- Email: [email protected]
On-line photochemical derivatization and flow-injection spectrophotometric determination of ergonovine maleate
95
Analytica Chimica Acta, 282 (1993) 95-100 Elsevier Science Publishers B.V., Amsterdam
On-line photochemical derivatization and flow-injection spectrophotometric determination of ergonovine maleate A. Mellado Romero and G. Gomez Benito Departamento de Q&mica, Colegio Universitario CEU, Universitat de Valencia, Moncaab, Vahcia (Spain)
J. Martinez Calatayud Departamento a2 Quhica Analitica, Universitat de Vakncia, c / . Dr. Moliner 50, 46100 Bwjmot, Valencia (@ah) (Received 7th December 1992; revised manuscript received 16th April 1993)
Abstral!t
A flow-injection manifold is proposed for the on-line photochemical derivatization of ergonovine maleate with application to pharmaceutical formulations. The procedure is based on the reaction with p-dhnethylaminobenzaIdehyde in presence of iron and sulphuric acid medium. The reaction rate is improved by irradiation of the solution with a mercury lamp (254.0 nm). The calibration graph is linear over the range 3-74 pg ml-’ ergonovine, with a mid-range R.S.D. of 1.7% and a sample throughput of 34 h-‘. The influence of foreign compounds was tested. Keywords: How injection; UV-Visible chemical derivatization
spectrophotometty;
Ergonovine is an alkaloid isolated from ergot. This and similar alkaloids have varying degrees of a-adrenergic blocking activity, a direct stimulating action on smooth muscle, especially that of the blood vessels and the uterus, and antiserotonin effects. They may be used in obstetrics to prolong uterine contractions in the later stages of labour and to check post-partum haemorrhaging
Dl.
Alkaloids; Ergonovine maleate; Pharmaceuticals;
Photo-
HC-COOH H!-COOH 14
GJWW2~C,H,O.,
Moht. = 441.48
Ergonovine maleate Correspondence to: J. Martinez Calatayud, Departamento de Quimica Arm&a, Universitat de Valencia, c/. Dr. Molder 50, 46100 Bmjasot, Valencia (Spain). 0003-2670/93/$06.00
Most of the currently used procedures for the determination of ergot alkaloids, including official recommendations in some pharmacopoeias
Q 1993 - Elsevier Science Publishers B.V. All rights reserved
96
A. Melha’o Romero et aL /AnaL Chim. Acta 282 (1993) 95-100
and in the AOAC Official Methods of Analysis [2], are based on reaction of the alkaloid with p-dimethylaminobenzaldehyde (PDAB), also known as the Van Urk reaction [31. Many modifications have been proposed to the original procedure. Smith [4] modified the reagent by dissolving PDAB in concentrated sulphuric acid instead of acidified ethanol and also found that exposure to light was necessary for full colour development. This exposure was unnecessary if a small amount of iron(II1) chloride [5] or hydrogen peroxyde was added to the reagent [6]. Michelon and Kelleher
[7] reported that nitrite enhanced the colour produced by the reaction between PDAB and ergot alkaIoids. The system ergonovine-PDBA in sulphuric acid medium was applied to the kineticspectrophotometric determination of nitrite ions [81; the reaction rate was strongly enhanced by the presence of nitrite or iron(II1). Linear calibration graphs were obtained for nitrite concentrations in the range 0.66-6 pg 1-l. The analytical potential of photochemical reactions arises from the inherent advantages of this type of reaction over ordinary chemical reactions. The advantages include the facility to change the nature of the reagent (the photon) by simply modifying the working wavelength, some reaction conditions, such as the lamp power or reactor configuration, can be easily changed to speed up the process concerned. There are a large number of light-sensitive pharmaceutical compounds and it would therefore be of interest to develop methods for their determination exploiting the features of photochemical processes. Implementing a continuous-flow assembly with photo-derivatization is an interesting trend [9,10]; the flow-injection (FI) determinations of reserpine [ll] and diferent phenothiazines [12-141 have been achieved with the drug derivatized on-line by photoreaction. This paper deals with the FI spectrophotometric determination of ergonovine on the basis of the reaction with PDAB in the presence of iron(II1) in an FI manifold provided with an on-line irradiation source.
a.
b.
EXPERIMENTAL
Fig. 1. Flow manifolds. (a) Assembly for preliminary experiments. L, 315 cm; S, 312.5 ~1; C, 3.4 ml min-‘. (b) Proposed FI assembly for ergonovine determination. C, carrier [4.2x 10e3 M PDAB-4.47 x lo-’ M Fe(III) in 30% sulphuric acid]; R, reagent [8.4~10-~ M PDAB-8.95~10~~ M Fe(III) in 30% sulphuric acidl; S, sample (ergonovine maleate in 30% sulphuric acid); Mz, mixing chamber; L, irradiation source (mercury lamp); P, peristaltic pump; D, spectrophotometric detector, Ret, recorder; W, waste. (c) Selected merging device.
Reagents and apparatus Analytical-reagent grade chemicals were used unless indicated otherwise. Ergonovine maleate (Sigma), p-dimethylaminobenxaldehyde (PDAB) (Scharlau), sulphuric acid (Panreac), sodium nitrite (Probus), iron(B) chloride hexahydrate (Probus, pure), phosphoric acid (Probus), sodium chloride (Panreac, ar), hydrochloride acid (Panreac), caffeine (Merck, pure), lactose (Poulenc) phenobarbital acid (Lasa, pure) cyclicine hydrochloride (Gayoso
A. Mellado Romero et al./AnaL Chim. Acta 282 (1993) 95-100
97
Wellcome, pure), paracetamol (Guinama, pure) and maleic acid (Doesder) were used. The stability of ergonovine and PDAB aqueous solutions was tested by means of periodic recording of absorption spectra. A 9.22 X 10m6 M ergonovine maleate aqueous solution, kept in a closed vessel, showed unchanged spectra during 45 days. In 30% sulphuric acid medium it remained stable during for about 2.5 h. The stability of the reagent solution (8.95 x 10m2 M iron(II1) plus 8.37 X 10T3 M PDAB in 30% sulphuric acid medium) was studied over 6 days and no changes were observed. The action of W irradiation on the system was studied by using the manifolds depicted in Fig. 1. The sample was irradiated whilst being circulated through a tube coiled around the lamp. The sample solution merged with a mixture of PDAB in sulphuric acid medium and iron(III), both at a flow-rate of 2.05 ml min-‘; the mixing chamber produced a homogeneous solution which, in turn, was injected, via a Rheodyne Model 5051 injection valve, into the carrier stream flowing at 4.85 ml rnin-‘. The carrier was a solution with the same composition as the reagent solution but with different concentrations. The sample-reagent mixture was irradiated with a
Philips T W mercury lamp (6 W) in the sample loop, the volume of which was 2008 ~1. After 1 min of irradiation the sample passed to the flowcell of a Hewlett-Packard Model 8452A diodearray spectrophotometer. Outputs at 542.0 nm were recorded. The PTFE tubing was of 0.8 mm i.d. and the peristaltic pump used was a Gilson Minipuls Model 2.
RESULTS AND DISCUSSION
Preliminary experiments were carried out by a batch procedure; the lamp was placed on a beaker containing the reaction mixture and spectra (from 380 to 820 nm) were recorded at different time intervals up to 20 min, the influence of the medium on the reaction rate was tested with sulphuric and phosphoric acid. Both acidic media were studied in the presence and absence of iron(II1) or nitrite ions; all results were obtained by comparing experiments carried out with irradiation with experiments without irradiating the mixture. Some of results obtained are shown in Fig. 2. All the recorded spectra showed the same profile, differences only being observed in the absorbance values.
(b)
Fig. 2. Development of the blue colour with time in the batch procedure. (a) Plot of absorbances at 554.0 MI: a = irradiation and 32.5% sulphuric acid; b = irradiation, 2.24 X lo-* M FefIII) in 15% sulphuric acid; c - no irradiation, 1.81 X lo-’ M nitrite in 32.5% sulphuric acid; d = irradiation and 32.5% phosphoric acid; e = irradiation and 15% sulphuric acid, f = irradiation, 2.24 x 10e3 M Fe(IIII in 32.5% phosphoric acid. (b) Spectra of the ergonovine-PDAB system vs. irradiation time: a = 0; b = 10; c=20,d==2S;e30, f = 40; g - 50 min. Concentration of Fe(IID, 2.24 X lo-* M, 32.5% phosphoric acid. Ergonoviue and PDAB final concentrations were 2.3 X 10e4 and 4.18 X lo-’ M, respect&k for all sohttions.
A. MeUado Romero et al. /Anal. Chim. Acta 282 (1993) 95-100
Sulphuric was found to be the more suitable acidic medium; the recorded absorbances were clearly higher than those ones obtained with solutions containing phosphoric acid. The comparison of absorbances in sulphuric acid against the corresponding in phosphoric acid were 22% and 5% higher in the presence of nitrite and iron(II1) ions, respectively. Some results are compared in Fig. 2b. Using the sulphuric acid medium, the absorbances were higher with irradiation alone than with irradiation in the presence of nitrite. The best results were observed in 15% sulphuric acid medium at 2.2 x 10m2 M iron(II1) and with irradiation; the absorbance was about 56% lower in the absence of iron(II1). The influence on the reaction rate and absorbances at the steady state was also tested by changing the’ order in which the reagents (and irradiation) were added. Systems in which iron0111 and ergonovine (in that order) were sucessively added to a solution of PDAB in sulphuric acid medium, the photon being the last added “reagent”, showed the best results. Two different lamps were tested (with the main wavelengths at 254 and 365 nm, respectively); the results were about 16% higher at 254 mn. The above results led to the following optimum conditions: sulphuric acid concentration, ca. 15%; main wavelength of irradiation, 254 nm; iron(II1) present, ca. 2.2 X 10m2 M; and PDAB concentration, ca. 4.18 x 10m3 M.
Unsegmented continuous-flow txpwiments Two different unsegmented continuous-flow manifolds were tested (see Fig. 1). Preliminary studies on the influence of various parameters were carried out: sulphuric acid was tested over the range 20-30%, the manifold conditions were varied and the irradiation time was changed from oto5min. The concentrations of iron(II1) and sulphuric acid had a marked effect. For instance, with 8.95 X 10m2 M Fe0111 the peak height absorbances were 0.30 and 0.15 for 3Q and 20% sulphuric acid, respectively; when the iron(II1) concentration was decreased to 1.79 x 10T2 M
the corresponding absorbances were 0.23 and 0.11. Some conclusions were drawn from the former continuous-flow experiments (the stopped-flow mode was considered): stopped flow for 1 min with and without irradiation resulted in absorbances of 0.29 and 0.14, respectively. The transient outputs increased with the time of irradiation; the stopped-flow interval (irradiation time) resulted in an increase in the transient signal 130% sulphuric acid and 8.95 x lob2 M Fe(III)]; the absorbances were 0.21, 0.29 and 0.35 for stopped-flow intervals of 0, 1 and 5 min, respectively. An increase in the lamp power from 6 to 30 W did not result in an increase in peak height. The FI manifold in Figure lb was selected for further work. The sample solution in the proposed manifold merges with the reagent stream [PDAB and iron(II1) in sulphuric acid medium], the resulting mixture is exposed to irradiation (mercury lamp, 254 nm, 6 W) in the sample loop, then the carrier leads the sample to the spectrophotometer flow cell. The optimization of both chemical and FI manifold parameters was carried out in a sequential procedure. Temperature was found not to be a critical parameter; the absorbances obtained varied from 0.50 to 0.54 when the temperature was increased from 6 to 46°C. The irradiation time was tested by varying the irradiation interval and the length of tubing coiled around the lamp. Figure 3 shows that irradiation times over 1 min are not critical.
Fig. 3. Influence on absorbanceof varyingthe irradiationtime by changing the length of the coiled tubing around the lamp: a=351; b-198, c=9Ocm.
A. Me&do Romeroet al. /Anal. Chim. Acta 282 (1993) 95-l@ TABLE 1 Optimization of chemical and FI parameters Parameter
Studied range
Selected value
Temperature CC) Reagent concentrations: PDAB (MI Fe(IIII (MI Sulphuric acid (%, w/v) Irradiation time (mitt) ’ Irradiated coil length (cm) Injection valve-detector distance km) Carrier flow-rate (ml min- ‘1 Wavelength (nm)
6-46
Ambient
3.4 x 10-3-1.3 x 10-s 1.79 x 10-2-35.8 x 10-s 10-50 OS-2 90-351 22-122 1.4-4.8 540.0-528.0
8.4 x 1O-3 8.95 x 1O-2 30 1 351 22 4.8 542.0
B The reported stopped-flow intervals were studied for each irradiated coil length tested.
Different sets of experiments were carried out in order to test the influence of flow rate, the type of merging device and the compositions of the reagent and carrier solutions. The compositions of the reagent and carrier solutions should be the same in order to avoid absoption differences due to the yellow colour developed in the mixture of reagents (without ergonovine). Owing to dilution of the reagent solution when it merges with the sample solution, the prepared concentrations in the reagent solution were twice those in the carrier. The selected values of concentrations given in Table 1 are those corresponding to the reagent solution. The merging device selected is depicted in Fig. lc. Bearing in mind the profile of the absorption spectra without a clear maximum absorption band, the monitoring wavelength was also optimized. The results of the optimization experimentes are given in Table 1. Analytical application The procedure was applied to the determination of ergonovine in pharmaceutical formulations. The calibration graph was linear over the range 3-74 pg ml-’ ergonovine. The calibration was carried out by means of three independent experiments on different days; the average equation was absorbance = ( - 1.7 X
x
lo-‘)
[ergonovine
+ 1.35
x
(kg ml-r)]
lo-’
were the absorbance is the corrected (from reagent absorbances) value; Mean results of three experiments were obtained and the calculated R.S.D. for the slope was 3.6%; the mean correlation coefficient was 0.9999 (n = 3). The reproducibility of the method and the sample throughput were studied by using a sample solution containing 29 Fg ml-’ of ergonovine. The calculated R.S.D. (17 replicates) was 1.7% and sample throughput was 34 h-‘. The tolerance of the method to foreign substances formulated together with the ergonovine was studied with sample solutions containing 29 ccg ml-’ ergonovine and various amounts of foreign compounds up to 1000 pg ml-’ or giving a relative error (in comparison with the output for pure ergonovine in distilled water) of up to about 3% (see Table 2).
TABLE 2 Infhtence of foreign compounds Compound
Concentration La ml-‘)
Relative error (%I
Paracetamol Caffeine Lactose Maleic acid Sodium chloride Phenobarbital Chloride acid Cyclicine (10% in ethanol)
750 1000 1000 loo0 loo0 500 750 750
0.2 1.9 2.3 0.9 0.1 0.1 0.3 0.4
A. MeUadoRomero et al /Anal. Chim. Acta 282 (1993) 95-100
Samples (injectable solutions) were prepared according to [l]: (a) sterile solutions of the drug at pH 3.0, adjusted potentiometrically with maleic acid; and (b) solutions of ergonovine maleate containing NaCl and HCl. Errors were calculated by comparing the results obtained with the amount added: (a) added, 200.0 pg ml-’ obtained 194 pg ml-’ and (b) added, 200.0 pg ml-‘, obtained 202 pg ml-’ calculated relative error, 3% and l%, respectivelly. Conclusion.9
A procedure for the determination of ergonovine (probably with application to other ergot alkaloids) in pharmaceutical formulations was developed. The procedure use milder conditions than the batch process (the sulphuric acid concentration was 65% in the former procedure). The present method allows the determination of ergonovine at lower concentrations than in a previously published non-selective FI procedure for ergonovine (linear calibration graph over the 150-250 pg ml-‘), based on the oxidation of the drug by the combined action of sodium periodate and hydrogen peroxide and spectrophotometric measurement [ 151.
REFERENCES 1 Martindale, The Extra Pharmacopoeia, Pharmaceutical Press, London, 6th edn., 1972. 2 S. Williams (Ed.) Official Methods of Analysis, of the Association of Official Analytical Chemists, Association of official Analytical Chemistry, Washington, DC, 14th edn., 1985. 3 H.W. van Urk, Pharm. Weekbl., 66 (1929) 473. 4 M.I. Smith, Public Health Rep., 45 (1930) 1466. 5 N.L. Allport and T.T. Cocking, Q.J. Pharm. Pharmacol., 5 (1932) 341. 6 E. Schulek and G. Vastag, Dan. Tidsskr. Farm., 13 (1939) 101. 7 L. Michelon and W.J. Kelleher, Lkrydia, 26 (1%3) 192. 8 J. Martinez Calatayud and C. Medina Hemandez, Michrochem. J., 43 (1991) 143. 9 J. Martinez Calatayud and C. Gomez Benito, Cienc. Pharm., 2 (1992) 57. 10 M.C. Mahedero and J.J. Aaron, Anal. Chim. Acta, 269 (1992) 193. 11 J. Martinez Calatayud and C. Gomez Benito, Anal. Chhn. Acta, 245 (1991) 101. 12 J. Martinez Calatayud and C. Gomez Benito, Anal. Chim. Acta, 256 (1992) 105. 13 A. Mellado Romero, C. Gomez Benito and J. Martinez Calatayud, Anal. Lett. 25 (1992) 1289. 14 D. Chen, A Rios, M.D. Luque de Castro and M. Valcarccl, Analyst, 116 (1991) 171. 15 J. Martinez Calatayud and S. Sagrado Vives, Pharmazie, 44 (1989) 614.
Analytica Chimica Acta, 282 (1993) 95-100 Elsevier Science Publishers B.V., Amsterdam
On-line photochemical derivatization and flow-injection spectrophotometric determination of ergonovine maleate A. Mellado Romero and G. Gomez Benito Departamento de Q&mica, Colegio Universitario CEU, Universitat de Valencia, Moncaab, Vahcia (Spain)
J. Martinez Calatayud Departamento a2 Quhica Analitica, Universitat de Vakncia, c / . Dr. Moliner 50, 46100 Bwjmot, Valencia (@ah) (Received 7th December 1992; revised manuscript received 16th April 1993)
Abstral!t
A flow-injection manifold is proposed for the on-line photochemical derivatization of ergonovine maleate with application to pharmaceutical formulations. The procedure is based on the reaction with p-dhnethylaminobenzaIdehyde in presence of iron and sulphuric acid medium. The reaction rate is improved by irradiation of the solution with a mercury lamp (254.0 nm). The calibration graph is linear over the range 3-74 pg ml-’ ergonovine, with a mid-range R.S.D. of 1.7% and a sample throughput of 34 h-‘. The influence of foreign compounds was tested. Keywords: How injection; UV-Visible chemical derivatization
spectrophotometty;
Ergonovine is an alkaloid isolated from ergot. This and similar alkaloids have varying degrees of a-adrenergic blocking activity, a direct stimulating action on smooth muscle, especially that of the blood vessels and the uterus, and antiserotonin effects. They may be used in obstetrics to prolong uterine contractions in the later stages of labour and to check post-partum haemorrhaging
Dl.
Alkaloids; Ergonovine maleate; Pharmaceuticals;
Photo-
HC-COOH H!-COOH 14
GJWW2~C,H,O.,
Moht. = 441.48
Ergonovine maleate Correspondence to: J. Martinez Calatayud, Departamento de Quimica Arm&a, Universitat de Valencia, c/. Dr. Molder 50, 46100 Bmjasot, Valencia (Spain). 0003-2670/93/$06.00
Most of the currently used procedures for the determination of ergot alkaloids, including official recommendations in some pharmacopoeias
Q 1993 - Elsevier Science Publishers B.V. All rights reserved
96
A. Melha’o Romero et aL /AnaL Chim. Acta 282 (1993) 95-100
and in the AOAC Official Methods of Analysis [2], are based on reaction of the alkaloid with p-dimethylaminobenzaldehyde (PDAB), also known as the Van Urk reaction [31. Many modifications have been proposed to the original procedure. Smith [4] modified the reagent by dissolving PDAB in concentrated sulphuric acid instead of acidified ethanol and also found that exposure to light was necessary for full colour development. This exposure was unnecessary if a small amount of iron(II1) chloride [5] or hydrogen peroxyde was added to the reagent [6]. Michelon and Kelleher
[7] reported that nitrite enhanced the colour produced by the reaction between PDAB and ergot alkaIoids. The system ergonovine-PDBA in sulphuric acid medium was applied to the kineticspectrophotometric determination of nitrite ions [81; the reaction rate was strongly enhanced by the presence of nitrite or iron(II1). Linear calibration graphs were obtained for nitrite concentrations in the range 0.66-6 pg 1-l. The analytical potential of photochemical reactions arises from the inherent advantages of this type of reaction over ordinary chemical reactions. The advantages include the facility to change the nature of the reagent (the photon) by simply modifying the working wavelength, some reaction conditions, such as the lamp power or reactor configuration, can be easily changed to speed up the process concerned. There are a large number of light-sensitive pharmaceutical compounds and it would therefore be of interest to develop methods for their determination exploiting the features of photochemical processes. Implementing a continuous-flow assembly with photo-derivatization is an interesting trend [9,10]; the flow-injection (FI) determinations of reserpine [ll] and diferent phenothiazines [12-141 have been achieved with the drug derivatized on-line by photoreaction. This paper deals with the FI spectrophotometric determination of ergonovine on the basis of the reaction with PDAB in the presence of iron(II1) in an FI manifold provided with an on-line irradiation source.
a.
b.
EXPERIMENTAL
Fig. 1. Flow manifolds. (a) Assembly for preliminary experiments. L, 315 cm; S, 312.5 ~1; C, 3.4 ml min-‘. (b) Proposed FI assembly for ergonovine determination. C, carrier [4.2x 10e3 M PDAB-4.47 x lo-’ M Fe(III) in 30% sulphuric acid]; R, reagent [8.4~10-~ M PDAB-8.95~10~~ M Fe(III) in 30% sulphuric acidl; S, sample (ergonovine maleate in 30% sulphuric acid); Mz, mixing chamber; L, irradiation source (mercury lamp); P, peristaltic pump; D, spectrophotometric detector, Ret, recorder; W, waste. (c) Selected merging device.
Reagents and apparatus Analytical-reagent grade chemicals were used unless indicated otherwise. Ergonovine maleate (Sigma), p-dimethylaminobenxaldehyde (PDAB) (Scharlau), sulphuric acid (Panreac), sodium nitrite (Probus), iron(B) chloride hexahydrate (Probus, pure), phosphoric acid (Probus), sodium chloride (Panreac, ar), hydrochloride acid (Panreac), caffeine (Merck, pure), lactose (Poulenc) phenobarbital acid (Lasa, pure) cyclicine hydrochloride (Gayoso
A. Mellado Romero et al./AnaL Chim. Acta 282 (1993) 95-100
97
Wellcome, pure), paracetamol (Guinama, pure) and maleic acid (Doesder) were used. The stability of ergonovine and PDAB aqueous solutions was tested by means of periodic recording of absorption spectra. A 9.22 X 10m6 M ergonovine maleate aqueous solution, kept in a closed vessel, showed unchanged spectra during 45 days. In 30% sulphuric acid medium it remained stable during for about 2.5 h. The stability of the reagent solution (8.95 x 10m2 M iron(II1) plus 8.37 X 10T3 M PDAB in 30% sulphuric acid medium) was studied over 6 days and no changes were observed. The action of W irradiation on the system was studied by using the manifolds depicted in Fig. 1. The sample was irradiated whilst being circulated through a tube coiled around the lamp. The sample solution merged with a mixture of PDAB in sulphuric acid medium and iron(III), both at a flow-rate of 2.05 ml min-‘; the mixing chamber produced a homogeneous solution which, in turn, was injected, via a Rheodyne Model 5051 injection valve, into the carrier stream flowing at 4.85 ml rnin-‘. The carrier was a solution with the same composition as the reagent solution but with different concentrations. The sample-reagent mixture was irradiated with a
Philips T W mercury lamp (6 W) in the sample loop, the volume of which was 2008 ~1. After 1 min of irradiation the sample passed to the flowcell of a Hewlett-Packard Model 8452A diodearray spectrophotometer. Outputs at 542.0 nm were recorded. The PTFE tubing was of 0.8 mm i.d. and the peristaltic pump used was a Gilson Minipuls Model 2.
RESULTS AND DISCUSSION
Preliminary experiments were carried out by a batch procedure; the lamp was placed on a beaker containing the reaction mixture and spectra (from 380 to 820 nm) were recorded at different time intervals up to 20 min, the influence of the medium on the reaction rate was tested with sulphuric and phosphoric acid. Both acidic media were studied in the presence and absence of iron(II1) or nitrite ions; all results were obtained by comparing experiments carried out with irradiation with experiments without irradiating the mixture. Some of results obtained are shown in Fig. 2. All the recorded spectra showed the same profile, differences only being observed in the absorbance values.
(b)
Fig. 2. Development of the blue colour with time in the batch procedure. (a) Plot of absorbances at 554.0 MI: a = irradiation and 32.5% sulphuric acid; b = irradiation, 2.24 X lo-* M FefIII) in 15% sulphuric acid; c - no irradiation, 1.81 X lo-’ M nitrite in 32.5% sulphuric acid; d = irradiation and 32.5% phosphoric acid; e = irradiation and 15% sulphuric acid, f = irradiation, 2.24 x 10e3 M Fe(IIII in 32.5% phosphoric acid. (b) Spectra of the ergonovine-PDAB system vs. irradiation time: a = 0; b = 10; c=20,d==2S;e30, f = 40; g - 50 min. Concentration of Fe(IID, 2.24 X lo-* M, 32.5% phosphoric acid. Ergonoviue and PDAB final concentrations were 2.3 X 10e4 and 4.18 X lo-’ M, respect&k for all sohttions.
A. MeUado Romero et al. /Anal. Chim. Acta 282 (1993) 95-100
Sulphuric was found to be the more suitable acidic medium; the recorded absorbances were clearly higher than those ones obtained with solutions containing phosphoric acid. The comparison of absorbances in sulphuric acid against the corresponding in phosphoric acid were 22% and 5% higher in the presence of nitrite and iron(II1) ions, respectively. Some results are compared in Fig. 2b. Using the sulphuric acid medium, the absorbances were higher with irradiation alone than with irradiation in the presence of nitrite. The best results were observed in 15% sulphuric acid medium at 2.2 x 10m2 M iron(II1) and with irradiation; the absorbance was about 56% lower in the absence of iron(II1). The influence on the reaction rate and absorbances at the steady state was also tested by changing the’ order in which the reagents (and irradiation) were added. Systems in which iron0111 and ergonovine (in that order) were sucessively added to a solution of PDAB in sulphuric acid medium, the photon being the last added “reagent”, showed the best results. Two different lamps were tested (with the main wavelengths at 254 and 365 nm, respectively); the results were about 16% higher at 254 mn. The above results led to the following optimum conditions: sulphuric acid concentration, ca. 15%; main wavelength of irradiation, 254 nm; iron(II1) present, ca. 2.2 X 10m2 M; and PDAB concentration, ca. 4.18 x 10m3 M.
Unsegmented continuous-flow txpwiments Two different unsegmented continuous-flow manifolds were tested (see Fig. 1). Preliminary studies on the influence of various parameters were carried out: sulphuric acid was tested over the range 20-30%, the manifold conditions were varied and the irradiation time was changed from oto5min. The concentrations of iron(II1) and sulphuric acid had a marked effect. For instance, with 8.95 X 10m2 M Fe0111 the peak height absorbances were 0.30 and 0.15 for 3Q and 20% sulphuric acid, respectively; when the iron(II1) concentration was decreased to 1.79 x 10T2 M
the corresponding absorbances were 0.23 and 0.11. Some conclusions were drawn from the former continuous-flow experiments (the stopped-flow mode was considered): stopped flow for 1 min with and without irradiation resulted in absorbances of 0.29 and 0.14, respectively. The transient outputs increased with the time of irradiation; the stopped-flow interval (irradiation time) resulted in an increase in the transient signal 130% sulphuric acid and 8.95 x lob2 M Fe(III)]; the absorbances were 0.21, 0.29 and 0.35 for stopped-flow intervals of 0, 1 and 5 min, respectively. An increase in the lamp power from 6 to 30 W did not result in an increase in peak height. The FI manifold in Figure lb was selected for further work. The sample solution in the proposed manifold merges with the reagent stream [PDAB and iron(II1) in sulphuric acid medium], the resulting mixture is exposed to irradiation (mercury lamp, 254 nm, 6 W) in the sample loop, then the carrier leads the sample to the spectrophotometer flow cell. The optimization of both chemical and FI manifold parameters was carried out in a sequential procedure. Temperature was found not to be a critical parameter; the absorbances obtained varied from 0.50 to 0.54 when the temperature was increased from 6 to 46°C. The irradiation time was tested by varying the irradiation interval and the length of tubing coiled around the lamp. Figure 3 shows that irradiation times over 1 min are not critical.
Fig. 3. Influence on absorbanceof varyingthe irradiationtime by changing the length of the coiled tubing around the lamp: a=351; b-198, c=9Ocm.
A. Me&do Romeroet al. /Anal. Chim. Acta 282 (1993) 95-l@ TABLE 1 Optimization of chemical and FI parameters Parameter
Studied range
Selected value
Temperature CC) Reagent concentrations: PDAB (MI Fe(IIII (MI Sulphuric acid (%, w/v) Irradiation time (mitt) ’ Irradiated coil length (cm) Injection valve-detector distance km) Carrier flow-rate (ml min- ‘1 Wavelength (nm)
6-46
Ambient
3.4 x 10-3-1.3 x 10-s 1.79 x 10-2-35.8 x 10-s 10-50 OS-2 90-351 22-122 1.4-4.8 540.0-528.0
8.4 x 1O-3 8.95 x 1O-2 30 1 351 22 4.8 542.0
B The reported stopped-flow intervals were studied for each irradiated coil length tested.
Different sets of experiments were carried out in order to test the influence of flow rate, the type of merging device and the compositions of the reagent and carrier solutions. The compositions of the reagent and carrier solutions should be the same in order to avoid absoption differences due to the yellow colour developed in the mixture of reagents (without ergonovine). Owing to dilution of the reagent solution when it merges with the sample solution, the prepared concentrations in the reagent solution were twice those in the carrier. The selected values of concentrations given in Table 1 are those corresponding to the reagent solution. The merging device selected is depicted in Fig. lc. Bearing in mind the profile of the absorption spectra without a clear maximum absorption band, the monitoring wavelength was also optimized. The results of the optimization experimentes are given in Table 1. Analytical application The procedure was applied to the determination of ergonovine in pharmaceutical formulations. The calibration graph was linear over the range 3-74 pg ml-’ ergonovine. The calibration was carried out by means of three independent experiments on different days; the average equation was absorbance = ( - 1.7 X
x
lo-‘)
[ergonovine
+ 1.35
x
(kg ml-r)]
lo-’
were the absorbance is the corrected (from reagent absorbances) value; Mean results of three experiments were obtained and the calculated R.S.D. for the slope was 3.6%; the mean correlation coefficient was 0.9999 (n = 3). The reproducibility of the method and the sample throughput were studied by using a sample solution containing 29 Fg ml-’ of ergonovine. The calculated R.S.D. (17 replicates) was 1.7% and sample throughput was 34 h-‘. The tolerance of the method to foreign substances formulated together with the ergonovine was studied with sample solutions containing 29 ccg ml-’ ergonovine and various amounts of foreign compounds up to 1000 pg ml-’ or giving a relative error (in comparison with the output for pure ergonovine in distilled water) of up to about 3% (see Table 2).
TABLE 2 Infhtence of foreign compounds Compound
Concentration La ml-‘)
Relative error (%I
Paracetamol Caffeine Lactose Maleic acid Sodium chloride Phenobarbital Chloride acid Cyclicine (10% in ethanol)
750 1000 1000 loo0 loo0 500 750 750
0.2 1.9 2.3 0.9 0.1 0.1 0.3 0.4
A. MeUadoRomero et al /Anal. Chim. Acta 282 (1993) 95-100
Samples (injectable solutions) were prepared according to [l]: (a) sterile solutions of the drug at pH 3.0, adjusted potentiometrically with maleic acid; and (b) solutions of ergonovine maleate containing NaCl and HCl. Errors were calculated by comparing the results obtained with the amount added: (a) added, 200.0 pg ml-’ obtained 194 pg ml-’ and (b) added, 200.0 pg ml-‘, obtained 202 pg ml-’ calculated relative error, 3% and l%, respectivelly. Conclusion.9
A procedure for the determination of ergonovine (probably with application to other ergot alkaloids) in pharmaceutical formulations was developed. The procedure use milder conditions than the batch process (the sulphuric acid concentration was 65% in the former procedure). The present method allows the determination of ergonovine at lower concentrations than in a previously published non-selective FI procedure for ergonovine (linear calibration graph over the 150-250 pg ml-‘), based on the oxidation of the drug by the combined action of sodium periodate and hydrogen peroxide and spectrophotometric measurement [ 151.
REFERENCES 1 Martindale, The Extra Pharmacopoeia, Pharmaceutical Press, London, 6th edn., 1972. 2 S. Williams (Ed.) Official Methods of Analysis, of the Association of Official Analytical Chemists, Association of official Analytical Chemistry, Washington, DC, 14th edn., 1985. 3 H.W. van Urk, Pharm. Weekbl., 66 (1929) 473. 4 M.I. Smith, Public Health Rep., 45 (1930) 1466. 5 N.L. Allport and T.T. Cocking, Q.J. Pharm. Pharmacol., 5 (1932) 341. 6 E. Schulek and G. Vastag, Dan. Tidsskr. Farm., 13 (1939) 101. 7 L. Michelon and W.J. Kelleher, Lkrydia, 26 (1%3) 192. 8 J. Martinez Calatayud and C. Medina Hemandez, Michrochem. J., 43 (1991) 143. 9 J. Martinez Calatayud and C. Gomez Benito, Cienc. Pharm., 2 (1992) 57. 10 M.C. Mahedero and J.J. Aaron, Anal. Chim. Acta, 269 (1992) 193. 11 J. Martinez Calatayud and C. Gomez Benito, Anal. Chhn. Acta, 245 (1991) 101. 12 J. Martinez Calatayud and C. Gomez Benito, Anal. Chim. Acta, 256 (1992) 105. 13 A. Mellado Romero, C. Gomez Benito and J. Martinez Calatayud, Anal. Lett. 25 (1992) 1289. 14 D. Chen, A Rios, M.D. Luque de Castro and M. Valcarccl, Analyst, 116 (1991) 171. 15 J. Martinez Calatayud and S. Sagrado Vives, Pharmazie, 44 (1989) 614.