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between the measured relative response factor of
polar metabolite and four physicochemical prop-
erties associated with ion evaporation.
13
These
properties include molecular volume, octanol-
water distribution coef
subsequently employed to enhance the coverage
of the urinary metabolome by Ramautar and
colleagues.
24
Approximately 900 molecular fea-
tures were detected in human urine by sheathless
CZE-ESI-MS, whereas only about 300 molecular
attributes were found with the use of a conven-
tional sheath-liquid interface (
Figure 1
). The inte-
gration of transient capillary isotachophoreis
(CITP) as an in-capillary preconcentration proce-
dure with sheathless CZE-ESI-MS provided
further improvement in detection sensitivity,
allowing the characterization of more than 1,300
molecular features in urine.
cient, absolute mobility,
and effective charge.
Sheath-Liquid versus Sheathless
Electrospray Interfaces
The coupling of CZE to ESI-MS can be
achieved via the use of sheath-liquid or a sheath-
less interface.
14
The sheath-liquid interface is
considered as a robust technique and has been
most widely employed for CZE-ESI-MS
measurements in metabolomics.
15
However,
intrinsic disadvantages of the sheath-liquid
include the dilution of the CZE ef
ANALYSIS OF PROTEIN
EXPRESSION LEVELS
USING CAPILLARY
ELECTROPHORESISeMASS
SPECTROMETRY
uent and the
potential
ow
inside the capillary, thereby negatively affecting
the achievable detection sensitivity and the
resulting separation ef
induction of a hydrodynamic
ciency and resolution.
Single-Dimension Capillary
Electrophoretic Separation
CZE-ESI-MS was employed for the analysis of
low molecular weight proteins (below 20 kDa)
and peptides for the discovery of biomarkers in
human urine. Samples were investigated from
patients suffering from a variety of diseases,
including ureteropelvic junction obstruction,
25
cancer,
26
e
28
vasculitis,
29
coronary artery dis-
eases,
30
e
32
kidney diseases,
33
e
35
lithium-induced
nephropathy,
36
graft-versus-host disease,
37
and
diabetes.
32
The CZE-ESI-MS data were generally
presented by plotting the measured molecular
masses against their migration times and
compared among healthy and diseased patients.
Known and potential new urine biomarkers
have been identi
In order
to enable the detection of
low-
abundance metabolites in body
fluids, a number
of research groups have therefore developed
various sheathless interfaces.
16
e
19
TheworkofJaninietal.
16
and Sanz-Nebot
et al.
17
have illustrated the potential of the
sheathless interface for the analysis of complex
peptide mixtures. Moini has fabricated a porous
capillary outlet by etching with hydro
uoric
acid and established the electrical contact by
protruding the porous tip from a stainless-
steel ESI needled
filled with static conductive
liquid.
18
Recently, Maxwell, Zhong, Chen,
and colleagues
20
e
22
designed an interface that
is based on a stainless-steel hollow needle
with a beveled sprayer tip for coupling CZE
or capillary isoelectric focusing (CIEF) with
ESI-MS.
A detailed analytical evaluation of a sheathless
interface on the basis of a porous tip
18
was con-
ducted by Busnel et al.
23
for the analysis of tryptic
digests of bovine serum albumin and Escherichia
coli cell
ed using subsequent MS/MS
experiments.
38,39
Although a variety of different
proteins/peptides was discovered, most of these
putative markers are derived from the most
abundant proteins in the body such as colla-
gen
d
mainly type
I,
II, and III, albumin,
-2-macroglobulin, and uromodulin.
40
lysate. Sheathless CZE-ESI-MS was
b
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