High-Accuracy Determination of Potassium and Selenium in Human Serum by Two-Step Isotope Dilution ICPMS

Table 6

Sources of uncertainty in the determination of Se.

Sources of uncertainty	Se
	Level 1		Level 2		Level 3
	Value (xi)	uc (xi)	Value (xi)	uc (xi)	Value (xi)	uc (xi)
Type A uncertainties
RY	0.020	0.0011	0.021	0.0013	0.024	0.0018
Rz	2.2	0.010	2.2	0.011	2.1	0.011
RBc	1.1	0.0053	1.0	0.0032	0.96	0.0042
RB	0.98	0.011	1.1	0.011	0.89	0.011
CB/	1.4	0.37	1.4	0.37	1.4	0.37
K	1.0	0.0068	1.0	0.0068	1.0	0.0068
Type B uncertainties
Cz/μg kg^−1	5.6 × 10^2	3.0	5.0 × 10^2	2.6	5.2 × 10^2	2.8
mzc/mg	7.1 × 10^2	0.040	7.0 × 10^2	0.040	7.5 × 10^2	0.040
mYc/mg	2.9 × 10^2	0.040	2.9 × 10^2	0.040	2.5 × 10^2	0.040
mY/mg	1.0 × 10^2	0.040	1.0 × 10^2	0.040	1.1 × 10^2	0.040
mx/mg	1.2 × 10^3	0.040	1.0 × 10^3	0.040	9.0 × 10^2	0.040
Combined type A and type B	—	2.5	—	3.4	—	4.2
Degrees of freedom (Veff)		5		5		5
Coverage factor (k)		2	—	2	—	2
Measured value/mg kg^−1	80.6	—	115.9	—	133.8	—
Expanded uncertainty (U ())	1.5		2.0		2.6