Running 5973 "standard" source at high temp

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

11 posts Page 1 of 1
Hi all,

I've seen lots of suggestions to run the source at high temp (280 C or so) to reduce tailing in high boiling PAHs. As far as I know our 5973 has the "standard" source rather than the inert, and the default max is set at 250 C. Furthermore Agilent's official documents suggest the range is up to 250 C. Will it damage the source to run it at 280 C? I spoke with a chemist at Restek who believed it wouldn't be a problem.

Thanks in advance
I think it shouldn't be a problem. Don't expect a lot less tailing by going from 250°C to 280°C, though.
Tailing is more about how dirty the source is than just the source temperature. A higher temperature will help keep it clean longer, but unless you can bake it at 300c or higher it will eventually get dirty and begin to tail for PAH analytes.

Another trick that will work is to run hydrogen carrier if you can do it and keep the sensitivity needed. If not you can install a simple three way valve so that when you are not running samples you can switch to hydrogen carrier and keep the source and column temp elevated and it will work to clean the system. The 5973s is compatible with hydrogen, but only some 5975s are, you have to check the magnets on the source to be sure on those. I currently run my 7000C with hydrogen and have little or not tailing and the source stays much cleaner. Tuning for DFTPP is possible but may require a little tweaking if you run hydrogen as the carrier for analysis, but I have yet to get it to pass consistently for BFB tuning for volatiles.
The past is there to guide us into the future, not to dwell in.
Try installing a 6mm drawout plate in place of the 3mm. This should improve heavy PAH tailing and resolution. It's worked well for us on a number of instruments.
Regards,

Christian
Thanks all for the replies. Our lab director is averse to running hydrogen, so unfortunately that's out. Restek did recommend the 6mm drawout plate in addition to a higher source temp. However, the instrument is still under warranty and they warned me that they wouldn't repair a source that was damaged by running too hot. I'll give the larger plate a shot.
cjm wrote:
Try installing a 6mm drawout plate in place of the 3mm. This should improve heavy PAH tailing and resolution. It's worked well for us on a number of instruments.


It might be common knowledge but I was not aware of this. After googling it seems you have a good point. Interesting post!

Is there a similar option for the EI extractor ion source on a 7000c?
To the OP: what are your chromatographic conditions? Column, oven ramp, flow, etc. There may be some changes we could make to your settings to improve the chromatography.

As for the 7000c, I'm not familiar with it. An interesting effect (that I still need to verify 100%) when switching from the 3mm to the 6mm drawout plate, is the repeller apexes of 69/219/502 when ramping all shifted to the right. On two instruments it's shifted far enough that we're running the repeller in the high 30s. It's not a problem, it's just different.
Regards,

Christian
cjm wrote:
To the OP: what are your chromatographic conditions? Column, oven ramp, flow, etc. There may be some changes we could make to your settings to improve the chromatography.

As for the 7000c, I'm not familiar with it. An interesting effect (that I still need to verify 100%) when switching from the 3mm to the 6mm drawout plate, is the repeller apexes of 69/219/502 when ramping all shifted to the right. On two instruments it's shifted far enough that we're running the repeller in the high 30s. It's not a problem, it's just different.


Seems there are three sizes of the drawout(extractor) opening for Agilent sources.(the 7000 uses the same as the 5977 for extractor source).

https://blog.restek.com/?p=13930

One thing that controls where the repeller peaks out is the emission current. If you lower the emission current you usually shift the maxima to the left leading to lower repeller voltages being needed. If you run the repeller ramp to the max and notice the abundance is still rising throughout the ramp, lower the emission current and you will reach a point where the ramps begin to peak and drop before maxing out the repeller voltage. This is where you can achieve maximum output from the repeller, and you can get longer life of the filaments if you can run at lower emission currents.
The past is there to guide us into the future, not to dwell in.
James, cjm, et al;

Thanks for the thoughts, I've been dragged into other projects and am just now coming back to this one. I haven't installed a new drawout plate yet. Tailing is significant enough to cause calibration issues with indeno(123-cd)pyrene and dibenz(ah)anthracene; the former tails into the latter, artificially inflating the higher cal points and throwing %RSD out of range for 8270. Narrowing the calibration range is an option for now, but I'm worried that the tailing will quickly get out of hand and affect separation of other compounds once we start running samples.

Conditions:

Column- Restek Rxi-5sil MS, 0.25mm, 0.25 um film, 30m w/ 5m guard
Constant flow mode at 1.2 ml/min (helium carrier)
Avg velocity 40 cm/sec

Inlet 290 C, drilled uniliner, no wool, 9.78 psi
Purge flow 60 ml/min
Purge time 0.15 min
Total flow 64.2 ml/min
1 uL injection

Oven program:
Init temp 50 C
Hold 0.5 min
15 C/min to 270 C
5 C/min to 320 C
(25.17 total run time)

MS transfer line 290 C
MS source 230 C
MS quad 150 C
I have problems with those two compounds also. One trick is to place a hold in the temperature program for about 30 seconds at 20C below the elution point of the compounds. Another it to increase the ramp rate just as they elute. The first can increase separation the second can improve peak shape. It makes for a complicated oven program but sometimes can help.

I switched to the same column phase but there is a special one they make that is 0.18mm ID, x 20m x 0.36um phase thickness that works really well for 8270 and Restek even has recommended conditions just for it. It is one of the few I have had recently that will separate those compounds. I think the thicker film helps.
The past is there to guide us into the future, not to dwell in.
I'd be interested to see your trace of those 2 compounds. We don't have them baseline separated either, and it depends on the state of the system. But we are fine in terms of QCs & proficiency tests. I always stress that it's important to integrate both calibration standards and samples/QCs consistently to the same extent. This can be a problem when only 1 of those compounds is present, or if it's a dirty waste water giving sharper peaks due to the matrix protecting active sites in the system.

On the original topic: have you tried increasing the inlet temperature? It could have a positive effect on those heavy PAHs.
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