Chromatography Forum

I'm trying to run a standard sample of tetradecane on a cool-on-column injection GC-MS with a DB-5 column. The air/water check and tune check both look fine. However, when I try to run a sample, there are no peaks. I'm injecting 1 uL and the oven starts at 40 C and ramps to 230 C at 10 C/min then holds for 7 min. At this point I'm thinking that the column is misaligned in the detector.

I had some P&T samples that unbeknownst to me, had tetradecanes in them. On a DB-VRX 20m x 0.18mm x 1um column max T 230C they came out very late and kept ghosting. I had to clean out my entire inlet system to get rid of it. I don't know where that would elute on a DB-5 column. I suspect you are not running your column to a high enough temperature. I suggest raising the final temperature to 300C or even 320C (I think that's the practical column limit) to get anything stuck on the column to elute.

Thanks for the reply. I'll try turning it up to 260 C and letting it run longer at that temp. That's the max of my other column. According to an approximate retention time sheet (which I know varies greatly system to system). It should elute around 15 min on a DB-1 with the same ramp.

The boiling point of your compound is higher than your final oven temperature. So you should indeed use a higher target temperature.

If you still don't see anything, post your detection settings.

Alright so I set the max temp to 260 and held there for 20 min after a ramp from 40C to 260C at 15C/min and still saw no peaks.

Here are my detector settings:

• EV Voltage -1247 (Relative)
• Scan from 50 to 550
• Threshold = 0
• Sampling Rate = 2^2

chokichi wrote:
Thanks for the reply. I'll try turning it up to 260 C and letting it run longer at that temp. That's the max of my other column. According to an approximate retention time sheet (which I know varies greatly system to system). It should elute around 15 min on a DB-1 with the same ramp.

Do you have two columns installed in the instrument that is limiting your upper temperature?

Yes, there are two columns. I now have a RTX-VMS (connected to FID) with a max of 260C the DB-5 (connected to MS) is much higher around 350 I think.

Could try taking off the other column and ramping to 320 and hold for 30 min or more to try to get a peak.
Also could inject a solvent (lower the scan range, get rid of your solvent delay if you have one, and slow the ramp) to check the column is aligned properly and that there isn't a septum core blocking it.

chokichi wrote:
Alright so I set the max temp to 260 and held there for 20 min after a ramp from 40C to 260C at 15C/min and still saw no peaks.

Here are my detector settings:

• EV Voltage -1247 (Relative)
• Scan from 50 to 550
• Threshold = 0
• Sampling Rate = 2^2

It would be interesting to post this chromatogram. More specifically: is there (some) bleed at the end?

Also:

- What is the concentration of tetradecane in the standard
- What is the solvent
- Do you have any other application on that GC-MS that does produce peaks?

I agree with the poster above: try to inject something that does give a peak. Any lower alkanes in the house?

I can post an image of the chromatogram later, but there is no column bleed at the end. It stays level for the duration of the run.

"What is the concentration of tetradecane in the standard?"
It just says 4 uL tetradecane on a 4 L jug of MTBE so it should be 1 ppm. I would like to make a new solution, but it seems we don't have tetradecane around anymore.

"What is the solvent?"
MTBE

"Do you have any other application on that GC-MS that does produce peaks?"
The MS hasn't been used in a while, so none at the moment.

I believe we have hexane. Our stock of chemicals is somewhat limited as we are working in a quarantine facility.

chokichi wrote:
Yes, there are two columns. I now have a RTX-VMS (connected to FID) with a max of 260C the DB-5 (connected to MS) is much higher around 350 I think.

I have a similar setup and switched to the Rxi-624SilMS column which has similar retention time and an upper temperature limit of 320C which makes it more compatible with the 5phase columns.

this is a photo I took of the MS output. I injected 1 uL of MTBE and overrided (overrode?) the solvent delay. Still nothing there.

Do you have any data files from past runs to compare it to? Did the baseline always remain level across a run, or did it increase with an oven ramp?

If this is abnormal, I would consider doing inlet maintenance and possibly cutting a few inches off the column.

Is your injector temperature really 112°C i see this inlet b temperature display showing 112°C??? Are you using a SSL with 112°C? Then i would suggest maybe everything is still in your liner...

sorry just saw now that you are using an on-colum injection method...nevermind than i have to think about it a bit longer