Question

1

A) When making the decision to use column chromatography as a separation method you must consider the sample size as well as well as its chromatographic behavior (ability to be separated using an appropriate stationary and mobile phase). How does increasing the diameter of your column affect the separation of compounds? How does increasing the column length affect the separation of the compounds

Answer 1

Column Chromatography is another common and useful separation technique in organic chemistry. This separation method involves the same principles as TLC, but can be applied to separate larger quantities than TLC. Column chromatography can be used on both a large and small scale. The applications of this technique are wide reaching and cross many disciplines including biology, biochemistry, microbiology and medicine. Many common pharmacueticals are purified by column chromatography.

1. In column chromatography resolution increases with increasing length, and decreases with increasing diameter.
So the answer to your question is that you will have worse separation if you increase the column diameter (From equation 1 shows Switching your analytical column to a smaller inner diameter can greatly reduce the required flow rate and add up to solvent savings).

Reduced column internal diameters result in smaller column volumes. It is this reduction in column volume that allows for an increase in detection limits.When the column volume is decreased the peak volume is decreased . This means that when the same sample volume is injected on a 4.6 mm i.d. column and a 3.0 mm i.d. column, the peak volume will be smaller on the 3.0 mm i.d. column. The same sample is more concentrated in the smaller peak volume and the result is enhanced sensitivity."

"The smaller the diameter the greater the efficiency and therefore the better the resolution. Reduce the diameter by half and the column efficiency doubles. As the diameter increases, the film thickness can increase to maintain the same phase ratio. The thicker the film, the greater the loading capacity. Overloading a column will always result in loss of resolution. If the column diameter is halved while maintaining the same film thickness, then the loading capacity will also be halved."

2. Efficiency is directly proportional to column length (Equation 2). Increasing column length will increase efficiency. Doubling column length increases resolution but only by a factor of 1.4. Short column lengths (30-50 mm) will give short run times and low backpressures and are ideal for gradient analyses. Longer columns (250-300 mm) will give greater resolution but with longer analysis times and at a greater cost.

According to Van deemters equation

N = L/H    N = Number of theoretical plates

                   L = Length of the column

                  H = Height equivallant of theoritical plate