I am in the market for a new mass spec and was told by Thermo that currently the standard cup configuration for CO2 clumped is 7 cups instead of 8 as I have in my current mass spec. The didn't tell me which cup is eliminated but I assume this is the mass 49 cup. I know some people run with only 48 as a check for sample cleanliness (but didn't realize it is that common as to become the new standard).
Those of you who have a 7 cup machine: do you simply not measure mass 49 and rely only on 48 or did you ever try measuring 49 by peak jumping or through interfering masses?
We have three machines with the 8 cups configuration here. I was just wondering if you were sure that it was the 49 cup that was removed? Could it not be the hydrogen cup, or the "middle" cup that we are not even sure here can be used to measure a signal? Have you asked Thermo if they would be able to configure your instrument to measure masses 44-49?
We do monitor mass 49, and we find it to be an excellent indicator of problems, in particular fractionation or pressure imbalance. So I would personally prefer to have both the 48 offset and 49 parameters in my tool kit.
we measure 48 and 49. The 49 signal in particular is a very sensitive indicator of contaminated samples. I'll also say that 48 is too but has to be measured precisely and accurately. For example we notice that contamination becomes a significant factor with small changes in D48. Usually, for carbonate samples we measure D48 in the range -0.1 to -0.4 per mille (no need for any linearity corrections). If D48 is greater than 0 per mille we discard the measurement. Swings in D49 are more significant and on the order of several tens of per mille. i.e. D49 is normally close to zero but if it swings to plus several tens of per mille again the sample is contaminated.
I don't know why Thermo can't configure an instrument just for 44-49 measurement, i.e. 6 faradays and sort out the linearity issue. I asked for a document to be circulated at the last clumped isotope meeting (I don't know if it was) that presents my thoughts on the origin of the non-linearity in the 253 and Isoprime instruments. Nu have supposedly found a workable solution. Certainly their solution fits with my description of the source of the problem. We certainly have never encountered linearity issues and believe we never will. Surely it is not beyond Thermo with their resources to find a solution too.
Paul, could you please upload this document you mentioned to the forum, perhaps in a new thread? If it was circulated I wasn't able to track it down.
Thank you. The document you mentioned was not circulated in Zurich. Can you circulate it online? Thermo think they have a solution to the negative baseline and therefore also to non linearity. I do not know details but hopefully those of you going to AGU will hear of it. In the meantime I got some more info about the cup configurations and 7 cups is CO2 44-49 (including 49, which I agree with Paul is needed. it in an indicator that is more sensitive and sometimes shows different contaminants than 48 and more often correlates with starange 47). the 7th cup is for N2 or SO2 - really non standard combination for clumped, so unlike what the Thermo representative in Israel told me, this is not likely the new clumped configuration.
I am guessing they have dropped the 49 cup and I wonder if this has anything to do with the apparent faulty power supply we may be dealing with. Since installation, our 253 produces errant noise about once per month in d13C, d18O, d47, D47, d48 etc etc. This noise is sample independent (could be a sample or a zero). Thermo continues to struggle with this. Thermo is currently building a custom 55 V power supply because they think the existing design is somehow insufficient for the 8 cup configuration. Perhaps they don't have a handle on the fix and have backed off the number of cups to 7 for shipping instruments until they figure out what is actually wrong. Something in their design changed so older instruments don't observe this. If what we are observing is a design flaw newer instruments should also be observing this. Our instrument began installation Nov 2012. I have tried to paste in an image showing example noise here:
Good luck with your new instrument.
Odd noise pattern. How are your trap and box currents? We had noise when the trap was too low - close to zero.
Also did you check your electreonics? We had a faulty x-focus plate. it took about 6 months after installation that the problems started and then for a year we has sudden and large sensitivity loss (we lost about 30% or the signal) approximately every 2 months. after a year we realized it was that the signal was extremely sensitive to minimal changes in x-focus. we replaced the X and einzel electronics plates and it got solved. so then we got a new plate for the X.
about the 7 cup configuration: it turned out that it is a setup for CO2 but not O2 (6 cups) plus another cup that enables doing also sulfur. I guess some people may want to add CO2 clumped cups for a sulfur machine, but whoever told me that this is the new standard configuration for clumped CO2 doesn't know much about it.