Usually it is recommended to grease the collection substrates to prevent particle bounce and charge transfer. In case of liquid aerosols or aerosols known to be free of bounce or charge transfer phenomena, greasing can be omitted.

In most applications it is difficult to estimate whether the particles are bouncy or not, so it is recommended to grease the substrates in any case in the beginning of the measurements. If bouncing occurs, it can be seen as particle deposits on top of the collection plates which are the lower surfaces of the jet plates.

2. What kind of grease should be used for greasing the collection substrates?

The most commonly used grease in impactor measurements is Apiezon-L, because of its purity and low evaporation properties. Dekati Ltd provides Apiezon- L for substrate greasing but other types of greases may also be used as long as the grease has low evaporation characteristics even in low pressures (min 100 mbar), a thin and smooth layer of used grease can be applied on the substrates, and the grease has low enough viscosity so that the air flow inside the impactor does not blow the grease out of the impactor collection substrates.

Before applying the grease on the substrates, it should be dissolved in a suitable solvent e.g. carbon tetrachloride, toluene or acetone. Dissolve the grease in the solvent by adding the grease in the solvent little by little until you reach grease/solvent ratio of about 1/20-1/30. The Apiezon-L grease is also available from Dekati as a spray.

3. What kind of collection substrates are used for chemical analysis?

Dekati provides polycarbonate collection substrates that are suitable for most chemical analysis methods. If the polycarbonate foils are not suitable, then any other collection substrate material can be used as long as the substrates are not too thick, have a smooth surface and are poreless. Consult your analysis laboratory for suitable substrates for specific methods.

4. What kind of filters should be used on the filter stage?

The filters used in the ELPI™ filter stage are Ø 47 mm in diameter. Many type of filters can be used, the requirements are that the filter should have high collection efficiency for particles smaller than 30 nm and it should not obstruct the flow inside the impactor too much. If the filter obstructs the flow too much, it may be impossible to reach 100 mbar above the filter stage (under the impactor stage # 1) depending on the efficiency of the pump. Certain filter-pump combinations have been tested by Dekati Ltd, and the results can be found in the technical note: Substrates and filters for sampling (available in Dekati customer pages or support@dekati.fi).

5. How often should I clean the impactor?

The cleaning interval of the impactor, and the charger, depends on the particle concentration and on the type of particles that are measured, i.e. how much particle matter is actually collected inside the impactor. The maximum recommended loading for one impactor stage is approximately 1 mg. If the impactor is not cleaned often enough, it will start affecting the calibrated D50 values and flow rate of the impactor.

6. What type of cleaning agent is recommended to be used with the impactor?

The impactor is made of stainless steel, so almost any type of solvent will do. The solvent has to dissolve the measured particles efficiently; for example salts dissolve easily to water, grease to non-polar solvents. The parts should be placed in an ultrasonic bath in the suitable solvent for several minutes. The last wash is recommended to be made with isopropanol, as it eases drying of the parts.

If the cleaning is not sufficient the flow rate through the impactor decreases since the particle deposits inside the nozzles in the jet plates obstruct the flow. This can be verified by adjusting 100 mbar below the first stage of the impactor and measuring the ELPI™ inlet sample flow with an external flow meter. If the measured flow rate is much lower that the flow rate mentioned in the calibration data sheet, the cleaning has not been sufficient and another solvents should be tested. The calibrated flow rate is in lpm in the conditions specified in the calibration data sheet.

7. How can I be sure that the flow through the impactor stays constant?

The flow through the ELPI™ impactor stays constant as long as the impactor does not leak, the impactor nozzles are not clogged and 100 mbar is adjusted under the first impactor stage. When 100 mbars is adjusted under the first impactor stage, the stage # 1 jet plate acts as critical orifice and controls the flow. Therefore no additional flow control is required.

8. How often should I clean the charger?

The required cleaning interval for the charger, and the impactor, depends on the sample particle concentration and length of the measurement. Usually it is recommended to clean the charger whenever the impactor is cleaned. In outdoor air quality measurements for example, cleaning the charger once a month is enough in most cases. Increase in the charger voltage can also be used as an indication for the need for charger cleaning.

9. What type of cleaning agent is recommended to be used with charger cleaning?

The solvent that is used for cleaning the impactor can be used for cleaning the charger as well. The solvent has to dissolve the measured particles efficiently; for example salts dissolve easily to water, grease to non-polar solvents. The parts should be placed in an ultrasonic bath in a suitable solvent for several minutes. The last wash is recommended to be made with isopropanol, as it eases the drying.

The charger is made of Teflon and stainless steel. These materials can stand many solvents. However, Teflon can easily build up static charge, so remember not to wear rubber gloves when handling the Teflon parts. In addition, some solvent molecules may stick to the Teflon surface making the Teflon part conductive even though it seems that all the solvent has evaporated. Therefore the agent should be washed carefully off with e.g. isopropanol and then allow the parts to dry before using the charger in the ELPI™.

10. Is it necessary to use a neutralizer before ELPI™ charger?

If particles have a strong positive charge before ELPI™ charger it might be necessary to neutralize the sample. Particle inherent charge can be seen when the ELPI™ charger is turned off. In this setup the ELPI measures the charge distribution of the particles. As a rule of thumb a neutralizer is needed if current values with charger OFF are positive and more than 50% of current values with charger ON. Negative charge before ELPI™ is usually not a problem as the ELPI™ charger is designed to produce sufficient amount of ions to compensate this negative charge and still give a maximum equilibrium charge to particles.

11. I made a measurement using wrong setup values in the ELPIVI software, can I fix the data?

The measured current values from all 12 stages, together with calculated distribution, are always saved in the ELPIVI data file. Therefore the results can be recalculated using the correct setup values. This is done in the data file filter section of the ELPIVI software. See more detailed instructions in the ELPIVI user manual in section 5.

12. What does correction mean in the data?

Correction refers to fine particle correction algorithm used in the calculation. In impactors diffusion causes particles smaller than the cut diameter of an impactor stage to be collected to some extent. As the concentration of particles in the gas in the vicinity of the collection plate approaches zero, diffusion creates a net flow of particles towards the collection plate. This phenomenon is characteristic to all instruments, not just the ELPI™. In gravimetric impactors this phenomenon has negligible effect on the result since the mass of these small particles is often insignificant compared to the actual impacted mass. However, when the particles are detected by their electrical charge, the effect can be significant due to the better sensitivity of the charge measurement.

To overcome these losses an algorithm is used to correct the ELPI™ reading, as the losses have been measured together with the stage collection efficiency curves. This algorithm is called correction algorithm and it is implemented in the ELPIVI software. The fine particle correction algorithm used in the software version ELPIVI4.0 corrects both for diffusion and space charge losses. The correction method is described in more detail in:

Virtanen, A., Marjamäki, M., Ristimäki, J., Keskinen, J. (2001). Fine particle losses in electrical low-pressure impactor, Journal of Aerosol Science, vol 32, pp 389- 401

13. I try to open the data file but I get an error message and no data is seen in the sheet. What can be the problem?

There are certain computer settings that need to be correct in order to enable the use of the ELPIxls. First, the computer decimal symbol must be set to dot (.) instead of comma (,). This can be changed in the computer control panel regional settings menu. Select numbers and decimal symbol, and change it to dot (.).

The other settings that need to be correct are Excel settings. When you start the Excel, select Tools à Macro à Security, and select security level medium or low. Also check that the calculation iteration is enabled in Tools à Options à calculations menu.

14. I’ve installed the filter stage in my ELPI™, now I cannot reach 100 mbar, what is the problem?

When filter stage is used the flow is pulled through the filter material. The flow is still controlled by adjusting 100 mbar below impactor stage # 1, which in the case of the filter stage means 100 mbar above the filter. This means that if the filter material is too thick, it will obstruct the flow too much and the pump is not powerful enough to reach 100 mbar above the filter.

First check that your pump is efficient enough to pull the flow through the filter material you’re using. Suitable filter/pump combinations can be found in the ELPI™ user manual section 8.6 and in the technical note ‘Substrates and filters for Dekati impactors’.

Check that there is only one filter on the filter stage. The filters are very thin so it is quite easy to accidentally place two filters on the filter stage instead of one. If there are two filters on the filter stage, they may together obstruct the flow too much.

15. Can the ELPI™ be used on an inclined plane?

ELPI™ can be used in vertical or horizontal position. The particles residence time in the impactor small enough so that the gravimetric force has negligible effect on particle trajectories. However, remember to make sure that the air circulation inside the ELPI™ cabinet is not affected (the power supply fan circulates the air inside the cabin).

If particles are collected on collection substrates for further chemical or gravimetric analysis, this result can be affected by the position of the ELPI™.