I would like to know how to set the splitless valve to get optimum sensitivity?
When GC goes to PreRun state for Splitless injection vent valve will be closed and flows to inlet go partly to septum purge and rest to column. Septum purge is usually around 3 ml/min and flow to column is one that has been set to be column flow. You then select purge flow and time when you want to purge excess material out from the liner. You want to start purging after liner has been flushed at least once by the flow. If you're using single tapered splitless liner with volume of 990 µl and 1 ml flow it takes almost minute to flow reach from top to bottom of the liner and after that you may start purging. Common purge start time is 2 minutes and purge flow is around 50 ml/min. Purge for few minutes and then return to gas saver mode.
There is also possibility of using Pulsed Splitless injection mode where you can elevate flow through liner during injection so that sample is transferred to column more efficiently. Select this mode and test it with two/three times normal flow to see if it helps. Other injection techniques to get more intensity are cold splitless and solvent vent. However these two need Multi Mode Inlet (MMI) as we need to be able to change inlet temperature during injection cycle.
Many years ago Chromacademy had a free split splitless demo which allowed you to enter all the needed data you are suggesting and it automatically did the calculations then displayed condition which would be optimal for the column used, carrier gas, Inlet type and size valve on /off times sweep and purge flows. Used it then and it worked great. Unfortunately my computer after 15 yrs blew up and I lost all me methods and setting. Have you ever tried the Demo? Do you know were or how to get it? I asked Chromacademy and I was told I would have to join as a prime member for a annual fee.
I haven't used this myself, but there's a free tool available on agilent.com that may do the calculations you need. Here's a screenshot of interface:
Does this look like it will help you? You can download it for free at this link: Agilent | GC Calculators
The software package you download there contains three programs:
Please let me know if this is useful, or if I'm totally wrong.
Thanks. - Josh
I haven't used that software but sounds interesting. Though it doesn't take too much time to calculate those values and we already know what linear velocity - which is shown in software as well - we want to use with capillary columns by interrogating van Deemter plot.
Josh already mentioned one of my all time favorite calculator, pressure/flow calculator. Other ones are really nice as well. You can calculate pressures for pulsed split/splitless with Vapor Volume calculator. Solvent vent calculator gives you parameters for Large Volume/Solvent Vent injections.
You could also try using the Pulsed Splitless to decrease the purge time and get the sample onto the column faster.
Pulse pressure time can 50-100% higher than the normal starting pressure for 0.25 or 0.5 minutes and then the normal column pressure/flow will resume.
If the Purge Time is set longer than the Pulse time, then the purge will happen after the Pulse time has elapsed. You could make the times the same however.
Also, on the 7890A and 7890B GC you can change the Septum Purge Mode to "Switched", and for the duration of the injection before the Purge time, the Septum Purge will be switched off so ALL the flow can go to the column. This is used when we perform the MSD Checkout at installs. It uses a combination of Switched Septum Purge and Pulsed Splitless to make sure no sample is lost through the septum purge vent, and help get the sample onto the column faster in a tighter band at the head of the column.
All good info given, one thing that is valuable and can improve reproducibilty is to switch the purge valve slightly before the end of the pressure pulse. For example if the pulse is 0.5 minutes long , switch the purge valve at 0.48. There is a problem with some versions of software that will not allow this setting. There is a work around that can be applied if you are using earlier versions of software . I have attached a work around document.
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