Detector sensor volume

Apparent Dispersion from Detector Sensor Volume... 

Figure 15. The Effect of Detector Sensor Volume on the Resolution of Two Solutes...

The introduction of small bore packed columns in LC [7] reduced the peak volume still further and placed an even greater strain on LC detector design. Due to the relatively lower sensitivity of LC detectors compared with that of GC detectors, the LC detector sensor volume was forced down to a level which, for present day technology, may well be the practical limit for many types of LC detectors. This interaction between detector design and column design continues to this day and probably will do so for many years to come. 

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... 

The detector responds to an average value of the total amount of solute in the sensor cell. In the extreme, the sensor volume or cell could be large enough to hold two... 

Figure 14. Effect of Sensor Volume on Detector Output...

The Separation of Two Solutes 4a Apart on a Mifcrobore Column Employing Detectors Having Different Sensor Volumes... 

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... 

A diagram of the micro argon detector sensor is shown in figure 3. This sensor was designed to have a very small "effective" volume so that it could be used with capillary columns where the flow rate may be as low as 0.1 ml/min or less. In the micro argon detector sensor, the anode is withdrawn into a small cavity about 2.5 mm in diameter. 

It is seen that the eluent is vaporized directly into the detector in an atomized form by means of a heated transfer tube situated in an oven held at 300°C. On entering the oven, the mobile phase and eluent are immediately vaporized, and the increase in volume resulting from the vaporization forces the vapor into the electron capture detector sensor (utilizing a nickel radioactive source). The sensor is also held at 300°C and a purge of nitrogen gas sweeps the vapor through the detector and out through a condenser coil. The electron capture detector... 

Some manufacturers have taken the minimum detectable concentration and multiplied it by the sensor volume and defined the product as the minimum detectable mass. This gives values that are very misleading. For example, a detector having a true sensitivity of 10 g/mL and a sensor volume of 10 /rL would be attributed to a mass sensitivity of 10 g. This is grossly incorrect, as it is the peak volume that controls the mass sensitivity, not the sensor volume. Conversely, if the peak volume does approach that of the sensor, then a very serious peak distortion occurs with loss of resolution thus, this way of specifying sensitivity remains meaningless. 

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