CE/Electrospray Ionization-MS Analysis of Underivatized d/l-Amino Acids and Several Small Neurotransmitters at Attomole Levels through the Use of 18-Crown-6-tetracarboxylic Acid as a Complexation Reagent/Background Electrolyte

Abstract

A new capillary electrophoresis/mass spectrometry technique is introduced for attomole detection of primary amines (including several neurotransmitters), amino acids, and their d/l enantiomers in one run through the use of a complexation reagent while using only approximately 1 nL of sample. The technique uses underivatized amino acids in conjunction with an underivatized capillary, which significantly reduces cost and analysis time. It was found that when (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (18-C-6-TCA, MW 440) was used as the background electrolyte/complexation reagent during the capillary electrophoresis/electrospray ionization-mass spectrometry (CE/ESI-MS) analysis of underivatized amino acids, stable complexes were formed between the amino acids and the 18-C-6-TCA molecules. These complexes, which exhibited high ionization efficiencies, were detectable at attomole levels for most amino acids. The detection limits of the AA/18-C-6-TCA complexes were on the average more than 2 orders of magnitude lower than that of the free amino acids in solution. In addition to lower detection limits under CE/ESI-MS, a solution of 18-C-6-TCA in the concentration range of 5-30 mM provided high separation efficiency for mixtures of l-amino acids as well as mixtures of d/l-amino acids. By using a solution of 18-C-6-TCA as the background electrolyte in conjunction with an underivatized, 130-cm-long, 20-microm-i.d., 150-microm-o.d. fused-silica capillary and by monitoring the m/z range of the amino acid/18-C-6-TCA complexes (m/z 515-700), most of the standard amino acids and many of their enantiomers were separated and detected with high separation efficiency and high sensitivity (nanomolar concentration detection limits) in one run. The solutions of 18-C-6-TCA also worked well as the CE/ESI-MS BGE for low-level detection of several neurotransmitters and some of their d/l enantiomers as well as for the analysis of amino acids at endogenous levels in lysed red blood cells.

Sensitive chiral analysis by capillary electrophoresis

“…Then, the ionization of the analytes, normally by ESI, is performed before the MS detection (usually by quadrupole or IT). Although in some works a low concentration of chiral selector is directly introduced in the MS obtaining satisfactory results [135], in most works the introduction of the chiral selector in the MS detector is avoided or chiral selectors compatible with MS detection, such us crown ethers [136,137] are employed. In fact, the introduction of nonvolatile chiral selectors in MS can affect the sensitivity and stability of this detector [138].…”

Capillary electrophoresis‐mass spectrometry: Recent advances to the analysis of small achiral and chiral solutes

“…Apart from one report on the use of neutral cyclodextrins (CDs) as chiral selectors in CZE-MS [72], most of the researchers for CE-MS focused on the analysis of charged chiral selectors, such as sulfated CDs [73][74][75] and crown ethers [76,77]. In addition, one report has appeared on the use of sugar derivatives as chiral selectors in nonaqueous EKC-MS [78].…”

Easily fabricated ion source for characterizing mixtures of organic compounds by direct analysis in real time mass spectrometry

“…18-Crown-6 ether is a proven host for primary amines, [83][84][85] with three of the oxygen atoms forming strong hydrogen bonds to a protonated amine group, which would stabilize the charge and increase the ease of detection with mass spectrometry. Additionally this interaction is known to be much stronger in the gas phase compared to the solution phase, making it an ideal host to test RADI.…”