Concentration of sample extract

Concentration of the solvent extract prior to analysis allows for lower detection limits required to meet regulatory requirements. Concentration of extracts may be achieved by  

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Poor recoveries, especially from tissue samples (54,59,63). Losses during concentration of sample extracts by evaporation (57). 

During extraction and cleanup, concentration of sample extracts can be performed using similar techniques as in the analysis of organochlorines, PCDDs, and PCDFs [1, 111]. Vacuum evaporation and evaporation under nitrogen flow are also typical concentration techniques in PCDE analyses. 

Many continuous extractions involving solid samples are carried out with a Soxhiet extractor (Figure 7.18). The extracting solvent is placed in the lower reservoir and heated to its boiling point. Solvent in the vapor phase moves upward through the tube on the left side of the apparatus to the condenser where it condenses back to the liquid state. The solvent then passes through the sample, which is held in a porous cellulose filter thimble, collecting in the upper reservoir. When the volume of solvent in the upper reservoir reaches the upper bend of the return tube, the solvent and any extracted components are siphoned back to the lower reservoir. Over time, the concentration of the extracted component in the lower reservoir increases. 

Analysis of the concentrated, purified sample extracts is effected by LC/MS or LC/MS/MS, as described in Section 2.1. 

Transfer the concentrated crop sample extract (strawberries, rice grain, barley grain and rice straw) into a 50-mL separatory funnel with a small volume of water. Extract the solution three times with 10 mL of a chloroform-methanol (3 1, v/v). Dry the chloroform-methanol layer with a small amount (about 8 g) of anhydrous sodium sulfate on a glass funnel and transfer the dried solution to a 100-mL separatory funnel. 

Soils in the North China Plain and Loess Plateau regions contained 0.04-3.01 mg/kg DTPA-extractable Zn with an average of 0.44 mg/kg. The concentrations of DTPA-extractable Zn in northern China are presented in Table 7.7. In the loessial soils of the Loess Plateau, 64% of the soil samples had less than 0.5 mg/kg of bioavailable Zn. The bioavailable Zn in the arid soils of North China varied from 0.08-11.84 mg/kg with an average of 1 mg/kg, with 41% of the soil samples having < 0.5 mg/kg of bioavailable Zn. The average amount of bioavailable Zn in calcareous soils was 0.35 mg/kg (trace - 1.12 mg/kg). The North China Plain and Loess Plateau are major Zn-deficient regions in China. Calcareous paddy soils frequently displayed Zn deficiency in rice. Zinc fertilizers have been applied to rice, maize, sorghum, wheat, cotton and fruit trees where bioavailable Zn was less than 0.5 mg/kg. 

Christman et al. [72] gave details of procedures for extraction, clean-up, and concentration of samples of soil prior to the determination of their content of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans by gas chromatography and by gas chromatography-mass spectrometry. Instrumental parameters are also included. Some typical results are tabulated. 

A method with LOQ at ppt levels was developed based on LLE followed by GC-AFID for the determination of trace concentrations of nitrobenzene, l-chloro-2-nitrobenzene and synthetic fragrances such as musk xylene (223) and musk ketone (224). The method was applied to study the distribution of these compounds in environmental samples of North Sea waters460. GC with atomic emission detection (AED) has been successfully applied to the determination of nitro musks in human adipose tissues, at ppb concentration levels. A clean-up procedure for nonpolar substances and element-specific detection with AED enabled for the first time target screening analysis for lipophilic nitro aromatic compounds. The lack of sensitivity of AED was compensated by higher concentrations of the extracts... 

Several strategies have been described for the preconcentration of sample components present at low concentrations. These techniques include zone sharpening,28-29 on-line packed columns,30 and transient capillary isotachophoresis (cITP).31-32 Other standard laboratory techniques are often used, including solid-phase extraction, protein precipitation, ultrafiltration, etc. Two important points to keep in mind when selecting a concentration protocol are the sample requirements of the method and the potential selectivity on relative concentrations of sample components. The latter point applies to purity and concentration analysis. 

In many cases, sample extracts are generally filtered, dried with desiccant, and concentrated before analysis. Concentration of the extract may allow for lower sample detection limits. Frequently, sample extracts must be concentrated to obtain detection limits low enough to meet regulatory action limits. 

The use of a volatile solvent, e.g., pentane, was not explored because of inherent limitations. Concentration of such extracts was not possible because of the volatility of the sample components. Therefore the maximum concentration factor that could have been achieved was limited by the partition coefficients of the compounds into the solvent used in the extraction. For most compounds this factor was estimated to be about 10 1. Furthermore, with CRMS and other general detectors, the solvent masking problem would still preclude observation of many compounds. Therefore, the method would be limited to detectors that are not responsive to the solvent used in the extraction. Recent work (3.4,5) has indicated that extraction with a volatile solvent is a viable approach for the analysis of a small set of compounds, e.g., the trihalomethanes, with an electron capture detector in drinking water samples where concentration factors of 10 1 or less are acceptable. 

Yoshikawa et al. 1986). The transfer of cresol from the aqueous hydrolysate to an organic solvent is accomplished by simple extraction with a volatile organic solvent such as methylene chloride or ethyl ether. Concentration of the extract by gentle removal of the solvent prepares the sample for the analysis stage. 

Extraction at 345 bar/40°C of a sample mixed with AA (1 1) removed lipid with the least amoimt of cholesterol. The average concentration of cholesterol extracted in the presence of AA at 345 bar/40°C (separated at 34.5 bar/40°C) was 60 mg/100 g of lipid extracted. At fractions beyond 6 kg of CO2 used where one-half of the lipid remained, the average concentration of cholesterol in the lipid was less than 40 mg/100 g of lipid extracted. This was a 70% reduction in the concentration of cholesterol in the extracted lipid. 

If analysis is to be attempted with a detection system of only moderate selectivity, a substantial cleanup procedure may be required in order to enhance the concentration of the extracted trace residue while decreasing die concentration of possible interfering substances in the sample matrix. This is die case with most of the relatively nonspecific physicochemical detection systems used in residue analysis. Occasionally a sample may be suitable for direct physicochemical analysis after an extraction and concentration step. However, the majority of edible animal products need extensive cleanup to separate the compounds of interest from animal lipids and other natural organic substances prior to detection. For such detection systems, there has been a general rule dictating diat the cleaner sample, the better the result obtained. 

Clarification with Carrez solutions can be used to eliminate interfering compounds (14,44,45). Purification, isolation, or concentration of the extract can also be performed by solid-phase extraction with C8 (53) or Cl8 (46,47,75). Hayakawa et al. (75) used Sep-Pak Cl8 cartridges for the separation of aspartame from its degradation products. The sample was applied to the Sep-Pak, and degradation products and aspartame were eluted with 10% and 30% methanol in acetate buffer, respectively. 

The extraction, purification, or concentration of samples for thaumatin determination will depend on the type of food matrix, as described in Sec. I.C. Purification can also be accomplished by partition (126). Ramsohoye and Kozlov (124) purified thaumatin on a cation-exchange cartridge (CM Sephadex C-50). After conditioning the cartridge with phosphate buffer, the sample is loaded and washed with buffer. Thaumatins are then eluted with 0.25 M sodium chloride. Mackenzie et al. (127) separated thaumatins on cellulose CM52. After equilibration of the column with buffer, pH 7.2, the sample was applied, washed with the buffer, and eluted with a gradient of 0 to 0.2 M NaCl in the same buffer. 

The carboxin is extracted from the sample with acetone in a Soxhlet extraction apparatus and, after concentration of the extract, is determined via gas-liquid chromatography using a nitrogen-selective detector. The presence of carboxin is confirmed by the use of a sulfur flame photometric detector. Recoveries ranged from 73 to 80% (barley) and 73 to 78% (wheat). 

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