Chapter 4 Temperature-Controlled Microchip HPLC System
4.6 Examples of Separation
4.6.2 Low Density Lipoprotein Separation
possible to use the pulsed-amperometric detection method where a multi-step electrical potential is input to the electrochemical sensor for non-derivatized amino acid detection [30].
4.6.2 Low Density Lipoprotein Separation
mildly oxidized LDL (LDL–), which usually represents up to 10% of plasma LDL.
Figure 4-25 shows the schematic illustration of LDL particles which has sizes of around 22 nm. So far, only the conventional desktop HPLC system has been used to separate LDL subfraction. Our goal is to carry out LDL separation using microchip HPLC system with a hope that in the future LDL diagnosis can be done using portable devices with much lower costs of time and money. As follows we demonstrate the first work using microchip ion-exchange chromatography [34] and conductivity sensing for LDL subfraction separation.
Table 4-1: Cardiovascular disease in the United States [32].
1,800,000 Rheumatic Heart Disease
4,400,000 Stroke
6,300,000 Angina (chest pain)
7,700,000 Acute Heart Attack
50,000,000 High Blood Pressure
Number of people effected in US
Disease
1,800,000 Rheumatic Heart Disease
4,400,000 Stroke
6,300,000 Angina (chest pain)
7,700,000 Acute Heart Attack
50,000,000 High Blood Pressure
Number of people effected in US
Disease
nLDL
Figure 4-25: Schematic illustration of nLDL/LDL- molecular structures.
4.6.2.2 Sample Preparation
Purified LDL samples suspended in PBS solution were extracted from human blood plasma using a two-step centrifugation process at the Atherosclerosis Research Unit at the University of Southern California and were stored at 8 0C. LDL concentration in the sample was 200 µg/mL (85% native LDL, 15% oxidized LDL).
Two types of buffer solutions were prepared. Buffer A: aqueous solution containing 20 mM TRIS with a pH value of 7.2. Buffer B: aqueous solution containing 20 mM TRIS and 1M NaCl with a pH value of 7.2. Buffer solutions were used for salt gradient elution as well as isocratic elution of LDL samples.
4.6.2.3 Conductivity Detection
Since native LDL and oxidized LDL 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 (∆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 [35]:
3
1 ( )
10 cell S E S
G C
K λ − λ −
∆ = − − (4.2)
where λS− and λE− are the equivalent conductance for the sample and eluent anions, CS is the concentration of the sample anion, and Kcell is the conductivity sensor cell constant. The conductance change ratio on sample elution is usually very small, only a few percents of the background conductivity as shown in (4.3):
( ) ( ) 1%
S E S S
Background E E E E
C C G
G C C
λ λ
λ λ
− −
+ −
∆ = − ≈ ≈
+ (4.3)
where CE 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 detector sensitivity. The conductance noise is mainly contributed from the temperature-fluctuation-induced background conductance change. In general, a 1 0C temperature change will result in a 2% conductance change [36]. Several approaches are available to increase the sensitivity for conductivity detection. First, suppressing the background conductance by using a suppressor column inserted after the separation column [37]. Second, increasing the sample plug concentration by using a sample preconcentration techniques [36]. Third, reducing sample injection volume and system dead volume. Fourth, reduce the sensor cell constant Kcell, which is especially achievable using MEMS technology [14]. The detection limit of conductivity detection can be found in 2.2.4.
The electrochemical sensor used for amino acid sensing in 4.6.1 was used here as a conductivity sensor for LDL. The difference is that AC signal instead of DC signal was input to the sensor for conductivity detection. Output current from the cell was sent to an op-amp circuit to convert the current signal into a voltage signal, which was recorded with the HP34970A data acquisition unit (RMS-DC mode). The sensor cell resistance during LDL separation was calculated and plotted against time to compose the chromatogram. Since LDL particles have a lower equivalent electrophoretic mobility than that of the anions in the eluent (chlorides), the conductivity of the sensor cell will drop when an LDL peak passes by. More details regarding the design principles and sensitivity of microchip conductivity sensor can be found in [35].
4.6.2.4 Separation Results and Comments
High-pressure-capacity HPLC microchips introduced in 4.2.2 were used here for LDL separation. Because of the large size of LDL particles (~ 22 nm in diameter), superficial porous anion-exchange particles (Hamilton PRP-X500) were used to pack the separation column to avoid serious band-broadening effect caused by LDL trapping in the stationary phase. As a reference, LDL separation was first carried out using the conventional HPLC system with salt gradient elution and UV detection as shown in Figure 4-26. LDL subfractions were separated by anion exchange chromatography into nLDL (native LDL) and LDL-/LDL2- (oxidized LDL).
Figure 4-27 shows the LDL chromatogram obtained by microchip HPLC system with isocratic elution and on-chip conductivity sensing. Sample volume was defined by the sample injection valve with a 20 µL sample loop (Rheodyne, Rohnert Park, CA). The preliminary data successfully demonstrates the separation and detection of 800 ng LDL (680 ng of native LDL and 120 ng of oxidized LDL). The limit of detection (LOD), calculated based on the signal-to-noise ratio in the chromatogram which is around 50, will be 80 ng assuming a signal-to-noise ratio of 5 is acceptable for analysis. The LOD of 80 ng corresponds to an original sample concentration LOD of 20 µg/mL if the sample injection volume is still 20 µL.
The relatively big band-broadening effect and long elution time observed in the microchip HPLC compared with the conventional HPLC were mainly caused by the large off-chip system dead/swept volume (~ 10 µL), large sample injection volume, as well as the weak elution strength of the mobile phase used for the isocratic elution. Those factors, however, can be improved with more efforts on the chromatography method development.
Figure 4-26: LDL separation obtained by conventional HPLC with salt gradient elution and UV detection.
Figure 4-27: LDL separation obtained by microchip HPLC with isocratic elution at room temperature and conductivity detection.
0 20 40 60 80 100
4.4 4.5 4.6 4.7 4.8 4.9
5.0
Flow rate:1 µL/min
Applied potential:
Sinusoidal 1Vpp,10 kHz Column ID: 50 µm
Column length: 1 cm LDL2- LDL-
Sensor Resistance (kohm)
Elution Time (min)
nLDL Elution: Isocratic
Eluent: 80%A + 20%B Sample volume:
680 ng (nLDL) 120 ng (LDL-+LDL2-) Stationary phase:
Hamilton PRP-X500
0 5 10 15 20 25
0.0 20.0k 40.0k
LDL2- Buffer A: 20 mM TRIS, pH 7.2
Buffer B: 20 mM TRIS; 1 M NaCl, pH 7.2 Elution: Salt gradient,
100%A, 0%B to 62%A, 38%B
Sample: 198 µg (nLDL), 2 µg (LDL-+LDL2-) Separation column: Hamilton PRP-X500 4.6 x 50 mm,7 µm beads
Flow rate: 1 mL/min LDL-
OD
280 nmElution Time (min)
nLDL