June, 19681 SOME ASPECTS OF INORGANIC TRACE ELEMENT ANALYSIS 117

Some Aspects of Inorganic Trace Element Analysis The following are summaries of the papers presented at a Meeting of the Society held on

November 15th, 1967, and reported in the December, 1967, issue of Proceedings (p. 180).

Recent Advances in Applications of Mass Spectrometry to Analysis BY R. K. WEBSTER

(-4 foiiiic Energy Research Establishment, Analytical Sciences Division, Harwell, Didcot, Bevkshire)

Two mass-spectrometric techniques are commonly used for trace analysis of solid inorganic materials : isotopic dilution based on thermal-emission mass spectrometry, and spark-source mass spectrography.

In thermal-emission mass spectrometry ions are produced by thermal surf ace ionisation, and measured by field scanning and electrical detection. The ionisation efficiency varies widely between different elements, so a known amount of a stable isotopic tracer must be added for each element to be determined, equilibrated with the sample, and the mixed element then separated chemically for isotopic analysis. Results for each element are calculated from the measured isotopic ratios. This isotopic-dilution method is precise and accurate, and in favourable cases can be extremely sensitive. For example, typical results for plutonium showed a coefficient of variation of about 0.2 per cent. for the measurement of about 0.1 pg, and caesium values, obtained by both stable isotopic dilution and neutron-activation analysis, showed an average agreement of 2 to 3 per cent. over the range 0-003 to 0.2 p.p.m. The need for a specific chemical separation has often limited the method to the determination of only one or two elements in any given sample, but this difficulty could be avoided by designing chemical separation schemes to isolate elements in groups suitable for isotopic analysis. As a first stage, the alkali metal, alkaline earth and rare earth groups can be separated by using cation-exchange chromatography and successive elution with 2.5 N hvdrochloric acid, 2.1 and 2.5 x nitric acid; each group is then further purified by anion-exchange chromatography

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118 [P~oc. SOC. Afialyt. Chew. for mass-spectrometric analysis. The procedure has been tested for potassium, rubidium, caesium, strontium, barium, cerium, europium, samarium and neodymium, and is satisfactory provided care is taken to avoid errors arising from impurities in the electromagnetically enriched tracers. This is insignificant when the concentrations of the various elements are similar, but can be important if large concentration ratios are involved (e.g., potassium-to- europium ratio in the standard diabase W-1 is greater than 4000).

In spark-source mass spectrography, samples are formed into electrodes and ionised by a high frequency spark. Ions for all masses, typically over the range 10 to 300 (Le . , covering boron to uranium), are recorded simultaneously with a photographic plate, and the concen- tration of each element calculated from the intensities of lines corresponding to one or more of its isotopes. The method provides a wide coverage of elements in a single analysis with good sensitivity (0.001 to 1 p.p.m. range), but its accuracy must be interpreted with caution. Unless similar standardised materials are available to calibrate the technique, it may be necessary to assume the same ionisation efficiency for all of the elements in a sample, and this assumption may introduce an uncertainty, by perhaps a factor of three. The spark- source technique can also provide a useful localised analysis, but for inhomogeneous samples the results may then not be representative of the sample as a whole, and this may introduce further errors. One method for avoiding these difficulties involves the dissolution of samples to promote homogeneity, and the addition of isotopic tracers as internal standards for elements of interest ( i e . , isotopic dilution). This method is attractive for ceramic and other non- conducting materials that inevitably need some prior treatment to form electrodes. For impurity concentrations greater than 10 to 20 p.p.m., electrodes can be prepared very simply by soaking solutions in porous graphite, followed by “freeze-drying”; tracer experiments with cobalt-60 have shown that this gives a homogeneous distribution of the sample in the graphite. The potential accuracy of the spark source - isotopic dilution technique was indicated by some results obtained for strontium and barium in three silicate samples; these showed a mean agreement of about 5 per cent. between duplicate measurements, and with results obtained by thermal-emission mass spectrometry. There was also some discussion of an empirical linearisation method for the interpretation of photographic plates, and of a preliminary version of a digitised photographic-plate reader.

SOME ASPECTS OF INORGANIC TRACE ELEMENT ANALYSIS

Some Applications of the Laser Microprobe to Analysis BY M. S . W. WEBB

(Atomic Energy Research Establishment, Analytical Sciences Division, Harwell, Didcot, Berkshive)

HIGH analytical sensitivity and good spatial resolution can be obtained by using the photon energy from an optical laser as a thermal source in spectrochemical analysis.

A commercially produced laser microprobe was described in which a 30-nanosecond pulse from a Q-switched ruby laser was focused by means of a metallurgical microscope on to a pre-selected area of the sample. The intense photon energy was absorbed, resulting in the vaporisation of a hemi-spherical area about 50 microns in diameter, and corresponding to a sample weight of 1 microgram. The energies of the atoms in the evaporated material were raised to excitation levels by causing the vapour cloud to short-circuit an electrical gap between two graphite electrodes on which there was a capacitive, standing voltage of 2500 volts. The “spark spectra” of the elements present in the sample were then recorded by conventional spectroscopy.

This technique had endowed optical spectroscopy with some facilities previously unobtainable, these included the generation of spectral emission without direct contact with, or preparation of, the sample; the derivation of a spectrum from a precisely selected area of the sample; and the direct analysis of non-conductors and refractory materials. In addition there was no restriction with atomic number of the elements determined (cf, electron microprobe).

The qualitative applications of the technique had been used for the examination of a wide range of materials including the identification of phases in phase mixtures and the characterisation of micro-inclusions in glass, alloys, geological and mineralogical specimens.

The absolute sensitivities of metals and some non-metals had been determined in a variety of matrices and these varied between and 10-l2 g. With care and esperience

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June, 19681 SOME ASPECTS OF INORGANIC TRACE ELEMENT ANALYSIS 119 reproducible results could be obtained and it was mentioned that coefficients of variation for single exposures of 5 per cent. had been obtained for zinc-to-tin ratios in British coin bronze.

The technique had been used to study one of the causes of embrittlement of steel under neutron irradiation. Boron-10, present in the steel, underwent an (np) reaction resulting in the production of helium which caused embrittlement. The distribution of boron in the austentite grains and various precipitates had been determined in niobium-stabilised steel, with the laser microprobe, standardisation being accomplished by using pellets made from the oxides of iron, chromium, nickel and niobium, to which known proportions of boron carbide had been added.

After a short period of use, it was considered that the laser microprobe was a powerful qualitative tool, and its potentialities as a semi-quantitative technique were useful. The equipment had not been fully assessed for quantitative analysis but the indications were that for some specific determinations quantitative results could be obtained.

Recent Advances in Neutron-activation Analysis

BY D. MAPPER (Atomic Energy Research Establishment, Analytical Sciences Division, Harwell, Didcot, Berkshire)

THE continuing importance of thermal neutron-activation methods, with reactor facilities, was emphasised. Pneumatic “rabbit” systems are particularly useful when dealing with the direct measurement of short-lived nuclides, such as aluminium-28, half-life 2.3 minutes, and vanadium-52, half-life 3.8 minutes. A laboratory rabbit system to BEPO and DIDO reactors (10l2 to 1013 neutron fluxes) was described, and mention was made of a very fast rabbit system with transit times of about 2 seconds into DIDO reactor (1014 neutron flux). These irradiation techniques generally require direct instrumental measurements on the irradiated sample. Nevertheless, to achieve the highest sensitivity for a large number of elements, radiochemical methods are often needed.

Radiochemical methods of separation have been considerably improved in recent years, and much thought has been directed to the question of automated procedures. The difficulties with such procedures are that they have to be individually designed for each type of sample likely to be encountered. The particular features of recent radiochemical techniques are the small amounts of carriers used; the use of solvent extraction; separations on a group basis, allowing final resolution to be achieved instrumentally; quantitative ion-exchange separations, avoiding the need for yield determinations ; and the use of substoicheiometric methods.

However, it has been in non-destructive or intact analysis of the irradiated sample that the most spectacular advances have occurred. The complex y-spectrum that is obtained with an NaI(T1) crystal and multi-channel analyser has been successfully resolved by using well established techniques of least-squares fitting with computers. This has been the most satisfactory way of processing the data from y-spectrometry, and various examples of this method were quoted. The recent development of Ge(Li) solid-state detectors has had a profound effect on the field of y-spectrometry, as the resolution obtained with such detectors is very much greater than that obtained with NaI(T1) crystals, although the efficiency, particularly of the smaller detectors, is not very high.

A system being developed in the Analytical Chemistry Group was briefly described. It consists of a 30-cc coaxial Ge(Li) detector in an ion-pumped cryostat cooled to liquid nitrogen temperature. The multi-channel analyser has been replaced with a PDP-8 data processor, which acts as the store for the output pulses from the ADC, which has been interfaced to the computer. The PDP-8 is a 4K computer with a 12-bit word length, and it has been pro- grammed to function as a 1000- or 2000-channel analyser, capable of storing 21R counts per channel. The y-spectra displayed on an oscilloscope, are permanently stored on a subsidiary magnetic tape system and the data can then be suitably processed. The magnetic tapes can then be processed directly on the larger IBM 360/65 computer, if least-squares fitting techniques are required to deal with complex spectra. Alternatively, manipulation of the data can be performed on the PDP-8 itself. The development of this system is still in its early stages but it offers several advantages over conventional multi-channel analysers.

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120 SOME ASPECTS OF INORGANIC TRACE ELEMENT ANALYSIS

Gamma-activation Analysis BY C. A. BAKER

[Proc. SOC. Analyt. Chew.

(Atomic Energy Research Establishment, Analytical Sciences Division, Harwell, Didcot, Berkshire)

THE principles of activation analysis are well known. They are to activate the material to be determined, separate the required activity chemically or instrumentally, count, correct for decay and compare with known standards. Activation can be carried out by using thermal neutrons, fast neutrons, charged particles and also with y-photons. Neutrons and y-photons have the desirable property of being uncharged and they can, therefore, penetrate deep into a sample. Thermal neutrons have been widely used for many elements but the activation products of (n,y) reactions on carbon, oxygen and nitrogen are unsuitable for activation analysis. y-Photons will produce (y,n) and (y,p) reactions on most elements; the energy thresholds are 10 to 20 MeV and the peak cross-sections occur a t 15 to 30 MeV. Photons of this energy can only be produced as bremsstrahlung, usually by stopping high energy electrons from an accelerator in a tungsten target. The product nuclides from y-irradiation of carbon, oxygen and nitrogen are particularly suitable for activation analysis and this is the main reason for the development of the technique.

The products from these light elements decay by positron emission and only O.51-MeV annihilation y-rays are observable by y-counters. In model systems the decay curve can be analysed to identify the components but generally fast, clean and efficient separations are necessary. The method has been used for determining carbon and oxygen in pure metals at the 1 p.p.m. level.

?/-Photons can be used to activate all of the elements in the periodic table (except hydrogen, helium and lithium), but unlike thermal neutron activation there are no large differences in cross-section from element to element. Roughly equal sensitivities can be achieved for all elements, and the technique is useful for the determination of major components in mixtures. Fortunately the heavier elements give rise to active species that decay with the emission of characteristic y-rays, which simplify identification. For example, a chromium - nickel - niobium stainless steel has been examined, and y-ray photopeaks were identified that corresponded to chromium-49 at 0.089 and 0.152 MeV, iron-53 and manganese-52% at 0.38 and 1.43 MeV and nickel-57 at 1.38 MeV. These activities can be used to determine chromium, iron and nickel, respectively.

Pure alumina has also been examined and the activities that were induced in the aluminium and oxygen were identified. Impure alumina, when activated, shows additional features in the y-spectrum, which have been identified as resulting from titanium, calcium, yttrium, iron, caesium and zirconium, present as impurities. Similar reactions are observed when silicate rocks are irradiated, and features can be recognised that correspond to aluminium, silicon, potassium, titanium, calcium, manganese and magnesium.

To improve the access to the accelerator for this type of irradiation a pneumatic transfer system has been designed and constructed.

The technique of y-activation analysis is now well established for the determination of the light elements, and there are many other fields in which it is proving of value. In particular, the analysis of impurities in a matrix that is unsuitable for thermal neutron activation, and intact analysis of alloys, are fields in which there could be substantial progress.

Activation Analysis with Charged Particles and Fast Neutrons BY T. B. PIERCE

(Analytical Sciences Division, Atomic Energy Research Establishnreiit, H a w e l l , Devkshwe)

,~SALYTICAL methods based on the irradiation of a sample with charged particles differ from those in which penetrating radiations, such as neutrons or y-photons, are used, in that the relatively low depth of penetration of charged particles limits interaction with a thick sample to a thin layer near to the surface. In this case, the gross element content of the sample can only be deduced from charged-particle data if the volume of the sample irradiated is charac- teristic of the sample as a whole ; however, charged-particle techniques offer sensitive methods of surface analysis. Moreover, if the diameter of the beam on target is carefully controlled

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June, 19681 RADIOCHEMICAL METHODS 121

and the particle energy is not too high some lateral resolution is also possible and specific areas of the sample can be selected and examined.

Information about the composition of the sample can be deduced from analysis of either prompt or delayed radiation emitted from the sample, but only methods based on the measurement of the radiation emitted during irradiation will be considered here.

Elastic scattering can be used to investigate surface layers, and the method is particularly satisfactory when thin films of a heavy element are present on a substrate of an element of lower atomic number, for particles scattered from the thin film have a higher energy than those scattered from the thick substrate. The technique has been used to examine gold films on silicon backings, as amounts of gold of less than lo-* g in beam can be rapidly determined, and the distribution of gold in films with maximum thicknesses of 20A has been investigated by scanning an a-particle beam across the sample.

Elemental analysis can be carried out by measuring the prompt y-radiation emitted during charged-particle irradiation. The Coulomb barrier restricts reaction of particles of low energy to elements of low atomic number, but most of the light elements can be determined by the technique, if the type of particle and the irradiating energy are chosen carefully.

Sensitivity a t the parts per million level is available for many elements under the most favourable conditions, i .e., when there is neither nuclear interference nor a high y-background in the spectral region of the measured peak. Boron, carbon, nitrogen and oxygen have all been determined as a result of deuteron bombardment, and concentrations of sodium, mag- nesium, aluminium and silicon have been measured by inelastic proton scattering. Again, small portions of the sample can be examined if the beam is carefully collimated, and thc distribution of magnesium, aluminium, silicon and iron across a number of geological speci- mens has been investigated.

Nuclear reactions induced by fast neutrons can be used to produce radionuclides from elements that cannot be satisfactorily determined as a result of radiative neutron-capture reactions. In addition, relatively inexpensive neutron generators have been produced, based on the reaction

2H + 3H + 4He + ln + 17.6 MeV, and sealed-tube units that can be installed in a small laboratory are now available. The most attractive aspect of fast-neutron activation analysis is that of analysis of the intact sample by y-ray spectrometry after irradiation, and the advent of high-resolution germanium counters has increased the potentialities of the technique. However, compared with con- ventional reactor activation analysis, nuclear interferences are more acute and neutron fluxes through the sample do not compare with reactor fluxes. By using a neutron generator based on the D-T reaction, several elements, including titanium, vanadium, chromium, manganese, cobalt, silicon and oxygen, have been determined in metallurgical samples.

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