Ionic liquids as advantageous solvents for headspace gas chromatography of compounds with low vapor pressure

Article abstract of DOI:10.1021/ac048737k

The potential of ionic liquids as solvents for headspace gas chromatography was investigated. Three compounds with boiling points above 200 °C were selected to demonstrate the feasibility of the concept described. 2-Ethylhexanoic acid, formamide, and tri-

Full text of DOI:10.1021/ac048737k

Anal. Chem. 2005, 77, 702-705
Ionic Liquids as Advantageous Solvents for
Headspace Gas Chromatography of Compounds
with Low Vapor Pressure
M. Andre,*,† J. Loidl,† G. Laus,‡ H. Schottenberger,*,§ G. Bentivoglio,§ K. Wurst,§ and K.-H. Ongania|
Sandoz GmbH, Biochemiestrasse 10, 6250 Kundl, Austria, Immodal Pharmaka GmbH, Bundesstrasse 44,
6111 Volders, Austria, Institute of Inorganic Chemistry, and Institute of Organic Chemistry, University of Innsbruck,
Innrain 52a, 6020 Innsbruck, Austria
chromatography (GC),13-15 for liquid-phase microextractions,16,17
and as modifying additives for eluents in liquid chromatogra-
phy.18,19
The potential of ionic liquids as solvents for headspace
gas chromatography was investigated. Three compounds
with boiling points above 200 °C were selected to dem-
onstrate the feasibility of the concept described. 2-Ethyl-
hexanoic acid, formamide, and tri-n-butylamine as ex-
amples of acidic, neutral, and basic analytes were dissolved
in acidic 1-n-butyl-3-methylimidazolium hydrogen sulfate
(1), neutral 1-n-butyl-2,3-dimethylimidazolium dicyana-
mide (2), and 2 containing 1,8-diazabicyclo[5.4.0]undec-
7-ene to adjust basic conditions. All analytes could be
determined with limits of detection and limits of quanti-
fication in the low-ppm concentration range.
Analysis of process-related residual reactants and solvents in
pharmaceuticals, as regulated by the official authorities, plays a
tremendous role in quality assurance and quality control. The
permanently demanded improvement of analytical performance
in chemical, clinical, and pharmaceutical laboratories relies on
either simplified sample preparation or thorough chemometric
automation. For example, multivariate analysis and classification
of the respective pharmaceuticals and intermediates using near-
infrared reflectance spectroscopy are powerful tools for analytical
automation20 but, in general, simplifying analytical sample prepara-
tion bears the highest potential of improving sample throughput
and time savings in an easy manner.
Headspace gas chromatography (HSGC) is a widely used
analytical technique for the determination of volatile substances
in solids and liquids.21-23 However, for compounds with a low
vapor pressure, the sensitivity in HSGC is rather limited. The
increase of the vapor pressure by increasing the thermostating
temperature is restricted by excessive pressure buildup in the vial.
A contemporary approach to overcome this problem is the use
of ILs as versatile solvents, since these low-melting organic salts
are easily synthesized or commercially available, respectively.4
They have ideal physicochemical properties for optimized head-
space conditions, as they are chemically inert, fairly good solvents
for organic substances with moderate to high polarity. They can
be tailored as application-specific systems with remarkably high
Ionic liquids (ILs) are salts1,2 or mixtures3 of salts that melt
below room temperature. They represent an important class of
innovative solvents. Their relevance is reflected by continuous
emergence of new applications, mainly in the field of homoge-
neous catalysis.4 In technical applications, they serve as superior
electrolytes for electric devices.5 Some utilization has been also
reported for various analytical purposes, e.g., for matrix-assisted
laser desorption/ionization mass spectroscopy (MALDI-MS),6 for
capillary electrophoresis (CE),7-12 as stationary phases for gas
* Corresponding authors. E-mail: max.andre@gx.novartis.com. fax: +43 5338
200 2463. E-mail: herwig.schottenberger@uibk.ac.at. fax: +43 512 507 2934.
† Sandoz GmbH.
‡ Immodal Pharmaka GmbH.
§ Institute of Inorganic Chemistry, University of Innsbruck.
|
Institute of Organic Chemistry, University of Innsbruck.
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702 Analytical Chemistry, Vol. 77, No. 2, January 15, 2005
10.1021/ac048737k CCC: $30.25 © 2005 American Chemical Society
Published on Web 12/16/2004

stability24,25 at temperatures even above 350 °C and exhibit
negligibly low vapor pressures.26 The relevance of these solvents
is reflected in a recent patent application.27 The intention of this
work was to evaluate their obvious potential and to demonstrate
the superiority of this concept for challenging analytical problems
in order to stimulate a broader use of ILs by the community of
analytical scientists.
EXPERIMENTAL SECTION
Reagents and Standards. Formamide (FA) [75-12-7], bp
210 °C, vapor pressure 1.87 Pa (0.014 Torr, 20 °C);28 racemic
2-ethylhexanoic acid (EHA) [149-57-5], bp 228 °C, vapor
pressure 4 Pa (20 °C);29 tri-n-butylamine (TBA) [102-82-9], bp
216 °C, vapor pressure 25.6 Pa (0.192 Torr, 25 °C);30 and
1,8-diazabicyclo[5.4.0]undec-7-ene [6674-22-2] (DBU) were pur-
chased from Aldrich. Ionic liquids 1-n-butyl-3-methylimidazolium
hydrogen sulfate (1), 1-n-butyl-2,3-dimethylimidazolium dicyana-
mide (2), and 1-n-butyl-2,3-dimethylimidazolium hydrogen sulfate
(3) were synthesized according to published procedures.31-33 The
precursor 1-n-butyl-2,3-dimethylimidazolium chloride exhibited
polymorphism, and X-ray structures of the two different types of
crystals are described (Figure S-1). Polymorphism in the crystal-
line state of imidazolium salts has been reported previously.34,35
A detailed description of the improved synthesis and analytical
characterization is given in the Supporting Information.
Apparatus. Two detection systems were used in this study:
a mass selective detector for FA and a flame ionization detector
for EHA and TBA.
System A. The analysis of FA was performed on a poly-
(ethylene glycol) fused-silica capillary column (J&W Scientific;
length 60 m, inner diameter 0.255 mm, film thickness 0.25 µm).
A CE Instruments gas chromatograph GC 8000 Top (Thermo
Electron) equipped with a Fisons mass selective detector MD 800
(Thermo Electron) was used. The carrier gas was helium (99.999%
purity) with a constant flow rate of 1 mL/min and a split ratio of
1:40. The temperatures of the capillary column and the injection
port were set to 200 °C. The mass selective detector used electron
impact (EI+, 750 V) ionization and was operated in the single ion
monitoring (SIM) mode, monitoring only the base peak of the
formamide mass spectrum (45 m/z). The temperature of the
column interface into the detector was 180 °C, and the temperature
of the ion source was 150 °C. For headspace sampling, a CTC
Figure 1. Chromatograms of production samples dissolved in ILs
applying high temperature thermostating HSGC: (a) penicillin G
benzathine spiked with 100 ppm formamide (*) in BMMI+ dicyana-
mide, thermostated at 180 °C, 15 min; (b) ampicillin sodium containing
0.4% 2-ethylhexanoic acid (*) in BMI+ hydrogensulfate, thermostated
at 150 °C, 15 min; (c) potassium clavulanate spiked with 100 ppm
tri-n-butylamine (*) in BMMI+ dicyanamide/DBU, thermostated at 150
°C, 15 min.
autosampler CombiPal was used (CTC Analytics AG), equipped
with a 2.5-mL airtight syringe (Hamilton). The syringe temperature
was set at 150 °C, fill speed at 0.5 mL/s, injection speed at 0.5
mL/s, pullup delay at 500 ms, preinject delay and postinject delay
at 500 ms, respectively, without needle flush. A 2.0-mL aliquot of
every sample solution containing 200 mg of matrix was filled into
a 10-mL headspace vial (Chromacol), and the vials were hermeti-
cally sealed. The headspace samples containing penicillin G
benzathine spiked with 100 ppm FA (based on the product) were
incubated at 180 °C for 15 min with shaking. The agitator speed
was set at 500 rpm. The injection volume was 1.0 mL (Figure 1a).
System B. The analyses of EHA und TBA were performed
on a dimethylpolysiloxane-phase fused-silica capillary column
(Agilent; length 30 m, inner diameter 0.53 mm, film thickness 2.65
µm). An Agilent gas chromatograph HP6890 (Agilent) equipped
with a flame ionization detector (FID) was used. The carrier gas
was helium (99.999% purity) with a constant flow rate of 4 mL/
min and a split ratio of 1:40. The temperatures of the capillary
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Analytical Chemistry, Vol. 77, No. 2, January 15, 2005 703

Table 1. Validation Results
slope
(area
repeatability
(RSD, n ) 6)
(%)a
mean
recovery
(%)
detection
intercept
(area)
correlation
LD
LQ
analyte
mode
ppm-1
)
coefficient r
(ppm)
(ppm)
2-ethylhexanoic acid
tri-n-butylamine
formamide
FID
FID
SIM
88.4
400.3
0.9994
0.9999
0.9998
9.6
2.0
10.9
95
93
101
22
8
13
65
24
39
0.0480
0.06019
155905
1.220 × 106
a At c ) 100 ppm.
column and the injection port were set to 250 °C, the temperature
of the FID was set to 250 °C. For headspace sampling, a Perkin-
Elmer headspace sampler HS40 was used (Perkin-Elmer). The
temperature of the transfer line was kept at 180 °C; the injection
pressure was set at 15 psi for 0.15 min. To minimize degradation
products, the incubation temperature has been optimized to 150
°C for ampicillin sodium containing 0.4% EHA (Figure 1b) and
for potassium clavulanate spiked with 100 ppm TBA based on the
product (Figure 1c). A 2.0-mL aliquot of every sample solution
containing 200 mg of matrix was filled into a 20-mL headspace
vial (Perkin-Elmer); the vials were hermetically sealed and
incubated for 15 min with shaking. The injection volume was 0.6
mL.
used in the drug synthesis, were chosen: (a) FA in benzylpeni-
cillin benzathine [1538-09-6], which is practically insoluble in
aqueous systems, freely dissolved in 1-n-butyl-2,3-dimethylimida-
zolium dicyanamide (BMMI+) dicyanamide as a neutral solvent,
(b) EHA in ampicillin sodium [69-52-3] dissolved in 1-n-butyl-
3-methylimidazolium (BMI+) hydrogen sulfate as an acidic solvent,
and (c) TBA in potassium clavulanate [61177-45-5] dissolved
in BMMI+ dicyanamide, containing an excess of DBU to generate
basic conditions.
Validation. To estimate the sensitivity of the applied system,
a limit test was determined without optimization of all instrumental
parameters for the three representative compounds dissolved in
pure ILs. A validation according to the current guideline of the
International Conference on Harmonization (ICH/Q2B) was
performed.38 Selectivity was assessed by identification using GC/
MS for FA and by standard addition for TBA and EHA, respec-
tively. An incubation time of 15 min was sufficient in all experi-
ments to ensure reproducible results. Blank runs have been
applied to check a possible carryover of analytes. The limit of
detection (LD) and the limit of quantification (LQ) were calculated
from the slope s and residual standard deviation σ of a linear
regression of the signal areas obtained at six concentration levels
(5, 10, 50, 100, 250, and 500 ppm based on the whole sample
solution) by the following equations, LD ) 3.3 σ/s, LQ ) 10 σ/s.
Recovery was calculated as found/expected value, and the
observed value was calculated from the calibration function. The
results, including repeatability at the 100 ppm level, are sum-
marized in Table 1. Representative chromatograms are shown in
Figure 1. Finally, we tested the use of a smaller volume of IL (0.5
mL) spiked at the 100 ppm level under otherwise identical
conditions and found the response to be comparable.
RESULTS AND DISCUSSION
Ionic Liquids. The ILs finally used in this study were 1-n-
butyl-3-methylimidazolium hydrogen sulfate (1) and 1-n-butyl-2,3-
dimethylimidazolium dicyanamide (2), respectively. The choice
of the anions such as hydrogen sulfate as a Bronstedt acid or
dicyanamide as a coordinating anion was governed not only by
the nature of the particular solutes but also by the affordability of
these room-temperature ionic liquids. The hydrogen sulfate,
especially, is very inexpensive. These salts can be dried at elevated
temperatures to virtually anhydrous solvation media, thus prevent-
ing hydrolysis of any reactive analytes. The choice of the
2-methylimidazolium cation in 2 was intentional, as carbene
formation could be avoided upon addition of strongly basic
additives. Furthermore, 1-n-butyl-2,3-dimethylimidazolium hydro-
gen sulfate (3) was also used and found to be equally suitable.
Controlled temperatures as high as 180 °C could be used without
any problems.
Analytes. As representative examples, three analytes with
boiling points above 200 °C were selected to demonstrate the
feasability of the HSGC approach described: EHA, FA, and TBA
as acidic, neutral, and basic compounds. For EHA and TBA, FID
was used as detection mode, whereas for FA, which exhibits very
low FID response, mass selective detection with SIM was used.
To the best of our knowledge, no successful application of HSGC
for the direct determination without derivatization or extraction
of EHA and FA has been published so far. The quantification of
FA by static HSGC at low temperatures reported in the literature
failed due to the lack of sensitivity.36 Only the quantification of
TBA with a vapor pressure significantly higher than the two other
compounds has been described.37
Since the compounds selected are solvents or reagents
frequently used in the pharmaceutical industry for the production
of active drug substances and since static HSGC is an official
method of the European and U.S. Pharmacopoeias, the enhanced
sensitivity of the analytical system would be of general importance
in routine quality control; the official limits of EHA and FA are
0.8% and 220 ppm, respectively; for TBA, the limit of 0.2% specified
as aliphatic amines in potassium clavulanate is applicable.39
Although the official limit for EHA is relatively high compared to
the sensitivity of the system presented, this compound still is a
Matrixes. To demonstrate the routine applicability, three
authentic production samples, spiked with the respective analyte
(38) http://www.emea.eu.int/htms/human/ich/quality/ichfin.htm (Q2B-Guide-
line on Validation of Analytical Procedures: Methodology).
(36) Brinkmann, K.; Ebel, S. Pharm. Ind. 1999, 61, 263-269.
(37) Tsukioka, T.; Ozawa, H.; Murakami, T. J. Chromatogr. 1993, 642, 395-
(39) Ph. Eur., 4th ed.; Council of Europe, Strasbourg Cedex, France, 2001, pp
40, 654, 3913 (Supplement 4.6), 4545 (Supplement 4.7).
400.
704 Analytical Chemistry, Vol. 77, No. 2, January 15, 2005

representative example for fatty acids, which are of great impor-
tance at low concentrations in many biological matrixes.
for thermogravimetric and differential scanning calorimetric
measurements.
CONCLUSIONS
SUPPORTING INFORMATION AVAILABLE
Optimized synthesis and characterization of the ionic liquids
1-3. This material is available free of charge via the Internet at
http://pubs.acs.org.
We have shown that ILs are well suited for trace analysis of
high-boiling, low vapor pressure residual solvents in pharmaceuti-
cal drug products. It was demonstrated that, by proper choice of
the IL, challenging analytical problems involving neutral, acidic,
and basic analytes can be successfully solved.
Received for review August 24, 2004. Accepted November
1, 2004.
ACKNOWLEDGMENT
We thank Dr. H. Kopacka for the NMR spectra, Dr. A. Zemann
for the capillary electrophoretic analysis, and Dipl.-Ing. E. Gstrein
AC048737K
Analytical Chemistry, Vol. 77, No. 2, January 15, 2005 705