An evaluation of the Nova 2 ionised calcium instrument

Many enzyme assays are linked to the formation or utilisation of NADH so that reaction rates can be monitored at 340 nm. The absorbance spectrum of naphthol green is sufficiently different from that of NADH to render calibration using this substance undesirable, and NADH or the particular chromophore used to monitor the reaction, e.g. p-nitrophenol, should be used. The new calibration procedure confirmed the extent of the disparity between the two instruments noted when performing enzyme analyses and supported the view that the problem was one of instrumentation, possibly electronic in origin. An electronic calibration procedure produced by the company was successful in bringing the instruments back into specification and cured the problem of the disparity between the 0.05 and 0.20 absorbance scales. Exactly what is accomplished when electronic calibration is carried out remains uncertain. Certainly such items as the ratio of the two absorbance scales and linearity across the recorder are correctly adjusted during this procedure, but the manner in which the photocell output is linked to true absorbance is not clear. In the light of the authors' experience, it is difficult to avoid the conclusion that electronic calibration merely succeeded in ensuring that both instruments investigated produced the same result. It may not ensure that the output from an instrument is linked to true absorbance. For this reason, a correction factor may still need to be applied to the results obtained after electronic and absorp-tiometric calibration has been carried out. It is suggested that results which are within + 5% of the true absorbance need not be corrected, while instruments operating outside of these limits should have a correction factor applied until they can be serviced and the error corrected. This may well depend on the type of analysis being performed on the instrument. The calibration procedure outlined could be performed at intervals of about six months. It can be adapted to calibrate any other type of reaction rate analyser and has been" used successfully in calibrating both the AKES (MSE Scientific proposal were generally adopted, better inter-laboratory agreement of enzyme results should occur. For future instruments, recommendations have been made which set very high specifications. [5] and which should render future calibration a more precise and rapid exercise.