DI-SPME

An SPME fiber can be exposed in two modes—by immersing it directly in the liquid sample to be analyzed (direct immersion SPME—DI-SPME), or by exposing it to the headspace (HS-SPME). In the latter case, the fiber is inserted into the headspace, above a liquid or solid sample. 

Sampling volatile analytes from samples having complex matrices usually takes place in the HS-SPME mode. This variant yields decidedly better results in the determination of aroma compounds59 and other volatile components.60 Moreover, HS-SPME prolongs the life of the fiber because it is not in direct contact with the sample. On the other hand, the direct extraction of less volatile compounds from solution is possible using DI-SPME. But in this case, the fiber deteriorates more quickly, increasing the cost of analysis. Headspace sampling is therefore employed whenever possible. 

SPME can either be performed by head-space extraction (HS-SPME) by placing the fiber in the vapour above a gaseous, liquid or solid sample, or by direct immersion extraction (DI-SPME), by immersing the fiber in a liquid sample. After a certain extraction time, the SPME needle is removed from the septum and inserted into the injection port of the GC or into the desorption chamber of the SPME-HPLC interface. The desorption is performed by heating the fiber in the GC inlet, or by pumping a solvent through the desorption chamber of the SPME-HPLC interface. The main advantages of SPME compared to LLE and solid phase extraction (SPE) are that no or little solvent is... 

There are several parameters to consider in SPME, such as extraction mode (HS-SPME or DI-SPME), type and thickness of fiber coating, extraction time, sample properties (analyte concentration, pH, buffer, temperature, agitation) and analyte desorption. Applications and optimization of SPME for food and agricultural samples are discussed in the excellent review articles (9J7). SPME fibers are commercially available from Supelco (Bellafonte, PA). 

Headspace sorptive extraction (HSSE) is a similar technique developed by Bicchi et al. in 2000 (79). In HSSE, a PDMS stir-bar is used for head-space sampling of volatile organic molecules. This technique also has similarities to HS-SPME. There are several review articles discussing and comparing HS-SPME, DI-SPME, SBSE and HSSE (10,80-82). 

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. 

In bio-analytical methods, both direct-immersion (DI-SPME) and head-space fiber (HS-SPME) have been applied with or without a derivatization step. Using the direct immersion approach, which means exposure of the fiber to the sample in solution, clenbuterol in urine and serum as well as citalopram, fluoxetine, and their main metabolites in urine, were determined without a derivatization step by HPLC analysis. 

More than one hundred papers reporting diverse applications to analyse wines were published until know. The majority of the referred methodologies use the headspace mode (HS-SPME) instead of the direct immersion mode (DI-SPME). In terms of performance, SPME showed comparable results to LLE or SPE. However, SPME is simpler and solvent-free, and uses smaller volumes of sample nevertheless, on the other hand, LLE had the possibility of carrying out simultaneously the extraction of several samples (Bohlscheid et al., 2006 Castro et al, 2008). When the interest is to obtain the maximum information about the volatile fraction of a wine, the coating DVB/CAR/PDMS seem to be the most suitable (Tat et al., 2005). On the other hand, for specific applications, the choice of a suitable solid-phase, depends on the class of compounds be analyzed, e.g. CAR/PDMS for volatile sulphides and disulphides (Mestres et al., 1999), on-fibre derivatization (PA) for the determination of haloanisoles and halophenols (Pizarro et al, 2007). 

For the determination of organotin compounds (tributyltin, triphenyltin, triethyltin, and tetra-ethyltin) a MAE is proposed before the normal phase (NP) HPLC/UV analysis [35], In organotin and arsenic speciation studies, hydride generation is the most popular derivatization method, combined with atomic absorption and fluorescence spectroscopy or ICP techniques [25,36], Both atmospheric pressure chemical ionization (APCI)-MS and electrospray ionization ESI-MS are employed in the determination of butyltin, phenyltin, triphenyltin, and tributyltin in waters and sediments [37], A micro LC/ESI-ion trap MS method has been recently chosen as the official EPA (Environmental Protection Agency) method (8323) [38] it permits the determination of mono-, di-, and tri- butyltin, and mono-, di-, and tri-phenyltin at concentration levels of a subnanogram per liter and has been successfully applied in the analysis of freshwaters and fish [39], Tributyltin in waters has been also quantified through an automated sensitive SPME LC/ESI-MS method [40],... 

FIGURE 13.9 Principle of SPME. (a) Extraction in a closed vessel by DI or the use of an SPME device, (b) Desorption of analytes from the fiber in the GC injection port. The graph in the middle corresponds to the amount of substance introduced in the GC. The signal due to the analytes increases with increasing hydropho-bicity and extraction time. 

Figure 5 shows a GC chromatogram with a large di-2-ethylhexyl phthalate (DEHP) peak obtained after HS-SPME of commercial PVC-tubing. Thermostatting time and temperature were 30 minutes at 80 °C. An ultrasonic sol-... 

Information for acertain parameter not available, if not given, na, not available DI, deionized water Cl, chemical ionization BCD, electron capture detector El, electron impact ionization FID, flame ionization detector MS, mass spectrometer MtBE, methyl tert-butylether MTBSTE, n-(tert-butyldimethylsilyl)-Wmethylfluoracetamide PDAM, 1-pyrenyldiazomethane PFBBr, pentafluorobenzyl bromide PEBOH, pentafluorobenzyl alcohol SPME, solid-phase microextraction TOPO, tri-n-octylphoshine oxide UASB, upflow anaerobic sludge blanket reactor. 

Suspended particulate matter and sediment samples were extracted in an ultrasonic bath, with n-hexane for 30 min. The extracts were purified as extracts from water samples RGBs were extracted using solid-phase SPME with a 100 /am poly(di-methylsiloxane) (PDMS) fiber 100... 

Boyd-Boland and Pawliszn reported the first application of SPME to the analysis of herbicide residues in 1995, for the simultaneous determination of nitrogen-containing herbicides in soU, water, and wine samples.Herbicides have been extracted following the three extraction modes (DI, HS, and in-tube), but direct insertion mode was the most used for these compounds. Krutz et al. have recently published an exhaustive review dealing with SPME for herbicide determination in environmental samples. 

A further extension of the SPME technique is coupling to HPLC (or HPLC-MS), which extends the method to (usually polar) compounds that are not amenable for GC analysis. This is also performed by DI sampling. After extraction, compounds bound to the fiber are extracted by a strong solvent. Note that the much simpler thermal desorption cannot be used, as the compounds to be studied are not volatile. This extraction takes place in a special extraction chamber, connected to a modified Rheodyne or Valeo valve of an HPLC system. To facilitate HPLC analysis, a special, so-called in-tube SPME device has been developed. With this technique, organic compounds in aqueous samples are directly extracted from the sample into the internally coated stationary phase of a capillary column and then desorbed by introducing a moving stream of mobile phase. 

The experiments were carried out on two different days using the same batch of popcorn, but freshly popped each time just before the experiment. The direct injection method results should represent the true nature of the sample as extracted by supercritical CO2. The aims of this experiment were to apply the use of the SFE SPME and SFE DI methods in volatile compound analysis for the first time and to evaluate the feasibility of using this technique for flavor analysis from difficult solid matrices using smaller sample quantities. 

Table 3 shows the list of compounds observed by both methods. Apparently, the most volatile compounds, pyrazines, aldehydes, etc., were observed more with the SFE SPME method than with the SFE DI method. Acetic acid, 2,5-dimethylpyrazine, 4-vinylguaiacol, and nonanal were the most abundant compounds observed with the SFE SPME method. On the other hand, palmitic acid, maltoxazine, nonanal, 2,3-dihydro-3,5-dimethyl-... 

TIC, total ion chromatogram SFE DI, supercritical fluid extraction direct injection SFE SPME, supercritical fluid extraction solid-phase microextraction. 

Figure 6 Relative amounts of pyrazine compounds extracted from 5.5 g of freshly popped nonflavored popcorn by SFE DI and SFE SPME procedures. Tetramethyl-pyrazine was not found. SFE DI, supercritical fluid extraction direct injection SFE SPME, SFE solid-phase microextraction.

Figure 2 GC/MS chromatograms of fruit juice beverage by (A) DCM extraction and (B) SPME (PDMS) liquid sampling. Peak identification is as follows (1) dichloromethane (2) ethyl butyrate (3) ethyl isovalerate (4) limonene (5) ethyl hexanoate (6) isoamyl butyrate (7) hexanyl acetate (8) cA-3-hexenyl acetate (9) hexanol (10) di -3-hexenol (11) cA-3-hexenyl butyrate (12) furfural (13) benzaldehyde (14) linalool (15) P-terpin-eol (16) butyric acid (17) 2-methyl butyric acid (18) a-terpineol (19) hexanoic acid (20) cw-methyl cinnamate (21)l-(2-furyl)-2-hydroxyethanone (22) furaneol (23) trans-methyl cinnamate (24) y-decalactone (25) dodecanoic acid (26) (hydroxymethyl)fur-fural.

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