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Assay Tutorials : Mar 15, 2011 ( )
Process Security for Liquid Handling
Software and Hardware Solutions Provide Full Traceability Should Errors Occur
Instrument Operational Qualification
Installation qualification/operational qualification has become a familiar term across a broad range of scientific sectors, referring to regulatory requirements for periodic verification of system performance. Operational qualification requirements for liquid-handling systems vary widely, depending on the instrument’s intended application, however the methods of verifying performance can be broadly divided into either gravimetric analysis or colorimetric-based technologies.
Traditionally, gravimetric techniques have been used for validation of liquid-handling equipment—both manual and automated—sequentially performing separate measurements for each pipetting channel. While this technique is still suited to some applications, it requires integration of a balance into the liquid-handling system. The trend toward high-density sample formats has also made gravimetric analysis impractical for use with many systems.
By contrast, colorimetric technologies offer a straightforward method for performance verification of all pipetting channels. Integrated multimode plate readers are now a common feature of automated liquid-handling installations, allowing the verification process to be fully automated, and even routinely scheduled. Several manufacturers offer colorimetric-based kits for liquid-handling instrument validation, the Artel MVS® (Multichannel Verification System) kit is an example (Figure 1).
Full sample traceability throughout processing is a key requirement for automated laboratory systems in a clinical setting. Once samples have been loaded onto the workstation, manual interaction with the instrument should be minimal (Figure 2). Software should generate a detailed log of all actions. For a complete record, it is not only necessary to confirm the presence of each sample at every step of the process, but also the information associated with each sample.
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.
Any form of liquid handling in a clinical or medical setting carries with it an inherent risk of sample carryover or contamination. Although disposable tips are routinely used for a majority of these applications—in both manual and automated pipetting protocols—system cleanliness must also be considered. The existence of small volumes of vaporized samples and/or airborne contaminants within the pipette channel pose a significant risk to sample integrity.
For air-displacement instruments, regularly scheduled decontamination and maintenance cycles are the only solution to this problem. Platforms utilizing liquid-displacement technologies can be automatically “flushed” with system liquid at scheduled intervals to ensure system cleanliness as well as reduce servicing requirements and instrument downtime.
Error Handling and Remote Monitoring
With a growing reliance on automated liquid handling and high-throughput techniques, the cost of a failed or aborted processing run is increasing almost exponentially. In addition to the loss of productivity, the loss of expensive reagents or compounds can represent a significant cost to laboratories. Error-handling protocols not only safe-guard against system errors but are also an important mechanism for dealing with complex liquid-handling requirements and avoiding unnecessary loss through operator errors.
The exact mechanisms of error handling vary widely between systems, however they all work on the basic premise of allowing the user to individually tailor the instruments response to any given error situation depending on the precise requirements of the application. By allowing the user to configure the platform for each error that may be encountered, the impact of the error can be minimized, maintaining productivity and reducing waste of samples and reagents.
For some applications, it is important that the user is able to take corrective action at the earliest opportunity. Remote-monitoring systems allow the operator to safe-guard such processes without having to physically supervise operation of the automated platform, offering increased walkaway time.
A variety of strategies for remote monitoring are now available. Tecan’s (www.tecan.com) CNS provides remote monitoring via an Intranet or Internet connection as well as utilizing push notification technologies using portable devices such as the iPhone®. These systems provide the operator with real-time monitoring of automated laboratory processes from multiple systems, allowing them to perform other tasks from remote locations during processing (Figure 3).
As well as providing users with a mechanism for remotely overseeing processing, network connectivity can play pivotal roles in sample management, data handling, and traceability. It is now common practice for large and high-throughput laboratories to use complex LIMS packages to organize workloads, track samples, and securely transfer data.
Barcode usage is often an integral part of this approach, allowing automated liquid-handling systems to identify individual samples and interrogate a central database to determine processing requirements. Seamless interconnectivity between the LIMS and the platforms also allows the creation of a detailed history for each sample, linking the sample identity to a log of the operations performed on the sample, the results obtained, and comments of events that occurred during processing. The direct communication between the platform’s software and the LIMS database provides a secure and validated audit trail, ensuring error-free data transfer and legislative compliance, as well as reducing the need for user interaction.
Wendy M. Lauber (email@example.com) is director of product management for the liquid handling and robotics business unit at Tecan Schweiz.
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