PAH sensitivity, replaced almost everything

Discussions about GC and other "gas phase" separation techniques.

9 posts Page 1 of 1
I am assuming the problem is on the GC side for now. The MS tune is low in air/water and has good abundance of 219 and 502 peaks. Then again there isn't much left to try but clean the source...

The issue is very poor sensitivity for the heavier PAH, like BGP, ICP...NAP and PHE are fine.
The 6 ring PAH area counts are about 50x lower than before.
The standard is new from Sigma and I made multiple dilutions to test.
This is an Agilent 5977 ms with 7890 gc

I replaced:
liner
column
gold seal
septum
cleaned the inlet metal with meoh
increased xfer line and source temps
= no improvement

anything left to try here?

thanks!

method settings:
Mode Pulsed Splitless
Injection pulse pressure 50 psi until 0.7 minutes
Purge flow to split vent 50 mL/min at 0.75 minutes
Septum purge flow mode Standard
Injection volume 1.0 µL
Inlet temperature 320 °C
Carrier gas Helium
Inlet liner Agilent 4 mm single taper, with glass wool
Quadrupole temperature 150 °C
Source temperature 320 °C
Transfer line temperature 320 °C
A deep source clean should sort you out. Have seen this many times with the heavier PAHs where all other analytes and tune looks ok.

edit - however I've seen a reduction in sensitivity of maybe 3-4x, never as low as 50x. Nevertheless, I'd still start with a good source clean.
Try cleaning or replacing the split line and filters.
Peter Apps
I do PAH's by HRGCMS and have come to realize that a clean source is an absolute necessity.
The difference is night and day, dirty source and the sensitivity is terrible and peak shape is marginal at best with lots of tailing.
Clean source, sensitivity is great and the peaks are needle sharp.

'Crabs
The tune still looks good in the high mass even with a dirty source. Is that because ptfba is much more stable than the PAHs and is less affected by the reactivity of the source?
Peter Apps wrote:
Try cleaning or replacing the split line and filters.


I may need to do this. Will this have an effect even though I am running pulsed splitless?

I cleaned the source and there has been no improvement in sensitivity.
Update:
the last two adjustments made a difference.
1. baking the new column properly
2. replacing the syringe, even though it was not old it had some corrosion on the needle that might have caused reactive issues

With these changes, the high mass compounds are now back and are in normal proportion to the other peaks.
The remaining problem is the area counts overall are about 5x lower than normal.

I don't usually consider bumping up the voltage to improve sensitivity since I assume the noise would increase as well. But with the emv at 1900 would it make sense to increase it at this point.
I tend to agree with Peter Apps - that you have a GC issue, not an MSD issue.
Are you running the standard 3mm drawout plate in the 5977 or the larger 6mm or 9mm ones?

For PAH the larger diameter ones help a lot. I run the 9mm in a 7000C with extractor source for full list semivolatile tests including the PAH, Acid and Base extractables. A dirty source does seem to affect the PAH much more than most other analytes.

The Split Vent Line also does make a difference especially when using splittless injection. When you imagine molecules in a gas phase at 250C, those molecules travel very fast and move all through the inlet and back again before going into the column inlet, not simply straight into the column like the little animations in training videos suggest, so active sites anywhere in the inlet, near the split vent and even in the vent line can cause problems.
The past is there to guide us into the future, not to dwell in.
9 posts Page 1 of 1

Who is online

In total there are 34 users online :: 0 registered, 0 hidden and 34 guests (based on users active over the past 5 minutes)
Most users ever online was 599 on Tue Sep 18, 2018 9:27 am

Users browsing this forum: No registered users and 34 guests

Latest Blog Posts from Separation Science

Separation Science offers free learning from the experts covering methods, applications, webinars, eSeminars, videos, tutorials for users of liquid chromatography, gas chromatography, mass spectrometry, sample preparation and related analytical techniques.

Subscribe to our eNewsletter with daily, weekly or monthly updates: Food, Environmental, (Bio)Pharmaceutical, Bioclinical, Liquid Chromatography, Gas Chromatography and Mass Spectrometry.

Liquid Chromatography

Gas Chromatography

Mass Spectrometry