Chirally sensitive collision induced dissociation of proton-bound diastereomeric complexes of tryptophan and 2-butanol

On the separation of enantiomers by drift tube ion mobility spectrometry

Racemic mixtures of twelve common α-amino acids and three chiral drugs were tested for the separation of their enantiomers by drift tube ion mobility spectrometry (IMS)-quadrupole mass spectrometry (QMS) by introducing chiral selectors into the buffer gas of the IMS instrument. (R)-α-(Trifluoromethyl)benzyl alcohol, (L)-ethyl lactate, methyl (S)-2-chloropropionate, and the R and S enantiomers of 2-butanol and 1-phenyl ethanol were evaluated as chiral selectors. Experimental conditions were varied during the tests, including buffer gas temperature, concentration and type of chiral selectors, analyte concentration, electrospray (ESI) voltage, ESI solvent pH, and buffer gas flow rate. The individual enantiomers yielded the same drift times and the racemic mixtures could not be separated. Energy calculations of the chiral selector-ion interactions showed that these separations are unlikely using 2-butanol as a chiral selector but they might be theoretically feasible depending on the chiral selector nature and the type of enantiomers. Several plausible explanations for not achieving separations were analyzed. Recommendations for potential enantiomer separations by IMS are proposed.

Buffer gas additives (modifiers/shift reagents) in ion mobility spectrometry: Applications, predictions of mobility shifts, and influence of interaction energy and structure

Ion mobility spectrometry (IMS) is an analytical technique used for fast and sensitive detection of illegal substances in customs and airports, diagnosis of diseases through detection of metabolites in breath, fundamental studies in physics and chemistry, space exploration, and many more applications. Ion mobility spectrometry separates ions in the gas-phase drifting under an electric field according to their size to charge ratio. Ion mobility spectrometry disadvantages are false positives that delay transportation, compromise patient's health and other negative issues when IMS is used for detection. To prevent false positives, IMS measures the ion mobilities in 2 different conditions, in pure buffer gas or when shift reagents (SRs) are introduced in this gas, providing 2 different characteristic properties of the ion and increasing the chances of right identification. Mobility shifts with the introduction of SRs in the buffer gas are due to clustering of analyte ions with SRs. Effective SRs are polar volatile compounds with free electron pairs with a tendency to form clusters with the analyte ion. Formation of clusters is favored by formation of stable analyte ion-SR hydrogen bonds, high analytes' proton affinity, and low steric hindrance in the ion charge while stabilization of ion charge by resonance may disfavor it. Inductive effects and the number of adduction sites also affect cluster formation. The prediction of IMS separations of overlapping peaks is important because it simplifies a trial and error procedure. Doping experiments to simplify IMS spectra by changing the ion-analyte reactions forming the so-called alternative reactant ions are not considered in this review and techniques other than drift tube IMS are marginally covered.

Low-Energy Collisions of Protonated Enantiopure Amino Acids with Chiral Target Gases

Here we report on the gas-phase interactions between protonated enantiopure amino acids (L- and D-enantiomers of Met, Phe, and Trp) and chiral target gases [(R)- and (S)-2-butanol, and (S)-1-phenylethanol] in 0.1-10.0 eV low-energy collisions. Two major processes are seen to occur over this collision energy regime, collision-induced dissociation and ion-molecule complex formation. Both processes were found to be independent of the stereo-chemical composition of the interacting ions and targets. These data shed light on the currently debated mechanisms of gas-phase chiral selectivity by demonstrating the inapplicability of the three-point model to these interactions, at least under single collision conditions. Graphical Abstract.