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5.0
nLDL
Flow rate:
1
Elution: Isocratic
Eluent: 80%A + 20%B
Sample volume:
680 ng (nLDL)
120 ng (LDL
−
+ LDL
2
−
)
Stationary phase:
Hamilton PRP-X500
7
LDL
−
4.9
L/min
Applied potential:
Sinusoidal
1Vpp, 10 kHz
Column ID: 50
μ
4.8
μ
m
LDL
2
−
4.7
Column length:
1 cm
μ
m beads
4.6
4.5
4.4
0
20
40
60
100
Elution Time (min)
Figure
16.19.
Chromatogram obtained from MEMS LC. The flow rate was con-
trolled by allied sinusoidal potential at 1Vpp and 10 kHz.The elution was
isocratic. The eluent contain 80% of buffer A (20mM TRIS, pH 7.2) and buffer B
(20mM TRIS, I M NaCl, pH 7.2. Hamilton PRP-X500 separation column was packed
with 7 mm beads. The LD sample contained 680 ng of nLDL and 120 ng of LDL
and LDL
2
as validated by HPLC.
16.4.4.1. Sensitivity of Conductivity Detection For nLDL and oxLDL.
Since nLDL and oxLDL are both charged molecules, it makes perfect sense to use
conductivity detection for sensing of those molecules. In conductivity detection,
the baseline conductance signal is generated by the eluent. When sample peak
enters the detector, a change of conductance
D
G can be measured. For anion
exchange ion chromatography, the magnitude of this change is expressed in the
following equation, assuming the samples and eluent are completely ionized [38].
1
10
3
K
cell
ðl
S
l
E
Þ
C
S
;
D
G
¼
ð
16
:
8
Þ
where
l
S
and
l
E
are the equivalent conductance for the sample and eluent
anions, C
S
is the concentration of the sample anion, and K
cell
is the conductivity
sensor cell constant. The conductance change ratio on sample elution is usually
very small, only a few percent change of the background conductivity:
G
Background
¼
ðl
S
l
E
Þ
C
S
D
G
C
S
C
E
1
ðl
E
þ
þ l
E
Þ
C
E
%;
ð
16
:
9
Þ
where C
E
is the concentration of eluent anions. The sensitivity of conductivity
detection is affected by the conductance noise level, sample injection volume,
system dead volume and the inherent detector sensitivity. The conductance noise
is mainly contributed from the temperature-fluctuation-induced background
conductance change. In general, 1
1
C temperature change will result in 2% of
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