# Plate Theory and Extensions - Quantitative Analysis from Retention Measurements > Page 48

_{ } (42)

where (X_{AB}) |
is the concentration of solutes (A) and (B) in the composite peak, |

(X_{A}) |
is the initial concentration of solute (A), |

(X_{B}) |
is the initial concentration of solute (B), |

(n_{(A)}) |
is the column efficiency for solute (A), |

(n_{(B)}) |
is the column efficiency for solute (B), |

(v_{A}) |
is the volume of mobile phase passed through the column in plate volume of solute (A), |

and (v_{B}) |
is the volume of mobile phase passed through the column in plate volumes of solute (B). |

Rearranging
(multiplying the top and bottom of the exponent by (n_{A}) and taking (n_{A}^{2})
inside the brackets),

(43)

Equation (43) gives the sum of the two peaks and thus produces produce a single envelope. As long as the retention times can be measured precisely, the data can be used to determine the composition of a mixture of two substances that, although having finite retention differences, are eluted as a single peak. This will be possible, if the standard deviation of the measured retention time is small compared with the retention time difference between the two solutes.