Absorption by a sample

The intensity of absorption by a sample varies with the length / of the sample according to the Beer-Lambert law ... 

A more widespread use of UV—Vis spectroscopy, however, has to do with determining the concentration of an unknown sample. As mentioned in Section 13.8B, the relationship A = eCl indicates that the amount of absorption by a sample at a certain wavelength is dependent on its concentration. This relationship is usually linear over a range of concentrations suitable for analysis. To determine the unknown concentration of a sample, a graph of absorbance versus concentration is made for a set of standards of known concentrations. 

Think About It Absorbance expresses the magnitude of light absorption by a sample. 

Attenuation of radiation as it passes through the sample leads to a transmittance of less than 1. As described, equation 10.1 does not distinguish between the different ways in which the attenuation of radiation occurs. Besides absorption by the analyte, several additional phenomena contribute to the net attenuation of radiation, including reflection and absorption by the sample container, absorption by components of the sample matrix other than the analyte, and the scattering of radiation. To compensate for this loss of the electromagnetic radiation s power, we use a method blank (Figure 10.20b). The radiation s power exiting from the method blank is taken to be Pq. 

Interference effects, which arise because of the extraordinary uniformity of thickness of the film over the spectrometer sample beam, superimposed on the absorption of incident light by parylene films, can be observed. Experimentally, a sinusoidal undulation of the baseline of the spectmm is seen, particularly in the spectral regions where there is Htde absorption by the sample. These so-called "interference fringe" excursions can amount to some... 

AATCC methods for determining water repeUency are AATCC 22 (spray test) and AATCC 70 (tumble jar dynamic absorption test). In the spray test, water is sprayed against the taut surface of the test specimen to produce a wetted pattern the size of which depends on the repeUency of the fabric. Evaluation is by comparing the pattern with a series of patterns on a standard chart. The latter method evaluates the percentage by weight of water absorbed by a sample after dynamic exposure to water for a specified period of time. 

Spectrometers are designed to measure the absorption of electromagnetic radiation by a sample. Basically, a spectrometer consists of a source of radiation, a compartment containing the sfflnple through which the radiation passes, and a detector. The frequency of radiation is continuously varied, and its intensity at the detector is compar ed with that at the source. When the frequency is reached at which the sample absorbs radiation, the detector senses a decrease in intensity. The relation between frequency and absorption is plotted as a spectrum, which consists of a series of peaks at characteristic frequencies. Its interpretation can furnish structural information. Each type of spectroscopy developed independently of the others, and so the data format is different for each one. An NMR spectrum looks different from an IR spectrum, and both look different from a UV-VIS spectrum. 

Spectrometer (Section 13.1) Device designed to measure absorption of electromagnetic radiation by a sample. 

Molar absorptivity (Section 14.7) A quantitative measure of the amount of UV light absorbed by a sample. 

The dependence of the in-phase and quadrature lock-in detected signals on the modulation frequency is considerably more complicated than for the case of monomolecular recombination. The steady state solution to this equation is straightforward, dN/dt = 0 Nss — fG/R, but there is not a general solution N(l) to the inhomogeneous differential equation. Furthermore, the generation rate will vary throughout the sample due to the Gaussian distribution of the pump intensity and absorption by the sample... 

The relationship in Equation 7-12 is more likely to hold when E and St are both present at low weight-fractions in the samples because x-ray absorption by the sample is then least affected by variations in the weight-fraction of E or St. Then a calibration curve is conveniently established by varying We at constant Wst (7.14, especially Figure 7-8). 

A special type of homogeneous measurements is found in a compositiorml table which describes chemical samples by means of the relative concentrations of their components. By definition, relative concentrations in each row of a compositional table add up to unity or to 100%. Such a table is said to be closed with respect to the rows. In general, closure of a table results when their rows or columns add up to a constant value. This operation is only applicable to homogeneous tables. Yet another type of homogeneous table arises when the rows or columns can be ordered according to a physical parameter, such as in a table of spectroscopic absorptions by chemical samples obtained at different wavelengths. 

The photodiode array detector (PDAD) measures absorption of light waves by a sample. This is considered the most powerful of the ultraviolet spectrophotometric detectors. The optical system focuses light from a deuterium source through the sample flow cell onto several photodiodes. These act as capacitators by holding a fixed amount of charge. When light strikes the photodiodes, they discharge a certain amount of current. 

A iH NMR spectrum is a graph of resonance frequency (chemical shift) vs. the intensity of Rf absorption by the sample. The spectrum is usually calibrated in dimensionless units called "parts per million" (abbreviated to ppm) although the horizontal scale is a frequency scale, the units are converted to ppm so that the scale has the same numbers irrespective of the strength of the magnetic field in which the measurement was made. The scale in ppm, termed the 6 scale, is usually referenced to the resonance of some standard substance whose frequency is chosen as... 

A quantitative measure of photon absorption by a molecule, expressed as the logio of the ratio of the radiant intensity Iq of light transmitted through a reference sample to the light I transmitted through the solution [/.c., A = log(/o//)]. Out-moded terms for absorbance such as optical density, extinction, and absorbancy should be abandoned. 

Fluorimeters use a photomultiplier (or PM) tube that is positioned perpendicular to the incident light beam to detect fluorescence emission by a sample. The photomultiplier has a light-sensitive surface (the photocathode) which is maintained at a negative potential of about 950-1500 volts. So-called photoelectrons are released from photocathode upon light absorption, and they are... 

Over the past decade two forms of vibrational optical activity have become established. One is called vibrational circular dichroism (VCD), the extension of electronic circular dichroism into the infrared vibrational region of the spec-tram. The first measurements of VCD were reported by George Holzwarth and co-workers at the University of Chicago in 1973 for crystals (3) and 1974 for neat liquids (4). In VCD one measures the small difference in the absorption of a sample for left versus right circularly polarized incident infrared radiation. The early stages of the development of VCD have been reviewed from several perspectives (5-8). 

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