We report on modulation transfer spectroscopy (MTS) of the ${}^{1}S_0\to {}^{3}P_1$ transition of ytterbium in a vapor cell. We stabilized our laser on the MTS error signal and measured the offset frequencies relative to modes of an ultralow expansion cavity. The isotope shifts were extracted through these measurements. The Zeeman splitting of ${}^{171}\mathrm{Yb}$ and ${}^{173}\mathrm{Yb}$, two fermionic isotopes with hyperfine structure, were also studied. Our results show significant deviations from linear Zeeman-effect calculations at intermediate magnetic fields of less than 60 G. Quadratic corrections to the Zeeman effect derived through second-order perturbation theory explain the observed nonlinearities. Such quadratic corrections are essential when using Zeeman-structure measurements to obtain the fermionic isotope shifts.

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• Fine & hyperfine structure
• Isotope shift
• Zeeman effect
• Cavity resonators
• Perturbative methods
• Spectroscopy