How to calculate System Suitability in Chromatography

System Suitability Testing in Chromatographic Analysis

How to calculate System Suitability in Chromatography

In my earlier post on generation of authentic chromatographic data I had emphasized the need for evaluation of system suitability before proceeding with analysis. Some factors contributing to system suitability failures in HPLC were discussed. The current post introduces you to system suitability parameters and their acceptance limits.


Resolution is a measure of the separation between two chromatographic peaks.

Well resolved peaks are basic requirement in both qualitative and quantitative estimations. Separation between closely spaced peaks is governed by affinity for the stationary phase.

Co-eluting compounds can be resolved by:

  • Change of mobile phase polarity
  • Increase of column length
  • Reducing particle size of stationary phase 
Resolution of Chromatographic Peaks

Resolution of Chromatographic Peaks

 R_S=\frac {tR_B - tR_A}{0.5 (W_A + W_B) }

Where tR_B and tR_A are retention times of peaks A and B

Peak widths W_A and W_B are obtained from the intersection of tangents with baseline

Resolution is considered complete if it equals or exceeds 1.5

Asymmetry or Tailing factor (A_s)

An ideal chromatographic peak should be of symmetrical Gaussian shape but due to various factors the shape often deviates. Peak tailing is the commonly observed peak deformation. It is mainly due to occurence of more than one mechanism of analyte retention. Tailing can be reduced by changing mobile phase pH or end-capping of stationary phase.

Assymetry factor

Assymetry factor

where A and B are peak widths at 10% of the height for leading and tailing ends of the peak

Ideal peak has As =1 but values in the range 0.9 – 1.1 are acceptable

Tailing becomes apparent when asymmetry factor As equals to or exceeds 1.2

As per USP definition the tailing is considered as the ratio of the widths a and b at 5% of peak height and is expressed as

T =  \frac {a+b}{2a}

 T should be less than or equal to 2 to satisfy the system suitability requirement.


Replicate injections of a standard preparation are used to ascertain if requirements of precision are met

Data from five replicate injections are used if requirement of relative standard deviation is less than 2%. Data from six replicate injections are used if the requirement of relative standard deviation is more than 2%.

Theoretical plates

The plate theory concept assumes that the chromatographic column comprises a large number of imaginary separation layers called theoretical plates. Equilibrium of the sample takes place between the stationary and the mobile phase in these imaginary plates. The analyte moves down the column by transfer of equilibriated mobile phase from one plate to the next.

Column efficiency is expressed in terms of theoretical plates(N).High resolution means greater number of plates in a given length of column

 N =16{[\frac{(t_R)}{W}}]^2 Where W is the peak at base


N = 5.54{[\frac{(t_R)}{W\frac{1}{2}}}]^2

Where  W_1_/_2 is peak width at half height where

 t_R is retention time and  W_1_/_2 is the peak width at half height

Theoretical plates should not fall below 2000

Retention factor (k’)

Retention factor (k’) or partition ratio or capacity factor is the relation of time spent by a compound in stationary phase to the time it spends in the mobile phase.

k’ is a unitless quantity

k' = \frac{t_r - t_m}{t_m}

Higher the value of k’ greater is the retention of a compound on a column

Ideally k’ should be greater than 2.0

About Dr. Deepak Bhanot

Dr Deepak Bhanot is a seasoned professional having nearly 30 years expertise beginning from sales and product support of analytical instruments. After completing his graduation and post graduation from Delhi University and IIT Delhi he went on to Loughborough University of Technology, UK for doctorate research in analytical chemistry. His mission is to develop training programs on analytical techniques and share his experiences with broad spectrum of users ranging from professionals engaged in analytical development and research as well as young enthusiasts fresh from academics who wish to embark upon a career in analytical industry.


  1. what do you mean by the factor in resolution formula 0.5 and in theatrical plates formula 16 or 5.54. how it was introduced in formula and what is use of that. i mean from where it came. please sir i am new for this profession.

  2. daud bangash says:

    how to calculate lower detection limit for hplc

    • Dr. Saurabh Arora says:

      Hi Daud, LOD is calculated by signal to noise ratios or from the slope and intercept of linearity studies. S/N is usually the preference.

  3. What are the factors responsible for the Relative Standard Deviation to exceed the limit?

    • Dr. Saurabh Arora says:

      Dear Sunil A host of factors can increase the RSD like injection volume, flow rates, sample prepration, temperature variation, evaporation of the stock solutions, gas quality variation You need to ID the one responsible by isolation and then solve it.

  4. what is difference between resolution and usp resolution in empower

  5. Why 5 or 6 replicate injection used in sst

  6. Why we don’t used 10 injection in sst instead of 5 or 6?

    • Hi Meena,
      If you can achieve the required precision in 5-6 injections why waste your time in making 10 injections. There is no guarantee that you will get improvements in 10, 15 or even 20 injections over your results so 6 is the optimum number .

      • There is a concept of 6 sigma and 3 sigma where in we need minimum 3-points for the agreement
        i mean to see that your system is precised enough for the analysis u need to meet the criteria of RSD either with 3-points or with 5-6 points

  7. What is RsD calculation in system suitability

  8. How to get sharp peak in phenomenox column?what s washing column procedure

  9. Hi sir this srikanth
    In system suitability parameters calculation
    Formulas in resolution0.5,railing factory 2,
    Teritacal plates 16…numbers is used could be explain sir……

  10. Why Assay specification is broad I.e 95% to 105%. But purity specification is NLT 99. 0%. What is the main reason

    • Dr. Saurabh Arora says:

      Hi Mahesh, the assay is for formulations where there are other ingredients and also the fact that on storage the assay might come down depending on stability. Thus the broader range.

  11. Hello Dr. Deepak,
    Is there any videos available to learn integration of peaks.

  12. hello sir,
    what is the system suitability requirement for bracketing standard and how many injections we should give .Because some companys are running sst 6 injections,check standard 2 injections and Two point claiberation 2 injections.And after 6 Injections of assay samples again they are giving two injections of two point it really needed.

    • Dr. Saurabh Arora says:

      This is usually based on the internal STP which is established during the method validation.

  13. Suresh Singaram says:

    hi sir, in my Assay analysis for Acarbose tablets,pre injection of resolution got exactly 1.5(limit NLT 1.5)but in main sequence got 1.42, why, can you explain the reason

  14. Sir tell me about ideal peak

  15. How we will set oven temperatur in gc

  16. How we will caleculate s/n in gc,
    Plese explain which is exat rt to caleculate SN

    • S/n ratio is defined as the lowest concentration at which the signal is 3 times the average noise level.There is no particular Rt defined for measuring it. You can use a reference compound whose Rt can be established through repeated runs

  17. Sudhir Bhosale says:

    What is the difference between Drug Purity and Drug Potency?

    Please explain

  18. Hello sir,
    Why we use Acetonitrile, and Methanol in preparation of HPLC mobile phase, why we not used remaining solvents and what is the reason

    • Solvent composition decides the overall polarity of the mobile phase. As the sample may contain more than one compounds of different polarities a mixture is generally used. The selected solvents should be fully miscible. Acetonitrile and Methanol are fully miscible with water as in caes of most aqueous phases.

  19. Hello sir,
    What are the units of vaccume, why some products are analysed in a vaccum oven.

    • Units of vacuum are torr. However, I have not come across any application which requires a vacuum to be maintained in the column oven. In any case since the fittings are airtight column oven pressure should not play any role in GC separations.

  20. Hi sir good morning
    Why we are not identify unknown peacks with there boiling point

    • Peaks are usually identified by retention times under the set of operating conditions using standard mixtures.The boiling points of compounds depend on purity. As purity can vary with sample matrix basing your judgement on boiling points can be misleading.

  21. How we will set split ratio

  22. suresh gajula says:

    What thing are keep in mind when we are integrate the chromotogram

  23. suresh gajula says:

    In lab online courses are only Hplc,Aas,safety…..?
    I want to join Gc direct classes

  24. suresh gajula says:

    Which operating conditions are show sequencel order rt based on there boiling point

    • It was difficult to make sense from the question but if I have understood it correctly you mean what operating conditions are needed to get retention time in a sequence matching the boiling points of components.From basic physics you will agree that the components will vaporize at different temperatures depending on their boiling points. As the column oven temperature increases the components begin to evaporate in a sequence of increasing boiling points.

  25. suresh gajula says:

    Good morning sìr
    What is defference between known and uknown impurity

  26. suresh gajula says:

    Please tell me sir types of sample and types of Gc columns

    • Samples constitute any matrix which contain compounds of your interest whose identity and amount needs to be determined. These can be synthetic mixtures, or specimens of pharmaceuticals, foods, environmental samples, forensic residues,etc.GC columns are of different types depending on their dimensions and packing material. You are advised to register for the free course on GC which will give you clarity on fundamental concepts.

  27. Sudhir Bhosale says:

    Why Analytical. method validation required

    • Analytical validation of a new method is important as it verifies that the method adopted will provide you reliable analysis. For more information you may browse through our articles on the topic which were published on 7th Feb 2017.

  28. Hi there, You have done an excellent job. I will definitely digg it and personally suggest to my
    friends. I’m sure they’ll be benefited from this site.

  29. isa hayatu Galadima says:

    Thank you very much sir. I have indeed gain alot from your knowledge. may God increase u in knowledge,meanwhile i want to learn more on hplc

    • gister for our free course on HPLC through the site. Click on About us on top menu and then click on Free courses in the drop down menu.You can select the HPLC free course and register for it by providing your name and mail ID in the dialog box.On submission you will get a link in your mailbox . Click on th link and your registration is complete. You will start receiving the successive modules after a gap of 3-4 days each.

  30. Lokendra Chauhan says:

    Hello sir..! What is the Gaussian peak?

    • A Gaussian peak is a perfect bell shaped peak without any distortions. It rises above a perfect horizontal baseline and drops back to the same baseline after reaching the maximum height.

  31. suresh gajula says:

    Helo sir!!
    What is defference between accuracy and precision,is there any formul??

  32. Hi sir gd mrng
    What can base use to
    Solvents are eluted in GC

  33. Lokendra chauhan says:

    Good morning sir..!!
    What is the effect on our assay or RS if talling factor will be out of limit… Why be use limit of talling..

    Ex:- our talling factor limit 1.5… And after calculations it will be found 1.8, Does it effect on our assay or rs?

  34. Adi Suwandi says:

    Good morning Sir
    What is the difference between checking standard and bracketing standard?
    And what is acceptance criteria for both?

  35. Dear sir in theorical plate formula is 16*(tr/w)2 so the 16 come from where?

  36. M Jani Basha says:

    Sir explain the difference between Assay and RS in HPLC ?

    • Assay gives you the amount of active ingredient present in a pharmaceutical formulation whereas RS or related substances give you amounts of impurities present. In HPlC chromatogram the main peak is the active ingredient ant the other small peaks represent the related substances.

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