A Practical Guide on Coupling a Scanning Mobility Sizer and Inductively Coupled Plasma Mass Spectrometer (SMPS-ICPMS)

The overall goal of this practical guide is to describe the different steps for coupling a scanning mobility particle sizer to an inductively coupled plasma mass spectrometer, and to explain how to use this analysis tool. The SMPS ICPMS instrumentation can help to answer questions in different environmental and technological applications, such as monitoring airborne or combustion emitted particles. We can now characterize synthesized engineered nano objects and study their fate.

The main advantage of this coupling strategy is to gain information on the size and the chemical composition of particles, simultaneously and online, with a time resolution of a few minutes. Based on previous attempts to set up the SMPS ICPMS combination, we started to develop this technique for various aerosol sources using a rotating disc diluter as the introduction system. This visual demonstration described the main step of the coupling strategy of the two instruments, as well as the different settings.

To couple the different instruments and to control the different gas flows, some modifications in the instrumental arrangements are needed. The main steps of the coupling concept are summarized here. Use conductive tubing with an inner diameter of 6.0 millimeters and an outer diameter of 12.0 millimeters to connect the different instrumental parts.

Install the rotating disc diluter between the aerosol source and the differential mobility analyzer, or DMA, where the particle size classification takes place. Split the classified aerosol at the DMA outlet in two fractions, one will be aspirated by the condensation particle counter, or CPC, the other is guided towards the inductively coupled plasma mass spectrometer, or ICPMS. Use a mass flow controller and a HEPA filter to provide particle free dilution argon to the rotating disc diluter.

Add another filter to the excess raw gas outlet of the diluter. Use an additional mass flow controller and filter to adjust the sheath gas flow introduced to the DMA. To adjust the DMA excess gas flow, mount a filter, mass flow controller, and vacuum pump, in series, at the DMA outlet.

Finally, connect an additional mass flow controller and filter to add particle free air to the CPC as makeup flow to reduce the amount of classified aerosol consumed by the CPC. For an example of using an aerosol generator for a suspension, prepare zinc oxide suspension from a commercial zinc oxide nano powder and polyacrylic acid as a stabalizer for the nano-particles. Dilute the prepared suspension to obtain a zinc oxide concentration of approximately 30 micrograms per milliliter.

Use the aerosol generator equipped with a nozzle and a silica gelled dryer to generate an aerosol from the particle suspension and to remove the water from the particles in the silica gel dryer. To do so, first fill the suspension or solution into the bottle and mount it on the aerosol generator. Then set the compressed air valve of the aerosol generator slightly above one bar.

This results in an aerosol flow behind the diffusion dryer of approximately one liter per minute. Finally, connect the dryer outlet to the inlet of the rotating disc diluter. Mass flow controllers are calibrated to gas mass flows under standard conditions.

Since volumetric flows are relevant for this type of measurements, all flows have to be manually verified, for example, by using a primary flow calibrator. First set the argon flow at the DMA sheath gas inlet to 3 liters per minute. Then set the rotating disc diluter temperature to 80 degrees celsius and set the evaporation tube temperature to 350 degrees celsius.

The flow rate of the classified aerosol leaving the DMA results from all the other flows into and out the DMA. The desired classified aerosol flow can be defined by carefully adjusting the excess gas. Adjust the dilution argon flow manually to obtain 0.6 liters per minute as flow of the diluted sample at the outlet of the rotating disc diluter.

Then, carefully adjust the excess gas mass flow controller to achieve a classified aerosol flow of 0.6 liters per minute, the same flow rate as that of the diluted poly dispersed aerosol at the DMA inlet. Next, place the flow calibrator between the DMA and the CPC. Adjust the CPC makeup airflow to reduce the flow rate of classified aerosol aspirated by the CPC to 0.18 liters per minute.

Check the remaining flow of classified aerosol to ensure that 0.42 liters per minute are directed to the ICPMS. Next, calculate the dynamic viscosity and the mean free path of argon at ambient temperature and pressure. Enter both the values in the SMPS software.

In the SMPS software, set the up and down scan durations of the DMA scanning cycle to 150 seconds and 30 seconds. Set the DMA maximum voltage to 4.5 kilovolts to prevent electric arching in the DMA, resulting in a covered particle size range of about 14 to 340 nanometers. Remove the conventional introduction system for liquid samples to directly introduce the dry aerosol into the ICPMS.

Add a conductive tube between the respective port of the DMA outlet and the ICPMS. Maintain the xenon flow constant for all measurements. Tune the other parameters in the ICPMS software, including ICP dilution gas and sampling depth to achieve a fixed xenon intensity.

Set the SMPS and ICPMS acquisition time to cover the desired total duration of the aerosol measurement. After setting the gas flows in the SMPS and the ICPMS parameters, run the measurement in the two instruments manually at the same time. Acquire blank signals during two scans of six minutes with the disc rotation speed set to zero.

Then set the speed to the desired value. Here we show the ICPMS signal of zinc isotope 66. Additionally, here we see the volume based particle size distribution.

This shows the strong correlation between the ICPMS and SMPS signals. Finally, see the text protocol for how to proceed with data analysis. Representative results of a zinc oxide suspension demonstrate that the volume based particle size distribution correlates well with the ICPMS signal.

SMPS data are originally measured in the number concentration regime. The particle size distribution appears shifted towards larger particles when compared to the number based particle size distribution. This is because the conversion from number based to volume based results, and stronger weighting of large particles in the volume regime.

The measurement of particles generated from an aqueous sodium chloride solution shows that keeping the experimental conditions constant results in steady state, time resolved, SMPS and ICPMS signals. The contribution of each element in the overall volume based particle size distribution is determined by the ICPMS signals. For the measurement of particles generated from the thermal treated copper chloride sample, by using a thermogravimetric analyzer, the correlation between the time resolved ICPMS signal of copper, and the volume based particle size distribution is obvious.

Chlorine signals from both particulate species, which are recorded as peaks, and gaseous species, which are recorded as a constant signal covering the entire measured particle size range can be discriminated by SMPS ICPMS. While attempting this measurement procedure, it's important to remember that depending on the sample aerosol particle and the gas metrics are compromised between the RDD dilution and the ICPMS sensitivity to the isotope of interest has to be found. There's a trade off between a high number of monitored elements and their isotopes low detection limits, a high size resolution, and the wide covered particle size range on one side, and the short scan duration, or high temporal measuring resolution.

After its development, this technique paved the way for researchers to explore nano objects regarding their fate, chemical composition, and size distribution. This is relevant to study gas quality as well as particle emissions, or exposure. We use this information for the further development of environmentally sound bioenergy and waste treatment technologies.

After watching this video, you should have a good understanding of how to establish a robust coupling of SMPS and ICPMS instruments, and how to carry out an accurate measurement.