I believe that everyone will find in daily tests that there are three parameters: tailing factor, symmetry factor, and asymmetry factor to evaluate the peak shape symmetry of a chromatographic peak. The currently used analysis software, the symmetry factor in the ChemStation workstation, the USP tailing factor in the Empower workstation, and the Chameleon workstation do not have the parameter of the symmetry factor, which is evaluated by the degree of asymmetry. What is the relationship between these three parameters and what is the difference? Today I will talk to you about it.

**Ideally, chromatographic peaks should have Gaussian characteristics:**

where χ is equal to (t-tR)/σ, t is time, σ=W/4, and y is peak height. True peaks in a chromatogram usually deviate slightly from a symmetrical Gaussian peak shape, often with more or less tailing. As shown below:

**Tailing factor**: Tailing factor is usually expressed as Tf, which is calculated at 5% of the peak height.

**Asymmetry factor**: Asymmetry factor is often expressed as As, calculated at 10% of the peak height.

**Symmetry factor:** Symmetry factor is commonly represented by S, and it has a reciprocal relationship with the asymmetry factor As.

**The relationship between As and Tf values can be roughly expressed as:**

**As≈1+1.5(Tf-1)**

Therefore, in general, the value of As is greater than the value of Tf to a certain extent.

**Relationship between peak tailing factor and asymmetry factor**

Peak-asymmetry factor (measured at 10% of peak height) | Peak-tailing factor (measured at 5% of peak height) |

1.0 | 1.0 |

0.00 | 1.2 |

1.6 | 1.4 |

1.9 | 1.6 |

2.2 | 1.8 |

2.5 | 2.0 |

Peak shape varies with asymmetry factor (As) and tailing factor (Tf). When As or Tf = 1.0, it corresponds to a perfectly symmetrical chromatographic peak, in which case the two chromatographic peaks can be well separated from each other. However, as peak tailing increases, so does the separation between them. In most cases the degree of peak tailing is not severe (Tf < 1.2) and generally goes unnoticed. This degree of tailing (Tf < 1.2) has negligible effect on the separation, unless a very large peak is followed by a very small peak.

**Discussion on the Specified Range of Tailing Factor, Symmetric Factor and Asymmetric Factor**

The Chinese Pharmacopoeia (CP) requires the tailing factor to be in the range of 0.95-1.05. There is an applicable premise, that is, the Chinese Pharmacopoeia stipulates that the tailing factor should be between 0.95-1.05 when the peak height method is used for quantification. If it is lower than 0.95, it is a delay. Peaks above 1.05 are tailing peaks.

When the European Pharmacopoeia (EP) and the British Pharmacopoeia (BP) stipulate the determination of related substances or content, unless otherwise specified, the chromatographic peak symmetry factor of the quantitative reference solution in the chromatogram should be 0.8~1.5.

The United States Pharmacopeia (USP) has a requirement for a tailing factor of no greater than 2.0 for some compounds.

The range of tailing factors is not specified in the Japanese Pharmacopoeia (JP).

Judging from the constraints on the scope of the tailing factor in the pharmacopoeia of various countries, there is no standard for determining the value range of the tailing factor, and specific problems need to be analyzed in actual chromatographic experiments.

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