Finding Volume in a Tube
Traditionally, the sample volume in a tube is found by knowing the tare weight of the tube, which is then subtracted from the gross weight of the tube, i.e., tube plus solubilized sample contents. This process is not only time consuming to perform manually, but also requires software that links the resulting data back to the sample inventory.
Technological advances have resulted in the introduction of instruments designed to assist with this method of volume determination. Typically, such instruments process a rack of tubes by selecting a single tube and scanning its bar code before then weighing it. The data is stored on file and linked back to the sample inventory. While these devices can also be used to pretare tubes, there are a number of limitations. Caps or septa, used to seal the tubes, can sometimes vary in weight by as much as 50 mg; this means that even with automated solutions, the weighing of a sample tube may not ensure that the volume information is accurate.
Another automated approach intended to address this issue utilizes acoustic sensor technology. Sensors, placed above the open necks of the tubes, allow the calculation of the distance to the upper surface of the liquid (and hence the volume in the tube) by measuring the time for the acoustic signal to reflect back from the meniscus.
While acoustic methods allow relatively high-speed volume auditing, there are again limitations when using this technique. For example, the signal from an acoustic sensor can be affected by several environmental factors that may lead to false information being returned. In addition, a calibration data set must be generated from previously measured sample volumes. Finally, and perhaps most significantly, this method only works with un-capped tubes, which may have significant implications for sample quality. Determining precipitates is also not possible using this technique.
Perhaps the simplest approach to tracking sample volume is to just maintain a calculated value in an inventory database, and decrement that value in line with the sample volume removed from the tubes during liquid-handling operations. This approach, which has been adopted by many groups, relies on the liquid handling being performed exactly as intended. However, blocked tips, or double aspirate/dispense cycles are not unknown, both of which will result in the tracked volume soon becoming inaccurate. The benefit of a closed-loop measurement process is thus clear.