appear as a single peak, albeit very distorted in shape. This extreme condition rarely occurs, but serious peak distortion and loss of resolution can quite often happen. This will be evident when the sensor volume is of the same order of magnitude as the peak volume. The problem can be particularly severe when columns of small diameter are being used. The situation is depicted in figure 7.   Figure 7 Effect of Sensor Volume on Detector Output Consider the elution profile of a peak eluted from a column 3 cm long, 3 mm I.D. packed with particles 3 m in diameter as shown in figure 7. If the peak is eluted at a (k') of 2, from figure 7 it is seen that the peak width at the base is about 14 ml wide. The sensor cell volume is 2.5 ml and the portion of the peak in the cell is depicted in the figure. The detector will obviously respond to the mean concentration of the slice contained in the 2.5 ml sensor volume. It is also clear that, if the sensor volume is increased, a

nbsp; The different effects of mass overload are now clearly revealed. Firstly, it must be emphasized that the curves in figure 8 represent the movement of the peak extremes and are in no way related to the peak shape. It is seen that the retention of the rear of the major peak, benzene, hardly changes with sample mass as this represents low concentrations of benzene and thus is eluted in the normal manner. However, the retention of the peak front is reduced progressively as the sample mass is increased. This results from both the formation of a non-linear adsorption isotherm and the increased elution strength of the mobile phase in contact with the benzene. In fact, as a result of solute–solute

tube, a different equation is necessary to describe the dispersion effect. Secondly, there will be a peak spreading which results from the finite volume of the sensor. If the sensor has a significant volume, the concentration measured will not be that entering the detector cell but the average concentration throughout the cell. Thus, the true profile of the peak can not be monitored. If the  sensor volume is significantly smaller than the peak volume the effect will merely give the peak an apparent dispersion. However, if the sensor volume becomes of the same order of magnitude as the peak volume, then the peak profile will be distorted and resolution will be lost.  In the extreme case two peaks could coexist in the sensor at one time and only a single peak will be represented.  The effect of viscous flow on dispersion will first be considered. Dispersion in Detector Sensors Resulting from Newtonian Flow Most sensor volumes are cylindrical in shape,

separation was carried out on a column 3 cm long, 4.6 mm in diameter and packed with a C18 reversed phase on particles 3 m in diameter. Courtesy of the Perkin Elmer Corporation Figure 32. The Separation of Some Aromatic Hydrocarbons The separation appears to be satisfactory and all the peaks appear to represent individual solutes; without further evidence, it would be reasonable to assume that all the peaks were pure. However, by plotting the adsorption ratio, , for the anthracene peak it becomes apparent that the peak tail contains an impurity as the clean rectangular shape of the peak top is not shown. The absorption ratio peaks are shown in figure 33

chromatogram was 24 cm long, 4.6 mm I.D. The mobile phase was tetrahydrofuran and the column was operated at a flow rate of 1 ml/min. The solute injected was benzene. The column used to produced the elution curves in the lower chromatogram was 1 m long, 1 mm I.D. and the same solvent was used at a flow rate of 40 ml/min Benzene was also used a the solute. It is seen that the reduction in cell volume has a dramatic effect on both peak width and peak shape. The large 25 ml cell causes significant peak asymmetry as well as excessive peak dispersion A result which is predicted by the work of Atwood and Golay (11) which is discussed below. It is seen that the large sensor cell has a disastrous effect on the band width of the solute eluted from the microbore column. Clearly, even cell volumes of 3 ml are too large for use with 1 mm I.D. columns and relatively few contemporary detectors have cell volumes less than 3 ml.    J. Chromatogr. 169(1979)51 Figure 17. Peak Profiles from

gross in error. Another serious source of error can arise when two peaks are unresolved, and the retention time of the maximum of the envelope is taken as the mean retention time of the two individual solutes. This measurement can only be accurate if the peaks are absolutely symmetrical and the two peaks are of equal height. The result of different proportions of each isomer on the retention time of the composite envelope is shown in Figure 11. It is quite obvious that the position of the peak maximum of the composite envelope is very different from the mean retention time of the individual peaks. Figure 11. A Composite Peak Formed by Two Closely Eluting Peaks of Different Size   Additionally, in the example given, the peaks were taken as truly Gaussian in shape. The peak maximum of the envelope are distorted to an even greater extent if the peaks are asymmetric. The retention time of a composite peak must never be assumed to have a specific relationship with those