Its my opinion that you have to weigh the scientific question that is
being addressed. Most standard isotopic studies using elemental analysis
can be addressed with isotopic data alone, not %C or %N. Take for example,
the study of collagen, which if it is pure, then the C/N or %C and %N are
largely invariant. Fossil collagen is another matter, of course. [I do not
suggest trying to study variations in %C and %N in modern collagens from
different sources. Don't go there. Its evil. We've been there before.]
If someone designs a study to determine the exact %C or %N in relationship
to isotopic composition, more work is needed, perhaps in line with what
Paul Brooks describes. In my experience, the burden of work is typically
on the person doing the experiment in the correct way rather than the
isotopic or elemental measurements. [Watch out for experiments using
squirrel monkeys. They can eat anything at any time (they are sneaky
beasts), and often without the approval of the monkey researcher.]
That said, I find that when elemental composition is required as much as
isotopic composition, the samples are best run at the same time and within
the same run as much as possible. Its very difficult to compare data from
two analysis times spaced 6 months apart for small differences (less than
1 per mil) in C or N isotopic compositions. In studies that I've
participated in, which required this level of care, we often ran the sets
of data over time, then when all of the experiments were finished with
initial assessment, we ran them again as a unit. Those situations
fortunately are rare.
To comment on the concept of "like vs like" for standards--that's a nice
idea if you're looking at one particular tissue, but its not the way I
think things should be done. There are isotopic standards, elemental
standards, and then there are samples. Adding a fish standard (?), a
squirrel monkey standard (?), can be an equally vexing issue. Who
determines the "standardness" of non-standard materials? What about when
you analyze a sample in which there is no standard material? Should we
give up on those? Of course not. We need to consider the research agenda
and plan accordingly. [Please send squirrel monkey standards to the
address below--if you have adequate material. I'm thinking of Round
Robining them if community response is high.]
The problem that I see comes from people who have little to no concept of
isotope bio/geochemistry and who have designed experiments that have
little chance in succeeding given the technology chosen. An example of
this is to HOPE that EA bulk analysis will solve a problem that requires
looking at the compound specific level. Often the reverse is true--zeroing
in on one particular molecule instead of the larger picture. That happens
frequently. [I avoid these situations like I avoid squirrel monkeys!]
That said, I've got a lot of isotopic and elemental data that while sound,
will never be published. Its "nice" but doesn't show anything and doesn't
prove much at all. [If any of you out there NEED some data so you can
write a manuscript, please let me know. It will come free of charge. If
you have any definitive data on the squirrel monkey (any individual
species will do), I'd like to know what you found.]
Trick or treat, Marilyn Fogel
> Paul raises most or all of the important issues related to CN EA work. Its
> an easy measurement to make and a very difficult one to calibrate
> properly, as outlined by Paul. I believe the general rule of thumb is,
> make your standards as similar as possible to your samples (i.e. fish
> calibrates fish, plant calibrates plant, soil etc etc). With all of the
> analytical sources of error, hows does one calibrate their in house
> standards to those reference materials that set the scale? It seems to me
> that USGS40 and 41 were an attempt to have more appropriate reference
> materials than NBS19 / LSVEC for d13C and IAEA-N-1 / USGS32 for d15N?
> Ultimately we all must be on VPDB and Air to report our d13C and d15N,
> respectively.
>
> andy
>
>
>
> Andrew Schauer
> ISOLAB
> Department of Earth and Space Sciences
> University of Washington
> Seattle, WA 98195
>
>
> 206.543.6327
> [log in to unmask]
>
>
>
> ----- Original Message ----
> From: stableisotopes <[log in to unmask]>
> To: [log in to unmask]
> Sent: Thursday, October 30, 2008 11:01:24 AM
> Subject: [ISOGEOCHEM] CN standards C/N ratio
>
> To ever analyst analyzing 15N and 13C with an elemental analyzer (EA)
> and isotope ratio mass spectrometer (IRMS):
>
> I have been following the thread (bottom of this email) on standards for
> 13C 15N calibration.
>
> For the purposes of my discussion:
>
> 1) I refer to the use of two standards with an equal mass of N or C but
> different different isotope ratios, for example USGS 40 and USGS 41, as
> "scaling" standards, for calibrating the instruments scale.
>
> 2) I refer to the same standard at different masses as "linearity"
> calibration or checking. This is the change of isotope ratio with size.
>
> 3) I refer to the same standard of the same mass analyzed periodically
> throughout the analysis as "drift" correction. This is to correct for
> drift with time.
>
> I agree completely that we need a set of two standards that have high
> and low 15N and 13C delta values so that we can scale the instruments.
> The problem is that so far the urea, Acetanilide and glutamic acids have
> C/N ratios of 6/1 or less, which is much lower than 90% of the samples
> that we analyze. Most of our samples are plant or soil material, with a
> C/N ratio of 15 to 30/1. Our experience is that the change of linearity
> with size of the combined EA/IRMS system can be significant and must be
> corrected for on every analysis. This should not be confused with
> linearity checks of the reference gas alone in the IRMS. I have found
> several researchers who assume that since their IRMS reference gas was
> linear compared to changes in size that different sized samples from the
> EA should be as well. When they checked the combined EA/IRMS response
> to different sizes of the same standard they discovered that the
> response was not linear, and size dependant. This effect seems to
> change as the EA and IRMS age, and may change from day to day.
>
> In addition, we have found that the EA/IRMS combination can drift over
> time, even with instruments with reference gas injection, so drift
> correction standards at regular intervals are required.
>
> Therefore we find it necessary to do analyzes with drift correction
> standards every 12 samples, 10 variable weight standards in the analysis
> to check for linearity, and additional standards to check scaling. We
> usually drift correct and check linearity with standard sets of NIST
> 1547 peach leaves, which we have found to be uniformly ground out of the
> bottle down to 0.2 mg weight, have a C/N ratio of 16, and are available
> in over 100g quantities. We then check the scaling with another suitable
> quality control standard at a different isotope ratio but similar
> material and C/N ratio to the samples we are analyzing. Even this amount
> of standards means that we have 30 standards in a typical overnight
> analysis of 90 unknowns, resulting in 120 total tins for analysis.
>
> If we were to try to use glutamic acid we would have to weigh out two
> sets of standards to get the N and C into the same range as the
> samples. This means that we would now have to weigh out 40 drift and
> linearity standards for the analysis, plus additional standards for
> scaling. This results in almost as many standards as samples in the
> samples tray and is completely unrealistic for most work.
>
> I have had several researchers explain that they get around this problem
> by; a) analyzing the samples first on a normal EA for %N and %C; b)
> weighing out two aliquots of the sample at specific weight for either C
> or N analysis so that for an analysis each sample set has very similar
> mass of N or C; c) analyzing the samples in two separate analyzes, one
> for N and one for C. The problem with this approach, apart from the
> fact that it does not lend itself to analysis of samples sent in by
> researchers not present at our facility, is that this requires three
> analyzes, %, dN and dC, for one sample.
>
> All our experience in ecosystem science work has shown that inevitably
> the noise level of the experiment is in the field sampling, and that to
> get the noise level of an experiment down one should take and analyze as
> many field samples as possible. Therefore it is much more productive to
> analyze three field samples each for a combined %, dN and dC than to
> analyze one sample three separate times for %, dN and dC.
>
> This raises the question as to how other labs deal with the problems
> described above, and if they have found suitable standard material with
> isotope ratios of N and C at different ends of the scale but with
> suitable C/N ratios.
>
> I am looking forward to input on this.
>
> Paul Brooks
>
>
>
>
> Arndt Schimmelmann wrote:
>> Dear Hilmar and those with interest in acetanilides and ureas with 13C
>> and/or 15N enrichment,
>>
>> I thank Ty Coplen for pointing out that USGS 40 and USGS 41 L-glutamic
>> acids already provide suitable reference materials with two different
>> 13C/12C and 15N/14N ratios for two-point-calibrations. These materials
>> should serve the purpose well for most labs. However, I would be happy
>> to send test aliquots of Indiana University's acetanilides and ureas
>> with 3 levels of 13C and/or 15N abundances to qualified laboratories
>> who wish to participate in an informal multi-laboratory comparison.
>> Please contact me directly and do not reply to the entire list.
>> Thank you for your interest,
>> Arndt Schimmelmann
>>
>>
>> Prof. Dr. Hilmar Förstel wrote:
>>> Dear Arndt Schimmelmann,
>>> Thanks for your comment in a public forum. Does it make sense to make
>>> a laboratory comparison? A official round robin test may need too
>>> much effort.
>>>
>>> Yours Hilmar Foerstel
>>>
>>>
>>>> Hello Lora and others,
>>>> The use of a single batch of isotopically known acetanilide for d13C
>>>> and/or d15N calibration is problematic because single-point
>>>> calibration does not account for differences in attenuation of
>>>> delta-scales among different mass-spectrometers (i.e. slope). We
>>>> should calibrate to the VPDB scale in a way that it correctly
>>>> reflects NBS-19 (+1.95) and L-SVEC (-46.6) values. This was
>>>> explained by Coplen et al. (2006) New guidelines for d13C
>>>> measurements. Analytical Chemistry 78 (7), 2439-2441. The rationale
>>>> is the same as for VSMOW and SLAP in hydrogen isotope systematics.
>>>> It follows that for internal calibration one should use pairs of
>>>> organic reference materials that express significant isotopic
>>>> differences. Our lab is currently developing acetanilides with 3
>>>> levels of 15N abundance (delta15N ca. +1.2, +19.6, and +40.6 permil;
>>>> for standards we use the IAEA- N-1 and IAEA-N-2 ammonium sulfates).
>>>> We could not obtain 13C-enriched acetanilide. Instead, we are
>>>> developing 3 batches of urea with different levels of 13C and 15N
>>>> abundances. A similar project is generating 4 nicotines with
>>>> different levels of 13C and 15N abundances for GC-IRMS. These
>>>> materials should be ready by the end of this year. Some compounds
>>>> are sufficiently characterized already now, but not yet listed on my
>>>> website (http://mypage.iu.edu/~aschimme/hc.html). Please contact me
>>>> if you have interest.
>>>> Best regards,
>>>> Arndt Schimmelmann
>>>>
>>>> Lora L. Wingate wrote:
>>>>
>>>>> I am using Acetanilide for a C/N weight percentage check standard
>>>>> on our new Costech EA coupled to a Delta V Plus. I am interested
>>>>> as to whether there is a consensus as to its value for d15N and
>>>>> d13C. I have run it alongside and calibrated it relative to IAEA
>>>>> and USGS standards, but I am finding that my Acetanilide values are
>>>>> different from one lab's data report that I had the opportunity to
>>>>> look over. I'd be interested in knowing what values other labs are
>>>>> getting. Or, if it is a moot point and Acetanilide isn't deemed a
>>>>> reliable isotopic standard, despite the reproducible results we
>>>>> obtain. Thanks for any insight, and off list replies are welcome.
>>>>>
>>>>>
>>>>>
>>>>> Lora L. Wingate
>>>>> University of Michigan
>>>>> Department of Geological Sciences
>>>>> Stable Isotope Laboratory
>>>>> 1100 North University
>>>>> 1013 C.C. Little Building
>>>>> Ann Arbor, MI 48109-1005
>>>> --
>>>> Arndt Schimmelmann, Ph.D.
>>>> Senior Scientist
>>>> Indiana University
>>>> Department of Geological Sciences
>>>> Biogeochemical Laboratories
>>>> 1001 East 10th Street
>>>> Bloomington, IN 47405-1405
>>>> Ph (812) 855-7645
>>>> home (812) 339-3708
>>>> FAX (812) 855-7961
>>>> e-mail: [log in to unmask]
>>>> personal home page:
>>>> http://www.indiana.edu/~geosci/people/faculty2.php?n=schimmelmann
>>>> home page of Biogeochemical Laboratories:
>>>> http://www.indiana.edu/~geosci/research/biogeochem/
>>>>
>>
>>
>
>
> --
> Mr. Paul D. Brooks,
> Dept. Integrative Biology MC3140,
> 3060 Valley Life Sciences Building,
> UC Berkeley, 94720-3140.
>
> [log in to unmask]
>
> phone (510)643-1748
> FAX (510)643-1749
>
> http://ib.berkeley.edu/groups/biogeochemistry/
>
>
Dr. Marilyn L. Fogel
Geophysical Laboratory
Carnegie Institution of Washington
5251 Broad Branch Rd., NW
Washington, DC 20015 USA
Phone: 202-478-8981
FAX: 202-478-8901
Website: http://fogel.gl.ciw.edu
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