Ion reaction time amplification

In general, ion reaction rates can be observed directly in a time-of-flight spectrometer with a time resolution comparable to that of the system, which is about 10-9 to 10 los. The rate measurement can achieve a much better time resolution by using an ion reaction time amplification method. With this method, very fast ion reactions can be measured with a time resolution much better than the time resolution of the system. It is with this method69 that the field dissociation reaction of 4HeRh2+ was measured with a time resolution of about 20 femtoseconds when the time resolution of the system was still only 1 ns. 

An ion reaction time amplification means that those ion reactions taking place in a very short time period 8r will have their detection stretched over a much longer period 8f so that the ion detector and the system electronics can adequately respond. The amplification factor can be defined as... 

Fig. 3.21 Geometrical scheme for ion reaction time amplification as discussed in...

This amplification factor is proportional to Lll, and is also dependent on the location where the ion reaction occurs. Maximum amplification occurs at x = /, A(l) = (m 2M)(L/l), or if the reaction occurs right after the parent ion has gained the full energy of the acceleration voltage. Equation (3.33) suggests that whenever possible one should measure the flight time distribution, or the energy distribution, of the ion species with the smaller mass. 

Two similar methods for phosphate ion which involve an amplification reaction, solvent extraction, and reverse solvent extraction were described at about the same time by Umland and Wiinsch (29) and Djurkin, Kirkbright and West (10, 30). An acidic solution containing the phosphate ion is treated with an excess of molybdate ion to form the phosphomolybdic acid, which is extracted into a water-immiscible solvent to free it of excess molybdate ion. The phosphomolybdic acid is then broken down and re-extracted by an aqueous basic solution and the molybdate ion determined colorimetrically through the use of 2-amino-4-chlorobenzenethiol or thiocyanate ion. The effective molar absorptiv-ities of the reagents are 359,000 at 710 m/x. for 2-amino-4-chlorobenzene-thiol and 150,600 at 470 m/x. for thiocyanate, which represent multiplication factors of 10 and 12, respectively, caused by phosphate molybdate ratio in the extracted phosphomolybdic acid. 

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