Filtering Issues

[Lab Falcon]

I'm currently developing a method to check benzoic acid in aqueous solutions by HPLC. I'm creating standards with sodium benzoate and filtering them through a syringe 0.2 micron nylon filter. I need to filter 0.2 microns with the current column I have and due to the aqueous sample solutions. I notice a large difference in peak area in standards that are either filtered or not. Why is there a large difference in the peak area? I would think that the benzoate would not be effected by the filter. Does anyone have any suggestions to fix this is issue?

[prepcolumn]

I'm currently developing a method to check benzoic acid in aqueous solutions by HPLC. I'm creating standards with sodium benzoate and filtering them through a syringe 0.2 micron nylon filter. I need to filter 0.2 microns with the current column I have and due to the aqueous sample solutions. I notice a large difference in peak area in standards that are either filtered or not. Why is there a large difference in the peak area? I would think that the benzoate would not be effected by the filter. Does anyone have any suggestions to fix this is issue?

You have to measure the volume that filter gets saturated. If the discard volume of sample is meaningful then you have to apply the method in that way. Otherwise you have to check filters that have different chemistry..

[unmgvar]

there is always the possibility that your membrane will get some of your compound stuck in it
nylon is not the most inert membrane these days. and you need to use the right membrane for the right compound and also the solution it is dissolved into.
and quality is also different from vendor to vendor.

[biochemist]

Yes, the others are right. I would :

a) Conduct a flush volume study of the nylon syringe filter - % recovery of first mL, vs second mL, vs third mL, etc..... If the recovery is bad (compared to a centrifuged sample) for all aliquots, you probably shouldn't use nylon. If it's only bad for the first few mL, incorporate this "flush volume" consistently in your method.

b) Change to a different filter membrane material. I use GHP or PVDF for most samples these days.

[Consumer Products Guy]

Can you dilute with some methanol and then use PTFE filters? That's what we do.

Benzoic acid is pretty insoluble in water, think some is precipitating and thus getting filtered out.