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Chromatographia

, Volume 80, Issue 5, pp 731–750 | Cite as

Applications of Hyphenated Liquid Chromatography Techniques for Polymer Analysis

  • Elena Uliyanchenko
Review
Part of the following topical collections:
  1. Young Investigators in Separation Science

Abstract

We find polymers everywhere in our daily activities, for example, as a part of consumer electronics products, healthcare devices, vehicles, etc. Analytical characterization of such materials is an important step towards understanding their properties and behavior in various applications. The increase of material complexity driven by highly demanding requirements for many applications necessitates the use of sophisticated analytical techniques to obtain sufficient insight into the structure of these materials. Coupling of liquid chromatography with other information-rich instrumental techniques becomes more and more important in the field of polymer characterization. Such combination can enable simultaneous separation, identification, and quantification of polymer sample components. In addition, it can provide information on interdependence of two polymer properties, e.g., molecular weight and chemical composition. Different hyphenated systems may be applied to address different problems in polymer research and development and a selection of the right technique may not be an easy and straightforward task. In this paper, the applications of LC-NMR, LC-IR, LC-Raman, LC-MS, LC-MALDI, LC × LC, and LC × Py-GC for polymer analysis are reviewed, their advantages and limitations are discussed, and practical challenges for the implementation of these techniques in a lab are addressed. Different hyphenated options are compared to facilitate selection of a suitable instrument for the particular problem at hand.

Keywords

LC-NMR LC-IR LC-Raman LC-MS LC-MALDI Two-dimensional liquid chromatography Hyphenated chromatography techniques Polymer analysis 

Introduction

We encounter polymer materials everywhere in our daily life. Many common items are made of plastic or have plastic components: computers, mobile phones, food packaging, cars, household items, and building parts. Modern polymers are replacing other materials, such as glass, metal, wood, etc, in different applications. Moreover, new highly demanding markets are open for plastics (e.g., aerospace, robotics, etc). To satisfy stringent requirements for such applications, plastic materials often need to incorporate multiple properties simultaneously, e.g., flame retardancy, toughness, good processability, good weatherability, etc. A single polymer can seldom meet all requirements for demanding applications. In most cases, this combination of properties is achieved by blending different polymers together, by producing copolymers and by including additive packages into the formulations. Final performance of the plastic material in the application depends a lot on the material structure at the chemical level. Even a simple homopolymer contains a mixture of macromolecules with different chain lengths (molecular-weight distribution, MWD) and often with different end-groups (functional-type distribution, FTD). Multicomponent material formulations that are used for the majority of real-life applications are much more complex in chemical nature. For example, in the case of copolymers, in addition to MWD and FTD, a chemical composition distribution (CCD) or block length distribution (BLD) will be present. Moreover, polymer chains may differ in molecular architecture and branching levels (Fig. 1).
Fig. 1

Polymer molecular distributions [91].

Reproduced by permission of the Royal Society of Chemistry

To facilitate the development of plastics with specific properties, the analytical characterization of material composition is essential. This allows an understanding of the mechanisms underlying the performance, generating knowledge on structure-properties relationships, predicting materials behavior, and improving material properties.

A number of instrumental analytical techniques can be applied for polymer characterization, including infrared spectroscopy (IR), nuclear magnetic resonance spectroscopy (NMR), Raman spectroscopy, matrix-assisted laser desorption/ionization mass spectrometry (MALDI), pyrolysis gas chromatography–mass spectrometry (Py-GCMS), etc. However, spectroscopy techniques are able to provide information merely on average chemical composition of the material and not on its distributions. This data cannot reflect the real complexity of polymer chain distributions in multicomponent formulations. MALDI suffers from severe mass discrimination and ion suppression phenomena that result in the fact that only part of the broad polymer distribution is observed in MALDI spectra. Py-GCMS involves macromolecule scission at high temperatures and can only provide information on combination of chemical fragments comprising original macromolecules. An application of liquid chromatography (LC) separation prior to the above-mentioned techniques can help address the described limitations. Separations based on one of the molecular distributions (e.g., MWD, FTD, and CCD) and in combination with the superior identification abilities of the above techniques make these hyphenated approaches very powerful tools for polymer analysis offering opportunities for in-depth characterization of material structure.

The goal of this paper is not to provide a comprehensive review of applications of hyphenated techniques in polymer analysis, as this can be found in several earlier publications [1, 2, 3], but to give a critical overview of existing technologies and their advantages and drawbacks for polymer characterization. This review is intended to offer polymer scientists guidelines for selection of a technique suitable to address a specific problem in polymer research and development. Recent advances in hyphenation of chromatography with other instrumental techniques will also be discussed and the reader will be referred to other detailed publications on each topic. The truly offline approach that includes fractionation of LC separation and analysis of fractions by other instrumental techniques is a relatively standard practice in polymer analysis. This procedure is straightforward, but very time- and labor-consuming and, therefore, cannot be applied for routine polymer analyses. This approach will not be the main focus of this review.

Hyphenation of Chromatography with Spectroscopy

Liquid Chromatography-Infrared Spectroscopy

Infrared spectroscopy (IR) is an instrumental analytical technique that measures vibrational bands of chemical substances [4]. It is based on the fact that all compounds absorb light with a specific frequency that matches the frequency of the vibrations of atoms or groups of atoms within the molecules. Most organic compounds absorb light in the infrared region generating a specific spectrum that allows their identification and quantification.

The usage of IR as a detector for chromatography has been proposed as early as the 1970s [5, 6]. In addition to qualitative information superior to that supplied by traditional LC detectors—ultra-violet (UV), refractive index (RI), and evaporative light scattering (ELSD)—IR also offers good quantification abilities. LC-IR approach is robust, simple, and cost effective compared to other hyphenated alternatives [7]. Therefore, LC-IR is an excellent choice when additional information about the components separated by LC is required.

Two approaches for LC-IR coupling exist: online coupling via a flow-cell and off-line couplings via a solvent-elimination interface. The flow-cell approach implies that the eluent from an LC system is transferred directly into the IR flow cell and the IR spectra are continuously recorded. Direct coupling through the flow cell offers a number of advantages. In this setup, all components eluting from the column are transferred and measured by IR. Because no additional manipulation with the sample is involved, oxidation and degradation of unstable compounds may be prevented [8]. This method is inexpensive, relatively quick, and requires low maintenance. Finally, an important advantage is the compatibility with high flow rates and buffer solutions in LC [7].

On the other hand, online LC-IR coupling through a flow cell has a number of serious limitations. The major drawback is the presence of intense IR signals from the LC eluent in the resulting IR spectra. This makes the detection of some compounds very challenging (when absorption bands of the analytes overlap with those of the eluent), jeopardizes sensitivity of IR measurements, and renders application of gradient in LC very difficult. Moreover, due to the limited time available for online IR measurements, the collection of multiple spectra to improve signal-to-noise ratio is often not possible [9]. Different ways to address these issues exist. Often highly concentrated samples need to be injected into an LC column to ensure sufficient IR signal. Chemometric methods are usually required to remove the signals from the solvents [10, 11, 12, 13]. When gradient is applied in LC, the algorithms for background correction become even more complex [14, 15, 16]. Because such mathematical correction can introduce some inaccuracy, the detection of minor components in the samples is often not feasible with the flow-cell approach. When designing online LC-IR experiments, the solvents need to be carefully selected. It is virtually impossible to find an LC eluent that is fully IR-transparent. However, for some applications, a mobile phase that shows low absorbance in the region of interest can be selected. One of such cases is the analysis of polyolefins in trichlorobenzene [9]. The use of expensive deuterated solvents is another option, because the absorbance for these solvents is shifted to lower wavenumbers. Finally, post-column extraction and solvent replacement can also be used [17], but this adds complexity to the system. Therefore, applications of LC-IR with flow-cell interface are mostly limited to the detection of the main sample components in isocratic mode.

A second approach, the solvent-elimination interface, was designed to overcome the limitations associated with the presence of solvent bands in the IR spectrum when using the flow cell. The sensitivity of this approach is significantly higher compared to the on-line method and can go down to nanogram level [7]. Moreover, the spectra, free of solvent bands, can be directly compared to the spectra from databases for reliable identification. The elimination of solvent can be performed using a thermospray interface, particle beam interface, electrospray interface, pneumatic nebulizers, and ultrasonic nebulizers, all well described in the review [18]. Among these devices, only few are commercially available. The first commercial nebulizer was based on a pneumatic principle and produced under the name LC Transform series 100 by Lab Connections [19]. Later, the ultrasonic nebulizer was commercialized as LC Transform series 300 [19]. LC transform 300 is one of the most commonly used interfaces in the literature for polymer analysis [20]. The operating principle of this device is as follows (Fig. 2): an eluent from an LC system is split and a fraction of it is transferred to an ultrasonic nebulizer nozzle that facilitates evaporation of the solvent and deposits the analytes in narrow focused bands on the rotating germanium disc situated below the nozzle. After sample deposition is completed, the disc is transferred to the IR to record the spectra for each LC fraction. In this way, a three-dimensional plot that includes LC elution time, wavenumber, and peak intensity can be obtained. These interfaces can be coupled to common Fourier transform infrared spectroscope (FTIR) instruments. Another commercially available alternative is a DiscovIR system by Spectra Analysis Inc. (currently Dani) that combines an FTIR detector with solvent-elimination interface that uses thermal nebulization and cyclone evaporator [21]. The advantage of this system is that the IR spectra are recorded with highly sensitive FTIR microscope in real time and that the deposition occurs under high vacuum and low temperatures facilitating the analysis of thermally unstable samples [7].
Fig. 2

Schematic setup of solvent-elimination interface for LC-FTIR

With the chromatographic solvent being fully eliminated in the offline approach, the IR sensitivity and spectral quality are greatly improved compared to the online flow-cell LC-IR. However, when performing LC-IR analysis with solvent elimination, one needs to ensure that experimental conditions allow for good solvent evaporation while having minimal effects on the analytes. The band-broadening during the deposition may compromise the sensitivity and it has to be taken into account. Parameters, such as eluent composition, evaporation rate, temperature, substrate speed, and analyte nature, are of a major importance. The optimization of experimental parameters may be a time-consuming and a challenging task. Failure to select suitable conditions may result in the loss of (part of) the analytes (e.g., due to evaporation with the solvent), occurrence of broad bands in the spectrum due to formation of amorphous deposits, sloping of the spectral baseline due to formation of thick crystal layer and light scattering effect, and shifts in the spectra due to changes in the compounds during or after the deposition [18]. When analyzing broadly distributed polymers, particular care has to be taken to prevent the low-molecular-weight part of the distribution from evaporating with the solvent [22]. This may distort the polymer molecular-weight information provided by SEC-IR. The main characteristics of two approaches for LC-IR coupling and their strengths and limitations in comparison with each other and with other hyphenated techniques are summarized in Table 1.
Table 1

Comparison of different liquid chromatography hyphenated techniques

Criteria

LC-IR

LC-NMR

LC-Raman

LC–ESI–MS

LC-off-line MALDI

LCxLC (UV/ELSD)

LC-Py-GCMS

Flow-cell

Solvent elimination

On-flow

Stop-flow

Loop storage

Bench-top NMR

Commercial availability of interface

Yes

Yes

Yes

Yes

Yes

No

No

Yes

Yes

Yes

Yes

Applicability to broad range of polymers

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Only polar and relatively low-molecular weight

Only highly polar to moderate polarity

Yes

Yes

Ease of operation

User-friendly

Time consuming method development

User-friendly

User-friendly

User-friendly

User-friendly

Complex as no commercial solution exists

User-friendly

Relatively easy, only need to optimize conditions for the interface

Conditions need to be optimized to match two dimensions

Needs tedious optimization of multiple experimental parameters

Sensitivity

Poor to moderate

High

Poor

Poor

Poor

Very poor

Poor

Very good

Good, but ion suppression may occur

Moderate (sample diluted after two separations)

High

Identification abilities

Limited due to solvent interferences

Good

Good if solvent signals do not interfere with analytes

Good if solvent signals do not interfere with analytes

Good if solvent signals do not interfere with analytes

Good if solvent signals do not interfere with analytes

Good, provided that solvent signal is eliminated

Moderate (requires accurate mass and TOF–TOF options)

Moderate (requires accurate mass and TOF–TOF options)

Poor

Good, provided that pyrolysis mechanism is understood

Quantification abilities

Good

Limited

Good

Good

Good

Limited due to very low sensitivity

Limited

Limited

Poor

Depend on detector

Moderate

Applicability to solvent gradient

No, unless complex mathematical corrections possible

Yes, but needs careful optimization

Possible, but very challenging

Possible, but very challenging

Possible, but very challenging

Not demonstrated

Yes, but need careful optimization

Yes

Yes

Yes

Limited

Dependence on eluent type

Flexible, ideally solvents that do not interfere with analyte signals

No buffers, only volatile eluents

Flexible, ideally solvents that do not interfere with analyte signals

Flexible, ideally solvents that do not interfere with analyte signals

Flexible, ideally solvents that do not interfere with analyte signals

Flexible, ideally solvents that do not interfere with analyte signals

Only volatile solvents when solvent elimination used

Only volatile eluents/additives

Flexible

Flexible, determined by the detector

Only volatile eluents

Flow rate compatibility

Up to high flow rates

Limited to moderate flow rates

Up to high flow rates

Up to high flow rates

Up to high flow rates

Limited to moderate flow rates

Up to moderate flow rates

Up to moderate flow rates with heated ESI

Limited: need solvent elimination or flow split

Flow rates in both dimensions need to be optimized

Limited

Time consumption

Fast, on-line

Relatively fast, semi-on-line

Fast, on-line

Relatively slow

Relatively slow

Fast, on-line

Relatively fast, semi-on-line

Fast, on-line

Relatively slow, off-line

Fast, on-line

Slow (stop-flow or at-line)

Cost

Affordable

Affordable

High

High

High

Affordable

High

Depends on the MS type

High

Affordable

Moderate

Application area (for polymers)

SEC, isocratic HPLC, identification of major components

All LC methods, identification of major and minor components

SEC, LCCC, identification and quantification of different properties

SEC, LCCC, identification and quantification of different properties

SEC, LCCC, identification and quantification of different properties

SEC, only proof of principle so far

Rarely used for polymers up till now

All LC methods, mainly for identification and MWD determination of oligomers

All LC methods, identification of eluted peaks

Different combinations of LC techniques, most common LC × SEC, determination of two polymer distributions

Limited applications, mainly coupling to SEC

Polymer analysis is one of the major application fields for LC-IR hyphenated systems as evidenced from multiple articles in the literature [20] as well as from vendor presentations stating availability of LC-IR instruments in a number of major polymer companies [23]. This is partially due to the fact that LC-IR is well compatible with isocratic elution, while SEC is the most common chromatographic technique using constant eluent composition during run. The other reason is that LC-IR combination offers solution to common problems in polymer research, viz. provides opportunity to obtain information on two polymer properties (e.g., molecular weight and chemical composition). Polymer analysis using LC-IR combination has been reviewed in a number of articles and book chapters [1, 2, 7, 20]. Here, only selected applications illustrating potential of such coupling and the most recent developments in the field of LC-IR will be covered.

A straightforward application of LC-IR is for deformulation of a polymer system, where components can be separated by LC and analyzed by FTIR to match their spectra with the library [21]. Another common application is coupling of SEC to IR for determination of chemical composition across the molecular weight. Recently, Prabhu et al. used SEC-IR coupled via LC Transform 300 to study the degree of grafting in maleic anhydride grafted polypropylene in relation to the molecular weight [24]. They observed the degree of grafting increasing at lower molecular weights. LC-FTIR can also be applied for a more in-depth study of chemical changes in materials upon chemical transformation, e.g., cross-linking, reaction, and degradation. Plass et al. described a study of PPG degradation using SEC-IR coupled via LC Transform 300 interface [7]. The SEC chromatogram was recorded using absorbance for CH stretching vibration and showed the presence of low-molecular-weight species in the aged samples. By plotting normalized intensities for the carbonyl signal, the olefinic signal, and the OH stretching mode (all—products of PPG degradation according to the mechanisms described in the literature) vs. molecular weight, the authors could conclude that PPG degraded by combined mechanism of peroxide-initiated chain scission and radical cleavage of C–C and C–O bonds.

Malanin et al. studied preferential solvation phenomenon in the system PEG–ethanol–chloroform using LC-RI and LC-flow-cell FTIR [25]. They demonstrated that the LC-FTIR system could provide improved accuracy for measurements of preferential solvation parameter and it allowed to gain a better insight into preferential solvation mechanism.

Piel et al. used online SEC-FTIR to study short-chain branching in polyethylenes [26]. They applied the bandpass filter to maximize the energy in wavelength region of interest passing to the detector and they used the background intensity lines for the background correction to effectively remove the interferences from the solvent. At these conditions, the SCB for several polyethylene samples could be determined with adequate accuracy and repeatability.

Analyzing literature on LC-IR characterization of polymers, it can be concluded that although LC-IR is a well-established approach, currently, it requires some hardware optimizations and certain expertise level to obtain meaningful results. The main problems in solvent-elimination approach are related to limited quantification abilities and to difficulty in selecting solvent-elimination conditions that are suitable across the entire separation range. Further technical developments in solvent-elimination interfaces may address this problem and make this technique more straightforward for less experienced users.

Similarly, flow-cell LC-IR approach has its own challenges. Although the setup is fundamentally comparable to that standardly used for LC-UV, obtaining meaningful data from online LC-IR can be rather difficult. The major challenge in flow-cell approach, the need for efficient eluent signal suppression, could be potentially solved in the future by incorporating special correction routines in the software. Recent work by Beskers et al. described a solvent suppression method for SEC-IR that is based on solvent signal subtraction together with mathematical drift correction and noise reduction [27]. Although the resulting analyte spectra could not be directly compared to the library references, they provided a good tool for identification of the unknown samples. The development and availability of chemometric tools with simple user interface that could help extract relevant data from the IR spectra, even when performing gradient LC, could be an important step forward in the proliferation of online LC-IR.

The other advances in LC-IR coupling are related to the hardware part. The sensitivity and signal-to-noise ratio are major problems in online LC-FTIR. It was shown that with some instrumental optimizations (using mercury cadmium detector, alignment of the optical path, and optimizing scan procedure) together with solvent suppression, the signal-to-noise ratio could be improved by a factor of 9000 and the FTIR could be made compatible with conventional SEC without column overloading [27]. Finally, the usage of quantum cascade lasers (QCL) seems to offer new possibilities for online LC-IR hyphenation. These lasers have high spectral power density and enable increased optical path length improving robustness and sensitivity [28]. They are compact and able to work at room temperatures that may bring analytical chemists closer towards LC-IR systems similar to commonly used LC-UV instruments.

Liquid Chromatography-Raman Spectroscopy

When an electron in the molecule is transitioned to a higher energy level by excitation with a photon, one of the ways to return to the initial state is by a Raman route [29]. The band intensities in IR and Raman are different, with Raman often offering a stronger signal for the bands that are weak in IR [30], making Raman a complementary technique to IR. Because Raman signal is less affected by the background signals of typical LC solvents (e.g., water), Raman could be a good alternative to IR detection. However, the inherently low efficiency of Raman scattering process prevents this technique from being commonly used in hyphenation with LC [31]. The concentrations required for Raman measurements are often not compatible with liquid chromatography. Several ways to address this issue and to realize LC-Raman coupling have been proposed in the literature. Although hardly any applications of LC-Raman for polymers have been published, the approaches used for low-molecular-weight samples should be applicable to macromolecules. Therefore, they will be briefly reviewed below.

One way to achieve improved signal in Raman is to apply surface-enhanced Raman spectroscopy (SERS), where several orders of magnitude increases in sensitivity are obtained by laser excitation of analytes adsorbed on metallic surfaces. To couple this technique to LC, a specially designed flow cell with a silver electrode can be used. Analytes passing the flow cell are absorbed on the electrode, their Raman spectra are recorded, and they are subsequently desorbed by varying electrode potential [32, 33]. Although this method could be potentially applied for semi-online coupling of SERS with LC, it has been only combined with flow injection analysis so far. Another approach is to add a colloidal silver solution to an LC eluent and to record spectra in the Raman flow cell [34]. LC chromatogram can also be immobilized on a TLC plate via a solvent-elimination interface [35, 36]. After that colloidal silver is applied on the TLC plate and Raman spectra are recorded. In addition to the above options, a direct online approach has been patented that involves separation of analytes on the metal-doped sol–gel medium and simultaneous detection through a window that is transparent for the excitation radiation as well as for the scattered radiation [37]. Although these approaches were shown to provide some useful data, they did not find routine applications because of method complexity and difficulty of handling colloidal metal solutions (limited stability, instrument clogging, etc).

An alternative approach to SERS is the use of liquid-core waveguide technology to enhance Raman sensitivity. Dijkstra et al. demonstrated the possibility to use this approach in combination with reversed-phase LC for a separation of a mixture of nitro-compounds [38].

A way to couple a confocal Raman microscope to LC via a solvent-elimination interface has been described by Surowiec et al. [39]. They used a piezo-actuated flow-through microdispenser that was able to deposit LC eluent on a spectroscopic plate in tiny droplets with diameter of a few tens of micrometers. The obtained deposit was analyzed by both FTIR and Raman and it was shown that Raman offered less reproducible, but more sensitive measurements than FTIR.

The feasibility of coupling a Raman microscope to SEC for polymer analysis has been assessed by Pitkanen et al. [40]. Although the coupling was not directly implemented, the authors studied the sensitivity of Raman towards typical concentrations encountered in analytical SEC experiments. Raman data provided accurate information on polystyrene–poly(methyl methacrylate), PS-PMMA copolymer chemical heterogeneity in SEC fractions. The authors concluded that continuous offline SEC-Raman experiments, similar to solvent-elimination SEC-FTIR, are feasible for qualitative and quantitative polymer analysis.

In spite of complementary data offered by Raman spectroscopy, the use of Raman as an LC detector has been very limited so far. LC-Raman coupling has been studied since 1990s, but only a limited number of publications on this topic appeared until now. Most studies available in the literature demonstrate only coupling feasibility, but do not describe real applications. With the progress in Raman instrumentation and solvent-elimination interfaces, the applicability of offline LC-Raman coupling should be revisited. This technique can offer distinct advantages over LC-IR for the separation of biopolymers and water-soluble synthetic polymers. It may also be advantageous for gradient LC analysis.

Liquid Chromatography-Nuclear Magnetic Resonance Spectroscopy

NMR spectroscopy is based on the phenomenon that certain atomic nuclei in a magnetic field can absorb electromagnetic radiation with a specific frequency that depends on the chemical structure of the compound (surroundings of the nuclei) [41]. NMR is a very useful characterization technique that can provide valuable qualitative information about the sample as well as reliable quantitative data without a need for separate calibration. NMR is commonly used in organic and polymer chemistry to study reaction mechanisms, to confirm formation of desired reaction products, to measure sample purity, etc. However, as all spectroscopic techniques, NMR can generally only provide an average information on the chemical composition of the analyzed sample and it has difficulties to distinguish compounds in a mixture especially without any prior knowledge on the components. Coupling of liquid chromatography to NMR offers a powerful characterization tool, because this combination allows to overcome the above limitation of NMR. LC separation prior to NMR detection ensures that fractions submitted to NMR represent compounds purified to a certain extent.

LC-NMR coupling has been employed for almost 40 years, and in spite of its high instrument cost and technical complications, it became a standard technique as evidenced by a number of books devoted to this topic [42, 43, 44] and by the availability of commercial coupling interfaces from major NMR manufacturers.

Several approaches can be used to couple LC and NMR:

  1. 1.

    On-flow coupling The LC is directly connected to NMR equipped with a flow probe. This setup often requires LC-flow-rate optimization. Low flow rates enable increasing number of NMR scans and improve sensitivity; however, they may not be optimal for LC separation. In addition, on-flow LC-NMR requires careful selection of mobile phase for LC. Ideally, the solvent signals should not overlap with the signals from the analytes. However, even if such solvents can be selected, much higher concentration of solvent compared to the analytes, may result in the loss of analyte response due to the limited dynamic range of the instrument [45]. The problem can be overcome by the usage of deuterated solvents in LC, that is a very costly option, or by applying sophisticated solvent suppression techniques [46]. The changes in the solvent composition as encountered during gradient-elution LC experiments provide an additional challenge in this respect. As a result of on-flow LC-NMR analysis, a two-dimensional representation of the studied sample (LC retention time vs NMR chemical shift) may be obtained.

     
  2. 2.

    Stop flow In this mode, the NMR starts data acquisition on the signal from LC as soon as the peak of interest is observed by an LC detector. The flow in the LC system is stopped during the NMR analysis and automatically started again after the analysis is completed. Solvent considerations similar to those for the on-flow experiments should be applied to this setup. Stop-flow mode allows more time for the NMR experiments, e.g., for collecting several one- and two-dimensional NMR spectra. The delay time between the LC detector and the NMR probe needs to be carefully accounted for. In addition, extended stop-flow time may result in enhanced band-broadening, especially for lower molecular-weight polymers that may adversely affect the LC resolution and detection sensitivity.

     
  3. 3.

    Loop storage The fractions of interest are collected in several storage loops and then transferred into the NMR after the LC analysis is completed. This setup operates similarly to the stop-flow mode, allowing additional NMR experiments to be performed for each fraction [47]. However, one has to consider that only a limited number of fractions can be collected with this setup that may influence the chromatographic resolution.

     
  4. 4.

    SPE interface In this case, the sample eluting from LC column is collected on a guard column. After that the LC mobile phase is removed by flushing the trapping column with a weak eluent. Finally, the analytes are back-flushed into an NMR probe with a strong eluent that is also a suitable solvent for NMR. This method has a number of advantages, e.g., the need for expensive deuterated solvents is minimized, the sample is concentrated, which helps to improve sensitivity [48]. However, the optimization of trapping conditions may be rather challenging. The author has no knowledge on application of this technique to analysis of synthetic polymers.

     
The applications of LC-NMR in polymer analysis have been reviewed by Hiller et al. [46]. The majority of publications in the literature describe coupling of SEC to NMR in the on-flow or stop-flow mode. That can be explained by SEC being the most common type of chromatographic separation for polymers and by the fact that using isocratic mobile phase simplifies somewhat the suppression of solvent signals in NMR. While SEC is separating based on MWD, NMR provides chemical composition information about each fraction that is especially beneficial, for example, for analysis of copolymers and polymer blends. SEC-NMR was applied for characterization of PS-block-PMMA copolymers [49]. The information on MWD of the copolymers and of each block could be obtained. In addition, microstructure of PMMA block could be revealed (Fig. 3). The MWD results were in agreement with those provided by multidetector SEC.
Fig. 3

On-flow SEC-NMR separation of PS-block-PMMA sample. Signals for PMMA microstructure: mm isotactic triads, mr atactic triads, rr syndiotactic triads. Solid line projection of PMMA, dashed line projection of PS.

Reprinted with permission from [49]. Copyright 2010 American Chemical Society

NMR can also be coupled to liquid chromatography at critical conditions (LCCC), an isocratic chromatography mode that enables separation based on end-groups or comonomer content while minimizing effect of the (second) repeat unit on the elution time. On-flow LCCC-NMR coupling was applied to measure molar masses of polyisoprene-block-polymethylmethacrylate (PI-b-PMMA) copolymers, and it was concluded that the results were in good agreement with SEC-NMR when internal referencing method was applied [50]. Hiller et al. also studied blends and copolymers of PI and PMMA by LCCC-NMR at critical conditions for both polymers (Fig. 4) [51]. These experiments offered the possibility to determine the MWD of the blend components and of the copolymers as well as the microstructure of PI and PMMA.
Fig. 4

LCCC-NMR of a blend of 1,4-PI and PMMA at: a critical conditions of PMMA, PI elutes in the SEC mode; b critical conditions for PI, PMMA elutes in the SEC mode.

Reprinted with permission from [51]. Copyright 2010 American Chemical Society

Recently Hehn et al. demonstrated the possibility to separate PMMA according to isotopic composition using on-flow LCCC-NMR [52]. LCCC on silica and C18 stationary phases was able to separate PMMA with 1H and 2H(D) isotopes, while NMR provided information on tacticity of the polymers. 1H-NMR allowed detection of 1H-PMMA, and 2H-NMR offered possibility to visualize 2H-PMMA.

The same group of authors showed the possibility to couple NMR to a gradient HPLC for separation of PMMA according to tacticity [53]. In general, the use of gradient in LC results in the changes in the chemical shifts and signal intensities of the solvent bands that complicate the use of conventional solvent suppression algorithms. To minimize the chemical shift changes, the authors added deuterated acetone to the acetone/dichloromethane eluent. This helped to stabilize the magnetic field and resulted in constant chemical shifts of the solvent signals [53].

One general problem for all LC-NMR coupling experiments is the need to inject large concentrations of sample onto the LC column due to inherent insensitivity of the NMR technique. Often analytical columns need to be overloaded, thus compromising the LC separation, or (semi-)preparative LC needs to be applied. Several NMR developments, such as high-field instruments and cryogenic probes, can improve the sensitivity helping to address this problem. Another aspect that prevents LC-NMR from being a commonly applied technique is the high cost of the instrument. The study published by Cudaj et al. shows high potential in this respect [54]. The researchers coupled a bench-top 20 MHz NMR instrument to an SEC separation for PS and PMMA using the conventional, non-deuterated solvents. They had to apply a preparative SEC column under overloading conditions as well as a solvent suppression procedure to obtain useful NMR signals. They demonstrated that at these conditions, the NMR data allowed for good detection and deformulation of the sample. Considering the much lower cost of this setup and continuous progress in developments of bench-top NMR systems with improved sensitivity, NMR has the potential to become a common option in the range of LC detectors.

Hyphenation of Chromatography with Mass Spectrometry

Liquid Chromatography-Mass Spectrometry

Coupling of LC to MS via electrospray ionization (ESI), atmospheric-pressure chemical ionization (APCI), or atmospheric-pressure photo-ionization (APPI) techniques are a very common approach used in many different application areas. Due to the limited mass range (usually up to several thousand Dalton), LC-MS is more useful for characterization of low-molecular-weight species and oligomers [55]. For some polymers that easily form multiply-charged ions, the higher molecular-weight species can be brought within the MS range. However, the spectra (especially in the case of broadly distributed polymers) become very complex and interpretation may be very challenging.

Among the three ionization techniques, ESI–MS is the most common approach. Some examples of the usage of LC–ESI–MS for polymer analysis will be described below. Hoteling et al. applied ultra-high-pressure LC (UHPLC) in combination with ESI–MS to characterize poly(dimethyl siloxane) oligomers [56]. Using UHPLC, they achieved efficient separation of the molecules by the chain length and the type of endgroup. High-resolution Orbitrap MS was used for accurate structure assignment of each separated peak. Gruendling et al. demonstrated the possibility to determine absolute molecular weights of polymers using SEC coupled to RI and ESI–MS [57]. They applied specially developed computer algorithm based on maximum entropy principle that allowed determination of molecular-weight distributions corrected for band-broadening for chains with different end-groups present within one sample. The same authors showed the possibility to use SEC-ESI–MS for determination of Mark–Houwink parameters for different types of polymers [58]. The obtained coefficients were especially accurate in the molecular-weight range below 20 kDa. Falkenhagen et al. demonstrated an interesting application of LC–ESI–MS for determination of critical conditions for polyethylene oxide polymers [59]. Using this technique, the critical conditions could be established using only one polymer sample.

APCI and APPI ionization may serve as complementary techniques for characterization of various polymer classes. In addition to commonly used ESI–MS, APCI-MS was coupled to LC to study PDMS samples used in medical applications [60]. LC-APCI-MS enabled detailed characterization of different end-groups present in the samples. APCI coupled to LC was also applied to study polyethylene oxide-block-polycaprolactone (PEO-block-PCL) copolymers [61]. Recently, Desmazieres et al. demonstrated an application of SEC-APPI-MS for characterization of PS. Although only low-molecular-weight species could be detected in the intact form, while higher molecular-weight chains fragmented at these conditions, the authors considered this combination very promising because of APPI ability to ionize apolar polymers and because of its compatibility with broad range of chromatographic solvents. More applications of LC-MS to polymer analysis can be found in a recent review by Crotty et al. [62] and in [63].

The main drawback of LC-MS with ESI/APCI/APPI ion sources is a limited mass range that becomes critical for analysis of polymers. For higher molecular-weight chains, matrix-assisted laser desorption/ionization mass spectrometry (MALDI) is considered to be a better option and this will be covered in more detail in the next section. The main differences between ESI and MALDI ionization techniques are summarized in Table 2.
Table 2

Comparison of ESI and MALDI ionization techniques

Characteristics

ESI–MS

MALDI

Ionization

Soft (electrospray)

Soft (laser)

Fragmentation

Common

Rare (e.g. loss of labile groups)

Multiple charges formed

Yes

Seldom

Adduct formation

Can be multiple

Usually single (due to added salt)

Spectra complexity

Complex (due to fragmentation, multiple charges, and adducts)

Relatively simple

Low M w analysis

Easy

Difficult due to matrix interferences

Upper M w limita

Few thousand Dalton (single charge)

<20 kDa higher resolution, >20 kDa lower resolution

Analyte polarity

Polar

From low-polar to polar

Coupling to LC

Yes, on-line

Off-line only

Solvent/additives compatibility

Only volatile

Any

Analysis of insoluble samples

Not possible

Possible

Quantification

Possible

Not possible

aExact values depend on the instrument, experimental conditions, and polymer type

Liquid Chromatography-Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry

MALDI analysis involves mixing of polymer of interest with a matrix (low-molecular-weight compound that absorbs the energy of the laser) and with ionization agent (salt) and transferring the mixture onto the MALDI target. After the solvent (if present) is evaporated, the plate is placed into the instrument and the sample is ionized by a laser. The obtained ions are separated in a time-of-flight analyzer based on their mass-to-charge ratio. MALDI is a soft ionization technique that is able to produce molecular ions for relatively large polymer chains. The upper molecular-weight cutoff depends on the instrument, on the experimental conditions and on the polymer properties. From the MALDI spectra, the information on monomeric units (chemical structure) and end-groups can be deduced. For polymers with low polydispersity (below 1.2), MALDI is also able to provide information on the absolute MWD.

MALDI itself suffers from severe ion suppression (easy-to-ionize molecules suppress ionization of other species in the sample) and mass discrimination (low-molecular-weight species give higher signal) [64]. These effects are especially significant when polydisperse (co)polymer samples are analyzed by MALDI [65]. Because of these phenomena together with MALDI spot inhomogeneity (the signal may depend on the exact position within the spot at which the spectrum is measured), MALDI is considered to be only a qualitative technique. In this respect, coupling of MALDI with LC offers important advantages. MALDI can provide valuable qualitative information about peaks separated by LC. In addition, because LC fractions have relatively uniform composition (in chemistry or molecular weight), the ion suppression and mass discrimination can be minimized within each fraction. Therefore, LC-MALDI may be applied in a quantitative way.

In most common MALDI systems, sample ionization occurs in a vacuum. Prior to ionization the fractions eluted from LC need to be mixed with matrix and often with salt. Therefore, MALDI is difficult to couple to LC online. In spite of several such attempts [66, 67, 68, 69, 70], the online coupling of LC with conventional MALDI is not typically applied due to technical complications and poor MALDI spectra generated by these methods. With the introduction of atmospheric-pressure (AP) MALDI, online coupling became more promising [71, 72]. Similar to ESI–MS, in this system, the desorption/ionization processes are separated from the high-vacuum zone of the instrument. Such setup has a number of advantages. It was shown that the analyte and matrix were homogeneously distributed within the droplets, therefore, minimizing the segregation effects. In addition, the AP MALDI does not require matrices, solvents, and analytes that are stable in vacuum. The limitations of this approach are poor compatibility with high LC-flow rates and possible decomposition of polymers in the source [63]. Daniel et al. demonstrated feasibility of online coupling of AP MALDI and HPLC for polymer analysis [73]. MALDI spectra of PEG 1000 with relatively high resolution could be obtained.

In contrast to online coupling, the offline coupling of LC and MALDI is more-developed and more-commonly applied. Several commercial spotting interfaces can be used for this purpose [74]. The operating principle of these devices may vary somewhat, but generally, they work as follows: the effluent from an LC system is split and a small fraction of it is mixed with a matrix (and salt) via a T-piece and then automatically spotted onto a MALDI target plate through a narrow-bore capillary. The capillary or a stage with the MALDI plate is moved in a predefined direction with a set speed that allows accurate deposition of the entire chromatographic run (or selected fractions) onto the target. Instead of mixing effluent with the matrix, the matrix may be applied onto the MALDI plate beforehand. Some modifications of spotting devices were developed, viz. “heated droplet interface” where partial solvent evaporation from the droplet was achieved by heating up the transfer tube [75], and “impulse-driven heated droplet” where a solenoid plunger was used to transfer each droplet onto the MALDI plate [76].

Weidner et al. used imaging MS to investigate homogeneity of the sample spots prepared by “dried droplet” method and they found that even though the spots looked visually homogeneous, a separation between the matrix and the polymer occurred [77]. Such segregation may cause repeatability problems in MALDI resulting in different signal intensities across the spot. Moreover, the authors observed mass separation of the polymer throughout the spot with larger molecular-weight chains accumulating at the outer edge of the spot [77, 78]. This effect can lead to inaccurate MWD measurements in the LC fractions. Gabriel et al. studied the segregation phenomenon in detail and concluded that the effect was mainly driven by slower crystallization of matrix compared to polymer [79]. The use of higher matrix concentrations helped to improve spot homogeneity.

Direct spraying of the sample onto the MALDI plate is another way to improve the homogeneity of obtained dried spots [77]. Solvent-elimination interfaces similar to those used for LC-IR coupling can be applied for LC-MALDI [80, 81, 82]. The solvent evaporation can be facilitated by ultrasonic nebulization or by using a carrier gas at elevated temperatures. The matrix can be added before spraying or a pre-coated plate can be used. This approach is compatible with high flow rates. However, it suffers from similar drawbacks as LC-IR: the solvent-elimination conditions need to be carefully optimized to ensure efficient solvent removal while preventing the analyte loss and decomposition.

Electrospray deposition (ESD) technique is an alternative to air-spray deposition [83]. It offers high signal intensity and a good spot homogeneity [84, 85]. In ESD interface, the sample is sprayed through the charged needle and the solvent evaporation is facilitated by elevated temperatures and a sheet gas. However, not all chromatographic solvents exhibit good spray formation in ESD, especially at high flow rates [63]. In addition, a possible polymer fragmentation at high voltages has to be taken into account.

SEC-MALDI coupling can be used to measure absolute molecular weight of a broad polymer without the need for calibration. SEC coupled to MALDI via a robotic interface was applied to determine absolute MWD for bisphenol A polycarbonate homopolymers and copolymers [86]. MALDI enabled determination of absolute Mp values of SEC fractions and these were used to calibrate the SEC separation. In addition, structural information about different end-groups present in the sample could be obtained.

MALDI can also be used as a second-dimension analysis after gradient-elution LC or LC at critical conditions of adsorption (LCCC) to obtain information on chemical composition or functionality-type distributions vs. molecular-weight distribution. LCCC was successfully coupled to MALDI via a spray interface for analysis of a mixture of copolyesters [87]. This yielded a two-dimensional plot showing a distribution of chemical heterogeneity vs. molecular weight. Similarly, PEO-polymethylene copolymers were studied by MALDI coupled to LC at near critical conditions [82]. MALDI provided information on the chemical composition and molecular weight of each fraction without a need for any calibration.

Another interesting application of LC-MALDI coupling was demonstrated by the group at BAM Institute in Germany. They applied MALDI imaging to visualize the traces of polypropylene oxide–polyethylene oxide (PPO–PEO) copolymer sample deposited after an LC separation by home-built ESD interface [88]. By constructing intensity plots for selected sample components throughout the imaged area, they obtained two-dimensional figures depicting the content of different species along the chromatographic run (Fig. 5).
Fig. 5

a Set of ion intensity plots of PPO/PEO copolymers (having constant PPO and varying PEO numbers) and b transformed data used for specific calibration.

Reprinted with permission from [88]. Copyright 2011 American Chemical Society

Other applications of LC-MALDI for polymer analysis were reviewed in [62, 63, 74, 89].

Hyphenation with Other Chromatographic Techniques

Two-Dimensional Liquid Chromatography

Coupling of one LC separation with another LC separation may be considered as a special form of hyphenation. Comprehensive two-dimensional liquid chromatography (LC × LC) where every fraction eluting from the first dimension is subsequently separated in the second dimension is especially useful for macromolecule characterization. LC × LC returns two-dimensional plots that incorporate information on two polymer distributions and on their mutual dependence. Based on the type of molecular distributions that need to be revealed, different separation modes can be selected for the first and second dimensions (Fig. 6). The most common combination includes gradient-elution LC separation based on chemical composition in the first dimension and size-exclusion chromatography separation based on molecular weight in the second dimension (HPLC × SEC).
Fig. 6

Polymer distributions and corresponding chromatography modes enabling determination of these distributions (TGIC temperature-gradient liquid chromatography, MTF molecular-topology fractionation, HDC hydrodynamic chromatography, for other abbreviations see text)

LC × LC has become an accepted technique for polymer characterization and publications on this topic regularly appear in the literature. LC × LC applications for macromolecules have been recently reviewed [2, 3, 90, 91]. In this paper, only selected (recent) applications illustrating potential of this technique will be discussed.

Malik et al. characterized poloxamers (triblock copolymers of poly(ethylene oxide), PEO and poly(propylene oxide), PPO) by LC × LC [92]. In the first dimension, the separation was performed on a silica column and based on the number of PEO units in the chains (critical conditions of adsorption for PPO), while in the second dimension, the separation was performed on a C18 column and based on the number of PPO units. The two dimensions were coupled through two switching valves, one equipped with two sample loops, and the other one—with two trapping C18 columns. Six first-dimension fractions were collected on one trapping column using the first valve. After that the second valve was switched to analyze the trapped sample in the second dimension. Using this approach, the authors could obtain a detailed compositional mapping of several commercial poloxamer samples (Fig. 7).
Fig. 7

2D NPLC × RPLC contour plots for: a Pluronic-10R5, b Pluronic-L35, c Pluronic-1740, and d Pluronic-17R4.

Reprinted from [92], with permission from Elsevier

Jeong et al. used normal-phase liquid chromatography coupled to SEC (NPLC × SEC) to study the purity of bicyclic PS synthesized by combined atom transfer radical polymerization and click chemistry [93]. NPLC was performed on a silica column and allowed separation based on PS topology. In combination with SEC in the second dimension, this allowed good separation between monocyclic, bicyclic, and branched PS.

Maiko et al. demonstrated separation of stereoregular poly(methyl methacrylates), PMMAs by LC × LC [94]. They applied solvent gradient on a carbon-based stationary phase (Hypercarb™) in the first dimension to separate a mixture of isotactic (it-) and syndiotactic (st-) PMMAs of different molecular weights (Fig. 8). In the second dimension, the samples were separated based on their size.
Fig. 8

Online LC × LC contour plot of quaternary blend (it-PMMA 4890 + it-PMMA 12000 + st-PMMA 7870 + st-PMMA 24000) showing the solvent-gradient LC (SGIC) and SEC projections. (ut-PMMA—PMMA peak with undetermined tacticity)

Reprinted with permission from [94]. Copyright 2013 American Chemical Society

Prabhu et al. characterized bimodal high-density polyethylene (BiHDPE) by high-temperature HPLC × SEC [95]. Using Hypercarb stationary phase in the first dimension, they achieved separation based on the 1-butene content and combined it with size-based separation in the second dimension. Under these conditions, the samples could be successfully separated into HDPE and linear low-density polyethylene (LLDPE) components. The authors managed to obtain quantitative information by coupling IR detector to their LC × LC system.

The main limitation of typical LC × LC is a need for tradeoff between analysis time and resolution. In a truly online approach, every fraction coming from the first dimension needs to be analyzed in the second dimension and total analysis time of all fractions should not exceed the first-dimension separation time. That implies that the more fractions sampled, the faster the second-dimension analysis should be. A high number of fractions are important to preserve the separation achieved in the first dimension. This poses stringent requirements for the second-dimension separation time. Unfortunately, there is a practical limit for the second-dimension separation speed that is determined by the requirements of sufficient resolution provided by the column, as well as by the maximum flow rate and pressure resistance of the column and the instrument. SEC is a type of chromatography that can be run relatively quickly as the sample elutes before the column void volume and it does not need any additional re-equilibration time (as compared to the gradient separations). However, even the fastest SEC analysis (with sufficient resolution) using wide-bore short columns specially designed for fast SEC, takes 2–3 min. This usually results in LC × LC separation of several hours and in using a liter or more of (organic) solvent per run. One way to improve this situation is to apply ultra-high pressure liquid chromatography (UHPLC) in a two-dimensional setup. UHPLC uses columns packed with small (sub-2 μm) particles at elevated pressures (above 400 bar). According to the chromatographic theory, this enables increasing analysis speed while obtaining the same or even higher efficiencies compared to the columns packed with larger particles [96]. When UHPLC is applied in both dimensions for analysis of polymers, the entire separation can be significantly shortened. An example of this is a separation of a mixture of PMMA and poly(n-butyl methacrylate), PBMA copolymers and homopolymers by LC × LC (Fig. 9) [97]. The separation by chemical composition in the first dimension was achieved by reversed-phase gradient-elution UHPLC; the separation by molecular weight in the second dimension was achieved using C18 UHPLC columns in the SEC mode (UHP SEC). The entire separation could be completed in less than 25 min while providing reliable data on molecular weight and chemical composition distributions of the entire sample. In addition to that the solvent consumption was reduced to only about 50 mL per analysis.
Fig. 9

UHPLC × UHP SEC separation of mixture of PMMA and PBMA copolymers and homopolymers. 15 PMMA homopolymers of different molecular weights between 15 and 100 kDa, 6 P(MAA-co-BMA) with 80/20 ratio of MAA to BMA and M w  = 80 kDa; 7 P(MAA-co-BMA) with 65/35 ratio of MAA to BMA and M w  = 20 kDa; 8 P(MAA-co-BMA) with 40/60 ratio of MAA to BMA and M w  = 50 kDa; 9 P(MAA-co-BMA) with 40/60 ratio of MAA to BMA and M w  = 50 kDa; 10 P(MAA-co-BMA) with 20/80 ratio of MAA to BMA and M w  = 110 kDa; 1113 PBMA homopolymers of different molecular weights between 19 and 100 kDa.

Reprinted with permission from [97]. Copyright 2012 American Chemical Society

Application of UHPLC in LC × LC setups for polymer analysis became even more viable with commercialization of UHP SEC technology by waters under the trade name APC (Advanced polymer chromatography). It has been already demonstrated that APC can be used for fast analysis of different types of polymers and oligomers [98, 99].

Another way to decrease analysis time in LC × LC is to perform analysis at elevated temperatures. At high temperatures, the mobile phase viscosity decreases considerably. This is especially noticeable when mixtures of organic solvent with water are applied (e.g., in gradient-elution LC) [100]. This has two main effects on chromatography [101, 102]: (1) the pressure drop across the column decreases and thus increase in flow rate is possible while maintaining the same pressure drop and (2) the diffusion coefficients of the analytes increase which, in turn, improves the C-term in van Deemter equation resulting in better efficiencies at high linear velocities. Both effects allow increasing analysis speed while maintaining good resolution. An example of HT-SEC applied in the second dimension for polymer analysis was demonstrated by Im et al. [103]. The authors could reduce the analysis time in the second dimension to about 1.5 min and the total analysis time to less than 1 h.

More approaches to speed up the second dimension in LC × LC are known: the use of monolithic stationary phases, superficially porous or non-porous columns, and several second-dimension columns in parallel. However, these approaches are not commonly applied to polymers, and therefore, they will not be discussed here.

LC × LC that is typically combined with UV or ELSD (and sometimes with RI) detection does not have strong identification abilities. In addition, quantification is often compromised, as it is, for example, the case for ELSD that shows a non-linear response with molecular weight and concentration. Therefore, several research groups tried to couple LC × LC with information-reach detectors. LC × LC coupling with NMR was reported for analysis of PEO [104]. LC × LC was also coupled to IR via a flow cell for analysis of styrene–methylacrylate copolymers [105] and via solvent-elimination interface for analysis of bimodal HDPE [95]. LC × LC coupling to MALDI was reported for the analysis of telechelic poly(caprolactone)s [106].

Coupling of Liquid Chromatography with Gas Chromatography

Macromolecules are usually not sufficiently volatile to be analyzed by GC. However, sample introduction via pyrolysis (Py) that is based on scission of macromolecules at high temperatures and subsequent analysis of fragments, can bring polymers within the GC application range. Combination of LC and Py-GC–MS is a very powerful tool for polymer characterization. LC can provide information on MWD or CCD of a sample and Py-GC–MS can offer compositional information about the LC fractions.

To characterize terpolymer consisting of methylmethacrylate, butylacrylate, and caprolacton, Py-GC–MS was coupled to SEC in a stop-flow mode using an automated interface that collects fractions from SEC and transfers them to a programmed-temperature vaporizing (PTV) injector [107]. Solvent elimination and pyrolysis occurred inside the PTV and the fragments obtained after Py were then analyzed by GC–MS. As a result, SEC elution profile for each monomer could be reconstructed and the conclusions on copolymer composition could be drawn. Similarly, PEO–PPO copolymer was analyzed by SEC-Py-GCMS to obtain average chemical composition (PEO content) across the molecular-weight distribution [108]. The same study [108] describes at-line coupling of RPLC with Py-GC–MS for analysis of PS-PMMA copolymer. The coupling was achieved via automated collection of LC eluent into the vials and subsequent reinjection of the fractions into Py-GC–MS instrument. In addition to polymer analysis, LC-Py-GC technique can be successfully used for additive characterization in polymers [107, 109].

In spite of very useful information supplied by LC-Py-GC–MS, this technique is not yet commonly used for polymer characterization. This is mainly due to the need for tedious optimization of multiple experimental parameters and for careful sequence programming to obtain useful data. In addition, this approach requires stop-flow mode or at-line characterization that makes this analysis rather time-consuming. Moreover, the quantitative properties of Py may need to be further improved [110].

Outlook

In modern industry, material development occurs very rapidly. Many plastic materials currently available on the market are very complex in formulation that allows them to incorporate multiple properties required for specific applications. Advances in material development drive progress in analytical characterization techniques. To understand complex composition and properties of plastics, a single analytical technique is no longer sufficient. A combination of several information-rich methods is often required. For polymers, hyphenation of chromatographic separation with other instrumental techniques offers information on several molecular properties or distributions, thus helping to visualize detailed material structure.

All hyphenated techniques described in this paper are complementary to each other and all of them have their advantages and limitations. Depending on a specific problem at hand, one or another technique may be preferred. For simultaneous quantitation and identification, LC coupled to FTIR or NMR could be of interest. When qualitative information on repeat units and end-groups is a main purpose, LC-MALDI could be suitable. If sample components are known and their efficient separation is required (e.g., to determine interdependence of two polymer distributions), LC × LC is a good option.

Most of these hyphenated techniques are still at the stage of development and further improvements in both the hardware and software are needed. For a number of systems, the interface between the two modules is a critical part and needs further optimizations. This is the case, for example, for LC-MALDI, LC-IR, and LC-Raman, where hardware developments are required to ensure efficient solvent elimination and homogeneous sample deposition. Poor sensitivity of spectroscopic techniques is another challenge and this can be addressed by advances in instrumental technology. On the software side, the improvements should be made to simplify the interface and make easier the process of obtaining meaningful data. This is especially critical when background correction is required, for example, to eliminate solvent signals in LC-IR or LC-NMR.

The research on hyphenated techniques for polymer analysis in academia is limited to a number of groups active in different fields of hyphenation. The reason for that might be the relatively high cost of the instruments and quite narrow application area. The progress in the hardware development is rather slow. The main efforts are being focused on the software side (e.g., improving background correction procedures) and on the application side (developing new methods for polymer analysis). In parallel, significant work is being done in the industry, where high-end and high-cost instrumentation is more accessible. Although only part of industrial research is publicly available via publications, patents, and conference proceedings, it is obvious that many hyphenated chromatography techniques are commonly used to solve everyday challenges in polymer R&D.

Notes

Acknowledgements

The author would like to acknowledge Christian Wold and Olivier Guise for their valuable suggestions on the content of this manuscript. The author also wants to thank Johannes Guenther for the useful discussions on the LC-NMR hyphenation.

Compliance with ethical standards

Conflict of interest

The author declares that she has no conflict of interest.

Ethical approval

This article does not contain any studies with human participants or animals performed by the author.

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© Springer-Verlag Berlin Heidelberg 2017

Authors and Affiliations

  1. 1.SABIC, Analytical TechnologyBergen op ZoomNetherlands

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