The advantages of electrosprays for the ionization of vapours vs. other more conventional approaches (e.g. radioactive sources) for mass spectrometry analysis have been long acknowledged in literature [1]. In this method, ions are not fragmented, enabling MS-MS analysis and reliable analyte identification. As a result, this peculiar ionization process has been generally dubbed secondary electro-spray ionization (SESI). Headed by Yale Professor Juan Fernandez de la Mora, SEADM speeded up the concept and applied it in late 2000’s for the detection of explosives, achieving sub-ppt limits of detection [2].
The ionization principle behind SESI is rather simple:
Neutral vapours + charger ions → Ionized vapours
Nevertheless, SESI technique is fundamentally impaired by inherent draw-backs (Fig. 2): a poor concentration of charging ions in the chamber (owing basically to the Coulombic repulsion of the electrospray cone as well as dilution due to penetration of counter-flow gas from MS) and a poor transmission, again given by the Coulombic repulsion effect. Any attempt to focus the ions (for instance by increasing sample flow rate, eventually reusing the flow provided by the MS) will lead to dilution and therefore to poor ionization efficiency.
Being well aware, through its more than 10 years-experience, of these inherent short-comings of conventional SESI, SEADM recently developed its Low Flow Secondary Electrospray Ionization Source, LFSESI [3,4], precisely targeting each and every problem of the former SESI technique by means of unique, patented two-electrode architecture (Fig. 3):
- Focusing electrode minimizes Coulombic repulsion, maximizing the concentration of charging ions in the chamber
- Impaction electrode avoids the penetration of counter-flow gas form the mass spec, augmenting even further the ionization efficiency vs. conventional SESI, and:
- Both electrodes achieve an optimum ion focus effect for optimum transmission.
Numerical simulation design works confirmed the soundness of the two-electrode approach (see Figure 4); besides, memory effects are eliminated due to devoted heating system (avoiding any condensation) and flow patterns are strictly streamlined, with no stagnant areas inside the device. As a result, LFSESI experimentally outperforms SESI in orders of magnitude for sample flow rates below 0.75 lpm, and by a factor of x2÷5 for flow rates above that figure (see Figure 5). Limits of detection are amply below 1 ppt. LFSESI is the only secondary electrospray vapour ionization technology patented up to date, and the only one who has demonstrated being able to overcome the fundamental problems of SESI.
Tens of customers are now benefiting from SEADM’s LFSESI around the world in multiple applications such as pharmacokinetics, microbiological studies, plant metabolism, breath analysis and many more. Learn more?
Take a look to our most recent presentation at the National Spanish Mass Spectrometry Congress (2017):
References
[1] Masamichi Y., Fenn J.B., “Electrospray ion source. Another variation on the free-jet theme”, J. Phys. Chem., 1984, 88 (20), pp 4451–4459
[2] Martínez-Lozano P., Rus J., Fernández de la Mora G., Hernández M., Fernández de la Mora J. “Secondary electrospray ionization (SESI) of ambient vapors for explosive detection at concentrations below parts per trillion” J Am Soc Mass Spectrom. 2009 Feb; 20(2):287-94.
[3] Vidal-de-Miguel G., Macía M., Pinacho P., Blanco J. “Low-Sample Flow Secondary Electrospray Ionization: Improving Vapor Ionization Efficiency” Anal. Chem., 2012, 84 (20), pp 8475–8479
[4] Barrios-Collado C., Vidal-de-Miguel G., Martinez-Lozano Sinues P., “Numerical modeling and experimental validation of a universal secondary electrospray ionization source for mass spectrometric gas analysis in real-time”, Sensors and Actuators B: Chemical, Vol. 223, 2016, pp. 217-225,
Patents
PATENT: G. Vidal de Miguel, “Ionizer for vapor analysis decoupling the ionization region from the analyzer”, USPTO 8,461,523 B2, Jun. 11, 2013
Figure 1 The so-called secondary electrospray ionization (SESI) involves bringing in contact a vapor containing gas and an electrospray cloud (the white feature to the right of the photo).
Figure 2 Fundamental problems of SESI technology leading to poor ionization rates and poor ion transmission to the MS
Figure 3 The new patented LFSESI architecture, available from SEADM, ensuring optimum ionization and transmission values
Figure 4. Design of the LFSESI source by means of devoted Multiphysics models; up: flow results for TNT vapor at 1 ppt; sample flow 0.05 lpm; concentration of charging ions [cc], neutral vapor [cv] and ionized vapor [ci]; medium: streamlined design to avoid any stagnant areas, (red and blue correspond to sample and MS counter flows, respectively); down: careful thermal design to minimize memory effects (increased by the presence of cold spots), ensure a uniform evaporation of the ESI droplets and ease manipulation.
Figure 5: Experimental performance of SEADM’s LFSESI vs. conventional SESI; DEA 23 fg/s; electrospray of H2O-HCOOH (0.1%)
