Uses Of Ionic Liquids In Analytical Chemistry
Uses of Ionic Liquids in Analytical Chemistry
A. Berthod, S. Carda-Broch* (samuel.carda@uv.es)
Laboratoire des Sciences Analytiques, CNRS, Université de Lyon 1, Bat CPE-308,
69622 Villeurbanne, France
ABSTRACT
Room temperature ionic liquids (RTILs) are salts with melting points close or below room temperature. They form
liquids in which ions are present. This fact produces interesting solvent properties. RTIL are able to dissolve some
apolar molecules as well as some very polar ones. They start to find original use in chemical analysis. Since some
RTILs are not soluble in water, they can be used in water/RTIL extractions. The distribution coefficients of a
variety of solutes were measured. Our results are presented and discussed along with the results of others. RTILs
were also used as electrolytes in capillary electrophoresis. Their low volatility makes them useful as solvent
working in high vacuum (MALDI matrixes) or high temperature (GC stationary phases). Examples of such uses
were developed and are also discussed.
1. INTRODUCTION
trifluoromethanesulfonate (CF
-
3SO3 ). The combination
of such cations and anions can lead to a large number of
Room temperature ionic liquids (RTILs) are ionic liquids that provide considerable flexibility in the
salts with melting points lower then 30oC. They look like
selection of the most suitable pair for a specific chemical
a classical liquid but they do not contain any molecules:
application.
they are made of ions. The structure of these liquids is
completely different from the structure of any other The main physicochemical properties of RTILs are: (i)
solvents made of molecules. The properties of a given
under an inert atmosphere, they remain liquid over a
solvent depend on the interaction between the solvent
temperature range of 200 to 300oC; (ii) they have
molecules. If there are strong interactions between the
practically no vapor pressure [5]; (iii) they are reported to
solvent molecules, the solvent is called a “polar” solvent,
have a wide window of electrochemical stability, good
e.g. water, methanol, ethanol. If the interactions between
electrical conductivity, high ionic mobility and excellent
the solvent molecules are weak, it is an “apolar” solvent,
chemical stabilities [6, 7]. With all these properties, it is
e.g. hexane, heptane, petroleum ether. Table 1 lists the
hoped that they can act as “green solvents” and they will
properties of some solvents as well as several polarity
replace volatile organic solvents in several chemical
index values: dipole moment, dielectric constant, reactions [1, 6].
Reichardt index and octanol/water partition coefficient
(as log P
oct). The great originality of RTILs is that they
are not made of molecules. Ions are present in the liquid
with an exactly equal number of positive and negative
ions so that the whole liquid is electrically neutral. Ionic
.
liquids are known for decades, but they were molten salts
at very high temperature, e.g. the melting points of
sodium, potassium, aluminum and calcium chloride are
respectively 801, 770, 190 and 782oC. Most organic
molecules are decomposed at these elevated
temperatures. The potential of the new solvent class of
ionic liquids at room temperature is actively investigated
as shown by Figure 1.
The first RTIL was discovered during World
War I, in 1914, looking for new explosives. It was ethyl
ammonium nitrate with a melting point of 12oC [1]. In
the eighties, Seddon and coworkers started to use RTILs
as nonaqueous polar-like solvents for electrochemical and
spectroscopic studies of transition metal complexes [2-4].
Typically, RTIL consists of nitrogen- or phosphorous-
containing organic cations and large organic or inorganic
anions [1]. Bulky organic cations such as N-alkyl-
Figure 1: Number of articles published worldwide per year on the
pyridinium and 1-alkyl-3-methylimidazolium are
subject “room temperature ionic liquids” (2002 = estimate)
combined with inorganic anions such as Cl-, Cl-/AlCl
(Chemical Abstracts, Current Contents and Medline databases).
3,
NO -
-
-
3 , PF6 and BF4 . Less common anions include bis
* on leave on a Marie Curie Fellowship from Area de Química Analitica,
(trifluoromethanesulfonyl) imide (CF3SO2)2N- and
Universidad de Jaime I, Castellón, Spain
1
2
t
a
rd
.5
.5
.5
.5
.5
.5
2
9
.5
0
65
35
46
11
60
50
26
11
40
44
65
23
1.
0.
76
33
27
15
34
62
20
10
10
65
Polarity
Reich
P oct
0
2
4
0
0
0
0
8
9
g
.2
.2
.3
15
88
78
97
83
.0
.3
.7
78
28
52
.8
30
30
38
18
34
54
70
.4
.6
-0
-0
-0
2.
0.
0.
1.
0.
-1
-1
-0
0.
4.
3.
-0
0.
1.
1.
3.
0.
0.
2.
-1
-0
Lo
n
t
3
6
ent.
∞
∞
∞
06
.1
.1
05
3
∞
∞
∞
3
01
01
∞
9
5
1
4
10
∞
∞
03
-
0.
20
44
0.
1.
3.
0.
0.
1.
1.
4.
0.
1.
%w/w
Water in
solve
t
n
5
1
4
4
lubility
0
3
electrical curr
∞
∞
∞
18
8
.5
81
9
∞
∞
∞
7
00
∞
7
8
8
24
05
∞
∞
07
-
ct
So
0.
7.
12
6.
8.
00
1.
4.
1.
Solve
0.
0.
0.
0.
in water
0.
C o
tant
15
.7
.5
28
.5
.8
9
34
.7
.7
.6
0
92
88
.7
.2
.1
5
.3
.3
6
38
.1
6.
20
37
2.
17
15
4.
4.
36
48
26
6.
1.
1.
32
15
13
4.
10
20
7.
2.
80
cond.
Dielectric
cons
poses, cond. = condu
e
74
69
44
0
75
64
15
15
86
30
66
88
0
08
87
76
70
32
76
10
75
31
87
Dipole
1.
2.
3.
1.
1.
1.
1.
3.
4.
1.
1.
0.
2.
2.
2.
1.
1.
3.
1.
0.
1.
i
ons
moment
deby
= decom
e
c.
e
,
d
f
some solvents at 20
c
tiv
1
9
8
7
4
1
1
1
9
3
7
0
9
2
2
4
0
5
9
3
1
0
2
4
7
6
4
9
8
4
8
8
9
7
8
9
9
5
5
6
7
2
6
9
3
0
1
0
index
37
35
34
50
39
39
48
35
43
47
36
37
38
37
32
37
35
36
42
38
40
49
33
41
liquid
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
Refra
C
y
ionic
o
8
8
3
9
6
5
1
0
iling
.1
.6
.1
.5
.2
.4
.3
.1
.4
.7
.5
.6
.2
4.
.1
11
56
81
80
11
99
61
34
15
18
78
77
98
68
64
79
11
55
97
66
11
10
dec.
ical properties o
Bo
19
point
C
hosphate, * dr
o
.7
7
8
6
5
4
5
6
3
7
7
4.
3.
5
8.
3.
0.
.5
3.
0.
5.
8
6.
4
6.
5
0
16
-8
-9
-4
5.
-8
-
115
-6
-
116
-6
18
-
114
-8
-9
-9
-9
-8
-8
-
108
-1
-
126
-
108
-9
Melting
point
ysico-Chem
t
y
1
33
36
65
95
78
58
23
2
2
2
45
41
32
55
43
60
27
2
3
55
59
0
scosi
cP
1.
0.
0.
0.
2.
3.
0.
0.
2.
1.
0.
0.
0.
0.
0.
0.
0.
7.
2.
0.
0.
1.
3
00*
Table 1 : Ph
Vi
y
l
imidazolium hexafluorop
y 3
9
0
2
6
0
7
9
3
9
5
9
1
4
9
1
5
1
1
7
4
8
7
8
2
cm
04
79
78
87
81
80
48
71
94
09
78
90
68
65
79
80
80
74
82
80
88
86
99
36
l
meth
Densit
g/
1.
0.
0.
0.
0.
0.
1.
0.
0.
1.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
1.
-
ht
9
0
0
0
4
60
58
41
78
74
74
74
73
78
46
88
86
32
72
88
60
72
92
18
Mole
cular
11
10
10
13
28
BMIM = buty
weig
e
o
ne
i
c
ide
id
one
her
t
l
l
am
ol
n
rm
her
l
itrile
lfox
l
ket
y
l
ket
y
l
et
e
ion 6 d
tano
tano
fo
a
n
e
ne
rofuran
en
ro
l
et
hy
hy
l
form
yl su
t
but
o
p
an
yd
lu
-PF
Solve
e
t
h
anol
Acetone
ethanol
hept
hexa
o
c
tano
pr
to
water
l
i
qui
Acetic acid
Benzene
et
Aceton
1
-
bu
2
-
bu
chlo
m
yl-
di
e
thy
e
th
ethyl acetate
1-
IM
e
t
h
y
l
et
tetrah
dim
d
i
m
m
e
t
h
y
l
i
s
obut
meth
BM
m
The physicochemical properties of RTILs reactions, electrochemistry and spectroscopy; (ii) non-
depend on the nature and size of both their cation and
molecular environment; and (iii) room temperature
anion constituents. Table 2 lists some data for several
chemistry. The very low vapor pressure of RTILs makes
RTILs formed by the 1-butyl-3-methyl imidazolium them possible candidates for matrixes in matrix-assisted
cation associated with different anions, and for various
laser desorption/ionization mass spectrometry (MALDI)
bis(triflyl)amide ((CF3SO2)2N-) ionic liquid salts with experiments. The MALDI matrix should absorb the light
different cations [1, 8-11]. This table shows that it is
energy, transfer it to the analyte and ionize it (adding a
difficult to relate the physicochemical properties of a
proton) so it can fly in the high vacuum the tube of time-
given RTIL to its chemical structure [8]. The polarity of
of flight (TOF) mass spectrometer. As some ionic liquids
RTILs was estimated using the solvatochromic effect of
are non-miscible with water, they could be also used in
the Reichardt’s dye (E N
T scale) [11]. A remarkable
countercurrent chromatography in a biphasic liquid
constancy was found for imidazolium based ionic liquids,
system, since in this technique both phases, stationary
all values falling between 64 and 68 [11, 12]. For
and mobile, are liquids.
comparison, the E N
T values the common solvents toluene,
acetone, acetonitrile, ethanol and methanol are
respectively 10, 36, 46, 65 and 76 [13] (Table 1). The
2. USES OF IONIC LIQUIDS IN
solubility of the RTILs in organic solvents depends on
CHEMICAL ANALYSIS
the dielectric constant, ε, of the solvent. Most RTILs are
fully miscible with solvents with a ε value higher than 6
(e.g. water, dimethylformamide, ethanol, acetone, Table
We thought that the unique properties of RTILs will be
1) [1]. However the water solubility of RTILs is highly
useful in chemical analysis. The main property is the
dependent on the anion. Chloride, bromide, original polarity of the RTILs used as solvent. The liquid
trifluoroacetate ionic liquids are water-soluble.
salts associate ionic interaction with hydrophobic
Hexafluorophosphate and bis(triflate) amide salts often
interaction (alkyl tail of the imidazolium ring). RTILs
form two phases with water [1].
are able to solubilize inorganic as well as organic
compounds. The thermal stability and low volatility of
RTILs will also be used in chemical analysis.
1-butyl-3-methyl imidazolium salts
gegenion m.p.
d
N Viscosity
Conduct
o
C
g/cm3
cP 20oC
ivity
S/m
BF -
-82 (g)
1.17
1.429
233
0.17
2.1. Liquid-liquid extractions with ionic
4
PF -6
-8
1.36
1.411
312
0.14
Cl-
65
1.10*
solid
solid
solid
liquids
CF3COO-
~-40 (g)
1.21
1.449
73
0.32
CF3SO3 -
16
1.29
1.438
90
0.37
(CF3SO2)N-
-4
1.43
1.427
52
0.39
C
~-40 (g)
1.33
1.414
182
0.10
Solute distribution in a biphasic liquid system
3F7COO-
C
-
4F9SO3
20
1.47
1.405
373
0.045
In liquid-liquid extraction, two immiscible or
bis-(trifluoromethyl sulfonyl) amide salts
1-methyl-3-methyl imidazolium
22
1.56
1.422
44
0.84
partially miscible solvents are used. Solutes will
1-et
hyl-3-methyl imidazolium
-3
1.52
1.423
34
0.88
1-ethyl-3-ethyl imidazolium
14
1.45
1.426
35
0.85
distribute or partition between the two liquid phases.
1-but
yl-3-methyl imidazolium
-4
1.43
1.427
52
0.39
1-isobutyl-3-methyl imidazolium
~-30 (g)
1.43
1.429
83
0.26
Different solutes will partition differently between the
1-but
yl-3-ethyl imidazolium
~-30 (g)
-
1.428
-
-
same liquid phases allowing to separate them. We used
1-met
hoxyethyl-3-methylimidazolium
~-30 (g)
1.50
1.429
54
0.42
1-methyl-2-methyl-3-ethylimidazolium
20
1.51
1.430
88
0.32
the 1-butyl-3-methyl imidazolium hexafluorophosphate
1-trifluoro ethyl-3-methyl imidazolium
~-30 (g)
1.66
1.409
248
0.10
1-ethyl-3-ethyl-4-methyl imidazolium
-22
1.43
1.430
36
0.62
(BMIM-PF
6) ionic liquid as a typical example of a RTIL
1-methyl-3-ethyl-4-methyl imidazolium
-3
1.47
1.427
37
0.66
forming a biphasic liquid system with water in liquid-
Data from Refs 1, 8-11; * supercooled liquid at 25oC ; (g) glass tran-
liquid extractions [22]. We synthesized our own ionic
sition, ~ approximate value (+/- 10oC)
liquid because it is difficult to find at a reasonable price.
We reacted butyl chloride with 1-methyl imidazole for 72
Table 2 : Effect of the nature of the anion on physicochemical properties
h at 70oC to obtain BMIM-Cl with a 95% yield. A
of 1-butyl-3-methyl imidazolium salts and of the cation of the bis
metathesis was done with hexafluorophosphoric acid at
(triflyl)amide salts (20oC).
room temperature producing two phases: BMIM-PF6 and
Since pioneering woks of Ford [14], Hussey [15]
an acidic aqueous solution. The distribution between the
and Seddon [16], the ionic liquids are actively studied by
ionic liquid and the aqueous phase of a large variety of
several researchers groups worldwide working in all compounds bearing different functionalities was studied
fields of chemistry. Recently, several researchers have
at several pH values [22].
reported uses for RTILs in chemical analysis. They have
been used as stationary phases in gas chromatography
Ionic liquid/water distribution coefficients
[17] and mobile phases in liquid chromatography [18],
We determined the water / BMIM-PF6
they were able to dissolve chiral selectors to make chiral
distribution coefficients for 45 aromatic compounds.
stationary phases [19] and they were used as unique
High performance liquid chromatography (HPLC) was
running electrolytes in the separation of phenolic the analytical technique used to measure the solute
compounds by capillary electrophoresis [20].
concentration in each phase. After a thorough
The RTILs are used in chemistry such as [21]:
equilibration (20 min under vigorous shaking), the two
(i) non-volatile solvents in organic synthesis, catalyzed
liquid phases are let standing for one night. 0.5 mL of the
aqueous upper phase and 0.5 mL of the ionic liquid lower
3
phase are collected separately. The RTIL phase is too
viscosity of the water-saturated ionic liquid phase was 10
viscous to be injected directly in the HPLC system. It
times lower than the pure ionic liquid. It can be
was diluted with 1 mL methanol to form 1.5 mL of fluid
generalized that the physicochemical properties of the
solution. 20 µL of the solution are injected in the HPLC
pure ionic liquid are significantly modified by the mutual
system (column 15 cm with octadecyl silane (ODS) saturation occurring in liquid-liquid extraction processes
bonded silica particles, methanol/water 70/30 mobile [23].
phase at 1 mL/min, UV detection at 254 nm). The peak
Heptane is an apolar solvent not soluble with
areas of the solutes were used to quantitate the solute
water (Table 1) nor with ionic liquids. A three-phase
concentrations in each phase. The ratio of (solute peak
system can be obtained with heptane, BMIM-PF6 and
area in the ionic liquid phase x 3) over (solute peak area
water. The ionic liquid phase can also be sandwiched
in the aqueous phase) gave the ionic liquid/water between a denser liquid phase such as chloroform and a
distribution coefficients.
lighter aqueous phase. A triphasic liquid system made of
BMIM-PF6, water and cyclohexane was used to perform
Comparison with the octanol/water distribution Heck reactions (coupling of alkenes with aryl halides or
coefficients
benzoic anhydride) in BMIM-PF6 [24].
The solute distribution between octanol and
water is used as a reference scale for hydrophobicity [13].
2.2. Additive in HPLC
Figure 2 shows the ionic liquid/water distribution
constants of the molecular forms of the aromatic
compounds plotted versus their respective octanol/water
Poole et al [18,25] studied the properties of
coefficients with a logarithm scale. A reference line with
tetraalkylammonium nitrate and thiocyanate ionic liquids
a slope of unity (i.e., the ionic liquid coefficient is exactly
in gas and liquid chromatography. These salts show very
equal to the octanol/water coefficient) is included in this
strong orientation and proton acceptor interactions with
plot. As can be seen in Figure 2, the ionic liquid/water
weak proton donor capacity and can be used in the
distribution coefficients of the amine-containing temperature range from around room temperature to 150-
compounds are slightly higher than the octanol/water
180oC, at which temperature they exhibit significant
coefficients. Conversely, the ionic liquid/water vapor pressure. Their viscosity is conveniently controlled
distribution coefficients of the acidic and phenolic by working at elevated temperatures or through dilution
compounds are significantly lower than the corres-
with a cosolvent.
ponding octanol/water coefficients. The neutral
In HPLC, they showed that ionic liquid
compounds and the ionizable compounds with both basic
containing mobile phases rapidly deteriorated the silica-
and acid functionalities show similar distribution based column packing (ODS). They used them as liquid
behavior in both the ionic liquid/water and octanol/water
mobile phase with unbounded silica packing. It was
systems.
shown recently that BMIM-PF could be used with pure
6
water to separate polar basic drugs. The screening of the
silanol groups by the imidazolium cation was not clearly
3
different from the screening obtained with amines (e.g.
2.5
triethylamine).
2
)
/w
1.5
2.3. Electrolyte in Capillary Electrophoresis
n
liq
(
io
1
g
P
In some capillary electrophoresis (CE) studies,
lo
alkylammonium salts have been used as electroosmotic
0.5
flow (EOF) modifiers [26-28]. Yanes et al [29] reported
0
the development of a fairly robust capillary
-0.50
0.00
0.50
1.00
1.50
2.00
2.50
3.00
electrophoretic method for the separation of polyphenols
-0.5
found in grape seed extracts that uses
log P(oct/w)
tetraethylammonium tetrafluoroborate (TEA-BF
4) as the
only electrolyte in the background electrolyte. They
COOH and/or OH
NH2
others
showed that the cation not only acted as an EOF modifier
but also played an active role through association with
Figure 2 : Ionic liquid/water distribution coefficients compared to the
octanol/water values (log scale). Crosses: amino-aromatic compounds,
polyphenols. The excellent reproducibility was attributed
open triangles: neutral compounds or compounds with both acidic and
to the coating of the capillary wall by the
basic substituents, filled-diamond: acidic and/or phenolic compounds.
tetraalkylammonium cations with a permanent charge
group not subject to pH-induced variations in ionization.
A polarity of ethanol with a low water solubility
Stalcup et al [20] used alkyl-methyl imidazolium
Our study showed that the polarity of the BMIM
ionic liquid salts with different anions as running
PF6 ionic liquid is comparable to that of ethanol, but electrolytes in CE to separate polyphenolic neutral
without being soluble in water or in apolar solvents. The
molecules. They found that the polyphenols eluted after
4
the EOF marker as if they were positively charged. The
for the special requirements of UV-MALDI detection.
separation mechanism relies on the association of the
The ionic matrix must have significant absorbance at the
polyphenols with the imidazolium cations either coating
desired wavelength, but also available protons. Most
the capillary wall or electrophoretically migrating in the
conventional ionic liquids that lack these properties are
bulk solution. The cation has a significant effect on the
ineffective as MALDI matrixes.
electropherogram acting on the retention times. The
anion has much less effect. The larger polyphenols were
In the quest for a good MALDI matrix, it is not
the most retained.
possible to predict:
(i) which salt will be efficient
(ii) which salt will be liquid.
2.4. Matrixes for MALDI-TOF MS
For protein MALDI detection [30], 38 combinations were
tested: 18 salts failed to produce any MALDI signal, and
20 salts were successful, of which only 9 were liquids.
In Matrix-assisted laser desorption/ionization We tried to find ionic matrixes that could be used in
time-of-flight mass spectrometry (MALDI-TOF MS), the
MALDI-TOF to determine deoxyribonucleic acid
role of the matrix is:
(DNA)-oligomers directly, improving the results obtained
(i) to absorb strongly the laser UV light and convert it in
for the 3-HPA conventional matrix. For DNA MALDI
enough heat so that the solute will be vaporized,
detection, 33 combinations were tested: 19 salts failed, 14
(ii) to ionize the solute so it can fly if an electrostatic field
salts produced a MALDI signal with DNA oligomers but
is applied.
none of them were liquid [31]. In most cases, an ionic
matrix produced greater spectral intensities than
The MALDI technique involves the process of
comparable solid matrixes. Since all ionic matrixes
desorption, dissociation and ionization of the analyte and
working were solids, the spectra that gave the best
the matrix under the condition of high laser energy intensity only could be obtained after several attempts to
density. The matrix should not be volatile, dissolve
find appropriate “hot spots” [31].
(liquid matrix) or cocrystalllize (solid matrix) with the
sample. It should protonate the solute, stifle both
chemical and thermal degradation of the sample and
produce homogeneous mixtures (10000 to 100000/1 2.5. Gas Chromatography Stationary Phases
matrix/solute ratio). These requirements make ionic
liquids possible candidates for MALDI matrixes. As
Armstrong et al [17] showed that RTILs could
liquids, they will produce much more homogeneous act as non-polar stationary phases in gas chromatography
mixtures of greater vacuum stability than any solid (GC) when separating volatile non-polar analytes such as
matrixes and they are good solvents for a variety of
the linear alkanes shown in Figure 3-A. Polar analytes
organic, inorganic and polymeric substances, etc, but they
that are proton-donor or acceptor molecules were
should protonate the sample.
separated by the RTIL GC column in a way similar to
what is obtained with an apolar classical GC stationary
Many conventional solid MALDI matrixes are
phase (e.g. dimethylpolysiloxane OV1 or OV101). The
acids such as sinapinic acid, α-cyano-4-hydroxycinnamic
unique point is that the same RTIL stationary phase was
acid, 2,5-dihydroxybenzoic acid and 3-hydroxypicolinic
also able to separate perfectly polar molecules with
acid (3-HPA). The classical ionic liquids failed to somewhat acidic or basic functional groups. These
produce signal in MALDI experiments because they were
molecules were highly retained similarly to what is
unable to protonate the solute. Associating the efficient
obtained with a polar classical GC stationary phase (e.g.
acid solid matrixes with bases producing liquid salts,
carbowax). Figure 3-B shows the separation of some light
useful ionic liquid matrixes were obtained for protein and
linear alcohols. Thus, molecules with proton-donor or
polymer molecular weight measurements. Armstrong et
acceptor characteristics tend to be spatially resolved, as a
al. [30] demonstrated that ionic liquids and solids may
group, from nonpolar analytes.
make the most useful MALDI matrixes. With ionic
matrixes, it is possible to combine the beneficial qualities
Inverse GC also is a good way to examine the
of liquid and solid matrixes. Ionic liquids produce a
nature of different ionic liquids. Preparing different
much more homogeneous sample solution (as do all capillary GC column with different ionic liquids, it is
liquid matrixes) yet have greater vacuum stability than
possible to establish some ionic liquid physico chemical
most solid matrixes. In most cases, an ionic matrix can
properties by studying the way a variety of solutes are
be found that produces greater spectral peak intensities
retained by the different columns at different temperature.
and lower limits of detection than comparable solid The chloride-containing ionic liquid interacted much
matrixes. Most ionic liquids readily dissolve biological
more strongly with proton-donor and acceptor solutes.
oligomers, proteins, and polymers. However, ionic The hexafluorophosphate-containing ionic liquid tended
liquids can vary tremendously in their ability to promote
to be somewhat less polar and interacted more strongly
analyte ionization. Both the cationic and anionic portion
with nonpolar solutes.
of the ionic matrix must be chosen with a consideration
5
A B
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Figure 3 : Gas chromatograms obtained on an ionic liquid coated
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To conclude this work, it is important to point
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temperature ionic liquid" has really unique properties.
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found in polar solvents or in apolar solvent. They were
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vapor pressure that minimizes the release of chemical in
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analytical chemists, we would like to temper too much
[28] Quang C., Khaledi M.G., (1993) Anal. Chem. 65:3354-
enthusiasm for such solvents. To select them to replace
3358.
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[29] Yanes E.G., Gratz S.R., Stalcup A.M. (2000) Analyst
make ionic liquid and the way to dispose of them should
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both be evaluated. In a global view, perfluorinated ions
[30] Armstrong D.W., Zhang L., He L., Gross M.L. (2001)
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may not be as "green" as too often stated.
[31] Carda-Broch S., Berthod, A., Armstrong, D.W. (2003)
Rapid Comm. Mass Spec. 17:553-560.
4. ACKNOWLEDGMENTS
SCB and AB thank the European Community for the Marie-
Curie Fellowship HPMF-CT-2000-00440 (Human Ressources).
6
