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Differential refractometer

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A diagram of the general front-view of a differential refractometer.

A differential refractometer (DRI), or refractive index detector (RI or RID) is a detector that measures the refractive index of an analyte relative to the solvent. They are often used as detectors for high-performance liquid chromatography and size exclusion chromatography. They are considered to be universal detectors because they can detect anything with a refractive index different from the solvent, but they have low sensitivity.[1]

Refractive index increment

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The refractive index increment,, often expressed as mL/g,[2] is the change in a solutions' refractive index vs concentration. A differential refractometer facilitates determining this term.[3] Typical light sources include Helium–neon laser, Argon-ion laser, and Sodium-vapor lamp.[4] There are two compartments or flow cells, one for the sample and the other for the reference solution.[5][4]

The optical wedge or prism sits after the cells and separates the light coming from the flow cells.[4] The difference in refractive index causes the light paths to reflect at different angles.[6][7] This difference is magnified by the optical wedge/prism.[8]

A detector that can measure of range of wavelengths, usually a Photodiode array,[4][8] measures the position of the two light paths. The detector quantifies the angle of refraction, which is proportional to the refractive index.

General Operation

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Common Differential Refractometer Brands

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There exist various brands of differential refractometers. Popular models include:

Instrument Calibration and Quality Control

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All refractive index detectors require calibration upon first setting up the instrument as well as periodic quality control.[9][10][11] Most manufacturer's recommend calibration with pure water and a sucrose calibration solution of a known refractive index.[12] Once the instrument is in calibration mode, the pure water acts as a zero baseline reading, while the sucrose solution compares its known RI to the output, and the machine is adjusted accordingly.[13]

After the pump has not been used for a while, it is necessary to purge the tubes of any contaminant air that has diffused into the channels. This is typically accomplished with isopropyl alcohol.[10]

Data Utility

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Differential refractometers are often used for the analysis of polymer samples in size exclusion chromatography. Other types of information that can be gathered from differential refractometers are:

Molecular Weight

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Since the molecular weight (or extent of polymerization) of a solute will correspond to a specific refractive index increment, the relationship between increasing solute weight and refractive index increment can be plotted to determine the exact molecular weight of an unknown solute.[14]

Interactions with Solvent

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Increasing addition of solute will alter the solvent's viscosity and polarizability, which cannot be measured by instruments that rely on low viscosity.[14] Since differential refractometer is an external tool,[15][16] the solvent viscosity does not pose a physical barrier to measurement, making them universal detectors.[17]

General Shape

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The shape of a solute will influence it's induced dipole.[18] This will affect the solvent polarizability, which affects the refractive index.[19]

Practical Considerations

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There are many practical factors that can affect the accuracy of a differential refractometer.

Solute Properties

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When solutes are added to a solvent, they change the solution's optical density. The size,[20] polarizability[19] and shape and molecular structure[20] of a solute all have effects on the refractive index of a solution. Generally, a Gaussian distribution is observed, although deviations occur.[20]

Temperature

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A controlled temperature is needed to ensure accurate measurements, as temperature affects many properties of a solution.[21] If the temperature changes between measurements, this variance will be reflected in the measured refractive index.[22]

Wavelength of Light

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Cauchy's equation and Sellmeier equation describe the effect of wavelength on refractive index of medium.

Applications

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The use of and results from differential refractometers are valuable in numerous fields of science, with its theory and function applied in various research directions, including drug analysis[23] and nanoparticle tracking.[24]

The nature of refractive indexes allows RIDs to be used in conjunction with additional analytical chemistry instruments. Following the use of other machines, differential refractometers can immediately (further) characterize compounds eluting from chromatographers, spectrometers, and detectors, including:

References

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  1. ^ Undergraduate Instrumental Methods of Analysis. James W. Robinson, Eileen M. Skelly Frame, George M. Frame II. Marcel Dekker, 2005, p. 810.
  2. ^ "Differential Index of Refraction, dn/dc" (PDF).
  3. ^ Light Scattering from Polymer Solutions and Nanoparticle Dispersions. Springer Laboratory. 2007. doi:10.1007/978-3-540-71951-9. ISBN 978-3-540-71950-2.
  4. ^ a b c d "Waters 2410 Differential Refractometer Operator's Guide" (PDF).
  5. ^ "Differential Index of Refraction, dn/dc" (PDF).
  6. ^ Barron, John. "Refractive Index (RI) and Brix Standards – Theory and Application" (PDF).
  7. ^ Kőrösy, F. (August 1954). "A Modified Differential Refractometer". Nature. 174 (4423): 269. doi:10.1038/174269b0. ISSN 1476-4687.
  8. ^ a b de Angelis, M.; Tino, G. M. (2005-01-01), "Optical Instruments", in Bassani, Franco; Liedl, Gerald L.; Wyder, Peter (eds.), Encyclopedia of Condensed Matter Physics, Oxford: Elsevier, pp. 159–175, doi:10.1016/b0-12-369401-9/00492-7, ISBN 978-0-12-369401-0, retrieved 2024-11-18
  9. ^ a b "2414 Refractive Index (RI) Detector". Waters. Retrieved November 4, 2024.
  10. ^ a b c "1260 Infinity II Refractive Index Detector". Agilent. Retrieved November 4, 2024.
  11. ^ a b "RefractoMax 521 Refractive Index Detector". ThermoFisher Scientific. Retrieved November 4, 2024.
  12. ^ Klongratog, B.; Suesut, T.; Nunak, N. (2013). "The Uncertainty in Sugar Solution Concentration Measurement Based on Density Approach". Advanced Materials Research. 811: 358–364. doi:10.4028/www.scientific.net/AMR.811.358. ISSN 1662-8985.
  13. ^ Charles, D. F.; Meads, P. F. (1955-03-01). "Measurement of Refractometric Dry Substance of Sucrose Solutions". Analytical Chemistry. 27 (3): 373–379. doi:10.1021/ac60099a013. ISSN 0003-2700.
  14. ^ a b Han, Ying; Li, Dejie; Li, Deqiang; Chen, Wenwen; Mu, Shu’e; Chen, Yuqin; Chai, Jinling (2020-02-05). "Impact of refractive index increment on the determination of molecular weight of hyaluronic acid by muti-angle laser light-scattering technique". Scientific Reports. 10 (1): 1858. doi:10.1038/s41598-020-58992-7. ISSN 2045-2322. PMC 7002679. PMID 32024914.
  15. ^ "Differential Index of Refraction, dn/dc" (PDF).
  16. ^ "Waters 2410 Differential Refractometer Operator's Guide" (PDF).
  17. ^ "Refractive Index Detection (RID)". www.shimadzu.com. Retrieved 2024-11-18.
  18. ^ "Induced Dipole Forces". www.chem.purdue.edu. Retrieved 2024-11-18.
  19. ^ a b Pachucki, Krzysztof; Puchalski, Mariusz (2019-04-30). "Refractive index and generalized polarizability". Physical Review A. 99 (4): 041803. arXiv:1902.05725. doi:10.1103/PhysRevA.99.041803. ISSN 2469-9926.
  20. ^ a b c Zhao, Huaying; Brown, Patrick H.; Schuck, Peter (May 2011). "On the Distribution of Protein Refractive Index Increments". Biophysical Journal. 100 (9): 2309–2317. doi:10.1016/j.bpj.2011.03.004. ISSN 0006-3495. PMC 3149238. PMID 21539801.
  21. ^ Lu, Jue Xi; Tupper, Connor; Gutierrez, Alejandra V.; Murray, John (2024), "Biochemistry, Dissolution and Solubility", StatPearls, Treasure Island (FL): StatPearls Publishing, PMID 28613752, retrieved 2024-11-18
  22. ^ Held, Daniela (December 5, 2017). "Tips & Tricks GPC/SEC: How to Treat Your RI Detector".
  23. ^ Al-Sanea, Mohammad M.; Gamal, Mohammed (2022-07-01). "Critical analytical review: Rare and recent applications of refractive index detector in HPLC chromatographic drug analysis". Microchemical Journal. 178: 107339. doi:10.1016/j.microc.2022.107339. ISSN 0026-265X.
  24. ^ van der Pol, Edwin; Coumans, Frank A. W.; Sturk, Auguste; Nieuwland, Rienk; van Leeuwen, Ton G. (2014-11-12). "Refractive Index Determination of Nanoparticles in Suspension Using Nanoparticle Tracking Analysis". Nano Letters. 14 (11): 6195–6201. doi:10.1021/nl503371p. ISSN 1530-6984. PMID 25256919.
  25. ^ Bruno, Alfredo E.; Krattiger, Beat (1995-01-01), El Rassi, Ziad (ed.), "Chapter 11 On-Column Refractive Index Detection of Carbohydrates Separated by HPLC and CE", Journal of Chromatography Library, Carbohydrate Analysis, vol. 58, Elsevier, pp. 431–446, doi:10.1016/S0301-4770(08)60516-3, ISBN 978-0-444-89981-1, retrieved 2024-11-08
  26. ^ LaCourse, William R. (2017-01-01), "HPLC Instrumentation", Reference Module in Chemistry, Molecular Sciences and Chemical Engineering, Elsevier, ISBN 978-0-12-409547-2, retrieved 2024-11-08
  27. ^ Antony, Airin; Mitra, J. (2021-03-08). "Refractive index-assisted UV/Vis spectrophotometry to overcome spectral interference by impurities". Analytica Chimica Acta. 1149: 238186. doi:10.1016/j.aca.2020.12.061. ISSN 0003-2670. PMID 33551061.
  28. ^ Endo, Yasushi; Tagiri-Endo, Misako; Seo, Hwan-Sook; Fujimoto, Kenshiro (2001-03-09). "Identification and quantification of molecular species of diacyl glyceryl ether by reversed-phase high-performance liquid chromatography with refractive index detection and mass spectrometry". Journal of Chromatography A. 911 (1): 39–45. doi:10.1016/S0021-9673(00)01240-1. ISSN 0021-9673. PMID 11269594.
  29. ^ Clement, A.; Yong, D.; Brechet, C. (April 1992). "Simultaneous Identification of Sugars by HPLC Using Evaporative Light Scattering Detection (ELSD) and Refractive Index Detection (RI). Application to Plant Tissues". Journal of Liquid Chromatography. 15 (5): 805–817. doi:10.1080/10826079208018836. ISSN 0148-3919.


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