United States
Environmental Protection
Agency
Environmental Monitoring Systems
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S4-85/062 Jan. 1986
ŁEPA Project Summary
Direct Calibration of GC/MS
Systems Using SRM
Gas Cylinders
G. D. Byrd, K. L. Richie, L. T. Sniegoski, and E. White V
A cryogenic trapping system has been
developed for use in calibrating GC/MS
systems for the analysis of volatile
organic compounds. This system pro-
vides for direct Standard Reference
Material (SRM) traceability on data
generated on gaseous samples. The
cryogenic trap is a coil of stainless steel
tubing immersed in a cryogen to trap
and preconcentrate organic species
present in a gaseous sample. The trap
also contains a heated injection port for
the addition of isotopically labeled com-
pounds for use in isotope dilution
measurements. The first part of this
research is concerned with the develop-
ment of isotope dilution mass spectrom-
etry (IDMS) as an independent method
for the quantification of analytes in
gaseous samples to be used as stand-
ards. Results are presented for the
determination of bromobenzene in nitro-
gen at nominal concentrations of 1 and
25 ppb. In the second part of the re-
search, a calibration curve method was
developed for using these standards in
auditing the performance of GC/MS
systems. This method is demonstrated
on multicomponent aromatic mixtures
in the 10 to 200 ppb concentration
range. The system using the calibration
curve method was evaluated at an
independent laboratory and compared
with a static dilution bottle calibration
curve method.
This Project Summary was developed
by EPA's Environmental Monitoring
Systems Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Summary
The goal of this project was to develop a
trapping system that can be used to
introduce volatile organic compounds
(VOCs) at low concentrations (ppb) from a
gas cylinder into a combined gas chromato-
graph/mass spectrometer (GC/MS) in-
strument. A trap has been constructed
that condenses the gaseous sample from
the cylinder using a small coil immersed
in a dry ice-ethanol bath. The coil is then
electrically heated to revolatize the sam-
ple. The gas is introduced into the GC/MS
with a syringe needle that is silver
soldered to the coil tubing so that the
effluent can be injected directly into the
instrument. A heated injection port also
allows introduction of liquid solutions
into the GC/MS. The performance of the
trap was evaluated using isotope dilution
mass spectrometer (IDMS) measure-
ments on samples that had been well
characterized by other methods. Re-
sponse factor determinations were made
using isotopically labeled analytes.
A GC/MS method based on establish-
ing a calibration curve was developed
that permits a direct comparison of the
concentrations of trace components from
two gas sources. This provided a means
of linking data from different laboratories
to a common SRM. The cryogenic trap
was used for this method, since it can be
employed with any GC system that con-
tains a septum inlet. The principle of the
calibration curve is to establish a plot of
response versus amount for a particular
analyte. From a measured volume of a
sample with an unknown concentration
of the analyte and its response, the
concentration can be calculated. SRV
gas cylinders containing the analytes of
interest are well suited as calibrant
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samples because of their certified values
for analytes. In order for this method to
work properly, a known relationship, pref-
erably linear, must be established over
the desired concentration range. This, in
turn, requires that reproducible amounts
of the sample be trapped and delivered to
the GC/MS for analysis. The procedure
was validated by measurements at an
independent laboratory on two cylinders
containing "unknown" concentrations of
benzene, chlorobenzene, and bromoben-
zene. A cylinder containing known con-
centrations of the same compounds was
used for calibration. All of the cylinders
had been previously prepared and ana-
lyzed at the National Bureau of Standards
(NBS) using the trapping system. The
combined average values for all the
measurements made on these cylinders
at both laboratories were within two
standard deviations of the preoared value
for all analytes except bromobenzene in
one cylinder. This value was slightly more
than three standard deviations outside of
the prepared concentration.
The components of the gas cylinders
were also analyzed by GC/MS using a
calibration curve generated by injections
of pure gaseous compounds from static
dilution flasks. This was the usual method
of calibration of the independent labor-
atory and afforded a way to compare the
results of the two ways to calibrate the
analytical system. The differences in the
relative standard deviations for the two
sets of measurements were not signif-
icant, indicating similar precision for both
laboratories. The average percent differ-
ence between delivered and measured
amounts-for the analytes was biased on
all cases with 11% to 35% more of the
compounds detected than was delivered.
G. D. Bryd, K. L. Richie. L T. Sniegoski. andE. White are with the National Bureau
of Standards, Gaithersburg, MD 20899.
Howard Crist is the EPA Project Officer (see below).
The complete report, entitled "Direct Calibration of GC/MS Systems Using SRM
Gas Cylinders."(Order No. PB86-110 715/AS; Cost: $9.95. subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S4-85/062
0000329 PS
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