NEW GUIDELINES RECOMMEND PHOTOMETRY
In 2005, ISO added Part 7 to Standard 8655 because of advancing technologies that overcome several limitations of gravimetry—such as susceptibility to evaporation errors,
difficulty in verifying the performance of individual channels in multi-channel devices, and the requirement of a temperature
and humidity controlled environment for accurate results. In this section, ISO formally approves photometry for assessment
of liquid delivery equipment performance. By relying on known light absorption properties at specific wave lengths, photometric
calibration can quickly and conveniently provide strong assurance of data integrity .
Two specific variants of photometric calibration are highlighted by the standard: single-dye and dual dye. As its name implies,
single-dye photometry measures light absorption in one colorimetric solution to verify volume. The dual-dye approach to calibration,
called Ratiometric Photometry, uses two highly characterized solutions to combat accuracy problems typically associated with
single-dye absorbance measurements, and yields results with uncertainty of less than one percent for volumes as low as 0.1
As the industry changes and laboratories encounter new challenges and solutions, it is likely that ISO will continue to advance
its standards. Currently, for example, ISO 8655 clearly applies to handheld pipettes. When these standards were prepared, ISO's focus did not extend to robotic pipettors,
which explains the exclusion of these liquid handling devices. The scope of the committee now includes a broader range of
laboratory equipment and is likely to move in one of two directions: write additional standards to guide calibration of automated
liquid handlers or revise existing standards to include them. It is important to note that 8655 Part 7 does approve "vertical beam photometry," which is useful in verifying robotic pipettors that dispense to microtiter plates.
Laboratories today must evaluate their processes from start to finish, because of the increasing number of regulations and
standards and the industry-wide focus on quality control. This priority is evident in the FDA's Quality System Inspection
Technique (QSIT) initiative. The program emphasizes corrective and preventive actions and requires laboratories to identify
where problems occur or recur, and to document corrective actions taken.
The high failure rate of liquid handling instrumentation is cause for concern, especially in light of the growing focus on
quality and the drastic consequences of failure. Forward-looking laboratories are taking measures to verify accuracy and precision
and maintain liquid handling quality control to facilitate compliance and produce quality products.
George Rodrigues, PhD, is senior scientific manager at ARTEL, 25 Bradley Drive, Westbrook ME 04092, 207.854.0860, fax 207.854.0867 email@example.com