A P P L I C A T I O N B R I E F

ICP - Mass Spectrometry

Authors:

Kyoko Kobayashi

Mitsuhiko Morimoto

PerkinElmer., Inc. Yokahama, Japan

Osamu Nagafuchi

University of Shiga Prefecture (Fukuoka Institute of Technology) Hikone, Shiga, Japan

Introduction Concern about air pollution has been growing rapidly, with most of the focus on gaseous pollutants, such as greenhouse gases, NOx, and SOx. However, airborne particulates, especially small ones, are rapidly gaining attention due to

their impact on human health. This is because smaller particles can be carried over long distances by wind and penetrate deep into the lungs, where contaminants can have a direct interaction with lung tissue and the associated blood vessels. Airborne particulates are generally classified into two size ranges: those with aerodynamic diameters less than 10 µm are categorized as PM10, and those with aerodynamic diameters less than 2.5 µm are referred to as PM2.5.

Throughout the world, PM2.5 regulations have been implemented. The World Health Organization (WHO), for example, has established guidelines for safe levels of PM2.5,1 and in 2008, the EU established their own limits of 25 µg/m3/annum.2 In Asia, many cities have high traffic densities and large populations in close quarters, which can have a significant impact on local and regional PM concentrations. As early as 2009, Japan implemented standards stating that the annual average PM2.5 concentrations must be less than 15 µg/m3, with daily concentrations of less than 35 µg/m3.3 This was followed in 2011 by the implementation of the “Guideline for Component Analysis of PM2.5” which states that soluble inorganic ions, organic carbon, elemental carbon, and various metal components must be analyzed.

PM10 and PM2.5 Air Pollution Monitoring and Source Apportionment in Asia Using the NexION ICP-MS

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In order to implement a regime to reduce the concentration of PM2.5, it is important to determine the origins of these particulates, hence the need to collect and analyze them.

Particle collection is accomplished using cascade impactors, which collect particles by size fraction on air filters with different pore sizes, placed at multiple locations within the city. By analyzing the elemental composition of each size fraction and correlating the results with meteorological data from the collection period, an understanding of particle composition, source, and mobility within the atmosphere can be gained. ICP-MS is often the analytical instrument of choice for such applications due to its low detection limits and wide linear dynamic range.

This work focuses on the analysis of inorganic components of PM2.5 and PM10 collected on air filters from different cities in Asia.

Experimental

Sample Collection and PreparationAir particulates were collected in Beijing, China (five samples), Seoul, South Korea (five samples) and Tokyo, Japan (two samples) from spring through to autumn using four-stage cascade impactors, as shown in Figure 1. Each collection period lasted 18 hours with air being drawn through the impactors at 3 L/min. The particles were collected on polycarbonate filters at each stage of the impactor.

After each collection period, the filters were removed and prepared for analysis. Since several different analyses were performed on the particles, each filter was divided into 12 sections, with five of the 12 sections being used for ICP-MS analysis. The other filter sections were used for ion chromatographic analysis of anions and IR analysis for organic components (not discussed in this application brief).

For ICP-MS analysis, the filters were digested in a microwave oven using 2.5 mL of concentrated nitric acid and 0.5 mL of concentrated hydrofluoric acid (both TAMAPURE AA-100 grade) in accordance with the microwave program shown in Table 1.

Figure 1. Schematic of a 4-stage cascade impactor used in this work.

Step Temp (°C) Ramp (min) Hold (min)

1 160 5 5

2 210 5 15

3 50 0 10

Table 1. Microwave Digestion Program.

Table 2. Instrumental Parameters.

Parameter Value/Composition

Nebulizer Glass Concentric

Spray Chamber Glass Cyclonic

Sample Uptake Rate 0.25 mL/min

RF Power 1600 W

Cones Pt

After removal from the microwave, the digestion vessels were cooled for 20 minutes in an ice water bath before being transferred to a hot plate and evaporated to near-dryness. Prior to analysis, the residue was re-dissolved in 10 g of 0.4 N nitric acid.

Meteorological Data CollectionAlthough not reported in this application brief, the likely sources of the trace metals and major ionic components in the aerosols were determined via a principal component analysis (PCA), by examining correlations between elements and by calculating the crustal enrichment factors of the elements. In addition, the major sources that affected PM10 and PM2.5 concentrations at the sampling site were identified using a Hybrid Single Particle Lagrangian Integrated Trajectory model (HYSPLIT, NOAA, Silver Spring, Maryland, USA) with the National Centers for Environmental Prediction Global Data Assimilation System (NCEP-GDAS, NOAA) meteorological dataset.

Instrumental AnalysisAll analyses were performed on a NexION® ICP-MS (PerkinElmer, Shelton, Connecticut, USA). The instrumental parameters are given in Table 2. During method development and validation, different analytical cell conditions and isotopes were investigated and the optimal values reported in Table 3. In Reaction mode, methane was used as a cell gas. For all isotopes, praseodymium (10 ppb) was used as an internal standard due to its absence from the samples and its similar first ionization potential to indium (In), where indium was not used since it was an analyte and found to be present at the sites.

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Element Isotope Cell Mode Methane Flow (mL/min) RPq

Lithium (Li) 7 Standard 0 0.25

Aluminum (Al) 27 Standard 0 0.25

Vanadium (V) 51 Reaction / CH4 0.6 0.7

Chromium (Cr) 52 Reaction / CH4 0.6 0.7

Manganese (Mn) 55 Reaction / CH4 0.6 0.7

Cobalt (Co) 59 Standard 0 0.25

Nickel (Ni) 60 Reaction / CH4 0.6 0.65

Copper (Cu) 63 Standard 0 0.25

Zinc (Zn) 64 Reaction / CH4 0.6 0.65

Arsenic (As) 75 Standard 0 0.25

Selenium (Se) 82 Reaction / CH4 0.6 0.65

Strontium (Sr) 88 Standard 0 0.25

Molybdenum (Mo) 98 Standard 0 0.25

Cadmium (Cd) 111 Standard 0 0.25

Indium (In) 115 Standard 0 0.25

Antimony (Sb) 121 Standard 0 0.25

Tellurium (Te) 130 Standard 0 0.25

Barium (Ba) 138 Standard 0 0.25

Lead (Pb) 208 Standard 0 0.25

Bismuth (Bi) 209 Standard 0 0.25

Table 3. Elements, Isotopes and Universal Cell Conditions

Results

Table 4 shows the average elemental concentrations in PM10 and PM2.5 fractions collected in Seoul, Beijing, and Tokyo over the sampling period. These results indicate that Beijing had the highest elemental concentrations for most analytes except copper (Cu), which was higher for both PM10 and PM2.5 in Tokyo. Vanadium (V) and barium (Ba) were found to be highest in PM2.5 fraction in Seoul. Seoul was found to have significantly lower PM10 concentrations for all elements than Tokyo, however

both cities were found to have similar PM2.5 concentrations for most analytes with the exception of Cu. The large standard deviation (sd) in the measurements can be explained by the filters being collected over three seasons in each city (spring, summer, autumn) where summer is the rainy season for each of the locations and will result in large changes in air quality due to the deposition of airborne particulates.

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Beijing Seoul Tokyo

Element PM10 PM2.5 PM10 PM2.5 PM10 PM2.5

mean sd mean sd mean sd mean sd mean sd mean sd

Li 0.97 0.88 0.65 0.36 0.06 0.04 0.29 0.02 0.59 0.55 0.58 0.09

Al 802 787 415 170 120 107 312 75.7 610 678 232 54.3

V 1.81 1.83 0.99 0.46 0.26 0.22 1.03 0.59 0.45 0.19 0.43 0.15

Cr 41.5 21.5 41.9 19.8 1.94 0.34 14.4 1.98 15.6 3.83 15.9 2.69

Mn 16.8 17.9 10.7 5.35 0.6 0.09 3.83 1.39 3.53 3.42 3.22 3.27

Co 0.59 0.33 0.36 0.1 0.04 0.01 0.1 0.05 0.12 0.09 0.1 0.06

Ni 32.4 23.6 18.4 9.37 5.53 3.23 10.7 3.35 30.2 36.2 14.3 11.2

Cu 10.5 2.8 39.5 72.6 0.93 0.55 6.63 4.64 67.9 89.9 62.7 85.7

Zn 76 56.8 102 43.8 2.16 0.11 21.5 6.36 28.5 27.0 51.0 42.7

As 2.86 2.97 2.92 2.33 0.02 0.00 0.31 0.03 0.14 0.08 0.09 0.07

Se 0.51 0.28 0.49 0.1 0.04 0.03 0.24 0.02 0.18 0.21 0.29 0.20

Sr 7.33 5.99 4.02 1.06 0.35 0.02 2.44 0.41 2.84 1.33 2.38 0.61

Mo 7.13 5.55 7.73 4.93 0.11 0.02 0.78 0.00 0.61 0.04 0.57 0.00

Cd 0.10 0.06 0.24 0.17 0.01 0.00 0.11 0.04 0.05 0.05 0.06 0.07

In 0.01 0.01 0.02 0.01 BD 0.00 0.01 0.00 BD 0.00 0.01 0.01

Sb 4.85 5.39 5.87 5.93 0.14 0.13 1.19 0.98 0.52 0.07 0.39 0.04

Te 0.02 0.02 0.01 0.01 --- --- 0.01 0.02 0.01 0.01 0.01 0.00

Ba 18.8 14.1 7.6 3.48 1.89 1.20 14.6 11.2 5.93 5.58 3.85 1.99

Pb 7.32 5.9 12.7 5.83 0.35 0.01 5.04 2.28 2.57 2.64 3.20 3.35

Bi 0.15 0.11 0.27 0.15 0.01 0 0.11 0.07 0.06 0.02 0.06 0.06

Table 4. Average Elemental Concentrations (ng/m3) for PM10 and PM2.5 Particles in Beijing, Seoul, and Tokyo over April-November.

BD = Below detection --- = No data available

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Correlating the collection locations and times with meteorological data,4 it was determined that the airborne particulates fell into three categories: natural origin, anthropogenic origin, and mixed source. PM10 fell into all three categories, while PM2.5 was almost completely from anthropogenic sources. Primary natural sources of the particles could be traced to the Gobi desert and sea-spray, whereas anthropogenic sources were thought to be primarily from iron, steel, semiconductor industries and fuel combustion in the areas. Further interpretation of the results is available elsewhere.4

Conclusion

This work has described the collection, sample preparation and inorganic elemental analysis of atmospheric PM2.5 and PM10 using the NexION ICP-MS. Particles were collected in three large Asian cities and analyzed by ICP-MS for inorganic elements. By applying advanced data interpretation algorithms and correlating with

meteorological data at each site, it was found that the particles originated from both natural and anthropogenic sources with PM2.5 originating almost exclusively from anthropogenic sources.

References

1. World Health Organization (WHO). 2006. WHO Air Quality Guidelines for Particulate Matter, Ozone, Nitrogen Dioxide and Sulfur Dioxide; http://www.euro.who.int/Document/E87950.pdf.

2. European Union (EU). 2008. https://ec.europa.eu/environment/air/quality/standards.html.

3. Ministry of Environment, Government of Japan (MoEJ). 2009. https://www.env.go.jp/en/air/aq/aq.html.

4. Nagafuchi O., Morimoto M. 2013. Potential Source Analysis for PM10 and PM2.5 in Beijing, Seoul and Tokyo, AAQR Conference Proceedings. Poster Presentation.