Preparative chromatography sample overload

Preparative chromatography involves the collection of individual solutes as they are eluted from the column for further use, but does not necessarily entail the separation of large samples. Special columns can be designed and fabricated for preparative use, but for small samples the analytical column can often be overloaded for preparative purposes. Columns can be either volume overloaded or mass overloaded. Volume overload causes the peak to broaden, but the retention time of the front of the peak... 

Detectors. The requirements for detectors for preparative work are different from those for analytical operation, For analytical work a very high sensitivity and a small volume with a very small distribution are essential- In preparative work, the concentrations of samples are important and overloading of the detection signal often occurs and may cause problems. The same principles of detection are used in both preparative and analytical work. The two detectors that have the widest range of applications and are the most often used in preparative chromatography are the UV absorption detector, with adjustable wavelength (suppression of too intense signals), and the refractive index (RI) detector. 

Since the goal of preparative chromatography is the production of large quantities of purified compounds, a large sample volume or a large mass of sample or a combination of both is usually applied to the column. This leads to additional effects not commonly encounter in analytical chromatography, such as shifts in retention times and additional broadening of the peaks. In the discussions in the remainder of the book, we tacitly implied the ab ce of such overload effects. For example, we assumed that the concentration of the analytes remains within the linear portion of the adsorption isotherm, while... 

Preparative Chiral Chromatography The Loading Capacity of a Column The Maximum Sample Volume Sample Volume Overload Sample Mass Overload Preparative Chromatography Apparatus Solvent Reservoirs Pumps... 

While an examination of the chromatogram, shows that the 10-mm diameter column was overloaded at the 50-mg sample the data in Table 10.1 indicate excellent recovery independent of sample or column size. In the preparative chromatography nonlinear effects caused by column overload are often observed, " and this affects the separation resolution as sample... 

Preparative chromatography involves theory of some sort in almost every aspect, whether it is for calculation of the performance of a column to determine if it is sufficiently well packed for prolonged use or for a full optimisation. Very often calculations are made to determine the column and particle dimensions which allow operation of a separation at the maximum operating pressure of the equipment to maximise the production from the column. Overloading usually is more controllable if one knows the saturation capacity of the column, since this can allow at least approximate calculation of the appropriate load to try in a separation. Once the sample... 

So far we have considered preparative-scale chromatography when the column is not overloaded and the desired amount of ,sample... 

In analytical applications of liquid chromatography the most common causes of peak asymmetry are mixed mechanisms of retention, incompatibility of the sample with the chromatographic mobile phase, or development of excessive void volume at the head of the column. In preparative applications of liquid chromatography and related techniques, column overload can also contribute to peak asymmetry. The causes of severe peak asymmetry in analytical applications should be identified and corrected because they are frequently accompanied by concentration-dependent retention, non-linear calibration curves and poor precision. In addition, peak asymmetry can significantly compromise column efficiency leading, in turn, to reduced resolution and lower peak capacity (see sections 2.5 and 2.6). 

---