Kryptoracemates

Racemic crystals normally crystallise in centrosymmetric spacegroups containing equal numbers of enantiomers. More rarely, racemates may crystallise in non-centrosymmetric space-groups having glide symmetry or, even more rarely, in space-groups devoid of a centre of inversion, having no rotary-inversion axes nor glide plane. The latter class of crystals form the subject of the present bibliographic review – a survey of kryptoracemic behaviour. The term kryptoracemic alludes to the presence of a hidden or non-crystallographic centre of inversion between two molecules that might otherwise be expected to crystallise in an achiral space-group, often about a centre of inversion. Herein, examples of molecules with stereogenic centres crystallising in one of the 65 Sohncke space-groups are described. Genuine kryptoracemates, i.e. crystals comprising only enantiomorphous pairs are described followed by an overview of non-genuine kryptoracemates whereby the crystal also contains other species such as solvent and/or counterions. A full range, i.e. one to six, stereogenic centres are noted in genuine kryptoracemates. Examples will also be described whereby there are more that one enantiomeric pair of molecules in the crystallographic asymmetric unit. A more diverse range of examples are available for non-genuine kryptoracemates. There are unbalanced species where in addition to the enantiomeric pair of molecules, there is another enantiomeric molecule present. There are examples of genuine co-crystals, solvated species and of salts. Finally, special examples will be highlighted where the counterions are chiral and where they are disparate, both circumstances promoting kryptoracemic behaviour

A microprocessor-controlled, multichannel fluorimeter for analysis of sea water

The multichannel fluorimeter described provides good sensitivity and rapid data acquisition. The advantages of multichannel fluorescence detection are discussed with special reference to the continuous monitoring of in vivo chlorophyll fluorescence in sea waters. Experiments on chlorophyll a determinations indicate a detection limit of 5 × 10-12 M with a linear range over at least three orders of magnitudes of concentration. © 1984

Optical output stabilization method for direct current arc lamps

A simple, effective technique for stabilizing the optical output of direct current (dc) arc lamps is described. The large output fluctuation due to arc wander in a commercially available lamp and power supply is minimized by the introduction of an alternating current (ac) waveform superimposed on the dc source voltage in conjunction with detector averaging. Arc stability is monitored indirectly by the detection of arc excited fluorescence from a standard sample. The monitored lamp output is typically maintained to within 1% relative standard deviation (RSD) by this method. Data are presented supporting the theory that arc wander is significantly reduced by the addition of an ac component to the dc lamp power. Various methods of ac introduction are discussed along with the design of a controllable oscillator circuit. The effects of variations in ac voltage and frequency on optical output stability are examined

Evaluation of ac stabilization of a DC arc lamp for spectroscopic applications

Spectroscopic applications of the recently described method of arc lamp stabilization by the addition of a small (\u3c2v) alternating current (AC) to a direct current (DC) arc lamp are discussed. A possible explanation for the improved arc stability is presented. Evaluation of arc formation and spatial behavior is monitored by a series of still photographs of the arc image. Observation of the electrode surfaces in similar photographs provide insight into the distribution of heat in the interelectrode region. A photodiode array imaging device is used to accurately detect lateral movement of the arc image over prolonged time periods. Advantages of AC stabilized lamps in spectroscopic applications are also presented. © 1987, Taylor & Francis Group, LLC. All rights reserved

Marine analysis using a rapid scanning multichannel fluoremeter.

The advantages of multichannel detection with an intensified linear photodiode array are discussed in reference to the continuous monitoring of oceanic phytoplankton populations by their in vivo fluorescencec.-from Author