Head Space Sample loop@ Calibration

[vdnsrinivas]

Hello,

I have a question regarding the Head space sample loop (1mL) is injected into GC. How can we know the efficiency and accuracy (calibration)of the sample loop @ analysis of every injection.

Thanks,
Sri

[chromatographer1]

You must inject the loop at the same pressure and temperature each time to get the same volume of gas each time.

(relative reproducibility)

Different loops may not be exactly the same volume. This difference must be determined experimentally.

(absolute reproducibility)

Rod

[vdnsrinivas]

Hi Rod,

Good morning!

Relative @How do we know each time 1ml of sample volume was injected into the system?

Thanks,
Sri

[Peter Apps]

Hello Sri

The answer to your question can be found by posing it in a different form - if a 1 ml loop is connected at both ends into a gas stream is there any way that a volume other than 1 ml can be transferred to the gas stream ? (temperature and pressure assumed constant, and no leaks of course).

Peter

[chromatographer1]

First you have to have a clear flow path. No blockages, partial or full and you have to have a flow rate through the loop that is permitted to flow for enough time to fully purge the loop.

If the flow rate through the loop was 5mL/min then a minimum of time the 1mL loop should have flow through it is a minimum of 12 seconds, 1/5 of a minute. But usually operators with any sense will leave the valve in inject position for most of the run cycle so the loop can be purged as much as it can be, be it 5 or 10 or 60 minutes.

The loop is only returned to fill position when the next sample is ready to be loaded.

If you have a loop of 1mL and your flow is 5mL per min, then having the loop in inject position for 6 seconds means you will only inject at best 0.5mL of sample. One half of the loop will still contain sample.

Your question is a valid one.

I hope this is satisfactory.

best wishes,

Rod

[Peter Apps]

Hi Rod,

Good morning!

Relative @How do we know each time 1ml of sample volume was injected into the system?

Thanks,
Sri

I suppose that you could use an internal standard and measure its peak area under conditions for which the loop volume was completely transferred to the gas stream. If the peak area was less than that value in an analytical run then an incomplete transfer of the loop volume is one of the possible causes.

An alternative might be to load the loop with a gas different from the carrier, and then detect and quantify it. I doubt that this would be practical unless the carrier flows were much higher and the loop volume much smaller than is usual - the signal would be very likely to go above the linear range of the detector. A way around that problem would be to load the loop with a dilute gas phase sample - but then we are back to the first suggestion in a slightly different form.

Why is it important that you inject exactly 1 ml ?

Peter

[Johnny Rod]

I think the question is more about precision than accuracy? The injection needs to be about 1ml to get the chromatography right, but that aside the precisioon is more important as it's a calibrate-at-time-of-use instrument. As Rod says, if the temp and pressure are repeatable, so will be the injection. It's a good suggestion about using an internal standard, this improves accuracy but also shows you the injector precision.

[vdnsrinivas]

Thanks all for making the concept clear!