Fourier transform infrared absorption spectra, measured at a sample temperature of 77 K, can be employed to assess substitutional carbon in electronic-grade silicon crystals without the need for a carbon-lean reference sample. The procedure proposed is based on the second derivative of the local mode absorption of C-12 at 607 cm(-1) which is calculated by numerical interpolation. The new method is particularly useful for low carbon concentrations of less than 100 parts per billion (ppb, 5 x 10(15)cm(-3)). The detection limit is better than 10 ppb. Baseline problems arising from the conventional difference method can be avoided. (C) 2003 The Electrochemical Society.