Analyst, May, 1968, Vol. 93, $9. 289-291 289 The Determination of Trace Amounts of Chromium Stable Isotope Dilution - Mass Spectrometry BY A. HEDLEY* (Bragg Laboratory, Naval Ordnance Irtspection Establishment, Shefield 9, Yorkshire) A simple chemical - mass-spectrometric method for the determination of chromium is described. The method has been applied to steels, copper-base and aluminium-base alloys. The conditions for the thermal ionisation of chromium are given, and the stable isotope-dilution technique is used for quantitative analysis over the range 0.003 to 0-2 per cent. The advantage of the isotope-dilution technique in not requiring quantitative extraction is clearly illustrated by the application of a single method to more than one type of alloy. A MASS-SPECTROMETRIC method for the determination of chromium at levels below 0.05 per cent. was evolved to check results obtained by chemical analysis for aluminium-base alloys.Further work showed that the same method could be applied satisfactorily to steels and copper-base alloys. It clearly illustrates one of the advantages of isotope dilution - mass spectrometry, as once isotope exchange has taken place there is no requirement for quantitative or “clean” separations. Blundyl reported on the efficiency of the extraction of chromium in the presence of smaller amounts of iron, nickel and copper than those used here. His results indicated that 100 per cent. extraction would not be achieved after one extraction under the conditions given in this paper. However, a second extraction could be performed if required.The mass spectrometer used was an A.E.I. MS-2-5 This is a 6-inch radius, 90” sector instrument fitted with a thermal-ionisation source for the analysis of solids. Ion detection is by a conventional plate collector, display being on a chart recorder. The techniques of thermal ionisation2v3s4 and isotope dilution3v4 have been adequately covered in earlier literature. The sample is dissolved in hydrochloric acid, oxidised with nitric acid and heated to fumes with sulphuric acid, and the chromium(II1) is then oxidised to chromium(V1) with cerium(1V) sulphate. The chromium(V1) is extracted into isobutyl methyl ketone in N hydrochloric acid and is then back-extracted into water. PREPARATION OF THE TRACER SOLUTION- Weigh about 2.5 mg of the oxide into a platinum crucible and fuse it with a 10-fold excess of a mixture of sodium carbonate and potassium nitrate (2 + 1).Leach the melt with de- ionised water and add, dropwise, a slight excess of lead nitrate solution (5 per cent w / ~ ) . Allow it to stand at room temperature for 30 minutes. Centrifuge the precipitate and wash it ten times with de-ionised water. Rinse the lead chromate out of the tube and dissolve it in nitric acid (sp.gr. 1.42). This solution when diluted to 50 ml has an approximate concen- tration of 1 ml equivalent to 30 pg of chromium-53. During the preparation there is the possibility that some enriched chromium may be lost and some natural chromium picked up. This would alter both the concentration and isotopic composition of the tracer solution. To determine these values, load the tracer solution on the source filament (described below) and measure sufficient beams to enable the com- position of the tracer solution to be calculated.Determine the strength of the solution by adding a known weight of natural chromium to a known volume of tracer solution and measure the isotopic ratio of peaks 53 : 52. The weight of artificially enriched chromium required to give the mixed isotopic composition can then be calculated by using the equation given below. The tracer, Cr,O, artificially enriched with chromium43 to a level of 97.5 per cent., was obtained from the Electromagnetic Separation Group, A.E.R.E., Harwell. EXPERIMENTAL *Address after June lst, 1968 : Admiralty Materials Laboratory, Holten Heath, Poole, Dorset. 0 SAC; Crown Copyright Reserved.290 HEDLEY: DETERMINATION OF TRACE AMOUNTS OF CHROMIUM [Analyst, vol.93 METHOD REAGENTS- Hydrochloric acid (Aristar, sp.gr. 1-18}. Hydrochloric acid, N. Nitric acid (Aristar, sp.gr. 1-42). Sulphuric acid, 50 per cent. v/v. Cerium(1V) sulphate solution, 10 per cent. wlv. Isobutyl methyl ketone. PROCEDURE- Weigh an appropriate amount of sample into a 125-ml conical beaker (Note l ) , add 0 6 m l of tracer solution, dissolve it in 1 O m l of hydrochloric acid (sp.gr. 1-18) and oxidise with a few drops of nitric acid (spgr. 1.42) (Note 2). Add 10 ml of sulphuric acid (50 per cent, v/v) and evaporate to fumes. Cool, dilute to about 50 ml, add 2 ml of cerium(1V) sulphate solution (10 per cent. w/v) and boil for 10 minutes.Cool the solution to room temperature and make it N in hydrochloric acid, before shaking it with 20 ml of isobutyl methyl ketone for 1 minute. Wash the organic layer with four 25-ml portions of N hydrochloric acid before back-extracting the chromium with two 10-ml portions of de-ionised water. Evaporate the aqueous layer to the smallest possible volume. NOTES- 1. An isotopic ratio of about 1 : 1 is preferred. If the approximate chromium content of the sample is known, a suitable sample weight can be calculated, keeping the amount of tracer used constant. 2. For copper alloys the addition of 5 ml of nitric acid (sp.gr. 1-42), after the addition of hydro- chloric acid, increases the rate of dissolution. MASS-SPECTROMETRIC PROCEDURE- Tests carried out with 2 pg of natural chromium indicated that the optimum filament was a triple filament assembly5 made of rhenium ribbon.Load the solution on to a side filament of the assembly by using a micro syringe, and dry the drops by passing a current through the filament. Place the source in the mass spectrometer and scan the mass range 51 to 54 with the centre filament at 6 amps and the side filament at 3 amps upwards. Once stable beams have been produced maintain these conditions and measure the isotopic ratio 53 : 52. A blank consisting of 0.1 g of the appropriate pure base metal, with 0-5 ml of tracer added, was carried through the full procedure. The blank had a value of about 5 p.p.m. when Aristar grade acids were used. CALCULATIOK- The two isotope peaks considered are chromium-52, the most abundant chromium isotope in nature, and chromium-53, purchased for use as a tracer.The ratio of the contents of chromium-52 and chromium-53 are measured. The chromium content can then be calculated from the following expression- E =- .-._.-. RO-R' R + l A t w - * x R ' - R RO+1 A0 s m where E = amount of chromium present in pep."., Ro = 53 : 52 ratio in the tracer material, R = 53 : 52 ratio in natural chromium, R' = 53 : 52 ratio in the sample - tracer mixture, A = atomic weight of natural chromium, A0 = atomic weight of the chromium in tracer form, t = atomic fraction of 52 and 63 in the chromium tracer, s = atomic fraction of 52 and 53 in natural chromium, w = volume of tracer added in millilitres, m = weight of sample taken in grams, and x = weight in pg per ml of chromium in the tracer solution.May, 19681 BY STABLE ISOTOPE DILUTION - MASS SPECTROMETRY 291 RESULTS Steels-Two B.C.S.standards, B.C.S. 322 and B.C.S. 271, were analysed, with the following results. B.C.S. 322 (0.039 per cent.) B.C.S. 271 (0.046 per cent.) 0.039, 0-039 0.044, 0.043 0.040, 0.039 0.043, 0.042 0.038, 0.039 0.043, 0.043 0.040 0.045, 0.043 Average 0.039 (1) Average 0.043 (3) Aluminium alloys-Three B.C.S. standards were analysed. B.C.S. 262 (0.06 per cent.) 0.061, 0.066 0.242 0.0029, 0-0027 0.062, 0.063 B.C.S. 263 (0.24 per cent.) B.C.S. 181/1 (0.01 per cent.) - 0.0031 Cofi+er alloys-No B.C.S. copper alloys containing a certified chromium figure are Therefore, to each 0-1 g of sample 35 pg of natural chromium were added, equivalent made.to 0.035 per cent. Three B.C.S. standards were used. B.C.S. 183/1 B.C.S. 207/1 B.C.S. 304 0.035 0.035 0-038 0-035 0.036 0.038 0.037 0.036 0.037 The high figures obtained for B.C.S. 304, an aluminium bronze, indicated that this alloy This was verified by analysing 1 g of the alloy in contained a small amount of chromium. duplicate, when the following figures were obtained. DISCUSSION The time taken for one analysis was between 14 and 2 hours but, as opposed to purely chemical methods in which the time taken for a batch does not increase greatly, the time taken with the mass spectrometer (30 to 45 minutes) is the same for each sample. This slight disadvantage is amply compensated for by the relatively simple method that can be applied to these three matrices.The method has no interference, as interferences have to be isobaric and no other elements have stable isotopes with masses 52 and 53. The parameters of the mass spectrometer preclude any doubly charged ions. B.C.S. 304: 0-0028, 0.0027 per cent. of chromium. CONCLUSION The results indicate that the method can be confidently applied to all three alloy types. There appears to be no objection to applying the method to other alloys that do not form insoluble sulphates or chlorides. The 3.53 per cent. of lead in B.C.S. 183/1 did not separate as sulphate with 0.1 g of sample. This paper is published with the permission of the Ministry of Defence (Navy Department). REFERENCES 1. 2. 3. Blundy, P. D., Analyst, 1958, 83, 566. Turnbull, A. H., “Surface Ionisation Techniques in Mass Spectrometry,” U.K. Atomic Energy Research Establishment Report R.4296, H.M. Stationery Office, London, 1963. Freegarde, M., “Mass Spectrometry-Applications of Isotope Dilution to the Analysis of Metals.” Paper presented at the Society for Applied Spectroscopy Conference, Cleveland, Ohio, October, 1964. Wilson, H. W., and Daly, N. R., J . Scient. Instrum., 1963, 40, 273. Inghram, M. G., and Chupka, W. A., Rev. Scient. Instrum., 1953, 24, 518. 4. 6. Received November loth, 1967