with respect to hexane is not as good as that of the FPD because of the greater number of interfering spectral emissions. These background emis sions also limit the excellent intrinsic sensitivity of the MPD, although de tection limits are commonly in the nanogram or picogram ranges. Thus, while its utility and potential as a multielemental detector are estab lished, it must be stressed that the MPD requires considerable optimiza tion for detection of individual analyte species. Rapid switching among a range of elements cannot, therefore, be achieved without sacrificing per formance. As with the FPD, most analytical studies using the MPD have been di rected to analysis for pesticides and drugs (24). However, recent concern about contamination of living orga nisms with methylmercury and dimethylmercury has seen applica-
These contribute significantly to the spectral background and necessitate the use of a monochromator for wave length selection in place of the sim pler narrow bandpass filters em ployed by commercial FPD's (22). Fewer interfering emission lines occur if helium is used as the carrier gas. owing to its high ionization potential, but its use requires vacuum equipment (22. 23). The MPD will not generally tolerate injection of large amounts of solvent which extinguish the plasma discharge {22). Also, the intense heat of the discharge often causes frosting of the quartz tube, thereby reducing sensitivity. In spite of its limitations, however, the MPI) offers a high degree of se lectivity for a wide variety of ele ments including carbon, fluorine, chlorine, bromine, iodine, phospho rus, and sulfur (Table IV). For most of t hese element s, t he select ivitν ratio
Microwave Cavity \
Quartz Discharge Tube
Photomultiplier Tube
Column
RADIOMETER COPENHAGEN
Amplifier
Injection Port
Çp un
0
Radiometer offers a single electrode for a multitude of purposes — the Ruzicka Universal Selectrode®. It can replace many singlepurpose electrodes with one electrode sensitive to many ions. The Selectrode® is sensitized by adding a small amount of electroactive material to its normally inert surface. This response may be altered whenever desired. It is ideally suited to ion studies, titration, quality control applications and as an educational tool for specific ion technology. Technical reports and literature available on request.
Recorder
Quartz , Discharge Tube
•-Microwave Cavity
Figure 2. Schematic of GC-MPD system (22)
Table III. FPD Detection Limits (19) Sample
Band or line. À
TiCI 4 AsCI 4 ZrCl4 Rh(hfa), 2 X 10-1" 2 X 10-8
2 3 2 7
3 2 6 2
Χ Χ Χ Χ
10-6 ΙΟ" 10 10-1» ΙΟ-»
X X X X
THE LONDON COMPANY
10-8 10-» 10-» 10-»
811 SHARON DRIVE CLEVELAND, OHIO 44145 (216) 871-8900
a R e s u l t s o b t a n e d w i t h B e c k m a n DU f l a m e p h o t o m e t e r . >> R e s u l t s o b t a i n 3d w i t h m o d i f i e d S p e c t r o n i c 20.
