Compton Scattering Interference in Fluorescent X-Ray Spectroscopy E. J. Brooks and 1. S. Birks, U. S. Naval Research Laboratory, Washington 25, D. C.
x-ray SpeCtrOSCOpy, the sample matrix contains mostly light elements, especially the elements below sodium (ll),Compton scattering may be strong enough to interfere with the analysis. Matrix materials such as the common plastics, hydrocarbon liquids, and many other materials fall in this class, A detailed explanation of the cause of Compton scattering may be found in any standard x-ray text (such as Compton and Allison’s “X-Rays in Theory and Experiment,” Van Yostrand, New York, 1943). I n x-ray spectroscopy, besides generating fluorescent x-rays, part of the primary radiation is scattered by the specimen. Some of the scattered radiation suffers a n energy loss-that is, a shift to longer wave length-resulting in an adjacent broad line beside each scattered primary wave length line. The lower the atomic number of the elements in the specimen, the greater the relative intensity of this modified or Compton scattering. Scattering of the fluorescent radiation also occurs, but because the secondary radiation is usually about two orders of magnitude weaker than the primary radiation, the effect is less noticeable. An example is shown in Figure 1 for a thick Lucite sample and primary rndiaN FLUORESCENT
I whenever
1
I
A
WLB,
I
I
j
I
11
I
35
Figure 1.
do
’ ’ DEGREES 29
,
,
,
I
45
Compton scattering from Lucite
Beside each line of scattered tungsten primary spectra is a low broad line due to Compton scattering. As examples of direct interference, positions A , B, and C where Ta Lp1, Ta La, and Hf La,,would fall if they mere present
tion from a tungsten target x-ray tube. At slightly longer wave length than each primary line (including a copper line from impurity in the target) there is a broad low peak due to Compton scattering. If one were trying to measure tantalum, for instance, there would be direct interference; for other neighboring elements, the effect would be a sloping background that might lead to erroneous assignment of intensity to the desired element lines. For a
different primary x-radiation, the Compton scattering would interfere with other elements; for a molybdenum target tube, for instance, the interference would be greatest a t N b K a but appreciable a t ZrKa as well. Thus interference in fluorescent x-ray spectroscopy may occur not only at the primary x-ray wave lengths but also at wave lengths corresponding to the Compton scattering associated with thp primary wave lengths.
Microhydrogenation Sample Technique for Volatile Compounds R. M. Engelbrecht, Research Department, Lion Oil Division, Monsanto Chemical Co., El Dorado, Ark. quantitative microhydrogenation of volatile compounds is troublesome from the standpoint of an adequate sample technique. The gelatin capsule technique of sample introduction to the hydrogenation media requires excessive time for dissolution of the capsule. Other difficulties are volatilization during the removal of air by sweeping out with hydrogen, volatilization while the hydrogenation apparatus is equilibrating, and the accurate weighing of milligram quantities of volatile compounds. The technique for sample introduction described circumvents these difficulties. HE
The sample holder is shown in Figure 1. D is a thin-walled capillary, 2.25 inches in length. Melting point capillaries were used in this work. The mark, E , is made with a file about 0.75 inch from the bottom to facilitate breaking when the system is ready for hydrogenation. The capillary is weighed empty on a semimicro analyt-
1556
ANALYTICAL CHEMISTRY
ical balance after the mark has been made. A hypodermic syringe was found to be the most efficient way of getting the sample into the capillary. The level of the sample should not go above the file mark. After the sample has been added to the capillary, the open end is sealed in a very hot flame. The capillary is reweighed on the semimicrobalance. F is a rather loosefitting glass tube placed over the capil-
Figure 1. Reaction vessel with modifications
,4. Reaction vessel B. Solid glass rod C. Rubber sleeve
D. Sample capillary E. File mark
F . Glass sleeve
lary when it is put into the reaction vessel; this should be of such length that it rests slightly above the file mark. The hydrogenation apparatus used in this FTork \vas purchased from the Arthur H. Thomas Co. The reaction vessel, A , supplied with this equipment is also shown in Figure 1. I n place of the stopcock on the side of the reaction vessel is placed a solid glass rod, B , and a tight-fitting rubber sleeve, C, attached t o both the vessel and the rod to hold the rod firmly in place. A mark made on the rod will aid in setting it a t the same point before and after breaking the capillary. T o determine the hydrogen number of a volatile compound, the stirring bar, solvent, catalyst, and capillary with sample are put into the reaction vessel in the order listed. The rubber sleeve, C, is always left on the reaction vessel. The solid rod, B , ITith some lubricating grease to facilitate positioning, is then inserted. The reaction vessel is loosely attached to the hydrogenation apparatus and flushed out with hydrogen for about 15 minutes. It is then firmly
attached to the apparatus. Khen equilibrium has been attained and the initial hydrogen volume is recorded, the sample capillary is broken by gently pushing the glass rod, B , against tlit: glass a.hic.ll exerts force agailist tile file inark arid ilreaks the capillary, The glass rod is al\vays \rit,hdra\vn to its original position, so as not to alter thr
initial hydrogen volume. The remainder of the hydrogenation is carried out in the conventional manner. This metliod of sample introchwtion wnrks very well for volatik samples that require a sample weight up to 30 mg. For volatile samples requiring a larger
sample size, introduction of the sample \Tith a hypodermic syringe works well. A gum rubber serum cap is placed over the stopcock opening and when initial equilibrium has been attained, the sample is inserted with syringe. The accuracy obtainable with this te.c.hnique is within 2%.
Two-Piece Centrifuge Crucible for Handling Microchemical Precipitates Cyrus Feldman and Janus Y. Ellenburg, Oak Ridge National Laboratory, Oak Ridge, Tenn. 7o
m of the operations in the isolation and treatment of a very small precipitate can seriously reduce analytical accuracy. If the precipitate is to be dried or ignited, considerable losses may occur in the transfer from centrifuge or filter to crucible. The danger of contamination, especially by common elements, becomes greater, the greater the amount of handling. If a precipitate is filtered on filter paper and ignited, the filter paper ash often contributes a blank of some elements greater than the amount originally present. It was felt that if a precipitate could be processed completely without changing containers, these losses and contamination would be greatly reduced or eliminated.
53
A device designed to make this possihle consists of a centrifuge tube which, after centrifugation, can be drained and separated into two parts. The lower part, which receives the precipitate, can be used as a container for drying, ignition, or dissolution. Leakage of liquid during centrifugation is prevented by the self-sealing action of centrifugal force on the beveled edge. A short rubber sleeve around the joint prevents accidental disassembly. The upper portion consists of a cylinder whose lower edge is beveled and ground. The lower portion has the shape of a crucible; its upper edge is beveled to fit the upper section. Its bottom may be flattened t o permit it to stand alone. A lid is provided to cover
the crucible during storage or ignition Before the device is used, ljoth beveled surfaces are coated lightly n i t h Silicone or Lubriseal stopcock grease. The pieces are pressed together and the joint is secured with a thin rubber sleeve. The centrifuge tube thus assembled is filled IT ith the liquid sample. After the wmplc has been rentrifuged and n ashcd, the supernatant liquor is decanted or drawn off until the liquid level is beloir the joint. The upper section is then removed. If desired, the remaining supernatant fluid can then be drawn off or evaporated, and the precipitate can be dissolved. dried, or ignited. The shape, size, and material of construction can be varied according to usage. The design shown fits a standard 50-ml. centrifuge cup. The models used in this laboratory are made from fused quartz, in order to permit ignition of precipitates in the crucible a t muffle temperatures. Glass. metal, plastic, or other materials can be used if thr subsequent treatment of the precipitate is to be less severe. The device was tested by dissolving a small quantity of acidified europium solution containing europium-154-5 tracer in approximately 50 ml. of water. This solution was transferred to the assembled crucible. Approximately 0.5 gram of sodium carbonate and 1.0 gram of sodium orthophosphate (XaoP04 10 H L O ) w r t l thrn added and dissolved. The solution nas centrifuged immediately. The supernatant solution was then dtcanted, the sample n as naihed thrrc
Table I. Distribution of Europium-1545 Tracer after Centrifuging, Washing, and Drying To of Eu Tracer Found 1:u Taken, i
I00 10
In
In supernatant
pircipitate ‘19 0 95 2
qolution 0 0 4 7
times with water, and the washings were combined with the supernatant solution. The centrifuge crucible was then disassembled and the precipitate was dissolved in acid. The recovery of eurnpium-154 tracer was measured with a gamma ray scintillation spectrometer. Table I s h o w that this device permitq precipitates as small as 10 t o 100 y to be isolated from gram quantities of salts, in a form suitable for neighing or igniting, with little or no low.
Reduced-Scale Auxiliary Recording of Infrared Spectra Fred Stitt and Glen F. Bailey, Agricultural Research Service, United States Department of Agriculture, Albany 10, Calif.
R
produced by commercial, direct-reading recording spectrophotometers are rather bulky. The need for reduced-scale infrared spectral curves has been met recording system for using an producing reduced-scale curve simultaneousb with the standard instrument record ( 1 , 2 ) . This work describes a ECORDS
simple, convenient auxiliary recording system in use with a Beckman Model IR-3 spectrophotometer and adaptable for use with other instruments. A Varian Model G-10 recorder, 1second full-scale response, with the same range as the recorder of the spectrophotometer (50 mv.), is used to prepare the reduced-scale record. Because the
input resistance of the auxiliary recorder is high compared to that of the spectroa input photometer signal can be used for both recorders without significantly affecting the spectrophotometer record. Under these conditions the transmittance scale of the smaller record is 5 inches long. This scale can be shortened by increasing the input signal range of the auxiliary VOL. 29, NO. 10, OCTOBER 1957
1557
