Fixed-wavelength photometer

Visual methods have been virtually displaced for most determinations by methods depending upon the use of photoelectric cells (filter photometers or absorptiometers, and spectrophotometers), thus leading to reduction of the experimental errors of colorimetric determinations. The so-called photoelectric colorimeter is a comparatively inexpensive instrument, and should be available in every laboratory. The use of spectrophotometers has enabled determinations to be extended into the ultraviolet region of the spectrum, whilst the use of chart recorders means that the analyst is not limited to working at a single fixed wavelength. 

The mobile phase consisted degassed distilled water containing 1.0 grams/liter of Aeorosol(B)-OT and varying amounts of sodium nitrate, NaNOj. The detector was a DuPont Model 840 UV photometer with a fixed wavelength of 254 nm. 

Photometers are more sensitive than spectrophotometers, are cheaper and more robust and are well suited to routine work where monitoring at 254 nm or some other fixed wavelength is acceptable. Spectrophotometers, however, allow tuning to the most favourable wavelength either to maximize sensitivity for a particular solute or to detune the response to other solutes. By allowing monitoring down to 190 nm, weakly absorbing or saturated compounds can be detected. 

Interference filters are used in photometers and spectrometers as fixed wavelength or tunable wavelength filters. An interference filter, equivalent to a Fabry-Perot etalon. 

Two types of photometers are used in HPLC— the fixed-wavelength filter photometer and the variable-wavelength spectrophotometer. These use a low-pressure mercury arc lamp as a light source, a transmission or interference filter to isolate a narrow band of wavelengths of light with which to excite the sample, and a photodiode to detect the light transmitted through the sample. 

Photometers operate as either fixed-wavelength or variable-wavelength detectors. Most fixed-wavelength UV... 

UV photometer (fixed wavelength) Measures absorbance of UV light Selective [Pg.159]

In principle, the same classification as with photometers, according to flexibility in choice of wavelength, can be used here, with the complication that both the excitation and emission wavelengths are of importance. The simplest system would involve fixed wavelengths for both excitation and emission the most complicated one would necessitate the use of two monochromators. 

Figure 10. Schematic diagrams for (a) simple fixed wavelength UV photometer-flow-through cell module and (b) variable wavelength double beam UV photometer-flow-through cell module.

These detectors may be divided into the three types shown in Section 1 of Table 2, namely (i) simple fixed wavelength, (ii) dispersion type (prism or grating) variable wavelength double beam UV rectrc hotometers and (iii) diode array UV photometers. Figure 10 consists of schematic diagrams of the first two types of detectors. 

---