| With the progress of science and technology,the role of trace components in the environment and materials has attracted more and more attention.Many low content substances,such as biomarkers and single cell metabolites,have become the key factors determining environmental safety,material properties and life processes.Accurate and sensitive measurement of trace characteristic substances is crucial for in-depth interpretation of relevant chemical and biological phenomena.Trace compounds have the characteristics of low content,many kinds and complex matrices,which are difficult to analyze accurately.Single mass analyzers can no longer sufficient for application needs.The quadrupole and ion trap tandem mass spectrometry combine their advantages.It can reduce the interference of the matrix,realize ion enrichment and multi-stage tandem mass spectrometry analysis,which is more suitable for the accurate measurement of low-content target substances in complex matrices.Currently commercial quadrupole and ion trap tandem mass spectrometers are connected in series with other dissociation devices,which are complex in structure and high in cost.It is necessary to improve the structure and simplify the device.In addition,there are still limitations in the analysis of complex mixture systems,especially when there are matrix ions with similar or the same mass-to-charge ratio(m/z)as the target ions.It is also necessary to study the precise ion manipulation techniques to reduce the space charge effect and solve the problem that it is difficult to accurately measure trace substances in complex matrices.In this paper,the quadrupole-linear ion trap tandem mass spectrometry platform and ion manipulation techniques were studied.The main contents are as follows:(1)Development of a quadrupole-linear ion trap tandem mass spectrometry platformIn order to realize the detection of trace substances in complex matrices,a quadrupole-linear ion trap tandem mass spectrometry platform was developed.Various components of the mass spectrometer were designed and selected.A five-stage vacuum system was developed to meet the transition from atmospheric pressure environment to high vacuum environment;Microliter and nanoliter electrospray ion sources were developed to meet the detection requirements of samples with high and low flow rates;An ion transport system was developed to realize the efficient transport of gas phase ions;A three-stage mass analyzer was simulated and designed to improve ion focusing and transmission;A dual-phase drive radio frequency(RF)high-voltage power supply was developed to drive the quadrupole and ion trap respectively to achieve the mass filtration of the quadrupole and the enrichment,fragmentation and detection of the ion trap.The direct coupling series technique of quadrupole and linear ion trap was studied.Matrix interference and space charge effects were reduced.The integration and performance test of the whole instrument were carried out.The mass range was 45.93-2271.45 amu;the resolution(FWHM)was less than 0.6 amu;the relative standard deviation(RSD)of the stability measurement signal intensity was 3.16%,which laid the foundation for the follow-up work.(2)Research on a gas-phase ion-selective enrichment technique based on the cylindrical quadrupole-linear ion trap(Q-LIT)tandem mass spectrometerAiming at the problem of difficult detection of low-content substances in complex matrices,a gas-phase ion selective enrichment technique based on Q-LIT was proposed.Auxiliary waveform signals were studied to realize the isolation,fragmentation and detection of ions.The timing control was studied.The ions were continuously enriched in the trap while they were selected by the quadrupole under gas phase conditions,which can effectively reduce the interference of the matrix and improve the signal intensity.The influence of space charge effect on mass resolution,mass shift and intensity were studied.The effectiveness of this method in reducing space charge effect was verified.The effects of the voltages of the ion transmission components and the ion accumulation time on the signal intensity were studied.The detection conditions were optimized.Compared with enrichment techniques of traditional ion trap,the signal intensities were increased by 8-22 times.The measured signal-to-noise ratio(S/N)of 1 pg reserpine was 15500.By calculation,the limit of detection(LOD)could reach 1.7 fg.The detection of low content substances in complex matrices and the accurate measurement of peptide impurities were achieved.(3)Research on a simultaneous fragmentation and accumulation technique based on Q-LITThere may be matrix ions with similar or the same m/z as the target ions in the complex matrices,which cannot be effectively distinguished by the quadrupole.Matrix ions are accumulated in the ion trap together with the target ions,interfering with the detection of the target ions.A simultaneous fragmentation and accumulation technique based on Q-LIT was developed.A composite waveform was studied.The specific m/z ions selected by the quadrupole were immediately fragmented upon entering the ion trap.Only the target product ions can be accumulated in the LIT.The matrix ions and their product ions were ejected from the trap.The fragmentation of precursor ions and the isolation and accumulation of product ions were performed simultaneously.The length of the operation sequence was shortened,and the duty cycle of the tandem mass spectrometry was improved.Compared with the gas-phase ion selective enrichment technique,the signal intensities were increased by 2-8 times.The increase of the ion accumulation time can further improve the signal intensity.Space charge effects were reduced.The detection of target substances in matrices with isomers was realized.It could be used for qualitative and quantitative analysis of substances in complex matrices,and could stably detect even low-abundance molecules in matrices that were difficult to be detected by commercial mass spectrometry.The concentration of 25-OH VD2 spiked in serum was detected.The linear correlation coefficient(R2)was 0.9978 in the range of 10–100 ng/m L,and the recovery rate of 25 ng/m L 25-OH VD2 was 107.5% ± 5.06%(n=3),the recovery rate of 70 ng/m L 25-OH VD2 was 97.1%±4.52%(n=3).Further application to the analysis of Mb trypsin-digested peptides demonstrated the ability of this method to be sensitive to biological samples.(4)Research on the simultaneous fragmentation and accumulation technique based on the quadrupole-dual pressure linear ion trap(Q-DPLIT)Aiming at the problem that the efficiency of ion trapping and detection in the ion trap cannot be optimized simultaneously under the same gas pressure,the hyperbolic quadrupole and dual pressure linear ion trap tandem(Q-DPLIT)control technique was developed.The resolution and transmission efficiency of the hyperbolic quadrupole and the cylindrical quadrupole were studied,and it was verified that the hyperbolic quadrupole has higher resolution and transmission efficiency.The effect of gas pressure on the trapping and detection capability of the ion trap was investigated.By changing the flow flux of helium introduced into the ion trap,the ability and resolution of trapping ions in single ion trap and dual pressure ion trap under different flow conditions were studied.It showed that the dual pressure trap had stronger ion trapping ability and higher resolution.The simultaneous fragmentation and accumulation technique based on the Q-DPLIT mass spectrometry platform was proposed to improve the sensitivity and accuracy of detection.0.01% of the target species in the isomer matrix can be detected linearly.Compared with the ion selective enrichment technique based on Q-DPLIT,the signal intensity of the target product ions measured by the simultaneous fragmentation and accumulation technique was improved by 1.3 to 10 times,and the linear correlation coefficient was improved from0.6693–0.9449 to 0.9942–0.9994.In the sensitivity test,the mean S/N of 1 pg reserpine was 197443,and the LOD reached 0.11 fg.The RSD of the stability of0.01% peptide impurities in the pure peptide detected for five times was less than 3%. |
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