5975C EPA 8270 Internal & Calibration Troubles (and more!)

[ajc92]

Hello all. Dealing with a frustrating situation(situations?) on my Agilent 5975C/6890.

They are two markedly different issues, so I'll go in chronological order.

Long story short, I'm trying to validate 8270E/625.1 - My first calibration looked amazing, but a few weeks later (after a routine PM visit) I have fronting peaks and possible ion suppression affecting linearity. Then, the lab decides to add another column for another GCMS method, so now we're using a 2-hole ferrule, switching between inlets/columns, and I've greatly lost sensitivity and have horrible peak shape.

MS: Agilent 5975C
GC: Agilent 6890A
Carrier gas: Hydrogen
Column: Rxi-SVOCms, 30 m, 0.25 mm ID, 0.50 µm
Split Ratio:10:1
Column flow: 1.2(constant flow)
Inlet Temp: 240
Source Temp: 230
Aux Temp: 280

Problem 1: Calibration Curve

8/21/24 - Installed brand new column, liner, seal & septa. Baked out overnight, then ran 20 or so high standards.

8/22/24 - 8/28/24 : DFTPP target tuned. (69~500K) 442 & 68/70 are high in DFTPP. Fine tuned to get 442 down. 68 and 70 are still right on the edge. I've deduced from the forums that could be a hydrogen carrier gas issue, but if anyone has any tips I'm all ears

8/29/24 : Ran a curve, second source, and mid-range iDOC samples. Curve had excellent low-end sensitivity and RSDs were <10 for all but the trouble compounds. 0.5ug/mL to 100ug/mL. In my 10 years of running 8270 I couldn't dream of a nicer curve.

9/13/24 : Engineer comes in for PM. Cleaned source, replaced pump oil, dusted fans tested filaments, tested tunes (I wasn't here for this)

9/16/24 : Other department switches out my column to run their method

9/23/24 : Switch back to my column, bake overnight

9/24/24 - 9/30/24 : Re-tuned (69~400k) and ran a curve. Slightly lower sensitivity, but horrible peak splitting (front shoulders). The compounds themselves are linear, but my internal standards drop significantly at the high points (basically at where the compounds ppm meets the IS ppm), skewing everything quadratic. I researched about this issue and tried the below:

Lowering EMV (no significant change)
Raising EMV (no significant change)
Various split ratios/flow rates (no significant change)
Alternative column using original GC parameters from august [RTX-5silMS 30mx.25umx.25mm] (variation in sensitivity [earlier eluters respond higher, later respond lower], much more column bleed, RT shifts of course, but no change)

I truly have no idea what caused my perfect calibration to fall out from under me. The only significant difference in both tunes is my august one had the entrance lens a smidge open at 7.5 - usually I've always kept it at 0. Other than intentional variations, all my GC parameters were identical.


Problem 2: Sensitivity with 2 columns into 1 transfer line

While I was trying to fix the above, the lab decided to put in a 2-hole ferrule at the transfer line and set up the GC with each dept's columns, so we don't need to constantly vent the instrument. My column got put in the back inlet, which I assume has never been used ever, so I replaced the seal/liner and went on my merry way.

10/9 - Re-tuned (69~400k) and ran DFTPP. Ions were in (though 68 still fights me), but high-not-failing benzidine tailing. Low responses. Ran a midpoint standard and my peak shape for my early eluters (1,4 dioxane, NDMA etc) is just tail city. Also, my responses are about 1/10th what they were during my previous problems above. Tried a few of my classic tailing fixers:

Cleaned split line (no change)
Tried different liner [Restek Topaz LP, had a Topaz Split prior] (even lower responses - but slightly less tailing)
and one thing I just noticed - Restek recommends 1.2mm inlet seals when using 2 columns. I've been using plain ol' 0.8mm ones. I wonder.

I'm positive this has something to do with having the other column going to the aux. I have it set to the same temperature/flow as my actual column. But I'm no engineer (barely a chemist if I'm honest ), so I'm not sure what direction to go in here to at least restore my peak shape so I can try fixing my calibration.


Sorry for the novel.
I've got chromatography and parameters if anyone thinks they need more detail.
I appreciate any and all ideas or discussion.

ajc

[cjm]

I think you're going to be fighting with the two hole ferrule.

What concentration DFTPP solution are you injecting? I ask only because I see you're doing a 10:1 split, so if you're injecting 1.0 uL of a 50 ug/mL standard, you're theoretically only getting 5 ng to the detector. Increasing this amount MAY help with consistency between runs. it may also help tailing evaluations as well.

I have no experience with H2 carrier on a MSD.

I would also consider raising the temp of your source to at least your inlet temp. We run inlet 250, quad 150, source 270 on our 8270/6890/5975 systems.

Constant changes to your system may be very difficult to troubleshoot.

[ajc92]

Constant changes to your system may be very difficult to troubleshoot.

That's what I keep telling these people!!
In another life I had two 5975Cs all to myself. Much easier.

I am injecting 50ug/ml DFTPP - I just dropped the split to 2:1 and much better on 68 & 70! I also had to re target tune it, for some reason the 69 was only about 100k when I looked at it a few hours ago. Now I just have to rope 442 back in. If only I could use it as the base peak for both methods.

I might try those temp parameters next. Going to run a spike mix and see how it looks. I appreciate the quick response!

[cjm]

2:1 split may not be consistent over time and may lead to higher than normal calibration RSDs, normally i would recommend min split 4:1. Just something to consider. We use a 10:1 pulsed split for 8270 with good results. the pulse and the higher split help minimize inlet residence time for problem compounds.

As far as the tune standard, I would generally recommend to inject as much as the method allows (50 ng DFTPP to the detector for 8270), taking into consideration any split ratios that may dramatically decrease the amount seen by the detector. If DFTPP starts FRONTING, that would indicate column overload and to back off on the concentration. You may find fewer instances of failure on ions like 68, 70, 197, 441.

[ajc92]

2:1 split may not be consistent over time and may lead to higher than normal calibration RSDs, normally i would recommend min split 4:1. Just something to consider. We use a 10:1 pulsed split for 8270 with good results. the pulse and the higher split help minimize inlet residence time for problem compounds.

As far as the tune standard, I would generally recommend to inject as much as the method allows (50 ng DFTPP to the detector for 8270), taking into consideration any split ratios that may dramatically decrease the amount seen by the detector. If DFTPP starts FRONTING, that would indicate column overload and to back off on the concentration. You may find fewer instances of failure on ions like 68, 70, 197, 441.

Thanks! Will try 4:1 with my next one. 442 is so very high - but I really cant bring my 414 or 502 much lower without them practically disappearing!

I've been putting off trying pulsed split since I've never used it with H2 before. Just tried it (5:1, pulse 30 til 0.6min (initial temp is 60)) and my early eluters climbed onto my solvent peak. Everything else looks quite nice though. I don't know much about modifying pulsed split, usually went off what I'd seen in Agilent applications. I was hoping to get away with not lowering my initial temp since it takes quite some time to get to lower temps. If you have any advice on this front, that would be excellent!

[ajc92]

Thanks! Will try 4:1 with my next one. 442 is so very high - but I really cant bring my 414 or 502 much lower without them practically disappearing!

Unfortunately it looks like this isn't helping much with 442, and pulsed split didn't help with the tailing in my early peaks - so back to the drawing board for today.

[cjm]

If you need to bring 442 down, I would make adjustments to the variable entrance lens offset for 414 and 502; assuming this is set to variable. manually adjusting this down after running a DFTPP.U should help drop 442 under 100% of 198.

[ajc92]

If you need to bring 442 down, I would make adjustments to the variable entrance lens offset for 414 and 502; assuming this is set to variable. manually adjusting this down after running a DFTPP.U should help drop 442 under 100% of 198.

That's usually my first go-to! I've also been bringing 219 & 131 up in an attempt to raise 198. The last four 5975Cs I've worked with liked 69/219 being around 55% and 69/502 being around 3%, but this instrument likes 219 higher (65-70%) and 502 lower (.5-1%) - I just don't like to drop 69/502 much lower or the 502 peak shape suffers.
Surprisingly, I was able to get it just below (95%~) consistently (today, at least) by switching my unused inlet from split to splitless. Now I'm on to trying to get better sensitivity for my 8270 compounds.

[MichaelVW]

I've wondered how much to be concerned about how 414 and 502 look, as long as the tune passes. All the ions for 8270 are ~300 or smaller.

I don't do 8270 anymore sand I can't remember what the 69 ion abundance is supposed to be for a tune... is 400k kind of high? I was using a 6890/5975. I remember losing linearity if I had the voltage too high.

Ideally I could calibrate from about 0.2 ppm to 30 ppm with most of the easy compounds. (For some of the difficult ones I'd have to do 5 ppm - 50 ppm or something. And for a few I'd use quadratic, unfortunately.) But if the abundance was too high, I'd have to cut either the top or the bottom end of the cal curve to make things linear.

[ajc92]

I've wondered how much to be concerned about how 414 and 502 look, as long as the tune passes. All the ions for 8270 are ~300 or smaller.

I don't do 8270 anymore sand I can't remember what the 69 ion abundance is supposed to be for a tune... is 400k kind of high? I was using a 6890/5975. I remember losing linearity if I had the voltage too high.

Ideally I could calibrate from about 0.2 ppm to 30 ppm with most of the easy compounds. (For some of the difficult ones I'd have to do 5 ppm - 50 ppm or something. And for a few I'd use quadratic, unfortunately.) But if the abundance was too high, I'd have to cut either the top or the bottom end of the cal curve to make things linear.

Fair point on 414 & 502. Just hope it isn't indicative of a bigger problem

400k is quite high, these target tunes end up around 2100 EMV which I consider pushing it - but it worked beautifully in my very first curve I did. The only three regressions I had were Benzoic Acid, 2,4-DNP, and 4-NP, which aren't shockers.

When I tried to calibrate after the PM, some strange ones started needing regressions. Lighter phthalates & isophorone are the ones I recall off the top of my head. It's due to those IS responses decreasing as the compound concentration increased. Still not sure how to fix this one.

I've tried lowering the EMV but I don't get enough response at my low points. My curve's 0.5ppm - 100ppm, and I'm starting to think about removing the high points altogether. I'm just doing R&D so I know what concentrations I'm spiking at, anyway. But I don't think EPA would like it if my IS concentration exceeds the high point of my cal curve, and I can't cut out my low points too much to meet 625.1 MDLs.

[ajc92]

I've wondered how much to be concerned about how 414 and 502 look, as long as the tune passes. All the ions for 8270 are ~300 or smaller.

I don't do 8270 anymore sand I can't remember what the 69 ion abundance is supposed to be for a tune... is 400k kind of high? I was using a 6890/5975. I remember losing linearity if I had the voltage too high.

Ideally I could calibrate from about 0.2 ppm to 30 ppm with most of the easy compounds. (For some of the difficult ones I'd have to do 5 ppm - 50 ppm or something. And for a few I'd use quadratic, unfortunately.) But if the abundance was too high, I'd have to cut either the top or the bottom end of the cal curve to make things linear.

Fair point on 414 & 502. Just hope it isn't indicative of a bigger problem

400k is quite high, these target tunes end up around 2100 EMV which I consider pushing it - but it worked beautifully in my very first curve I did. The only three regressions I had were Benzoic Acid, 2,4-DNP, and 4-NP, which aren't shockers.

When I tried to calibrate after the PM, some strange ones started needing regressions. Lighter phthalates & isophorone are the ones I recall off the top of my head. It's due to those IS responses decreasing as the compound concentration increased. Still not sure how to fix this one.

I've tried lowering the EMV but I don't get enough response at my low points. My curve's 0.5ppm - 100ppm, and I'm starting to think about removing the high points altogether. I'm just doing R&D so I know what concentrations I'm spiking at, anyway. But I don't think EPA would like it if my IS concentration exceeds the high point of my cal curve, and I can't cut out my low points too much to meet 625.1 MDLs.

[MichaelVW]

Ah ok, I forgot you were running split mode. I always did splitless, so 400k seemed too high (40 ppm would cause peaks to be so large they'd get cut off and be flat on top).

But I don't think EPA would like it if my IS concentration exceeds the high point of my cal curve, and I can't cut out my low points too much to meet 625.1 MDLs.

Can you use a different (smaller) IS concentration? 8270E allows it.

You're doing R&D? Like treatment technologies or something?

[ajc92]

Ah ok, I forgot you were running split mode. I always did splitless, so 400k seemed too high (40 ppm would cause peaks to be so large they'd get cut off and be flat on top).

But I don't think EPA would like it if my IS concentration exceeds the high point of my cal curve, and I can't cut out my low points too much to meet 625.1 MDLs.

Can you use a different (smaller) IS concentration? 8270E allows it.

You're doing R&D? Like treatment technologies or something?

I've always been taught that split was best for GC/MS. Usually I'd only run splitless for FID methods. Maybe, if I run splitless I can reduce my EMV and still be able to detect my low standards.

I was thinking about trying a lower concentration IS.. Maybe just dilute all my standards by 2

I'm trying to validate alternative test procedures, focusing on reducing/eliminating DCM usage in sample prep. Maybe bring it to the EPA if I can check all the ATP boxes.

[MichaelVW]

Yeah, everything I'd read from Restek or whoever would say that split is better. But whenever I tried it for semivolatiles, I didn't see much improvement. Or at least, it didn't work for some of the analytes at the required LOQs for compounds on the RCRA UTS table.

I may have been doing something wrong or just didn't have a clean enough system, though.

[ajc92]

Circling back to this as I'm back in the weeds with it! But in a much better place.

We removed the two-column two-hole ferrule setup back to a single column and the peak shape looks to be where it was before that. I'd lost my 441 in DFTPP (on top of the high 442 and 68/70) - But then I tried switching my polarity from Pos to Neg to fix most of those issues - and it did!

I just ran a calibration and still have remains of my first issue. While my internal standard responses are more consistent than before, which is a great improvement, my high standards still are fronting and I had to put compounds on regressions that I'd rather not. If I raise the split more, I lose sight of my low standard, so I'm considering just calibrating in a shorter range.