Peak Purity in Openlab CDS 2.3version

Dear All,

we are using openlab CDS2.3 Version and i have done some trial with force degradation of API and using the openlab CDS 2.3 version gave some weird result of peak purity and spectra with standard and degraded sample. Using 50% threshold value in the standard sample solution i am still getting peak purity failing and while in the degraded sample shows more pure then in standard which is not making any sense. I have attached the screen short to see if any one can help with the following point. There are 2 pic attached in the first pic sample (API) with 0.5M HCl and in the second pic it pure standard SST solution. further in the 2nd pic i overlaid the SST and sample solution if you see the spectrum is not matching at all.  

  1. what is the purity in term of regulations
  2. how we can set up the threshold value in Oenlab CDS.
  3. will the sample concertation will have an impact on peak purity.  Regard
  • Hello,


    I have added tags to this post to increase visibility.


    1) I don't know what regulations you are referring to, so the best way I can think to address this questions is to just provide information on how OpenLab CDS calculates the impurity evaluation. See below:


    Where Threshold and Match Factor are calculated as below:


    2) The only real way for a user to impact the threshold calculation is by manipulating the Impurity sensitivity set for the compounds in the Processing Method:


    3) Sample concentration should not have any impact on impurity as long as concentration does not impact a compound's spectra. 

  • Dear Royboyle,


    Thank you for you email, i am still having issue with peak purity at 50% sensitivity the peak purity of standard is failing and when i change the sensitivity to 40% then it passes, i am bit confused now what is the standard guideline regarding the peak purity threshold and sensitivity value set up. I am not sure why for the standard solution the peak purity is failing can any one please guide me. this not taking anywhere. 

    What is the legal requirement in term of regulation how much we can change the sensitivity if you changed to some how 10% or 20% off-course it will pass the peak purity. 


  • Hi Ryoboyle,

    Thank you so much for quick reply, is the threshold means purity.  

  • Personally, all I can tell you how is how OpenLab CDS calculates results. I do not know what regulatory requirements your lab needs to conform to so I cannot weigh in on that side of things. 

  • Hello,


    See below for some guidance that can be found in the CDS help files. There is no standard threshold you can apply to all systems as it will be dependent on your methodology. Remember that peak purity only a measure of spectral similarity not overall sample purity. If you have a degraded sample that would only change the peak purity of the API if the degradants eluted at the same RT as the main peak and had a markedly different spectra. Typically in methods looking at degradation your are more interesting in the chromatogram as a whole than specific peak purity. Unless you know your degradation products column interactions would lead to coelution with the main peak. 


    Marty Adams


    Check for UV impurities

    The peak impurity check, or peak purity is a 3D-UV feature in OpenLAB CDS allowing you to verify if the peaks can be considered as pure (no impurity underneath), or impure if there is any impurity across the peak absorbing at the defined UV wavelengths.

    A sensitivity percentage is used to adjust the spectra purity threshold towards the considered instrument or application. The result is highly depended on noise, column, solvent, matrix etc. Therefore, the returned match factor must always be reviewed by a chemist.

    General approach

    To check for UV impurities:

    1. Run a pure standard that contains the compounds of interest.

      Depending on the type of samples and your application, you may choose to spike a blank with the pure standard compounds.

    2. Do an impurity check calculation for that standard.

      Compared to ChemStation, there is a simplified set of parameters. You only need to adapt the sensitivity (globally or per compound) so that the peaks of interest are shown as pure. The sensitivity basically shifts the threshold curve up or down.

    3. Adapt the sensitivity, and check the purity ratio curve in the Peak Details window.

    4. Use this processing method to analyze your samples.

    Be aware that a UV impurity check has its limits and always needs to be evaluated by a chemist.


    Any spectra functionality has dependencies with the instrument accuracy as well as the chromatographic application and its way to process the data. Here are examples of influencing parameters when computing a UV peak purity:

    • Signal-to-noise is a generic parameter that is influenced by many system variables. As a generic observation, the lower the signal-to-noise, the higher the contribution of the noise. The peak purity considers and retrieves the noise from the peak start and end. However, with a low signal-to-noise (for example, lower than the limit of detection), the calculation of the noise is difficult and will lead to fluctuating peak purity results. An opposite effect can be observed working at too high concentration reaching the detector upper range limit. In this case, the absorbance of the components will fluctuate, and the peak purity will not be accurate.

    • Instrument health and maintenance: Pump accuracy, solvent degasser, injector carryover, column bleed and detector linearity are factors that influence spectral analysis.

    • Solvent cutoff and quality: Every solvent manufacturer delivers the UV specifications of the solvent batch purchased. This solvent cutoff needs to be considered in the wavelength bandwidth taken for the spectra purity calculation, especially when working with a solvent gradient.



    Default recommended values



    50 % for conventional, routinely maintained, or new instrument after instrument checkout

    75 % for state-of-the art chromatography

    Decrease the default value by 10 to 20% for each of the following point not fulfilled:

    • Instrument is maintained routinely (for example: 1 month for 75% daily use) and system performance tests are performed daily.

    • HPLC grade solvent quality or higher.

    • Solvent is routinely exchanged if not used.

    • Peaks are higher than the LOD x 2.

    • Solvent injection blank run does not show any peak across the chromatogram.

    Wavelength Bandwidth

    According to solvent cutoff (L0 + 5 nm)

    According to compound spectral absorptions (if known)

  • Hi Sir,


    I have just checked with one compound and the peak purity is 986.1 with sensitivity at 40% and then i started  changing  the sensitivity to from 100% to 10% and at 50% the peak showed red sign and then at 40% and 43% it showed whole Green and when i changed to 10% see if it effect the purity but the peak purity still stayed at 986.1. and when changed to 100% it showed red sing again but peak purity still the same. Can you please give you point of view, what is their relation with peak threshold and when the peak shows Green then why the peak purity is still 986.1 i am expecting at 990% or 995% peak purity. 

  • Hi Sir,


    Can you please give me an idea how i would know that the system is taking 5 spectra per peak, is there any way to find out. Please 

  • Ho to find out the match factor value to be compared to threshold value to find out that the peak is pure or impure. 

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