High-quality, precision liquid-handling instruments have tended to give scientists throughout the drug-discovery, testing,
and production processes a sense of confidence in their data. However, the large amount of resources dedicated to drug development,
the long US Food and Drug Administration approval process, and the numerous recalls and legal actions plaguing well-known
drug companies suggest that more attention should be paid to quality assurance. In particular, liquid-handling processes—the
core of pharmaceutical laboratory operations—demand the application of robust, rigorous, science-based methods and tools to
ensure data quality.
Table I: Strengths and weaknesses of verification methods.
In life-science laboratories, which commonly include technologically advanced instruments, scientists often have various tools
to complete everyday tasks such as liquid-handling quality assurance. Several options are available to laboratories for calibrating
liquid-handling instrumentation and measuring the efficacy of liquid-handling processes. Each option has its own applications,
benefits, and drawbacks. The optimal technology for a laboratory application depends on factors such as the volume of liquids
to be quantified, the type of instrumentation used, and the applicable regulatory and quality standards. Also to be considered
are the laboratory environment, tolerance for risk, required calibration frequency, and the demands of the laboratory's processes.
This article will compare gravimetry, fluorometry, single-dye photometry, and ratiometric photometry (common means for verifying
liquid-handling instrumentation) and provide data and guidance regarding best applications of each.
Laboratories have traditionally relied on gravimetry to measure the performance of liquid-handling devices. This method uses
a balance to weigh liquid volumes. The balance reports a weight, and that weight is converted to mass and then to volume using
conversion factors that may be found in tables, calculated from formulas, or produced by software packages.
Gravimetry has several advantages, including the wide availability of weighing devices in most laboratories. In addition,
gravimetry is a well-accepted technology. It is recognized by national and international regulatory agencies, including the
International Organization for Standardization (ISO), the College of American Pathologists, and ASTM International. Published
standard methods of gravimetry include ASTM E1154 and ISO 8655-6 (1, 2). Gravimetric calibration can also be traced to national
standards, thus facilitating regulatory compliance and standardization.
Gravimetry is frequently the method of choice for measuring device performance when handling larger volumes. For example,
a 1000-μL aliquot weighs approximately 1 g and can be weighed reliably on a modern laboratory analytical balance. The current
trend in laboratories toward handling small liquid volumes with automated devices illustrates one major drawback of this method:
as volumes decrease, weighing becomes more challenging for several reasons.
First, measuring small liquid volumes requires specialized balances that produce measurement results to five or six decimal
places on the gram scale. Such balances are delicate, require a stable platform to limit vibration, and are not as portable
as the less-sensitive models used for measuring large liquid volumes. These requirements often make microgram balances unsuitable
for use on the deck of automated liquid handlers. Illustrating the need for sensitivity, ISO 8655-6 requires that volumes
of 10 μL or less be measured on a six-place (microgram) balance (2).
Because microgram balances take time to settle, gravimetric calibration can also be time-consuming. In addition, gravimetry
is affected by various environmental conditions, including evaporation and static electricity. As volumes become smaller,
these error sources become more significant.
For example, modern dispensing equipment can deliver volumes so small that they can evaporate in seconds. Obtaining adequate
resolution for small volumes requires a highly sensitive balance with complicated evaporation traps, static eliminators, and
vibration dampeners. Other methods for controlling for evaporation can be complicated. One method is to measure the evaporation
rate and correct for the resulting volume variation. Alternatively, the humidity in the room can be increased or a draft shield
built to prevent air from flowing over the testing area. These steps add time and complexity to the measurement process.