Brookhaven - BI-MwA Molecular Weight Analyzer


Molecular Weight Analyzer

Features at a glance

Multiangle, laser light scattering detector for SEC/GPC or stand alone use

Proteins & polymers

Polysaccharides & biopolymers

Study protein aggregation, oligomerization, & complex formation

Mn, Mw, Mz, Rg, A2

Small footprint, stackable

RI and Viscosity detectors optional

Absolute Molecular Weights

Quickly, easily, and accurately determine absolute molecular weights of proteins and polymers. Eliminate SEC/GPC column calibration and improve data quality. Rapidly reduce uncertainty with no assumptions required and true extrapolation to zero angle. Best of all, the BI-MwA has the highest performance/price ratio of any light scattering detector used for molecular weight determination.

The BI-MwA Molecular Weight Analyzer is simple to use, but incorporates sophisticated features. Inject your sample into the low-volume, 7-angle flow cell. The sample is illuminated by a temperature stabilized, precision power-controlled diode laser. The ultra-stable, high-sensitivity, low-noise CCD detector automatically collects the scattered light. Then, the software extrapolates the data to zero angle for the absolute molecular weight determination.

There are two ways to use the BI-MwA: Determine the molecular weight distribution (flow mode); or determine the average molecular weight (batch-mode). Macromolecular samples often have a range of molecular weights. In some cases, this distribution is quite narrow, and in other cases, it is very broad and/or multimodal. Variations in the distribution can indicate the presence of impurities or aggregation. However, for other applications, the average molecular weight is sufficient to characterize your sample. Sometimes the distribution is of interest, and sometimes the overall average is sufficient. Either way, the BI-MwA is the right tool for the job.

Molecular Weight Distribution (Flow Mode)

When the distribution of molecular weights in your sample is important, consider the BI-MwA as a chromatography detector for your SEC/GPC system. In this way, you can readily determine the true molecular weight distribution. Many average values unavailable from a batch mode measurement can be calculated from these distributions. Better yet, the molecular weight distribution is determined while the problems of column calibration are avoided.

The way it was:

Size exclusion chromatography (SEC), also known as gel permeation chromatography (GPC), uses columns to separate polymer samples. In the standard method, columns are calibrated to obtain a relationship between molecular weight and elution volume. However, the column-sample interaction depends on not just the size (molecular weight) but also the chemistry of the sample. Therefore, an accurate column calibration requires standards over a range of molecular weights with exactly the same chemistry and structure (e.g., branching) as the sample. With a few exceptions, such standards are difficult or impossible to obtain, especially for new or unique materials.

A better approach:

The BI-MwA provides a method of determining absolute molecular weights without resorting to any assumptions about the sample or column calibration. Simply attach the BI-MwA and an appropriate concentration detector such as the BI-DNDC and the ParSEC GPC software to the end of the SEC column. The column is then used to separate the species of interest and the BI-MwA gives full information on the molecular weight of each fraction. Figures 1 and 2 show the results of a measurement of the molecular weight distribution of a dextran sample by SEC.

MwAF 1

Figure 1 Data from an SEC system equipped with a BI-MwA and the ParSEC GPC software.
Sample concentration and molecular weight of dextran are plotted as a function of elution volume.
No column calibration was required to obtain this plot.

MwAF 2

Figure 2 Molecular weight distribution of calculated from the data in figure 1.

Average Molecular Weight (Batch Mode)

When the average molecular weight is sufficient to characterize your sample, use the BI-MwA in batch mode to easily determine average properties. The BI-MwA permits routine determination of the weight average molecular weight, Mw, the z-average radius of gyration, Rg, and the second virial coefficient, A2. Readily prepare Zimm, Berry, or Debye plots to calculate these parameters with the optional BI-ZPMwA software.

Using the BI-MwA to determine average molecular weight

Obtain molecular weights in batch mode by preparing dilute solutions of known concentration. Follow the steps in the software to enter concentration values and inject solutions. The software will automatically evaluate the data. With the click of a mouse, data are analyzed and presented. Data review and analysis are under user control. Results are clearly presented on screen and in printed reports. See one example of batch mode data in Figure 3.



Figure 3 Zimm plot of a polystyrene sample in toluene. The results are in excellent agreement with
expectations. The weight average molecular weight, Mw, radius of gyration. Rg, and second virial coefficient
A2, are rapidly and easily obtained.

Samples are unaffected by measurement and can be recovered for future analysis.

User-Friendly Application Software

The application software is easy-to-learn, convenient to use, and maximizes user productivity.

The BI-MwA can be used with industry-leading, SEC-system software, which allows rapid measurement setup, user customizable output, and reliable data collection and analysis.

The batch-mode software can generate Zimm, Debye, and Berry plots. Figure 3 is an example of a Zimm plot obtained with the BI-MwA. For complex samples, both linear and higher order polynomial fits can be performed. All fitting is coupled with sophisticated statistical analysis. Finally, the software incorporates algorithms for taking advantage of the flow system to remove artifacts due to contaminants (dust). Thus, good information can be recovered even from imperfect samples.

Why Seven Angles?

Absolute molecular weight is determined from the scattered intensity at zero angle. Unfortunately, it is impossible to measure scattered light at zero angle since transmitted light will also be measured. Measurement at one or more nonzero angles and extrapolation to zero angle is necessary for molecular weight determination. A single-angle light scattering instrument relies on extrapolation based on one data point; occasionally, it works. In addition, measurement of the change in scattering with angle is necessary to determine radius of gyration. Therefore, a multi-angle instrument is far better than a single-angle instrument.

When comparing multi-angle instruments, keep in mind that the precision of the light scattering results is approximately proportional to the square root of the number of angles. Thus, results from a seven-angle instrument such as a BI-MwA are significantly more precise than results from a two- or three-angle instrument. The seven-angle design offers the best performance/price ratio in the industry.

Optimal Design, Small Footprint, Rugged Construction

The BI-MwA design eliminates the pitfalls of similar instruments. In order to avoid artifacts and drift due to trapped bubbles, the flow path is vertical, not horizontal. The patented cell is more easily flushed clean when samples are changed.

The sample cell can withstand pressures up to 3.5 MPa, a much higher pressure than other instruments of this type. Therefore, the BI-MwA is a perfect complement to an SEC system where high pressures often lead to premature instrument failure.

In addition to the cell design, the overall mechanical design is also optimized for maximum performance. The footprint (21 cm wide x 38 cm deep) conserves valuable bench space. In addition, the internal partition between the electronics and the liquids minimizes the extent of damage and the cost of repair if a leak occurs.

Contact us for complete assistance in setting up a customized SEC system.

Join a large and growing community of users by contacting Brookhaven Instruments about your application.

Molecular Weight Range: < 103 to > 109 Daltons, depending on dn/dc and concentration. [High end for dendrimers and other compact structures.]


Angles: 7, nominally 35, 50, 75, 90, 105, 130, and 145 degrees.

Fiber: Low numerical aperture, integral to cell.

Maximum Pressure: 3.5 MPa (500 psi) maximum.

Fittings: Standard HPLC Inlet/Outlet on front panel.

Volume: Cell, 100 µL nominal; Scattering, 20 nL nominal.

Laser: Temp. stabilized w/prec. power control, 35 mW, 635 nm, vertically polarized.

Cell: PEEK std., options on request.


Control: Integrated, dedicated, powerful microcontroller. USB driven.

Detector: CCD, ultra-high sensitivity and spatial uniformity.

Analog Inputs: 4 standard, with 16 and 24 bit resolution, 15 additional analog channels (optional) with 24-bit resolution. Computer selectable gain adjustment. Suitable for use with most common RI, UV, viscometer, thermocouple, thermistor, injector, pressure gauge and pump outputs.

Power Requirements: 100/115/220/240 VAC, 50/60 Hz,25 Watts

Dimensions: Size: 195(H) x 210(W) x 380(D) mm; Weight: 5.5 kg

Certificates: CE Certified

U.S. Patent # 6,052,184

A policy of continued improvement may lead to specification changes.

BI-MwATC: Temperature control, ambient to 80 ± 0.015 °C.

BI-MwAXB: Expansion board allows 15 additional, 24 bit, analog inputs.

BI-DNDC: Differential Refractometer (RI detector).

BI-ViSC: Viscometer for monitoring polymer solutions in TDSLS.

BI-ACOMP Software for Automatic Continuous Online Monitoring of Polymerization (formerly TDSLS).

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