Trace contaminations of n-heptane in synthetic air is measured in the parts-per-billion (ppb) range using near infrared photoacoustic detection. We describe the fundamental theory used in the design of the photoacoustic cell for trace gas analysis and determine the detection limit of the cell. On the basis of a modified procedure from the American Clinical and Laboratory Standards Institute for the case that it is practically impossible to produce perfectly blank samples, a residual detection limit of 3 ppm n-heptane is confirmed. We investigate theoretically the impact of changing the buffer diameter on the window generated signal and find good correlation with previously reported experimental results.