diluted in carbon tetrachloride is recorded from 2.5 to 5 microns on absorbance (semilog) paper. The ratio of the areas of the absorption peaks at 4.05 (0-D) and 3.00 microns (0-H) is the ratio of D to H in the original sample. H20
+ HC(OCHJ3 K: HCOOCH, + 2 CH30H
(1)
The accuracy of the analysis is limited only by the accuracy with which these peak areas can be measured. Results whose coefficient of variation is 0.8% can be obtained by multiplying the peak height by the width at half-height. The rather intense absorption peaks between 3.4 and 3.6 microns, the C-H of the methanol and the methyl formate, do not interfere. Inorganic impurities will have no effect
on the absorption at 3.00 and 4.05 microns. The C-H of organic impurities will be under the C-H of the methanol and methyl formate. The 0-H of alcohols, sulfonic acids, etc. and the N-H of amines, amides, etc. will have much the same D to H ratio as the water and are measured with the 0-D and 0-H of the methanol introducing a very small and usually negligible error. The results obtained will be mole per cent heavy water in water, which is the same as volume per cent heavy water in water, but is not the same as weight per cent heavy water in water. RECEIVED for review January 29, 1969. Accepted April 4, 1969.
Some Simple Techniques to Prepare Powder Samples for Spark Source Mass Spectrometric Analysis Ping-Kay Hon Argonne National Laboratory, 9700 S. Cass Ace., Argonne, Ill. 60439 POWDER SAMPLES are often encountered in spark source mass spectrometric analysis. Briquetting is normally used to compress the sample powder into suitable size electrodes. If the sample powder is pressed directly into a plain die, contamination may often be a serious problem. This is because the die surface contacting the powder is not optically smooth and fine powder particles may imbed in the scratches and cause cross sample contamination. The A. E. I. technique is accomplished by drilling a small hole in a polyethylene plug which is filled with the sample powder and then the whole thing is compressed in a special die. A good size electrode free of contamination from the die is achieved. Rather strong electrodes can also be obtained if the sample powder is very fine and has good compacting properties. For sample powder having relatively poor compacting properties, a good, strong electrode is no longer obtained. This is because most of the force in the pressing is absorbed by the polyethylene itself and only a small fraction is transmitted to and absorbed by the powder. In addition, special care is also needed in drilling and cleaning the polyethylene plug. In our laboratory we have developed a very simple technique for compressing the sample powder without contamination from the die. Sample powder is filled into a small polyethylene tubing (about 1.5 mm i.d. and 3 mm 0.d.). The filled tube length is cut to 20 mm and compressed to a 4,0005,000 pound load equivalent to a pressure of 64,000-80,000 psi in a Seidel mould (Ringsdorff Carbon Corp., East McKeesport, Pa.) with a 2 x 20-mm cross section plunger. The diameter of the tubing can be very flexible as long as it fits into the mold without too much difficulty. The length should be slightly shorter than that of the mold to avoid a crooked electrode. A good strong electrode can be obtained even for powders with relatively poor compacting properties and which require an effective high pressure of many thousand pounds per square inch in order to get good compression. The wall of the polyethylene tubing is quite thin and the applied force is effectively transmitted to the powder. The polyethylene tends to expand slightly to its original shape after the pressure is released. This property is ideal for extracting the electrode from the tubing. To fill the small polyethylene tubing, the 1148
ANALYTICAL CHEMISTRY
powder is put into a small plastic vial. By striking the open end of the polyethylene tubing in and out of the vial, the powder will “walk” into the tubing automatically (Figure 1, A-I). A small rod with one end flattened may be used to push the powder into the tubing and pack it more densely. Both ends of the sample-filled tubing need not be sealed because the resulting electrode can be broken in two to give fresh uncontaminated ends. When a sample powder is sensitive to air and moisture, it has to be loaded into the tubing and extracted under inert atmospheric conditions. After loading the powder, both ends of the tubing can be sealed by means of a pair of long nose pliers and a soldering gun (Figure 1, C-2, 3). Then the compression process can be performed in air. The need for putting a press inside a glovebox, usually neither feasible nor desirable, is eliminated. For small amounts of sample, the powder is first put into the tubing and pushed to the middle. Both ends of the tubing are then filled with a supporting pure metal or graphite powder. After compression the electrode is cut at the center and extruded with the sample-tipped ends out first (Figure 1, B-3) to obtain two electrodes. Silver electrodes tipped with copper powder have been made by this technique. When the electrodes were sparked end to end the densities of the silver lines on the resulting plate were not significantly different from those obtained with unsupported copper electrodes for 1 x lo+ coulomb of exposure (the copper tipping was about 1 to 1.5 mm on each end of the electrodes). Thus, by this procedure, it is possible to produce a sample tip not contaminated by the supporting material. If the Seidel mold is not available, a simple mold as illustrated in Figure 2 can be easily constructed. It merely requires bolting a few metal bars together. In compressing, the total force exerted on the side walls is not very large. This certainly depends on what and how much material is being compressed. If the substance is a fluid or if it exhibits some fluid characteristics under high pressure-e.g., polyethylene and fine graphite powder-the applied force will be evenly transmitted in all directions. The total force on the side walls, in fraction of the applied force, will be proportional to the fractional area of the whole system being compressed. If the substance is a solid or in solid form at the final state of
3
0
.-.
,
,
.
.~ .,
.
6. D
3 Figure 1. Schematic diagrams showing various techniques to make different electrodes A. General; 8. For sample tipping; C. For air or moisture sensitive samples; D. For poor electrical conductant or self binding materials
compression, then the force will be mainly transmitted in the direction of the force applied. When thin-wall plastic tubing is used for holding the sample and in the final state of compression the sample becomes solid, then the force transmitted to the side walls of the die is mostly from the thin skin of the plastic which has a relatively small effective area toward the side. Hence, the total force on the side walls of the die will he much less than that in compressing a large piece of polyethylene plug for the same pressure applied. The screws in the mold are sufficiently strong to hold the metal bars together even if high pressure is applied to the center piece. X The illustrated mold (Figure 2) was made from two 1" stainless-steel bars for the side plates, and four '/a" X 1" steel bars for the plunger, bottom piece, and the end plates. A large hexagon head steel screw was used to hold the side plates together and to sandwich the bottom piece. The
Figure 2. A simple die design for making electrodesfrom powdered sample A.
Assemble view. B. Disassemble view
mold has been used for a load of 6,000 pounds equivalent to a pressure of 48,000 psi on the plunger whose cross section was '/s" x 1". For semiconducting- or nonconducting.materials. it is almost !standard procedure to mix the sample powder with pure 1graphite or a metal powder and then compress into elect rodes t, n T-AL:.-.AL->LL I: --..--.... (i-o,. 111 LIUS I I I ~ L I I U U L W S U ~ ~ UL I U L ~IMLCLUL ~ u a y wrrribute more than 30% of the ions in the beam when the sample is sparked. High purity is therefore required for the supporting material, otherwise correction for impurity concentration from the supporting material is necessary and difficult. Kashuba and Morrison (7) packed the sample powder into pure graphite cups when they analyzed magnesium oxide powders. This also has a laree and erratic ion contribution from the Iyaphite rod. We have developed a simple technique to L.-:-,
I
rometry," R. M. Elliott, Ed., Macmillan, New York, N. Y., p 141.
:hastagner, Appl. Specrrosc., 19, 33 (1965). +.Evans, Jr., and G. H. Morrison, ANAL.CHEM.,40, 869 ).
(4jP. F. S. Jackson and J. Whitehead, Analyst (London), 91, 418 (1966). ( 5 ) E. B. Owens in "Advances in Mass Spectrometry," W. L. Mead, Ed., Elsevier, Amsterdam, 1966, p 197. (6) H. J. Svec and R. J. Conzemins, ANAL.CHEM.,40,1379 (1968). (7) T. Kashuba and G. H. Morrison, Cornell University, Ithaca, N. Y.,private communication, 1966. VOL. 41, NO. 8,JULY 1969
-
1149
Figure 3. Orientation of the special sample electrodes minimize, as much as possible, ions generated from the supporting materials. The sample powder is first prepressed, at finger tight pressure, into a thin layer in a Seidel mold. Silver powder is put on the top and then compressed at high pressure (Figure 1,D). The sample and the silver are thus mechanically and strongly bound. To spark the sample the electrodes are arranged in such a way that two electrodes overlap about 2-3 mm at the ends and have the sample sides facing each other (Figure 3). Because the silver is bound closely to the sample, which is less than 1 mm thick, sparks can be initiated primarily at the sample part and ions produced from the silver base are minimal. This technique has been used
to analyze uranium sulfide and uranium selenide powders. At 30 X coulomb of exposure level the densities of the silver lines were less than at an exposure of 3 X 10-10 coulomb from a pure silver powder electrode. Therefore, the ion contribution from the silver base was less than 1%. At this low exposure level, impurity lines were not detected on the pure silver plate. Hence, it can be safely assumed that all the coulomb exposure level on impurity lines seen at 30 X the sample plate were entirely from the sample powder. This makes the calculations to deduce the sample impurity concentrations much simpler. Thus a less pure silver powder (4N instead of 6 N ) can be used as the supporting material. This technique is particularly useful for small amounts of samples which have relatively poor self-binding properties. Mixing the sample with electrical conducting powder will not only dilute the sample and present a uniform mixing problem, but also change the characteristics of the matrix if large amounts of supporting material are added. RECEIVED for review February 6, 1969. Accepted April 17, 1969.
SimuI taneous Spectrophotometric Determination of Nitrosy1 Bromide and Bromine Chiya Eden, Hans Feilchenfeld, and Shalom Manor Department of Physical Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel
NITROSYL BROMIDE (NOBr) partially dissociates to nitric oxide and bromine; this dissociation is one of the reasons why this simple molecule has been little investigated; even its boiling point was unknown until recently. In order to facilitate research in this field, a spectrophotometric method has been developed which allows the simultaneous determination of bromine and nitrosyl bromide in the presence of nitric oxide without disturbing the equilibria. EXPERIMENTAL
Determination of the Spectrum of Nitrosyl Bromide. The spectrum of nitrosyl bromide was determined by means of the apparatus shown in Figure 1. Liquid bromine which had previously been washed with potassium bromide solution dried and distilled, was introduced into the side arm A of the apparatus, and the opening B was closed in the flame. The apparatus was outgassed and sealed off at C while the bromine was cooled with liquid air. The bromine in A was then warmed to 5 "C at which temperature its vapor pressure was about 85 mrn Hg. The side arm A with the liquid bromine was then sealed off at D,and the apparatus was removed from the vacuum line by cutting below the stopcock at E . The bromine concentration was determined exactly by inserting the 1-cm optical cell F into the compartment of a "Cary 14" spectrophotometer, the lid of which had been replaced by a thermostat. After the absorbance of the bromine vapor had been measured, the apparatus was reattached to the vacuum line and evacuated up to the breakseal G. The bromine was condensed by cooling H with liquid air and the breakseal opened. From a reservoir on the vacuum line about 2 mmole nitric oxide were transferred to H ; the amount was estimated from the pressure drop in the reservoir. This represented a large excess of nitric oxide over bromine, and ensured that all bromine would be converted to nitrosyl bromide. The apparatus was sealed off at I , the liquid air removed, and the reactants were allowed to warm up and react. The 1150
ANALYTICAL CHEMISTRY
spectrum was then determined as described above for the temperatures at 26.5 "C, 40.6 "C, and 50 "C. At the end of the experiment, in order to determine the exact volume, both the apparatus and the remainder of the breakseal were opened at one end and weighed with air and with water. RESULTS AND DISCUSSION
The absorbance at 416 nm of the bromine vapor at 25.5 "C before addition of nitric oxide was found to be 0.525. Because E = 170 ( I ) , it followed that the bromine concentration was 3.09 mM. By sealing off at I, the vapor volume had been reduced from 29.95 ml to 17.37 ml; the final bromine concentration was therefore 5.32 m M and the nitric oxide concentration about 110 mM. From the equilibrium data (2) it could be calculated that less than 0 , 0 4 z of the bromine introduced remained unreacted. Therefore, the concentration of nitrosyl bromide was 10.65 mM. The molar absorptivity (Figure 2) was calculated from the absorbance data which were the same for the three temperatures measured. The knowledge of the molar absorptivity of nitrosyl bromide allowed the simultaneous determination of bromine and nitrosyl bromide. Nitric oxide did not absorb in this region. The frequencies chosen were the respective maxima of absorption for nitrosyl bromide, 338 nm, and for bromine, 416 nm. The molar absorptivities at these wavelengths were:
NOBr Br2
338 nm
416 nm
82.9 2.8
51.7 170.0
(1) A. A. Passchier, J. D. Christian, and N. W. Gregory, J . Phys.
Chem., 71, 937 (1967). (2) C . M. Blair Jr., P. D. Brass, and D. M. Yost, J. h e r . Chem. SOC.,56, 1916 (1934).
