2,4,6-Trihydroxyacetophenone on zeolite surface: Correlation between electronic relaxation and fragmentation on mass spectra

Quantitative analysis of bisphenol A eluted into canned tomato and mackerel by matrix-assisted laser desorption ionization mass spectrometry using anthracene as matrix

Quantitative analysis of bisphenol A (BPA) was performed by matrix-assisted laser desorption ionization mass spectrometry. It was found that BPA was ionized as deprotonated species when anthracene was used as the matrix. A peak of deprotonated BPA and a peak assignable to epoxy resin were observed on analysis of liquids in canned tomato and mackerel samples. In addition, many identical peaks were observed from the liquids in both cans, indicating that epoxy resin was degraded and BPA was eluted into the canned tomato and mackerel during the storage period. It was suggested that the mackerel heat-treatment process and the acidity of tomato were responsible for the elution of BPA. Using bisphenol B (BPB) as the internal standard, the concentrations of BPA were determined to be 0.55 ± 0.05 and 1.72 ± 0.13 ng/µL (µg/mL) for the canned tomato and mackerel samples, respectively. These canned products were imported goods, and their BPA levels exceeded the safe concentration recommended by The Can Manufacturers Institute of Japan. The results indicate that consumers should exercise caution when consuming canned products particularly those manufactured overseas, which have different safety standards.

Ammonium Ion Exchanged Zeolite for Laser Desorption/Ionization Mass Spectrometry of Phosphorylated Peptides

α-Cyano-4-hydroxycinnamic acid (CHCA), an organic matrix molecule for matrix-assisted laser desorption/ionization mass spectrometry, was adsorbed to NH4 (+)-type zeolite surface, and this new matrix was used for the detection of low-molecular-weight compounds. It was found that this matrix could simplify the mass spectrum in the low-molecular-weight region and prevent interference from fragments and alkali metal ion adducted species. CHCA adsorbed to NH4 (+)-type ZSM5 zeolite (CHCA/NH4ZSM5) was used to measure atropine and aconitine, two toxic alkaloids in plants. In addition, CHCA/NH4ZSM5 enabled us to detect phosphorylated peptides; peaks of the protonated peptides had higher intensities than the peaks observed using CHCA only.

Contribution of thermal energy to initial ion production in matrix-assisted laser desorption/ionization observed with 2,4,6-trihydroxyacetophenone

RATIONALE

Although several reaction models have been proposed in the literature to explain matrix-assisted laser desorption/ionization (MALDI), further study is still necessary to explore the important ionization pathways that occur under the high-temperature environment of MALDI. 2,4,6-Trihydroxyacetophenone (THAP) is an ideal compound for evaluating the contribution of thermal energy to an initial reaction with minimum side reactions.

METHODS

Desorbed neutral THAP and ions were measured using a crossed-molecular beam machine and commercial MALDI-TOF instrument, respectively. A quantitative model incorporating an Arrhenius-type desorption rate derived from transition state theory was proposed. Reaction enthalpy was calculated using GAUSSIAN 03 software with dielectric effect. Additional evidence of thermal-induced proton disproportionation was given by the indirect ionization of THAP embedded in excess fullerene molecules excited by a 450 nm laser.

RESULTS

The quantitative model predicted that proton disproportionation of THAP would be achieved by thermal energy converted from a commonly used single UV laser photon. The dielectric effect reduced the reaction Gibbs free energy considerably even when the dielectric constant was reduced under high-temperature MALDI conditions. With minimum fitting parameters, observations of pure THAP and THAP mixed with fullerene both agreed with predictions.

CONCLUSIONS

Proton disproportionation of solid THAP was energetically favorable with a single UV laser photon. The quantitative model revealed an important initial ionization pathway induced by the abrupt heating of matrix crystals. In the matrix crystals, the dielectric effect reduced reaction Gibbs free energy under typical MALDI conditions. The result suggested that thermal energy plays an important role in the initial ionization reaction of THAP.