I decided to write this article about charged aerosol detection as it appears to be a topic that many people are interested in and have lots of questions to ask – whenever we do a webinar that involves charges aerosol detection we are always guaranteed lots of questions. Charged Aerosol Detection, or CAD as it is commonly referred too, is a unique detection technology offered by Thermo Fisher Scientific. CAD is an evaporative detector that offers sensitive, universal detection with a near-uniform response which makes the technology amenable to a wide-range of applications. The technology is made available in two options; The Thermo Scientific™ Corona™ Veo™ which can be connected to any HPLC or UHPLC system from any vendor and the Thermo Scientific™ Vanquish™ CAD optimized for the Thermo Scientific™ Vanquish™ UHPLC platform.
How does CAD Work?
Essentially it is a three-step process: First, the detector converts the analyte molecules eluting from the column into dry particles. The number of particles increases proportionally with the amount of analyte. Secondly, a stream of positively-charged gas collides with the analyte particles. The charge is then transferred to the particles—the larger the particles, the greater the charge. Finally, the particles are transferred to a collector where the charge is measured by a highly-sensitive electrometer. This generates a signal in direct proportion to the quantity of analyte present. This process can be seen in this short whiteboard video.
Five Reasons to Choose CAD
I personally believe there are numerous compelling reasons to invest in CAD technology, but here are my top five:
See what others are missing – there are many detector options available, but have limitations and biases. UV-based detectors are very common, but cannot detect analytes lacking a chromophore; whilst mass spectrometry struggles to detect analytes that ionise poorly. If your sample contains analytes that lack chromophores and/or ionise poorly then you are in danger of missing these analytes and not gaining a truly representative picture of your sample. As CAD measures the charge transferred to the particles it offers near-universal detection any non-volatile and most semi-volatile analytes, allowing you to see the full range of analytes in your sample.
Quantitative data – the technique provides a consistent analyte response independent of chemical structure and the charge is proportional to the amount of analyte present allowing for relative quantitation without the use of reference standards. This is a big advantage if reference standards are not available and helps reduce costs. One previous limitation of evaporative detectors was that the quantitation accuracy was affected by the changing composition of the mobile phase, however this can be overcome by the use of an inverse gradient and is described in the excellent article by my colleague Jenny-Marie Wong.
Broad range of applications – due to the near-universal detection capabilities of the CAD the range of applications and markets that utilise the technology is very broad -- ranging from both small and large molecule pharmaceuticals, foods and beverages, chemicals, environmental applications through to polymers and lipids. Whatever your application, I’m fairly confident that someone has developed a method based on CAD technology. To view the broad range of CAD applications, I suggest taking a look at this Charged Aerosol Detection Bibliography.
Great performance – in comparison to other universal detectors such as refractive index (RI) and evaporative light scattering detectors (ELSD), CAD offers superior sensitivity and linearity. Depending on the chromatographic conditions, typically low ng limits of detection can be achieved and the dynamic range is in the order of 104 to 105 – whereas ELSD is typically 102-103.
In summary, there are many great reasons to choose charged aerosol detection for your liquid chromatography and I am sure that the users of CAD could easily add many more. It would be great to hear your experiences of CAD technology in the comments below.