ARTIFACTS IN FOURIER TRANSFORM MASS SPECTROMETRY

这个子项目是许多研究子项目中利用 资源由NIH/NCRR资助的中心拨款提供。子项目和 调查员(PI)可能从NIH的另一个来源获得了主要资金， 并因此可以在其他清晰的条目中表示。列出的机构是 该中心不一定是调查人员的机构。 最近(Mathur等人，2008年)为傅立叶变换离子回旋共振质谱仪(FT-ICR MS)设计了一种新的、灵敏度更高的前置放大器，发现了一些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了比较(Mathur等人，2009年)。 选择C60作为感兴趣的化合物，使用定制的7T MALDI FT-ICR质谱仪获得其质谱图。模拟光谱是使用MATLAB脚本生成的。用三个无阻尼正弦函数对C60的前三个同位素进行了模拟。研究发现，放大器增益或偏置的不平衡会导致模数转换器(ADC)的过载，从而导致质谱图中出现谐波。通过仿真证实了这一观察结果，其中限幅数字暂态的一侧导致在仿真频谱中出现多个谐波峰。由于欠采样(混叠)而产生的折叠峰对于高次谐波峰也很明显。进一步表明，ADC的严重过载(即使在放大器完全平衡且没有DC偏移的情况下)也可能导致离子同位素分布的扭曲，这将使质谱图的自动解释变得困难。最后，ICR信号上的射频干扰(RFI)噪声会导致ADC的不平衡和/或过载，还可能导致同位素峰值和RFI噪声之间的相互调制，导致不良伪影。因此，确定了几种常见信号处理伪影的来源，并将其与放大器和/或数字转换器的RFI噪声和饱和度相关联。在这种情况下，快速傅里叶变换(FFT)产生对应于实信号峰值和RFI频率的谐波和混合频率的频谱伪峰。虽然这项研究是使用FT-ICR MS数据完成的，但必须强调的是，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Orbitrap和FT离子陷阱。

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.