UV detector output

Both the capillary LC and NMR are controlled by the interface software, which enables the operator to use the UV-detector output for peak selection. Only peaks of interest can be subjected to NMR analysis, while minor or unimportant compounds can be directed to waste. NMR acquisition can take place in either on-flow or stop-flow mode. The combination of capillary LC and NMR is suitable for sample-limited applications (e.g., proteomics) and allows for low nanograms detection. 

Figure 3. Gel permeation chromatograms of (A) unexposed Micro-posit 2400-17 and (B) Microposit 2400-17 exposed to 418 mJ/cm. The UV detector output at 250 nm is shown.

The pressure sensitivity of a detector will be one of the factors that determines the long term noise and thus can be very important. It is usually measured as the change in detector output for unit change in sensor-cell pressure. Pressure sensitivity and flow sensitivity are to some extent interdependent, subject to the manner in which the detector functions. The UV detector, the fluorescence detector and the electrical... 

In a typical UV detector, the current is allowed to flow for a very short period of time before the voltage is reduced and the current stopped. Thus the output of the sensor tube is a series of voltage pulses, the frequency of which is proportional to the intensity of the UV sensed by the detector. The closer a fire is to the detector, the higher the output frequency, and the smaller the flame size that is needed to actuate the system. 

A flow synthesis of peptides using a macroporous polymer resin soon followed additionally incorporating an in-line UV detector, which constantly monitored the progress of the reaction by analyzing the output of the column the reaction was recirculated until it reached completion. However, backpressures of500-1000 psi were encountered, making these systems only suitable for metal-encased columns [93],... 

FIGURE 13. Comparison of pulsed GC-AA output continuous UV detection output. The pulsed output was obtained using a 0.85 ml injection at 382-ppm trimethylarsine. The power supply dial was set at 2000 °C and the pulse program was set to give a 7-s pulse on a 30-s interval. The background corrector of the detector was not employed and, as indicated by the arrow, molecular absorption of the 193,7-nm EDL-As can be detected. The continuous output was obtained using 0.20 ml of 2536-p.p.m. trimethylarsine with the furnace at ambient temperature and very low attenuation of signal to the recorder... 

If necessary, a classical detector can be attached to the column outlet, preceeding the fraction collector. UV detectors are the easiest to install and use. A variable-wavelength detector allows selection of the monitoring wavelength. Such detectors should be equipped with the appropriate cell. Analytical cells, used on HPLC systems, have path lengths of 6-10 mm and are not suitable for use with preparative open columns, as the detector output will quickly become saturated. A more generally useful cell for open column detectors would be a cell with a path length of less than 0.5 mm. 

The solutes chosen were (S) and (R) 4-benzyl-2-oxazolidinone. The apparatus consisted of an LC pump, a 1 pi internal loop sample valve and the column was situated in a thermostatically controlled oven. The oven could be operated at temperatures between 5°C and 85°C with a precision of 0.2°C. The mobile phase was passed through a preheater, consisting of 1 m length of coiled stainless steel tube, 0.010 in. ID., situated in the thermostat, between the sample valve and the column. The column outlet was connected directly to a UV detector operating at 220 nm. The eluent from the detector was passed to a 50 ml Grade A burette and the retention volumes were measured directly in ml of mobile phase. The burette was read with an accuracy of 0.02 ml. The output of the detector was displayed on a digital meter and an electronic recorder/integrator. 

One or more detectors is attached to the output of the columns. For routine analysis of linear homopolymers, this is most often a Differential Refractive Index (DRI) or a UV detector. For branched or copolymers, however, it is necessary to have at least two sequential detectors to determine molecular weight accurately. Branched polymers can be analyzed using a DRI detector coupled with a "molecular weight sensitive" detector such as an on-line viscometer (VIS) or a low-angle laser light scattering (LALLS) detector. 

A low volume (0.2 pi) Valeo sample valve was employed with one end of the open tube connected directly to the valve and the other connected directly to the sensor cell of the detector. The UV detector was the LC 85B manufactured by Perkin Elmer, and specially designed to provide low dispersion with a sensor volume of about 1.4 pi. The total variance due to extra-column dispersion was maintained at all times less than 2% of that of the elution curve of the solute. The open tube was 365.4 cm long with a radius of 0.0184 cm it was enclosed in a 1 cm I.D. plastic tube through which thermostatting fluid passed to maintain the temperature 25°C 0.2°C. The detector output was passed to a high speed Bascomb Turner recorder... 

Detectors can be broadly classified into two types. Bulk Property detectors which function by measuring some bulk physical property of the column eluent (e.g. dielectric constant or refractive index) and Solute Property detectors which function by measuring a physical and/or chemical property that is characteristic of the solute only (e.g. UV absorption). This classification is not completely precise, for example, the UV detector, which is usually classed as a solute property detector, when used with an ethyl acetate-heptane solvent mixture as the mobile phase will give a constant background signal due to UV absorption by the ethyl acetate. Furthermore, any fluctuation in ethyl acetate content of the mobile phase will appear as noise on the detector output. It follows that the UV detector, although a solute property detector, behaves as a hybrid between a bulk property detector and a solute property detector under some conditions of use. 

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