Passive optodes

In the passive mode, the optical device measures the variation in fluorescence characteristics (intensity, lifetime, polarization) of an intrinsically fluorescent analyte. The optical device can have different optical configurations involving in most cases an optical fiber (passive optode) (Figure 10.44). 

Figure 17-d. Comparison of normalized absorbance for different dyes. TB Thymol Blue BPB Bromophenol Blue BCG Bromocresol Green NR Neutral Red BTB Bromothymol Blue PR Phenol Red PP phenolphthalein. (a) Passive optodes (b) optode with Amberlite XAD-4 without membrane (configuration as in Figure 17-3 c). 

Dye Theoretical pA", pK measured with passive optode Semi-active optode References... 

Passive optodes shown in Figure 17-34 are used for Raman or fluorescence spectrometry. 1 vo types of monofiber optode have been described [166], as shown in Figure 17-34a. They require the use of a pierced mirror and suitable optics to separate the source laser and fluorescence emission beams at the fiber-measuring instrument junction. With a single fiber the observation of fluorescence is divided into four zones in order to calculate the effective optical path (/,). For a fiber with radius rg and numerical aperture A and observation in a medium with refractive index n , this path was evaluated by Deaton [191] as... 

Figure 17-34. Passive optodes in fluorescence and Raman spectrometry for process control.

Ihble 17-3. values for passive (without immobilization of dyes) and semi-active (active-extrinsic) optodes... 

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