Standard-Free Quantitation of Mixtures Using Clusters Formed by Electrospray Mass Spectrometry

Abstract

Ion abundances in electrospray ionization mass spectra depend on many factors, including molecular hydrophobicity, basicity, solution composition, and instrumental parameters. A recently introduced method that uses nonspecific cluster ion abundances to obtain solution-phase molar fractions of analytes directly from ESI mass spectra without using standards was evaluated using solutions containing 0.03% to 24% L-threonine, D-threonine, L-leucine, L-lysine, L-glutamic acid or diglycine with L-serine as a major component. Because of the propensity of serine clusters to exhibit “magic” numbers, which can be chirally selective, these experiments provide a rigorous test of this standard-free cluster quantitation method, which requires that clusters form statistically from analytes in solution. For each of these solutions, the compositions of clusters containing ≥ 32 molecules reflect the solution molar fractions of each component. From the abundances of these larger clusters, the solution molar fraction can be determined to better than 10% accuracy over nearly three orders of magnitude in concentration. In contrast, the ionization/detection efficiency of the individual amino acids differs by as much as a factor of 460 in these experiments. The protonated octamer incorporates some molecules statistically but efficiently excludes other molecules that have significantly different properties or chirality. This standard-free quantitation method may be most advantageous for rapidly characterizing mixtures, such as products of chemical synthesis, which contain unknown products or molecules for which suitable standards are not readily available.

Simultaneous Quantitation of Amino Acid Mixtures using Clustering Agents

“…Even though specific cluster formation occurs, the sum total composition analysis over a wide range of cluster sizes more accurately reflects solution molar fraction than data for an individual cluster size. This suggests that cluster formation may occur stoichiometrically, if not statistically, at each cluster size, which is consistent with these clusters being predominately formed by a charged residue mechanism, as has been reported previously [46, 50]. Although this method is clearly not as accurate as techniques using traditional standards, this method offers a significantly more reliable indicator of solution composition than the abundances of individual protonated molecules and provides rapid, albeit rough, quantitative information even from relatively complex mixtures.…”

Fragmentation of the tryptophan cluster [Trp9–2H]2− induced by different activation methods

“…Non‐covalent amino acid clusters have been the subject of intense scrutiny over the past two decades. Diverse areas of interest have included: (i) the use of fragmentation of proton‐bound dimers to determine the gas‐phase proton affinities1 and gas‐phase acidities2–4 of amino acids via Cooks's kinetic method:5–7 (ii) establishing whether amino acids can exist in their zwitterionic form within clusters;8–10 (iii) the search for magic numbers, which has most notably revealed the homochiral serine octamer;11–13 (iv) the use of silver amino acid cluster precursors to assembly silver cluster and silver‐hybrid cluster cations in the gas phase for subsequent reactivity14, 15 and spectroscopic studies;16, 17 (v) peptide bond formation within amino acid clusters;18 (vi) analytical applications aimed at establishing the chirality of substrates using metal clusters of amino acids;19, 20 and using large non‐specific clusters of known concentrations of an amino acid with an analyte to provide ‘standard‐free quantitation’ of the analyte;21, 22 (vii) formation and ionisation of neutral, non‐covalent amino acid complexes23 including electron attachment to clusters embedded in helium nanodroplets;24, 25 and (viii) studies aimed at examining the formation26–31 and fragmentation reactions of multiply charged clusters using combinations of electrospray ionization (ESI) or its variants together with collision‐induced dissociation (CID)30 and electron‐induced dissociation (EID) 31…”

Quantitative analysis of biogenic polyamines in distilled drinks by direct electrospray ionization tandem mass spectrometry using a nanocontainer

“…However, if the host‐guest binding is selective and supramolecular structures survive the electrospray ionization (ESI) process, leading to stable gas‐phase complexes, the resulting ESI‐MS or ESI‐MS/MS signals can be used for analytical purposes. This concept has been applied to direct injection (infusion) analysis of small molecules using metal phthalocyanins, low molecular weight explosives and carboxylic acids using cyclodextrins, and drugs by following the formation of dimers and clusters …”