Ion Chromatography - Inductively Coupled Plasma Interfaces (LC/ICP-AAS) 3
After K. E. Freniere, V. A. Fassel and D. E. Eckels 59(6)(1987)879.
Figure 49B. The Separation of Some Silicone Polymers by Size Exclusion Chromatography Monitored by ICP-AAS
A mixture of silicone polymers (covering a wide range of molecular weights) could be separated and quantitatively assayed by the technique. The system was extremely stable, and the variation of the average peak area was less than 10% for 180 individual analyses continuously run over a period of 120hr. A 10mg sample (200ml of a solution containing 50ppm of the silicone polymer) was analysed repetitively 13times over a 2week period. The average peak area varied by less than 3.7%.
The LC/ICP-AAS tandem instrument has been employed to monitor many elements of environmental or toxicological significance. Inorganic arsenic compounds (known carcinogens) are metabolized in the body (and in the process detoxified) by methylation to monomethyl arsonic acid and dimethyl arsonic acid before renal excretion. Consequently, the determination of those metabolites in urine is a convenient way of monitoring inorganic arsenic exposure. Dietary intake of arsenic is largely from seafood as the arsenicals arsenobetaine arsenocholine. Intake of seafood produces a minor increase in hydride generating arsenic compounds and a significantly greater excretion of total arsenic. The LC/ICP-AAS tandem arrangement has also been used for the determination of arsenic and selenium as described by LaFrenier et al.(7), who employed a direct injection nebulizer to introduce the column eluent into an ICP atomic spectrometer. They used a standard ICP torch, (Plasma-Therm. Inc.,) driven by a Model HFS-5000D: 27.12 MHz generator. The monochromator was the McPherson Model 2051, and the spray system was a direct injection nebulizer, manufactured by Ames Laboratory Inc. The nebulizer flow rate was 200ml/ min., and the auxiliary argon flow 600 ml/min. An example of a separation obtained by the authors is shown in figure 50.
After K. E. LaFreniere, V. A. Fassel and D. E. Eckels, Anal. Chem., 59(6)(1987)879, Copyright 1987 American Chemical Society
Figure 50. The Separation of Some Arsenic Species Employing Ion Pairing Reagents
A reversed phase column was used, with an ion-pairing reagent present in the mobile phase. The column (Whatman Partisil 5 ODS 3) was 25cm long and 4.2mm I.D, which retained the solutes by dispersive and ionic interactions. The mobile phase consisted of 5mM tetrabutylammonium phosphate in water. The flow rate was 0.75ml/min, and 15%v/v of the eluent passed to the nebulizer and into the plasma. A 200ml sample of a solution contained 10mg/ml of each arsenic species. As a consequence, each peak represents the injection of 2mg of the arsenic compound.
Employing the same chromatographic column, but with a mobile phase that consisted of 90% 5mM tetrabutylammonium phosphate in water and 10% methanol, the authors easily separated a selenite from a selenate. The flow rate was again 0.75ml/min, and about 15% of the eluent was passed to the nebulizer. Each peak represented a mass of 0.6mg and the wavelength monitored to detect the selenium was 196.1nm. The signal to noise ratio appeared from the chromatogram to be about 20 and so the lower limit of detection for the different selenium species appeared to be about 60ng.
The combination of the ion chromatograph with a spectrometer complicates and already complicated analytical instrument but, nevertheless, offers unique advantages in speciation analysis and. in particular, can provide extremely high sensitivities.