Autosamplers

Nano Quantity Analyte Detector (NQAD™) A New Alternative In Sensitive HPLC Detection

Oct 21 2009

Author: Charles E. Bille, David Crowshaw on behalf of Grace

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High performance liquid chromatography (HPLC) is a common and well-established technology for a host of quantitative techniques employed by analytical chemists to separate compounds (analytes) in solutions.
The most common type of detector used in HPLC today is the UV absorbance type. Use of UV detectors is limited to analytes that contain a chromophore. To detect analytes with a weakly responding or no chromophore, chemists must rely on alternate detection systems. These systems include refractive index, conductivity, mass spectrometry, and aerosolbased detectors.

To address a broad range of issues affecting the performance of alternative detection systems, Quant Technologies of Blaine, MN (www.quanttechnologies.com) have introduced the NQADTM, or Nano Quantity Analyte Detector.

Background
The NQADTM’s operation is partly based on original techniques from early work on Condensation Nucleation Light Scattering Detection (CNLSD) by Dr. J. A. Koropchak and colleagues at Southern Illinois University
Carbondale and a number of papers have been published detailing its development 1,2,3,4,5,6,7.

Aerosol-based HPLC detectors all initiate their detection methods on similar principles. They begin by continuously nebulizing the column eluent. The mobile phase is evaporated from the droplets, which leave particles suspended in air that consist of chemicals in the eluent that have a lower volatility than that of the mobile phase. When a non-volatile analyte elutes from the column, the size of the particle increases. At this point, in an Evaporative Light Scattering Detector (ELSD), a photometric measurement of the aerosol cloud occurs. As the size of the residue particles increases, the level of light reflected by (or absorbed by) the aerosol changes. These detectors convert the light scattering levels to an analog output 4. The sensitivity of this detection technique is limited by the amount of background photodetector noise 8,9. Another type of aerosol-based detector uses Charged Aerosol Detection (CADTM). Rather than utilizing light scattering measurements with the aerosol cloud, this type of detector measures the increase in particle size by the ability of the aerosol to carry an electrical charge. The resulting electrical charge is measured using an electrometer and converted to an analog output signal. The sensitivity of this technique can be limited by background noise in the electrometer and
can also be susceptible to drift 9. A rather significant limitation that has been seen with charged aerosol detection methodology is increased noise when sampling mobile phases with high pH 9.

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