The Determination of Specific Refractive Index Increment (dn/dc)

The specific refractive index increment dn/dc is an essential parameter for determining molecular weight with Static Light Scattering (SLS). Its value is clarified by observing Debye's constant in the SLS equation, where the dn/dc is a squared term. This means that any inaccuracy in the specific refractive index increment will greatly affect the molecular weight results. This is the case both when SLS is employed in batch mode (goniometer system), and in flow mode (MALS Detector in GPC/SEC).

Summary

This article presents how dn/dc can be established with high precision when aided by a specialized dn/dc refractometer and software.

Overview

Example of a dn/dc Instrument on the Market and its Characteristics

The dn/dc from TESTA Analytical Solutions e.K. is a deflection type refractometer that can be bought for use in either batch or GPC/SEC mode. In batch mode, the precise refractive index increment, dn/dc, is determined. This value is necessary as a parameter in molecular weight measurements using light scattering. In GPC/SEC mode, dn/dc is already established, and the instrument is operated as a concentration detector for GPC/SEC applications.

What is Important?

So as to enable the utmost flexibility, all key elements can be programmed by the user. The dn/dc can be set using the front panel controls and read using the LED readout or by an external computer. It contains a microprocessor, which ensures handling of all functions of the instrument are user-friendly, in addition to refractive index and temperature data transfer to a private computer via a standard serial interface. The most important feature is that warm-up time for the dn/dc is only one hour, which is significantly less than other examples on the market.

Optical path.

Figure 1. Optical path. Image Credit: TESTA Analytical Solutions

Key Features

  • 9 measurement ranges
  • Instrument warms up in one hour
  • Sensitivity (2.5 x 109 ΔRIU)
  • Active temperature control to 80 °C
  • Autozero
  • Reference cell flush
  • Wavelength selection: 620, 535 or 470 nm
  • DN/DCW Windows™ software

    Measurement Principles

    Determination of dn/dc

    To determine dn/dc, a calibration of the batch mode instrument with a solvent/solute combination with known dn/dc, typically KCl in water is necessary. The calculated constant is not solvent reliant, and it may be used for all additional measurements.

    Accessories and Software

    In batch mode, the instrument is accompanied by BIDN/DCW software to be used with an external PC. A syringe kit with PEEK™ and Teflon® tubing, fittings, and syringes is included with the dn/dc instrument.

    Screenshot of the BI-DNDC Differentil Refractometer Software.

    Figure 2. Screenshot of the BI-DNDC Differentil Refractometer Software. Image Credit: TESTA Analytical Solutions

    Sample

    It is possible to recover and reconstitute the sample if necessary. Usually, five or more concentrations, which range from 0.1 to 10 mg/mL, are advised. Due to only needing 0.5 ml of liquid to flush out the previous sample, new instrument readings may be performed promptly and efficiently.

    Determination of Concentration

    In this mode, the dn/dc serves as an extremely sensitive concentration detector. Additionally, with the correct wavelength and the BI-MwA, the dn/dc becomes part of a system that eliminates GPC/SEC column calibration.

    Conclusion

    Light-scattering experiments will be simplified with the release of the DNDC Differential Refractometer by TESTA Analytical Solutions. The new, differential refractometer can warm-up in just one hour and is significantly cheaper than other models on the market.

    An accessory to the current line of TESTA GPC/SEC instruments, the dn/dc is a deflection type refractometer that delivers key information for on-going research and/or monitoring activities. The instrument is accompanied by sophisticated Windows™ based software that can calculate the exact refractive index increment (dn/dc) in either static or dynamic mode. In molecular weight determination by light scattering an accurate value of dn/dc is critical.

    Additionally, in dynamic mode, the dn/dc can be used as a concentration detector for HPLC and GPC applications. Due to it being easy to program and user-friendly, the dn/dc may be controlled and read using the LED readout or by an external computer. It is also possible to transfer refractive index and temperature data to a PC via a standard serial interface. A syringe kit with tubing, fittings, and syringes is also included with the instrument. Additional features include nine measurement ranges, active temperature control up to 80 °C, autozero, reference cell flush and sensitivity of 2.5 x 10-9 RIU – equivalent to 1.5 ng fructose per ml H2O.

    This information has been sourced, reviewed and adapted from materials provided by TESTA Analytical Solutions.

    For more information on this source, please visit TESTA Analytical Solutions.

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