Indi an Journal of Chemical Techno logy Vol. II. March 2004, pp. 170-1 77

Ro]e of matrix material in the characterization of high-purity copper by fl ame and graphit~ fu rnace AAS

Sanj ukta A Kum ar, Manisha B Sangli kar, M ahze bin S Shaikh & M Sudersanan*

Analytical Chemistry Div., Bhabha Atomic Research Centre, Trombay. Mumbai 400085. India

Received 2 Muv 2003; rel'ised received 26 Septelllber 2003; accepted 5 Fehm arv 2004

Ma trix effec t in the determi nat ion of trace metall ics in high- pur ity copper has bee n evaluated for est imation of Ag. Cr. Fe. Ii. Pb, Sb ,lIld Zn by FAAS and GFAAS . Matrix effec ts were less significant fur FAAS while it was qu ite ill1 po n:lIlt fo r GFAAS. Methods have bec n stand ardized fo r de term ination of varioll s trace elcmcn ts ann separation of matri x clement and pre-concentration of the Jna lytes. For separat ion of ma trix two differe nt procedures were standa rdized: (i) co-precipit at ion of meta l hydroxides using La as carri er and (ii ) electro depos ition . To va lidate the mcthod , the recovery of' mctal ions were tes ted and were found to be satisfac tory. The methods have bee n app li ed to the determ inati on of these impurities in high-purity copper samples. For the det erm inati on o f' silver, a ~el ec t i ve

depos ition method was ado pted in order to avoid co-deposition of co pper. These three di lTe ren t modes or' matri x separati on were found to be complementary to each other for determin ati on of' varioll s criti cal trace clemen ts in hi gh­purit y e lectrol yti c copper.

IPC Code: C25C Keywords: Copper, Silver, Flame fu rnace, Graphite furnace, Matri x efrec ts, Co-precipit ati on, Electrodeposit ion

Recently, there has been a concerted effo rt to anal yse high-purity meta llic materials with high sensiti vity and precision. This is due to the reali zation of the deleterious ro le of trace metal ions in affecting the properties of high-purity materi als of strategic importance. Copper is one of the most important components in electronic de vices and the presence of trace level of impurities can affect their conducti vityl. High-purity copper used in modern e lec tri cal in struments is produced by electro lytic refining. It contains impurities ranging from 0. 1 to 20 ).l g/g, whi ch may lower th e e lec tri ca l conductivity and enhance the so ftening temperature2•

High-purity copper wires used fo r sound recording, high fidelity sou nd reproducti on, etc. contain sil ver and sulph ur, which should not exceed 0.1 ppm (4.98x I 015

ato lll s!c lll ' ) and 0.0 I ppm ( I .68x I 0 1' atoms/cm' )

respecti vely'. It is, therefore, very important to quanti fy the most common impuriti es present at trace levels in high-purity copper. Non-meta llic constituents like S and H are analyzed by elemental analys is while fo r metall ic impurit ies AAS tec hniques are generall y employed.

" For corres pondence (E-mail: headacd @magnll Jl1 .barc.e rnet.in : F;t\: + 9 122 5505 151 )

Although impurities in ppm to ppb levels can be determined by GFAAS, matri x separati on is des irab le fo r the accurate and precise determination of trace impuriti es in pure me ta ls . S ince th e nature and concentrati on levels of impurities may vary with the nature of samples, it is also des irable to in ves tigate the feas ibility of analys is without matrix remova l, as it simplifies analytica l meth odology and also minimizes the contribution due to the blank . For separation of copper matrix e lectro-depos iti on~ - X is an attrac tive option because of its simplicity. Electro-depos ition of copper fr om a soluti on co nt a inin g ove r 99 % co pp er concentration by application of selecti ve potential result in currents which arc often very high()x . So it is difficult to control the exact depos ition potenti al and needs a frequ ent attenti on unl ess a good potenti os tat is employed. Keeping the current fixed is easy and simple and can be applied for routine analys is. In thi s paper a fixed current electro-depos ition method was fo ll owed fo r matrix removal. The result s obtai ned are described in thi s communication.

Sepa rati on of ana ly tes as the ir hyd rox ide prec ipitates using a su itable carrier has been enip loyed for the prior separat ion of trace impuri ties in some

KUMAR el nl. : CHARACTERIZATION OF HIGH PU RITY COPPER 171

mate ri a l s') · I~ . Kochi et a/. ~ have described the estimation

of Ag and Sb by ICP-MS after separation of copper

matri x in a s imilar mann e r. The co-precipitati on

procedure has been evaluated for the estimation of trace

ana lytes in copper by FAAS and GFAAS . The results

are a lso desc ribed in thi s paper.

S il ve r is one of the essenti al constituents of copper

ores . S in ce silver gets e lec trodepos ited alon g with

copper, it is o ft en present in ppm leve ls in hi gh-purity

co pp e r pre pa re d b y e lec tro -de positi o n . A

spec troph otometric method I) has been reported fo r the

dete rmin ati on of Ag in coppe r amalgam. An AAS

procedure for Ag has a lso been reported after se lec tive

separati on of Ag by adsorption on sulphydryl cotton 10.

Se lecti ve ex traction of Ag by thi oethe r followed by

determinati on by FA AS is al so reported 17 . In thi s paper

an in ves tigation was carri ed out on the e ffect of different

depos ition potentia ls rang ing from 0 .15 to 0 .2 V ve rsus

SCE fo r se lecti ve separati on of Ag from coppe r in

nitri c ac id medium in presence o f 50 mg o f tartari c

ac id so as to selecti ve ly depos it s il ver in presence of a

large excess of coppe r. Thi s procedure was adopted for

the determination of s il ver in hi gh-purity copper.

Other separation techniques like ion exchange with hi g hl y se lec tive co mpl ex in g res in (c he lax 100),

adso rpti on and ex tracti on have been described but a re

not genera ll y applicable lx for trace ana lys is in hi gh­

purity mate ri a ls in view of limitati ons due to blank and

inc reased sample process ing which increase the ri sk of

contamination.

Direct determination o f trace e lements by GFAAS,

in presence of copper matri x is subject to errors without modifi cati o n sl ~. Hence, in the present pape r an effo rt

has been made fo r the es timati on o f Ag, Cr, Fe, Ni ,

Pb, Sb and Zn in hi gh-purity copper afte r se lec ti ve

removal of copper. The influence of matri x mate ri a l

was eva luated using FAAS and GFA AS . Three matri x

separati on procedures were adopted and were found to

be co mplementary to each othe r fo r thi s analys is .

Experimental Procedure

Instrumenta tion

For analys is of sampl es and standards, a Chemito

AA 203 in strument was used fo r Flame Atomic Abso rpti on S pec tro ph oto me try, and GBC 906 A A

Atomi c Absorpti on Spectroph otomete r w ith deute rium

co rrect io n was used f o r El ec troth e rm a l Atomi c

Abso rpti on Spectrophotometry.

Reagents

Hig h-purity copper samples were received from

Hindustan Copper Limited , Ta loja. The va ri ous ac ids

and reagents were o f ana lytical grade . De ioni sed wate r,

co llected from a Branstead Easy pure RF compact ult ra

p ure w at e r sys te m , w a s u sed thr o u g ho u t t he

expe riments. All g lasswares were kept soaked in a 107c nitric ac id bath . Before use they were ri nsed w ith

de mine ra li sed wate r. S tanda rd solut ions of trace meta ls

we re procured from commerc ia l sources.

Sample dissolution

The coppe r cathode samples were subjec ted to a

washing procedure, before di ssoluti on . Firs t these were

washed with petro leum ethe r to e nsure the removal of

any oil o r g rease, followed by a washing w ith 10%

hydrochlori c ac id . The second step ensures the removal

of any ox ide layer. In third step the sam ples we re washed with demine rali sed water fo ll owed by. acetone .

Afte r thi s all the samples were a ir-dried.

I g of the dried sam ple was take n in a 250 mL

beaker. To it 8 mL of I : I nitric ac id was added. After

the initi a l reacti on had ceased, the beake r was warmed

fo r a while to complete the di ssoluti on. The soluti on in

the beaker was evaporated till a s light blui sh prec ipitale

appeared and then it was coo led and demine ral ised

wate r was added . If the so lut ion was not c lear, then I

to 2 drops of nitric ac id was added to get a clear

solution . Afte r thi s the soluti on was diluted up to the

required volume w ith demine rali sed water.

Stock solutions

Stock solutions fo r s il ver, chromium, iro n, nicke l,

lead, antimony and z inc were prepa red by approp ri ate dilution of each 1000 mg L·1 standa rd soluti on. Working

standards for FAAS and GFAAS of these e le ments

were prepa red by success ive diluti on of these slock

soluti ons. A 40 mg/mL copper stock so luti on was made by

taking 2 g of eac h sampl e, di ssolving it in 16 rnL of

I : I nitri c ac id . Fina ll y, the soluti on was diluted to 50

mL in a vo lumetric n ask by de mine rali sed water. A

soluti on with I mg/mL in lanthanum was prepa red by

taking O. 1173 g o f La,O and d isso lving it in 8 m L o f - .\

I: I nitri c ac id . T he so luti on was diluted to 100 mL in a volumetric fl ask by demine ra li sed water. 0. 1 g/mL

ammonium chloride solution was made by taking desired

amount of so lid and di sso lving in appropri ate amount

of de ioni sed water.

172 INDIAN J. CHEM. TECHNOL.. MARCH 2004

Results and Discussion

Analysis without ma trix separation - Role of copper in analysis

Analys is without matrix separation is an attractive method. since it requires m inimum sample pretreatment. Due to this, blank values w ill be low and a good detec tion limit can be o btai ned. Analys is by thi s procedure with various modi fica ti ons']O-"] has been

repo rted. In order to eval uate the feas ibility of thi s procedu re for the anal ysis of high-purity copper, studies

we re ca rri ed out using sy nthetic sampl es by FAAS and GFAAS. Si nce impuriti es levels were low, to get a s ignificant absorbance, sample soluti ons were prepared \Vith 20 mg/mL copper concentration. Standard addition recovery experimen ts were carri ed out to invest igate any matrix effect. Addit ions of standards were done in

the beaker, befo re the add iti on of ni tri c acid for sample cli:-,so lutlon to investigate any losses during sample process ing. Ex peri ments we re carri ed out in triplicate LO ensure the reproduc ibility.

The analyt ical paramete rs and wavelength used for determinations are given in Tab le I . The resu lts obtained for the copper samples are summari zed in Table 2.

For anal ysis in GFAAS, the manufac turer's ins tructions were fo llowed with respect to temperature programme.

From Table 2, it is observed that , in case of FAAS matrix effect was not pron ounced . Howe ver in case of

GFAAS a pronounced matrix e ffect was obse rved because of the lower recovery in a ll cases except Ni.

To investigate the matrix e ffect, the s lopes of the calibration curve for matrix matched standard solutions and aqueous standard we re compared , in the case of both FAAS and GFAAS . In case of G FA AS , since a matrix e ffect was indicated from standa rd additi on recovery experiments , concentrations were taken so as to get significant absorbance values for matrix-matched standards. A 40 mg/mL stock copper so lution was used

Table I- Wavelenglh used for de lerminalions

Element s An ~

Cr Fe Ni Ph Sb

Vhvclength 3211 . 1 359.3 248.3 232. 0 2833 2 17.6

In nm

Tab le 2-Results for analysis of copper sa mpl es. without matri x separa ti on by FAAS

Elcments

Ag Cr Fe

Ni Pb Zn

Cone. in sample (~ g/mL)

NO" NO NO NO NO 0.268

Spike added Cone. obtained (~g/mL) in s piked so lution

(~g/mL)

2 1. 80

5 5 .1 0

5 4.48

5 4 .90 5 5.24

1.35

Results for ana lys is of copper. without matrix separation by GFAAS

Elemcnt s Cone. in samp le Spike added Cone. obtained

(~ g/mL) (~g/mL) in spiked so lution

(~g/I1lL)

!\g 87 100 143

Cr 1.35 50 15.2

Fe NO 50 22.5

Ni 1.08 50 47 .S

Ph 1.60 100 60.7

Sb NO 50 5.30

"N O: Not OClectcd. "SD : Standard Devi ation

Recovery obtained

(~g/mL)

1.80 5. 10 4.48 4.90 5.24

1.08

Conc. recovered (~g/I1lL)

56 13.85 22 .5 46.7

59.1 5.30

Zn

2U.X

SO" (n=3)

0.18 0.17 0.25 0 .1

0.19 0.01

SOh

(n=3)

5.1 6.2 4 .9 6 .3

5 .2 6.1

KUMAR e/ al. . CHAR ACTER IZATION OF HIGH PUR ITY COPPER

1.2

1.0 (!) () C

0.8

Cd ..0 0.6 "-0 0.4 en

..0 <t: 0.2

0.0

0.6

0.5 (!) ()

C 0.4

Cd ..0 0.3 "-0 0.2 en

..0 <t: 0.1

0.0

1.4

1.2

(!) 1.0 ()

C 0.8 Cd

..0 0.6 "-o en 0.4

..0 <t: 0.2

0.0

- Aqu eou s standard -- Matrix matched standard

... •

0 20 40 60 80 100

Sb conc .(ng/mL)

- Aqu eou s standard -- Matrix matched standard

/ 0 10 20 30 40 50

Ni conc .(ng/mL)

- Aqu eous standard ~ --M atri x match ed standard /

o 10 15 20 25

Cr conc.(ng/mL)

0.25

- Aqueous standard 0.20 -- Matrix matched standard

(!) () C 0.15

Cd ..0 "-o en

..0 <t:

0. 10

0.05

0.00

1.0

(!) 0.8

()

C 0.6 Cd

..0 o 0.4

en ..0 ~.2 <t:

0.0

0.6

0.5

w () 0.4 C Cd ..0 0.3 "-o 0.2 en

..0 <t: 0.1

0.0

o 20 40 60 80

Pb cone. (ng/mL)

- Aqueous standard __ Matrix m atched s tandard

o 10 20 30 40

Fe conc.(ng /mL)

- Aqu eous stan dard -- Matrix matc hed s tandard

o 20 40 60 80

Ag conc .(ng/mL)

100

50

100

Fig. I a- Comparison of absorbance for aqueous and matri x matched stand ards to illustrate the matri x effec t in GFAAS

to prepare a so lution with matrix content at 20 mg/mL le ve l fo r matrix-matched s tandards . Res ults are presented in Fig. I (a & b).

In case of FAAS, the re was a lmost no change in the slope fo r aqueous and matri x matched standards . However in case of GFAAS a large decrease in s lope fo r matrix matched standards in compari son to aqueous stanJarus were fo und fo r many e leme nts except fo r nid.c! This can be due to the reason that, in case of

FAAS the samples gets d iluted by fue l and ox idants , whereas in case of GFAAS the 20 mg/mL so luti on gets

further concentra ted during the va ri o us s tages o r

temperature prog ramme. Because of th is. the spec ies of interest may be fo rming inte rmetallic compou nds with the matri x e le ment , wh ich in turn can preve nt co mple te atomi zati on o f the spec ies.

Since a matri x e ffec t was observed in case of analysi s in GFAAS wi th 20 mg/mL co, pe' so lu tion.',

174 INDIA 1. CHEM. TEC H OL, MARCI-l 2004

0.06

0.05

Q) 0.04 U

C C\J 0.03

..0 "--0 0.02 (j)

..0 0.01 <t: 0.00

0. 14

0.12

~ 0.10

C C\J

..0 "--o (j)

..0 <t:

0.08

0.04

0.02

0.00

- Aqueous standard - Matrix matched standard

Cr conc .(/lg/m L)

- Aqueous standard - Matrix matched standard

0.

06

1

L-~~ __ ~ __ ~ __ ~ ___ ,~ 10

Ni conc.( /lg/mL)

- Aqueous standard - Matrix matched standard

Q) 0. 14 U 0.12 C C\J 0.10

..0 "-- 0.08 0 0.06 (j)

..0 0.04 <t: 0.02

0.00

0.4 0.6 0.8 1.0

Zn conc.(/lg/mL) Fi g. I b- Com pari son of absorbance fo r aqueous and matrix

matched standards to illu strate the matri x effect in FAAS

it became necessa ry to optimize th e matrix concentrati on up to which an accurate analysis without matrix separati on can be carri ed out. To work out this, a seri es of so luti ons were made wi th increas ing concentrations of matri x and fix ed concentration of different metal ions, separately. Results are shown in Fig. 2.

For elements except Ni , a suppress ion in signal was fo und with increase in matrix concentration . It was fo und that, for an accurate analys is without matri x separation of lead and antimony, a maximum of I mg/

0.30

0.25 Q)

U 0.20 C C\J

.D 0. 15

"--0 0. 10 (j)

..0 <t: 0.05

0.00 1

0. 14

0.12

0.10 Q)

u c C\J 0.08

.D "-o (j)

.D <t:

0 .06

0.04

0.02

0.00

120

::J' 100 E C,

80 c t.i c 60 0 u Q) 40 >-<ii 20 c <t:

0

130

120

::J' 110

E 100 C, 90 C

t.i 80 c 0 70 U Q) 60

>- 50 co C 40 <t:

30

- Aqueous standard • - Matrix matched standard

, 10

Fe conc. (/l g/mL)

- Aqueous standard - Matrix mat ched standard

10

Pb Conc .(/lg/mL)

------ S b ----Fe --pb

I I

0 5 10 15 20

mg/mL Copper ----Ni

~

~ I

0 5 10 15 20

mg/mL Copper

Fig. 2-Recovery of analyte wi th increasi ng concentrati on of

matrix element

KUMAR el al . . CHARACTERIZATION OF HIGH PURITY COPPER 175

mL copper was permi ss ible. For iron it was 3 mg/mL and for chromium it was 6 mg/mL. For nickel there was a fluctuation in the signal within ±20% range. Since it is known that GFAAS itself has a 10% preci sion , the fluctuation in signal in case of nickel could not be fully assigned to matrix effect.

Analysis after matrix separation

Matrix separation by co-precipitation

Sample was dissol ved as mentioned previously. 50 mL of sample solution containing 20 mg/mL copper was taken in a 250 mL beaker. To it 10 mL of I mg/ mL lanthanum solution was added followed by 10 mL of 0.1 g/mL ammonium chloride so lution . Then ammoni a so lution was add ed s low ly to it with continuous stirring until ammonia sme ll comes from the sampl e solution. The sample solution was then digested on a preheated wate r bath for 30 min . The res idue was filtered by Whatman 542 filter paper. It was washed with ammonia solution. The res idue was then di ssolved in I: I nitric acid . The so lution thus obtained was evaporated to about 2 mL and afte r

cooling, was diluted up to 25 mL by deminerali sed water in a volumetric flask. Each mL of thi s so lution represents nominally the impurities present in 40 mg of copper. The colour of the soluti on does not indicate the presence of any copper which means that if at all any copper is present, it must be below a concentration level of I mg/mL. The co-precipitation method provides a successful separation of copper matrix. Addition recovery experiments were used to validate the method . Experiments were done in quad ruplica te. Standard deviations were calculated for the four values. Results are shown in Table 3.

Copper is known to form a strong soluble complex

with ammoni a and hence remain s in solution whereas

chromium, iron and lead precipitate as the ir hydroxides

along with lanthanum . Silver, nickel and zinc also have

tendency to form soluble ammonia complex and hence

their recoveries were found to be poor.

While thi s procedure is suitabl e for Cr, Fe and

Pb, it was not possible to est imate s ilve r, ni cke l and

zinc by thi s matrix separation method and hence

alternative method has to be followed for the ir analysis.

Matrix separation by electro-deposition

Samples were dissolved as described previously .

Here the nitric acid used throughout the process of

making so lution was boil ed prior to use. This was

done to expel the nitrou s ac id as the nitrite ion is

reported2J to interfere during e lectrodepos ition.

The electrodepositi o n was carri ed out o n a

cylindrical pl atinum gauze e lec trode at a fixed current

of 2 amperes for 1.30 h, with constant s tirring . The

time was set such that around 99% copper will get

deposited. Since the depos iti on was performed at a

constant current, complete deposition of copper may

cause a change in the voltage to the deposition potential

of next element and so 99% deposit ion of copper is a

safer procedure to avo id co-depos ition of other metallic

species .

Satisfactory e lectro-depos ition of copper was not

possible f ro m pure nitri c acid medium. El ec tro­

deposition in presence of sulphuric acid"' has been

employed to ensure sati sfactory deposi ti on. Hence 50

mL of the 20 mg/mL copper so lution was taken in a

250 mL beaker. To it 2 mL of sulphuric acid was

add e d . Th e vo lume of the solution was made

approximately to 150 mL so th at max imum portion of

the e lectrodes will be dipped in the so lution . Then

e lectro-deposition was carried out. After 90 min the

solution was taken out. Volume was reduced to about

0 .5 mL and finally made up to 10 mL with 59'0 nitric

ac id. By this a five times pre-concentration was achieved

and each mL of the solution represents the impurities

present in 100 mg of copper. By thi s method addi ti on

Table 3- Addition recovery for co-precipiiati on method of matrix separat ion

Element s Cone. added Cone. recovered Standard deviation Re marks (Il g/ ITI L) (ll g/mL) (n=4)

An "

2 0.045 Loss or alla lyte <r 5 5.290 0.30 Recovery is OK Fe 5 5.250 0.21 Recovery is OK Ni 5 0.000 Loss of analyte Pb 5 4 .700 0.24 Recovery is OK Zn 0.150 Loss or analyte

176 INDIAN J. CHEM. TECHNOL.. MARCH 2004

recove ry of zinc was seen and found to be not

satisfactory (Table 4) . This may be attributed to the

fact that , sUlphuric acid causes chemical interferences during the an alysis by AAS method .

Sulphuric acid was, therefore, replaced by 50 mg

tartaric acid. This method was found to be satisfactory.

Addition recovery was carried out to validate the

method. Experiments were done in quadruplicate. Values

are given in Table 5 which indicate a satisfactory recovery of zinc.

Standard addition recoveries were found to be

good for all elements except silver and antimony. Silver

is more electro-positive than copper and hence tends to

deposit with copper. Sb can also be lost from solution,

by co-deposition or owing to the partial reduction to

stibine gas that may be generated at the cathode24.

Copper was also analyzed in the final solution and found to be below I mg/mL.

Selective depositio1l of silver

To determine silver, selective electrodeposition was followed. Attempt was made to deposit silver on the cathode leaving copper matrix in the electrolytic solution . The electrolytic solution was made as before for copper deposition . This solution was electrolyzed at different potentials above the deposition potential of copper in order to avoid interference from copper and to see the recovery of silver and amount of co-deposited copper. The deposition was carried out for 2 h. Results are given in Table 6 .

As potential goes on decreasing, it was found that the copper deposition was inc reas ing in the experimental solution. However at 0 . 15 V, the copper concentration was found to be within I mg/mL. So for estimation of silver in copper, deposition at 0 .15 V was selected. By this procedure a satisfactory deposition of silver without much interference from copper was achieved.

Table 4--EtTeet of Sulphuric acid and Tartaric acid on Zinc signal

Reagents Cone. of Zn added (llg/mL) Conc. Zn recovered (llg/mL)

Sulphuric ac id Tartaric ac id

Elements

Cr Fe Ni Pb Sb Zn

Voltage applied (V)

0.20

0. 19

0. 18

0. 17

0. 16

0.15

5 5

1.32 4.97

Table 5-Addition recovery studies for electro-deposition method of matrix separation

Cone. added (llg/mL)

2 5 5 5 5 100 ng/mL I

Cone. recovered <llg/mL)

0.00 5.54 4.46 5.03 4.30 48.8 ng/mL 0.91

Table 6-EtTect of varying potential on deposition of silver

Silver added in llg/mL

2

2

2

2

2

2

Silver recovered in Ilg/mL

2.00

2.03

1.98

1.95

2.00

1.99

Standard deviation ( n = 4)

0.21 0.29 0.12 0.16 4.64 0.038

KUMAR 1'1 al. : CHARACTERIZATION OF HIGH PURITY COPPER 177

Conclusion Estimation of Ag, Cr, Fe, Ni, Pb, Sb and Zn in

sub-ppm level, in high purity copper was not possible in presence of matrix using GFAAS due to matrix effect. Separation of matrix was necessary for obtaining accurate and precise values for the trace impurities. All the three methods described above for matrix separation were found to be complementary to each other in determination of these elements.

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