Mass Accuracy and Isotopic Abundance Measurements for HR-MS Instrumentation: Capabilities for Non-Targeted Analyses

Abstract

The development of automated non-targeted workflows for small molecule analyses is highly desirable in many areas of research and diagnostics. Sufficient mass and chromatographic resolution is necessary for the detectability of compounds and subsequent componentization and interpretation of ions. The mass accuracy and relative isotopic abundance are critical in correct molecular formulae generation for unknown compounds. While high-resolution instrumentation provides accurate mass information, sample complexity can greatly influence data quality and the measurement of compounds of interest. Two high-resolution instruments, an Orbitrap and a Q-TOF, were evaluated for mass accuracy and relative isotopic abundance with various concentrations of a standard mixture in four complex sample matrices. The overall average ± standard deviation of the mass accuracy was 1.06 ± 0.76 ppm and 1.62 ± 1.88 ppm for the Orbitrap and the Q-TOF, respectively; however, individual measurements were ± 5 ppm for the Orbitrap and greater than 10 ppm for the Q-TOF. Relative isotopic abundance measurements for A + 1 were within 5% of the theoretical value if the intensity of the monoisotopic peak was greater than 1E7 for the Orbitrap and 1E5 for the Q-TOF, where an increase in error is observed with a decrease in intensity. Furthermore, complicating factors were found in the data that would impact automated data analysis strategies, including coeluting species that interfere with detectability and relative isotopic abundance measurements. The implications of these findings will be discussed with an emphasis on reasonable expectations from these instruments, guidelines for experimental workflows, data analysis considerations, and software design for non-targeted analyses.

Non-targeted screening approaches for contaminants and adulterants in food using liquid chromatography hyphenated to high resolution mass spectrometry

“…Croley, Non-targeted screening approaches for contaminants and adulterants in food using liquid chromatography hyphenated to high resolution mass spectrometry, J. Chromatogr. A (2015), http://dx.doi.org/10.1016/j.chroma.2015.08.059 Adapted from [41,58] with permission.…”

“…Both q-TOF and Orbitrap mass analyzers generally have mass accuracies below 5 ppm [54][55][56][57][58] and have been routinely employed in the analysis of food. While the specified measured mass accuracies for these instruments are sufficient for non-targeted analysis, impaired data quality can occur, especially when analyzing complex sample matrices [58]. This can result in the generation of incorrect molecular formulae and/or detected compounds that are not extracted from the data.…”

“…Q-TOF mass analyzers can be susceptible to mass accuracy errors at low and high ion abundance [54,55,58], while measured mass accuracies by Orbitrap mass analyzers can worsen due to peak coalescence and ion suppression [41,59]. An example of peak coalescence is shown in Fig.…”

“…Applying a lock mass calibration can provide smaller mass accuracies, especially for instruments that are susceptible to large mass accuracy drift [56,58,60]. If less than 3 ppm mass error is routinely achievable, this mass window can be used in a targeted search for analytes in a molecular database.…”

Product ion isotopologue pattern: A tool to improve the reliability of elemental composition elucidations of unknown compounds in complex matrices

“…Furthermore, the first and second isotopic peaks of a small molecule generally show low abundance and therefore are prone to isobaric interferences. [12] Nevertheless, carbon-atom-based isotopic abundances have been used to process data produced by unit-mass resolving triple-quadrupole instruments. [3] Isobaric issues caused by coeluting matrix compounds were significantly reduced by the use of Q-HRMS technology.…”