Reagent assessment for detection of ammonium ion-molecule complexes

Ammonia (NH3) is an important chemical target in sensor applications such as trace explosives detection of ammonium nitrate (NH4NO3) and environmental monitoring. Ion-molecule reagent chemistries show potential to increase sensitivity in detection systems relying on atmospheric pressure ionization (API) of reagent-ammonium (M + NH4(+)) complexes. Gas-phase reagent selection assessment is based on mass spectrometric (MS) determination of binding constants relative to competitive ions and critical energies for ion-molecule complex dissociation.

Eight ammonium complexation reagents were identified and gas-phase ion-molecule interactions were studied using electrospray ionization. Binding constants were determined, in Log(K), using the competition method for one host molecule with three guests (NH4(+), Na(+), and K(+)) in single quadrupole MS. Critical energy determination was based on calibration of threshold activation voltage using collision-induced dissociation (CID) tandem mass spectrometry (MS/MS).

This assessment informs selective binding affinity and intrinsic ion-molecule critical energy for dissociation. Relative NH4(+) binding affinity was highest for sucrose and 4-tert-butylcalix[6]arene, while 4-tert-butylcalix[6]arene and methyl acetoacetate showed the highest preferential binding of NH4(+) versus Na(+) and K(+). The intrinsic critical energy for NH4(+) binding was highest for crown ethers, tetraglyme and methyl acetoacetate.

An MS-based framework was developed to quantitatively assess API ion-molecule reagent chemistries based on ammonium selectivity versus competing ions, and intrinsic ammonium binding strength and complex survivability for detection. Methyl acetoacetate is an attractive ammonium reagent for vapor-phase API techniques given its high vapor pressure, preferential selectivity, and high critical energy for dissociation.