While our
group at the Geophysical Laboratory forged ahead with measuring the isotope
compositions of individual compounds using liquid chromatography methods, these
were tedious, time-consuming techniques, and really forced us to measure what
Tom Hoering termed “five well-chosen samples.” John Hayes and his students were
pioneering continuous flow methods that worked directly with gas
chromatography-IRMS (Matthews and Hayes, 1978). At the same time, both of the
major IRMS companies, Finnigan MAT and VG Instruments, were perfecting versions
of an interface that allowed compounds in a stream of He to enter the IRMS
source at pressures that could be handled by the source turbo pump. Computer
software was also being drafted to handle ion counts from distinct peaks rather
than static measurements from dual inlet systems. Finnigan’s team included John
Hayes (Indiana University), Martin Schoell (Chevron), Kate Freeman (Indiana
University graduate student), Willi Brand (Finnigan), and Bob Dias (Chevron).
They worked with software engineer Margaret Ricci (Penn State) to produce
Finnigan’s first commercial model of the gas chromatograph-combustion system
(GC-C-IRMS). At VG, Jeanette Jumeau, Steve Macko (University of Virginia), and
Michael Engel (University of Oklahoma) developed a parallel instrument.
Competition for customers was fierce!
I received NSF funding for one of these
instruments in 1990, and Tom Hoering and I traveled to see both companies’
products. In the VG factory in Manchester, England, the PRISM IRMS system in
their demo lab was not operational. We spent two days looking at the GC-C set
up, but were unable to determine how it worked. We took a second trip to John
Hayes’ lab where the Finnigan instrument was demonstrated by Kate Freeman, who
has now been elected to the National Academy of Science. Kate started her earth
science career as an undergraduate intern at the Geophysical Laboratory,
washing glassware for me and working on some organic extractions for Tom, so we
knew her well. The 252 IRMS was a real winner, and the backing of people that
we knew well, Freeman and Hayes, sealed the deal for us. Kate recalls, “I
remember showing the instrument to Tom. It was an exciting moment for me (and a
bit terrifying). Tom walked into the room and immediately disappeared. We were startled but quickly found him
crouching behind the instrument to see the pumps and guts of the thing. “
The first system I received was
relatively primitive compared to the GC- Isolink systems used today. There were
four needle valves for controlling GC flow that needed to be fiddled with
daily, if not before every sample. Combustion reactors were expensive, $1000 in
1992, and broke easily when they were installed. Jeff Silfer (Michael Engel’s
Ph.D. student) joined the Geophysical Laboratory for a short time as a postdoc
and helped get our system running. It worked slightly better than the old
liquid chromatography methods, and we could measure hydrocarbons, fatty acids,
and other molecules that liquid chromatography could not easily separate. Much
of our work at that time was discovery-based and unpublished. Our questions
were very simple: what is the variation in the carbon isotope of certain
compounds in a plant? How do the carbon isotope values compare between species?
Silfer brought his amino acid methods (Silfer et al., 1991) to the Geophysical
Laboratory and we were soon off and running on CSIA of amino acids, methods I
still use today. Upwards of 40% of presentations at the 2018 Isotop Ecology
(IsoEcol) meeting included amino acid isotope measurements.
Diane O’Brien, a postdoc from Stanford
who came to the Geophysical Laboratory for long stretches of time, furthered
CSIA-AA with a series of papers on dietary resources for butterflies (2002,
2003, 2003, 2005). During the caterpillar life stage of butterflies and moths,
plants are their only source of nutrition for these insects. Once they hatch
into butterflies and moths, the adults only consume sugary nectar that contains
carbon, but no nitrogen. While the adults can make all of their nonessential
amino acids from nectar, they must rely on their essential amino acids from the
plants consumed during the caterpillar life stage. Based on controlled feeding
experiments, O’Brien confirmed that this was in fact the case, demonstrating
that sugars were turned into nonessential amino acids, while essential amino
acids were routed through the butterflies’ various life stages.
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