ESR AND TL STUDIES OF IRRADIATED ANATOLIAN LAUREL LEAF (Laurus Nobilis L. ) Semra Tepe Çam1,*, Canan Aydaş1, Birol Engin2, Ülkü Rabia Yüce1, Talat Aydın1,
Mustafa Polat3
1Turkish Atomic Energy Agency, Sarayköy Nuclear Research and Training Center, 06983 Saray, Ankara, Turkey
2Dokuz Eylül University, Faculty of Science, Physics Department,35160 Buca, İzmir,Turkey
3Hacettepe University, Physics Engineering Department, Beytepe, Ankara, Turkey
*Corresponding author. Tel.:+90 312 8101500/1711; fax:+90 312 8154307
E-mail address: stepe06@gmail.com (S.Tepe Çam)
Abstract
Laurel leaf (Laurus nobilis L.) samples originated from Turkey were analyzed by Electron Spin Resonance (ESR) and Thermoluminescence (TL) techniques before and after gamma irradiation. Unirradiated (control) loreal leaf samples exhibit a weak ESR singlet centered at g = 2.0020. Besides this central signal, two weak satellite signals situated about 3 mT left and right to it in radiation-induced spectra. The dose-response curve of the radiation-radiation-induced ESR signal at g = 2.0187 (the left satellite signal) was found to be described well by a power function.Variation of the left satellite ESR signal intensity of irradiated samples at room temperature with time in a long-term showed that cellulosic free radicals responsible from the ESR spectrum of loreal leaves were not stable but detectable still after 100 days. Annealing studies at four different temperatures were used to determine the kinetic behaviour and activation energy of the radiation-induced cellulosic free radicals responsible from the left satellite signal (g = 2.0187) in laurel leaves. TL measurements of the polymineral dust isolated from the laurel leaf samples allowed distinguishing between irradiated and unirradiated samples.
1. Introduction
Electron Spin Resonance (ESR) is a spectroscopic method that detects unpaired electrons (e.g. free radicals). For the detection of irradiated food samples, ESR method is one of the most successfully applied physical methods (Raffi and Stocker, 1996;Douifi et al., 1998 and references therein). It has become increasingly popular all over the world because of its nondestructive nature, specificity, rapidity, and simplicity to detect radicals in irradiated foods. ESR has been utilized to detect the presence of radiation-induced free radicals in dry plant samples containing cellulose for a long time (Raffi and Agnel, 1989,Polo’nia et al., 1995; Desrosiers et al.,1996; Yordanov et al., 1998; Raffi et al., 2000; Korkmaz and Polat, 2001; Delincée and Soika, 2002; Bayram and Delincée, 2004; Polovka et al., 2006; Yordanov et al., 2009). Besides foods containing crystalline sugar (EN 13708, 2001) and bone (EN 1786,1997), this technique has also been accepted as a standard method (EN Protocol EN 1787, 2000) for irradiated foods containing cellulose in the EU community.
Identification of irradiated plant products containing cellulose is based on the detection of a characteristic pair of satellite ESR lines found at the symmetric positions on both sides of the single non-specific central ESR signal due to gamma induced cellulose free radicals in studies mentioned above. The spacing of this radiation-induced signal pair is about 6 mT. In some cases only single ESR signal can be observed after irradiation (Raffi et al., 2000; Yordanov and Gancheva, 2000;Engin et al., 2011). This signal can not be distinguished from the naturally present single signal even in Q-band spectrometry (Yordanov and Aleksieva, 2004). In this case, recently, a new approach, based on thermal treatment and ESR saturation have been used for detection of previous radiation treatment (Yordanov and Gancheva,2000; Yordanov et al., 2005; Engin et al., 2011).In addition to cellulose-like signals, carbohydrate free radical signals were also observed in some irradiated dry plants (Franco et al., 2004; Yordanov et al., 2009; Aleksieva et al., 2011).
In case ESR could not give useful information about the radiation history of the dry plant sample, other techniques such as thermoluminescence (TL) analysis have to be used. TL is an analysis method that meaures accumulated radiation dose on the some materials containing crystalline minerals by trapped charge carriers following irradiation. TL method has been tested for detection of dry plant samples containing silicate minerals (quartz, feldspar, etc.) by several authors and was adopted as a European Standard method for detecting irradiated foods from which silicate minerals can be isolated (Yordanov et al., 1998; Raffi et al., 2000; EN1788, 2001; Bayram and Delincée, 2004;Engin, 2007; Correcher and Garcia-Guinea, 2011).
A laurel leaf is a leaf from the laurel tree, known formally as Laurus nobilis L. These leaves have been utilized by humans for thousands of years, and they have a wide range of uses.The EN 1787 and EN1788 standards cover spices, herbs, shells and stones of the foods. In this category, Laurel leaf,
advantage in traditional herbal medicine (Polovka and Suhaj, 2010). The laurel leaf is also used as a spice in flavoring of soups, stews, meat, fish and sauces. So far, in the literature, only antioxidant properties of extracts prepared from unirradiated ground laurel leaves as well as from those gamma irradiated were studied by ESR spectroscopy for γ-irradiation detection (Polovka and Suhaj, 2010).
As a continuation of our earlier started dry plant investigations (Çam and Engin, 2010), this study presents the examination of unirradiated and gamma irradiated laurel leaves by ESR and TL techniques. The aim of the present work is, (i) to investigate the feasibility of the application of EN 1787 standard to loreal leaf, (ii) to construct ESR dose-response curve of radiation-induced signals, (iii) to investigate decay characteristics of radiation-induced ESR signals at room and high temperatures and lastly (iv) to investigate the feasibility of the application of EN 1788 standard to loreal leaf. Based on EN 1787 and 1788 standards, the present paper reports the first results obtained by the ESR and TL techniques on Turkish laurel leaf (Laurus Nobilis L.).
2. Experimental
2.1. ESR measurements
The loreal leaf samples were provided from the local markets in Ankara (Turkey) and they had not been irradiated. All samples originated from Turkey (Mediterranean region). The leaves were broken into small pieces suitable for inserting in the ESR sample tubes. Their water content was determined by thermogravimetric measurements and found to be less than 10 % by weight. Loreal leaves were sealed in polyethylene bags and irradiated with 60Co gamma rays at ambient conditions using gamma
cell with a dose rate of 1 kGy/h at the Sarayköy Establishment of the Turkish Atomic Energy Authority (TAEA) in Ankara. The radiation doses were 0.5, 1, 2, 3, 4, 5, 7 and 10 kGy in order to get dose-response curve. The uncertainty in radiation doses was nearly ±3%. The absorbed dose at the sample location was checked by Fricke chemical dosimeter. Unirradiated loreal leaf (control) samples were also prepared in the same way for comparison purposes.The irradiated and unirradiated (control) samples were stored in the dark in the normal laboratory conditions ( about 21 ± 2°C and 25 ± 3% relative humidity) until further use. All ESR measurements were performed at least 24 h after irradiation in order to avoid any interference by the radiation-induced short-lived free radicals. Unirradiated (control) and irradiated loreal leaf samples were placed in standard pyrex ESR tubes with inner diameter 4.0 mm not exhibiting any ESR signal, and ESR measurements were performed on samples open to air at room temperatute using a Bruker e-scan and Bruker EMX131 X-band spectrometers . The sample weight was ~ 200 mg for all the ESR measurements and all the samples were positioned in exactly the same manner in the cavity to avoid any errors in the g factor and signal intensities arising from changes in the cavity-filling factor. The spectrometer parameters used were
central field 348.5 mT, modulation amplitude 0.29 mT, microwave power 0.6 mW and scan range 10 mT. Each sample was measured three times and the average of three such measurements was used for each data point.Thus, the maximum experimental error was estimated to be about ± 4 %. The strong pitch (g = 2.0028) was used as a standard sample for measuring g-factor. During this work, the evolutions of the radiation-induced ESR signal with the applied microwave power, received radiation dose, storage time and temperature were followed from recorded experimental spectra by measuring the peak-to-peak height of characteristic resonance peak.The intensity of the radiation-induced ESR signal is reported in arbitrary units (a.u.).
A long-term decay feature of the radiation-induced resonance signal at room temperature was performed over a period of 100 days using a sample irradiated at a dose of 10 kGy. The kinetic feature of the radiation-induced resonance signal at high temperatures (313, 333, 353 and 373 K) was also investigated by using the samples irradiated at a dose of 5 kGy. After irradiation process, the laurel leaves were transferred to the water baths at temperatures mentioned above, then their ESR spectra were recorded regularly over a time interval of 0-80 min after cooling them to room temperature following predetermined heating times. All spectral evaluations were performed by comparing the results with those obtained from spectra recorded at room temperature before heating. The activation energy value of the radiation-induced radical specie was calculated from Arrhenius plot.
2.2. TL measurements
The laurel leaf samples (~300 g) were packed as whole in polyethylene pouches and sealed under air atmosphere for irradiation at ambient temperature. Irradiation was carried out with 60Co gamma source
(dose rate 1 kGy/h). All samples were stored under normal laboratory conditions (in dark) before and after irradiation. For TL analysis, adequate amounts of inorganic dust (silicate minerals) need to be isolated from the foods. Sufficient mineral grains could be isolated using a density separation step applying high density sodium polytungstate solution according to EN 1788 (2001) European Standard for investigation of loreal leaf samples whether unirradiated (control) or irradiated. The silicate weight used for all the TL measurements was 5 mg. No special care was used to obtain dust grains in unique size.
The first TL glow curves (TL1) for polyminerals isolated from irradiated and unirradiated (control) laurel leaf samples were recorded. The already measured minerals were re-irradiated at 1 kGy radiation dose for the purpose of normalization and the second TL glow curves (TL2) were obtained. TL measurements were carried out using a Risø TL/OSL reader (TL/OSL-DA-20) in the 70-500°C range with a heating rate of 6°C/s under continuous nitrogen flux to reduce spurious TL signals. As typical, the sample preparation and TL recording procedures take circa, 72 h.
3. Results and discussion
3.1. ESR study
3.1.1. Room temperature ESR spectra of unirradiated (control) and γ- irradiated laurel leaf
Unirradiated (control) loreal leaf samples exhibit a weak ESR singlet centered at g = 2.0020 ± 0.0003 with peak-to-peak line width (ΔHpp) of 0.89 ± 0.02 mT (Fig.1a). Irradiation produces a significant
increase in this single signal intensity of samples.However, any change was not observed in g factor and peak-to-peak line width. This singlet was observed in various foods, and is attributed to the semiquinone-like free radicals produced by the oxidation of polyphenolic compounds present in plants (Raffi and Agnel, 1989; Polovka et al., 2006).The stable free radicals were always detectable in the unirradiated (control) loreal leaf samples. Two weak satellite peaks on the sides of the main single ESR resonance signal with a separation of ~6.08 mT were also observed at g = 2.0187 and 1.9840 for loreal leaf samples irradiated at a dose of 5 kGy (Fig.1b). These signals originating from a triplet spectrum (hyperfine splitting a = 3 ± 0.1 mT) whose central line is overlapped on the initial central line are attributed to the cellulose-derived radical (Raffi and Agnel, 1989; Raffi et al., 2000; Yordanov et al., 2009; Çam and Engin, 2010). In Fig.1b, the two cellulose satellite lines spacing ~6.08 mT which shown with arrows are clearly visible. Their appearance is considered to be unambiguous evidence of the radiation treatment of the sample under investigation and is used for irradiated food detection in the EN 1787 standart. Therefore, the left satellite ESR signal (at g = 2.0187) intensity was adopted throughout this work to follow the evolution of ESR spectra as a function of microwave power, absorbed dose, temperature and time. A second intense singlet must be present in irradiated loreal leaves in order to explain the relatively high signal intensity increases of the main ESR singlet with irradiation, as also pointed out by Raffi et al.(2000) and Yordanov et al.(2009).However, the origin of this strong signal is unknown. It may be attributed to radiation-induced free radicals of phenols, quinones, etc., present in the plant (Yordanov et al., 2009).
On the other hand, three weak additional lines were also observed in the ESR spectrum of 10 kGy gamma irradiated loreal leaf samples.These lines marked with asterisks in Fig.1c.They are attributed to the carbohydrate-derived radicals (Yordanov et al., 2009). According to Yordanov et al.(2009), these signals may also be taken into consideration when previous radiation processing must be identified by the method of ESR spectroscopy.
3.1.2. Variations of the peak height with microwave power
Variations of the signal intensities of unirradiated(control) (at g = 2.0020) and irradiated (at g = 2.0187) samples with microwave power at room temperature were studied first in the range of 0.01-25 mW. The results are given in Fig.2. All measured intensities experienced continuous increases up to nearly 4mW then they started to saturate. Above 4mW, while the signal intensities (at g = 2.0187) of irradiated samples experienced similar decreases, the control signal (at g = 2.0020) showed a measurable increase (Fig.2). These differences in microwave saturation features of the signal intensities of unirradiated (control) and irradiated samples indicated the difference between the origins of natural (control) and radiation-induced radicals. Basing on the results given in Fig.2, 0.6 mW was adopted as microwave power in the rest of the work to avoid any saturation effect on measured intensities.
3.1.3. Dose-response curve
Laurel leaf samples irradiated with doses of 0.5, 1, 2, 3, 4, 5, 7 and 10 kGy were used to construct the dose-response curve. Variations of the cellulose-derived radical signal intensity at g = 2.0187 of the laurel leaf with the absorbed radiation dose are given in Fig.3.As it is seen from this figure, the signal strength increases with applied dose. The g factors and line width did not change in the studied dose range (0.5-10 kGy). Some functions were used to describe the variation of ESR signal intensity with the absorbed radiation dose and the best correlation was obtained for the power function ( I = aDb).In
this function, I and D stand for the ESR signal intensity and absorbed radiation dose in kGy, respectively, and a and b represent the rate of radical production and/or radiation yield upon irradiation. The constants a and b were calculated as 0.301 and 0.664 respectively, with a correlation coefficient of r2 = 0.9884 from fitting procedure. Theoretical dose response curve relevant to the
measured peak intensities was also calculated using parameters values given above. It is also represented in Fig.3 as dashed line with its corresponding experimental counterparts. It is seen that the agreement between experimental and theoretical data is fairly good.
3.1.4. Fading study and annealing at high temperatures
Fading study was performed at room temperature (in the dark) in a storage period of 100 days by using a loreal leaf sample irradiated at a radiation dose of 10 kGy. The ESR spectra of the sample were recorded in regular time intervals over the storage period. The left satellite ESR signal intensity (at g = 2.0187) variation at room temperature with storage time was given in Fig.4. It can be seen that the signal intensity decreases rapidly in the first 10 days 24 h after the irradiation and tends to be stable
until the end of the storage period. A decrease of ~ 92% was observed at the end of the storage period. At that period the remaining signal intensity was approximately eight times of the unirradiated spectra noise level. The stability of this remaining signal determines the potential of the ESR technique for detection of irradiated loreal leaf samples. The experimental room temperature decay data can be explained best by the sum of two first-order decay functions [I = I0Aexp(-kAt) + I0Bexp(-kBt)]. I and I0A
+ I0B represent signal intensity at any time (t) after irradiation and at zero, respectively, and kA,B
represent reaction rate constants. The reaction rate constants kA and kB were calculated as 0.845 and
0.020 day-1, respectively, from fitting procedure. This showed that the decay in ESR signal intensity at
room temperature followed two reaction steps having first-order kinetic. The first reaction (kA) has a
high and the second one (kB) has a slow reaction rate constant. In other words, the free radical species
responsible from the first reaction are unstable compared to the species responsible from second reaction at normal laboratory conditions ( about 21 ± 2°C, 25 ± 3 % humidity). The theoretical decay curve calculated using the parameter values given above was also presented in Fig.4 as dashed line. The g factor and line width of the ESR signal at g = 2.0187 did not change during the storage period. It must be emphasized here that the changes in the ESR signal intensity with storage time were observed when the samples were stored at normal laboratory conditions in the dark without any special conditioning before and after irradiation.
The decrease in signal intensity at high temperatures would be expected to originate from decay of the magnetic units contributing to the ESR spectrum of irradiated loreal leaf samples. To get deep insight into the decay process at high temperatures, loreal leaf samples irradiated at a dose of 5 kGy were kept in water baths at temperatures of 313, 333, 353 and 373 K for predetermined times between 1 and 80 min. After the annealing times at each temperature, the samples were left to cool down to room temperature and then, their ESR spectra were recorded at room temperature. The peak-to-peak height of the left satellite ESR signal (at g = 2.0187) was determined at each annealing time for each temperature. Variations of signal intensity vs. to the time at different annealing temperatures were given in Fig.5. As it is expected, the decrease in signal intensity increases with increasing annealing temperature. As the long-term decay at room temperature, the signal intensity decreased during the annealing times, which were best fitted to the sum of two first-order decay functions for each isotherm. The theoretical decay curves calculated by using the reaction rate constant values obtained from fitting procedure were also given in Fig. 5 as dashed lines. As seen from this figure, the agreement between the experimental and calculated decay data was fairly well. The reaction rate constant (k) is expected to exhibit an exponential dependence on the temperature of the type [ k(T) = k0exp(-E/RT)], where E is the activation energy, R is the gas constant, k0 is the frequency factor and T
is the absolute temperature ( Masterton and Slowinski,1969). If so, the ln (k) versus 1/T plot should give a straight line whose slope is proportional to the activation energy. The reaction activation energy values of EA = 27.3 ± 4.9 (for kA) and EB = 42.1 ± 3.6 kJ/mol (for kB) were calculated from these plots
(Fig.6) for the two cellulose-derivative radical signal components, responsible from the left satellite ESR signal ( at g = 2.0187) of irradiated loreal leaf samples. Similar activation energy values were also obtained for another dry plant samples ( Polat and Korkmaz,2008; Çam and Engin, 2010).
3.2. TL study
TL analysis has been done in accordance with the EN 1788 (2001). According to EN 1788 (2001), TL glow curves (TL1) of the irradiated polymineral samples exhibit a peak between 150 and 250 °C, whereas in unirradiated samples (control) low level natural radioactivity causes TL signal (TL1) above 300 °C.As shown in Fig.7a and b, these criteria were fulfilled for unirradiated and irradiated (5 kGy) laurel leaf samples. Also, as seen in Fig. 7a and b, TL1 intensity of 5 kGy irradiated laurel leaf is approximately 500 fold higher than TL1 intensity of control sample. Therefore, the detection of irradiation treatment just on the basis of first glow curve (TL1) is possible.
It is recommended in EN 1788 that also the TL glow ratio (TL1/ TL2) is applied for evaluation to verify the reliability of detection results. TL glow ratios, integrated in the temperature range of 150-250°C, from irradiated samples are typically greater than 0.1 whereas those from unirradiated samples are usually below 0.1. In our case, the TL ratios of irradiated and unirradiated (control) samples were 3 (> 0.1) and 0.05 (<0.1), respectively. So, the ratio criteria were satisfied for unirradiated and irradiated (5 kGy) laurel leaf samples.
It is important to know the stabilities of the traps correlated with the TL peaks since these reflect the storage capacities of the traps. Thus, fading study was performed under normal laboratory conditions (in dark) in a storage period of 35 days by using 5 kGy irradiated laurel leaf samples. Three days after irradiation, some irradiated samples were read out for getting the reference glow curves (at zero time). The remaining samples were read out at different elapsed time after irradiation. The change in the first TL glow curve (TL1) intensity of 5 kGy irradiated laurel leaf was plotted as a function of storage time (Fig.8). This curve was get by the integrated area from 70 to 500°C. The fading in 35 days at room temperature was quite rapid; the TL signal intensity decreased about 89% of the initial value (3 days after the irradiation). This is an unwished for a good dosimetric material. Therefore, polyminerals isolated from laurel leaf could not suitable for dose estimation. On the other hand, if the irradiation time of the polymineral samples is known exactly, the irradiation dose may be estimated taking into account the room temperature fading. Similar high fading results at room temperature were also obtained for another polymineral samples isolated from different food samples (Favalli et al., 2006; Furetta and Zaragoza, 2007). According to these studies, the continuous trap distribution may be responsible from high thermal fading behaviour at room temperature. As the elapsed time from
irradiation increases the traps are emptied from the shallowest ones to the deepest, confirming a trap distribution as being responsible for the main peak.
.
4. Conclusion
Identification of gamma-irradiated Turkish laurel leaves was carried out by using electron spin resonance (ESR) and thermoluminescence (TL) techniques. From the data presented in this paper, the following conclusion can be drawn:
1. Unirradiated loreal leaf samples exhibited a weak ESR singlet at g = 2.0020. Gamma irradiation caused a significant increase in signal intensity of this single central line without any changes in g-factor and peak-to-peak line width. Two satellite lines at left and right of this single central line were observed at g = 2.0187 and 1.9840 with a separation of 6.08 mT after irradiation at ambient conditions.The discrimination of unirradiated and irradiated loreal leaf samples seems to be possible just by comparing their ESR spectra at room temperature which is compatible with EN 1787, 2000.
2. The intensity of the left satellite cellulose-like ESR signal at g = 2.0187 increased with the increasing of gamma radiation dose.The power function described best this variation in the studied dose range (0.5-10 kGy).
3. At room temperature, the post-irradiation decrease in cellulose-like signal intensity (g = 2.0187) was about ~92% during the 100 days. However, long-term decay data shows that ESR technique could be used to detect irradiated loreal leaf samples at least during the 100 days after irradiation process. Long-term decay data at room temperatute were best fitted to the sum of two first-order decay functions.
4. Annealing at high temperatures showed that the radiation-induced cellulose-like signal at g = 2.0187 in loreal leaf samples was very sensitive to temperature and it decreased with the increase in annealing temperature. The thermal decays of this signal can be described best by the sum of two first-order decay functions. This may support the existence of at least two cellulose-derivative radical signal component.The activation energy values of these contributing radical species were calculated as EA = 27.3 ± 4.9 and EB = 42.1 ± 3.6 kJ/mol
from ln(k)-1/T plots.
5. TL investigation of the silicate minerals isolated from the laurel leaf samples allowed to discriminate clearly irradiated and unirradiated samples and so EN1788 standard can also be applied to laurel leaf.
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