7
Complementary Supercritical Fluid
Chromatography - Mass Spectroscopy
The high dissolving power of supercritical fluids for some polymers and the high
efficiencies of capillary columns make supercritical fluid chromatography (SFC)
particularly suitable for analyses of oligomeric and polymeric materials. The separation
of 42 styrene oligomers has been reported [1, 2]. The separation of every oligomer
in a mixture is routinely possible, in contrast to size exclusion chromatography in
which resolution is much lower. For example, Figure 7.1a shows the separation
of a polydimethysiloxane mixture. Not only are the individual linear oligomers
resolved, but also a series of small peaks is assigned to branched-chain oligomers.
The analysis of numerous other polymers and surface-active agents by SFC had
been reported, including methylphenylsiloxanes, styrene and other vinyl aromatic
polymers, polyolefins and waxes, polyethers, polyglycols (underivatised, since analysis
temperatures are well below the decomposition temperature), polyesters, and more
polar polymers such as epoxies (Figure 7.1b) and isocyanates.
Such analyses generally involve density programming to bring out oligomers at fairly
regular intervals. Simultaneous temperature programming also extends the range of
compounds eluted and gives greater chromatographic efficiencies by increasing solute
diffusion coefficients in the mobile phase.
For very high molecular weight polymers, hydrocarbon (e.g., pentane) mobile phases
modified with polar additives such as alcohols and ethers are required. Many such
separations have been reported by SFC using packed columns, often with gradient
elution, but such an approach poses special problems in capillary SFC because of the
low flow rates. Capillary SFC with solvent programming is a likely future growth area.
A number of advantages accrue from combining the separating power of capillary
SFC with the explicit structural information of mass spectroscopy (MS). Most of the
common ionisation modes have been shown to be compatible with SFC, including
electron impact, chemical ionisation, and charge exchange [3]. The variety of structural
data available from SFC with negative-ion chemical ionisation detection has been
demonstrated: while methane reagent gas gave mainly the methane positively charged
ion plus I ion (M + I), both methane with carbon dioxide and ammonia with carbon
dioxide as reagent gases gave many more fragment ions.
159
Chromatography Mass Spectroscopy in Polymer Analysis
Recorder response
(a)
(b)
Time (minutes)
Figure 7.1 (a) SFC chromatogram of polydimethylsiloxane. Conditions: column,
20 m x 50 µm id SB-methyl; mobile phase: CO2 at 120 °C with asymptotic
density programming, detector: flame ionisation detection (FID); and (b) SFC
chromatogram of epoxy acrylate oligomers. Conditions: column: 20 m x 50 µm
id, SB-biphenyl-100; mobile phase: CO2 at 70 °C with linear density programming,
detector: FID. Source: Author’s own files
160
Complementary Supercritical Fluid Chromatography - Mass Spectroscopy
By changing the density of the supercritical fluid, different fractions may be selectively
extracted from the complex mixture or simple matrix. On decompression, the
extracted solutes are precipitated and may be collected from injection into a gas
chromatography or SFC for analysis. Figure 7.2 shows a simple apparatus for on-line
supercritical fluid extraction (SFE)/SFC: solutes extracted from the sample matrix
are deposited from the end of a restrictor into the internal loop of the microinjection
valve of the capillary SFC. The valve loop contents are subsequently switched into
the SFC column by means of liquid or supercritical carbon dioxide.
SFE-SFC Interface
Solutes aspirated
from restrictor into
cooled valve
Extraction
vessel
CO2
Heated restrictor
housing
Frit
restrictor
Oven
CO2
50um capillary
column
Figure 7.2 Apparatus for coupled supercritical fluid extraction.
Source: Author’s own files
SFC has been used to determine oligomers in polyethylene glycol [4] and low molecular
weight, high-density polyethylene wax [5]. Carbon dioxide, propane and propanemodified carbon dioxide have been studied as eluents.
SFC coupled to time-of-flight – secondary ion mass spectroscopy has been applied to
the fractionation of polydimethylsiloxane oligomers in the molecular weight range
of 1,000–10,000 [6].
Nerin and co-workers [7] used supercritical fluid extraction coupled to MS to
determine potential migrants from paper and board intended for food packaging.
161
Chromatography Mass Spectroscopy in Polymer Analysis
One attractive approach in this context is the use of SFE with carbon dioxide as the
supercritical fluid, which can provide a fast and efficient extraction of a wide range
of compounds. SFE has been shown as an excellent extraction system of different
matrixes [8-19].
References
1.
E. Klesper and W. Hartmann, Journal of Polymer Science, Part C: Polymer
Letters Edition, 1977, 15, 12, 707.
2.
E. Klesper and W. Hartmann, Journal of Polymer Science, Part C: Polymer
Letters Edition, 1977, 15, 1, 9.
3.
N.M. Frew, C.G. Johnson, R.H. Bromund in Supercritical Fluid Extraction
and Chromatography, Eds., B.A. Charpentier and M.R. Sevenants, American
Chemical Society Symposium Series No.366, ACS, Washington, DC, USA,
1988, p.208.
4.
K.D. Bartle, T. Boddington, A.A. Clifford, N.J. Cotton and C.J. Dowle,
Analytical Chemistry, 1991, 63, 20, 2371.
5.
J.C. Via, C.L. Braue and L.T. Taylor, Analytical Chemistry, 1994, 66, 5, 603.
6.
B. Hagenhoff, A. Benninghoven, H. Barthel and W. Zoller, Analytical
Chemistry, 1991, 63, 21, 2466.
7.
L. Nerin, E. Asensio and C. Jiménez, Analytical Chemistry, 2002, 74, 22,
5831.
8.
EN 1186-13, Materials and Articles in Contact with Foodstuffs – Plastics Test Methods for Overall Migration at High Temperatures, 2002.
9.
C. Jimenez, Internal Report, CPS University of Zaragoza, 2000.
10. J.L. Hedrick, L.J. Mulcahey and L.T. Taylor, Mikrochimica Acta, 1992, 108,
3-6, 115.
11. I.J. Barnabas, J.R. Dean and S.P. Owen, Analyst, 1994, 119, 11, 2381.
12. S. Bøwadt and S.B. Hawthorne, Journal of Chromatography A, 1995, 703,
1/2, 549.
162
Complementary Supercritical Fluid Chromatography - Mass Spectroscopy
13. T.L. Chester, J.D. Pinkston and D.E. Raynie, Analytical Chemistry, 1996, 68,
3, 487.
14. I.A. Stuart, J. MacLachan and A. McNaughton, Analyst, 1996, 121, 1, R11.
15. J.L. Synder, R.L. Grob, M.E. McNally and T.S. Oostdyk, Analytical
Chemistry, 1992, 64, 17, 1940.
16. K. Schafer and W. Baumann, Fresenius’ Zeitschrift für Analytische Chemie,
1989, 332, 884.
17. EPA Method 3560, Supercritical Fluid Extraction of Total Recoverable
Petroleum Hydrocarbons, 1996.
18. EPA Method 3561, Supercritical Fluid Extraction of Polynuclear Aromatic
Hydrocarbons, 1996.
19. E. Anklam, H. Berg, L. Mathiasson, M. Sharman and F. Ulberth, Food
Additives and Contaminants, 1998, 15, 6, 729.
163
Chromatography Mass Spectroscopy in Polymer Analysis
164