ior a pressure diflcrentid across t h c vaive seat of 2C inn1 ot mercury (200
tunes sampie pressures normally encountered in analytical mass spectrometry). No leak was detected at normal sample pressures, Glass fiber notably decreases the usual thermal degradation of Teflon at 350” C . Five grams of Teflon-glass fiber in an evacuakd glass ampoule a t 350” C. for 1 week decreased in weight by only 0.03 f 0.01%. The minute amount of volatile materia! fornietl anaiyzed 96y0 water plus carbon dioxide and 4y0 of a fluorine-conhining mawrial of apparent mass 100. The heated pump-out valve has given very dependable service in routine operation for almost 2 years a t 200” to 350” C. Tne Teflon-glass fiber valve seat now in use shows no sign of failure after 8 months. Satisfactory instrument, background is readily reattained after each sample introduction snd has never shown a mass peak attributable to decomposition of the
Teflon-containing mntpriai. Lack of :: “cold spot’” in the vnive ~ssemblyis shown by linearity oi peak heighi with sample size at 300” C. for the mass 632 peak of a CeHd& compound. Figure 2 shows a gallium-covered orifice for introduction of samples into a heated inlet system. The glass plug is removed and galiiur;, flows through the orifice until tube 2 fills to the point shown. The sampic pipet of %gage hypodermic tubing is pushed through the gallium and tlic liquid sample flows or evaporates through the orifice into the iniet system. After the mass scan is finished, the gallium in A is collected in the recovery tube and poured into the upper portion of the orifice assembly. KOcontamination of succeeding samples by entrainment of previous samples in the gallium has been noted. Continued. re-use permits routine operation with a supply of about 100 grams of gallium. Periodic cleaning by vacuum filtration through a medium porosity porous disk removes accumulation of gallium oxides and
other forqy: matter, which niny plug the orificc. or cause an air leak because of insuffirient wetting of the glass walie of the upper portion of the orifice assembly. Supplemental electrical heaters around the top portion of the orifice assembly and the lower portion of A outside the oven assure that the gallium in contact with the liquid sample is a t the set temperature of the iniet system. The gallium-covered orifice avoids the sample-fractioiiating difficulties of sample introduction by porous disks covered with liquid metal. R e p e a t ability of mass spectrometer peak heigh-cs for successive sampie introductions through the gallium-covered orifice normally is to + 2 to 3y0. No difficulty attributable to reaction with gallium has been detected for a wide variety of organic compounds containing carbon, hydrogen, and oxygen. ASTM E-14 Meeting OR Mass Spectrometry, New Orleans, La., June 1958.
A New Standardization Technique for X-Ray Absorption Measurements James 0. Hibbits, Stancil S. Cooper,’ and Mary R. Menke, Aircraft Nuclear Propulsion Department, General Electric Co., Cincinnati 15, Ohio
by x-ray absorption measare rapid and nondestructive, although nonspecific. The latter difticulty becomes relatively unimportant when the desired constituent is of considerably higher atomic number than other elements present, as is usually the case when uranium is the element of interest. Preliminary work t o establish a procedure for determining uranium by x-ray absorption met with &ifEeult.y because of changes in instrument response. Bartlett [ANAL. CHEM. 23, 701 (1951)j has also reported the necessity for frequent recalibration. A Genera! Electric Model SO43351 G1 x-ray photometer was employed. The instrument, having a General Electric FS source with CS125 tube (45 bvp.), is similar t o that described by Lambert [U S. Atomic Energy Cornrn.. Project h p t . HW-3(?190 (?953)J. It wa.. the authors’ opinion that cse of hluminun, plates a5 pseudcjuraniurn standaras wouid be more aesirabie than frequenb meparatior of standard crraniwa solutions, 11: view oi the uncertainty of their stability with time.
23.90 28.98 24.18 29.33 23.93 28.93 24.18 29.32 562 23.92 28.98 470 29.31 24.13 562 23.86 470 28.95 23.86 28.81 466 Av.“ 28,96 23.90 Average of Operator I data only.
Two operators, each using a different cell for the standard solutions but the same reference cell, obtained x-ray absorption readings on succeeding days. The attenuation readings for the standard solutions, when plotted, yielded very nearly a straight line, inasmuch as the uranium concentration range was not very great (23.99 to 27.95 mg. of UOZ per gram of solution). The data from the standard uranium solutions were used t o obtain the equation of this line by the method of least squares. The corresponding “uraniuni equivalence” values for the two aluminum blocks were then obtained by appropriate substitutiou into these equations (Tabie I).
that their suri,ces were as nearly parallel as possible and notched so that they would fit reproducibly in the sampic holder. The “uranium equivaience” of the aluminum plates was then determined bi- x-ray absorption meascrrments of the plates and of four standarC uranium so1ut:ons. The uranium s3!11tions were contained in I-crn. round borosilicate glass cellc with Core1 windows. Brass plates with maphiiir:; hoies were placed above thc sample anci reference ;rulutions to prevent absorption by the c d 1 side wslls.
During the 4 days these measurements were being made, variations in instrument response of the order of Rere &served. However, the uranium equh aience values remaiped csscntia11.constan.. T r i above data indicete that aluminuE2 a~&s of the proper thickness can i)e stindardiaed against, st,u,dard urar?iunl solutions an6 thereifrm uyed in rdaw o; these solutions sa-~rngtime anci efiort Tablt I indicates ti182 the glass ~ 1 1ws e d in x-ray absorption work are not inrerc~hangesble. ever, tlioi,gh the cells are from matched pairs.
NALYSES
14 urements
Two alumrnum plaws. 0.0992 and0.113 inru t&&, WWP prepared, machined so . Decewec.
1748
ANALYTKA? CHEMISTRY
Table
I.
Uranium Equivalences o f Aluminum Plates Uranium
Equivalence, Mg. UOZ/G.
OP-
Day erator
I I1 I II I I1 I I1
Solution
Cell
A~s
A1113
470 562
470 ~..
