ASTM E406-81(1996)
(Practice)Standard Practice for Using Controlled Atmospheres in Spectrochemical Analysis
Standard Practice for Using Controlled Atmospheres in Spectrochemical Analysis
SCOPE
1.1 This practice provides general recommendations relative to the use of gas shielding during and immediately prior to specimen excitation in optical emission spectrochemical analysis. It describes the concept of excitation shielding, the means of introducing gases, and the variables involved with handling gases.
1.2 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific precautionary statements are given in Section 9.
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Standards Content (Sample)
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Designation: E 406 – 81 (Reapproved 1996)
Standard Practice for
Using Controlled Atmospheres in Spectrochemical
Analysis
This standard is issued under the fixed designation E 406; 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.
1. Scope dance with Practice E 406.
1.1 This practice provides general recommendations rela-
6. Concepts of Excitation Shielding
tive to the use of gas shielding during and immediately prior to
6.1 Control of Excitation Reactions:
specimen excitation in optical emission spectrochemical analy-
6.1.1 Nonequilibrium reactions involving variable oxidation
sis. It describes the concept of excitation shielding, the means
rates and temperature gradients in the analytical gap produce
of introducing gases, and the variables involved with handling
spurious analytical results. The use of artificial gas mixtures
gases.
can provide more positive control of excitation reactions than
1.2 This standard does not purport to address all of the
is possible in air, although air alone is advantageous in some
safety concerns, if any, associated with its use. It is the
instances.
responsibility of the user of this standard to establish appro-
6.1.2 Methods of introducing the gas require special con-
priate safety and health practices and determine the applica-
sideration. Temperature gradients in both the specimen and the
bility of regulatory limitations prior to use. Specific precau-
excitation column can be controlled by the cooling effect of the
tionary statements are given in Section 9.
gas flow. Also, current density can be increased by constricting
2. Referenced Documents the excitation column with a flow of gas.
6.1.3 Control of oxidation reactions is possible by employ-
2.1 ASTM Standards:
ing nonreactive or reducing atmospheres. For example, argon
E 135 Terminology Relating to Analytical Chemistry for
can be used to preclude oxidation reactions during excitation.
Metals, Ores, and Related Materials
A gas may be selected for a particular reaction, such as
E 416 Practice for Planning and Safe Operation of a Spec-
nitrogen to produce cyanogen bands as a measure of the carbon
trochemical Laboratory
content of a specimen. Oxygen is used in some instances to
3. Terminology
ensure complete oxidation or specimen consumption. In point-
to-plane spark analysis, a reducing atmosphere can be provided
3.1 For definitions of terms used in this practice, refer to
by the use of carbon or graphite counter electrodes in combi-
Terminology E 135.
nation with an inert gas or by the use of special circuit
4. Significance and Use
parameters in ambient air.
6.2 Effects of Controlled Atmospheres:
4.1 An increasing number of optical emission spectrometers
6.2.1 Numerous analytical advantages can be realized with
are equipped with enclosed excitation stands and plasmas
controlled atmospheres:
which call for atmospheres other than ambient air. This
6.2.1.1 The elimination of oxidation during point-to-plane
practice is intended for users of such equipment.
spark excitation can significantly reduce the so-called “matrix”
5. Reference to this Practice in ASTM Standards
effects and compositional differences. This can result in im-
proved precision and accuracy.
5.1 The inclusion of the following paragraph, or suitable
6.2.1.2 The use of argon or nitrogen atmospheres in point-
equivalent, in any ASTM spectrochemical method, preferably
to-plane procedures can increase instrument response so that a
in the section on excitation, shall constitute due notification
wide range of concentrations can be covered with one set of
that this practice shall be followed:
excitation parameters, but because of the increased back-
X.1 Gas Handling—Store and introduce the gas in accor-
ground, small losses in the detection limit can result from
oscillatory high voltage spark excitation. Which effect occurs
This practice is under the jurisdiction of ASTM Committee E-1 on Analytical
depends on wavelengths used.
Chemistry for Metals, Ores and Related Materials and is the direct responsibility of
Subcommittee E01.20 on Fundamental Practices.
Current edition approved June 26, 1981. Published October 1981. Originally Schreiber, T. P., and Majkowaki, R. F., “Effect of Oxygen on Spark Excitation
e1
published as E406 – 70. Last previous edition E406 – 70 (1986) . and Spectral Character,” Spectrochimica Acta, Vol 15, 1959, p. 991.
2 5
Annual Book of ASTM Standards, Vol 03.05. Bartel, R., and Goldblatt, A., “The Direct Reading Spectrometric Analysis of
Annual Book of ASTM Standards, Vol 03.06. Alloy Cast Iron,” Spectrochimica Acta, Vol 9, 1957, p. 227.
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NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 406
6.2.1.3 Various forms of the Stallwood jet are used in d-c opaque to radiation in this region and must be replaced, for
arc procedures. One gas or a mixture of gases can be used with example, by argon, to permit transmission of these wave-
this device depending on the particular analytical problem. lengths. Commercial vacuum spectrometers are equipped with
Mixtures of 70 % argon and 30 % oxygen, or 80 % argon and gas-shielded excitation stands. In these instruments, a flat
20 % oxygen are routinely used to eliminate cyanogen bands, specimen often is used to seal the excitation chamber. Other
reduce background intensity, and promote more favorable shapes can be accommodated if a special holder is constructed
volatilization. Certain gases enhance intensity at various wave- to also seal the chamber. Such holders are commercially
7 10
lengths. The precision and accuracy achieved for most ele- available.
ments with d-c arc procedures employing controlled atmo-
8. Variables Concerned with Gas Handling
spheres are significantly better than when ambient air is used.
Such improvement is of particular value in trace analysis.
8.1 Gas Purity—Gases used in excitation shielding must be
6.2.1.4 Self-absorption of analytical lines can be reduced by of consistent purity. While total impurities as high as 50 ppm
employing a suitable gas flow around or across the excitation
may not affect analytical results when nitrogen is used, most
column; the flow of gas sweeps away the cooler clouds of suppliers can furnish inert gases with total impurity levels of 30
excited vapor which cause the self-absorption. In argon, the
ppm or less.
diffusion of ions out of the excitation column is comparatively 8.1.1 Gases that have been packaged by means of water or
slow, and this also decreases self-absorption.
oil-lubricated compressors are to be avoided because of pos-
sible contamination by moisture or organic species or both.
7. Means of Introducing Atmospheres
Industry practice is to produce and store the major inert gases,
7.1 Design Considerations—Design of a device for excita-
for example, argon and nitrogen, in liquid form. In general, the
tion shielding involves the following: (1) degree of shielding
terms “water pumped” and “oil pumped” are only classifica-
needed, (2) type of excitation to be employed, (3) speed of
tions and do not relate to the types of compressor lubrication.
specimen handling, (4) constructional simplicity, and (5) cost.
The major inert gases are usually packaged directly from the
7.2 The purpose of the shield dictates its complexity; a
liquid phase through impeller pumps and head exchangers.
totally enclosed system would be superfluous when a simple jet
However, helium is not liquefied and is packaged under
would suffice. The excitation employed dictates the choice of
pressure immediately after purification. Additional pressure, if
materials. With spark excitation, a plastic shield can frequently
needed, is furnished by nonlubricated diaphragm pumps. Some
be used, but a more refractory material, such as alumina or
small producers using gaseous liquefaction plants still employ
heat-resistant glass, is usually necessary when employing an
oil or water compressors for packaging under pressure. There-
arc. Speed and ease of specimen handling are important design
fore, conditions of manufacture and purity must be evaluated
considerations for routine operation. Construction should be
locally in light of the laboratory requirements.
simple, employing easily obtainable materials and as few parts
8.1.2 Those instruments with enclosed gas-shielded excita-
as possible. Provision should be made for conveniently clean-
tion stands usually employ a pointed counter electrode of
ing the interior.
thoriated tungsten, copper, silver, or other metal. Because the
7.3 Enclosed Chambers and Other Devices—The method of
excitations used usually are polarized oscillating sparks where
introducing the atmosphere is determined by the intended
the current does not pass through zero, additional purification
purpose. For example, a t
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