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Home > Magazine > Liquid Chromatography
Condensation Nucleation Light Scattering Detection
by Derek Oberreit, Quant Technologies LLC
IntroductionHigh performance liquid chromatography (HPLC) is a common technique used by analytical chemists to separate compounds (analytes) in solution. In HPLC a solvent is pumped under high pressure through a column of tightly packed particles and a small sample of analytes is injected into the solvent at the column inlet. The analytes pass through the column at different rates depending on how they interact with the solvent (mobile phase) and the column packing (stationary phase). The time it takes for a known amount of analyte to pass through the column is measured along with the detector response. This information is then used to help determine the quantity of the analyte in a test sample. Detection of compounds as they exit the column (elute) is most commonly done using a UV type absorbance detector. By measuring the amount of absorbed UV light the chemist can get an estimate of the amount of analyte that eluted the column and at what time after the injection.
UV detectors are limited to analytes that contain a chromophore. To detect analytes that do not contain a chromophore, chemists must rely on alternate detectors. These include refractive index, conductivity, mass spectrometry and aerosol-based detectors. The new CNLSD (condensation nucleation light scattering detector, Figure 1) discussed here is an aerosol-based detector.
Background
Figure 1. | Aerosol-based HPLC detectors all work on the same principle. They start by continuously nebulizing the column effluent. The mobile phase is evaporated from the droplets, which leaves particles that consist of chemicals in the effluent that have a lower volatility than the mobile phase. The size of the particles is constantly measured by a detector. When a non-volatile analyte elutes the HPLC column the particles left after evaporation will increase in size. The detector measures the increase in the size of particles and converts it to an analog output.
The main advantage of aerosol-based detectors is their ability to be used with any analyte that is less volatile than the mobile phase. This near-universal compatibility sets aerosol-based detectors apart from most detection techniques. There are several considerations that are worth noting. The sensitivity is inversely proportional to the amount of non-volatile residue present in the mobile phase. Since the detectors rely on measuring a difference in the size of the aerosol, the less the analyte contributes to the background aerosol size, the less percent change it will make on the detector. Furthermore, aerosol-based detectors are not able to differentiate compounds that elute at the same time (co-elute). The chemist must be aware of this during method development and when comparing aerosol-based detectors to UV based.
Early types of aerosol-based detectors used a photometer measurement of the aerosol cloud. This technique is referred to as ELSD (evaporative light scattering detector). As the size of the residue particles increases the level of light reflected by (or absorbed by) the aerosol increases. These detectors convert the light scattering levels to an analog output. The sensitivity of this detection technique is limited by the amount of background photodetector noise. They are also sensitive to photodetector signal drift.
A recent addition to the aerosol-based detector family is the Corona Cad charged aerosol detector. This instrument measures the increase in particle sizes by measuring the increase in 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 is limited to the background noise in the electrometer and is also susceptible to drift.
A CNLSD detects the change in the aerosol size by measuring the increase in the number of particles counted using a Water-based Condensation Particle Counter (WCPC). The WCPC condenses water vapor onto particles and grows them to a size that can be detected individually using an optical sensor. The WCPC detector in the CNLSD will only condense vapor onto particles that are above a certain size, particles below this size are not counted. As the particle sizes increase the number of particles that the WCPC detects increases. The number of particles counted by the WCPC detector is then converted to an analog output signal. The CNLSD technique was originally developed by Dr. John Koropchak at Southern Illinois University Carbondale.
Performance
 Figure 2.
LC parameters
Mobile phase: ACN H20 Gradient Flow rate: 1.0 mL/min
Column: Prevail Carbohydrate Es 5 m, 250 3 4.6 mm,
Alltech.
Column temp: Room Temp
Run time: 15 min
Detector: CNLSD
Click to enlarge. | Similar to other aerosol based HPLC detectors, the sensitivity of the CNLSD is limited by the level of background non-volatile residue that co-elutes with the analyte. However, the CNLSD is able to measure smaller changes in the size shift of the dry aerosol distribution. The dynamic range of the detector spans from greater than 1 part per thousand to below 1 part per billion. The lower end sensitivity is limited by the purity of the mobile phase and sample. The CNLSD has a linear response over several orders of magnitude. An example of the linearity of the CNLSD response is shown in Figure 2.
The CNLSD measures 14× 6× 13 and weighs less than 14 lbs. It is compatible with most solvents used in HPLC.
Derek Oberreit, senior development engineer at Quant Technologies LLC, may be contacted at cnlsd@quanttechnologies.com or by phone at 763-398-0456.
Company’s Other Products
Quant Technologies LLC
1463 94th Lane NE
Blaine MN 55449
Phone: 763-398-0456
Fax: 763-398-0480
http://www.CNLSD-Quant.com
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