Materials Science and Engineering Laboratory FY 2004 ... - NIST

over 40 u. Figure 2 shows that calibration was again
an important issue in this interlaboratory comparison.
The instrument calibration varied by as much as 20 u,
even though the end groups were known.
Figure 2: The average molecular mass of PSH end groups for
each laboratory. The red line represents the actual molecular
mass of the end groups.
Figure 3 shows a plot of the “Theoretical Mixture
of PSOH” polymer fraction assuming 7 % PSH polymer
in the PSOH material from the NMR data. The plot
is compared to MALDI-MS data from some of the
participating laboratories. The agreement is generally
good, but the data show a wide range of variability
among the laboratories. Deviation of the data from the
expected percentages was found widest at the extremes
of concentration of 95:5 and 10:90 PSOH: PSH, but
even at the 50:50 ratio the deviation was as high as 10 %.
Mixtures of PS with Different End Groups
Figure 3: The fraction of PSOH determined by MALDI compared
with the gravimetric data.
The agreement and standard deviation of data are
shown in Table 1. The fraction of PSOH polymer in
each mixture is given in the 2nd column. This column
assumes the PSOH-material is 7 % PSH polymer and
93 % PSOH polymer. Seven percent is the midpoint
of the range assigned by the NMR. It is seen from both
Figure 3 and Table 1 that the agreement between the
MALDI-MS and the gravimetric results for the fraction
of PSOH polymer in each mixture is good when judged
by the standard deviation of the MALDI results.
Technical Highlights
The key to “getting it right” in MALDI-MS, especially
for Mix A and Mix E, is to optimize the instrument. This
Table 1: The fraction of PSOH polymer by mass
and determined by MALDI
By MALDI MALDI
Mass Average Std Dev
Mix A 0.93 0.933 0.051
Mix B 0.716 0.746 0.086
Mix C 0.481 0.47 0.041
Mix D 0.284 0.259 0.051
Mix E 0.086 0.117 0.2
was seen in the higher variation in the fraction of PSOH
determined for Mix A and Mix E, where a minor component
may be lost in the noise. The result, which indicates
significant variation in the fraction of PSOH for instrument
mode, may be due to the resolution difference between
reflectron and linear mode. In order to better understand
the optimization of the MALDI-MS, we have begun
to study the influences of the instrument and sample
preparation parameters on the MALDI-MS signal.
We are using an orthogonal fractional factorial design
to study detector voltage, laser energy, delay time,
and ion optic voltages, as well as matrix and polymer
concentration. Once the effects of all parameters are
understood, optimization will be easier to obtain.
These results also recapitulate the need for an unbiased,
automated peak integration routine when the S/N is poor.
We will continue to refine our MassSpectator software as
a benchmark method to perform unbiased peak integration.
For More Information on this Topic
S.J. Wetzel, K.M. Flynn, B.M. Fanconi,
D.L. VanderHart (Polymers Division, NIST);
S. Leigh (Statistical Engineering Division, NIST)
C.M. Guttman, S.J. Wetzel, K.M. Flynn,
B.J. Fanconi, W.E. Wallace, and D.L. VanderHart,
“International Interlaboratory Comparison of Mixtures
of Polystyrenes with Different End Groups,”
Proceedings of the 52nd ASMS Conference on
Mass Spectrometry and Allied Topics, Nashville, TN.
C.M. Guttman, S.J. Wetzel, W.R. Blair,
B.M. Fanconi, J.E. Girard, R.J. Goldschmidt,
W.E. Wallace, and D.L. Vanderhart, Analytical
Chemistry 73(6), 1252–1262 (2001).
S.J. Wetzel, C.M. Guttman, and K.M. Flynn, “The
Influence of Matrix and Laser Power on the Molecular
Mass Distribution of Synthetic Polymers obtained by
MALDI,” Int. J. Mass Spectrometry, in press.
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