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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.

Spectral Overlap:

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

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;

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;

(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.

Figure 8.3:

Sloping background correction

Figure 8.4:

Curved background correction

Figure 8.5:

Spectra for 100 μg/mL As and 0.1, 1.0, 10, and 100 μg/mL Cd