ARTIFACTS IN FOURIER TRANSFORM MASS SPECTROMETRY

该子项目是利用 由NIH/NCRR资助的中心赠款提供的资源。子项目和 研究者（PI）可能从另一个NIH来源获得主要资金， 因此可以在其他CRISP条目中表示。列出的机构是 中心，不一定是研究者的机构。 最近的工作（Mathur等人，2008）对傅里叶变换离子回旋共振质谱（FT-ICR MS）的新的、更高灵敏度的质谱设计进行了研究，揭示了许多不包含有用化学信息的伪峰（光谱特征）。为了确定这些伪影的原因并消除它们，将这些严重失真的光谱与类似失真的信号模型进行比较（Mathur等人，2009年）。 选择C60作为目标化合物，其质谱使用定制的7 T MALDI FT-ICR质谱仪获得。使用MATLAB脚本生成模拟光谱。使用三个无阻尼正弦函数来模拟C60的前三个同位素。结果表明，放大器的增益或偏置不平衡会导致模数转换器（ADC）过载，从而导致质谱中出现谐波。这一观察结果通过模拟得到证实，其中数字瞬态的一侧的限幅导致模拟频谱中出现多个谐波峰值。由于欠采样（混叠）导致的折叠峰对于高阶谐波峰也很明显。进一步表明，ADC的严重过载（即使放大器完全平衡，没有DC偏移）也可能导致离子的同位素分布失真，这将使质谱的自动解释变得困难。最后，ICR信号上的射频干扰（RFI）噪声导致ADC的不平衡和/或过载，也可能引起同位素峰值和RFI噪声之间的互调，从而导致不期望的伪影。因此，由此确定了若干常见信号处理伪影的源，并将其与放大器和/或数字化仪的RFI噪声和饱和度相关联。在这样的条件下，快速傅里叶变换（FFT）产生对应于真实的信号峰值和RFI频率的谐波和混合频率的频谱伪像峰值。虽然这项研究是使用FT-ICR MS数据完成的，但重要的是要强调这些伪影是数字化和FFT过程所固有的，因此与任何基于FT的MS仪器相关，包括轨道阱和FT离子阱。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Recent work (Mathur et al., 2008) on a new, higher sensitivity preamplifier design for Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed a number of artifact peaks (spectral features) which do not contain useful chemical information. In order to determine the cause of these artifacts and eliminate them, these severely distorted spectra were compared with similarly distorted signal models (Mathur et al., 2009). C60 was chosen as the compound of interest, whose mass spectra were obtained using the custom built 7T MALDI FT-ICR mass spectrometer. Simulated spectra were generated using a MATLAB script. Three undamped sinusoidal functions were used to model the first three isotopes of C60. It was found that imbalance in the gain or bias of the amplifier could lead to overloading of the analogue to digital converter (ADC) which resulted in appearance of harmonics in the mass spectra. This observation was confirmed by simulation, where clipping of one side of the digital transient led to appearance of multiple harmonics peaks in the simulated spectra. Fold-over peaks due to under-sampling (aliasing) were also apparent for higher order harmonics peaks. It was further shown that severe overloading of the ADC (even when the amplifier is perfectly balanced with no DC offsets) could also cause distortion in the isotopic distribution of ions, which would make the automatic interpretation of the mass spectra difficult. Finally, radio-frequency interference (RFI) noise riding on the ICR signal causing imbalance and/or overloading of the ADC may also induce intermodulations between isotopic peaks and the RFI noise, leading to undesirable artifacts. Thus, the source of several common signal processing artifacts was thereby determined and correlated to RFI noise and saturation of the amplifier and/or the digitizer. Under such conditions, the fast Fourier transform (FFT) generates spectral artifact peaks corresponding to harmonics and mixing frequencies of the real signal peaks and RFI frequencies. While this study was done using the FT-ICR MS data, it is important to stress that these artifacts are inherent to the digitization and FFT process and thus are relevant to any FT-based MS instrument, including the orbitrap and FT ion trap.