Encoded Photometrics Infrared (EPIR) spectroscopy replies upon a photometrically simple, yet rugged and efficient design, where the incoming infrared beam from the sample is imaged on to a diffraction grating based spectrograph.

The dispersed radiation from the grating is imaged across an aperture above the surface of a rotating encoder disk. The encoder disk has a series of reflective tracks, which are spatially located within the dispersed grating image to correspond to the wavelengths and wavelength regions used for the analysis. Each track has a pattern that produces a reflected beam with a unique sinusoidal modulation for each individual wavelength.

The reflected beams are brought to an image on a single detector, which generates a signal that forms a discrete interferogram. The intensity contribution for each wavelength component is obtained by applying a Fourier transform to the interferogram.

The encoded photometrics based instrument is designed to encode the analytical information in the same way as an interferometer, but without the environmentally sensitive components of an interferometer.

EPIR spectrometry is proposed as a valid and advantageous alternative to other IR wavelength generation devices such as NDIR and FTIR spectrographs and spectrometers for two main reasons. Firstly, it retains desirable attributes of these technologies such as the size and cost of NDIR and the abundant spectral information produced by FTIR. Secondly, it leaves their typical drawbacks behind, such as the limited spectral information with NDIR and the lack of ruggedness of FTIR.

EPIR spectroscopy is based upon the use of an encoder disk onto which all wavelengths in a given region of interest are uniquely modulated, and the final spectral intensities computed via a Fourier transform. Current technology allows up to 256 user-defined

micro-regions of the spectrum to be measured simultaneously by a single detector (multiplexing advantage), hence providing the opportunity for simultaneous quantitative analysis of several components as well as the implementation of complex chemometrics methods.

For this experiment, an Advanced Photometrics Inc (API) Spectral Engine configured for the 3–5µm region providing a data point every 15nm was used. A glow-bar source and a lead selenide detector with single stage thermoelectric cooling were used. The instrument was set to collect 100spectra per second, scanning at 100Hz.

A first set of experiments was completed using a 15cm path length gas cell. They consisted of the analysis of three common low-molecular weight hydrocarbons: methane, ethane and propane.

The goal was to test the EPIR spectrometer’s capacity to resolve three homologue species in a mixture, despite the high degree of similarity between the spectra of the three compounds and the significant band overlapping typically observed in the 3–5µm region.

The second set of experiments relied upon the use of a five-metre folded pathlength cell and focused on the determination of the threshold of sensitivity, or level of detection (LOD), of the EPIR spectrometer as a function of scanning time for four homologue hydrocarbon gases – methane, ethane, propane, and butane – and carbon monoxide (CO) present in a mixture.

Data were collected using API’s proprietary data collection software. Final data treatments and graphing were completed using IGOR Pro (see www.wavemetrics.com)

Conclusion

The API Spectral Engine clearly demonstrated both the spectroscopic resolution of closely related chemical compounds in a mixture and the sensitivity required to complete fine analytical measurements (tens ppb levels) in a time of only 1–5minutes.

If additionally considering if rugged design, capability of remaining operational in vibrating environments and over a larger than expected range of temperatures, this marks this instrument as a serious candidate for industrial, on-site chemicals sensing applications.

For more information about Aspectrics and the EP-IR, visit www.aspectrics.com