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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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