The Mass Sensitivity of a Chromatographic System

The mass sensitivity of a chromatographic system, which includes column, sample valve, connecting tubes and detector, is defined as that mass of solute m that will provide a peak hei t that is twice the noise level. Consider a chromatographic peak, with a height equivalent to twice the noise level, being sensed by a detector with a maximum sensitivity of Xp (g/ml). The peak volume can be taken as 4oc(o c the volume standard deviation of 

the mass sensitivity of the chromatographic system depends exclusively on the detector sensitivity, column 

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The mass sensitivity of a chromatographic system is that mass of solute (m) that will provide a peak with a height equivalent to twice the noise level. 

The concentration sensitivity of a chromatographic system (XJ is defined as that which will provide a peak with a height equivalent to twice the noise level and can be obtained directly from the system mass sensitivity. If the minimum detectable mass is dissolved in the maximum permissible sample volume [9] (that sample volume that will limit the increase in sample variance to 10% of the column variance), then this solute concentration will constitute the minimum detectable sample concentration. 

It is seen from equation (20) that the minimum detectable mass, or mass sensitivity of a chromatographic system, where the column has been designed to have the optimum radius for the detector employed, is directly proportional to the extra column dispersion and the detector concentration sensitivity. It follows that detector dispersion is as important as detector sensitivity in its influence on the overall chromatographic mass sensitivity where the chromatographic system has been optimized with respect to the radius of the column. The effect of extra column dispersion and in particular, detector dispersion on the overall mass sensitivity of the chromatogaphic system is not generally appreciated or completely understood. As the total extra column dispersion is the integral of a variety of sources, the distribution and nature of the various sources of dispersion will now be considered in some detail. 

The maximum and minimum flow rate available from the solvent pump may also, under certain circumstances, determine the minimum or maximum column diameter that can be employed. As a consequence, limits will be placed on the mass sensitivity of the chromatographic system as well as the solvent consumption. Almost all commercially available LC solvent pumps, however, have a flow rate range that will include all optimum flow rates that are likely to be required in analytical chromatography... 

The dispersion effect of the sample volume was discussed in Chapter 4 and little more needs to be said about it. It will be seen later that the sample volume controls both the concentration and the mass sensitivity of the chromatographic system and thus, should be made as large as possible. This means that all other sources of band dispersion must be kept to an absolute minimum to permit the maximum possible sample volume to be used. A better understanding of the causes of band dispersion has resulted in... 

Thus (m ), the mass sensitivity of the chromatographic system depends on the detector sensitivity, column dimensions, column efficiency and the capacity factor of the eluted solute. However, irrespective of the column properties, the mass sensitivity is still directly related to the detector sensitivity. It will also be seen that the column radius will depend on the extracolumn dispersion, much of which arises from the detector connecting tubes and sensor. It follows that the design of the detector and its sensitivity has a major influence on the mass sensitivity of the overall chromatographic system. 

The chromatographic system as a whole including the detector also has a defined sensitivity. The mass sensitivity of the chromatographic system depends not only on the detector sensitivity, but also on the column dimensions. The concentration sensitivity of the chromatographic system, on the other hand, depends solely on the detector sensitivity providing the sample is placed on the chromatographic system in the maximum permissible sample volume. The detector sensitivity also controls the maximum capacity factor at which a solute can be eluted. 

The analytical specifications must prescribe the ultimate performance of the total chromatographic system, in appropriate numerical values, to demonstrate the performance that has been achieved. The separation of the critical pair would require a minimum column efficiency and the number of theoretical plated produced by the column should be reported. The second most important requisite is that the analysis should be achieved in the minimum time and thus the analysis time should also be given. The analyst will also want to know the maximum volume of charge that can be placed on the column, the solvent consumption per analysis, the mass sensitivity and finally the total peak capacity of the chromatogram. The analytical specifications can be summarized as follows. 

The most important detector specification is probably detector sensitivity as it not only defines the minimum concentration of solute that can be detected but also allows the overall mass sensitivity of the chromatographic system to be calculated. The detector sensitivity also places a limit on the maximum (k ) (capacity factor) at which a solute can be eluted from a chromatographic column. In order to calculate the mass sensitivity or the maximum (k ) value, the detector sensitivity must be available in concentration units, e.g. g/ml. Moreover, if all detector sensitivities were given in units of g/ml, then all detecting devices, functioning on quite different principles, could then be rationally compared. 

The sensitivity of a detector is not the minimum mass that can be detected. This would be the system mass sensitivity, which would also depend on the characteristics of the apparatus as well as the detector and, in particular, the type of column employed. During the development of a separation the peaks become broader as the retention increases. Consequently, a given mass may be detected if eluted as a narrow peak early in the chromatogram, but if eluted later, its peak height may be reduced to such an extent that it is impossible to discern it from the noise. Thus detector sensitivity quoted as the minimum mass detectable must be carefully examined and related to the chromatographic system and particularly the column with which it was used. If the data to do this are not available, then the sensitivity must be calculated from the detector response and the noise level in the manner described above. 

The importance of the extra column dispersion now becomes apparent, as equation (26) shows that the minimum detectable mass Increases linearly with the extra column dispersion. It Is also becomes obvious that it is of little use designing a detector for increased sensitivity (Xp) if this is achieved (as is often the case) at the expense of increased extra column dispersion (oe). Conversely, if the chromatographic system is designed to have very low extra column dispersion, a proportional reduction in the minimum detectable mass will be achieved even if the actual detector concentration sensitivity remains the same. It follows, that in the design of an optimized column for a particular analysis, the extra column dispersion will determine both the radius of the column and the mass sensitivity that will be available. 

The sensitivity or MDC of a detector is not the same as the minimum mass that can be detected. This would be the system mass sensitivity, which will depend on the characteristics of the column and the chromatographic properties of the solute, as well as the detector specifications. In all chromatographic systems, the peak becomes broader as the retention increases. Con-... 

The detector is another of the critical components of a high pressure liquid chromatograph, and in fact, the practical application of liquid chromatography had to await a good detector system. Many types of detectors are now on the market. The four most common, the ultraviolet absorption (uv), fluorescence, refractive index (RI), and electrochemical (EC) detectors, will be discussed as well as the newer light scattering mass sensitive detector. 

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