Single Drop Micro extraction

SDME is based on the preconcentration of target analytes in a small volume (0.5-3 mL) of organic solvent, aqueous solutions or ionic liquid (IL). In this sense, SDME is considered to be the basic liquid-liquid microextraction technique (Sarafraz-Yazdi and Amiri 2010; Nerin et al. 2009; Psillakis and Kalogerakis 2002; Xu et al. 2007; Lambropoulou et al. 2007) .

Equilibration times, which range from seconds to hours, can happen in SDME, depending on the target analyte. In any case, regardless the sampling regime (i.e. equilibrium or kinetic), the extraction process is not usually exhaustive. In those cases in which the amount of extracted compound is negligible, several extractions can be performed on the same sample, as mentioned before for sorptive extraction (Jeannot et al. 2010).

Table 4.4 CPE application to the determination of priority or emerging pollutants in water samples during the period 2000-2010

Analysis

Apparent

Analyte

Matrix

Surfactant

technique

LOD

recovery (%)

Precision (%)

Reference

C1P

River water

0.5% Triton X-114

HPLC-UV

1.7-5.0 ng/L

40-100

4-10

Seronero et al. (2000)

Estrogens

Water

o.25% Triton X-114

HPLC-UV

0.23-5 ng/L

81.2-99.5

8.1-12

Wang et al. (2006)

Hydrazine

Drinking water River water

DAB

UV

0.08 ng/L

98.5-105

1.22

Zarei (2007)

OPPs

Wastewater Groundwater

POLE Genapol X-080

HPLC-UV

0.61-2.6 ng/L

62.6-84.5

0.5-3.2

PadrĂ³n Sanz et al. (2004)

PAHs

Water

1.0%PEG/PPG-18/18 dimethicone 1.0% PEG-12 dimethicone

HPLC-UV

93.6-100

Yao et al. (2007)

PAHs

Aqueous solution

1.0% PEG/PPG-18/18 dimethicone

HPLC-UV

69-100

Yao and Yang (2008b)

PAHs

Seawater

1.0% POLE

HPLC-FLD

1.0150 ng/L

32.6-115

<10.4

Pino et al. (2002)

PAHs

Water

2% Tergitol TMN-6

HPLC-UV

79.3-92.6

Yao and Yang (2007)

PAHs

Water

10% Tergitol TMN-6

HPLC-UV

Yao and Yang (2008a)

PAHs

Water

5% Triton X-114

HPLC-FLD

0.6-1.8 ng/L

Pongpiachan (2009)

PAHs

Aqueous solution

Tergitol 15-S-5 LE-203, Brij 30

HPLC-UV

80-100

Hung and Chen (2007)

PAHs

Aqueous solution

3% Tergitol 15-S-7

HPLC-FLD

80-96

Bai et al. (2001)

PAHs

Aqueous Solution

1% Tergitol 15-S-5

HPLC-FLD

Li et al. (2004)

PAHs

Aqueous solution

Tergitol 15-S-9 Neodol 25-7 Tergitol 15-S-7

HPLC-UV

0.1 mg/L

Hung et al. (2007)

PAHs

Seawater

1.0% POLE/Brij 30

HPLC-FLD

32.4-231 ng/L

72-99

<8.07

Delgado et al. (2004)

PAHs

Aqueous solution

Triton X-114 SDSA

GC-FID

0.9-9.9 ng/L

92-105

Sikalos and Paleologos (2005)

PBDEs

Water

Triton X-114

GC-MS

1-2 ng/L

>99.9

<8.5

Fontana et al. (2009)

(continued)

(continued)

Table 4.4 (continued)

Analysis

Apparent

Analyte

Matrix

Surfactant

technique

LOD

recovery (%)

Precision (%)

Reference

PCDDs

Water

5% POLE

12.80 ng/L

-

-

Padron Sanz et al. (2002)

PEs

Effluent water

0.25% Triton X-l 14

HPLC-UV

1.0-3.8 ng/L

86-103

1.9-3.9

Wang et al. (2007)

Phenol

Wastewater

Monodisperse nonionic surfactants Polydisperse nonionic surfactants

CE-UV

Taechangam et al. (2009)

Phenolic

Rainwater

PONPE 10

MEKC-UV

0.10-0.20 (.ig/L

>89.5

Stege et al. (2009)

Compounds

River water

Phenols

Seawater/depurated waste water

HPLC-UV

0.6-3.5 ng/L

-

-

Santana et al. (2004)

Triazine

Aqueous solution

Triton X-l 14

CE-UV

-

-

-

Carabias-Martinez

herbicides

et al. (2003)

4.3.2.1 Single Drop Microextraction Modes

So far, up to seven different modes of SDME are found in the literature (DI, HS, continuous flow (CF), drop-to-drop (DD), directly suspended droplet (DSD), liquid-liquid-liquid (LLL) and a combination of LLL and DSD). However, DI-SDME and HS-SDME are the most frequently found in the bibliography (38% and 41%, respectively), since they do not require extra-equipment (i.e., pump in CF) and are applied to the widest range of analytes (Sarafraz-Yazdi and Amiri 2010; Xu et al. 2007; Lambropoulou et al. 2007; Jeannot et al. 2010).

In DI-SDME the solvent drop is in direct contact with the aqueous solution and, therefore, the extraction solvent must be immiscible in water. Therefore, in DI-SDME the acceptor solvent is limited to non-polar or slightly polar organic solvents and ILs. In this sense, DI-SDME is limited to semi-volatile non-polar or slightly polar compounds, since volatile compounds are better extracted in the HS mode. The most used solvents are n-hexane and toluene followed by, in many cases, GC analysis. When LC is used after DI-SDME, solvent exchange from non-polar to polar is necessary, unless an IL was used for pre-concentration (Jeannot et al. 2010).

In HS-SDME the acceptor solvent is placed in the headspace of the extraction vial and, in this sense, there are no restrictions on the nature of the extraction solvent. However, solvents with a high vapor pressure (mostly preferred for GC analysis) evaporate too quickly during HS-SDME and, thus, low vapor pressure solvents are preferred. In this sense, the vapor pressure of the solvent is the primary consideration in order to study the best acceptor phase rather than the extraction efficiency. The organic solvents mainly used are 1-octanol, hexadecane, dodecane and decane, but polar solvents, including ILs, aqueous solutions and even pure water, can be used. Therefore, HS-SDME is applicable to both polar and non-polar compounds, volatile or semi-volatile (Xu et al. 2007; Lambropoulou et al. 2007; Jeannot et al. 2010). Additionally, non-volatile matrix interferences are reduced (Xu et al. 2007; Jeannot et al. 2010), as mentioned for HS-SPME and HSSE. GC is also the preferred separation technique in HS-SDME.

The first works on SDME were carried out by Liu and Dasgupta (1996), where a micro-drop of organic immiscible solvent was suspended in a larger aqueous drop, and at the same time by Jeannot and Cantwell (1996), who suspended a micro-drop at the end of a Teflon rod. The problem with these two approaches was that the extraction and injection were performed separately. Thus, Jeannot and Cantwell suggested the use of a microsyringe to suspend the drop (Jeannot and Cantwell 1997).

Both DI-SDME and HS-SDME can be performed in the dynamic mode and, in this case, not only the sample is stirred but also the extracting solvent is in motion in order to improve mass transfer and, thus, extraction efficiency (He and Lee 1997). The dynamic mode can, as well, be performed with both unexposed and exposed drops. In the unexposed or in-syringe mode (see Fig. 4.8), the extraction solvent with a small volume of the sample (1-3 mL) is withdrawn into the syringe needle, held for a time and then the sample is expelled. This cycle is repeated from 30 to 90 times. The in-syringe mode should be termed dynamic LPME since a drop configuration is not involved. In the exposed-drop, the drop is exposed to the sample, withdrawn into

Fig. 4.8 Dynamic liquid phase micro-extraction (LPME): (a) the acceptor solvent within the syringe plug, (b) small volume of the sample is withdrawn into the syringe and held for a time and (c) the acceptor phase is enriched with the analytes and the sample expelled

the needle for a specified time and expelled out to the needle tip again (Xu et al. 2007; Lambropoulou et al. 2007; Jeannot et al. 2010). During dynamic HS-LPME vapor pressure problems are overcome and the selection of organic solvent is more flexible than previously mentioned for static HS-SDME (Xu et al. 2007).

4.3.2.2 Ionic Liquids

As previously mentioned, ILs are found among the acceptor phases used in SDME. ILs are low-temperature melting salts that form liquids composed entirely of ions. Although the term IL was firstly used to include all types of thermally stable organic and inorganic melts, the emphasis has changed to low-melting point, organic, air stable salts with wide temperature-stable liquid ranges. The main types of room temperature ILs are alkylammonium, tetraalkylammonium, tetraalkylphosphonium, 1,3-dialkylimidazolium, and N-alkylpyridinium salts formed with weak nucleophilic anions such as bis(trifluoromethylsulfonyl)imide, hexafluorophosphate, tetrafluo-roborate, perfluoroalkylsulfonate, etc. According to some authors ILs can be considered as environmentally friendly (Poole and Poole 2010; Aguilera-Herrador et al. 2008b). The most direct form of analysis of ILs extracts is LC but the coupling to GC is more troublesome since ILs should not enter the capillary column used in GC. For the coupling of IL-SDME with GC different approaches have been used. For instance, Aguilera-Herrador et al. (Aguilera-Herrador et al. 2008b) developed an interface which consisted in an injection zone, a removable unit and a transfer line for the direct coupling of ionic liquid based SDME (IL-SDME). Zhao et al. (2009a)

used a homemade tube that was introduced in a glass injector insert of a GC for the IL-HS-SDME of chlorobenzene derivatives. Finally, Chisvert et al. (2009) used a commercially available TDU for TD-GC-mass spectrometry (TD-GC-MS) analysis of chlrobenzenes in water samples. Some applications of ILs in SDME are included in Table 4.5.

4.3.2.3 Single Drop Microextraction (SDME) Development and Applications

During SDME development the most frequently studied variables are common to other extraction techniques such as inert salt addition, pH adjustment, agitation of the sample, extraction solvent nature, extraction time or volumes of donor and acceptor phase solutions (Psillakis and Kalogerakis 2002; Lambropoulou and Albanis 2007) . High stirring rates improve mass transfer. However, in DI- SDME high stirring rates cause droplet instability (Psillakis and Kalogerakis 2002; Xu et al. 2007). Organic drop volume is a characteristic variable during SDME optimization. The use of a large organic drop results in an increase of the extraction efficiency, whereas larger drops are more difficult to manipulate and less reliable. Besides, large injection volumes cause band broadening in capillary GC. In this sense, approx. 1 mL organic drop volume is commonly used (Psillakis and Kalogerakis 2002) . Extraction temperature is studied in HS-SDME in order to accelerate the mass transfer from the sample to the headspace. However, high temperatures decrease the organic solvent-headspace distribution coefficient and, therefore, a consensus is needed. In more sophisticated systems HS-SDME can be performed with solvent cooling (Yamini et al. 2004).

During SDME it is advisable to wash the syringe several times with the extraction solvent in order to eliminate air, since the presence of air bubbles can change the rate of extraction and give raise to non-reproducible results. Besides, precision is improved when the drop is placed in a reproducible and stable position (Psillakis and Kalogerakis 2002) .

During SDME development it should be considered that there are two types of GC syringes, those where the plunger of the syringe is a wire inside the needle itself, and those where the plunger is a wire inside the glass barrel of the syringe. In the latter case, there is an extra or dead volume of solvent contained within the needle that is always retracted and not taken into account. This dead volume should be taken into account when conducting extraction rate experiments where the calibration standards are directly prepared into the solvent (Psillakis and Kalogerakis 2002).

Applications of SDME to priority or emerging pollutants are included in Table 4.5.

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