"Hi there, I am currently doing some analysis of triclosan in soap. The method requires a blank baseline to commence the run. The mobile phase and diluent are the same.(acetonitrile-1%acetic acid in water) 70:30v/v. However, as the run progresses, I am getting a peak that is above the limit of quantification in the blank at the same retention time as the standard peak.
The column is C18. I thought perhaps carryover, so incorporated a stringent needle flush. I prepared a vial of acetonitrile and injected it, and the baseline was clear and then I prepared a vial containing mobile phase and injected it and the peak was present. I prepared fresh mobile phase and injected and had clear baseline, after 8 hours, peak was again present.
I cannot imagine that the mobile phase is going off. Can anyone help?"
I like to think that we're experts in triclosan assay in soap, as we've been doing this for decades, back when triclosan suppliers were monkeying with extracting the triclosan with hexane, blowing off solvent, reconstituting in another solvent, then trying GC or HPLC. First of all: bar soap or liquid soap? Anyway, the HPLC portion is exactly the same. If bar soap, blend the sample with DMF as that solvent excludes soap itself (if acetontrile itself allows decent blending, you can use that), then filter; use 2.5 grams sample per 100 ml solvent. For liquid soaps, also make up 2.5% sample in methanol, DMF, or your mobile phase, then filter. Yes, we do extract with organic solvent (and make up the triclosan standards in that same solvent), then we use mixed organic-aqueous as the mobile phase. We also use RP-18 and isocratic acetonitrile-water-acetic acid, and we use UV detection at 280 nm, and 5 ul injections, as there's plenty of sensitivity. We actually use "older" Type A RP-18 because the procedure has been used here so long. We use Agilent equipment, and have no issues at R&D, production, or contract manufacturers. We do not use a needle wash. As to how blank must your baseline be before you can run - tough to define "zero". Our autosampler rotor composition is either tefzel or vespel. After a series of maybe 50 samples, we wash out the column with high acetonitrile to be ready for next set. You state that after 8 hours you see a peak at the triclosan elution time: is that after 8 hours of just pumping or after 8 hours of injecting standards and samples in a sequence? Have you tried eluting the triclosan under isocratic conditions then starting a gradient to clean off the column after each injection? That should not "violate" your current test procedure, as you would still be using the exact-same HPLC conditions for quantitation. If you do this, (1) your run times will be longer and (2) you'll need to program in SUFFICIENT re-equilibration time between injections. You could also try substituting methanol for acetonitrile, but will need to bump up the methanol percent a little, but I don't think that will take care of things. We leave our acetontrile out in the reservoirs and let the quaternary pump mix that with the acetic acid-water. Can you post chromatograms and what wavelength you are currently using? Also, I am not familiar with the term "the mobile phase is going off", do you mean selective evaporation from the reservoirs? Do you pre-mix your mobile phase? Are you making claims for the triclosan like "antibacterial" so there's FDA regulation as an OTC pharmaceutical, or is this just for R&D or for internal QC?
