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Analyst, Febrztary, 1977Communications

141

Material for publication as a Comm unicatio n must be o n a n urgent matter andbe of obvious scienti;fic imp ortan ce. R ap id it y of public ation i s enhanced i fdiagram s are omitted, but tables and form ula e can be included. Communic a t ionsshould not be simple claims fo r prior i ty: this facil i ty for rapid publication i sintended for brief descriptions of work that has progressed to a stage at which itis likely to be valuable to workers faced with similar problems. A f u l l e r pape rm a y be offeered subsequently, if justijied by later work.Manuscripts are not subjected to the usual examination by referees andinc lusion of a Com mun ication is at the Edito r's discretion.Novel Sampling System for the Direct Analysis of Powders byAtomic-absorption SpectrometryKeywords Powders ;min erals ; sa mp ling; atomic-absorpt ion spectrometry

The analysis of soils, rocks, minerals, cement and similar materials by atomic-absorption spectro-metry has received widespread attention. These materials, however, usually require time-con-suming and often complex dissolution procedures prior to their analysis and the possibility of usinga simple, direct method of analysis for this type of material would therefore be particularly welcome.Willis' has recently studied the factors that govern the analysis of powders by flame atomic-absorption spectrometry using a slurry procedure. After grinding solid samples, then dispersingand suspending them in an aqueous medium, by using constant agitation, i t was possible to nebulisethe samples directly and to determine the concentrations of several elements. Unfortunately, thereare two limitations to this technique: in order to achieve a satisfactory atomisation efficiency' it isnecessary to grind the solid samples to a particle size of less than 10 pm and secondly constantmixing of the suspended material, using either ultrasonic agitation or a magnetic stirrer, is requiredduring the analysis.

Firstly, by usingelectrothermal atomisation (Perkin-Elmer HGA 74 and Model 360 atomic-absorption spectrometer)rather than flame atomisation, it is possible to tolerate larger particle sizes for the samples. Thisadvantage is borne out by work carried out on the direct introduction of powdered solid samples toelectrothermal atomisers.2-5 The limiting factor often becomes dependent more on the ability toachieve homogeneous sampling from the suspension. Secondly, by using a thixotropic thickeningagent it is possible to achieve a suspension which is stable for several days but which can still berepeatedly and reproducibly sampled.The thixotropic thickening agent used in this work was a commercially available material,Viscalex HV30 (Allied Colloids Ltd. , Bradford), which is an acrylic copolymer containing carboxylgroups, supplied in the form of an acidic, low-viscosity emulsion. Dilution and neutralisation ofthis emulsion produces a highly viscous gel over the pH range 6-10, The production of thisviscous gel together with the incorporation of the powdered sample unfortunately introduces a largenumber of air bubbles, which makes reproducible sampling of the suspension extremely difficult.The bubbles can be removed, however, by ultrasonic agitation of the gel, for about 60 s, so that thetrapped bubbles rise to the surface as a froth. An alternative and more convenient procedure is toadd a commercial de-foaming agent (e.g.,NOPCO NPZ, supplied by Diamond Shamrock ChemicalCo., Morristown, N.J., USA) to the gel during preparation, which considerably reduces the amountof air trapped in the gel.Fig. 1 illustrates the variation in signal response for chromium in a rock sample (0.1 g ) suspendedin various concentrations of Viscalex HV30 (0.5, 1.0 and 2.0% V /V in a volume of 100 ml). Thesuspensions were analysed by atomic-absorption spectrometry using electrothermal atomisation,over a period of several days, without agitation of any type. The results show th at the 2% V /Vconcentration was stable for the period of testing; a 5% V /V concentration, while also providing avery stable suspension, could not be sampled satisfactorily owing to its high viscosity.Checks on the reproducibility of sampling from a 2% V /VViscalex HV30 gel produced the follow-ing results. The average mass of 20 consecutive samples taken with a 5O-pl micropipette was0.0466 g with a relative standard deviation of 0.01 (the specific gravity of the gelat 20 "Cwas 0.999).

In the work outlined here, these two problems have been largely overcome.

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142 COMMUNICATIONS Analyst, Vol. 102There is a reduction in sample size, therefore, of approximately 7% compared with aqueoussamples.

Recommended ProcedurePlace up to 1 g of sample in a200-ml beaker together with 1 ml of a 1% m/V solution of sodium hexametaphosphate (a wetting

agent) and mix the two together with a glass rod. Add 2 ml of Viscalex HV30, dilute to approxi-mately 50 ml with water, add 0.1 ml of NOPCO NPZ and then a few drops of ammonia solution toneutralise the mixture. Stir well with the glass rod for 1-2 min to disperse the sample as the gelforms. Either dilute the solution to approximately 100 ml in the beaker or transfer i t into a100-ml calibrated flask prior to dilution with water. Finally, stir or shake the sample to effectefficient mixing.

Grind the sample to pass a 325-mesh (,

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February, 1977 COMMUNICATIONS 143th at a micro-sampling cup system is used for nebulisation. It is probable that a less viscous 1%V /V Viscalex HV30 gel would be preferable. A t this concentration the reduction in sensitivitycompared with aqueous standards is of the order of 30%. Using this instrumental set-up, one ofthe problems described by Willisl is overcome, i.e., the need for constant agitation of the sample.The major requirement that th e sample particle size must be below 10 prn is still essential, however,in order to obtain a high atomisation efficiency.

In summary, the use of a thixotropic thickening agent enables the advantages of the slurryprocedure to be exploited to the full. Lengthy sample dissolution procedures, which are requiredfor normal methods of analysis, are avoided. Multiple determinations can be achieved from a singlesample weighing compared with the multiple sample weighings that are required for direct solidanalysis.

This work is published by permission of the Directors of Tioxide International Limited.References

1.2.3 .4.5.

Willis, J , B., Analyt. Chem., 1975, 47, 1752.Langmyhr, F. J., and Thomassen, Y . , 2. A d y t . Chem. , 1973, 264, 122.Langmyhr, F. J. , Stubergh, J . R., Thomassen, Y . , Hanssen, J . E., an d Dolezal, J., Analytica Chim.Langmyhr, F. J., and Rasmussen, S., Analytica Chim.Acta, 1974, 72, 79.Gong, H., and Suhr, N. H., Analytica Chim. d a , 1976, 81, 297.

A c t a , 1974, 71, 35.Received December 14th, 1976

Tiox ide International Limited,Stockton-on-Tees,Cleveland C.W. FullerI. Thompson

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