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4 LC/ MS of Polymers

4 LC/ MS of Polymers

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Selected Publications on the Characterization of Oligomeric/

Polymeric Samples by ESI or APCI Mass Spectrometry

Combined with On-Line Separation






Amino resins

Phenolic resins

Poly(propylene imine)


Oligomeric surfactants

































8, 42, 89, 96




79, 80

74, 80








SEC: size-exclusion chromatography; RPLC: reversed-phase liquid

chromatography; GPEC: gradient polymer elution chromatography;

CE: capillary electrophoresis; SFC: supercritical-fluid chromatography; EI: electron ionization; PB: particle beam.

LC techniques for synthetic polymers can be categorized according to their

mode of operation. Figures 4.9 to 4.11 illustrate different modes that may

afford separation of oligomers and their mixtures according to specific molecular properties for an oligomeric surfactant Triton X-100 [octylphenoxypoly(ethoxy)ethanol] as an example. Figure 4.9 shows the total ion current

(TIC) chromatogram, along with the contour plot (m/z on the axis vs. chromatographic elution time on the horizontal axis, and shaded areas in the x-y

plane indicate ESI ions with intensity exceeding the threshold), for a normalphase separation by gradient elution. In this mode of LC, the oligomers are

separated, but the elution of different oligomeric series (I-IV) that reflect

chemical heterogeneities overlap.

A reversed-phase LC-ESI-MS analysis of Triton X-100 is shown in Figure 4.10,

which displays separation according to chemical heterogeneity practically

independent of molecular size.The chromatographic resolution of oligomeric

mixtures may also rely on liquid adsorption chromatography (LAC, performed usually on silica gel as a stationary phase) and gradient polymer

elution chromatography (GPEC). In LAC, all sample components initially

prefer to adsorb on the surface of the stationary phase, and the increase in

the percentage of a strong solvent (displacer) results in the sequential elution

according to the change in the adsorption equilibria involving the analyte


molecules, the stationary phase, and the mobile phase. To date, no application of LAC coupled with ESI/APCI mass spectrometry has been reported.

©2002 CRC Press LLC


TIC chromatogram (bottom chart) and contour plot for the LC-ESI-MS analysis of Triton X-100,

an oligomeric surfactant, by using gradient normal-phase chromatography (2 mm i.d. cyanopropylsilica column, from 95/5 hexane/dichloromethane to 50/40/10 hexane/dicholomethane/

methanol in 20 min, 200 µL/min flow rate, no effluent split, nebulizer-assisted electrospray),

and the oligomer series identified (I-IV). The m/z values of the peaks that belong to the major


oligomer series (I) follow the formula [M + NH4 ] = 224 + 44n, where n is the number of ethoxy


units, and doubly charged [M + 2NH4] ions are also present. (Courtesy of PE Sciex, Foster

City, CA)

©2002 CRC Press LLC


TIC chromatogram (bottom chart) and contour plot for the LC-ESI-MS analysis of Triton X-100,

by using gradient reversed-phase chromatography [2 mm i.d. octadecylsilica column, 20/80 to

50/50 acetonitrile/10 mM ammonium acetate in 20 min, 200 µL/min flow rate, 3:1 effluent split,

nebulizer-assisted electrospray]. See Figure 4.9 for the oligomer series (I-IV) separated. (Courtesy of PE Sciex, Foster City, CA)

In GPEC (also known as high-performance precipitation liquid chromatography),84–87 the oligomeric mixture is dissolved in a good solvent of the

analyte and injected onto the column equilibrated with a poor solvent (“nonsolvent”) as an initial mobile phase that results in the precipitation of the

polymer on the top of the column. An increasing percentage of the good

solvent during gradient elution will redissolve the oligomer molecules

according to both their molecular weight and chemical composition. Ideally,

the stationary phase does not have an effect on the separation process.

Therefore, GPEC may be performed on nonpolar stationary phases such as


octadecylsilica (ODS) bonded phase. GPEC/ESI-MS has been used to char89

acterize dipropoxylated bisphenol A/adipic acid polyesters. When liquid

chromatography at the critical point of adsorption (LCCC) is used, the chromatographic elution becomes independent of the molecular weight and only

depends on chemical heterogeneity. LCCC requires specific solvent composition and temperature; thus, method development is critical and may be

tedious. Experimentally less demanding gradient separations in which the

method is tuned for a mass-independent elution have been developed

(“pseudo-LCCC”; the analysis of Triton X-100 presented in Figure 4.10 may

be considered as an example) and used on-line with ESI mass spectrometry

for the analysis of alkylated poly(ethylene glycol) and terephthalic acid/

neopentyl glycol polyester resin. Because the separation does not significantly decrease the polydispersity of the analyte in this hyphenated technique,

ESI mass spectrometry is useful mainly for identification of the oligomers, and

as a support to LC method development and LC-based quantification.

©2002 CRC Press LLC

SEC, which is also known as gel permeation chromatography (GPC), is

the most commonly used method for determining polymer molecular


weight distributions (MWD). This LC method separates compounds based

on their hydrodynamic volume in solution; larger molecular size materials,

higher molecular weight, eluting first followed by the smaller molecules of

lower molecular weight. The commonly used differential refractive index

(RI) detector provides, again, very little information about the chemical

composition, and molecular weight information obtained by the technique

is highly dependent on the accuracy of the calibration procedure. Although

a well-defined relationship exists only between the hydrodynamic volume

(not molecular weight) of the solute and its retention volume (VR), the common logarithm of relative molecular weight (log Mr) is correlated to VR in

practice. Well-characterized, narrow molecular weight distribution oligomer

and polymer calibrants of similar chemical composition provide the most

accurate results. Such calibrants are usually unavailable, and narrow molec91

ular weight polystyrene standards are often used. Besides, the mechanism

of separation in SEC may involve solute-solvent-packing interactions that


are not strictly dependent on molecular size, and such interactions may

lead to systematic errors in estimation of the molecular weight relying on

calibration curves obtained by polystyrene standards when measuring polymers other than polystyrene. The SEC analyses of oligomeric mixtures may

suffer the most from structure-dependent interactions. Oligomers of dissimilar chemical composition can also assume significantly different hydrodynamic volumes depending on their conformation in solution, even though

their Mr is identical.

ESI mass spectrometry is compatible with the SEC conditions applied to

the routine analysis of synthetic oligomers and polymers, and the coupling

offers specific benefits in terms of obtaining chemical composition information


and accurate molecular weight calibration.

Figure 4.11 shows the GPC/

ESI-MS analysis of Triton X-100. [No cationizing agent is added to the tetrahydrofuran mobile phase; therefore, the major ions represented in the

contour plot are the protonated octylphenoxypoly (ethoxy)-ethanol oligomers

with m/z = 207 + 44n.] Most GPC/ESI-MS applications have relied on the

pre-column or post-column addition of a cationizing agent, most commonly

NaI which has good solubility in the mobile phase. ESI mass spectrometry

can directly handle effluents from analytical (7.8-mm i.d.) SEC columns with

very little (<1%) of that effluent required (spectra are recorded from ∼10 ng of

sample during elution). This approach results in a significant decrease in the

polydispersity of the analyte entering the ion source of the mass spectrometer, as demonstrated in Figure 4.11; therefore, problems associated with the

ESI-MS analysis of polymeric samples with broad molecular weight distribution (discrimination according to cationization efficiency as a function of

molecular weight, bias based on detection efficiencies, choice of experimental

conditions, and so on) are eliminated or greatly reduced. With the mass

spectrometer continually acquiring spectra as the molecules elute from the

SEC, an on-line absolute molecular weight detector is employed for polymers

©2002 CRC Press LLC


TIC chromatogram (bottom chart) and contour plot for the SEC-ESI-MS analysis of Triton X-100

(30 cm ì 7.8 mm i.d. PLGel 3-àm Mixed-E column, tetrahydrofuran mobile phase at 1.0 mL/

min, effluent split 1:100). Major ions are the protonated oligomers.

that have been size-separated by the SEC. From the mass spectra, the elution

profiles of individual oligomers are determined from the reconstructed

selected ion current as a function of elution time (tR) or elution volume, VR.

Because ESI uses only a very small fraction of the effluent, conventional SEC

detectors (RI, UV, and the like) can be operated parallel with mass spectrometry. The peak apex for each selected ion chromatogram or selected oligomer

profile (the sum of ion intensities over different charge states) is used for an

accurate elution volume for the given oligomer mass, which is then used to

generate a calibration curve as shown in Figure 4.12. To better demonstrate

the effect of using an SEC calibration obtained by coupling with ESI mass

spectrometry vs. calibration with narrow-dispersity polystyrene standards,

an octylphenoxypoly(ethoxy)ethanol sample with higher average molecular

weights and broader molecular distribution (Igepal), compared to Triton X100, was used as an analyte. In Table 4.3, a comparison of quantitative

molecular weight distribution data obtained by direct ESI, analytical SEC

with polystyrene calibration, and SEC after accurate ESI mass spectrometric

calibration is presented. A recent development in GPC/ESI-MS includes

miniaturization of the column (µSEC) that offers various advantages to the

technique, such as low eluent consumption, low cost per column, reduced

maintenance requirement, ability to interface to other chromatographic

©2002 CRC Press LLC



a) UV chromatogram (λ = 254 nm) and [M + nNa] selected-ion traces for octylphenoxypoly

(ethoxy)ethanol oligomers separated on three SEC columns (30 cm × 7.8 mm i.d., 1000 Å, 500 Å,

and 100 Å UltraStyragel) in series. The selected-ion trace for the triply charged n = 50 oligomer

was obtained by summing m/z 824 to 826; the selected-ion trace for the doubly charged n = 35

oligomer was obtained by summing m/z 895 to 826; and the trace for the singly charged n = 20

oligomer was obtained by summing m/z 1108 to 1110 through the duration of the chromatogram. b) Calibration curves for octylphenoxypoly(ethoxy)ethanol: polystyrene vs. on-line ESI

mass spectrometry. (Reprinted with permission from Ref. 94. Copyright ©2000 American

Chemical Society.)

techniques (multidimensional LC) and the possibility of coupling to ESI mass


spectrometry without the need for flow splitting. In addition, better chromatographic performance can be achieved with microcolumns when compared to

conventional-bore systems, which enables a better separation of sample constituents or significantly reduced time of analysis with separation power

identical to conventional SEC columns. Newer mass analyzers such as



orthogonal acceleration time-of-flight (oa-TOF) and FT-ICR instruments

have been introduced into GPC/ESI-MS of polymers. Larger oligomers, such

©2002 CRC Press LLC


Molecular Weight Averages and Polydispersity of an

Octylphenoxypoly(ethoxy)ethanol Oligomeric Surfactant

by Direct ESI Mass Spectrometry, SEC with Polystyrene

Calibration, and SEC with Calibration Via On-Line ESI Mass


ESI mass spectrometry, no separation

SEC, polystyrene calibration

SEC, calibration by on-line ESI-MS














as poly(methyl methacrylate) (PMMA) up to 9000 Da as [M + 5Na] ions, and

minor impurities can be easily detected due to the extended mass range and

high duty cycle of the oa-TOF analyzer, compared to GPC/ESI-MS on a

quadrupole instrument or a quadrupole ion trap. Selected oligomer profiles

for the sodiated (1+ through 5+ charge states) ions were also generated for

a commercial, narrow molecular-weight-distribution PMMA sample, and

they were used for obtaining a calibration curve and calculating accurate

molecular-weight distribution data. In addition, GPC/ESI-FT-ICR mass

spectrometry of PMMA allowed for an unequivocal end-group determination and characterization of a secondary distribution due to the formation

of cyclic reaction products. A GMA/BMA copolymer with a broad molecular-weight distribution, where fractionation and high resolving power were

required for adequate characterization, has also been analyzed by this

hyphenated technique, and sodiated GMA/BMA oligomers in excess of

9 kDa were detected. End-groups resulting from the polymerization process

were positively identified, and GPC/ESI-FT-ICR also allowed the accurate

determination of the molecular weight distribution data.



In this chapter, the principles, instrumentation, and application of atmosphericpressure ionization (principally ESI) mass spectrometry to synthetic oligomers

and polymers are discussed through selected representative examples. The

technique has proven potential in this application area from the structural

and compositional characterization discussed here to, perhaps, preparative mass


spectrometry to generate monodisperse synthetic polymers. Direct use of the

technique is appropriate for the qualitative analysis of samples with moderate

complexity and molecular weight due to the phenomenon of multiple charging characteristic of ESI, and it is also well-suited under these conditions

for sophisticated structural studies involving ultra-high resolution or tandem mass spectrometry. ESI mass spectrometry of mixtures with broad

molecular weight distribution should benefit a prior separation, reducing

©2002 CRC Press LLC

the polydispersity of the analyte. The advantage of hyphenated LC/MS for

obtaining information about chemical composition, resolution of overlapping charge envelopes in the ESI mass spectra of polymers, SEC calibration,

and complex mixture analysis have been highlighted.


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