Suppose you’re doing an HPLC assay of a basic pharmaceutical
and find a 66% recovery for the API. What happened? Did the drug not extract
completely? Did degradation occur? These types of scenarios take up valuable
laboratory time and resources with investigations, troubleshooting, and re-run
analyses. In this case, it may be simply due to the autosampler glass. Regular
glass has silanols on the surface that can bind with basic analytes and
therefore lead to lower recovery. However, this problem can be prevented with some
simple considerations.
In this example using a basic test analyte (cetylpyiridinium chloride), we can see how
use of either an ammonium acetate buffer (red trace) or a formic acid additive
(blue trace) can reduce the effect of analyte loss compared to DI water alone
(green trace). Using 0.1% formic acid,
the silanols become protonated and neutral, which prevents ionic interactions
with the analyte from occurring. This is the more effective of the two
additives. Ammonium acetate helps the problem by a different approach. The
ammonium ion competes with the analyte for the silanols groups, and so fewer
sites are available for analyte loss to occur.
Here we use a different
strategy for reducing the analyte loss. Reduced Surface Activity (RSA™) glass
is made with almost no surface silanols and we can see a major improvement for
analyte recovery compared to regular glass. For best results, you can use a
combination of RSA glass vials and a formic acid diluent. For solubility
reasons though, sometimes you might have to use an ammonium acetate diluent. In
that case, RSA glass shows significantly better recovery than regular glass.
When analysts plan a sample preparation procedure,
they may put much thought into the extraction method, dilution procedures, and
so on. However, the role of the vial is often overlooked. Vials are inexpensive and disposable, so they can
be seen as relatively unimportant in the analysis process. This data shows how
they are not inert and can significantly skew your analytical results. A
careful selection of vial and diluent is all it takes to prevent your
laboratory from experiencing these kinds of problems.
