Continuous-scan FT-IR spectrometer

The frame rate of the FPA directly determines the time resolution of a PA-IR spectrograph, while its size can Hmit the achievable bandwidth/resolution and the field of view. Maximum frame rates vary between lOOps and 17 ms per spectrum, depending on FPA type and size. Such time resolutions are similar or significantly better than those possible with continuous-scanning FT-IR spectrometers. It should be pointed out that the Hmiting factor is often not the detection of the IR radiation, but rather the time necessary for the read-out and data transfer. Perfectly optimized electronics could eventually enable a 70 ps time resolution. 

The fact that conventional continuously scanning FT-IR spectrometers can measure an interferogram in a time of 1 second or less makes them excellent tools to follow transient or time-varying processes. Let us first examine some of the instrumental aspects of FT-IR spectrometers used for time-resolved spectrometry. 

Synchronous Time-Resolved Measurements Using a Continuous-Scan FT-IR Spectrometer... 

Figure 20.4 Schematic illustration of the relationship between the interferometer scan and data acquisition in synchronous time-resolved mesisurements using a continuous-scan FT-IR spectrometer.

In time-resolved measurements described in Sections 20.3.3.1 and 20.3.3.2, the timeresolving process is synchronized with the timing of sampling of the interferogram. In this respect, both of them may be called synchronous time-resolved measurements. By contrast, a method (called asynchronous time-resolved measurements) that does not need the synchronization of these two timings although still using a continuous-scan FT-IR spectrometer has been reported [15]. 

Protein Adsorption. The development of medical implant polymers has stimulated interest in the use of ATR techniques for monitoring the kinetics of adsorption of proteins involved in thrombogenesis onto polymer surfaces. Such studies employ optical accessories in which an aqueous protein solution (93) or even ex - vivo whole blood (94-%) can be flowed over the surface of the internal reflection element (IRE), which may be coated with a thin layer of the experimental polymer. Modem FT-IR spectrometers are rapid - scanning devices, and hence spectra of the protein layer adsorbed onto the IRE can be computed from a series of inteiferograms recorded continuously in time, yielding ah effective time resolution of as little as 0.8 s early in the kinetic runs. Such capability is important because of the rapid changes in the composition of the adsorbed protein layers which can occur in the first several minutes (97). 

As the speed of electronics increased, it became possible to use rapid-scanning interferometers at their lowest scan speed for hyperspectral imaging. The first report of this type of instrument was made by Snively et al. [8] at Purdue University in 1999. Several manufacturers of FT-IR spectrometers claim to have implemented continuous scan imaging for the first time, implying that they developed the idea. Perhaps they intended to say that they implemented it on their company s interferometer for the first time, but Lauterbach s group should be given credit for the original implementation. 

Commercial analyzers mainly intended for laboratory use have been adapted with optical fibers to most spectrophotometers (Photonetics Sodety). Assemblies of fibers on spec-trofluorimeters [82,199] and Raman [167,177,193,200], infrared (IR), near-IR [189] mid-IR [201], and Fourier transform (FT) IR [202,203] spectrometers have already proved their value for remote spectroscopic sensing. However, these units are only partially adequate for continuous in situ process control because of their weak dynamic range after coupling of fibers and a maladjusted numerical aperture with respect to optical fibers. Photodiode-array instruments, without spectral scanning, are nevertheless the best for on-line control. This class includes the series of Hewlett-Packard HP 8450-8452 spectrophotometers and the Fluo 2(X)1 spectrometer (Dilor Society) [175] for fluorimetric and Raman determinations. 

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