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The following example is for an application where a submitter has been obtaining minor levels (0.1 to 1.0 %) of Cr in an

alloy containing roughly equal amounts of Fe and Ni. The laboratory where this alloy is analyzed uses a procedure where 0.2

grams of the sample is dissolved in 5 mL of a 1:1 HNO3 / HCl mixture and diluted to 1000 mL with DI water. The analyst is

informed that a limit of detection (LOD = 3SD

) of 1 ppm Cr based upon the original sample and the ability to quantify the

Cr to within ±10 % relative at the 10 ppm level is an absolute minimum requirement.

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calculation and determines that using the most sensitive Cr line and the current procedure, the lowest possible detection limit

is 4 ppm and a more realistic estimation would be ~ 4 times the IDL or ~ 16 ppm. The analyst then pulls up the following

spectra, instrument detection limits, and linear regression data which were obtained on their radial view instrument about

four years ago when installed using pure single element solutions as described above.

The 205.552 nm Cr line was found to be the most sensitive of the 16 Cr lines originally characterized with an IDL of 4.0 ppm

= [ (0.0008 μg/mL Cr IDL) x 1000 ] / 0.2 based upon original sample size and dilution as described above. However, the

spectrum of a 0.1 ppm Cr standard shows significant interference from both Ni and Fe at a concentration of 100 ppm making

the line useless at low ppm Cr levels (see Figures 7.1 and 7.2).

The analyst then begins the relatively simple process of identifying a Cr line with the most sensitivity that is spectrally clean.

Figures 7.3 and 7.4 show the line identified using the same scan data shown for the 205 Cr line. The 267.716 nm Cr line looks

clean at the current dilution factors and has an IDL of 0.0016 μg/mL Cr which increases the detection limit to somewhere

between 8 to 32 ppm.

The good news is that the 267.716 line looks spectrally clean and the possibility of increasing the sample size while lowering

the final volume by a factor of 100 is possible (i.e., 2 grams sample up to 100 mL using 20 mL of 1:1 HCl/ HNO

). The

concentrations of the Fe and Ni in the final solution would be ~ 10,000 μg/mL each. This capability was confirmed when

40,000 μg/mL solutions of both Fe and Ni were scanned as shown in Figure 7.5. These spectral data indicate a realistic

detection of << 1 ppm Cr.

Figure 7.1:

Spectra of pure 100 ppm Fe and Ni solutions, 0.1 ppm Cr

and a water blank at the 205.552 nm Cr wavelength

Figure 7.2:

IDL, BEC and regression data for the 205.552 nm Cr line

Figure 7.3:

Spectra of pure 100 ppm Fe and Ni solutions, 0.1 ppm Cr

and a water blank at the 267.716 nm Cr wavelength

Figure 7.4:

IDL, BEC and regression data for the 267.716 nm Cr line