PDMS-coated capillary columns

Figure 6 EIC of a human serum sample using (a) tandem ZIF-8 molecular sieve platform (ZIF-8 based SPME followed by ZIF-8 based GC separation) with MS detection. GC separation was carried out on the ZIF-8-coated capillary column (20 m long x 0.25 mm i.d.) at a He flowrate of 1 mLmin" using temperature programming (80°Cfor 1 min, then 10°Cmin to290°C). 1 hexane 2 heptane 3 octane 4 nonane 5 decane 6 undecane 7 dodecane 8 tridecane (b) combination of commercial PDMS/DVB SPME fiber with HP-5 capillary (30-m longx0.25-mm i.d.) with MS detection. GC separation was carried out at a He flow rate of 1 mLmin" using temperature programming (30 °C for 3 min, then 5°Cmin to 250 °C). (Reprinted with permission from Ref 56. Copyright (2011) American Chemical Society.)...

Dynamic Solid Phase Extraction The technique of dynamic solid phase extraction (SPDE) can be regarded as similar to NTD, where the rimer wall of the needle is coated with polymer. Lipinski introduced such a concept and attached a short section of metal capillary column coated with typical PDMS polymer to a gas-tight syringe [87]. The SPDE system is presented in Fig. 14.7 [88]. 

High-resolution analysis of the entire suite of C2-C12 NMHCs by conventional one-dimensional gas chromatography is typically performed with various siloxane-coated fused-silica capillaries or PLOT columns. Designing a single GC analysis which will adequately separate the entire complex mixture is very difficult to achieve, because ethene, acetylene, and ethane are difficult to separate, even with a 100 m siloxane-coated column. Furthermore, separations with a column of this length take approximately 70 min. The PLOT columns can completely resolve the C2-C3 NMHCs, but resolution of the Cg-Ci2 NMHCs is poor. If the sample is analyzed separately in a 60 m siloxane column, mounted in one GC and a PLOT column mounted in another, the analyses can be completed in approximately 45 min. The preconcentrated sample is transferred to a 60 m X 0.32 mm i.d. fused-silica capillary column coated with a 1 /rm-thick film of poly-dimethylsiloxane (PDMS) and a 30 m X 0.53 mm i.d., PLOT column coated with alumina to resolve the C4-C12 and C2-C3 NMHCs, respectively. The oven temperature for the PDMS column is held at — 50°C for 2 min, then increased to 210°C at 8°Cmin and thereafter to 250°C at... 

SPME capillary gas chromatography (SPME-GC) can be used for the extraction of organometal compounds after these have been derivatized to a sufficiently volatile form (see also organotin speciation). A silica fiber coated with polydimethylsiloxane (PDMS) is brought into the (headspace) of the sample. After exposure, the fiber is inserted into the GC injection port and the compounds are thermally desorbed for subsequent analysis. This method has higher sensitivity compared to the injection of solvent on a capillary column (usually 1 fil) but requires the use of standard addition as a calibration method. After GC separation, analysis can be performed by furnace atomization plasma emission spectrometry (FARES)." ... 

Solid-phase micro-extraction (SPME) first became available to analytical researchers in 1989. The technique consists of two steps first, a fused-silica fiber coated with a polymeric stationary phase is exposed to the sample matrix where the analyte partitions between the matrix, and the polymeric phase. In the second step, there is thermal desorption of analytes from the fiber into the carrier gas stream of a heated GC injector, then separation and detection. Headspace (HS) and direct insertion (DI) SPME are the two fiber extraction modes, whereas the GC capillary column mode is referred to as in-tube SPME. The thermal desorption in the GC injector facilitates the use of the SPME technology for thermally stable compounds. Otherwise, the thermally labile analytes can be determined by SPME/LC or SPME/GC (e.g., if an in situ derivatization step in the aqueous medium is performed prior to extraction). Different types of commercially-avarlable fibers are now being used for the more selective determination of different classes of compounds 100 /rm polydimethylsiloxane (PDMS), 30 /rm PDMS, 7 /rm PDMS, 65 /rm carbowax-divinylbenzene (CW-DVB), 85 /rm polyacylate (PA), 65 /rm PDMS-DVB, and 75 /rm carboxen-polydimethyl-siloxane (CX-PDMS). PDMS, which is relatively nonpolar, is used most frequently. Since SPME is an equilibrium extraction rather than an exhaustive extraction technique, it is not possible to obtain 100% recoveries of analytes in samples, nor can it be assessed against total extraction. Method validation may thus include a comparison of the results with those obtained using a reference extraction technique on the same analytes in a similar matrix. 

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