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ment allows the use of a simple flow-injection system, and the samples (e.g. serum or beverages) need no preparation.

The Diaz-Garcia group found the probe they were seeking in the form of a palladium complex (palladium-tetrakis(l-methyl-4-pyridyl)porphine) that shows intense RTP in the presence of dsDNA in aqueous solution. Time-resolved discrimi­nation of the weak signals, which might come from RNA, single-stranded DNA (ssDNA), or other nucleotides or surfac­tants, is easily done. The approach re­quires the exclusion of oxygen from the system, which is accomplished with so­dium sulfite. The nature of the DNA-palladium-porphine complex is not yet known, a question that should be answered by X-ray analysis after crystals are obtained.

So far, only preliminary experiments have been performed with the DNA-palladium complex, but they show that this strategy is promising. An opttcal sensing device that incorporates the complex is the

next step for Diaz-Garcia's group. "This optical sensor will be the first for nucleic acids based on RTP," she says. Besides a sensor for dsDNA another application could be the detection of ssDNA If ssDNA in the sample intercalates wiih ssDNA immo­bilized on an optical fiber, ,he palladium com­plex will emit a phosphorescence signall

A look into the future? Diaz-Garcia is full of ideas: "A more demanding scientific dream consists in die possibility of using RTP probes for flow cytometry. Howeverr the few microseconds during which a sin­gle sample, such as a cell, traverses the light beam in a flow cytometer imposes some constraints on the luminescing probe; long emission lifetimes are less useful. But RTP systems are simple with regard to sample preparation and the reagents needed. Therefore, it will be necessary to alter the flow cytometer by introducing a pulsed excitation source and a synchro­nized detector, for example."

Veronika R. Meyer

A cellular litmus test A color-changing organic ring with a lan-thanide metal at its center could act as a cellular litmus test, according to research­ers at the University of Durham (U.K.). David Parker usually designs clinical imag­ing and targeting agents but realized that his chemical know-how could help in de­signing a pH-responsive probe molecule.

Parker and his team have built a tetra-azacyclododecane derivative that com­plexes a single europium (III) ion; the red luminescence from the metal ion is switched on only under acidic conditions and switched off at basic pH. Additionally, reacting this complex with methyl iodide alters its structure slightly and makes it respond to hydroxide ions in the same way, such as a molecular equivalent of a litmus strip. The work is described in the Oct. 15 issue of Chemical Communications (1997, p. 1777).

According to Parker, chemically robust single-component luminescent sensors are needed, not only for analyzing ionic compo­nents of industrial and environmental sam­ples but also for studying ionic changes in living cells. For example, changes in local pH often accompany tumor growth or indi­cate a failed oxygen supply to an organ.

When they excited the complex with radiation at 375 nm at pH <5, antenna groups "switched on" the europium ion so it glowed red. The strength of the lumi­

nescence, says Parker, is at least 500-fold stronger than that seen at neutral pH, when the antennae are switched off. This strength, Parker says, creates a "genuine pH switch." The pH at which the effect occurs can be controlled by simple changes in the molecular structure of the antenna. In the presence of hydroxide ions above pH 10, the red luminescence from europium is switched off.

A deliberate, but slight, delay in emis­sion from the complex, says Parker, allows them to remove background cellular fluo­rescence. The team is now figuring out how to incorporate the europium complex into a fiber-optic sensor for use in medical cellular imaging. Sensor scientist James Ingle of Oregon State University adds, "This certainly has many potential uses for micro-scale pH measurements," although he points out that "the range of pH over which the 'switch' turns on and off would be criitcal."

David Bradley

NEWS FROM THE FALL ACS NATIONAL MEETING

Alan Newman reports from Las Vegas.

Analytical chemistry and the Chemical Weapons Convention The recently approved Chemical Weapons Convention (CWC), which stipulates the destruction of these weapons, includes "the most intrusive verification regime in arms control," according to Lt. Col. Den­nis Perry, head of the U.S. Department of Defense's Defense Special Weapons Agency. That verification regime is put­ting new demands on analytical chemistry for techniques and instrumentation that are portable and specific.

CWC provides for regular on-site in­spection of the signatories' chemical weapon storage, production, and destruc­tion siies. All the enalytical lquipment is brought to the sites by the inspectors. Because these sites include private indus­trial facilities, analyses must be specific for CWC-related analytes to avoid charges of industrial espionage.

In addition, there can be severe time restraints on the analyses. If a country charges another of violating the CWC, it can trigger a "challenge inspection." Ac­cording to Perry, these inspections are limited to 108 h from arrival to departure. "Practically speaking, this total amounts to about 96 h of on-site work, which is fur­ther limited because the work is tied to the site's schedule."

As a result, Perry's agency is funding the development of equipment that is lightweight, provides fast analyses, and is specific for the chemical warfare agents. Moreover, safety is a key concern, and the methods of choice (ideally nondestruc­tive) must minimize handling of hazard­ous materials.

Currently, CWC inspectors are using GC/MS methods developed jointly by Fin­land and the United States to identify nu­merous chemical warfare agents and their degradation products in matrices such as soil and water. According to Marjatta Rautio of the University of Helsinki, low-resolution GC/MS is being used to qualita­tively measure these enalytes at concentra­tions of around 1 ug/mL in water or 1 ug/g in soils. Solid-phase extraction is used for collecting the analytes, but "the technique is time-consuming," says Rautio. Perry, ,oo, sees this approach as limited saying "We need to look beyond these methods"

Macrocycles function as cellular pH probes.

656 A Analytical Chemistry News & Features, November 1, 1997