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The analyte detection challenge
One of the biggest challenges liquid chromatographers currently face with analyte detection is no single method can reliably measure all compounds.
Often, analytes respond more strongly to one form of detection than another or do not respond at all. What is most needed is the ability to detect a wide range of analytes — from small molecules to antibodies — with a response that delivers accurate quantitation.
For complex separations where multiple analytes in a sample are incompatible with UV detectors and mass spectrometers (MS), like when compounds lack a chromophore or cannot ionize, you can turn to a universal detection approach called Charged Aerosol Detection.
Thanks to the application versatility, the Charged Aerosol Detector (CAD) is now a preferred universal detector for both routine and complex separations, driven by the need for sensitive, near-universal analyte response and standard-free quantitation.
The CAD can measure all analytes in the two samples shown above. Other detectors are more limited in scope. For example, mass spectrometry (MS) requires that analytes form gas phase ions (A) while response by a UV detector depends upon the nature of the chromophore (A and B).
HPLC-CAD learning center
Related resources
The CAD can detect all non-volatile and many semi-volatile compounds, but with a differing response:
- Non-volatile components give a uniform analyte response irrespective of the chemical structure that allows for standard relative quantitation
- Semi-volatiles show a non-uniform response due to the reduced signal depending on the volatility and LC method conditions and this effect becomes more prominent, especially at lower concentrations
CAD benefits
The most desired feature of a near-universal detector with a uniform analyte response like the CAD is the ability to quantitatively measure compounds incompatible with UV-Vis and MS detection and relative amounts of compounds when certified standards are not available for a single calibrant quantification.
By meeting the flexibility and performance needs of analytical R&D studies and the simplicity and reproducibility demands for manufacturing quality control in QA/QC studies, the CAD gives you more confidence in every LC analysis.
The CAD can also increase the efficiency of your analytical operations and may open up entirely new possibilities by uncovering a range of analytes unseen by other detection methods.
Major benefits of using the CAD include:
- An exact relative response for all non-volatile compounds independent of analyte structure
- Excellent sub-nanogram sensitivity coupled with a wide dynamic range for unrivaled performance
- Single calibrant for the quantification of multiple analytes when individual standards are not available
CAD vs. ELSD
Both the CAD and Evaporative Light Scattering Detector (ELSD) are evaporative aerosol detectors able to detect non-volatile and many semi-volatile compounds. But how the particles are detected differs between the two technologies and this difference can significantly impact detector performance.
The CAD response depends on particle charge, and the detector can measure smaller charged particles and detect less analyte, which equals better sensitivity.
In contrast, the ELSD response depends on particle size, and the efficiency of light scattering greatly decreases as the particle size and amount of analyte decrease.
For ELSD, you need bigger particles, and more analytes present than the CAD.
The CAD has other numerous advantages over the ELSD like:
- CAD response is not affected by the optical properties of an analyte like refractive index
- Near-uniform analyte response and standard free quantitation
- A single nebulizer works for all flow rate ranges
- Better sensitivity and a wider dynamic range
- Less complex response curves
UV-Vis detectors (UVD) require the analyte to have a chromophore for detection but the problem is not all compounds have a sufficiently strong chromophore. For these compounds, you can use low wavelength ultra-violet detection, but selectivity issues can arise when distinguishing between target versus unknown peaks.
This approach also lacks sensitivity and is only compatible with certain solvents. For instance, you cannot easily detect a peak for an unknown when the compound absorbs below the cut-off wavelength of the eluent, like acetonitrile at 190 nm.
With the CAD you can easily detect target compounds and impurities regardless of solvent absorbance characteristics leading to higher detection sensitivity with better baselines.
Refractive index detection (RID) is universal but has shortcomings like incompatibility with gradient elution and an extensive startup time of easily several hours.
The CAD, on the other hand, works with both isocratic and gradient separations, is more sensitive, and requires less time for equilibration.
Although mass spectrometry (MS) detection is powerful the technology only works if your compounds form gas-phase ions. The response might suffer from ion suppression and is difficult to quantify without isotope standards.
Unlike MS, the CAD response is independent of ion formation, which gives you the freedom to quantify analytes with just a surrogate standard.
HPLC-CAD publications
This collection of publications is a resource to demonstrate the analytical capabilities of the Charged Aerosol Detector by highlighting the breadth and scope of the different analytical applications found in the literature.
Charged Aerosol Detection delivers versatility and allows chromatographers to use traditional HPLC, UHPLC, and nano-flow separations. In many cases, the CAD can eliminate the need for derivatization or sample pretreatment, providing dilute-and-shoot simplicity.
Scientists in diverse areas like bio/pharmaceuticals, food and beverage, natural products, fundamental research, and industrial and environmental testing rely on this detection method for complete analyses.
Pharma and biopharma
Pharmaceutical · biopharmaceutical · biotechnology
These publications cover analysis of small and large APIs, counterions, excipients and adjuvants, liposome characterization, QC library measurement, degradants/impurities, analyte purity, and stability.
Biochemical research
Biochemical research · life science
These articles cover many disciplines relevant to the study of living organisms including the fields of biochemistry, biology, microbiology, and physiology.
Natural products
Botanicals · herbals · natural products · traditional medicines
Articles include the measurement of compounds found in plant extracts, many of which are used in traditional medicines. Some highlight CAD for analyte quantitation when reference standards are unavailable.
Food and nutrition
Foods · supplements · functional foods · consumer products
Includes publications describing the use of the CAD for measurement of analytes like carbohydrates and lipids, plus metabolomic approaches for determining product authenticity and adulteration.
Industry and environment
Industrial · environmental · agricultural · fuels sector
This section spans many industries including agriculture and farming, petrochemical, environmental, and the production and use of plastics and polymers.
Clinical research
Health · disease
Includes topics such as tissue drug measurement in fresh and post-mortem samples, effects of drugs, and how disease progression affects metabolism.
HPLC & UHPLC analysis
Chromatography · method development · detector performance
Articles include fundamentals, operating principles, detector optimization, detector comparison, method development, and evaluation of columns.
CAD testimonials
Dive into the technical capabilities of our unique HPLC/UHPLC detector. Hear from industry experts in pharma/biopharma and food/beverage how you can easily apply the CAD in your analyses along with tips, tricks, pointers, and advice from our guest speakers.
Access our full CAD symposium webinar series on-demand here.
FAQ
Both CAD and ELSD are evaporative aerosol detectors able to detect non-volatile and many semi-volatile compounds. But how the particles are detected differs between the two technologies. CAD measures particle charge while ELSD measures the ability of the particle to scatter light.
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