Near-infrared spectroscopy limitations

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). 

Ferrari M.MottolaL, Quaresima V. Principles, techniques, and limitations of near infrared spectroscopy. Canadian Journal of Applied Physiology 2004, 29, 463 187. 

Individual near-infrared absorption bands have absorptivities on the order of 10 1 Al /mm/mM for peak absorption bands in aqueous matrices.30 Such low absorptivities limit detection to the major components within skin tissue. As a general rule of thumb, substances must be present at concentrations above 1 mM to be quantified by near-infrared spectroscopy. Although such low absorptivities greatly restrict the number of possible analytes one can measure in clinical samples, the inability to measure chemicals present below millimolar concentrations enhances selectivity by rendering measurements insensitive to many different types of endogenous molecules. Only the major chemical components of these biological samples must be considered for selectivity purposes. 

Functional Near-Infrared Spectroscopy Potential and Limitations in Neuroimaging Studies... 

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. 

Among the analytical applications using near-infrared spectroscopy have been the following determinations water in hydrocarbons water in alcohols water in carboxylic acids alcohols in hydrocarbons alcohols in acids acids in hydrocarbons acids in anhydrides amines in hydrocarbons benzene in hydrocarbons and olefins in hydrocarbons. By differential techniques it is possible, for example, to lower the sensitivity limits (detectability) by another factor of 10 over the usual limits. The usual limit for water in alcohols ranges from 0.05 to 0.2 %. Examples of special interest to biochemists are the applications of Klotz and Frank (1965) of near-... 

Near infrared spectroscopy can be used to analyse the concentrations of all the components of a PVC mixture. Unlike conventional methods for the analysis of mixtures, this technique is not limited to one critical component. NIR, after calibration, can analyse all the components in a single measuring operation. This technique is therefore especially suitable for analysing dry blends comprising several components. 

Figure 8.14 Near-infrared spectra of four commercial spirits (a) Scotch whiskey (40% ethanol) (b) gin (47% ethanol) (c) vodka (50% ethanol) (d) Bourbon (55% ethanol) [12], From McClure, W. F. and Stanfield, D. L., Near-Infrared Spectroscopy of Biomaterials , in Handbook of Vibrational Spectroscopy, Vol. 1, Chalmers, J. M. and Griffiths, P. R. (Eds), pp. 212-225. Copyright 2002. John Wiley Sons Limited. Reproduced with permission.

In addition, both producers and users of reclaimed fibres should know the material composition of the blend of reclaimed fibres in question. It can be defined by means of qnantitative chemical analysis, which, however, will entail corresponding cost. There are systems allowing quick definition which are based on near-infrared-spectroscopy. They are limited to qualitative information of the fibre materials contained in a blend. One well-known example is the sorting of end-of-life carpets by the kinds of polymers contained in their piles. - ... 

Near-infrared spectroscopy has been used to determine the activity of the brain using oxygenation levels. More particularly, a series of articles by Boas et al. [65, 66] discusses the limitations that single-point NIR has in the analysis of focal brain activation compared to diffuse activation. Researchers found that it is possible using NIR spectroscopy to... 

In a prospective study in 32 elderly patients, ASA grades 1-3, who received spinal anesthesia with bupivacaine 10-17.5 mg, baseline blood pressure variabDity and near-infrared spectroscopy reduction predicted hypotension with high sensitivity (0.73 and 0.90 respectively) and specificity (0.78 and 0.64, respectively) [32. However, heart rate, systemic vascular resistance index, baroreceptor sensitivity, and heart rate variability were of limited predictive value. 

Raman spectroscopy has enjoyed a dramatic improvement during the last few years the interference by fluorescence of impurities is virtually eliminated. Up-to-date near-infrared Raman spectrometers now meet most demands for a modern analytical instrument concerning applicability, analytical information and convenience. In spite of its potential abilities, Raman spectroscopy has until recently not been extensively used for real-life polymer/additive-related problem solving, but does hold promise. Resonance Raman spectroscopy exhibits very high selectivity. Further improvements in spectropho-tometric measurement detection limits are also closely related to advances in laser technology. Apart from Raman spectroscopy, areas in which the laser is proving indispensable include molecular and fluorescence spectroscopy. The major use of lasers in analytical atomic... 

Of the analytical techniques available for process analytical measmements, IR is one of the most versatile, where all physical forms of a sample may be considered - gases, liquids, solids and even mixed phase materials. A wide range of sample interfaces (sampling accessories) have been developed for infrared spectroscopy over the past 20 to 30 years and many of these can be adapted for either near-lme/at-lme production control or on-line process monitoring applications. For continuous on-line measurements applications may be limited to liquids and gases. However, for applications that have human interaction, such as near-line measurements, then all material types can be considered. For continuous measurements sample condition, as it exists within the process, may be an issue and factors such as temperature, pressure, chemical interfer-ants (such as solvents), and particulate matter may need to be addressed. In off-line applications this may be addressed by the way that the sample is handled, but for continuous on-line process applications this has to be accommodated by a sampling system. 

Scheinost, A.C. Chavernas, A. Barron,V. Tor-rent, J. (1998) Use and limitations of second-derivative diffuse reflectance spectroscopy in the visible to near-infrared range to identify and quantify Fe oxide minerals in soils. Clay Min. Soc. 46 528-536... 

Tunable operation with bandwidth-limited sub-10-fs pulses in the visible (550-700 nm) and near infrared (900-1300 nm) was also performed by changing the seed delay with respect to the pump after increasing the seed chirp [10]. The NOPA is one of most useful light sources for ultrafast spectroscopy in the present stage on an extremely short time scale. 

If near-infrared diode lasers have low-noise characteristics similar to those of mid-infrared diode lasers, and thus minimum absorbances of 10 5 or less are possible, then an approximate detection limit can be calculated for an absorption experiment. For a 200-m optical path, the calculated detection limit is 5 x 1010 molecules/cm3, which is well above levels of H02 expected to be found in the atmosphere. An absorption experiment in this spectral region apparently would require extremely long optical path lengths, and, indeed, a calculation with a 5-km path yields a calculated detection limit of 2 x 109 molecules/cm3, still rather high for tropospheric measurements. Other issues associated with the use of diode lasers in absorption spectroscopy are discussed in the next section. 

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