The development of CE-MS and CEC-MS interfaces based on noncontinuous electrospray and chip-based microinjector

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Several approaches based on noncontinuous spray and chip-based microinjector have been developed to overcome the clogging problem in sheathless CE-MS, and to increase the throughput in CE and CEC-MS. To develop a more practical and sensitive CE-MS interface, a sheathless pulsed–ESI interface has been developed for coupling capillary electrophoresis (CE) with ion trap mass spectrometer (MS). In sheathless CE-MS, because the EOF in CE is about 50~250 nL/min, in order to meet the optimal flow rate of the sprayer, the orifice of the sprayer has to be tapered down to ~20 µm or less o.d. Nevertheless, the susceptibility of breaking or clogging of the tip during coating or sample analysis limits the application of the tapered tips having small orifices. The use of noncontinuous spray mode allows the use of a sprayer with a larger orifice thus alleviate the problem of column clogging during conductive coating and CE analysis. A pulsed ESI source operated at 20 Hz and 20% duty cycle was found to produce the optimal signals. For better signals, the maximum ion injection time in the ion-trap mass spectrometer has to be set to a value close to the actual spraying time (10 ms). Using a sprayer with 50 µm o.d., more stable and enhanced signals were obtained in comparison with continuous CE-ESI-MS under the same flow rate (150 nL/min). The utility of this design is demonstrated with the analysis of synthetic drugs by capillary electrophoresis-mass spectrometry (CE-MS). For increase the throughput in CE-MS, a multiplex electrophoresis–mass spectrometry using four low flow sheath liquid ESI sprayers has been developed. Because of low sample dilution and the producing of smaller droplets, low flow interface is known to outperform conventional sheath liquid interface in sensitivity and the tolerance of salts. In a low flow interface, the sprayer is very close to the MS entrance for better ion transmission and because of the limited space between the sprayer and the entrance aperture of the ESI source, multiplex can not be achieved with the conventional rotating plate approaches. Based on noncontinuous spray for each sprayer, the multiplex low flow system was achieved by applying ESI potential sequentially to the four low flow sprayers, resulting in only one sprayer being sprayed at any given time. The synchronization of the scan event and the voltage relays was accomplished by using the data acquisition signal from the ion trap mass spectrometer. By synchronization, the ESI voltage was provided to the sprayers sequentially according to the corresponding scan event. With this design, a four-fold increase in analytical throughput was achieved. To perform fast analysis in CEC-MS approach, an approach to perform chip-based packed CEC-MS was proposed. This fast CEC-MS approach was based on a chip-based microinjector and a short fritless CEC column. Unlike integrated CEC chip, a chip-based poly-(dimethylsiloxane) (PDMS) microinjector was incorporated with a short fritless packed CEC column. By taking the advantages of small sample plug produced in channel cross intersection, automation in sample injection and separation, and fast separation because of a short CEC column, the proposed approach not only preserved the merits of chip analysis (fast separation and automation) but avoid the difficulty in packing ODS particles inside a chip. For better ESI sensitivity, this device was coupled with MS using a low flow sheath liquid interface. The potential and limitation of this device were evaluated in the analysis of a peptide mixture. To develop a more reliable sample loading method and to increase the throughput in sampling, a flow injection sampling method has been implemented for fast CEC-MS analysis. Because of the high back pressure of the CEC column, sample from the syringe pump will flow only into the sampling and waste channel. Therefore, a sample plug was formed according to the length of the sampling channel. With the incorporation of a six-port valve and a syringe pump to the chip microinjector, sample was delivered to the sampling channel at a flow rate of 1.56 µL/min. This simple and semi-automation system allows rapid sampling and high sample throughput. The potential and limitations were demonstrated in the analysis of peptides and protein digestion.