POLARIZABILITY AND INDUCTIVE EFFECT CONTRIBUTIONS TO SOLVENT-CATION BINDING OBSERVED IN ELECTROSPRAY IONIZATION MASS SPECTROMETRY

Authors: Striegel, Andre; PIOTROWIAK, PIOTR - UNIV. OF NEW ORLEANS; COLE, RICHARD - UNIV. OF NEW ORLEANS; Boue, Stephen

Submitted to: Journal of American Society for Mass Spectrometry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/19/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: For field crops such as cotton, determining which type of cotton is appro- priate to plant in certain geographical areas is crucial. One would not wish to use a cotton variety developed at mild latitudes with a humid climate in an area where the plant would be subjected to severe heat and water stress. Once the appropriate plant type has been selected or developed, equally important is the optimizing of other factors, such as harvest time. The quality of cotton bolls, etc., is jeopardized by picking them too early or late after the plant flowers. In order to determine the variables mentioned above, a number of methods have been developed which measure different properties of cotton, corn, etc. One method involves measuring the molecular weight of principal components of these crops. To do so, one must usually dissolve cotton, starch, etc. This is not always an easy task. One of the effective solvents for this type of work has been nin use for more than twenty years, although exactly how it works is still not fully understood. It is a mixed solvent system, consisting of both a solvent and a salt. The work presented systematically investigates the influence that each half of the solvent-salt pair exerts on the other half. This directly relates to previous work done by our group showing how this solvent interacts with molecules which serve as models for the principal component of cotton, i.e., cellulose. A better understanding of how the dissolution of cellulose, starch, etc., is effected, would allow for more accurate molecular weight determinations. This would help in developing a clearer picture to follow when choosing appropriate field conditions for crops and also to those who manufacture cotton and starch products. All who use products from these plants will benefit from this research.

Technical Abstract: Electrospray ionization (ESI) mass spectrometry has been used in conjunction with computer modeling to investigate binding tendencies of alkali metal cations to low molecular weight solvents. Intensities of peaks in ESI mass spectra corresponding to solvated alkali cations were found to decrease with increasing ionic radii of the alkali cations (Li>Na>K approx Cs) in either dimethyl acetamide (DMAc) or dimethy formamide (DMF). When LiCl was added to an equimolar mixture of DMF, DMAc, and dimethyl propionamide (DMP), the intensities of gas-phase [Solvent+Li]+ peaks observed in ESI mass spectra decreased in the order DMP>DMAc>>DMF. This ranking correlates with the relative strength of the inductive effect exhibited by the alkyl group adjacent to the carbonyl function on each solvent. Electron donation thus appears to play a role in the stabilization of the lithium cation attached to the carbonyl oxygen. This proposition is supported by computer modeling studies of electrostatic potentials. The modest changes in the electrostatic potentials, however, appear to be insufficient to fully account for the mass spectrometric data. Contributions from the increasing polarizability and the augmented ability to dissipate thermal energy with increasing size of the solvent molecule are postulated to act in conjunction with the inductive effect to more completely explain the ESI mass spectra. These experiments suggest a combined role of the inductive effect, polarizability, and solvent molecular size in determining relative intensities of solvated cation peaks in ESI mass spectra of equimolar mixtures of homologous solvents.