COMPARATIVE EVALUATION OF ANALYTICAL METHODS FOR SIMULTANEOUS DETERMINATION OF NISOLDIPINE AND ITS PHOTODEGRADATION PRODUCTS

PLEASE SCROLL DOWN FOR ARTICLE

This article was downloaded by: [Università della Calabria]On: 2 September 2008Access details: Access Details: [subscription number 772811864]Publisher Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

Analytical LettersPublication details, including instructions for authors and subscription information:http://www.informaworld.com/smpp/title~content=t713597227

COMPARATIVE EVALUATION OF ANALYTICAL METHODS FORSIMULTANEOUS DETERMINATION OF NISOLDIPINE AND ITSPHOTODEGRADATION PRODUCTSClaudio Vetuschi a; G. Ragno b; M. Veronico a; A. Risoli b; A. Gianandrea a

a Farmaco-chimico Department, University of Bari, Bari, Italy b Scienze Farmaceutiche Department, Universityof Calabria, Rende (CS), Italy

Online Publication Date: 08 May 2002

To cite this Article Vetuschi, Claudio, Ragno, G., Veronico, M., Risoli, A. and Gianandrea, A.(2002)'COMPARATIVE EVALUATION OFANALYTICAL METHODS FOR SIMULTANEOUS DETERMINATION OF NISOLDIPINE AND ITS PHOTODEGRADATIONPRODUCTS',Analytical Letters,35:8,1327 — 1339

To link to this Article: DOI: 10.1081/AL-120006669

URL: http://dx.doi.org/10.1081/AL-120006669

Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf

This article may be used for research, teaching and private study purposes. Any substantial orsystematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply ordistribution in any form to anyone is expressly forbidden.

The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae and drug dosesshould be independently verified with primary sources. The publisher shall not be liable for any loss,actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directlyor indirectly in connection with or arising out of the use of this material.

©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

PHARMACEUTICAL ANALYSIS

COMPARATIVE EVALUATION OF

ANALYTICAL METHODS FOR

SIMULTANEOUS DETERMINATION

OF NISOLDIPINE AND ITS

PHOTODEGRADATION PRODUCTS

Claudio Vetuschi,1,* G. Ragno,2 M. Veronico,1

A. Risoli,2 and A. Ginnandrea1

1Farmaco-chimico Department of University,via E. Orabona 4, 70126 Bari, Italy

2Scienze Farmaceutiche Department, University ofCalabria, 87036 Rende (CS), Italy

ABSTRACT

Simultaneous determination of Nisoldipine and two photolyticproducts in pharmaceutical preparations and rawmaterial wasperformed by two spectrophotometric methods, utilizing aderivative procedure and a multivariate analysis based onclassical least square algorithm. In order to improve the pre-diction ability of CLS analysis, a selection procedure of theuseful wavelengths has been performed. Both methods werecompared with a conventional gas chromatographic method.The recovery values were estimated to be within 96 and 106%and RSD values below 5%. The LOD for both photoproductswas 1%.

ANALYTICAL LETTERSVol. 35, No. 8, pp. 1327–1339, 2002

1327

DOI: 10.1081/AL-120006669 0003-2719 (Print); 1532-236X (Online)Copyright & 2002 by Marcel Dekker, Inc. www.dekker.com

*Corresponding author: E-mail: [email protected]

Downloaded By: [Università della Calabria] At: 17:27 2 September 2008

©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

Key Words: Nisoldipine; Photodegradation; Derivativespectrophotometry; UV Multivariate analysis; Gas chroma-tography

INTRODUCTION

Nisoldipine (NSD), (�) 3-isobutyl-5-methyl-1,4-dihydro-2,6-dimethyl-4-(2-nitrophenyl)-pyridine-3,5-dicarboxylate, like the analogous 1,4-dihy-dropyridine calcium channel blockers, when exposed to light, degrades tonitrosophenylpyridine (NTS) and, in a smaller amount, to nitropyridine(NTR) derivatives, both lacking in pharmacological activity.[1,2] Moreover,the nitropyridine analogue NTR represents one of the main metabolitesof the drug.[3]

N

NO2

H3COOC COOCH2CH(CH3)2

H3C CH3H

H3COOC COOCH2CH(CH3)2

X

NH3C CH3

hν

NSD NTR X= NO2

NTS X= NO

Nisoldipine has been analysed in pharmaceutics by HPLC[4] andvoltammetry.[5] A gas chromatographic method has been proposed fordetermination of NSD in plasma.[6]

No spectrophotometric methods for nisoldipine analysis are reportedyet in literature. Actually, the products present absorbance spectra closelyoverlapped, making very difficult to define an analytical procedure inordinary spectrophotometry (Fig. 1).

Therefore derivative spectrophotometry has been tried out in differentorders to overcome the problems of interference due to the spectral overlap,defining a procedure for the simultaneous determination of the threeproducts by using the first-order derivative spectra.

A further spectrophotometric procedure, based on a CLS multivariatecalibration method, has been applied on the absorption (zero-order) andderivative spectra. In order to improve the prediction ability of analysis,both the instrumental parameters and the wavelengths selection have beenperformed, in agreement with a procedure defined in a previous paper.[7]

1328 VETUSCHI ET AL.

Downloaded By: [Università della Calabria] At: 17:27 2 September 2008

©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

The proposed spectrophotometric methods were applied successfullyto the analysis of synthetic ternary mixtures and drug commercialformulations. The results were statistically compared with those obtainedby a new gas chromatographic procedure, developed as a referencemethod.

MATERIALS AND METHODS

All laboratory procedures were conducted in a dark room providedwith a red lamp (60watt), kept at a distance of about 2m from thesamples.

Apparatus

The zero- and first to fourth-order derivative spectra were carried outin 10mm silica quartz cells using a Perkin-Elmer Lambda 15 Spectropho-tometer, over the wavelength region 190–500 nm.

The default instrumental conditions were fixed as: slit width nm 1; timeresponse sec 1; scan speed nmmin�1 240; data interval nm 1. The optimiza-tion of these parameters was performed by investigating within the follow-ing values: slit width 0.25–1–2–3–4; time response 0.5–1–2–5–10; scan speed30–60–120–240–480; data interval 0.1–0.2–0.5–1.

Figure 1. Absorbance spectra of ethanolic solutions of 10.32 mgmL�1 nisoldipine

NSD (�), 10.40mgmL�1 nitropyridine product NTR (i) and 10.80mgmL�1

nitrosopyridine product NTS (œ).

METHODS FOR DETERMINATION OF NSD, NTS, AND NTR 1329

Downloaded By: [Università della Calabria] At: 17:27 2 September 2008

©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

PECSS 4.1 package software by Perkin-Elmer was used for spectraacquisition.

For gas chromatographic analysis a Hewlett Packard Model 5890Series II, equipped with an ‘‘on column’’ injector and a flame ionizationdetector, was used. The samples were separated on a HP5 (30m� 0.53mm)wide-bore column, with a film thickness of 0.88 mm. The oven temperaturewas increased from 200 to 225�C with a rate of 3� min�1, then to 240�Cwith a rate of 5� min�1; the detector temperature was 300�C. Nitrogenwas used as carrier gas at a pressure of 225 kPa. Under these conditionsNSD, NTR and NTS presented retention times of 18.1� 0.2, 12.2� 0.1and 3.6� 0.2min, respectively. The retention time of internal standard2-(2,4-cichlorobenzyl)-4-(1,1,3,3-tetramethylbutyl)-o-phenol (Clofoctol)resulted to be 14.0� 0.1min. Chromatographic data were processed withthe software ‘‘Chemstation 5.0’’ by Hewlett Packard in peak-area mode.

Multivariate Analysis Software

The CLS UV multivariate method was performed by using the Perkin-Elmer ‘‘PMULT 1.0’’ software package. The procedure consists of acalibration step based on an algorithm which enables the absorptivitiesof the single components to be acquired at the concentrations used forreference solutions. The calibration procedure can be performed using upto 15 reference solutions after verifying that all components obey Beer’slaw. In a successive prediction step the component concentrations of anunknown mixture are calculated.

The software produces, for each component, a calibration report withthe following parameters:

(a) PRESS (Prediction residual errors sum of squares), calculatedas the difference between the real and the calculated concen-trations, squared and summed, over all references for eachcomponent.

(b) SNST (Sensitivity), which represents the response of the instru-ment to the components. Its value increases with that of thedata points.

(c) SLCT (Selectivity), with a value between 0 and 1, expresses theabsorbance of a single component compared with the wholeabsorbance.

These parameters represent a guide to select the most suitable instru-mental conditions and wavelength ranges to enable a successful analysis tobe obtained.

1330 VETUSCHI ET AL.

Downloaded By: [Università della Calabria] At: 17:27 2 September 2008

©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

Materials

Nisoldipine and its photodegradation products were kindly suppliedby Smithkline-Beecham Farmaceutici (Milan, ITALY). The internalstandard was obtained as reported in literature.[8] PTFE membrane filters,0.45 mm pore size, by Whatman (England). All the solvents were analyticalgrade and ethanol 95� was spectrophotometric grade.

Standard Solutions

Fifteen ethanol standard solutions were prepared of pure drug andwith mixture concentrations ranged between: NSD 14.70 and 24.60 mgmL�1;NTR 2.72 and 13.58 mgmL�1; NTS 1.04 and 13.05 mgmL�1. For theseranges each component was demonstrated to obey Beer’s law and the con-tributions of each component to the total absorbance in all the mixtureswere also shown to be additive. The mixtures concentration values arereported in Table 1.

For GC analysis, concentrations were within the range 0.1–15.0 mgmL�1 for all the products. All injections were 1 mL.

Table 1. Concentrations of the Calibration Solutions.

Values Are Expressed as mgmL�1

Reference NSD NTR NTS

1 0.00 0.00 10.442 0.00 10.86 0.00

3 19.60 0.00 0.004 19.60 0.00 10.445 19.60 10.86 0.00

6 14.70 2.72 13.057 14.70 5.43 10.448 14.70. 13.58 2.61

9 19.60 5.43 5.2210 19.60 8.14 2.6111 19.60 10.86 10.4412 24.50 3.26 20.9

13 24.50 4.34 1.0414 24.50 8.14 2.6115 24.50 10.9 4.18

METHODS FOR DETERMINATION OF NSD, NTS, AND NTR 1331

Downloaded By: [Università della Calabria] At: 17:27 2 September 2008

©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

Laboratory Samples

Synthetic mixtures were made in order to obtain component levelswithin the ranges reported in ‘‘Standard solutions.’’ These mixtures wereused to perform an external validation, by establishing the accuracy andprecision of the methods.

Pharmaceuticals

For the analysis of NSD specialties, five tablets were weighed andreduced to a fine powder. An amount corresponding to the average ofone tablet was accurately weighed, stirred and made up to a volume of10mL with ethanol. The suspension obtained was filtered with a PTFE0.45 mm membrane and 1mL of this filtrate was then diluted to 10mLwith ethanol.

Zadipina tablets (Bayer, Italy) pharmaceutical formulations wereobtained commercially.

RESULTS AND DISCUSSION

The absorption spectra of nisoldipine and its two photodegradationproducts present a serious spectral overlap (Fig. 1), making impracticablethe analysis of their mixtures by direct zero-order spectrophotometry.The spectral interference was especially critical for both the photoproductsNTR and NTS, whose absorption curves result totally covered by NSD,especially in the presence of relatively high concentration of the parentcompound.

Therefore alternative spectrophotometric techniques were investigated.

Derivative Spectrophotometry

As a first approach the derivative spectrophotometry was applied,because of the selectivity resolution enhancement of the spectral curvesobtained by means of this technique. In fact, by examining the first tofourth-order derivative spectra, some signals resulted to be correlated tothe concentrations of the pure products.

In the first-order derivative spectrum the simultaneous presence ofthree signals was pointed out, whose amplitudes were demonstrated to be

1332 VETUSCHI ET AL.

Downloaded By: [Università della Calabria] At: 17:27 2 September 2008

©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

proportional to each product (Fig. 2), whereas the derivative curves ofthe other components corresponded to the zero-crossing wavelengths,with absorbance values equivalent to zero.

In particular the minimum at 389 nm was proportional to NSD,because the two other components did not present absorbance above370 nm. The concentration of photoproduct NTR exhibited linear responseat wavelength 280 nm, corresponding to the zero-crossing wavelength ofNSD and NTS. The same condition was valid for NTS determinationby using the wavelength at 335 nm. The persistence of the zero-crossingconditions has been validated for the concentration ranges investigatedfor any ratio of concentrations.

Table 3 shows the calibration data in the determination of theproducts.

Multivariate Spectrophotometry

As a second spectrophotometric technique, a multivariate methodbased on a Classical Least Squares (CLS) procedure has been developed.In this method a training set of mixtures of known composition are recorded,so to elaborate a calibration model for a chosen range of wavelengths.

Figure 2. First-order derivative spectra of ethanolic solutions of 10.32 mgmL�1

nisoldipine (�), 10.40 mgmL�1 nitropyridine product NTR (i) and 10.80mgmL�1

nitrosopyridine product NTS (œ).

METHODS FOR DETERMINATION OF NSD, NTS, AND NTR 1333

Downloaded By: [Università della Calabria] At: 17:27 2 September 2008

©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

Table

2.

PRESSValues

andPercentageErrors

ontheReference

SolutionConcentrationsObtained

byMultivariate

Analysis,

UsingtheSelectedWavelength

Zones

andInstrumentalParameters

Derivative

Order

NSD

NTR

NTS

Wavelength

Range

PRESS

%Mean

Error

Wavelength

Range

PRESS

%Mean

Error

Wavelength

Range

PRESS

%Mean

Error

0212–238

0.1036

1.76

222–232

0.1503

2.06

258–272

0.0566

1.92

1215–240

0.1148

1.98

262–290

0.1935

2.87

280–300

0.1201

3.48

2220–238

0.1275

2.33

264–288

0.1654

2.43

282–298

0.0724

2.33

3220–232

0.0921

1.45

267–283

0.0859

1.29

295–305

0.0360

1.03

4225–236

0.0846

1.36

266–282

0.1118

1.52

275–295

0.0522

1.84

1334 VETUSCHI ET AL.

Downloaded By: [Università della Calabria] At: 17:27 2 September 2008

©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

The model is then used to predict the unknown concentrations of an analo-gue mixture.

CLS is considered a full spectrum method, involving the whole rangeof wavelengths or quantifying all the components by using the same range ofwavelengths. Thus, wavelengths with noise or irrelevant information arealso used in the elaboration of a calibration model, resulting in less precisionin the prediction phase. The definition of an unambiguous selection of thebest range of wavelengths is still a problem.[9–12]

A recent procedure, based on the minimum PRESS criterion, consist-ing of a procedure to select the optimal set of wavelengths and instrumentalconditions, for each component, has been proposed.[8] This selection pro-vides to restrict the wavelength intervals to those that captures only thewavelengths with the maximum spectral information, thus eliminating anyunnecessary ones.

In accordance with this procedure, an accurate selection of thewavelength regions has been performed, obtaining good improvementof the results, especially when the procedure was applied in the derivativespectra (Table 2). The best results were obtained in the third-order der-ivative mode, by using in the calibration model the following wavelengthzones: 220–232 for NSD; 267–283 for NTR and 295–305 for NTS(Fig. 3).

Figure 3. Third-order derivative spectra of ethanolic solutions of 10.32 mgmL�1

nisoldipine NSD (�),10.40 mgmL�1nitropyridine product NTR (i) and10.80mgmL�1 nitrosopyridine product NTS (œ). The selected intervals for multi-

variate analysis are pointed out as grey zones.

METHODS FOR DETERMINATION OF NSD, NTS, AND NTR 1335

Downloaded By: [Università della Calabria] At: 17:27 2 September 2008

©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

Gas Chromatography

Finally a sensitive gas chromatographic method has also beendefined. Under the reported conditions NSD, NTR, NTS and I.S. pres-ented retention times 18.9� 0.1, 12.2� 0.1, 9.6� 0.1 and 14.0� 0.1min,respectively (Fig. 4).

The peak area ratios between analytes and internal standard wereplotted against the concentration of the products, obtaining the regressionrelationships reported in Table 3. Calibration curves were prepared bydetermining the peak area responses from known amounts of standardsolutions, and fitted to the linear equations by least-squares regression.

Figure 4. Chromatogram of NSD and its photoproducts NTR and NTS in the

presence of internal standard (SI).

1336 VETUSCHI ET AL.

Downloaded By: [Università della Calabria] At: 17:27 2 September 2008

©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

Since many dihydropyridines present thermal degradation,[13] thestability of NSD under the reported GLC instrumental conditions wasinvestigated. For this aim samples of NSD standard solutions were injected,resulting in all the cases the drug recovery to be not below 96% and photo-products NTR and NTS within 1–3%. It was pointed out the on-columninjector to be essential to keep the thermal degradation to a minimum.

This gas chromatographic method, as well as in pharmaceuticalsystems, can be used for the simultaneous determination of the productsin biological samples.

Table 4. Accuracy and Precision Results for NSD, NTR and NTS by Assay onLaboratory Samples and Commercial Formulations

MethodDerivativeOrder

NSD NTR NTS

%Recovery RSD

%Recovery RSD

%Recovery RSD

Derivativespectrophotometry

1 98.60 2.46 96.06 3.34 106.66 3.48

Multivariate analysis 0 102.56 1.12 98.23 2.58 96.58 3.111 98.02 1.10 98.53 1.88 102.58 2.152 98.45 2.15 98.38 2.45 97.82 3.453 99.96 0.85 101.19 1.25 100.56 1.85

4 104.03 3.32 105.05 4.81 97.47 3.32Gas chromatography – 106.90 3.65 106.59 4.34 95.86 4.02

All the recovery values are means of six determinations.

Table 3. Calibration Graphs for NSD, NTR and NTS Assay by Derivative

Spectrophotometry and Gas Chromatography

Method Analyte Signal Slope InterceptCorrelationCoefficient

LinearityRange

(mgmL�1)

1st Derivativespectro-photometry

NSD 1D389 �16.340 �0.154 0.9983 0–30NTR 1D280 �6.238 �0.646 0.9975 0–14NTS 1D335 �14.482 �0.867 0.9943 0–14

Gas chromato-graphy

NSD A18.9/AI.S 7.06� 10�4 8.55 0.9989 2–40NTR A12.2/AI.S 4.72� 10�4

�2.82 0.9934 2–40NTS A9.6/AI.S 6.13� 10�4 5.13 0.9932 2–40

METHODS FOR DETERMINATION OF NSD, NTS, AND NTR 1337

Downloaded By: [Università della Calabria] At: 17:27 2 September 2008

©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

All the proposed methods have been applied to the analysis of labora-tory mixtures obtaining the results shown in Table 4. The limit of quantita-tion for both NTR and NTS resulted to be 0.3 mgmL�1, calculated on aNSD 30 mgmL�1 solution, equivalent to an impurity level of 1%. When themethods were applied to the analysis of NSD pharmaceutical preparations,the assay results were in accordance with the drug declared amount, result-ing NTR and NTS found to be not more than 1%.

To summarize, the two spectrophotometric methods described can beapplied successfully to the analysis of nisoldipine pure or formulationswith good accuracy and precision. Both procedures present advantagesin time and costs in respect to a separation technique, such as gas chroma-tography, proving to be very rapid in estimating simultaneously the drugand both photodegradation products.

ACKNOWLEDGMENTS

This research was supported by grants from M.U.R.S.T. (Italy).

REFERENCES

1. Yan, T.; Wu, Y.; Zhao, J.; Nie, H.; Yuan, F.; Tang, H.; Jin, S.Preparation and Structure of Photochemical Decomposition Productof Nisoldipine. Yaowu Fenxi Zazhi 1989, 9, 10–12.

2. Takahashi, K.; Noda, H.; Noda, A. Photostability of Nisoldipine.Kyushu Yakugakkai Kaiho 1993, 47, 37–43.

3. Scherling, D.; Karl, W.; Ahr, H.J.; Wehinger, E. Pharmacokinetics ofNisoldipine III. Biotransformation of Nisoldipine in Rat, Dog, Monkeyand Man. Arzneim. Forsch/Drug Res. 1988, 38, 1105–1110.

4. Barbato, F.; Cappello, B.; Grumetto, L.; Morsica, P. Analysis ofCalcium Channel Blocking Dihydropyridines bu High PerformanceLiquid Chromatography. Farmaco 1992, 48, 417–426.

5. Alvarez-Lueje, A.; Naranjo, L.; Nunez-Vergara, L.J.; Squella, J.A.Electrochemical Study of Nosoldipine: Analytical Application inPharmaceutical Forms and Photodegradation. J. Pharm. Biom. Anal.1988, 16, 853–862.

6. Van Harten, J.; Lodewijks, M.T.M.; Guyt-Scholten, J.W.; VanBrummelen, P.; Breimer, D.D. Gas Chromatographic Determinationof Nisoldipine and One of its Metabolites in Plasma. J. Chromatogr.1987, 423, 327–333.

1338 VETUSCHI ET AL.

Downloaded By: [Università della Calabria] At: 17:27 2 September 2008

©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

7. Vetuschi, C.; Ragno, G.; Losacco, V. Applicazione Della Spettrofoto-metria UV Derivata e Della Gascromatografia Alla Determinazione diun Antibatterico in Supposte. Boll. Chim. Farm. 1988, 7/8, 193–197.

8. Vetuschi, C.; Ragno, G. Improvement of Multivariate UV Analysis bySelecting Wavelengths and Instrumental Parameters. Anal. Lett. 2001,in press.

9. Garrido Frenich, A.; Jouan-Rimbaud, D.; Massart, D.L.;Kuttatharmmakul, S.; Martinez Galera, M.; Martinez Vidal, J.L.Wavelength Selection Method for Multicomponent Spectrophoto-metric Determinations Using Partial Least Squares. Analyst 1995,120, 2787–2792.

10. Liang, Y.Z.; Xie, Y.L.; Yu, R.Q. Accuracy Criteria and OptimalWavelength Selection for Multicomponent SpectrophotometricDeterminations. Anal. Chim. Acta 1989, 222, 347–357.

11. Martens, H.; Naes, T. Multivariate Calibration. Wiley: Chichester.1989.

12. Salamin, P.A.; Bartels, H.; Forster, P. A Wavelength and Optical PathLength Selection Procedure for Spectroscopic MulticomponentAnalysis. Chemom. Intell. Lab. Syst. 1991, 11, 57–62.

13. Beresford, A.P.; Macrae, P.V.; Stopher, D.A.; Wood, B.A. Analysisof Amlodipine in Human Plasma by Gas Chromatography.J. Chromatogr. 1987, 420, 178–183.

Received August 3, 2001Accepted April 9, 2002

METHODS FOR DETERMINATION OF NSD, NTS, AND NTR 1339

Downloaded By: [Università della Calabria] At: 17:27 2 September 2008

©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

Downloaded By: [Università della Calabria] At: 17:27 2 September 2008