The radioactivity detector

The radioactivity detector has an obvious application in metabolic studies. Unfortunately it is necessary when using this detector to trade resolution and speed for sensitivity. The response of the radioactivity detector is a function of the total amount of radioactivity in the detector cell, which means that the detector cell should be as large as possible. On the other hand a large cell volume will cause dispersion of the chromatographic peaks, and a compromise must therefore be found. The response is also a function of the residence time of the solute in the cell, calling for slow pumping velocity of the mobile phase, thus giving increased time of analysis. 

Detection by on-line coupled liquid chromatography-mass spectrometry (LC-MS) is at present at an experimental stage. The technique and instrumentation in LC-MS is improving rapidly at present, and recent reviews on the subject should be consulted for up-to-date information. 

The use of radioactive tracers in the study of reaction mechanisms has been steadily increasing over the last two decades. Radioactive tracers have been used to elucidate the mechanism of complex laboratory reactions, in photosynthetic chemistry and, in particular, to follow metabolic pathways of substances, synthetic and natural, in both plants and animals. In fact, the first reported use of an in-line radioactivity detector fitted directly to a gas chromatograph was in a paper by James and Piper (51) who developed the detector to study the synthesis of lipids, glycerides and fatty acids in plant tissue. James and his 

In 1964, Sjoberg and Agren (53) described a dual in-line UV absorption and radioactivity detecting system. This type of system is of great practical value as the output from the 

Homogeneous radioactivity detectors are to be preferred in analytical LC where the recovery of the sample is not important but sensitivity and versatility is essential. Nevertheless the coupling of a homogeneous radioactivity monitor to a liquid chromatograph will still require some compromise between the sensitivity of the monitor and the speed and resolution obtained from the liquid chromatograph. 

Radioactivity detectors are, at present, not frequently used in LC separations but may well find increasing use in the expanding fields of biotechnology. To date its most common application appears to be for monitoring biosynthetic pathways in 

---

Regardless of cell size, the faster you push a radioactive peak through the cell, the smaller that peak will appear to the radioactivity detector (Figure 5). Thus low level samples cannot be run at high flow rates. With the usual 4.6mm x 25cm HPLC columns, flow rates of 0.5-1.0ml/min. are used routinely with no difficulty. 

Analog versus Digital. Most detectors produce analog (continuous) signals that must be digitized before they can be manipulated by a digital computer. The main exception is the radioactive detector. 

The Radioactivity Detector and Some Lesser Known Detectors... 

An example of the use of the radioactivity detector to monitor some alkylethoxylate urinary metabolites (9) is shown in figure 6. 

An Example of the Use of the Radioactivity Detector to Monitor Some Urinary Metabolite Derivatives... 

The Katherometer and Some of the Less Well Known Detectors The Simple Gas Density Balance The Radioactivity Detector... 

During this period, many improvements have also been made in older traditional techniques. Noteworthy here is the positive impact HPLC had on conventional liquid chromatography regarding equipment and stationary phases and the significantly enhanced sensitivity of the radioactivity detectors for monitoring column effluents. 

The following three detectors have some use, or potential use, in biochemical analysis, namely the refractive index detector, the radioactivity detector, and the mass spectrometer. 

Finally it should be mentioned that the bile acid nature of a peak can be made likely from isotope studies in vivo using simultaneous mass and isotope (i C or less preferably H) determination with radio-gas chromatography. The radioactivity detector, however, requires fairly high specific activities and one may have to collect the compound for conventional isotope determination. 

An example of the use of the radioactivity detector to monitor some alkylethoxylate urinary metabolites by Mackey et al. (58) is shown in Figure 31. The separation was achieved by the use of a reversed-phase column and an acetonitrile-acetate buffered-water mobile phase. The radioactive metabolites are clearly seen. Although the metabolites can be separated by LC and with the aid of the radioactivity detector, the peaks can be monitored there could remain a serious problem of identification. 

---