Background

When comparing silica gel there are chromatographic parameters that are important to consider.  These parameters are retention factor (k), selectivity (α), and efficiency (N).  These factors combined are responsible for the overall peak resolution (Rs).  While these can very on a case by case basis with specific compounds, the separation of acetophenone and 4-methoxyacetophenone is well documented.

Definitions

Retention Factor (k) – Related to the interaction of the silica and the compound, a larger k value typically indicates the compound spent longer interacting with the silica.

Selectivity (α) – The ability of the chromatographic system to separate or chemically distinguish different compounds in a sample. The higher the value the more selective the system is. If α is equal to 1, the compounds co-elute.

Efficiency (N) – The theoretical plate number or efficiency is a measure of the peak dispersion on the column, the higher the N, the more efficient the column is.

Resolution (Rs) – The resolution relates all three of the above parameters (retention factor, selectivity, and efficiency) to describe the separation of two identified peaks.

Practical Examples

The following chromatograms were obtained using a 40-gram chromatography column packed with the specific media of choice.  The flow rate was maintained at 30mL/min for the equilibration step and the chromatography separation. The sample tested was 100mg/mL of acetophenone and 100mg/mL of 4’-methoxyacetophenone for a total concentration of 200mg/mL for the test sample.  For the 1% loading testing, 2mL of the test sample or 400mg is loaded onto the column, for 5% loading 10mL or 2g of sample is loaded.

In comparing the Technical, Standard, and Enhanced grades there are distinct changes in selectivity, resolution, and efficiency even if the chromatograms look similar. As expected, the Enhanced exhibits the best selectivity and resolution of all three, however, the column efficiency is lower.  The Enhanced grade has a lower water content that affects packing.  The 40930H or Standard grade exhibits the better efficiency over the Technical grade.  As expected, the lowest performing is the Technical grade due to the wider range of product specifications.

When comparing the different silicas for higher loading, the Standard and Enhanced grades work significantly better than the Technical grade.  The peaks are unable to be separated on the 40930 material, whereas baseline resolution is almost obtained using the 40930H and 40930M materials.  The major difference between the Enhanced and Standard is the overall efficiency is better for the 40930H but a better selectivity is shown with the Enhanced 40930M material.

When improved separations are needed there are other options available. These silicas are manufactured to very strict standards and have the lowest trace metal content of all Sorbtech granular silicas. When comparing the 52500 with the 52300 there is a dramatic increase in selectivity, peak resolution, and overall column efficiency. When both are compared to the 40930 and the 40930M all parameters are improved.

Loading can elucidate the major differences in using the 52500 Premium grade and the 52300 Premium grade.  While selectivity is identical for both silicas, the smaller particle size of the 52300 Premium silica shows far superior resolution and efficiency.  The efficiency of the 52300 is why the peaks are significantly narrower.  The two peaks of the 52500 chromatogram are never fully separated unlike the separation obtained using the 52300 silica. If the two Premium silicas (52500 and 52300) are compared to either the 40930H or the 40930M, all the observed parameters are improved yielding superior separations.

Overall, most separations will work well with the 40930 series. The 40930H is recommended over the 40930 to dramatically improve efficiency and peak resolution, as well as for increased loading onto the silica.  The 40930M is recommended as the optimal choice of the 40930 series when it comes to a lower water content, it has the lowest water content of all three and the highest selectivity and peak resolution.

52500 and 52300 are unsurpassed for separations of an industrial scale and/or a pharmaceutical nature. It is the best quality granular silica due to lower metals content and a much narrower particle distribution; this is evident with the improved selectivity, resolution, and efficiency.  For enhanced loading, 52300 is the preferred silica as baseline separation can still be achieved even at 5% loading.