Chromatography Forum

Dear all,

I'm wondering what the benefits of a CAD (charged aerosol detector) compared to an ELSD (evaporative light scattering detector) are, besides the usual aspects such as: higher sensitivity, broader dynamic range, higher consistency of response, better peak reproducibility.

In short: why would you choose a CAD instead of an ELSD? Is it worth the price you pay for it (typical price of a CAD is 3 times higher compared to an ELSD, as far as I know)?

Please post comments/advices!

I used both detector. You have listed all important benefits of CAD over ELSD. In my opinion, better sensitivity and better peak reproducity are very important for quantitative analysis.

We have an ELSD and recently bought a CAD as well. One issue with the ELSD was that we could not detect some phospholipids we use in our research which on the other hand are easily be detected by the CAD. Also sensitivity of CAD is much higher compared to ELSD.

Regards

Mukul

Mashtikar:

I am curious...why do you suppose that the CAD worked better than the ELSD for detection of phospholipids.

Thanks
Hajdaei

I do have both ELSD and CAD. I have tried hard to get the sensitivity claimed by the manufacture. I found my problem is that my noise is 10 to 100 times larger than the specification e.g. 10 mV v.s. <1 mV.

Besides concurrence with JelmerD's question, XL and mashtikar, do you have comparison data showing that CAD is better than ELSD? What kinds of application you are referring to? Could you please provide some details?
Thanks in advance,

Hello, I was using Acclaim Trinitiy P1 column for pharmaceutical counterion analysis. Initially I used a Sedex 85 ELS detector. When Corona ultra was available, I did some comparison for inorganic ions. The Figure 1 in the following link shows that CAD gives significantly higher sensitivity than Sedex 85 for Na and Cl ions under the same chromatographic conditions. If you are interested in reading the rest content in the same file, you will see Acclaim Trinity P1 with CAD is a useful combination for pharmaceutical API and counterion analysis.
http://www.dionex.com/en-us/webdocs/707 ... 239-02.pdf

The high background noise in the CAD is most likely silica bleed (J. Pharm. Biomed. Anal., 50 (2009) 809–814).

I have not used the CAD myself but all people I talk to agree with XL (even though he works for Dionex that sell the Coronas) that it is more sensitive. For counter ion analysis the most commonly used column is our polymeric ZIC-pHILIC with the Corona CAD (J. Pharm. Biomed. Anal., 50 (2009) 809–814), and the standard silica ZIC-HILIC using ELSD (LCGC North America, 24 (2006) 776-785).

Links to these papers can be found at:
http://www.sequant.com/scientificpapers

Hi XL and Bintang,

I am sorry I am very analytical.

As I mentioned before I have two CAD with me. If we were asked we also claimed that CAD has better sensitivity. But when I get in the lab to try work out a high sensitive method I found it is difficult to control those noise.

Yes, I agree the noise may be caused by one or combinations of column, mobile phase, CAD parameters, lack of an additional pulse dampener, and so on. But if ESA, Dionex, or now Thermo can not provide a feasible method development instruction this CAD is not very useful for impurity analysis.

Thanks

Excel,

You have brought up a very good point. Indeed, for "high sensitive" analysis everything needs to come together right. Everything includes organic solvent, water, buffer salt, pH, gas source, column, etc. Very often, column bleed is the reason for high background noise of CAD because it is a more sensitve detector. Usually sillica based amino columns give high background noise while modern C18 columns give minimal nosie level. There are ways to identify the root cause of the high noise - mobile phase, column, or the detector. My expereince is that with clean mobile phase and good quality of column, CAD consistently provides better sensitivity and reproducity compared to ELSD.

I have read that the CAD requires "particles that can be charged in the detector". My question is: are there any compounds that can´t be charged in the detector? Which ones? In these cases, Do I have to use the ELSD instead the CAD?
Thanks

Very often high noise is attributed to switching from TFA to ammonium formate or acetate, since instrument traps buffer. We have dedicated system for analyses which involve TFA and ammonium buffers. It usually takes few hours to flash system completely. Part of the noise can also be solubilization of silica when running with MeOH and hydrolysis of the stationary phase. In general CAD is 5-10 times more sensitive than ELSD.

...regarding buffers. Wash system extensively between TFA and ammonium containing buffers. Be careful not to make inadvertently ammonium trifluoroacetate. It is not volatile and will contaminate CAD.