Chromatography Forum

Hi everyone.
I have been doing some method development with splitless injection.
My solvent is hexane and the next compound is bp 170degC.
I use 30m RTX5 x 0.25mm x 0.25um 50degC for 1 min then to 280degC at 4degC/min. Flow 1ml/min. Split open at different experimental times:

I have noticed poor early peaks shapes at split open 60 to 90secs but better shapes at 30secs.
In your experience is 30-45secs too early for this split to open??
Will other higher boilers disappear with the lower split shut time??

Thanks
WK

A general rule of thumb for the minimum splitless time is to calculate the time required for the liner to be swept 3 times, i.e. liner volume divided by flow rate x 3. But if the solvent peak is not encrouching on the early eluters, you may as well just use a longer time, like 2 minutes.

What size liner and what injection volume are you using? If the expansion volume is much greater than the liner volume, a good portion of what you inject will backflush into lines and take a while to work there way onto the column. This could give bad peak shapes for the early eluters.

Second possibility: can you start your program at 40C instead of 50C. A difference of 10C will generally make a big difference in how well the analytes focus.

You different splitless time change the sample transfer to the column

A splitless injection with a splitless period of only 30s duration,with a commonly used 1mL/min gas flow rate,is effectively a split injection,that's why the peak shape of your early eluting peaks are better.

Reducing the temp from 50 to 40°C will increase solvent recondensation,and accelerates sample transfer to the column ,providing a satisfactory tranfer even with 40s splitless period and 1.5-2 mL flow.

Thanks for the ideas.
I have tried 40degC and peak shapes are still poor.
I have 0.1microlitre injection with PE PSS liner which is 70mm (50mm effective) and 2mm id. 250degC injector He approx 10psi.
The approx. liner volume is between 160-220uL.
Dean Rood suggests 1-1.5 liner volumes of carrier gas until purge on.
I left the purge off for 30secs and got superb peak shape throughout the run and sensitivity. I would think in this case 30secs does not consitute a split injection.
I should not have backflash under these conditions (Restek online calculator).
Any thoughts?
WK

Solvent focusing requires oven starting temp should be about 20 degrees below the boiling point of the solvent, in this case, hexane has a B. P. of 69 degrees, 50 degrees of start temp for the oven program is just right.

0.1 UL injection of hexane equals about 20 UL sample in the liner at 250 degrees and 10 psig. Based on the liner volume of 220 UL and flow rate of 1 ml/min, hold time of 30 secs is more than enough and no backflush.

If the injection volume is large, pressure pulsed injection is one way to shorten the hold time.

Just some thoughts.

I am a little surprised that you are using splitless injection with a 0.1uL volume. It seems to me that your concentration is high enough that you could be doing split injections. Is the poor peak shape a result of overloading the column?

I tend to take a pragmatic approach to things like splitless times, and if 30 seconds give you the sensitivity and reproducibility you need then it is a good value for your analysis. As a rule of thumb I try to keep the splitless time as short as possible to minimize the amount of solvent on column. 30 to 45 seconds is usually good enough, especially considering the volume of your liner.

I agree too with a 2mm id liner 30s should be ok.
But in your first post you didn't mention the type of liner,therefore i tried to respond for a general or most common 4mm id liner

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