1.2.1 Correlating Absorbance to Concentration
For the majority of metal analysis performed by AAS, the absorbance A of a beam of photons is measured by a detector. In order for this signal to be useful, this physical process must be correlated to the concentration of the analyte of interest. This can be accomplished by Beer’s law
A = ε b C
where ε is the molar extinction coefficient (a measure of the absorbance of a particular chemical or chromophore), b is the cell path length (usually 1.0 cm), and C is the molar concentration of the analyte. Beer’s law, as shown above, is the result of some mathematical simplifications that are only applicable over absorbance ranges from 0.000 (100 percent transmission) to approximately 2 (1 percent transmission). This limitation is usually enforced by sample preparation techniques that concentrate or dilute the sample to values within the acceptable range. Also note that Beer’s law only governs absorption. Emission measurements are not limited from 0.0 to 2.0 “absorbance” units. In emission spectrometry the intensity of the emitted light is typically measured in units of counts per time.
The difficult task of measuring low concentrations of an analyte requires the reduction of instrumental noise to a minimum level (Section 2.2.10). Ideally, instruments would only measure the exact wavelength that the given analyte absorbs. However, the light that reaches the detector is not always spectrally pure due to various imperfections in instrumental components and due to interactions with instrument components. The slit width of the monochromator (a common instrument component used to select for a specific wavelength of electromagnetic radiation) limiting the bandwidth must be wide enough to allow enough radiation in to produce a sufficient signal (over the noise), but be narrow enough to provide sufficient wavelength resolution. If the entrance slit is too wide, other wavelengths can create background noise in the signal, creating a non-linearity in the calibration. These two conflicting needs are balanced by selecting a proper exit slit width in the monochromator.
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