Emulsion Calibration

There are many methods that can be used to relate the response of a photographic emulsion to the intensity of the light striking the emulsion and there are several different graphical techniques that can be used to present the data. The problem of relating spectral line intensity to the concentration 

Relative intensities of spectral lines, if they are accurately known, also can be used for photographic emulsion calibration. For this purpose, it is necessary that the relative spectral line intensities be well established and that they exhibit no self-absorption. The lines should have approximately the same excitation energies. The spectral lines should lie in the same general spectral region since emulsion contrast (emulsion gamma) varies with wavelength. 

Dieke and Crosswhite studied the feasibility of using selected iron lines for emulsion calibration, and later Crosswhite extended the study. If the iron lines are selected from those with a common lower energy level, consistent results are possible. Table 8-1 includes a series of iron lines from 3157.0 to 3248.2 A all having a common lower energy level (z D), selected from the more extensive tables of Crosswhite, which can be used for emulsion calibration. Ahrens and Taylor give a shorter table of relative intensities of iron lines that also can be used. 

A Intensity Log intensity Line, A Intensity Log intensity 

It is recommended that a metallic arc not be used for calibration purposes, because of the danger of self-absorption. One satisfactory method is to use iron oxide diluted with carbon to minimize the possible self-absorption effect of the metal electrode iron arc. 

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Attempts to obtain quantitative results entail emulsion calibration, microphotometry of line densities, corrections for ion mass and ion energy, and computer handling of the data. Data interpretation is discussed in detail by Kennlcott (2.) and Owens (10). Results obtained by this procedure vary from accuracies of 5% to deviations of orders of magnitude from true values. 

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. 

FIGURE 7-7. Typical photographic emulsion calibration curve. 

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. 

A two-line method of emulsion calibration has been described by Churchill it makes use of a pair of spectral lines whose intensity ratio is known. The spectral line pair selected must possess certain characteristics, as follows ... 

Data from Figure 8-6 to Be Used to Plot 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. 

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

FIGURE 8-8. A photographic emulsion calibration curve plotting percentage transmittance versus relative intensity on a log—log scale. 

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. 

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... 

Emulsion Calibration Data Using a Seven-Step Filter and the Germanium Emission Line at 3039 A... 

FIGURE 8-10. A photographic emulsion calibration curve using a seven-step neutral filter and the germanium spectral emission line at 3039 A. 

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. 

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. 

For ion detection, several approaches are possible. Classical spark source mass spectrometry has developed the use of photographic plates. Very hard emulsions with low gelatin content are required. Emulsion calibration is similar to that described in optical emission spectrography, but usually varying exposure time are applied instead of using optical step filters. Provided an automated microdensitometer is used, mass spectrography is still a useful tool for survey analysis of solids down to the sub-pg/g level. 

Spectrograms are recorded on photographic plates and the log spectral line intensity ratio Na 3303.0/Pd 2763.1 evaluated. Emulsion calibrations of the two wavelengths are established using the microphotometer transmittance data from adjacent steps of the spectrograms. 

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