Optic spectrophotometers

Kapany N.S., Silbertrust N., Fiber optics spectrophotometer for in-vivo oximetry, Nature 1964 208 138. 

Optical spectrophotometers work in different modes to measure optical density, absorbance, or transmittance. 

This is indeed a very large value to be measured by an optical spectrophotometer (see Section 1.3). 

Instrnmentation for UV-vis process analysis falls into fonr categories scanning instruments, diode-array instrnments, photometers, and fiber-optic spectrophotometers with photodiode-array (PDA) and charge-conpled device (CCD) detectors. The former two are more typically enconntered in at-line or near-line applications, whereas the latter two are better snited to actnal on-line analyses. 

Both PDA and CCD fiber-optic spectrophotometers may be combined with a fiber switch or multiplexer to access multiple probes or cells in a sequential fashion. Multiplexers are available to select from among 2, 6, 9, or even more fibers/probes. 

The UV visible measurements are performed using a commercial HR 2000 high-resolution fibre optic Ocean Optics spectrophotometer equipped with a halogen light source HL2000. The detector is a linear silicon CCD array. The spectral resolution measured with a Hg-Ar calibration lamp is better than 0.2 nm. 

On the other hand, there are measurements in which signal and noise cannot be directly filtered and the signal has to be transposed onto a carrier wave to be shifted away from the noise frequencies (modulation). Then an amplifier is tuned to the frequency of the carrier wave and the amplified original signal is finally recovered (demodulation). The use of a chopper in optical spectrophotometers is a common example of this process [i]. Ref [i] Horowitz P, Hill W (2001) The art of electronics. Cambridge University Press, Cambridge... 

In forward optic spectrophotometers, electromagnetic radiation of a specified wavelength is passed through sample solutions. Because radiation from source is polychromatic (consists of light of all wavelengths), the various wavelengths of radiation must be separated... 

Fig. 1 (A) Components of a typical UV-vis spectrophotometer (forward optics). (B) Components of a reverse optics spectrophotometer.

Forward optic spectrophotometers are either singlebeam or double-beam spectrophotometers. The singlebeam instruments can be either very simple or expensive depending on the sophistication desired or needed. Simple single-beam instruments have poor stability and excessive drift. These advantages are eliminated in systems equipped with a high-resolution monochromator with adjustable slits, controlled by microprocessors for rapid data acquisition and evaluation of data. 

Reverse Optic Spectrophotometers-Photodiode Array Detectors... 

The linear optical spectra of the thin solid samples, depending on the experiments, were measured on the spectrophotometer Hewlett Packard 8453, fast fibre optics spectrophotometer Avantes AVS-SD2000 and fast fibre optics spectrophotometer Calibri VMK Optoelectronics, Novosibirsk (http //www.vmk.ru/). Fast spectrophotometers allow the measurement of 40 spectra per second. Sample heating for determination of the J-aggregates thermal stability was done in a thermostatic optical chamber with a constant heating rate of 1.5 deg/ min. 

Most multiparametric procedures are restricted to two active components, and the main challenge is in using different reagents and detection wavelengths. The use of photodiode array spectrophotometers and, more recently, multichannel fiber optic spectrophotometers facilitates this approach. Solenoid valves, to control the flow system, have proved to be very useful in the development of techniques for simultaneous determination of Zn, Fe, Cu, Ca, and Mg in pharmaceutical drugs. Derivative... 

As the complex reaches the column, it is retained at the top of the packing and its absorbance monitored with a fiber-optic spectrophotometer. The output is a curve where the peak height is proportional to the nitrite concentration in the sample. This procedure affords detection limits in the sub-ppm range. 

Traditional flow cells require a holder for placement. The type of holder to be used is dictated by the particular detector. With conventional spectrophotometers, the holder can be one of the models used in batch mode. Fiber-optic spectrophotometers, however, require a modified cell holder, which is available in various forms. Figure 5.15 shows a cell holder to which a light emitting head powered by a current-based source and an optical fiber connected to a CCD spectrophotometer are screwed on each side. In Figure 5.4(b), the cell holder is directly connected to the spectrophotometer, avoiding the need for an extra optical fiber. 

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