Further Readings

1. C. Maczka et al., "Evaluating Impacts of Hormonally Active Agents in the Environment," Environmental Science and Technology ( 1 March 2000): 136A; G. M. Solomon and T. Schletter, "Environment and Health: 6. Endocrine Disruption and Potential Human Health

Implications," Canadian Medical Association Journal 163 (2000): 1471; S. H. Safe, "Endocrine Disruptors and Human Health: Is There a Problem?" Environmental Health Perspectives 108 (2000): 487.

2. P. H. Jongbloet et al., "Where the Boys Aren't: Dioxin and the Sex Ratio," Environmental Health Perspectives 110 (2002): 1.

3. World Health Organization, "Global Assessment of the State-of-the-Science of Endocrine Disruptors" (2007): available at www.who.int/ipcs/publications/ new_issues/endocrine_disruptors/en/

4. P. A. Darnreud et al., "Polybrominated Diphenyl Ethers: Occurrence, Dietary Exposure, and Toxicology," Environmental Health Perspectives 109 (supplement 1) (2001): 49.

5. R. Renner, "The Long and Short of Perfluorinated Replacements," Environmental Science and Technology 40 (2006): 12.

6. K. S. Betts, "Perfluoroalkyl Acids: What Is the Evidence Telling Us?" Environmental Health Perspectives 115 (2007): A250.

Websites of Interest

Log on to www.whfreeman.com/envchem4/ and click on Chapter 12.

Environmental Instrumental Analysis II!

Electron Capture Detection of Pesticides

Chlorine-containing organic compounds of the type that have been discussed in the preceding chapters usually occur in the environment at very small concentrations, but they can be detected and quantified by techniques such as the one discussed in this box.

The widespread occurrence of pesticides in the environment makes their detection an important task, but their often-low concentration makes this job difficult. One of the solutions to detecting very small amounts of environmentally important chemicals is to use very sensitive chromatographic detectors. In the case of methane, this is accomplishèd with the flame ionization detector (see Environmental Instrumental Analysis Box II).

The most common gas chromatographic (GC) detector used for halogen-containing pesticides is the electron capture detector (ECD). Since many important pesticides contain chlorine, a detector system that responds to molecules that contain this element is the key to sensitive biospheric analysis. Examples of target chlorinated pesticides are DDT (and its breakdown product DDE), lindane, and chlordane. The only chlorine-containing compounds unsuitable for this technique are those whose high boiling points make them unsuitable for gas chromatographic analysis.

The electron capture detector, like all GC detectors, is located at the end of the chromatographic column (see Environmental Instrumental Analysis Box II) located in a temperature-controlled (and programmable) oven. When analytes (compounds that have been separated by the chromatographic process)

exit the column, they enter the ECD and are detected.

The principle upon which the ECD works involves the disruption of a detector's electronic standing current by the arrival in the ECD of an analyte containing electron-loving (electrophilic) atoms as halogens, which is the basis for the ECD signal. The standing current is generated by a piece of radioactive nickel-63 fixed on the wall of the detection chamber. This unstable element (half-life 92 years) continuously emits beta particles (j3 particles, high-energy electrons from nuclear decay, as described in Chapter 9) at a relatively constant rate. The GC carrier gas used in this analysis is usually a mixture of helium and a small amount (say, 5%) of another volatile compound, such as methane, at a constant concentration. Because the carrier gas mixture is homogeneous, a constant ratio of helium and CH4 flows into the ECD. The flow of /3 particles from 63Ni collides with some of the methane molecules in the carrier gas and create a "cloud" of slow (or thermal) electrons in the detection chamber. This cloud creates an electrical potential between the two electrodes placed in the detection chamber; the resultant current is amplified and sent to the computer (or integrator). Since this constant standing current is present whenever the detector is on and the carrier gas is flowing, the computer receives a constant detector signal. The figure on the top of next page shows the major components of the ECD.

The ECD's standing current changes when an analyte arrives in the ECD from the end of the GC column after chromatographic

(continued on p. 538)

Environmental Instrumental Analysis III

Electron Capture Detection of Pesticides (continued)

Waste/ trap

Electrodes

Analyte

Computer <-

GO column entrance separation: Target compounds decrease the standing current because some of the electrons are captured by electrophilic atoms that are present in the analyte. The more of the compound that arrives, the larger the decrease in current. The computer measures the amount of this decrease and correlates detector signal with analyte concentration; however, unlike the positive FID signal (in which more analyte means more signal), the ECD signai is "upside down"—its information is a measure of missing signal. The result is, however, the same: The amount of each target analyte can be sensitively and reproducibly determined by the ECD. Furthermore, like other chromatographic systems, the time that each compound exits the column and generates the detector signal can be used as a means of identification if other analyses and chemical standards are used.

Among many other applications, the ECD has been used to measure the presence and amount of DDT and the related substance DDE (Chapter 10) in the tissue of Mexican free-tailed bats (Tadarida brasiliensis). These animals absorb DDT and DDE from their diet of insects that have been exposed to DDT in the environment. Although the DDT content of the bats is very low, the breakdown product £>,f>'-DDE remains detectable. The figure below shows two (superimposed) chromato-grams resulting from ECD analysis of carcasses of female hats collected from two southwestern U.S. caves, Carlsbad Caverns, New Mexico, and Vickery Cave, Oklahoma (Thies and McBee, 1994). Although no DDT itself was detected in either animal, the DDE content of the bat carcass (all tissue except brain and intestines) from Vickery Cave was approximately 41.9 jjlg DDE per gram of total fat.

Seabirds collected in the Barent Sea (in the Arctic, above 75°N) have also been analyzed using this instrument. A number of chlorinated pesticides were determined for two bird species whose diets were known to consist

-

1 DDE

- Vickery Cave,

Oklahoma

Carlsbad Caverns,

-

New Mexico

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I

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i t i -I t t- r

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6 8 10 Time (minutes)

cL Black guillemot .

HHHHHHHHHHBH

0 20 40 60 80

Mean contribution to total chlorinated pesticides and metabolites

Hi HI /7-HCH ■ oxychlordane Hi ds-chlordane

H /ra/7.s'-nonachlor p,//-DDE Mirex entirely of polar codfish. DDTs metabolite DDE was again found, along with six or seven chlorinated pesticides. The mean DDE concentrations were 608 (±43) ppb for Black guillemot and 1168 (±231) ppb for Kittiwake, expressed as mass of DDE per mass of bird lipid (Borga ct al., 2007). The figure above shows the mean distribution of seven chlorinated compounds examined using this method.

By comparing the ratio of the concentration (in these birds' fat) of a specific chlorinated pesticide such as ds-chlordane to that of a highly bioaccumulated polychlorinated biphenyl (PCB 153), workers have been able to determine a relative measure of bioaccumulation for the chlorinated pesticides and metabolites examined. In comparing the bird species Black guillemot and Kittiwake, ds-chlordane was better eliminated by both species when compared with the DDT metabolite DDE. But for two other bird species examined, DDE was very slowly eliminated and, in fact, is bioaccumulat-ing in relation to the PCB-153 standard in those species. This result means that the biotransformation of chlorinated compounds is highly bird-species-specific.

References: K. Borga, H. I lop, J. I J. Skaare, H. Wnlkers, and G. W. Gabrielsen, "Selective Bioaccumulation of Chlorinated Pesticides and Metabolites in Artie Seabirds," Environmental Pollution, 145 (2007): 545- 553.

M. L. Thies and K. McBee, "Oross-Placental Transfer of Organochlorine Pesticides in Mexican Free-Tailed Bats from Oklahoma and New Mexico," Archives of Environmental Contamination and Toxicology 27 (1994): 239 242.

Chemistry-Based Animations, 2006. http://www.shsu .ed u/~ chm_tgc/so u nds/sound. htm 1.

Gas Chromatography/Mass Spectrometry (GC/MS)

Environmental Instrumental Analysis IV

Analytical identification of volatile compounds in samples taken from the environment often relies on this extremely powerful technique. The heart of this method is mass spectrometry, a method of identifying molecules by their unique '[fingerprints. "

As we saw in our discussion concerning FID identification (page 537), gas chromatography (GC) is a very powerful tool for separating the components of a mixture. In identifying the structure of individual compounds in a separated mixture, one of the most powerful gas chromatography detectors is the mass spectrometer (MS); it is one the few GC detectors whose analytical signals actually probe the structure and elemental composition of the molecules it analyzes. Since the MS can be used as a stand-alone analytical tool, the combination of GC and MS is called a "hyphenated technique": gas chromatography/ mass spectrometry, GC/MS.

Like the FID and ECD, the mass spectrometry detector is positioned at the end of the gas chromatographic column and analyzes compounds one by one, as they exit the GC column in the gas phase. The MS can be divided into three parts: the ionization/fragmentation source, the mass analyzer, and the mass detector.

As the molecules enter the low-pressure ionization/fragmentation chamber of the mass spectrometer, they are bombarded with high-energy electrons, which causes many of the molccules to lose an electron, to form free-radical cations. Only charged molecular fragments are accelerated from there to the mass analyzer; all un-ionized particles are sucked out, by the vacuum system, to waste. A simple example of ionization is the formation of CH4+ cations from methane molecules. This ion has a charge of +1 and a mass of 16, giving a mass/charge (m/z) ratio of 16.

After their formation, the ions are separated from one another according to their m/z ratio in the mass analyzer and then enter the mass detector. In the figure on the top of next page, depicting a 70-electron-volt ionization source, the GC column effluent enters on the right, ionization occurs where the stream of electrons cross its :path, and ionized fragments are accelerated by the charged plates and exit from the ionization chamber and into the mass analyzer on the left. The intensity of the mass detectors signal versus m/z is recorded in each mass spectral scan, the mass spectrum.

The chromatographic plot for GC/MS is called the total ion chromatogram; it displays the total ion current on the y axis versus time on the x axis. The individual peaks correspond to different compounds in the mixture that were separated by the GC. This plot is comparable to the FIDs chromatogram, which has the FID signal on the y axis and time on the x axis. (In contrast to GC/MS, the FID technique requires chemical standards—known compounds—whose retention-time data can be compared to the unknown compounds for identification purposes.) In the figure on the bottom of next page, the ionization source has been miniaturized and shown above and feeding molecular fragments to the quadrupole mass analyzer. The mass detector is shown below the mass analyzer. An example of a GC/MS total ion chromatogram is shown at the right in the figure.

To mass analyzer

High-vol tage sou ice

To vacuum

Heated filament (electron source)

Charged slits/plates accelerate and focus ions

Column from GC via transfer line

(continued on p. 542)

Environmental Instrumental Analysis IV

Gas Chromatography/Mass Spectrometry (GC/MS) (continued)

The CH4+ species that is generated from methane is know as the molecular (M +) or parent ¿on; from its m/z ratio of 16, one can determine the molecular weight of the molecule, a parameter that would aid in the identification of this compound if it were an unknown. Because the electrons that impact the molecules have more than enough energy to cause the formation of the molecular ions, some of them generally hreak down by cleavage of bonds to form fragment ions having lower m/z ratios. In the case of CH4, since there are only C—H bonds, the fragmentation pattern is very simple, producing CH3" (m/z = 15), CH2+ (14), CHT (13), and finally CT (12). Such fragmentation patterns yield more information about the molecular structure of the parent compound; its structural formula can often then be identified, particularly when the fragmentation pattern is compared to a library of GC/MS patterns from known molecules.

For instance, the mass spectral fragmentation patterns of two chlorinated phenol isomers—which, of course, have the same molecular weight—can be used to differentiate between them. For example, although 2,3,4 trichlorophenol and 2,4,5 trichlorophenol have vanishingly small differences in boiling point and other physical properties, their electron impact mass spectra are significantly different since to some extent they fragment differently, and therefore they can be used to discriminate between these two isomers. Databases containing electron impact mass spectra are available free online (NIST, 2005) and are for sale from GC/MS instrument manufacturers.

The figure below shows the mass spectrum, reconstructed from data in the NIST

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database, for f>,p'-DDE, the environmental breakdown product of £>,£>'-DDT The formulas corresponding to the three most intense mass spectral peaks in the DDE mass spectrum are shown above their appropriate m/z values. The C]4HgCl4+ peak corresponds to the parent ion, whereas the other two labeled peaks correspond to fragment ions formed by the loss of two or of all four chlorine atoms. The fragmentation pattern for other DDE isomers, such as o,p'-DDE, would have different relative intensities of the ion peaks.

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

P. Janos and P. Aczel, "Ion Chromatographic Separation of Selenate and Selenite Using a Polyanionic Eluent," Journal of Chromatography A 749 (1996): 115 122.

MST Chemistry WebBook, 2005. http://webbook.mst .gov/chemistry.

Interventions available today could lead to decisive gains in prevention and treatment—if only the world would apply them

By Claire Panosian Dunavan

Claire Panosian Dunavan, "Tackling Malaria/' Scientific American, December 2005, 76-83.

Long ago in the Gambia, West Africa, a two-year-old boy named Ebrahim almost died of malaria. Decades later Dr. Ebrahim Samba is still reminded of the fact when he looks in a mirror. That is because his mother—who had already buried several children by the time he got sick—scored his face in a last-ditch effort to save his life. The boy not only survived but eventually became one of the most well-known leaders in Africa: Regional Director of the World Health Organization.

Needless to say, scarification is not what rescued Ebrahim Samba. The question is, What did? Was it the particular strain of parasite in his blood that day, his individual genetic or immunological makeup, his nutritional state? After centuries of fighting malaria—and conquering it in much of the world—it is amazing what we still do not know about the ancient scourge, including what determines life and death in severely ill children in its clutches. Despite such lingering questions, however, today we stand on the threshold of hope. Investigators are studying malaria survivors and tracking many other leads in efforts to develop vaccines. Most important, proven weapons—principally, insecticide-treated bed nets, other antimosquito strategies, and new combination drugs featuring an ancient Chinese herb—are moving to the front lines.

In the coming years the world will need all the malaria weapons it can muster. After all, malaria nut only kills, it holds back human and economic development. Tackling it is now an international imperative.

Continue reading here: Villain in Africa

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