Dear Participants to Willi Brand's H2 Ring Test,
In an attempt to study linerity of hydrogen
isotope-ratio mass spectrometers and improve the
comparibility of hydrogen isotopic data among
laboratories, Willi proposed that gaseous
hydrogen reference materials be prepared and
distributed to interested laboratories. He
encouraged Messer Griesheim, a gas manufacturer
in Germany, to support this program, and about
50 laboratories have agreed to participate.
Before these reference materials are distributed
it is important to determine that they are
isotopically homogeneous. I agreed to perform
this task. We received 7 each of the three
different gases on May 24. Below are the results
of the investigation.
The isotopic composition of the gases are
approximately:
Gas A 0 per mill
Gas B -400 per mill
Gas C -700 per mill
The label says that the Gas Range is 0 (or
-400 or -700) v.s.SMOW (I would have chosen VSMOW,
but that's a discussion for another day). Please
note that the key word here is "Range." These
values are only approximate and should be
determined by the 50 laboratories to the best of
their ability. Forms are provided with the gases
for reporting isotopic results to Willi.
Each gas is provided in a light weight, thin-walled
aluminum tank with a volume of 1 liter. One valve
is provided for three tanks. Gas pressure is about
12 bars.
The aluminum tanks and valve were easy to use.
Users should be advised that there is no need to
purge the valve as suggested on the side of each
tank if one conscientiously evacuates the dead
volume prior to sampling.
Users are advised to place a valve as close as
possible to the valve on the tank in order to
minimize the amount of gas that is extracted from
a tank and expanded into their mass spectrometer.
To help minimize the amount of hydrogen extracted,
I put solid glass rod inside of my 1/4 inch tubing
to minimize internal volume. I replaced the 1/4
inch Swaglok stainless steel front ferrule on the
valve outlet with a Teflon ferrule. Teflon provides
leaktight service for repeated connections.
We have checked each tank for contamination by
comparing the ion gauge reading of the sample
with that of the working standard. We have seen
no measureable contamination--detection level is
about 2 percent.
We recorded the pressure of the gas taken from
each tank. To my surprise, they are not the
same. At least two of the tanks I received were
leaking hydrogen through the tank closure device
(similar to the top of a conventional spray can).
One of these (Gas A, tank 2) had dropped in
pressure from about 12 bars initially to 9 bars
by the time we analyzed it on May 30. Gas C,
tank 1, was leaking slightly when I received it,
but the leak was not sufficient to affect its
isotopic composition (see results below).
Users can identify tanks that have (or are)
leaking in two ways. The first is by measuring
the pressure of gas in the tank when the tank
arrives by using the gauge on the valve
provided. If the pressure is less than 10 bars,
then the tank may have leaked and the user might
want to consider obtaining a replacement. Second,
it will be impossible to obtain an excellent
vacuum when pumping on a leaking tank.
The isotopic results follow.
Tank Pressure Delta H-2 (relative to mean)
# bars standard deviations are 1 sigma
Gas A
1 12.2 +0.1 ± 0.3 6 analyses on 3 aliquots
2 9.0 +0.8 ± 0.4 4 analyses on 2 aliquots
3 11.8 +0.1 ± 0.4 4 analyses on 2 aliquots
4 10.9 -0.2 ± 0.4 4 analyses on 2 aliquots
5 12.5 0.0 ± 0.4 4 analyses on 2 aliquots
6 13.1 +0.2 ± 0.3 8 analyses on 2 aliquots
7 13.1 -0.2 ± 0.3 4 analyses on 2 aliquots
Mean (excluding Tank 2) 0.0
Tank Pressure Delta H-2 (relative to mean)
# bars standard deviations are 1 sigma
Gas B
1 12.9 +0.1 ± 0.6 5 analyses on 3 aliquots
2 12.1 -0.4 ± 0.4 4 analyses on 2 aliquots
3 12.1 -0.1 ± 0.5 4 analyses on 2 aliquots
4 12.9 +0.3 ± 0.7 4 analyses on 2 aliquots
5 12.7 -0.4 ± 0.6 4 analyses on 2 aliquots
6 12.6 -0.4 ± 0.6 3 analyses on 2 aliquots
7 13.1 +0.6 ± 0.3 5 analyses on 2 aliquots
Mean (of all 7 tanks) 0.0
Tank Pressure Delta H-2 (relative to mean)
# bars standard deviations are 1 sigma
Gas C
1 13.0 +0.1 ± 0.7 10 analyses on 3 aliquots
2 13.0 -0.1 ± 0.4 4 analyses on 2 aliquots
3 12.2 +0.2 ± 0.9 4 analyses on 2 aliquots
4 13.2 +0.2 ± 0.9 4 analyses on 2 aliquots
5 13.2 -0.3 ± 0.5 4 analyses on 2 aliquots
6 13.2 0.0 ± 0.9 4 analyses on 2 aliquots
7 13.2 +0.1 ± 0.2 4 analyses on 2 aliquots
Mean (of all 7 tanks) 0.0
Of the tanks of Gas A hydrogen, Tank 2 is the most
enriched in H-2 and is statistically different than
the other 6--it lies outside (but just barely) the
mean plus 2 sigma standard deviation. Evidently
H-1 leaks out faster than H-2, even though the
pressure is relativey high. The remaining 6 tanks
of Gas A are statistically identical.
All 7 tanks of Gases B are statistically
identical within our measurement accuracy.
All 7 tanks of Gases C are statistically
identical within our measurement accuracy.
>From these results my conclusion is that Messer
Griesheim has succeeded in producing three very
useful and desirable isotopically homogeneous
gaseous reference materials. I suspect that
these gases will be extremely useful over time
to the laboratories that have or are able to
purchase them.
When analyzing these gases, you may want to confirm
that your H3+ contribution coefficient is correct in
your software. An easy way to do this is employ a
working standard and sample with greatly different
isotopic composition. For example, you might analyze
Gas C versus Gas A. Measure the delta valve with
sample and standard voltages matched. Next, increase
the sample voltage by 50%. Analyze again. The delta
values should be identical within experimental error.
If not, see Isotope Geoscience, v. 72, p. 293-297,
1988, (Equation 18), for an algorithm to calculate
the "correct" delta value.
Good Luck with your measurements,
Tyler B. Coplen
U.S. Geological Survey
431 National Center
Reston, VA 22092
Tel: 703-648-5862
Fax: 703-648-5274
E-mail: [log in to unmask]
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