Topics - Flow-Cell
Flow cell is the term usually given to the sensor volume of a chromatography detector (it may also be given to the detector sensor volume used for other techniques but its use is mostly restricted to chromatography). In LC, the cell volume must be small (preferably less than 8 l) so that it can only contain a fraction of the volume of the smallest eluted peak (ca 0.3 x peak volume). To reduce peak dispersion (peak spreading), the cell length must be much greater than the cell diameter. Typically the cell would be about 10 mm long and about 0.8 mm ID giving a cell volume of about 5l. In cells used for light absorption, the connections to the cell must be at either end and constructed so as to direct the stream of mobile phase from the column onto the face of the window. This arrangement requires the flow to reverse and produce strong mixing and, thus, minimize peak dispersion. The arrangement of the exit tube should be the same so that the stream of mobile phase axial to the cell must turn and exit through the tube oriented in the opposite direction, again facilitating mixing and reduction of peak dispersion. For GC detectors (e.g. the thermal conductivity detector), and for a packed column, the sensor cell can be much larger than in LC as the mobile phase flow rate is also much larger and the peak volume also much greater. Volumes of 0.5 to 1 ml can often be tolerated, but this is not so for GC capillary columns. In most capillary GC instruments the FID is used as the detector which is not concentration sensitive but mass sensitive and, thus, the volume of the sensor is not important. If, however, a concentration sensitive detector is used with a capillary column then, as the peak volumes can be only a fraction of a micro-liter, the restrictions on flow cell volume are extremely severe. Detectors that require flow cells should be avoided, if possible, when using capillary columns.