I agree with Dr. Gibbs.  The method he described in his paper is a better way to calibrate the isotopes values from GC-C-IRMS.  I have noticed the interference from internal standards when we do CSIA over years.

River

--- On Sun, 5/16/10, Max Gibbs <[log in to unmask]> wrote:

> From: Max Gibbs <[log in to unmask]>
> Subject: Re: [ISOGEOCHEM] preparation of CRMs for PLFA analysis via GC-IRMS
> To: [log in to unmask]
> Received: Sunday, May 16, 2010, 4:12 PM
> Hello Alice
> 
> I have been working on PLFA in soils as biomarkers for
> determining the
> provinance of sediment. You might find the method I used
> useful.
> 
> Gibbs, M. (2008).Identifying source soils in contemporary
> estuarine
> sediments: a new compound specific isotope method.
> Estuaries and Coasts,
> 31:344-359.
> 
> I no longer use an internal standard because of the
> possibility of
> interfering with a naturally occurring FA. Instead I
> include a mixed
> standard of pure 6 FAs, which have been treated the same as
> the soil
> extracts, and confirm with GC-MS if in doubt.
> 
> Cheers
> 
> Max
> 
> Dr Max Gibbs
> Limnologist, Environmental Chemist
> NIWA, PO Box 11-115
> Gate 10 Silverdale Rd
> Hamilton 3251
> New Zealand
>  
> Phone: +64 7856 1773
> Fax: +64 7856 0151
> Cell: 027 604 1449
> Email: [log in to unmask]
> 
> 
> >>> Alice Chang <[log in to unmask]>
> 15/05/2010 11:05 a.m. >>>
> Hello Isogeochemists:
> 
> 
> 
> I have students who are preparing soil PLFA samples for
> analysis on
> GC-C-IRMS. The samples are currently natural abundance but
> eventually
> there
> will be enriched samples. I have not done this kind of
> sample
> preparation
> before, or analyzed these kinds of samples. We have a
> couple of
> certified
> reference materials from Indiana University:
> 
> 
> 
> 1)      Icosanoic acid methyl ester (C20:0)
> #2 (d13C = -30.68 +/- 0.02
> per
> mil vs VPDB)
> 
> 2)      Icosanoic acid methyl ester (C20:0)
> #X (d13C = -6.91 +/- 0.04
> per
> mil vs VPDB – 13C enriched).
> 
> 
> 
> These CRMs will be used to anchor values as part of the
> two-point
> isotopic
> calibration. We chose C20 because it is not one of the C
> chains we see
> in
> our PLFA chromatograms (as determined by GC-MS).
> 
> 
> 
> Questions:
> 
> 1)      Has anyone used these CRMs as part
> of their PLFA analysis (or
> any
> analysis) protocol?
> 
> 2)      If so, how are they prepared for GC
> analysis? Should they be
> prepared the same way as the PLFAs (i.e., principle of
> identical
> treatment)
> even though they are already in a fatty acid state? Or
> should they
> simply be
> dissolved in hexane (at what concentration?), ready to be
> analyzed?
> 
> 3)      If the CRMs are prepared in a
> similar way to our samples (some
> steps
> include transesterification, methylation, etc.), how might
> that affect
> the
> isotopic value (i.e., will the values still be close to
> “certified”
> or will
> there be some fractionation)?
> 
> 4)      Should the CRM solutions be added
> into each PLFA sample, or
> analyzed
> several times throughout the run as stand-alone solutions
> (what about
> the
> enriched CRM – see point 6)?
> 
> 5)      If the CRMs are added into the
> sample, and since both CRMs are
> C20,
> should two sets of samples be produced? Otherwise it would
> be difficult
> (if
> not impossible) to separate the C20 peaks on the
> chromatogram.
> 
> 6)      The #X CRM is enriched: what is the
> best way to set up a sample
> list
> to avoid memory effects in the GC-IRMS? I assume any (all)
> samples
> containing enriched material should be run after the
> natural abundance
> material.
> 
> 
> I would appreciate any suggestions on protocol or
> references to
> published
> techniques. Thank you.
> 
> Alice
> 
> -- 
> Alice Chang
> Stable Isotope Facility
> Department of Forest Sciences
> University of British Columbia
> Vancouver BC Canada
> 
> NIWA is the trading name of the National Institute of Water
> &
> Atmospheric Research Ltd.
>