Two methods for determining Hg in coal fly ash using atomic absorption were used

Question

Two methods for determining Hg in coal fly ash using atomic absorption were used to see if both methods would give the same results. In the first method only HNO3 is used for digestion, while in the second method a mixture of HNO3 and HCl is used. The following concentrations of Hg (in ppb) are determined by analyzing a standard multiple times.

Concentrations (HNO3 only):             161 165 160 167 166

Concentrations (HNO3 and HCl): 159 145 140 147 143 156

Determine whether there is any significant difference between methods at a 95% confidence level.

Explanation / Answer

No. of tests are different in each case.

We use t-test to propose the existence of a significant difference between methods, if any.

Mean when only HNO3 is used = 163.8

SD in this case s1 = 3.1145

Mean in case 2 = 148.33

SD in this case S2 = 7.528

Pooled estimate, S = [(4*3.1145^2 + 5*7.528^2)/(5+6-2)]^(0.5) = 5.983

t-value of the data given = |x2-x1|/S(1/n1 + 1/n2)^(0.5) = (163.8 - 148.33)/5.983*(1/5+1/6)^(0.5) = 4.2700

Consulting t-table for 9 degrees of freedom,

t at 5% sig. = 2.262

t at 1% sig. level = 3.250

Since 2.262 < 4.270 there is a significant difference between the two methods used. and measures must be taken to find out the errors arising from either of the tests

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