Concept: Full-Spectrum Analysis

This article explains the technical advantages of full-spectrum (dispersive) acquisition in spectroscopic applications. This information pertains to any Applied Analytics product which uses the nova II UV-Vis/SW-NIR Spectrophotometer.

Dispersive 'Full-Spectrum' Analysis

The full-spectrum acquisition performed by Applied Analytics spectrophotometers is known as dispersive because a holographic grating physically separates the received light signal and focuses each constituent wavelength onto a corresponding photodiode within the array, allowing for individual measurement of light intensity at each discrete wavelength.

By contrast, 'non-dispersive' methods use filters to isolate single wavelengths and therefore destroy all data present at any other wavelength.

In the nova II, each photodiode contributes one data point to a 'full spectrum' comprised of all of the data points simultaneously, ranging in wavelength from the shortest ultraviolet wavelength to the longest NIR wavelength. To review the optical assembly of the spectrophotometer, read Spectrophotometer Principle of Operation.

The Advantage of Collateral Data

Any single photodiode measurement is vulnerable to noise, signal saturation, or unexpected interference. This susceptibility to error makes a lone photodiode data point (as used by a photometer) an unreliable indicator of one chemical's absorbance.

As accepted in the lab community for decades, the best way to neutralize this type of error is to use collateral data in the form of 'confirmation wavelengths,' i.e. many data points at many wavelengths instead of a single wavelength. Consider the example of measuring SO₂ in a sample fluid:

In the figures above, each diamond represents a single photodiode and data point. The nova II registers absorbance at each integer wavelength within the 265-295 nm measurement range and produces an SO₂ absorbance curve. After being calibrated on a full spectrum of pure SO₂, the spectrophotometer knows the absorbance-concentration correlation for each measurement wavelength; the system can average the modeled concentration value from each wavelength to completely eradicate the effect of noise at any single photodiode.

The spectrophotometer visualizes the SO₂ absorbance curve in this manner and knows the expected relation of each data point to the others in terms of the curve's structure. This curve analysis enables the system to automatically detect erroneous results at specific wavelengths, such as when a single photodiode is saturated with light.