Kinesis Solutions Techtips
The need to measure and optimise flow rate
Welcome to this month’s techtip from Kinesis Solutions "The need to measure and optimise flow rate". If you have any questions regarding this techtip or any others please contact us through our website www.kinesis-solutions.co.uk
The relationship between flow rate and resolution is often misunderstood. This can be very bad news for users as carrier gas flow rate through new 0.25mm ID columns can vary by up to 16%, due to small variations in column ID (0.245mm – 0.255mm).
To really understand the impact this can have, let’s briefly review why flow rate is important.
The molecules of your sample will enter and exit the stationary phase (partition) and will spend a different amount of time in the stationary phase compared to the mobile phase (carrier gas). The different sample molecules will spend the same amount of time in the mobile phase, but different amounts of time in the stationary phase. This is what produces the separation.
If the molecules spend too much time in the column, they will diffuse (peak broadening) and if they diffuse into different sample molecules there will be a loss of resolution. If the molecules don’t spend enough time inside the column, they won’t interact with the stationary phase long enough to achieve a separation.
These factors are described in the Van Deemter curve (see below), where the lower the HETP (height equivalent of a theoretical plate) the better the column efficiency. Optimising carrier gas velocity will maximise efficiency of the column and, hence, maximise resolution.
The optimum value will depend on the nature of the carrier gas, column diameter, column length, film thickness and other variables, and is easily measured (carrier gas velocity (cm/sec) = length of column (cm) divided by the hold-up time (seconds) of a non-retained solute).
Now, if you have an old 30m column that you have had to trim a dozen times, you may have reduced its length to just 25m. If you then install a new 30m column without adjusting the head pressure, you could have a reduction in carrier gas velocity, due to the possible variation in internal diameter and the extra length. This could be sufficient to cause the resolution problems captured below.
For this reason, it is very important to check & adjust the carrier gas velocity, when installing a new column, or trimming the inlet end of an existing column.
