Chromatography Forum

I was asked to increase the sensitity of our instrument so we can simplify our sample preparation.
We are running 2 very different machines with on column injection and a PTV injector. The column is a 5 HT or select mineral oil (which is more or less the same I believe) with 15mx0.32mm ID, 5m guard (0.53 ID), detection is FID. One is run in split (1:5) and the other GC has no split exit.

Nevertheless, I injected 3 microliters instead of 1 microliter and I got more about 4 times more response. I expected 3xArea in the best case, actually I was ready to see about the double Area.

When it happened at the first machine, I thought of a positive effect of my temperature program (I also increased injection temperature to 50°C, we are injecting n-hexane).

But now it happened at the second GC as well so I think there is a mechanism behind it. Did you have similar experiences? How much do you inject with on column?

Are you really doing on-column injections - in other words does the tip of the needle enter the column (or guard column) and deposit the sample there ?

I doubt it because you cannot do split injections on-column and a PTV injector is not an on-column injector (unless you have some fancy liners with precise internal tapers).

As usual I have to ask for details on what you are doing - you say you increased the temperature to 50C but you don't say what you increased it from, are you really doing splitless injections and/or using something like a Uniliner, if so are you using the PTV to programme the inlet temperature. What are the flows and pressures and they programmed ? What are you injecting; composition and concentration ?

This is very likely to be some kind of solvent effect but the more you don't tell me the more I can't explain.

Peter

The thing is, at one we are doing on column with a metal insert, and a special syringe tip, which needs the 0.53mm guard column. At this instrument, the split is connected to the gas supply, so the split outlet is actually also an inlet. the only outlet of the insert is the septum purge (and the column). We had injected at 40°C at constant flow of 80cm/sec (25,9 kPa). Now we inject at 50°C (for faster n-hexane evaporation). The oven and PTV temperature program are the same, heating up to 320°C at a 22°C/min rate.

At the other instrument, we use a normal syringe tip, which fits into a 0.53 mm ID also, but I dont know exactly how deep it injects. Nevertheless, here we use a glass liner (Agilent Crosslab, Liner SPI 0.8mm 530ID) which is thouroughly connected to the column. Here we have a split outlet, the split is 5 throughout the whole measurement. We always injected at 50°C here and we heat with 80°C/min up to 360°C. The PTV heats after the injection at 50°C, with 150°C/min up to 320°C. pressure is about the same, we got 94 cm/sec here, although it is constant flow with 3ml/sec

pff I dont know, there are surely more details to mention but you can see those 2 injections are fairly different.

You changed both injection volume and injection temperature, what happens if you change only one thing at a time - which is good experimental practice ?.

And you still have not told me what you are injecting - what is dissolved in the hexane ???

Peter

Sorry, didnt have time to post during weekend.

I am injecting Alkanes, mixtures of Alkanes and chemically similar things, its not defined, you might know it under H53 or EN ISO 9377 or EN 14039. my standard is a mixture of mineral oils.

"You changed both injection volume and injection temperature, what happens if you change only one thing at a time - which is good experimental practice ?" - I did that at one instrument and got the same result. Can do that at the other instrument, but 40°C injection temperature was wrong even with 1µl injections, from my point of view.

I got a lot of questions concerning that method, if someone is also working on it, I would like to know their injection technique. It seems to me that there is optimization potential.

Sorry, didnt have time to post during weekend.

I am injecting Alkanes, mixtures of Alkanes and chemically similar things, its not defined, you might know it under H53 or EN ISO 9377 or EN 14039. my standard is a mixture of mineral oils.

"You changed both injection volume and injection temperature, what happens if you change only one thing at a time - which is good experimental practice ?" - I did that at one instrument and got the same result. Can do that at the other instrument, but 40°C injection temperature was wrong even with 1µl injections, from my point of view.

I got a lot of questions concerning that method, if someone is also working on it, I would like to know their injection technique. It seems to me that there is optimization potential.

No I don't know it under those names, so you have to tell me.

So you got the same 4 times increase in peak area with only a change in temperature ??!! And a four times increase in area with a three times increase in injection volume ?

Peter

Hexane boiling point is 68.7C so either starting temperature is going to condense the hexane on the head of the column. The higher temperature is probably allowing it to evaporate more quickly so it gives less interference with the whole process so maybe that is giving better areas than expected.

Try injecting at 70C and see if the area increases or decreases when not solvent focusing.