The Science of the Total Enuironment, 60 (1987) l-16 Elsevier Science Publishers B.V., Amsterdam ~ Printed in The Netherlands

1

APPLICATION OF DIFFERENTIAL PULSE ANODIC STRIPPING VOLTAMMETRY TO THE DETERMINATION OF HEAVY METALS IN ENVIRONMENTAL SAMPLES*

I’. OSTAPCZUK, P. VALENTA, H. RUTZEL and H.W. NURNBERG

Institute of Applied Physical Chemistry, Nuclear Research Center (KFA) Jiilich, P.O. Box 1913, D-51 70 Jiilich (Federal Republic of Germany)

(Received May 13th, 1986; accepted June 12th, 1986)

ABSTRACT

A reliable procedure for the determination of the trace metals Cd, Cu, Ni, Pb and Zn in liquid and solid environmental samples by an advanced voltammetric method has been developed. A convenient method of sample pretreatment is wet digestion in a HNO,/HClO, mixture. The simultaneous voltammetric determination of Cd, Cu, Pb and Zn is made at pH 2 by differential pulse anodic stripping voltammetry (DPASV); the simultaneous determination of Ni and Co at pH 9 after adsorptive accumulation of the respective complex with dimethylglyoxime at the electrode is made by adsorption differential pulse voltammetry (ADPV). The particular advantages, of the vol- tametric approach in food control for heavy metals are high sensitivity, good precision and accuracy, the possibilities for the simultaneous determination of groups of metals and low cost instrumentation.

INTRODUCTION

Reliable, sensitive and convenient analytical methods are an essential prere- quisite for tasks in research and in routine consumer protection. Until now the analytical determination of toxic metals in food has been in the (domain of atomic absorption spectroscopy (AAS) with flame, or at lower trace metal levels using the electrothermal mode (ETAAS) [l-4]. Voltammetry has been used comparatively rarely in food analysis. However, the high sensitivity and selectivity of the electrochemical approach, combined with inexpensive instru- mentation (Table 1) and the possibility of the simultaneous determination of several metals make it eminently suited for this task. Because of the low concentrations often encountered, differential pulse anodic stripping voltam- metry (DPASV), or, for Ni and Co, adsorption voltammetry (AV) or adsorption differential pulse voltammetry (ADPV), will usually be the voltammetric meth- ods of choice [l-9]. For the determination of heavy metals in drinking water, sample pretreatment is limited to filtration and an acidification procedure [I’/]. Only for water samples with a high level of dissolved organic matter (DOM) is efficient photolytic decomposition of inert metal complexes by UV irradiation

*Dedicated to the memory of Professor Dr Hans Wolfgang Niirnberg.

004%9697/87/$03.50 0 1987 Elsevier Science Publishers B.V.

TABLE 1

Comparison of equipment costs and determination time for voltammetry and electrothermal atomic absorption spectroscopy

Analytical method

Equipment costa Analysis time (DM) (min)

ETAAS

DPP/DPASV/ADPV DPASV ADPV swv

Basic equipment Complete equipment with Zeeman compensation

Basic equipment Microprocessor controlled

polarographic analyzer

5-7 x 104

12-17 x lo4 206 1.52.5 x lo4 4oc

5.5 x 104 2oc

al$ U.S. = 2.50DM; 1DM = 0.43 $ US. = SO.29 b Determination of one element. ‘Determination of four elements.

necessary [lo]. All stages of water sample pretreatment in the laboratory (filtration, acidification, UV irradiation) can be included in a fully automated photodigestion device [ll]. For wine and certain fruit juices with low sugar content, photolytic decomposition of dissolved organic material by UV irradia- tion from a mercury lamp (150 W, 254 nm) with addition of H,O, is sufficient [8]. This digestion procedure has the special advantage that it introduces no analytical contamination risk. For all other types of food, complete digestion is usually necessary. Various digestion techniques are used to mineralize the organic matrix in food samples. Wet digestion with various acid mixtures is a cheap, practical and widely applicable method for all laboratories [6, 7, 9, 121.

The main aim of this work was to establish a reliable, routine voltammetric analytical procedure for the determination of heavy metals in food. An in- dependent, convenient and versatile alternative for heavy metal trace analysis in food is thereby provided enabling either analytical quality control of AAS data or to be used as a routine method. A universally applicable digestion procedure for all food types, adapted for subsequent voltammetric determina- tion of the trace metals Cd, Co, Cu, Ni, Pb and Zn, was developed and evaluated by application to a large variety of important constituents of the food basket.

ANALYTICAL PROCEDURE

The usual analytical procedure consists of three stages: sample pretreat- ment, adapted to the respective food type, wet digestion and the simultaneous voltammetric determination of Cd, Cu, Pb and Zn by DPASV and Ni and Co by linear scan or, at low heavy metal levels, adsorption differential pulse voltam- metry (AV or ADPV). According to recent assessments [13], of particular advantage for the determination of heavy metals at levels normally found in food is the application of the square wave mode instead of the differential pulse

3

mode. This mode is, however, only incorporated in some of the more recent and expensive microprocessor-controlled instruments (see Table 1).

Sample preparation

Before digestion, an aliquot of the sample has to be taken in such a way so as to avoid any risk of heavy metal contamination. Manipulations are carried out using plastic implements, e.g. knives and pincettes with Teflon-covered pincers [9, 121. Dry food is crushed between dry polyethylene foil to prepare a sample for digestion (no significant Zn contamination was observed). Other dry food types, such as rice and flour, are weighed directly into quartz digestion dishes. For food samples with high water contents, e.g. vegetables and fruits, and also liquid food, e.g. juices, beer and milk, prior drying is recommended by heating in a quartz dish between 100 and 15O’C for 3660 min until constant dry weight (DW) is obtained. The drying has the advantage that, in the subsequent wet digestion, the necessary amount of HCIO, can be reduced to O.lml for samples having originally 0.24.5 g fresh weight (FW). The 24 h freeze-dried sample can then be homogenized by grinding between dry polyethylene foil. All these pretreatment manipulations before digestion should be carried out using a clean bench with controlled and filtered laminar air flow. The operator should wear precleaned protective polyethylene gloves.

The chemicals and reagents used have to meet the following quality con- ditions. All acids were Merck “Suprapur” substances. Standard solutions containing 1 g 1-l of metals were prepared from Merck, Titrisol, and deionized water from a Milli-Q-Purification System. For nickel and cobalt determination, 2M ammonia buffer of pH9.2-9.4 was prepared by weighing appropriate amounts of 25% NH, (Merck, Suprapur), 30% HCl and deionized water into a polyethylene bottle. Dimethylglyoxime (DMG) (0.1 M stock solution) was prepared by dissolving appropriate amounts in 96% ethanol (both of purity p.a.).

Wet digestion procedure

To a solid or liquid sample of 0.245 g or 0.5 ml, respectively, when no prior treatment is necessary, or to a sample from the drying procedure, 0.1-0.5ml 70% HClO, and O&l.Oml65% HNO, are added in a quartz dish (10ml volume) covered with a quartz watch glass. The amount of digestion acid is adjusted within the stated ranges according to sample type and weight. The amount of 65% HNO, needed also depends upon the digestion time. The first stage of the digestion procedure consists of gentle heating to 100°C; the sample dissolves with foaming, the extent of which depends on the type of sample; the color of the solution becomes dark green to brown. Rapid heating will require more HNO, and may cause foaming problems. When foaming has finished and the dissolution of the sample is complete, heating at 2OO“C is continued until evolution of nitric oxides ceases and a light yellow or colorless liquid is formed.

If the solution is dark brown, or black, further addition of HNO, is necessary. This HNO, addition has to be repeated until a colorless solution results.

The digestion of fats (butter, margarine, vegetable oils) follows in principle the same pattern, but is more difficult and requires more HClO,, e.g. 1-2 ml for a 0.2 g sample. The temperature has to be controlled and must not exceed 200°C, as the oxidation by HClO, is rather vigorous. If the temperature becomes too high and the HClO, level is too low the sample can become carbonized. With fats a dark brown or black solution is formed and the above treatment, by addition of appropriate amounts of HNO,, has to be applied until a colorless solution is obtained.

Only if the trace metal level in the sample is < 10 pg kg ’ FW is the digestion solution to be evaporated to dryness. The resulting white crystalline residue is then dissolved in 2 ml of water and 5 ~1 of 70% HClO,. For samples with higher metal contents the colorless digestion solution is transferred with some water into a 10ml volumetric flask and diluted with additional water.

Due to the small amount of HClO, required this digestion procedure is without risk of an explosion. Nevertheless, it should be carried out in a fume hood behind a protective glass screen. The HClO, containing vapour has to be suctioned off by a water pump to avoid damage to the plastic parts and interior of the fume hood. This wet digestion procedure is suitable for all types of food and biological material [7, 9, 141 when voltammetry is to be applied for heavy metal determination. This digestion procedure ensures the necessary complete mineralization and excludes interference by organic substances which remain dissolved in the analyte. This is not the case with pressurized wet digestion, which is frequently applied to the AAS determination of heavy metals, as it leaves dissolved organic break-down products in the analyte. This constitutes no problem for AAS, but if voltammetry is to be applied, the solution resulting from pressurized digestion has to be treated subsequently by UV irradiation or by heating with HClO, to decompose the dissolved organic digestion products [15]. A simpler digestion approach for wines and fruit juices with low sugar content using UV irradiation has been described [8].

Voltammetric determination

Voltammograms were recorded in the differential pulse mode [lo] with a PAR 174 A Polarographic Analyzer connected to a Metrohm cell (EA 875-20). Potentiostatic control of the electrode potential was established by means of the three electrode system consisting of a hanging mercury drop electrode (HMDE, Metrohm E 410, area 2.2mm”) as the working electrode, a coiled Pt-wire as the counter electrode and an Ag/AgCl electrode as reference elec- trode connected to the analyte by a salt bridge.

In the voltammetric cell, 20 ml of deionized water, 0.04 ml of 70% HClO,, as supporting electrolyte, and 0.5 ml of analyte solution resulting from the HClO,/ HNO, wet digestion procedure are de-aerated for 10 min with purified nitrogen (99.999%). Cadmium, Cu, Pb and Zn are determined simultaneously by DPASV.

H

YU :D uk ,‘C”

0 10

10

50

500

5000 fi

20 NG co 2c NC PB

!OO NL NI 1000 NC cu

1nooo N‘ ZN

1 A/ -I -F1--l, , 4.7

E/V -1.2

IN= lOJ,LIYG/KG CO=14&JG/KG Cu=9,5hnc/nr PB=SJ,tk/KG

NI=~QJG/KG

NI = 100 NG

! 0.5~ A

-1.2 0.8 0.2 0.7 E/V '.2

Fig. 1. Determination of heavy metals in wheat bran. Sample weight, 0.0582g; DPASV, pulse height = 25mV. scan rate 5mVs-‘, clock time = 0.5 s; deposition time: Zn, 20 s (Ed = 1.2 V); Cd, Pb and Cu, 180 s (Ed = - 0.8V); Ni 60s (Ead = - 0.7 V). (1) Sample curve; (2)‘second standard addition curve.

6

The heavy metals are preconcentrated simultaneously at the HMDE by deposi- tion as amalgams. This is achieved by adjusting the deposition potential of _ 1.2V for 26min and stirring the solution with a magnetic stirrer (700rpm) to enhance mass transfer to the HMDE. After turning off the stirrer, and a rest period of 30s anodic stripping is performed in the differential pulse mode (DPASV) with the following parameters: pulse height, 50mV; pulse duration, 56ms; time, 0.5 s; and scan rate, 5 mV s I. If necessary, the sensitivity of the instrument can be altered so that the different responses between zinc and cadmium and between lead and copper can be detected. The measurement is repeated once before adding small volumes (50-150~1) of standard solution. Two standard additions are sufficient to determine the concentration. The concentrations of the metals are calculated from the intercept of the linear regression lines with the x-axis. An example is given in Fig. 1.

Nickel (and cobalt) determination is performed subsequently by adsorption voltammetry (AV) [6, 71 in the same solution after addition of 0.5ml of 2M ammonia buffer to adjust the pH to 9.2 and 50 ~1 of a 0.1 M solution of di- methylglyoxime in ethanol. The nickel- and cobalt-dimethylglyoxime com- plexes which are formed are at first accumulated by adsorption of the HMDE surface at a potential of - 0.7 V. The adsorbed Ni and Co complexes are then reduced by scanning (scan rate 20 mV s -‘) with a linear voltage scan to more negative potentials. At very low nickel and cobalt concentrations ( < 10 ~1 kg ‘) the differential pulse mode has to be applied with the parameters given above, i.e. by adsorption differential pulse voltammetry (ADPV). Figure 1 shows an example of the simultaneous determination of all treated heavy metals.

RESULTS AND DISCUSSION

Accuracy

The accuracy of the described digestion and determination procedure was tested by analysis of NBS standard reference materials. The values determined ‘are in good agreement with the certified values (Table 2).

Determination of sensitivity and precision

The determination of the sensitivity of the voltammetric procedure after wet digestion is not limited by the methodological potential of the voltammetric methods themselves, but by the digestion blanks values which vary with the amount of acid used. In 1 ml of digestion mixture (HClO,/HNO,, 1:2) the blank values were: long Zn, 0.1 ng Cd, 0.3 ng Pb and 0.08ng Ni, with a day-to-day fluctuation of < 20%. For many types of food the trace metal concentrations are > 1OOpg kg-’ and therefore the digestion blank value is negligible.

TABLE 2

Determination of Zn, Cd, Pb, Cu and Ni in NBS Standard Reference Materials

Samplea Element

Orchard Leaves Zn NBS 1571 Cd

(n = 5) Pb CU Ni

Bovine Liver Zn NBS 1577 Cd

(n = 6) Pb cu Ni

Spinach Zn NBS 1570 Cd (n = 6) Pb

cu Ni

an = number of independent determinations.

Found values Certified values

(mgW’) (mgW’)

24.7 i 0.7 25 * 3 0.11 i 0.01 0.11 + 0.01 44.4 * 1.7 45 + 3 11.1 t 0.4 12 -t 1 1.28 + 0.17 1.3 + 0.2

126.6 + 7.0 130 i 10 0.31 k 0.02 0.27 i 0.04 0.28 i 0.03 0.34 ? 0.08

197.6 k 7.6 193 + 10 0.15 i 0.01 0.155 (22)

49.1 ? 0.7 50 * 2 1.58 k 0.06 (1, 5) 1.29 & 0.08 1.2 k 0.2 11.1 * 0.4 12 * 2 5.78 k 0.20 (6)

APPLICATIONS

The developed, and extensively tested analytical procedure with vol- tammetric determination has been applied to a large number of food types. Although the number of specimens investigated of each food type is rather limited, the data presented provide an estimate of the magnitude of heavy metal levels to be expected in the food types studied and show that in this respect different food groups can be distinguished. The number (n) of specimens inves- tigated is given in the tables; three independent samples from each specimen were analyzed.

Cereals and cereal products

Table 3 contains the results for cereals and cereal products, except rice for which the results are listed in Table 4. Levels of Cd are comparatively high in wheat grain and wheat products including noodles. A particularly high Cd level is observed in wheat bran. In rice (Table 4) the Cd levels are usually an order of magnitude lower, except for rice from the U.S.A. This is either a consequence of intensive fertilizer application or due to higher Cd pollution in the rice growing area. In Japan the Cd level in rice is about 0.1 mg kg-‘. Only the Cu content of wheat bran is significantly higher, while the rice from north-east Brazil contains rather low concentrations of Cu (see Tables 3 and 4).

The Pb-level in bread is more than double that in wheat grain and flour and is probably caused by some contamination during baking. The levels of Cd and

8

TABLE 3

Heavy metals in cereals and cereal products

Specimen n Metal (pgkg ‘)

Wheat grain Wholemeal bread Wheat/rye mixed

bread Wheat bread Wheat flour Wheat bran

Full Diet With milk sugar

Noodles

Zn Cd Pb CU Ni

19300 54 12 3000 570 12100 26 27 1540

18600 29 43 2140 113 19500 50 46 2160 3860 39 19 840

107400 148 58 9560 640 73800 66 37 13300 590 84300 52 28 10800 1010 11300 62 22 4650 44

Pb in cereals and cereal products should be considered in the context of the average daily intake allowed in the WHO recommendations. For adults with a typical body weight of 70 kg a daily intake of 70 ,ug Cd and 430 pg Pb is recom- mended; for children the recommended intake is lower.

Vegetables

Table 5 presents data on vegetables, together with data for grass, clover, sugar beet and sugar. Vegetables, as well as grass and clover for comparison, were collected from gardens in a heavily Pb- and Cd-polluted area close to a lead smelter at Stolberg and from an unpolluted agricultural area at Jiilich which exhibits, like many other rural regions, the lowest heavy metal deposi- tion from the atmosphere in the Federal Republic of Germany. In general, the data reflect the potential of some vegetables to accumulate heavy metals and show that, at locations with severe heavy metal pollution through deposition from the atmosphere, the heavy metal levels in vegetables can be orders of

TABLE 4

Heavy metals in rice

Cultivation area

n Metal (pg kg- ‘)

Zn Cd Pb CU Ni

North-east Brazil

France, Camarque Bali U.S.A.

1 10800 2.3 15.5 945 200 1 14000 1.7 11.6 1700 120 1 11300 5.1 17.4 1800 160 1 9000 21.6 14.6 1200 340

9

TABLE 5

Influence of environmental burden of heavy metals on their contents in vegetables and plants

L pecimen” $ n Metal (pgkg-‘)

Zn Cd Pb CU Ni

F’otatoes (a) 3 11000 210 18 1500 118 (b) 3 1590 8 3 330 66

Rhubarb (a) 3 1520 68 296 164 18 (b) 3 1220 2 13 105 14

Leek (a) 3 6570 136 290 402 65 Parsley (b) 2 12600 42 425 5350 380 Lettuce, garden (b) 1 3370 46 68 611 38

greenhouse 1 4150 69 5 285 59 Tomato, greenhouse 1 770 9 1 231 17 Grass (a) 17 19300 172 6200 2680 183

(b) 3 14300 18 230 1960 170 Clover (a) 20 18000 164 5600 2000 175

(b) 3 13800 21 250 780 93 Sugar beet (b) 3 3290 39 36 503 23 Sugar 1 105 1 65 55 12

(a) = sample collected near a lead smelter at Stolberg. (b) = sample collected in Jiilich.

magnitude higher, which is apparent in plants with large leaf surfaces, e.g. lead in garden lettuce is higher than in greenhouse lettuce (see Table 5). The Federal Health Office has recommended tolerable levels of 1.2 mg Pb kg-’ and 0.1 mg Cd kg-l in vegetable leaves [l]. Only the Cd concentrations in leek from polluted areas are higher (Table 5). For potatoes the recommended values are 0.2mgPbkg-’ [l]. In Cd-polluted areas this metal accumulates in potatoes. Generally, it can be stated that the consumption of vegetables from Cd- and Pb-polluted areas, especially without careful mshing, results in an un- necessarily high intake of both metals.

The same applies with respect to Cd and Pb in grass and clover. Both plant species show a remarkable accumulation potential for Pb (see Table 5).

A comparison of the heavy metal concentrations in sugar and s’ugar beet shows only low lead contamination during sugar production (see Table 5). The levels of the other elements are lower in sugar than in sugar beet.

Fruits

Rather low heavy metal levels are to be expected in fruits, as Table 6 reflects. The differences in the heavy metal content between the skin and pulp of grapes are exceptional; the heavy metal levels in grape pulp correspond in magnitude to the average levels found in wines [5]. An exception is Pb which, on average, has significantly higher levels (by about 2 orders of magnitude) in wine than in

10

TABLE 6

Heavy metals in fruits

Fruit n Metal (pgkg ‘)

Zn Cd Pb cu Ni

Apples New Zealand

Apples, France Apples, Germany Pears, Italy Grapes, Italy

Skin Pulp

Orange juice

3 178 0.5 5.1 138 8.8 3 89 0.5 0.7 212 8.2 3 189 2.4 3.3 169 32.6 3 326 0.9 3.7 445 75.7

1 6160 1.1 23.0 5640 24.5 1 1120 0.12 0.86 1023 8.1 2 223 1.0 1.76 180

grape pulp and, typically, about a factor of ten or more than in the grape skin. According to our extensive investigations [3] the typical average levels in wine are (pgl-l): Cd, 1.5; Pb, 130; Cu, 400 (but can be up to 4000); and Ni, 60. Heavy metal levels in beer are rather low. The investigation of a number of different German beers yielded the following ranges (pglll): Zn, 540; Cd, 0.4-1.7; Pb, 0.7-5.5; Cu, 555; and Ni, 2.510. Although these ranges are low, only their lower values overlap with the respective heavy metal levels in drinking water.

Food types of animal origin

Table 7 provides an estimate of the heavy metal levels in some common food types of animal origin. For comparison margarine has also been included as an important type of fat in the food basket of man. Generally the heavy metal levels are rather low, particularly in milk, due to the very small transfer factors from cattle fodder, grass and clover, into milk.

The same is true for meat, as shown by a recent study on beef and veal [9]. Only for the detoxification and storage organs, i.e. the liver and kidney, have high heavy metal levels been found. This seems to be a common feature for very different kinds of animals, as extended studies on pelagic and benthic fish [12] and recently on mussels and oysters have shown [16]. In the muscle of fish, Hg is usually present, > 90% of which is in the form of methylmercury, which, in toxicological terms, plays a significant role. The Hg burden of large fish such as tuna and swordfish is particularly high [12, 171. The other heavy metals, among them the toxic Cd and Pb, reach elevated levels in the liver and kidneys [12]. Similar findings have been established for mussels and oysters, although it must be taken into account that specimens of these sea foods are consumed whole.

From the data for fats in Table 7, compared with milk the levels of Cd and Pb are somewhat higher in butter, whereas the content of Zn and Cu is lower

11

TABLE 7

Heavy metals in common food types of animal origin _

Specimen n Metal (pg kg-’ )

Zn Cd Pb Cu Ni

Milk 3.5% fat 10 3730 0.1 1.83 40.3 4.4 Milk 0.3% fat 10 3940 0.1 5.50 49.4 10.7 Butter 3 309 2.27 9.84 3.36 11.6 Margarine 3 1270 42.0 164.1 58.0 610 Smoke-dried 1 13350 1.97 11.3 380 6.3

ham Pork 1 0.27 7.1 300 4.3

by an order of magnitude. Noticeably higher levels of Cd, Pb and Ni .are found in margarine, than in butter. For Ni, and possibly for Cd and Pb, the levels in margarine are probably associated with input during the production process.

Mushrooms

Substantial levels of Cd, Pb and Cu are contained in wild mushrooms picked from forests and fields, as shown in Table 8. Mushrooms are potent heavy metal accumulators [18, 191. There are remarkable differences between the heavy metal contents of the stem and the cap, which amount, for example for BoEetus edulis, to a 5-fold higher heavy metal enrichment in the cap compared with the stem. The most frequently consumed cultivated mushroom (champignons) have

TABLE 8

Heavy metals in mushrooms (FW)

Specimen n Metal (pg kg-‘)

Zn Cd Pb cu Ni

Yellow bolete (Boletus edulis)

Stem Cap Gill

Chanterelle (Cantharellus cibarius)

Field mushroom (Agaricus Camp.)

Cultivated champignons

Fresh Canned

1 16706 680 406 5300 660

3 6100 150 41 3000 60

3 2800 15 6 1500 5 3 3406 8 7 1500 9

30600 440 200 13500 440 19200 1920 320 4800 105 27706 3010 580 6200 174 45400 11600 1730 20600 240

12

TABLE 9

Heavy metals in baby food

Specimen n Metal (pgkg ‘)

Zn Cd Pb CU Ni

Instant milk

powder Instant baby

food Banana Fruits Semolina Orange Rusk Pear

Whole milk food Caramel Chocolate Fruits Carrot flakes

2 11800 0.13 11.7 160 43.1

1 7280 15 42.3 1450 71.4 1 11 21 910 1 33 12 1300 1 36 4 1300 1 31 27 2000 1 14 320 640

1 22 28 1400 1 26 27 2500 1 38 34 1400 1 115 100 3600

rather low contents of Cd and Pb, as they are grown on unpolluted soil in greenhouses where no input of heavy metals by deposition from the atmosphere is possible. The consumption of not more than 20s250g of wild mushrooms weekly is recommended by the Federal Health Office. This recommendation is based on the preliminary permissible intake of cadmium published by WHO (5OOpg week-‘), for which an absorption of 5% of the consumed Cd by the digestive tract is assumed.

Baby food

A special food type is instant baby food preparations. Heavy metal levels found in four types are given in Table 9. The concentration of Cd is remarkably high, except for instant milk powder, because cereals (see Table 9) are a basic constituent of instant baby foods. This would explain the elevated levels in the various fruit preparations compared with the Cd content of fresh fruit (Table 6). The Cu levels are also comparable with those in cereals and cereal products (Table 2) and in many cases, the same applies to the Pb levels.

The instant baby food containing pears has a rather high Pb level which indicates that Pb contamination has possibly taken place during production.

In general, the levels of Cd found in the randomly selected baby food samples seem to be undesirably high. In this context it should be borne in mind that the WHO recommendation for a tolerable average daily intake of 70mg Cd refers to an adult of 70 kg body weight. Taking into account the lower body weight of babies the tolerable average daily Cd intake is also correspondingly lower. The Cd content of some of the baby food products investigated is such that a baby would ingest the tolerable lower levels of Cd with a daily consumption of 200 g of baby food.

TAB

LE

10

Hea

vy

met

als

in p

ipe

and

ciga

rett

e to

bacc

o

Spec

imen

n

Met

al

(mg

kg-‘)

Zn

Cd

Pb

CU

N

i co

Pipe

tob

acco

I

3 33

.4

* 1.

1 0.

70

* 0.

04

1.33

+

0.19

15

.2

i 0.

5 4.

62

+ 0.

36

1.03

i

0.03

II

3

33.6

+

1.7

0.49

*

0.04

0.

75

* 0.

04

6.26

&

0.1

9 1.

60

t 0.

11

0.53

f

0.04

II

I 3

18.4

-t

3.1

0.

22

* 0.

05

0.58

f

0.01

5.

86

+ 0.

32

1.43

+

0.28

0.

93

+_ 0.

09

Cig

aret

te

toba

cco

I 3

41.0

*

3.7

1.03

z!

z 0.1

1 1.

91

* 0.

14

9.35

+

0.70

2.

24

i 0.

95

0.56

i

0.07

II

3

29.8

?

0.6

1.43

t

0.06

2.

20

i 0.

07

10.9

&

0.3

3.

36

-t 0

.20

0.79

i-

0.03

II

I 3

47.8

f

3.1

1.29

+

0.01

2.

03

?r 0

.37

21.0

+

1.8

3.67

+

0.16

0.

60

i 0.

08

Y

TAB

LE

11

Hea

vy

met

als

in t

ea,

coffe

e an

d co

coa

Spec

imen

n

Met

al

(mgk

g-I)

Zn

Cd

Pb

cu

Ni

co

Tea I II

II

I

Cof

fee

I (in

stan

t)

II

III

Coc

oa

I

3 41

.0

i 2.

2 0.

028

* 0.

003

0.81

i

0.07

22

.9

+ 1.

7 8.

67

_+ 0.

62

0.67

?

0.10

3

27.0

+

1.5

0.03

2 ?

0.00

2 0.

83

? 0.

05

20.4

-c

1.0

6.

62

-t 0

.66

0.28

i

0.02

3

24.0

f

1.4

0.01

2 i

0.00

1 0.

52

i 0.

07

30.6

?

1.9

5.57

*

0.53

0.

18

i- 0.

01

3 2.

91

i 0.

19

0.00

5 *

0.00

1 0.

053

+ 0.

007

0.23

_t

0.0

1 0.

75

F 0.

07

0.31

*

0.04

3

3.83

?r

0.1

1 0.

004

f 0.

001

0.07

1 *

0.00

2 10

.9

? 0.

8 0.

47

+ 0.

07

0.11

+

0.02

3

5.34

f

0.11

0.

005

i 0.

001

0.04

1 t

0.00

3 13

.2

+ 0.

05

0.32

i

0.01

0.

14

* 0.

01

3 58

.1

i 5.

5 0.

16

? 0.

005

0.29

*

0.02

31

.6

i 0.

7 10

.6

+ 0.

7 1.

31

* 0.

09

A permanent control of the levels of toxic heavy metals in baby food products by reliable and convenient analytical methods is therefore urgently needed.

Tobacco, cigarettes, coffee and tea

Heavy metal contents of pipe-tobacco and cigarettes are shown in Table 10. The high Cd level results in a large cadmium concentration in the blood, urine, liver and kidneys of smokers than non-smokers [20, 211.

Nickel compounds can be toxic; a large number of insoluble inorganic Ni compounds cause lung and nasal cancers [22]. The very high Ni level i-n tobacco smoke, caused by reducing conditions and elevated temperatures dur:ing smok- ing, can be a hazard to long-term smokers, but more investigations on this subject are necessary. The highest level of Ni, and all other metals, was found in cocoa (see Table ll), which may reflect the natural elemental composition of cocoa beans or may be a consequence of enrichment in preparative stages. The Ni levels in tea are about 20-100 time larger than in other plants (Tables 5 and 11); the Cd and Pb levels in tea are similar to those observed in various common vegetable samples. Even lower cadmium and lead concentrations are found in coffee. In instant coffee (Table 11) the Cu level is 50 times lower than in coffee grain, but for Ni and Co a slight enrichment was observed. The concentration of heavy metals in tea leaves and coffee grain cannot provide information about the intake of these metals by drinking tea and coffee. Table 12 shows the heavy metal concentrations in a normally prepared water extract of tea and coffee. The concentrations of all the metals in the water phase, with the exception of nickel in tea, are lower than the tolerable limits for the metals in drinking water [23]. The nickel components in tea leaves are soluble in water (see Table 12) unlike those in coffee grains. Similar concentration relationships in extracts of tea and coffee were found for Zn and Cu. Cadmium and lead concentrations in drinks are very low, lower than the concentrations in some drinking water samples [24].

TABLE 12

Water extract of heavy metals from tea and coffee

Element Tea IV Coffee IV

Raw Solution % of total Raw Solution % of total material 4.94 g tea in amounts in material 10.7 g coffee in amounts in (mgkg-‘) 250 ml water solution (mgkg-‘1 100 ml water solution

(mgkg-‘) (mg kg-‘)

Zn 26.0 0.283 55.0 3.55 0.197 51.5 Cd 0.034 0.0001 17.4 0.002 0.00008 46.9 Pb 0.708 0.002 15.4 0.056 0.030 53.8 cu 23.0 0.083 18.3 11.2 0.218 18.1 NI 6.52 0.106 82.2 0.481 0.016 32.0 CO 0.192 0.0015 40.3 0.159 0.0075 60.7

16

Relatively high Zn and Cu levels were found in all the food samples inves- tigated. The daily requirement for an adult is 15mg of Zn and 5mg of Cu [22] and food is the fundamental source of these essential heavy metals.

CONCLUSIONS

The voltammetric approach to the determination of heavy metals in food samples after wet digestion is a very reliable, sensitive and accurate method. Simple sample pretreatment, the low cost of the electrochemical equipment (see Table 1) and the possibility of simultaneous determination of metal groups are the greatest advantages of voltammetry. The rapid progress in microcom- puter techniques has enabled the construction of fully automated voltammetric equipment which reduces the determination procedure for four elements (Zn, Cd, Pb and Cu) to about 20min (see Table 1).

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