Ionic liquids enable electrospray ionisation mass spectrometry in hexane

Article abstract of DOI:10.1039/b606938j

Addition of a lipophilic ionic liquid to non-polar, hydrocarbon solvents (such as hexane and toluene) permits electrospray ionisation mass spectrometric analysis of dissolved analytes. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b606938j

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Ionic liquids enable electrospray ionisation mass spectrometry in
hexane{
Matthew A. Henderson and J. Scott McIndoe*
Received (in Cambridge, UK) 16th May 2006, Accepted 5th June 2006
First published as an Advance Article on the web 16th June 2006
DOI: 10.1039/b606938j
commercially-available ionic liquid [PR1 R2]+[N(SO2CF3)2]2 (1)
Addition of a lipophilic ionic liquid to non-polar, hydrocarbon
3
(R1 = hexyl, R2 = tetradecyl). An ionic liquid (IL) was chosen for
solvents (such as hexane and toluene) permits electrospray
several reasons: neat ILs themselves may be electrosprayed with
the intention of characterising dissolved analytes10 (though it is
likely that at least some molecular solvent is required to facilitate
the process);11 a recent paper has shown that ILs can be
electrosprayed successfully from a benzene solution;12 and there
is a huge selection of commercially available ILs with a wide
variety of properties, including miscibility with hexane. ILs have
previously found utility in mass spectrometry as matrices for
matrix-assisted laser desorption ionisation (MALDI).13
ionisation mass spectrometric analysis of dissolved analytes.
Electrospray ionisation mass spectrometry (ESI-MS) has made
possible the swift characterisation of a huge range of thermally
labile, polar molecules, most notably biomolecules but also
pharmaceuticals,1 coordination complexes,2 supramolecular
assemblies3 and organometallic catalysts.4 The ESI-MS process
transfers ions, pre-formed in solution, into the gas phase. Neutral
compounds, unless they interact with charged species such as H+
or Na+, are essentially invisible to ESI-MS. A limitation of the ESI
technique has been choice of solvent; it is generally only reasonably
volatile, polar solvents that provide a stable spray and satisfactory
spectra. Water, methanol and acetonitrile are most commonly
employed, frequently with organic acids added to enhance the
formation of protonated species. Less polar solvents such as
CH2Cl2 provide good spectra under the right conditions,5 but we
are unaware of any reports of ESI mass spectra having been
successfully recorded in non-polar solvents such as hexane. We
report herein a convenient means of enabling ESI-MS to be
performed in a variety of non-polar hydrocarbon solvents, by
simply adding a small amount of a lipophilic ionic liquid.
ESI-MS{ of pentane, hexane and cyclohexane solutions of 1 do
not provide a stable spray (manifested by spectra that consist
entirely of noise) until a concentration of y1025 M is reached, at
which point excellent spectra of the species [CnAn21]+ (where C =
cation and A = anion, positive ion mode, see Fig. 1) and
[Cn21An]2 (negative ion mode) (n = 1 or 2) are obtained.
Benzene and toluene require slightly lower concentrations of the
ionic liquid (see the Supporting Information for full details), and
for more polar solvents (e.g. dichloromethane, diethyl ether,
The main problem that prevents non-polar solvents being used
in ESI-MS may simply be lack of conductivity. ESI-MS is
fundamentally an electrochemical process, with the same number
of ions being discharged at the capillary as are generated in the
source.6 Non-polar solvents may shut down this process due to
high resistance, a problem which is dealt with in electrochemistry
by the addition of a supporting electrolyte.7 Supporting electrolytes
in conjunction with electrochemical processes in electrospray have
garnered attention before, but the emphasis has been on reducing
the negative aspects of their presence, viz. suppression of a desired
analyte by a buffer.8 Signal suppression by an abundance of
electrolyte does not, however, appear to be a significant problem in
some cases, e.g. the ESI-MS characterisation of organometallic
complexes.9
We reasoned that a supporting electrolyte may well improve
the solvent characteristics with respect to the electrospray
process. A range of non-polar solvents become amenable to
electrospray upon addition of a suitable salt; we used the
Department of Chemistry, University of Victoria, P.O. Box 3065,
Victoria, BC V8W 3V6, Canada. E-mail: mcindoe@uvic.ca;
Fax: +1 (250) 721-7147; Tel: +1 (250) 721-7181
{ Electronic supplementary information (ESI) available: General experi-
mental details. ESI-MS of 1 at various concentrations in pentane,
cyclohexane, hexane, toluene and benzene. Synthesis and characterization
of 2. ESI-MS of 3 in toluene. See DOI: 10.1039/b606938j
Fig. 1 Positive-ion ESI mass spectra of 1 at differing concentrations in
cyclohexane. Each spectrum was collected over 1 minute (60 summed
spectra). A stable spray is obtained at a threshold of y1025 mol L21
.
2872 | Chem. Commun., 2006, 2872–2874
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fluorobenzene), we were untroubled in collecting good spectra of
the IL down to the detection limits of the instrument; there
appeared to be no minimum concentration of IL required for a
stable spray.
A further advantage of 1 is its high molecular weight, resulting
in aggregates (in the positive ion mode, [CnAn21]+, in the negative
ion mode [Cn21An]2) spaced 763 m/z apart, leaving wide, clear
windows for analytes in which the salt-derived ions do not
interfere. Only two ions from the ionic liquid appear in most
spectra: the bare ion and the first aggregate. The high molecular
weight, lipophilic salt [PR1 R2]+[BArF
2
]
(ArF = 3,5-bis{trifluor-
3
4
omethyl}phenyl) (2)§ contains a very non-coordinating anion and
may prove to be a good choice when examining highly reactive
systems. It is even better than 1 in terms of limiting interference in
the mass spectrum, as the gap between aggregates is 1346 m/z, and
in practice, the higher aggregates are barely detectable (Fig. 2).
This observation is in stark contrast to simple salts such as sodium
Fig. 3 Positive-ion ESI-MS of [Rh(COD)(PPh3)2]+ in cyclohexane/1.
Inset: expansion of isotope pattern match for the analyte.
iodide, in which the aggregates form with such ease that the series
+
[NanIn21
]
and [Nan21In]2 are routinely used for instrument
calibration. ESI-MS studies of imidazolium-based ionic liquids
reveal similarly extensive supramolecular assemblies of the IL.14
2 is soluble in toluene but phase-separates with hexane; however,
it is sufficiently soluble in hexane to enable the electrospray process
to work well. Another readily-prepared salt suitable for enabling
ESI-MS in non-polar solvents is [(C2H4O)6Na]+[BArF 2 (3), made
]
4
by adding a stoichiometric amount of Na[BArF4] to a solution of
18-crown-6 (see Supporting Information for spectrum).
Of course, enabling a stable spray in a non-polar solvent means
that solvent may be used in routine fashion for the characterization
of charged solution species. Adulterating solvents for enhanced
spray and ionization characteristics is nothing new; acids are
routinely added, solvent mixtures are frequently employed,
buffered solutions are sprayed, with the intention of either
improving sensitivity or convenience. Here, the salt acts as an
enabling additive in a similar fashion, and provided the analyte of
interest does not have an ion coincident in m/z with the salt,
analysis can proceed without difficulty. An example of this can be
seen in Fig. 3 for [Rh(COD)(PPh3)2][BF4]. The spectrum, whilst
dominated by peaks associated with 1, provides high quality data
on the analyte of interest. If so desired, spectra uncomplicated by
the IL may be obtained by background subtraction of a blank.
The IL acts to assist collection of ESI-MS in an additional way:
ionic complexes of low solubility dissolve much more readily in the
IL-adulterated solvent, presumably because they undergo counter-
ion exchange. An especially obvious example of this behavior may
Fig. 4 Negative-ion ESI-MS of [NEt3H][HFe3(CO)11] in hexane/1 (n =
8–11). Inset: while the compound is hexane-insoluble (left), it dissolves
readily when 1024 mol L21 of 1 is added (right).
be observed for [HNEt3][HFe3(CO)11],15 which is insoluble in
hexane but dissolves readily in hexane containing 1 (Fig. 4).
JSM thanks the Canada Foundation for Innovation (CFI), the
British Columbia Knowledge Development Fund (BCKDF) and
the University of Victoria for a New Opportunities Fund Award,
and the Natural Sciences and Engineering Research Council
(NSERC) of Canada for a Discovery Grant. We thank Dr
Andrew Weller (Bath) for the Na[BArF4] and Sarah Jackson
(Victoria) for a sample of [Rh(COD)(PPh3)2][BF4].
Notes and references
{ ESI-MS data were collected on a Waters1 Micromass1 Q-Tof micro2
mass spectrometer with Z-spray electrospray source. Samples were infused
from a 250 mL gas-tight syringe at 10–40 mL min21, via a syringe pump.
Instrument settings were unexceptional: capillary voltage 2900 V, cone
voltage 20 V, source temperature 100 uC, desolvation gas temperature
200 uC. Nitrogen was used as the desolvation gas.
§ Synthesis of 2. Trihexyltetradecylphosphonium chloride (0.143 g,
0.28 mmol) was dissolved in toluene (4 mL) and added to a solution of
sodium tetra-(3,5-bis(trifluoromethyl)phenyl)borate (0.243 g, 0.28 mmol) in
toluene (6 mL) with two drops of acetonitrile. The mixture was stirred for
one hour, filtered through a Celite plug and washed with toluene. The
solvent was removed and the residue dried under vacuum at 50 uC for
3 hours to provide 2 (42%, not optimized). 1H NMR (CDCl3): d (ppm) 7.68
(s, 8H, ortho-H); 7.51 (s, 4H, para-H); 1.92, 1.56, 1.42, 1.23, 0.87, 0.85 (all
multiplets, 68H, CH2 and CH3). ESI-MS: (+ve) 483.5 m/z [PC32H68]+; ESI-
MS (2ve) 863.1 [BC32H12F24]2.
Fig. 2 Negative-ion ESI-MS of 2 in toluene. The region between 2000
and 2400 m/z has been enhanced in intensity by a factor of 500.
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