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A N A LY ST. APRIL 1989.

VOL.

114

355

Co mp a r i so n o f S o m e W e t D ig e s t io n an d D r y A sh in g Me th o d s fo r

Vo l t am m et r i c T race E lem en t Ana lys i s

Samuel B Adeloju

Centre for Industrial Research and Advanced Technology University

of

Western Sydney Nepean

Kings wood

2

750, Australia

Several wet digestion and dry ashing methods were compared for the precise and accurate determination of

some trace elements i n biological and environmental materials. The wet digestion methods were generally

faster than the d ry ashing methods, but required the use of large amounts of reagents and, therefore, gave

higher blank contributions for some elements. The main advantages of the dry ashing method were the lower

blank levels, improved lower) background current and its ability to handle considerably larger amounts of

sample. However, careful dissolution of the sample ash in

a

suitable reagent was necessary. Under suitable

conditions, both decomposition methods allowed the reliable voltammetric determination of trace elements

in biological and environmental materials wi th relative standard deviations of between 1and 3 . The ultimate

choice of decomposition method was influenced by the amount

of

sample available, the nature of the sample,

the sample matr ix and the analysis time available.

K e y w o r d s : Digestion methods; trace elements; voltammetry; biological and enviro nmenta l materials

The determination of inorganic elements in biological and

environmental materials at t race and ul tra-trace levels has

now

become a major aspect of various diagnost ic and

monitoring programmes. For this reason, several analyt ical

techniques have been developed o ver the last two decades that

are capable of determining th e elements reliably at these

levels . Such methods include numerous variants of electro-

analyt ical methods. for example, anodic s tripping voltam-

met ry (AS V), ca thod ic s t r ipp ing vo ltammetry (CS V), d if -

ferent ial-pulse polarography (DPP). square-wave voltani-

metry (SWV) and the more recent approach of adsorpt ive

cathodic s tripping voltammetry (ACSV). These techniques

are now being used more

in

the determinat ion of t race

elements in biological and environmental materials owing to

their simplicity, improve d selectivity an d high sensitivity.

1 - 1 1

Un fortu natel y , the direct determinat ion of the elemen ts in

these sample matrices is not feasible because

of

t h e en o rm o u s

matrix effect usually enco unter ed at the t race a nd ul tra-trace

concentrat ions of the analytes . In general . the accurate

voltammetric determinat ion of t race elements in these sample

matrices requires a com plete an d thorough decomposition of

the organic ma tter , while also demanding quanti tat ive reten-

t ion of the analytes in s tates or forms am enab le to quantifica-

The two types of decomposit ion method commonly

emp loyed for the determ ination of trace elem ents in biological

and environm ental m aterials are wet digest ion and dry ashing.

Although the lat ter is well ac cepte d, the wet digest ion method

is mo re com monly em ployed with most analyt ical techniques.

The preference for th is type of decomposit ion method is based

on the reduced danger of losses at the lower operating

tempera tu res . On the o ther hand , the d ry ash ing method

requir es the use of ashing aids and/ or careful manipulation of

the ashing temperatures

to

minimise or prevent the loss of

analyte.3.11.12 Ev iden tly, th e use

of

various reagents, both as

ashing aids and for the sample ash dissolut ion, represents the

major source of contamination in this decomposit ion m ethod .

Similarly. the use of relatively larger amounts of reagents in

the wet digest ion meth ods represents an even greater source

of contamination.

Concern about the problems of contamination and loss of

elements associated with both types of decomposition method

led to the development of other methods based on closed

systems, such

as

wet decomposit ion in pressure bombs,

combustion in oxygen flasks, and in oxygen bombs and o ther

combustion systems, as well as direct dissolution with tetra-

t ion .

1.3-5

alkylainmonium hydroxide.

'3.14

Ho wev er, none of these

methods have completely el iminated the associated blank

contribut ions resul t ing from the use of the reage nts for ei ther

the decomposition or the sample ash dissolution.

The suitability of wet digestion and dry ashing methods is

frequently investigated for various analytical techniques such

as

atomic absorpt ion spectrometry, neutron act ivat ion anal-

ysis , f luorimetry and spectrophotometry. Unfortunately .

s imilar investigat ions are rare for vol tammetric t race element

analysis. Consequently, there have been increasing reports of

inconsis tencies between voltammetric resul ts and those

of

othe r analytical techniques. In general , the d em and for careful

and complete decomposition of biological and environmental

materials is m ore crit ical for vol tamm etric techniques owing to

their reliance on the presence of analytes in states or forms

that are sui table for accurate determinat ion. In part icular, the

possible inhibition of th e electro de processes by the presenc e

of organic residues in the decomposed samples is of major

concern.

In this study, several wet digestion and dry ashing methods

have been compared for the voltammetric determinat ion

of

arsenic, b ismuth, cadmium, cobalt , copper, lead, nickel ,

selenium, vanadium and zinc in biological and environmental

materials . The methods considered are those readily acces-

sible to mo st analytical lab oratorie s: (i) direct dry ashing: (ii)

dry ashing with sulphuric acid a s an ashing a id; (iii) dry ashing

with nitric acid

as

an ashing aid; (iv) dry ashing with

magnesium nitrate as an ashing aid; (v) wet digestion with

nitric acid only ; (vi) wet digestion with a m ixture of nitric and

sulphuric acids: and (vii) wet digestion with a mixture of nitric

acid and potassium persulphate.

Animal Muscle, Bovine L,iver, Orcha rd Leaves and Oyster

Tissue were chosen as test materials because of the diversity of

their sample matrices . The influences of the nature of the

sample, the amount of sample avai lable, the sample matrix

and the available analysis time on the choice of decomposition

method for the reliable voltammetric analysis of trace

elements were also examined.

Experimental

Reagents and Standard Solutions

All acids and the amm onia solut ion used were of A ris tar grade

(BDH, Poo le , Dorse t , UK); a l l o ther reagen ts were o f

analyt ical-reagent grade. Th e amm onia - amm onium chloride

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ANALYS'I ' .

A P R I L 1989. V O L. 114

buffer solut ion, dimethylglyoxime (dm gH? ), s tandards and

dist i l led, de-ionised water were prepared as described pre-

\

iously. h

Instrumentation

All experiments were performed by differential-pulse voltam-

met r ic methods on an EG

:

G

Princeton Applied Research

(Princeton, N J, 1J SA ) microprocessor-control led instrument

as described previously.3-- A m edium-size mercury dr op

having a surface area of

0.015

cmz was used

in

all instances.

Glassware

All glassware and polyethylene bottles were soaked

in

2 M

nitric acid for at least 7 d , washed thre e times with distilled,

de-ionised water, soaked in dis t i l led, de-ionised water and

finally soaked in 0.1 M hydrochloric acid until ready for use.

Biological Standard Reference Materials SKM s)

Animal Muscle ( IAEA RM M-4) was ob ta ined f rom the

Analytical Quality Co ntrol Services of the Interna tional

Atomic Energy Agency ( IA EA ) in Vienna , Aus t ria . Bov ine

Liver (NBS S R M 1577a). Oyster Tissue (N BS S RM 1566a)

and O rchard Leaves (N BS SR M 1571) were ob ta ined f rom the

US Nat ional Bureau of Standards (NBS) , Wash ing ton , DC.

All materials were t rea ted as recommended by the suppliers .

Digestion Methods

1. We t d iges t ion

( a )

W i t h H N O j only ( W l ) .

Transfer

0 . 3

g (o r

10

ml) of

sample and

1 0

nil of nitric acid

(65%)

into a 125-ml

Erlenmeyer flask, insert a pre -cleaned glass funnel and heat

on a hot-pla te at approxim ately 290 C until nitrogen oxide

fumes are just given off. Repeat the digestion with two

separate addit ions

of

10 ml of nitric acid, cooling th e tlask for

about 2 min between each addition. With the final addition of

ni tr ic acid , continue heat ing at the same tem pera ture unti l the

nitrogen oxide fumes are completely evolved. Cool the flask

again for abou t 2 min an d rinse the funnel with a small volume

o f dist il led. de-ionised water in to the flask. Fo r the determ ina-

tion of selenium, add 3 ml of 37% hydrochloric acid (1 l ) ,

replace the funnel and then heat th e mixture at a temperature

setting of approximately 210 C on the hot-plate for at least 30

min to conv ert al l the selenium to selenium(1V). Cool to room

tem pera ture, r inse the funnel again into the flask with water

and transfer the contents in to a 25-ml cal ibrated flask, making

up to the ma rk with dis t i lled, de-ionised water.

( b )

W i t h H N 0 3 H 2 S 0 4 W2).

Transfer 0.3 g (o r 10 ml) of

sam ple , 10 ml of nitric acid (6.5%) an d 1 ml of sulphur ic acid

(98%)

in to a 125-ml Erlen mey er flask and carry out the

digestion a s described in (a) [ including the t re atm ent with

HCI

for the conversion of selenium to selenium(IV)]. In this

instance, repea t the digest ion with three s epa rate addit ions of

nitric acid and continue heating after the final addition of

nitric acid until the nitrogen oxide fumes a nd the sulphite mist

have d i sapp eared .

(c)

W i t h

FINO3 - K2S2O8 3). Transfer 0 .3 g (or 10 ml) of

sam ple. 10 ml of co nc ent rat ed nitric acid an d 4 ml of potassium

persulphate (10% mlV) into a 125-ml Erlenmeyer flask and

repeat the digestion as described in (a) [including the

treatment with HC1 for the conversion of selenium to

selenium(IV)].

In all of these wet digestion m eth od s, care was taken to

avoid sample charring because this can result in reductive

conditions that might lead to the loss of selenium via the

formation of volat i le hydrogen selenide. For the arsenic

determina t ion , convers ion of the elem ent to arsenic(II1) was

carried out by the method de scribed by Sadana7 after the

nitrogen oxide fumes ( and sulphite mist) had disappeared.

2 . Dry ushirig

(a )

Direct dry ushing without an ashing nid

( 0 1 ) . A c cu -

rately weigh

0.5

g of sample into a pre-cleaned silica dish and

heat gently on a hot-plate to volatilise as much moisture and

organic matter as possible. From t ime to t ime, spread the

sample out with a pre-cleaned silica rod to accelerate drying.

Wh en the sample is fairly dry

(30-60

min) transfer th e dish to a

temperature-control led muffle furnac e at

450 C.

Leave in the

furnace for

8

h (o r overnight) to complete the decomposit ion

step. Remove from the fu rnac e, leave to cool and add

3

ml of

6 M hydrochloric acid. Wa rm gently on a h ot-plate to dissolve

the sample ash and to extract the elements from any insoluble

residue. Transfer the contents quantitatively into a 10-ml

calibrated tlask by washing th e dish with distilled, de-ionised

water an d mix thoroughly to en sure homogeneity .

(b )

Wi th sulphuric acid as a n ushing aid

( 0 2 ) . Accurately

weigh 0.5 g of sample into a pre-cleaned silica dish and add

5 ml of 20%

V/V

sulphuric acid (3.6 I). Heat gently on a

water-bath for 30 min and trans fer to a hot-plate, heating until

the white fumes cease to be evolved and the residue is

completely dry. Transfer the dish to the muffle furnace at

500

C and leave overnight or f or at least X h to complete the

decomposit ion. Dissolve the residue an d make up the solut ion

as described in 2(a). If the sample contains carbonaceous

residue, leave i t to set t le before removing an al iquot for

analysis.

(c)

Wirh nitric acid

us

an ushing aid

(0 3 ) . T h e p roc ed ure

was s imilar to tha t in 2(b) except that

5% VIV

nitric acid was

adde d instead

of

sulphuric acid and the ash ing was carried out

at 4.50 C.

(d)

Wi th magne sium ni trate as an ushing aid

( 0 4 ) . T h e

procedure was s imilar to that in 2(b) except that the sample

was first digested with nitric acid and

5

ml of 80% mi

magnesium nitrate solution before dry ashing in the muffle

furnace a t 500 C for 30 min as described previously by

Holak.8 The ashed sample was dissolved accordingly.

Voltammetric Determinations

The determinat ions of arsenic, cadmium, co balt , copp er,

lead , nickel , selenium and zinc were pe rformed as de5cribed

previously by the author and co-workers . '. ' Bismuth and

vanadium were determined as described by Velghe and

Claeysg and van den Berg and Huang.10 respectively.

Working Area

This work was carried out in clean air conditions controlled at

a tempe ratur e of 22.5

k

0.5 C. All sample preparat ions were

performed in a class-100 clean room, and the analytical

measurem ents in a class-1000 clean room .

Results and Discussion

Nature

of

Sample

Material

Th e resul ts summarised in Table

1

indicate th at th e suitability

of ei ther the wet digest ion or dry ashing methods for the

voltam metric determ ination of the trace elem ents in biological

and environmental materials was dependent on both the

nature of the sample material and the element(s) of in terest .

Although

n o

single meth od was ideal for all sample materia ls,

the physical s tate

of

the sample was an important considera-

tion in the selection of a suitable digestion method. For

example, the decomposition of liquid samples could only be

performed adequately by wet digestion, except in the

instances for which the initial freeze-drying of the liquid

samples permitted the use of the dry ashing methods.

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114

457

Table

1. Suitabili ty

of

direct decomposition methods for the voltammetric determination of tracc elements in biological and environmental

materials

Decomposition Sample Elemental Vo t am me t r c

Element method

t

YPe stat e techniq ue'

Arsenic

Bismuth

C ad m u m

Cobalt

. .

C o p p e r .

.

Lead

.

.

Nickel . .

Selenium

Vanadium

Zinc

. .

. . . .

. . . .

. . . .

. .

. .

. .

. .

. . . .

. .

. .

. . . .

. . . .

. .

. .

W 2 >

W 3 >

W1

W 2 = D 4

W 2 >

W I = W 3

D l - D 2 - D 3 = W 2

W 2 > W 1 = W 3

W 2 = D 2 > D 1= D 3

W2

>

w3

> w1

D 1 > D 3 = D 2 > W 2

W2

>

W 1 =

W 3

D1 > D 2 = D3, W2

w2

>

W 3

>

W1

D 2 > Dl =D3 , W2

W 2 > W 3 > W 1

D l > D 3 = D 2

>

W2

W 2 > w3 >

W1

W 2 = D3

W 2 > W 3 > W 1

W 2 = D 2

W 2 > W 1 = W 3

W 2 = D 2 > D 1 = D 3

Liquid

Solid

Liquid

Solid

Liquid

Solid

Liquid

Solid

Liquid

Solid

Liquid

Solid

Liquid

Solid

Liquid

Solid

Liquid

Solid

Liquid

Solid

DPCSV at

H M D E

D PA SV a t

H M D E

D PA SV a t

H M D E

D PA C SV , D PP a t

H M D E i D M E

D PA SV a t

H M D E

D PA SV a t

H M D E

D PA C SV . D P P a t

H M D E I D M E

DPCSV at

I I M D E

D P A C S V , D P P a t

H M D E i D M E

DPASV at

H M D E

* D PC SV = dif ferentia l-pulse cathodic s t r ipping vol tammetry; DP ASV = differential-pulse anodic stripping voltammetry; DPACSV

=

diffcrential-pulse adsorptive catho dic stripping voltam metry ; DP P = dif ferentia l -pulse polarography; H M DE

=

hanging mercury dro p e lect rode;

a n d D M E = dropping mercury e lect rode.

Ho we ver , with solid sample materia ls, both types of digestion

me thod w ere general ly sui table for the decomposit ion without

the nee d for any prior t reatm ent . In terms of the voltammetric

analysis. th e criteria used for determ ining the suitability of the

digest ion methods was based on the completeness of the

decomposit ion, as de term ined by the presence of the residual

organic matrix and the recovery of the t race elements . The

order of preference indicated for the different digest ion

methods in Table 1 represents

95-100 %, 9(&95 /0, 85-90%

an d

70-85 /0

recovery of the elements for the fi rs t , second,

third an d fourth preferenc es, respect ively . O n the basis of th is

information the two preferred decom posit ion methods for

the rel iable voltammetric determinat ion of t race elements in

liquid and solid

biological ienvironmental

materials were the

wet d iges tion wi th H N 0 3 -

H2S03

and th e direct dry ashing

without an ashing aid .

Digestion Time

An important consideration for the selection of digestion

methods for the determination of trace elements in biological

and environmen tal materials is the required digestion t ime. A s

expected, the wet digestion methods w ere considerably more

rapid, requiring about 2-3 h compared with at least

8

h for

complete decomposit ion by the dry ashing methods. Gener-

ally, the required decomposition time for both types of

decomposit ion method increased with increasing amount of

sample digested. Hence, the analysis t ime avai lable was a

ma jor factor influencing the choice of a part icular decom po-

sit ion m ethod for the rel iable determinat ion of trace elements

by

voltammetry or othe r analyt ical techniques.3 .4 Iowever,

the preference for a part icular me thod might be influenced by

the extent of the blank contribut ions that may resul t from the

digest ion. In s i tuat ions where the extent of the blank

contribut ion was not considered to be

a l imitat ion and the

analysis time available w as sh or t, prefer ence w as given to the

wet digest ion methods. This , in some instances, had serious

consequences on the reliability of the voltammetric analysis,

part icularly with respect to the completeness of the sample

decomposit ion and the resul t ing background currents asso-

ciated with th e residual acid from the wet digestion methods.

Nevertheless , i t is worth noting that these effects were reduced

considerably when wet digestion of the samples was carried

out over a longer period

(8-10

h) .

Blank

Contributions

General ly , the extent of contamination from the sample

decomposit ion process was greater for the wet digest ion

methods in terms of the num ber and am ounts of reagents used

than for dry ashing. particularly

if

the decomposition was

performed under conventional laboratory condit ions.

Ho we ver, th is did not cause a major problem under clean

laboratory condit ions because the blank contribut ions were

easily maintained at very low and constant levels. In this

regard, an at t ract ive feature of the dry ashing methods was

their ability to deco mpo se relatively large r amoun ts of sample

(u p t o 10 g) compar ed with wet dige stion, without significant

increases in the decomposit ion t ime and the blank con tribu-

tions. The decomposition of such large amounts of sample by

dry ashing proved useful in reducing the effect of the higher

and variable blank contributions obtained in a conventional

laboratory. Nevertheless , s t r ingent control and maintenance

of the furnace was required in order to avoid or reduce the

irregular an d variable blank contribut ions. This control a nd

maintenance required an initial firing of the furnace

u p

to

800

Co n a regular basis to remove any residual t race elements

from the digest ion chambe r, prior to the introduction of the

sample for decomposit ion at lower temperatures . Table

2

gives the typical residual blank contributions which resulted

befo re and a fter furnace condit ioning. Evidently , prior

conditioning of the furna ce was effective in reducing the blank

contributions significantly and was used prior to all the dry

ashing methods discussed in this paper. In comparison, the

blank contribut ions from th e wet digestion methods given in

Table

3

were general ly higher.

As

can be seen from these data

the blank contribut ions for these digestion methods increased

with the amounts and numb er

of

reagents used for the sample

decomposit ion.

Determination of Volatile Trace Elements

Selenium and arsenic are two of the highly volatile trace

elements that were s tudied. Owing to their volat il i ty, the use

of magnesium n itrate a s an ashing aid for the dry ashing

of

the

samples for their determinat ion was mandatory.

11.12

In its

absence, considerable losses of these elements were experi-

enced and the approach was considered unsuitable in terms of

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A N A L Y S T , A P R I L

1989. V O L .

114

Table

2.

Typical blank contr ibutions from dry ashing in a muffle

furnace based on m ethod D1 and sa mple dissolution w ith 0.1

M

HCI

Blank con t r ibu tion ' / B lank con t r ibu t ion t i

E lement Llg I - 1 181

Arsen ic . . . . . .

Bismuth . . . . . .

C a d m i u m

. . . . . .

Cobalt . . . . . .

Copper

. . . . . .

Lead . . . . . .

Nickel . . . . . .

Selen ium . . . . . .

Vanadium

. . . .

Zinc

. . . . . . . .

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A N A LY ST, A PR IL 1989 ,

V O L . 114

459

t

4

2

u

0.8

1

o

1.2

bl

T

1

0.8 1 o 1.2 0.8 0.9 1.0 1.1

- E N

Fig.

3. Compar ison of (a) direct dry ashing without an ashing aid

with b )wet digestion with HNO, and (c) wet digestion with HNO , -

K2S20, for the determination of Ni and Co in Oyster Tissue

by

D PA C SV . a ) 1,O; 2 , 6pg

1 - 1

of Nip lus 3 pg - of C o ; 3 , + 1 2 p g l

of Ni plus 6 pg1 of Co . b ) nd (c) 1 O ; 2 , + 2 pgl-1 of Niplus

1

pgl 1

of C o ;

3,

+ 4 pg

1 - 1

of Ni plus 2

pg

1 - 1 of Co;

4, + 6 pg

1-l of Ni plus 3

pg 1

of Co. Accumulat ion t ime ( t ' , ) =

60 s ;

scan rate,

4

m V s - l ;

[dmgH2] , 2 x

10-1

M ; 0 . 3

g

of

sample in 50 ml

Th e requ irem ent for the complete decomposit ion of biolog-

ical and environmental materials for the cathodic s tr ipping

voltammetric determinat ion of selenium and arsenic is furth er

demonstrated by the resul ts given in Table

4.

It can be seen

that the selenium concentrat ions obtained for the urine

sample by wet d iges tion wi th the H N 0 3 H 2 S 0 4 a nd H N 0 3

K2S208 mixtures and by dry ashing with Mg(N03)2

as

an

ashing aid were co mpa rable . Th e lower selenium concentra-

t ion obtained by wet digest ion with H N 0 3 only was due to the

incomplete decomposit ion of some of the organic matrix .

Similar observat ions have been made by Neve

et

a1.15

who

found tha t wet d iges tion with H N 0 3 on ly gave er roneous

resul ts for the d eterminat ion of selenium by atomic absorpt ion

spectrometry because

of

the incomplete mineral isat ion of

some organic selenium com poun ds including selenium deriva-

t ives , which a re the m ain metaboli tes of the elem ent in urine.

With voltammetric techniques, the adsorpt ion of the incom-

pletely digested o rganic matrix inhibi ts the electrode process

and dis torts the response d ue t o the catalysis of th e evolut ion

of hydrogen. T he com plete destruct ion of the organic matrix

by wet digestion with HNO? only seems to be l imited by the

low boiling-point

of

th is acid . R epe ated digestion of the urine

sample with addit ional ni t r ic acid did no t improve th e recovery

or precis ion of the wet digest ion procedure for the voltam-

metric determinat ion of selenium.

Determination of Other Elements

Of all the digestion m eth ods investigated, the direct dry ashing

method without an ashing aid proved to be the most sui table

for the accurate dete rmin at ion of bismuth, cadmium, cobalt ,

copper, lead, nickel , vanadium and zinc. This method was

part icularly advantageous in reducing the extraneous addi-

t ions that resul t from the use of reagents

as

ashing aids or for

wet digestion.

Complete recovery of thcse elements was

accomplished at an ashing temperature

of

450

C.

Although

the other dry ashing and wet digest ion methods were also

ad eq ua te , careful co ntrol of the a nalytical blank variability

was necessary to e nsure that th e resul ts were rel iable. Th e

data given in Table 3 indicate that the blank contribut ions

from the wet digest ion with acid mixtures were general ly

0.8 1 o 1.2

- E N

Fig.

4

Determinat ion of Ni and Co in Orchard Leaves by DPA CSV

after decomposition

by

direct dry ashing without an ashing aid. 1 , O ; 2.

+2.5 pg

1 - 1 of Ni plus

0 . 5 pg 1 1

of

Co;

3 , + S . 0

pg

1

I of

Ni plus

1.0

pg 1 - of C o ;

4,

+7.5 pg

1 - 1

of N i plus 1.5

pg

1 - 1 of Co; [drngH,], 1 X

10

4

M ;

0.2

g of sample in 50

ml.

Oth er condi t ions

as

in

Fig.

3

0.85

0.65 0.45

- E N

Fig.

5. Determinat ion of Cd and

P b

in Oyster Tissue bv DP AS V

after decomposition by wet digestion with

HNO,

- H2S04.1 , O ; 2 , + 2 ;

3, + 4 pg 1-1; f = 120 s ; scan rate, 4 m V

s-1 ;

0 . 3 g of sample in 250 ml

higher than for dry ashing in the muffle furnace (T able

2).

Fur ther, th e resul t ing sensit iv i ties and background currents

for the voltammetric determinat ion of the elements were

depe nden t on the n ature of the chosen decomposition

method. Fig. 3 shows that the background current for the

determination of nickel and cobalt in Oyster Tissue digested

by the direct dry ashing technique w ithout an ashing aid was

considerably lower than those obtained with the wet digest ion

methods. Such high background currents can affect the

accuracy of the voltamme tric peak measureme nts an d, hence,

reduce the rel iabi l i ty of the method. However, unl ike wet

d iges tion with H N 0 3 on ly [F ig.

3 b)]

the inclusion of

potassium persulphate in the digest ion mixture [Fig.

3(c)]

reduced the background current in the region of posi t ive

potential. Such a reduction in the background current is

undoub ted ly due to improved decomposi tion wi th the H N 0 3

K 2 S 2 0 Xmixture, as is already evident from the selenium

results in Table

4.

Further, previous evidencelh.17 has indi-

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V O L .

114

Table

5 .

Comparison of the wet digestion method using H N 0 3 -

H,SOI with the direct ashing method without an ashing aid for the

voltatnmetric determination

of

C d . P b , C o a n d Ni in Oyster Tissue

Wet Certified

Vol tammetr ic diges t ion/ Dry ashing*/ value-i./

Element technique

pgg P g g -

ELsg

Cadmium

.

.

D P A S V

3.14k0.12

3.22k0.17 3.5 kO.3

Lead . . .

.

D P A S V

0.51

1

.02

0.50

k

0.05 0.48

t

0.04

Cobalt . .

D P A C S V 0.32

k

0.04 0.37k 0.04

0.4

Nickel . .

D P A C S V 1.12 .05

1 . 10 i 0.04 1.03k

0 .19

Triplica te determinat ions

i

mean devia t ion)

The er ro r i s the s tandard de via t ion.

Non-certified value.

Table 6. Comparison between open and closed wet digestion

procedures using HNOi - H,S04 for the CSV determination

o f

se e n u

n

Animal Muscle/ Bovine Liver/

M ode of digestion pgg I o f S e pgg

I o f S e

Closed

. . .

.

. .

0.273 t 0.005

1.08 .01

O pen . . . . . . . .

0.288 -t 0.002

1.13k 0.03

Certified values+

.

.

. .

0.28

k

0.03

1 . 1

50.1

* The e r ro r is the mean deviation based on triplicate determina-

The error is the standard deviation based on the results obtained

tions.

by various ins ru me n a1 tec h n q ues

.

cated that the adsorption of organic residues might inhibit the

electrode process a nd dis tort the response du e to the catalysis

of hydrogen evolut ion. Hence, the effects of the catalysed

hydrogen evolut ion would be expected to be more pro-

nounced in samples with high residual acids. Generally,

incomplete sample decomposit ion would resul t in the reten-

tion of unu sed a cids, in addition to undigested soluble organic

residues.

Fig.

3

also shows that the sensitivities obtained for nickel

and cobalt were general ly much improved for the sample of

Oyster Tissue digested by the direct dry ashing method

without an ashing aid. Th e lower sensitivity obtain ed with wet

digestion with

H N 0 3

only was due part ly to incomplete

decomposit ion an d, possibly, part ly to the presence of residual

acid in the sample solution which required neutralisation with

higher concentrations of ammonia buffer. Previously, it was

demonstrated that the sensitivities of nickel and cobalt are

affected by such high conce ntration s of amm onia buffer. T he

completeness of the decomposition by wet digestion was

improved with the use

of

t h e H N 0 3

K 2 S 2 0 8

mixture. Similar

effects on the sensi tiv i t ies of t he voltam metric determ inat ion

of bism uth. cop per, le ad, vanadium and zinc in biological and

environmental materials were observed for the different

d iges t ion methods . The d ry ash ing method was , therefo re ,

more sui ted for the voltammetric determinat ion of bismuth,

cobalt , copper, lead, nickel , vanadium and zinc in these

materials in terms of sensi t iv i ty , background current ,

com-

pleteness of sample decomposition and reliability of the

resul ts . Fig . 4 shows that the dry ashing method was also

ade quate for the voltammetric determinat ion of nickel and

cobalt in Orchard Leaves.

The anodic s tripping voltammetric determinat ion of cad-

mium in biological and environmental materials showed the

least depe nden ce on the type of decomposit ion method used.

General ly , the wet digest ion and dry ashing methods were

equally adequate for the determinat ion of th is element ,

indicating that cadm ium is not as s trongly bound to the organic

matrix as are selenium, arsenic or zinc. However, for dry

ashing , careful dissolution of t he sam ple in a suitable acid was

necessary to obtain reliable results by the voltammetric

technique. In this regard, the dissolution of the ash in

hydrochloric acid was m ore satisfactory th an that in nitric acid

in terms of peak resolutio n, sensitivity and reliability of the

results.i This did not create any problems w i t h the wet

digest ion methods, provided that thc sample was digested

carefully

as

described

in

th is paper. Nevertheless , the back-

ground currents of the sample decomposed by wet digestion

were very high

as

shown

in

Fig. 5 for the determination of

cadmium and lead in the sample of Oyster Tissue. In

comparison, the background currents for samples decom-

posed by the dry ashing methods w ere general ly much low er,

as dem onstrated previously .-? The data given in Table

5

provide a more direct comparison between the

t w o

preferred

decomposit ion m ethods for the rel iable voltammetric determi-

nation of cadmium , lea d, nickel and cobalt in the Oy ster

Tissue sample. Both sets

of

results agree favourably with the

certified values and hence verify their suitability for the

voltammetric determinat ion of these elements in similar

biological and environmental materials.

Decomposition of a

Complex

Sample

Matrix

On e of the complex sample matrices encountered i n this study

was that of a sediment sample containing a highly siliceous

component. Neither the wet digestion nor the dry ashing

methods described here al lowed com plete decomposit ion to a

form sui table for the voltammetric determinat ion of the t race

elements. It was necessary with both types of digestion

method to decomp ose the sam ple further with small al iquots

(1-2 ml) of hydrofluoric acid. For the dry ashing meth ods,

i t

was necessary to add som e nitric acid (2-5 ml) in addition to

the hydrofluoric acid t o al low adeq uate dissolut ion of the ash.

This additional step proved successful for the reliable quantifi-

cation of the trace elements in the sediment sample by

voltammetric techniques with both types of digestion.

However ,

i t

was necessary, for both methods of decomposi-

t ion, to perform the dissolut ion s tep

in

a PTFE vessel at

200

C in order to avoid react ion between the acid and the

digestion vessel. This precaution also avoided serious con-

tamination of the sample resulting from such a reaction.

Modes

of

Wet Digestion

Typically, the wet digestion of a sample can be performed

in

one

of

two modes, namely, in an open system, as in this

s tudy, or in

a

closed system , which usually involves carrying

out the digestion under refluxing conditions or i n pressure

bombs. For ubiquitous elements such as copper, lead and zinc,

the closed mode of wet digestion was beneficial in reducing

atmospheric contamination and minimising the use of

reagents . However, for non-ubiquitous elements such as

selenium and arsenic. the benefit of the closed mode over the

open mode was marginal. This is supported by the results

given in Table

6,

which compares the wet digestion of some

biological materials with the H N 0 3 -

HzSOj

mixture in the

open and closed mo de for th e cathodic s tripping voltammetric

determination of selenium. Evidently, both modes of wet

digestion we re equally suitable fo r reliable deter mina tion of

the element . The s ignificant difference between the two

modes was that the amount of reagent used in the closed

system was a bou t half that used in the open m od e, which

represents a significant decrease in the reported blank

contributions (Table

3 ,

particularly for the ubiquitous ele-

ments . Nevertheless , the t ime required for the complete

decomposition of the samples in the closed system for the

voltamm etric determ inations of the trace elem ents was at least

twice that required for the open system, depending on the

amount of sample used. Ultimately, the critical factors

influencing the choice of mode in real situations were the

analysis time available and the conc entratio ns of the elem ents

to be determined

in

the sa mple , part icularly with regard to the

extent of blank contribut ions that can be toler ated.

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A N A LY S T. A PR IL 1989. V O L .

113

46

1

Conclusion

The rel iable voltammetric determinat ion of t race elements in

biological and environmental materials require5 careful con-

siderat ion of the choice of decomposit ion method. In par-

t icular, factors such as the nature

of

the sample, the sample

matrix , the amount of sample, the analysis time available,

blank contributions, the resulting sensitivity and the back-

ground current m ust be given due considerat ion. For the two

most suitable decomposition methods in this study, namely

wet digestion with

HNO? H2S04

and direct dry ashing

without an ashing aid , the precis ion for the voltammetric

determination in liquid and solid samples was in the range

1-3%.

The author is grateful to the Graduate Studies and Research

Committee at Deakin Universi ty for providing the research

grant for th is work and he acknowledges the use

of

the

U n ve rs t

y

CIe an Labor a to r

y

C o m p 1ex.

3 .

4.

5.

6 .

7 .

8 .

9.

10.

11.

12.

13.

13.

1s.

16.

17.

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Paper 8103122C

Received August

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Accepted October 31st , I988

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