Talrose V L, Person M D, Whittal R M, Walls F C, Burlingame A L, Baldwin M A
Mass Spectrometry Facility, Department of Pharmaceutical Chemistry, University of California, San Francisco, CA 94143-0446, USA.
Rapid Commun Mass Spectrom. 1999;13(21):2191-8. doi: 10.1002/(SICI)1097-0231(19991115)13:21<2191::AID-RCM774>3.0.CO;2-I.
Although the ionization/desorption mechanisms in matrix-assisted laser desorption/ionization (MALDI) remain poorly understood, there is a clear difference between the energy absorption processes in the ultraviolet (UV) and infrared (IR) modes of operation. UV-MALDI demands an on-resonance electronic transition in the matrix compound, whereas results presented here support earlier work showing that a corresponding resonant vibrational transition is not a requirement for IR-MALDI. In fact, data from the present study suggest that significant absorption of radiant energy by a potential matrix impairs its performance, although this observation is at variance with some other reports. For example, sinapinic acid, with an IR absorption maximum close to the 2.94 micrometer wavelength of the Er-YAG laser, has been little used as an IR-MALDI matrix. By contrast, succinic acid, with much lower IR absorption and no history of use in UV-MALDI as it has no UV absorption at the wavelength of common UV lasers, has become widely recognized as a good general purpose matrix for IR-MALDI. Despite reports by others that glycerol is an effective matrix for IR-MALDI, we find that glycerol, which also absorbs strongly at 2.94 micrometer, is useful only if applied as a very thin film. Thus the cumulative evidence for the role of the matrix in IR-MALDI appears confusing and often contradictory. Water has been postulated to be a major contributor to the absorption of energy in IR-MALDI. Consistent with this, we find that samples dried from D(2)O, which does not absorb at 2.94 micrometer, give spectra of inferior quality compared with the same samples from H(2)O. Similarly, samples dried under vacuum, that probably contain less water than those dried in the open laboratory, give weaker and more erratic spectra. Another potential participant in energy absorption and energy transfer is the surface of the metal support, an alternative mechanism for IR-MALDI, for which some evidence is presented here.
尽管基质辅助激光解吸/电离（MALDI）中的电离/解吸机制仍未得到很好的理解，但在紫外（UV）和红外（IR）操作模式下的能量吸收过程存在明显差异。紫外-MALDI要求基质化合物发生共振电子跃迁，而此处给出的结果支持了早期的研究工作，即相应的共振振动跃迁并非红外-MALDI的必要条件。事实上，本研究的数据表明，潜在基质对辐射能的显著吸收会损害其性能，尽管这一观察结果与其他一些报告不一致。例如，芥子酸的红外吸收最大值接近铒钇铝石榴石激光的2.94微米波长，很少被用作红外-MALDI基质。相比之下，琥珀酸的红外吸收要低得多，且由于在常见紫外激光波长下没有紫外吸收，因此没有用于紫外-MALDI的历史，但已被广泛认为是一种很好的通用红外-MALDI基质。尽管其他人报告称甘油是一种有效的红外-MALDI基质，但我们发现，同样在2.94微米处有强烈吸收的甘油，只有在制成非常薄的薄膜时才有用。因此，关于基质在红外-MALDI中作用的累积证据显得令人困惑且常常相互矛盾。水被认为是红外-MALDI中能量吸收的主要贡献者。与此一致的是，我们发现，用重水（D₂O）干燥得到的样品，由于重水在2.94微米处不吸收，与用普通水（H₂O）干燥得到的相同样品相比，其光谱质量较差。同样，在真空下干燥的样品，其含水量可能比在开放实验室中干燥的样品少，给出的光谱较弱且更不稳定。另一个可能参与能量吸收和能量转移的是金属载体表面，这是红外-MALDI的另一种机制，本文给出了一些相关证据。
