International Interlaboratory Comparison of Mixtures of
Polystyrenes with Different End Groups
Introduction
We conduct interlaboratory comparisons as measures of the
current practice in a specific area of polymer mass spectrometry.
This enables us to gauge the needs of the community for standard
methods and materials, to begin a dialog on these needs, and
to steer our own research. This interlaboratory comparison,
on mixtures of of two polystyrenes with different end groups,
is of particular interest to industry where understanding and
controlling the chemistry of reactive pre-polymers in a major
concern.
Approach
Five gravimetric mixtures of two polystyrenes with matched
molecular mass distributions but with different end groups,
-H and –OH, were created. The test for the participants
was, without knowing the gravimetric values, to determine the
mass fraction of each polymer in each mixture. Challenges include
different ionization probabilities of the two polymers, and
finding small peaks in background noise for species that are
in the minority in the mixture. Fourteen labs from 5 countries
participated: 6 industrial, 4 academic, and 4 government.
Results
Individual labs’ agreements with gravimetric result
could be quite poor
Collectively agreement is better due to cancellation of uncertainties
Systematic uncertainty for 50:50 mixtures a surprise
Raw data (not shown) showed wide miscalibrations of the mass
axis, some on the order of 20 u when <1 u is expected
Lessons Learned
Statistically significant variation among laboratories even
for this “easy” mixture.
Errors in mass calibration an important issue. This was unanticipated
Optimization of instrument for signal to noise ratio the most
important factor in “getting it right”
Recapitulates the need for an unbiased, automated peak integration
routine when the S/N is poor
Actions Taken
We will strengthen our effort to develop protocols to help
practitioners optimize instrument parameters
We will promote MassSpectator as a benchmark method to perform
peak integration
Contributors:
Charles M. Guttman, Stephanie J. Wetzel, Kathleen M. Flynn,
David L. Vanderhart, Bruno M. Fanconi, William E. Wallace
Characterization and Measurement Group
Polymers Division
Materials Science and Engineering Laboratory
