Feldhaus D, Menzel C, Berkenkamp S, Hillenkamp F, Dreisewerd K
Institute of Medical Physics and Biophysics, University of Münster, Robert-Koch-Str. 31, D-48149 Münster, Germany.
J Mass Spectrom. 2000 Nov;35(11):1320-8. doi: 10.1002/1096-9888(200011)35:11<1320::AID-JMS66>3.0.CO;2-S.
The dependence of the signal intensity of analyte and matrix ions on laser fluence was investigated for infrared matrix-assisted laser desorption/ionization (IR-MALDI) mass spectrometry using a flat-top laser beam profile. The beam of an Er : YAG laser (wavelength, 2.94 microm; pulse width, 90 ns) was coupled into a sapphire fiber and the homogeneously illuminated end surface of the fiber imaged on to the sample by a telescope. Three different laser spot sizes of 175, 350 and 700 microm diameter were realized. Threshold fluences of common IR matrices were determined to range from about 1000 to a few thousand J m(-2), depending on the matrix and the size of the irradiated area. In the MALDI-typical fluence range, above the detection threshold ion signals increase strongly with fluence for all matrices, with a dependence similar to that for UV-MALDI. Despite the strongly different absorption coefficients of the tested matrices, varying by more than an order of magnitude at the excitation laser wavelength, threshold fluences for equal spot sizes were found to be comparable within a factor of two. With the additional dependence of fluence on spot size, the deposited energy per volume of matrix at threshold fluence ranged from about 1 kJ mol(-1) for succinic acid to about 100 kJ mol(-1) for glycerol.
利用平顶激光束轮廓，对红外基质辅助激光解吸/电离（IR-MALDI）质谱中分析物和基质离子的信号强度对激光能量密度的依赖性进行了研究。将铒钇铝石榴石激光（波长2.94微米；脉冲宽度90纳秒）的光束耦合到蓝宝石光纤中，并用望远镜将光纤均匀照明的端面成像到样品上。实现了三种不同的激光光斑尺寸，直径分别为175、350和700微米。常见红外基质的阈值能量密度测定范围约为1000至数千焦耳每平方米，具体取决于基质和照射区域的大小。在MALDI典型的能量密度范围内，高于检测阈值时，所有基质的离子信号都随能量密度强烈增加，其依赖性与紫外MALDI相似。尽管所测试的基质吸收系数差异很大，在激发激光波长下相差一个多数量级，但对于相同的光斑尺寸，发现阈值能量密度在两倍的因子范围内具有可比性。考虑到能量密度对光斑尺寸的额外依赖性，在阈值能量密度下，每体积基质沉积的能量范围从琥珀酸的约1千焦每摩尔到甘油的约100千焦每摩尔。
