Real Time Online Correction of Mass Shifts and Intensity Fluctuations in Extractive Electrospray Ionization Mass Spectrometry Yong Tian, † Miao Yu, † Jian Chen, ‡ Chunxiao Liu, † Jianbo Shi,* ,† Huanwen Chen,* ,‡ and Guibin Jiang † † State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China ‡ Jiangxi Key Laboratory for Mass Spectrometry and Instrumentation, College of Chemistry, Biology and Material Sciences, East China University of Technology, Nanchang, Jiangxi 330013, China * S Supporting Information ABSTRACT: Real time online calibration of mass shift and intensity fluctuation to improve the accuracy of measurements for identification and quantitation in trace mass spectrometric analysis was demon- strated using extractive electrospray ionization mass spectrometry (EESI-MS). The signals of authentic compounds (e.g., lysine (Lys), proline (Pro), and histidine (His)) spiked into the extractive solution for the EESI process were used as the references to calibrate the signal of analytes (e.g., methionine (Met)) in the untreated sample solution. The intensity of the analyte signal was recorded simultaneously with the reference signals. The analyte signals at a given time point were calibrated on the basis of these correlation factors and real time signal response of the reference. The calibrated signal of Met at 10 μgL -1 was improved with a better signal-to-noise ratio (S/N from 2.3 to 4.3), better linearity (R 2 from 0.9758 to 0.9980), and reduced relative standard deviation (RSD from 9.8% to 6.0%). The shift of mass-to-charge ratio of Met signal between the detected and theoretical values was decreased from 247 ± 133 to -7 ± 167 ppm for 50 min of detection using a linear ion trap mass analyzer and was reduced from -0.27 ± 0.60 to -0.12 ± 0.23 ppm for 50 min of detection using an Orbitrap mass analyzer (P = 95%). This method has been validated using a certified standard amino acids solution (GBW(E)100062) and applied for quantitative detection of amino acids in chicken feed, urine, nutritional drink, and facial mask samples, showing that the method is useful to improve the accuracy of mass spectrometric analysis. M ass spectrometry (MS) is increasingly used for trace detection in multiple disciplines such as element analysis, 1 clinical chemistry, 2 metallomics, 3 proteomics, 4 metabolomics, 5 lipidomics, 6 etc. due to its unparalleled capability for identification and quantification of the analytes. Normally, efforts have been spent to improve the sensitivity and resolution power of MS instruments to acquire more accurate and reliable results. 7-9 To date, most commercial mass spectrometers are routinely calibrated prior to use for sample analysis to guarantee the accuracy and precision of the measurement, 10 especially for cases (e.g., analysis of biological, geological, and environmental samples) where matrix effects are notably detected. Because the spectral baseline shift is normally encountered, 11,12 the long-term accuracy and reliability of the analytical results can be affected after a long operation time. This problem is particularly serious for biological analyses using large amount of samples, e.g., metabolomics, 11,13 causing misinterpretation of experimental data. External calibration, standard addition, and internal standards are commonly used to obtain quantitative results. 12 However, the instrument drift is hardly corrected using these methods in practical applications. 14 External calibration performed with MS instruments with high frequency should be able to reduce the instrument shift, but it is too laborious to carry out, especially in cases where online real time monitoring or a large batch of sample analysis is required. 10 Isotope labeled internal standards are also widely used to correct the signal variation caused by differences in sample composition and instrument drift. 15,16 Unfortunately, the isotope labeled compounds are usually expensive and not commonly available for all analytes. For certain compounds, the naturally occurring isotopes of the analyte and nonlinear signal response would also influence the signal of the internal standard. 11,17,18 Eckers et al. 19 and Wolff et al. 20 developed the LockSpray for accurate mass measure- ments. By switching the reference and sample spray alternately into MS, the mass value of the analytes could be calibrated by the reference for both MS and tandem mass spectrometry (MS/MS) measurements. 19-22 However, the mass intensity correction by two sprayers was not reported to our knowledge. Therefore, it is highly desirable to develop a new method for Received: August 25, 2015 Accepted: November 23, 2015 Published: November 23, 2015 Technical Note pubs.acs.org/ac © 2015 American Chemical Society 11962 DOI: 10.1021/acs.analchem.5b04372 Anal. Chem. 2015, 87, 11962-11966