Metrohm 746 VA Trace Analyzer User Manual

Page 488

6.9 Content calculation

746 VA Trace Analyzer / 747 VA Stand

6-25

eff,n

–

eff,s

Difference in the mass concentrations between

the n

spiked sample and the original sample

solution (calculated from the known volumes

V.sample

and

V.add

as well as

std

)

Determination of the calibration curve for the standard

addition
The parameters for the two possible model functions

y = a + bx

Straight line

y = a + bx + cx

Nonlinear curve of 4

degree

are determined by the procedure described in section 6.9.3 with y =

and

x =

eff

–

eff,s

by weighted least squares minimization. These parameters are

displayed on the "RESULTS/Calibrations" page and have the following

meaning:

Y.reg/offset

Intercept of calibration curve

Slope

Slope of calibration curve in the linear region

Nonlin.

Non-linearity factor

Calculation of the mass concentration

ρρ

eff,s

A requirement for the use of the standard addition is that when

eff

= 0 the

evaluation quantity

= 0. If 0 is substituted for these two quantities in the

calibration function, the sought mass concentration

eff,s

can be calculated

from the following two equations:

eff,s

– a = 0

for y = a + bx

eff,s

+ b

eff,s

– a = 0

for y =a + bx + cx

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.

std.add.

linear

nonlin

Display of the standard addition curve
The curve shown above

eff

–

eff,s

is not displayed and plotted by the

VA Trace Analyzer. Instead, it displays and plots

eff

in which the

coordinate origin is shifted to the left by the amount

eff,s

. The parameter a in

this representation denotes the value estimated from the curve for the sam-

ple and is thus designated by

Y.reg

ρρ

eff,s

Sample

Standard solution

ρρ

eff

–

ρρ

eff,s