4.3 Calculation at Variable Pathlengths (longitudinal
measurement}
Certain (centrifugal) analyz.ars
measure longitudinally
through the cuvettes. The pathlength thus becomes proportio–
nal to the total assay volume in the cuvette. Dilution of the
assay medium in the cuvettes with an inactive fluid eauses a
corresponding elongation of the pathlength, and the absorban–
ce remains constant. The net effect is that the calculation
factor becomes proportional to the sample volume and indepen–
dent of the sample volume fraction in the cuvette. (Ref. 34,
CK). In the example below the sample volume is 30 pL.
Symbols Type of quantity
a
Area of cuvette end
Example
0.25
x
10- 4
Uni
t
m2
v
d
Sample volume
Equals decrease in absor–
bance at 340
nm
per seeond
30
Other symbols as given above
b=
b=
b=
b=
d x a
E
X
v
d
x
25
189000
=
d
x
0.25
x
10-
4
630
x
30
x
l0-
6
d
x
0.001323
x
kat/L
d
x 1323 pJcat/L
Calculated for a Measuring Time of
60
s:
b=
d
x
1323 U/L
b=
d
x 22·.os Jlkat/L
52
x
10-
6
L
x
x
-l
s
-l
2
s
x m
m
2
x mol-l x
L
mol x s-l
L
Klinisk kemi
i
Norden 2: supp/, 1990
