You are correct in that the standard N2 gas configuration does not use m/z 30.
However, you can define your own gas configuration that does include it. In
Isodat, choose the "Editors" button at the top, select "Ratio editor"; in the
"Evaluation" list box, click on "N2", then click the "copy" button below. Give
it a different name in the dialog box (e.g. 30N2), and click "OK". Then, find
that name in the "Evaluation" list box and click on it.Then in the "Nominator"
dialog box, click "30N2", and in the "Divisor" box, click "28N2"; the button
"Add ratio 30N2/28N2" in the center will now be un-highlighted; click on it.
You will see the two ratios, 29/28 and 30/28 in the lower left-hand box. When
this looks right, you can click "save". Next, back at the "Editors" button,
select "Gas Configuration Editor"; when that dialog box opens, select "Add".
Give it a name (I use 15N label, as I use this configuration for labelled
material), and select the new "30N2" gas configuration as the "Template"; then
"Save & Close". Use this gas configuration for your analyses (selected in the
first page of the Method). This then, should solve your problem.
If you have samples that are already run, and you need to recalculate them
manually, that may be answered by someone else; I am currently trying to figure
out exactly how Isodat calculates ratios by reading the Isodat script for
different project using CO2. It isn't straightforward, as the ratios are
standardized and corrected for other ion influences. I can't even reproduce the
ratios that they provide. One possiblity to consider, though; the standrd for
N2 is air, and is defined as zero. The offset you see may be an offset caused
by whatever you as using as the "standard" in your formula; that is, the
isotope ratio in that substance (I assume N2 in a tank) isn't the "real" number,
but a working value (leading to a somewhat circular calculation for your air N2
In general, the modified gas configuration has worked well for my labelled
samples. In some cases, a very heavily labelled compound can cause very high
m/z 30 readings (> than 12-13V); in this case, it isn't possible to get a valid
value. The correction in this case is to swap out the resistors on the cups to
reduce the sensitivity and better represent the actual ion ratios. This is
somewhat drastic. however, the resistor ratios as installed reflect the
expected natural abundances, and if you are far from there, then changing the
resistors is the way to go. If your instrument isn't dedicated to 15N2, this
means a lot of swapping, not a good idea in itself.
As to your second question (Trace GC). I use my Trace/PLOT in exactly this
capacity; CO2 in headspace (although it is for water equlibration). The first
peak is N2 gas in your sample; it forms NO2 in the source, combining with O2 or
H2O and as such has a meaningless m/z 46 value. If you don't get good
separation, this will ruin your CO2 values, and can be an issue at normal
atmospheric N2 partial pressures (70%); if you incubate with a He/O2
atmosphere, this problem can be significantly reduced. The peak shape may not
be that bad, if you magnify it. The peaks are usually less than strictly
Gaussian, anyway; they are usually "skewed-normal" or "gaussian with a drift",
as the molecular movement is biased by the carrier gas movement, so the
"right", trailing tail is longer than the "left", leading tail. Some of the
distortion is related to the nature of ion formation in the source.
David R. Vann, Ph.D.
Forest Biogeochemistry and Physiology
Department of Earth & Environmental Science
The University of Pennsylvania
240 S. 33rd St.
Philadelphia, PA 19104-6316
Quoting Melissa Bautista <[log in to unmask]>:
> Dear List Members
> As I understand it, the Isodat software does not include m/z30 in its
> calculation of del15N/14N and thus this step has to be done off-line. Using
> the the peak areas from Isodat and the equation del =
> [(Rsample/Rstandard)-1]*1000, I get a value of 63 per mil for air. I feel
> I'm making a simple mistake somewhere, but can't think where. Can someone
> please point me in the right direction?
> Also, I've just started measuring CO2 on a Trace GC Ultra with a 25m
> Poraplot Q column. The samples are headspace gas from sediment core
> incubations. The GC method I'm using is isothermal (35C). There are two
> peaks, the first of which is all m/z46 and tends to be non-Gaussian, the CO2
> peak elutes second at about 150s. Would the first peak be NO2? Why would the
> peak shape be so poor?
> Thanks in advance for the help!
> Melissa D. Bautista
> IRMS Research Technician
> Centre for Coastal Biogeochemistry
> Southern Cross University
> P.O. Box 157 Military Road
> Lismore NSW 2480 Australia
> phone: +61 266 269 565
> mobile: +61 431 928 278
> email: [log in to unmask]
> audaces fortuna iuvat