Chromatography Forum

Virtual Colleagues,

I have been assigned a project/method which uses an evaporative light scattering detector for the detection of phospholipids. From my initial survey of the literature, vendor and peer-review, I have some doubts as to the viability of developing a stability indicating method with such a detector. In particular, I am certain about detectability (LOQ/LOD).

I would appreciate more information/guidance concerning such an endeavor.

Kind regards,
Robotjock

Hi,

Please define "Stability indicating method". I have heard the term so many times, but as far as I remember I found no definition. A method can be selective, accurate and precise; To indicate stability not only a method (or two) are nessecary, but also a stability study.
I am just curious.

Alex

Alex,
You're correct. A stability-indicating(SI) method presupposes a stability study and stability samples. It is a common method name in the pharma industry, along with others such as Assay method and In-process method.

If a SI method is developed correctly, it will be selective, precise and accurate for degradation products which may occur in the material being studied. Which is why I need to learn more about detectabiliy using an ELS detector.

regards,
Robotjock

Some general comparisons. First off, I'll assume that you will be monitoring the appearance of degradants (rather than the loss of the API). Second, I'll assume that you are comparing ELSD with UV (rather than, say, MS):

- ELSD response is essentially independent of structure
- UV response depends on the structure of the chromophore

- ELSD response has a narrow linear range
- UV tends to have a wide linear range

- ELSD requires the use of volatile buffers
- UV requires the use of UV-transparent buffers

Taking that further:

- If you have a reasonable chromophore, UV will probably be more sensitive than ELSD.

- If you have standards for all your degradants, then UV will probably be at least as accurate as ELSD.

- If you do not have standards for the degradants, then ELSD may be more accurate than UV, (because the UV quantitation in that case assumes that degradant and API have the same chromophore).

- If you don't have a chromophore, then ELSD wins.

I guess that phospholipids won't really absorb on UV so ELSD is probably the best detector choice...

I have done phospholipids with a UV and ELSD in series. For the fully saturated lipids, the ELSD is more sensitive than UV. The unsaturated lipids and their peroxidation products are more sensitive by UV.