In-process monitoring of pipetting transfers is essential for sample tracking. The first action required to ensure accurate and reproducible liquid transfer is that of liquid level detection (LLD). Determination of liquid levels for samples and reagents is accomplished by a number of means, including capacitive sensing, pressure sensing, sound waves, and infrared interrogation. Determining the initial volume of the sample is important for several reasons.
First, it is crucial to avoid aspiration of air to ensure that the correct volume of liquid is transferred, particularly for quantitative measurements. In addition, the correct volume of liquid can be aspirated with minimal penetration of the pipette tip into the liquid, reducing the risks of carryover, contamination, and sample overflow.
The most commonly used LLD technologies on automation platforms utilize capacitance or pressure-based sensing. Capacitive liquid level sensing detects changes in electrical conductance as the probe comes into contact with the liquid. This technology requires the pipetting tip and liquid to be electrically conductive. By contrast, pressure-based detection works through continuous monitoring of the air pressure within the pipetting channel, sensing the small change in pressure caused by contact with the liquid surface.
Pressure-based monitoring of pipetting also offers advantages for in-process assessment, allowing the changing pressure curve within the pipette tip to be tracked during aspiration and dispensing operations. By comparing the measured pressure curve with the expected curve in real time, the platform can identify any irregularities during an aspiration or dispense, including errors in liquid handling caused by bubbles or foaming as well as tip occlusions caused by solid materials that may be present in biological samples.
Many liquid-handling instruments on the market use a combination of technologies. Instruments able to combine LLD modes offer advantages in terms of functionality through enabling advanced tasks such as liquid-liquid extractions.
The security provided by pressure and capacitive-based sensing is sufficient to ensure peace of mind and legislative compliance for a majority of applications. If further assurance of pipetting accuracy is required, however, some instrument manufacturers offer “liquid arrival” verification systems based on the gravimetric techniques. By integrating a precision balance into the platform, each liquid-dispensing operation can be independently verified by the corresponding increase in sample container mass. This verification at dispense point is particularly relevant in clinical diagnostics and blood pooling applications, where patient safety is a primary concern.