Picosecond imaging detector The investigation of various molecular dissociation processes in experiments such as molecular ion beams depends on fragment-imaging techniques that record the final kinetic energy release in the dissociation process. The value of the kinetic energy is typically deduced by measuring the impact positions and times of the molecular fragments on the surface of the detector. As a result, the detector's time and position resolution are important for obtaining meaningful data. For example, simple diatomic molecular ions with an energy of a few MeV have a maximum time difference be-
Linking isocyanates and asthma Toluene diisocyanate (TDI), a ubiquitous organic used in the polymer industry, has been implicated in allergic asthma among workers. However, understanding its role in the disease is difficult because of a lack of data on exposure and its accumulation in body tissues. Recent results published by Swedish scientists {Analyst t997, 122,151) could help researchers follow TDI through the body and trace a path from exposure to disease. TDI is used in the manufacture of polyurethane foams, adhesives, and printing plates; annual production is ~ 9 million tons. Asthma triggered in workers exposed to isocyanates is of increasing worldwide concern although demonstrating the link in each particular case is difficult because of current testing inadequacies. Gunnar Skarping and colleagues (University Hospital, Lund, Sweden) have studied the adducts formed between TDI and blood proteins such as albumin and hemoglobin. They found that albumin is the major repository for TDI and as such can be used as a marker for biological sampling of workers exposed to the organic. Exposure to TDI is usually measured by analyzing urine samples by GC/MS for the corresponding amine metabolites 2,4-toluene diamine (IDA) and the 2,6-isomer, its chemical cousin. Skarping says that TDI levels in urine samples from workers vary consider-
tween the impact of two fragments on the order of a few nanoseconds. Z. Amitay and D. Zajfman of the Weizmann Institute of Science (Israel) described a new multiparticle three-dimensional imaging detector, which is capable of ~ 100 ps time resolution and ~ 100 urn position resolution (using a standard charge-coupled camera focused on a phosphor screen). The detector takes advantage of a new generation of multianode photomultipliers that allow simultaneous position and time measurements for several particles. The system can be easily implemented in ultrahighvacuum applications, requires no special anode design, and can be used in various experimental conditions. (Rev. Sci. Instrum. 1997 68 1387-92)
ably day to day and reflect only the most recent TDI exposure rather than overall exposure over weeks and months, which could be more important in monitoring chronic respiratory disorders. Skarping's group separated and fractionated plasma from exposed workers into 200 fractions using ion-exchange chromatography and sulfuric acid to hydrolyze protein adducts to release the corresponding TDA. They then measured levels of 2,4-TDA and 2,6-TDA using various techniques, such as capillary GC and chemical ionization MS, and totaled the quantities from the fractions. Gel electrophoresis and an electroimmunoassay were run with each fraction to pinpoint which proteins were the source of the amines. From these results, Skarping and his collaborators demonstrated that the albumin fraction is the main source of TDA and so must act as the main repository for TDI. The fraction containing immunoglobulin, he says, was devoid of TDA "The concen-
Polyurethane foam production—bad for workers' health?
Schematic drawing of the three-dimensional imaging detector. (Adapted wiih permission of the American Institute of Physics.)
tration of TDA in hydrolyzed plasma was more stable and reflects exposure during the past few weeks or months." The team's studies should help in understanding the health effects of TDI, as well as provide a more rellable marker of exposure. "The biochemistry of isocyanate-induced asthma is still not fully understood," Skarping explains, "but it is believed that isocyanatemodified proteins are involved. Albumin might serve as a scavenger for TDI and thus reduce its harmful effects." Toxicologist Ian Shaw of the University of Central Lancashire (U.K.) explains that the research follows pioneering work by other researchers. "Skarping's results could be a very useful way to monitor important toxic chemicals and exposure in the workplace," he says. Swedish labor unions are now pressuring the government to act, and increased awareness of the problem could have a more global effect on legislation. At present, 20 ppb exposure is the acceptable ceiling in the United States, but even incidental levels followed by reexposure have been reported as fatal. Exact numbers of exposed workers are unknown, however, and Skarping worries that if there are problems in the developed world despite safety mcsit can be presumed that in developing nations using isocyanates things will be fcir worse Shaw points out ,We can develop wonderful techniques for monitoring toxic compounds but whether anyone uses them—-oarticularly in the undeveloped world is a different matter " David Bradley
Analytical Chemistry News & Features, June 1, 1997 3 3 9 A
