Sudden elevated baselines towards the end

[dcc]

Hi, I am new to gas chromatography. I was preparing for some measurements and did a standard run to check if everything is well. But the chromatograms look like this:



It's similar for the standards, only that you also have analyte peaks. Could this be caused by septum bleed/coring? Septum flush is running at 3ml. I switched the liner and the septum, but it persists. What could I do to fix this? Thanks.

[Peter Apps]

Welcome to the forum.

The more you tell us, the more we can help.

What instrument is this on - make and model. Column type (stationary phase) and dimensions. What detector are you using. Conditions and settings; gas flows and pressures, temperatures of each heated zone, how are you doing your injections, and what kind of samples.

Peter

[James_Ball]

Definitely need more info, but just looking at that chromatogram since the jumps are so rapid it almost looks like an electronic or gain setting problem. If they jump up at exactly the same time in each run I would be looking more at a signal setting than contamination.

[Peter Apps]

Definitely need more info, but just looking at that chromatogram since the jumps are so rapid it almost looks like an electronic or gain setting problem. If they jump up at exactly the same time in each run I would be looking more at a signal setting than contamination.

Either that or an MS running SIM shifting to different ions was my guess.

Peter

[tkubowicz]

Hello

As I see on signal plot you're running it in SIM mode. Check your ions groups and ranges. Perhaps it is just range change.

Regards

Tomasz Kubowicz

[dcc]

Thanks and sorry. I will have to look a bit more into the method, but it's an Agilent 7890B coupled to a 5977A MS (software Masshunter B7 for aquisition, quantand qual analysis). Ion source is an EI, detector a Quadrupol. Column is an Agilent HP-5MS (30m length, diameter 0.25mm, coated with 0.25µm PDMS, temperature range -60 - 325, peak 350°C). We are measure the 16 EPA PAHs and 7 PCBs. Standards and blanks are liquid (in Hexane), samples are measured via SPME using a liner exchange system. Our injector is a cold injection system, cooled by a Gerstel UPC Plus (coolant 50:50 ethanol, millipore water).

Temperature gradient is 50, 190, 250 and 300°, I think (got to take a look at the times) but I would have to check again, pressure usually between 73kPa at standby and 135kPa during runs.

Switched the septa of the liners, same for the septa of the washing and waste containers. Chromatogram recovered meanwhile, but I got to keep watching then.

As for ion groups and ranges, recently adjusted them using scan data. Regularly testing them with standards as well.

Thanks for the pointers, everyone.

[M-KW]

Looks pretty normal to me for SIM-chromatogram combined to a TIC... I guess you hit a m/z of the column bleeding...

Give us your time segments and the corresponding m/z for each.

[dcc]

Sorry for the late reply, took the data of our column oven and the time + m/z segments:

Column oven:

Initial 60°C, hold time 15min, to run time 15mins
Ramp 1, increase 15°C/min, hold time 2mins, target 195°C, to run time 26mins
Ramp 2, increase 15°C/min, hold time 0, target 225°C, to run time 28mins
Ramp 3, increase 5°C/min, hold time 0, target 260°C, to run time 35mins,
Ramp 4, increase 20°C/min, hold time 10mins, target 300°C, to run time 47mins

SIM time segments and target ions

18-21mins, Naphthalene, m/z 128 and 127.1
21-24mins, Acenaphthylene, Acenaphthene, m/z 152.1, 153.1, 154.1
24-26mins, Fluorene, m/z 166, 165.1
26-27.1mins, Phenanthrene, Anthracene, m/z 176, 178.1
27.1-28.6mins, PCB28, PCB52, m/z 220, 255.9, 257.9, 291.9
28.6-31.25, Fluoranthene, Pyrene, PCB101, PCB118, m/z202.1, 203.1, 253.9, 325.9
31.25-33.75, BaA, Chrysene, PCB 138, PCB 153, m/z 226.1, 228.1, 289.9, 359.9
33.75-39, BbF, BkF, BaP, PCB180, m/z 250.1, 252.1, 323.9, 393.8
39-47, InP, DbA, BghiP, m/z 276.1, 277.1, 278.1

So yeah, it looks like it may be column bleeding, I guess. Chromatogram looked similar again, standards looked contaminated with 'ghost peaks'. A septa change "showed" an improvement. But could it be, that column bleeding shows up periodically or would it show in every chromatogram?

[M-KW]

I can see that you have indeed a change in SIM-Monitoring at the sudden jump.

Actually I think this a matter of the zooming.

Did you check your standards? The higher the standard, the lower seems the bleeding, although if controlled carefully, it is the same height. I see that your bleeding is at about 8* 10^4 counts. Maybe you can look at different chromatograms and find out the background noise for the last segment and tell us if that differs a lot.

I found these helpful:
https://www.agilent.com/cs/library/Supp ... F05001.pdf
http://blog.restek.com/?p=10706

[Peter Apps]

Don't get too fixated on column bleed - septa are made of the same substance as siloxane stationary phases, and give a similar bleed picture. If changing the septum improves things, then your problem is a least partly due to septum bleed.

Peter

[lpnian]

how is the mass spectrum after 39 min, would you please show us , so we can check what contamination it is ?