Application. Gas Chromatography March 1998

Transcription

1 Micro Liquid-Liquid Extraction with the Agilent 7683 Automatic Liquid Sampler Application Gas Chromatography March 1998 Authors Matthew S. Klee Agilent Technologies, Inc. 285 Centerville Road Wilmington, DE USA Abstract Micro liquid-liquid extraction (MLLE), which uses smaller samples and requires less sample handling than standard liquid-liquid extraction (LLE), lowers the cost of analysis by reducing both the amount of solvent necessary to prepare a sample and the amount of waste requiring disposal. Moreover, with the Agilent 689 Plus gas chromatograph (GC) system, MLLE results are as sensitive as results from standard LLE. Introduction Liquid-Liquid Extraction Liquid-liquid extraction (LLE) is one of the oldest and most widely used sample preparation processes. LLE relies on the partition of analytes between two immiscible liquids. For the most effective separation and concentration, analytes of interest should show a significant preference for one liquid over the other. The degree of separation of a given analyte between two liquids is a function of several factors, including: Polarity of the solute Relative polarity of the two solvents Ionic strength ph Temperature Presence and concentration of additional analytes Time Mixing/contact between the bulk liquids The concentration of an analyte in a given extraction solvent is related to its original concentration in the starting solvent, its affinity for the extraction solvent relative to the original solvent, and the relative volumes of the two solvents. This is expressed by: C a = K C b V b /V a where K is the partition coefficient (a constant that depends on the analyte and solvents), C a andc b are the concentrations of the analyte in solvents a and b, and V a and V b are the respective volumes of solvents a and b. Maximizing concentration of a given analyte (for increasing sensitivity of analysis at a constant injection volume) in solvent a requires a large K and a small volume of solvent a compared with solvent b. As a rule of thumb, to maximize concentration of an analyte in a given solvent (maximum K), the polarity of the solvent should be very similar to the polarity of the analyte of interest. For example, analytes found in petroleum samples are usually nonpolar and, therefore, favor hydrocarbon solvents. In contrast, analytes containing acidic or basic functional groups prefer water or alcohols as solvents. LLE is widely used in many arenas of analysis, among them environmental, forensic, food, process,and pharmaceutical. Common examples of LLE in sample preparation for environmental analysis include semivolatiles in water, pesticides in food, and PAHs in water. LLE is performed for severalreasons: To increase the concentration of analytes of interest so as to increase the sensitivity of the analysis

2 To isolate analytes of interest from other analytes, contaminants, or interferences in the sample To transfer analytes from a problematic solvent (for example, water) to a preferred one (called "solvent exchange") There are several benefits of LLE: There are a wide variety of solvents from which to choose, giving analysts great flexibility in optimizing selectivity. Fresh solvent is added to each sample, avoiding any chance of carryover or cross contamination. The concepts and approaches to LLE are well understood and predictable. Table 1. Miscibilities and Relative Polarities (p') of Several Common LLE Solvents. 1,2 The larger the value of p', the higher the polarity and the affinity for polar solutes. X implies that the pair of solvents is immiscible and, therefore, potentially useful for LLE. O implies partial miscibility. Table 1 lists several common solvents, their relative polarities, and their mutual miscibilities. In choosing the appropriate solvent for a given analysis, analysts can use information such as this to help select an extraction solvent with the best polarity and solvent strength to yield the desired result. The range of choice in selectivities and solvent strengths is much larger for liquids than for alternative extraction/concentration techniques using bonded stationary phases such as with solid-phase extraction (SPE) or solid-phase micro extraction (SPME). In the table, an "x" indicates that the solvent pair is immiscible and, therefore, a potential match for LLE. An "o" indicates that the solvent pair is partially miscible but that it might still be useful for LLE. Micro Liquid-Liquid Extraction Micro liquid-liquid extraction (MLLE) is a simple implementation of LLE that can be accomplished with the * Saturated alkanes such as pentane, cyclohexane, hexane, trimethylpentane, isooctane, etc. n/a = not available 7683 automatic liquid sampler (ALS). Because the depth of the syringe needle into the ALS vial can be adjusted, the sample can be removed from either the top or the bottom layer of two immiscible liquids, offering flexibility and reduced sample preparation. The first step is to select an appropriate solvent for extraction. Then, approximately.5-ml of solvent 1 is added to 1-mL of sample in the standard 2-mL ALS sample vial. The vial is capped, shaken, and then the two layers are allowed to separate after the vial is placed in the ALS tray or turret. Discussion Large-Volume Injection in Combination with MLLE Interest in increasing analytical measurement sensitivity has driven the development of several largevolume injection techniques. (3-5) Larger injections lead to proportionally lower detection limits. The 7683 ALS can be used to inject large volumes (typically 25 to 1 ml). To remove the large amount of solvent before it reaches the column, where it could distort peak shape and obscure analyte peaks, the 689 Plus gas chromatograph (GC) system 2

3 offers two inlet choices: a programmable temperature vaporizer (PTV) inlet in solvent vent mode for complex matrices or a cool on-column (COC) inlet with solvent vapor exit for cleaner matrices such as drinking water. References 3, 4, and 5 provide further explanation of large-volume injections with the 689 Plus GC. 1. ml Water sample +.5 ml Pentane ((( Vortex ))) Sample withdrawn and Injected Applications of Micro Liquid-Liquid Extraction Figure 1. Typical quick MLLE procedure to extract analytes from water. Halocarbons in Drinking Water Halocarbons are produced as by products in the disinfection of drinking water. The concentration of halocarbons in drinking wateris regulated in most parts of the world. Halocarbons in drinking water can be determined quickly by using MLLE with pentane for the extraction (figure 1) and a gas chromatograph with an electron capture detector (GC/ECD) for the analysis. 2 Figure 2 shows a chromatogram from the MLLE of common halocarbons at concentrationsnear the World Health Organization regulatory limit (1 ppb total). Figure 3 illustrates the linearity of response achieve with the 689 Micro-ECD. Hz Figure 2. CHCl 3 CHBrCl 2 CHBr2 Cl CHBr 3 Pentane MLLE of 5 ppb trihalomethanes in water. Sample preparation: 1 ml of water +.5 ml of pentane, vortex vial, auto inject 1 ml of supernatant (splitless). GC conditions: 3 m 53 mm 2.65 mm Agilent HP-1, 35 C (1) C/min. Detector: 689 Micro-ECD. min Area 18, Area = * Amt , Correlation: , 12, 1, 8, 6, 4, 2, 5 Amount [ppb] 1 Figure 3. Dichlorobromomethane calibration curve for pentane/water extraction of trihalocarbons showing excellent linearity with the 689 Micro-ECD. 3

4 Figure 4 presents the results of MLLE in an analysis of drinking water, showing chloroform concentration that grossly exceeds the regulatory limit. Leaking Underground Storage Tanks Hz ppb Chloroform 11 ppb CHBrCl 2 Many states in the U.S. require the monitoring of gas stations for leaks in their underground storage tanks. A quick extraction of some soil (in water) from various points around the storage tanks, or from water sources near the site, can indicate if a leak is present and from where it is coming. Figure 5 shows the result of a cyclohexane MLLE of diesel fuel from water. Profiles from the chrmatogram are used to identify the type and specific source of the fuel. Total flame ionization detector (FID) area response under the profile is used to determine the concentration of diesel fuel in the water sample. PAHs in Water 4 4 ppb CHBrCl 2 1 ppb CHBr Figure 4. Trihalocarbons determined in actual drinking water by the MLLE method. pa 3 min Many common polynuclear hydrocarbons (PAHs) are carcinogens. As such, their concentrations in drinking and wastewater are regulated. Nitromethane is a unique solvent that can extract aromatic compounds selectively. 6 Nitromethane is denser than cyclohexane; so in this case, an aliquot of the bottom layer in the vial is injected for analysis. Figure 6 shows the resulting chromatogram for nitromethane extract of PAHs from cyclohexane C 14 C 16 C 18 C 2 Figure 5. MLLE extraction of 1 percent diesel oil in water, extracted into cyclohexane. Column: 3 m 32 mm.25 mm Agilent HP-5. Inlet: 25 C, 1 ml splitless injection. Detector: FID, 25 C. Resulting area RSD: 2 to 3 percent. 6 4

5 Amphetamines Figure 7 shows how MLLE can be used to identify and quantify illicit drugs such as amphetamines. The procedure involves dissolution of the material in water, ph adjustment, and MLLE. The procedure is fast and effective. The simple extraction procedure is as follows: 1. Transfer and weigh 5 to 1 mg of sample into a 2-mL sample vial. (Use more sample if the content of amphetamines in the sample is less than.1 percent) 2. Pipette 1 ml of phosphate buffer (ph 1) into the vial and mix (by shaking) to dissolve the amphetamines. 3 Pipette.5 ml of methylenechloride into the vial and vortex twice (5 to 1 seconds each time), or shake, to extract the amphetamines from the buffer. 4. Place the vial in the tray of the 7683 ALS. Inject 1 ml of methylene chloride extract (bottom layer) into the GC for analysis. Figure 6. MLLE extraction of 1 ppm PAHs extracted selectively from cyclohexane into nitromethane. Column: 3 m 32 mm.25 mm Agilent HP- 5. Inlet: 25 C, 1 ml splitless injection. Detector: FID, 25 C. RSD: 1 to 2 percent. 7 Conclusion Micro liquid-liquid extraction is a fast and effective means to analyze a wide range of samples. This technique also reduces the cost of analysis by requiring less solvent than standard liquid-liquid extraction. Moreover, MLLE can be accomplished with the standard Agilent 689 Plus GC with an Agilent 7683 ALS. Figure 7. FID analysis of amphetamine and methamphetamine. MLLE extraction of 1 mg/ml of each component in water. Resulting area repeatability was 1 percent RSD. Column: 3 m.25 mm id.25 mm Agilent HP-5MS. Oven: 12 C (2 min) to C/min, to 3 C (2 min) at 25 C/min. Inlet: 25 C. 1 ml injection split 5:1. Detector: FID, 3 C. 5

6 References 1. Snyder, L. R., and Kirkland, J. J., Introduction to Modern Liquid- Chromatography, 2 nd ed., John Wiley & Sons, Inc., High Purity Solvent Guide, James T. Przybytek, ed., Burdick & Jackson Laboratories, David, F., Sandra, P., and Klee, M. S., "Analysis of Trihalomethanes by Micro Liquid-Liquid Extraction and Capillary Gas Chromatography with the 689 Micro ECD, " Agilent Technologies, Application Note , Publication No. (23) E, April Wilson, B., Wylie, P. L., and Klee, M. S., "Large-Volume Injection for Gas Chromatography Using a PTV Inlet," Agilent Technologies, Application Note , Publication No. (23) E, March Wilson, B., Nixon, D., and Klee,M. S., "Large-Volume Injection for Gas Chromatography Using COC-SVE," Agilent Technologies, Application Note , Publication No. (23) E, April David, F., Sandra, P., and Klee, M. S., "Large-Volume Cool On-Column Injection Using the G2399A Solvent Vapor Exit Kit," Agilent Technologies, Application Note , Publication No. (23) E, April David, F., Sandra, P., and Stafford, S., "Use of Variable Sampling Depth for Micro Liquid-Liquid Extraction with the G1513A Injector: Sequential Analysis of Hydrocarbons and PAHs," Agilent Technologies, Application Note , Publication No.(23) E, May Agilent shall not be liable for errors contained herein or for incidental or consequential damages in connection with the furnishing, performance, or use of this material. Information, descriptions, and specifications in this publication are subject to change without notice. Copyright 2 Agilent Technologies, Inc. Printed in the USA 4/ E