Ultraviolet detectors range

The photoconductive detector is primarily used in the visible-infrared region rather than the ultraviolet—visible range. 

The reason for such difficulties is the GPC mechanism itself. We do not separate by molar mass but by the size of the solvated molecules. Different solvation of chemical unlike molecules results in breaking the M sequence of the calibration curve this becomes visible especially in the low molar mass range. Sometimes such difficulties can be circumvented if a specific detector is used, e.g., if the sample absorbs in the ultraviolet (UV) range and the disturbing peaks are UV transparent. 

Despite their higher sensitivity and relative cheapness compared with ultraviolet detectors, amperometric detectors have a more limited range of applications, being often used for trace analyses where the ultraviolet detector does not have sufficient sensitivity. 

Headspace analysis has also been used to determine trichloroethylene in water samples. High accuracy and excellent precision were reported when GC/ECD was used to analyze headspace gases over water (Dietz and Singley 1979). Direct injection of water into a portable GC suitable for field use employed an ultraviolet detector (Motwani et al. 1986). While detection was comparable to the more common methods (low ppb), recovery was very low. Solid waste leachates from sanitary landfills have been analyzed for trichloroethylene and other volatile organic compounds (Schultz and Kjeldsen 1986). Detection limits for the procedure, which involves extraction with pentane followed by GC/MS analysis, are in the low-ppb and low-ppm ranges for concentrated and unconcentrated samples, respectively. Accuracy and precision data were not reported. 

Such solvent systems continued to be used even though the lack of solubility of triacyl-glycerols with carbon numbers greater than 46 in this mobile phase has been noted. The solvent gradients that would be required for optimum separations of complex triacylglycerol mixtures are not compatible with RI detection. Therefore, ultraviolet detectors have also been used, but the range of mobile phases is limited, since TGs absorb only in the far-UV range. 

The analytical procedure is outlined in Fig. 4.6. Reproducible retention times (which varied less than 2% over a nine-month period) eliminated the need for column reequilibrium. Although some coeluting PAHs (eg anthracene and phenanthrene) had been placed into different fractions, it was clear that no single ultraviolet wavelength was capable of resolving all of the PAHs within a fraction. The sensitivities of the ultraviolet detectors, defined as a signal-to-noise ration of 2, ranged from 0.25 to lng IT1. 

Apparatus (See Chromatography, Appendix IIA.) Use a high-performance liquid chromatograph operated at room temperature with a 10-p.m particle size, 30-cm x 4-mm (id), C18 reverse-phase column (jxBondapak C18 column, Waters Corp., 34-T Maple Street, Milford, MA 01757, or equivalent). Maintain the Mobile Phase at a pressure and flow rate (typically 2.0 mL/min) capable of giving the required elution time (see System Suitability in High-Performance Liquid Chromatography). Use an ultraviolet detector that monitors absorption at 254 nm (0.2 to 0.1 AUFS range). 

Procedure Prepare a series of THI-DNPH Standard Solutions serially diluted from the Stock THI-DNPH Solution. Pipet 1, 2, and 5 mL, respectively, of the Stock THI-DNPH Solution, into separate 10-mL volumetric flasks, and dilute to volume with absolute, carbonyl-free methanol. Prepare a standard curve by injecting 5 p-L of the Stock THI-DNPH Solution, and the serially diluted THI-DNPH Standard Solutions into a 250-mm x 4-mm (id), 10-lm LiChrosorb RP-8 HPLC column (Alltech Associates, Inc., or equivalent) fitted with an ultraviolet detector set at 385 nm. The mobile phase is 50 50 (v/v) methanohO.l M phosphoric acid. Inject 5 pL of sample into the column. Adjustments in the mobile phase composition may be needed as column characteristics vary among manufacturers. At a mobile phase flow rate of 2 mL/ min and column dimensions of 250 x 4.6 mm, elute THI-DNPH at about 6.3 0.1 min. Measure the peak areas. Calculate the amount of THI in the sample from the standard curve. (For THI limits greater than 25 mg/kg, prepare a series of Standard THI-DNPH Solutions in a range encompassing the expected THI concentration in the sample.)... 

The basic near range sensors are the infrared and ultraviolet detectors, often coming together as IR/UV-sensor, in most cases in a line-scanner (LS) assembly. The IR/UV-LS is a passive bi-spectral remote sensor that is sensitive in the thermal infrared (TIR) between 8 and 14 pm and in the near ultraviolet (NUV) between 0.32 and 0.38 pm. At an aircraft altitude of 300 m its swath width amounts to approximately 500 m. This sensor on the one hand is used to measure the thermal emission of the sea surface in the TIR and on the other hand serves for the detection of highly reflecting... 

One of the major problems with packed columns is the pressure drop across the column, CO2 is the only practical SF for these systems. In contrast, a wider range of SFs can be used with open tubular capillary columns, e.g. ammonia and hydrocarbons. SF chromatography requires equipment capable of generating mobile phase pressures of up to lOOOOpsi (69 MPa), together with suitable detectors. Ultraviolet detectors are used with carbon dioxide since it is transparent in this region of the spectrum. 

Ultraviolet detectors. Ultraviolet detectors function by monitoring the light absorbed by the solute molecules from the incident beam. Ultraviolet detectors are the most commonly used type with LC systems, they are not appreciably flow or temperature sensitive, have a good dynamic linear range, but are, however, selective. The absorbance is proportional to concentration and obeys the Beer-Lambert Law which is defined as follows ... 

The fundamental properties of fluorescence make this a particularly attractive basis for an HPLC detection system [27], for whereas photometers depend upon the measurement of fairly small differences between the intensity of a full and slightly attenuated beam the measurement of fluorescence starts in principle from zero intensity. At sufficiently low values of concentration (<0.05 absorbance) then the intensity of fluorescence is directly proportional to concentration with a linear range of three to four decades. Consequently, fluorescence detectors are more selective and sensitive than ultraviolet detectors in LC by a factor of 10 giving noise equivalent sensitivities of better 1 ngmP. ... 

A UV/Vis absorbance detector can also be used if the solute ions absorb ultraviolet or visible radiation. Alternatively, solutions that do not absorb in the UV/Vis range can be detected indirectly if the mobile phase contains a UV/Vis-absorbing species. In this case, when a solute band passes through the detector, a decrease in absorbance is measured at the detector. 

Many kinds of detectors have been designed, ranging from the widely used, cheap but robust flame ionization (GC) or ultraviolet absorption type (LC) to the much more exciting and informative, if much more expensive, mass spectrometer. 

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