I really think you have water in your H3PO4. Try using some new (99% or
higher) acid to check this. This makes sense because water in the H3PO4
will not affect your d13C values, but it will exchange with the oxygen
in the carbonate during digestion. I just had the same problem last month.
On 5/24/11 1:57 PM, Joe Lambert wrote:
> Hi all,
> We have recently started experiencing poor reproducibility with respect
> to d18O with our Gasbench-Delta+ setup. I was curious if anyone had
> suggestions for a possible solution. Below is our setup:
> ~10-year old Delta Plus
> ~4-year old GasBench
> 95% of sample throughput is carbonates (manual acid injection) with the
> remaining samples being d13Cdic or d18O of water.
> typical 1-sigma reproducibility for d13C based on NBS-19 (n=15 per 80
> samples) = +/- 0.05 ‰ (before recent problems)
> typical 1-sigma reproducibility for d18O based on NBS-19 (n=15 per 80
> samples) = +/- 0.10 ‰ (before recent problems)
> Method setup for carbonates: peaks 1-3 reference, peaks 4-13 sample, peak 14
> reference with peaks 3 and 14 assigned values within the method
> Reactions are stable at 25.0 ºC.
> It is easy to get Standard On/Offs below 0.10 for d13C and 0.08 for d18O (10
> REF peaks).
> Over the last several weeks our reproducibility has slipped to the 0.10 to
> 0.20 ‰ range for d18O (sometimes worse). The numbers for carbon still look
> very good with errors always better than 0.10 ‰. We have cleaned the source,
> replaced the filament, re-tuned 4-5 times, baked out the GC column (150 ºC
> overnight) multiple times without any improvement in the O errors. There
> are no indicates of problems with either the flushing or sampling needles.
> We performed a test where we measured the isotopic composition of CO2 from
> our He+0.3% CO2 cylinder (typically for water equilibration) and the error
> for d18O was +/- 0.10‰ (n=96).
> I would think that issues related to the source or tuning can be ruled out
> due to the good performance of C and the good numbers from the
> before-mentioned CO2 experiment.
> However, with that said we do have problems with linearity. Within a single
> sample, as the sample peaks get smaller the d18O values become more
> 18O-depleted by up to 0.8‰ at times. We had somewhat similar problems with
> this in the past but we circumvented the issue by making the last reference
> peak (#14) a second anchor point. Between samples, although less
> pronounced, there is also a weak relationship between d18O value and sample
> weight/peak amplitude/peak area.
> My gut tells me we have a problem with water, but would water associated
> with the carbonate samples/acid cause linearity issues no matter how
> well-tuned the source is? We’ve never dealt with significant linearity
> issues in the past so I’m at a bit of a loss in how to attack the problem.
> The H2O background of the He stream is less than 300 mV (mass 18.0 centered
> on cup 2). We have an old water trap in the He line but not sure if it does
> any good or not. Again, baking out the GC doesn’t help. I’ve heard problems
> with the nafion traps could result in the behavior we are experiencing. I
> do know that He is flowing out the vent of the trap, although we currently
> have no good means of measuring the flow rate (suggestions?).
> I assume it is entirely possible that we are creating the water from each
> carbonate/orthophosphoric acid reaction and it is not properly being
> scrubbed from the He stream on a sample-to-sample basis. With time the H2O
> is removed from the line thus giving good background numbers and good tests
> with dry CO2 experiments.
> An additional thought is that the nafion traps are OK but our NBS-19 has
> adsorbed too much water for the traps to handle. A few months before our
> problems started our dessicator stopped working and the southeast USA is a
> very humid area even inside research buildings. Yet, I’m not even sure if
> storing within a dessicator is essential.
> We’ve been dodging tornadoes left and right down here in Alabama so isotopes
> have been our last concern but we’d certainly like to get back on track here
> at our lab.
> Thanks in advance for any comments or suggestions you might have.
> Alabama Stable Isotope Laboratory
Iowa State University
Department of Geological& Atmospheric Sciences
Room 12, Science I
Ames, IA 50011-3212, USA
Email: [log in to unmask]
Phone: (515) 294-5142
Fax: (515) 294-6049
Stable Isotope Laboratory and Research Group
Phone: (515) 294-6090