How to calculate System Suitability in Chromatography

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

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

Where and  are retention times of peaks A and B

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

Resolution is considered complete if it equals or exceeds 1.5

Asymmetry or Tailing factor ()

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

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 = 

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

Precision

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

 Where W is the peak at base

or

Where  is peak width at half height where

 is retention time and  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

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

Ideally k’ should be greater than 2.0