T&nfa, Vol. 37, No. 8, QQ, 825430, 1990 Printed in Great Britain. All rights rescrv&

~39-91~~ $3.00 + 0.00 Copyright@ 1%t&gamOnQWSQk

DEVELOPMENT OF A SLURRY ATOMIZATION METHOD FOR WE DETERMINATION OF CADMIUM IN FOOD

SAMPLES BY ELECTROTHERMAL ATOMIZATION ATOMIC-ABSO~ION SPECTROMETRY

Stir LYNCH and DAVID LI~LWO~

Department of pure and Applied Chemistry, University of Strathclyde, 29.5 Cathedral Street, Glasgow Gl IXL, Scotland, U.K.

(Received 14 December 1988. Reoised 20 February 1990. Accepted 27 February 1990)

Summary-Matrix modifiers have been compared for the determination of cadmium in food&r& by ETA-AAS with the sample injected in the form of a slurry. Addition of 800 pg/ml Pd stabilized cadmium to a similar extent as did ammonium dihydrogen phosphate, but avoided the increase in background signal associated with the latter. An analytical procedure was developed, based on palladium matrix m~fi~tion, platform atomization with a pi-ato~~tion cooling step and integrated absorbance measurements. The method allowed the analysis of milk, liver and olive leaf slurries at concentrations up to at least 50 mg/ml by direct calibration with aqueous standards. The accuracy of the analytical results was within 15% and the detection limit for cadmium in analysis of a 50 m&ml slurry was 10 rig/g..

Various attempts have been made to eliminate the tirn~~nsu~ng sample preparation pro- cedures traditionally associated with the analysis of solid samples by electrothermal atomization atomic-absorption spectrometry (ETA-AAS). Two novel sample intr~uction methods have been studied. In the first, solid samplesL-‘O are weighed on a microbalance and transferred to a furnace. Although, with care, samples may be introduced into a furnace accurately and precisely, the procedure is time- consuming and susceptible to environmental contamination, and calibration is difficult. The second approach used is the in~oduction of samples as a suspension or a slurry.t’-28 Although the accuracy and precision of this type of sample-introduction depend on the stability and homogeneity of the slurry, the method has several advantages. Full automation can be achieved by using a stirring device compatible with co~er~ally available auto~mplers.28 Chemical modifiers can be added to the slurry diluent, allowing intimate contact with the ana- lyte. Samples may also be injected reproducibly onto the same location within the furnace tube.

Cadmium is a difficult element to determine accurately by use of solid sampling. This is mainly due to the low thermal stability of most cadmium salts, which prevents the thermal re- moval of interfering matrices prior to atomiz- ation, resulting in a loss of analytical sensitivity

and necessitating use of the standard-additions ~libration method. Several chemicals which form cadmium compounds of greater tbermal stability have been added as modifiers. The most popular is phosphate,‘82e’4 often as one of its ~rnoni~ salts, to form cadmium pyro- phosphate. Other modifiers used include ammo- nium fluoride,3s forming cadmium Auoride, Arnold persulphate36 or thiourea,37 forming cadmium sulphide, selenium,38 forming cad- mium selenide, and palladium salts,3p which presumably form an intermetallic species.

In this work ammonium dihydrogen phos- phate and palladium nitrate were compared as modifiers for the determination of cadmium in food slurries. Addition of magnesium nitrate with these modifiers was also evaluated.““’ Subs~uently, an analytical procedure was de- veloped which used platform atomization with Zeeman background correction. This enabled direct vibration with aqueous standards to be used for the analysis of a range of foodstuffs.

EXPERIMENTAL

All atomic-absorption measurements were performed with a Perkin-Elmer Zeeman 5000 spectrometer equipped with an HCA 400 fur- nace programmer and an AS40 autosampler. The Zeeman background correction system

825

826 SEAN LYNCH and DAVID LITEFSOHN

was used throughout this study. Measurements were performed at the 228.8 nm resonance line of cadmium with a spectrometer bandpass of 0.7 nm and use of a hollow~ath~e lamp (Cathodeon Ltd., Cambridge, England) oper- ated at 8 mA. Integrated absorbance measure- ments were made throughout.

Non-grooved pyrolytically coated graphite tubes were used with pyrolytically coated platforms, unless otherwise stated.

Slurry samples (20 yl) were injected either manually after manual shaking or by using the AS40 autosampler together with the integral magnetic stirring device described previously.”

The tube-wall tem~ratures over the pro- grammed range of 80~2~’ were checked with an Ircon 1100 optical pyrometer. The measured temperatures were within 100” of the programmed temperatures.

The food samples used in optimization studies (cadmium content not certified) were obtained as finely ground dry powders from the Ministry of Agriculture, Fisheries and Food, Food Analysis Laboratories, Norwich, U.K. The certified reference materials (CRMs) analysed to determine the accuracy of the pro- posed procedure were (i) Bovine Liver CRM 185, (ii) Milk Powder CRM 150, (iii) Milk Powder CRM 15 1 and (iv) Olive Leaves CRM 062 (Community Bureau of Reference, Brussels, Belgium).

Preparation of slurries

A 0.1-0.5 g portion of powdered food sample was accurately weighed, appropriate reagents were added to it and the mixture was diluted to 10 or 20 ml with distilled water. Chemical modifiers were added during this preparation, ensuring good mixing with the sample. Addition of 5 mg of antifoaming agent per ml of slurry prevented foaming. Concentrated ammonia so- lution was added to give a 5% v/v con~ntration (5 ml per 100 ml of slurry). The suspension was then shaken vigorously for 15 min to achieve thorough dispersion of the milk powder or animal tissue,

Reagents

AnalaR grade reagents (BDH Chemicals Ltd., Poole, England) were used throughout this study unless otherwise stated. Distilled water was used in the preparation of samples and solutions.

Concentrated nitric acid PRONALYSAR grade, (May and Baker Ltd., Dagenham, England).

Con~n~ated a~onia solution, ARISTAR grade (BDH).

Antifoam B emulsion (Sigma Chemical Company, St. Louis, U.S.A.).

Palladium(I1) nitrate solution, 10% w/v (Johnson Matthey PLC, Royston, England).

Cadmium nitrate solution, 1000 mg/l., pre- pared in 1M nitric acid.

Procedures

Choice of mod~~r. The major criterion used in selecting a modifier was its ability to produce a more thermally stable cadmium compound from both slurry samples and standards. The thermal stability was determined by making six measurements of the time parameters of the signal, rloX and rpepk, defined as the time re- quired for the atomic-absorption signals to reach 10% of the peak value and the peak absorbance, respectively. An increase in the rlow value indicates an improvement in the thermal stabilization of cadmium, as during the initial stages of the ato~zation step the temperature of the tube increases with time.

Conditions for direct analysis. Recovery of added cadmium was used as an initial indicator of the accuracy of analysis, and was defined as:

% Recovery = - a-bXlOO c- d

where a, b, c and d are the mean values of at least three absorbance signals for Q, 20 ~1 of the slurry sample plus 20 ~1 of 2 ng/ml Cd solution, b, 20 ,ul of the slurry sample, c, 20 ~1 of 2 ng/ml Cd solution, and d, 20 ~1 of an appropriate reagent blank mixture. Care was taken to ensure that all atomic absorption measurements were within the linear dynamic range of the inst~ment response.

Analysis of samples. The certified reference materials (CRMs) were analysed as follows.

(1) Standard solutions containing 0, 1, 2, 3,4 and 5 ng/ml Cd were atomized in duplicate. Linear least-squares regression analysis was ap- plied to the average of the integrated ab- sorbances.

(2) Samples and appropriate blanks were prepared in triplicate and three aliquots of each were atomized.

Determination of cadmium in food 821

(3) The cadmium content of each sample and blank replicate was determined by interpolation from the calibration graph. Sample concen- trations were corrected for the blank before calculation of the mean and the 95% confidence interval for the cadmium content in each sample.

RESULTS AND DISCUSSION

Comparison of modifiers

The optimum amount of palladium to be used was determined before different modifiers were compared.

Plots of 210% for a 2 ng/ml Cd solution and a 10 mg/ml kale slurry charred at 500” and atomized at 1800” from the tube wall, are shown in Fig. 1. A palladium concentration of 800 pg/ml in the sample gave an acceptable approach to maximization of the Cd signal and was used in all subsequent investigations. The effects of various modifiers are compared in Table 1 in terms of Q,% for samples charred at 700” and atomized from the tube wall at 1800”. The NH,HzP0,-Mg(N0,)2 mixture gives the best performance for the cadmium nitrate solutions. The performances of NH,H,PO, or Pd alone were comparable, but inferior to that of the NH,Hz PO,-Mg(NO,), mixture.

The corresponding results for the food slurries are rather interesting. The thermal stabilization varies with the type of sample and the type of modifier. Palladium gave significantly better performance than the phosphate-magnesium nitrate mixture for all slurry samples, except the bovine liver, and comparable performance to NH4H2P04 or the Pd-Mg(NO,), mixture. A combination of 800 pg/ml Pd, 10 mg/ml NH4H2P04 and 10 mg/ml

0.25

c .- .-.A

0.20

0.15 /

.,.6

% .

in

/ s A

p 0.10

/ ODS A

./ A I I I 1

0 200 400 600 600

Concentration of palladium (mgA.1

Fig. 1. Influence of palladium concentration on stability of cadmium in terms of the peak-time parameter r,OK. A, 2 pg/l. aqueous solution of cadmium; B, 10 mg/ml kale slurry.

Mg(NO& was examined in an attempt to combine the favourable performance of the NH., H, P04-Mg(NO& mixture for cadmium nitrate solution with that of Pd for slurry samples, but significant improvement was found only for bovine liver, and the performance was poorer for the milk slurries. Similar trends were observed for the rpcak values.

Char temperature curves for cadmium in aqueous solution and slurries prepared from samples of lettuce, kale, brussels sprouts and fish are shown in Fig. 2 for use of 800 pg/ml Pd as the modifier. The results indicate that with Pd added, the thermal stability of cadmium in slurries is relatively insensitive to differences in the matrix composition. No significant reduction in the cadmium signal was observed for char temperatures up to at least 800” for each of the samples studied. A conservative charring temperature of 750” was chosen for all subsequent studies. Addition of palladium as modifier avoided the high background signal associated with the vaporization of phosphate

Table 1. T,~% values obtained for cadmium with different modifiers; samples charred at 700” with wall atomization at 1800” (95% confidence limits in parentheses)

T,~% values, set

Modifier 2 ng/ml Cd

(aqueous solution) Kale

Slurries (10 mg/ml)

Milk Fish Bovine liver

No modifier co.01 N.S. N.S. N.S. N.S. 10 mg/ml NH,H,PO, 0.14( kO.02) 0.05(+0.02) 0.16(&0.01) 0.13( + 0.02) O.lO(~O.02) 10 mg/ml NH,H,PO,-

10 mg/ml Mg(NO,), 0.25( * 0.04) 0.04( f 0.02) 0.09( f 0.03) 0.04( f 0.03) 0.07( f 0.02)

800 mg/l. Pd 0.15(+0.01) 0.09(_+0.01) 0.14(*0.01) 0.11(+0.03) 0.07(*0.01) 800 mg/l. Pd-

10 mg/ml Mg(NO,), 0.10(~0.01) 0.09(_+0.02) 0.16(*0.01) 0.13(+0.02) 0.13(*0.02)

800 mg/l. Pd- 10 mg/ml NH,H,PO, 0.17(&0.03) 0.08(+0.02) O.lO(~O.02) o.ll(+o.ol) 0.12(&0.02) 10 mg/ml Mg(NW,

N.S. = No signal.

828 SEAN LYNCH and DAW LI~LEJOHN

0.5r la 1 12q- Ia)

0.4 - C

6

0.3 -

A

0.2 -

0.3 -

fi I v-

r: Ic O

I I I I I

1.2 (b) 8 120 (b)

1.1 F: P r

0.7

0.6 E

0.5 0 0.4

0.3

0.2

0.1

0 2 400 600 600 1000 1200

Tsmperatura (*C 1

Fig. 2. Graphs of integrated absorbance signal for cadmium as a function of charting temperature. A, 1 ng/ml aqueous solution of cadmium; B, 60 mg/ml brussels sprout slurry; C, 20 mg/ml kale slurry; D, 60 mg/ml fish sluny; E, 40 mg/ml lettuce slurry; F, 20 mgjmi bovine liver slurry; platform

atomization at 1700”.

salts in the furnace. The background signals for 20 ,cll of 10 mg/ml NH,H,PQ, obtained with platform atom~ation and the tern~~t~e programme shown in Table 2, were 1.4 ab- sorbance and 6.0 absorbance. set for peak height and peak area measurements, respec- tively. The background signal for 20 ~1 of 800 pg/ml Pd did not deviate sibilantly from the background measured during atomization of a water blank.

Palladium was therefore selected as a modifier for the determination of cadmium in slurries, on the basis of its comparatively good stabilizing effect and the absence of background signal enhancement when it was vaporized with the analyte.

60

000* 1290 1400

Temperature PC )

Fig. 3. Recovery of added cadmium at different atomization temperatures. A, 40 mg@l milk slurry; B and C, 10 and 20 mg/mt bovine liver slurries, respectively; D, 40 mg/ml brussels sprout slurry; E, 40 mg/ml fish slurry. The rest of the temperature programme is as given in Table 2,20 ~1 of 2 ng/ml Cd added to 20 ~1 of the slurry in the atomizer tube.

Optimization of atomization stage

The effect of a~om~tion temperature on the recovery of added Cd for slurries prepared from a range of food samples is illustrated in Fig. 3. The recovery of the signal for 20 ~1 of 2 ngjml cadmium solution was used to assess the extent of matrix interferences when the food slurries were analysed with calibration by use of aqueous solutions of cadmium nitrate. A pre- atomization cooling step to room temperature was used to enhance the delay in atomization. An atomization temperature of 1350” allowed recoveries close to 100% to be achieved for the four food samples studied and hence was used for subsequent investigations. Above this temperature there was a general decrease

Table 2. Furnace programme for analysis of samples with platform atomization (nitrogen used throughout)

Temperature, Ramp time, Hold time, Gas flow, Step “C see see mljnin

DV :: 5 40 300

Pretreat 30 20 Cool 20 Atomize 1350 t!r

20 :: 10 0

Clean 2400 1 2 300

Determination of cadmium in food 829

in recovery although the rate of this decline depended markedly on the sample matrix. Comparatively good recoveries of cadmium were achieved over the temperature range 1350-2150” for milk slurries. However, recoveries decreased to 50% at atomization temperatures of approximately 1750, 1950 and 2150” for fish, bovine liver and brussels sprout slurries, respectively.

The general decrease in recovery with increas- ing atomization temperature may be explained as follows. As the atomization temperature is increased, it takes longer for the tube to achieve the ideal condition of temporal isothermality, if the heating rate is unaltered. Indeed, at high temperature settings cadmium atom production will mostly occur while the tube temperature is still increasing, which results in a shorter atom residence time and lower integrated absorbance compared to measurements made at lower tem- peratures. As the rate of cadmium atom pro- duction was observed to be greater from the food slurries than from an aqueous solution of cadmium nitrate, significant differences in the average cadmium atom residence time can occur for the standard solutions and slurries, when vaporization takes place under non-isothermal conditions. In addition, greater temporal reso- lution between the atomic absorption and the background signals occurred at lower atom- ization temperatures. This implies better resolu- tion in the vaporization of the analyte and matrix species and may explain, to some extent, the superior recovery obtained at the relatively low atomization temperature of 1350”.

The effect of slurry concentration on the recovery is shown in Table 3, for a variety of food slurries, and an atomization temperature of 1350”. Recoveries in the range 90-110% were achieved for milk and brussels sprout samples up to the maximum slurry concentra- tion studied, of 100 mg/ml. Similar recoveries were achieved for fish slurries up to a slurry concentration of 60 mg/ml. The high level of

Table 3. Recovery as a function of slurry concentration, for platform atomization at 1350” after a cooling step

Slurry concentration,

mglm[

10 20 40 60

100

Recovery, %

Bovine Brussels Fish Milk liver sprout

105 99 96 103 107 96 97 100 96 108 - 96 95 103 - 91 82 107 - 92

cadmium present in the bovine liver sample prevented the analysis of slurry concentrations above 20 mg/ml. Though the range of slurry concentrations over which close to quantitative recoveries are achieved is impressive, the accu- racy and precision of slurry sample deposition into a graphite tube has been shown to deterio- rate for some foodstuffs at slurry concentrations above approximately 50 mg/ml.** Therefore, the highest slurry concentration that can be used for accurate analysis based on the use of aqueous standards may vary for different samples.

Analysis of samples

The furnace conditions optimized for the determination of cadmium in slurry samples by direct calibration with aqueous standards are given in Table 2. A slurry concentration of 40 mg/ml was used for the analysis of the CRMs, except for Bovine Liver CRM 185, as its higher cadmium content required the use of a 10 mg/ml slurry. The cadmium concentrations determined by the proposed method (Table 4) are in close agreement with the certified values, although the 95% confidence limits are generally poorer than those of the certified values.

Good sensitivity was achieved by the pro- posed method. The cadmium characteristic mass for aqueous solutions was 0.60 pg/O.O044 absorbance. set and this was within a factor of 2 of the value of 0.35 pg/O.O044 absorbance. set reported previously” for aqueous solutions. For the food types analysed in this study, the cadmium detection limit (2~) is 10 rig/g (dry weight), based on a 50 mg/ml slurry.

CONCLUSIONS

Palladium stabilizes cadmium to a similar extent as does the commonly used NH4H2P04 modifier for a variety of food slurries, but gives

Table 4. Determination of cadmium in CRMs by the proposed method

Cadmium concentration, pg/g

Sample Proposed method* Certified

Bovine Liver 0.30 + 0.04 0.298 & 0.025 CRM 185

Milk Powder 0.024 f 0.006 0.0218 + 0.0014 CRM 150

Milk Powder 0.114 f 0.030 0.101 f 0.008 CRM 151

Olive Leaves 0.106 f 0.010 0.10 * 0.02 CRM 062

*Based on triplicate analysis of three specimens of each CRM; results quoted with 95% confidence intervals.

830 SEAN LYNCH and DAVID LITTLEJOHN

a much lower background signal. A magnesium nitrate-palladium mixture did not give an additional improvement in the thermal stability of cadmium in aqueous solution or food slurries.

The direct dete~ination of cadmium in food slurries was achieved by using. pa~a~um matrix modification with platform atomization, a char temperature of 750”, a pre-atomization cooling step and integrated absorbance measurements. Accuracy to within 15% was achieved for the CRMs analysed. Quantitative recoveries were achieved for some slurries up to a concentration of 100 mg/ml, but a maximum slurry concen- tration of approximately 50 mg/ml is recom- mended for general use.

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