Splitting between MS and FID - Quant by FID response factor

[Karen01]

I have a situation where I am dealing with a process that generates a very large number of hydrocarbons (alkanes, alkenes aromatics) some branched, some cyclic, some linear that vary by as much as 18 carbons ... I need to get as much info as possible about the compounds present, their structures, (Carbon #, linear, branched, cyclic, aromatic) and their amounts/proportions quickly.

What I have is an Agilent 7890 that has a 5975 MS and an FID detector with a non purged splitter.

What I would like to do is split the effluent between the FID and the MS, use the MS to ID the peaks (assuming i get good enough seperation so that the library matches can be counted on to be reasonably accurate most of the time) and then use FID response factors by carbon number and compound class to give reasonable estimates of the amount of each compound, and also calculate statistics by class and carbon # for the sample.

I don't think there is a way to get reasonable ballpark quantitation by MS alone without standards, or using a single external standard (is that a good assumption?), but I think using using a single external standard by FID with response factors by class and carbon number should give ballpark numbers.

While I have done effluent splitting before, it was aways in much simper situations with standards for all the analytes. In that case any changes in the split between detectors during a run were irrelevant, as long as they were reproducible between runs. In this case it is not possible to have standards for most of the compounds produced.

My thoughts of how this might be done are:

I will need to use a long slow temperature program to get the separation. I think that means this HAS to be in constant flow mode as I need to always acurrately know the time offset between FID and MS peaks to make the correspondence.

I think I also need constant flow to keep the split between the 2 detectors constant (I assume the same flow means the same pressure at the end of the splitter so the split would stay constant)

Are my assumptions correct and is what I am thinking reasonable? If so what would be the major pitfalls?

Thanks for any feedback

- karen

[Peter Apps]

Hi Karen

You need a way of ensuring that the split stays constant during the temperature programme. This might be as simple as ensuring that after the split and in the oven there is the same length of uncoated silica going to the FID and the MS - if the lengths are different the change in viscosity with temperature will change the relative flow resistance between the two paths. The worst design is a splitter right at the tip of the transfer line, because then only the line to the FID is affected by the temperature programme.

Alternatively you need a heater on the line to the FID at the max temp of the programme.

Peter

[Karen01]

Thanks Peter...

I had not thought about the change in viscosity... that helps clarify things... Does the MS being under vacuum and the FID being at atmospheric complicate things further?

In any case if that was the only limitation I could make the length (and ID) of two lines the same and see how that works...

But thinking a bit more about this that may be problematic for another reason. The GCMS was originally purchased before I started here... and it is a diffusion pump system, which I've been told means the MS can only take about 1mL/min flow, and the unpurged splitter needs needs at least 3 to 4 mL per minute input, so most would have to go to the FID ... so the lines could not be of equal length...And in any case that column flow would be out of the optimum range for the column, and the separation of the large number of components needs to be optimal.

I guess the only practical way this could work would be to use a system with Turbo pump MS with a purged splitter. *I THINK* a purged splitter could keep the split constant between the detectors during the temperature program.

Maybe I need to see if management is willing to spring for a new GCMS system...

Thanks again,
- karen

[rb6banjo]

Here's how they do it in my GCMS-O system. I split the effluent at the end of the second column of a Dean's switching system (nonpolar first column, can make heartcuts to a polar second column). The polar column goes to the MS but the effluent is split between the MS and a PID (atmospheric pressure detector just like a flame). Effluent from the PID is swept out with humidified air to the nose cone for the olfactory part of the analysis:

http://www.sisweb.com/ms/sis/opensplt.htm

Mine is actually an unequal split - more to the nose - because the system is really designed to track down off aromas. Generally, it's more important to actually isolate the off aroma. You can always do other things to enhance the mechanical detection once you know where/what the bad actors are in your chromatogram.

You might call someone at SIS and see if this will do what you're wanting.

[Peter Apps]

Hi Karen

Adjusting split leg lengths can get tricky - the vacuum in the MS tends to bias the plit in that direction, but if you increase the length of silica to the MS to balance the flows it introduces a time lag that can make it difficult to align the FID and MS chromatograms.

That unpurged split begins to sound like a re-purposed 1/16 inch
swagelok ! You could replace it with a press-fit Y-splitter or with one of the zero dead volume Ts.

If your management really is likely to cough up for new instruments (a consumation devoutly to be wished) then take a look at what r6banjo suggests, or I think that Agilent has some out of the box self-adjusting plumbing.

Peter

[GasMan]

Karen,

It is highly recommended to use the purged splitter whenever one of the detectors operates at sub ambient pressure. The danger with the un purged splitter, especially with the MSD/FID combination is that if you get the two split restrictors wrong, it is possible for the MSD to suck in hydrogen from the FID. I am not a mass spec user, but I understand that hydrogen is not too kind to mass spec systems.

Gasman

[James_Ball]

To handle the vacuum of the mass spec I would use a smaller ID line going into the mass spec, with larger line going to the FID.

Doing a quick and dirty calculation with a pressure/flow calculator I get 1m of 0.18 tubing under vacuum with head pressure of 1psi gives 401cm/sec flow rate while 0.73m of 0.53 tubing at atmospheric with head pressure of 1psi gives 295cm/second.

This gives you a hold up time of 3.39seconds to the FID with the 0.53 tubing and 2.49 seconds to the MS with the 0.18 tubing.

Playing with the numbers I can get to where 1m 0.18 tubing is 2.49 seconds to MS and 0.86m 0.53 tubing is 2.51 seconds to the FID.

Of course things will vary with the parameters used in your run, but if you go with 1m of 0.18 to the MS you could probably adjust the 0.53 tubing length to the FID until you get almost the same retention times. Plus with 1m of 0.18 tubing the splitter will not be seeing much of the vacuum at all. I had only a few cm of 0.18 before a splitter where I joined two columns together before a MS and even at 1ml/min at each column the instrument could not calculate the correct head pressure to give the desired flow because there was excess pressure in the splitter instead of vacuum. With that much tubing before the MS the splitter should see the same pressure as the exit of the column at atmospheric pressure would see so there should not be any reverse flow from the FID to worry about.

[Karen01]

I spoke with my manager and said I should get a quote for a new GCMS system (turbo pump single quad) that can take a purged splitter, and I might be able to get it.

Assuming I do, this had better work reasonably well...

BTW I wonder if I can get a system with both a purged splitter and Dean's Switch installed (in case a future project comes along that could benefit from heart cutting comes along)

I have never used heart cutting , but it seem to be big these days

- karen

[Peter Apps]

I spoke with my manager and said I should get a quote for a new GCMS system (turbo pump single quad) that can take a purged splitter, and I might be able to get it.

- karen

Can I come and work in your company ? !!

Peter