1、Designation: E 406 81 (Reapproved 2003)Standard Practice forUsing Controlled Atmospheres in SpectrochemicalAnalysis1This standard is issued under the fixed designation E 406; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year
2、 of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers general recommendations relativeto the use of gas shielding during and immediately prior tos
3、pecimen excitation in optical emission spectrochemical analy-sis. It describes the concept of excitation shielding, the meansof introducing gases, and the variables involved with handlinggases.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. I
4、t is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:E 135 Terminology Relating to Analytical Chemistry forMetals, Ores, and Related
5、Materials2E 416 Practice for Planning and Safe Operation of a Spec-trochemical Laboratory33. Terminology3.1 For definitions of terms used in this practice, refer toTerminology E 135.4. Significance and Use4.1 An increasing number of optical emission spectrometersare equipped with enclosed excitation
6、 stands and plasmaswhich call for atmospheres other than ambient air. Thispractice is intended for users of such equipment.5. Reference to this Practice in ASTM Standards5.1 The inclusion of the following paragraph, or suitableequivalent, in any ASTM spectrochemical method, preferablyin the section
7、on excitation, shall constitute due notificationthat this practice shall be followed:X.1 Gas HandlingStore and introduce the gas in accor-dance with Practice E 406.6. Concepts of Excitation Shielding6.1 Control of Excitation Reactions:6.1.1 Nonequilibrium reactions involving variable oxidationrates
8、and temperature gradients in the analytical gap producespurious analytical results. The use of artificial gas mixturescan provide more positive control of excitation reactions thanis possible in air, although air alone is advantageous in someinstances.6.1.2 Methods of introducing the gas require spe
9、cial con-sideration. Temperature gradients in both the specimen and theexcitation column can be controlled by the cooling effect of thegas flow. Also, current density can be increased by constrictingthe excitation column with a flow of gas.6.1.3 Control of oxidation reactions is possible by employ-i
10、ng nonreactive or reducing atmospheres. For example, argoncan be used to preclude oxidation reactions during excitation.A gas may be selected for a particular reaction, such asnitrogen to produce cyanogen bands as a measure of the carboncontent of a specimen. Oxygen is used in some instances toensur
11、e complete oxidation or specimen consumption. In point-to-plane spark analysis, a reducing atmosphere can be providedby the use of carbon or graphite counter electrodes in combi-nation with an inert gas4or by the use of special circuitparameters5in ambient air.6.2 Effects of Controlled Atmospheres:6
12、.2.1 Numerous analytical advantages can be realized withcontrolled atmospheres:6.2.1.1 The elimination of oxidation during point-to-planespark excitation can significantly reduce the so-called “matrix”effects and compositional differences. This can result in im-proved precision and accuracy.6.2.1.2
13、The use of argon or nitrogen atmospheres in point-to-plane procedures can increase instrument response so that awide range of concentrations can be covered with one set of1This practice is under the jurisdiction of ASTM Committee E01 on AnalyticalChemistry for Metals, Ores and Related Materials and
14、is the direct responsibility ofSubcommittee E01.20 on Fundamental Practices.Current edition approved June 10, 2003. Published July 2003. Originallyapproved in 1970. Last previous edition approved in 1996 as E 406 81(1996).2Annual Book of ASTM Standards, Vol 03.05.3Annual Book of ASTM Standards, Vol
15、03.06.4Schreiber, T. P., and Majkowaki, R. F., “Effect of Oxygen on Spark Excitationand Spectral Character,” Spectrochimica Acta, Vol 15, 1959, p. 991.5Bartel, R., and Goldblatt, A., “The Direct Reading Spectrometric Analysis ofAlloy Cast Iron,” Spectrochimica Acta, Vol 9, 1957, p. 227.1Copyright AS
16、TM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.excitation parameters, but because of the increased back-ground, small losses in the detection limit can result fromoscillatory high voltage spark excitation. Which effect occursdepends on wavelengt
17、hs used.6.2.1.3 Various forms of the Stallwood jet6are used in d-carc procedures. One gas or a mixture of gases can be used withthis device depending on the particular analytical problem.Mixtures of 70 % argon and 30 % oxygen, or 80 % argon and20 % oxygen are routinely used to eliminate cyanogen ban
18、ds,reduce background intensity, and promote more favorablevolatilization. Certain gases enhance intensity at various wave-lengths.7The precision and accuracy achieved for most ele-ments with d-c arc procedures employing controlled atmo-spheres are significantly better than when ambient air is used.S
19、uch improvement is of particular value in trace analysis.6.2.1.4 Self-absorption of analytical lines can be reduced byemploying a suitable gas flow around or across the excitationcolumn;6the flow of gas sweeps away the cooler clouds ofexcited vapor which cause the self-absorption. In argon, thediffu
20、sion of ions out of the excitation column is comparativelyslow, and this also decreases self-absorption.7. Means of Introducing Atmospheres7.1 Design ConsiderationsDesign of a device for excita-tion shielding involves the following: (1) degree of shieldingneeded, (2) type of excitation to be employe
21、d, (3) speed ofspecimen handling, (4) constructional simplicity, and (5) cost.7.2 The purpose of the shield dictates its complexity; atotally enclosed system would be superfluous when a simple jetwould suffice. The excitation employed dictates the choice ofmaterials. With spark excitation, a plastic
22、 shield can frequentlybe used, but a more refractory material, such as alumina orheat-resistant glass, is usually necessary when employing anarc. Speed and ease of specimen handling are important designconsiderations for routine operation. Construction should besimple, employing easily obtainable ma
23、terials and as few partsas possible. Provision should be made for conveniently clean-ing the interior.7.3 Enclosed Chambers and Other DevicesThe method ofintroducing the atmosphere is determined by the intendedpurpose. For example, a totally enclosed chamber is necessaryfor excitation at all pressur
24、es other than atmospheric.87.3.1 Shielding devices for point-to-plane spark analysisrange from simple jets to more sophisticated dual flow designs.Frequently, these same devices are also suitable for use witharc excitation provided they can withstand the associated hightemperatures.87.3.2 Effective
25、shielding for point-to-plane spark analysis inconventional excitation stands can be accomplished by the useof a chamber around the counter electrode. The gas is directedinto the chamber and its outward flow envelops the counterelectrode, analytical gap, and excited area of the specimen.Several varia
26、tions of such a device are commercially avail-able.87.3.3 Optical and excitation shielding is necessary withvacuum emission instruments for spectra below 2000 . Air isopaque to radiation in this region and must be replaced, forexample, by argon, to permit transmission of these wave-lengths. Commerci
27、al vacuum spectrometers are equipped withgas-shielded excitation stands. In these instruments, a flatspecimen often is used to seal the excitation chamber. Othershapes can be accommodated if a special holder is constructedto also seal the chamber. Such holders are commerciallyavailable.98. Variables
28、 Concerned with Gas Handling8.1 Gas PurityGases used in excitation shielding must beof consistent purity. While total impurities as high as 50 ppmmay not affect analytical results when nitrogen is used, mostsuppliers can furnish inert gases with total impurity levels of 30ppm or less.8.1.1 Gases tha
29、t have been packaged by means of water oroil-lubricated compressors are to be avoided because of pos-sible contamination by moisture, organic species, or both.Industry practice is to produce and store the major inert gases,for example, argon and nitrogen, in liquid form. In general, theterms “water
30、pumped” and “oil pumped” are only classifica-tions and do not relate to the types of compressor lubrication.The major inert gases are usually packaged directly from theliquid phase through impeller pumps and head exchangers.However, helium is not liquefied and is packaged underpressure immediately a
31、fter purification. Additional pressure, ifneeded, is furnished by nonlubricated diaphragm pumps. Somesmall producers using gaseous liquefaction plants still employoil or water compressors for packaging under pressure. There-fore, conditions of manufacture and purity must be evaluatedlocally in light
32、 of the laboratory requirements.8.1.2 Those instruments with enclosed gas-shielded excita-tion stands usually employ a pointed counter electrode ofthoriated tungsten, copper, silver, or other metal. Because theexcitations used usually are polarized oscillating sparks wherethe current does not pass t
33、hrough zero, additional purificationof even the liquid argon may be necessary to achieve the propersampling. The purification can be accomplished by passing thegas through a reducing atmosphere furnace, containing tita-nium, at 427C (800F) to remove oxygen and moisture, orother purification such as
34、molecular sieves may be used. Inaddition, ample exit ports for the gas must be provided toremove debris. For each enclosed excitation stand, there existsa critical flow rate and pressure. These must be determined inorder to achieve proper sampling and excitation.NOTE 1Some specimens are inherently d
35、ifficult to excite; for ex-ample, NBS Ductile Irons Nos. 1142 and 1142a, and NBS Leaded SteelNo. 1169. Some users have found that even with apparently goodspecimens, one in fifty burns might be bad (superficial) for no obviousreason. A superficial burn produces a whitish film on the surface of thesp
36、ecimen and the intensities obtained for the analytical and internal6Stallwood, B. J., “Air-Cooled Electrodes for the Spectrochemical Analysis ofPowders,” Journal of the Optical Society of America Vol 44, No. 171, 1954.7Baker, M. R., Adelstein, S. J., and Vallee, B. L., “Physical Basis of LineEnhance
37、ment in Argon and Krypton,” Journal of the Optical Society of America,Vol46, 1956, pp. 138140.8Available from both Spex Industries, Inc., 3880 Park Ave., Edison, NJ 08820,and Angstrom, Inc., Box 248, Belleville, MI 48111.9Available from Thermo Jarrell Ash, 8 E. Forge Parkway, Franklin, MA 02038.E 40
38、6 81 (2003)2standard lines are generally inadequate. For some instruments, this mayalso result in a long integration time.8.2 Packaging and Storage:8.2.1 High-pressure gas cylinders are more common, butliquid-state containers are also used. Depending on the gas,high-pressure welding-size cylinders,
39、when full, will containfrom 15 to 18 MPa (2200 to 2650 psi). Widespread availabilityand long storage life favor the use of the high-pressurecylinders. However, they are relatively inefficient and deliveronly 5700 to 6500 L (200 to 230 ft3) of gas per cylinder so thatthe space and handling considerat
40、ions may be important.Although only slightly larger than the high-pressure cylinder, astandard 30-gal liquid container will deliver 10 to 14 times thevolume of gas. For example, 1 gal of liquid nitrogen isequivalent to 2630 L (93 ft3) at 20C and atmospheric pressure;thus, a 30-gal container will del
41、iver 2630 L 3 30 or 97 000 L(2790 ft3) of nitrogen gas.8.2.2 Liquid gas containers cannot be stored for extendedperiods because of the necessity for venting. Due to heat lossesthrough the container, equilibrium conditions are not possiblebetween the liquid and gaseous phases; consequently, thecontai
42、ner continuously exhausts excess pressure so that signifi-cant amounts of gas are lost over long periods.8.2.2.1 Installation of liquid gas cylinders so that they standon a scale to monitor their weight and where they can beconnected with purge lines to permanent copper tubing leadingto the spectrom
43、eter is desirable. One instrument manufacturerrecommends a scale capacity of 453 kg (1000 lb). The liquidcontainers should be returned with a minimum of 11 kg (25 lb)above tare weight, and the high-pressure cylinders should bereturned with a minimum of 172 kPa (25 psi).8.2.3 If high purity is necess
44、ary, liquid containers offer thegreatest degree of consistency. Such containers are recom-mended particularly when unidirectional condensed dischargesare used, because impurities in the gas suppress the “energetic-type burn.” High-voltage spark discharges using graphitecounter electrodes are not, in
45、 general, as dependent on gaspurity.8.2.3.1 It is strongly recommended that an arrangement bemade with the gas supplier whereby the user will have theexclusive use of certain tanks. Either by recording the serialnumbers or using a distinctive mark, the same tanks can alwaysbe identified and set asid
46、e for the user.8.3 Flow SystemA basic gas flow system for excitationshielding contains the following components: (1) a two-stageregulator with pressure gages, (2) a flow-metering valve, (3)aflow indicator, and (4) tubing for gas transport. The basicfunction of the system is to deliver to the shieldi
47、ng device aconstant flow of gas at a given pressure.8.3.1 Incorporating a dual-stage regulator provides pressurereduction and simultaneous monitoring of the cylinder pressureand the outlet pressure. The outlet pressure should be main-tained at a definite level since this determines the pressure atth
48、e shielding device.8.3.1.1 Dual-stage regulators are commercially available;but, care must be exercised to obtain the proper type for the gasto be used. Adapters are available that will allow the use of oneregulator with several different gases; however, this practicepresents serious safety hazards
49、and danger of cross-contamination. Some regulators introduce significant amountsof impurities through leakage and degassing. Special regulatorsare available for handling high-purity gases. Because of agingand adsorption, metal diaphragms are preferred over rubberones.8.3.2 The metering valve controls the flow of the gas andshould be precise in operation and sized to handle the intendedflow rate. Frequently, it is incorporated in the flowmeter toprovide an integral unit capable of controlling and indicatingflow rates. Flowmeters may be calibrated for specific gases, orthey ma
