直接电离质谱结合扩口毛细管用于毒品检测研究

Study of Direct Ionization Mass Spectrometry with a Flared Capillary for Drug Detection

直接电离质谱具有快速、原位、实时离子化等优势,被广泛用于毒品、爆炸物、食品添加剂等成分的检测。直接电离源与质谱真空的耦合通常是通过毛细管或分离锥等窄小导孔实现的,导致离子传输效率大幅降低,成为提高仪器灵敏度的主要瓶颈。本研究基于脉冲直流电喷雾离子源与快筛质谱联用平台,探究了扩口和平口毛细管中喷雾电压、喷针位置、喷针内径、毛细管入口内径对仪器性能的影响。流体仿真和离子轨迹模拟结果表明,扩口毛细管因径向气流分布范围宽,扩大了可聚焦喷雾羽流面积,提高了直接电离源到质谱锥口之间的离子传输效率。与传统的平口毛细管相比,入口内径6 mm的扩口毛细管大气压接口搭配20μm喷针内径,可将10μg/L甲基苯丙胺离子信号强度提高3.3倍,甲基苯丙胺、3,4-亚甲双氧甲基苯丙胺、氯胺酮、可卡因、O^(6)-单乙酰吗啡等5种常规毒品在毛发中的信噪比均有明显提升。本研究表明,采用扩口毛细管无需对仪器进行大规模修改,易与直接电离源相结合,可显著提高仪器灵敏度。

Direct ionization mass spectrometry has the advantages of rapid,in-situ and real-time ionization,which has been widely used for the detection of drugs,explosives,food additives,etc.The coupling between direct ionization source and mass spectrometer vacuum is usually achieved with a narrow baffle hole such as capillary or skimmer,which can result in a significant reduction in the ion transmission efficiency.It is well known that this has become a key bottleneck for the improvement of instrument sensitivity.This study was carried out based on a rapid screening mass spectrometry platform coupled with a pulsed direct current electrospray ionization(pulsed-dc ESI) source,which has the advantages of low sample volume required,high ionization efficiency,no cross-contamination risk,and so on.The instrument performances with flat and flared capillaries under different spray voltages,emitter positions,inner diameters of emitter,diameters of capillary inlet were investigated.For the pulsed-dc ESI emitters with the inner diameter of 5,20,50 and 100 μm,the highest ion intensity could be achieved with the 20 μm emitter for all capillaries.Besides,compared with the flat capillary,the ion intensity of flared capillary was more stable to the variation of radial distance between emitter and capillary inlet.By simulating the gas flow and ion trajectories of various capillaries,it was found that the air stream around capillary inlet dominated the ion transmission.For the flat capillary,the gas flow pointed inward only in the area of very close to the porthole.As a contrast,the gas velocities of flared capillaries increased gradually from the entrance of conical mouth to the narrowest part of capillary,of which the directions always pointed to the capillary's porthole.Due to the larger radial focusable airflow area,the flared capillaries,especially the flared capillary with an inlet diameter of 6 mm could evidently improve the ion transmission efficiency between the direct ionization source and the mass spectrometer.In the simulation,the enhancement of ion transmission efficiencies by fared capillaries was more prominent for high mass-to-charge ratio ions,of which the motion usually was more sensitive to gas flow because of lower mobility.Compared to the conventional flat capillary,the ion signal intensity of 10 μg/L standard methamphetamine solution by the flared capillary of 6 mm in the inlet diameter had increased 3.3 times.The signal-to-noise ratios of five conventional drugs,such as methamphetamine,3,4-methylenedioxymethamphetamine,ketamine,cocaine,and O^(6)-monoacetylmorphine in hair matrix were also boosted to certain degrees.Especially for ketamine,the signal-to-noise ratio of the flared capillary to the flat capillary was up to 3.3.The results showed that the adoption of a flared capillary can significantly increase the detection sensitivity of direct ionization screening platform,which does not require any complex modification to the instrument.