Optimal Conditions for CSR_LVSI

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

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Referring to this post Optimal conditions for Concurrent Solvent Recondensation – Large Volume Splitless Injection (CSR_LVSI). I have summerized as follows....
[1] Hold flow constant for width of solvent peak
[2] Hold oven temp below solvent B.P. for splitless interval
[3a] At end of splitless interval, ramp Temp to 10C over solvent B.P.
[3b] Hold Temp 10C over solvent B.P. for solvent peak interval
[4] Ramp Temp and Flow for normal program from that point onward

I did a few calculations and for the column the article is using, the injection pressure is around 36 psi. This means that the calculated B.P. for MeCl is actually not 40C but closer to 65C (using this quick online calculator. So, the 50C cited in the method above is actually not sufficient to boil off the solvent.

I have done this with my GC-FID system for DRO.
analytical column: HP-5 50m x 0.20um x 0.33um column
pre-column: 5m of 0.53 deactivated silica Rxi guard column
Flow Program: 5.4mL/min (hold 2.40min) to 5.0 mL/min at 10 mL/min
Oven Temperature 60C (hold 1.0 min) to 80C at 60C/min (hold 1.1 min) 2.40min total
Program: to 320 @ 20C/min (hold 3 min) total time 18.60 min
Carrier Gas Hydrogen, constant flow 5mL/min

Inlet Mode: Splitless for 1.0 min then split at 100 mL/min
Inlet Temperature: 280C
FID Temp: 350C
Samples are in n-hexane B.P. 69C

For my program and hardware; the B.P. is predicted to be 100C at 60psi which is rather higher than the 80C I have set up. But 100C seems a bit high. So, in light of the referenced link, I'm thinking 80C is OK for my system just like 50C is OK for the MeCl. The solvent peak comes on at 1.35 min and is back to baseline at 2.40 min. And that has guided my settings. I am getting my 1000 ppm reference spike back just fine using 1uL injections and with 5uL injections, the same standard is giving me 5000 ppm with surrogates to match. DRO (C10-C28) runs between 3.4 min and 13.3 min. In the same way, my 10,000 ppm standard at 1uL is giving me 50,000 ppm with 5uL. Even my 100,000 ppm at 1uL is giving me 500,000 ppm at 5uL.

I have a 50uL gastight SGE syringe on backorder and plan to go on up to a 20uL injection at the end of the month when it finally ships.

Does anyone have something to say about the boiling point discrepancy? Am I thinking about this wrong?

P.S. I realize that column is a bit long but it was free. I've been running for some time routinely with it with just the DB-5 column, 1uL injections, the 60psi pulsed split, splitless for 1 minute then 5mL/min flow for a 21.2 min run.
There is nothing magical about a boiling point, it is just the point on the curve of vapour pressure vs temperature that happens to coincide with the ambient pressure. If they have gas flowing over them, solvents evaporate plenty quick enough for practical methods at temperatures well below their boiling points.

Peter
Peter Apps
I did a few calculations and for the column the article is using, the injection pressure is around 36 psi. This means that the calculated B.P. for MeCl is actually not 40C but closer to 65C (using this quick online calculator. So, the 50C cited in the method above is actually not sufficient to boil off the solvent.

Does anyone have something to say about the boiling point discrepancy?


just to concur with Peter's comment

If they have gas flowing over them, solvents evaporate plenty quick enough for practical methods at temperatures well below their boiling points.


Just think about water - BP 100°C but will evaporate completely at room temperature. It is the vapour pressure and not just boiling point that determines volatalisation

I apologise if I am not understanding your post correctly
Regards

Ralph
Another thing to remember is the pressure is the pressure in the inlet. The further you progress down the column the lower the pressure will become. Since there is no "restriction" at the end of the column such as a plug, the pressure will have a gradient down the whole length of the column. The boiling point even a meter in will probably not be affected that much by the pressure on the inlet.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
Another thing to remember is the pressure is the pressure in the inlet. The further you progress down the column the lower the pressure will become. Since there is no "restriction" at the end of the column such as a plug, the pressure will have a gradient down the whole length of the column. The boiling point even a meter in will probably not be affected that much by the pressure on the inlet.
I thought of this also and it feels intuitively correct but I had to ask. I don't want to be missing a trick when I get the 50uL syringe and start doing 20uL injections.
GOM wrote:
just to concur with Peter's comment

If they have gas flowing over them, solvents evaporate plenty quick enough for practical methods at temperatures well below their boiling points.


Just think about water - BP 100°C but will evaporate completely at room temperature. It is the vapour pressure and not just boiling point that determines volatalisation

I apologise if I am not understanding your post correctly
I agree with this also. I expect that the quick bump up of temperature during the solvent peak is effective to focus the content of the guard column whether sub-boiling or not. Especially if I only care about DRO and don't care about GRO.

Taking the time to do this optimization actually shortened the width of my solvent peak and took a couple minutes off my run time. So its a win win.
LALman wrote:
GOM wrote:
just to concur with Peter's comment

If they have gas flowing over them, solvents evaporate plenty quick enough for practical methods at temperatures well below their boiling points.


Just think about water - BP 100°C but will evaporate completely at room temperature. It is the vapour pressure and not just boiling point that determines volatalisation

I apologise if I am not understanding your post correctly
I agree with this also. I expect that the quick bump up of temperature during the solvent peak is effective to focus the content of the guard column whether sub-boiling or not. Especially if I only care about DRO and don't care about GRO.

Taking the time to do this optimization actually shortened the width of my solvent peak and took a couple minutes off my run time. So its a win win.


Good to know, we are about to begin work doing the same thing with our DRO.
The past is there to guide us into the future, not to dwell in.
Little update here. I've been using a 60 psi pressure pulse for my DRO since I started. I've found a couple of things out.

(1) Pressure pulse or not, DRO peak defining mix still comes out very close to same retention times.
(2) Pressure pulse results in all defining mix peaks having the same height and also better formed at base. Without pulse C10 to C25 peaks are progressively higher as carbon number increases and also have diffusion broadened bases for the early peaks.
(3) Pressure pulse puts more on the column.
(4) Dropping the flow before the solvent peak ends bad idea. It causes loss of the early part of the DRO. Perhaps flowing back into the inlet because of lower pressure while guard column is boiling off.

I had a series of column breaks with this 50m column and switched to an HP-5ms 30m x 25mm x 25um. I used the RESTEK method translator and went from 5mL/min @ 60 psi to 2.5mL/min @ 13.8 psi. Still using a pressure pulse but only 24 psi. I got a good calibration from 25 to 100,000 ppm. I have a cluster of C23-C36 peaks showing up which did not go away after cleaning inlet, trimming column, changing gold seal, gaskets, liner. My first calibration was clean down to 25 ppm but now the 25 and 50 are compromised by these peaks.

I think its parafilm residue. I foolishly thought I could seal my DRO peak locations mix vial from evaporating by using parafilm over the top. Got it on gloves of course and probably transfered it to inlet parts as I was putting in the new hardware. Can I clean the inlet parts or do I have to do a complete change out of everything again? Any suggestions for getting rid of it would be welcome.
Found the RESTEK blog How Dirty Are You and there is the parafilm front and center and hexane is the best solvent. Oy Vey!
I would think hexane rinse and a good baking would remove the parafilm residue.

I have been playing around with pressure pulses on other methods and it can be tricky to get the best timing and pressure, but once you do, the results are great.
The past is there to guide us into the future, not to dwell in.
Yesterday I disassembled and cleaned the inlet with hexane/acetone mixture. This did not eliminate the problem. I've also run both split and splitless and the peaks are still there.

The RESTEK Cat#24799 50R-HP-0.63 SGE injector syringe arrived today. Its SGE Part# 004665 50R-AG-0.63 This has a removable 26 gage needle and has a steel plunger. Half the plunger length is the barrel width, the other half is thinner presumeably to make attachment easy and reduce inertial mass. Since this is not a gas tight syringe, I hope it will prove adaquate. Apparently the gas tight model is SGE#004668 and also has a dual 26/23 gage fixed needle. I think on thge whole, being able to replace the needle ($22 for two pack) makes this one more desireable.
That is the syringe I am using, except with a 23gauge needle to be compatible with a Merlin Microseal (can't use the dual taper needles with Merlin or Gerstel/Agilent septumless head). They seem to work well, but you will want to take the plunger out ever once in a while and wipe it down with a kemwipe and solvent as it gets a little sticky over time, probably from dust getting into it at the top.

I have to replace a needle because I wasn't paying attention and installed a cyclo liner upside down :roll: The needle didn't bend surprisingly but it did rough up the tip enough that it will cut the septum or Merlin if I try to use it.
The past is there to guide us into the future, not to dwell in.
I tried the new syringe and with 25uL injections, I see about 5X the trash peak intensity that I had with 5uL. Suspecting solvent...

I put a 2" square of Parafilm in 4mL of hexanes and sonicated then centrifuged the result. Then I diluted that another 80X (50uL/4mL) and shot it for reference. The peaks I am seeing are not parafilm. The worst two peaks are at C22 and C24 decreasing from there. But Parafilm rises to a peak at C29 and C30.

Pest grade acetone gives a clean run! So, now I know this it a solvent contamination issue. I suspect my dispenser but I am checking my reagent bottles now also. It appears that UV/Pest grade Hexane fresh from bottle, has baseline trash before C10 and between C11 to C12 but higher peaks are nearly clean. So, dispenser is contaminated, darn it to heck.

Fisher Pest Grade Hexanes used for O&G and redistilled gives a clean baseline run with a single very very small peak between C25 and C26. I used a steric acid and hexadecane spike for O&G. Neither of those could be a fit. But this is clearly clean enough to do DRO work! Cleaner than the UV/Pest grade right out of the bottle.
That is the downside to large volume injections. It not only increases the sensitivity to the target analytes, but also amplifies any impurities in your prep.

I am dealing with it here trying to switch 8270/625 analysis to 100ml extractions and LVI, we have to make sure the the whole process is cleaner since any source of contamination makes a 10x larger problem than before. It isn't as simple as extract less, inject more, which is what the accounting people want. :)
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
That is the downside to large volume injections. It not only increases the sensitivity to the target analytes, but also amplifies any impurities in your prep.

I am dealing with it here trying to switch 8270/625 analysis to 100ml extractions and LVI, we have to make sure the the whole process is cleaner since any source of contamination makes a 10x larger problem than before. It isn't as simple as extract less, inject more, which is what the accounting people want. :)
When I went from 1uL to 5uL that began to be apparent. But as you say it really came home when I went up to 25uL.

I'm having a problem with spit peaks with this 25uL injection. My solvent peak goes from 1 min to 3.25 min. The void time for my column is 32 seconds. So I am injecting at 60C, purging the inlet at 1 minute, also ramping 60C/sec to 80C. Then holding 80C until 2.75 minutes.

After a few tries its apparent...
Its because my column flow is 2.5mL/min and the pressure pulse at 24psi makes a flow of 5.4mL/min during the pulse. With 1uL and 5uL injections under these conditions, peak splitting was not apparent. But with 25uL the peaks for C18-C25 are splitting. So, I have tried the whole run with the same 24psi pressure pulse and then a flow of 5.4mL/min. This shrank my solvent peak down to 2 minutes so I made appropriate timing adjustments. The peaks are no longer split [Edit: but are only 2.5X instead of] 5X the area of the 5uL peaks.

Edit: It turns out that a pressure pulse is no good for CSR if the pressure does not come off to let the solvent boil away. So, I kept the 24psi pressure pulse for the 1 minute until purge vent opened and flow dropped to 2.5mL/min as before. Then at same time oven heating began (2.75min), I ramped flow 4mL/(min/min) up to 5.4mL/min when time reached 4 min. This actually gave the combination of 5X increase in signal and unsplit peaks.
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