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Owing to the narrowly distributed density of states, even small amounts of chemical impurities can cause relatively distinct midgap states and significantly reduce the charge carrier transport in liquid crystals. However, the effects of chemical impurities on charge carrier transport have not yet been explained sufficiently because the discovery of electronic conduction in liquid crystals is relatively recent. In this report, we investigate impurity effects in various mesophases of model smectic liquid crystals (2-phenylnaphthalene derivatives) doped with a terthiophene through transient photocurrent measurements. We discuss the effects of chemical impurities on the carrier mobility in various mesophases as a function of temperature and impurity concentration. This study provides guidelines for purity assessment of liquid crystals as organic semiconductors.