Chromatography Forum

Hi all,

I am having a little trouble with my TD-GC-MS at present. The system I am currently using is an Agilent 7890 B Gas Chromatograph coupled to an Agilent 5977 A Mass Spectrometer. I am currently running a dual inlet/column system with the rear inlet and column connected to a Markes UNITY2 thermodesorption unit for analysis of VOCs trapped on sorbent tubes, while the front inlet and column are reserved for liquid injections. Both of the columns are connected to a deans switch that outputs into a single MSD transfer line.

The issues I am having appear when trying to run an MS tune, where it states that I have an air leak. I have two methods (one for liquid injections on the front column and one for TD injections on the rear column) that switch the column pressures depending on the method used. When the front column is in use the front column pressure is set to 10 PSI and the rear column pressure is set to 4 PSI. The reverse is true when the rear column is in use. I have attached the tune files below:

Front Column Tune



Back Column Tune



I'm not sure why I am failing the air/water check when using the rear column but not the front column.

Any help would be appreciated.

Cheers,

Joe

Having a TD system can be tricky. Did you do a leak check on the TD system ? If there is a leak there, you might see a leak in your GC-MS. I had a bunch of leaks with my TD-GC-MS system and the best way to solve them is to get a leak checker and check every single point on the GC_MS where a leak can happen. If you don't have a leak checker, you can use the old methanol Q-tip trick and monitor for methanol.



Let me know how it goes
Loay

Having a TD system can be tricky. Did you do a leak check on the TD system ? If there is a leak there, you might see a leak in your GC-MS. I had a bunch of leaks with my TD-GC-MS system and the best way to solve them is to get a leak checker and check every single point on the GC_MS where a leak can happen. If you don't have a leak checker, you can use the old methanol Q-tip trick and monitor for methanol.



Let me know how it goes
Loay

Hi Loay,

I have leaked checked every possible point on the both the GC-MS and the TD unit. I have even connected the TD directly to the MS and there is no leak originating from the TD unit.

Cheers,

Joe

Hello Joe

Initially I suspected the thermal desorber, but you have eliminated that. Next likely culprit from the point of view of lots of connections that might be leaking or gas flows that might not be quite right is the Deans switch. The easiest troubleshooting is probably to change flows and pressures at the switch to see if that makes a difference - or connect each column in turn direct to the MS.

Peter

Hello Joe

Initially I suspected the thermal desorber, but you have eliminated that. Next likely culprit from the point of view of lots of connections that might be leaking or gas flows that might not be quite right is the Deans switch. The easiest troubleshooting is probably to change flows and pressures at the switch to see if that makes a difference - or connect each column in turn direct to the MS.

Peter

Hi Peter,

I fear you may be correct with regards to the deans switch as I have connected both columns directly to the MS one at a time this afternoon, which seemed to eliminate my air leak.

Looking at the price of a new deans switch, I'm not convinced my lab will invest. This is a bit of a long shot, but is there a way of configuring the GC-MS in order to bypass the deans switch?

Cheers,

Joe

Hi Joe

There has been quite a bit of discussion about connecting two columns to an MS and if it is working right some kind of gas switching arrangement based on a Deans switch is the easiest to set up and operate.

How often do you switch columns and/or inlets ? - sometimes the best solution is the simplest; have both columns connected to the inlets, with gas flowing all the time, but connect only one of them to the MS. I do not know how it goes on your Agilent but on a Varian/Bruker 320 I leave the nut on the column when I remove the column from the transfer line, and then just thread the column back in and tighten the nut a touch when I reconnect.

Peter

Hi Joe

There has been quite a bit of discussion about connecting two columns to an MS and if it is working right some kind of gas switching arrangement based on a Deans switch is the easiest to set up and operate.

How often do you switch columns and/or inlets ? - sometimes the best solution is the simplest; have both columns connected to the inlets, with gas flowing all the time, but connect only one of them to the MS. I do not know how it goes on your Agilent but on a Varian/Bruker 320 I leave the nut on the column when I remove the column from the transfer line, and then just thread the column back in and tighten the nut a touch when I reconnect.

Peter

Hi Peter,

I was having a bit of a think about my predicament and was wondering why increases in nitrogen and oxygen are occur when using the back column? Surely if it is the deans switch then the air leak would be present regardless of which column is in use? Or is it a consequence of column pressures?

I would say that, on average, we swap between liquid and TD injections about once a month. If I were to only connect one column at a time to the MS I would probably leave the MS transfer line as it is, but disconnect it from the deans switch and connect the column directly to the transfer line using press-fit capillary column connectors like this: http://www.anthias.co.uk/techtips/tech- ... -connector.

Not sure if that's a wise idea?

Cheers,

Joe

Hi Joe

A Deans switch operates by moving gas flow from one side to the other - so if you have a leak on one side you only see the air if the gas is flowing from that side - which can give the impression that the leak is in one column or inlet.

For one column change a month I would definitely dispense with the Deans switch. You will save a bit of gas as well.

Peter

Hi Joe

A Deans switch operates by moving gas flow from one side to the other - so if you have a leak on one side you only see the air if the gas is flowing from that side - which can give the impression that the leak is in one column or inlet.

For one column change a month I would definitely dispense with the Deans switch. You will save a bit of gas as well.

Peter

Hi Peter,

Sorry for the delayed response, I have been attending a conference in the middle of the English countryside, where they don't seem to have discovered broadband yet.

I will definitely dispense with the deans switch - thanks for your advice.

Cheers,

Joe

Hi Joe

What is this "broadband" of which you speak ?

Peter

Hi Joe

What is this "broadband" of which you speak ?

Peter

I think it's some new-fangled version of dial-up internet...

Joe

Hi Joe/Peter

Wonderful, that made me smile and reminded me of a time when I was trying to pay an electricity bill over the phone.

When asked to input the amount that I wished to pay on the keypad my son chose that moment to log into the then dial up internet on the extension !

I nearly ended up paying in excess of £8,000,000

Regards

Ralph

Hi Joe/Peter

Wonderful, that made me smile and reminded me of a time when I was trying to pay an electricity bill over the phone.

When asked to input the amount that I wished to pay on the keypad my son chose that moment to log into the then dial up internet on the extension !

I nearly ended up paying in excess of £8,000,000

Regards

Ralph

The wonders of modern technology.

Peter