Polymer division banner Polymer division home page Research areas link Research projects link Research facilities link Staff contact link Search link NIST link Polymer division home page Materials Science and Engineering Laboratory
Polymers Main Page > Research Highlights > Detail
   

Technical Highlights
 

MALDI-MS Interlaboratory Comparison of Polymer Mixtures
Polymer synthesis often results in impure products, containing multiple end groups, due to the synthesis process itself or the presence of impurities. Identification and quantification of these end groups and impurities is important because they influence polymer properties. An interlaboratory comparison of a mixture of two polystyrenes, one terminated with a proton, and the other terminated with a hydroxyl group, was performed using matrix-assisted laser desorption-ionization mass spectrometry (MALDI-MS). This interlaboratory comparison will be used to quantify the accuracy and repeatability of MALDI-MS in the identification of multiple end groups as a result of polymer synthesis.
 
Charles M. Guttman and William E. Wallace
 
A MALDI- MS interlaboratory comparison of mixtures of synthetic polymers of the same repeat unit and with closely matching molecular masses but with different end groups was sponsored by NIST. This interlaboratory comparison was designed to determine how well a group of laboratories engaged in MALDI-MS analyses of synthetic polymers agree on the percent composition of each polymer in the mixture. In an earlier interlaboratory comparison, NIST studied the molecular mass distribution (MMD) and the moments obtained by MALDI on polystyrene (PS) and compared the results with those obtained by classical methods. Interlaboratory comparisons enable us to gauge the needs of the community for standard methods and materials, to begin a dialog with the community on these needs, and to steer our own research. The interlaboratory comparison on mixtures of two polystyrenes with different end groups is of particular interest to industry where understanding and controlling the chemistry of reactive pre-polymers is a major concern.
 
Five gravimetric mixtures of two polystyrenes with matched molecular masses but with different end groups, -H and -OH, were created. The challenge for the participants was, without knowing the gravimetric values, to determine the mass fraction of each polymer in each mixture. Difficulties to overcome 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 laboratories.
 
The PS syntheses were both initiated with butyl initiator. One was terminated with H (PSH) and the other was terminated with CH2CH2OH (PSOH). These end groups are separated by 44 u and 60 u in the mass spectrum and are well resolved (Fig. 1). Five mixtures of these were made up with mass ratios between approximately 95:5 to 10:90 PSOH: PSH. Nuclear magnetic resonance (NMR) and Fourier-Transform infrared (FTIR) spectroscopies conducted at NIST confirmed the gravimetric mass ratio. The existence of a small amount of PSH in the as-received PSOH material was evident in our MALDI-MS analysis. NMR gave between (4 to 9) % PSH polymer in the PSOH-material. This is close to the value obtained from the initial MALDI-MS analysis performed at NIST.
 
Figure 1: MALDI mass spectrum of polystyrene mixture.
Figure 1: MALDI mass spectrum of polystyrene mixture.
 
Each participating laboratory was asked to perform MALDI-MS on each of the five mixtures using two distinct protocols: one using all-trans retinoic acid with defined concentrations of matrix:analyte:salt and a second protocol defined by the user as their preferred method for analyzing polystyrene. Each laboratory was asked to report two repeats of each protocol for each mixture to check for intralaboratory variability. Each laboratory was asked to report the estimated ratio of the mass of PSH to the mass of PSOH in each mixture and return data sets listing mass versus the integrated peak area of the mass spectra.
 
The analysis of the data revealed a strong variation among laboratories. This was seen in our first interlaboratory comparison as well. The effect of instrument manufacturer on the data was not significant. But the difference between acquiring mass spectra in the linear and the reflectron mode of operation resulted in significant variation in the end group fractions. The analysis of variance showed no significant influence of the sample preparation protocol on the fraction of PSOH in each mixture.
 
The accuracy of the instrument calibration of each laboratory was assessed by calculation of end group mass. In our first interlaboratory comparison, calibration varied more widely than anticipated. We would expect the calibration of most time-of-flight mass spectrometers in this mass range to be off by less than 3 u. Some laboratories showed calibrations off by 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 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 %.
 
Figure 3: The fraction of PSOH determined by MALDI compared with the gravimetric data.
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 mid point of the range assigned by the NMR. It is seen from both Fig. 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.
 
By Mass
MALDI
MALDI
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
Table 1: The fraction of PSOH polymer by mass and determined by MALDI.
 
The key to "getting it right" in MALDI-MS, especially for Mix A and Mix E, is to optimize the instrument. This 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 ," in 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, D.L. Vanderhart, Analytical Chemistry 73(6), 1252-1262 (2001).
 
S.J. Wetzel, C.M. Guttman, 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.
 
 
 
 
 
 
 
 
 
NIST Material Science & Engineering Laboratory - Polymers Division