Table 15–2: LC-EC conditions for analysis of norepinephrine (continued)
Parameter
Description
Mobile phase
H
3
PO
4
50 mM, citric acid 50 mM, 20 mg/L
EDTA, 100 mg/L octane sulphonic acid (OSA),
pH=3.1 with KOH, 5% methanol
Sample
1.0 µmol/L norepinephrine, 20 µL injection
Temperature
30 °C
Cell
Flow cell with 3 mm GC WE, SB REF with 50
µm spacer
E cell
800 mV (vs. Ag/AgCl, filled with saturated KCl)
I
cell
approx. 3 nA
In the literature, several ways are described to determine the detection limit. In principle, it does
not matter which definition of detection limit is used, as long as the definition is precise.
In this manual, the concentration detection limit (c
LOD
) for a given compound is defined as the
analyte concentration that results in a signal that is 3 times the standard deviation of the noise:
Where sigma-noise is 0.2 x peak-to-peak noise and c
A
is the concentration of analyte injected.
The
Example S/N ratio for norepinephrine (Page 127)
image shows a typical S/N ratio for a flow
cell with 2.74 mm WE. In this example, the concentration detection limit for norepinephrine based
on three times the sigma-noise is 11 pmol/L (refer to the "LC-EC conditions for analysis of
norepinephrine" table). Expressing the performance of a detection system by only the peak
height makes no sense.
A system can easily be changed in a way that increases peak height. However, if the noise
increases similarly, it has the same effect as switching a recorder to a higher sensitivity: peaks
appear higher but the S/N ratio is the same. Expressing the limit of detection in an absolute
amount (in picomoles) without mentioning the injection volume makes a good comparison
between different systems difficult.
December 16, 2021, 715007395 Ver. 00
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