what is the background level?
The background level refers to the spectra that can be measured when no sample is being introduced in the ionizer.
An ideal analyzer should provide a signal corresponding to the introduction of the sample, and no signal when no signal is introduced. However, the ionizer produces ions even when no sample is introduced:
- Primary ions produced by the electrospray. This includes charging ions and other contaminants in the electro-spary liquid. If a sample molecule has the same molecular mass as the ions produced by the spray, SUPER SESI will not be able to detect it because the ionizer is already saturated with these types of ions.
- Memory effects. Low volatility species tend to adhere in the internal walls of the ionizer. These species are then released back in the gas phase until they are fully depleted. This results in a signal that lasts longer than the actual exposure to the sample.
The background level depends on the previous history of the ionizer, and can change over time. The memory effects for each species is different, with lower volatility species having longer memory times.
The Limit of Detection (LoD) depends on the sensitivity (see ionization efficiency), and the background level for each species. In order to detect a chemical species in a sample, the signal produced by it when the sample is introduced must stand out from the background.
why is it important?
Having a low background level is required to detect biologically relevant molecules in the gas phase.
- Low volatility species tend to have strong memory effects, which lead to high background levels that deteriorate the LoD. In the gas phase, low volatility species are present in very concentrations, which require very high ionization efficiency. As the ionization efficiency is improved, LoD is more and more determined by background levels.
- The ionization efficiency of SUPER SESI is sufficient to produce clear signals even for very low volatility species, but that doesn't mean that they can be always detected. In the high mass range of volatile compounds (200 to 700 Da), the main factor defining the LoD is the background level.
Having a well characterized and known background level is required to differentiate signal variations caused by the sample from other variations caused by the ionizer.
- The fact that the background level can change over time because it depends on the previous history of the ionizer can pose some difficulties to analyze the data. In a long bio-marker discovery clinical study, background changes can be a serious source of confounding results.
- When monitoring real time metabolic changes, the time resolution of the analysis is defined by the memory effect time of each species. Improving memory effects enables better time resolved in-vivo metabolic studies.
what affects the background levels?
The background level may depend on many factors. This is a list of some of the most important:
- Exposure: a specific molecular mass may appear in the background of the ionizer only if the ionizer is exposed to it. As obvious as this may sound, it is important to bear it in mind. SUPER SESI is designed to analyze the composition of breath, and we will be breathing on it as we take it out of its box!! Another important source of contamination is the bare hand. SUPER SESI should be handled with clean gloves, not because it is dangerous to the skin, but to avoid contaminating it.
- Surface available: memory effects depend on the interaction between the volatile and the internal surfaces. More surface means more memory effects. The electrode-less configuration was indeed developed to dramatically reduce the available surface. The type of surface available is also very important. All surfaces exposed to the volatile are coated with analytical grade passivized silica to minimize adsorption.
- Temperature: The memory effect can be dramatically reduced by rising the temperature. For this, eliminating any cool spot in the gas-path is crucial. SUPER SESI transfer line is uniformly heated for this reason. The core is machined in a compact block to provide a uniform temperature. Since Sciex counter-flow is not heated, SUPER SESI for Sciex incorporates an extra heater to provide a uniform temperature distribution.
The temperature in the ionization region is limited by the boiling point of the electrospray liquid. As a result, this region is more susceptible to be contaminated. For this reason, minimizing the surface area in the ionization region is very important.
- Purity of the electro-spray liquid: the liquid can be contaminated during solution preparation, storage, or it can be contaminated if high purity grade solvents are not used. It is strongly advisable to use high purity grade water and formic acid. SUPER SESI is designed to minimize chances of contamination during spray handling.
- Purity of the gas. If the gas supplied to the ionizer are contaminated, these contaminants will appear as background signals.
- Fragmentation: fragmentation might occur if the electrospray is not properly formed and a corona discharge with a high energy region is formed instead. This can happen if the electrospray voltage is too high. To prevent the formation of coronas, SUPER SESI incorporates a dampener that drops the voltage if a corona discharge if formed. In addition, the narrow capillaries used in SUPER SESI require lower voltages that are not sufficient to initiate a discharge.
- Cleaning: As obvious as it sounds, cleaning the ionizer improves the background levels.
how SUPER SESI is designed to reduce background levels
- Coating: the sample gas only encounters stainless steel coated with analytical grade passivized silica in its path to the ionization chamber. The clean gas inlet and all outlets are also coated to minimize contamination in the event that the gas accidentally flows backwards. Adsorption is dramatically reduced.
- Improved sensitivity
- Improved washing time
- Improved memory effects
- Electrode-less SESI: By eliminating the electrodes, this configuration greatly reduces the available surface onto which contamination can accumulate. Reducing the surface in the ionization region is particularly important because this region is particularly vulnerable to contamination:
- The local temperature is limited by the boiling point of the electro-spray liquid.
- The areas exposed to the ectrospray plume cannot be passivized because ions would otherwise accumulate onto the coated surfaces, thus destabilizing the electro-spray and changing the electrostatic configuration in an uncontrolled fashion.
- Reversed flow: A small fraction of the sample flow is outputted through a secondary outlet located in the back of the electro-spray.
- This causes the gas to flow backwards in the ionization chamber. As a result, contaminant released by the inner walls of the ionizer are dragged away from the ionization region.
- These contaminants do not get ionized, and cannot contribute to the background levels of the SUPER SESI.
- Cleaning flow: When no sample is introduced in SUPER SESI, the clean curtain gas fills the ionizer. Before this, the curtain gas is passed through a charcoal filter to ensure that no VOCs originating from the gas lines of the laboratory of the mass spectrometer enter the ionizer.
- By default, SUPER SESI is cleaning itself in normal operation. Contaminants are washed by the clean gas.
- This prevents room air from entering the ionizer, thus reducing the exposure.
- Conventional cleaning: SUPER SESI can be easily disassembled for further cleaning, and incorporates a steam generator to steam-clean it (this feature is only available Generation 2).
If, by any chance, SUPER SESI gets contaminated beyond what can be automatically self-cleaned, SUPER SESI can be cleaned the old way: with solvents, soap, or other cleaning solutions (this will depend on the specific contaminant that made it into the ionizer). Cleaning can be boring, but it is the best way to reduce the background levels. Once again, mum was right!
- SUPER SESI Is designed to make this easier. SUPER SESI can be easily opened so that the internal surfaces of the ionizer are easily accessible and cleaned.
- This operation requires to stop the SUPER SESI. Considering all steps (including waiting for the temperatures to reach a steady state15 min to cool down, and 15 min to fully warm up), this operation can be completed in 1 hour. Depending on your application, you may want to clean with a solution of water and methanol every week, or even every day.
- Steam-desorption cleaning: Steam-desorption is widely used to regenerate active adsorvents. In SUPER SESI, this mechanism is used to clean the internal surfaces without even needing the open the ionizer.
- The advantage of steam-desorption cleaning over conventional cleaning is that it is much faster.