Food Biophysics

, 6:527

LIBS: A Quality Control Tool for Food Supplements

Authors

  • Rahul Agrawal
    • Centre of Food TechnologyUniversity of Allahabad
  • Rohit Kumar
    • Laser Spectroscopy Research Laboratory, Department of PhysicsUniversity of Allahabad
  • Shikha Rai
    • Laser Spectroscopy Research Laboratory, Department of PhysicsUniversity of Allahabad
  • Ashok Kumar Pathak
    • Laser Spectroscopy Research Laboratory, Department of PhysicsUniversity of Allahabad
    • Laser Spectroscopy Research Laboratory, Department of PhysicsUniversity of Allahabad
  • Gyanendra Kumar Rai
    • Centre of Food TechnologyUniversity of Allahabad
ORIGINAL ARTICLE

DOI: 10.1007/s11483-011-9235-y

Cite this article as:
Agrawal, R., Kumar, R., Rai, S. et al. Food Biophysics (2011) 6: 527. doi:10.1007/s11483-011-9235-y
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Abstract

In the present paper the ability of calibration free laser induced breakdown spectroscopy (CF-LIBS) as a quality control tool to monitor the composition of different minerals present in food supplement samples belonging to Indian brands (brand-A and brand-B) has been demonstrated. LIBS spectra of these two food supplements (brand-A and brand-B) available in the form of tablet have been recorded. As reported by manufacturers of these two food supplements, LIBS spectra of brand-A contains the spectral signatures of minerals like Ca, Mg, C, P, Zn, Fe, Cu, and Cr whereas LIBS spectra of brand-B shows the presence of spectral lines like Ca, Mg and C. The spectral signatures of Na and K are also found in both brands whereas spectral signature of Ti is observed only in brand-B but these elements are not mentioned on the nutritional label of the brands. The quantitative analysis of mineral contents in food supplements has been done using CF-LIBS for brand A and brand B to verify the content of the minerals reported by the manufacturer of the food supplements. Our results show that Ca and Mg are the main matrix elements of these brands. The concentration of minor and trace elements estimated using CF-LIBS technique is found in agreement with the reported nutritional values of both the brands. The concentration of major elements Ca and Mg are also estimated from Atomic Absorption Spectroscopy which is in close agreement with CF-LIBS result.

Keywords

LIBSCF-LIBSFood supplementsMinerals

Introduction

The Food and Drug Administration (FDA) regulates food supplements as a category of foods, but not as drugs. Food Supplement is also known as nutritional supplement/dietary supplement and is used to provide nutrients like vitamins, minerals, amino acids, antioxidant etc. Codex Alimentarius Commission [an organization that is sponsored by the Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO)] has included dietary minerals as a category of food supplement. There have been a lot of studies focusing on elemental compositions of food and dietary supplements to assess their quality for public consumption 14. Traditionally, dietary supplements such as cod liver oil, iron tablets and multivitamins were taken to ensure the adequacy of the diet. They were taken to ensure that our diet contained enough essential nutrients to prevent overt deficiency diseases and to ensure that we did not suffer other more subtle adverse effects of marginal nutrient inadequacy 5. The people take supplements in the hope that they will have additional health benefits like prevention of the risk of developing age-related diseases (heart disease, cancer and boost the immune system). It is difficult to put an exact figure on the number of minerals that are essential nutrients. Several minerals such as calcium, iron and zinc are required in relatively large (mg) quantities per day. Some others are clearly essential but are required in much smaller (microgram) quantities (such as chromium, selenium, copper etc.) and these are often referred to as the trace elements. The new legislation based upon the new EU Food Supplements Directive lists fifteen minerals (Calcium, Chromium, Copper, Fluoride, Iodine, Iron, Magnesium, Manganese, Molybdenum, Potassium, Selenium, Zinc, Chloride, Phosphorus, and Sodium) that may be used in food supplements 6.

The motive of this paper is to demonstrate the capability of CF-LIBS technique as a real-time and rapid quality control tool for monitoring minerals provided in the form of food supplements/medicinal supplements without any sample preparation as well as to validate the nutritional label mentioned on packaging material because food labels act as an important educational tool for the consumers about health requirements of specific foods 712.

Material and Methods

Food supplement tablets of two brands were purchased from the market of Allahabad, India. The experimental setup used in the present study is shown in Figure 1. Nd: YAG laser operating at 532 nm wavelength and capable of delivering a maximum energy of 425 mJ over a pulse duration of 4 ns, sample stage, optical fiber cable having collecting lens inclined at an angle 45° with the direction of the incident laser beam, and a spectrometer with CCD detector (Ocean Optics, LIBS 2000+) having fixed gate delay of 1.5 μs are the components of the present LIBS experiment. A 15 cm focal length lens was used to focus the laser beam on the tablets of food supplements. LIBS spectra of these tablets were recorded in the spectral range 200–500 nm in open atmosphere using LIBS 2000+ spectrometer having resolution 0.1 nm and analyzed using OOILIBS software. The average spectra of 50 laser shots were recorded to enhance the signal-to-noise ratio. The laser pulse energy and pulse repetition rate were optimized and better signal to background and signal to noise ratio was observed at 20 mJ energy and at 10 Hz repetition rate. The energy of laser pulse was measured with energy meter Genetec-e Model UP19K-30H-VM-DO.
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Fig. 1

Experimental Setup of Laser Induced Breakdown Spectroscopy

Preparation of Samples and Standards for AAS

Duplicate samples (1.00 g) of both samples were weighed in digestion tube and treated with 5 ml of concentrated nitric acid (HNO3). A blank sample was prepared applying 5 ml of nitric acid into empty digestion flask 13. The flasks were heated for 2 hours in digestor at 450oC (pelican equipments, Chennai, India) until a clean solution was obtained. After digestion few drops of concentrated hydrochloric acid (HCl) was added. The solution was heated gently. The residue was again subjected to digestion and filtrate was collected. After cooling, the solution was filtered with Whatman No. 42 filter paper. Filtrate was transferred into a 100 ml of volumetric flask and make-up the volume by adding distilled deionized water. Working standard solutions of calcium (Ca), magnesium (Mg) and iron (Fe) were prepared from the stock standard solutions containing 500 ppm of element in normal concentration of nitric acid. Calibration and measurement of elements in food supplements were done using atomic absorption spectrophotometer (model: A-Analyst 700, Perkin Elmer, USA). The calibration curves were prepared for each element individually applying linear correlation. A blank reading was also taken and necessary correction was made during the calculation of concentration of different elements.

Results and Discussion

Spectral Analysis of Food Supplements

LIBS spectra of food supplement brand A in the spectral range 200 to 500 nm is shown in Figure 2 which clearly demonstrate the presence of atomic lines of numerous minerals like calcium (Ca), magnesium (Mg), carbon (C), phosphorous (P), zinc (Zn), iron (Fe), copper (Cu), chromium (Cr), sodium (Na), and potassium (K) while atomic lines of calcium (Ca), magnesium (Mg), titanium (Ti), sodium (Na),and potassium (K) are present in the LIBS spectra of brand B (Figure 3a and b).These results clearly show the presence of the above minerals in these food supplements. The molybdenum, selenium and iodine in brand-A sample are not observed in LIBS spectra and it may be due to inappropriate addition of minerals in sample as compared to the amount reported on the label of food supplement.
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Fig. 2

LIBS spectra of food supplement brand A in the spectral range 200 to 500 nm

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Fig. 3

a LIBS spectra of food supplement brand B in the spectral range 200 to 500 nm, b LIBS spectra of food supplement brand-B showing spectral lines of Ti

CF-LIBS Technique

The algorithm of CF-LIBS 14,15 was proposed by Ciucci et al. 16 and is based on the assumptions of stoichiometric ablation, local thermal equilibrium, and optically thin plasma. The following sections show that how these assumptions are valid for the laser-produced plasma in the present experiment.

Stoichiometric Ablation

In this experiment, laser ablation of the solid food supplement tablets was achieved using Nd:YAG laser having 20 mJ energy at 532 nm laser wavelength, 4-ns pulse duration, 9 mm beam waist. We have focused the laser beam on the surface of the sample using converging lens of 15 cm focal length. At the focal point of the sample the spot size ‘D’ is calculated using the following formula
$$ {\text{D}} = {4}\lambda {\text{f}}/\pi {\text{d}} $$
(1)

(where ‘λ’ is the wavelength, ‘f’ is the focal length of the lens and ‘d’ is the laser beam diameter) and the value of D is ≈ 11 μm. The calculated power density at focal spot is ≈ 5 × 1012 W-cm−2 that satisfies the condition of stoichiometric ablation 1719.

Optically Thin Plasma

Before using the intensity of an atomic line of any elements for calculating its concentration in the tablets one must be certain that the plasma is optically thin i.e. the radiation emitted by an excited atom in the plasma is not reabsorbed by another atom in a lower energy state. The measure of self-absorption effect can be verified by comparing the intensity ratio of two interference free emission lines from a species, having the upper energy levels as close as possible to the products of the ratio of their transition probabilities, the ratio of their upper level degeneracies and the inverse ratio of their wavelengths. We have chosen Ca 370.6 nm and 373.6 nm for this calculation.

The intensity ratio of Ca-II (370.6 nm)/Ca-II (373.6 nm) is equal to 0.51 whereas the product of ratio of their transition probabilities, the ratio of their upper levels degeneracy and the inverse ratio of their wavelengths is 0.52. Similarly the intensity ratio of Mg-I (383.2 nm)/Mg-I (383.8 nm) and Mg-II (279.5 nm)/Mg-II(280.2 nm) are equal to 0.52 and 1.97 respectively whereas the product of ratio of their transition probabilities, the ratio of their upper levels degeneracies and the inverse ratio of their wavelengths are 0.54 and 2.00 respectively which clearly satisfy the condition of optically thin plasma.

Local Thermal Equilibrium (LTE)

Measurement of Plasma Temperature and Electron Density:

The plasma temperature has been determined using the familiar form of the Boltzmann plot 20,21 given below
$$ { \ln }\;\frac{{I_{\lambda }^{{ki}}}}{{{A_{{ki}}}{g_k}}} = - \frac{{{E_k}}}{{{K_B}T}} + { \ln }\frac{{{C_S}F}}{{{U_S}T}} $$
(2)
where ‘KB’ is the Boltzmann constant, ‘Us(T)’ is the partition function, ‘Aki’ is the transition probability, ‘gk’ is the statistical weight for the upper level ‘Ek’, ‘T’ is the temperature and ‘F’ is a constant depending on experimental conditions. The measured intensities of spectral lines and other spectroscopic data, obtained from NIST data 22,23 were used to obtain the Boltzmann plots. We have obtained the Boltzmann plot (Figure 4) using atomic lines of different elements like Ca, Zn, and Mg. The calculated plasma temperature from Boltzmann plot is 1.25 × 104 ± 8.96% K which shows that the plasma is in local thermal equilibrium in the present experiment.
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Fig. 4

Determination of plasma temperature using Boltzmann plot

The plasma emission is not a direct consequence of the photo excitation but it is due to impact excitation by thermal electrons and this requires an electron density that ensures a high collision rate whose lower limit for LTE is given by following relation 19,20.
$$ {{\text{N}}_{\text{e}}} \geqslant {1}.{6} \times {1}{0^{{{12}}}}{\left[ {\text{T}} \right]^{{{1}/{2}}}}{\left[ {\Delta {\text{E}}} \right]^{{3}}} $$
(3)

Here, Ne (cm−3) is the electron density, T (K) is the plasma temperature, and ΔE (eV) is the difference between neighboring states with allowed transition.

The electron density (Ne) is calculated by the measurement of the full width at half maximum (FWHM) of Stark broadened Lorentzian line shape of the atomic line (422.7 nm) of Ca I (Figure 5). The observed line shape has been corrected by subtracting the contribution of the instrumental width which is 0.05 nm.
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Fig. 5

Lorentzian plot for the atomic line Ca-I 422.7 nm

$$ \Delta \lambda = \Delta {\lambda_{\text{observed}}} - \Delta {\lambda_{\text{instrument}}} $$
(4)
As the contribution of ionic broadening is very small as compared to the Stark broadening due to plasma electrons and ions 19,20 therefore the relation between electron density and FWHM (Δλ) of Stark broadened line can be expressed by following expression
$$ \Delta \lambda \approx {\text{2w}}\left( {{{\text{N}}_{\text{e}}}/{1}{0^{{{16}}}}} \right) $$
(5)

Where w is electron impact width parameter 1921. The impact parameter w is 7.18 × 10−3 Å for Ca 422.7 nm, and FWHM is 0.43 nm.

The calculated value of Ne is of the order of ≈ 1018 cm−3, which is greater than the lower limit of Ne ≈ 1016 cm−3 for LTE. In this way all the essential criteria for CF-LIBS has been verified.

CF-LIBS Algorithm

The steps involved in CF-LIBS program under MATLAB environment are as follows:
  • Analysis of LIBS Spectra.

  • Checking of total number of species present in LIBS spectrum.

  • Calculation of Plasma temperature by the Boltzmann plot of each species

  • Calculation of Partition functions for each species present in the sample.

  • Calculation of F.

  • Calculation of species concentration by formula in the MATLAB program.

The concentration Cs of species is calculated from the intercept of the Boltzmann plot which is proportional to the logarithm of the species concentration times the experimental factor F. In order to remove the unknown experimental factor F we have used the normalization condition for species concentration Cs of the sample, i.e.,
$$ \sum {{\text{C}}_s} = {1} $$
(6)
The calculated value of minerals concentration observed using CF-LIBS are found in close agreement as reported on nutritional labeling of food supplements (Tables 1 and 2). To verify the results of CF-LIBS we have also measured the concentrations of Calcium, Magnesium, and Iron in brand-A and B using Atomic Absorption Spectrometry (AAS). The results obtained from CF LIBS and AAS are shown in Table 1 and Table 2 for brands A and B respectively. Table 1 and Table 2 clearly show that the results obtained from CF-LIBS and AAS are in close agreement.
Table 1

Concentration of minerals present in Food Supplement Brand A

Elements and their amount mentioned in nutritional Facts of BRAND A (%)

Elements found in LIBS Spectra

Concentration By CF-LIBS (%)

Concentration By AAS (% ± % error)

Ca

53.85

Ca

55.54

52.48 ± 3.83

Mg

26.92

Mg

20.06

25.65 ± 8.22

Zn

4.04

Zn

3.59

Cu

0.54

Cu

1.15

P

12.12

P

9.41

Fe

1.88

Fe

2.24

1.85 ± 0.54

Cr

0.03

Cr

0.57

Mn

0.54

Mn

2.65

Na

Na

2.05

K

K

2.75

Se

0.02

Mo

0.02

I

0.04

Table 2

Concentration of minerals present in Food Supplement Brand B

Elements and their amount mentioned in nutritional Facts of BRAND B (%)

Elements found in LIBS Spectra

Concentration By CF-LIBS (%)

Concentration By AAS (% ± % error)

Ca

65

Ca

64.53

64.22 ± 2.33

Mg

35

Mg

33.85

36.15 ± 4.97

Ti

0.26

Na

0.59

K

0.75

Conclusion

The present work successfully demonstrates the capability of CF-LIBS technique for rapid as well as online Quality control tool. Though the nutritional elements put on the label of food/mineral supplement is in close agreement with the LIBS analysis but still some of the elements which are present in the samples such as Ti, Na, K in brand B and Na and K in brand A has not been reported by the manufacturers.

Acknowledgement

Financial assistance from the BRNS, BARC, Mumbai (no. 2009/37/30/BRNS/2063) is gratefully acknowledged. Mr. Rahul Agrawal is grateful to Centre of Food Technology, University of Allahabad for Financial Assistance. Mr. Rohit Kumar is grateful to BRNS-BARC for Financial Assistance as JRF. Mrs. Shikha Rai is grateful to CSIR for Financial Assistance as SRF. Mr. Ashok Kumar Pathak is grateful to UGC, New Delhi (No.F.27-99(TF)/2009(NRCB) for granting Teacher Fellowship Award under FIP Scheme.

Copyright information

© Springer Science+Business Media, LLC 2011