ORIGINAL PAPER Simultaneous spectrophotometric

Chemical Papers 63 (3) 336–344 (2009) DOI: 10.2478/s11696-009-0003-0

ORIGINAL PAPER

Simultaneous spectrophotometric determination of minoxidil and tretinoin by the H-point standard addition method and partial least squares Maryam Bordbar*, b Ali Yeganeh-Faal, c Jahanbakhsh Ghasemi, a Mohammad Mahdi Ahari-Mostafavi, d Nahid Sarlak, a Mohammad Taghi Baharifard a Department b Department c Department

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a

of Chemistry, Islamic Azad University, Qom-Branch, Qom, Iran of Chemistry, Hamadan payame Noor University, Hamadan, Iran

ofChemistry, Faculty of Science, Razi University, Kermanshah, Iran

d Department

of Chemistry, Lorestan University, Khoram Abad, Iran

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Received 16 May 2008; Revised 16 August 2008; 4 September 2008

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A simple, sensitive and selective spectrophotometric method for simultaneous determination of tretinoin and minoxidil using partial least square (PLS) calibration and H-point standard addition method (HPSAM) is described. The results of the H-point standard addition method show that minoxidil and tretinoin can be determined simultaneously with the concentration ratio of tretinoin to minoxidil varying from 2 : 1 to 1 : 33 in mixed samples. A partial least squares multivariate calibration method for the analysis of binary mixtures of tretinoin and minoxidil was also developed. The total relative standard error for applying the PLS method to eleven synthetic samples in the concentration range of 0–10 µg mL−1 tretinoin and 0–32 µg mL−1 minoxidil was 2.59 %. Both proposed methods (PLS and HPSAM) were also successfully applied in the determination of tretinoin and minoxidil in several synthetic pharmaceutical solutions. c 2008 Institute of Chemistry, Slovak Academy of Sciences Keywords: minoxidil, tretinoin, partial least squares, H-point standard addition method, simultaneous determination

Introduction Hair follicles have a complex and dynamic structure that can contribute significantly to the passive transport of compounds across the skin (Weiner, 1998; Rougier & Lotte, 1993). Minoxidil (Fig. 1) is widely used for stimulating hair growth after topical administration (Lauer, 1999). Tretinoin (all-trans retinoic acid; vitamin A acid) (Fig. 1) has been widely used in the local treatment of several skin disorders (Boyd, 1989). Its effectiveness in the treatment of male-pattern baldness when co-administered with topical minoxidil solution is also acknowledged (London*Corresponding author, e-mail: [email protected]

Wong & Hart, 1990). It was reported that percutaneous minoxidil absorption was enhanced by tretinoin as a result of increased stratum corneum permeability (Ferry et al., 1990). Concentration of minoxidil in hamster ear follicles is widely used in the evaluation of new formulations. Because only a small fraction of the applied dose is normally delivered through skin by the trans-follicular route (Lauer & Lieb, 1995), a sensitive analytical technique is needed to evaluate various topical minoxidil formulations. Various analytical techniques for the determination of minoxidil, including capillary electrophoresis (Fanali et al., 1987), electro-analysis (Ar-

M. Bordbar et al./Chemical Papers 63 (3) 336–344 (2009)

O H 2N

–

N+

NH2 N

O

N

OH

Tretinoin

Minoxidil

Fig. 1. Structures of minoxidil and tretinoin.

PLS modeling is a powerful multivariate statistical tool (Haaland & Thomas, 1988a) and can be performed by easily accessible statistics software. Other advantages of robust multivariate methods, such as PLS, can be performed by ignoring the concentrations of all other components except the analyte of interest. The basic concept of PLS was originally described by J¨ oreskog and Wold (1982) and Gerlach et al. (1979); and consequently, different algorithms for PLS modeling were developed (Frank et al., 1984; Geladi & Kowalski, 1986; Despagne et al., 1997; Spiegelman et al., 1998). PLS is a full-spectrum method and therefore enjoys the signal averaging advantages of other full-spectrum methods such as the principle component regression (PCR), and the classical least squares (CLS) method. PLS also has characteristics and advantages of the inverse least squares (ILS) method limited in the number of spectral frequencies that can be included in the analysis (Haaland & Thomas, 1988b). In this project, two different methods (HPSAM and PLS) for simultaneous determination of minoxidil and tretinoin are reported on. The aims of this research were to present a sensitive and selective chemical system for simultaneous determination of minoxidil and tretinoin, and to compare the applicability of HPSAM and PLS in simultaneous analysis of binary mixtures.

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cos et al., 1991), and differential-pulse polarography (Amankwa et al., 1983) have been reported. Methods for the quantitation of tretinoin in biological fluids include radioimmunoassay (RIA) (Royer et al., 1977), GLC (Van de Vaart et al., 1982), and highperformance liquid chromatography (HPLC) (Tracqui et al., 1995; Law & Appleby, 1996; Gaillard & Pépin, 1997), differential pulse polarography (Wang, 2000), and ion-pair extraction (Trotta et al., 2003). Also simultaneous determination of minoxidil and tretinoin by first derivative spectrophotometry has been reported (Mahrous et al., 1992; Chen et al., 2000). Quantitative analysis often involves spectrophotometric resolution of two or more components mixtures with partially overlapped spectra. The greater the extent of overlapping, the more difficult is the resolution rendered. Not surprisingly, this topic has been the subject of a number of chemometric studies originally intended for the resolution of binary mixtures and extended to three or more components (Hargis, 1988; Brown et al., 1988). Although the standard addition method could remove the error resulting from the sample matrix, it cannot remove the constant error resulting from other components in the system. Bosch-Reig and CampínsFalcó (1988) presented a new technique, the H-point standard addition method (HPSAM), based on the principle of dual wavelength spectrophotometry and standard addition methods. They further studied its principle and application (Bosch-Reig et al., 1991, 1992a, 1992b, 1992c, 1993, 1994a, 1994b, 1994c, 1995). The greatest advantage of HPSAM is that it can remove the errors resulting from the presence of interferent and blank reagents. So, HPSAM can determine two components simultaneously (Campíns-Falcó et al., 1998; Safavi et al., 2000). The requirements for the method are: to work at only two wavelengths where the analytical signal for one of the species (interferent) are identical and for the other one (analyte) as different as possible. By plotting the analytical signals vs. the added analyte concentration, two straight lines with a common point H and with coordinates (CH , AH ) were obtained; CH is the unknown analyte concentration and AH is the analytical signal affected by the interferent species.

337

Experimental Reagents and chemicals Minoxidil (2,6-diamino-4-piperidinopyrimidine-1oxide) (Merck, Germany), propylene glycol, ethyl alcohol, potassium hydrogen phosphate (K2 HPO4 ), potassium dihydrogen phosphate (KH2 PO4 ) (Merck, Germany), and tretinoin (all-trans retinoic acid; 3,7dimethyl-9-(2,6,6-trimethyl-1-cyclohexenyl)-nona-2, 4,6,8-tetraenoic acid) (Roche, Switzerland) were of analytical reagent grade. Stock tretinoin solution containing 200 µg mL−1 of tretinoin was prepared by dissolving 0.01 g of this reagent in mixed solvent (62 mass % of ethanol, 38 mass % of propylene glycol) and adjusting the volume to 50.0 mL with mixed solvents in a volumetric flask. Stock minoxidil solution containing 2 mg mL−1 of minoxidil was prepared by dissolving 0.01 g of this reagent in mixed solvents and adjusting the volume to 5.0 mL with mixed solvents in a volumetric flask. All of the solutions were prepared fresh every day. Instrumentation and methods UV-Visible absorbance digitized spectra were collected on a JENWAY 6505 spectrophotometer, using a 1.0-cm quartz cell. Measurements of pH were made using an EDT GP 353 pH-meter equipped with a glasssaturated calomel combined electrode. PLS program

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0.8 0.7

Absorbance/a.u.

0.6 0.5 0.4 0.3 0.2 0.1 0 200

225

250

275

300

325

350

375

400

425

450

Wavelength/nm

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was written in MATLAB version 7 (Mathworks Inc.) and was run on a DELL personal computer (Pentium IV processor) with the operating system Windows XP. Individual calibration: Appropriate volume of the minoxidil and tretinoin stock solutions were added to a 5 mL volumetric flask and filled up to the mark with mixed solvents (20 mass % of buffer solution, pH = 5.0; 20 mass % of ethanol; 60 mass % of propylene glycol). Buffer solution (pH = 5.0) was prepared using potassium hydrogen phosphate and potassium dihydrogen phosphate in doubly distilled water. The final concentration of minoxidil and tretinoin had to be between 0.59–32 µg mL−1 and 0.11–10 µg mL−1 , respectively. For each measurement, about 3 mL of the above solution were transferred to a spectrophotometric cell and absorbances were measured after 5 minutes from preparation against a reagent blank at 348 nm and 297 nm for tretinoin and minoxidil, respectively. PLS calibration: The calibration and prediction sets were designed using 30 and 11 binary mixtures of the cited drugs. The concentrations of minoxidil and tretinoin in calibration and prediction solutions were in the range of 0–32 µg mL−1 and 0–10 µg mL−1 , respectively. Each calibration or prediction solution was prepared as described in the individual calibration and, after 5 min, the spectrum of each solution was recorded against its reagent blank in the range of 260–400 nm with a wavelength interval of 1 nm. H-Point standard addition method: Synthetic samples containing different concentration ratios of minoxidil and tretinoin were prepared, and standard additions of tretinoin (up to 10 µg mL−1 ) were made. Simultaneous determination of minoxidil and tretinoin with HPSAM was performed by measuring the absorbance at 293 nm and 301 nm (when standards of tretinoin were added) for each sample solution. The concentration range of minoxidil and tretinoin for the construction of HPSAM calibration graph was 3–20 µg mL−1 and 0.6–6 µg mL−1 , respectively. For the analysis of real samples, the same procedure was repeated with 3 mL of synthetic samples.

Results and discussion Tretinoin has been widely used in local treatment of several skin disorders (Boyd, 1989). Its effectiveness in treatment of male-pattern baldness when coadministered with topical minoxidil solution (LondonWong & Hart, 1990) is also known. It was reported that percutaneous minoxidil absorption was enhanced by tretinoin as a result of increased stratum corneun permeability (Ferry et al., 1990); therefore, the system seems to be appropriate for simultaneous indirect spectrophotometric determination of minoxidil and tretinoin. Fig. 2 shows spectral features of minoxidil, tretinoin and their mixture. According to this figure, minoxidil and tretinoin spectra overlap; thus, univariate calibra-

Fig. 2. Absorption spectra of 14 µg mL−1 minoxidil (dashed line); 4 µg mL−1 tretinoin (dashed–dotted line); mixed reagents (dotted line), and blank (solid line) at pH = 5.

tion methods cannot be applied in their simultaneous determination; PLS and HPSAM were used to obtain quantitative information from spectral data. In order to find optimum pH for the determination of minoxidil and tretinoin by the spectrophotometric method, the influence of pH, in the range of 2–10, on their spectra was investigated separately. The results showed that pH in the range of 2–10 had significant effect on the spectra of minoxidil and tretinoin. At pH 5, the spectrum of minoxidil and tretinoin showed appropriate spectral overlapping and the spectra of minoxidil and tretinoin showed significant increase in the maximum absorbance (Fig. 3). So, to achieve higher sensitivity and stability, pH 5 was selected as optimum for simultaneous determination of minoxidil and tretinoin. Calibration To verify the validation of the Beer’s law, four calibration graphs were constructed over the whole range of wavelengths. These calibration graphs were prepared for fixed amounts of minoxidil and tretinoin separately. The appropriate correlation coefficients obtained (in the range of 0.9998–0.9999) indicate that no interaction between the two drugs exists hence it does not affect linear correlation prevailing between the absorbance and concentration of each drug. Thus, chemometric methods based on factor analysis, such as PLS, seem to be suitable for use in this system. Individual calibration graphs were constructed with several points (Fig. 4) as absorbance versus drug concentration in the range of 0.59–32 µg mL−1 for minoxidil and 0.11–10 µg mL−1 for tretinoin, and evaluated by linear regression. The intercepts on the ordinates were negligible in two graphs. Limits of detection calculated as 3s0 /m (Perez-Bendito & Silva, 1998), where s0 and m are the standard deviation of

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2

Absorbance/a.u.

Absorbance/a.u.

3.5 9 10

1

5

7

0.5

b

1.2

2 2.5

1.5

10

1.4

a

1

5

4.4

0.8

7

2

2.5

3.5

0.6

9

0.4 0.2

4.4 0 257

277

297

317

0 257

337

297

Wavelength/nm

337

377

417

Wavelength/nm

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Fig. 3. Absorption spectra of a) minoxidil and b) tretinoin at different pH. Table 1. Results of several analysis experiments of minoxidil-tretinoin mixtures at different concentration ratios by HPSAM Drug content/(µg mL−1 )

0.4844 0.4773 0.4051 0.3723 0.2880 0.2436 0.4125 0.3660 0.3789 0.3411 0.5476 0.5346 0.4019 0.3581 0.3514 0.2986 0.4539 0.3704

+ + + + + + + + + + + + + + + + + +

0.0533Ci 0.0413Ci 0.0542Ci 0.0414Ci 0.0599Ci 0.0451Ci 0.0551Ci 0.0417Ci 0.0552Ci 0.0420Ci 0.0518Ci 0.0387Ci 0.0550Ci 0.0416Ci 0.0570Ci 0.0427Ci 0.0590Ci 0.0449Ci

0.9982 0.9987 0.9997 0.9998 0.9989 0.9981 0.9987 0.9990 0.9990 0.9985 0.9998 0.9996 0.9988 0.9990 0.9997 0.9989 0.9999 0.9998

ut

= = = = = = = = = = = = = = = = = =

A

A301 A293 A301 A293 A301 A293 A301 A293 A301 A293 A301 A293 A301 A293 A301 A293 A301 A293

r

Nominal

Found

Tretinoin

Minoxidil

Tretinoin

Minoxidil

0.60

18.00

0.59

18.06

2.50

10.00

2.56

9.94

3.00

3.00

3.00

3.08

3.40

8.00

3.47

7.99

2.80

8.00

2.86

7.97

1.00

20.00

0.99

19.9

3.20

8.00

3.27

8.03

3.80

4.60

3.69

4.50

6.00

3.00

5.92

2.91

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A vs. C equation

the intercept on the ordinate and the slope of the calibration graph, respectively, for 0.17 µg mL−1 of minoxidil and 0.033 µg mL−1 of tretinoin.

Fig. 2 shows the absorption spectra for minoxidil, tretinoin, and their mixture. To select the appropriate wavelength pair for HPSAM, the following principles were applied: at the selected wavelengths, the analyte (tretinoin) signals had to be linear with concentrations of the interferent (minoxidil) signal remaining constant with the changing analyte concentration; the analytical signal obtained from a mixture of the analyte and the interferent should be equal to the sum of the individual signals of the two species. In addition, the difference in slopes of the two straight lines

1.4 1.2

Absorbance/a.u.

H-Point standard addition method

1.6

1 0.8 0.6 0.4 0.2 0 0

5

10

15

20

25

30

Concentration/(µg mL–1)

Fig. 4. Individual calibration graphs for minoxidil () at 297 nm (r = 0.9999) and tretinoin ( ) at 348 nm (r = 0.9999).

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Table 2. Results for five replicate analyses of minoxidil–tretinoin mixtures by HPSAM Drug content/(µg mL−1 ) A vs. C equation

= = = = = = = =

0.2880 0.2436 0.2856 0.2421 0.2857 0.2436 0.2829 0.2399

+ + + + + + + +

0.0599Ci 0.0451Ci 0.0591Ci 0.0448Ci 0.0588Ci 0.0447Ci 0.0598Ci 0.0452Ci

0.9989 0.9981 0.9994 0.9994 0.9997 0.9990 0.9970 0.9972

Found

Tretinoin

Minoxidil

Tretinoin

Minoxidil

3

3

3

3.08

3

3

3.04

2.97

3

3

3.07

2.94

3

3

2.95

3.01

3.02 0.05 1.72

3 0.06 2.02

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Mean value Standard deviation RSD/%

0.7

0.6

0.6

0.5 0.4 0.3 0.2 0.1 0 -2

0 2 4 Amount of tretinoin added/(µg mL−1)

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-4

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Absorbance/a.u.

Nominal

6

8

A

Fig. 5. Plots of H-point standard addition method for fixed minoxidil concentration (4.00 µg mL−1 ) and different tretinoin concentrations: 4.8 µg mL−1 ( ), 2.4 µg mL−1 ( ), and 3.4 µg mL−1 ().

•

measured at the two wavelengths selected (λ1 and λ2 ) should be as large as possible in order to reach good accuracy and sensitivity (Campíns-Falcó et al., 1998). When tretinoin was selected for the analyte, selecting several pairs of wavelengths was possible. Considering that the higher the value of the slope increment the smaller the error of the analyte concentration, 293 nm and 301 nm were chosen as the best wavelength pair. In order to determine accuracy of the method, several synthetic mixtures with different concentration ratios of minoxidil and tretinoin were analyzed using the proposed HPSAM. The results are given in Table 1. These results show that by applying the proposed HPSAM method, minoxidil and tretinoin can be simultaneously determined accurately in the concentration ratio of minoxidil to tretinoin from 33 : 1 to 1 : 2. Figs. 5 and 6 show the H-point standard addition plots for several synthetic test solutions. As can be seen from these figures, employing this wavelength

Absorbance/a.u.

A301 A293 A301 A293 A301 A293 A301 A293

r

0.5 0.4 0.3 0.2 0.1 0

-3

-1

1

3

5

7

−1

Amount of tretinoin added/(µg mL )

Fig. 6. H-Point standard addition method for fixed tretinoin concentration (2.4 µg mL−1 ) and different minoxidil concentrations: 4.0 µg mL−1 ( ), 6.8 µg mL−1 ( ), and 8.0 µg mL−1 ().

•

pair, CH (concentration of tretinoin) was independent of the minoxidil concentration, and AH (absorbance proportional to minoxidil concentration) was independent of tretinoin concentration. To check the repeatability of the method, four replicate experiments with minoxidil were done (Table 2). Then, concentration of the interferent was calculated in each test solution by the calibration method using standard solution and ordinate value of the Hpoint (AH ). The relative standard deviation (RSD) for four replicate measurements of 3.0 µg mL−1 mixtures of minoxidil and tretinoin were 1.72 % and 2.02 %, respectively. Partial least square regression The ability of the partial least squares (PLS) calibration to resolve an overlapped spectrum was examined by selecting calibration and prediction sets. A number of 30 binary mixtures were selected as the

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Table 3. Composition of calibration set

positions of calibration and prediction standards are summarized in Tables 3 and 4, respectively. A total of 140 data points were recorded between 260 nm and 400 nm. The number of latent variables (factors) for each element was determined by the cross-validation method, leaving out one sample at a time (Haaland & Thomas, 1988b). The prediction error was calculated for each element of the prediction set. This error was expressed as prediction residual error sum of squares (PRESS). PRESS was calculated for the first latent variable building PLS modeling in the calibration step. Then, another factor was added and PRESS was calculated again. Because of using PLS1 modeling, for one to 10 latent variables (used in PLS modeling), calculations were repeated for each component in the prediction set sample solutions and the corresponding PRESS values were determined. Fig. 7 shows a plot of PRESS against the number of factors for each individual component. Two variables presenting minimum PRESS were selected for minoxidil and tretinoin, respectively. The results obtained by applying the PLS1 constructed for each action of the 11 prediction samples are summarized in Table 4. The results obtained show that PLS as a full spectrum chemometric approach gives accurate prediction results in simultaneous determination of minoxidil and tretinoin with overlapping spectra. The prediction error for a single component in mixture was calculated as relative standard error (RSE) of the prediction concentration (Otto & Wegscheider, 1985; Blanco et al., 1994)

Drug content/(µg mL−1 )

6.80 8.00 4.00 6.80 10.00 6.80 5.20 8.00 6.80 6.80 8.00 10.00 5.20 8.00 10.00 6.80 4.00 10.00 4.00 5.20 4.00 8.00 8.00 5.20 5.20 8.00 6.80 6.80 10.00 8.00

3.00 4.80 7.60 6.40 8.00 9.20 4.00 8.40 10.00 10.00 4.50 8.40 9.20 3.52 7.60 2.44 6.00 6.40 3.00 8.40 6.40 5.60 4.80 3.40 2.44 2.44 4.00 6.48 4.00 3.00

ho

Minoxidil

n

ut

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Tretinoin

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Sample

A

calibration set for model construction (Table 3). For the evaluation of the constructed model, a prediction set of 11 samples was selected randomly. The com-

RSEs /% =

cj − cj )2 j=1 (ˆ × 100 n 2 j=1 (cj )

The total prediction error of the n-th sample was

Table 4. Prediction set composition, values predicted by PLS and statistical parameters Amount added/(µg mL−1 )

Amount found/(µg mL−1 )

Tretinoin

Minoxidil

Tretinoin

Minoxidil

Tretinoin

Minoxidil

3.00 8.40 3.40 4.00 4.80 7.60 2.44 6.40 5.52 3.40 5.60 – – –

10.00 8.00 4.00 4.00 4.00 5.20 5.20 5.20 10.00 6.80 5.20 – – –

3.01 8.42 3.34 3.97 4.77 7.63 2.39 6.37 5.51 3.43 5.59 – – –

10.09 7.85 4.41 4.37 4.31 5.16 5.12 5.11 10.09 6.61 5.07 – – –

100.3 100.2 98.2 99.2 99.4 100.4 98.0 99.5 99.8 100.9 99.8 99.6 0.60 2.59

100.9 98.1 110.2 109.2 107.8 99.2 98.5 98.3 100.9 97.2 97.5 101.2 3.30 2.59

Recovery/%

Sample

1 2 3 4 5 6 7 8 9 10 11 Mean recovery Singlea RSE/% Totalb RSE/%

a) Calculated by Eq. (1); b) calculated by Eq. (2).

(1)

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different effects of the interfering compounds on each analyte. Among the interfering compound tested, azelaic acid did not interfere in concentrations 900 times higher than those of the analytes. However, progesterone showed interference already at concentrations 10 times higher than those of the analytes. According to these results, the simultaneous determination method shows sufficient selectivity.

0.05

0.5

0.04

0.4

0.03

0.3

0.02

0.2

0.01

0.1

Analysis of synthetic mixtures

0

Both proposed methods (PLS and HPSAM) were applied in the determination of minoxidil and tretinoin in several synthetic samples prepared to match the composition of some drugs. In HPSAM, to determine minoxidil compound for women, tretinoin solution was added to the real sample solutions to expose the ratio of minoxidil to tretinoin in the range of 1 : 2 to 33 : 1. At last, the added amount would be excluded from the final result. The results are shown in Table 5. The good agreement between these results and known values indicates successful application of the proposed methods for simultaneous determination of minoxidil and tretinoin in various samples. The results shown in Table 5 also indicate that outputs of the PLS method were better.

0 0

2

4

6

8

PRESS

PRESS


10

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Latent Variable

Fig. 7. PRESS vs. the number of latent variables for minoxidil () and tretinoin ( ).

calculated as follows m n RSEt /% =

(ˆ cij − cij )2 i=1 m j=1 n 2 i=1 j=1 (cij )

× 100

(2)

ho

where n is the number of the prediction sample, cj the concentration of the component in the j-th mixture, cˆj the predicted concentration, cij is the concentration of the i-th component in the j-th sample and cîj is its prediction. Table 4 also shows reasonable single and total relative errors for such a system.

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Interference analysis

A

For interference analysis, the influence of several compounds was tested including those frequently accompanying minoxidil and tretinoin in real samples. The effect of interfering compounds at different concentrations on the absorbance of a solution containing 4 µg mL−1 of each analyte was studied. A compound was considered to interfere when its presence resulted in a variation of more than 5 % in the absorbance sample. This increment of absorbency was evaluated at two wavelengths corresponding to the maximum absorption of minoxidil and tretinoin, to establish the

Conclusions Two methods (PLS and HPSAM) for simultaneous determination of minoxidil and tretinoin without any sample pretreatment were described. The most important characteristics of these methods are: (i) HPSAM and PLS regression allow rapid, accurate and simple resolution of minoxidil and tretinoin mixtures; (ii) HPSAM can be used in complex samples with matrix effects because the standard addition method is capable to remove these effects; partial least squares regression cannot be used in these cases; (iii) PLS method determined minoxidil and tretinoin simultaneously in a wide range relative to HPSAM; (iv) capabilities of these two powerful chemometric methods (PLS and HPSAM) to analyze binary mixtures with high order of spectral overlapping were compared.

Table 5. Actual composition and calculated concentration of minoxidil and tretinoin in synthetic mixtures of some drugs Founda Drug

Composition of synthetic mixture/%

PLS Tretinoin

HPSAM Minoxidil

Tretinoin

Minoxidil

Minoxidil compound (0.5 %) Minoxidil (0.5), tretinoin (0.025) 0.024 ± 0.001 0.498 ± 0.013 0.024 ± 0.002 0.491 ± 0.008 Minoxidil compound Minoxidil (3), tretinoin (0.025), 0.025 ± 0.001 2.970 ± 0.050 0.024 ± 0.002 3.100 ± 0.20 for women azelaic acid (1.5), progesterone (0.25) a) Mean ± S.D. (four replicates).


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