ViscoStar III

ViscoStar: The next generation online differential viscometer.

Highly Sensitive

A highly sensitive, on-line differential viscometer used in conjunction with SEC-MALS-IV to determine the size and conformation of all types of biopolymers, synthetic polymers and even proteins and peptides.

Multiple Novel Technologies

The ViscoStar incorporates multiple novel technologies to provide the highest sensitivity, stability and solvent compatibility of any available viscometer for GPC. Its ease-of-use and serviceability make it the perfect companion for Wyatt's DAWN light scattering and Optilab refractive index detectors.

White Paper

Download "ViscoStar: Innovations in Online Viscometry for GPC".

Another path to polymer and protein characterization: SEC-MALS-IV

When coupled with size-exclusion chromatography and a concentration detector such as the Optilab, the ViscoStar derives the intrinsic viscosity (IV) of polymers. An inherently valuable physico-chemical property in itself, IV is best utilized together with molar mass information from multi-angle light scattering (SEC-MALS-IV) to derive the hydrodynamic radius, conformation, and branching ratio of a macromolecule in solution.

Key benefits of the ViscoStar

  • High sensitivity, equivalent to 100 ng of 100 kDa polystyrene in THF at 1.0 mL/min injected onto a standard GPC column
  • High dynamic range of over 135,000:1. The full measurement range corresponds to over 13 mg/mL of 100 kDa polystyrene in THF
  • Extended solvent compatibility with all-316 stainless steel transducers
  • Temperature range from 4 °C to 70 °C for polymers or proteins that require a range of temperatures
  • Minimal band broadening thanks to fast pressure transducers
  • Pump pulse suppression to below 4 Pa (hardware alone) or 1 Pa (hardware + software) without loss of chromatographic resolution
  • Automated capillary bridge tuning with no moving parts, to guarantee a perfectly balanced bridge, every run
  • Enhanced thermal stability with drift of <2.5 Pa/hour

SEC-IV without MALS?

Yes, as much as we love MALS for optimal polymer analysis, it's true that there are occasions when it just won't fit the bill.

Universal calibration is a parallel technique to SEC-MALS for determining molecular weight, commonly used to characterize linear polymers. While universal calibration overcomes some of the problematic issues associated with column calibration by reference molecules, it does not account for non-ideal column behavior caused by chemical solute-column interactions or hyperbranching.

Universal calibration is particularly-well suited to replace MALS for:

  • Strongly fluorescent polymers, in particular those not adequately addressed by a DAWN™ with long-wavelength (785 nm) laser or a standard DAWN (685 nm) with fluorescence-blocking filters.
  • Weakly-scattering polymer/solvent systems with very low dn/dc values

The Mark-Houwink-Sakurada relationship makes use of empirical relationships between intrinsic viscosity and molar mass, specific to each polymer and solvent, to estimate molar mass from SEC-IV measurements. Of course, the best way to determine MHS coefficients is to measure them using SEC-MALS-IV and ASTRA!

Go to the Advanced Technology tab to learn more about the unique features of the ViscoStar.

Viscostar pump pulse 

Patented pulse compensation technology in the ViscoStar eliminates pump fluctuations. At the hardware level, the bridge is impedance matched for dynamic balance, greatly reducing the effect of pump pulses. An additional layer of software filtering eliminates residual pump pulses.

ViscoStar measurements of PLGA and carboxymethyl cellulose

The ViscoStar provides the highest available dynamic range and sensitivity in online viscometry. This graph shows the intrinsic viscosities of 1) poly(lactic co-glycolic acid), or PLGA, in THF and 2) carboxymethyl cellulose, or CMC, in aqueous mobile phase, both measured with a ViscoStar and Optilab, spanning a range from tens to thousands of mL/g. The same instruments readily measure intrinsic viscosity values in the low single digits with excellent signal-to-noise, even at very low concentrations.

viscostar-measurement of protein size as hydrodynamic radius

Three IgG proteins with similar MWs eluted at different elution volumes. To understand their different elution properties, online DLS is typically used to measure the hydrodynamic radius, Rh, of a protein. However, online DLS did not provide reliable insights, in this case due to the relatively high experimental uncertainty of the DLS. The precise Rh values obtained with a MALS detector and the ViscoStar III, on the other hand, clearly revealed the causes. IgG B eluted later than IgG A due to its more compact structure; IgG C had a slight interaction with the SEC column.

Superior measurements of intrinsic viscosity for macromolecular size and conformation

Click here to request a copy of our ViscoStar brochure.

Innovations in the ViscoStar

Exquisite sensitivity, stability and robustness require more than just incrementally better engineering. They are made possible by the innovative research and development carried out in Wyatt Technology's R&D center. Here are the primary innovations incorporated in this instrument:

Thermal bridge tuning - The four arms of the bridge must be perfectly balanced in order to make the most sensitive measurements. And yet accumulation of dirt or sample on the capillary walls can skew the balance set during production. Thermal bridge tuning in the ViscoStar accomplishes this balance quickly and robustly, without sliding seals or other mechanical means, guaranteeing perfect balance every run.

Pump pulse compensation - Even a well-balanced bridge is inherently susceptible to systematic noise due to dynamic pressure pulses generated by the chromatography pump. Complete impedance matching symmetrizes the dynamic properties of the capillary bridge and suppresses pump pulses in the DP transducer by 100x. And the ViscoStar does it without introducing the artificial band broadening that is used by other online viscometers to smear out pump pulses at the cost of reduced chromatographic resolution.

Pump pulse elimination - ASTRA implements a unique algorithm for calculating intrinsic viscosity from the IP and DP transducers that eliminates the contribution of pump pulses in the IP transducer. An additional, proprietary software filter eliminates residual DP pump pulse levels without any loss of resolution.

Dual-Peltier thermal control - Wyatt's ViscoStar is the only online viscometer that offers heating and cooling of the sample via Peltier-based temperature regulation. Its dual-Peltier control eliminates internal temperature gradients, bringing signal drift down to just 2.5 Pa, 50x less than competing instruments.

System pressure protection - The highly sensitive pressure transducers utilized in the ViscoStar must be protected from overpressure due to high flow rates and/or tubing clogs which might occur in the instrument, or even in downstream instruments. The ViscoStar incorporates active protection with a system pressure sensor and bypass valve. Besides offering full protection, this arrangement has the added benefit of providing a positive warning to the user and data logging of protection activation.

ViscoStar 3 
          capillary bridge

Without delay columns, the impedance of the capillary bridge would be fully balanced. The pulse compensation element matches the additional impedance of the delay columns, eliminating the dynamic effect of pump pulses on the DP transducer. Thermal bridge tuning further improves measurements by guaranteeing a perfectly balanced bridge with every run.

Dual-Peltier thermal control of the ViscoStar

A dual-Peltier design eliminates internal temperature gradients, improving stability while enhancing the bridge's isolation from the adverse effects of room-temperature variations.


The ViscoStar may be used with any* GPC or HPLC-SEC system as long as the sample does not need to be heated above 70 °C in order to flow through 0.010" (0.25 mm) i.d. tubing. Combine with an Optilab for the concentration measurements needed in viscometry calculations.

ASTRA supports Universal Calibration as well as molar mass determination using Mark-Houwink-Sakurada coefficients.


Combine the ViscoStar with a DAWN or miniDAWN MALS detector for an even more versatile combination that can determine Mark-Houwink-Sakurada parameters and other measures of conformation and stability. ASTRA handles all the calculations you need for full characterization of your polymers.

*Certain HPLC pumps or injectors generate pressure pulses that can interfere with sensitive differential viscosity measurements. While the ViscoStar overcomes most of these, it is always best to start with a pulsation-free pump. Contact Us for information on optimal GPC systems.

Differential Viscosity  
Sensitivity (minimum detectable sample) 0.1 μg of 100 kDa polystyrene in THF at 1 mL/min, under typical GPC conditions
Differential pressure noise < 0.05 Pa
Differential pressure range 6.9 kPa, protected against overpressure
Dynamic range 135,000:1
Linearity 0.5% of full scale
Differential pressure drift < 2.5 Pa/hr
Capillary bridge autobalance Thermal, leak-free mechanism
Sample shear rate 5000 Hz with water at 1 mL/min
Max. flow rate Aqueous: 1.5 mL/min; THF: 3 mL/min
Delay volume options 16.2, 10.8 or 5.4 mL, with additional factory-installed option for 32.4 mL1
Max. permissible pressure inlet to outlet 690 kPa
Wetted Materials 316 stainless steel, PTFE, zirconium silicate, Kalrez®, Inconel, 26-1 Stainless Steel
Temperature Control  
Range 4 ºC to 70 ºC
Stability <0.005°C
Ambient Range 0 ºC to 40 ºC, 20% to 70% RH
Digital Output 20 bits @ 10 Hz
Auxiliary I/O  
   Inputs Autoinject contact closure, Alarm In, Analog In: 2 differential inputs, 18 bits resolution, range -10 V to +10 V
   Outputs Autoinject Retransmit, Alarm Out, Recycle Out
On-board Computer  
   Communications Ethernet
Front Panel Display 7" (178 mm), high-resolution, multi-touch display for viewing/setting signals, instrument settings and diagnostics.
Dimensions 58 cm (L) x 36 cm (W) x 18 cm (H)

1 Volume of fluid through the ViscoStar until the retained sample elutes. Actual volume of fluid in the column is half this value.

Host PC requirements may be found in Computer Requirements.

Specifications subject to change without notice.

Other Viscometers

microViscostar, molar mass determination, molar mass measurement, molar mass analysis, molecular weight measurement, molar mass characterization

microViscoStar - Similar to the ViscoStar but designed specifically for use with UHPLC/APC.