Chromatography Forum

Dear Members

I need to perform the HPLC analysis of samples of dissolution study of a lipophilic drug. The dissolution media need to contain 0.5% w/v of Sodium lauryl sulfate. I can at maximum dilute sample by equal volume of solvent in consideration of drug concentration in diluted samples. Thus reducing SLS to 0.25% w/v. Which probably is very high concentration. Injecting these samples change the peak shape badly specially downward (a sort of shoulder).

SLS possibly (in fact it does) accumulate on the column (C18). Distorted peaks are observed with the 'pure drug (without SLS) injected after 'samples containg SLS'.

Similar is the case with the hydrolytic degrdation product of drug.

I got to know 'cleaning method for SLS for column' in a post in this forum. However, SLS is practically making method not feasible for samples. I googled for similar problem and got this link.

http://dissolution.com/vbulletin/archiv ... t-258.html

I am putting a part of that in this forum as under.

"Use of surfactants in hplc is well known-called soap chromatography or dynamic ion exchange chromatography. The SLS will be adsorbed on a C18 column totally changing its mechanism. It is almost impossible to regenerate the column afterwards. Also as you are slowly adding SLS from the injection of diss. media it will never stabilize. Best approach is add SLS to the mobile phase [you will have to redevelop the chromatography]-this will solve the problem. But-put the column aside and use it only for this application. Also note equilibration time is about twice normal when you have SLS in the eluent."

Considering the our case its not possible to keep the column aside as we need to perform very few studies compared with average column life time span.

I look forward for possible solution for such case.

I remember previously we developed a method containing Sodium octyl sulfonate as ion pairing agent and injected samples of 'same drug containg SLS' (in fact dissolution samples only). We had not observed any peak distortion in that case.

I look for the comments of the forum Members.

re dedicating a column: you have to weigh the cost of the column against the cost of your time searching for an alternative.

Apparently Cost of Column is much higher than time of searching alternatives.

First, let me contradict some of the advice from the dissolution archive. Both JMB and I have ample experimental evidence that SLS can be quantitatively removed from a modern RP or polar-embedded RP column. I will comment that many brands of SLS have UV-detectable, hydrophobic contaminants, and you need to find a source of clean SLS. You might try the good half of their advice, and develop conditions with SLS in the mobile phase. You might be surprised how much SLS a column can hold; in my experience even a saturated column can be successfully cleaned.

Clearly, neither you nor your superiors will be satisfied by simply me saying so. Doubts are ease to raise and hard to dispell. To prove it to yourself, run a system-suitability test (something really touchy) before and after SLS exposure and cleaning. You can even bring an old column out of retirement just for this exercise.

There are other possibilities. If you don't mind adding another step to the procedure, you can probably work out an SPE cleanup prior to analysis.

Dear MarK

It may not be difficult to remove the SLS as you said. Thats about the cleaning the SLS.

What if because of SLS peak is distorting.

Can You name some brands of SLS you found suitable for HPLC. I remember your post stating you do this all the time.

I do this all the time. It takes a wash first with 90% methanol for 3 column volumes followed by acetone for 3 volumes at 1 mL/min. The first removes the SDS, the second removes the SDS impurities.
_________________
Mark Tracy
Senior Chemist
Dionex Corp.

[/quote]

I have found Merck OmniPur to be satisfactory.

To solve the peak distortion problem, one approach is to saturate the column with SLS by putting it in the mobile phase, as suggested in the dissolution archive. That raises the issues of cleaning and dedicating a column.

Personally, I think you should seek an approach that prevents SLS from getting on the column in the first place.

Dear Mark

Can you comment why the same peak problem was not observed when sodium octane sulfonate was used in the mobile phase.

To me apparently SLS will adsorb stronger than octane sulfonate.
Or
Is it possible that Once the column is pre-saturated with sodium octane sulfonate, SLS can not bind strongly to negatively charged sulfonate and C18 phase is poentially not available for binding due to pre-saturation with sulfonate.
In that case What should be the cleaning procedure for octane sulfonic acid?

It should be a nice learning exercise for me to try with SLS on old out of use column.

I can speculate. The column is already saturated with octanesulfonate, rendering it negatively charged. Laurylsulfate still adsorbs to the column, but displaces a roughly equal amount of octanesulfonate. The effect on retention is quite small until a very large number of samples have been injected.

In contrast, without IP-agent in the mobile phase, the column starts with no adsorbed anions. When you inject SLS, it saturates the first few mm of the column and alters its retention mechanism.

0.25% w/v solution injection into column with 20 microliter loop means 50 microgram of SLS injected. Considerably good amount.

Just a thought, octane sulfonate in continuous mobile phase may give good binding competition and contiuously keep SLS eluting.
Just a wild guess or thought or just biasing the favor of octane sulfonate.

...SLS can be quantitatively removed from a modern RP or polar-embedded RP column.

Mark, what is the border line between modern and ‘old-fashioned’ RP column? Why could the SLS only be removed from modern column?

I specified 'modern' because I have only tested SLS removal from columns made from Type-B high-purity, low-acidity, spherical silica with a good particle size distribution. I have no experience with the older silica, but in principle, it should work.

In addition to use of Solid phase extraction, I wonder if it is possible to precipitate SLS out of solution by addition of a divalent cation, like Ca2+ or Mg2+. Then just filter.

As a historical note, SLS was the first commercial synthetic detergent for laundry and dishwashing. It was introduced precisely because it does not precipitate the Ca and Mg in hard water.