APR September/October 2020 - 82

QC CORNER

Why analyte binding to syringe filters must be
studied during filter validation for QC testing
Vivek Joshi, Ph.D.

Wayne K. Way, Ph.D.

Senior R&D Manager, Applications Development
MilliporeSigma

Pharma Analysis & QC Strategy
MilliporeSigma

Many pharmaceutical QC tests such as dissolution

so analyte binding is typically low and recoveries high.

testing, content uniformity, assay, and blend uniformity

Nylon membranes, on the other hand, contain amino and

require sample filtration prior to HPLC or UHPLC analysis.

carboxylic acid functional groups as well as amide bonds

Since quantifying analytes is critical for these tests,

which can interact with acidic or basic analytes through

method validation should include filter validation studies

electrostatic and hydrogen bonding, leading to high

to evaluate analyte loss to the membrane. Different

analyte binding and low recoveries. As many APIs are either

membrane filters can bind analyte to varying degrees

acidic or basic, they can prove difficult to quantify. Analyte

depending on membrane type, the nature of the analyte

recovery with nylon syringe filters can be improved by

and analyte concentration.

simply saturating the filter with sample, so the contained

The easiest method for determining analyte binding

analytes will effectively block the filter's binding sites.

to a syringe filter is to filter the sample through it and

These assumptions on binding propensity were confirmed in

subsequently collect and analyze various filtrate fractions.

a study of ours on analyte binding from a multi-component

Comparing these samples with a centrifuged sample

migraine formulation using three different Millex® syringe

that represents 100% recovery will provide information

filters with hydrophilic PTFE, hydrophilic PVDF, or nylon

on analyte binding as well as the volume required for

membranes. The formulation contained an acidic (acetyl

filter saturation.

salicylic acid), neutral (caffeine), and basic (acetaminophen)

Physico-chemical properties
of analytes and membranes

API. While acetyl salicylic acid and acetaminophen did not

Analyte binding mostly depends on the physico-

filters containing a nylon membrane. However, quantitative

chemical properties of the membrane and the analyte.

recovery was obtained when the nylon filters were saturated

These

properties

can

lead

to

bind to the hydrophilic PTFE or PVDF membranes due to
their lack of binding sites, both APIs did bind strongly to the

various

secondary

interactions between analyte and membrane, such
as electrostatic interactions, hydrogen bonding, and
hydrophobic interactions.
Nylon, hydrophilic PTFE and hydrophilic PVDF are materials

82 |

with analyte by filtering a higher volume of sample to block
the limited number of binding sites on the membrane.
As expected, neutral caffeine did not bind to any of the
three membranes.

commonly used for membrane filters in pharmaceutical

The effects of analyte concentration

QC testing. Hydrophilic PTFE and PVDF membranes have

The analyte saturation point of a filter also depends on

very few functional groups that can interact with analytes,

its concentration. Generally, as the analyte concentration

| September/October 2020

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APR September/October 2020

Table of Contents for the Digital Edition of APR September/October 2020
