The offset or basal value of 13CO 2/ 12CO 2 ratio determinations for breath-gas analysis using non dispersive infrared (NDIR)-technology shows a nonlinear dependence on sample O 2 and CO 2 content, which is empirically described and corrected with polynomial functions. The corresponding coefficients are usually determined by regression and measurements at nodes, which cover the entire working range. A sparse grid causes interpolation problems and dense grids increase the calibration effort. As a way out the covariance of coefficients of earlier calibrations is estimated using a ‘random coefficient’ approach and decomposed into 4–6 principal components. Each component describes the device variability and refers to a polynomial, which covers the entire working range. An actual calibration surface is explained by an overlay of these components, which obviates a systematic variation of the interferences and allows to design a plausibility check. When applied on the two gaseous interferences it gives a root-mean-squared error in 13CO 2/ 12CO 2 ratios smaller 10 − 5 over the entire working range. With only a few coefficients to be identified by regression, the approach is robust against random measurements errors and paves the way for flexible on-line δ 13C metabolic measurements with NDIR technology under increased O 2 content.