Analytical Chem istry - DePauw University

Chapter 7 Collecting and Preparing Samples339Table 7.9 Minimum pH for Extracting 99%of an Aqueous Metal Ion Using4.0 mM Dithizone in CCl 4 (V aq = V org )Metal IonMinimum pHHg 2+ -8.7Ag + -1.7Cu 2+ -0.8Bi 3+ 0.9Zn 2+ 2.3Cd 2+ 3.6Co 2+ 3.6Pb 2+ 4.1Ni 2+ 6.0Tl + 8.7Source: Kolthoff, I. M.; Sandell, E. B.; Meehan, E. J.; Bruckenstein, S.Quantitative Chemical Analysis, Macmillan: New York, 1969, p. 353.7HSeparation Versus PreconcentrationTwo common analytical problems are: (1) matrix components that interferewith an analyte’s analysis; and (2) an analyte with a concentration that is toosmall to analyze accurately. We have seen that we can use a separation tosolve the first problem. Interestingly, we often can use a separation to solvethe second problem as well. For a separation in which we recover the analytein a new phase, it may be possible to increase the analyte’s concentration.This step in an analytical procedure is known as a preconcentration.An example from the analysis of water samples illustrate how we cansimultaneously accomplish a separation and a preconcentration. In the gaschromatographic analysis for organophosphorous pesticides in environmentalwaters, the analytes in a 1000-mL sample may be separated fromtheir aqueous matrix by a solid-phase extraction using 15 mL of ethyl acetate.21 After the extraction, the analytes in the ethyl acetate have a concentrationthat is 67 times greater than that in the original sample (assumingthe extraction is 100% efficient).7IKey Termscentrifugation composite sample coning and quarteringconvenience samplingdensity gradientcentrifugationdialysisdistillation distribution ratio extractionextraction efficiency filtrate filtration1000 mL≈ 67 ×15 mLAs you review this chapter, try to define akey term in your own words. Check youranswer by clicking on the key term, whichwill take you to the page where it was firstintroduced. Clicking on the key termthere, will bring you back to this page sothat you can continue with another keyterm.21 Aguilar, C.; Borrul, F.; Marcé, R. M. LC•GC 1996, 14, 1048 –1054.