Dear Pratigar and fellow GC-Isolink HTC users,
Given the interest in this thread I thought I would add my tuppence worth though from a condition side of things our way does not differ that much from what Marilyn, Patrick and Pratigar have described.
To condition the HTC reactor we initially opted for repeated injections of n-hexane with all the necessary precautions. Since our instrument was hybridized to an ion trap MS for simultaneous compound peak identification, we eventually moved from n-hexane injections to injections of a mixed alkane standard. This way we combined HTC conditioning with a "crude" GC and GC column test since the alkane mix was run using a basis GC programme unlike the isothermal n-hexane runs.
2H scale calibration: On this subject I beg to differ with Pratigar's comments. Standards for 2H scale calibration of GC-IRMS analyses have been (and still are) available from Arndt Schimmelmann for about 10 years (if not longer).
In the meantime, directly GC amenable 2H standards available as 3 chemically identical isotopologues (of low, middle and high 2H abundance covering a range of >300 ‰) have been reported in Anal. Chem. (2016), 88, 4294-4302. Standards reported in this article include compounds such as n-hexadecane (-116.2; -10.2; +381.4) or C20-FAME (-183.9; -4.9; +348.3) and these are available from USGS, Reston.
Admittedly, the practical side of 2H scale calibration of GC-IRMS measurements is not without its challenges but they are not insurmountable. Here are my thoughts. Apologies to those of you to whom the words grandma, eggs and suck come to mind. ;-)
1. Combining standards with sample is not an option in most cases due to the risk of peak overlap.
2. Considering compounds are thermolytically converted at temperatures of 1400°C+ there is no pressing need for standards being 100% matched to sample compounds by way of chemical composition. [However, N-rich compounds are an exception].
3. If one's standards are chemically identical but isotopically different one has to bite the bullet and inject single standard solutions separately (at least 2 end-points; better still 2 end-points plus 1 QC in analogy to VSMOW/SLAP plus GISP). These standard solutions should be injected in triplicate at the beginning, the middle and the end of a batch run so their "raw" delta values measured against the H2 cylinder can be checked for signs of instrument drift.
4. If one has two sets of chemically different standards at one's disposal (e.g. hexadecane and C20-FAME) one can of course prepare standard mixtures which will reduce the number of standard injections within a batch run.
5. Ideal scenario: there is no drift in raw d2H values over the course of a batch run. In this case, one can average the results for the end-point standards and use measured/accepted as x/y data pairs to determine the correction equation. Using the mid-point standard as "unknown" one can QC if the correction equation yields valid results.
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Pratigya Polissar
Sent: 27 April 2017 13:01
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] Isolink HTC conditioning
I am fascinated by the range of conditioning approaches we all use for GC HTC reactors. Methane or hexane are the two sources of carbon but the amount varies and interestingly, the frequency varies as does apparently the reactor life. I would be very curious to hear more about peoples experiences and best practices. Our lab's experience is below.
On our system we condition with 2 x 1ul injections of hexane in straight mode, run an alkane standard such as A5 overnight and then start running samples. We do not recondition the reactor. We are using the Thermo alumina tube reactors with the welded connection to a silicosteel capillary on the oven size. A reactor typically lasts 2-3 weeks at 1420 C. The end occurs not because of leaks but because of a very large size effect and systematic offset. Reconditioning very temporarily fixes this but not long enough to make it worth our while. Our typical external precision that includes analytical uncertainty as well as uncertainty in realizing the VSMOW scale is ±3 per-mil (1s) calculated as in Polissar and D'Andrea (2014 GCA). We do not do a multi-point calibration as would be best because appropriate standards are not available. (I think to do this properly would require several standard mixtures containing many peaks, each standard having very similar dD values for a!
ll peaks and the different standards having very different dD values).
We run a lot of isotopic standards and a drift sample throughout our analysis period. These are run at multiple concentrations every day to track any size effects along with systematic offsets etc.
We find that in the first few days to week after conditioning:
* standards such as A5 'behave' and have relatively low obs-exp variations across peaks (see Polissar and D'Andrea, 2014 GCA for an example).
* size effects are small (this is the time to run those small samples). Small samples are slightly more positive than large samples (by ~a few per-mil). Cutoff for size is ~150 ng on-column which equates to ~20 V-s on our system.
After ~ a week:
* standards such as A5 do not behave and have high obs-exp variations across peaks. This may occur because Mix A5 has large peak to peak isotope differences (by design, Sessions et al., 2001 GCA) that are affected by memory in the HTC reactor (c.f. Wang and Sessions, 2008 GCA). We observe a compression of the isotope scale in Mix A5 at this time that is consistent with memory effects.
* drift samples/sample replicates behave and give identical values to those from the beginning of the reactor
* size effects become larger and samples must be run at higher concentrations. On our system the size effects become negative at this time. The effect can be large, ~10 per mil for small samples. Cutoff for size is ~250 ng on-column which equates to ~35 V-s on our system.
After ~2-3 weeks:
* size effects become unmanageable, even for very large injections (cutoff would need to be >500 ng on column)
* throw the reactor away and start again
Because the reactor is at its 'best' in the first week, we have tried running reactors for only a week and then replacing them. For this, we used empty alumina tubes connected with valco fittings inside the GC (with the transfer capillary fed into the reactor ~3-4 cm) rather than the expensive thermo reactors. This worked pretty well although requires a lot of hands on work as we changed the reactor every 4-6 days and it is more prone to leaks. If we have samples that are large it is easier to just run a thermo reactor for 3 weeks.
Marilyn, FYI we have completely removed the sled, multifunction valve cluster, and 4-port on the isolink from the flow path and simply have a valco tee in the oven for the backflush vent capillary (and replaced the multifunction valve cluster @#$% with a SGE valve for the backflush vent, a la the GCC III). To switch from H to C requires changing some fittings but the decrease in problems and increase in precision are well worth it. For C measurements we oxidize continuously with a slow bleed of 1% O2 in helium introduced through a 2nd tee on the oven side of the reactor. The increase in precision and accuracy of d13C with this bleed was substantial although it wouldn't work for d15N measurements.
Lamont Associate Research Professor
Division of Biology and Paleo Environment Lamont-Doherty Earth Observatory of Columbia University
61 Route 9W, Palisades, NY 10964
845.365.8400 (phone) • 845.365.8150 (fax) [log in to unmask] • www.ldeo.columbia.edu/user/polissar
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