Split

Chromatography Forum: GC Archives: Split
Top of pagePrevious messageNext messageBottom of pageLink to this message  By MJ on Wednesday, July 28, 2004 - 05:47 am:

Hi
My velocity at the detector end (FID-HP-5890 seriesII) is around 10ml/min. I cannot figure out how to reduce it to 1ml/min. I am trying to get a split of 1:35.

I want to know which one of the following should I try tuning or rather how to introduce the split precisely-
(a)Total Flow controller
(b)Column Head Pressure Control
(C) Septum purge

I have 36ml/min at the split vent and around 4 ml/min at the purge vent.
I am using a Bubble flow meter to measure.

Bye


Top of pagePrevious messageNext messageBottom of pageLink to this message  By GC user on Wednesday, July 28, 2004 - 09:30 am:

The recommended flow rate depends on the column dimensions. A flow rate of 1 mL/min in a 0.53 mm column is an inappropriate as a flow rate of 10 mL/min in a 0.25 mm column. Determine the appropriate flow rate for the column, set the head pressure to achieve that flow rate, then adjust the total flow controller to reach the desired split ratio. The septum purge is higher than what I would use, although 4 mL/min is usable.

Some column flow and split combinations are not possible to achieve, for example 4 mL/min with a 30 m x 0.53 mm column require such a low head pressure that you will not be able to run a 100:1 split and control the head pressure at the desired value.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By MJ on Wednesday, July 28, 2004 - 09:49 am:

The column is 0.53mm*30 m. Do I first measure the detector flow rate and then establish the split by adjusting the total flow or is it the other way?


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Consumer Products Guy on Wednesday, July 28, 2004 - 10:22 am:

Using the HP utility flow calculator software for your column dimensions, a column flow of 1.00 ml/min will require a 1.3 psi head pressure (I'm assuming about 150C temperature); however, this gives a linear velocity of only 10.3 cm/sec, which is quite low (I would normally keep linear velocity between 30 and 75 cm/sec).

Here's what I would do, similar to poster above:
(1) use HP flow calculator (http://www.chem.agilent.com/cag/servsup/usersoft/main.html#flowcalc205) to determine appropriate flow rate and get the head pressure from that
(2) set the head pressure knob to that pressure to achieve that flow rate
(3) adjust the total flow controller knob to reach the desired split flow (split flow divided by column flow or total flow will give the approximate split ratio.
(4) set purge vent rate to maybe 2 ml/min.
Pressures this low are not the easiest things to set, FYI.


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