NIR: An Enabling Technology
Near-infrared (NIR) spectroscopy is fulfilling its promise for rugged, robust process analysis. A vibrational spectroscopy, NIR uses light between the visible and IR regions. Typically, one fiber-optic probe interrogates the sample or process while another reads the absorbance. As with IR, any molecule whose vibrational modes entail changes in dipole moment is suitable for NIR analysis. But where IR measures fundamental vibrations, NIR hones in on lower-absorbing overtone or combination bands.
“This is an advantage,” notes Todd Strother, Ph.D., an applications scientist at Thermo Fisher Scientific. Strongly absorbing IR specimens must be diluted, which makes IR a poor real-time analysis method. “NIR is different. Since materials don’t absorb as much, you can measure them directly.” With NIR it is possible to position probes directly inside a bioreactor (or, for that matter, a powdered sample) and obtain a spectrum.
NIR absorbances are not as straightforwardly identified with particular chemical groups or their concentrations in solution. “Analytes in bioprocesses don’t follow Beer’s law,” observes Dr. Strother. “They absorb differently and nonlinearly at varying concentrations.”
Applying NIR in bioprocess monitoring, therefore, requires that operators first construct spectral training sets. Using independent primary analysis methods, with the aid of mathematical algorithms, operators can then assign qualitative and quantitative attributes to spectral peaks, connecting them to analytes of interest, e.g., glucose, lactate, ammonia, and others. Once the training set is constructed for a particular fermentation or cell culture the primary method need not be used again.
NIR analyzers can provide virtually instantaneous readouts of all analytes of interest, which makes them ideal for process analytic technology. Most upstream NIR probes operate inside bioreactors and can undergo CIP/SIP. It is also possible to construct an NIR system for disposable processing by taking advantage of the penetration of NIR radiation through thin plastic.
NIR was popular in the early 1990s for analyzing microbial fermentations. “That died down somewhat but is picking up again,” says Denise Root, marketing manager at FOSS NIRSystems. “Now, for every two mammalian cell fermentation projects we get, we hear from one microbial cell-based manufacturer.”
One NIR benefit is its flexibility with respect to reactor interface. Root says her company’s probes fit through all standard ports, but dissolved oxygen ports are a favorite.
FOSS’s principal bioprocess product is the XDS Process Analytics™ MicroBundle Analyzer, which uses 80 optical fibers—40 for illumination and 40 for collection. The device can monitor up to nine channels (analytes) in a single reactor, or nine separate reactors/fermentors at a time. The company has recently completed a special design for a single-use reactor as well.
Finesse Solutions, which specializes in measurement and control products for upstream bioprocessing, manufactures a line of sterilizable and single-use sensors. On the sterilizable side there is the TruDO Optical dissolved oxygen sensor with a built-in transmitter, TrupH (pH), TruDO (dissolved oxygen), and a NIR-based TruCell2 (cell density).
The single-use TruFluor™ dissolved oxygen and temperature sensor consists of a disposable sheath, optical reader, and transmitter. The sheath is inserted in a disposable bioreactor bag port and irradiated with the bag to preserve a sterile barrier. All finesse sensors are precalibrated, RFID-tagged, and meet USP Class 6 standards for cGMP operation.
Bioreactor pressure has become a critical measurement since the advent of polymer-based, single-use process bags. “People spend $5,000 to $10,000 on a disposable bioreactor and $100,000 on media, and if you don’t keep an eye on pressure the bag can explode,” explains Mark Selker, Ph.D., CSO. At “Interphex,” Finesse introduced TruTorr™, a single-use pressure sensor for measuring headspace pressure in single-use containers to prevent overpressure damage.