WATERS CORPORATION 3465, Обзор и руководство по обслуживанию

Страница: 155 / 191

Table 16–2: LC-EC conditions for analysis of norepinephrine
Column ODS-2, 3 µm, 100 x 4.6 mm
Flow rate 1.0 mL/min
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
o
C
Flow cell VT-03 flow cell with 3-mm GC WE mounted
with 50-µm spacer
E cell 800 mV (vs. Ag/AgCl, filled with saturated KCl)
I
cell
Approx. 3 nA
Industry literature describes several ways to determine the detection limit. In principle, it does not
matter which definition of detection limit is used, as long as the definition is precisely described.
In this manual, the concentration detection limit (cLOD) for a compound is defined as the analyte
concentration that results in a signal that is three times the standard deviation of the noise:
where sigma-noise is 0.2 x peak-to-peak noise and cA is the concentration of analyte injected.
Figure 16–3: Typical S/N ratio for norepinephrine measured with a VT-03 glassy carbon flow cell
(peak height: 80 nA, peak-to-peak noise: 1.5 pA) (Page 156)
shows a typical S/N ratio of a VT-03
glassy carbon 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 so that a larger peak height is obtained. 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 (that is, in picomoles) without mentioning
the injection volume makes comparison between different systems difficult.
December 16, 2021, 715007395 Ver. 00
Page 155