Think Small: Low-Cost Optical Spectral Measurements for Chemical Sensing

Sep 30, 2006

John P. Coates
The term spectral engine is not new and is normally intended to indicate the integration of optics and electronics to form a single measurement package. In the system described in this column, the term is extended to include the close-coupling of the measurement system to the sample via nonimaging optics.

Emil W. Ciurczak
This column focuses on the spectral engine concept and its role as an integrated component into continuous in-line/on-line measurement systems. One obvious mode of integration is the combination of the spectral sensor within a process analytical measurement system, and this is discussed in terms of the New Sample Sensing Initiative (NeSSI) miniaturized sampling systems, pioneered by the Center for Process Analytical Chemistry (CPAC), which is the industry–academia consortium based at the University of Washington, Seattle. Other forms of integration can include combinations with other small-scale sampling systems and microfluidic devices. Examples of applications are provided ranging from water quality measurements to the compositional measurements of organic sample streams.

In 2000, I discussed the concept of a small-scale "microspectrometer" based upon the integration of a spectrally separating optical filter, known as a linear varible filter (LVF), with a photodiode array (1). This device, known as the microPac, was developed and introduced by Optical Coatings Laboratories, Inc. (OCLI), now a division of JDS Uniphase (Santa Rosa, California). These systems featured visible and near-infrared (NIR) filter devices optically coupled to a commercial silicon photodiode array. At that time, the device was offered as a tool for instrument developers. For some, one issue was that there were a number of miniaturized spectrometer systems on the market that offered good performance at competitive prices. When viewed as an instrument platform, the microPac did not seem to offer any advantage over the existing microspectrometers, primarily because of cost and size issues.

The role of LVF devices, also termed variable filter array (VFA) devices, has been pioneered by Wilks Enterprises (South Norwalk, Connecticut) (2) for infrared spectroscopy applications. Wilks has always taken the position that high resolution is not necessary for dedicated analyses and has demonstrated this with the VFA analyzer systems. The Wilks VFA instruments operate with a typical spectral resolution in the range 40–50 cm-1 . Wilks has contended that most analyses can be performed at low resolution and that optical filters are adequate for such measurements. The microPac microspectrometer was similar in concept but had its focus on the visible and NIR and provided a spectral resolution (based upon spectral bandpass) in the range of 4–8 nm. Likewise, adequate measurements could be made in the visible and NIR with this degree of spectral resolution.

When the original microPac was introduced, it was recognized that it should be viewed as something different than a traditional instrument; the biggest opportunity for this type of integrated device was for spectral sensing. Spectral sensing means a component that can be integrated into a measurement system for monitoring chemical composition, rather than a standalone instrument. In the broadest sense, spectral sensing can be performed by instruments; however, instruments are not necessarily convenient or cost effective for true sensing applications. In an ideal world, spectral sensing should be performed by small, low-cost dedicated devices that are fully integrated with sample handling, data acquisition, and data handling. Such systems ideally are compact, maybe being no more than 1–2 in. wide and a few inches in length. They should be in a format that can be introduced into a process without major impact on the operation of the process. Note that the word "process" can be taken in the broadest sense and can include traditional industrial production processes, but it also can include environmental, biological (medical), and consumer-related processes such as water condition or quality monitoring.