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 ¹³C, ¹⁵N, and ³⁴S. 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.

中文翻译：

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使用同位素比质谱法表征Orbitrap质量分析仪的光谱精度

耦合到Q Exactive Plus的红外基质辅助激光解吸电喷雾电离（IR-MALDESI）源已广泛用于生物组织切片的非目标质谱成像（MSI）分析。尽管Orbitrap是高分辨率和精确质量（HRAM）的质量分析器，但仅这些属性不能单独用于可靠鉴定在复杂生物基质中观察到的未知分析物。光谱精度（SA）是质谱仪准确测量同位素分布的能力，当与高质量测量精度（MMA）一起使用时，可以促进阐明单一元素组成。为了研究不同离子种群对Orbitrap SA和MMA（咖啡因溶液，四肽MRFA）的影响，然后使用Q Exactive Plus对八个不同的自动增益控制（AGC）目标进行了Ultramark分析。还使用同位素比质谱法（IRMS）对来自相同批号的相同化合物进行了单独分析，以准确确定同位素的丰度。¹³ C，¹⁵ N和^34S。我们证明了在最佳的绝对离子丰度下，Orbitrap可用于精确计算质量不同的样品中的碳，氮和硫。此外，通过使用预期的（IRMS）和实验的（Orbitrap）同位素分布来计算Pearson卡方检验，凭经验确定了高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目标MSI研究中，可通过快速筛选绝对离子丰度足够高以准确估计原子数的同位素分布来缩短识别列表。