The Development Of Photoionization Sources For Time-of-flight Mass Spectrometry And Its Application

Time-of-flight mass spectrometry?TOFMS?technique has attracted extensive attention in the field of analysis and detection because of its powerful capability in qualitative and quantitative analysis.As one of the most commonly used ionization techniques for TOFMS,photoionization?PI?is characterized by the unique advantages of simple spectrum,good versatility and high ionization efficiency.In this thesis,some new photoionization techniques have been developed for TOFMS to improve the sensitivity and expand the detectable range of analytes.Successful applications have been achieved in rapid analysis of industrial catalysis,environmental science,food quality and other fields.1)Development and application of RF-discharge photoionization sourceAs a key component of a PI source,the vacuum ultraviolet?VUV?light source plays a crucial role in the ionization efficiency and ionizable range of analytes.The most commonly used light sources include lasers,synchrotron radiation sources and rare gas discharge lamps,etc.However,the high price and bulky size of lasers and synchrotron radiation sources limit their practical application.The photon flux and photon energy of the traditional discharge lamps are relatively low,restricting the detection sensitivity and the range of detectable compounds of PI mass spectrometry?MS?.Therefore,a novel RF-discharge ion source based on a windowless radio frequency discharge lamp was developed.High-purity argon?Ar?gas was employed in the lamp to generate high-intensity VUV beam with the photon energies of 11.6eV and 11.8 eV,and the photon flux of about¹0¹⁵ photons/s,overcoming the shortage of traditional VUV lamp and achieving high-sensitivity detection of analytes.The stability of the ion source was greatly enhanced using the windowless structure design,and the relative standard deviation?RSD?of continuous working for 11 hours was as low as 1.19%.The limits of detection?LODs?of ethylene and propylene were down to 16.98 and 9.64 ppbv/0.5 s,respectively.The initial stage of methanol to olefin?MTO?reaction was real-time monitored with a high temporal resolution of 0.5 s,and thus online analysis of the generation sequence of ethylene and propylene was realized,which provided data support for the mechanism research of the first C-C bond formation.2)Development and application of HPPI and rear-source CIDAccording to the Lambert-Beer law and the ideal gas state equation,the ionization efficiency can be largely improved by increasing the ion source pressure,and thus the detection sensitivity can be improved.A high-pressure photon ionization?HPPI?ion source was developed by elevating the ion source pressure to kPa.The ion transmission system,composed of a radio frequency?RF?-only quadrupole and direct current?DC?electrostatic optical lens,was designed and optimized to achieve a high ion transmission efficiency,and the achieved detection sensitivities for common VOCs were down to pptv^ppbv.However,molecular mass is not a unique indicator for analyte identification,the inability to obtain molecular structure information is a major issue for soft ionization sources.Due to the poor fragmentation of HPPI source,it is difficult to obtain the structural information of the analytes.A feasible solution for this problem is to break the molecular ions into pieces of fragments with characteristic fragmentation pattern for each species through collision-induced dissociation?CID?,and the fragmentation can aid in the differentiation of isomers and isobars if the fragment ions are unique to the analytes.As different degrees of fragmentation can be achieved for analyte molecules with different dissociation energies by collision-induced dissociation?CID?,it is expected to achieve qualitative analysis of the compounds with same molecular weight.Based on the technique of rear-source CID combined with HPPI-TOFMS,a method for rapid online measurement of biogenic volatile organic compounds?BVOCs?was developed.In order to quantitatively analyze each constituent in BVOCs isomer mixtures,a quantitative analysis algorithm has been designed by using the least square method.Six monoterpene isomers were clearly distinguished according to their unique fragmentation patterns under a series of controlled CID energies.The developed method based on HPPI-MS was further utilized in the rapid online analysis of monoterpene isomers of real plant samples,which usually contains large quantities of isomeric compounds.3)Development and application of HPPI-CI ion sourceHPPI is a kind of threshold photoionization technology,thus it is impossible to ionize the analytes with ionization energies higher than the photon energy.To further expand the detectable range of PI sources,a high-pressure photoionization-chemical ionization?HPPI-CI?combined ion source was developed for TOFMS.By introducing photoelectron-induced O₂⁺cation chemical ionization,the detectable range of analytes was effectively broadened.The volatile fingerprint mass spectra of thirty-one green tea samples of four sub-types?tender chestnut-like,tender-like,chestnut-like and roasted chestnut-like?were obtained based on this method.Combined with multivariate statistical analysis techniques,such as Principal component analysis?PCA?,Partial least squares discrimination analysis?PLS-DA?and Hierarchical cluster analysis?HCA?,an objective and rapid analysis method was developed to differentiate the types of green teas.The classification efficiency was evaluated by analyzing the Receiver operating characteristic?ROC?curve.The observations and the robustness were assessed using 200 permutation tests.The results showed that HPPI-CI TOFMS combined with multivariate statistical analysis method could be used for clustering analysis of different types of samples,exhibiting high prediction accuracy for unknown samples,thus providing a rapid and objective new idea for differential analysis in disease screening,quality identification and other fields.4)Development of ion mobility-mass spectrometryIt is a challenge for MS alone to distinguish between isobaric molecules,including isobars and isomers.Therefore,the coupling of appropriate pre-separation method with MS to form a two-dimensional analytical technique,can effectively improve the analytical capability of MS.Ion mobility spectrometry?IMS?is an ultra-fast gas-phase ion separation technology,which has the respond time of millisecond and the ability to separate isomers.Therefore,the use of IMS as a fast gas phase separation technique before MS,can make up for the deficiency of MS and form a powerful analytical method.A hyphenated technique of ion mobility-mass spectrometry?IMMS?based on the HPPI source,the inverse ion mobility spectrometry and a home-made TOFMS for online analysis of small molecule VOCs was developed with elaborate design in the overall structure,the HPPI-IMS interface and the ion transmission interface of IMS-MS.The operation mode and control system of the hyphenated IMMS were explored,and finally,an online two-dimensional IMMS platform was constructed for the rapid online separation and detection.This work provides a new thought and a reliable analytical tool for complex small molecule VOCs mixtures.