Dispersion detector sensor volume

Apparent Dispersion from Detector Sensor Volume... 

The maximum allowable dispersion will include contributions from all the different dispersion sources. Furthermore, the analyst may frequently be required to place a large volume of sample on the column to accommodate the specific nature of the sample. The peak spreading resulting from the use of the maximum possible sample volume is likely to reach the permissible dispersion limit. It follows that the dispersion that takes place in the connecting tubes, sensor volume and other parts of the detector must be reduced to the absolute minimum and, if possible, kept to less than 10% of that permissible (i.c.,1 % of the column variance) to allow large sample volumes to be used when necessary. 

Most sensor volumes, whether in LC (e.g., a UV absorption cell) or in GC (e.g., a katharometer cell), are cylindrical in shape, are relatively short in length and have a small length-to-diameter ratio. The small length-to-diameter ratio is in conflict with the premises adopted in the development of the Golay equation for dispersion in an open tube and, consequently, its conclusions are not pertinent to detector sensors. Atwood and Golay [12] extended the theory of dispersion in open tubes to tubes of small length-to-diameter ratio. The theory developed is not pertinent here as it will be seen that, with correctly designed cells, that dispersion from viscous sources can be... 

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... 

The sensor volume of a detector can cause dispersion and contribute to the peak variance in two ways. Firstly there will be dispersion resulting from the viscous flow of fluid through the cell sensor volume, which will furnish a variance similar in form to that from cylindrical connecting tubes. Secondly, there will be a peak spreading which results from the finite volume of the sensor. If the sensor has a significant volume, it will not measure the instantaneous concentration... 

Extra-column dispersion can arise in the sample valve, unions, frits, connecting tubing, and the sensor cell of the detector. The maximum sample volume, i.e., that volume that contributes less than 10% to the column variance, is determined by the type of column, dimensions of the column and the chromatographic characteristics of the solute. In practice, the majority of the permitted extra-column dispersion should... 

The main sources of broadening are the solute dispersion in the connecting tubes, the effective working volume of the detector and the dynamics of the sensor, of the electronic circuitry and of the recording device. 

It is clear that reducing dispersion to these low limits places strict demands on detector design. The solutes are actually sensed in the sensor cell or sensing volume of the detector but must be carried to... 

The first conductivity detector was developed by Martin and Randall as long ago as 1951 [1]. Improved cell designs have been described by Harlan [2], Sjoberg [3] and more stable and sensitive electronic circuits have been Avinzonis and Fritz [4] and Berger [5]. Scott et al. [6] inserted electrodes in the wall of a column to monitor the progressive band dispersion along a packed LC column. Keller [7] described a bipolar electrical conductivity detector and Kornilova et al. [8] describe a electric conductivity sensor for use in LC having a volume of 0.1 pi. 

The detector cell is 3 mm long terminated at one end by a cylindrical quartz window and at the other by a plano-convex quartz lens that disperses the transmitted light over a significant portion of the light sensitive area to the photocell. The total volume of the sensor cell was about 2.5 pi. Next to the quartz windows are two stainless steel discs separated by a 3 mm length of Pyrex tube. The mobile phase enters and leaves the detector cell through radial holes in the stainless steel discs that connect to the central orifice of the disc. The stainless steel... 

The open-tubular column is, by far, the most popular type of GC column in use today. As a result of its small internal cross section, however, extracolumn dispersion can become a serious problem. This means that open-tubular columns must be used with special types of injector and reduced volume connectors, and certain detectors must have specially designed sensor cells to avoid impairing column performance. 

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