Transmittance, percent

Multiplying the transmittance by 100 gives the percent transmittance (%T), which varies between 100% (no absorption) and 0% (complete absorption). All methods of detection, whether the human eye or a modern photoelectric transducer, measure the transmittance of electromagnetic radiation. 

A sample has a percent transmittance of 50.0%. What is its absorbance SOLUTION... 

The sensitivity of an atomic absorption line is often described by its characteristic concentration, which is the concentration of analyte giving an absorbance of 0.00436 (corresponding to a percent transmittance of 99%). Eor example. Table 10.11 shows a list of wavelengths and characteristic concentrations for copper. 

The transmittance of a solution is found to be 85.0% when measured in a cell whose pathlength is 1.00 cm. What is the percent transmittance if the pathlength is increased to 10.00 cm ... 

An example of an absorption spectrum—that of ethanol exposed to infrared radiation—is shown in Figure 12.12. The horizontal axis records the wavelength, and the vertical axis records the intensity of the various energy absorptions in percent transmittance. The baseline corresponding to 0% absorption (or 100% transmittance) runs along the top of the chart, so a downward spike means that energy absorption has occurred at that wavelength. 

The spectrophotometer used has two scales-absorbance (log scale, 0-2) and percent transmittance (linear scale, 0-100). Most readings are taken from the transmittance scale and then converted to absorbance through the relationship... 

Historically, in the infrared region spectra have been represented as percent transmittance versus wavenumber as shown in Fig. 5.2. 

Fig. 5.5. Illustration of the method to determine the percent transmittance of a peak in an infrared spectrum.

Transmittance and percent transmittance are two parameters that can be displayed on the readout. 

The conversion of absorbance (A) to transmittance (T) or percent transmittance (%T), the reverse of the above, may also be important. 

What is the percent transmittance given that the absorbance is... 

What is the concentration of an analyte given that the percent transmittance is 70.3%, the path-length is 1.0 cm, and the molar absorptivity is 8382 L moF1 cnF1 ... 

Quantitative analysis procedures using infrared spectrometry utilize Beer s law. Thus only sampling cells with a constant pathlength can be used. Once the percent transmittance or absorbance measurements are made, the data reduction procedures are identical with those outlined in Chapter 7 (preparation of standard curve, etc.). 

Reading the percent transmittance from the recorded IR spectrum for quantitative analysis can be a challenge. In the first place, there can be no interference from a nearby peak due to the solvent or other component. One must choose a peak to read that is at least nearly, if not completely, isolated. 

Secondly, the baseline for the peak must be well defined. The baseline for the entire spectrum is the portion of the trace where there are no peaks. It does not usually correspond to the 100% T line. Thus, two percent transmittance readings must be taken, one for the baseline (corresponding to where the baseline would be if the peak were absent—a blank reading) and one for the minimum percent transmittance, the... 

Obtain a liquid sampling cell from your instructor and fill it with the 20% solution. Place this cell in the FTIR instrument, and obtain the transmittance spectrum according to the instructions specific to your instrument. Record the percent transmittance value at the tip of the 817 cm-1 peak and at the baseline. Repeat with the other three standards, the unknown, and the control sample. 

Convert all percent transmittance readings to absorbance. Subtract the absorbance at the baseline from the absorbance at the tip of the 817 cm"1 peak in each spectrum. 

What percent transmittance range and what absorbance range are considered to be the optimum working ranges for spectrochemical measurements ... 

We have spoken frequently in this chapter about sensitivity and detection limit in reference to advantages and disadvantages of the various techniques. Sensitivity and detection limit have specific definitions in atomic absorption. Sensitivity is defined as the concentration of an element that will produce an absorption of 1% (absorptivity percent transmittance of 99%). It is the smallest concentration that can be determined with a reasonable degree of precision. Detection limit is the concentration that gives a readout level that is double the electrical noise level inherent in the baseline. It is a qualitative parameter in the sense that it is the minimum concentration that can be detected, but not precisely determined, like a blip that is barely seen compared to the electrical noise on the baseline. It would tell the analyst that the element is present, but not necessarily at a precisely determinable concentration level. A comparison of detection limits for several elements for the more popular techniques is given in Table 9.2. 

A transmission spectrum is a plot of transmittance or percent transmittance vs. wavelength. An emission spectrum is a plot of emission intensity vs. wavelength. 

The optimum working range for percent transmittance (to avoid instrumental deviations from Beer s law) is between 15 and 80%, which corresponds to an absorbance range of 0.10 to 0.82. 

Spectra are sometimes recorded in units of transmittance (I) or percent transmittance (%T), which do not scale in a linear manner with chemical concentration ... 

Spectra of solid samples are usually recorded in the units of reflectance (R) or percent reflectance (%/ ), which is analogous to percent transmittance in that reflectance equals the ratio of the reflected radiation to the incident radiation. With diffuse reflectance, the reflected signal is attenuated by two phenomena absorption (coefficient k) and scattering (coefficient s). Lollowing the Kubelka-Munk theory, these two coefficients are related to the reflectance of an infinitely thick sample, by... 

Figure 3-1 Scale of a Bausch and Lomb Spectronic 20 spectrophotometer. Percent transmittance is a linear scale and absorbance is a logarithmic scale. 

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