Near infrared spectroscopy is a technique with tremendous potential in coming days. The method uses near IR radiations for analysis of any kind of material, (transparent or opaque) clinical, pharmaceutical, agricultural, industrial, food and paint etc. However because of weak and broad bands, this technique has not been used routinely as a spectroscopic technique. So much work has been carried out on this technique to study its usefulness in different kind of samples and found very effective and economic for the said. Interpretation of NIR data is somewhat complicated than IR data and requires thorough knowledge of vibrating molecules. This article may help in opening the gateways and promoting the readers toward the use of NIR technique in pharmaceutical analysis.
Near-infrared spectroscopy (NIRS) is a fast and nondestructive technique, which provides multi-constituent analysis of virtually any matrix. Historically, the discovery of the NIR region in 1800 is ascribed to Herschel who separated the electromagnetic spectrum with a prism and found out that the temperature increased markedly towards and beyond the red, i.e. in the region that is now called the near-infrared 1. NIR spectroscopy gained wide acceptance after the work initiated in 1900 by Coblentz, researcher who first time obtained absorbance spectra of pure substances and verified their usefulness for the identification of organic functional groups.
Although a number of NIR experiments were carried out in the early 1920s; but NIR spectroscopy was not popularly used before the mid to late1960s 2. It was Karl Norris, Department of Agriculture U.S, recognized the potential of this analytical technique and introduced modern NIR Spectroscopy into industrial practice 1. Over the last twenty years, NIR spectroscopy has exploded from an obscure method for measuring certain food, agricultural components1 and, gained wide acceptance within the pharmaceutical industry for raw material testing, product quality control and process monitoring. This technique can be applicable for liquid, slurry, and powdered or solid sample.
NIR spectroscopy shows advantages of an easy sample preparation without any pretreatments, the possibility of separating the sample measurement position and spectrometer by use of fiber optic probes, and the prediction of chemical and physical sample parameters from one single spectrum 1.
NIR spectroscopy shows its best use in medical research where it can be used for non- invasive assessment of the brain function through on intact skull in human subjects by detecting changes in blood hemoglobin concentrations associated with neural activity known as optical topography 3. NIR spectroscopy is the measurement of absorbed light directed on sample in the wavelength region of 780 to 2500 nm. It provides qualitative and quantitative information as a result of interaction of near-infrared electromagnetic waves with sample constituents.
NIR spectroscopy is typically used for quantitative measurement of organic functional groups, especially O-H, N-H, and C=O. Typical applications of NIR include pharmaceutical, medical diagnostic (including blood sugar and oximetry), food and agrochemical quality control, as well as combustion research 4.
A NIR spectrometer is generally composed of Light source (tungsten halogen lamp) Monochromator, Sample holder or Sample presentation interface and a detector (silicon, lead sulfide and indium gallium arsenide). The basic NIR spectrophotometer is shown in Figure 1. A number of optical configurations exist that can be used to separate the polychromatic NIR spectral region into monochromatic frequencies 5.
Figure 1: A block diagram of Basic NIR Spectrophotometer.
Tungsten halogen lamp is usually used as light source in NIR spectroscopy, since it is small and rugged. Common incandescent or quartz halogen light bulbs, light emitting diodes (LEDs) may be used. LEDs offer advantages over other source in the sense of greater lifetime, spectral stability and reduced power requirements, selected frequencies, thus, covering only a narrow spectral range of 50–100 nm 5.
Grating monochromator is used to measure the full visible and NIR spectrum may be in transmittance or reflectance mode offering versatility to instrument. It is mainly used for research or when a wide range of different applications is required. Dispersive monochromator used in NIR instruments is the acousto-optically tunable filter (AOTF), which offers advantages of mechanical simplicity (i.e. no moving parts) and their wavelength stability over grating instruments 5.
Sample holders can be glass or quartz and typical solvents are CCl4 and CS2, proper sample presentation is of utmost importance especially in case of measuring solid samples, since scatter effect stray light induced by variations in packing density of powders or sample positioning of tablets or capsules may cause large sources of error in the spectra 6.
Appropriate NIR measuring mode will be dictated by the optical properties of the samples as shown in Figure 2. Transparent materials are usually measured in transmittance (800-1100 nm). Turbid liquids or semi-solids and solids may be measured in diffuse transmittance, diffuse reflectance or transflectance, depending on their absorption and scattering characteristics 1.
On-line samplers offer advantages of low cost, rapid, accurate and stable calibration. There are three types of NIR On-line analyzer 5; Remote (no contact) sensor, By-pass sampler, Fiber-optic probe.
Transparent samples are measured in glass/quartz cuvettes with typical optical paths varying from 1 to 50 mm. carbon tetrachloride (CCl4), is used as best reference substance for transmittance and transflectance measurements which shows no absorption bands in the whole NIR region. However, this toxic substance must be avoided unless it can be safely conditioned in a sealed flask.
Recently, this type of measurement has been found to be appropriate for quantitative determination of the active principle of pharmaceutical tablets because the longer optical path, resulting from internal scattering, can provide information which is better correlated with the average sample content than the surface dominated diffuse reflectance signal 2.
Figure 2: NIR measuring modes.
Where A, B shows transmittance; C shows diffuse reflectance and D, E shows transflectance.
The most frequently employed detectors for the NIR spectral region are based on silicon, lead sulfide (PbS) and indium gallium arsenide (InGaAs) photoconductive materials. In particular, the latter possess a very high detectivity and a very high response speed. Together with high-powered radiation sources (a tungsten coil or a halogen lamp is employed by the majority of manufacturers) these detectors can impart a very high signal-to-noise ratio for NIR measurements.
The choice of detector used depends primarily on the range of wavelengths to be measured. Detector types employed in NIR include
· Silicon: - fast, low noise, small and highly sensitive from the visible region to 1100 nm
· Lead sulfide (PbS): - slower, but very popular since they are sensitive from 1100 to 2500 nm, provide good signal-to-noise ratio.
· Indium gallium arsenide (InGaAs): - Most expensive, sensitive in the range 800-1700 nm.
Here filters are used as wavelength selectors, commercially available for dedicated applications. For example, an instrument for determination of the quality parameters of gasoline (Zeltex Inc.) employs 14 interference (Fabri-Perrot) filters and 14 LED (Light Emitting Diode) sources in the NIR region. Other applications include identification of polymers for recycling purposes, protein, moisture and oil in agricultural samples. Filter instruments are designed for a limited range of routine analyses, either in the laboratory or on-line.
Using Light Emitting Diodes (LED) in the field , price and size of the instrument can be reduced, produce NIR radiation with a band width of about 30-50 nm. LEDs function as both the light source and the wavelength selection system, typically cover the range 400–1700 nm. They have the advantages that the measurement is very fast (e.g. one spectrum per second) and noninvasive. These features are particularly useful where a high sample throughput or ultra-rapid on-line measurements are required.
Acousto-Optical Tunable Filters (AOTF)7 are modern scan spectrophotometers offer advantage of constructing instruments with no moving parts, very high scan speeds, broad NIR spectral region, random access to any number of wavelengths . The scan speed is usually limited by the detector response time. An AOTF comprises a crystal of TeO2 through which a plane traveling acoustic wave is generated at right angles to the incident light beam.
These are instruments based on grating monochromators, offer advantage of longer life, relatively low cost, when compared with other scanning instruments employing modern technologies. The main limitations are the slow scan speed and a lack of wavelength precision, which deteriorates for long term operation due to mechanically driven mechanism fatigue. Also, the presence of moving parts limits the use of dispersive instruments in the field and in more aggressive environments. It is mainly employed in the control of sugar and alcohol production, also on a truck to monitor the content of protein, oil and humidity in real time during grain harvest.
These instruments are based on the use of interferometers and Fourier transform to recover the intensities of individual wavelengths in the NIR region are, undoubtedly, the instruments combining most of the best characteristics in terms of wavelength precision and accuracy, high signal-to-noise ratio and scan speed (although slower than AOTF based instruments) 8. Typical wavelength accuracy is better than 0.05 nm and the resolution can achieve values below 1 nm in the NIR region, at the cost of decreasing the scan speed. On the other hand, the spectrophotometer is not as robust as an AOTF-based instrument, which is assembled without any moving parts. These instruments can tolerate some environmental vibrations without losing wavelength precision and photometric reproducibility; price is comparable with the AOTF-based spectrophotometer.
The main advantages of NIR spectrometry are 1, 9
· Give data of the composition of sample
· No need of sample preparation, requires relatively small quantity of sample
· A rapid, inexpensive, non-destructive and multi-parametric method
· Lower cost of the NIR instrument, more robust instrument because its optical parts are not harmed by environmental humidity.
· Possibility of using intact samples presented directly to the instrument without any pre-treatment.
· No hazardous waste is created since NIR analysis requires no solvents or reagents
· Allows several constituent to be measured concurrently, suitable for in-line and on-line analysis
· NIR analysis depends on less-precise reference method
· Indirect method that requires calibration
NIR spectroscopy is based on the vibrational spectroscopy behaves as a wave with the properties of simple harmonic motion. The NIR spectrum originates from radiation energy transferred to mechanical energy associated with the motion of atoms held together by chemical bonds in a molecule. . NIR covers part of electromagnetic spectrum in the wavelength range 780 nm to 2500 nm. The technique is based on the measurement of reflected or transmitted light by sample. First of all, the sample to study must be enlightened by a light source with a wavelength range between 800 to 2500 nm. Then the light reflected or transmitted by the sample is collected by detector and transformed into a spectrum. In this way NIR spectrophotometer play key role in identification and quantization of sample 10.
NIR spectra of samples comprise broad bands arising from overlapping absorptions corresponding mainly to overtones and combinations of vibrational modes involving C-H, O-H and N-H chemical bonds. These chemical bonds between atoms in molecules vibrate and this vibration behaves as a simple harmonic motion. When the frequency of the radiation matches that of the vibrating molecule, there will be a net transfer of energy from the radiation to the molecule which can be measured as a plot of energy versus wavelength called a spectrum as shown in Figure 3.
Figure 3: A typical spectrum of a powdered cereal sample (upper curve) and some of its constituent absorptions.
NIR absorption bands are typically broad, overlapping and 10–100 times weaker than their corresponding fundamental mid-IR absorption bands 11. NIR spectral data is very difficult to use directly and requires data analysis methods in order to develop multi-linear statistics models to predict wanted outlet data. Finally these methods require calibration, which requires lot of time, and then also it gives good results indicating good efficiency of the sensors.
The principal attractiveness of the technique, its direct and non-invasive nature, has been the driving force towards its application in agriculture and in process analysis; pharmaceutical research is one of the growing area for the scope of NIRS in future.
NIRS can be used for non-invasive assessment of the brain function through on intact skull in human subjects by detecting changes in blood hemoglobin concentrations associated with neural activity. This application is sometimes called optical topography (OT) in which NIRS is used for functional mapping of the human cortex 3. It is commonly used for medical diagnostics, in particular for oximetry (the measurement of oxygen levels in the blood) and for blood sugar determination 3.
Recycling carpet requires a fast, reliable and accurate means of identifying the carpet face fiber type entering recycling streams. Plastic recycling faces one huge problem, plastic types must not be mixed for recycling. Even a small amount of the wrong type of plastic can ruin the melt. Whether it is a carpet or plastic, classification and identification of recyclable material is necessary which can be achieved using optic spectrophotometer 12.
By using optics spectrometers, single seeds can be measured or bulk samples of seeds can be measured . FT-NIR can be used to determine moisture, protein, oil in a wide range of seed grains, beans etc. The same sampling configuration also works for analyzing various components such as protein, fat, moisture, fiber etc. in meat, cheese and animal feeds 12.
· Moisture, protein, oil content in soybean, corn, rice, etc.
· Fat, protein, lactose and solid content in milk
· Fat, protein and moisture in meat and cheese
· Protein, carbohydrate, fiber and moisture in animal feeds
Many researchers reported the NIR technique was useful to detect multi information in both chemical and physical properties of wood materials. It is used as online measurement techniques during paper- making process control.
The ash content is a conventional expression for the residue on ignition from a solid sample according to certain testing procedures. The ash content from papers can be correlated with their NIR spectra.
Thickness of paper can be determined by a special testing procedure with a precision micrometer. The thickness values acquired from conventional methods can be correlated with NIR spectra from the same papers.
Each porous material like wood, textile, leather and paper contains water in the form of vapor in larger pores and in the form of liquid in the narrow capillaries of the structure. The absolute moisture content is expressed in % and the percentage of water in the paper related to the mass of the material. The absolute moisture is constant value, mainly used as a measured and controlled variable during paper manufacturing. It is possible to investigate if the absolute moisture from papers can be correlated with their NIR-spectra.
In dental composites vinyl resin is used. Near infrared (NIR) spectroscopy, which uses clinically relevant sized specimens, to provide information on both vinyl group conversion and water uptake on the same composite specimen, influence of the type and amount of filler phase of the composite on conversion and water absorption.
NIR spectroscopy is widely used for determination of constituents in food & feed analysis such as fresh raw milk composition analysis, determination of eugenol content in clove oils, protein, oil, ash, moisture and particle size in flour, fermentation, microbiology and study of microorganism in food products2,5,6
Food Applications include 15, 16, 17
Determination of protein content, moisture, and hardness of wheat. NIR is applicable to the analysis of moisture, protein, fat, starch, sugars and fiber in intact cereal foods such as bread, biscuits, cake mixes, breakfast cereals, pasta and snack foods.
NIR has a key role in the analysis and process control of dairy products. It offers flexibility in the analysis of protein, moisture, fat and lactose contents in a wide range of dairy products including: liquid milk, dried whole milk, skim milk and whey powders, cream, traditional and processed cheese. Milk powders are analyzed on-line using a powder analyzer, which enables the moisture content to be controlled.
Fresh fruits and vegetables are graded by shape, size and color. Objective, nondestructive methods of sorting enable growers and packers to market a consistent product over an extended season. The nondestructive sorting of fruit for ripeness is done by the on-line determination of its sugar content. Using NIR technique sugars, fibers and titratable acidity in juices, various quality parameters in carrots, apples, tomatoes and straw- berries (e.g. carotenoid content, fruit sugar content, peel firmness, compressive firmness, dry matter content) can be determined 18.
NIR fiber-optic interactance probes can be used to measure nondestructively the protein, moisture and oil contents of whole fish. In view of meat quality evaluation, the use of NIRS appears more promising when categorizing meat into quality classes on the basis of meat quality traits for example discriminating between feeding regimes, discriminating fresh from frozen-thawed meat, discriminating strains, etc 12, 19.
NIR is also used foranalyzing the nutritive value of fresh silage. The nutritive value of silage is most commonly assessed on dried, ground material but volatile compounds are lost during the drying process. NIR analysis of fresh silage would avoid the need for drying and grinding, and overcome the problem of volatile losses 20.
NIR applications in the confectionery industry include the determination of moisture in granulated sugar and chocolate crumb, protein and oil in cocoa powder and fat in whole chocolate.
In the brewing industry, NIR is widely used to monitor the fruit quality, alcohol content of wine, beer and original gravity of beer using online flow-through cells. It is also used for determination of alcohol, sugar and water content in beverages.
The applications of NIR for authenticity testing of coffee, fruit pulps, milk powders, orange juice, pig carcasses, rice, sausages, sugars, vegetable oils, wheat grain and wheat flour have been reviewed by Downey 21. These applications are based on the principles of discriminant analysis in which the problem is to compare the spectrum of the test sample with a reference library. The library should contain examples of known authentic and adulterated samples. e.g. non-dairy fat in milk products.
NIR spectroscopy has been used for assessing grain and soil qualities and has proven to be a rapid, convenient means of analyzing many soil constituents at the same time. The goals of this study were to analyze the potential of NIR spectroscopy to estimate soil nitrogen content (N), phosphorus content (P), potassium content (K), organic matter content (OM), and pH and to combine these predictions with geographic information systems (GIS) 22.
NIR spectrometers can be used for Fuel Ethanol Industry for the following applications:
· Monitoring incoming grain: moisture, starch, fat and protein contents
· Fermentation monitoring: ethanol, glucose, starch, glycerol, lactic acid and other carbohydrates in fermented mash.
· QC analysis of by-products: percent solids in syrup 12.
Vibrational spectroscopy (NIR) has been widely used for the analysis and characterization of polymers. Infrared spectra of polymers give insight at the molecular level as to the orientations and conformations of the polymer chains 12.
The absorption bands of O-H, C-H, N-H and S-H bonds of distinct compounds are identified in the NIR spectral region and used as a first approach for qualitative analysis 11, 24. NIR spectroscopy is not suitable for structure elucidation. However, it has been widely employed for fast and direct access to identify starting products used, for example, by the pharmaceutical industry. Qualitative use of the NIR spectral information is helpful for quality control of pharmaceuticals 25, 26.
Supervised or unsupervised methods are used for the identification and classification of sample 27.In the supervised method each spectrum in the spectral set used for training the identification/classification algorithm is attributed to a given class. In the unsupervised method no a priori assumption is made about the number of classes present in the sample set. The algorithm must identify (or help to identify, with the aid of the user) how the number of groups within the samples can be distributed (cluster analysis), classify the samples of the training set and, at same time, provide the model for further classification of unknown samples.
NIR spectroscopy is not very sensitive, hence mainly used to determine major constituents in the sample. In general, the detection limit is about 0.1% (m/m), although, for some specific applications and under favorable characteristics of the sample matrix and analyte, NIR can reach lower values 23, 28.
Infrared spectroscopy is used in astronomy for studying the atmospheres of cool stars where molecules can form. The vibrational and rotational signatures of molecules such as titanium oxide, cyanide and carbon monoxide can be seen in this wavelength range and can give a clue towards the star's spectral type 3.
In theaerospace field frequent determination of thickness of layers deposited over large surfaces is needed which can be done using phizer device 29.
In forensics field, for the purpose of narcotic identification to take immediate action on the spot NIR handheld technology is useful 29.
In pharmaceutical, handheld analysers are valuable for field inspections to determine original formulation from counterfeits 29. A handheld and compact NIRS instrument is shown in Figure 4. NIR based tobacco moisture sensor instrument gives accurate and reliable moisture measurement for quality control in manufacturing processes 30.
Figure 4: Handheld analyser used in various applications.
NIR analysis for the pharmaceutical industry provides increased economic benefits, improved tests and analysis on the dock, in the lab and on the process line. Samples are often measured through glass vials, containers or plastic bags. NIR testing is completely non-destructive and requires no sample preparation, No hazardous waste is created, not requires solvents or reagents, which results into reduced cost of routine testing by 50% to 95%, depending on the application.
NIR spectroscopy via fiber optic probes is rapidly becoming a standard method for the accurate identification and validation of both solid and liquids raw materials in their containers providing unprecedented speed and flexibility. Complete identification software guides the user through the library creation process and provides single click identification even at the loading dock. There is no need to label; sent sample to the laboratory. FT-NIR is an ideally suited spectroscopic technique for process measurements because of its ability to rapidly perform remote measurements via high efficiency quartz fiber optics 31, 32.
Utilizing Process Analytical Technologies (PAT) for Pharmaceutical Industry
· Raw material identity and purity check at the receiving dock
· Real time monitoring of blend uniformity, blend ratio and drying processes
· In-line control of solvent recovery
· Quality control of final product for potency, excipient level and moisture content
· Whole tablet analysis in both transmission and diffuse reflectance modes
· Polymorph analysis with fluorescence free FT-Raman
· Determination of homogeneity of blending processes in the petrochemical, pharmaceutical and food industries
· Polymorphism screening of new drug substances using high-throughput screening
NIR spectroscopy is used for
· Monitoring residual moisture in the active pharmaceutical ingredient32 and for pharmaceutical formulations.
· Checking blend uniformity 33, drug content in tablets, or content uniformity 34.
· Identification of tablets in bulk and non-invasively inside individual blister pack cells using several sample presentations including a fibre-optic probe 35.
· The method of choice for content uniformity of dosage forms in the pharmaceutical industry is high-performance liquid chromatography (HPLC), although ultraviolet (UV) spectroscopy is also used. NIR spectroscopy is an alternative to these destructive time consuming methods, which require large amounts of toxic and expensive solvents.
NIR spectroscopy was used for real time monitoring of a blending process. NIR spectrometer has been interfaced to a 16 quart V-blender. Data acquired in real time, during blending will be presented 36.
NIR has been used for
· In-line Process Analysis of Residual Moisture in a fluid bed granulator–dryer
· Determine moisture in freeze-dried injectables by non-invasively measuring spectra through the bases of product vials 37.
· Real Time Monitoring of Blend Ratio, Blend Uniformity, Drying Process.
· In-line testing, which places probes in constant contact with drug product
· Give predictive values for dissolution, content uniformity, assay, moisture, and hardness.
· On/at-line assurance of dissolution rates using analytical data correlations.
· Monitoring of fermentation processes, and products.
Chemical imaging can be used in many industries to solve avariety of problems in the laboratory, quality control, and quality assurance or process environments. The applications of NIR Chemical Imaging include the following 38:
· Non-destructive determination of pharmaceutical tablet component distribution and concentrations, intra- and inter-tablet uniformity data
· Provides an intuitive understanding of the relationship between the components,
· Extent of ingredient blending, domain size distributions, agglomeration of component particles, presence of polymorphic forms and trace contaminants.
· Assessment of pharmaceutical blend uniformity
· Determining domain and particle size and distribution of chemical components in heterogeneous mixtures 38.
The NIR technique can be applied to…
· Measure the mixing efficacy or the internal structure of a solid sample, such as a pill.
· Quality Control of Potency, Excipient Levels and Moisture Content.
· Suitable for chemists developing new compounds 39.
· NIR analysis of drugs include analysis of Amoxicillin for Potency, Trihydrate, & Moisture determination, Ampicillin/Ampicyllin for Potency & Moisture, Mouthwash for Glycerine & Alcohol, Cough Syrup for Acetaminophen.
Today's competitive industrial environment requires manufacturers to continuously strive to improve product quality while reducing manufacturing costs. Near-infrared spectroscopy is well suited to process analysis, helping achieve these goals. It shows tighter control over the manufacturing process by optimizing use of materials and reduces or eliminates the production of off-specification material, thus saving reprocessing or disposal costs.
Although variable in results, studies dealing with the predicting ability of NIR spectroscopy to determine sample chemical properties show its good potential to replace analytical procedures, which can be time-consuming, expensive and sometimes hazardous to health or environment. NIR technology incorporate all the benefits brought by the evolution of correlated fields such as Chemometrics, new materials for optical components, new sensors and sensor arrays, microcomputers and microelectronics. Technology provides better knowledge of raw materials, manufacturing parameters and their impact on finished product quality. This will result in a more robust process, better products, more uniform dissolution results, and a huge cost savings for the manufacturer. In spite of its great potential, the practical use of NIR Spectroscopy may well be limited by the fact that it needs a laborious calibration for every purpose.
On the other hand, universal, non-invasive and non-destructive nature of the NIR spectroscopy, its expeditiousness, and the robustness of the NIR spectrophotometers commercially available today may overcome the disadvantages indicated herein. The number of scientific papers and the successes of international congresses on the theme are evidence of this fact. Undoubtedly, NIR has come to stay and its ability of quickly incorporating the advances in the correlated fields ensures it an extensive and fruitful field for research and development.
Near infrared (NIR) spectroscopy has become one of the most dynamic developments in modern analytical Chemistry. NIR spectroscopy is also currently being used in branches of Cognitive psychology as a partial replacement for fMRI techniques. Dispersive NIR is the most common technique, which uses a spectrometer and a multi-channel detector to measure and record the NIR spectra. MEMS-based spectrometer (Micro-Electro-Mechanical Systems) makes easy measurement on the site without needing to bring the sample to the laboratory.
NIR based moisture analyzers and transmitters, employ true Smart Sensor Technology, next generation optics and stable electronics, to meet the increasingly stringent quality controls required by today's manufacturing industries, and to provide innovative process control solutions for maximum on-line manufacturing productivity, cost efficiency and profitability.
Over the last twenty years, NIR spectroscopy has exploded from an obscure method for measuring certain food and agricultural components to a technique that is finding application in all areas of chemical analysis. Development has not stopped and the latest application areas, such as environmental analysis and chemical imaging, promise much in the future.
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Miss Smita Patil
Dept. of Pharmaceutical Chemistry,Govt.College of Pharmacy, Karad, Dist: Satara, 415 124, M.S. Date: 23.05.2007
Appasaheb BirnaleCollege of Pharmacy, South Shivajinagar, Sangli, Maharashtra, India, 416416.
Department of Pharmaceutical Chemistry, Government College of Pharmacy, Karad, Maharashtra, India, 415124.