However, to simplify the calculation and accommodate non-Gaussian peaks, the following calculation methods are used in actual practice.
1. Tangent Line Method
This also presents a problem if the peak is distorted, so that it has multiple inflection points.
2. Half Peak Height Method
(LCsolution allows selecting the coefficient via the [Column Performance] setting, where the calculation method for 5.54 is "JP" and for 5.55 is "JP2."
For broader peaks, the half peak height method results in larger N values than other calculation methods.
3. Area Height Method
4. EMG Method (Exponentially Modified Gaussian)
Comparison of Calculation Methods
Therefore, the four calculation methods were compared using chromatograms. Profile A shows a typical chromatogram (with some tailing), whereas profile B shows a chromatogram with significant tailing. The theoretical number of plates calculated using the four methods are indicated in the table below. Results for N varied even for chromatogram A. Also, peaks with more significant distortion, such as at peak 1 in profile B, can result in N values that differ by many times.
A key factor for performing reliable quantitative analysis is whether or not separation is possible, so there is a common opinion that a calculation method that judges broader peaks, such as with tailing, more severely is more practical. However, unfortunately, there seems to be no consensus on opinions regarding N and W.
Consequently, if a certain method is already being used for evaluation, then to achieve correlation, it is probably preferable to keep using the same method.
|A (roughly typical peak)||B (significant tailing)|
|Half Peak Height Method||15649||20444||20389||22245||5972||7917||-||9957|
|Tangent Line Method||14061||18516||20309||21447||5773||7692||5795||9707|
|Area Height Method||13828||19207||17917||21020||4084||7845||6217||8641|
Shimadzu's LC workstation software is able to output performance reports using any of the methods indicated above - 1. tangent line, 2. half peak height (5.54), 2'. half peak height (5.55), 3. area height, or EMG. We recommend recording the corresponding column performance results along with analytical results!