News
PNA biosensors show promise When it comes to DNA sequence-specific biosensors, there may be a better material than complementary DNA. Joseph Wang and co-workers at New Mexico State University, the Academy of Sciences of the Czech Republic, and the Panum Institute (Denmark) have made DNA biosensors with peptide nucleic acid (PNA) probes. PNA is a structural DNA analogue with an uncharged AL(2-aminoethyl)-glycinebased pseudopeptide backbone that forms complementary pairs with normal DNA The PNA-DNA pairs have higher thermal stability and can be formed at low ionic strengths. PNA also shows a higher specificity in the recognition of DNA sequences and allows the use of shorter probes. The authors compared the electrochemical hybridization signals of 15-base DNA and PNA oligomers adsorbed on a carbon paste electrode in a variety of buffer concentrations and a range of temperatures. The signals were based on the binding of Co(phen)|+ to the surface hybrid (either DNA or PNA), and the results indicate that the PNA biosensors were applicable to a wider variety of conditions. To test the PNA biosensor, the authors used various noncomplementary DNA sequences, including a 15-mer with a single-base mismatch, with the complementary strand. The noncomplementary strands interfered with the hybridization signal of the DNA biosensor far more than with that of the PNA biosensor. (J. Am. Chem. Soc. 1996,118, 7667-70)
Effect of buffer concentration (a) and temperature (b) on the PNA-DNA (n) and DNA-DNA (•) hybridization at carbon paste electrodes. 652 A
Multichannel FT-IR
Hiro-o Hamaguchi and Mamoru Hashimoto of the Kanagawa Academy of Science and Technology (Japan) and the University of Tokyo (Japan) have constructed a multichannel FT-IR spectrometer for single-event, timeresolved spectroscopy that can obtain a time resolution of ~ 5 ms without mirror scanning. The system uses a triangle common-path interferometer and a 1024element PtSi multichannel detector to cover the mid-IR range from 4500 cm-1 to 2500 cm"1 with spectral resolution of 13 cm"1. They have used the multichannel FT-IR to measure the solid-solid and solid-liquid phase transitions of two alkanes, octadecane and nonadecane, with time resolutions of 5-50 ms. (Appl. System configuration of the multichannel FT-IR spectrometer. Spectrosc. 1996, (Adapted with permission from the Society for Applied 50,1030-33) Spectroscopy.)
Various types of time-resolved IR spectroscopy instruments have been developed for investigating transient molecular phenomena, with resolutions as high as 380 fs for repeatable transient phenomena. For measurements of unrepeatable transient phenomena such as phase transitions or explosions, however, time resolution has been limited to ~ 20 ms because of the restrictions of rapid-scan FT-IR, the only applicable technique.
Glow-discharge FT-ICRMS In recent years, many types of ion sources, including EI, CI, MALDI, and ESI, have been used with FT-ICRMS. However, transporting ions from the relatively high-pressure ion source to the FT-ICR analyzer cell located in the center of a high-field (often superconducting) magnet can be a challenge. Formation of the ions outside the magneticfieldgreatly facilitates access to the ion source for repairs and modifications, but the intricacy of the transfer optics adds dramatically to the expense and complexity of the system and can lead to decreased sensitivity. J. R. Eyler and colleagues at the University of Florida, Brigham Young University, and Oak Ridge National Laboratory have developed a glow-discharge source that works within the high magneticfieldof the FT-ICR mass spectrometer. The researchers used a concentric-tube vacuum chamber and optimized the diameters of the tubes for maximum conductance of the differentially pumped regions within the vacuum chamber and probe. They found that the optimal operating voltage,
Analytical Chemistry News & Features, November 1, 1996
The GD FT-ICR mass spectrometer. (Adapted with permission from the American Society for Mass Spectrometry.)
pressure, and current of the internal GD source are significantly lower than those of normal glow discharges and that the optimal sputter rate was only l/30th of that of a GD source operated outside the magnetic field. In addition, the relative proximity of the source to the FT-ICR cell leads to higher ion transport efficiency, resulting in detection limits (25 pg for ^Ni) much better than those obtained with the external source and comparable to those obtained with other forms of GDMS. (J. Am. Soc. Moss Spectrom. 1996, 7,923-29)
