Transflection spectroscopy

M. Blanco and M.A. Romero, Near infrared transflectance spectroscopy. Determination of dexketoprofen in a hydrogel, J. Pharm. Biomed. Anal., 30, 467-472 (2002). 

One hundred and thirty-eight (138) oil samples were analyzed with visible (vis) and near-infrared (NIR) transflectance spectroscopy. Forty-six of them were Greek pure extra virgin olive oils and the same oils adulterated with 1% (w/w) and 5% (w/w) sunflower oil. However, no significant difference was found between the spectrum of pure sunflower oil and that of olive oil, which can be detected by the naked eye. Olive and sunflower oils differ in their composition principally in their content of linoleic and oleic acids. Accordingly, typical figures for olive oil were quoted at 12.3% and 66.3%, respectively, while for sunflower oil the corresponding mean values of 66.2% and 25.1%, respectively. 

Three types of external reflection spectra can be measured with the microscope optics in the reflection mode shown in Figure 1.2. In the first (which is of increasing popularity for mid-infirared spectroscopy), transflection spectroscopy, a sample... 

Another type of transflectance spectroscopy that is commonly used for measurements of near-infrared spectra falls somewhere between specular reflection and diffuse reflection spectroscopy (see Chapter 16). Because overtone and combination bands are weak, anomalous dispersion is usually negligibly small in the near infrared. It is quite common for the NIR spectra of liquids to be measured by transflection using the same optical geometry used for diffuse reflection. In this case, the sample is loaded in a cell whose rear face is a ceramic disk. Radiation that has passed through the sample is scattered back through the sample, emerges from the window, and is focused onto the detector. This is a particularly useful way of measuring the spectrum of turbid samples, such as milk. 

NIR spectroscopy is probably the most successful technique for the development of qualitative and quantitative methods in the pharmaceutical industry. NIR spectra contain both chemical and physical information from samples (solid and liquid). Spectra can be acquired off-line in three different modes transmittance, reflectance and transflectance. In all cases, the spectra are obtained in a few seconds without or minimum sample pretreatment. Multivariate data analysis techniques are usually needed for the development of the... 

The final type of measurement that can be made with the microscope in its reflection mode is diffuse reflection (DR) spectroscopy. Today, very few appHca-tions of mid-lR microspectroscopy of neat samples are available, because for mid-IR DR spectrometry the samples should be diluted to a concentration of between 0.5 and 5% with a nonabsorbing diluent (e.g., KBr powder) to preclude band saturation and severe distortion by reflection from the front surface of the particles. However, this mode has substantial application for NIR measurements, where sample dilution is not needed. Because the absorption of NIR radiation by most samples is rather weak, they must either be at least 1 mm thick or be mounted on a reflective or diffusing substrate, such as a ceramic or Teflon disk. In the latter case, the spectrum is caused by a combination of diffuse reflection, transflection and front-surface reflection (hopefully with diffuse reflection being the dominant process). 

Correction of dispersive line shape artifact observed in diffuse reflection infrared spectroscopy and absorption/reflection (transflection) infrared micro-spectroscopy. Vib. Spectrosc., 38, 129-32. 

For ethanol analysis by transmission NIR spectroscopy, good results were obtained, with SEC = 0.16%, simple correlation coefficient (r) = 0.998, and standard error of prediction (SEP) = 0.16% with first-derivative values of the optical data obtained at the wavelength of 1672 nm. The ethanol content of low-alcohol beers defined as < 1 v/v% could also be measured by NIR to the same accuracy, but this might not be sufficient for alcohol-free beers defined as <0.05%, although, for ethanol analysis by transflectance NIR, good results were also obtained with SEC = 0.08%, R = 0.999, and SEP =0.16% with first-derivative values of the optical data obtained at the wavelengths of 1672 and 1212 nm. Ethanol can be measured accurately and precisely in beer by both transmission and transflectance NIR methods. 

S. A. Halsey. The use of transmission and transflectance near infrared spectroscopy for the analysis of beer. J Inst Brew 91 306-312, 1985. 

The greatest advantage of NIR spectroscopy is the ease of sample handling. Reflectance, transmission, or transflectance can be used. Classical spectroscopy requires physical separation of the... 

The methods of presenting samples such as a tissue or isolated single cell for study in an FT-IR microscope have to date been predominantly confined to transmission and, the so-called, transflection sampling techniques. The latter is actually a reflection-absorption technique vide infra). Of increasing recent interest is use of the so-called ATR sampling technique for the analysis of tissue samples. ATR is an abbreviation for attenuated total reflection and is an internal reflection spectroscopy technique. On the horizon are perhaps nearfield techniques. Each of these will now be considered in turn. 

Figure 4.2 Schematic diagrams of some reflection spectroscopy techniques (a)-(c), internal reflection spectroscopy (a) and (b), single-reflection ATR (c), multiple internal reflection (MIR) (d) transflectance . Notes (i) collimated beam with ATR hemicylinder (ii) the component of specular reflectance (dotted line) will be superimposed on the double-pass transmission spectrum of the film in the transflectance measurement.

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