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Direct determination of lead in sweet fruit-flavored powder drinks by electrothermal atomic absorption spectrometry
SPECTROCHIMICA ACTA PART B
E LS EV I E R
Spectrochimica Acta Part B 53 (1998) 601-611
Direct determination of lead in sweet fruit-flavored powder drinks by electrothermal atomic absorption spectrometry I~der C. Lima 1, Francisco Jos6 Krug*, Marco A.Z. Arruda 2 Centro de Energia Nuclear na Agrieultura, Universidade de Sgto Paulo, P. Box 96, CEP 13400-970, Piracicaba-SP, Brazil
Received 6 November 1997; revised 23 December 1997; accepted 23 December 1997
Abstract
A simplified method for direct determination of lead in sweet fruit-flavored powder drinks, syrups and honeys by electrothermal atomic absorption spectrometry without sample digestion is proposed. Samples were dissolved in water, acidified to 0.2% (v/v) HNO> and directly injected into an end-capped transversely heated graphite atomizer (THGA). Building up of carbonaceous residue inside the atomizer was effectively precluded for sugar solutions not exceeding 8.0% (m/v) when a heating program with two pyrolysis steps (600 and 1000°C) was carried out without air-ashing. Under these conditions one atomizer supported about 250 firings. Among various chemical modifiers tested, better recovery and repeatability results were obtained with a 5 gg Pd + 3 #g Mg(NO3)2 mixture. Tests carried out with individual concomitants containing up to 1.0 #g Na, K, Ca or C1, and up to 10.0 gg phosphate or sulphate, and several mixtures of these six concomitants, did not reveal significant interferences on lead atomization. Characteristic mass and detection limit based on integrated absorbance were 15 and 11 pg Pb, respectively. The relative standard deviation based on 10 measurements for typical samples (20-60 ng g-l Pb) was always lower than 5.5%. The detection limit of 7.0 ng g-1 Pb attained the Codex recommendation for the maximum allowed lead contents in the sugar samples. Application of t-test to the results obtained by the proposed direct analysis, and the official method adopted by Food Chemical Codex, demonstrated that there were no significant differences at the 5% probability level. © 1998 Elsevier Science B.V. Keywords: Pb; Sugar; Transverse heated graphite atomizer; Electrothermal atomic absorption spectrometry
1. I n t r o d u c t i o n Overall exposure to lead is of public health concern because of several hazardous effects that may occur to humans. Lead poisoning may provoke irritability, anorexia, malaise and headache. Intoxication progress may * Correspondingauthor. Fax: +55 19 4294610; e-mail: [email protected] Graduate student from Departamento de Qufmica, Universidade Federal de S~o Carlos. 2 Permanent address:'Instituto de Qufmica, Universidade Estadual de Campinas, Campinas-SP, Brazil.
led to attacks of abdominal pain until coma and death [1]. Approximately 80% of total human lead intake is supplied by diet. Thus, there is a current interest in the determination of this element in foods and foodstuffs [2]. An important class of foods consists of sugar and related products, such as sweets, confectioneries, sweet fruit-flavored powder drink, syrups, and other products which are commercially available, as well as used in the home for sweetening and baking [3]. According to the Codex Alimentarius Commission [4], the maximum lead concentration allowed in sugar varies from 0.1 /~g g-~ in dextrose and fructose to
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E.C. Lima et al./Spectrochimica Acta Part B 53 (1998) 601-611
Table 1 THGA heating program Step
Temp. (°C)
Ramp (s)
1 2 3 4 5 6
150 180 600 1000 1800 2400
5 5 5 5 0 1
Hold (s) 25 25 5 20 5 4
Argon flow rate (ml rain -I) 250 250 250 250 0 250
Program time, 105 s; injection temperature, 100°C.
0.5/zg g-~ in glucose syrups, sucrose and highfructose corn syrups, Morris et al. [5] developed the first determination of lead in sugar by electrothermal atomic absorption spectrometry, employing yeast fermentation of the sugar in order to decompose the sample matrix. Recently, a method proposed by MiUer-Ilhi [6], and subsequently evaluated [7] for use with modern graphite furnace atomic absorption spectrometers, was adopted by the Food Chemical Codex FCC [4] for lead determination in sugar samples. It is based on the analysis of partially decomposed sugar samples with HNO3-H202, and employs graphite tubes with platform atomization, Mg(NO3)2 as chemical moditier, and pyrolysis assisted by oxygen, which serves to oxidize the organic compounds as well as to stabilize the analyte [6,8]. A direct method for lead determination in sugar were developed by Miller-Ilhi and Greene [9], where a sample solution containing 50% (m/m) of sugar in 5.0% (v/v) HNO3 was delivered into the platform, using Mg(NO3)2 as chemical modifier, with a long pyrolysis step assisted by oxygen, Although the reported methods [4,6,9] present robustness and good analytical characteristics for lead determination in sugar samples for routine analysis, a method capable of running a larger number of samples per day, with a longer atomizer lifetime, was needed. The aim of the present work is to propose a simplifled procedure for the direct determination of lead in sweet fruit-flavored powder drinks, honeys and syrups without sample digestion, by using ETAAS with endcapped THGA, capable of attaining the Food Chemical Codex recommendation for the maximum allowed lead concentration in sugar samples. The strategy was to avoid air-ashing during the pyrolysis step,
which was possible by decreasing the sugar content of the sample solution and developing a suitable heating program with Pd+ Mg(NO3)2 as chemical modifier.
2. Experimental
2.1. Instrumentation A Perkin-Elmer 4100ZL atomic absorption spectrometer with a longitudinal Zeeman-effect background correction system, furnished with standard (Part no. B050-4033) and end-capped THGA (Part no. B3000653) were employed. Measurements were made at 283.3 nm using a Perkin-Elmer electrodeless discharge lamp EDLII system. Sample and modifier aliquots of 20.0 and 10.0/A, were taken from polypropylene cups and delivered into the tube by means of an AS-71 autosampler from the same manufacturer. Unless otherwise stated, argon (AGA) as purge gas, and the heating program shown in Table 1, were used throughout. Pyrolysis assisted by oxygen experiments were carried out with air (AGA). All measurements were made with at least three replicates and based on integrated absorbance.
2.2. Reagents, reference solutions, samples High purity deionized water obtained by a Milli-Q water purification system (Millipore) was used throughout. Analytical reagent-grade nitric and hydrochloric acids were distilled in quartz sub-boiling stills (Kiirner, Germany). Lead (1000 mg 1-1 stock solution) was prepared from Pb(NO3)2 (Johnson & Matthey), by dissolving 0.7992 g in 1.0% (v/v) HNO3. Analytical calibration
[LC. Lima et al./SpectrochimicaActa Part B 53 (1998)601-611
curves were obtained within 0.00 and 20.0 ng m1-1 of lead in 0.2% (v/v) HNO3. The calibration was checked periodically after every 15 measurements with 10.0 ng ml -I of lead solution, The effect of concomitants was investigated with solutions containing up to 100 mg 1-1 Na (NaC1, Johnson & Matthey), 150 mg 1-1 K (KC1, Johnson Matthey), 200 mg 1-1 Ca (CaCO3, Johnson & Matthey), 1000 mg 1-1 C1 (HC1, Merck), 1000 mg 1-l S-sulphate (H2SO4, Suprapur Merck), and 1000 mg 1-1 P-phosphate (NHaH2PO4, Suprapur Merck), in 0.2% (v/v) HNO3. The following chemical modifier solutions were prepared from NHaH2PO 4 salt (Suprapur Merck), 10.0 g 1-I Pd solution (Pd(NO3)2 Merck) and 10.0g 1-1 Mg(NO3)2 solution (Merck): (1) 0.03% (m/v) Mg(NO3)2 + 0.50% (m/v) NHnH2PO4; (2) 0.05% (m/v) Pd + 0.03% (m/v) Mg(NO3)2; (3) 0.20% (m/v) Pd + 0.05% (m/v) Mg(NO3)2; (4) 0.052.00% (m/v) Mg(NO3)2; (5) 0.05-0.15% (m/v) Pd. Sweet fruit-flavored powder drinks (80-85% sucrose) provided from different manufactures, honeys and syrups (70-75% total sugar and 30-25% moisture) [ 10] were purchased at local supermarkets, 2.3. Materials
All solutions were stored in polypropylene bottles (Nalgene). Plastic bottles, autosampler cups and glassware materials were cleaned by soaking in 20% v/v) HNO3 for 24 h, rinsing five times with MiUi-Q water and kept until dry in a Class 100 laminar flow hood. 2.4. Analysis of sweet fruit-flavored powder drinks, honeys and syrups 2.4.1. Official method adopted by Food Chemical Codex Samples were prepared according to methods described elsewhere [4,6], and the resulting solutions were analyzed by ETAAS by delivering 20.0/zl of the digest and 5/xl of the chemical modifier, 2.0% (m/v) Mg(NO3)2, into the atomizer, using the recommended heating program with oxygen ashing during the pyrolysis step [4,6]. 2.4.2. Direct analysis Four grams of samples were accurately weighed (to
603
0.1 mg), completely dissolved in water, and acidified to 0.2% (v/v) HNO3, and the volume was made up to 50 ml. Then 20.0/xl of these samples and 10/~1 of the chemical modifier solution (0.05% (m/v) Pd + 0.03% (m/v) Mg(NO3)2) were directly delivered into the end-capped THGA atomizer. The heating program in Table I was used, and all determinations were carried out in triplicate.
3. Results and discussion 3.1. General considerations
For optimizing the amount of sugar in the sample solution introduced into the atomizer, preliminary tests were performed with solutions containing 4.016.0% (m/v)of the carbohydrate. For sugar contents higher than 8.0% (m/v) a build-up of carbonaceous residue inside the atomizer was observed. With 16.0% (m/v), tube lifetime was limited to 100 firings, and the measurement precision was poor (R.S.D. >-10%). As a compromise between atomizer lifetime, precision and sensitivity, it was decided to work with samples containing up to 8.0% (m/v) in sugar. Also, the limit of solubility of sweet fruit-flavored powder drinks in water is about 8% (m/v), due to other constituents present on their formulation. These experiments were performed by adding palladium plus magnesium, or ammonium phosphate plus magnesium. Pyrolysis was carried out in two steps: ramping from 180 to 600°C and then from 600 to 1000°C. The first step was needed for minimizing the amount of carbonaceous residue building up inside the atomizer. After a 5-s holding time at 600°C, smoke was ceased and deposition of carbonaceous residue was not perceptible. The pyrolysis temperature was then increased to 1000°C for better separation of other concomitants. In this situation, the background was minimized and each graphite tube supported 240-260 firings. For comparison, when the suggested method was employed for direct lead determination in sugar using the same recommended conditions [9] (pyrolysis step at 750°C assisted by oxygen and 60/xg Mg(NO3)2 as chemical modifier), tube lifetime did not exceed 105 firings, and the background was more than doubled for the same sample solution containing 8.0% (m/v) sugar (Fig. 1). So, it was concluded that under the proposed
604
E.C. Lima et aL/Spectrochimica Acta Part B 53 (1998) 601-611
0.250 (A) f~ Analyte S i g n a l .......
.8 < 0
.
.
.
nor rto e t esampsinsuita em um
_
~../,.....~
_....
.
. Time (s)
As an ultimate consequence, the variable analytical cost was remarkably decreased for lead determination in the samples employing transversely heated graphite atomizers [ 11].
. 5.0
for lead atomization, a minimum amount of nitric acid was added to the sample solution. For undigested samples, acidity was kept in 0.2% (v/v) HNO3, although acidity as high as 5.0% (v/v) could be supported by the THGA. No significative effect on lead integrated absorbance was found, when the acidity was varied within this concentration range.
0.250
(S) -
c,
I /
<
/\ /,f.; .~, 0
AnalySi~gnal
.............aa~ground ,...,
/ "" -time (s)
5.0
Fig. 1. Peak profile for 300 pg Pb in 8.0% (m/v) lemon-flavored powder drink (acidity kept 0.2% (v/v) HNO3). (A) Proposed method
(integrated absorbance, 0.112s; integrated background, 0.039 s):
heating program shown Table 1; modifier, 5~g Pd + 3/~g Mg(NO3)2; (B) Oxygen-ashing (integrated absorbance, 0.102 sl
integrated background, 0.100s): heating program described else-
where [9]; modifier,60/~gMg(NO3)2. experimental conditions air-ashing should be avoided in the pyrolysis step. It is also important to mention that, when a pyrolysis assisted by oxygen was used [6,9], a subsequent long cooling step (61 s)under argon atmosphere was required to eliminate all the oxygen inside the graphite furnace, in order to avoid the atomizer lifetime diminishing even further. As a consequence, the total heating program was very long (211-213 s) [6,9], limiting the sample throughput to approximately 15 h -~. With the proposed simplified and faster procedure (105-s heating program) it was possible to avoid a carbonaceous residue building up inside the atomizer, without air-ashing, for sample solutions containing up to 8.0% (m/v) sugar. By using the heating program shown in Table 1, a sample throughput of 30 h-1 and a 2.5-fold increase in the tube lifetime were obtained,
3.2. Modifiers
Among various chemical modifiers tested, a mixture containing 5/~g P d + 3 #g Mg(NO3)2 was the most suitable for obtaining reproducible signals as well as good lead recoveries. By comparing the pyrolysis temperature curves (Fig. 2) obtained with the tested modifiers (Pd, Pd + Mg(NO3)2, NH4H2PO4 + Mg(NO3)2, Mg(NO3)2, and Mg(NO3)z with air-ashing during the pyrolysis step), it can be seen that significant differences were found when 400 pg lead was atomized from aqueous solution (0.2% (v/v) HNO3) and from an 8.0% (m/v) spiked sample. In aqueous solutions, all modifiers stabilized lead to at least 1000°C. On the other hand, lead was thermally stabilized only up to 700°C by magnesium nitrate in the presence of 8% (m/v) sample solution. Taking into account that oxygen could increase analyte thermal stability [8], an experiment was carried out with Mg(NO3)2 in combination with air-ashing (heating program described elsewhere [9]), and it was verified that lead stability increased to 800°C (Fig. 2(B)), which is in agreement with the observations made by Miller-Ilhi and Greene with a longitudinally heated graphite atomizer [9]. It should be stressed that during an evaluation study of the method proposed for lead determination in sugar, Miller-Ihli reported that the maximum attainable pyrolysis temperature had to be decreased from 750 to 550-600°C when magnesium nitrate was employed with air-ashing in the THGA atomizer [7]. Without modifier (presence of only 0.2% (v/v) HNO3), the lead analytical signal vanished at temperatures above 700°C (Fig. 2(A)). Probably, during the decomposition of organic matrix, lead was carried away from the
[~.C. Lima et al./Spectrochimica Acta Part B 53 (1998) 601-611
605
1.2
(A)
¢-
-e 0
<
"0 0
0.8 0-------0~0
04
\o
e\
m
0.0
O I
600
'
|
900
'
|
1200
'
1500
Temperature (°C)
1,2
(B)
E m O
0.8
O ~
' =
~
0.4
m
Z
0.0 I
600
'
I
'
900
I
1200
'
1500
Temperature (°C) Fig. 2. Pyrolysis temperature curves of 400 pg Pb in presence of: (A) no modifier. (B) 60 #g MG(NO3)2. Atomizationtemperature, 1800°C. (-O-) 0.2% (v/v) HNO3; ( - 0 - ) 8.0% (m/v) sweet fruit-flavoredpowderdrink; (-&-) 8.0% (m/v) sweet fruit-flavoredpowder drink (with airashing during pyrolysis [9]). atomizer by the fumes [5,12]. Palladium alone or Pd + Mg(NO3)2 were more effective for lead determination in sugar matrix, supporting up to 1100°C in the pyrolysis step (Fig. 2(C,D)). With NH4H2PO4 + Mg(NO3)2 this temperature should not exceed
900°C (Fig. 2(E)). This 200°C difference was also observed by Lynch and Littlejohn [13]. It is important to point out that, for other concentrations of Mg(NO3)2, Pd + Mg(NO3)2 and Pd, similar electrothermal behavior of lead was observed.
[~.C. Lima et al./Spectrochimica Acta Part B 53 (1998) 601-611
606
1.2
(C)
¢O O)
08
e---~~e~e------e~e o-
~
o
____-
-o
~ o
°
o
0.4
,w O
Z
0.0 I
'
U
600
'
900
I
'
1200
1500
Temperature (°C)
8
1.2 o
(o)
C
-E o
o~O 0.8-
•o
.~
o-
O~o o
-o
~ -c
~-~
%
0.4
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-
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I
600
900
'
I
1200
°
1500
Temperature (*C) Fig. 2 (continued). (C) 5/~g Pd. (D) 5/~g Pd + 3/xg Mg(NO3)2. Atomization temperature, 1800°C. ( - O - ) 0.2% (v/v) HNO3; ( - O - ) 8.0% (m/v)
sweet fruit-flavoredpowderdrink. Better lead peak profiles were achieved with the mixture Pd + Mg(NO3)2 when compared to Pd alone. With the addition of magnesium nitrate, a more uniform distribution of palladium throughout the platform should be obtained, allowing a better stabilization of the analyte, leading to sharper absorbance peaks
with a slight increase in the sensitivity, as also suggested by Qiao and Jackson [14]. For palladium masses higher than 15.0 ~g, higher blank levels appeared, decreasing the measurement repeatability. Best results of chemical modification in the sugar matrix, with good sensitivity and reproducible signals,
607
E.C. Lima et al./Spectrochimica Acta Part B 53 (1998) 601-611
@
.2
(E)
f-
_<~~ 0.8 ~
0
0 -~-
~~0
--------O~-----O.
0.4
Z
0.0 I
600
'
I
'
900
I
1200
'
1500
Temperature (°C) Fig. 2 (continued). (E) 3 ~g Mg(NO3)2 + 50 ~g NH4H2PO4. Atomization temperature, 1800°C. ( - © - ) 0.2% (v/v) HNO3; (-@-) 8.0% (m/v)
sweet fruit-flavoredpowderdrink. were achieved with 5.0/zg Pd + 3.0 #g Mg(NO3)2 or 3.0 #g Mg(NO3)2 + 50.0/~g NH4H2PO4. These two mixtures were then chosen for further experiments. Atomization studies were performed with these modifiers but, for simplicity of presentation, they were withdrawn from Fig. 2. The temperature that promoted the highest atomic signal without memory effects for further injections was 1800°C when 5.0/~g Pd + 3.0/~g Mg(NO3)2 was used. 3.3. R e c o v e r y tests
The correct choice of chemical modifier was decisive for obtaining good recoveries. The mixture 3.0/~g Mg(NO3)2 + 50.0/zg NHaH2PO4 gave rise to inconsistent lead recoveries ranging from 57 to 99% for different samples (Table 2), which means that the efficiency of this modifier varies according to the chemical composition of the samples. For these reasons this modifier was ruled out. The 5.0 #g Pd + 3.0/zg Mg(NO3)2 mixture gave rise to better yields (90.2-106% recoveries, average 97.1% (4.2%), with good repeatability (R.S.D. --5.9%) for all tested samples (Table 2). Consequently, this chemical modifier was chosen to improve this work.
3.4. Interference studies
The effects of the individual main concomitants, which can be found in the samples [15,16], on the atomization of 100-400 pg of lead are shown in Table 3 The results are presented as the ratio between the absorbances of lead atomic signals in the presence and absence of the concomitants. Although these observations were isolated, the whole set of data gave an indication that most concomitants would not be expected to interfere with the analyte atomization. It should be pointed out that no significant differences were observed between the slopes of lead analytical curves obtained in the absence or presence of these concomitants, within 100-400 pg Pb range. Up to 1/~g Na, K, C1 or Ca, and 10 ~g of phosphate or sulphate, did not cause remarkable variations (higher than - 10%) on the lead atomic signals. With an atypical content of 10/~g CI-, a signal suppression of 30% was observed. Mixtures of inorganic concomitants normally expected in real samples did not affect the atomization of 400 pg Pb. In principle, no interferences caused by inorganics should be expected on lead determination in sugar matrix. The results in Table 4 show that there
608
E.C. Lima et al./Spectrochimica Acta Part B 53 (1998) 601-611
were no significant interferences from mixtures containing 2-10-fold the normal content [ 15,16] of those concomitants, 3.5. Analytical characteristics
A linear range up to 20.0 ng m1-1 Pb was obtained by using the established conditions. It was observed that the end-capped THGA atomizer always provided an increase of 60% on the integrated absorbance when compared to standard THGA, as previously observed [17]. End-capped tubes were then used throughout. Reproducible characteristic mass (15.0 _+ 1.0 pg Pb) based on integrated absorbance values (uncertainty based on 15 average results obtained in different days) was attained using end-capped THGA. Each tube supported up to 240-260 firings with 20 #1 of 8.0% (m/v) sample solution plus 10/~1 chemical modifier, 0.05% (m/v) P d + 0.03% (m/v) Mg(NO3)2. The detection limit of 0.55 ng ml -~ Pb (11 pg for 20 #1 of sample solution injected) or 7.0 ng g-1 Pb, was calculated after 20 consecutive measurements of the blank solution (0.2% (v/v) nitric acid) according to IUPAC [18]. The performance of end-capped THGA was practically the same for at least 240 firings over the entire period of analysis. The relative standard deviation of measurements for typical sampies containing 20-60 ng g-t Pb was always lower than 5.5% (n = 10), and the slope of the analytical calibration curve did not change more than 10% in 8 h of continuous operation. Recalibration was properly done after about 100 firings. 3.6. Analysis o f sweet fruit-flavored powder drinks, honeys and syrups
Several sweet fruit-flavored powder drinks from different manufacturers, honeys and syrups purchased at local supermarket were analyzed by the proposed method. The calibration was run against aqueous reference solutions. The results obtained with the proposed method and the reference method [4,6] showed good agreement (Table 5). Applying the t-test, no significant difference was found between the results at 5% probability level, which is an indication of the accuracy of the proposed direct method. Furthermore, after employing the analyte additions method, negligible variations
were foundin the slope of the resulting linear regression curves obtained with the samples and aqueous reference solutions, emphasizing that aqueous standards could be employed for calibration. Also, the lead concentrations obtained by the analyte addition method were in agreement with those obtained by direct calibration, which was another indication of the robustness of the proposed method. Although the achievable detection limit of 7.0 ng g-1 Pb (8.0% (m/v) sample solution) is a little worse than the previously reported values of 3.3 ng g-1 Pb (15% (m/v) sample solution) [6], and 0.9 ng g-i (50% (m/m) sample solution Pb) [9], and 5.0 ng g-1Pb (15% (m/v) sample solution) [4], it was still quite adequate for determining the analyte in the samples, taking into account the maximum allowed concentration of 100-500 ng g-l Pb in several sugar samples [4].
4. Conclusion Although the FCC method for lead determination in sugar and syrups samples presents good reproducibility and robustness for longitudinally heated atomizers [6,7], as well as transversely atomizers [7], it is possible to improve it for samples with similar chemical composition, at least when end-capped transversely heated graphite atomizers are employed. The sampling throughput was increased by a factor of 2 by using palladium plus magnesium nitrate as chemical modifier and a suitable heating program. The tube lifetime was at least 2.5-fold longer than with the already existing direct method [9]. Consequently a remarkable decrease in analytical variable costs could be obtained for transversely heated atomizers [11]. It is beyond the scope of this work to make an evaluation of this method in comparison with other graphite atomizers, but this method presents a strong analytical potential for application to large-scale analysis of sugars and similar confectioneries.
Acknowledgements The authors are grateful to G. Schlemmer (Perkin-Elmer) for donation of end-capped THGA, to Elias A.G. Zagatto (CENA-USP) for critical comments, to Uelinton Guaita (CENA-USP) for his
E.C. Lima et al./Spectrochimica Acta Part B 53 (1998) 601-611
609
Table 2 Recoveries (%) of lead in sweet fruit-flavored powder drinks, honeys and syrups Modifier
Sample
Recovery (%)
Lead added (pg): l 1 1 1 2 2 2 2 2 2
Strawberry a Tropical fruits Honey Syrup 1 Strawberry a Tropical fruits Orange Syrup 1 Honey 1 Honey 2
100 91.2 82.9 58.4 79.8 92.8 96.8 96.5 98.4 94.2 99.2
- 9.3 +- 3.5 ± 19 _-_ 10 _ 5.1 ± 2.9 ± 4.8 ± 5.6 ± 4.8 ± 5.1
200
300
400
99.2 ± 4.8 69.8 --- 5.0 60.4 _+ 14 78.2 ± ll 90.2 ± 5.6 106 ± 2.5 95.3 ± 1.8 94.3 ± 5.4 91.2 _+ 5.2 94.4 ± 5.2
95.2 ± 3.1 82.2 _+ 13 57.2 + 15 76.2 ± 14 98.0 4- 4.1 98.1 _+ 1.1 98.2 _ 1.9 101 + 5.7 97.2 ± 5.9 96.2 ± 5.4
94.4 ± 1.3 75.8 ± 3.0 57.9 ± 18 74.4 _+ 12 104 ± 4.3 100 _+ 2.2 97.6 ± 0.9 102 ± 5.2 93.7 ± 4.9 95.8 ± 5.7
"Sweet fruit-flavored powder drink. Modifier 1, 3.0/xg Mg(NO3)2 + 50.0 p.g NH4H2PO4; modifier 2, 5.0 #g Pd + 3.0/zg Mg(NO3) 2.
Table 3 Effects of individual concomitants on lead atomization (Relative Absorbance) [Pb] (pg)
K 100 200 300 400 Na 100 200 300 400 Ca 100 200 300 400 CI1O0 200 300 400
POll00 200 300 400 SO 2100 200 300 400
Mass of concomitants (#g) 0
0.050
0.100
0.250
0.500
1.000
10.0
1.00 1.00 1.00 1.00
0.93 1.02 0.96 0.98
0.97 0.96 0.98 0.98
0.97 1.02 0.97 0.98
1.03 1.05 0.99 1.01
1.07 1.04 0.99 1.01
-----
1.00 1.00 1.00 1.00
1.10 1.00 1.07 1.03
1.00 1.02 0.99 0.98
0.97 1.02 0.98 0.98
0.97 0.95 0.97 0.97
0.97 0.97 0.98 0.98
-----
1.00 1.00 1.00 1.00
1.03 0.98 0.98 0.99
1.07 1.05 0.96 0.98
1.10 1.02 0.90 0.97
1.03 1.05 0.98 0.98
1.07 1.00 0.98 0.98
-----
1.00 1.00 1.00 1.00
0.90 0.95 1.00 0.97
0.93 0.93 0.98 1.04
1.07 1.02 1,00 1.03
0.93 0.98 0.98 0.97
0.97 0.93 0.95 0.95
0.71 0.68 0.72 0.69
1.00 1.00 1.00 1.00
1.01 0.99 1.01 1.04
1.02 1.02 1.03 1.01
1.01 1.04 1.05 1.03
1.03 1.04 1.08 1.02
1.04 1.06 1.06 1.04
1.05 1.07 1.07 1.06
1.00 1.00 1.00 1.00
1.06 1.07 1.08 1.10
1.06 1.07 1.09 1.09
1.06 1.00 1.08 1.07
1.06 1.03 1.09 1.07
1.13 1.02 1.08 1.05
1.10 1.06 1.06 1.02
610
E.C. Lima et al./Spectrochimica Acta Part B 53 (1998) 601-611
Table 4 Effects of mixed concomitants on the atomization of 400 pg of lead Mass of concomitants (#g) 0.50 1.00 0.50 1.50 1.00 0.25
K K K K K K
+ + + + + +
0.25 Na 0.50 Na 0.50 Na 1.00 Na 0.50 Na 0.50 Na
+ + + + + +
0.50 Ca 0.50 Ca 1.00 Ca 1.50 Ca 2,00 Ca ZOO Ca
Relative absorbance + + + + + +
0.50 PO 3- + 1.00 PO43- + 1.50 PO 3- + 0.50 PO43- + 1.50 PO]- + 1.50 PO43- +
0.25 SO~- + 0.50 SO24- + 1.00 SO ]- + 0.50 SO 2- + 1.00 $042- + 1.50 SO42- +
0.50 0.25 0.25 0.50 0.50 0.25
CICICI CICICI-
1.01 1.04 1.05 0.96 0.98 0.99
Table 5 Lead determination in sweet fruit-flavored powder drinks, honeys and syrups by direct analysis and wet digestion by ETAAS Sample
Proposed method (ng g-i)
Powder drink Pineapple 1 a Pineapple 2 a Orange a Lemon a Mango Passion fruit a Strawberry 1 a Strawberry 2 a Strawberry 3 a Peach a Grape 1 ~ Grape 2 a Grape 3 a Honey 1 Honey 2 Syrup 1 Syrup 2
26.7 43.9 21.7 < 7 69.1 26.4 25.3 39.1 42.2 21.9 25.0 40.2 27.4 54.2 < 7 29.3 45.3
_~ 2.9 ~ 2.6 _~ 3.3 __. 4.0 _ 1.4 + 1.6 -+ 2.2 _+ 3.5 _+ 3.0 _ 2.3 _+ 0.4 _+ 1.3 +- 10 -+ 1.9 _ 2.9
Official method [4] (ng g-i)
27.1 46.2 20.3 < 4 70.8 28.2 23.9 41.4 38.7 22.2 24.7 41.2 29.2 52.4 < 4 28.1 46.8
_+ 3.5 --_ 3.0 -_+ 3.1 _+ 2.8 _+ 1.4 - 2.7 _+ 3.0 4- 2.9 ___2.0 ± 0.5 _+ 1.2 _+ 1.3 + 3.1 _+ 2.4 _+ 4.5
Values are mean + standard deviation (n = 3). "Sweet fruit-flavored powder drink. h e l p f u l a s s i s t a n c e in t h e w e t d i g e s t i o n p r o c e d u r e , a n d tO I o l a n d a A . R u f f i n i ( C E N A - U S P ) f o r t e c h n i c a l s u p port. W e a l s o t h a n k F u n d a q ~ o d e A m p a r o ~ P e s q u i s a do Estado de S~o Paulo (FAPESP) for financial support (Grants 1995/5782-7 and 1996/6845-5), and the Conselho Nacional de Desenvolvimento Cientffico e Tecnol6gico
( C N P q ) f o r g r a n t s to E . C . L . , F . J . K a n d
M.A.Z.A.
References [1] K.L. Nuttal, Evaluating lead exposure in the laboratory, Lab. Med. 26 (1995) 118-123.
[2] WHO, Toxicological Evaluation of Certain Food Additives and Contaminants, the 30th Meeting of the Joint F A t - W H O Expert Committee on Food Additives, Cambridge University Press, Cambridge, UK, 1987, pp. 223-230. [3] M.A. Clarke, Sugar in food processing, Int. Sugar JNL 99 (1997) 114-126. [4] Food Chemical Codex, 4th ed., FCC IV, General Test & Assays, 1996, pp. 763-765. [5] N.M. Morris, M.A. Clarke, V.W. Tripp, F.G. Carpenter, Determination of lead, cadmium, and zinc in sugar, J. Agric. Food Chem. 24 (1976) 45-47. [6] N.C. Miller-Ilhi, Graphite furnace atomic absorption method for the determination of lead in sugars and syrups, J. Assoc. off. Anal. Chem. 77 (1994) 1288-1292. [7] N.C. Miller-Ilhi, Evaluation of a graphite furnace atomic absorption method developed for the determination of lead in sugars, J. Agric. Food Chem. 43 (1995) 923-927.
[~.C. Lima et al./Spectrochimica Acta Part B 53 (1998) 601-611
[8] L. Hahn, G. Miiller-Vogt, W.J. Wendl, Role of oxygen in the determination of oxide-forming elements by electrothermal atomic absorption spectrometry. Part 1. Effect of oxygen on the reaction of thallium, J. Anal. At. Spectrom. 8(1993)223-227. [9] N.C. Miller-Ilhi, F.E. Greene, Direct determination of lead in sugars using graphite furnace atomic absorption spectrometry, At. Spectrosc. 14 (1993) 85-89. [10] P. Sporns, Present and future challenges for the Alberta honey industry, Agric. For. Bull. 5 (1982) 37-39. [11] D.L. Massart, B.G.M. Vandeginste, S.N. Deming, Y. Michotte, L. Kaufman, Costs in Data Handling in Science and Technology--Chemometrics--A Textbook, vol. 2, Elsevier, Amsterdam, 1988, pp. 137-147. [12] H.Huffman Jr., J.A. Caruso, Analysis of lead in evaporated milk by tameless atomic absorption spectroscopy, J. Agric. Food Chem. 22 (1974) 824-827. [13] S. Lynch, D. Littlejohn, Palladium as chemical modifier for the determination of lead in food slurries by electrothermal atomisation atomic absorption spectrometry, J. Anal. At. Spectrom. 4 (1989) 157-161.
611
[14] H. Qiao, K.W. Jackson, Mechanism of modification by palladium in graphite furnace atomic absorption spectrometry, Spectrochim. Acta Part B 46 (1991) 1841-1859. [15] R.M. Awadallah, M.K. Sherif, A.E. Mohamed, F. Grass, Determination of trace elements in egyptian cane sugar by neutron activation analysis, J. Radioanal. Nucl. Chem. 92 (1985) 7-25. [16] R.M. Awadallah, M.K. Sherif, A.E. Mohamed, F. Grass, Determination of trace elements in Egyptian cane sugar (Deshna Factories) by neutron activation, atomic absorption spectrometrometric and inductively coupled plasma-atomic emission spectrometric analysis, J. Radioanal. Nucl. Chem. 98 (1986) 49-64. [17] N. Hadgu, W. Frech, Performance of side-heated graphite atomizers in atomic absorption spectrometry using tubes with end caps, Spectrochim. Acta Part B 49 (1994) 445-447. [ 18] Commission on Spectrochemical and Other Optical Procedures for Analysis, Nomenclature, symbols, units and their usage in spectrochemical analysis--lI. Data Interpretation, Spectrochim. Acta Part B 33 (1978) 241-245.
E LS EV I E R
Spectrochimica Acta Part B 53 (1998) 601-611
Direct determination of lead in sweet fruit-flavored powder drinks by electrothermal atomic absorption spectrometry I~der C. Lima 1, Francisco Jos6 Krug*, Marco A.Z. Arruda 2 Centro de Energia Nuclear na Agrieultura, Universidade de Sgto Paulo, P. Box 96, CEP 13400-970, Piracicaba-SP, Brazil
Received 6 November 1997; revised 23 December 1997; accepted 23 December 1997
Abstract
A simplified method for direct determination of lead in sweet fruit-flavored powder drinks, syrups and honeys by electrothermal atomic absorption spectrometry without sample digestion is proposed. Samples were dissolved in water, acidified to 0.2% (v/v) HNO> and directly injected into an end-capped transversely heated graphite atomizer (THGA). Building up of carbonaceous residue inside the atomizer was effectively precluded for sugar solutions not exceeding 8.0% (m/v) when a heating program with two pyrolysis steps (600 and 1000°C) was carried out without air-ashing. Under these conditions one atomizer supported about 250 firings. Among various chemical modifiers tested, better recovery and repeatability results were obtained with a 5 gg Pd + 3 #g Mg(NO3)2 mixture. Tests carried out with individual concomitants containing up to 1.0 #g Na, K, Ca or C1, and up to 10.0 gg phosphate or sulphate, and several mixtures of these six concomitants, did not reveal significant interferences on lead atomization. Characteristic mass and detection limit based on integrated absorbance were 15 and 11 pg Pb, respectively. The relative standard deviation based on 10 measurements for typical samples (20-60 ng g-l Pb) was always lower than 5.5%. The detection limit of 7.0 ng g-1 Pb attained the Codex recommendation for the maximum allowed lead contents in the sugar samples. Application of t-test to the results obtained by the proposed direct analysis, and the official method adopted by Food Chemical Codex, demonstrated that there were no significant differences at the 5% probability level. © 1998 Elsevier Science B.V. Keywords: Pb; Sugar; Transverse heated graphite atomizer; Electrothermal atomic absorption spectrometry
1. I n t r o d u c t i o n Overall exposure to lead is of public health concern because of several hazardous effects that may occur to humans. Lead poisoning may provoke irritability, anorexia, malaise and headache. Intoxication progress may * Correspondingauthor. Fax: +55 19 4294610; e-mail: [email protected] Graduate student from Departamento de Qufmica, Universidade Federal de S~o Carlos. 2 Permanent address:'Instituto de Qufmica, Universidade Estadual de Campinas, Campinas-SP, Brazil.
led to attacks of abdominal pain until coma and death [1]. Approximately 80% of total human lead intake is supplied by diet. Thus, there is a current interest in the determination of this element in foods and foodstuffs [2]. An important class of foods consists of sugar and related products, such as sweets, confectioneries, sweet fruit-flavored powder drink, syrups, and other products which are commercially available, as well as used in the home for sweetening and baking [3]. According to the Codex Alimentarius Commission [4], the maximum lead concentration allowed in sugar varies from 0.1 /~g g-~ in dextrose and fructose to
0584-8547/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PII S05 84-8547(98)00085-8
602
E.C. Lima et al./Spectrochimica Acta Part B 53 (1998) 601-611
Table 1 THGA heating program Step
Temp. (°C)
Ramp (s)
1 2 3 4 5 6
150 180 600 1000 1800 2400
5 5 5 5 0 1
Hold (s) 25 25 5 20 5 4
Argon flow rate (ml rain -I) 250 250 250 250 0 250
Program time, 105 s; injection temperature, 100°C.
0.5/zg g-~ in glucose syrups, sucrose and highfructose corn syrups, Morris et al. [5] developed the first determination of lead in sugar by electrothermal atomic absorption spectrometry, employing yeast fermentation of the sugar in order to decompose the sample matrix. Recently, a method proposed by MiUer-Ilhi [6], and subsequently evaluated [7] for use with modern graphite furnace atomic absorption spectrometers, was adopted by the Food Chemical Codex FCC [4] for lead determination in sugar samples. It is based on the analysis of partially decomposed sugar samples with HNO3-H202, and employs graphite tubes with platform atomization, Mg(NO3)2 as chemical moditier, and pyrolysis assisted by oxygen, which serves to oxidize the organic compounds as well as to stabilize the analyte [6,8]. A direct method for lead determination in sugar were developed by Miller-Ilhi and Greene [9], where a sample solution containing 50% (m/m) of sugar in 5.0% (v/v) HNO3 was delivered into the platform, using Mg(NO3)2 as chemical modifier, with a long pyrolysis step assisted by oxygen, Although the reported methods [4,6,9] present robustness and good analytical characteristics for lead determination in sugar samples for routine analysis, a method capable of running a larger number of samples per day, with a longer atomizer lifetime, was needed. The aim of the present work is to propose a simplifled procedure for the direct determination of lead in sweet fruit-flavored powder drinks, honeys and syrups without sample digestion, by using ETAAS with endcapped THGA, capable of attaining the Food Chemical Codex recommendation for the maximum allowed lead concentration in sugar samples. The strategy was to avoid air-ashing during the pyrolysis step,
which was possible by decreasing the sugar content of the sample solution and developing a suitable heating program with Pd+ Mg(NO3)2 as chemical modifier.
2. Experimental
2.1. Instrumentation A Perkin-Elmer 4100ZL atomic absorption spectrometer with a longitudinal Zeeman-effect background correction system, furnished with standard (Part no. B050-4033) and end-capped THGA (Part no. B3000653) were employed. Measurements were made at 283.3 nm using a Perkin-Elmer electrodeless discharge lamp EDLII system. Sample and modifier aliquots of 20.0 and 10.0/A, were taken from polypropylene cups and delivered into the tube by means of an AS-71 autosampler from the same manufacturer. Unless otherwise stated, argon (AGA) as purge gas, and the heating program shown in Table 1, were used throughout. Pyrolysis assisted by oxygen experiments were carried out with air (AGA). All measurements were made with at least three replicates and based on integrated absorbance.
2.2. Reagents, reference solutions, samples High purity deionized water obtained by a Milli-Q water purification system (Millipore) was used throughout. Analytical reagent-grade nitric and hydrochloric acids were distilled in quartz sub-boiling stills (Kiirner, Germany). Lead (1000 mg 1-1 stock solution) was prepared from Pb(NO3)2 (Johnson & Matthey), by dissolving 0.7992 g in 1.0% (v/v) HNO3. Analytical calibration
[LC. Lima et al./SpectrochimicaActa Part B 53 (1998)601-611
curves were obtained within 0.00 and 20.0 ng m1-1 of lead in 0.2% (v/v) HNO3. The calibration was checked periodically after every 15 measurements with 10.0 ng ml -I of lead solution, The effect of concomitants was investigated with solutions containing up to 100 mg 1-1 Na (NaC1, Johnson & Matthey), 150 mg 1-1 K (KC1, Johnson Matthey), 200 mg 1-1 Ca (CaCO3, Johnson & Matthey), 1000 mg 1-1 C1 (HC1, Merck), 1000 mg 1-l S-sulphate (H2SO4, Suprapur Merck), and 1000 mg 1-1 P-phosphate (NHaH2PO4, Suprapur Merck), in 0.2% (v/v) HNO3. The following chemical modifier solutions were prepared from NHaH2PO 4 salt (Suprapur Merck), 10.0 g 1-I Pd solution (Pd(NO3)2 Merck) and 10.0g 1-1 Mg(NO3)2 solution (Merck): (1) 0.03% (m/v) Mg(NO3)2 + 0.50% (m/v) NHnH2PO4; (2) 0.05% (m/v) Pd + 0.03% (m/v) Mg(NO3)2; (3) 0.20% (m/v) Pd + 0.05% (m/v) Mg(NO3)2; (4) 0.052.00% (m/v) Mg(NO3)2; (5) 0.05-0.15% (m/v) Pd. Sweet fruit-flavored powder drinks (80-85% sucrose) provided from different manufactures, honeys and syrups (70-75% total sugar and 30-25% moisture) [ 10] were purchased at local supermarkets, 2.3. Materials
All solutions were stored in polypropylene bottles (Nalgene). Plastic bottles, autosampler cups and glassware materials were cleaned by soaking in 20% v/v) HNO3 for 24 h, rinsing five times with MiUi-Q water and kept until dry in a Class 100 laminar flow hood. 2.4. Analysis of sweet fruit-flavored powder drinks, honeys and syrups 2.4.1. Official method adopted by Food Chemical Codex Samples were prepared according to methods described elsewhere [4,6], and the resulting solutions were analyzed by ETAAS by delivering 20.0/zl of the digest and 5/xl of the chemical modifier, 2.0% (m/v) Mg(NO3)2, into the atomizer, using the recommended heating program with oxygen ashing during the pyrolysis step [4,6]. 2.4.2. Direct analysis Four grams of samples were accurately weighed (to
603
0.1 mg), completely dissolved in water, and acidified to 0.2% (v/v) HNO3, and the volume was made up to 50 ml. Then 20.0/xl of these samples and 10/~1 of the chemical modifier solution (0.05% (m/v) Pd + 0.03% (m/v) Mg(NO3)2) were directly delivered into the end-capped THGA atomizer. The heating program in Table I was used, and all determinations were carried out in triplicate.
3. Results and discussion 3.1. General considerations
For optimizing the amount of sugar in the sample solution introduced into the atomizer, preliminary tests were performed with solutions containing 4.016.0% (m/v)of the carbohydrate. For sugar contents higher than 8.0% (m/v) a build-up of carbonaceous residue inside the atomizer was observed. With 16.0% (m/v), tube lifetime was limited to 100 firings, and the measurement precision was poor (R.S.D. >-10%). As a compromise between atomizer lifetime, precision and sensitivity, it was decided to work with samples containing up to 8.0% (m/v) in sugar. Also, the limit of solubility of sweet fruit-flavored powder drinks in water is about 8% (m/v), due to other constituents present on their formulation. These experiments were performed by adding palladium plus magnesium, or ammonium phosphate plus magnesium. Pyrolysis was carried out in two steps: ramping from 180 to 600°C and then from 600 to 1000°C. The first step was needed for minimizing the amount of carbonaceous residue building up inside the atomizer. After a 5-s holding time at 600°C, smoke was ceased and deposition of carbonaceous residue was not perceptible. The pyrolysis temperature was then increased to 1000°C for better separation of other concomitants. In this situation, the background was minimized and each graphite tube supported 240-260 firings. For comparison, when the suggested method was employed for direct lead determination in sugar using the same recommended conditions [9] (pyrolysis step at 750°C assisted by oxygen and 60/xg Mg(NO3)2 as chemical modifier), tube lifetime did not exceed 105 firings, and the background was more than doubled for the same sample solution containing 8.0% (m/v) sugar (Fig. 1). So, it was concluded that under the proposed
604
E.C. Lima et aL/Spectrochimica Acta Part B 53 (1998) 601-611
0.250 (A) f~ Analyte S i g n a l .......
.8 < 0
.
.
.
nor rto e t esampsinsuita em um
_
~../,.....~
_....
.
. Time (s)
As an ultimate consequence, the variable analytical cost was remarkably decreased for lead determination in the samples employing transversely heated graphite atomizers [ 11].
. 5.0
for lead atomization, a minimum amount of nitric acid was added to the sample solution. For undigested samples, acidity was kept in 0.2% (v/v) HNO3, although acidity as high as 5.0% (v/v) could be supported by the THGA. No significative effect on lead integrated absorbance was found, when the acidity was varied within this concentration range.
0.250
(S) -
c,
I /
<
/\ /,f.; .~, 0
AnalySi~gnal
.............aa~ground ,...,
/ "" -time (s)
5.0
Fig. 1. Peak profile for 300 pg Pb in 8.0% (m/v) lemon-flavored powder drink (acidity kept 0.2% (v/v) HNO3). (A) Proposed method
(integrated absorbance, 0.112s; integrated background, 0.039 s):
heating program shown Table 1; modifier, 5~g Pd + 3/~g Mg(NO3)2; (B) Oxygen-ashing (integrated absorbance, 0.102 sl
integrated background, 0.100s): heating program described else-
where [9]; modifier,60/~gMg(NO3)2. experimental conditions air-ashing should be avoided in the pyrolysis step. It is also important to mention that, when a pyrolysis assisted by oxygen was used [6,9], a subsequent long cooling step (61 s)under argon atmosphere was required to eliminate all the oxygen inside the graphite furnace, in order to avoid the atomizer lifetime diminishing even further. As a consequence, the total heating program was very long (211-213 s) [6,9], limiting the sample throughput to approximately 15 h -~. With the proposed simplified and faster procedure (105-s heating program) it was possible to avoid a carbonaceous residue building up inside the atomizer, without air-ashing, for sample solutions containing up to 8.0% (m/v) sugar. By using the heating program shown in Table 1, a sample throughput of 30 h-1 and a 2.5-fold increase in the tube lifetime were obtained,
3.2. Modifiers
Among various chemical modifiers tested, a mixture containing 5/~g P d + 3 #g Mg(NO3)2 was the most suitable for obtaining reproducible signals as well as good lead recoveries. By comparing the pyrolysis temperature curves (Fig. 2) obtained with the tested modifiers (Pd, Pd + Mg(NO3)2, NH4H2PO4 + Mg(NO3)2, Mg(NO3)2, and Mg(NO3)z with air-ashing during the pyrolysis step), it can be seen that significant differences were found when 400 pg lead was atomized from aqueous solution (0.2% (v/v) HNO3) and from an 8.0% (m/v) spiked sample. In aqueous solutions, all modifiers stabilized lead to at least 1000°C. On the other hand, lead was thermally stabilized only up to 700°C by magnesium nitrate in the presence of 8% (m/v) sample solution. Taking into account that oxygen could increase analyte thermal stability [8], an experiment was carried out with Mg(NO3)2 in combination with air-ashing (heating program described elsewhere [9]), and it was verified that lead stability increased to 800°C (Fig. 2(B)), which is in agreement with the observations made by Miller-Ilhi and Greene with a longitudinally heated graphite atomizer [9]. It should be stressed that during an evaluation study of the method proposed for lead determination in sugar, Miller-Ihli reported that the maximum attainable pyrolysis temperature had to be decreased from 750 to 550-600°C when magnesium nitrate was employed with air-ashing in the THGA atomizer [7]. Without modifier (presence of only 0.2% (v/v) HNO3), the lead analytical signal vanished at temperatures above 700°C (Fig. 2(A)). Probably, during the decomposition of organic matrix, lead was carried away from the
[~.C. Lima et al./Spectrochimica Acta Part B 53 (1998) 601-611
605
1.2
(A)
¢-
-e 0
<
"0 0
0.8 0-------0~0
04
\o
e\
m
0.0
O I
600
'
|
900
'
|
1200
'
1500
Temperature (°C)
1,2
(B)
E m O
0.8
O ~
' =
~
0.4
m
Z
0.0 I
600
'
I
'
900
I
1200
'
1500
Temperature (°C) Fig. 2. Pyrolysis temperature curves of 400 pg Pb in presence of: (A) no modifier. (B) 60 #g MG(NO3)2. Atomizationtemperature, 1800°C. (-O-) 0.2% (v/v) HNO3; ( - 0 - ) 8.0% (m/v) sweet fruit-flavoredpowderdrink; (-&-) 8.0% (m/v) sweet fruit-flavoredpowder drink (with airashing during pyrolysis [9]). atomizer by the fumes [5,12]. Palladium alone or Pd + Mg(NO3)2 were more effective for lead determination in sugar matrix, supporting up to 1100°C in the pyrolysis step (Fig. 2(C,D)). With NH4H2PO4 + Mg(NO3)2 this temperature should not exceed
900°C (Fig. 2(E)). This 200°C difference was also observed by Lynch and Littlejohn [13]. It is important to point out that, for other concentrations of Mg(NO3)2, Pd + Mg(NO3)2 and Pd, similar electrothermal behavior of lead was observed.
[~.C. Lima et al./Spectrochimica Acta Part B 53 (1998) 601-611
606
1.2
(C)
¢O O)
08
e---~~e~e------e~e o-
~
o
____-
-o
~ o
°
o
0.4
,w O
Z
0.0 I
'
U
600
'
900
I
'
1200
1500
Temperature (°C)
8
1.2 o
(o)
C
-E o
o~O 0.8-
•o
.~
o-
O~o o
-o
~ -c
~-~
%
0.4
\o,
-
Z
0.0 U
I
600
900
'
I
1200
°
1500
Temperature (*C) Fig. 2 (continued). (C) 5/~g Pd. (D) 5/~g Pd + 3/xg Mg(NO3)2. Atomization temperature, 1800°C. ( - O - ) 0.2% (v/v) HNO3; ( - O - ) 8.0% (m/v)
sweet fruit-flavoredpowderdrink. Better lead peak profiles were achieved with the mixture Pd + Mg(NO3)2 when compared to Pd alone. With the addition of magnesium nitrate, a more uniform distribution of palladium throughout the platform should be obtained, allowing a better stabilization of the analyte, leading to sharper absorbance peaks
with a slight increase in the sensitivity, as also suggested by Qiao and Jackson [14]. For palladium masses higher than 15.0 ~g, higher blank levels appeared, decreasing the measurement repeatability. Best results of chemical modification in the sugar matrix, with good sensitivity and reproducible signals,
607
E.C. Lima et al./Spectrochimica Acta Part B 53 (1998) 601-611
@
.2
(E)
f-
_<~~ 0.8 ~
0
0 -~-
~~0
--------O~-----O.
0.4
Z
0.0 I
600
'
I
'
900
I
1200
'
1500
Temperature (°C) Fig. 2 (continued). (E) 3 ~g Mg(NO3)2 + 50 ~g NH4H2PO4. Atomization temperature, 1800°C. ( - © - ) 0.2% (v/v) HNO3; (-@-) 8.0% (m/v)
sweet fruit-flavoredpowderdrink. were achieved with 5.0/zg Pd + 3.0 #g Mg(NO3)2 or 3.0 #g Mg(NO3)2 + 50.0/~g NH4H2PO4. These two mixtures were then chosen for further experiments. Atomization studies were performed with these modifiers but, for simplicity of presentation, they were withdrawn from Fig. 2. The temperature that promoted the highest atomic signal without memory effects for further injections was 1800°C when 5.0/~g Pd + 3.0/~g Mg(NO3)2 was used. 3.3. R e c o v e r y tests
The correct choice of chemical modifier was decisive for obtaining good recoveries. The mixture 3.0/~g Mg(NO3)2 + 50.0/zg NHaH2PO4 gave rise to inconsistent lead recoveries ranging from 57 to 99% for different samples (Table 2), which means that the efficiency of this modifier varies according to the chemical composition of the samples. For these reasons this modifier was ruled out. The 5.0 #g Pd + 3.0/zg Mg(NO3)2 mixture gave rise to better yields (90.2-106% recoveries, average 97.1% (4.2%), with good repeatability (R.S.D. --5.9%) for all tested samples (Table 2). Consequently, this chemical modifier was chosen to improve this work.
3.4. Interference studies
The effects of the individual main concomitants, which can be found in the samples [15,16], on the atomization of 100-400 pg of lead are shown in Table 3 The results are presented as the ratio between the absorbances of lead atomic signals in the presence and absence of the concomitants. Although these observations were isolated, the whole set of data gave an indication that most concomitants would not be expected to interfere with the analyte atomization. It should be pointed out that no significant differences were observed between the slopes of lead analytical curves obtained in the absence or presence of these concomitants, within 100-400 pg Pb range. Up to 1/~g Na, K, C1 or Ca, and 10 ~g of phosphate or sulphate, did not cause remarkable variations (higher than - 10%) on the lead atomic signals. With an atypical content of 10/~g CI-, a signal suppression of 30% was observed. Mixtures of inorganic concomitants normally expected in real samples did not affect the atomization of 400 pg Pb. In principle, no interferences caused by inorganics should be expected on lead determination in sugar matrix. The results in Table 4 show that there
608
E.C. Lima et al./Spectrochimica Acta Part B 53 (1998) 601-611
were no significant interferences from mixtures containing 2-10-fold the normal content [ 15,16] of those concomitants, 3.5. Analytical characteristics
A linear range up to 20.0 ng m1-1 Pb was obtained by using the established conditions. It was observed that the end-capped THGA atomizer always provided an increase of 60% on the integrated absorbance when compared to standard THGA, as previously observed [17]. End-capped tubes were then used throughout. Reproducible characteristic mass (15.0 _+ 1.0 pg Pb) based on integrated absorbance values (uncertainty based on 15 average results obtained in different days) was attained using end-capped THGA. Each tube supported up to 240-260 firings with 20 #1 of 8.0% (m/v) sample solution plus 10/~1 chemical modifier, 0.05% (m/v) P d + 0.03% (m/v) Mg(NO3)2. The detection limit of 0.55 ng ml -~ Pb (11 pg for 20 #1 of sample solution injected) or 7.0 ng g-1 Pb, was calculated after 20 consecutive measurements of the blank solution (0.2% (v/v) nitric acid) according to IUPAC [18]. The performance of end-capped THGA was practically the same for at least 240 firings over the entire period of analysis. The relative standard deviation of measurements for typical sampies containing 20-60 ng g-t Pb was always lower than 5.5% (n = 10), and the slope of the analytical calibration curve did not change more than 10% in 8 h of continuous operation. Recalibration was properly done after about 100 firings. 3.6. Analysis o f sweet fruit-flavored powder drinks, honeys and syrups
Several sweet fruit-flavored powder drinks from different manufacturers, honeys and syrups purchased at local supermarket were analyzed by the proposed method. The calibration was run against aqueous reference solutions. The results obtained with the proposed method and the reference method [4,6] showed good agreement (Table 5). Applying the t-test, no significant difference was found between the results at 5% probability level, which is an indication of the accuracy of the proposed direct method. Furthermore, after employing the analyte additions method, negligible variations
were foundin the slope of the resulting linear regression curves obtained with the samples and aqueous reference solutions, emphasizing that aqueous standards could be employed for calibration. Also, the lead concentrations obtained by the analyte addition method were in agreement with those obtained by direct calibration, which was another indication of the robustness of the proposed method. Although the achievable detection limit of 7.0 ng g-1 Pb (8.0% (m/v) sample solution) is a little worse than the previously reported values of 3.3 ng g-1 Pb (15% (m/v) sample solution) [6], and 0.9 ng g-i (50% (m/m) sample solution Pb) [9], and 5.0 ng g-1Pb (15% (m/v) sample solution) [4], it was still quite adequate for determining the analyte in the samples, taking into account the maximum allowed concentration of 100-500 ng g-l Pb in several sugar samples [4].
4. Conclusion Although the FCC method for lead determination in sugar and syrups samples presents good reproducibility and robustness for longitudinally heated atomizers [6,7], as well as transversely atomizers [7], it is possible to improve it for samples with similar chemical composition, at least when end-capped transversely heated graphite atomizers are employed. The sampling throughput was increased by a factor of 2 by using palladium plus magnesium nitrate as chemical modifier and a suitable heating program. The tube lifetime was at least 2.5-fold longer than with the already existing direct method [9]. Consequently a remarkable decrease in analytical variable costs could be obtained for transversely heated atomizers [11]. It is beyond the scope of this work to make an evaluation of this method in comparison with other graphite atomizers, but this method presents a strong analytical potential for application to large-scale analysis of sugars and similar confectioneries.
Acknowledgements The authors are grateful to G. Schlemmer (Perkin-Elmer) for donation of end-capped THGA, to Elias A.G. Zagatto (CENA-USP) for critical comments, to Uelinton Guaita (CENA-USP) for his
E.C. Lima et al./Spectrochimica Acta Part B 53 (1998) 601-611
609
Table 2 Recoveries (%) of lead in sweet fruit-flavored powder drinks, honeys and syrups Modifier
Sample
Recovery (%)
Lead added (pg): l 1 1 1 2 2 2 2 2 2
Strawberry a Tropical fruits Honey Syrup 1 Strawberry a Tropical fruits Orange Syrup 1 Honey 1 Honey 2
100 91.2 82.9 58.4 79.8 92.8 96.8 96.5 98.4 94.2 99.2
- 9.3 +- 3.5 ± 19 _-_ 10 _ 5.1 ± 2.9 ± 4.8 ± 5.6 ± 4.8 ± 5.1
200
300
400
99.2 ± 4.8 69.8 --- 5.0 60.4 _+ 14 78.2 ± ll 90.2 ± 5.6 106 ± 2.5 95.3 ± 1.8 94.3 ± 5.4 91.2 _+ 5.2 94.4 ± 5.2
95.2 ± 3.1 82.2 _+ 13 57.2 + 15 76.2 ± 14 98.0 4- 4.1 98.1 _+ 1.1 98.2 _ 1.9 101 + 5.7 97.2 ± 5.9 96.2 ± 5.4
94.4 ± 1.3 75.8 ± 3.0 57.9 ± 18 74.4 _+ 12 104 ± 4.3 100 _+ 2.2 97.6 ± 0.9 102 ± 5.2 93.7 ± 4.9 95.8 ± 5.7
"Sweet fruit-flavored powder drink. Modifier 1, 3.0/xg Mg(NO3)2 + 50.0 p.g NH4H2PO4; modifier 2, 5.0 #g Pd + 3.0/zg Mg(NO3) 2.
Table 3 Effects of individual concomitants on lead atomization (Relative Absorbance) [Pb] (pg)
K 100 200 300 400 Na 100 200 300 400 Ca 100 200 300 400 CI1O0 200 300 400
POll00 200 300 400 SO 2100 200 300 400
Mass of concomitants (#g) 0
0.050
0.100
0.250
0.500
1.000
10.0
1.00 1.00 1.00 1.00
0.93 1.02 0.96 0.98
0.97 0.96 0.98 0.98
0.97 1.02 0.97 0.98
1.03 1.05 0.99 1.01
1.07 1.04 0.99 1.01
-----
1.00 1.00 1.00 1.00
1.10 1.00 1.07 1.03
1.00 1.02 0.99 0.98
0.97 1.02 0.98 0.98
0.97 0.95 0.97 0.97
0.97 0.97 0.98 0.98
-----
1.00 1.00 1.00 1.00
1.03 0.98 0.98 0.99
1.07 1.05 0.96 0.98
1.10 1.02 0.90 0.97
1.03 1.05 0.98 0.98
1.07 1.00 0.98 0.98
-----
1.00 1.00 1.00 1.00
0.90 0.95 1.00 0.97
0.93 0.93 0.98 1.04
1.07 1.02 1,00 1.03
0.93 0.98 0.98 0.97
0.97 0.93 0.95 0.95
0.71 0.68 0.72 0.69
1.00 1.00 1.00 1.00
1.01 0.99 1.01 1.04
1.02 1.02 1.03 1.01
1.01 1.04 1.05 1.03
1.03 1.04 1.08 1.02
1.04 1.06 1.06 1.04
1.05 1.07 1.07 1.06
1.00 1.00 1.00 1.00
1.06 1.07 1.08 1.10
1.06 1.07 1.09 1.09
1.06 1.00 1.08 1.07
1.06 1.03 1.09 1.07
1.13 1.02 1.08 1.05
1.10 1.06 1.06 1.02
610
E.C. Lima et al./Spectrochimica Acta Part B 53 (1998) 601-611
Table 4 Effects of mixed concomitants on the atomization of 400 pg of lead Mass of concomitants (#g) 0.50 1.00 0.50 1.50 1.00 0.25
K K K K K K
+ + + + + +
0.25 Na 0.50 Na 0.50 Na 1.00 Na 0.50 Na 0.50 Na
+ + + + + +
0.50 Ca 0.50 Ca 1.00 Ca 1.50 Ca 2,00 Ca ZOO Ca
Relative absorbance + + + + + +
0.50 PO 3- + 1.00 PO43- + 1.50 PO 3- + 0.50 PO43- + 1.50 PO]- + 1.50 PO43- +
0.25 SO~- + 0.50 SO24- + 1.00 SO ]- + 0.50 SO 2- + 1.00 $042- + 1.50 SO42- +
0.50 0.25 0.25 0.50 0.50 0.25
CICICI CICICI-
1.01 1.04 1.05 0.96 0.98 0.99
Table 5 Lead determination in sweet fruit-flavored powder drinks, honeys and syrups by direct analysis and wet digestion by ETAAS Sample
Proposed method (ng g-i)
Powder drink Pineapple 1 a Pineapple 2 a Orange a Lemon a Mango Passion fruit a Strawberry 1 a Strawberry 2 a Strawberry 3 a Peach a Grape 1 ~ Grape 2 a Grape 3 a Honey 1 Honey 2 Syrup 1 Syrup 2
26.7 43.9 21.7 < 7 69.1 26.4 25.3 39.1 42.2 21.9 25.0 40.2 27.4 54.2 < 7 29.3 45.3
_~ 2.9 ~ 2.6 _~ 3.3 __. 4.0 _ 1.4 + 1.6 -+ 2.2 _+ 3.5 _+ 3.0 _ 2.3 _+ 0.4 _+ 1.3 +- 10 -+ 1.9 _ 2.9
Official method [4] (ng g-i)
27.1 46.2 20.3 < 4 70.8 28.2 23.9 41.4 38.7 22.2 24.7 41.2 29.2 52.4 < 4 28.1 46.8
_+ 3.5 --_ 3.0 -_+ 3.1 _+ 2.8 _+ 1.4 - 2.7 _+ 3.0 4- 2.9 ___2.0 ± 0.5 _+ 1.2 _+ 1.3 + 3.1 _+ 2.4 _+ 4.5
Values are mean + standard deviation (n = 3). "Sweet fruit-flavored powder drink. h e l p f u l a s s i s t a n c e in t h e w e t d i g e s t i o n p r o c e d u r e , a n d tO I o l a n d a A . R u f f i n i ( C E N A - U S P ) f o r t e c h n i c a l s u p port. W e a l s o t h a n k F u n d a q ~ o d e A m p a r o ~ P e s q u i s a do Estado de S~o Paulo (FAPESP) for financial support (Grants 1995/5782-7 and 1996/6845-5), and the Conselho Nacional de Desenvolvimento Cientffico e Tecnol6gico
( C N P q ) f o r g r a n t s to E . C . L . , F . J . K a n d
M.A.Z.A.
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