The Emulsion Calibration Curve

A variation of the two-line method of emulsion calibration is to use a neutral step filter with an accurately known intensity ratio. The percentage transmittances of the light and dense portions of a series of spectral lines covering a range of transmittances from very light to very dense is obtained. From these data a preliminary curve is obtained from which the calibration data can be determined in the same manner as in the two-line method. The lines selected for the two-step method should lie in the same spectral region as the analysis line. 

Three different methods that are commonly used to present graphically emulsion calibrations will be described. These include the H and D curve, the percentage transmittance curve, and the Seidel function curve. 

The H and D calibration curve, in addition to being useful to obtain intensity vs. line density data, also provides a quick measure of emulsion contrast since the slope of the straight line portion of an H and D curve is defined as the emulsion gamma. 

The H and D curve is a plot of spectral line density vs. relative intensity. It is necessary therefore to convert the % T values of Tables 8-4 to density values. In the case of percentage transmittance, the relation 

FIGURE 8-7. /yand D photographic emulsion calibration curve (From data of Table 8-4). 

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Data from Figure 8-6 to Be Used to Plot the Emulsion Calibration Curve... 

The most precise method to correct for background is to convert all %T values, or densities, to relative intensities based on the emulsion calibration curve. Background relative intensity can then be subtracted from the line plus background intensity to produce a relative intensity for the spectral line. The same calculation also is required on the internal standard line if background is present adjacent to the internal standard line. 

Concentration is the independent variable, and the relative intensity or intensity ratio is the dependent variable. Some analytical curves are plotted on logarithmic coordinates, but computer curve-fitting procedures readily allow wide-range rectilinear calibrations, as well as calculation of the concentration values for unknown samples. For very accurate work, emulsion calibrations are repeated for each spectrum by adopting the sample spectrum as the source of the emulsion calibration. 

Detection limits by the Harvey method are basic to use of the method and Harvey defines detectability as the ability to see a spectrum line on a film preparatory to making density measurements. Harvey s method requires that a background appear in the spectrum, since spectral line intensities are compared to background intensity. Harvey has observed that a spectral line intensity about one and one-half times greater than background is necessary for the line to meet his definition of detectability. Background intensity thus serves the function of an internal standard. To obtain spectral line intensities from line densities also requires the preparation of an emulsion calibration curve relating intensity to density. 

To illustrate a typical calculation based on equation (7-10), an emulsion calibration curve is needed. Figure 7-7 is a typical calibration curve using percentage transmission as the ordinate and relative intensity as the abscissa. 

Line intensities are determined by use of a photoelectric densitometer and the germanium line at 2417.4 A is used as an internal standard. Intensity ratios are determined from the appropriate emulsion calibration curve. Standard comparison samples are prepared by mixing the elements as oxides or carbonates and are then buffered as described above. 

FIGURE 8-9. A photographic emulsion calibration curve using the Seidel function plotted versus relative intensity on a log—log scale. 

A third method of preparing an emulsion calibration curve is to plot the Seidel function, log[( o/ ) — 1] against log intensity, where Jq is the galvanometer deflection through the clear plate and d is the deflection through the spectral line. If the galvanometer is adjusted to read 100 through the clear... 

The movable vertical scale and the horizontal slide rule scales are convenient to plot emulsion calibration curves and working curves. The working strip board permits the preparation of intensity ratio scales for a series of different elements. Any calculating board uses the principles of emulsion calibration and working curve preparation that were presented in earlier sections of this chapter. The board makes the plotting of the data more rapid and convenient and is especially useful for routine repetitive analysis and multielement analysis, especially when one emulsion calibration curve is sufficient. 

Concentrations are expressed in moles of olefin groups per liter and absorptivities in liters per mole-cm. The concentration of the side vinyl group in the sodium polymer is cvi and for the trans-vinylene group, 15.3 — Cvi similarly for the emulsion polymer, cv> is vinyl, and 15.8 — cv2 is trans concentration. Solving these equations gave molar absorptivities and concentrations from which calibration curves were constructed, relating corrected absorbance at 910 and 967 cm."1 to concentration of side vinyl and trans groups, respectively. 

One drawback with NMR is that the liquid signal from water in o/w emulsions has to be subtracted to obtain the true SFC. This can be done by analyzing emulsions and fat blends with no tendency to supercooling and making calibration curves. Another possibility is to measure reference samples without fat and calculate the true SFC by subtracting the signal from the water and water-soluble components9. 

The contrast (y) of a photographic emulsion also is dependent on wavelength, where the contrast generally increases as the wavelength increases. A calibration curve for an emulsion, therefore, is only correct at the particular wavelength at which it was obtained. Spectral sensitivity curves for some commonly used spectral plates and films are shown in Figure 6-8. 

Another method used to present graphically emulsion calibration data is to plot log percentage transmittance vs. log relative intensities. This type of plot is shown in Figure 8-8, using the data from Table 8-4. This method of presenting emulsion calibration data is widely used and convenient for the preparation of analytical working curves. 

The calibration curves plotted from certain steroids after having sprayed with sulphuric acid, do not pass through the origin a small part of the film emulsion is always slightly altered for various reasons during development. The slopes of the calibration curves of the different individual compounds (steroids) differ as a result of differences in the intensity of the light emitted from the spots [316, 317]... 

A calibration curve for the monophasic FI-CL method was drawn using wheat germ oil diluted in chloroform for LOOH concentrations between 0.1 and 4.8 pmol mL for an injected sample of 50 pL. Analysis of the heated wheat germ oil samples by both the IT and monophasic FI-CL methods showed an overall increase in LOOH concentration with the heating time increase. These two methods did show slight differences in the trend of LOOH concentrations but were similar to those observed when using the emulsion system. 

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