ASTM E1747-95

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
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Designation: E 1747 – 95
Standard Guide for
Purity of Carbon Dioxide Used in Supercritical Fluid
Applications
This standard is issued under the fixed designation E 1747; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
The rapid commercial development of carbon dioxide for use in supercritical fluid extraction (SFE)
and supercritical fluid chromatography (SFC) has hastened the need to establish common purity
standards to be specified by specialty gas suppliers. As a consequence of its isolation from
petrochemical side-streams or as a by-product of fermentation or ammonia synthesis, carbon dioxide
contains a wide range of impurities that can interfere with analytical quantification or instrument
operation. This guide is intended to serve as a guide to specialty gas suppliers for testing the suitability
of carbon dioxide for use in SFC and SFE applications.
1. Scope D 2504 Test Method for Noncondensable Gases in C and
Lighter Hydrocarbon Products by Gas Chromatography
1.1 This guide defines purity standards for carbon dioxide to
D 2820 Test Method for C Through C Hydrocarbons in the
ensure the suitability of liquefied carbon dioxide gas for use in
Atmosphere By Gas Chromatography
SFE and SFC applications (see Guide E 1449 for definitions of
D 3670 Guide for Determination of Precision and Bias of
terms). This guide defines quantitation, labeling, and statistical
Methods of Committee D-22
standards for impurities in carbon dioxide that are necessary
D 3686 Practice for Sampling Atmospheres to Collect Or-
for successful SFE or SFC laboratory work, and it suggests
ganic Compound Vapors (Activated Charcoal Tube Ad-
methods of analysis for quantifying these impurities.
sorption Method)
1.2 This guide is provided for use by specialty gas suppliers
D 3687 Practice for Analysis of Organic Compound Vapors
who manufacture carbon dioxide specifically for SFE or SFC
Collected by the Activated Charcoal Tube Adsorption
applications. SFE or SFC CO products offered with a claim of
Methods
adherence to this guide will meet certain absolute purity and
D 4178 Practice for Calibrating Moisture Analyzers
contaminant detectability requirements matched to the needs of
D 4532 Test Method for Respirable Dust in Workplace
current SFE or SFC techniques. The use of this guide allows
Atmosphere
different SFE or SFC CO product offerings to be compared on
E 260 Practice for Packed Column Gas Chromatography
an equal purity basis.
E 355 Practice for Gas Chromatography Terms and Rela-
1.3 This guide considers contaminants to be those compo-
tionships
nents that either cause detector signals that interfere with those
E 594 Practice for Testing Flame Ionization Detectors Used
of the target analytes or physically impede the SFE or SFC
in Gas Chromatography
experiment.
E 697 Practice for Use of Electron-Capture Detectors in
1.4 The values stated in SI units are to be regarded as the
Gas Chromatography
standard.
E 1449 Guide for Supercritical Fluid Chromatography
1.5 This standard does not purport to address all of the
Terms and Relationships
safety concerns, if any, associated with its use. It is the
E 1510 Practice for Installing Fused Silica Open Tubular
responsibility of the user of this standard to establish appro-
Capillary Columns in Gas Chromatographs
priate safety and health practices and determine the applica-
2.2 CGA Publications:
bility of regulatory limitations prior to use.
2. Referenced Documents
Annual Book of ASTM Standards, Vol 05.01.
2.1 ASTM Standards:
Discontinued—See 1992 Annual Book of ASTM Standards, Vol 05.01.
Annual Book of ASTM Standards, Vol 11.03.
1 5
This guide is under the jurisdiction of ASTM Committee E13 on Molecular Annual Book of ASTM Standards, Vol 05.02.
Spectroscopy and is the direct responsibility of Subcommittee E13.19 on Chroma- Annual Book of ASTM Standards, Vol 14.02.
tography. Available from Compressed Gas Association, Inc., 1725 Jefferson Davis
Current edition approved Sept. 10, 1995. Published December, 1995. Highway, Arlington, VA 22202-4100.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 1747
CGA P-1 Safe Handling of Compressed Gases in Contain- 4. Purity Specifications for SFE or SFC Grade CO
ers
4.1 This guide proposes the following minimum purity
CGA V-7 Standard for Hydrogen Piping Systems at Con-
specifications for CO for each of the classes of contaminants,
sumer Locations
based on the demands of currently practiced SFE or SFC
CGA P-9 The Inert Gases: Argon, Nitrogen and Helium
techniques.
CGA V-7 Standard Method of Determining Cylinder Valve
4.1.1 Liquid-Phase Contaminants Specification:
Outlets Connections for Industrial Gas Mixtures
4.1.1.1 SFE grade carbon dioxide is intended to be used as
CGA P12 Safe Handling of Cryogenic Liquids
an extraction solvent from which a significant concentration of
G6 Carbon Dioxide
self-contained contaminates is possible because relatively large
HB-3 Handbook of Compressed Gases
(>50 g) amounts of carbon dioxide may be used. Because each
impurity cannot be identified, a known amount of internal
3. Classification
reference compounds (for example, HD and HCB) will be used
3.1 This guide covers the following four different classes of
during the analysis to quantify contaminants on a relative
compounds:
weight basis. Total contaminant levels will be expressed in ng
3.1.1 Liquid-Phase Contaminants—These are materials dis-
of contaminant per g of CO and defined as that amount of
solved in the CO liquid phase that can be volatilized below
impurity that will produce a detector signal at the “typical”
300°C and resolved chromatographically using a gas chroma-
detection limits for an FID or ECD found in 1.0 g of CO . The
tography (CG) column; and detected by either a flame ioniza-
1-g amount of carbon dioxide was selected as a convenient
tion (FI) or electron capture (EC) detector (D). Species
mass from which the chemist could relate carbon dioxide
representative of this class include moderate (100 to 600)
contamination levels with the amount of carbon dioxide
molecular weight hydrocarbons and halocarbons (oils and
required for his/her analysis by a simple ratio.
lubricants).
4.1.1.2 SFC grade carbon dioxide is intended to be used as
NOTE 1—Liquid-phase contaminant levels are defined in terms of the
a mobile phase material transferred directly from a chromato-
lowest limit of detector response (LLDR) for FIDs or ECDs only, because
graphic column to a detector (FID or ECD) without pre-
they are the primary detectors used with SFE or SFC techniques.
concentration (see Practice E 355). Accepted internal reference
However, the purification procedures used by the gas supplier to remove
compounds (for example, HD and HCB) will be used as
FID- and ECD-responsive contaminants are assumed to be effective for
surrogate contaminants. Contaminant levels will be expressed
contaminants responsive to other (for example, NPD, MS, IR, UV, etc.)
detectors. in ng of contaminant per g of CO and will be defined as that
Because a wide variety of contaminants are found in liquid-phase CO
amount which will produce a detector signal 20 times greater
as a consequence of its source, full speculation of every impurity by the
than the “typical” detection limit for FID and 25 times greater
gas supplier is impractical. All liquid-phase contaminants are therefore
than an ECD at the lowest detectable limit for a single peak. A
quantified relative to two representative internal primary reference stan-
total of 200 times the lowest detectable limit will be set for all
dards: hexadecane (HD or C H ) for the FID and hexachlorobenzene
16 34
contaminants for a specific detector.
(HCB or C Cl ) for the ECD. Contaminant limits are defined on a mass
6 6
basis for single peaks and for the sum of all detector responses. 4.1.1.3 When specifying a FID response for SFE, the
maximum amount of any one contaminant (that is, one peak in
3.1.2 Moisture—Although water is sparingly (<0.1 %
the chromatogram) will be 1 ng/g of liquid-phase CO . This is
weight) soluble in liquid-phase CO , more than 10 ppm of
equivalent to 1 ppb on a mass basis, or 1 ppb w/w. The
moisture may result in physical interference resulting from ice
maximum amount of all FID-responsive contaminants (that is,
formation during SFC or SFE applications. A maximum limit
the sum of all peaks in the chromatogram) will be 10 ng/g of
of 1 ppm of water in the carbon dioxide will be considered
liquid-phase CO or 10 ppb w/w. Contaminant concentrations
acceptable.
are expressed in terms of the equivalent response for hexade-
3.1.3 Gas-Phase Contaminants—Gaseous, noncondensible
cane, the internal standard, regardless of the actual identity of
molecules released upon vaporization of liquid CO may act as
the contaminant.
interferences during SFC applications; this is less of a problem
4.1.1.4 When specifying an FID response for SFC, the
in SFE applications. Species representative of this class include
generally accepted LLDR for a FID is 0.25 6 0.1 ng for a
oxygen and light hydrocarbons, such as methane, ethane, and
single component with a signal-to-noise ratio of 3:1. Therefore,
propane. A combined maximum concentration in the gas phase
“20” 3 0.25 ng = 5 ng to the detector (one peak), and
of 10 ppm will be considered acceptable.
“200” 3 0.25 ng = 50 ng total detector response. If all 5 ng of
3.1.4 Nonvolatile—Materials that leave a nonvolatile (boil-
the contaminant comes from1gof liquid-phase carbon
ing point >250°C) residue following the vaporization of liquid
dioxide, the single component impurity level would be 50 ppb.
CO , such as small particles and high-boiling solutes, are
This assumes that1gof carbon dioxide arrives at the detector
detrimental to both SFE and SFC applications. Species repre-
at one time, and the density of the CO is 1 g/mL. Under
sentative of this class include nonchromatographicable hydro- 2
typical SFC conditions of ;400 atm and 75°C, less than 0.1 g
carbons or halocarbon oils, greases, and inorganic particles (for
of CO actually reaches the FID when using a 0.25 mm inside
example, silica). A maximum concentration of 1 ppm will be 2
diameter column with a 15-s wide peak. Therefore, the
considered acceptable.
contamination level acceptable for SFC applications would be
less than 16 ppb on an absolute basis for a single peak (see
Poole, C. F., and Poole, S. K., Chromatography Today, Elsevier, 1991, p. 86. Practice E 594).
E 1747
TABLE 1 Proposed Minimum Specifications for SFE and SFC
4.1.1.5 ECD Detector—For SFE, the maximum amount of
CO
any one contaminant (that is, one peak in the chromatogram)
Maximum Single Total
will be 0.2 ng/g of liquid-phase CO . This is equivalent to 0.2
2 Contaminant
Concentration Concentration
ppb w/w, or 200 ppt w/w, on a mass basis. The maximum
Liquid-phase (SFE)
amount of all ECD-responsive contaminants (that is, the sum
FID responsive 1 ppb w/w 10 ppb w/w
of all peaks in the chromatogram) will be 2 ng/g of liquid-
ECD responsive 0.2 ppb w/w 2 ppb w/w
phase CO or 2 ppb w/w. Contaminant concentrations are Liquid-phase (SFC)
FID responsive 5 ppb w/w 50 ppb w/w
expressed in terms of the equivalent response for hexachlo-
ECD responsive 1 ppb w/w 10 ppb w/w
robenzene, the internal standard, regardless of the actual
Moisture . 1 ppm m/m
Gas phase
identity of the contaminant (see Practice E 697).
Oxygen . 5 ppm m/m
4.1.1.6 For SFC applications, the ECD is >5 times more
THC . 5 ppm m/m
sensitive than the FID, assuming two halogen atoms per Nonvolatile . 1 ppm w/w
molecule. Therefore, the total concentration of a single ECD
impurity is proposed to be 1 ng/g of CO or 1 ppb. The total
5. Gas Handling and Safety
amount of ECD impurities considered acceptable is 10 ng/g of
5.1 The safe handling of compressed gases and cryogenic
CO or 10 ppb.
liquids for use in chromatography is the responsibility of every
4.1.2 Higher-Purity Materials—The specifications and
laboratory. The Compressed Gas Association, Inc. (CGA), a
methodology proposed in this guide can be used to certify CO
member group of specialty and bulk gas suppliers, publishes
materials with higher-purity specifications. To certify such
the following guidelines to assist the laboratory chemist in
materials, gas suppliers must vary (increase) the quantity of
establishing a safe work environment: CGA P-1, CGA V-7,
CO collected and adjust the quantity of internal standard used
CGA P-9, CGA V-7, CGA P12, G6, and HB-3.
for calibration. Contaminant concentrations are expressed in
terms of the equivalent responses for the internal standards
6. Representative Analysis Method for Liquid-Phase
recommended above and reported on a mass basis relative to
Contaminants
the mass of CO collected. The applicable detector must be
6.1 Contaminants dissolved in the liquid phase of CO are
specified.
the most critical to the success of an SFE or SFC experiment.
4.1.2.1 Minimum-purity CO contains a total of 10 ng of
The literature provides a wide variety of analytical methods for
FID-responsive contaminants per g of CO (10 ppb w/w), with
detecting liquid-phase trace contaminants, any of which can be
no single FID-responsive contaminant greater than 1 ng/g (1
used by gas suppliers as long as the method can achieve the
ppb w/w). Higher-specification CO , for example, may contain
detectability and statistical requirements recommended in this
a total of 1 ppb w/w of FID-responsive contaminants, with no
guide.
single contaminant greater than 0.1 ppb w/w.
6.2 Adsorbent Concentration Method— Outlined below is a
4.1.2.2 Gas suppliers are free to manufacture materials with
representative method for liquid-phase contaminants, referred
purity specifications as stringent as they choose. SFC and SFE
to as the adsorbent concentration method.
practitioners may use the purity reporting standards defined
6.2.1 The method is included to develop the quantitation
here as a basis for needs assessment and product comparison.
and statistical calculations discussed in Section 8; however,
No “grading” nomenclature is recommended in this guide.
this guide does not mandate its use.
4.1.3 Moisture Specification—The maximum amount of
6.2.2 Apparatus:
moisture acceptable in the carbon dioxide is 1 ppm (mole or 6.2.2.1 Gas Chromatograph—The procedure requires a gas
volume basis).
chromatograph equipped with both an FID and an ECD. The
LLDR for the FID must be
...