Transmittance quantitative analysis

The single beam instrument measures directly the amount of energy transmitted by the sample. They give the most accurate transmittance measurements and is particularly helpful for quantitative analysis. It has simpler and more reliable systems than double beam. 

Absorbance then is a parameter that increases linearly with concentration and is important for quantitative analysis. If the analyst measures transmittance, he or she must convert it to absorbance via Equation (7.12)... 

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. 

In usual practice, there are two methods that are frequently employed for the determination of the transmittance ratio in quantitative analysis namely ... 

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. 

Modern infrared (IR) spectroscopy is a versatile tool applied to the qualitative and quantitative determination of molecular species of all types. Its applications fall into three categories based on the spectral regions considered. Mid-IR (MIR) is by far the most widely used, with absorption, reflection, and emission spectra being employed for both qualitative and quantitative analysis. The NIR region is particularly used for routine quantitative determinations in complex samples, which is of interest in agriculture, food and feed, and, more recently, pharmaceutical industries. Determinations are usually based on diffuse reflectance measurements of untreated solid or liquid samples or, in some cases, on transmittance studies. Far-IR (FIR) is used primarily for absorption measurements of inorganic and metal-organic samples. 

For quantitative analysis of a single component, a strong absorption band that is relatively free of overlapping bands or interference is selected from the IR spectrum. The intensity of the band is measured either in units of percent transmittance or absorbance. 

An FTIR spectrum is commonly expressed as a transmittance spectrum, in which vibration band peaks point downward. It can also be expressed as an absorbance spectrum, in which vibration band peaks point upward (Figure 9.21). For quantitative analysis, an absorbance spectrum should be used because the peaks of a transmittance spectrum are not linearly proportional to concentration. 

Ideally, the intensity (/o) of infrared radiation incident upon a sample cell is reduced (to f) by the absorption of the samples. Actually, some of the incident energy is scattered by the sample and this scattered energy makes the Beer-Lambert law inaccurate, especially at high values of absorbance [4]. The baseline method for quantitative analysis is an empirical method used to establish a calibration curve of log (/q//) versus concentration. Infrared absorption bands may overlap neighboring bands or may appear on a sloping background, so transmittance is measured in practice as shown in Figure 8.13. The absorbance. A, is determined from measurements of / and 7o, then a calibration curve of absorbance versus concentration is plotted. 

UV-visible spectroscopy is used for quantitative analysis, studies of reaction rate and mixtures, identifying compounds and as detectors for HPLC. Quantitative analysis requires calibration curves to adequately characterize the variation of concentration and absorbance (or transmittance). Reaction rates may be derived by following the variation of the concentration of a compound in a vessel with time-on-stream. The total absorbance is the sum of the individual absorbances. 

Of all the techniques of preparation used, a 10- to 20-per cent solution of the substance provides the best IR spectra with the least disturbance. It proves most practical to fill the solution into detachable, sealed liquid cuvettes. The path length is set by means of spacers. The windows consist of plane-parallel NaCl plates (16 pm transmittance limit) or KBr plates (25 pm transmittance limit). In quantitative analysis, the plane-parallelism of the layers and plates must be checked and the path length measured by interferometry with an empty cuvette. 

Solvents employed in paint manufacture can be qualitatively and quantitatively determined using liquid cells or from the spectra obtained from a few drops of the solvent, previously distilled, deposited between two crystals of an alkyl halide (in this case, for quantitative purposes an internal standard or the bands quotient approach must be employed). From these spectra the presence of several solvents in the formulation can be established and, if the mixture is not too complex, quantitative determination of all solvents can be carried out without chromatographic separation. Attenuated total reflectance (ATR) measurements offer a simple and fast alternative to the use of transmittance cells for qualitative and quantitative analysis. For this purpose, the solvent fraction obtained from the paint is placed over the surface of an ATR crystal (usually ZnSe). Figure 4 shows the spectra obtained for pure solvents and the solvent fraction of one polyester type paint. 

Reflectance spectroscopy is commonplace for samples that cannot be prepared for transmittance measurements. However, reflectance measurements must be carefully conducted as the reflected beam is not only indicative of the composition of the sample but is also affected by surface conditions at the sample plane. This makes the reflectance spectra, though indicative of material chemistry, difficult to interpret and generally less useful for quantitative analysis. Since the polarization of the beam is maintained for reflectance, especially specular reflectance methods, examination of orientation at polymer surfaces using reflection techniques is attractive [10]. Reflection-absorption modes involve the transmission of the infrared beam through the sample and subsequent reflection to pass through the sample again. Usually, sample preparation is difficult for such experiments and they... 

Sometimes the passage of light through the cuvette is described in terms of transmittance, or transmission (T) T = 1/4 and is generally expressed as a percentage. It is important to note, however, that only absorbance, not transmittance, is linearly proportional to the chromophore concentration. In quantitative analysis, where it is required to obtain the concentration of substance, therefore, absorbance is more conunonly used. The relation between these two parameters is given by the following ... 

The following statements hold true for what is most often termed Beer s law (1) The relationship between transmittance and concentration is nonlinear, and (2) the relationship between absorbance and concentration is linear. Beer s law is the common basis for quantitative analysis. Knowledge of Beer s law allows us to calculate the maximum theoretical dynamic range for an instrument using a few simple mathematical relationships. 

Frequently, however, especially for quantitative analysis, it is desirable to express this data in absorbance units rather than in percent transmittance. The absorbance is given by... 

Analysis for the purpose of accurately determining the quantity of a chemical species existing in a sample is called quantitative analysis. Quantitative infrared spectroscopic analysis mainly deals with the intensity of an infrared absorption band. In this chapter, basic aspects of quantitative spectroscopic infrared analysis for a target substance (the analyte) in solution samples are described. The subjects to be described include the characteristics of a Fourier transform infrared (FT-IR) spectrometer, the relation between percentage transmittance and absorbance, Lambert-Beer s law on the relationship between the intensity of an infrared band and the concentration of a sample, the use of a working curve in quantitative analysis, and the origins of deviations from Lambert-Beer s law. 

The inherent disadvantage of the optical null mechanism was the attenuation of the reference beam, so that accuracy and response time became worse as sample transmittance (%T) decreased. Linearity of response depended on how well the comb had been machined. Theoretically it was impossible for the equipment to measure 0 %T. Scanning had to be slow to avoid excessive pen lag. Reliable quantitative analysis was difficult though possible with heed. 

Spectral changes observed with ATR measurements are smaller than from transmittance, making ppm-order quantitative analysis problematic. However, further progress in semiconductor manufacturing requires more sensitive analysis of cleaning solutions for silicon wafer surfaces. We attempt FUV transmittance spectroscopy to determine SC-1 concentfations on the order of ppm [6]. 

The absorbance, A, is measured as the peak height or peak area in a spectrum. Percent transmittance spectra (%T) should not be used for quantitative analysis since the relationship between %T and concentration is complex and nonlinear. The concentration, c, in Beer s Law is the concentration of analyte in the sample. The analyte must be an absorbing species that is, it must have at least one peak in the infrared spectrum to be analyzable. The concentration units used in Beer s Law can be whatever is convenient for the analyst such as moles/liter, parts per million (ppm), weight percent, etc. The predicted concentrations in the unknown sample will be in the same units as the standards used to obtain the calibration. The pathlength in Equation 5.1 is the thickness of sample seen by the infrared beam as seen in Figure 5.1. For solids and liquids analyzed in the mid-infrared, pathlengths on the order of microns are typical. 

The basic law for spectroscopic quantitative analysis is Beer s law. This shows how sample concentration is related to a measure of radiation intensity in a spectrometer. Consider a sample in solution, held in a cell of uniform thickness that transmits the monochromatic radiation of interest. Let the intensity of the radiation entering the sample be 7o and the intensity of the radiation that has passed through the sample be I. Then the transmittance T is given by... 

The baseline method of quantitative analysis involves selecting an absorption band of the substance under analysis that does not fall too close to the bands of other matrix components. The value of the incident radiant energy Pq is obtained by drawing a straight line tangent to the spectral absorption curve at the position of the sample s absorption band. The transmittance P is measured at the point of maximum absorption. The value of log P(/P) is then plotted against eoncentration. 

Quantitative analysis may be performed directly on the thin-layer plate in a variety of ways by visual comparison by measurement of spot areas by the transmittance of spots that are colored, are charred, or that absorb ultraviolet light by reflectance or by fluorescence. Alternatively, the zone of absorbent containing the constituent can be scraped from the plate and extracted. The extract is then analyzed by... 

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