Abstract: The present invention provides a method for determining a background count rate in liquid scintillation counting. The method comprises measuring external standard spectra of a sample, determining, from the external standard spectra, a triple to double coincidence ratio and a quench parameter, determining, based on the triple to double coincidence ratio and the quench parameter, a background reference parameter, and determining, based on the background reference parameter, the background count rate from a background reference curve.

Abstract: The present invention relates generally to the field of biochemical laboratory. More particularly the invention relates to the improved and more efficient instrumental features of equipment used as e.g. fluorometers, photometers and luminometers. The object of the invention is achieved by providing an optical measurement instrument where there is an interface (218, 223, 233a, 233b, 238) for a changeable optical module (240), the interface being adapted for at least one excitation beam and at least two emission beams. This allows performing various types of measurements by changing an optical module. The change of module and related parameters can be performed automatically controlled by software. It is also possible to easily upgrade the instrument for new types of measurements by just providing the instrument with a new optical module and the related software.