Martens J, Ens W, Standing K G, Verentchikov A
Department of Physics, University of Manitoba, Winnipeg, Canada.
Rapid Commun Mass Spectrom. 1992 Feb;6(2):147-57. doi: 10.1002/rcm.1290060215.
Heavy molecular ions with energies in the range 10-20 keV and masses from 276 u to 132,000 u, produced by matrix-assisted laser desorption, were used as primary projectiles to produce secondary-ion spectra from a variety of surfaces in a tandem time-of-flight mass spectrometer. In the negative mode the ratio of electron emission to secondary-ion emission was found to decrease rapidly with increasing projectile mass. Ion emission was found to dominate for primary ions larger than approximately 10,000 u. Positive or negative molecular ions and cations were observed from several organic targets of masses up to 1140 u (gramicidin S) for incident projectiles up to mass 132,000 u, i.e., for projectile speeds down to approximately 7000 m/s. Other ions characteristic of the target were also observed for these projectiles. Thus, large polyatomic ions can cause secondary-ion desorption even at very low velocity. The background ions of both polarities are similar to those found in keV particle bombardment by monatomic projectiles. The same ions are observed for all the projectiles; most can be identified with hydrocarbon background. The relative intensities of the background positive ions are largely independent of projectile, and for both polarities the ratio of the ions characterizing the target to those forming the background is approximately constant for all the projectiles. These results strongly suggest that the background ions come from the usual layer of organic impurities attached to the target surface. No direct evidence for surface-induced dissociation was observed in this mass and energy range.
通过基质辅助激光解吸产生的能量在10 - 20 keV范围内、质量从276 u到132,000 u的重分子离子，被用作初级射弹，在串联飞行时间质谱仪中从各种表面产生二次离子光谱。在负模式下，发现电子发射与二次离子发射的比率随着射弹质量的增加而迅速降低。对于大于约10,000 u的初级离子，发现离子发射占主导。对于质量高达132,000 u的入射射弹，即对于低至约7000 m/s的射弹速度，从质量高达1140 u（短杆菌肽S）的几个有机靶标中观察到正或负分子离子和阳离子。对于这些射弹，还观察到了靶标的其他特征离子。因此，即使在非常低的速度下，大的多原子离子也能引起二次离子解吸。两种极性的背景离子与在keV单原子射弹轰击中发现的离子相似。对于所有射弹都观察到相同的离子；大多数可以确定为烃类背景。背景正离子的相对强度在很大程度上与射弹无关，并且对于两种极性，表征靶标的离子与形成背景的离子的比率对于所有射弹大致恒定。这些结果强烈表明背景离子来自附着在靶标表面的通常的有机杂质层。在这个质量和能量范围内没有观察到表面诱导解离的直接证据。
