Optimizing Extraction Conditions

The sample volume also has an effect on both the rate and recovery in SPME extractions, as determined by extraction kinetics and by analyte partition coefficients. The sensitivity of a SPME method is proportional to n, the number of moles of analyte recovered from the sample. As the sample volume (Vs) increases, analyte recovery increases until Vs becomes much larger than the product of K, the distribution constant of the analyte, and Vf, the volume of the fiber coating (i.e., analyte recovery stops increasing when KfeVf Vs) [41]. For this reason, in very dilute samples, larger sample volume results in slower kinetics and higher analyte recovery. 

Extraction temperature can also be an important factor, especially in headspace SPME analyses. However, in SPME, unlike in GC headspace analysis, increasing the temperature in SPME can result in a maximum usable temperature for the method (i.e., going from 25°C to 30°C may result in a reduction in sensitivity [42], 

The sample matrix may also be modified to enhance extraction recovery. This is typically done by either dissolving a solid sample in a suitable solvent, usually water or a strongly aqueous mixture, or by modifying the pH or salt content of a solution. Modifying the pH to change the extraction behavior works the same way in SPME as it does for classical liquid-liquid extraction. At low pH, acidic compounds will be in the neutral form and will be extracted preferentially into the fiber coating at high pH, basic compounds are extracted favorably. Neutral compounds are not affected appreciably by solution pH. 

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Lopez-Avila and Benedicto combined SEE with ELISA to determine sulfamethazine in powdered milk. Various conditions were tested in order to achieve quantitative extraction of sulfamethazine. Variations in extraction pressure, temperature, extraction period, and the presence of organic modifier resulted in extraction efficiencies of 0-92%. Once optimal extraction conditions had been developed, a commercially available ELISA was utilized to determine sulfamethazine concentrations. The LOD was 2.5 pgkg and satisfactory recoveries were obtained at levels from 5 to 15 pgkg-i. 

It has been concluded from the data that the optimal extraction conditions are methanol-water (8 2, v/v) as extracting agent, boiling temperature and 15min extraction time [154],... 

Tzeng et al studied the SCCO2 extractions with addition of 16.25% ethyl alcohol as a co-solvent to obtain scopoletin and artemisinin (1) from A. annua. A two-factor central composite experimental design was adopted to determine the optimal extraction conditions. Two-hour ethanol-modified SCCO2 extractions was more efficient than 16 h-Soxldet hexane extraction to provide pure artemisinin (1). ... 

Figure 7.6a gives the response surface of the partition coefficient of sulphacetamide. It can be seen that optimal extraction conditions of sulphacetamide are binary compositions of methylene chloride and methyl tert.-butyl ether. It can also be observed that the partition coefficient is nearly constant at the binary axis methylene chloride/chloroform. Therefore, small variations in the binary compositions of methylene chloride and chloroform will not significantly change the partition coefficient. In other words binary compositions with methylene and chloroform yield robust extractions for sulphacetamide. This conclusion is confirmed by the robustness plot of the partition coefficient of sulphacetamide (Figure 7.6b). This plot also shows that under conditions where the partition coefficient is optimal (binary mixtures of methylene chloride and methyl tert.-butyl ether), the robustness of the partition coefficient reaches a maximum value.

Greco [40] determined optimal extraction conditions for the recovery of nitrogen-containing aromatic compounds from soil. 

The analyst s function is to optimize extracting conditions so that the distribution of solute between phases lies far to the right in equation (2.1) and the resulting value of Kd is large, indicating a high degree of extraction from phase A into phase B. Conversely, if KD is small, less chemical X is transferred from phase A into phase B. If KD is equal to 1, equivalent concentrations exist in each phase. 

The interlot or intersample differences can also cause variations in the recovery for an analyte and its internal standard. For example, in a method based on liquid-liquid extraction for p-hydn)x y-a(< >r vastatin, the recovery of internal standard varied from 67.19 to 89.99 % (1.5-fold) for the four subjects tested despite the fact that the ratios of analyte to IS were relatively independent of subject sources, i.e., no impact on the quantitation [36], It should be borne in mind that a method is usually optimized aiming the maximum recovery for an analyte, i.e., not for any matrix components. In case where a matrix component causes matrix effect and the optimal extraction conditions happen to be an unreliable extraction condition for the matrix component, variable IS response is very likely. 

Plant Material Compound(s) of Interest Extraction Solvent Optimal Extraction Conditions Reference... 

Another expected advantage of the technique is a good control of the whole extraction/separation process. It is worth recaUing that, with the classical supercritical fluid extraction process, the choice of optimal extraction conditions is often... 

Supercritical fluid extraction (SFE) processes can be sealed-up from lab-seale or pilot-scale results aeeording to a simple procedure (27,28) At first, small-scale experiments lead to the optimal extraction conditions through a scanning of different pressures, temperatures, solvent ratios, and eomposi-tion. Then, the scale-up method will depend on the meehanism eontrolling the extraction ... 

NIST certified reference material SRM 1649a (urban dust-organics) was applied to evaluate ASE method for the determination of selected PAHs, OCPs and PCBs under the optimized extraction conditions. The concentrations, expressed as mass fractions, for 22 PAHs, 35 PCBs (some in combination) and 8 OCPs have been... 

In fact, the theory can be used to optimize extraction conditions with a minimum number of experiments, but without the need to repeat calibration tests at the new conditions. 

The effects of pH in the oxidative destruction of folates has been reviewed by several authors (6,10,28). Each vitamer has its own unique pH stability, and it is therefore sometimes difficult to optimize extraction conditions for all folate forms in a single extraction. The pH of the extractant, presence of oxygen, the temperature used, and in addition, buffer type, can all affect the efficiency of folate extraction (8). Several folate derivatives can be altered during extraction. The presence of phosphate accelerates conversion of 10-HCO-H4-folate to (1) 5,10-CH+-H4-folate and (2) 5-HCO-H4-folate at pH values below pH 7 during extraction. Also, 5,10-CH2-H4 folates can be converted to H4 folate during extraction (29). As the use of ascorbic acid reduces 5-CH3-H2-folate to 5-CH3-H4-folate, the former can be quantified only in the 5-CH3-H4-folate pool (30). Thus, extraction conditions can affect the vitamer distribution and therefore need always to be carefully considered and fully reported. The chosen extraction conditions are dependent on the purpose of the analysis to be carried out. 

From the above, it is obvious that the design engineer has to evaluate optimized extraction conditions and to select the best way of separation in order to obtain a viable process. In the following sections, various design possibiUties are described in more detail. 

The optimal extraction eonditions aceording to Golmakani et al. study were as follows 180 C, 207 bar, 15 min and eflianol + ethyl lactate (50 50 v/v) in the case of PLE, predicting 21.5% of total yield and 63.9% of GLnA recovery. For GXLs extraction optimal extraction conditions were 40"C, 300 bar, 90 min and C02/ethanol with 50% ethanol, and the predicted results were 7.0% of total yield and 28.0% of GLnA recovery. These results were experimentally eorroborated (at least three times) by the authors and the prediction was pretty close to the mean real extraction process. Therefore, different extraction alternatives using ethyl lactate as alternative solvent were successfully applied to produce enriched fractions in y-linolenic acid. 

Small scale experiments should be performed first to achieve optimal extraction conditions through a scanning of different process parameters such as extraction pressures, temperatures, solvent flow... 

Perform the scaling-up SFE under the optimized extraction condition. 

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