Carbon dioxide flow injection analysis

The classical manifold architecture in Fig. 8.22, upper was exploited in the pioneering work incorporating GD in flow injection analysis for the spectrophotometric determination of total carbon dioxide in blood plasma [265]. Details of the separation unit are shown in Fig. 8.23, left. The donor stream with the sample zone was acidified and the released CO2 diffused through the membrane towards the acceptor stream, which was an alkaline cresol-red indicator solution. Analyte collection resulted in a transient lowering of the pfi of this stream and hence a transient modification to the monitored absorbance. The recorded peak height was proportional to the CO2 content in the injectate. 

Linares, P., M. D. L. DeCastro, and M. Valcarcel. 1989. Simultaneous determination of carbon-dioxide and sulfur-dioxide in wine by gas-diffusion flow-injection analysis. Anal. Chim. Acta 225 443-448. 

Schachl K, Alemu H, Kalcher K, Jezkova J, Svancara I, Vytfas K (1997) Amperometiic determination of hydrogen peroxide with a manganese dioxide-modified carbon paste electrode using flow injection analysis. Analyst 122(9) 985-989... 

When on-column injection is used the end of the transfer capillary is inserted into the column inlet or retention gap where decompression of the supercritical fluid occurs. Carbon dioxide gas exits through the column and the seal made between the restrictor and septum (unless a closed injector is used). The analytes are focused by cold trapping in the stationary phase. The transfer line must be physically removed from the injector at the completion of the extraction to establish the normal carrier gas flow for the separation. Analyte transfer to the column is virtually quantitative but blockage of the restrictor is more conunon and involatile material accumulates in the injection zone eventually degrading chromatographic performance. The on-column interface is probably a better choice for trace analysis of relatively clean extracts with modest fluid flow rates than the split interface. When optimized both the on-column and split interfaces provide essentially identical peak shapes to those obtained using conventional solution injection. 

Figure 5 Fast SFC analysis of the S-enantiomer of clevidipine after hydrolysis into its corresponding acid. Column and conditions Chiralpak AD 50 x 4.6 mm ID at 30°C, mobile phase carbon dioxide with 28% of 2-propanol, flow rate 4.0 ml min backpressure 150 bar, UV detection at 240 nm. Sample preparation and work-up 2 mg of S-clevidipine substance was dissolved in 0.5ml of methanol followed by 50 pi of 1 mol r sodium hydroxide. After 10 min, 1 ml of water and 50 pi of sulfuric acid, 1 mol l was added and 0.5ml of dichloromethane as extraction medium. After brief vortexing and centrifugation 5 pi of the lower organic phase was loaded manually and injected. Upper trace sample, lower trace sample with 0.1% of S-acid added. (Reproduced with permission from Gyllenhaal O (2001) Fast enantioselective separation of clevidipine and a dihydropyridine substituted acid by SFC on Chiral pak AD. Fresenius Journal of Analytical Chemistry 369-. 54-56 Springer.)...

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