SPME-HPLC

Similar to the SPME-CC desorption/injection system, SPME and HPLC desorption/injection can be combined. The most common way to do this is to use a six-port injection valve (as described in detail in Chapter 3). In SPME-HPLC, the injection loop is replaced by a desorption chamber. 

The SPME syringe is introduced into the desorption chamber having the six-port injection valve in the load position. The fiber is positioned and sealed to avoid leakage from the system under pressure. This step is followed by valve turned to inject position with desorption into the mobile phase and subsequent transfer to the analytical column. 

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Lord and Pawliszyn" developed a related technique called in-tube SPME in which analytes partition into a polymer coated on the inside of a fused-silica capillary. In automated SPME/HPLC the sample is injected directly into the SPME tube and the analyte is selectively eluted with either the mobile phase or a desorption solution of choice. A mixture of six phenylurea pesticides and eight carbamate pesticides was analyzed using this technique. Lee etal. utilized a novel technique of diazomethane gas-phase methylation post-SPE for the determination of acidic herbicides in water, and Nilsson et al. used SPME post-derivatization to extract benzyl ester herbicides. The successful analysis of volatile analytes indicates a potential for the analysis of fumigant pesticides such as formaldehyde, methyl bromide and phosphine. 

GC injection port) may well be. This was clearly demonstrated by a comparison between cryotrapping/direct injection, cryotrapping/SPME, and solid-phase (Tenax-GC) extraction for sampling of odorous sulfur compounds [71], Thermally labile compounds are likely to break down in the GC injection port/column/transfer line. Instead of SPME-GC, the recently developed SPME-HPLC [72] might be more applicable to analysis of such thermally unstable compounds. 

The main characteristics of on-line SPME-HPLC(-MS) are shown in Table 7.18. Most of the SPME fibres are compatible with HPLC solvents. SPME combined with HPLC provides a means by which simple, rapid concentration of analytes can be achieved together with a means of introduction of the concentrated analytes to the HPLC system. This eliminates the need for larger injection volumes, and avoids derivatisation if the analytes were to be detected by GC. An advantage of the SPME method over LLE methods is the absence of a solvent peak in chromatograms obtained after extraction by SPME. SPME is not suitable for organic solutions. As SPME is a microextraction technique, coupling to ft, HPLC may be envisaged. 

Applications The scientific literature on this relatively new approach is still quite limited. SPME-HPLC-MS is suitable for quantitation of polar and semipolar organic compounds from aqueous solutions. 

Table 7.18 Main features of on-line SPME-HPLC(-MS)...

It is important that any method for surfactant analysis maintains the same oligomer distribution in the extracted samples. LLE and SPE are generally combined with chromatographic methods for separation and resolution of non-ionic surfactants into their ethoxamers. An alternative is the use of SPME-HPLC, recently reported by Chen and Pawliszyn [141]. Alkylphenol ethoxylate surfactants such as Triton X-100 and various Rexol grades in water were determined by means of SPME-NPLC-UV (at 220 nm) [142]. Detection limits for individual alkylphenol ethoxamers were at low ppb level. 

Boyd-Boland and Pawliszyn [77] pioneered the SPME analysis of APEOs by SPME-HPLC using normal-phase gradient elution with detection by UV absorbance at 220 nm. The Carbowax-template resin (CW-TR) and Carbowax-divinylbenzene (CW-DVB) fibres allowed the analysis of APEO with a linear range of 0.1-100 mg L 1. The former coating produced the best agreement between the distribution of ethoxymers before and after extraction. This CW-TR fibre provided a limit of detection for individual AP ethoxamers at the low ppb level. The determination of NP in water by SPME-GC (FID) was accomplished by Chee et al. [78] using a polydimethylsiloxane (PDMS) fibre. The linear range was between 1 and 15 mg L 1 with an estimated detection limit of 0.1 mg L-1. 

Aranda and Burk [81] have established an SPME-HPLC-FL method and on-line derivatisation to determine AEs (Brij 56) in water. The surfactant was extracted with a PDMS-DVB fibre and pre-column derivatisation with 1-naphthoyl chloride in the presence of 4-(dimethyl-amino)pyridine as catalyst. The method has a limit of detection of 0.1 mg L-1. 

Ceglarek et al. [89] used the CW-TR fibre to analyse LAS in influent and effluent wastewater samples of a WWTP applying SPME. The optimised conditions included the addition of 0.5 g mL 1 of ammonium acetate to 3 mL of sample, extraction by immersion of the fibre (2 h) and static desorption (15 min) in isopropanol/methanol (1 1). The extracted LAS were analysed by SPME-HPLC-FD and LC-ESI-MS. The former was not suitable for quantifying LAS because of its limited extraction efficiency, whereas LC-ESI-MS showed a linear range from 0.5 to 100 xgL 1, with detection limits of 0.5 p.g L 1 for each individual homologue of LAS. The CW-TPR fibre also extracted alkylether sulfates (AESs) but not under optimised conditions. 

Aulakh, J.S., A.K. Mailk, V. Kaur, et al. 2005. A review on solid phase micro extraction—High performance liquid chromatography (SPME-HPLC) analysis of pesticides. Crit. Rev. Anal. Chem. 35 71-85. 

Insertion/introduction of the needle into the GC port, depression of the plunger, and thermal desorption of the analytes. Alternatively, the analytes are washed out of the fiber by the HPLC mobile phase via a modified HPLC six-port injection valve and a desorption chamber that replaces the injection loop in the HPLC system. The SPME fiber is introduced into the desorption chamber, under ambient pressure, when the injection valve is in the load position. The SPME-HPLC interface enables mobile phase to contact the SPME fiber, remove the adsorbed analytes, and deliver them to the separation column. Analytes can be removed via a stream of mobile phase (dynamic desorption) or, when the analytes are more strongly adsorbed to the fiber, the fiber can be soaked in mobile phase or another stronger solvent for a specific period of time (e.g., 1 min) before the material is injected onto the column (static desorption) (Fig. 6). 

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... 

While SPME started as a solvent-free extraction system for GC analysis, it can now also be used to introduce samples into an HPLC apparatus. A new SPME-HPLC interface, Fig. 1.12, allows the use of an SPME fiber to sample nonvolatile analytes such as nonionic surfactants in water, and elute the analyte into the solvent mobile phase used for the HPLC analysis. The sampling process and elution are shown schematically in Fig. 1.10. HPLC and its applications are covered in Chapter 13. 

Figure 1.12 A new SPME-HPLC interface. Reprinted with permission of Sujaelco, Bellefonte, PA 16823, USA (www.sigma-aldrich.com).

Lu et al. described the trace determination of sulfonamide residues in meat with a combination of SPME and LC-MS. Fiber coated with a 65 p,m thickness of polydimethylsiloxane/divinylbenzene (PDMS/DVB) was used to extract sulfonamides at optimum conditions. Analytes were desorbed with static desorption in an SPME-HPLC desorption chamber for 15 min and then determined by LC-MS. The linear range was 50-2000 p-g/kg, with RSD values below 15% (intra-day) and 19% (inter-day) and detection limits 16-39 pg/kg. Some meat samples collected from the local market contained residues of sulfonamides ranging from 66 to 157 pg/kg. The results demonstrated that the SPME-LC-MS system could effectively analyze residues of sulfonamides in meat products. 

Gaurav, V, A. Kaur, A. Kumar, A. Kumar Malik, and P. K. Rai. 2007. SPME-HPLC A new approach to the analysis of explosives. Journal of Hazardous Materials 147 691-697. 

Figure 1.12 An SPME-HPLC interface. (Reprinted with permission of Supeico, Bellefonte, PA. www.sigma-...

A solventless technique of solid-phase microextraction (SPME) has been also employed for HPLC determination of MC in a natural Microcystins sp. bloom in a freshwater, where three dominant MC variants MC-LR, -YR, and -RR were quantified. For this purpose a measuring system with commercial SPME-HPLC interface was employed. Microcystins were sorbed from acidified solutions using SPME fibers with carbowax/templated resin and polydi-methylsiloxane/divinylbenzene coating and desorbed at dynamic mode with HPLC eluent, which was used in isocratic elution mode and consisted of water and methanol with 0.05% TFA. For each toxin partition equilibrium was achieved within 60 min and example response obtained in SPME-HPLC system is shown in Fig. 6. The detection limits for all examined MC for 5 ml samples were reported at about 7 ppb. 

Penalver et al. [178] have applied SPME coupled to HPLC with ECD and UV detection to determine the 11 phenolic compoimds considered priority pollutants by the US EPA. In this work, 85 mm polyacrylate fibers were used to extract the analytes from the aqueous samples. They have compared static and d)mamic desorption modes. They found out that the static desorption showed better recoveries for the phenolic compounds. The authors evaluated the performance of the SPME-HPLC-UV-ECD with river water and waste-water samples. The method enabled the determination of phenolic compounds at low levels in these water samples. Figure 16.5 shows [178] the chromatograms for the Ebro river water sample spiked with 1 mg/L for ECD, and 0.1 mg/L for UV detection. 

The SPME device not only combines extraction and concentration but also directly transfers the absorbed compounds into a GC injector. These features of HS-SPME provide major advantages over previous headspace techniques. Coupling to GC, GC-MS (including ion-trap), split/splitless and on-column injection or desorption of the analytes in an SPME-HPLC interface have been described. A significant difference in sensitivity between direct and headspace sampling can occur only for very volatile analytes. HS-SPME introduces some selectivity into the extraction technique as only analytes with sufficient vapour pressure at room temperature are detected. An obvious drawback of HS-SPME is that semi- and non-volatiles will not be present in detectable amounts in the headspace. In combination with GC this is actually advantageous and enables faster equilibration than sampling from liquid [992]. 

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