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Hi Todd,
For the nitrogen problem I can highly recommend using a longer packed molsieve column - 1.2 to 1.5 metres. I am using a home-made 1.5 m column with 80/100 grit 5A molsieves from Grace and can advertise 200 s of separation between N2 and CO. I have two monitoring gas shots between them.
The back-pressure is high, about 1.7 bar for approx 90 ml/min flow rate, but has not been a problem. The student I put onto the packing job was most diligent, vibrating and jolting the packing into the straight tube all morning. After this we coiled the tube, which no doubt further compresses the packing.
These are available commercially - appeal to the list for a part number?
We happily get good numbers for dual H and O with this column (thermo tcea). The 29/28 ratios for the N2 peak, and the 29/28 ratios for the CO peak both look so repeatable that I wonder if I could not be getting d15N and d13C as well given the right monitoring gases.


> -----Original Message-----
> From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
> Behalf Of BROWN, TODD M [AG/1005]
> Sent: Thursday, 9 October 2014 6:45 a.m.
> To: [log in to unmask]
> Subject: Re: [ISOGEOCHEM] Oxigen measurments in leaves and wood cellulose
> 
> One of the issues I have run into when using benzoate for the standards
> is that it does not contain any nitrogen. I am using a Thermo TCEA so the
> first peak I get when running leaves is N2 (28, 29). This tails badly and
> I am never convinced that it separates 100% from the CO. So as the column
> efficiency decreases with time a little more of it seeps into the CO
> peak. The only solution I have to this is to bake out the column after
> every run to restore the column to peak efficiency. I have been toying
> with the idea of making two benzoate house standards, determining their
> values using IAEA601 and 602 standards, then adding a small amount of
> imidizole to mimic the nitrogen in leaves. This way my standard would
> have the same amount of carryover from the nitrogen as the samples.
> 
> Todd Brown
> Monsanto
> 700 Chesterfield Pkwy
> Chesterfield MO 63017
> Office 636 737-6397
> FAX 636 737-4844
> 
> -----Original Message-----
> From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
> Behalf Of Wolfram Meier-Augenstein (aps)
> Sent: Wednesday, October 08, 2014 4:57 AM
> To: [log in to unmask]
> Subject: Re: [ISOGEOCHEM] Oxigen measurments in leaves and wood cellulose
> 
> Dear Jose,
> 
> In line with a previous response to your question I can only stress the
> importance of using the two benzoate reference materials (IAEA-601 and
> IAEA-602) as organic scale anchors to normalize your d18O values to the
> VSMOW scale.  You could also consider using reference waters (VSMOW,
> GISP, SLAP2) enclosed in silver tubes as available from USGS Reston's
> Stable Isotope Lab.
> 
> http://isotopes.usgs.gov/lab/referencematerials.html
> 
> With regards to matrix matched QC materials, you could use IAEA-CH-3
> cellulose and monitor its longitudinal d18O value.  However, given the
> questions regarding its homogeneity, you could equally follow advice
> already given and purchase pure cellulose from e.g. Sigma-Aldrich (they
> do an "acid washed, from spruce" cellulose for column chromatography,
> which should be fine clean and homogeneous).
> 
> Two more matters that I consider important are (a) make sure your samples
> are dry and (b) make sure your samples stay dry.
> 
> (A) Instead of monitoring your lab's humidity level (and its isotopic
> composition), keep your 18O samples, QC standards and reference materials
> away from moisture / humidity by storing them in a desiccator together
> with a strong drying agent (phosphorous pentoxide).
> 
> (B) To keep your 18O samples, QC standards and reference materials away
> from moisture / humidity while sitting in the autosampler waiting their
> turn to be analysed, use a Costech Zero-Blank autosampler since this
> autosampler can be purged with He and hermetically sealed so the sample
> tray and its contents are isolated from ambient.
> 
> 
> Kind regards,
> 
> Wolfram
> 
> 
> -----Original Message-----
> From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
> Behalf Of "José Manuel Moreno Rojas, PhD"
> Sent: 07 October 2014 16:27
> To: [log in to unmask]
> Subject: [ISOGEOCHEM] Oxigen measurments in leaves and wood cellulose
> 
> Dear all,
> 
> I have been analyzing samples for groups working on tree rings (d13C)
> even if it is not my field of expertise (food, fraud,...). Now, those
> colleagues are interested on going deeper on their contact with
> "isotopes" and they showed their interest on analyzing d18O. Since my
> experience is linked to other type of samples I had several doubts:
> - which are the international stds people use for that? any WS to buy
> somewhere?.
> - which advices are given for this kind of analyses? any issues to take
> into account for having accurate results?.
> 
> Thank you.
> 
> Regards,
> 
> José Manuel
> 
> 
> José Manuel Moreno Rojas, PhD
> Researcher
> Postharvest Technology and Food Industries Area Centro "Alameda del
> Obispo"
> Instituto de Investigación y Formación Agraria y Pesquera (IFAPA) Avda.
> Menéndez Pidal, s/n. 14071 Córdoba, Spain.
> tel.: +34 671532758; fax:  +34957016043
> email: [log in to unmask]
> 
> 
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