Faculty of Design, Kyushu University, 4-9-1, Shiobaru, Minami-ku, Fukuoka 815-8540, Japan.
Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.
Anal Chem. 2022 Oct 25;94(42):14691-14698. doi: 10.1021/acs.analchem.2c03077. Epub 2022 Oct 13.
Barbiturates are highly susceptible to dissociation in mass spectrometry (MS) because of their long side chains combined with a nonaromatic ring consisting of several carbonyl and amine groups. As a result, they exhibit extensive α-cleavage and subsequent rearrangement, making the identification of these compounds difficult. Although a library of electron ionization MS (EIMS) is available, most barbiturates have very similar fragment patterns. Accordingly, it would be desirable to develop a technique for soft ionization, providing a molecular ion and large fragment ions as well. In this study, a molecular ion was clearly observed, in addition to large fragment ions, for a variety of barbiturates based on multiphoton ionization MS (MPIMS) using a tunable ultraviolet femtosecond laser as the ionization source (fs-LIMS). This favorable result was achieved when the optimal laser wavelength for minimizing the excess energy remaining in the ionic state was used. An examination of the photofragmentation pathways suggested that an H atom in the side chain was abstracted by an oxygen atom in the carbonyl group in the ring structure thus initiating fragmentation and subsequent rearrangement. Barbiturates that are substituted with alkyl groups (amobarbital and pentobarbital) had narrower spectral regions for optimal ionization than the other barbiturates with alkyl and alkenyl groups (butalbital and secobarbital) and more with alkyl and phenyl groups (phenobarbital). All of the barbiturates studied provided unique mass spectral patterns in fs-LIMS, which was useful for the reliable identification of these compounds in practical trace analysis.
巴比妥酸盐在质谱（MS）中非常容易发生离解，因为它们的长侧链与由几个羰基和胺基组成的非芳香环相结合。结果，它们表现出广泛的α-裂解和随后的重排，使得这些化合物的鉴定变得困难。尽管有电子电离 MS（EIMS）的库，但大多数巴比妥酸盐的碎片模式非常相似。因此，理想的情况是开发一种软电离技术，既能提供分子离子，又能提供大的碎片离子。在这项研究中，基于使用可调谐紫外飞秒激光作为电离源的多光子电离 MS（MPIMS），除了大的碎片离子之外，还可以清楚地观察到各种巴比妥酸盐的分子离子。当使用优化的激光波长以最小化离子态中剩余的过量能量时，获得了这种有利的结果。对光解途径的研究表明，侧链中的 H 原子被环结构中的羰基中的氧原子夺取，从而引发了碎片和随后的重排。取代有烷基（阿洛巴比妥和戊巴比妥）的巴比妥酸盐比具有烷基和烯基（丁巴比妥和异戊巴比妥）的其他巴比妥酸盐具有更窄的最佳电离光谱区域，并且具有更多的烷基和苯基（苯巴比妥）。在 fs-LIMS 中，所有研究的巴比妥酸盐都提供了独特的质谱图谱，这对于实际痕量分析中这些化合物的可靠鉴定非常有用。
