Determining Heavy Metals in Particulate Matter

Primary particulate matter is generated from various sources. A part of the air pollution caused by particulate matter has anthropogenic origin and occurs not only due to the burning of fossil fuels and wood, but also due to swirling and abrasion. Particulate matter pollution can also be caused due to natural sources such as sea spray, erosion, volcanic eruptions, and desert dust (Figure 1).

Figure 1. Sources of particulate matter: industry, burning of wood and fossil fuels, volcanic eruptions, sea spray, desert dust and erosion.

Chemical reactions involving materials of agricultural origin, e.g. SO_x, NH₃, NO_x, volatile organic compounds, and waste gases released from combustion processes lead to the formation of secondary particulate matter in the atmosphere. When we inhale air, innumerable airborne particles enter our body, and based on their sizes, they may be deposited in the respiratory tract or in the gas exchange zone (the pulmonary alveoli).

Particle Sizes

A mixture of particles with a diameter of <10 µm is called particulate matter or PM₁₀ (i.e. thoracic dust). These particles are so small that they can easily enter the lungs via the windpipe. Similarly, a mixture of dust particles with diameters of less than 1 and 2.5 µm are respectively known as PM₁ and PM_2.5 (i.e. respirable dust). Dust particles with diameters of less than 0.1 µm are known as ultrafine particles.

Sampling

Conventional sampling systems are equipped with filters to collect particulate matter from the air as well as dust precipitation that settles on the ground. The collected particulate matter is dissolved using acid digestion. However, this technique is highly time-consuming, and can also result in inaccurate analysis results as a result of contamination.

There are other sampling systems available that can overcome such disadvantages and are considered to be viable alternatives to the filter techniques. Particle-Into-Liquid Sampler, or PILS (Figures 2), can directly transfer aerosols, and consequently particles, into an aqueous sample solution. The collection process takes around 15 minutes to ensure that an adequate sample solution is provided for the voltammetric analysis.

The acquired measuring solution can be transferred to the analyzer for voltammetric or ion-chromatographic determination without the need for additional sample preparation steps. The semi-continuous measurement involved in this process enables the determination of anions and cations with a higher temporal resolution, allowing the tracking of rapid changes in the composition of aerosols.

Figure 2. The PILS (Particle-Into-Liquid Sampler) transfers aerosols, and therefore also particles, directly into an aqueous sample solution that can be analyzed voltammetrically by a 797 VA Computrace without any further sample preparation steps needed.

The size of the particles to be analyzed can be limited to a diameter of less than 1, 2.5 or 10 µm by using an upstream separator (cyclone, impactor). The PILS can directly transfer particles of all sizes (from PM₁ to PM₁₀₎ into the aqueous sample solution.

Schematic Setup of a PILS Sampling System

Ambient air is drawn inside by a vacuum pump at a rate of 1 m³/h and is transported via supersaturated steam generated from ultrapure water in the PILS. As shown in Figure 3, in the steam atmosphere, the aerosol particles develop into droplets and are collected by an impactor plate at the end. As a result, adequate sample solution can be produced in a short period of time for subsequent analysis.

Figure 3. Schematic setup of a PILS sampling system.

The PILS can be combined with a voltammetric measuring system, such as the 797 VA Computrace (Figure 4), to quantify heavy metals such as cadmium, zinc, lead, copper, cobalt, nickel, and so on (Figure 5) in particulate matter at a temporal resolution of nearly 30 minutes.

Figure 4. A 797 VA Computrace.

Figure 5. Semi-continuous voltammetric determination of heavy-metal concentrations (Cd, Pb and Cu) in ambient air with the help of a PILS sampling system.

Example of a Measurement of Cadmium, Lead and Copper in Ambient Air

For the given example, the heavy metal concentration in the ambient air was continuously monitored for three days. Once the sample was generated using PILS, the concentrations of the heavy metals such as copper, lead, and cadmium were determined voltammetrically. There was a clear indication of a steady increase in the concentrations of heavy metals. As the ambient air is cleaned after a spell of rain, there was a noticeable decrease in the measured concentrations of copper and lead.

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.