真伪大黄的二维相关红外光谱

采用红外光谱法并结合二维相关光谱(two-dimensional correlation spectroscopy)分析技术，对药用西宁大黄和伪品华北大黄进行了无损快速鉴别研究。西宁大黄和华北大黄在一维谱图上差别不显著，而在二维谱图上显示出较大的差别。在1200-1700cm-1波段范围内，西宁大黄在同步图上有两个较强的自动峰，分别在1460和1560cm^-1附近，而华北大黄只有一个较强的自动峰，在1460cm^-1附近；同样在1030-1170cm-1波段范围内西宁大黄有两个较强的自动峰，分别在1060和1080cm^-1附近，而伪品大黄也只有一个较强的自动峰，在1080cm^-1附近。两个波段的异步谱也显示出较大的差别。结果表明：二维相关谱可以提高谱图的分辨率，增加谱图的识别能力，可用于药材真伪品的鉴别。该法快速、准确，为客观评价中药材的来源提供了一种新的方法。     

Optimization for quantitative determination of four flavonoids in Epimedium by capillary zone electrophoresis coupled with diode array detection using central composite design

Herba Epimedii (family Berberidaceae), Ying-Yang-Huo in Chinese, is a famous Chinese herbal medicine. Flavonoids are thought to be the major active components in it. A capillary zone electrophoresis (CZE) separation were developed for simultaneous determination of four flavonoids including icariin, epimedin A, epimedin B and epimedin C in Epimedium. The effects of the experimental variables on CZE had been optimized by using central composite design (CCD). The best separation of four flavonoids could be obtained using 50 mM borate buffer (pH 10.0) containing 22% acetontrile as modifier, while separation voltage was 15 kV and temperature was at 25 degrees C. The method developed is accurate, simple and reproducible, which could be used for quality control of Epimedium and its medical preparations.     

Herba Epimedii: anti-oxidative properties and its medical implications

Herba Epimedii is a Chinese herbal medicine with proven efficacy in treating cardiovascular diseases and osteoporosis, and in improving sexual and neurological functions. This efficacy is found to be related to the potent anti-oxidative ability of Herba Epimedii and its flavonoid components, with icarrin as the main effective constituent, along with polysaccharides and vitamin C. These ingredients have been proven to be effective against oxidative-stress related pathologies (cardiovascular diseases, Alzheimer's disease and inflammation) in animal rodent models and in vitro studies. Their anti-oxidative properties are found to be related to an inductive effect on endogenous free-radical scavenging enzymes such as catalase and glutathione peroxidase and the inherent electron-donating ability of flavonoids.     

Thermal-Dependent dehydration process and intramolecular cyclization of lisinopril dihydrate in the solid state

The pathway of dehydration and intramolecular cyclization of lisinopril dihydrate in the solid state was investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and a combination of thermal analyzer with Fourier transform infrared microspectroscopy (thermal FT-IR microscopic system). The results indicate that the dehydration from the solid-state lisinopril dihydrate had a two-step process from dihydrate to monohydrate at 76 degrees C and then from monohydrate to anhydrate at 99-101 approximately C, which could be clearly observed from the above three methods. Only the thermal FT-IR microscopic system could give vital information on diketopiperazine (DKP) formation via intramolecular cyclization in anhydrous lisinopril. A new peak at 1670 cm(-1) assigned to the carbonyl band of DKP formation was clearly evidenced. The water of reaction byproduct was liberated at a temperature >157 degrees C and appeared on the IR spectra near 3200-3400 cm(-1). Moreover, the peak at 1574 cm(-1) assigned to carboxylate shifted to 1552 cm(-1) due to the DKP formation. The peak at 1670 cm(-1) related to the DKP formation changed slightly in intensity from 147 degrees C and significantly near 157 degrees C. DSC and TGA methods were poor for use in supplying information on DKP formation in lisinopril. The thermal FT-IR microscopic system is useful from the view point that it can quickly and directly show the solid-state stability of drug.     

Surface phase transition of C12E1 at the air/water interface: a study by dynamic surface tension, external RA FT-IR, and 2D IR correlation methods

The surface conformational states of the Gibbs monolayer of ethylene glycol mono-n-dodecyl ether (C(12)E(1)) at the air/water interface was studied using dynamic surface tension, external reflection-absorption FT-IR spectroscopy (ERA FT-IR), and two-dimensional infrared (2D IR) correlation methods at constant temperature. The dynamic surface tensions were measured at different bulk concentrations of C(12)E(1), and it was observed that a constant surface tension region appears at approximately 38.5 mN m(-1) in a dynamic surface tension profile at concentrations higher than 11 micromol kg(-1). This constant surface tension region corresponds to the surface phase transition from liquid expanded (LE) to liquid condensed (LC). Two sets of ERA FT-IR spectra were collected, one at different bulk concentrations but after equilibrium time (equilibrium measurements) and another at constant bulk concentration (m = 16 micromol kg(-1)) but at different times (dynamic measurements). The first set of these measurements show that the peak area increases in the range of 11 < m < or = 16 micromol kg(-1), which means the increase in the number of surfactant molecules at the air/water interface. Also, the wavenumber of antisymmetric CH(2) stretching decreases gradually from approximately 2923 cm(-1) (for 10 and 11 micromol kg(-1)) to approximately 2918 cm(-1) (for m > or = 16 micromol kg(-1)) with increasing concentration. The wavenumbers of 2923 and 2918 cm(-1) were assigned to LE and LC phases, respectively, and the decrease of wavenumber in the concentration range of 11 < m < or = 16 micromol kg(-1) were correlated to the surface phase transition (LE --> LC), or in other words, in the mentioned concentration range, two phases coexist. The dynamic ERA FT-IR measurements at 16 micromol kg(-1) also confirm the surface phase transition from LE to LC. The 2D IR correlation method was applied to the both equilibrium and dynamic IR spectra of the C(12)E(1) monolayer. The synchronous correlation maps show two strong autopeaks at approximately 2922 and approximately 2851 cm(-1) and also show a strong correlation (cross-peaks) between antisymmetric CH(2) stretching (nu(a)) and symmetric CH(2) stretching (nu(s)). The asynchronous correlation maps show that both observed bands of nu(a) and nu(s) in one-dimensional IR split into two components with the characteristic of overlapped bands, which reveals the coexistence of two phases (LE and LC) at the interface at 11 < m < or = 16 micromol kg(-1). The synchronous and asynchronous maps that were obtained from dynamic IR spectra closely resembled the equilibrium map.     

Differentiation of Herba Cistanches by fingerprint with high-performance liquid chromatography-diode array detection-mass spectrometry

Herba Cistanche (Rou Cong Rong in Chinese), dried succulent stems of Cistanche deserticola or C. tubulosa, is a famous Chinese herbal medicine and has been recorded in the Chinese Pharmacopoeia. In recent years, another two non-official species, C. salsa and C. sinensis have also been used as Herba Cistanche in some regions of China. To investigate the possibility of using these two non-official species as alternatives to the official species, a high-performance liquid chromatography-diode array detection-mass spectrometry (HPLC-DAD-MS) fingerprint method was developed to comparatively analyze the crude herbs of these four species. The fingerprint of C. deserticola, a historically certified species of Herba Cistanche, serves as 'standard pattern' for comparing the similarities with the other species by means of similarity and Principle Component Analysis. Additionally, 18 characteristic peaks in the fingerprints were identified by comparing their retention times, UV spectra and ESI-MS data with those of the reference substances and/or the data in the literatures. The comparative results demonstrate that the fingerprints of C. tubulosa and C. salsa possess high similarity to the standard pattern, suggesting that these two species may be used as alternative species; while that of C. sinensis has low similarity (0.053 correlation coefficient) to the standard pattern, indicating that it cannot be used as the substitute of the official herb. However, the varying fingerprint patterns among the samples of C. deserticola collected from various habitats illustrate that the quality consistency of crude herbs is still a problem worthy of serious concern.     

Simultaneous determination of nine flavonoids and qualitative evaluation of Herba Epimedii by high performance liquid chromatography with ultraviolet detection

A simple high performance liquid chromatographic method has been developed for the determination of nine flavonoids in Herba Epimedii, including quercetin-3-O-glucoside, epimedin A, hexandraside A, epimedin B, epimedin C, icariin, icariside I, icariside II, and icaritin. The chromatographic separation was performed on a Diamonsil C(18) column (5 microm, 250x4.6 mm) with gradient elution by acetonitrile-0.5% acetic buffer (adjusted to pH 5.00 with triethylamine). Methodological validation gave acceptable linearities (r(2) >0.9992) and recoveries (ranging from 98.9 to 102.4%). The limits of detection of these flavonoids ranged from 16.3 to 83.3 ng. The results indicated that the contents of flavonoids in Herba Epimedii varied significantly from habitat to habitat with contents ranging from 0.05 to 39.0 mg/g. Twenty five Herba Epimedii samples prepared from five different botanical materials were investigated by the established method, and the results showed the influence of the botanical material and the habitat on the quality of Herba Epimedii. The proposed method is simple, effective, and suitable for the evaluation of this traditional Chinese medicine.     

Diffusion and structure of water in polymers containing N-vinyl-2-pyrrolidone

Poly(N-vinyl-2-pyrrolidone) (PVP), a water-soluble polymer, is known for its excellent biocompatibility. It is generally recognized that the properties of polymers may be profoundly affected by the structure of water absorbed in them. In this study, Fourier transform infrared (FT-IR) in attenuated total reflection (ATR) and transmission mode was performed to examine the diffusion and structure of water in PVP and its copolymers. The obtained spectra were analyzed using two-dimensional (2D) IR with the aid of density functional theory (DFT) calculations. The 2D IR of time-resolved FT-IR/ATR spectra shows that type II water between 3300 and 3500 cm(-1) occurs earlier during the water absorption process, which is also demonstrated by transmission FT-IR at the initial stage of water absorption. Conversely, type II water changes last when desorption takes place. Results from DFT calculations indicate that type II water might be monomeric or dimeric water molecules interacting with a carbonyl group in the pyrrolidone moiety. Furthermore, it is found that vibrations less than 3300 cm(-1) (type I water) arise from water molecules involved in a carbonyl group interacting with more than two water molecules. It is reasonable that the transmission FT-IR spectra of film with an extra low water amount hardly show vibration bands below 3300 cm(-1); however, this region is distinct in the FT-IR/ATR spectra of fully swollen film. In addition, vibration bands between 3800 and 3500 cm(-1) (type III water) are assigned to free water or water with relatively weak hydrogen bonding, as supported by the transmission FT-IR spectra of polyacrylonitrile (PAN) and the calculation results. Therefore, the diffusion process and the structures of water in PVP and its copolymers can be successfully accessed on the basis of the 2D IR analysis and DFT calculations.     

Pharmacokinetics and oral bioavailability of epimedin C after oral administration of epimedin C and Herba Epimedii extract in rats

Epimedin C, an ingredient of Herba Epimedii, has potential for treatment of cardiovascular disease and bone loss. However, there is still no sensitive analytical method to monitor epimedin C in biological samples. The goal of this study was to develop a sensitive and reliable method based on a LC‐MS/MS for evaluating the pharmacokinetics of epimedin C after administration of Herba Epimedii in rat. Electrospray ionization in positive‐ion mode and multiple reaction monitoring were used to identify and quantitate active components. Analytes were separated by a reverse‐phase C₁₈ column. Liquid–liquid extraction using ethyl acetate, evaporation and reconstitution was used to plasma sample preparation. Mass transition of precursor ion → product ion pairs were monitored at m/z 823.4 → 313.1 for epimedin C and m/z 237.1 → 178.9 for carbamazepine (internal standard). A calibration curve gave good linearity (r > 0.999) over the concentration range 2.5–500 ng/mL. Pharmacokinetic data demonstrated that there was rapid distribution and slow elimination after epimedin C administration (1 mg/kg, i.v.). Oral bioavailabilities of epimedin C in the pure compound and in the Herba Epimedii were around 0.58% and 0.13%, respectively. The result suggests that other herbal ingredients of Herba Epimedii may suppress the oral bioavailability of epimedin C. Copyright © 2013 John Wiley & Sons, Ltd.     

Infrared microspectroscopic identification of marker ingredients in the finished herbal products based on the inherent heterogeneity of natural medicines

Finished herbal products (FHPs) are preparations made from one or more herbs. The first stage in assuring the quality, safety, and efficacy of FHPs is to identify the herbs in the products. A new simple and quick method is developed in this research to detect the marker ingredients in FHPs. The inherent chemical heterogeneity of herbs and FHPs makes it possible to resolve different ingredients, without any additional separation or labeling, by infrared microspectroscopic imaging. Therefore, multiple marker ingredients in FHPs can be recognized directly and simultaneously by the infrared microspectroscopic identification method. As an example, all six kinds of herbs in Liuwei Dihuang Wan are identified through the following steps: (1) Each herb is characterized by infrared spectroscopic imaging, then the spectra of the main ingredients are calculated by the combination of principal component analysis, independent component analysis, and alternating least squares. (2) One marker ingredient is chosen for each herb. Ten typical pixels, the spectra of which best match the calculated spectrum of the marker ingredient, are selected by partial least squares target. The average spectrum of the typical pixels is taken as the marker spectrum. (3) Correlation coefficients between the typical pixel spectra and the marker spectrum are calculated. The acceptance correlation threshold is determined through the beta distribution function and then validated by positive and negative samples. (4) Using the above marker spectra and correlation criteria, herbs in the model mixture and the commercial product are identified. Good recognition results reveal the potential of the infrared microspectroscopic identification method in the quality control of herbs and FHPs.

Analysis and identification of different animal horns by a three-stage infrared spectroscopy

In this study, a new method, a three-stage infrared spectroscopy (Fourier transform infrared spectroscopy (FT-IR) integrated with second derivative infrared spectroscopy and two-dimensional correlation infrared spectroscopy (2D-IR)) was developed to analyze the organic and inorganic compositions of three different horns (Cornu Antelopis, Cornu Bubali and Pulvis Cornus Bubali Concentratus). In IR spectra, all the three horns had their own macroscopic fingerprints especially for those compositions containing amide groups, CH groups and Ca(3)(PO(4))(2). Their second derivative spectra amplified the differences and revealed the potentially characteristic IR absorption bands 1350-400 cm(-1) to be investigated in 2D-IR. Subsequently, many covered characteristic fingerprints were disclosed in 2D-IR spectra in the range of 1350-400 cm(-1) and the three horns were therefore effectively discriminated. Meanwhile, the analysis results of inorganic constituents were verified by atomic spectroscopy. Furthermore, thirty different horn samples including ten of each horn were also successfully classified by soft independent modeling of class analogy (SIMCA). It was demonstrated that the above three-stage infrared spectroscopy could be applicable for quick, non-destructive and effective analysis and identification of very complicated and similar mixture systems (e.g. traditional Chinese medicines).     

Discriminating five Polygonatum medical materials and monitoring their chemical changes associated with traditional process by FT-IR spectroscopy coupled with multivariate analysis

Several Polygonatum species are important medicinal materials as tonic to cure disorders in China. Because of their different medical effects, it is desired to distinguish them at species level. In addition, to ensure and control their medical quality, it is also important to monitor their chemical changes associated with traditional process. Taking the advantages of Fourier transform infrared spectroscopy (FT-IR) and multivariate analysis, we developed a convenient, fast and reliable approach to discriminate and quality control these materials. Despite similar absorption patterns, each species also presented spectral differences, especially on the FT-IR fingerprint range of 1800-600 cm⁻¹. Second derivative method obviously enlarged those differences and then showed more species-specific features. These spectral differences could be used as powerful discriminating points to distinguish them. PCA results showed that each species separated clearly with their biological replicates grouped together, which indicated that the variance between species is greater than within species, therefore, these species could be distinguishable. Using this approach, the five herbal materials were discriminated successfully in their raw, processed and ethanol extracted formats. On the other hand, visual inspecting infrared spectra of samples from 1 to 9 process steps, absorbance near 1737, 1259, 817 and 780 cm^-1 increased gradually but decreased gradually at 927 cm⁻¹. Besides, spectral contour near 1050 cm⁻¹ changed sharply with process treatment. These spectral changes indicated that hydrolyzing polysaccharides into oligo- and mono-saccharides, especially glucose and fructose, are the main chemical changes associated with traditional process. This is consistent with the traditional experience that the processed materials are dark as night and sweet as malt sugar. Meanwhile, our results also indicated that their chemical constituents changed profoundly after process, which might be the chemical basis for raw and processed materials have different medical effects. Based on absorbance at 817 and 780 cm⁻¹ and the color, taste, smell of processed materials followed by energy efficacy, raw materials had to be processed more than 21 h to ensure their quality. This research shows the potential of FT-IR spectroscopy coupled with multivariate analysis to discriminate different herbs and to monitor chemical changes with process and then control their quality. This could be very helpful to ensure the quality, safety, and efficacy of herbs on clinical practices.     

Application of two-dimensional near-infrared correlation spectroscopy to the discrimination of Chinese herbal medicine of different geographic regions

Fructus Lycii is a traditional Chinese medicinal herb. The objective of this paper was to apply two-dimensional (2D) near-infrared (NIR) correlation spectroscopy to the discrimination of Fructus Lycii of four different geographic regions. Generalized 2D-NIR correlation spectroscopy was able to enhance spectral resolution, simplify the spectrum with overlapped bands, and provide information about temperature-induced spectral intensity variations that was hard to obtain from one-dimensional NIR spectroscopy. The 2D synchronous and asynchronous spectra showed remarkable differences within the range of 4950-5700cm(-1) between samples of different geographic regions. Using NIR instead of IR made the 2D approach more convenient and fast, and it can be applied to more area like process control. This approach can also be applied analogously to the discrimination of other Chinese herbal medicine of different geographic regions.     

Development and Validation of a LC–ESI–MS Assay for Determination of Icariin in Rat Plasma after Administration of Herba Epimedii

This paper reports a sensitive, specific, and stable chromatographic procedure with selective detection (electrospray mass spectrometry in selected-ion-monitoring mode) combined with simple and efficient sample preparation for determination of icariin in rat plasma after administration of Herba Epimedii. Separation of the analyte, possible endogenous compounds, and constituents of Herba Epimedii were accomplished on a 250 mm × 2.0 mm i.d. C₁₈ column by use of a rapid gradient. Ionization of icariin and clarithromycin (internal standard) was achieved by use of the electrospray interface in positive-ion mode. Conditions such as ionization mode, type of organic modifier, eluent additives, and Q-array potential were optimized to achieve good sensitivity and specificity of icariin detection. Response was a linear function of concentration over the range 0.2–20 ng mL⁻¹. The method is accurate and precise; within-batch and between-batch precision (CV) are <15%, and accuracy (RE) is better than ±15%. The method can be used for analysis of icariin in plasma after administration of Herba Epimedii or of traditional Chinese medicinal preparations containing Herba Epimedii.     

PH-dependent coordination of metal-lisinopril complex investigated by attenuated total reflection/Fourier transform infrared spectroscopy

In order to simulate the in vivo binding behavior of angiotensin-converting enzyme (ACE) inhibitors to the zinc-containing active center of ACE, the in vitro interaction between lisinopril and zinc or nickel ions was investigated in aqueous solutions of different pH by using attenuated total reflection (ATR)/Fourier transform infrared (FT-IR) spectroscopy with second-derivative IR spectral analysis. The results indicated that the lisinopril dissociation process occurred in a stepwise fashion during increase in pH. The IR peaks at 1642 cm(-1) (carbonyl stretching of tertiary amide) and at 1582 cm(-1) (asymmetric COO- stretching) for lisinopril in solution at pH 3.5 shifted to 1606 and 1586 cm(-1) after addition of Ni2+ ions, respectively, but there was no marked changes in IR spectra of lisinopril after addition of Zn2+ ions. When the Zn2+ ions were added to lisinopril solution at pH 5.0, the peak at 1642 cm(-1) also shifted to 1604 cm(-1) and the peak at 1582 cm(-1) shifted to 1586 cm(-1), similar to the changes at pH 3.5 after adding Ni2+ ions. However, the peaks at 1582 and 1642 cm(-1) both shifted to 1599 cm(-1) after addition of Ni2+ ions at pH 5.0 or at pH 7.3. The peak at 1576 cm(-1) also shifted to 1599 cm(-1) after addition of Zn2+ ions to lisinopril solution at pH 7.3. Different coordination sites or types (chelating, bridging or pseudounidentate complex) between lisinopril and Zn2+ or Ni2+ ions were proposed, based on the separation value between v(as) (COO-) and v(s) (COO-), and the shifting of carbonyl groups. Coordination of the secondary amine in lisinopril to metal ions was also evidenced.     

Determination of Quercetin and Rutin in Selected Herbs and Pharmaceutical Preparations

Optimization of the extraction time of rutin and quercetin from herbal plant (Inflorescentia tiliae, Violae tricoloris herba, Anthodium arnica, Flos sambuci, Herba rutae, and Herba hyperici) is described. Determination of aforementioned flavonols was performed by pharmacopoeial, spectrophotometric, and chromatographic methods. The highest quercetin concentration was obtained for Inflorescentia tiliae whereas Herba hyperici contains the highest amount of rutin. The methods were tested on pharmaceutical preparations. The obtained results were compared by statistical test.     

Simultaneous determination of seven flavonoids in Epimedium using pressurized liquid extraction and capillary electrochromatography

Herba Epimedii (known as Yinyanghuo in China) is one of the commonly used Chinese medicines. Flavonoids are considered as its active components. In this study, a CEC method was developed for the simultaneous determination of seven flavonoids, including hexandraside E, kaempferol-3-O-rhamnoside, hexandraside F, icariin, epimedin A, B, and C, in Epimedium using baicalein as internal standard (IS). The influence of relevant parameters such as buffer concentration, pH, and proportion of ACN was investigated and optimized. Baseline separation was obtained using a Hypersil C18 capillary (3 microm, 100 microm/25 cm) with a mixture of 20 mM phosphate buffer (pH 4.0)/ACN (70:30 v/v) as mobile phase running at 30 kV and 25 degrees C in 20 min. All calibration curves showed good linearity (r2 >0.9992) within test ranges. The LOD and LOQ were lower than 8.6 and 42.8 microg/mL, respectively. The RSDs of intra- and interday for relative peak areas of seven analytes were less than 3.1 and 4.4%, and the recoveries were 95.2-103.3%. Samples of different Epimedium species were analyzed using the validated method, which is useful for quality control of Epimedium and its medical preparations.     

Research on processing medicinal herbs with multi-steps infrared macro-fingerprint method

How to apply rapid and effective method to research medicinal herbs, the representative of complicated mixture system, is the current study focus for analysts. The functions of non-processed and processed medicinal herbs are greatly different, so controlling the processing procedure is highly important for guarantee of the curative effect. Almost, the conventional criteria of processing are based on personal sensory experience. There is no scientific and impersonal benchmark. In this article, we take Rehmannia for example, conducting a systematic study on the process of braising Rehmannia with yellow wine by using the multi-steps infrared (IR) macro-fingerprint method. The method combines three steps: conventional Fourier transform infrared spectroscopy (FT-IR), second derivative spectroscopy, and two-dimensional infrared (2D-IR) correlation spectroscopy. Based on the changes in different types of IR spectra during the process, we can infer the optimal end-point of processing Rehmannia and the main transformations during the process. The result provides a scientific explanation to the traditional sensory experience based recipe: the end-point product is "dark as night and sweet as malt sugar". In conclusion, the multi-steps IR macro-fingerprint method, which is rapid and reasonable, can play an important role in controlling the processing of medicinal herbs.     

Vibrational spectra and potential energy distributions for 4,5-dichloro-3-hydroxypyridazine by density functional theory and normal coordinate calculations

The solid phase FT-IR and FT-Raman spectra of 4,5-dichloro-3-hydroxypyridazine have been recorded in the regions 4000-400 cm(-1) and 3500-100 cm(-1), respectively. The spectra were interpreted with the aid of normal coordinate analysis following a full structure optimization and force field calculations based on the density functional theory (DFT) using the standard B3LYP/6-31G* and B3LYP/6-311+G** method and basis set combinations. The DFT force field transformed to natural internal coordinates was corrected by a well-established set of scale factors that were found to be transferable to the title compound. The IR and Raman spectra were predicted theoretically and compared with the experimental spectra.     

Reaction kinetics of solid-state cyclization of enalapril maleate investigated by isothermal FT-IR microscopic system

To investigate the reaction kinetics of the solid-state degradation process of enalapril maleate, a Fourier transform infrared microspectroscope equipped with thermal analyzer (thermal FT-IR microscopic system) was used. The isothermal stability study was conducted at 120-130 degrees C for 1-2 h and changes in the three-dimensional plots of the IR spectra of enalapril maleate with respect to heating time were observed. The study indicates that the bands at 1649, 1728, and 1751 cm(-1) assigned to intact enalapril maleate gradually reduced in peak intensity with heating time. However, the peak intensities at 1672 and 1738 cm(-1) (due to enalapril diketopiperazine (DKP) formation) and at 3250 cm(-1) (corresponding to water formation) gradually increased with heating time. The solid-state diketopiperazine formation and the degradation process of enalapril maleate via intramolecular cyclization were found to be simultaneous. The isothermal decomposition curves were sigmoidal and were characterized by induction and acceleration periods, indicating the presence of autocatalytic solid-state decompositions. Moreover, the power-law equation (n = 1/4) was found to provide the best fit to the kinetics of decomposition. This isothermal FT-IR microscopic system was easily used to investigate the degradation of enalapril maleate and the concomitant formation of DKP. The solid-state reaction of enalapril maleate required an activation energy of 195+/-12 kJ/mol to undergo the processes of decomposition and intramolecular cyclization.