Question

Describe how the retention time of a peak is calculated if dichloromethane is injected with the sample

Answer 1

I need some more info about the whole experiment so I can get a better idea about it. However, I'll tell you how to do it, with the equations I have and the theory of retention time:

First to all, retention time isvthe time taken for a particular compound to travel through a column to a detector. This time is measured from the time at which the sample is injected to the point at which the display shows a maximum peak height for that compound. Different compounds have different retention times. For a particular compound, the retention time will vary depending on:

• the boiling point of the compound. A compound which boils at a temperature higher than the column temperature is going to spend nearly all of its time condensed as a liquid at the beginning of the column. So high boiling point means a long retention time.

• the solubility in the liquid phase. The more soluble a compound is in the liquid phase, the less time it will spend being carried along by the gas. High solubility in the liquid phase means a high retention time.

• the temperature of the column. A higher temperature will tend to excite molecules into the gas phase - either because they evaporate more readily, or because they are so energetic that the attractions of the liquid no longer hold them. A high column temperature shortens retention times for everything in the column.

Now, the retention time (tr) has relation with the number of theoretical plates, n, which is a dimensionless number related to the ratio between the retention time, tr, and the width of the peak containing the compound. If the peaks are reasonably symmetric, it can be assumed that they are Gaussian in shape. In this case, n is found from:

n = 5.45(tr/W_1/2)

The peak width at half height, W_1/2, is found by drawing a line vertically from the peak maximum to the baseline, measuring half-way up the peak, drawing a horizontal line, and measuring the length of the horizontal line. The retention time, t_r, is measured at the point where the vertical line drawn through the maximum intersects the baseline. Both t_r and W_1/2 must be measured in the same units. Since the measurement is usually made from a recorder chart, the units are usually in cm, mm, or in. n varies depending on the compound as well as the column packing material. So a column does not have a single n value. n also varies with the flow rate, and the column length. It is good practice to specify the column conditions and the compound used to determine n.

If you need something else, you can tell me in a comment and I'll gladly answer it for you.