FAQ

SDI – FAQ 

1. Is it necessary to grease the collection substrates and why?

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

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

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

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

1. Is it necessary to grease the collection substrates and why?

Usually it is recommended to grease the collection substrates to prevent particle bounce. In case of liquid aerosols or aerosols known to be free of bounce greasing can normally 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 135 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 and 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.

The cleaning interval of the impactor depends on the sample concentration and 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 through the impactor.

5. 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 135 mbar below the first stage of the impactor and measuring the SDI 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. 

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

The flow through the SDI impactor stays constant as long as the impactor does not leak, the impactor nozzles are not clogged and 135 mbar is adjusted under the first impactor stage. When 135 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.