## 1 Introduction  
Emission spectral analysis is the most widespread express method to estimate elemental composition [1, 2, 3, 4, 5] in various industries [6, 7, 8]. Most of the modern emission spectrometers utilize the so-called charge-coupled devices (CCDs) as an optical radiation-recording system [9, 10, 11, 12, 13, 14, 15], and different types of gas-discharge plasma are used for optical excitation [5, 16]. Setting up emission spectrometers includes, above all, calibration graphs that plot the dependence of an element's analytical-line intensity on its concentration in an analyzed sample, or an inverse function that plots the concentration dependence on intensity. To determine the impure-element content in a sample, it is necessary to measure the analytical-line intensity of this element. For applications using a CCD recording system, this means summing the digitized charge values of individual CCD pixels, on which the image of the analytical line of a selected impure element falls. At this point it is necessary, if possible, to eliminate the signal, which corresponds to the intensity of the plasma background radiation independent of the concentration of a given impure element (it may be plasma electron bremsstrahlung, molecular bands of plasma-forming gases, etc.). In modern spectrometers, this is performed, as a rule, according to the following algorithm [17, 18]. In the corresponding spectrometer's software window, the pixels on which the image of the analytical line falls are specially marked (see Figure 1). As an example, Figure 1 shows the software window of the SPAS-05 emission spectrometer produced by Active Co. Ltd., Saint Petersburg, Russia, in the CI 193.09 nm analytical carbon-line area (see Appendix A for brief technical spectrometer specifications). The CCD pixels on which the image of this line falls are within the yellow rectangle. In this case, to subtract plasma background radiation to the left and right of the analytical line (sometimes only on one side), an area in the spectrum that is free from the spectral lines of all elements is selected. Then, the set of intersection points of the right and left reference frames (see purple lines Figure 1) and the plasma spectrum envelope are connected by a polynomial of a degree (see the red line Figure 1), which we will refer to as the cutoff line. The analytical signal below this line is considered the plasma background and is neglected when calculating. Under this approach, the unknown spectral shape of plasma radiation at the analytical-line location cannot ultimately coincide with the envelope shape and, thus, the plasma background is inaccurately accounted for. This leads to the fact that at zero concentration of the given impurity, the intensity of its analytical line as measured by the spectrometer is non-zero. This, in turn, causes significant and uncontrollable device-to-device distortions of the calibration curves in the area of low impurity concentrations, which further results in a slope decline in the calibration curve for the case of the intensity dependence on concentration, (or an increase for the case of the inverse function), and a forced increase in the degree of the polynomial that is approximating the calibration curve. These effects lead to a lower detection limit and an increase in the RMS deviation of the concentration measurements. They are, inter alia, one of the factors that impede the replication of the calibration curves of one device in other devices of the same type. This paper is dedicated to the development of this algorithm in order to accurately evaluate the actual background at the element's analytical-line location using standard data captured by the CCD recording system, thus eliminating the above-listed deficiencies.