DETERMINATION OF GOLDIN GOLD JEWELLERY ALLOYS

BY ICP SPECTROMETRY*

M. Brill

K-H. Wiedemann

W C. Heraeus GmbH,Hanau, Germany

As a result of recent research and development worksupported by World Gold Council, Inductively Coupled

Plasma Spectrometry (ICP) is rapidly emerging as a seriousand viable alternative to the centuries-old cupellation or fire

assay method for the determination of goldin gold jewellery alloys.

INTRODUCTION

No sector of industry makes such great demands on theperformance of the methods for the analysis of ele-ments as that of precious metal chemistry. Since onlyat a relatively few places in the world is the whole rangeof precious metals activities to be found — from primarywinning, manufacture of typical products, through torecycling — only a few specialists have experience withall the analytical methods available today for the analy-sis of precious metals.

Consequently, the impact of new spectrometrictechniques has been felt only during the last few years.But today, in addition to the widespread use of largespectrometers for quality control, spectrometry is now

beginning to play an important role in the finenessanalysis of gold alloys and other precious metals usedin the jewellery sector.

Cupellation or Fire AssayThe determination of the gold content in gold jewel-lery alloys has hitherto been carried out with unrivalledprecision by the 'cupellation' or fire assay method [1].The basis of this method is that the gold jewellery alloyspecimen for analysis is diluted with silver and, to-gether with added lead, is melted in a cupellation

* This paper is based on an earlier version originally published inGerman in Metag 1991, 45(7), 656-62.

Gold Bull., 1992, 25 (1) 13

crucible (magnesium oxide) so that the lead is progres-sively converted into lead oxide. In this process, the leadoxide takes up the oxides of the base metal part in theform of a molten glass that drains away from the hotmetal bead and diffuses into the porous crucible. Themetal button remaining after this cupellation processessentially consists of gold and the excess of addedsilver. The silver is extracted from the flattened noblemetal button in nitric acid leaving the gold. The goldcontent of the sample is then accurately determined byweighing.

From time immemorial, the cupellation techniqueoutlined above has been the most precise methodavailable for the determination of gold content subjectto the proviso that the material to be investigatedsatisfies the ideal conditions for the application of thetechnique, as is the case for gold-silver-copper alloys.

However, the rapid development of new materialswith the use, for example, of platinum metals in gold-containing alloys, is continuously presenting the ana-lyst with new problems for the adaptation of theirtraditional methods. In the most extreme cases this hasled to the abandoning of these methods.

It is not surprising that in view of technical progressand the opening up of the international markets, vari-ous efforts have been made to ensure internationalharmonization of the analytical methods used for thetesting and designation of goods made of preciousmetals. The ISO (International Organization forStandardization) Technical Committee TC 174: Jew-ellery, and the corresponding CEN (Comite Europeende Normalisation) Committee TC 283: Precious Met-als Applications in Jewellery and Associated Products,are working on the preparation of international stand-ards for jewellery, while the analytical techniques thatcoul be standardized for the determination of finenessare being reviewed by the corresponding WorkingGroup WG1.

SpectrometricMethods

Because of its high performance in the assay of standardgold jewellery materials, the specification in ISO/TC174 N87 [2] for the cupellation method has met withinternational approval as referee method for the deter-mination of gold. On the other hand, new develop-

ments in the field of ICP (inductively coupled plasma)solution spectrometry indicate that this method couldwell become a viable alternative to cupellation. Thehigh precision of the ICP method makes it equallysuitable for the determination of precious metal con-tents of gold (N71), platinum (N67) and palladium(N69) in the corresponding jewellery alloys.

The use of spectrometric methods to analyse forthe elements present, and hence determine the finenessof precious metal alloys in weight percent, can beconsidered only if the method could be relied upon togive results with variation coefficients of well under1 %. In the particular case of coloured and white goldjewellery alloys, it is clear from the ISO-TC 174 work-ing document N 50 [3] that, for the analytical deter-mination of the gold in gold-based alloys, greater pre-cision is specified than for the main components ofcommon base metal alloys. At the same time, thisdocument also raises the question of the reliability ofthe determination of gold in the parts per thousandrange of accuracy.

In a draft document, ISO/TC 174 WG1 N 71 [4],the ICP method for the determination of gold in goldjewellery alloys was considered as being the sole spec-trometric method capable of serving as an alternativeto cupellation. The required precision with a repro-ducibility of about one part per thousand (1%0) goldcan be achieved by the ICP method provided thefollowing conditions are met:1. Simultaneous measurement of the gold I line,

267.6 nm, and the yttrium II line, 371.0 nm, us-ing yttrium as the internal reference element.

2. Buffering the strong hydrochloric acid-based aque-ous solution of the gold in the presence of copperchloride and sodium nitrate.

3. Use of precision weighing to ensure exact aliquotsof the solution of the gold jewellery sample and theadded amount of the yttrium reference solution.

4. Measurement of the sample and calibration solu-tions with both prepared at the same time.

By taking these precautions, it has been possible toestablish that cupellation is not more than five timesbetter than ICP spectrometry.

The ICP method, as described in this paper, hasthe following important advantages with respect tocupellation:— It permits the simultaneous determination of sil-

ver and palladium as well as gold.

14 Gold Bull., 1992, 25 (1)

is

XRFPM—ALLOYS

ICPAu-SOLUTIONS

PM-SOLTYMN—s-

ETAPM-TRACES

01111Ww.

— Gold can be determined in materials for whichcupellation cannot be used directly.

— It is well suited for automated analyses.Undoubtedly, the confirmation of the suitability of theaqueous solution medium for the ICP determinationof platinum, rhodium and iridium, as described below,is also of great interest. It is only with extreme difficultythat these precious metals can be determined by gravi-metric techniques.

ICP Spectrometry

There can be little doubt that for a comparison ofmethods for the determination of gold in gold jewelleryalloys for the purpose of fineness analysis no techniquescould be more appropriate than cupellation and ICPspectrometry (Figure 1).

Whatever the purpose of the analysis of gold —analytical quality control, hallmarking, recycling — inthe comparison of available methods for gold jewelleryalloys, cupellation with lead provides an accurate andreproducible gold value that is little short of 'disquiet-

ing'. The high analytical performance of this methodis unequalled, arising from the noble character of golditself and the consequent marked tendency of theelement to prefer the metallic state. It is therefore notsurprising that cupellation is the obvious choice ascriterion for comparison with a spectrometric substi-tute method. In other words, this means that in the caseof gold it is somewhat venturesome to invite compari-son with other methods.

The reasons for considering other methods areelucidated elsewhere [5]; as the underlying theme inthe following account the question in the foregroundis under which conditions the performance of ICPspectrometry, which in principle is known, as regardsthe development of a comparable determination ofgold and also all the other precious metals — can berealized.

Here, it is evident that with the measuring signalof the gold ICP emission internally standardized bymeans of a foreign element such as yttrium, a repro-ducibility of between 0.01 % and 0.1 % is effectivelyattainable. It is encouraging to consider that for therival technique of cupellation of white gold, the draft

Analytical Scheme forPrecious Metals

Figure 1

Gold Bull., 1992, 25 (1) 15

Ag -75

-Cu\5

Zn max 20Sn, In, Ga each max 4

Coloured Gold%o

Ag Cu

750 0-20 5-25

585 5 - 35 5 - 35

375 5 -15 45 - 50

(333) 5-40 25 - 60

White Gold%o

750

Ag

0-10

Cu

0 -10

Pd(Ni)

10 - 20

585 0 -25 5-30 5-20

- 25 375 0-35 5-50 5-20

(333) 0-35 10 - 50 5-25 75

Gold-Jewellery Elemental Composition[%] for Orientation

Figure 2

ISO standard on the reproducibility of cupellationspecifies 'only' one part per thousand of gold [3].

Inevitably, the idea arises in this case of using thedissolved jewellery sample to determine silver and pos-sibly also palladium at the same time. As a workinghypothesis in the development of the solvent medium,the extent to which it is also suitable for the measure-ment of palladium, platinum, rhodium and iridiumshould always be borne in mind. These elements ex-hibit roughly the same sensitivity in their ICP emis-sions as gold. Precious metal chemists will also appre-ciate this from the viewpoint that alternative gravimet-ric methods for rhodium and iridium cannot reason-ably be considered as practicable. Incidentally, it maybe mentioned here that preliminary draft ISO stand-ards have been prepared for the ICP determination ofpalladium [6] and platinum [7] in the correspondingjewellery alloys on the same basis.

EXPERIMENTAL

The Sample Material & the Requirementsfor the Analysis of Gold

The supposition that it will be possible to determinerepresentative gold values with small samples of goldjewellery alloys [8] is supported by the following ex-perimental facts: the arrangement of the elements Au,Ag, Cu, Pd and Ni in the periodic system is reflectedin their chemical and physical properties in that inliquid mixtures at the concentration ratios of jewelleryalloys they exhibit mutual solubility. Solidificationfrom the melt is dominated by the main componentsAu, Ag and Cu, and by means of the continuous castingtechnique, for example, material segregation duringsolidification can to a large extent be prevented. Thegenerally applied additional mechanical working of theductile alloys is finally a further factor contributing to

16 Gold Bull., 1992, 25 (1)

material homogeneity so that even extremely smallsample weights allow accurate and representative de-termination of the gold content. The logical outcomeof this is found in the normal practice of cupellation ofgold, where, with samples as small as 0.125 to 0.250 g,reproducibility of the determination of gold to betterthan 1%0 can be achieved. As well as the generalizedmaterial structure of the alloys the colours that may beobtained are also of interest for jewellery applications(Figure 2).

The preliminary draft standard for gravimetricanalysis of gold [3] differentiates as regards the requiredreproducibility between coloured gold (0.5%0) andnickel-containing white gold (1%o) and this is readilyachievable by means of cupellation. The reproducibil-ity requirement of 1Too for nickel-containing whitegold provided an opportunity for the analysts eager topromote the ICP method to use it as a basis for thesystematic use of the method. The fact that palladium-containing white gold alloys were covered by the result-ing draft standard on the ICP method [4], gave rise tothe additional objective of avoiding the obstacles en-countered when determining gold by the cupellationmethod in the presence of nickel and palladium.

Procedure for the Determination of Goldin Goldfewellery Alloys Using ICP(Inductively Coupled Plasma) Spectrometry

Since the cupellation and ICP spectrometry methodsfor the analysis of gold in gold jewellery alloys aredescribed in the ISO preliminary draft standards, ref-erences [3] and [4], experimental details can be omittedhere. This account may also be taken as a commentaryon the ISO drafts.The simplicity of the ICP method for the determina-tion of gold is illustrated by the following:- the chemicals required (Table 1),- the preparation of calibration and sample solu-

tions (Table 2),- the ICP emission lines to be measured for gold and

yttrium (Au I: 267.6 nm; Y II: 371.03 nm),- the model used for the numerical evaluation

(Table 3).It can be seen at once that the decisive criterion for theaccuracy of the ICP technique corresponds to thereproducible determination of the quotient of the lineintensities of IAu267.6nm / 1371.0nm . For 750%0 goldthis coefficient can be determined at a probability of95 % with a variation coefficient or RSD (Relative

Table 1:Chemicals Used

for the ICP Method of Gold

Table 2:Measuring Solutions

for ICP Spectrometry of Gold

Gold Bulk, 1992, 25 (1)

17

Table 3:The Numerical Evaluation of Gold

by ICP Spectrometry

Standard Deviation) of s 0.1 % under simultaneousmeasurement of the two lines with an integration timeof 5 s for each with the emission spectrometer. As iswell known, to obtain optimum accuracy with the ICPmethod the absence of additive interference of possibleaccompanying elements an the gold and yttrium linesis a decisive prerequisite.

Table 4 is a synopsis of the research on possibleinterfering elements. It shows that the selected pair ofAu and Y lines in conjunction with the usual compo-sition of gold jewellery alloys but also in the presenceof numerous elements — for example in the case of golddental alks [9] — can allow the determination of goldwithout interference. These facts and not least thecomposition of the solute medium used in the ICPexcitation constitute the core of the analysis method.

The individual components of the ICP measuringsolution, their concentrations and their probable ef-fects are described below (Table 5).

Au3+ 30 to 100 mg/I

This corresponds to the range of the gold concentra-tions to be measured. The ICP signal for gold is to a

large extent linearly dependent on the gold concentra-tion in this range.

Y3+ 20 mg/I

This concentration of the internal reference elementyttrium equally as about 70 mg/1 gold give rise to asignal-to-background ratio of about 100:1. To ensurethe utmost accuracy, it is important to make sure thatthe precisely weighed quantity of 20.000 g yttriumstandard solution is added to the calibration and sam-ple solutions. Precision weighing is also used for possi-ble division of a sample solution into aliquot parts.Thus, dilution of the solutions to the end volume ofthe measuring solution does not affect the accuracy.This procedure ensures that the requirement of highaccuracy can be taken care of by simple means. Exces-sive care for what in practice will generally be a volumemeasurement in great demand is thus quite unneces-sary.

Cu++ (CI)" 10 000 mg/I

The relatively high copper concentration in itself al-ready justifies the idea of a buffer solution. In thestrongly acid solution medium, the high concentration

18 Gold Bull., 1992, 25 (1)

Table 4:Determination of Gold by ICP Spectrometry

Interfering Elements (10 % in Sample)

of copper ions additionally produce the oxidation po-tential necessary for the stabilization of Au 3+. With thehigh proportion of copper there is, moreover, thepossibility for a so-called quartation if metal samplesdo not immediately go into solution with acids. In thiscase, the sample (100 mg) is melted with 5 g of copper.The resulting alloy of high copper content is then easilydissolved in the HCl/HNO3 mixture. For adjustmentof the copper concentration, copper chloride is addedto bring the copper content of the measuring solutionup to 10 000 mg/l. In the case of gold jewellery alloys,such a quartation is generally not necessary. The metalchips dissolve in the HCl/HNO3 acid mixture whenthe following procedure is followed: after attackingwith a little HNO3, the HCI part is added; as theremaining HNO3 is then added bit by bit the sample

Table 5:Components of the Measuring Solution

to Determine Gold by ICP

dissolves on heating. (The quartation type procedureis mainly of importance for certain platinum and pal-ladium based materials that sometimes do not readilydissolve in aqua regia.)

HCI (32 %) 50 vol.%

The final measuring solution is thus effectively 16 %hydrochloric acid. This makes it possible for some50 mg/1 of silver to be held in solution with preferentialformation of the AgC12 complex. In this respect, thesolutions have long term stability so that the silver canbe determined by the ICP method at the same time asgold in the jewellery alloy solution.

Na+ (NO3) 1000 mg/I

The sodium fraction allows for unforeseeable alkaliconcentrations in the solution of the precious metalsamples. However, in any event, the relatively highalkali concentration has a stabilizing effect on thetemperature and electron density of the ICP plasma.

Despite the fact that this solution medium appearsto be strategically optimum for gold and silver, solu-tions made in this way are not sufficiently stable in viewof the extremely high requirements for the measuringaccuracy of the gold intensity. As is not unusual fortransition metals it seems probable that the root causeof this is a progressive complex-ligand exchange at theAu+++ central atom ion. To compensate for this effect,it has been established that the calibration and samplesolutions should be prepared at the same time if the

Gold Bull., 1992, 25 (1) 19

CUPwith proof assay'

915.8 N =

750.5 N = 5

585.5 N 4

333,6 N = 4s 0,3r %o

without proof assay

759.6 N = 2714.6695.6661.3596.8500.9348.1288.3

°Apo Au

Table 6:

Repeatability in Determining Gold [0104

by ICP and Cupellation

Type ICP

917 915.2 N = 2

750 751.0585 586.9333

recycled jewellery material

332.8

No 5883 762.5 N =5231 714.65900 695.7

5901 661.65884 597,95238 499.8

5222 347.7

5229 288.0s= O.73",00 hu

utmost accuracy of the method is to be obtained. Asregards the precious metals palladium, platinum, rho-dium and iridium, this effect does not show itself soclearly. It has proved best to subject the calibration andsample solutions to the shortest possible measuringcycles and to replace the calibration solution by afreshly prepared solution after one week.

Table 7:Cupellation Results of Gold

in White Gold Containing Nickelafter Scorification

Au clop

Type FoundN.-12 SN=I2

335+1

336.11

0.20

590+1

590.72

0.14

750+i

750.77

0.23

Synthetic - without proof assay

750 E0 749.77

0.23

The ICP Results for Gold& Evaluation in Comparison

with Cupellation

The possibility of choosing cupellation as the compari-son method for the evaluation of the ICP analysis isparticularly fortunate. For gold jewellery alloys, cupel-lation is to be regarded as an absolute method. Al-though there can be no doubt about this, an availableset of gold determinations in nickel-containing whitegold yields the listing given in Table 7. In addition,values are shown for synthetic material which confirmthe accuracy of the gold value obtainable by cupellation[10, 11].

The high quality of the cupellation method thusmakes it ideal for comparison with the ICP spectrome-try of gold. Table 6 shows corresponding comparativedeterminations carried out on characteristic colouredgold jewellery alloys [12]. At the same time, a few goldresults from routine analysis of recovered jewellerymaterial are reproduced. On the basis of the corre-sponding standard deviation indicated it is seen that

20 Gold Bull., 1992, 25 (1)

Spent Time DeterminingGold in Jewellery

Weighingsm ple

DissolutionHNO3Cl

ICI ,Auer, I

Weighingsample solution

f Cu' -Buffer

st standar,

Conditioning

DilutionI-ICl/H. 0-; volume

Measuringcalibration-. alculaliofl

Preliminary Assay.:, ,A; Ag, Pd, ,li

Weighing + Inquartationsample lq 1

proof assay Au A

Sim Pd. N. Cu

Ag 3x Au

EncapsulationPh foil

0.- S or(Iification1100'C

'111,ye low gold

► Cupellationtiou C2.0101-1FENN::

Au Ag 0-'d-button

PartingAu Ag-cornet

I HNO, (1 2)-, HNO3 031

HNO3 (13)Au-cornet

WeighingAu-cornet. sample MAu-cornet, proof B

Figure 3

roughly speaking cupellation cannot be more than 5times better than ICP. Considered outside the contextof the wide range of conditions in which analysis ofgold has to be applied, this statement could easily beseen as disadvantageous to ICP. Moreover, it is sup-ported by the fact that the cupellation of gold — par-ticularly in the case of Au/Ag/Cu jewellery — can becarried out in as little as 120 minutes. The attempt toextend this analysis to alloys of more complex compo-sition multiplies the effort considerably for cupellation— palladium is a good example of this. Also, one hasonly to consider that in the field of analysis of gold-containing dental alloys [9] the fact that the repeatabil-ity attainable immediately with ICP of about 1 %o goldrepresents an excellent result for the determination ofgold. For comparison of the time required for the twomethods the individual operations are represented insimplified form in Figure 3.

A little background should further elucidate theperformance of ICP spectrometry.

Because of the extensive dynamic range and repro-ducibility on the one hand and the powerful detection

of the precious metals on the other, ICP emission isbreaking into the domain of 'fineness analysis' to asignificant extent. The truth of this statement for vari-ous precious metals is still being examined for themodel case of gold which from the chemical aspect isby no means the least favourable.

In Figure 4, the variation coefficients for each offive consecutive measurements of the Au/Y intensityratio are plotted against the corresponding relativecumulative frequency. The gain in reproducibility asthe gold and yttrium concentrations are increased canbe clearly seen. By giving up the high linearity a furtherreserve in the dynamic range is available to increase stillfurther the quality of the measurement. Also notewor-thy is the fact that by modification of the stabilizingcircuit of the high frequency generator using the latesttechnology it is possible to considerably improve thereproducibility of the measuring process. It should berecalled at this point that the exceptionally promisingreproducibility although it certainly provides a neces-sary quality of ICP gold determination is however byno means a sufficient condition. For checking the

Gold Bull., 1992, 25 (1) 21

99,9

— 70 Au, 20 Y (5.3.90)

1000 Au, 100 1'

- 1000 Au, 20 Y

• 70 Au, 100 Y

70 Au, 20 Y

I Au 267RSD [%0 of the average (I Y 371

at four different conditions

CA u, Cy (mg/1)

0,1 0,2 0,3 0,4 0,5 0.6 0,7 0,8 0,9 1,0 1.1 1,2 13 1,4

RSD

99

5

Figure 4:Reproducibility Analysis for the ICP Determination of Gold

accuracy, multiple determinations and parallel meas-urements on known test specimens must not be dis-pensed with.

Table 8:

Determination of Gold and Silver in Jewellerywith ICP of 5 mg Samples

The performance of the ICP method as regardssensitivity is shown in Table 8. On the basis of accu-rately weighed 5 mg samples of the most current jew-ellery alloys with 750, 585 and 333 %o gold in eachcase the gold and silver content was simultaneouslydetermined by the ICP method. Nevertheless, usingthe original method 50 ml of the measuring solutionwere still available. Even with 5 ml of measuring solu-tion or 0.5 mg of the jewellery sample it is possible thata meaningful analysis for gold and silver may be carriedout under these conditions; this gives rise to the con-cept of an 'ICP precision touchstone'.

Equipment

Evidently, the spectrometer configuration used is ofmajor interest [13, 14]. It is shown in the simplifiedblock diagram of Figure 5. The diagram also indicatesthe particular measuring lines and other relevant lines.The spectrometer can thus be described as a simulta-neous-sequential emission spectrometer [15]. For ap-praisal of the choice of optical parameters of the indi-

22

Gold Bull:, 1992, 25 (1)

•

"'40g9tObleir7. *V'

to r

ICPStand

HFGenerator

SIMSpectrometer

Pt 265 9 IGDL

Video StandDisplay Scanner

GDLKeybord PunterSource

Y 371.0

Au 267 6Aci 338.3Pd 340.5

SEQ• Spectrometer

Lightbeam4-

t

4••• nn••••••nn•11\

t

SamplerUnit

Table 9:The Technical Specifications

of the Simultaneous ICP Spectrometer

Schematic Principle of theSpectrometric Configuration

Figure 5

Gold Bull., 1992, 25 (1) 23

24 Gold Bull., 1992, 25 (1)

Figure 6:

One of the Authors (K -H. Wiedemann) Calibrating the ICP Spectrometer

Table 10:The Technical Specifications of the Sequential ICP Spectrometer

Table 11:Physical Parameters of the ICP Light Source

cated measuring lines of gold and yttrium the corre-sponding data for the simultaneous spectrometer areemphasized in Table 9 and those for the monochroma-tor part in Table 10.

It has proved extremely useful to supply the meas-uring solutions to the ICP torch by means of a sam-pling apparatus. In this way it is possible, for example,without great difficulty, to measure the sample solu-tion so that it is bracketed by successive calibrationsolutions, thereby appreciably increasing the accuracyof the value obtained. The vessels containing the sup-plies of calibration solutions are connected by tubingcommunicating with the appropriate sample tubes.The physical arrangement is shown in Figure 6. Re-garding the use of this multifunction spectrometerarrangement, automatic ICP spectrometry representsthe optimum in unattended laboratory time.

Finally, the physical data of the actual plasmasources are reproduced in Table 11.

Conclusions & Prospects

Comparison of the performance of the two analysismethods for gold — cupellation and ICP spectrometry— can serve to weigh the arguments for the use of oneor other method in the field of jewellery alloys. Thiscomparison is summarized in critical form in Table 12.Notwithstanding the outstanding quality of the cupel-lation method it should not be overlooked that for

various reasons its position as the universal method forgold is controversial. It would be unwise in this con-nection to lose sight of the outstanding performance ofcupellation of gold [16]. It should be remembered thatthe docimastic collection of precious metals — in par-ticular, gold [17] — is of general importance in thecontext of the analysis of complex recovered material.It will be clear that this account aims to show that ICPspectrometry is also outstandingly well suited for theanalysis of gold and that it represents a valuable com-plementary component of the analytical strategy ofprecious metal chemistry.

Why the performance range of ICP spectrometryis only gradually being appreciated is certainly to beascribed to the apparently greater specificity of the ICPmethods. Apart from the mostly quickly checked ad-

Table 12:Comparison of the Performance

of the two Methods

Gold Bull, 1992, 25 (1)

25

ditive interference effects, such as the line interference,the element signals appear to undergo multiplicativeinterference so that the radiation performance of thecentral atom ion is affected by the different effects ofcomplex ligands. The kind of coupling of the highfrequency power and possible 'atomization mechanics'of the solution do not, however, seem to be involvedin this chemical-thermochemical effect. This and, forexample, also the widespread misconception that forinternal standardization only atom line pairs or ion linepairs lead to a successful result — Au 267.6 is an atomline and Y 371.0 an ion line [18] — makes one wonderwhether the state of knowledge of ICP has alreadyreached its achievable level. A further factor is thereforeadded to this unclear situation, namely, that on thebasis of better knowledge of the solution chemistry, themeasuring strategy and the undoubted further devel-opment possibilities of experimental techniques, ICPspectrometry will in the future be capable of analysingelements with virtually any desired degree of precision.

References

1 W. Walchli and P. Vuilleumier, 'Assaying Gold by Cu-pellation', Aurum, 1987(No. 29), 56-64; Gold Technol-ogy, 1991(No. 3), 19-27.

2 International Organization for Standardization, Doc.ISO/TC 174 N87, June 1988

3 International Organization for Standardization, Doc.ISO/TC 174 N50, June 1988

4 International Organization for Standardization, Doc.ISO/TC 174 WG 1 N 71, Milano, Oct. 1989

5 M. Brill, `Bericht fiber das Geschaftsjahr 1988',Fachvereinigung Edelmetalle e.V. Dusseldorf, pp. 63-84 (1989)

6 International Organization for Standardization, Doc.ISO/TC 174 WG 1 N 69, Milano, Oct. 1989

7 International Organization for Standardization, Doc.ISO/TC 174 WG 1 N 67, Milano, Oct. 1989

8 Heraeus Edelmetalle GmbH, Edelmetall-Bijouterie-Legierungen, Hanau, 1986

9 Heraeus Edelmetalle GmbH, Edelmetall-Dental-legierungen, Technische Daten, Hanau, 1986

10 K. Aldinger, 'Personal Communication', Pforzheim,1989

11 H. Knosp, 'Personal Communication',Pforzheim, 1989

12 H.-M. Laschow, 'Personal Communication', Hanau,1989

13 M. Brill, 'Proceedings International Precious MetalsConference (IPMI)', Brussels 1987, 369-400

14 M. Brill, TTB-Bericht des 78. Seminars', Physikalisch-Technische Bundesanstalt, Braunschweig, 1989, 187-206

15 Applied Research Laboratories SA. Manuel 3580/1186,Ecublens, 1984

16 The Gold Institute, Washington D.C., The Fire Assayof Gold', January 1985

17 E. E. Bugbee, Textbook of Fire Assaying', ed. J. Wiley& Sons, Inc., New York, 1940

18 M. Brill, P. Cassagne, 'European Winter Conference onPlasma Spectrochemistry', Dortmund, January 1991

26 Gold Bull., 1992, 25 (1)