News
The fundamental review on mass spectrometry failed to make the traditional June 15 issue and will be published instead in the August 15 issue oo Analytical Chemistry. We apologize for the eelay.
Endocrine disrupters in the air Endocrine disrupters seem to be the latest danger in the familiar problem of air pollution. Small airborne particles—with a mean aerodynamic diameter of less than 10 microns—have long been a concern because they enter the lungs and become trapped by alveoli. In urban areas, these tiny particles have been implicated in premature deaths, and children are known to be especially sensitive to them. Previous work to extract and study these respirable particles has focused on the identification of mutagenic polycyclic aromatic compounds. ButT. R. Zacharewski and colleagues at the University of West Ontario (Canada), Michigan State University, McMaster University (Canada), Agriculture and Agri-Food Canada, and CanTox (Canada) looked instead for endocrine disrupters, which are thought to be linked to health problems such as reproductive abnormalities, immune-system deficiencies, and increased cancer rates. In some cases endocrine disrupters act by mimicking naturally occurring hormones; in others by blocking or diminishing their activities. The effects sometimes involve nuclear steroid receptors but may occur through other pathways The antiestrogenic effects of dioxin for example are mediated by the aryl hydrocarbon (Ah) receptor
In this study, the researchers used an in vitro assay with a luciferase reporter gene. The authors found significant estrogenic and dioxin-like activity in the crude extract and nonpolar fractions of the samples. In particular, they found three polycyclic aromatic hydrocarbons (PAHs) that induced dioxin-like effects via the Ah receptors and estrogenic effects via estrogen receptors. Three other PAHs produced only dioxinlike effects. Although the authors acknowledge that in vitro assays cannot replace in vivo methods, they note that the ability to screen large numbers of compounds quickly and inexpensively and to identify potential mechanisms of action makes in vitro testing a valuable technique. {EnviSci Technol 1998 32 1853-60) 502 A
Temperature is no problem Temperature variations can really mess up a scanning tunneling microscope (STM) experiment. Modest thermal drifting can distort the image, and severe thermal drifting of the surface towards or away from the tip can lead to a situation in which the change in surface to tip distance exceeds the span of the piezoelectric actuator. As a result, Schematic of the temperature-programmable STM: Scanner A rests on support block H, radiation shield numerous STM designs can incorE protects piezo element B from thermal radiation porate some type of thermal behavfrom sample C, sample holder D is clamped down by ior optimization. However, these designs do not allow researchers to leaf springs G, images based on tip F. (Adapted with permission. Copyright 1998 American Institute of image a particular surface area of Physics.) a non-semiconductor sample while varying the temperature over an appreciapensated piezoelectric scanner was comble range bined with a new sample stage that defines a M. S. Hoogeman and colleagues at the fixed center from which thermal expansions FOM-Institute for Atomic and Molecular in all three directions are forced outwards. Physics and at Leiden University (both in The microscope operates over a sample The Netherlands) introduce a novel STM temperature range of 60 to at least 850 K. design that allows a selected area to be kept Performance of the system was demonin the microscope's view while the temperastrated with several surfaces, including ture is varied over a range of several hunimaging a particular surface region of dred degrees. To maintain an extremely low Au(110) over a range of more than 270 K. drift with the scanner, a thermal-drift com(Rev. Sci. Instrum. 1998, 69,2072-80)
Distinguishing s t r a i n s of E. coli MALDI MS could be a powerful way to identify bacteria, but some of the subjectivity and guesswork must be taken out of the spectral analysis. James P. Reilly and Randy J. Arnold of Indiana University describe a modified correlation approach that allows them to distinguish 25 common laboratory strains of E. coli bacteria using their MALDI mass spectra. For the cross-correlation technique, the data from two smoothed and calibrated mass spectra are entered as separate files with mass and peak intensity information. A mass range is selected and divided into intervals. For E. coli identification, the mass range 3.5-10 kDa is divided into 13 intervals of 500 Da. For each interval, the program calculates interval cross-correlation and autocorrelation values for the two spectra. Each cross-correlation value is normalized by dividing it by the average of the auto-correlation values. The composite
Analytical Chemistry News & Features, August 1, 1998
correlation index is then defined as the product of the normalized interval correlation values. Even when the method is used to compare similar spectra, the composite correlation index is smaller for the comparison of spectra from different strains than it is for comparison of spectra from the same strain. If the same mass range is divided into smaller intervals, the method becomes more selective, and the composite correlation index for mismatched spectra drops precipitously as the number of intervals increases. However, the mass range and number of intervals must be chosen such that each interval has an appropriate number of significant peaks. Otherwise the composite correlation index becomes ridiculously small and virtually meaningless Although it is usually possible to Select a. match by comparison frorri a QTouD of mismatches there is no obvious value of the comDosite correlation index (e e 0 500) that unambiguously signals a match (Rabid Commun Maw Stiertrom
