Department of Chemistry and Molecular Education, Technology, and Research Innovation Center (METRIC), North Carolina State University , Raleigh, North Carolina 27695, United States.
Precision Engineering Consortium, Department of Mechanical and Aerospace Engineering, North Carolina State University , Raleigh, North Carolina 27606, United States.
Anal Chem. 2018 Feb 6;90(3):1897-1906. doi: 10.1021/acs.analchem.7b03983. Epub 2018 Jan 11.
Infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI) source coupled to the Q Exactive Plus has been extensively used in untargeted mass spectrometry imaging (MSI) analyses of biological tissue sections. Although the Orbitrap is a high-resolution and accurate-mass (HRAM) mass analyzer, these attributes alone cannot be used for the reliable identification of unknown analytes observed in complex biological matrices. Spectral accuracy (SA) is the ability of the mass spectrometer to accurately measure the isotopic distributions which, when used with high mass measurement accuracy (MMA), can facilitate the elucidation of a single elemental composition. To investigate the effects of different ion populations on an Orbitrap's SA and MMA, a solution of caffeine, the tetrapeptide MRFA, and ultramark was analyzed using a Q Exactive Plus across eight distinct automatic gain control (AGC) targets. The same compounds from the same lot numbers were also individually analyzed using isotope ratio mass spectrometry (IRMS) to accurately determine the isotopic abundance of C, N, and S. We demonstrated that at optimum absolute ion abundances the Orbitrap can be used to accurately count carbons, nitrogens, and sulfurs in samples with varying masses. Additionally, absolute monoisotopic ion abundances required for high SA were empirically determined by using the expected (IRMS) and experimental (Orbitrap) isotopic distributions to calculate the Pearson chi-square test. These thresholds for absolute ion abundances can be used in untargeted MSI studies to shorten an identification list by rapidly screening for isotopic distributions whose absolute ion abundances are high enough to accurately estimate the number of atoms.
红外基质辅助激光解吸电喷雾电离（IR-MALDESI）源与 Q Exactive Plus 耦合已广泛应用于生物组织切片的非靶向质谱成像（MSI）分析。尽管轨道阱是一种高分辨率和精确质量（HRAM）质量分析器，但仅凭这些属性无法可靠地识别在复杂生物基质中观察到的未知分析物。光谱准确性（SA）是质谱准确测量同位素分布的能力，当与高质量测量精度（MMA）结合使用时，可以促进单一元素组成的阐明。为了研究不同离子群体对轨道阱 SA 和 MMA 的影响，使用 Q Exactive Plus 对咖啡因、四肽 MRFA 和 ultramark 的溶液进行了分析，跨越了八个不同的自动增益控制（AGC）目标。来自相同批号的相同化合物也分别使用同位素比质谱（IRMS）进行了分析，以准确确定 C、N 和 S 的同位素丰度。我们证明，在最佳绝对离子丰度下，轨道阱可用于准确计数不同质量样品中的碳原子、氮原子和硫原子。此外，通过使用预期的（IRMS）和实验的（轨道阱）同位素分布来计算 Pearson 卡方检验，经验确定了高 SA 所需的绝对单同位素离子丰度。这些绝对离子丰度的阈值可用于非靶向 MSI 研究，通过快速筛选具有足够高的绝对离子丰度的同位素分布，从而缩短鉴定列表，以准确估计原子的数量。
