Mass spectrometry (MS) plays an important role in chemical analysis which is currently being enhanced by the increasing demand for rapid trace analysis in the areas of public safety, forensics, food safety, and pharmaceutical quality assurance, amongst others. These demands constitute an impetus to simplify MS instrumentation and methodologies. This in turn has resulted in the development of miniaturized instrumentation [1][2][3] and the invention of ambient ionization methods in which samples are examined without preparation in their native state. [4][5][6][7][8][9][10][11][12] The ambient ionization methods include spray-based, [13][14][15][16][17][18][19][20] plasma-based, [21,22] and laser-assisted methods. [23][24][25][26][27] Like other ambient methods, desorption electrospray ionization (DESI) [14] has the advantages of simple instrumentation, rapid and sensitive analysis, and broad applicability. In situ mass spectrometry requires both portable instruments and simple ionization/sampling methods. In our laboratory, this requirement has been met by fitting the small ion trap based Mini 10 and 11 instruments [1] with ambient ionization sources. The performance of the combined system is limited by the low pumping speed of small mass spectrometers and the large nebulizing gas and solvent volumes that must be handled. This problem was addressed by the development of the discontinuous atmospheric pressure interface (DAPI). [28,29] The DAPI interface is opened briefly to admit a bolus of ions, solvent vapor and gas, then closed while the neutrals are pumped away before the trapped ions are mass analyzed. The system operates well in spite of a duty cycle of just 1 %. [28,29] Further improvement should be achievable by synchronization of the experiment (Figure 1) which requires the use of an inductive method to charge the primary microdroplets. This allows droplet creation to be synchronized with the opening of the sample introduction system (and also with the pulsing of the nebulizing gas). Synchronized inductive DESI shows good performance: 1) over 100-fold improvement in sensitivity (Figure 1c and 1d) while still using the 1:100 DAPI duty cycle, 2) reduced solvent spray flow rate from ca. 5 mL min À1 to 0.5 mL min À1 , 3) reduced nebulizing gas usage from ca. 2 L min À1 to 0.2 L min À1, 4) improved sampling efficiency by a factor of 100, and 5) quasi-simultaneous recording of positive and negative ion spectra using a pulsed monopolar ion source. These capabilities are based on accurate control of charged droplet creation by placing an electrode near a spray emitter (typically 2-5 mm distant) and pulsing it repetitively to high positive potentials (5-7 kV, 50-3000 Hz, pulse width 0.2-2 ms). The pulsed positive voltage was applied to a metal tube (inner diameter (i.d.) 250 mm), covering an inner silica capillary which served as the spray emitter tip (i.d. 50 mm). Electromagnetic induction produces high electrical fields in the DESI source that result in bursts of charged droplets. Precise synchronization with the ...

Mass spectrometry (MS) plays an important role in chemical analysis which is currently being enhanced by the increasing demand for rapid trace analysis in the areas of public safety, forensics, food safety, and pharmaceutical quality assurance, amongst others. These demands constitute an impetus to simplify MS instrumentation and methodologies. This in turn has resulted in the development of miniaturized instrumentation [1][2][3] and the invention of ambient ionization methods in which samples are examined without preparation in their native state. [4][5][6][7][8][9][10][11][12] The ambient ionization methods include spray-based, [13][14][15][16][17][18][19][20] plasma-based, [21,22] and laser-assisted methods. [23][24][25][26][27] Like other ambient methods, desorption electrospray ionization (DESI) [14] has the advantages of simple instrumentation, rapid and sensitive analysis, and broad applicability. In situ mass spectrometry requires both portable instruments and simple ionization/sampling methods. In our laboratory, this requirement has been met by fitting the small ion trap based Mini 10 and 11 instruments [1] with ambient ionization sources. The performance of the combined system is limited by the low pumping speed of small mass spectrometers and the large nebulizing gas and solvent volumes that must be handled. This problem was addressed by the development of the discontinuous atmospheric pressure interface (DAPI). [28,29] The DAPI interface is opened briefly to admit a bolus of ions, solvent vapor and gas, then closed while the neutrals are pumped away before the trapped ions are mass analyzed. The system operates well in spite of a duty cycle of just 1 %. [28,29] Further improvement should be achievable by synchronization of the experiment (Figure 1) which requires the use of an inductive method to charge the primary microdroplets. This allows droplet creation to be synchronized with the opening of the sample introduction system (and also with the pulsing of the nebulizing gas). Synchronized inductive DESI shows good performance: 1) over 100-fold improvement in sensitivity (Figure 1c and 1d) while still using the 1:100 DAPI duty cycle, 2) reduced solvent spray flow rate from ca. 5 mL min À1 to 0.5 mL min À1 , 3) reduced nebulizing gas usage from ca. 2 L min À1 to 0.2 L min À1 , 4) improved sampling efficiency by a factor of 100, and 5) quasi-simultaneous recording of positive and negative ion spectra using a pulsed monopolar ion source.These capabilities are based on accurate control of charged droplet creation by placing an electrode near a spray emitter (typically 2-5 mm distant) and pulsing it repetitively to high positive potentials (5-7 kV, 50-3000 Hz, pulse width 0.2-2 ms). The pulsed positive voltage was applied to a metal tube (inner diameter (i.d.) 250 mm), covering an inner silica capillary which served as the spray emitter tip (i.d. 50 mm). Electromagnetic induction produces high electrical fields in the DESI source that result in bursts of charged droplets. Precise synchronization with the ...