in black and white
Main menu
Home About us Share a book
Biology Business Chemistry Computers Culture Economics Fiction Games Guide History Management Mathematical Medicine Mental Fitnes Physics Psychology Scince Sport Technics

Chromatografy Methods for Environmental - Ando D.J.

Ando D.J. Chromatografy Methods for Environmental - Wiley publishing , 2003. - 265 p.
Download (direct link): chromatography2003.pdf
Previous << 1 .. 69 70 71 72 73 74 < 75 > 76 77 78 79 80 81 .. 83 >> Next

Response 5.3
While a different mass of sample is taken, ranging from 0.5-25 g, most methods involve some form of liquid-solid extraction, i.e. a known mass of sample is extracted with a solvent. A range of solvents have been used, including acetic acid. The most common separation technique is gas chromatography, with this requiring the conversion of butyltin into a volatile form. This has been most commonly camed out with either NaBH4 or NaBEt4, or with a Grignard reagent (butyl-, ethyl- or pentylmagnesium chloride). A range of detection systems have been used for gas chromatography, such as mass spectrometry or flame photometric detection, as well as atomic spectroscopy, e.g. quartz-furnace atomic absorption spectroscopy.
Response 5.4
While a different mass of sample is taken, ranging from 0.15-2 g, most methods involve ultrasonic extraction, i.e. a known mass of sample is extracted with a solvent by using ultrasonic agitation. The most common solvent system is methanol and water. The most commonly used separation technique is high performance liquid chromatography, with anion-exchange being the most popular. Atomic spectroscopy, coupled with high performance liquid chromatography, was the method of choice for this analysis.
Chapter 6
Response 6.1
By changing the pH of the solution it is (potentially) possible to achieve selectivity between Sn and Pb. Tin can only form an APDC complex in the pH range 2-8, while Pb can do the same thing between pH 2-14 (see Table 6.1). Therefore, if the Pb complex was formed at pH values >8, no similar complex for Sn should be formed. Thus, selectivity between Pb and Sn should be possible.
Responses to Self-Assessment Questions
Chapter 7
Response 7.1
First of all, it is important to identify what the acronyms represent. The following have been used: aMAE, atmospheric microwave-assisted extraction; SFE, supercritical fluid extraction; pMAE, pressurized microwave-assisted extraction; PFE, pressurized fluid extraction; ASE, accelerated solvent extraction; MSPD, matrix solid-phase dispersion.
The conventional techniques are Soxhlet, Soxtec, ultrasonic and shake-flask, while the instrumental techniques are aMAE, SFE, pMAE and PFE (ASE).
Matrix solid-phase dispersion (MSPD) is a relatively new technique, with, as yet, only a limited number of applications.
Response 7.2
A typical extraction efficiency-time profile is shown below in Figure SAQ 7.2. As the extraction time is increased, a greater recovery of analyte is achieved. However, the maximum recovery is achieved in the first time-domain (0 to 0.251); thereafter, smaller quantities of analyte are recovered. In the end (t), it may not be possible to remove all the analyte from the matrix. In practice, therefore, a compromise extraction time is often chosen.
Extraction time
Figure SAQ 7.2 A typical extraction efficiency-time profile for the removal of an analyte from a matrix.
Response 7.3
A comprehensive evaluation of the various techniques, based on the suggested guidelines, is presented below in Table SAQ 7.3. Hopefully, your own survey will have contained most of the details given in this table.
Table SAQ 7.3 Comparison of the various techniques used for the extraction of organic analytes from solid matrices“
Feature/ Soxhlet Soxtec Ultrasonic MSPD SFE MAE PFE
Description of Utilizes cooled Also known as Sample is Sample is mixed Utilizes Utilizes Utilizes high
method condensed automated covered with with a supercritical microwave temperature (100°C)
solvents passing Soxhlet. organic dispersant, CO2, with or radiation to and pressure
over the sample Soxtec places solvent, and e.g. Cl 8 without heat solvent. (2000 psi) to extract
contained in a the sample then a sonic media, and organic Can be carried analytes. Solvent and
thimble to into the horn is placed then placed in modifier, to out either at analytes are flushed
extract analytes. boiling inside the an empty extract elevated from the extraction
Uses specialist solvent and container with solid-phase analytes. pressures or vessel by using a
glassware and then flushes the solvent and extraction Pressures up atmospheric small volume of fresh
heating clean solvent sample cartridge. to 680 atm pressure solvent and a N2
apparatus over it. Faster than Soxhlet Analytes eluted with appropriate solvent and temperature up to 250° C can be used. Analytes collected in solvent purge. Fully automated
Sample 10 10 1-5 1-5 1-10 2-5 Up to 30 g
mass (g)
Extraction 6, 12 or 24 h Reduced time Typically Should be 30 min-1 h 20 min (plus 12 min
time compared to Soxhlet, i.e. 2-4 h 5-15 min, but repeated for up to three times possible in under 30 min 30 min cooling and pressure reduction)
Solvent type Acetone:hexane As Soxhlet As Soxhlet Requires CO2 (plus Typically, Acetone:hexane
(1:1 (vol/vol)); optimization organic acetone:hexane (1:1 (vol/vol)) or
acetone:DCM modifier). (1:1 (vol/vol)). acetone; DCM
(1:1 (vol/vol)); Tetra- The solvent(s) (1:1 (vol/vol)) for
Previous << 1 .. 69 70 71 72 73 74 < 75 > 76 77 78 79 80 81 .. 83 >> Next