Metrohm 757 VA Computrace - Manual (Page 129)

5.8 Data processing and evaluation
757 VA Computrace – Software
119
Std.dev.(n) Standard deviation of the individual value
EV(n) = s(n)
c(n) – c(s) Difference in the mass concentrations
between the spiked sample
n and the
original sample solution
3. Determination of standard addition curve
For the calculation of the linear standard addition curve, the
parameters
a and b of the linear regression curve y = a + bx
are calculated by weighted least square minimization with
y = EV and x = c – c(s). The weight factor for each point is
the standard deviation obtained from the replications. The pa-
rameters
a and b are displayed in the RESULTS window and
have the following meaning:
a = Y.reg/offset Intercept of std.add. curve
b = Slope
Slope of std.add. curve
4.
Calculation of mass concentration c(s)
A requirement for the use of the standard addition is that
when
c = 0 the evaluation quantity EV = 0. If 0 is substituted
for these two quantities in the calibration function, the sought
mass concentration
c(s) can be calculated from the equation:
c(s) = a / b
In the graphical representation of the standard addition curve,
the sought mass concentration on the x-axis is given by the
distance between the zero point and the intersection point
with the calibration function.
EV
0
c(s)
c - c(s)
Y.reg/offset
Sample
Standard additions
Slope
5.
Calculation of result deviation MC.dev.
The total deviation of the calculated substance mass concen-
tration Mass.conc. is determined using a linear error calcu-
lation. Independent of the number of measurements, the total
deviation MC.dev. is always calculated in a way that
Mass.conc. ± MC.dev. gives the range in which the mass
concentration may be expected with a probability of 68.3%.