Fluorescent Sample for Focusing an Electron Microprobe Beam Larry C. Hall,l National Aeronautics and Space Administration, George C. Marshall Space Flight Center, Huntsville, Ala.
4 used to adjust anti focu> is commonly the elec-
-
Z I X C SUI.FIDI: SCREES
tron h a m in elecatron microprobe analyzer.. Such scrcens are usually prepared by placing a rolloidial suspension of zinc sulfide on a inetal sanil)le and letting this dry. The intensity of fluorescence is difficult to control with the result that the sput size is very large and bright. l-se of uranium glass gives a very small: weak spot n-hich is unstable as a result of the large electrostatic charge that build- up on the glass surface. -1 conducting sample that gives a 3- to 5-micron fluorescing blue spot when a 30-kv. electron beam is properly focused has been prepared from potassium bromide in the5e lahoratories. PROCEDURE
Three grams of pot a-&m bromide and 0.1 gram of uranyl acetate (both J. T. Baker) were mixed and ground to pass a 200-mesh screen. Disks (0.25 in.
thick) were prepared in an .1pplied Research Laboratories Model 3502 briyuetting press using a 0.5411. diaineter niold and a pressure of 40,000 liounds. The disks were then mounted in diallyl phthalate copper metallurgical molding resin, ivhirh i b obtainable froni the Metallurgical Sup1)Iy Co. (P.0. Box 12037, Houhton, 'resass). h setting temperature of 150' C. and pressure of were used. The 1-inch diameter blocks were then fine ground ending with 600-grit silicon carbide paper. Methanol was used for washing and lubrication. The above sequence was used also for pure KBr disks. The samples were stored in a desiccator when not used.
ance between a pure KBr and a uranylKBr disk was very slight. The main distinction was that the uranyl sarnlile did not seem to leave trarks on the surface as readily as the pure Kl3r sample. Residual surface moisture seeiiis to be the niain factor in tracking since this phenomenon doe? not become noticeable after operating the probe for 10minutes. .1blue fluorescing s1)ot was produced in both samples. h t 30 kv. a well defined, somewhat diffuse halo of about 9 microns was observed. In the center of this halo, a much more intense spot of about 3 microns could be discerned. A spot of less than 3 microns was found at 20 kv. With care, the Kehnelt image of the filament can be obtained. This image is dim when the filament is not centered and heroine? noticeably brighter upon centering. 1 Present address, Ikpartment of Chemistry, Yanderbilt 1-niversity, Nashville, Tenn.
RESULTS
The samples can be handled readily in air and do not absorb water to any appreciable extent. A Sorelco -%AIR,3 electron probe analyzer was used at 30 and 20 k v . The difference in perform-
Dangers in Use of "High Purity" Spectrographically Analyzed Standards Stanley S. Yamamura, Atomic Energy Division, Phillips Petroleum Co., Idaho Falls, Idaho
SIXCE spectrographically analyzed standards with high purity specifications have become readily obtainable from commercial sources, many scientists, including a often use them in stoichioiiietric ctieniicd standards-a purpose not intended. This has been found to be a dangerous practice. Spectrogral)hically analyzed standards are 1)we only with re-pect to spectroscol)ically detectable inipurities and can contain .significant amounts of the elenlent-, carbc 11, hydrogen. nitrogen, ox!-gcn, and sulfur that normally are not detected. Ccnseyuently, there ran be ctonsidrixble difference? between the stated I w i t y for a coniliound and the actual purity. Sevpral exani~)le; of obaei,vetl differences nil1 be noted t o point out the dangw associated with the uriguardrd use of spectrographic standard> and to emphasize the need for caution. In addition, sinil)le precauticina1.j- measures which may be used to avoid caostly errors will bc discussed. OYCT the past several years> allproximately 25 sl~ectloyra~ihically analyzed standard-. l)i~incipallymetali: and niet a1 osic!cs, with purity silecifications ranging from 99.9 to 99.999% ha\-? been analyzed titrimetrically with
each was stated to be 99.97,. In addition, as in the case of neodymium oxide and metallic zinc, there can he differences among supposedly similar products of different nianufactiirers. The basic cause for all such differences is thought to be the presence of spectroscopically nondetectable elements, carbon, hydrogen. nitrogen, oxygen, and sulfur, as noted above. The weight
established (ethylenedinit rilo) tetraacetic acid (EDT.1) methods in our laboratories. .is shown in Table I, serious differences have been noted between the stated purity and the observed purity determined by EDTA\ titrat,ions. For example, the observed purities for europium oxide, indiuni oxide, and neodymium oxide were 93.1, 95.6, and 85.6T0, respectively, though
Table I,
Results of Analyses of Spectrographically Standardized Substances
Substance Europium oxide
Stated purity, c 99 9
Observed purity, c /Ou.
93 1
Indium oxide
99.9
95.6
Xeodymium oside
99, 9+
85 6
9+ 9994 999 9s
at 950-1000" C., ignited oxide assayed 99.87, 3.8'T xeight loss observed on ignition at 750-800' C. Ignited oside assayed 99.15 Obtained frum manufacturer A . 0.75% weight loss observed at 110" C. 13.3LT weight loss observed on ignition at 9.i0-1000° C. Ignited oxide assayed 99.65; Obtained from manufac,turer B Obtajned from nianufacturer C Obtained from manufacturer I)
99 1 100 0 90 0 Zinc oxide 100 0 Based on a titrimetric determination of the metallic component u l t h estahl~shed
Zinc metal
99 99 99 99
Iternarks 6 . i % weight 1oys observed on ignition
EDTA methods
VOL. 36, NO. 13, DECEMBER 1964
251 5
loss observed for various metallic oxides upon ignition and the increased assay value for the ignited oxides tend to substantiate this. -4s shown in Table I, each of the three oxides, europium oxide, indium oxide, and neodymium oxide, showed appropriate weight losses on ignition, and. in all cases, the observed purities of the ignited oxides were better than 99%. When using spectrographically analyzed standards, there are several precautionary measures which could be used to guard against unnecessary and costly errors. The following are recommended : 1. Independent assay of all spectrographic standards by other reliable analytical methods 2 . Drying or ignition of all spectrographic standards before use 3. Cleansing of all metallic spectrographic standards to remove surface films 4. Use of proper weighing techniques
*All spectrographic standards should be assayed independently whenever possible. Quite frequently, the chemicals contain an EDTA-titratable
metal ion. For these chemicals, the readily available EDTA methods of analysis (2) which are reliable to k0.5% or better are recommended. Spectrographic standard chemicals are often blended with other chemicals in such forms as alloys, powder mixtures, and solutions. With appropriate manipulations (for example, pH control, selective masking, and preliminary separations) EDTA methods can be used to analyze such multicomponent samples ; however, for the sake of simplicity and economy, the unaltered spectrographic standards should be analyzed initially. The iniportance of proper pretreatment and proper weighing techniques (precautionary measures 2 , 3, and 4) is aell recognized, but all too frequently these important details are overlooked. Before use, all spectrographic standards should be dried at the highest permissible temperature below that a t which decomposition, chemical reaction, or volatilization occurs. Simple drying in a conventional oven at 110-160" C. may not be adequate. As noted in the remarks for neodymium oxide, Table I. the observed weight loss was lesq than 1% a t 110" C., while the ohserved total
weight loss was 13.3y0a t 950-1000° C. DuvaI's comprehensive work "Inorganic Thermogravimetric -4nalysis" ( I ) , is especially useful as a guide to the selection of appropriate temperatures. ,111 metallic standards should be cleaned before use-particularly the reactive metals, such as aluminum, zinc, and zirconium which are oxidized readily. Finally, proper weighing techniques should be used to reduce the absorption of common contaminants such as carbon dioxide and water. Diligent use of these precautionary measures will eliminate many unnecessary and costly errors which may be encountered in the use of spectrographically analyzed standards. LITERATURE CITED
(1) Duval, C., "Inorganic Thermogravimetric Analysis," 2nd ed., Elsevier, Kew York, 1963. (2) Meites, L., "Handbook of Analvtical Chemistry," Section 111, pp. 167-284, McGraw-Hill, New York, 1963. WORKdone under Contract X o . A T ( 10-1)205 yith the U. S. Atomic Energy Commission.
Apparatus for Recording Kinetics of Absorption or Evolution of Small Volumes of Gases at Constant Pressure by liquid Systems L. R. Mahoney, R. W. Bayma, A. Warnick, and C. H. Ruof, Scientific Laboratory, Ford Motor Co.3 Dearborn, Mich. is engaged in a the kinetics of compounds in liquid solutions (4,5 ) . An improved apparatus has been developed which permits the aut'oinatic recording of the volume of oxygen gas absorbed at constant pressure and temperature as a function of time and which is readily adaptable to wide ranges of rates of absorption. This apparatus is equipped with a servo-controlled pressure stabilizer and can be used without modification for recording the rates of evolution of gases as well. The present apparatus is somewhat similar to that described by Edgeconibe and Jardine ( 2 , 3). Their apparatus, however, was designed for rapid, relatively large, changes in volume and utilized a glass spiral manometer which was sensitive to vibrations of the building causing a "noise" level of about 100 to 500 microns. The present apparatus has an overall accuracy of k 17 microns and utilizes a rugged, capacitive differential micromanometer which is insensitive to building vibrations. With the particular gas absorption cell design and volumes of solution dexribed below, it is possible to record the volume changes accompanying the oxidation of small amounts of rare and expensive HIS
LABORATORY
Tdetailed study of oxidation of organic
2516
ANALYTICAL CHEMISTRY
Figure 1. Gas absorption apparatus
i ,
materials and to determine the slow initial rates encountered in highly inhibited systems. The average uncertainty amounts to about 0.5 p1. per minute. DESCRIPTION
OF
THE APPARATUS
The apparatus, Figure 1, consists of a reaction cell, a means for evacuation and charging of the cell to the desired pressure, the micromanometer and associated electronics, the infusion-withdrawal pump system, and the linear potentiometer-recorder system. Gas Absorption Cell. The cell, Figure 2 , consists of a borodicate glass vessel surrounded by a water jacket which is supplied from a constant temperature (60.00' i 0.02" C.)
circulating pump. A second water jacket a t the top of the cell is held constant at 25.00" 0.02" C. by a similar pump: this cold jacket prevents excessive evaporation of solvent during the evacuation. The 2-nim i.d. capillary lines of the apparatus are i n d a t e d from localized transients in the teniperature of the air-conditioned laboratory by foamed plastic (.Arniaflcx 2 2 , *Armstrong Cork Co.). Micromanometer Pressure Transducer and Associated Electronics. A sensitive capacitive sensor and associated electronics are available commercially ( 1 ) . For greatw long term stability, the follon.ing micromanometer, a.c. bridge, and servo-system have been constructed in this labora-
*
