INTRODUCTION:

Tobacco (Nicotiana tabacum) belongs to the Solanaceae family and the plant is considered to be a good source of a large number of bioactive substances. Recently, the chemical compositions of tobacco have attracted considerable attentions in the world^1-4. Solanesol, a 45-carbon, all-trans-nonaprenol (Fig.1), was first isolated from flue-cured tobacco⁵. Solanesol itself can be used as antiulcer and hypertension treating agent^{6, 7}. In addition, solanesol is a necessary medical intermediate in the industrial synthesis of coenzyme Q₁₀^8–10, which is an excellent medicine in cardiovascular disease, cancer, atherosclerosis and so on^11–15. Solanesol is in fact found in many plants from the Solanaceae family, one member of which is the Nicotiana genus. Other members of the family known to contain solanesol include tomato plants, potato plants, egg plants and pepper plants¹⁶.

However, it was reported that the content of solanesol in tobacco was considerably higher than that in other plants and thus this plant represented the most convenient source for large scale isolation of solaneso^17–19. So it is very important to determine the content of solanesol in tobacco. Many analytical methods including of column chromatography, thin-layer chromatography (TLC), gas chromatography (GC) and high performance liquid chromatography (HPLC) have been documented for the determination of solanesol^20–28. All of the methods above suffered from some limitations, such as, column chromatography method had low recovery, the precision obtained by TLC was poor, the GC method was complicated by the interference of solanesenes, produced from the pyrolysis of solanesol at high temperatures in the GC oven and the breakdown of solanesol hindered the direct quantification of solanesol. The HPLC methods above had poor selective and sensitivity compared with the LC–MS/MS methods proposed in the research study.

At present study, the liquid chromatography – mass spectrometry (LC–MS/MS) has been accepted by more and more people as a useful method for identification and determination of compounds^29,30.

Fig. 1. Structure of solanesol.

Especially, it is very effective in the analysis of compounds from complex samples because of its low detection limit, high sensitivity and the possibility for short run time^31–33. Signal suppression or enhancement of the target extracts by matrix components is a common phenomenon in LC–MS/MS analysis and should be considered³⁴. Moreover, interfering matrix components can affect accuracy of the proposed method and may be lead to some compromising results³⁵. In order to avoid the problems related with matrix effect, some authors preferred to select the optimization of sample preparation, the optimization of the chromatographic system and MS/MS detection^36–38. In addition, some authors refered to the necessity of implementation of the standard addition method as a form of eliminating matrix effects^39,40. Solanesol is a nonaprenol containing a linear hydrocarbon chain, which is made up of nine isoprenoids. CoQ10 is an ubiquinone, whose side chain is made up of 10 isoprenoids. Structurally, solanesol and CoQ10 have some similarities. LC–MS/MS method with MRM have been reported for the determination of CoQ10 in Nicotiana tabacum⁴¹. The aim of this study is to propose a validated LC-MS/MS method with multiple reaction monitoring (MRM) for separation and determination of solanesol in tobacco. Regarding that matrix components in tobacco are complex, in order to avoid the problems related with matrix effect, each step mentioned above (sample preparation, chromatographic system and MS/MS detection) was carefully optimized in the study. At the same time, the standard addition method was used as a quantification method to further minimize the matrix effect in the study. We have found this technique to be suitable for the quantitative determination of solanesol in tobacco. Based on this work, the contents of solanesol in various samples are determined and compared in this research paper.

MATERIALS AND METHODS:

Equipments:

LC–MS/MS analysis was performed on an API3000 (Applied Biosystems) triple-stage quadrupole mass spectrometer equipped with an atmospheric pressure chemical ionization (APCI) interface and an Agilent 1100 series HPLC from Agilent technologies (Agilent). The Agilent HPLC system consisted of a G1312A HPLC binary pump, a 7725i manual injector. A reverse phase Symmetry RP18 column (5µm, 93A°, 66% porosity, 3.9mm ×150 mm, Waters, USA) was used.

Reagents and materials:

Acetonitrile and isopropanol were of HPLC grade (Ranbaxy). Ammonium acetate was HPLC grade (Ranbaxy). Methanol, ethanol, acetone and hexane were of analytical grade (Ranbaxy, India). Solanesol standard (>95%) was purchased from Sigma (USA). Tobacco samples (Nicotiana tabacum) were collected from Guntur, Andhra Pradesh, India., for the determination of solanesol by LC–MS/MS.

LC–MS/MS conditions:

Chromatographic analysis was carried out by a Symmetry RP18 column. Column temperature was maintained at 26°C. The mobile phase was a mixture of acetonitrile and isopropanol (1:1, v/v) containing 2mM Ammonium acetate. Elution was performed at a flow rate of 1.0 ml/min. The injection volume was 5µl. For operation in MS/MS mode, a mass spectrometer fitted with an APCI source interface was used for analysis. Ionization of solanesol was achieved in the positive ionization mode. The infusion experiment was performed using a single syringe pump ‘11’. MRM was performed with 150 ms dwell time. Nitrogen was used for the nebulizing, curtain and collision gas. The mass spectrometer was programmed to monitor the precursor ion [(M−H₂O)+H]⁺ at m/z 613.7 via the first quadrupole filter (Q1), the product ion at m/z 69.2 was monitored via Q3. Finally, all MS parameters were manually fine-tuned to obtain the highest MRM signals. Under the above conditions, the ion source was thermally stabilized for 30min. before injection. Peak areas obtained from the MRM were utilized for the quantification of solanesol. The optimized MS parameters for the detection of solanesol were listed in Table 1.

Preparation of standard solutions:

Solanesol standard was purchased from Sigma, USA. Then, stock solution of solanesol was prepared by accurately weighing 7.50 mg of powder and dissolving this in 10 ml of the mobile phase. The stock solution was stored at 4ºC. From the stock solution, working standards were prepared by dilution with mobile phase. All standard solutions were filtered through 0.45µm membrane filter (Millipore).

Preparation of sample solution:

Sample solution was prepared by the method of Ref.⁴². For the plant sample, plant material was grinded to a fine powder. Subsequently, 2 g of the fine powder was placed in a closed stainless steel vessel and 20 ml of methanol were added. The optimized extraction was carried out in an ultrasonic washer for 20 min. at 25°C. The extract was centrifuged at 12,000 rpm for 8 min. After filtering, the supernatant was extracted with hexane. The above procedure was repeated for three times and the hexane phases were combined. Combined hexane phase was concentrated to dryness and the dried extract was dissolved in the chromatographic mobile phase. After filtering, the supernatant was injected directly.

NEB: Nebulizing gas; CUR: curtain gas; CAD: collision-activated dissociation; IS: ion spray voltage; TEM: temperature; AUX: auxiliary gas; DP: declustering potential; EP: entrance potential; FP: focusing potential; CEM: channel electron multiplier; CE: collision energy; CXP: collision cell potential; MRM: multiple reaction monitoring.

Table 1: Parameters of mass spectrometric conditions

Parameters	Value
NEB	14
CUR	12
CAD	5
IS (V)	4000
Ion Source TEM (ºC)	350
AUX (psi)	70
Q1 mass (precursor ion [(M-H₂O)+H]⁺)	613.7
Q3 mass (product ion)	69.2
DP (V)	60
EP (V)	10
FPV (V)	400
CEM (V)	2000
CE (V)	35
CXP (V)	8
MRM (amu)	613.7/69.2

Recovery studies:

The recovery experiment of solanesol was performed by adding solanesol standards to the extract solutions of tobacco samples respectively. Several portions of the same volume of extract solutions were, respectively, spiked with the same volume of solanesol standard solution at different mass concentrations. All samples were vortex-mixed and filtered through a 0.45 µm Millipore filter. Samples were determined three times by LC–MS/MS. The recovery was calculated as follows:

Recovery (%) = (A- B) x 100%

where A is the amount detected, B the amount of extract without added standard, C is the added amount of the standard.

RESULTS AND DISCUSSION:

Optimization of sample preparation:

Fresh tobacco leaves were used as extraction material. Anhydrous ethanol, methanol and 95% ethanol were chosen as extraction solvent. At the same time, different extraction methods (ultrasonic, Soxhlet and reflux) were compared. Solanesol in fresh tobacco leaves were extracted according to the procedure described in Section 2.5. The extracts obtained by the above three extraction methods were injected, respectively. The extraction yields of solanesol in tobacco leaves were determined under the LC–MS/MS conditions of Section 2.3. The results are shown in Fig. 2.

In Fig. 2, the extraction yield of solanesol obtained with 95% ethanol is obviously low, that obtained with anhydrous ethanol is higher and that obtained with methanol is the highest. Therefore, methanol was considered a safe and more effective solvent for extraction of solanesol from tobacco leaves. It can also be seen from Fig. 2 that the extraction yield of solanesol obtained by ultrasonication is highest, that obtained by Soxhlet is lower and that obtained by reflux is obviously lowest. In addition, the extraction completeness by Soxhlet extraction depends to a large extent on the extraction time, the extraction completeness of ultrasonication is almost independent on time. Ultrasonic extraction gave better result within 20 min. than Soxhlet extraction did within 240 min. Ultrasonic extraction needs a shorter time compared to reflux and Soxhlet extraction. With regard to extraction yields and time, methanol was regarded as extraction solvent, ultrasonic extraction was considered to provide effective conditions for the extraction of solanesol from tobacco was used in the following tests.

Fig. 2. Effect of different sample preparation methods on extraction yield of solanesol.

Optimization of chromatographic conditions:

It is vital to select an appropriate mobile phase to ensure that the peak of solanesol could be separated and well resolved within a reasonable analysis time. In order to assure optimal chromatographic conditions for solanesol, the mobile phase systems were optimized. The organic solvent, methanol or acetonitrile is commonly used as one component of the mobile phase. Since solanesol readily dissolves in isopropanol, various proportions of methanol–isopropanol and acetonitrile–isopropanol were initially tested as mobile phases. The results showed that the solanesol responses increased with increasing percentage of methanol or acetonitrile. However, because of acetonitrile’s lower viscosity as compared to methanol, from the standpoint of pressure, acetonitrile was chosen as one component of the mobile phase. In succession, mixtures of acetonitrile and isopropanol in different ratios were tested. Eventually, it was found that acetonitrile–isopropanol (1:1, v/v) gave the best separation of solanesol.

It was reported that addition the ionizing agents to mobile phase had the greatest effect on the ionization of compound and the improvement of sensitivity^43–46. To enhance the sensitivity and ionization efficiency, different percentages of ammonium acetate were used in the mobile phase. The results showed that solanesol was completely separated and the peak shape of solanesol was better when the concentration of ammonium acetate was in the range of 2–8 mM. However, when the concentration of ammonium acetate was below 2 mM, the ionization was poor. Considering both ionization and sensitivity, the optimized ammonium acetate concentration was 2 mM. As a result, a mixture of acetonitrile and isopropanol (1:1, v/v) containing 2mM ammonium acetate was confirmed as the optimum mobile phase. Under these conditions, the retention time of solanesol was 2.2 ± 0.1 min.

Fig. 3. Positive-ion Q1 mass spectrum of solanesol standard

Optimization of MS/MS detection conditions:

Electrospray ionization (ESI) and atmospheric pressure chemical ionization techniques were, respectively, tested in positive and negative ion mode. The results showed that APCI in positive mode was superior to APCI in negative mode and to ESI in positive or negative mode. Solanesol was determined with much better sensitivity on using the APCI source in positive mode. Thus, the APCI source in positive mode was chosen for the detection of solanesol. Infusion experiments were carriedout to examine ionization and fragmentation patterns of the solanesol standards using a syringe pump. First, a standard solution of solanesol was chosen to obtain a constant signal in the Q1 scan mode. A full scan spectrum of the solanesol standard was acquired with a scan range of 500–700 amu, a dwell time of 1.5 s and a step size of 0.1 amu. The declustering potential (DP) was optimized using the quantitative optimization function of Analyst 1.4 to achieve the highest signal response. The main ions were observed at m/z 613.7 [(M−H₂O)+ H]⁺ with smaller signals visible at m/z 648.7 [M+NH₄]⁺ and 631.9 [M+H]⁺ when the data were acquired in the Q1 scan mode ( Fig.3). Compared with NH₄ ⁺ adducts at m/z 648.7 and the protonated [M+H]⁺ ion at m/z 631.9, the [(M−H₂O)+H]⁺ ion at m/z 613.7 showed the highest intensity signals, the [(M−H₂O)+H]⁺ ion at m/z 613.7 was chosen as precursor ion of solanesol. Secondly, we used product ion scans to look for the most abundant product ion. The collision energy (CE) was optimized to achieve highest sensitivity. The product ion spectrum obtained, which is shown in Fig. 4, consisted of an intense ion at m/z 69.2 and a second intense ion at m/z 81.1 when CE was 35V. The two base ions at 69.2 and 81.1 were characteristic ions of polyprenol^47,48. Among the product ions, that at m/z 69.2 was the most abundant and was therefore chosen for the quantitative determination of solanesol. Finally, the precursor/product ion pair of m/z 613.7/69.2 was chosen for the MRM scan. MRM was performed with 150 ms dwell time. Peak areas obtained from the MRM of solanesol standards were utilized for the quantitative determination of solanesol. Sample solutions of tobacco leaves extract (prepared according to the procedure described in Section 2.5) were injected directly, separated, and detected under the optimum condition mentioned earlier (Section 2.3).

Fig. 4. Production ion spectrum and fragmentation pattern of solanesol standard.

The MRM chromatogram of solanesol in tobacco leaves is shown in Fig. 5. It can be seen from Fig. 5 that the retention time of solanesol was 2.24 min. The analysis procedure can be finished in a shorter analysis time. The results show the proposed method to be fast.

Fig. 5.LC–MS/MS chromatogram of solanesol.

Method of standard addition:

Quantification was based on the standard addition method with the tobacco extract solution spiked with solanesol standards of three different concentrations. The detector response can then be plotted against the added concentration of solanesol. This is referred to as a standard addition curve. The unknown solanesol concentration can be found by extrapolating the best fit line to the x-axis intercept. That intercept will be the unknown solanesol concentration. If the equation of the best fit line is written in the form y = mx + q (y, peak area, counts; x, added solanesol concentration, µg/ml) then the x axis intercept is equal to the y-axis intercept (q) over the slope (m).

Table 2: Recovery of solanesol (n=3)

Samples

Solanesol added (μ g)

Found(µg)

Recovery (%)

R.S.D. (% )

Sample-1

0.0

30.0

150.0

750.0

127.6

157.4

275.3

867.2

---

99.67

98.47

98.61

2.1

2.2

1.8

1.9

Sample-2

0.0

7.5

15.0

30.0

18.8

26.2

33.7

48.4

---

98.67

99.33

98.67

2.1

2.0

1.9

2.0

Sample-3

0.0

1.5

3.0

7.5

1.9

3.4

4.8

9.3

---

98.53

97.72

98.67

1.7

2.5

2.0

2.7

Sample-4

0.0

3.0

7.5

15.0

7.0

10.0

14.4

21.9

---

98.67

99.23

99.41

2.0

2.4

2.5

2.0

Sample-5

0.0

1.5

3.0

7.5

4.6

6.1

7.6

12.1

---

98.37

98.97

99.36

2.6

2.3

2.0

2.3

Sample-6

0.0

1.5

3.0

7.5

2.8

4.3

5.8

10.3

---

98.86

99.23

99.45

2.7

2.2

1.9

2..5

Table 3: Determination of solanesol in tobacco n=3

Samples	Standard addition curve^a	Regression coefficient (R²)	Content of solanesol (mg/g)	R.S.D (%)
Sample-3	y = 15. 57x + 81.14	0.9998	0.089	1.2
Sample-2	y= 15. 52x + 792.91	0.9997	0.869	1.4
Sample-1	y = 15. 50x + 5292. 5 9	0.9995	5.908	1.6
Sample- 4	y = 15.60x + 301.04	0.9996	0.325	1.8
Sample-5	y=15.72x + 191. 53	0.9997	0.215	1.6
Sample- 6	y = 15.72x + 117.03	0.9995	0.129	2.1

^ay, peak area (counts); x, added solanesol concentration (μg/ml).

Method validation:

Linearity:

Six standard solutions of solanesol with concentrations between 0.075 and 750µg/ml were determined by LC–MS–MS with MRM. Each individual standard with a certain concentration was consecutive injected for three times. The regression equation for solanesol (peak area, A, counts, versus concentration, C, μg/ml) was as follows:

A = 15.54C + 1.52 (R2 = 0.9997)

The regression equation was found to be linear in the ranges of 0.075–750μg/ml with the good correlation coefficient (R2 = 0.9997).

LOQ and LOD:

For the establishment of LOQ and LOD, 750 μg/ml of standard solutions of solanesol were gradually diluted with mobile phase. Each individual standard with a certain concentration was consecutive injected for three times. LOQ of 5.0 ng/ml was obtained for solanesol (S/N = 10). LOD of solanesol was 1.5 ng/ml (S/N = 3). Compared with other analytical methods, LOD obtained by the proposed method was lower than some results by column chromatography, thin-layer chromatography, gas chromatography and high performance liquid chromatography methods^20–28. The result showed the proposed method was highly sensitive.

Precision and recovery:

The intra-day and inter-day precisions (expressed as the relative standard deviation, R.S.D.) for peak area were determined by repeated analysis (n = 5). The result showed that intra-day and inter-day R.S.D.s for peak area were, respectively, 0.89 and 1.12%. Compared with other analytical methods^25–28, the precision obtained by the proposed method was better. To validate the propose method, the recoveries were obtained by the procedure described in Section 2.6 and the results were listed in Table 2. It was found that the recoveries of solanesol were closer to 100% of and corresponding R.S.D.s were all no more than 2.7%.

The determination of solanesol in tobacco

The contents of solanesol in tobacco samples were prepared and analyzed by the procedure described in Sections 2.5 and 2.3. The calculated contents of solanesol by standard addition method described in Section 3.4 were summarized in Table 3.

The results from Table 3 show that the contents of solanesol are different samples of tobacco and the content of solanesol.

CONCLUSIONS:

In the present study, a LC–MS/MS method for the determination of solanesol in tobacco has been presented. The method makes analysis procedure be finished in a shorter analysis time with good recovery, precision and sensitivity. The contents of solanesol in different tobacco samples were analyzed and compared using the method. At the same time, this method provides a reference for the analysis of solanesol in other samples.

ACKNOWLEDGEMENTS:

The authors express sincere thanks to Dr. V. Krishnamurthy, Director, Central Tobacco Research Institute, Rajahmundry, Andhra Pradesh, India and Sri. B. Bhanumurthy, Managing Director, M/s. Bio-Pharma Laboratories (P) Ltd., Guntur, Andhra Pradesh, India for encouragement and permission to communicate the results for publication.

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Received on 30.04.2011 Modified on 25.05.2011

Asian J. Research Chem. 4(7): July, 2011; Page 1125-1130