA and 10nm for Merck-Hitachi, Flow cell = 10mm for 996PDA and 5mm for Merck-Hitachi….

Regarding Tom’s question about the data system, I am using Millennium software for Waters and Maestro version 2.4 for Merck-Hitachi.