Improving the Ionization Efficiency

What is the ionization efficiency?

The ionization efficiency enables the sensitivity, which is required to detect species at very low concentration.

The ionization efficiency of each chemical species is different, and it is defined as the ratio of ions delivered to the mass spectrometer, over molecules introduced in the ionizer. It measures the percentage of the molecules introduced in the ionizer that are ionized and transferred to the mass spectrometer.

 

Why is it important?

Having a high ionization efficiency is required to detect biologically relevant molecules in the gas phase.

  • The amount of molecules that pass from the lining of the lung into the gas phase, or from a cell culture to the heads-pace above it, depends on the volatility of each substance. Low volatility species have low vapor pressures, and are evaporated at very low concentrations, even if they are abundant in the condensed phase.
  • The more biologically specific metabolites are usually larger molecules, which tend to have lower vapor pressures. As a result, the concentration of biologically relevant molecules in the gas phase is very low.

Knowing that the ionization efficiency is predictable over time is important to measure dynamic changes in the metabolism

  • Weather measuring a circadian rhythm, or the metabolic response to a predefined stimuli, either in breath or in an in-vivo analysis, being confident that the observed signal variations are attributable to the biological system being studied and not to unexpected variations in the ionization efficiency is crucial.

 

what affects the ionization efficiency?

The concentrations of ions in a SESI plume is uniform and proportional to the concentration of vapors (see ionization mechanism for more details). This counter-intuitive result eases the analysis of what affects the ionization efficiency. Below is a short list of some of the key aspects that affect the ionization efficiency.

  • The charge transfer reaction rate. In positive mode, SESI ionizes the molecules with higher proton affinity than the water clusters (the proton affinity of water in the gas phase is 697 kJ/mol). In special applications targeting known compounds, other charging solutions can be used to narrow the species that are ionized.
    The reaction rate in SESI can be very high for ionizable species because the high pressure keeps the charging ions, the moisture, and the molecules very densely packed.
  • The actual concentration of the sample molecules in the ionization region. If not carefully designed, the sample flow required to carry the vapors to the ionizer, and the flow of gas sampled by the MS, can greatly dilute the molecules of interest, thus reducing the amount of ions produced.
  • The transmission of ions to the mass spectrometer. For this, the ionization region must communicate with the mass spectrometer. However, if not carefully designed, this opening can lead to gas mixing and dilution of the neutral molecules. A properly designed SESI avoids dilution while maximizing the passage of ions from the ionization region to the analyzer.
  • The humidity level, which has a great effect on the reaction rate.

 

Historical perspective

- Pioneering developments: The first SESI sources consisted of a simple chamber located at the inlet of the mass spectrometer where the spray was mixed with the stream of gas and the vapors of interest. This configuration was subjected to very strong dilution of neutral and ionized species, requiring up to 5 lpm of gas to operate. Despite this, the charge transfer reaction rates in SESI are very high and this was sufficient to prove the power of the technique in the early stages of SESI development.

- Low Flow SESI: The LF-SESI was developed with one key criteria in mind: improving the ionization efficiency so as to improve the sensitivity of the detector. The fist LF-SESI was integrated in an explosives detector that used ion mobility and mass spectrometry analysis [***]. A subsequent version was designed to be directly coupled with a mass spectrometer alone [***]. The main goals of these developments were:

  • To reduce the flow gas that carries the sample so as to reduce dilution of the neutral molecules (hence the name Low Flow).
  • To improve the flow of ions passing to the mass spectrometer.

In the LF-SESI, this was accomplished by incorporating two extra electrodes that prevented turbulent mixing of the different gas streams, and produced strong electric fields to guide the ions towards the MS. This greatly improved the ionization efficiency, but at the cost of increasing the area onto which low volatility species could accumulate. The result was an improved ionization efficiency and a worsened background contamination levels, which deteriorated the Limits of Detection over time.
 

SUPER SESI, Generation 1

- Electrode-less SESI:
Developing this configuration became possible only once the numerical methods that simulate the ionizer configuration were mature enough. Like the LF-SESI, the electrode-less SESI configuration reduces dilution of neutral molecules while maximizing the transmission of ions. However, this is accomplished by carefully controlling the flow and the electrostatics configurations.

  • In the regions were the Reynolds is high enough to produce turbulent structures, the geometries are streamlined and the boundary layers continuously accelerated.
  • The detachment line is a well defined by an edge in the orifice that communicates the ionizer and the mass spectrometer.

The electrode-less configuration was implemented in the first generation of SUPER SESI, which was designed for Thermo and Sciex instruments. Each geometry was specifically tailored to each instrument to ensure that the flow is smooth and laminar.

  • Although the electrode-less SESI is not immune to turbulence because there are no walls or electrodes, it is laminar because it is carefully designed.
  • The result is a high ionization efficiency without the burdens caused by the extra electrodes.
     
 
 Detail of the SUPER SESI core coupled with a Thermo Instrument. The simulation results are superposed.

Detail of the SUPER SESI core coupled with a Thermo Instrument. The simulation results are superposed.

 Detail of the SUPER SESI core coupled with a Sciex Instrument. The simulation results are superposed.

Detail of the SUPER SESI core coupled with a Sciex Instrument. The simulation results are superposed.

SUPER SESI, Generation 2

- Toroidal vortex locking:
Our previous simulation method failed to predict the performance of the ionizer when the carrier gas was below 0.2 lpm, as the measured ionization efficiency fall very rapidly below the values predicted by the numerical models. This happened to the LF-SESI and the electrode-less SESI configurations.

The friction between the moving ions and the neutral gas accelerates the gas and forms a toroidal vortex right in the ionization region. Paradoxically, this rather intuitive result became apparent to us only when we were able to incorporate this effect in our numerical model simulating the ionizer! The new simulations showed two things:

  • With our previous design, even with a stable configuration, the toroidal vortex suctions the clean gas into the ionization region when the flow of sample gas is below 0.2 lpm.
  • This dilutes the neutral molecules in the ionization region, reducing the ionization efficiency.

SUPER SESI second generation has been designed with the new simulation method. The internal geometry of the new SUPER SESI incorporates two main features:

  • The internal geometry of SUPER SESI locks this vortex in a fixed position with a smooth cavity that improves its stability.
  • The sample gas inlet has been relocated so that the vortex suctions the flow towards the ionization region.

This results in an improved ionization efficiency at reduced flow rates.

 
 Streamlines and neutral vapor concentration distribution in a SUPER SESI (second generation), showing the toroidal vortex filled with neutral molecules. This configuration is stable and filled with neutral molecules with flows as low as 0.05 lpm.

Streamlines and neutral vapor concentration distribution in a SUPER SESI (second generation), showing the toroidal vortex filled with neutral molecules. This configuration is stable and filled with neutral molecules with flows as low as 0.05 lpm.

 Streamlines and sample ion concentration distribution in a SUPER SESI (second generation).

Streamlines and sample ion concentration distribution in a SUPER SESI (second generation).

towards a fully predictable ionization efficiency

SUPER SESI provides means to control all the parameters that have an impact on the ionization efficiency, including:

  • Temperatures of operation. The temperatures of the sample line, the core, and the curtain gas are independently controlled.
  • Electrospray voltage, and back-pressure, Due to the tolerances in the internal diameter of the capillary, the actual flow passing through the capillary might vary when different capillaries are used. We cannot control the internal diameter of the capillaries, but SUPER SESI incorporates a nano-amperementer that measures the current emitted by the spray which is proportional to the flow of liquid, and can be adjusted by adjusting the electrospray back-pressure and the voltage. Indeed, the current of emitted ions is more relevant to the ionization process than the liquid flow itself.
  • Position of the electrospray tip. The position of the electrospary tip is important to ensure that the toroidal vortex is properly locked. In addition, since the concentration profile of charging ions depends on the position of the tip, and this is important under charge competition conditions, this position can be very important when dealing with charge competition effects.
  • Pressure and flow rate of the sample gas and the curtain gas. SUPER SESI is designed to operate under a wide range of flow rates. Nevertheless, being able to control this is helpful to ensure that the flow configuration within the ionization chamber is always known and predictable.

About humidity

The humidity level of the sample gas carrying the molecules of interest into the ionization chamber is the main parameter influencing the ionization efficiency. The humidity level of alveolar breath samples is very well defined (dew point equals body temperature).

  • For breath samples, measuring the alveolar section of the exhalation is enough to ensure a reliable & predictable ionization efficiency.
  • For in-vivo analysis, regulating the temperature and humidity level of the sample and the carrying gas is important to ensure a reliable & predictable ionization efficiency. DynaSAMPLER is designed to do just that.