Journal of Radioanalytical and Nuclear Chemistry, Vol. 262, No. 2 (2004) 355–362
Trace element determination in tomato puree using particle induced X-ray emission and Rutherford backscattering E. Romero-Dávila,1 J. Miranda2* 2 Instituto
1 Facultad de Química, de Física, Universidad Nacional Autónoma de México, Apartado Postal 20-364, 01000 México, D.F., Mexico
(Received March 5, 2004)
Particle induced X-ray emission (PIXE) and Rutherford backscattering spectrometry (RBS) were used to determine the concentrations of trace elements in samples of 12 tomato puree brands sold in the Mexican market. While RBS offered information about the main elements present in the matrix, PIXE gave results on trace elements. As a whole, data for 17 elements (C, N, O, Na, Mg, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, and Zn) were obtained. To evaluate the results, a comparison with brands from USA, Japan, Colombia, and Chile was carried out, using tomato purees produced following the domestic technology recipe. Additionally, the results were considered in the light of the Codex Alimentarius and the Mexican standard. It was found that all of the brands fall within the limits established by these standards, being of the same order of magnitude as the foreign brands.
* E-mail:
[email protected]
percent, of natural tomato soluble solids.” Furthermore, “processed tomato concentrate is the product prepared by concentrating the liquid obtained from substantially sound, mature red tomatoes (Lycopersicum esculentum P. Mill). Such liquid is strained or otherwise prepared to exclude skins, seeds, and other coarse or hard substances in the finished product. Salt and other suitable seasoning ingredients may be added. The product is preserved by physical means. The concentration shall be 8% or more natural tomato soluble solids but not dehydrated to a dry powder or flake form.” In 1998, Mexico produced more than 2.106 tons of tomato, from which more than 105 tons were processed as tomato puree. Total income for the latter were almost 80 million US dollars, including national and export markets.19 Although there is a national standard for the characteristics of tomato puree, including the contents of several metals like Cu, Zn, As, Sn, and Pb, the information regarding other elements is largely unknown. The Codex Alimentarius standard mentions only possible contamination by Pb or Sn, specifying their maximum concentrations. Moreover, only traditional methods, like AAS or colorimetry have been used to determine the contents of those elements. Although reliable, these methods are rather cumbersome for their application, as they provide information about one element at a time only. In this regard, the use of alternative techniques should be considered, as is the case of XRF or PIXE, together with RBS. Therefore, PIXE and RBS were applied in this work to analyze the elemental concentration in Mexican and some foreign commercial tomato puree, in order to verify if the Codex Alimentarius or the Mexican national standard requirements for elemental concentrations are fulfilled, and to obtain new information about the contents of other elements. The results may be useful to establish new standards for the measured elements, both internationally or locally.
0236–5731/2004/USD 20.00 © 2004 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest Kluwer Academic Publishers, Dordrecht
Introduction The study of trace elements in food is an important issue, because of the possible effects they may have on human health.1,2 Normally, analytical methods such as atomic absorption spectrometry (AAS), inductively coupled plasma mass spectrometry (ICP-MS), and other traditional chemical procedures have been applied to determine the elemental contents of the samples. However, it has been found that nuclear analytical methods are especially advantageous for trace element determinations in many scientific fields, including food analyses.3 Therefore, techniques like neutron activation analysis (NAA) and particle induced X-ray emission (PIXE) are expected to provide good results for food chemistry. This is particularly true for PIXE,4 as it is a multielemental technique, non-destructive, and very sensitive, requiring small amounts of material for analytical purposes. X-ray fluorescence (XRF) is another powerful method in this area. Additionally, PIXE can be complemented with other methods, such as Rutherford backscattering spectrometry (RBS), which provides information about light elements (like C, N, O, Na, and Mg), that are not easily be detected by X-ray spectrometric methods.5 PIXE has been successfully used to measure trace elements in soy sauce,6–8 alfalfa,9 bean sprout,10 powder milk,11 plant leaves,12 mate,13 barley roots,14 Basella alba leaves,15 and several kinds of fods.16 Moreover, KOCSONYA et al.17 determined Pb contents in wine, although they did not take full advantage of PIXE multielemental analysis capabilities. Tomato puree is a product with a high consumption in the Mexican market. It is defined by the Codex Alimentarius standard Codex Stan 57-1981.18 According to this, “tomato puree is a tomato concentrate that contains not less than 8 percent, but less than 24
E. ROMERO-DÁVILA, J. MIRANDA: TRACE ELEMENT DETERMINATION IN TOMATO PUREE
Experimental Samples from 12 brands sold in Mexican markets were acquired (three different lots for each one), including one with USA origin; three more purchased in the USA, one from Japan, one from Colombia, and two more from Chile, for a total of 19 brands. Three pellets were prepared for each product. Moreover, as a reference, a tomato puree was prepared following the Mexican Consumer Federal Attorney recommended procedures.20 This was done to have a sample expected to preserve better the nutrition properties, due to a reduced manipulation of the tomato puree. Table 1 shows a description of each sample, including their packing type. To obtain the samples to be analyzed, 200 ml of high purity ethanol were added to 150 g of tomato puree extracted from each container. The mixture was shaken to homogenize, and then allowed to stand for about 15 minutes. Afterwards, the sample was heated with an electric oven at 50 °C for 2 hours in air, or until totally dried. This temperature prevents volatilization of some elements. Finally, the resulting material was ground in an agate mortar, and a 10 mm diameter pellet was pressed, with a stainless steel pelletizer, using a 5 t/m2 pressure. The mass of each pellet was around 1 g. The masses were measured using a model SC-6010 Ohaus balance, with a 600 g range and a 0.1 g resolution. The pellets were analyzed with particle induced Xray emission (PIXE) using the Instituto de Física 3 MV Pelletron accelerator. An external beam setup was employed, which consisted of an 8 µm Al window and a Canberra Model GL0055P LEGe detector with a 2026 Canberra amplifier. Figure 1 shows the outline of the experimental setup.21 The resolution of the detector was 155 eV at 5.9 keV. Spectra were collected in an Oxford Tennelec PCA3 multichannel card with the help of an Oxford Tennelec multiplexer-router. X-ray spectra were deconvoluted using the QXAS computer code.22 To obtain elemental concentrations from the X-ray peak areas, the PIXEINT computer program23 was used. The detector efficiency was determined using pellets of the International Atomic Energy Agency certified reference materials IAEA-SL-1 and IAEA-Soil-7. Furthermore, a third pellet of the National Institute of Standards and Technology Tomato Leaves 1573a standard reference material was used for quality assurance procedures. Beam energy at the sample surface was 2.63 MeV, which resulted from the production of a 3 MeV beam at the accelerator, crossing the Al foil and a 10 mm air path before reaching the sample.
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Additionally, RBS analyses were carried out in vacuum using a 0.7 MeV proton beam produced by the 0.7 MV Van de Graaff accelerator, at the Instituto de Física, UNAM. Backscattered protons were registered with a 25 mm2 PIPS particle detector, placed at an angle of 155° with respect to the incoming beam direction. An Ortec 142A preamplifier and a Canberra 2022 amplifier were employed to process the signals from the detector. Spectra were then collected using an Ortec MCA card and, later on, analyzed using the RUMP computer code.24 The main sources contributing to uncertainty in PIXE results were the concentrations in the certified reference materials, the X-ray production cross sections, mass absorption coefficients, and ion stopping powers,4 while for RBS, ion stopping powers, the PIPS detector solid angle and the detection angle were the major sources. The resulting uncertainties are quoted in Table 3, which were calculated using the method recommend by Mexican standard NMX-CH-140-IMNC-2002.25 In this study, type B uncertainties are more important than those of type A.
Table 1. Description of tomato puree samples
a b
Sample
Origin
Type of packing
M1 M2 M3 M4 M5 M6 M7 M8 M9 M 10 M 11 M 12 M 13 M 14 M 15 M 16 M 17 M 18 M 19 M 20
Mexico Mexico Mexico Mexico Mexico Mexico Mexico USAa Mexico Mexico Mexico Mexico USA USA USA Colombia Chile Chile Japan Mexicob
Can Can Can Can Tetrapack Can Tetrapack Can Can Can Tetrapack Can Can Can Can Glass container Can Can Metallic bag Glass container
Imported to Mexican market. Prepared following the Mexican Consumer Attorney recipe. 20
E. ROMERO-DÁVILA, J. MIRANDA: TRACE ELEMENT DETERMINATION IN TOMATO PUREE
Fig. 1. Schematics of the experimental setup
Fig. 2. Typical RBS spectrum of a tomato puree sample, obtained in the Japanese market (sample M19)
Results and discussion Figure 2 displays a typical RBS spectrum. With this technique it was possible to quantify the elements that compose the organic matrix of the samples. In particular, C, N, O, and Na were measured with this method. Mg, P, and S were observed in a few samples, too. Moreover, PIXE gave results on the contents of Cl, K, Ca, Ti, V, Cr, Mn, Fe, Cu, and Zn, which were present in trace levels. Figure 3 shows a PIXE spectrum for a tomato puree sample. Table 2 contains the minimum detection limits for all the elements measured with PIXE, on a dry basis. These figures represent the minimum amount that gives rise to a peak area equal to three times the
background below the peak.26 Table 3 presents the results for the elemental concentrations measured in every brand with RBS, representing the average of the three lots (on a wet basis), together with the fraction of dry mass. Data about this fraction, after extraction of water from the sample, showed that samples 5 and 9 lie completely outside of the range allowed for the definition of tomato puree, showing that they are highly diluted in water. Furthermore, it is impossible for the samples to reach the range of tomato paste, as defined by the Codex Alimentarius, cited above.18 The remaining samples agree, within the uncertainty, with the definition of tomato puree.
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Fig. 3. Typical PIXE spectrum of a tomato puree sample, obtained in the Chilean market (sample M18). The Ar peak correspond to atmospheric argon
Table 2. Minimum detection limits for PIXE analysis of tomato puree samples, on a dry basis Element
Minimum detection limit
P S Cl K Ca Ti V Cr Mn Fe Cu Zn
0.1% 0.2% 0.01% 0.01% 17 mg/kg 1.3 mg/kg 0.7 mg/kg 0.7 mg/kg 0.9 mg/kg 1.2 mg/kg 1.2 mg/kg 0.8 mg/kg
Concentrations measured with PIXE for Cu and Zn lie within the allowances of the Mexican standard, which are not mentioned in the Codex Alimentarius. Both elements are considered essential to life. Zn helps in the synthesis and degradation of carbohydrates, nucleic acids, lipids and proteins, and it has been shown to play a role in genetic expression.1 Furthermore, Cu is present in many metalloenzymes, and may have influence on other enzymes not containing Cu. High amounts of this element in food are very rare, and are usually related to accidental or intentional ingestion from Cu containers. The roles of Cu and Zn in human nutrition are closely related, as Cu concentrations are reduced by an excess of Zn.1 Several elements that are not considered in the standards were found, as is the case for Ti, V, Cr, and Mn. None of them were measured in levels considered as toxic for human consumption.1 The latter two
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elements are considered as micronutrients,1,27 which contributed to the growth of the plant, subsequently, they are also micronutrients for human consumers. The presence of V is explained because plants take it readily from soil through the roots; V has an important role in photosynthesis, lipid metabolism and N2 fixation.27 This element is not considered toxic when ingested by humans,2 being harmful only when inhaled. In contrast to this, Ti has not been reported in any study of trace elements of tomato puree. Furthermore, its role as a nutrient is still not fully understood.27 However, it may play a role in photosynthesis and N2 fixation.2 Regarding Cr, its toxicity depends on the valence state (+2 to +6), affecting solubility and reactivity. Trivalent Cr has a low toxicity level, and the absorption depends on the presence of other chelating elements, like Fe or Zn. Cr is necessary for normal metabolism of carbohydrates and lipids.2 Mn is an essential element for plants and, therefore, was expected to be found in tomato puree. In the case of man, Mn is widely distributed in the body, although in low concentrations.2 Furthermore, the foreign brands showed a very similar behavior to the Mexican products, one of the USA tomato purees presented the highest contents for Fe, with the “reference” tomato puree (M20) having comparable Fe concentrations. The latter sample, additionally, had concentrations not very different for all the elements. Na and Cl in commercial samples must be related to the NaCl added to the tomato puree, a compound allowed as an additive by the international and Mexican standards.
E. ROMERO-DÁVILA, J. MIRANDA: TRACE ELEMENT DETERMINATION IN TOMATO PUREE Table 3. Dry mass fraction and elemental concentrations measured in tomato puree (wet basis) a,b Sample M1 M2 M3 M4 M5 M6 M7 M8 M9 M 10 M 11 M 12 M 13 M 14 M 15 M 16 M 17 M 18 M 19 M 20 Sample M1 M2 M3 M4 M5 M6 M7 M8 M9 M 10 M 11 M 12 M 13 M 14 M 15 M 16 M 17 M 18 M 19 M 20 Sample M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15 M16 M17 M18 M19 M20
Dry mass, % 6.40 8.12 7.89 8.02 4.49 7.96 6.10 5.71 2.97 8.63 7.98 6.97 6.09 8.58 12.1 7.67 6.95 9.25 6.00 13.3 Mg, 300 1.0 480 ND ND 1.0 ND 1.0 220 ND 140 1.0 ND ND 280 80 1800 ND 1210 1530
Ca, 28 20 30 12 40 30 11 11 6 18 52 18 118 41 52 65 31 57 189 1140
C, 4.5 5.0 4.7 5.1 3.6 5.2 4.3 3.6 2.1 5.5 4.0 4.4 4.0 5.9 10 5.2 4.5 6.0 4.0 8.7
mg/kg (30) (0.08) (5)
P, ND 0.054 0.12 0.13 0.051 0.029 0.065 0.038 0.022 0.038 0.180 0.080 0.102 0.081 0.022 0.088 0.012 0.073 0.010 0.34
(0.1) (0.1) (20) (14) (0.07)
(30) (9) (200) (10) (15)
mg/kg (3) (2) (3) (1) (4) (3) (1) (1) (1) (2) (5) (2) (12) (4) (5) (7) (3) (6) (19) (120)
Ti, 0.20 0.20 0.80 0.20 0.40 0.20 0.20 0.20 0.10 0.20 1.3 0.60 2.6 0.70 0.10 0.60 0.10 0.80 ND ND
mg/kg (0.02) (0.02) (0.08) (0.02) (0.04) (0.02) (0.02) (0.02) (0.01) (0.02) (0.1) (0.06) (0.3) (0.07) (0.01) (0.06) (0.01) (0.08)
% (0.4) (0.5) (0.5) (0.5) (0.4) (0.5) (0.4) (0.4) (0.2) (0.6) (0.4) (0.4) (0.4) (0.6) (1) (0.5) (0.4) (0.6) (0.4) (0.9)
N, 0.07 0.29 0.44 0.24 0.23 0.13 0.01 0.32 0.06 0.30 0.04 0.25 0.05 0.05 0.01 0.10 0.26 0.33 0.05 0.10
%
% (0.007) (0.03) (0.04) (0.02) (0.02) (0.01) (0.001) (0.03) (0.006) (0.03) (0.004) (0.03) (0.005) (0.005) (0.001) (0.01) (0.03) (0.03) (0.005) (0.01)
S, 0.024 0.036 ND 0.017 ND 0.046 ND ND ND ND ND 0.033 ND ND ND ND ND 0.034 ND 0.095
(0.005) (0.01) (0.01) (0.005) (0.003) (0.006) (0.004) (0.002) (0.004) (0.018) (0.008) (0.010) (0.008) (0.002) (0.009) (0.001) (0.007) (0.001) (0.03) V, ND 0.08 0.32 0.08 0.27 0.16 0.12 0.06 0.030 0.09 0.40 0.14 0.80 0.26 ND 0.15 ND 0.37 ND 0.13
mg/kg (0.01) (0.03) (0.01) (0.03) (0.02) (0.01) (0.01) (0.003) (0.01) (0.04) (0.01) (0.09) (0.03) (0.02) (0.04) (0.01)
% (0.002) (0.004) (0.002) (0.005)
(0.003)
(0.003) (0.009)
O, 1.5 2.35 2.2 2.3 0.29 2.3 1.6 1.6 0.7 2.4 3.2 2.0 1.6 2.4 1.6 2.0 2.0 2.7 1.6 3.5
% (0.2) (0.2) (0.2) (0.2) (0.029) (0.2) (0.2) (0.2) (0.08) (0.2) (0.3) (0.2) (0.2) (0.2) (0.2) (0.2) (0.2) (0.3) (0.2) (0.4)
Cl, 0.138 0.016 0.17 0.050 0.061 0.089 0.021 0.087 0.002 0.011 0.23 0.052 0.120 0.045 0.120 0.13 0.056 0.032 0.030 0.052 Cr, 0.06 ND 0.10 0.10 0.14 0.10 ND 0.10 ND 0.10 0.16 0.10 0.30 0.17 0.12 0.15 ND 0.10 ND ND
% (0.014) (0.002) (0.02) (0.005) (0.006) (0.009) (0.002) (0.009) (0.0002) (0.001) (0.02) (0.005) (0.01) (0.004) (0.01) (0.01) (0.006) (0.003) (0.003) (0.005)
mg/kg (0.01) (0.01) (0.01) (0.01) (0.01) (0.01) (0.01) (0.02) (0.01) (0.03) (0.02) (0.01) (0.02) (0.01)
Na, 0.14 0.06 0.14 0.12 0.14 0.11 0.05 0.04 0.01 0.09 0.27 0.08 0.06 0.01 0.02 0.01 0.01 0.05 0.01 0.17
% (0.01) (0.01) (0.02) (0.02) (0.04) (0.02) (0.01) (0.01) (0.002) (0.01) (0.03) (0.01) (0.01) (0.001) (0.002) (0.001) (0.001) (0.01) (0.002) (0.02) K, 0.045 0.028 0.045 0.013 0.058 0.020 0.007 0.008 0.008 0.024 0.062 0.047 0.19 0.12 0.029 0.049 0.019 0.074 0.18 0.11 Mn, 0.10 0.10 0.10 ND 0.10 0.080 ND ND 0.10 0.10 0.16 0.10 0.30 ND ND 0.23 ND 0.10 ND 1.3
% (0.004) (0.003) (0.004) (0.001) (0.006) (0.002) (0.001) (0.001) (0.001) (0.002) (0.006) (0.005) (0.02) (0.01) (0.003) (0.005) (0.002) (0.008) (0.02) (0.01) mg/kg (0.01) (0.01) (0.01) (0.01) (0.008)
(0.01) (0.01) (0.02) (0.01) (0.03)
(0.02) (0.01) (0.1)
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E. ROMERO-DÁVILA, J. MIRANDA: TRACE ELEMENT DETERMINATION IN TOMATO PUREE Table 3. continued Sample M1 M2 M3 M4 M5 M6 M7 M8 M9 M 10 M 11 M 12 M 13 M 14 M 15 M 16 M 17 M 18 M 19 M 20
Fe, 0.20 1.7 0.30 0.20 0.30 0.70 0.10 0.60 0.30 0.70 0.20 0.30 2.4 0.90 0.20 0.60 0.70 0.40 0.20 2.1
mg/kg (0.02) (0.17) (0.03) (0.02) (0.03) (0.07) (0.01) (0.06) (0.03) (0.07) (0.02) (0.03) (0.2) (0.09) (0.02) (0.06) (0.07) (0.04) (0.02) (0.21)
Table 4 gives the concentrations of the elements measured with PIXE for the 1573a NIST standard reference material, given with certified values. As is seen in this table, most of the elements were in fair agreement with the certified values, a fact that supports the accuracy of the analytical methods. Additionally, an underestimation of the concentrations for Ca and Fe was obtained in this analysis, which might be reflected in the results for the tomato puree measurements. Nevertheless, these elements are not considered in the national or international standards for tomato puree, so no consequences of the aforementioned underestimation can be contemplated, except for analytical purposes. Table 5 displays the maximum elemental concentrations allowed in tomato puree, as specified by the Mexican Standard NMX-F-33-1982,28 and the Codex Alimentarius18 for Cu, Zn, As, Sn, and Pb. It should be pointed out that the latter standard only includes Sn and Pb. The official standards are based on analysis using traditional chemical analysis methods. As can be seen after comparison of Tables 2 and 4, none of the tomato puree samples had concentrations of those elements above the limits established in the standard. As, Sn, and Pb were not detected. This means that the products have suffered no contamination during the production, packing, storage, and distribution to the final consumer. Even those samples which were canned showed no effect of the materials composing the container. As a further comparison, Table 6 shows concentrations of several elements, measured in fresh tomatoes, using traditional chemical analysis techniques.27 It is
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Cu, 0.10 ND 0.20 ND 0.10 0.10 ND ND ND ND 0.20 0.10 1.0 0.10 0.10 0.10 0.70 0.10 0.10 ND
mg/kg (0.01) (0.02) (0.01) (0.01)
(0.02) (0.01) (0.1) (0.01) (0.01) (0.01) (0.07) (0.01) (0.01)
Zn, 0.10 ND 0.16 0.10 0.18 0.10 ND ND ND 0.10 0.16 0.10 0.61 0.26 0.12 0.15 0.70 0.18 0.10 0.13
mg/kg (0.01) (0.02) (0.01) (0.02) (0.01)
(0.010) (0.02) (0.01) (0.06) (0.03) (0.01) (0.01) (0.07) (0.02) (0.01) (0.01)
apparent that the elemental contents measured in this work are of the same order of magnitude as those given in Table 6. This fact reinforces the statement that the tomato puree samples analyzed in this work showed no metal contamination. A word must be added regarding the combined use of PIXE and RBS as compared with other multielemental techniques. PIXE and RBS are accelerator-based methods, a fact that certainly complicates their routine use by industry. In this regard, techniques like XRF are advantageous, as they have a lower cost and are easier to establish in a factory. Using state-of-the-art XRF spectrometers, it is possible to study almost the same elemental range with similar detection limits. The application of the ion beam analysis (IBA) techniques PIXE and RBS in this work allows a more complete elemental analysis. The results from RBS are necessary to determine accurately the organic matrix of the samples, required in PIXE quantitative analysis with PIXEINT.23 Although in this work, two separate analyses were done for these techniques, at present day it is possible to carry out the analyses simultaneously, thus reducing analytical time and improving accuracy. Moreover, other IBA techniques can be applied, like elastic recoil detection analysis (ERDA) or nuclear reaction analysis (NRA), offering quantitative information on other light elements (H, Li, B, Be), that cannot be measured with XRF. It should also be mentioned that the relative cost of ion accelerators has been decreasing in recent years.
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Table 4. Concentrations measured in 1573a NIST standard reference material Element N K Ca P Mn Fe Cr Zn a
Technique
Measured
Certified
RBS PIXE PIXE PIXE PIXE PIXE PIXE PIXE
3.9 (0.39)a % 2.6 (0.31)% 3.8 (0.46)% 0.20 (0.03)% 201 (20) mg/kg 290 (30) mg/kg 3 (1) mg/kg 26 (2) mg/kg
3.03 (0.15)% 2.70 (0.05)% 5.05 (0.09)% 0.216 (0.004)% 246 (8) mg/kg 368 (7) mg/kg 1.99 (0.06) mg/kg 30.9 (0.7) mg/kg
Numbers between parentheses represent standard uncertainties.
Table 5. Maximum allowed elemental concentrations according to Mexican and International standards, wet basis 28 Element Pb As Sn Cu Zn
Maximum limit NMX-F-33-1982, mg/kg 0.36 0.1 250 5.0 5.0
Table 6. Elemental concentrations (in mg/kg) measured in fresh tomatoes, using analytical methods other than PIXE or RBS, in wet and dry basis27 Element Cu Sr Ba Zn Cd Hg B Al Zr As V Cr Mo Br Mn Fe Co Ni
Wet 0.65 NDa ND 1.4 0.02 1b ND ND 0–1.79 0.46b ND 0.004 0.042; 0.024 ND 1.1 3 3.2b 0.03
Dry 8.8 9 2 26 0.18 34, 3.1b 6 20 ND 9–120b 0.5* 0.074 0.82 10 12 58 62–200b 0.43–0.48
a
ND: Not measured. b Concentrations in µg/kg.
Conclusions The results given in this work showed that the use of accelerator-based techniques is advantageous for analysis of widely consumed food products, as is the case of tomato puree. They provide reliable and fast results for many elements simultaneously, as compared to traditional chemical analytical methods. This work is not aimed to present PIXE as a substitute for other
Maximum limit Codex Alimentarius, mg/kg 1.5 Not specified 250 Not specified Not specified
techniques which may be easier to implement in an industry, such as XRF, but rather to encourage the use of X-ray spectrometric and IBA methods. It was found that no Mexican or foreign brands presented elemental concentrations above the limits of the Codex Alimentarius, nor those of the Mexican standard. The inclusion of tomato purees from several countries suggests that standards, either international or local, should be revized to include more elements which may have a role in nutrition or because of its toxicity. No elements were found in levels that may be considered as toxic for human consumption. The elemental contents were also similar to those of the “reference” tomato puree, which was expected to preserve the nutritional properties of the product, because of shorter periods of storage and less manipulation. * The authors acknowledge the technical assistance of Mr. K. LÓPEZ, Mr. F. J. JAIMES, and Mr. J. C. PINEDA for accelerator operation, as well as Mrs. Jaqueline CAÑETAS-ORTEGA, for sample preparation. This work was partially supported by CONACYT (Contracts F036-9109 and G-0010-E).
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