Splitless mode analysis

[Dinesh Hegde]

I am working on an Agilent GC system in a splitless mode for headspace analysis and i am not getting any peaks. Please help me out regarding parameters to be given for this mode like inlet purge time, inlet purge mode etc.

[zokitano]

Hi,

What analyses are you running? What are the HS conditions?
If you are getting no peaks, that maybe is a result of inadequate pressures of the carrier gas in HS and your GC. Make sure that the HS carrier gas pressure is greater than GC carrier gas pressure

Best regards

[Peter Apps]

Before we can help you out, you have to help us by giving the operating conditions that you are running at the moment, and what you are trying to analyse in what matrix.

Peter

[Dinesh Hegde]

The analysis is of residual solvents(Acetone, Methanol, Hexane, Ethyl Acetate, Diisopropyl ether, Acetonitrile) in an API matrix. The solvent in N-methyl Pyrrolidone. The GC / HS gases are adequate for analysis. The column is DB-624 (30 m x 0.53 mm x 3µm film). Inlet temp-220◦C, Det. temp-250◦C, oven temp 50◦C for 6 min nd then a rise at a rate of 15◦C per min to 200◦C for 3 mins.

[Peter Apps]

Hi Dinesh

You say that you are doing headspace analysis, but you do not give any conditions for taking the headspace sample, so how are we supposed to help ??

Peter

[chromatographer1]

No peaks, well that means no sample is reaching the column inlet.

Your vials may not be sealed properly. No pressure in the vial means it will not fill the loop thus no sample will ever get to the column.

Your flow from the sample loop may be inadequate to bring the sample to the column inlet before the system is flushed clean.

If you are sampling for 30 sec and the flow rate is 2 cc/min and you have 2cc dead volume between the loop and the column you will never get any sample onto the column.

I understand that you may not know how to do what you are doing. Please convey the pressures, the flows, and the sampling parameters of your headspace system in order that others on the forum may assist you.

best wishes,

Rod

[AICMM]

Dinesh,

Divide and conquer. First thing to verify is if you are getting peaks without the headspace sampler. Assuming your headspace sampler is connected to the inlet, do a manual injection of some solvent or the standard mix directly into the GC injection port and verify you are getting peaks in the GC system. If you get peaks and they are reasonably resolved and decent shape, try to divide the headspace sampler in half and see if you can isolate the problem within that system.

Best regards.

[GaryR]

Dinesh

We have seen similar problem. We have PE HS samplers hooked up to Agilent 6890 GCs.
Occasionally the GC inlet does not recognise that HS is connected and carrier flow is not switched off at inlet.
Try re-initialising the inlet on the GC and make sure your carrier is coming from HS sampler and not GC.

Regards
GaryR

[GaryR]

Dinesh

Just out of interest, why are you doing splitless injections with headspace sampling? What levels of solvents are you looking for (low %, or ppm)?

Improvements can be made in sensitivity without having to resort to splitless injections.

Regards
Gary

[Chromatocrat]

Let me give some background about Splitless injection.

<B>Note that there are severe limitations for using Splitless mode with the headspace sampler. </B>

With a Splitless mode injection on the headspace there is a significant volume of tubing for the sample to go through before it reaches the inlet. With the low flows that are typical for a Splitless injection it takes a very long time for the sample to get on the column. Because of this you will encounter two problems:

1. Severe band broadening of the sample on the column. (Broad peaks)
2. Loss of active compounds due to the length of time the sample is in contact with heated pneumatic parts.

Another consideration is the Split/Splitless inlet may be unstable due the low flows and greatly increased volume added to the system by the headspace pneumatics.

Splitless mode can be used when the column flow is 20 to 30 ml/min with a 530um column. Also the 6890 GC needs to have Auto Prep Run enabled in order to have the sequence run correctly. To do that you must enable it from the front panel of the 6890 GC. To enable the function:

1. On the front panel of the 6890 GC press the Config key.
2. Scroll the display down to Instrument and press the Enter key.
3. At Auto prep run on the display press the On key to enable the feature.

If you need more information, kindly let me know
1. what model of Agilent GC & headspace sampler you have?
2. How Headspace is connected to GC? Through septum OR directly in line with carrier gas?


regards