ICP_Operations_Guide_2016

Figure 8.3: Sloping background correction

demonstrates that care was taken to avoid The Re line on the long wavelength side of the Zn 213.856 nm line and that a straight line that accurately determines the background intensity in the peak area is obtained. Figure 8.3 shows a sloping but linear background. If the instrument only allows for selection of background points then intensities are taken at set wavelengths, averaged and subtracted from the peak intensity. Here, background points must be taken equal distance from the peak center in order to make an accurate correction. Again, a linear fit was used. Curved backgrounds are encountered when the analytical line is near a high intensity line, as is the case shown in Figure 8.4 below. In this case an algorithm estimating a curve (parabola) was used. For some instruments, depending upon design and software, this type of correction can be very difficult. This is a case where the 589.592 nm Na line would allow for the easier linear correction without loss in sensitivity. For purposes of demonstration the interference of the As 228.812 nm line upon the Cd 228.802 nm lime will be used. In this example, the analysts is attempting to determine the feasibility of measuring Cd in the 0.05 to 100 μg/mL range with 100 μg/mL As present. The analyst would like to have both elements present in the calibrations samples as well as make accurate Cd determinations in unknown samples. The analyst would also like to estimate the detection limit for Cd under these conditions. As discussed in part 7 of this guide, spectra collected at the Spectral Overlap:

Figure 8.4: Curved background correction

time of the establishment of a given instrument in the laboratory can save significant time later. In this case, we will be using spectra collected just after the instrument was installed. It is true that the instrument has aged and it’s performance characteristic may be different (better or worse), but the analyst can still call upon the aid of these data to gain some insight into the feasibility of making a given determination. Consequently, Figure 8.5 shows the spectra for solutions containing 0.1, 1.0 10 and 100 μg/mL Cd along with the spectrum of a 100 μg/mL As solution. Table 8.1 contains intensity data collected from Figure 8.5. This table shows: (A) the concentration of Cd; (B) the relative concentration of As to Cd; (C) the net intensity of the corresponding Cd concentration with no As present; (D) the estimated standard deviation of measurement of Cd; (E) the net intensity of 100 ppm As at the 228.802 nm wavelength; Figure 8.5: Spectra for 100 μg/mL As and 0.1, 1.0, 10, and 100 μg/mL Cd

(F) the estimated standard deviation for measurement of As; (G) the estimated standard deviation of the combined signals for As at 100 ppm and Cd at the concentrations given; (H) the uncorrected relative error for measuring Cd 228.802 nm with 100 ppm As present, and; (I) the best-case relative errors for correcting the Cd intensity to account for 100 ppm As.

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