FOR OPTICAL PURITY ANALYSIS AND SEPARATION OF OPTICAL ISOMERS A Proven HPLC Method... Developed by Pirkle*. the method is • Sensitive.. .Only nanograms of sample required •Unequivocal...Trace amounts of one enantiomer determinable in presence of great excess of the other • Insightful... Chirai separations observable even in complex mixtures • Preparative...Up to 100 mg per pass with ' 0 mm I.D column
From A Reliable Column Source... Regis is a pioneer and leader in developing HPIC columns based on the Pirkle* concept of chirai separation Regis chirai columns are • Efficient...Average 40.000 plates ne- mete' • Versatile.. .Separate many classes of aromatic compounds such as alcohols hydroxyls sulfoxides hydantoins. and succinimides • Guaranteed . . . If for any reason. yoc are not satisfied with ttie per'ocmance of your Regis column sarpiy return it for a full refund
'
HOW SUPPLIED:
λ
Ionic D-Phenyl Glycine (Either 25 cm χ 4.6 mm I.D. or 25 cm χ 10 mm I.D.) Covalent D-, L-, or D, t-Phenyl Glycine (Either 25 cm χ 4.6 mm I.D. or 25 cm χ 10 mm I.D.) Ionic L-Leucine (25 cm χ 4.6 mm I.D.) Covalent L-Leucine (25 cm χ 4.6 mm I.D.) *Dr. W.H. Pirkle, University of Illinois Literature available upon request
REGIS
CHEMICAL COMPANY 8210 Austin Avenue Morton Grove, Illinois 60053 USA Phone: 1 (312) 967-6000 Telex: 910-223-0808 CIRCLE 183 ON READER SERVICE CARD
make it ideal for in vivo monitoring or analysis of small sample volumes. The small size can also be used to advan tage in making multiple-ion sensor ar rays. The main technological problem that has prevented wide-scale use of CHEMFETs is related to encapsula tion of the device. No traces of mois ture or ionic contaminants can be al lowed to penetrate beyond the ion-se lective coating or instability results. Problems with ion-selective coating adhesion and device encapsulation continue to be studied by numerous groups, and steady progress is being made. A very sensitive detector for hydro gen can be made if the chemically sen sitive coating of the CHEMFET is a thin film of palladium. The Pd-gate CHEMFET was first reported by Lundstrom et al. in 1975 (9). Hydro gen can reversibly adsorb into the hot palladium (~150 °C), which causes a shift in the work function difference between the palladium and the silicon of the transistor. The change in work function results in an altered electric field in the transistor's gate region and a corresponding shift in observed drain current. Detection of hydrogen at levels well below 1 ppm is possible. The detection of other gases such as H2S, NH 3 , and CO has also been re ported. In addition to detecting ions in solu tion and reducing gases, CHEMFETs offer tremendous potential for immu nological and enzymatic assays. Jana ta (7) has conducted several very in teresting investigations in this area. While several mechanisms of interac tion are possible between the immo bilized enzyme or antibody coating and the CHEMFET, interactions that cause a change in interfacial charge density generally provide measurable interactions. The small size of the CHEMFET is particularly attractive if expensive enzymes or antibodies are to be used in a disposable device. Ion-controlled diodes. The ioncontrolled diode (or gate-controlled diode as it is sometimes called) was first described by Zemel in 1975 (10). This device is a combination of a p-n junction and a metal oxide semicon ductor capacitor in which the junction makes contact with the inversion layer of the capacitor. Like the CHEMFET, the device can be used as an ion sensor by application of a suitable ion-selec tive coating to the gate surface. Varia tions in the effective gate voltage re sulting from the interaction of solu tion ions with the membrane produce changes in the properties of the inver sion layer. In operation, the ion-con trolled diode behaves as a variable dis tributed RC element, so that changes in the inversion layer can be followed with an admittance bridge. A more de
9 2 A · ANALYTICAL CHEMISTRY, VOL. 56, NO. 1, JANUARY 1984
tailed discussion of the operating mechanism of this device can be found in Reference 11. The ion-controlled diode has several interesting features in addition to being small, rugged, and inexpensive. The electrical contacts to this device are on the side opposite the selective coating. This means that encapsu lation of the device to protect all but the selective coating from the solution is greatly simplified. Furthermore, the device can be used in a mode whereby the gate voltage is held constant (with respect to the reference electrode) and the driving frequency is varied to keep the inversion layer capacitance con stant. In this mode, a digital output signal is obtained. Measurements of potassium and hydrogen ion concen trations with this device have been re ported (11). Schottky diodes. A device that has shown considerable sensitivity as a gas sensor is the Schottky diode (Figure 3). In its simplest form a Schottky diode consists of a small area of metal in contact with a semiconductor. This contact exhibits a rectifying behavior and a characteristic nonlinear cur rent-voltage dependence. It has been demonstrated that if the metal used in the Schottky diode is palladium, then a very sensitive detector of hydrogen results. As in the case of the Pd-gate CHEMFET, the mechanism of opera tion relies on a change in the work function of the metal caused by the adsorption of hydrogen. Gas-sensitive metal-semiconductor Schottky diodes have been made with Pd-TiC>2, Pd-CdS, Pd-ZnO, and PbS-Si. Metal-insulator-semiconductor Schottky diodes also have been made with Pd-SiCVSi structures (12). In operation, the reverse-bias diode cur rent exhibits significant variation with exposure to ppm levels of hydrogen gas. Although these devices are very small (e.g., 1 mm 2 ), simple, and sensi tive, the palladium devices seem to be restricted to detecting low-molecularweight gases such as H 2 , H 2 S, NH 3 , and CO with poor selectivity. The use of organic semiconductors offers con siderable untapped potential for fabri cation of vapor-sensitive Schottky diodes, which can be functionalized and made selective to higher-molecu lar-weight organic vapors. Chemiresistors. It has been known for some time that semiconductors ex hibit characteristic electronic conduc tivities that are strongly affected by ambient gases and vapors (13,14). In the case of organic semiconductors the use of such compounds as vapor sen sors has been hindered by the rather high resistivities of the material (e.g., 10 8 -10 15 ohm-cm). To obtain measur able electronic current flow it was sometimes necessary to apply hun-
