News from Analytica '98: Glucose monitoring gets under the skin
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NEWS FROM HPLC 98
Britt Erickson reports from St. Louis
Rapid, simultaneous detection of triazine herbicides Atrazine is one of the most widely used herbicides in the world. Methods for de-tecting it in water and soil are challenging because of its many degradation products. In addition, related triazine herbicides, such as simazine and cyanazine, are often found in the sample matrix. David S. Hage and co-workers at the University of Ne-braska have developed a new method based on high-performance affinity chro-matography for the simultaneous detection of atrazine and its related compounds.
The method relies on an immunoaffinity column, which contains a covalently at-tached monoclonal antibody (mAb) that is specific for triazine compounds. The tri-azine herbicides are extracted from water samples upon passing through the immu-noaffinity column. A set of standard re-versed-phase (RP) HPLC columns is then used to reconcentrate and separate the ex-tracted solutes. According to Hage, the method requires much smaller sample vol-umes than traditional GC methods and it
offers a lower detection limit of 0.1 ppb for a 250-uL sample injection. This limit is well below the EPA maximum allowance of 3 ppb in drinking water.
Sample throughput can be increased by running samples simultaneously, says Hage. Because the system contains both an immu-noaffinity column and standard RP columns, you can have multiple samples, one on each column, at the same time, he explains. The direct analysis of samples can be completed in as little as 12 min for ramples with hpp levels of triazine compounds and in 30 min for those with pptr levels, he says. .n addi-tion, no sample pretreatment, other than simple filtration, is required.
The method has been used for monitor-ing triazine herbicide contamination in sur-face and river waters. In addition, it has been used to study the degradation rates of atrazine in groundwater and for evaluating new purification strategies in water treat-ment plants. With a different mAb, the method can also be used for the measure-ment of other contaminants in the environ-ment. Hage says that they are currently de-veloping a similar method for the detection of alachlor, another widely used herbicide.
ANDI moving ahead In an ongoing effort to develop data stan-dards for the instrumentation industry, a subcommittee of the American Society for Testing and Materials (ASTM) is closing in on guidelinesdubbed the Analytical Data Interchange Protocols (ANDI)for mass spectrometry and chromatography. At a May 4 meeting in Attanta, the committee finalized the eraft for rhromatography. Those standards should be available on the ASTM web site (http://www.astm.org) soon, says Lynn Matthews, president of Thru-put Systems, Inc. (Orlando, FL) and chair of the subcommittee.
The standards are designed to allow manipulation and archiving of data long past the life span of the instrument that collected it. The plan calls for specifications to handle raw data, results, the full data-processing method, the full chemical method, and good laboratory practice (GLP) information. Only specifications for raw data and results have been completely defined in the current round of protocols, says Mike McConnell, director of engi-neering at PE Nelson and a. technical co-chair of an early ANDI committee on chromatography.
Glucose monitoring gets under the skin Elevated glucose is not just a symptom of diabetes; it is a cause of the pathology, says Mark Shults of the University of Wisconsin Medical Center and the com-pany Markwell Medical (Racine, WI). The open-chain aldehyde form of glucose reacts nonenyzmatically with proteins throughout the body. After years of expo-sure to such glycosylated proteins, many organs, including the eyes, kidneys, and heart, are severely damaged. Although insulin can be given to lower gluose lev-els, it must be given cautiously to avoid driving glucose too low, which results in serious insulin reactions.
One proposed solution to this di-lemma involves an implantable glucose sensor that could provide a "rich stream" of glucose data to the patient so that insulin can be used optimally. That goal is moving closer to reality. Shults and his co-workers have taken their
amperometric glucose sensors based on glucose oxidase chemistry, implanted them in dogs, and obtained clinically relevant data for as much as four and a half months. They have received clearance from the U.S. Food and Drug Administration to be-gin limited human clinical trials with up to five individualss
One of the most difficult aspects of de-signing an implantable glucose sensor is in tricking the body to use its defense mecha-nisms in the sensor's favor. Any time an object is introduced into the body, the body produces a "foreign body capsule", or FBC, that is intended to isolate the intrusive ob-ject. The FBC needs its own blood supply as it develops, but, unless something is done to sustain the process, angiogenesis (the creation of new blood vessels) stops. The FBC then loses its vascularization, becoming a tough, fibrous membrane with such poor blood circulation that the loses its analyte.
The researchers have developed an outer layer for the sensor membrane from a pro-
prietary material, the texture of which seems to promote angiogenesis. In addi-tion, the sensor membrane must have a stable layer that can protect the underly-ing layers from macrophages mat at-tempt to degrade the sensor. This biopro-tective layer can be constructed of poly-mers such as PTFE and polypropylene.
When the sensor is operating in vitro, the 90 % response time to an increase in glucose levels is 3-5 min. In vivo ii is snly slightly slower, indicating a good blood supply to the sensor. The sensor re-sponds over the clinical range of interest, 50-500 mg glucose/100 mL ((.8-28 mM). The fully implanted sensor transmits data using radiotelemetry.
Interestingly, most sensor failures have little to do with the reaction chem-istry. The prototype sensors are not in hermetically sealed packages, which limits long-term in vivo operation of electronics. This problem could be over-come by using the same techniques as in pacemaker construction.
Analytical Chemistry News & Features, July 1, 1998 447 A