split/splitless ratio.

[oscarBAL]

Hello all,

I need your help because i need to put clear a disscusion with the people that giveme technical service to my GC.

I have two GC Perkin Elmer Clarus 500, new Brand, both equipments have split splitless injectors, I don't know if some body have one of this. I am ussing capillary column in my GC's; they have an electronic gas controller for all the pneumatic system.
When I want to work splitless using my column the equipment never comes to stabilize the gas flux on the siplit and column, so they turn off, the pneumatic system (I love the equipment that work just like they want!! )
well the people from technical service say that that is normal, that I can not work splitless with a capillary column.
Is this right????
because only when I programate high presion (over 15 psi) and work with split ratio over 15:1 is when I achieve to regulate my GC
They Say that I just can work Splitless with widebore column i.d>0.5mm
is that true?
I realy need to know if they are right, because I am starting to doubt!!

I am working with column i.d 0.25 mm and 30 mts.

also Iwant to know: when Iam analizing residues, wich is the biggest volume that I can Inject, and do not saturate my column?

Thanks in Advance.

Oscar

[Rolandas Plausinaitis]

Hi,

You didn't mention temperatures of injector and column, which could also help to give some more specific advice.

Generaly you SHOULD be able to work in splitless mode.

The catch is that electronic pressure control systems have "operating range" - limits of flow and pressure they can provide (some GC manuals give details on that). If you are using low pressure, instrument will have problem to maintain very low or very high flows. Splitless mode will require low flow so you should give enough pressure (maybe more than separation would require). Depending on what temperatures/gas you are using 15 psi could be quite realistic value - not "high" at all.

Other solution would be "pulsed injection" which is available on most new instruments (have no idea if yours also). In this mode high pressure will be maintained only for (splitless) injection time and then lower pressure value for the rest of analysis in split mode with higher total flow.

Good luck!

[oscarBAL]

Thanks Rolandas,
You have reason, 15 psi is not a hight pressure, but even a 18 or 20psi I have the same problem, I think that at 18 psi I might work with split ratio less than 10:1, but I can't.

My temperature on the injector is 250C, in the column is 150C, and my GC don't have pulsed injection system.
I want to know if this is normal as the Perkin Elmer techniccian say?


Thanks in advance.

[Rolandas Plausinaitis]

Hi,

I'm also geting feeling that something is wrong either with method setings or the instrument.

From what I know 0 split is used only with the columns over 0.53 mm, your support people are correct about that. But splitless injection is normal common injection technique when split is set to 0 only for short period of time (0.5- 2 min). Sinice injector is called split/splitless it certainly should handle that with normal capillary column.

You should try to check if all the "funny" proprietary modes (prevent and similar) are switched off.

Other thing which should be checked if septum purge is set correctly and working. If septum purge is too low PPS will be not stable at low flow setings. Also most electronic pressure systems in fact do not close split but re-route that flow trough purge injector exit.

That all suggestions what I have now.

Unfortunately I only worked with Agilent and Shimadzu, so don't have experiences from PE instruments.

Good luck!

[Ron]

It is hard for me to believe that PE can't operate in splitless mode with narrow bore columns. I know that Shimadzu, Agilent, Thermo, and Varian GCs are routinely operated in splitless mode with narrow bore columns. There may be a parameter set wrong, or there may be some kind of problem with the instrument.

Older GCs did reroute flow during splitless operation, so the total flow had to be set fairly high for a proper splitless injection. Newer GCs such as the Shimadzu GC-2010 and Agilent 6890 close the split vent and operate in a true splitless forward pressure regulated mode. I am not familiar with the way the PE injection port works, but if it can't do splitless injection with a 0.25 id column you probably have the wrong equipment.

[2152812]

Oscar:
I have same problem with Clarus500. The column and other settings similar to yours were tried. I can say for sure that it is such piece of shit, both hardware and software. PE folks tried to instal a piece of smaller ID capillary either before or after the column in order to increase back pressure (so that the PPC con function properly)... all did not help at all! So I sort of gave up trying lower split ratios.

I would highly recomend people not to consider buying GC from PE

[Okkie]

When I read everything OK, you use a 30m column, id 0.25mm at 150°C

If you have 15kPa, youre flow is 0.1 with Helium, or 0.18 ml/min, assuming you don't use a MS.

I can imagion that the machine have a lot of problems with that.
maybe if you go to flow 0.5 = 70 kPa He of 40 kPa H2, the machine is in working range.

Also youre flow is lower than the optimum flow according to van Deemter. It is better to have a (too) higher flow than a (too) low flow. In both cases you are not working optimal but at high flow you work faster and you don't loss much resolution.

Okkie

[oscarBAL]

Thanks to all, I apreciate too much your help, finally, yes I have to increase the flow.
2152812, I know very good people in PE, but you have reason!!, I had posted here so many problem with my GC's even now still I have problems, I never has received in this forum an answer from they, I read post from people that work for others companies, I believe that they should read this forum and see what is happening, after all not only the pharmaceutical laboraoties use PE.
we also, and some times is hard.
is just my opiniom

Thanks again!