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Example 1-2:

A least- squares analysis of calibration data for the determination of lead based on its flame emission spectrum yielded the equation

S = 1.12 Cpb + 0.312

Where the Cpb is the lead concentration in ppm and S is the measurement of the relative intensity of the lead emission line. The following replicate data were then obtained:

Calculate: (a) the calibration sensitivity;

(b) The analytical sensitivity at 1 and 10 ppb of Pb

( c ) the detection limit

Conc. Pn (ppm) No.of replications Mean value of S s 10.0 10 11.62 0.15 1.00 10 1.12 0.25 0.000 24 0.0296 0.0082

Explanation / Answer

a. The slope of the given straight line corresponds to the calibration senstivity. In this case, the equation of the line is y = mx + c which is S= 1.12Cpb + 0.312. So, the slope m is 1.12 = calibration senstivity.

b. analytical senstivity = m/ s (given)

At 1 ppb , analytical senstivity = 1.12/0.25 = 4.48

At 10 ppm, analytical senstivity = 1.12/0.15 = 7.46

c. detecction limit Cm = (Sm - Sbl)/m

where Sm = Sbl - ksbl = 0.0296 - (3)0.0082 = 0.054

Cm = 0.054 - 0.0296 / 1.12 = 0.022ppm of lead

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