Dual-wavelength measurements

N. Okui and E. Okada. Wavelength dependence of crosstalk in dual-wavelength measurement of oxy- and deoxy-hemoglobin. Journal of Biomedical Optics, 10(l) 011015-l-011015-8, 2005. 

Dual wavelength measurement with the BHRP was straightforward, because it does not have filters. The skulls of the mice and rats were exposed and small holes were drilled at the frontal cortex and olfactory lobes in order to insert the BHRP. Since the probe has a high spatial resolution and was kept in contact with the tissue surface during the experiment, the spectral changes at exactly the same point on the brain could be compared. 

The final step in the development of a method is to establish the requirements for speed versus the necessary specificity. In short, a method may perform better in terms of specificity if a broad spectral region is used in a comprehensive che-mometric model, but the duration of data acquisition may not be suitable for the application. The opposite is also true, where a single- or dual-wavelength measurement may meet the ideal speed target, but fail in terms of specificity. Various instrumental platforms are available on the market, and it is imperative that a balance between these parameters be achieved for a method to ever reach deployment. For laboratory-based methods, specificity is often much more important than speed, reinforcing the need for a hyperspectral imaging system, if the instrument is to be used for quality control, then speed should be considered in the development of the method. It is important to bear in mind that speed and specificity are not mutually exclusive indeed, they can often both be achieved if the method is properly targeted to the question at hand. 

I.Y. Eom, P.K. Dasgupta, Frequency-selective absorbance detection refractive index and turbidity compensation with dual-wavelength measurement, Talanta 69 (2006) 906. 

Fiber optic pH sensors have distinct advantages over pH electrodes. They are small, not interfered by electromagnetic flelds and have remote sensing capability. They can be used in extreme environments, such as deep-water analysis, chemical reactors, or wastewater. Moreover, they can be intrinsically referenced due to the dual wavelength measurement capabiUty and do not require a reference electrode [90]. Optical pH sensors also pave the way for imaging applications, whereas pH electrodes only enable punctiform pH measurements. Sensors for pH determination are also of high significance in environmental and marine research because they provide the basis for CO2 sensors. 

Ratzlaff, K. L., and bin Darus, H., Optimization of Precision in Dual Wavelength Spectrophotometric Measurement, Anal. Chem. 51, 1979, 256-261. 

As mentioned above, in all photophobic action spectra measured so far additional peaks and/or shoulders occur at 565 and/or 615 nm, besides the chlorophyll a peaks, indicating the participation of phycobiliproteins and, hence, PS II in the active light absorption (Fig. 4). Moreover, activity of both PS I and PS II was confirmed by inhibitor experiments (see reviews by Nultsch 10°),Nultsch and Hader102), and Hader47)) and also with the aid of a dual wavelength technique39 40 101), described in detail below. 

Another feature is that the absence of spectral change precludes ratiometric measurements. However, dual-wavelength Cl sensors have been constructed. For instance, in compound A-6 (Figure 10.30), 6-methoxyquinolinium (MQ) as the Cl -sensitive fluorophore (blue fluorescence) is linked to 6-aminoquinolinium (AQ) as the Cl--insensitive fluorophore (green fluorescence), the spacer being either rigid or flexible. 

Chance and associates employed spectrophotometry on intact mitochondria or submitochondrial particles to investigate both the sequence of carriers and the sites of phosphorylation. Using the dual wavelength spectrophotometer, the light absorption at the absorption maximum (Aniax) of a particular component was followed relative to the absorption at some other reference wavelength (Aref). The principal wavelengths used are given in Table 18-6. From these measurements the state of oxidation or reduction of each one of the carriers could be observed in the various states and in the presence of inhibitors. The... 

ABLE (AirBome Lidar Experiment) is a Nd-Yag high energy lidar for the measurement of aerosols and PSC. It will operate in either a dual wavelength configuration 532 nm or 355 nm emissions or at 532 nm with dual polarisation detection capacity. It will detect aerosols, PSC and tropospheric clouds. 

Two UV detectors are also available from Laboratory Data Control, the UV Monitor and the Duo Monitor. The UV Monitor (Fig.3.45) consists of an optical unit anda control unit. The optical unit contains the UV source (low-pressure mercury lamp), sample, reference cells and photodetector. The control unit is connected by cable to the optical unit and may be located at a distance of up to 25 ft. The dual quartz flow cells (path-length, 10 mm diameter, 1 mm) each have a capacity of 8 (i 1. Double-beam linear-absorbance measurements may be made at either 254 nm or 280 nm. The absorbance ranges vary from 0.01 to 0.64 optical density units full scale (ODFS). The minimum detectable absorbance (equivalent to the noise) is 0.001 optical density units (OD). The drift of the photometer is usually less than 0.002 OD/h. With this system, it is possible to monitor continuously and quantitatively the absorbance at 254 or 280 nm of one liquid stream or the differential absorbance between two streams. The absorbance readout is linear and is directly related to the concentration in accordance with Beer s law. In the 280 nm mode, the 254-nm light is converted by a phosphor into a band with a maximum at 280 nm. This light is then passed to a photodetector which is sensitized for a response at 280 nm. The Duo Monitor (Fig.3.46) is a dual-wavelength continuous-flow detector with which effluents can be monitored simultaneously at 254 nm and 280 nm. The system consists of two modules, and the principle of operation is based on a modification of the 280-nm conversion kit for the UV Monitor. Light of 254-nm wavelength from a low-pressure mercury lamp is partially converted by the phosphor into a band at 280 nm. 

In spectrophotometric analyzers, interference filters are selected for desired wavelengths, as determined from the spectral relationship curves. Photodetectors are least sensitive in the blue end of the spectrum. This can be dealt with by using prefilters or narrow spectral ranges, which are calibrated for more sensitivity. Improvements in spectrophotometers include a flashed xenon light source with dual-beam measurement. Dual-beam machines measure the spectrum of both the light source and the reflected light for each measurement. 

For all the above reasons, it is to be preferred to measure two solute properties in one detector, especially if both measurements can be performed simultaneously. An example of this is the application of dual-wavelength absorption detection in LC. The application of this technique for the purpose of selectivity optimization has been investigated by Drouen et al. [584]. For the purpose of peak assignment or recognition, ratio recording may be used. The principle of this technique is based on Beer s law and may be explained from the following equation for the absorption ratio Ra ... 

Langner M, Hui SW. Merocyanine interaction with phosphatidylcholine bilayers. Biochim. Biophys. Acta. 1993 1149 175-179. Ross E, Bedlack RS, Loew EM. Dual-wavelength ratiometric fluorescence measurement of the membrane dipole potential. Biophys. J. 1994 67 208-216. 

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