Molar mass determination
- Solutions for molar mass determination
- PSS products and services for molar mass determination
- Theoretical background molar mass determination
GPC/SEC is a reliable, precise, and fast method to measure the molar mass averages Mn, Mw, Mz, the polydispersity index PDI and the complete molar mass distribution (MMD) of soluble macromolecules.
There are several GPC/SEC techniques that mainly differ by the added detectors and calibration options:
- conventional GPC/SEC with a concentration detector (RI, UV, ELSD) and a narrow/broad/integral calibration curve constructed from matching molecular weight reference standards and materials
- GPC/SEC-light scattering with a concentration detector and a light scattering detector. If only a RALLS (right angle 90° laser light scattering) detector is available, in most cases a viscometer is needed to overcome the limitations of 90° light scattering (Triple detection approach)
- GPC/SEC-viscometry with a concentration detector and a viscometer and a universal
calibration curve constructed from any
molecular weight reference standards and materials
Another approach to determine only molecular weight averages is to use (batch) light scattering or (batch) osmometry. Static light scattering in batch mode needs only a light scattering detector to yield reliable and very precise Mw values. Osmometry on the other hand allows the determination of Mn values for samples. However, since without GPC/SEC on-line fractionation is missing, only molecular weight averages are available. The very important distribution information can not be measured.
PSS offers complete turn-key systems for GPC/SEC with all detection methods as well as GPC/SEC service analysis in our analytical service department.
The turn-key system is based on the modular SECcurity GPC System including light scattering, viscometry, and Triple detection with WinGPC UniChrom GPC/SEC software. PSS decided for the modular approach because of the following advantages:
- No compromises just to fit everything in a single box: Each component can be addressed precisely for its special technical needs resulting in highest detector sensitivity and stability.
- Complete path of expansion: Additional components can be added any time to meet changing needs.
- Highest flexibility in a constantly changing analytical environment: Exchange or replace single components to adress new analytical tasks with other instruments or reduce workload by changing solvent only for single components, not for complete systems.
GPC/SEC columns are the heart of a GPC/SEC system. Best performance and reliable results can only be achieved with perfectly matching columns. PSS offers the widest range of column materials for aqueous and organic solvents alike. A large and growing number of column applications with separation examples for all types of macromolecules shows the applicability and separation power of the PSS columns. For new materials or samples where no GPC/SEC method is available the PSS column experts offer column selection and/or method development.
To increase the precision of GPC/SEC measurements reliable and matching calibration standards and instrument validation standards are required. PSS offers a wide range of molecular weight GPC/SEC calibration standards to allow precise measurements even for small budgets.
For Mw and/or Mn determination PSS offers stand-alone instruments as well as service analysis in our analytical service department. The PSS light scattering detector SLD7000 can be used without any modifications as batch multi angle laser light scattering detector (MALLS). With the corresponding MALLS software, not only Mw but also the radius of gyration, <Rg>, z and the second virial coefficient A2 can be measured. For highest precision dn/dc determination instruments are also available.
Molar mass averages and molar mass distribution
Synthetic materials, polysaccharides, and also some proteins do not exhibit a single definite molar mass, unlike low-molecular-weight substances. They consist of mixtures of chains with different numbers of repeating units, with each chain having its own molar mass.
The molar mass of a macromolecule is obtained by averaging the molar mass of the different chains by number (Mn) or by weight (Mw). However, even if values for Mn, Mw and Mw/Mn — the polydispersity index (PDI) — are available, macromolecules are still not characterized comprehensively.
Macromolecules can have the same averages but still show significantly different physical properties. This is because they have a different molar mass distribution (MMD) which means the fractions of the defined molar masses are different.
The molar mass averages and the MMD influence the macroscopic properties of the materials. Therefore reliable, precise, and fast determination of the averages and the MMD is required for QC/QA and R&D alike.
What is the difference between a GPC/SEC chromatogram and a molar mass distribution?
GPC/SEC chromatograms show the fractions and the concentration change with molecular size in solution for the sample, but this information is superimposed by the parameters of the analytical equipment. For example, if the same sample is measured in two separate laboratories on two different instruments using different-sized columns, the resulting chromatograms will obviously be different. Without previous knowledge nobody will assume that the chromatograms represent the same sample. If the instrument is properly calibrated, using any kind of calibration (for example, conventional, universal, light scattering), and the samples are correctly evaluated, the influence of the equipment is eliminated. MMDs are obtained that are independent from the instrument and allow inter-laboratory comparison.
Unfortunately, many high performance liquid chromatography (HPLC) data systems that also perform GPC/SEC calculate “molar mass diagrams”, that do not eliminate the influence of the instrument. The difference between a molar mass diagram and a molar mass distribution is mainly the y-axis. MMDs have a y-axis w(log M), where the mass fractions w in constant molar mass increments (log (M)) are shown. “Molar mass diagrams” have the same y-axis as the chromatogram, normally the detector signal intensity. This can make an inter-laboratory comparison extremely difficult. The determination of fractions above or below certain molar masses, for example below 500 g/mol, can also be faulty. The figure clearly shows that both, peak position, which relates to the molar mass, and peak width, which relates to the PDI, can be wrong.
Why is conventional GPC/SEC a relative method?
For the determination of molar mass averages and molar mass distributions conventional GPC/SEC with RI, UV, or ELS detection needs a calibration curve. In most of the cases calibration curves are constructed using reference polymer standards of one polymer type with a narrow molecular weight distribution (PDI close to 1). PSS offers carefully assembled calibration kits of up to 12 standards with different (and precisely determined) molar masses over a broad molecular weight range for fast and easy GPC/SEC calibration.
Since GPC/SEC separates according to the hydrodynamic volume and not to molar mass, the resulting calibration curve is only valid for chemically matching unknown samples. For example a Poly(styrene) calibration curve constructed from
Poly(styrene) molar mass standards is only valid
for Poly(styrene) samples and unknowns. For
Poly(methyl methacrylate) unknowns conventional GPC/SEC with a Poly(styrene) calibration curve yields apparent molecular weights. These molar masses can be compared to each other providing valuable information, but a precise and absolute molar mass determination is not possible.
Several approaches have been developed to overcome this limitation and to allow accurate molar mass determination for all samples:
- calibrate with matching molar mass standards. Profit from the large selection of all kinds of reference materials offered by PSS.
- use GPC/SEC-viscometry and evaluate data using a universal calibration curve (see: H. Benoît, Z. Grubisic, P. Rempp, D. Decker, J.G. Zilliox, J. Chem. Phys. 63, 1507 (1966)). A universal calibration curve is valid for all types of unknowns.
- use GPC/SEC-light scattering to measure the molar mass of homopolymers on-line
What does GPC/SEC-light scattering offer?
GPC/SEC-light scattering is an absolute method. The light scattering detector allows the direct measurement of the molar mass, when the refractive index increment (dn/dc) is known. For highest precision of light scattering experiments PSS recommends to measure the dn/dc off-line using dn/dc instruments.
There are several static light scattering techniques, that are used on-line coupled to GPC/SEC or as stand-alone (off-line, batch) techniques:
Low angle (laser) light scattering
Molar mass information based on a single angle
Right angle (laser) light scattering
Molar mass information for isotropic scatterers (Macromolecules with molar masses below 150 000 Da or many proteins); use viscometer to overcome this limitation (Triple detector approach)
Multi angle (laser) light scattering
Molar mass information, radius of gyration, branching information based on the simultaneous measurement of multiple angles at the same time