Quantitative Near-infrared Spectroscopy

Principles and Characteristics NIRS is a secondary technique requiring calibration against other techniques. The primary method therefore limits the precision and accuracy obtained using NIR. The accuracy of the NIR technique is also dependent upon the validity of the calibration data set. FT-NIR spectroscopy allows development of high-precision quantitative methods. However, the higher the precision, the more difficult will be the calibration development and instrument transfer processes. Use of NIR for quantitation is an area of great innovation. 

Since the vibrational intensities of characteristic near-infrared bands are only slightly dependent on the state of the system, NIR is well suited to quantitative analysis up to the high pressures and temperatures of extruders. Moreover, in spectroscopic measurements covering an extended NIR wavenumber range, overtone and combination modes with 

The use of the Kubelka-Munk equation for quantitative analysis by diffuse reflectance spectroscopy is common for measurements in the visible, mid-IR and far-IR regions of the spectrum, but not in the near-IR region. As has been pointed out [187, 188], almost all near-IR diffuse reflectance spectra have been converted to log(l// ) R = reflectance of the sample relative to that of a non-absorbing sample). The use of log(l// ) instead of the K-M function provides a more linear relationship between reflectance and concentration. Olinger et al. [189] explain this behaviour by the effective penetration depth of the beam, which is very short, when absorption is strong. For many of the algorithms developed to achieve multicomponent determinations from the diffuse reflectance spectra of powdered samples, a linear dependence of band intensity on analyte concentration is not absolutely mandatory for an analytical result to be obtained. 

Jansen [190] has compared FT-NIR diffuse reflection and diffuse transmission measurements for quantitative analysis of powder mixtures. Diffuse transmission as a mode for powder measurements has been quite rarely considered in the past but appears to have some advantages, especially when it comes to low concentrations. 

Hall et al. [151] have demonstrated the feasibility of using NIRS to measure additive levels in PP pellets obtained from two process streams. Analysis was performed directly on the polymer pellets with no sample preparation. In a rugged spectroscopic model, variations in the polymer pellet size or shape, and sample and packing density, which will affect the effective path length of the NIR radiation in the sample, should be accounted for. Even when the chemical identity of the additive is unknown, the availability of a NIR spectrum permits assignability of spectral features (i.e. at 2172 nm) within the matrix that can be used for quantitative purposes. 

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The tiny absorbance values associated with quantitative near-infrared spectroscopy render these measurements extremely sensitive to slight variations in spectrometer alignment. Indeed, slight differences in incident radiant power between the sample and reference spectra create small positive or negative offsets along the absorbance axis. Such offsets are commonly observed for near-infrared spectra of aqueous solutions, as is apparent in the spectra presented in Figure 13.3. 

Infrared spectra of fats and oils are similar regardless of their composition. The principal absorption seen is the carbonyl stretching peak which is virtually identical for all triglyceride oils. The most common appHcation of infrared spectroscopy is the determination of trans fatty acids occurring in a partially hydrogenated fat (58,59). Absorption at 965 - 975 cm is unique to the trans functionaHty. Near infrared spectroscopy has been utilized for simultaneous quantitation of fat, protein, and moisture in grain samples (60). The technique has also been reported to be useful for instmmental determination of iodine value (61). 

The phase composition of glycine crystal forms during the drying step of a wet granulation process has been studied, and a model developed for the phase conversion reactions [88], X-ray powder diffraction was used for qualitative analysis, and near-infrared spectroscopy for quantitative analysis. It was shown that when glycine was wet granulated with microcrystalline cellulose, the more rapidly the granulation... 

Near-infrared spectroscopy is quickly becoming a preferred technique for the quantitative identification of an active component within a formulated tablet. In addition, the same spectroscopic measurement can be used to determine water content since the combination band of water displays a fairly large absorption band in the near-IR. In one such study [41] the concentration of ceftazidime pentahydrate and water content in physical mixtures has been determined. Due to the ease of sample preparation, near-IR spectra were collected on 20 samples, and subsequent calibration curves were constructed for active ingredient and water content. An interesting aspect of this study was the determination that the calibration samples must be representative of the production process. When calibration curves were constructed from laboratory samples only, significant prediction errors were noted. When, however, calibration curves were constructed from laboratory and production samples, realistic prediction values were determined ( 5%). 

Near infrared spectroscopy (NIRS), a technique based on absorption and reflectance of monochromatographic radiation by samples over a wavelength range of 400-2500 run, has been successfully applied for food composition analysis, for food quality assessment, and in pharmaceutical production control. NIRS can be used to differentiate various samples via pattern recognitions. The technique is fast and nondestructive method that does not require sample preparation and is very simple to use compared too many other analytical methods such as HPLC. The drawback of NIRS, however, is that the instrument has to be calibrated using a set of samples typically 20-50 with known analyte concentrations obtained by suitable reference methods such as FIPLC in order to be used for quantitative analyses. Simultaneous quantification of the... 

A.D. Trafford, R.D. Jee, A.C. Moffat and P. Graham, A rapid quantitative assay of intact paracetamol tablets by reflectance near-infrared spectroscopy, Awa/yrf, 124, 163-168 (1999). 

C. Coffey, B.E. Cooley and D.S. Walker, Real time quantitation of a chemical reaction by Hber optic near-infrared spectroscopy, Anal. Chim. Acta, 395 (3), 335-341 (1999). 

S.C. Harris and D.S. Walker, Quantitative real-time monitoring of dryer effluent using fiber optic near-infrared spectroscopy, J. Pharm. Sci., 89(9), 1180-1186 (2000). 

Y. Ren, Y. Gou, Z. Tang, P. Liu and Y. Guo, Nondestructive quantitative analysis of analgin powder pharmaceutical by near-infrared spectroscopy and artificial neural network technique. Anal. Lett., 33, 69-80 (2000). 

R. Cinier and J. Guihnent, Quantitative analysis of resorcinol in aqueous solution by near-infrared spectroscopy, Vib. Spectrosc., 11, 51-59 (1996). 

M. Alcala, J. Leon, J. Ropero, M. Blanco and R.J. Romanach, Analysis of low content drug tablets by transmission near infrared spectroscopy selection of calibration ranges according to multivariate detection and quantitation limits of PLS models, J. Pharm. Sci, 97(12), 5318-5327 (2007). 

Reeves, J.B. Ill and McCarty, G.W. (2001) Quantitative analysis of agricultural soils using near infrared reflectance spectroscopy and a fibre-optic probe. Journal of Near Infrared Spectroscopy 9, 25-34. 

In many pharmaceutical companies, quality control departments already use NIRS to identify formulations. Figure 23 illustrates a PLS calibration for the active content determination in a low-dose tablet. Once identity testing is passed, it is straightforward to consider as a next step the determination of active content in intact tablets. Thus, qualitative and quantitative analysis can be performed by acquiring a single NIR spectrum per sample. Two analytical techniques are replaced by one—nondestructive—NIR measurement. For this purpose near-infrared spectroscopy is a fast and powerful alternative to traditional analysis, which only remains necessary as reference analytics. 

The investigation of the 300 MHz spectrum of poly(3-methyl-l -butene) indicates that the conclusions drawn by previous workers (2—4) concerning the structures of the crystalline and amorphous polymers are essentially correct the crystalline polymer being almost entirely of the 1,3-structure and the amorphous polymer being a mixture of both 1,2- and 1,3-structures. Further, it has indicated that this method is useful for analysis of the composition of the polymer. Quantitative composition determination, however, has not been carried out, since it is felt that the accuracy of the previous estimates utilizing near infrared spectroscopy were satisfactory. 

Trafford, A.D. Jee, R.D. Moffat, A.C. 8t Graham, P., A Rapid Quantitative Assay of Intact Paracetamol Tablets by Reflectance Near-Infrared Spectroscopy Analyst 1999, 124, 163-168. 

Blanco, M. Eustaquio, A. Gonzalez, J.M. etal., Identification and quantitation assays for intact tablets of two related pharmaceutical preparations by reflectance near-infrared spectroscopy Validation of the procedure J. Pharm. Biomed. Anal. 2000, 22, 139-148. 

Sun, S., Du, D., Zhou, Q., Leung, H.W. and Yeung, H.W. (2001) Quantitative analysis of rutin and ascorbic acid in compound rutin tablets by near-infrared spectroscopy Anal. Sci. 17, 455 58. 

Yang, N., Cheng, Y. and Qu, H. (2003) Quantitative determination of mannitol in Cordyceps sinensis using near infrared spectroscopy and artificial neural networks Fenxi Huaxue 31, 664-668. 

Pudelko-Koerner, C. (1998) Quantitative near-infrared reflectance spectroscopy of sennosides from Sennae fructus angustifoliae in in-process and quality control including method validation Pharmazeutische Industrie 60, 1007-1012. 

Although composed of weak and overlapping spectral features, near-infrared spectra can be used to extract analytical information from complex sample matrices. Chemical sensing with in-line near-infrared spectroscopy is a general technique that can be used to quantify multiple analytes in complex matrices, often without reagents or sample pretreatment.7-9 Applications are widespread in the food sciences, agricultural industry, petroleum refining, and process analytical chemistry.10-13 These activities demonstrate that near-infrared spectroscopy can provide selective and accurate quantitative measurements both rapidly and nondestructively. 

In the current study, we tested the applicability of near infrared spectroscopy coupled with correlation analysis to quantitation of the dependence of montmorillonite spectra on two of the structural features known to be of importance in clay surface chemistry -hydration and exchangeable iron. These clays were... 

Near-infrared spectroscopy (NIR) works in the 800 nm-2.5 pm (12,500— 4,000 cm-1) range. The advantage of NIR is that it can typically penetrate much farther into a sample than the mid-infrared radiation (30-1.4 pm, 4,000-400 cm-1)- It can be used for the quantitative measurement of organic functional groups of soil organic matter, especially O—H, N—H, and C=0 (Siesler et al. 2002). In addition, the structural modifications under the effect of chemical treatments (e.g., acidic treatments) can also be studied by NIR (Madejova et al. 2009). 

Dreassi, E. Ceramelli, G. Corti, P. Massacesi, M. Perruc-cio, P.L. Quantitative fourier transform near-infrared spectroscopy in the quality control of solid formulations. Analyst 1995, 120, 2361-2365. 

Near-infrared spectroscopy was used to quantitatively determine the phase composition of a compound capable of existing in two polymorphic forms even though the NIR spectra were fairly similar.79 Using a five-wavelength calibration model, nearly 100% recovery was obtained in a series of spiked calibration samples, with relative standard deviation values ranging from 0.1% to 0.9%. The authors concluded that the use of NIR spectroscopy... 

Near-infrared spectroscopy has been used to quantitate sulfathiazole Form-I and Form-Ill in binary physical mixtures in which one form was the dominant component.81 The spectra of each form exhibited sufficient differences that unique wavelengths of absorbance were easily attributable to each form. Excellent linearity in calculated versus actual compositions were obtained over the concentration range of 0-5% for either Form-I in Form-Ill, or for Form-Ill in Form-I. After considering appropriate calibration models, a limit of quantitation of approximately 0.3% was ultimately deduced. 

C. Pudelko-Korner, Quantitative Near-Infrared-Reflection-Spectroscopy of Sennosides in Sennae fructus angustifoliae in Processing and Quality Control as well as Validation of these Methods, Pharma Technol. J., 19(1), 57-67 (1998). 

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