The Use of a Dual Dye Photometric Calibration Method to Identify Possible Sample Dilution from an Automated Multichannel Liquid-handling System

A significant difference between the reported volumes delivered by a fluid-filled automated liquid-handling system with eight stainless steel fixed tips was observed when a gravimetric method and a dual dye photometric method were used to measure delivered volumes between 20 and 200 μL. A series of experiments, using the default pipetting parameters, was performed which demonstrated that the difference in the reported volume was due to a dilution effect by the system liquid of the liquid handler and did not indicate an error in the delivered volumes. This dilution effect led to a decrease in the reported volume by the dual dye method, which alerted the user to the problem of sample dilution. In contrast, the gravimetric method provided the expected volume, and therefore did not alert the user to the problem. Without optimization of pipetting parameters, the dilution issue can be significant, because the resultant change in the concentration of the compound(s) of interest in the sample could lead to an unidentified error in the sample assay.