United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S4-86/006 Aor. 1987
Project Summary
A Gravimetric Technique for the
Preparation of Accurate Trace
Organic Gas Standards
G. C. Rhoderick, W. F. Cuthrell, and W. L. Zielinski
An accurate procedure based on
microgravimetry has been used for the
preparation of volatile, hazardous or-
ganic chemicals in a nitrogen matrix in
pressurized gas cylinders at analyte
concentrations ranging from 10 ppb to
10 ppm, by mole. In this technique, the
organics of interest are individually
weighed into separate glass capillary
tubes using a microanalytical balance.
The tubes are sealed, and subsequently
broken in a fixed line connected to an
evacuated cylinder. A known weight of
pre-analyzed matrix gas (nitrogen) is
then used to pressurize the cylinder,
and the concentrations of the organics
are calculated on a molar basis relative
to the number of moles of the matrix
gas.
A number of these gravimetric primary
mixtures have been prepared and
analytically intercompared using gas
chromatography (GC) with flame ioniza-
tion detection (FID). Excellent agree-
ment has been found between analyte
concentration values prepared gravi-
metrically and concentration values
determined by analysis.
This paper will focus on a description
of the microgravimetric technique and
the analytical system, the estimation of
specific uncertainties associated with
the preparation of these mixtures, and
how these uncertainties are used to
assign a net uncertainty to the final
analyte concentration. Particular atten-
tion is given to mixtures at the 10 to
150 ppb level. A brief description of
how the overall network of gravimetric
primary standards provides long term,
consistent data quality for trace organic
gas mixtures is included.
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 In-
formation at back).
Discussion
Compressed gas standards of multi-
component volatile toxic organic com-
pounds in the low part-per-billion (PPB)
range are prepared by a special grayi-
metric procedure developed at the Na-
tional Bureau of Standards using extreme
caution. By this procedure, the targeted
concentrations are obtained by quantita-
tively mixing known weights of each of
the organics with a known weight of pure
nitrogen matrix gas in a new precleaned
aluminum cylinder, with the resulting
concentrations of the organics expressed
in nanomoles/mole (ppb) of the total
mixture.
A brief description of the procedure
follows: A glass capillary tube (approxi-
mately 20 mm long by 1.6 mm OD), for
which one end had been sealed and the
other had been drawn to a fine open tip,
is weighed empty. The desired organic is
drawn into the tube by air displacement,
following which the tube is centrifuged
to force the organic liquid to the sealed
end of the tube. The tube is then sealed
and weighed to obtain the weight of the
organic. The tube is then inserted into a
teflon sleeve attached to a tared, evalu-
ated aluminum cylinder. The valve of the
cylinder is slightly opened and the cap-
iliary tube is cracked open inside the
teflon sleeve and the organic is trans-
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ferred into the cylinder with mild heating
from a hot-air gun. This process is re-
peated for each organic added to the
desired mixture, following which the
cylinder is pressurized with dry, clean
nitrogen and weighed.
A series of six standard five-component
organic mixtures were prepared in this
manner at concentrations ranging from
about 5-150 ppb by mole to ascertain the
linearity of the overall concentration
range and the feasibility of preparing and
analyzing such mixtures. The five compo-
nents are identified in Table 1. The
standards were analyzed by gas chro-
matography using a flame-ionization
detector (FID), with the detector response
plotted vs. gravimetric concentration in a
linear regression fit. The concentrations
predicted from the linear regression plot
agreed well with the gravimetric con-
centrations, indicating the presence of
negligible random errors in the prepara-
tion of these standards. The mean of the
differences for any one fit (e.g., benzene)
(Table 1) represents the imprecision of
the gravimetric preparation for that
organic.
The total uncertainty of the concentra-
tion of any of the organics at the 95%
confidence level is determined by two
times the square root of the sums of the
squares of the imprecision of analysis
(obtained by replicate analyses) and the
imprecision of gravimetric preparation.
While the total uncertainties of the con-
centrations of the organics is quite ac-
ceptable for these low concentrations,
the total uncertainties of the concentra-
tions for the halogenated organics pre-
sent in these mixtures could be further
reduced by the use of an electron capture
detector (rather than an FID) due to the
markedly greater signal obtainable for
halogonated organics with this detector.
To illustrate: the total uncertainty at 95%
confidence for chloroform when analyzed
by FID was 10%, but was reduced to 2%
using an electron capture detector.
Gravimetrically prepared standards en-
compassing more than 30 volatile organic
compounds (all designated by EPA) have
been prepared under this program, repre-
senting a systematic network of over-
lapping concentrations ranging from the
low part-per-million level to the low-ppb
level. Such standards have typically
shown excellent long-term stability at
these low concentrations over several
years of reanalyses. Stable concentrations
of some of these organics have been
gravimetrically prepared to as low as one
ppb and NBS-traceable standards have
been provided to EPA that contain as
many as nine organic compounds in the
same mixture. Work currently is under-
way to prepare a mixture at the 10 ppb
level containing in excess of 15 volatile
organic compounds. The standard output
of this program currently serves to provide
the basis for data quality assurance and
traceability of national ambient air and
hazardous waste incineration monitoring
efforts.
Table 1. Comparison of Gravimetric and Analyzed Concentrations
Component
Benzene
Vinyl chloride
Chloroform
Carbon tetra-
chloride
Tetrachloro-
ethylene
Cylinder
AAL-11133
CAL-7493
CAL-8746
AAL-12029
AAL-7001
AAL-7009
AAL-11133
CAL-7493
CAL-8746
AAL-12029
AAL-7001
AAL-7O09
AAL-11133
CAL-8746
AAL-7001
CAL-7493
AAL-12029
AAL-7009
AAL-11133
CAL-7493
CAL-8746
AAL-12029
AAL-7O01
AAL-7009
AAL-11133
CAL-7493
AAL-12029
AAL-7001
CAL-8746
AAL-7O09
Gravimetric
Cone., ppb
99.9
46.8
38.2
37.0
15.1
7.6
152
86.0
b
24.5
20.6
5.54
101
27.3
19.9
19.8
16.1
4.68
107
35.1
25.9
23.5
16.2
6.76
132
27.6
13.6
13.3
10.3
3.71
Analyzed
Cone., ppb
99.9
46.7
39.0
36.3
...'
7.56
152
85.1
23.0
24.8
20.7
5.54
101
28.2
19.9
20.5
15.8
4.59
107
32.5
..."
23.6
a
6.68
132
27.7
13.7
12.2
11.2
3.57
Percent
Difference'
0
-0.2
+2.1
-1.9
...
-0.5
x = 0.9
O
-1.0
...
+1.2
+0.5
O
x = 0.5
O
+3.3
O
+3.5
-1.9
-1.9
x = 1.8
O
-7.4
...
+O.4
...
-1.2
x = 2.2
0
+0.4
+0.7
-8.3
+8.7
-3.8
x= 3.6
" Particular organic showed interference by a trace impurity in the chromatogram.
b Analysis gave a reliable, consistent value which was different from the gravimetric value.
c Percent difference calculated from: [Analyzed cone. - gravimetric cone.] x 1OO, divided b]
gravimetric cone.; the mean percent differences (x) represent the imprecision of f/J«
concentration.
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G. C. Rhoderick. W. F. Cuthrell, and W. L Zielinski are with the National Bureau
of Standards. Gaithersburg, MD 20899.
Howard Crist is the EPA Project Officer (see below).
The complete report, entitled "A Gravimetric Technique for the Preparation
of Accurate Trace Organic Gas Standards," (Order No. PB 87-145 736/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/S4-86/006
0000329 PS
4GENCr
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