| Proton transfer reaction mass spectrometry(PTR-MS)is an analysis method for the detection of volatile organic compounds(VOCs),which has the advantages of high sensitivity,self-quantitation,and real-time detection.Until now,PTR-MS has wide applications in various fields,including atmospheric science,food science,biological medicine,and public safety.Detection sensitivity is one of the important points in PTRMS.The concentration of the target VOCs is usually very low,at pptv or even ppqv levels,such as the detection of low olfactory threshold substances,detection of semiVOCs,and searching of disease makers.So improving the detection sensitivity in PTRMS is a hot research point in PTR-MS,which is also beneficial to the development of many research fields.In this paper,according to the sensitivity enhancement technology of PTR-MS,starting from the drift tube of PTR-MS instrument and the ion guide between drift tube and mass spectrometer,the new sensitivity enhancement technology of focusing quadrupole ion funnel drift tube PTR-MS,PTR-MS of PTR4 drift tube,PTR-MS of DC funnel ion lens and so on is developed.The effectiveness of the novel device is proved by theoretical simulation and experiment,and the sensitivity of PTR-MS is improved effectively.The main contents are summarized as follows:1.The research on the novel focusing quadrupole ion funnel.Radio frequency(RF)focusing technique is the most common focusing technique in the drift tube,and the typical application is RF ion funnel.Our laboratory also had some researches on the RF ion funnel in the early stage,which successfully improved the sensitivity of the PTRMS.This paper combines the RF focusing technique and the redesign of the drift tube to develop a new ion focusing technique in the drift tube-focusing quadrupole ion funnel to further improve the sensitivity of PTR-MS.Firstly,the ion trajectory,kinetic energy and electric field distribution in the quadrupole ion funnel were investigated by SIMION simulation,and then the ion transmission and soft ionization effect in the drift tube were obtained.Further experimental studies were carried out to investigate the fragmentation of limonene in the drift tube to reflect the change of collision energy with voltage,the change of the ratio of reagent ions with voltage,the voltage response of 12 VOCs,and the comparison of sensitivity and detection limit between quadrupole ion and DC drift tube and ion funnel.Finally,a quadrupole ion funnel was used to detect VOCs generated by food waste for a long time.The results show that the ion transmission efficiency of the quadrupole ion funnel is obviously better than that of the DC drift tube and the ion funnel.The sensitivity of the quadrupole ion funnel is 14-88 times higher than that of the DC drift tube,and 1.7-4.8 times higher than that of the ion funnel.2.The research on the PTR4 drift tube.In chapter 2,although the sensitivity of PTR-MS is effectively improved by the focusing quadrupole ion funnel,it also has some disadvantages:The mean electric field intensity in most areas of the drift tube is very low,which results in the generation of a large number of water cluster ions.Therefore,water cluster ions can only be reduced by reducing the water vapor flow rate into the ion source.As a result,a large amount of sample gas will flow into the ion source and discharge to generate impurity ions.Some impurity ions would have a charge transfer reaction with the VOCs and generate molecular ion peaks,which could complicate spectrometry analysis.In order to reduce the impurity ions in the focusing quadrupole ion funnel,a novel drift tube PTR4 was developed,which has the same structure as the focusing quadrupole ion funnel and a different voltage loading method.First,the ion trajectory and ion kinetic energy in PTR4 reaction tube were investigated by SIMION simulation,and then the ion transmission efficiency and soft ionization effect in the drift tube were obtained.Further experimental studies were carried out to investigate the change of the ratio of reagent ions with voltage,the voltage response of four VOCs,and the sensitivity comparison between PTR4 drift tube and DC drift tube.Finally,PTR4 reaction tube was used to monitor the common VOCs in the chemical laboratory for 12 hours.The results show that the PTR4 drift tube can effectively improve the sensitivity under the condition of high purity of reagent ion.The proportion of total reagent ions in the PTR4 reaction tube is increased from 70.5%to 90.5%compared with the quadrupole ion funnel drift tube,and the sensitivity is 15-113 times higher than that of the DC drift tube.3.The research on the DC funnel ion lens.In this paper,we developed a DC funnel ion lens as the ion guide between drift tube and mass spectrometer.Firstly,SIMION simulation was used to investigate the electric field distribution and ion trajectory in the DC electric field funnel ion lens,focusing on the ion transmission efficiency with different incident angles of initial ions.Further experimental studies were conducted to investigate the reagent ion distribution,voltage response and sensitivity improvement effect.Finally,the instrument was used to study the impact of the water vapor flow on the collision energy.The results show that the DC electric field funnel ion lens can focus the ions with a wider incidence angle,compared with traditional ion lens,the sensitivity is improved by 35%to 61%.The final application research shows that the water vapor flow has an impact on the collision energy in the drift tube,and it is not comprehensive to measure the collision energy in the drift tube only by E/N. |
