Methods Of Measuring Refractive Indexes - Wyatt Technology Optilab rEX User Manual

Chapter 1: About the Optilab rEX

1.1 Methods of Measuring Refractive Indexes

For the past four decades, virtually all differential Refractive Index (dRI)
detector manufacturers have used a deflection-based instrument design to
measure the refractive index difference between a reference fluid and a
sample fluid. Wyatt Technology's Optilab DSP stood alone as an interfero-
metric-based instrument with the ability to cool samples below ambient
temperatures. When the Optilab DSP was introduced, an interferometric-
based design was the most sensitive method available for measuring dRI.
However, technology does not remain motionless. Significant advances
over the past five years in nanoscale semiconductor fabrication tech-
niques, linear photodiode array technology, and computer speed have
allowed Wyatt Technology to develop a novel deflection-based dRI instru-
ment that surpasses all current measurement methods.
The Optilab rEX (refractometer with EXtended range) is a deflection-
based instrument that pioneers the use of an array of light sensitive detec-
tors in the measurement of dRI. It has a linear array of 512 photodiodes,
rather than the traditional two photodiodes used by other deflection-based
instruments. This photodiode array allows for unprecedented sensitivity
and range.
With most RI instruments, users must choose a gain setting, which essen-
tially means choosing between good sensitivity over a limited range,
reduced sensitivity over a large range, or some compromise between the
two. The Optilab rEX has no gain settings; the maximum sensitivity exists
over the entire range. Tiny signals and huge signals may be measured
within the same data run. The ability to see both large and small signals
may be quantified by the instrument's dynamic range (defined as range
divided by noise). The dynamic range of the Optilab rEX is 2.35 million,
literally orders of magnitude greater than any other RI instrument on the
market today.
In addition to the photodiode array, the Optilab rEX contains a new flow
cell design that allows the measurement of a fluid's absolute Refractive
Index (aRI) in addition to the dRI between two fluids. The aRI of a fluid is
vital for characterizing a system and is a necessary input parameter for
light scattering measurements that determine the molecular mass of mol-
ecules in solution.
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Optilab rEX User's Guide (M1500 Rev. F)