Detection of carcinogenic chromium in synthetic hair dyes using laser induced breakdown spectroscopy M. A. Gondal,* Y. W. Maganda, M. A. Dastageer, F. F. Al Adel, A. A. Naqvi, and T. F. Qahtan Laser Research Group, Physics Department, King Fahd University of Petroleum and Minerals, Box 504, Dhahran-31261, Saudi Arabia *Corresponding author: [email protected] Received 15 October 2013; revised 31 December 2013; accepted 8 January 2014; posted 30 January 2014 (Doc. ID 199462); published 7 March 2014
A laser induced breakdown spectroscopic (LIBS) system, consisting of a pulsed 266 nm laser radiation, in conjunction with a high-resolution spectrograph, a gated intensified charge coupled device camera, and a built-in delay generator were used to develop a sensitive detector to quantify the concentration of toxic substances such as chromium in synthetic hair dyes available on the local market. The strong atomic transition line of chromium (Cr I) at 427.5 nm wavelength was used as a fingerprint wavelength to calibrate the detection system and also to quantify the levels of chromium in the hair dye samples. The limit of detection achieved by our LIBS detection system for chromium was 1.2 ppm, which enabled us to detect chromium concentration in the range of 5–11 ppm in the commercial hair dyes available on the local market. The concentrations of chromium in the hair dyes measured using our system were validated using a standard analytical technique such as inductively coupled plasma mass spectrometry (ICPMS), and acceptable agreement (nearly 8%) was found between the results obtained by the two methods (LIBS and ICPMS). This study is highly significant for human health, specifically for people using synthetic hair dyes for changing the color of their hair. © 2014 Optical Society of America OCIS codes: (020.0020) Atomic and molecular physics; (140.3530) Lasers, neodymium; (300.6365) Spectroscopy, laser induced breakdown. http://dx.doi.org/10.1364/AO.53.001636
1. Introduction
Synthetic hair dyes are widely used cosmetic products in the modern world by people of all age groups to change their hair color and to improve their physical appearance; therefore they play an important role in shaping the quality of our lives [1]. However, heavy metals such as chromium, cadmium, and arsenic are common elements in synthetic hair dyes as color additives [2,3]. These toxic metals, exceeding the permissible safe levels, damage body organs, disrupt the nervous system, and have adverse effects on human metabolism [4]. Chromium enters the body through dermal, gastrointestinal, and inhalation 1559-128X/14/081636-08$15.00/0 © 2014 Optical Society of America 1636
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exposure. The toxicity of chromium causes mutations and chromosomal damage in human cells [3,5]. Besides being a carcinogen, prolonged exposure to chromium has adverse systemic effects such as damaging the kidney, liver, and blood forming organs [6–11]. Considering the extent and frequency of contact of the dyes with the permeable human skin, hair dyes and their ingredients penetrate into the body and therefore are deemed hazardous to humans’ health. The safe permissible limit for chromium is 1 ppm [12,13]. Hence control and easy detection of toxic metals in synthetic hair dyes is of great significance. To detect toxic metals in various cosmetic products, methods such as Hg-based stripping voltammetric analysis, the screen printed silver electrode technique, chromatography methods, and atomic absorption spectrometry, which are time
consuming, expensive, laborious, and require a lot of sample preparations, have been extensively used by researchers [14–18]. On the contrary, laser induced breakdown spectroscopy (LIBS) is a far superior technique as compared to the conventional methods. It is cost effective, requires little or no sample preparation, and enables rapid sample analysis. The basic principle of LIBS is based on the spectral analysis of radiation emitted from the plasma generated by focusing a high-power pulsed laser beam on the target surface. The characteristic emission from the plasma provides information about the elements present in the target material. The atomic emission wavelengths and their intensities are compared with standard atomic line references and calibrated against samples of known concentration to determine the chemical composition of a sample qualitatively and quantitatively [19]. In our study, we have developed a sensitive laser induced breakdown spectrometer for the detection and quantification of chromium concentration in the synthetic hair dyes available on the local market. In order to achieve the best limit of detection, a good signal-to-noise ratio (S/N) of the LIBS system should be achieved. Besides other experimental parameters, the vital parameters that affect the sensitivity of the LIBS detection system are the delay between the excitation and the detection, the sampling gate width, the laser fluence, and the sampling geometry. All these parameters were optimized in order to achieve a good S/N. The choice of these parameters was validated using the Mcwhirter criterion [20], which proved that the laser induced plasma (LIP) generated by our LIBS system was optically thin, and in local thermodynamic equilibrium (LTE), a condition which is very fundamental and must be satisfied in LIBS applications. This study will also be important to provide baseline data that will help to determine the levels of chromium toxicity of synthetic hair dyes. 2. Experimental Methods A.
LIBS System
A pulsed fourth-harmonic Nd:YAG laser (Spectra physics, Model GCR100) of wavelength 266 nm operating in Q-switch mode and delivering a maximum pulse energy of 50 mJ with a pulsewidth of 8 ns at a 20 HZ repetition rate was used as the excitation source. A convex lens of focal length 20 mm was used to focus the collimated laser beam onto the surface of the sample to create the plasma spark. In order to minimize the pitting on the sample and the consequent signal reduction (due to change of the focal point of the laser beam), the sample was placed on the XY translator and moved at a constant speed during laser ablation. A fiber optic cable with a miniature lens was positioned at an appropriate distance from the focal volume of the plasma with an optimum 45° with respect to the sample surface in order to collect the light emitted by the plasma spark.
The plasma light was then directed to the spectrometer (1200 line∕mm grove density), which was connected to an intensified charge coupled device (ICCD, Andor-50SI) for processing. The entire detection system is interfaced with a computer having software that generates the optical emission spectrum. A slit width of 100 μm, gate width 200 μs, number of accumulations 20, and exposure time of 4 s were used as the spectrometer parameters. In the study of emission intensity as a function of laser fluence, laser energy for a single pulse of spot diameter 0.1 mm was measured using an energy meter (Ophir model 300). B. Sample Preparation
Three hair dye samples of different quality, brand, and price range were procured from different cosmetic shops within the kingdom of Saudi Arabia. These samples were in powder form, which blows out of the sample holder when a high-power pulsed laser beam is incident on it, and therefore it is not trivial to analyze using the LIBS technique. Hence a special form of preparation to transform the fine powder into solid compact pellets was required for easy and effective analysis. The base matrix (fine powder) was mixed thoroughly with potassium bromide (KBr) as the binding material in appropriate ratios as per the standard procedure and grinded using agate pestle and motor thoroughly to ensure the homogeneity of the mixture. The homogeneous mixture was then compacted using a 10 bar pellet press having a stainless steel cylindrical die with a diameter of 20 mm and thickness of 2 mm. To avoid moisture, humidity effects, and any form of contamination, the pellets were wrapped in a clean aluminum foil and safely stored in a desiccator ready for analysis. To calibrate the LIBS system, solid pellet stoichiometric samples with different concentrations of chromium (Cr) in parts per million (ppm) of 40, 60, 80, and 100 ppm were prepared by homogeneously mixing chromium (II) sulfate with the base matrix (synthetic hair dye powder). In order to further confirm the measurements obtained using our LIBS system, inductively coupled plasma mass spectrometry (ICPMS), a conventional technique, was also used. In this case each powder sample was digested by the addition of nitric acid (HNO3 ) and hydrogen peroxide (H2 O2 ). The resultant solution was refluxed at 95°C, for 5 h, and the exothermic reaction was allowed to complete. The solution was then diluted to a desirable final volume and analyzed using an inductively coupled plasma mass spectrometer. 3. Results and Discussion A. Optimization of the Time/Gate Delay for Sensitive Detection
At the early stages of plasma formation (below 400 ns after excitation) the typical LIBS spectrum is basically a continuum (background noise), which is due to blackbody radiation of the plasma and elastic 10 March 2014 / Vol. 53, No. 8 / APPLIED OPTICS
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Fig. 1. LIBS signal intensity of 427.5 nm Cr spectral line as a function of time/gate delay.
collisions of electrons with the ionic species (Bremsstralung). Concurrently, broadened ionic lines and weak atomic lines are superimposed on the continuum and often overlap. After some appropriate time, the plasma expands and cools down, resulting in the emission of well-defined characteristic atomic transition lines of the neutral and ionized elements present in the sample [20]. Hence the delay between the laser pulse trigger and acquisition of the spectrum has to be optimized in order to minimize the background noise and maximize the intensity of the emission line of interest. In our study, the intensities of the spectral marker line (Cr 427.5 nm) were obtained for different time/gate delay times in the range of 600–1200 ns. From Fig. 1, the intensity increases and reaches a maximum at 800 ns and then drops. Hence 800 ns was selected as the optimum time/gate delay. It is worth noting that the optimum time/gate delay depends on the transitional probability and the lifetime of the upper level of the analytical spectral line [21]. Therefore different elements have different intensity temporal evolutions. B.
Optimization of the Excitation Source
The laser fluence is an important parameter in plasma generation and plays a vital role in the sensitivity of the LIBS system due to the fact that it is proportional to the emission intensity of an analyte when the plasma is optically thin. The emission intensity as a function of laser fluence for the spectral marker line (Cr 427.5 nm) using the optimal time/ gate delay (time between the laser trigger and acquisition of the spectrum) of 800 ns was studied in order to optimize the LIBS signal intensity and is depicted in Fig. 2. From Fig. 2, the intensity initially increases linearly with the incident laser fluence. This phenomenon is attributed to the increase in the amount of ablated material and to the increase in the electron temperature. For laser fluence values higher than 24 Jcm−2 , the emission intensity reaches saturation, mainly due to the absorption of the laser beam by the plasma formed in front of the target of the sample, a process known as plasma shielding 1638
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Fig. 2. LIBS signal intensity of 427.5 nm Cr spectral line as a function of laser fluence.
[20]. Also self-absorption can be used to account for this effect. Hence, in our case 24 Jcm−2 is the optimum laser fluence for our LIBS system. It is worth noting that the most prominent mechanism responsible for the plasma absorption at such high laser fluence is inverse Bremsstralung, whereby a free electron absorbs a laser photon [20]. However, the saturation can also be explained by assuming the formation of a self-regulating regime near the target surface at such higher laser fluence levels [22]. At low plasma temperature, the absorption of laser photons by the plasma becomes high, resulting in less sample ablation and a consequent decrease in the density of the ionic species. This behavior as a result increases the absorption of the laser photons by the plasma, hence increasing the temperature of the plasma. On the other hand, when the absorption of the laser energy is less, the process is reversed. It has also been theoretically proven that the density and temperature of the plume adjust in such a manner that the plasma absorbs the same amount of laser radiation to maintain a selfregulating regime [20]. C.
Local Thermodynamic Equilibrium Condition
In order to calibrate a LIBS system using spectral line intensities, the LIP should be optically thin (re-absorption and absorption of the incident radiation by the plasma is negligible) and in LTE. In a transient system such as plasma produced in LIBS, the LTE condition holds if the free electrons in the plasma have a Maxwellian distribution. It is worth mentioning that the electron velocity distribution for a relatively dense plasma with a low temperature (ne > 1016 cm−3 , kT < 5 eV) is nearly always Maxwellian [22]. Also, collision, excitation, and de-excitation processes should dominate over the radiative processes for the LTE condition to be valid. Along the boundaries of the plasma, number densities are low and movement is very rapid; therefore LTE is not a good assumption. However, deeper into the plasma volume where conditions change more
slowly and collisions occur more frequently, this assumption is valid [20]. Clearly, for LTE to hold, the electron number density must be sufficiently high, and hence it is worthwhile to check the minimum density condition for LTE using the Mcwhirter criterion [20] as given in Eq. (1): ne ≥ 1.4 × 1014 T 1∕2 ΔE3 cm−3 ;
(1)
where ne is the critical electron density and its dependence on the largest energy gap (ΔE) between two adjacent levels of the considered species, i.e., the energy of the shortest wavelength transition used in the temperature determination. T is the plasma temperature as mentioned before. In our study plasma temperature (T) and electron density (ne ) were explicitly determined using the Boltzmann plot and Stark broadening, respectively. For optically thin plasma in LTE, its temperature is obtained using Eq. (2): ki Iα λ E Cα F ; ln − k ln K BT gk Aki U α T α
(2)
where K B is the Boltzmann constant, Uα (T) is the partition function, Aki is the transitional probability, g is the statistical weight for the upper level, Ek is the excited level energy, T is the plasma temperature, and F is a constant depending on experimental conditions. A plot of magnitude of the component on the left side of Eq. (2) as a function of the value for the upper energy levels (Ek ) for the spectral lines under consideration yields a Boltzmann plot that is used to determine the electron temperature. The main sources of error when using Eq. (2) arise from using inaccurate values of Aki, imprecision in the recorded intensities or choosing transitions having upper levels with a small energy difference [22,23]. However, the use of logarithmic relations significantly reduces the influence of error. For instance, an error of 20% in the argument I ki λki ∕gk Aki reduces to about 5% when calculating the logarithm of such as an argument [24]. In our experiment the electron temperature was estimated using the atomic emission line intensities of chromium (Cr) observed in the LIP. It is worth noting that the spectral lines selected were in close spectral proximity, well resolved, strongly intense, and with well-known transitional probabilities and upper energy levels. The required parameters for the Boltzmann plot as obtained from the NIST database and Griem [23,25] are summarized in Table 1. Figure 3 shows the Boltzmann plot on which Table 1.
Spectroscopic Data for Chromium Spectral Lines [24]
Wavelength (nm)
gk
Aik s−1
Ek ev
425.433 427.481 428.973 460.742
9 7 5 7
3.15E 07 3.07E 06 3.16E 07 2.5E 06
2.913 2.899 2.889 3.698
Fig. 3. Boltzmann plot for plasma temperature determination.
data were fitted with the least-square approximation, and the slope of the plotted curve yielded a plasma temperature (T) of 5286 850 K. In a LIP, a spectral line is broadened due to Stark, Doppler, instrumental, and natural broadening mechanisms. In our case instrumental broadening was minimized by setting the detection system at its maximum resolution. It is worth mentioning that in a low-temperature, high-density plasma generated in our study, the Stark broadening is dominant and therefore was used to estimate the electron density by determining the full width at half-maximum (FWHM) of the broadened profile for the chromium atomic transition spectral line (Cr 427.5 nm). Stark broadening is due to collisions of the electrons with charged species resulting in both broadening of the line and shifting in the peak wavelength [26–30]. A Stark broadened profile is described by a Lorentzian function, and Eq. (3) relates its FWHM with the electron density: Δλ1∕2 2W
ne ; 1016
(3)
where W is the impact parameter ne is the electron density, and Δλ1∕2 is the FWHM. To estimate the electron density of the plasma, the line profile for chromium atomic spectral transition at 427.5 nm was used as shown in Fig. 4. The data points were fitted with a Lorentzian fit using origin8.0 software to yield a profile with a FWHM of 0.412 nm. The electron impact parameter w is obtained from Griem [23], and therefore the electron density is 1.68 × 1018 cm−3 . In our case the electron density determined using a gate/time delay and laser fluence of 800 ns and 24 Jcm−2 , respectively, as the optimal values is 1.68 × 1018 cm−3 , implying that ne ∼ 1018 cm−3 and kT ∼ 0.56 eV, which explicitly justifies that the free electron velocity is Maxwellian. Also the estimated minimum electron density is 1.98 × 1017 cm−3 , which is lower than the actual electron density; hence the plasma generated by our LIBS system is optically thin and in LTE. 10 March 2014 / Vol. 53, No. 8 / APPLIED OPTICS
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Fig. 4. Lorenztian fit for spectral line 427.5 nm for time/gate delay and laser fluence of 800 ns and 24 Jcm−2 , respectively.
Fig. 6. Intensity as a function of wavelength for different chromium concentrations.
D.
(in ppm) by adding known concentrations of chromium sulfate in the base matrix (synthetic hair dye) and analyzing them using the LIBS system. The spectrometer was adjusted to a center wavelength of 427.5 nm, and the spectra were recorded for different chromium concentration levels. It was observed that the intensity of the spectral line at 427.5 nm grows with the increase of chromium concentration at the same spectral position without any shift, and the effect of concentration on the Cr-I 427.5 nm line is shown in Fig. 6. This result strongly confirms the presence of chromium (Cr) in the synthetic hair dye. In LIBS, atomic emission intensity of a spectral line is used for analysis and quantification of a particular element in any given sample. The integrated intensity I z of a spectral line occurring between the upper energy level k and the lower energy level i for a species in the ionization stage z in optically thin plasma and in LTE is given as
Detection of Chromium in Synthetic Hair Dye Samples
Besides achieving the LTE for optically thin plasma, the detection system was optimized with respect to the temporal parameters, laser fluence, and other experimental parameters such as optical alignment and sample positioning. These optimized conditions ensured the best S/N, and a typical LIBS spectrum of the synthetic hair dye sample #1 in the 400–500 nm wavelength range is depicted in Fig. 5. The identification of the atomic transition lines in Fig. 5 is carried out using the National Institute of Standards and Technology (NIST) spectral data. In Fig. 5, besides other atomic transitions, four intense and persistent atomic transition lines of chromium (Cr) were identified, of which the spectral marker line at 427.5 nm (marked within a small rectangular cell) was used for the calibration and quantification. Other elements detected are copper (Cu), calcium (Ca), titanium (Ti), iron (Fe), and potassium (K). To further consolidate the presence of chromium in the synthetic hair dye and to confirm the emission wavelength at 427.5 nm, free from any spectral shift, we used pellets of different chromium concentrations
Fig. 5. Typical LIBS spectrum for hair dye in the 400–500 nm wavelength range for a time/gate delay and laser fluence of 800 ns and 24 Jcm−2 , respectively. 1640
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−Ek;z hc nz Iz ; A L g exp K BT 4πλki;z ki;z Pz k;z
(4)
where h is the Planck constant, c is the speed of light, L is the characteristic length of the plasma, Aki;z is the transitional probability, and λki;z is the transition line wavelength. The index z refers to the ionization stage of the species, K B is the Boltzmann constant, T is the plasma temperature, Ek;z and gk;z are the energy and degeneracy of the upper level k, respectively, nz is the number density, and Pz is the partition function of the species in the ionization stage z [31]. A plot of intensity as a function of concentration of the analyte yields a calibration curve [32–34] as shown in Eq. (4). If the curve is linear, then one can determine an unknown concentration of an analyte at any intensity provided the analyte concentration exists within the dynamic range of the curve and the same experimental conditions are observed. The LIBS system was calibrated in order to quantify the
concentration of chromium present in the cosmetic hair dye samples. The atomic transition line 427.5 nm of chromium (Cr) was selected as the spectral marker line for quantitative analysis because it is isolated, has no interference with other spectral lines in the spectrum, and is strongly intense compared to other spectral lines as observed in Fig. 5. In order to validate the homogeneity of the stoichiometric samples, measurements were made at different points on the sample surface. To further enhance the precision, a few laser shots were applied on the sample surface prior to the actual measurements and to eliminate the effect of laser pulse fluctuation during the analysis. The spectrometer was then adjusted to a center wavelength of 427.5 nm, and LIBS spectra for all the stoichiometric samples were obtained. Intensities corresponding to the atomic transition line 427.5 nm for each sample were recorded, and a linear calibration curve was established by plotting the intensity (a.u) as a function of chromium concentration (ppm) as illustrated in Fig. 7. Achieving the optimum experimental condition, discussed earlier, the typical LIBS spectra for all the synthetic hair dye samples were recorded in the 426–444 nm wavelength range, and they are shown in Fig. 8. Now that we have the calibration curve in hand, the intensities corresponding to the atomic transition line of Cr I at 427.5 nm were used to determine the chromium concentration in the synthetic hair dye samples. The concentrations measured by our LIBS system are in the range of 5–11 ppm, which is above 1 ppm set by the Environmental Agency and other regulatory authorities [13]. These LIBS results of the synthetic hair dye samples were confirmed using ICP spectrometry, and both measurements were in good agreement. This work clearly demonstrates that LIBS can be applied for online rapid analysis of toxic elements in cosmetic products. E.
Detection Limit
The estimation of the limit of detection is very significant for any analytical instrument. A detection limit
Fig. 8. Typical LIBS spectra showing chromium concentration variation in all the cosmetic hair dye samples.
is the lowest amount of concentration of an analyte that can be reliably detected by the analytical instrument. The calculation of the detection limits is based on the noise of the background, and in our case we define the noise of the background as the standard deviation σ B of the experimental data over a spectral range free from dynamic peaks. The limit of detection is then given as the concentration yielding a net line intensity equal to two times the standard deviation [34–36]: σ LOD 2 B ; S
where S is the slope of the calibration curve, which is the ratio of the intensity to the concentration. The limit of detection of chromium with our LIBS detection system was estimated to be 1.2 ppm, using the above equation. F.
Precision and Accuracy
Precision is the relative standard deviation (RSD) of the test results obtained under reproducibility conditions and is given by S RSD 100% ; M
Fig. 7. Calibration curve for chromium using 427.5 nm Cr spectral line.
(5)
(6)
where S is the standard deviation and M is the mean. It is important due to the fact that there may be plasma perturbations basically due to the sampling techniques involved, the sample homogeneity, and the target surface condition, which affect the level of reproducibility of the measurements by the LIBS system [36,37]. The RSD value of the measurements obtained by our LIBS system decreased with respect to the number of accumulations, but no improvement was observed after 20 accumulations. It is worth mentioning that typical values of RSD for LIBS 10 March 2014 / Vol. 53, No. 8 / APPLIED OPTICS
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Table 2.
Concentrations of Chromium Present in Cosmetic Hair Dye Samples
Concentration (ppm) Sample 1 Element
Sample 3
Wavelength (nm)
Transition
LIBS
ICP
LIBS
ICP
LIBS
ICP
LIBS Limit of Detection (ppm)
427.4
3d5 6 S4s to 3d5 6 S4p
11
9.8
9
8.1
5
4.6
1.2
Chromium
are in the range of (1%–10%) [12]. In our case the RSD value was 1.6%; hence highly repeatable and reproducible results were obtained. We adopted the following to improve S/N: first, we improved the number of data accumulations. As we know, the S/N will improve by the factor SQRT (n) with an increased number of accumulations (n). Also we cooled the ICCD sensor to -20°C to reduce the dark current of the sensor. We also reduced the noise by subtracting the noise background from the measured signals by deconvolution methods, and also using a black screen dump to absorb the scattered and reflected laser light from optics components. In statistical terms, the S/N is the reciprocal of the RSD. The estimated RSD for 11 ppm of chromium is about 0.016, and this amounts to an S/N of 65 at such a low concentration. The accuracy of an analytical system is defined as how close the measured experimental values are to the acceptable ones [36,37]. It depends on the sample composition, homogeneity, surface condition, and particle size. Therefore a sample matrix can affect the amount of material ablated and hence the intensity of the signal. Residual error was calculated between the values obtained using the LIBS system and ICP spectrometry. Both values are comparable yielding a residual error in the range of 0.12–0.40, which is acceptable for any good analytical system. 4. Conclusion
A laser induced breakdown spectrometer for detection of chromium in locally available synthetic hair dye was developed using a pulsed laser beam of 266 nm wavelength as the excitation source and a spectrometer equipped with a gated ICCD camera. The atomic transition lines were identified using spectroscopic data published by NIST, and a chromium (Cr) atomic transition spectral line of wavelength 427.5 nm was used as the marker line. Our calibrated LIBS system with a limit of detection of 1.2 ppm was able to detect chromium concentration levels in the range of 5–11 ppm. The Cr concentration detected with our LIBS system was validated using a standard method such as ICP spectrometry, and the results obtained by both methods were comparable as presented in Table 2. The results obtained by both methods indicate that the concentration of Cr present in hair dyes is above the acceptable permissible limit of 1 ppm set by the Environmental Protection Agency (EPA) and other environmental agencies [13]. 1642
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This study is part of project nos. RG1201-1 and 1201-2 funded by the Deanship of Scientific Research (DSR), King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia. The support provided by the Department of Physics and Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia, is also acknowledged. References 1. M. Denli, D. Rusen, D. Zelihagul, D. Tucer, A. Mehtap, and B. Volkan, “Effect of long term use of hair dyes on the DNA damage in health female subjects,” Med. J. Kocatepe 3, 57–62 (2002). 2. I. C. Nnorom, J. C. Igwe, and C. G. Oji-Nnorom, “Trace metal of facial (make up) cosmetics commonly used in Nigeria,” African J. Biotech. 4, 1113–1138 (2005). 3. I. Al-Saleh, S. Al-Enazi, and N. Shinwari, “Assessment of lead in cosmetic products,” Regul. Toxicol. Parmacol. 54, 105–113 (2009). 4. P. Apostoli, “Elements in environmental and occupational medicine,” J. Chromatogr. B 778, 63–97 (2002). 5. G. Saxena, G. M. Kannan, N. Saksenad, R. J. Tirpude, and S. J. S. Flora, “Lead induced oxidative stress and DNA damage using comet assay in rat blood,” J. Cell Tissue Res. 6, 763–768 (2006). 6. “Some metals and metallic compounds,” IARC Monographs on the Evaluation of Carcinogenic Risk of Chemicals to Humans, Vol. 23 (International Agency for Research on Cancer, 1990). 7. “Health assessment document for chromium,” United States Environmental Protection Agency, final report No. EPA600/ 8-83-014F (1984). 8. “Chromium, nickel and welding,” in IARC Monographs on the Evaluation of Carcinogenic Risk of Chemicals to Humans, Vol. 49 (International Agency for Research on Cancer, 1990), pp. 463–474. 9. R. Shrivastava, R. K. Upreti, P. K. Seth, and U. C. Chaturvedi, “Effects of chromium on the immune system,” FEMS Immunol. Med. Microbiol. 34, 1–7 (2002). 10. D. P. Bruyzee, G. Hennipmann, and W. G. Van Ketel, “Irritant contact dermatitis and chromium passivated metal,” Contact Dermatitis 19, 175–179 (1988). 11. H. Brieger, “Zur Klinik der akuten Chromatvergiftung,” Zeitchrift fur experimentelle pathologie und Therapie 21, 393–408 (1920). 12. G. J. Nohynek, R. Fautz, F. Benech-Kieffer, and H. Toutain, “Toxicity and human health risk of hair dyes,” Food Chem. Toxicol. 42, 517–543 (2004). 13. M. A. Gondal, Z. Seddigi, M. M. Nasr, and B. Gondal, “Spectroscopic detection of health hazardous contaminants in Lipstick using laser induced breakdown spectroscopy,” J. Hazard. Mater. 175, 726–732 (2010). 14. G. Gillian, G. Duyckaerts, and A. Disteche, “Direct and simultaneous determination Zn, Pb, Cd, Cu, Sb, Bi dissolved in sea water by differential pulse anodic stripping voltammetry with a hanging mercury drop electrode,” Ann. Chim. Acta 106, 23–27 (1979). 15. Y. Shih, J.-M. Zen, A. S. Kumar, Y.-C. Lee, and H.-R. Huang, “Determination of the toxic lead level in cosmetic hair dye formulations using a screen printed silver electrode,” Bull. Chem. Soc. Jpn. 77, 311–312 (2004).
16. T. Tande, “Simultaneous determination of Cr (III) and Cr (VI) in water by reversed phase HPLC, after chelating with sodium diethyldithiocarbamate,” Chromatographia 13, 607–610 (1980). 17. I. T. Urasa and S. H. Nam, “Direct determination of Cr (III) and Cr (VI) with ion chromatography using direct current plasma emission as element selective detector,” J. Chromatogr. Sci. 27, 30–37 (1989). 18. C. Tonetti and R. Innocenti, “Determination of heavy metals in textile materials by atomic absorption spectrometry: verification of the test method,” AUTEX Res. J. 9, 66–70 (2009). 19. T. Hussain and M. A. Gondal, “Monitoring and assessment of toxic metals in gulf war oil spill contaminated soil using laser induced breakdown spectroscopy,” Environ. Monit. Assess. 136, 391–399 (2008). 20. S. S. Harilal, C. V. Bindhu, V. P. N. Nampoori, and C. P. G. Vallabhan, “Temporal and spatial behavior of electron density and temperature in a laser produced plasma from YBa2Cu3O7,” Appl. Spectrosc. 52, 449–455 (1998). 21. P. Stavropoulos, C. Palagas, G. N. Angelopoulos, D. N. Papamantellos, and S. Couris, “Calibration measurements in laser induced breakdown spectroscopy using nanosecond and picosecond lasers,” Spectrochim. Acta B 59, 1885–1892 (2004). 22. A. P. Thorne, Spectrophysics, 2nd ed. (Chapman & Hall, 1998). 23. H. R. Griem, Principles of Plasma Spectroscopy (Cambridge University, 1997). 24. F. J. Gordillo-Vazquez, M. Camero, and C. Gomez-Alaixandre, “Spectroscopic measurements of the electron temperature in low pressure radiofrequency Ar/H2/C2H2 and Ar/H2/C2H4 plasmas used for synthesis of nanocarbon structures,” Plasma Sources Sci. Technol 15, 42–51 (2006). 25. NIST Atomic spectra database, http://www.nist.gov/physlab/ data/asd.cfm. 26. M. Milan and J. J. Laserna, “Diagnostics of silicon plasmas produced by visible nanosecond laser ablation,” Spectrochim. Acta B 56, 275–288 (2001). 27. M. Qian, C. Ren, D. Wang, J. Zhang, and G. Wei, “Stark broadening measurement of electron density in an atmospheric pressure argon plasma jet with double power electrodes,” J. Appl. Phys. 107, 063303 (2010).
28. Y. J. Hong, G. C. Kwon, G. Cho, H. M. Shin, and E. H. Choi, “Measurement of electron temperature and density using stark broadening of the coaxial focused plasma for extreme ultraviolet lithography,” IEEE Trans. Plasma Sci. 38, 1111–1117 (2010). 29. W. T. Y. Mohamed, “Study of the matrix effect on the plasma characterization of six elements in aluminum alloys using LIBS with a portable Echelle spectrometer,” Progr. Phys. 2, 42–49 (2007). 30. K. J. Grant and G. L. Paul, “Electron temperature and density profiles of excimer laser-induced plasmas,” Appl. Spectrosc. 44, 1349–1354 (1990). 31. V. K. Unnikrishnan, K. Alti, V. B. Kartha, C. Santhosh, G. P. Gupta, and B. M. Suri, “Measurement of plasma temperature and electron density in laser induced copper plasma by time resolved spectroscopy of neutral atom and ion emissions,” Pramana J. Phys. 74, 983–993 (2010).] 32. M. A. Gondal, M. A. Dastageer, A. A. Naqvi, A. A. Isab, and Y. W. Maganda, “Detection of toxic metals (lead and chromium) in talcum powder using laser induced breakdown spectroscopy,” Appl. Opt. 51, 7395–7401 (2012). 33. M. M. Naser and M. A. Gondal, “Detection of hazardous pollutants in chrome tanned leather using locally developed laser induced breakdown spectrometer,” Environ. Monit. Assess. 175, 387–395 (2011). 34. M. A. Gondal and T. Hussain, “Determination of poisonous metals in waste water collected from paint manufacturing plant using laser induced breakdown spectroscopy,” Talanta 71, 73–80 (2007). 35. M. A. Ismail, H. Imam, A. Elhassan, W. T. Younisss, and M. Harith, “LIBS limit of detection and plasma parameters of some elements in two different metallic matrices,” J. Anal. At. Spectrom. 19, 489–494 (2004). 36. T. Hussain and M. A. Gondal, “Detection of toxic metals in waste water dairy products plant using laser induced breakdown spectroscopy,” Bull. Environ. Contam. Toxicol. 80, 561–565 (2008). 37. M. A. Gondal, M. H. Shwehdi, and A. A. Khalil, “Application of LIBS for determination of ionic species (NaCl) in electrical cables for investigation of electrical breakdown,” Appl. Phys. B 105, 915–922 (2011).
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A laser induced breakdown spectroscopic (LIBS) system, consisting of a pulsed 266 nm laser radiation, in conjunction with a high-resolution spectrograph, a gated intensified charge coupled device camera, and a built-in delay generator were used to develop a sensitive detector to quantify the concentration of toxic substances such as chromium in synthetic hair dyes available on the local market. The strong atomic transition line of chromium (Cr I) at 427.5 nm wavelength was used as a fingerprint wavelength to calibrate the detection system and also to quantify the levels of chromium in the hair dye samples. The limit of detection achieved by our LIBS detection system for chromium was 1.2 ppm, which enabled us to detect chromium concentration in the range of 5–11 ppm in the commercial hair dyes available on the local market. The concentrations of chromium in the hair dyes measured using our system were validated using a standard analytical technique such as inductively coupled plasma mass spectrometry (ICPMS), and acceptable agreement (nearly 8%) was found between the results obtained by the two methods (LIBS and ICPMS). This study is highly significant for human health, specifically for people using synthetic hair dyes for changing the color of their hair. © 2014 Optical Society of America OCIS codes: (020.0020) Atomic and molecular physics; (140.3530) Lasers, neodymium; (300.6365) Spectroscopy, laser induced breakdown. http://dx.doi.org/10.1364/AO.53.001636
1. Introduction
Synthetic hair dyes are widely used cosmetic products in the modern world by people of all age groups to change their hair color and to improve their physical appearance; therefore they play an important role in shaping the quality of our lives [1]. However, heavy metals such as chromium, cadmium, and arsenic are common elements in synthetic hair dyes as color additives [2,3]. These toxic metals, exceeding the permissible safe levels, damage body organs, disrupt the nervous system, and have adverse effects on human metabolism [4]. Chromium enters the body through dermal, gastrointestinal, and inhalation 1559-128X/14/081636-08$15.00/0 © 2014 Optical Society of America 1636
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exposure. The toxicity of chromium causes mutations and chromosomal damage in human cells [3,5]. Besides being a carcinogen, prolonged exposure to chromium has adverse systemic effects such as damaging the kidney, liver, and blood forming organs [6–11]. Considering the extent and frequency of contact of the dyes with the permeable human skin, hair dyes and their ingredients penetrate into the body and therefore are deemed hazardous to humans’ health. The safe permissible limit for chromium is 1 ppm [12,13]. Hence control and easy detection of toxic metals in synthetic hair dyes is of great significance. To detect toxic metals in various cosmetic products, methods such as Hg-based stripping voltammetric analysis, the screen printed silver electrode technique, chromatography methods, and atomic absorption spectrometry, which are time
consuming, expensive, laborious, and require a lot of sample preparations, have been extensively used by researchers [14–18]. On the contrary, laser induced breakdown spectroscopy (LIBS) is a far superior technique as compared to the conventional methods. It is cost effective, requires little or no sample preparation, and enables rapid sample analysis. The basic principle of LIBS is based on the spectral analysis of radiation emitted from the plasma generated by focusing a high-power pulsed laser beam on the target surface. The characteristic emission from the plasma provides information about the elements present in the target material. The atomic emission wavelengths and their intensities are compared with standard atomic line references and calibrated against samples of known concentration to determine the chemical composition of a sample qualitatively and quantitatively [19]. In our study, we have developed a sensitive laser induced breakdown spectrometer for the detection and quantification of chromium concentration in the synthetic hair dyes available on the local market. In order to achieve the best limit of detection, a good signal-to-noise ratio (S/N) of the LIBS system should be achieved. Besides other experimental parameters, the vital parameters that affect the sensitivity of the LIBS detection system are the delay between the excitation and the detection, the sampling gate width, the laser fluence, and the sampling geometry. All these parameters were optimized in order to achieve a good S/N. The choice of these parameters was validated using the Mcwhirter criterion [20], which proved that the laser induced plasma (LIP) generated by our LIBS system was optically thin, and in local thermodynamic equilibrium (LTE), a condition which is very fundamental and must be satisfied in LIBS applications. This study will also be important to provide baseline data that will help to determine the levels of chromium toxicity of synthetic hair dyes. 2. Experimental Methods A.
LIBS System
A pulsed fourth-harmonic Nd:YAG laser (Spectra physics, Model GCR100) of wavelength 266 nm operating in Q-switch mode and delivering a maximum pulse energy of 50 mJ with a pulsewidth of 8 ns at a 20 HZ repetition rate was used as the excitation source. A convex lens of focal length 20 mm was used to focus the collimated laser beam onto the surface of the sample to create the plasma spark. In order to minimize the pitting on the sample and the consequent signal reduction (due to change of the focal point of the laser beam), the sample was placed on the XY translator and moved at a constant speed during laser ablation. A fiber optic cable with a miniature lens was positioned at an appropriate distance from the focal volume of the plasma with an optimum 45° with respect to the sample surface in order to collect the light emitted by the plasma spark.
The plasma light was then directed to the spectrometer (1200 line∕mm grove density), which was connected to an intensified charge coupled device (ICCD, Andor-50SI) for processing. The entire detection system is interfaced with a computer having software that generates the optical emission spectrum. A slit width of 100 μm, gate width 200 μs, number of accumulations 20, and exposure time of 4 s were used as the spectrometer parameters. In the study of emission intensity as a function of laser fluence, laser energy for a single pulse of spot diameter 0.1 mm was measured using an energy meter (Ophir model 300). B. Sample Preparation
Three hair dye samples of different quality, brand, and price range were procured from different cosmetic shops within the kingdom of Saudi Arabia. These samples were in powder form, which blows out of the sample holder when a high-power pulsed laser beam is incident on it, and therefore it is not trivial to analyze using the LIBS technique. Hence a special form of preparation to transform the fine powder into solid compact pellets was required for easy and effective analysis. The base matrix (fine powder) was mixed thoroughly with potassium bromide (KBr) as the binding material in appropriate ratios as per the standard procedure and grinded using agate pestle and motor thoroughly to ensure the homogeneity of the mixture. The homogeneous mixture was then compacted using a 10 bar pellet press having a stainless steel cylindrical die with a diameter of 20 mm and thickness of 2 mm. To avoid moisture, humidity effects, and any form of contamination, the pellets were wrapped in a clean aluminum foil and safely stored in a desiccator ready for analysis. To calibrate the LIBS system, solid pellet stoichiometric samples with different concentrations of chromium (Cr) in parts per million (ppm) of 40, 60, 80, and 100 ppm were prepared by homogeneously mixing chromium (II) sulfate with the base matrix (synthetic hair dye powder). In order to further confirm the measurements obtained using our LIBS system, inductively coupled plasma mass spectrometry (ICPMS), a conventional technique, was also used. In this case each powder sample was digested by the addition of nitric acid (HNO3 ) and hydrogen peroxide (H2 O2 ). The resultant solution was refluxed at 95°C, for 5 h, and the exothermic reaction was allowed to complete. The solution was then diluted to a desirable final volume and analyzed using an inductively coupled plasma mass spectrometer. 3. Results and Discussion A. Optimization of the Time/Gate Delay for Sensitive Detection
At the early stages of plasma formation (below 400 ns after excitation) the typical LIBS spectrum is basically a continuum (background noise), which is due to blackbody radiation of the plasma and elastic 10 March 2014 / Vol. 53, No. 8 / APPLIED OPTICS
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Fig. 1. LIBS signal intensity of 427.5 nm Cr spectral line as a function of time/gate delay.
collisions of electrons with the ionic species (Bremsstralung). Concurrently, broadened ionic lines and weak atomic lines are superimposed on the continuum and often overlap. After some appropriate time, the plasma expands and cools down, resulting in the emission of well-defined characteristic atomic transition lines of the neutral and ionized elements present in the sample [20]. Hence the delay between the laser pulse trigger and acquisition of the spectrum has to be optimized in order to minimize the background noise and maximize the intensity of the emission line of interest. In our study, the intensities of the spectral marker line (Cr 427.5 nm) were obtained for different time/gate delay times in the range of 600–1200 ns. From Fig. 1, the intensity increases and reaches a maximum at 800 ns and then drops. Hence 800 ns was selected as the optimum time/gate delay. It is worth noting that the optimum time/gate delay depends on the transitional probability and the lifetime of the upper level of the analytical spectral line [21]. Therefore different elements have different intensity temporal evolutions. B.
Optimization of the Excitation Source
The laser fluence is an important parameter in plasma generation and plays a vital role in the sensitivity of the LIBS system due to the fact that it is proportional to the emission intensity of an analyte when the plasma is optically thin. The emission intensity as a function of laser fluence for the spectral marker line (Cr 427.5 nm) using the optimal time/ gate delay (time between the laser trigger and acquisition of the spectrum) of 800 ns was studied in order to optimize the LIBS signal intensity and is depicted in Fig. 2. From Fig. 2, the intensity initially increases linearly with the incident laser fluence. This phenomenon is attributed to the increase in the amount of ablated material and to the increase in the electron temperature. For laser fluence values higher than 24 Jcm−2 , the emission intensity reaches saturation, mainly due to the absorption of the laser beam by the plasma formed in front of the target of the sample, a process known as plasma shielding 1638
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Fig. 2. LIBS signal intensity of 427.5 nm Cr spectral line as a function of laser fluence.
[20]. Also self-absorption can be used to account for this effect. Hence, in our case 24 Jcm−2 is the optimum laser fluence for our LIBS system. It is worth noting that the most prominent mechanism responsible for the plasma absorption at such high laser fluence is inverse Bremsstralung, whereby a free electron absorbs a laser photon [20]. However, the saturation can also be explained by assuming the formation of a self-regulating regime near the target surface at such higher laser fluence levels [22]. At low plasma temperature, the absorption of laser photons by the plasma becomes high, resulting in less sample ablation and a consequent decrease in the density of the ionic species. This behavior as a result increases the absorption of the laser photons by the plasma, hence increasing the temperature of the plasma. On the other hand, when the absorption of the laser energy is less, the process is reversed. It has also been theoretically proven that the density and temperature of the plume adjust in such a manner that the plasma absorbs the same amount of laser radiation to maintain a selfregulating regime [20]. C.
Local Thermodynamic Equilibrium Condition
In order to calibrate a LIBS system using spectral line intensities, the LIP should be optically thin (re-absorption and absorption of the incident radiation by the plasma is negligible) and in LTE. In a transient system such as plasma produced in LIBS, the LTE condition holds if the free electrons in the plasma have a Maxwellian distribution. It is worth mentioning that the electron velocity distribution for a relatively dense plasma with a low temperature (ne > 1016 cm−3 , kT < 5 eV) is nearly always Maxwellian [22]. Also, collision, excitation, and de-excitation processes should dominate over the radiative processes for the LTE condition to be valid. Along the boundaries of the plasma, number densities are low and movement is very rapid; therefore LTE is not a good assumption. However, deeper into the plasma volume where conditions change more
slowly and collisions occur more frequently, this assumption is valid [20]. Clearly, for LTE to hold, the electron number density must be sufficiently high, and hence it is worthwhile to check the minimum density condition for LTE using the Mcwhirter criterion [20] as given in Eq. (1): ne ≥ 1.4 × 1014 T 1∕2 ΔE3 cm−3 ;
(1)
where ne is the critical electron density and its dependence on the largest energy gap (ΔE) between two adjacent levels of the considered species, i.e., the energy of the shortest wavelength transition used in the temperature determination. T is the plasma temperature as mentioned before. In our study plasma temperature (T) and electron density (ne ) were explicitly determined using the Boltzmann plot and Stark broadening, respectively. For optically thin plasma in LTE, its temperature is obtained using Eq. (2): ki Iα λ E Cα F ; ln − k ln K BT gk Aki U α T α
(2)
where K B is the Boltzmann constant, Uα (T) is the partition function, Aki is the transitional probability, g is the statistical weight for the upper level, Ek is the excited level energy, T is the plasma temperature, and F is a constant depending on experimental conditions. A plot of magnitude of the component on the left side of Eq. (2) as a function of the value for the upper energy levels (Ek ) for the spectral lines under consideration yields a Boltzmann plot that is used to determine the electron temperature. The main sources of error when using Eq. (2) arise from using inaccurate values of Aki, imprecision in the recorded intensities or choosing transitions having upper levels with a small energy difference [22,23]. However, the use of logarithmic relations significantly reduces the influence of error. For instance, an error of 20% in the argument I ki λki ∕gk Aki reduces to about 5% when calculating the logarithm of such as an argument [24]. In our experiment the electron temperature was estimated using the atomic emission line intensities of chromium (Cr) observed in the LIP. It is worth noting that the spectral lines selected were in close spectral proximity, well resolved, strongly intense, and with well-known transitional probabilities and upper energy levels. The required parameters for the Boltzmann plot as obtained from the NIST database and Griem [23,25] are summarized in Table 1. Figure 3 shows the Boltzmann plot on which Table 1.
Spectroscopic Data for Chromium Spectral Lines [24]
Wavelength (nm)
gk
Aik s−1
Ek ev
425.433 427.481 428.973 460.742
9 7 5 7
3.15E 07 3.07E 06 3.16E 07 2.5E 06
2.913 2.899 2.889 3.698
Fig. 3. Boltzmann plot for plasma temperature determination.
data were fitted with the least-square approximation, and the slope of the plotted curve yielded a plasma temperature (T) of 5286 850 K. In a LIP, a spectral line is broadened due to Stark, Doppler, instrumental, and natural broadening mechanisms. In our case instrumental broadening was minimized by setting the detection system at its maximum resolution. It is worth mentioning that in a low-temperature, high-density plasma generated in our study, the Stark broadening is dominant and therefore was used to estimate the electron density by determining the full width at half-maximum (FWHM) of the broadened profile for the chromium atomic transition spectral line (Cr 427.5 nm). Stark broadening is due to collisions of the electrons with charged species resulting in both broadening of the line and shifting in the peak wavelength [26–30]. A Stark broadened profile is described by a Lorentzian function, and Eq. (3) relates its FWHM with the electron density: Δλ1∕2 2W
ne ; 1016
(3)
where W is the impact parameter ne is the electron density, and Δλ1∕2 is the FWHM. To estimate the electron density of the plasma, the line profile for chromium atomic spectral transition at 427.5 nm was used as shown in Fig. 4. The data points were fitted with a Lorentzian fit using origin8.0 software to yield a profile with a FWHM of 0.412 nm. The electron impact parameter w is obtained from Griem [23], and therefore the electron density is 1.68 × 1018 cm−3 . In our case the electron density determined using a gate/time delay and laser fluence of 800 ns and 24 Jcm−2 , respectively, as the optimal values is 1.68 × 1018 cm−3 , implying that ne ∼ 1018 cm−3 and kT ∼ 0.56 eV, which explicitly justifies that the free electron velocity is Maxwellian. Also the estimated minimum electron density is 1.98 × 1017 cm−3 , which is lower than the actual electron density; hence the plasma generated by our LIBS system is optically thin and in LTE. 10 March 2014 / Vol. 53, No. 8 / APPLIED OPTICS
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Fig. 4. Lorenztian fit for spectral line 427.5 nm for time/gate delay and laser fluence of 800 ns and 24 Jcm−2 , respectively.
Fig. 6. Intensity as a function of wavelength for different chromium concentrations.
D.
(in ppm) by adding known concentrations of chromium sulfate in the base matrix (synthetic hair dye) and analyzing them using the LIBS system. The spectrometer was adjusted to a center wavelength of 427.5 nm, and the spectra were recorded for different chromium concentration levels. It was observed that the intensity of the spectral line at 427.5 nm grows with the increase of chromium concentration at the same spectral position without any shift, and the effect of concentration on the Cr-I 427.5 nm line is shown in Fig. 6. This result strongly confirms the presence of chromium (Cr) in the synthetic hair dye. In LIBS, atomic emission intensity of a spectral line is used for analysis and quantification of a particular element in any given sample. The integrated intensity I z of a spectral line occurring between the upper energy level k and the lower energy level i for a species in the ionization stage z in optically thin plasma and in LTE is given as
Detection of Chromium in Synthetic Hair Dye Samples
Besides achieving the LTE for optically thin plasma, the detection system was optimized with respect to the temporal parameters, laser fluence, and other experimental parameters such as optical alignment and sample positioning. These optimized conditions ensured the best S/N, and a typical LIBS spectrum of the synthetic hair dye sample #1 in the 400–500 nm wavelength range is depicted in Fig. 5. The identification of the atomic transition lines in Fig. 5 is carried out using the National Institute of Standards and Technology (NIST) spectral data. In Fig. 5, besides other atomic transitions, four intense and persistent atomic transition lines of chromium (Cr) were identified, of which the spectral marker line at 427.5 nm (marked within a small rectangular cell) was used for the calibration and quantification. Other elements detected are copper (Cu), calcium (Ca), titanium (Ti), iron (Fe), and potassium (K). To further consolidate the presence of chromium in the synthetic hair dye and to confirm the emission wavelength at 427.5 nm, free from any spectral shift, we used pellets of different chromium concentrations
Fig. 5. Typical LIBS spectrum for hair dye in the 400–500 nm wavelength range for a time/gate delay and laser fluence of 800 ns and 24 Jcm−2 , respectively. 1640
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−Ek;z hc nz Iz ; A L g exp K BT 4πλki;z ki;z Pz k;z
(4)
where h is the Planck constant, c is the speed of light, L is the characteristic length of the plasma, Aki;z is the transitional probability, and λki;z is the transition line wavelength. The index z refers to the ionization stage of the species, K B is the Boltzmann constant, T is the plasma temperature, Ek;z and gk;z are the energy and degeneracy of the upper level k, respectively, nz is the number density, and Pz is the partition function of the species in the ionization stage z [31]. A plot of intensity as a function of concentration of the analyte yields a calibration curve [32–34] as shown in Eq. (4). If the curve is linear, then one can determine an unknown concentration of an analyte at any intensity provided the analyte concentration exists within the dynamic range of the curve and the same experimental conditions are observed. The LIBS system was calibrated in order to quantify the
concentration of chromium present in the cosmetic hair dye samples. The atomic transition line 427.5 nm of chromium (Cr) was selected as the spectral marker line for quantitative analysis because it is isolated, has no interference with other spectral lines in the spectrum, and is strongly intense compared to other spectral lines as observed in Fig. 5. In order to validate the homogeneity of the stoichiometric samples, measurements were made at different points on the sample surface. To further enhance the precision, a few laser shots were applied on the sample surface prior to the actual measurements and to eliminate the effect of laser pulse fluctuation during the analysis. The spectrometer was then adjusted to a center wavelength of 427.5 nm, and LIBS spectra for all the stoichiometric samples were obtained. Intensities corresponding to the atomic transition line 427.5 nm for each sample were recorded, and a linear calibration curve was established by plotting the intensity (a.u) as a function of chromium concentration (ppm) as illustrated in Fig. 7. Achieving the optimum experimental condition, discussed earlier, the typical LIBS spectra for all the synthetic hair dye samples were recorded in the 426–444 nm wavelength range, and they are shown in Fig. 8. Now that we have the calibration curve in hand, the intensities corresponding to the atomic transition line of Cr I at 427.5 nm were used to determine the chromium concentration in the synthetic hair dye samples. The concentrations measured by our LIBS system are in the range of 5–11 ppm, which is above 1 ppm set by the Environmental Agency and other regulatory authorities [13]. These LIBS results of the synthetic hair dye samples were confirmed using ICP spectrometry, and both measurements were in good agreement. This work clearly demonstrates that LIBS can be applied for online rapid analysis of toxic elements in cosmetic products. E.
Detection Limit
The estimation of the limit of detection is very significant for any analytical instrument. A detection limit
Fig. 8. Typical LIBS spectra showing chromium concentration variation in all the cosmetic hair dye samples.
is the lowest amount of concentration of an analyte that can be reliably detected by the analytical instrument. The calculation of the detection limits is based on the noise of the background, and in our case we define the noise of the background as the standard deviation σ B of the experimental data over a spectral range free from dynamic peaks. The limit of detection is then given as the concentration yielding a net line intensity equal to two times the standard deviation [34–36]: σ LOD 2 B ; S
where S is the slope of the calibration curve, which is the ratio of the intensity to the concentration. The limit of detection of chromium with our LIBS detection system was estimated to be 1.2 ppm, using the above equation. F.
Precision and Accuracy
Precision is the relative standard deviation (RSD) of the test results obtained under reproducibility conditions and is given by S RSD 100% ; M
Fig. 7. Calibration curve for chromium using 427.5 nm Cr spectral line.
(5)
(6)
where S is the standard deviation and M is the mean. It is important due to the fact that there may be plasma perturbations basically due to the sampling techniques involved, the sample homogeneity, and the target surface condition, which affect the level of reproducibility of the measurements by the LIBS system [36,37]. The RSD value of the measurements obtained by our LIBS system decreased with respect to the number of accumulations, but no improvement was observed after 20 accumulations. It is worth mentioning that typical values of RSD for LIBS 10 March 2014 / Vol. 53, No. 8 / APPLIED OPTICS
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Table 2.
Concentrations of Chromium Present in Cosmetic Hair Dye Samples
Concentration (ppm) Sample 1 Element
Sample 3
Wavelength (nm)
Transition
LIBS
ICP
LIBS
ICP
LIBS
ICP
LIBS Limit of Detection (ppm)
427.4
3d5 6 S4s to 3d5 6 S4p
11
9.8
9
8.1
5
4.6
1.2
Chromium
are in the range of (1%–10%) [12]. In our case the RSD value was 1.6%; hence highly repeatable and reproducible results were obtained. We adopted the following to improve S/N: first, we improved the number of data accumulations. As we know, the S/N will improve by the factor SQRT (n) with an increased number of accumulations (n). Also we cooled the ICCD sensor to -20°C to reduce the dark current of the sensor. We also reduced the noise by subtracting the noise background from the measured signals by deconvolution methods, and also using a black screen dump to absorb the scattered and reflected laser light from optics components. In statistical terms, the S/N is the reciprocal of the RSD. The estimated RSD for 11 ppm of chromium is about 0.016, and this amounts to an S/N of 65 at such a low concentration. The accuracy of an analytical system is defined as how close the measured experimental values are to the acceptable ones [36,37]. It depends on the sample composition, homogeneity, surface condition, and particle size. Therefore a sample matrix can affect the amount of material ablated and hence the intensity of the signal. Residual error was calculated between the values obtained using the LIBS system and ICP spectrometry. Both values are comparable yielding a residual error in the range of 0.12–0.40, which is acceptable for any good analytical system. 4. Conclusion
A laser induced breakdown spectrometer for detection of chromium in locally available synthetic hair dye was developed using a pulsed laser beam of 266 nm wavelength as the excitation source and a spectrometer equipped with a gated ICCD camera. The atomic transition lines were identified using spectroscopic data published by NIST, and a chromium (Cr) atomic transition spectral line of wavelength 427.5 nm was used as the marker line. Our calibrated LIBS system with a limit of detection of 1.2 ppm was able to detect chromium concentration levels in the range of 5–11 ppm. The Cr concentration detected with our LIBS system was validated using a standard method such as ICP spectrometry, and the results obtained by both methods were comparable as presented in Table 2. The results obtained by both methods indicate that the concentration of Cr present in hair dyes is above the acceptable permissible limit of 1 ppm set by the Environmental Protection Agency (EPA) and other environmental agencies [13]. 1642
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