Chapter 28: Q21P (page 792)
How does solid-supported liquid-liquid extraction differ from solid-phase extraction?
The difference between solid supported liquid-liquid extraction and solid –phase extraction is explained.
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:(a) Describe the steps in QuEChERS and explain their purpose.
(b) Why is an internal standard used in QuEChERS?
(c) What is displayed in the total ion chromatogram in Figure 28-22?
(d) What is displayed in the extracted ion chromatogram in Figure 28-22? What is the difference between an extracted ion chromatogram and a selected ion chromatogram? Which would have greater signal-to-noise ratio?
(e) What mass spectrometric method could be used to obtain even greater signal-to-noise ratio from the same QuEChERS extract?
The following wet-ashing procedure was used to measure arsenic in organic soil samples by atomic absorption spectroscopy: A 0.1- to \({\bf{0}}.{\bf{5}} - \)g sample was heated in a \({\bf{150}} - {\bf{mL}}\) Teflon bomb in a microwave oven for \(2.5\;{\rm{min}}\) with \(3.5\;{\rm{mL}}\)of\(70\% \,\,\,{\rm{HN}}{{\rm{O}}_3}\). After the sample cooled, a mixture containing \(3.5\;{\rm{mL}}\)of \(70\% \,\,\,{\rm{HN}}{{\rm{O}}_3},1.5\;{\rm{mL}}\) of\(70\% \,\,{\rm{HCl}}{{\rm{O}}_4}\), and \(1.0\;{\rm{mL}}\) of \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\)was added and the sample was reheated for three \({\bf{2}}.{\bf{5}} - {\bf{min}}\) intervals with 2 -min unheated periods in between. The final solution was diluted with \(0.2{\rm{M}}\,\,\,{\rm{HCl}}\)for analysis. Why was \({\rm{HCl}}{{\rm{O}}_4}\) not introduced until the second heating?
When you flip a coin, the probability of its landing on each side is \(p = q = \frac{1}{2}\)in Equations 28-2 and 28-3. If you flip it \(n\)times, the expected number of heads equals the expected number of tails \( = np = nq = \frac{1}{2}n.\)The expected standard deviation for \(n\)flips is\({\sigma _n} = \sqrt {npq} \). From Table 4-1, we expect that \(68.3\% \)of the results will lie within \( \pm 1{\sigma _n}\) and \(95.5\% \)of the results will lie within\( \pm 2{\sigma _n}\).
(a) Find the expected standard deviation for the number of heads in \({\bf{1000}}\) coin flips.
(b) By interpolation in Table 4-1, find the value of \(z\)that includes \(90\% \)of the area of the Gaussian curve. We expect that \(90\% \)of the results will lie within this number of standard deviations from the mean.
(c) If you repeat the\({\bf{1000}}\)coin flips many times, what is the expected range for the number of heads that includes\(90\% \) of the results? (For example, your answer might be, "The range \({\bf{490}}\) to \({\bf{510}}\) will be observed \(90\% \)of the time.")
In 2002, workers at the Swedish National Food Administration discovered that heated, carbohydrate-rich foods, such as french fries, potato chips, and bread, contain alarming levels \((0.1to4\mu {\rm{g}}/{\rm{g}})\) of acrylamide, a known carcinogen\(36\).
After the discovery, simplified methods were developed to measure ppm levels of acrylamide in food. In one procedure,\(10\;{\rm{g}}\) of pulverized, frozen french fries were mixed for \(20\;{\rm{min}}\)with\(50\;{\rm{mL}}\) of \({{\rm{H}}_2}{\rm{O}}\)to extract acrylamide, which is very soluble in water \((216\;{\rm{g}}/100\;{\rm{mL}}).\)The liquid was decanted and centrifuged to remove suspended matter. The internal standard \(^2{{\rm{H}}_3}\)-acrylamide was added to\(1\;{\rm{mL}}\) of extract. A solid-phase extraction column containing \(100{\rm{mg}}\)of cation-exchange polymer with protonated sulfonic acid groups\(\left( { - {\rm{S}}{{\rm{O}}_3}{\rm{H}}} \right.)\) was washed twice with 1 -mL portions of methanol and twice with \(1 - {\rm{mL}}\)portions of water. The aqueous food extract \((1{\rm{mL}})\)was then passed through the column to bind protonated acrylamide \(\left( { - {\rm{NH}}_3^ + } \right)\)to sulfonate \(\left( { - {\rm{SO}}_3^ - } \right)on\)the column. The column was dried for\(30\;{\rm{s }}at\)\(0.3\)bar and then acrylamide was eluted with\(1\;{\rm{mL}}\) of \({{\rm{H}}_2}{\rm{O}}.\)Eluate was analyzed by liquid chromatography with a polar bonded phase. The chromatograms show the results moni- tored by ultraviolet absorbance or by mass spectrometry. The retention time of acrylamide is different on the two columns because they have different dimensions and different flow rates.
(a) What is the purpose of solid-phase extraction prior to chromatography? How does the ion-exchange sorbent retain acrylamide?
(b) Why are there many peaks when chromatography is monitored by ultraviolet absorbance?
(c) Mass spectral detection used selected reaction monitoring (Figure 22-33) with the \(m/z72 \to 55\)transition for acrylamide and \(75 \to 58fo{r^2}{{\rm{H}}_3}\)-acrylamide. Explain how this detection method works and suggest structures for the ions with \({\rm{m}}/{\rm{z}}72\)and 55 from acrylamide.
(d) Why does mass spectral detection give just one major peak?
(e) How is the internal standard used for quantitation with mass spectral detection?
(f) Where does \(^2{{\rm{H}}_3}\)-acrylamide appear with ultraviolet absorbance? With mass spectral selected reaction monitoring?
(g) Why does the mass spectral method give quantitative results even though retention of acrylamide by the ion-exchange sorbent is not quantitative and elution of acrylamide from the sorbent by \(1\;{\rm{mL}}\) of water might not be quantitative?
Chromatograms of acrylamide extract after passage through solid-phase extraction column. Left: Phenomenex Synergi Polar-RP 4- \(\mu {\rm{m}}\)column eluted with 96:4 \((vol/vol){{\rm{H}}_2}{\rm{O}}:{\rm{C}}{{\rm{H}}_3}{\rm{CN}}.\)Right: Phenomenex Synergi Hydro-RP 4- \(\mu {\rm{m}}\)column eluted with \(96:4:0.1(vol/vol/vol){{\rm{H}}_2}{\rm{O}}:{\rm{C}}{{\rm{H}}_3}{\rm{OH}}:{\rm{HC}}{{\rm{O}}_2}{\rm{H}}.\) (Data from L. Peng. T. Farkas, L. Loo, \({\rm{J}}.\)Teuscher, and \({\rm{K}}.\)Kallury, "Rapid and Reproducible Extraction of Acrylamide in French Fries Using a Single Solid-Phase Sorbent," Am. Lab. News Ed, October 2003, p. 10.)
(a) Explain how dispersive liquid-liquid microextraction reduces the use of solvent in comparison with liquid-liquid extraction.
(b) What is the purpose of the disperser solvent, which is used in much greater volume than the extraction solvent?
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