Percentage transmittance

The detector must be sensitive to the radiation falling on it, and the spectrum is very often displayed on a chart recorder. The spectrum may be a plot of absorbance or percentage transmittance (IOO///0 see Equation 2.16) as a function of frequency or wavenumber displayed linearly along the chart paper. Wavelength is not normally used because, unlike frequency and wavenumber, it is not proportional to energy. Wavelength relates to the optics rather than the spectroscopy of the experiment. 

The scales of spectrophotometers are often calibrated to read directly in absorbances, and frequently also in percentage transmittance. It may be mentioned that for colorimetric measurements I0 is usually understood as the intensity of the light transmitted by the pure solvent, or the intensity of the light entering the solution /, is the intensity of the light emerging from the solution, or transmitted by the solution. It will be noted that ... 

All infrared spectrophotometers are provided with chart recorders which will present the complete infrared spectrum on a single continuous sheet, usually with wavelength and wavenumber scales shown for the abscissa and with absorbance and percentage transmittance as the ordinates. More advanced instruments also possess visual display units on which the spectra can be displayed as they are recorded and on which they can be compared with earlier spectra previously obtained or with spectra drawn from an extensive library held in a computer memory. These modern developments have all led to quantitative infrared spectrophotometry being a much more viable and useful analytical procedure than it was just a few years ago. 

Fig. 5.2. Typical format, percentage transmittance (%T) versus wavenumber, for infrared spectra. This is a spectrum of benzoic acid.

Each dip in the spectrum is called a band or peak. The transmittance is 0 % if all the radiation is absorbed and with no absorption the transmittance is 100%. The absorption of infrared radiations can be expressed either in terms of wave length (A.) or in wave number (y). Mostly infra red spectra of organic compounds are plotted as percentage transmittance against wave number. The relationship between wave length and wave number is as follows ... 

The quantitative analysis of pharmaceutical substances may be achieved by emperical-ratio method either by plotting percentage transmittance against wavelength or by plotting the log T1 o/T1 against concentration as illustrated in Figure 22.4. 

The values for 7 and 70 cannot be measured in absolute terms and the measurements are most conveniently made by expressing / as a percentage of / . This value is known as the percentage transmittance (T) and only shows a linear relationship with the concentration of the test substance if the logarithm of its reciprocal is used. It is therefore more convenient to report the measurements initially as this logarithmic function of 7 and 70, a parameter which is known as absorbance (A) ... 

It is possible to assess the proportion of stray light by measuring the amount of radiation transmitted by samples that are optically opaque at the wavelength to be assessed but that transmit radiation of other wavelengths. The instrument is set to zero and 100% transmittance in the normal way and the opaque substance introduced into the sample compartment. The amount of light transmitted by the sample, measured in percentage transmittance, is... 

If the concentration is doubled what would be the absorbance and the percentage transmittance of the new solution ... 

The percentage transmittance of light at 250 nm through a certain aqueous solution is 20.0% at 25°C. The experimental cell length is 1.0 cm, and the concentration of the solution is 5 X 10 " mol L . Calculate the absorbance. Calculate the molar absorption coefficient. 

Measure the percentage transmittance of the tube using the blanked spectrophotometer. 

Stir the unboiled chloroplasts with the transfer pipet.Transfer 3 drops to tube 3. Immediately cover with Parafilm and mix the tube. Measure the percentage transmittance of the tube using the blanked spectrophotometer. 

Infrared spectra are represented in terms of a plot of percentage transmittance versus wavenumber (cm-1). In its most common form, infrared spectroscopy makes use of Fourier transformation, a procedure for interconverting frequency functions and time or distance functions. Fourier-transform IR (FTIR) spectroscopy allows the rapid scanning of spectra, with great sensitivity, coupled with... 

Figure 9.18 Absorption of the light by a homogeneous material and representation of percentage transmittance as a function of the material s thickness. The Ught reaching the sample can he reflected, diffused, transmitted or ahsorhed. Here only this last fraction is taken into account.

Figure 10.2 Mid-IR spectrum of a polystyrene film. The typical representation of an infrared spectrum with a linear scale abscissa in cm (see formula 10.1) for an easier observation of the right-hand section and with percentage transmittance along the ordinate. The transmittance is sometimes replaced by the absorbance A(A = — log T). The scale, in cm , (or kaysers) is linear in energy E = hc/X) and will decrease from left to right (i.e. from high to low energy).

Figure 12.14 X-ray densitometry - examples of percentage transmittance for two films made of light elements. A 7 0,m film of beryllium is often used for making windows of energy detectors. For a radiation of 1 keV, (e.g. Na Ka) it is noteworthy that the attenuation achieved by this film is still of 50 per cent.

Instruments used for quantitative measurements may incorporate a transmittance, a percentage transmittance or an absorbance scale. If the molecular weight of the substance is unknown, representing the... 

First, we will examine some of the basic features of an infrared spectrum. Early infrared instruments recorded percentage transmittance over a linear wavelength range. It is now unusual to use wavelength for routine samples and inverse wavelength units are used. This is the wavenumber scale, and the units used are cm"i. The output from the instrument is referred to as a spectrum. Most commercial instruments present the spectrum with the wavenumber decreasing from left to right... 

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