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dry particle. In fact, many of these particles are large enough to be individually sensed and counted. After water is condensed on the particles, the particles are streamlined so that they pass through a laser beam one by one. When a particle passes the laser beam, it will scatter light. A photodetector will count every time it sees scattered light. What is important about this step, is that only some particles are large enough to be detected (Figure 2).
What the WCPC detector will see and what it will not see is dependent on a factor parameter called the CPC detection efficiency curve as shown in light blue in the graph (Figure 1). Anything that lies to the right of this CPC detection efficiency curve is large enough for the detector to see it. Anything that lies to the left of the CPC detection efficiency curve is too small to be detected. Since background particles that only contain mobile phase residue are small, only a small fraction of these particles cross the CPC detection efficiency curve and will be detected (shown in black). These particles contribute to the baseline of the chromatogram. When there is analyte and mobile phase residue present, the particles are much larger in diameter. Because of this shift in size, many more of these particles are large enough to cross the CPC detection efficiency curve and be detected. These particles are show in yellow and will contribute to the analyte signal on the chromatogram.
How does WCN detect in LC? WCN starts the same way as any other aerosol detector on the market. First, the mobile phase is continuously nebulized as it comes off the column. Then, it is evaporated leaving
Figure 3: WCPC Linear range (Uracil)
dry residue suspended in air or nitrogen. The dried residue then undergoes an additional step called Water Condensation Nucleation. In this step, the dry particles are swept through a chamber of water saturated air or nitrogen. This causes water to condense on the surface of the particle, effectively growing the particles to a size in which they can be individually sensed and counted. WCN is highly sensitive because individual particles are counted. This is similar to counting photons (if you could do that) rather than measuring light intensity such as in the ELSD.
Enhanced user benefits The Water Condensation Particle counting technology makes a highly useful tool. In Universal detection the limit of detection of these detectors is always quoted as amounts on column rather in amounts/volume.
Sub nanogram sensitivity is possible, when using columns with low bleed and pure solvents, making it the most sensitive detector
in the field today. This, coupled with a wide dynamic range, a linear range that generally spans 3-4 orders of magnitude (Figure 3) and reproducibility less than 2%, even at low concentrations making WCPC ideal for a wide range of application areas, particularly for the pharmaceutical, food/beverage, polymer and cosmetic industries.
One of the major benefits of aerosol based detectors is their ability to detect compounds that do not have a chromophore such as lipids, polymers, surfactants, carbohydrates, and impurities/degradation products. Detecting these compounds without derivatization greatly reduces sample preparation time and allows for less complicated chromatography.
WCPC technology is uHPLC and HPLC compatible, Figure 4 is an example of a PEG separation on HPLC and uHPLC. Baseline resolution was still achieved when the peaks were separated in approximately 1 minute as opposed to the 7 minutes required for the HPLC separation. uHPLC minimizes sample analysis time, and can minimize waste consumption by up to 80%.
WCPC detectors are also SFC compatible. Generally, to be successful with SFC applications, the nebulizer temperature should be heated to 40C in order to keep a stable baseline during the adiabatic expansion of the CO2 gas. Later models of WCPC detectors are most suited for SFC applications since these models allow control of the nebulizer temperature.
Figure 4: WCN Compatibility with HPLC and UHPLC Systems
Gradient elution is often an important requirement to separate compounds of interest. However, for some aerosol based detectors, the response is mobile phase dependent. This results in a higher response with a higher organic concentration. Ultimately, when estimating the concentrations of sample components using these detectors,
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