ASTM E406-81(2003)
(Practice)Standard Practice for Using Controlled Atmospheres in Spectrochemical Analysis
Standard Practice for Using Controlled Atmospheres in Spectrochemical Analysis
SIGNIFICANCE AND USE
An increasing number of optical emission spectrometers are equipped with enclosed excitation stands and plasmas which call for atmospheres other than ambient air. This practice is intended for users of such equipment.
SCOPE
1.1 This practice covers 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 concerns, 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.
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Designation:E406–81 (Reapproved 2003)
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 X.1 Gas Handling—Store and introduce the gas in accor-
dance with Practice E 406.
1.1 This practice covers general recommendations relative
to the use of gas shielding during and immediately prior to
6. Concepts of Excitation Shielding
specimenexcitationinopticalemissionspectrochemicalanaly-
6.1 Control of Excitation Reactions:
sis. It describes the concept of excitation shielding, the means
6.1.1 Nonequilibriumreactionsinvolvingvariableoxidation
of introducing gases, and the variables involved with handling
rates and temperature gradients in the analytical gap produce
gases.
spurious analytical results. The use of artificial gas mixtures
1.2 This standard does not purport to address all of the
can provide more positive control of excitation reactions than
safety concerns, if any, associated with its use. It is the
is possible in air, although air alone is advantageous in some
responsibility of the user of this standard to establish appro-
instances.
priate safety and health practices and determine the applica-
6.1.2 Methods of introducing the gas require special con-
bility of regulatory limitations prior to use.
sideration. Temperature gradients in both the specimen and the
2. Referenced Documents excitationcolumncanbecontrolledbythecoolingeffectofthe
gas flow.Also, current density can be increased by constricting
2.1 ASTM Standards:
the excitation column with a flow of gas.
E 135 Terminology Relating to Analytical Chemistry for
6.1.3 Control of oxidation reactions is possible by employ-
Metals, Ores, and Related Materials
ing nonreactive or reducing atmospheres. For example, argon
E 416 Practice for Planning and Safe Operation of a Spec-
can be used to preclude oxidation reactions during excitation.
trochemical Laboratory
A gas may be selected for a particular reaction, such as
3. Terminology
nitrogentoproducecyanogenbandsasameasureofthecarbon
content of a specimen. Oxygen is used in some instances to
3.1 For definitions of terms used in this practice, refer to
ensure complete oxidation or specimen consumption. In point-
Terminology E 135.
to-planesparkanalysis,areducingatmospherecanbeprovided
4. Significance and Use
by the use of carbon or graphite counter electrodes in combi-
nation with an inert gas or by the use of special circuit
4.1 An increasing number of optical emission spectrometers
parameters in ambient air.
are equipped with enclosed excitation stands and plasmas
6.2 Effects of Controlled Atmospheres:
which call for atmospheres other than ambient air. This
6.2.1 Numerous analytical advantages can be realized with
practice is intended for users of such equipment.
controlled atmospheres:
5. Reference to this Practice in ASTM Standards
6.2.1.1 The elimination of oxidation during point-to-plane
spark excitation can significantly reduce the so-called “matrix”
5.1 The inclusion of the following paragraph, or suitable
effects and compositional differences. This can result in im-
equivalent, in any ASTM spectrochemical method, preferably
proved precision and accuracy.
in the section on excitation, shall constitute due notification
6.2.1.2 The use of argon or nitrogen atmospheres in point-
that this practice shall be followed:
to-plane procedures can increase instrument response so that a
wide range of concentrations can be covered with one set of
This practice is under the jurisdiction of ASTM Committee E01 on Analytical
Chemistry for Metals, Ores and Related Materials and is the direct responsibility of
Subcommittee E01.20 on Fundamental Practices.
Current edition approved June 10, 2003. Published July 2003. Originally Schreiber, T. P., and Majkowaki, R. F., “Effect of Oxygen on Spark Excitation
approved in 1970. Last previous edition approved in 1996 as E 406 – 81(1996). 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.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E406–81 (2003)
excitation parameters, but because of the increased back- electrode, analytical gap, and excited area of the specimen.
ground, small losses in the detection limit can result from Several variations of such a device are commercially avail-
oscillatory high voltage spark excitation. Which effect occurs able.
depends on wavelengths used. 7.3.3 Optical and excitation shielding is necessary with
6.2.1.3 Various forms of the Stallwood jet are used in d-c vacuum emission instruments for spectra below 2000 Å.Air is
arc procedures. One gas or a mixture of gases can be used with opaque to radiation in this region and must be replaced, for
this device depending on the particular analytical problem. example, by argon, to permit transmission of these wave-
Mixtures of 70 % argon and 30 % oxygen, or 80 % argon and lengths. Commercial vacuum spectrometers are equipped with
20 % oxygen are routinely used to eliminate cyanogen bands, gas-shielded excitation stands. In these instruments, a flat
reduce background intensity, and promote more favorable specimen often is used to seal the excitation chamber. Other
volatilization. Certain gases enhance intensity at various wave- shapes can be accommodated if a special holder is constructed
lengths. The precision and accuracy achieved for most ele- to also seal the chamber. Such holders are commercially
ments with d-c arc procedures employing controlled atmo- available.
spheres are significantly better than when ambient air is used.
8. Variables Concerned with Gas Handling
Such improvement is of particular value in trace analysis.
6.2.1.4 Self-absorption of analytical lines can be reduced by
8.1 Gas Purity—Gases used in excitation shielding must be
employing a suitable gas flow around or across the excitation
of consistent purity. While total impurities as high as 50 ppm
column; the flow of gas sweeps away the cooler clouds of
may not affect analytical results when nitrogen is used, most
excited vapor which cause the self-absorption. In argon, the supplierscanfurnishinertgaseswithtotalimpuritylevelsof30
diffusion of ions out of the excitation column is comparatively
ppm or less.
slow, and this also decreases self-absorption. 8.1.1 Gases that have been packaged by means of water or
oil-lubricated compressors are to be avoided because of pos-
7. Means of Introducing Atmospheres
sible contamination by moisture, organic species, or both.
7.1 Design Considerations—Design of a device for excita- Industry practice is to produce and store the major inert gases,
for example, argon and nitrogen, in liquid form. In general, the
tion shielding involves the following: (1) degree of shielding
needed, (2) type of excitation to be employed, (3) speed of terms “water pumped” and “oil pumped” are only classifica-
tions and do not relate to the types of compressor lubrication.
specimen handling, (4) constructional simplicity, and (5) cost.
7.2 The purpose of the shield dictates its complexity; a The major inert gases are usually packaged directly from the
liquid phase through impeller pumps and head exchangers.
totallyenclosedsystemwouldbesuperfluouswhenasimplejet
would suffice. The excitation employed dictates the choice of However, helium is not liquefied and is packaged under
materials.With spark excitation, a plastic shield can frequently pressure immediately after purification. Additional pressure, if
be used, but a more refractory material, such as alumina or needed, is furnished by nonlubricated diaphragm pumps. Some
heat-resistant glass, is usually necessary when employing an small producers using gaseous liquefaction plants still employ
arc. Speed and ease of specimen handling are important design oil or water compressors for packaging under pressure. There-
considerations for routine operation. Construction should be fore, conditions of manufacture and purity must be evaluated
simple, employing easily obtainable materials and as few parts locally in light of the laboratory requirements.
as possible. Provision should be made for conveniently clean- 8.1.2 Those instruments with enclosed gas-shielded excita-
tion stands usually employ a pointed counter electrode of
ing the interior.
7.3 EnclosedChambersandOtherDevices—Themethodof thoriated tungsten, copper, silver, or other metal. Be
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