Handling of Unknown peaks by HPLC method in cleaning validation samples

Just wondering if anyone can give me some kind of direction in handling unknown peaks detected during testing of cleaning validation samples (swabs), i am looking at alterante methods like TOC to address unknown peaks.


just a correction i am not looking at TOC as an alternate method.

Dear Mr. cs2009,

This is a frequently asked question by many who are involved in cleaning validation activities. The answer might not be as simple as the question itself. This is an issue which is related to how analytical methods are developed and should be dealt with before the execution of CV (i.e. before actual swab or rinse sampling starts). I try to highlight some of the factors which could cause unknown peaks to appear in the HPLC chromatograms when testing CV samples.


  • [b]Use of contaminated analytical solvents/HPLC vials/rinse solvent/swab solvent[/b]: This issue can be handled by running “blanks” i.e. without the target residue.
  • [b]Swab material[/b]: Again as highlighted above, any peak arising because of “swabs” can be captured by analyzing samples obtained after sonicating blank swabs with a suitable solvent (usually the one which is used for taking swab samples from the equipment surfaces).
  • [b]Residues from other products &/or cleaning agent[/b]: The commonly made mistake when developing analytical methods (e.g. HPLC) is that the methods are developed & validated solely based on the target residue without taking into account the interferences which may be caused by other substances such as excipients, cleaning agents and residues (including active ingredients, excipients, degradants, if any) from other products. It is a common practice to use methods used for testing of active ingredients in raw materials or finished products with slight modification (such as LOD or LOQ made lower than MACO) in order to save time & effort (easy to use already validated method). Then what should be done? How to develop methods? One of the ways to solve the issue is to use a non-specific method such as TOC (no issue of known or unknown peaks). However, TOC has its own limitations e.g. it can’t detect molecules which don’t consist organic carbon etc. Another way is to anticipate possible residues which could give rise to unknown peaks (when using HPLC) and use them (by spiking studies) to develop & validate HPLC methods. This might not be easy when you are dealing with multiple products (but it is still the best way to understand the nature & chemistry of the residues). Easiest and feasible way in that case (of multiple products) is to take few blank samples (may or may not contain the target residue) from worst case locations of the equipment surfaces (using the same sampling solvent), analyze and evaluate the results. It would then be easy to determine any possible unknowns eluting at the same retention time (as that of the target residue). When developing and validating the method use spiked concentrations of these blank samples rather using standard solution. This way it will be easier to figure out which residue is going to interfere in the HPLC analysis. Although it is desirable (as per PIC/S or other guidelines) to have a specific method for the analysis of the target residue but it will be an advantage if the method can also detect residues other than the target residue (provided other residues don’t interfere with the analysis of the target residue).

    Hope the reply will be helpful.


  • We are a big manufacture, more than 100 products produced. It is very hard to do investigation for unknown peak. We often has some unknown peaks not matching with detergent, placebo, surface or swab blank and provius 3 to 7 products active. What is the requirment to investigate unknown for CV from reguletory point of view? At what level we should do investigation?

    Dear Mr. b0b,

    I can suggest you three things:

    A. Make the HPLC method as specific as possible (although I am not in the favor of developing/using too specific HPLC methods). If you consider your HPLC method to be specific enough (i.e. nothing else can be done to make it more residue specific) then in that case simply estimate the amount of target residue and neglect other peaks, as you are interested in estimating the amount of worst-case residue not any other residue (this case is applicable only when the relative peak areas are small, see point no. C).
    B. Second approach is to use more universal (non-specific) methods for residue analysis such as TOC (provided the residues are organic in nature), NIR etc.
    C. Even if you want to go ahead with current HPLC method and are interested in knowing/understanding more about unknown peaks then you have to consider the unknown peaks as impurities and try to limit them in formulations as per the ICH Q3 guidelines (i.e. consider reporting threshold, identification threshold and qualification threshold for describing unknown residues). If the amount of residue(s) detected = identification/qualification threshold limit, then you have to identify/qualify the residue. In order to do that you may have to use advanced analytical techniques such as LC-MS, NMR etc. for identification/qualification of residues.