Calibration for determining monomer ratios in copolymers by electrospray ionization mass spectrometry

Abstract

Monomer ions appear in electrospray mass spectra of poly(styrene sulfonate-co-maleic acid) as the cone voltage (CV) is changed from −20 to −100 V. For a 1:1 styrene sulfonate (SS): maleic acid (MA) copolymer at CV = −100 V, the SS:MA monomer ion intensity ratio r 1:1 ( i m/ z 183 / i m/ z 115 ) 1:1 is 18.50 ± 0.64, reflecting the relatively high acidity of SS (p K a 0.7, versus 1.92 for MA). When sampling a 3:1 SS:MA copolymer, r 3:1 is 84.64 ± 1.83. The ratio of these ratios ( r 3:1/ r 1:1) polymer is 4.57 ± 0.18, significantly higher than the value expected [(3:1)/(1:1) = 3] based on the relative (nominal) monomer ratios in the copolymers. This compromises the use of one polymer sample as a standard for assessing the monomer ratio in the other at this CV. Use of monomer mixtures as standards is also precluded at CV = −100 V due to extensive fragmentation when sampling free MA monomers. Use of a lower CV and consideration of fragments as well as the monomer ions leads to better performance. At CV = −40 V, total polymer ion intensity ratios ( Σ i m / z ⁡ 183 + i m / z ⁡ 119 + i m / z ⁡ 80 i m / z ⁡ 115 + i m / z ⁡ 71 ) are 42.60 ± 2.01 and 13.11 ± 0.87 for the 3:1 and 1:1 copolymers, respectively. The ratio of these ratios ( Σ 3:1/ Σ 1:1) polymer is 3.25 ± 0.29, in good agreement with the nominal composition ratio. Similar results were obtained at CV −40 to −90 V. Using monomer mixtures as calibrants at CV = −40 V also provides accurate quantitation of the copolymer monomer ratios, provided that the total intensity ratios are used. Comparison of monomer and polymer breakdown diagrams provides the insight needed to determine the appropriate CV for quantitation with either polymer or monomer standards. Similar results are obtained for a second ionic copolymer system (2-acrylamido-2-methylpropane sulfonic acid-co-acrylic acid). Once optimum conditions are achieved, monomer ratio determination can proceed very quickly, suggesting potential utility for routine quality monitoring.