Considerations in Identification and Qualification of Spectrophotometer Equipment for Microbial Fermentations

The authors outline qualification procedures for a process-critical piece of equipment.
May 01, 2011
Volume 24, Issue 5


Biotechnology processes depend on key equipment to aid in process decisions. The lifespan and support period for equipment can sometimes come to an end in the midst of commercial production. When this happens, alternative equipment must be identified and qualified to replace the original equipment without impacting the process outputs. This article provides the approach used during a recent exercise to identify a replacement spectrophotometer used to make process decisions for microbial fermentations. Three spectrophotometer models were tested in these experiments. First, instrument and process linear ranges were determined for each spectrophotometer. Next, a calibration model (margin of error) was used to determine the accuracy and precision of each spectrophotometer. These results were used to select the replacement spectrophotometer. Finally, the selected spectrophotometer was evaluated in bench-scale fermentations in order to confirm the capability of the instrument to serve as a viable replacement.

Spectrophotometers measure optical density (OD) at a particular wavelength, and are used in microbial fermentation processes to monitor culture growth, make process decisions (e.g., initiate feeds and induce production at specified OD), and to measure performance parameters (final OD). With the current company-wide platform spectrophotometer (Model A) being phased out by the vendor, the authors set out to identify a replacement spectrophotometer for microbial fermentation processes. Ideal characteristics for the replacement spectrophotometer included the following:

  • An expanded measurement linear range relative to Model A.
  • Minimal to nonexistent bias relative to the OD output of Model A.
  • Similar or better measurement precision relative to Model A.

The second criterion was especially important because the replacement would then be "like-for-like," meaning that minimal updates to existing manufacturing procedures and process-related documents would be required.


Spectrophotometer technical specifications

Table I: Technical specifications of test spectrophotometers.
Three test spectrophotometers, manufactured by different vendors, were evaluated. Table I compares the technical specifications of the test spectrophotometers (Models B, C, and D) to those of the current spectrophotometer (Model A).

Spectrophotometer models A and C have a tungsten/deuterium light source that provides continual reading of sample absorbance. Model C is the next generation model of spectrophotometer to Model A. Spectrophotometer models B and D use a xenon light source that does not continually read the sample absorbance, and operate by a press-to-read method. All spectrophotometers have a small footprint, which is desired because of space constraints in large-scale manufacturing areas. The normalized model A linear range is 0.2–1.6 dilution OD.

Spectrophotometer test solutions

National Institute of Standards and Technology (NIST) OD standards were purchased from GFS Chemicals in the 0.2–12.0 normalized OD test range. NIST OD standard solutions were used to determine the instrument linearity of each spectrophotometer.

Culture broth samples were taken from bench-scale, fed-batch fermentations using an Escherichia coli expression system. The process linearity experiment samples were frozen prior to testing to allow all samples to be analyzed at one time. All dilutions were performed using a saline diluent.

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