Flow rate and interfacing considerations

The present pneumatically assisted ESI interface is optimised around flow rates of 50-300 pl7min. The use of analytical columns 3. 6 mm i.d. with flow rates between 0.5 to 2 ml/min and narrow bore columns of 1-2 mm id with flow rates of 0.2-0.5 ml7min is routine in most pharmaceutical laboratories for HPLC analysis [24]. Capillary LC columns, because of their limited commercial availability and special practical considerations are used more where there is limited sample available or when sensitivity issues are present [25]. 

Electrospray ion sources display all the characteristics of being concentration dependent. It is therefore normal to split the eluent flow after the UV detector, such that around 300 pl/min goes to the mass spectrometer and the remainder to waste. Care needs to be taken to ensure that there is minimal band broadening caused by the split. It should be possible to minimise band broadening by careful choice of tubing size and positioning of the split point. 

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