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Environmental Instrumental Analysis VI

Inductively Coupled Plasma Determination of Lead

The analysis of heavy metals in environmental samples is now routinely accomplished by the spectroscopic method discussed in this box.

As discussed in the text, anthropogenic lead contamination in the environment in North America and Europe has been decreasing since tetraethyllead and tetra-methyllead were phased out of gasolines in the 1970s and 1980s. Presently, major sources of environmental Pb are residual lead in aerosol particles and dust near roadways (sometimes originating in decaying structures originally painted with lead-based paints), smelting ash, and lead pipes in plumbing (plumbum is Latin for "lead"). Other sources include automobile battery manufacture and disposal as well as cigarette smoke. In the developed countries, exposure to lead is dropping, but in developing countries this exposure is increasing (Ahmed and Ishiga, 2006).

Like some of the other toxic metals discussed in this book, lead concentrations can be determined quickly and accurately using an atomic emission technique called inductively coupled plasma spectroscopy (ICP).

Each chemical element has a unique atomic structure with electrons in well-defined (quantized) energy levels. The movement of electrons between these levels, which requires the absorption or emission of energy, is also well defined, and therein lies the key to atomic emission spectroscopy. If the atoms in a sample are excited using a very-high-energy source—such as a flame, spark, or plasma— many of the atoms' electrons will be excited to higher energy levels. Almost immediately, these excited-state electrons will relax by returning to the ground state; but that return is accompanied by the emission of a photon whose energy corresponds to the difference between the excited-state and ground-state energy levels. And just as the energy of the promotion is well defined, meaning that only specific energies can be absorbed by a particular atom, the energy released by this relaxation—and the emitted photon containing that energy—is very specific to the atom. Since photon energy is related to wavelength (Chapter 1), a means of elemental detection can be based on detecting the light emitted from a sample after the atoms in it are excited by some means; That light is characteristic of the atoms excited in the sample.

In the case of ICP, the excitation source is a very-high-temperature plasma (see Chapter 16 for a definition of plasmas). Light emitted by sample atoms injected into the plasma is collected via lenses and mirrors and focused onto a diffraction grating. This grating separates individual wavelengths in space and focuses the light on a photomultiplier tube (PMT) or other detector that converts light into electronic signals. The wavelength of the light identifies the elements in the sample that emitted the photons, and the intensity of the light as measured by the PMT specifies the concentration of that element in the sample. Atomic identity and amount are the two parameters determined by emission spectrometry that enable it to be used as an analytical tool in environmental analysis.

The specific components of an ICP spectrometer depend on which of two basic designs is used. The first type is called a sequential spectrometer and the second a simultaneous spectrometer. The sequential spectrometer uses only one PMT (detector) and requires a process of scanning through the emission wavelengths to determine multiple elements in one sample. This scanning process is usually accomplished by very exacting rotation of the diffraction grating, which separates the light emissions. This rotation is carefully controlled by a computer so that the signal generated by the PMT can be correlated with the wavelength of light that falls on it; i.e., the computer knows by the grating position—which it is controlling—which wavelength is striking the PMT Sequential ICPs require time to scan and sample the light for each element's emission, but the cost of a sequential instrument is modest compared to the second, more powerful spectrometer design, the simultaneous 1CP spectrometer.

Simultaneous spectrometers have a similar configuration to the simpler sequential instruments—they both have a plasma source (also called a torch) and a monochromator to separate the light into individual wavelength components. However, in the simultaneous instrument the spatially separated wavelength beams are simultaneously dispersed onto a plane, by a grating and a prism, and focused on a detector similar to that of a digital camera. This detector, called a charge-coupled device (CCD), can simultaneously record the intensities of each of the different wavelengths for all of the elements emitting in the hot environment of the plasma. In this way, many

Signals to computer

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Environmental Instrumental Analysis VI

Inductively Coupled Plasma Determination of Lead (continued)

elemental emission wavelengths can be detected individually—but all at one time— without having to scan between them; therefore, many chemical elements can be determined in the same sample simultaneously. Once again, the wavelength identifies the element emitting it, and the light's intensity conveys the concentration of that element in the sample injected into the plasma.

Street dust is a measure, to some degree, of the amount of lead to which people in cities become exposed via inhalation. It is thought that most of the lead in urban dust originated in the past from the particulates released by the combustion of leaded gasoline. With that in mind, a comparison has been made of the lead determined in dust samples taken from the streets of Manchester, England, over time (Nageotte and Day, 1998). Samples of dust, dirt, and soil were scraped off streets with a spatula and analyzed by atomic absorption spectrometry (AAS) and by a modified ICP technique. In AAS, a high-temperature flame or hot carbon tube is used to atomize the element of interest (lead, in our case) in a dust sample that has been dissolved in an acid solution. Once the lead ions have been atomized (converted to elemental Pb) and vaporized by the high temperature, a source lamp with a specific wavelength is shone through the vaporized atoms. Since only specific energies can be absorbed by Pb atoms (since the gaps between the atomic energy levels are quantized and specific to that element), the amount of absorption of the source lamp's light can be used as a measure of the amount of lead in the beam. A PMT on the opposite side of the sample atoms from the source lamp records a decrease in source-lamp light intensity when the sample is introduced. The larger the absorption, the more lead atoms are present in the sample. AAS therefore significantly differs from ICP: One is an absorbance method and the other depends on emission.

The following table shows abridged results from both the English studies, in 1975 and 1997. Lead amounts are reported for three different sampling categories: high-traffic areas, low-traffic areas, and areas where children played (Nageotte and Day, 1998). The significant improvement in lowered lead levels from 1975 to 1997 certainly seems to stem from England's phase-out of leaded gasoline.

Category

ppm Pb (number of samples)

1975

1997

>100 cars/hour

1001 ±40 (180)

577 ±53 (17)

<10 cars/hour

993 ± 186 (53)

536 ±93 (13)

Playgrounds, parks, gardens

1014 ±206 (49)

572 ± 77 (47)

Childrens' blood lead levels are also a measure of exposure to this toxic metal. The U.S. Centers for Disease Control (2006) notes that lead-associated intellectual deficits are found in children with less than 100 ¿¿g/L of lead in their blood. In many developing countries, leaded gasoline and paint are still common. A meta-study incorporating 315 published papers and involving 11,272 children in provinces all over China (in the period 1994-2004) reported a weighted mean of 92.9 fxg Pb/L of blood in children of ages 1 to 12 years. Blood concentration levels of lead in children living in industrial areas were significantly higher than those in suburban or rural areas (Wang and Zhang, 2006).

References: E Ahmed and H. Ishiga, "Trace Metal Concentrations in Street Dusts of Dhaka City, Bangladesh," Atmospheric Environment 40 (2006): 3835-3844.

Chemistry-Based Animations, 2006: http://www.shsu .edu/~chm_tgc/sounds/sound.html.

S. M. Nageotte and J. P. Day, "Lead Concentration and isotope Ratios in Street Dust Determined by Electrochemical Atomic Absorption Spectrometry and Inductively Coupled Plasma Mass Spectrometry," Analyst 123 (1998): 59-62.

U.S. Centers for Disease Control (2006), www.cdc. gov/nceh/ lead.

S. Wang and J. Zhang, "Blood Lead Levels in Children, China," Environmental Research 101 (2006): 412-418.

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