Report
Solid-Phase i Microextraction Solid-phase microextraction integrates sampling, extraction, concentration, and sample introduction into a single step
Zhouyao Zhang Min J. Yang Janusz Pawliszyn University of Waterloo 844 A
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n analytical process typically consists of several discrete steps (e.g., separation, quantification, and data analysis), each of which is critical for obtaining accurate and reproducible results. A sample preparation step is often necessary to isolate the components of interest from a sample matrix, as well as to purify and concentrate the analytes. Despite advances in instrumentation and microcomputer technology, however, many sample preparation practices are based on nineteenth-century technologies. For example, the commonly used Soxhlet extraction was developed more than 100 years ago. Efforts to introduce novel sample preparation concepts were frequently ignored by practitioners, who were explicitly required by law to use the approved traditional methods. Problems associated with these traditional sample preparation methods, such as the use of toxic organic solvents and multistep procedures that often result in loss of analytes during the process, frequently make sample preparation the major source of error in an analysis and prohibit integration with the rest of the analytical process. Renewed awareness of the pollution and hazards caused by hydrocarbons, including ozone depletion and carcinogenic effects, has resulted in international initi-
Analytical Chemistry, Vol. 66, No. 17, September 1, 1994
atives to eliminate the production and use of the organic solvents on which many current sample preparation methods depend. This phasing out of solvents is poised to induce a major change in analytical methodology (1) and also presents an opportunity for the scientific community to formulate practical alternatives to exisiting sample preparation methods. The major hurdle that keeps practitioners from adopting a new analytical method is the expense in terms of both capital cost and training requirements. An ideal sample preparation technique should be solvent-free, simple, inexpensive, efficient, selective, and compatible with a wide range of separation methods and applications. It should be able to be used to simultaneously separate and concentrate the components and should allow on-site extraction and analysis. In this Report we provide an overview of solvent-free techniques for sample preparation and describe how solid-phase microextraction (SPME) can integrate sampling, extraction, concentration, and sample introduction into a single step, resulting in high sample throughput. Solvent-free techniques
Although extraction techniques that use little or no organic solvent have been available for some time, only recently have 0003-2700/94/0366-844A/$04.50/0 © 1994 American Chemical Society
A Solvent-Free Alternative for Sample Preparation solvent-free sample preparation techniques begun to attract widespread attention, primarily because of regulatory pressures to reduce the use of toxic organic solvents. Solvent-free operations can be classified according to the separation medium. Most techniques fall into one of three categories: gas-phase, membrane, and sorbent extraction. Gas-phase extraction. These methods include static headspace sampling, purge and trap, and supercriticalfluidextraction. The headspace approach has been widely used for analysis of volatile compounds because the extracting phase (air or nitrogen) is compatible with most instruments and is suitable for field operations. Static headspace, which is probably the simplest solvent-free sample preparation technique, has been used for several decades to analyze volatile organic compounds (VOCs) in food, beverage, clinical, and other samples (2). The sample is equilibrated with its headspace, and a small volume of the headspace is then directly injected into a gas chromatograph for analysis. Because of the lack of any concentrating effect, however, static headspace suffers from low sensitivity. In addition, it cannot achieve exhaustive extraction and requires careful calibration. Dynamic headspace analysis, of which the commonly used purge-and-trap
method (3) is an example, uses a multiplepartition concept and allows quantitative removal of VOCs. Carrier gas is first bubbled through an aqueous sample to purge VOCs from the matrix; these compounds are then collected using a cold trap or sorbent trap. Drawbacks include foaming, cross contamination, and stripping flow rates that are incompatible with on-line operation. Although headspace methods are limited to VOC analysis, they can be used in combination with thermal desorption for analysis of solid samples and less volatile compounds. By heating the sample to an elevated temperature, volatile analytes can be thermally desorbed from solid samples and less volatile species in aqueous or solid samples can be better partitioned into the gas phase (4). Increasing the temperature is particularly important when analyzing samples containing solids such as fly ash or clay soil, which tend to strongly bind organic analytes. However, thermally labile analytes and the high moisture content of the desorbed gas mixture frequently prevent the use of thermal desorption. Because supercriticalfluidspossess both gas-like mass transfer and liquid-like solvating characteristics, supercritical fluid extraction (SFE) is an attractive solvent-free sample preparation technique
L
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Report (S). However, SFE requires an expensive, high-pressure delivery system and a large amount of high-purity carbon dioxide. Because of the need for heavy equipment, on-sitefieldanalysis is still difficult. Membrane extraction. Membrane extraction involves two simultaneous processes: polymeric extraction from the sample matrix and gas extraction from the polymeric phase. Despite three decades of development in analytical membrane technology, limited information is available on its use in sample preparation for chromatography. Although supported membrane sheets were used in many early methods, hollow fibers have been used in most recent membrane extraction techniques (6). The hollow fiber is self-supporting, and—compared with conventional headspace methods—also has a significantly higher surface-area-to-volume ratio, which allows greater mass transfer rates and thus a more efficient extraction. Membrane extraction can be directly combined with MS (7) or GC (8) to perform continuous monitoring. In membrane extraction a low-pressure stripping gas is typically used for VOC analysis; however, the use of high-pressure stripping gas makes it possible to apply the membrane method for the analysis of semivolatile components (9). Disadvantages of membrane techniques include slow response of the membrane to changes in concentration and a very limited capability to analyze polar compounds because of a lack of commercially available polar hollow-fiber membranes. This situation is likely to change in coming years, however, considering the rapid progress in polymer research. Sorbent extraction. The concept of using sorbent material to extract trace organic compoundsfroman aqueous sample was developed several years ago (10), and sorbents can now be used to extract organic compoundsfrommatrices such as water, air, and soil. One of the most important features of this technique is the concentration of analytes by the sorbent By using a sorbent that has a strong affinity for organic compounds, such compounds can be enriched from an otherwise very dilute aqueous or gaseous sample. Many sorbents are available for extracting different groups of organic compounds with various degrees of selectivity. 846 A
Solid-phase extraction (SPE) is one of the most commonly used sorbent extraction techniques. Analytes are extracted together with interfering compounds by passing an aqueous matrix through a plastic cartridge containing dispersed sorbent on a particulate support. A selective organic solvent is normally used to remove interferences first, and then another solvent is chosen to wash out the target analytes. SPE has a number of attractive features compared with traditional solvent extraction. It is quite simple, is inexpensive, can be used in the field, can be automated, and uses relatively little solvent. Particle-loaded membranes (known as Empore extraction disks), introduced in 1990, further improve extraction effi-
silicafiberor wires made of appropriate materials. The cylindrical geometry of the resulting SPME system (13) allows rapid mass transfer during extraction and desorption, prevents plugging, and facilitates handling and introduction into analytical instruments. SPME
Solid-phase microextraction consists of two processes: partitioning of analytes between the coating and the sample and desorption of concentrated analytes into an analytical instrument. In thefirstprocess, the coated fiber is exposed to the sample and the target analytes are extracted from the sample matrix into the coating. Thefiberwith concentrated analytes is then transferred to an instrument for desorption, followed by separation and quantitation. A cleanup step can be incorporated into the process by using selective solvents, similar to SPE; however, to date this additional step has not been necessary because of the selective nature of the available coatings. Thus far, SPME applications have focused on extracting organic compoundsfromvarious matrices such as air, water, and soils, followed by directly transferring them into a gas chromatograph injector where they are thermally desorbed, separated on the column, and quantified by the detector. For these applications, a fused-silica fiber coated with a gas chromatographic stationary phase, such as poly(dimethylsiloxane), is used. The techniques reciency, reduce the use of solvent, and decrease plugging in SPE (11). SPE does quired to produce thesefibersand their coatings have been well developed for have some limitations, however, such as manufacturing optical fibers, and the low recovery, which resultsfrominteraction between the sample matrix and ana- fused-silicafiberitself is chemically inert and very stable even at high temperature. lytes, and plugging of the cartridge or The small size and cylindrical geometry blocking of the pores in the sorbent by of thefiberallow it to be incorporated into solid and oily components, which results a syringe-like device (Figure 1), which in low breakthrough volume and low capacity. Although the use of Empore disks can be operated like an ordinary syringe has reduced these negative effects (12), and easily accommodated in a gas chromatograph injector. these problems, along with high blank values and batch-to-batch variation of the SPME preserves all the advantages of sorbents, persist. Because SPE is a multi- SPE such as simplicity, low cost, easy autostep approach involving concentration of mation, and on-site sampling and, at the the extract, it is limited to semivolatile same time, eliminates the disadvantages of compounds with boiling temperatures sub- SPE such as plugging and the use of solstantially above those of the solvents. vents. No special thermal desorption module is used, and no modification of the One solution to these limitations is to gas chromatograph is needed. SPME with improve the geometry of the sorbent by thermal desorption completely elimicoating it on afinerod such as a fused-
SPME preserves all of the advantages of SPE while eliminating the disadvantages ofplugging and solvent use.
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nates organic solvents from extraction and injection, and it integrates both pro cesses into a single step. The geometry of SPME enables placement of the sorbent (the fiber coating) into a sample (such as gaseous or aqueous matrices) or the headspace above the sample to extract ana lytes. Because SPME is a static extraction process, large surface area is no longer as critical as in SPE. Smooth liquid coat ings can be used, eliminating the problem of plugging. Furthermore, by sampling from headspace, SPME can extract ana lytes from very complex matrices such as sludge. As shown in Figure 1, the fused-silica fi ber is connected to a stainless steel tubing that is used to increase the mechanical strength of the fiber assembly for re peated sampling. The stainless steel tub ing is then contained in a specially de signed syringe. During SPME, the fiber is first withdrawn into the syringe needle, then lowered into the vial by pressing
down the plunger. The fiber coating is ex posed for a predetermined time to ex tract analytesfromtheir matrix. Once sam pling is completed, the fiber is directly transferred into a gas chromatograph in jector. Analytes are thermally desorbed from thefibercoating and quantitatively analyzed by GC. SPME can be performed manually or by an autosampler. Because the SPME device is basically a syringe, anyone who can use a syringe properly can carry out SPME. The only difference between a nor mal autosampler and a SPME autosam pler is that the plunger movement and tim ing must be carefully controlled to per form absorption and desorption correctly. If SPME is used to sample in the field and then is taken back to the laboratory for analysis, it is important to prevent loss of analytes during transport. The needle opening of the SPME device can be sealed by using a piece of septum and/or by cooling the needle.
Plunger
- Barrel - Plunger retaining screw - Z-slot Hub viewing window
Adjustable needle guide/depth gauge 'Tensioning spring " Sealing septum
Septum piercing needle Fiber attachment.tubing Fused-silica fiber-\
Figure 1 . SPME device.
Theoretical aspects of SPME
The principle behind SPME is the parti tioning of analytes between the sample matrix and the extraction medium. If a liquid polymeric coating is used, the amount of analyte absorbed by the coating at equilibrium is directly related to its concentrations in the sample
WW. Wt+v.
(i)
where η is the mass of an analyte ab sorbed by the coating; Vf and Vs are the volumes of the coating and the sample, respectively; Kis is the partition coefficient of the analyte between the coating and the sample matrix; and C0 is the initial con centration of the analyte in the sample. Equation 1 clearly indicates the linear rela tionship between the amount of analytes absorbed by thefibercoating and the ini tial concentration of these analytes in a sample. Because the coatings used in SPME have strong affinities for organic com pounds, Kts values for targeted analytes are quite large, which means that SPME has a very high concentrating effect and leads to good sensitivity. In many cases, however, Kfs values are not large enough to exhaustively extract most analytes in the matrix. Instead, SPME, like static headspace analysis, is an equilibrium sam pling method and, through proper cali bration, can be used to accurately deter mine the concentration of target analytes in a sample matrix. As Equation 1 indi cates, if Vs is very large (Vs » KfsV^, the amount of analyte extracted by the fiber coating η = tffe7fC0
(2)
is not related to the sample volume. This feature, combined with its simple geome try, makes SPME ideally suited for field sampling and analysis. Because the fiber can be exposed to air or dipped directly into a well, lake, orriver,SPME reduces field analysis time by combining sam pling, extraction, concentration, and in jection into a single uninterrupted pro cess. The speed of extraction is controlled by the mass transport of the analytes from the sample matrix to the coating. This pro cess involves convective transport in an air Analytical Chemistry, Vol, 66, No. 17, September 1, 1994 8 4 7 A
Report or liquid sample, the rate of desorption of analytes from the solid surface when par ticulate matter is present, and diffusion of analytes in the coating (14). In direct SPME sampling, the mass transfer rate is determined by the diffusion of analytes in the coating if the sample matrix is per fectly agitated. When the mass transfer rate is deter mined by the diffusion of the analyte in the coating, for most analytes equilibrium is achieved in < 1 min (14). Rapid extraction is ensured because the coating is very thin—typically between 10 and 100 μπι. In practice, this limit can be achieved for gaseous samples because of large diffu sion coefficients. For aqueous samples, however, this case is possible only when using very vigorous agitation methods such as sonication (15). For more practi cal agitation methods such as magnetic stirring, the equilibration time is much longer and is determined by diffusion through a thin static aqueous layer adja cent to the fiber. This thin layer of water, which surrounds the fiber, is very difficult to remove even when water is stirred rap idly to enhance the mass transfer of ana lytes; the analytes must diffuse through the water before they can be absorbed by the fiber coating. Placing a fiber directly into a sample to extract organic compounds works well for gaseous samples and relatively clean wa ter samples. If we want to sample analytes from a solid matrix or from a wastewater sample with grease, oil, and high molecu lar weight humic acid, however, direct SPME sampling may not work well; sam pling analytes from the headspace above the sample matrices is necessary. Thus SPME can be used to extract organic com pounds from virtually any matrix as long as target compounds can be released from the matrix into the headspace (16). For volatile compounds, the release of analytes into the headspace is relatively easy because analytes tend to vaporize once they are dissociated from their ma trix. For semivolatile compounds, the low volatility and relatively large molecu lar size may slow the mass transfer from the matrix to the headspace and, in some cases, the kinetically controlled desorp tion or swelling process can also limit the speed of extraction, resulting in a long ex traction time. 848 A
in water, headspace SPME sampling is faster than direct SPME. In headspace SPME, the mass transfer from water to headspace can be speeded up rapidly by constantly stirring the water sample to generate a continuously fresh surface. The mass transfer of volatile com pounds from the headspace to the fiber coating is very fast because of large diffu sion coefficients of analytes in the gas phase, and volatile compounds transfer more efficiently from water to headspace to coating than from water directly to coat ing (16). When the limiting mass transfer step is the mass transport of analytes from ma trix to headspace, the extraction time profile curve is characterized by an initial rapid rise (associated with partitioning of analytes originally present in the gas eous headspace) followed by a section with a smaller slope (determined by slower mass transport of analytes from the matrix). This rate is determined by the convective and diffusive transport of ana lytes in the sample matrix (16) and/or by the slow desorption kinetics as in SFE (17). Because partition coefficients are tem perature dependent, there is usually an optimum temperature for headspace SPME. As the temperature rises, more analytes are released from the matrix to the headspace, a process that results in high analyte concentrations in the headfrom the matrix, and at the same time space and favors SPME extraction. How speeds up the mass transport of analytes. ever, at high temperature, coating headIn headspace SPME, three phases space partition coefficients decrease, and (coating, headspace, and matrix) are in the matrix-dependent optimum extrac volved, and the chemical potential differ ence of analytes among the three phases is tion temperature is determined by the in teractions among analytes, coating, and the driving force that moves analytes from their matrix to the fiber coating. For matrix. With a good coating, headspace SPME can be used to extract both volatile aqueous samples, the headspace/water partition coefficients (Khs) are directly re and semivolatile compounds from their matrices. lated to the analytes' Henry's constants, which are determined by their volatility Although SPME is mainly an equilib and hydrophobicity. rium extraction technique, it has the abil ity to perform exhaustive extraction. If the In an aqueous matrix, most com coating/matrix partition coefficient, Kis, pounds have quite small Khs values (< 0.25), and the capacity of the headspace is very large (KisVt » Vs), the amount of for trapping analytes is small. As a result, analyte absorbed by the coating is the sensitivity of headspace SPME is al η = C0VS, and exhaustive extraction is achieved. most the same as that of direct SPME. The loss in sensitivity is important only when the target analytes partition well Extraction into the headspace (large Khs), and/or a SPME has two important functions: ex large headspace volume is used. For VOCs tracting analytes and desorbing them into
When the matrix adsorbs analytes more strongly than the extracting me dium does, the analytes partition poorly into the extraction phase. Because of the limited amount of the extraction phase in SPME (as in SPE), the extraction will have a thermodynamic limitation. In other words, the partition coefficient (Kis) is too small, resulting in poor sensitivity. If the coating has a stronger ability to adsorb an alytes than the matrix does, it is only a matter of time for a substantial amount of analytes to be extracted by thefibercoat ing, and only kinetics plays an important role during extraction. One of the most ef ficient ways to overcome the kinetic limi tation is to heat the sample to higher tem peratures, which increases the vapor pressure of analytes, provides the energy necessary for analytes to be dissociated
One of the most efficient ways to overcome the kinetic limitation is to heat the sample.
Analytical Chemistry, Vol. 66, No. 17, September 1, 1994
analytical instruments. Most important experimental parameters (speed, sensitivity, accuracy, and precision) are determined largely during extraction. Desorption, which is closely related to the efficiency of chromatographic separation and the precision of quantitation, has a great influence on the quality of data obtained and on full utilization of the potential of SPME. Because different groups of analytes can be extracted by different types of sorbents, a variety of sorbents have been used for SPME. For organic compounds, the basic principle of "like dissolves like" applies. Polar coatings such as polyacrylate and carbowax extract polar compounds such as phenols and carboxylic acid very effectively, whereas nonpolar coatings such as poly(dimethylsiloxane) retain hydrocarbons very well. A gold coating works for detection of mercury, and an ion exchanger extracts metal ions. As shown in Table 1, SPME can be used to analyze a wide range of compounds in various matrices through proper optimization or modification of SPME procedures. Sensitivity. Several factors can influence the amount of analytes extracted by SPME: the volume of the coating, the coating characteristics, derivatization of target analytes, modification of matrices, heating the sample, and cooling the coating. As indicated by Equation 1, the amount of analyte extracted by the fiber coating is directly proportional to the volume of the coating, and sensitivity improves as the volume of the coating increases, by increasing the thickness of the coating, the length of thefiber,or both. For nonpolar and moderately polar organic compounds, conventional coatings are capable of extracting at least a few picograms of analyte from low part-pertrillion samples, a level that typically constitutes the detection limits specified by regulatory agencies. This amount is sufficient for identification and quantitation with an ion trap mass spectrometer. Because both matrix and coating are competing for analytes, the affinity of coatings for target analytes is crucial in SPME sampling. Nonpolar compounds are more likely to be extracted by nonpolar coatings, and vice versa. However, for a polar coating to extract polar compounds from
Table 1 . SPME techniques Approaches Direct SPME
Headspace SPME
Artalytes
Matrices
Routine
Most compounds
Gaseous and liquid
In situ chemical derivatization In situ redox
Polar compounds
Routine
Volatile and semivolatile compounds Volatile and semivolatile compounds with small partition coefficients Polar compounds
Heating/cooling
In situ chemical derivatization
water, it must have a much stronger affinity for the analytes than water does. For example, poly(dimethylsiloxane), a nonpolar coating, extracts nonpolar compounds such as BTEX (benzene, toluene, ethylbenzene, and xylene isomers) and PAHs (polyaromatic hydrocarbons) from water very effectively but cannot extract polar compounds such as phenol and its derivatives (Figure 2a). Polyacrylate, a more polar coating, extracts phenol and its derivatives quite well but does not provide high sensitivity when extracting BTEX compounds (Figure 2b). The coating/water partition coefficients of BTEX for poly(dimethylsiloxane) are much larger than those for polyacrylate, whereas the opposite is true for phenol and its derivatives. For compounds with large molecular sizes (such as PAHs), large Kis values can be obtained using the poly(dimethylsiloxane) coating. In fact, we can achieve quantitative extraction of these analytes from an aqueous matrix because of their very large coating/water partition coefficients (18). Derivatization can be used to reduce the polarity of polar compounds such as phenols and carboxylic acids and increase their coating/water partition coefficients as well as improve the chromatographic separation. In SPME, polar analytes can be derivatized by derivatizing the analytes in their aqueous matrix (e.g., convert phenols into acetate derivatives) and then doing SPME sampling (19) or by doping the fiber coating with an appropriate derivatization reagent. During the parti-
Inorganic ions Any matrix
tion, polar analytes are simultaneously extracted and derivatized to less polar analogues that are amenable to GC analysis. In addition to improving sensitivity, the doping method allows integrated response when the derivatization reaction is slow. Because the amount of reagent in the coating is much higher than the equilibrium concentration of the acids, the reaction has pseudo-first-order kinetics, and the amount of derivative accumulated by the fiber is proportional to the integral of the acid concentration in air during the time of exposure. For example, highly polar propionic and butanoic acids can be derivatized within the fiber coating into the pyrenyl ester, which has a high affinity for the coating and can be easily analyzed by GC or HPLC. Because the partition coefficients of analytes (Kis) are partially determined by the interaction between target analytes and the matrix, the nature of the matrix can be modified to influence the coating/matrix partition coefficients of the analytes. By adding a salt (e.g., NaCl or NaS04) to the aqueous samples, the ionic strength of water can be increased, thereby increasing the partitioning of polar organic compounds (but not ions) into the polymer coating. Because the neutral forms of analytes are more efficiently extracted by the non-ionic polymeric coatings, the pH of the aqueous sample must be adjusted to prevent ionization of the analytes. Soil samples present special problems for SPME extraction, because they cannot be directly extracted; analytes must first
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there is usually an optimum temperature for SPME. If a sampling can be heated to a (a) high temperature and a low coating tem perature can be maintained at the same , o-Xylene time, K{s values and thus sensitivity in Κ = 794 crease dramatically. By modifying the s SPME device and using liquid C02 as a ϋ coolant, the sample can be heated to 250 C °C while maintaining the coating tempera ο 2,4-Dichlorophenol ture at below 20 °C. By using a high tem Κ =4.6 β perature, additional water, and a cooled fi ber, quantitative extraction can be achieved for toluene, ethylbenzene, and xylene isomers, which have small parti tion coefficients and are impossible to ex tract quantitatively under room-tempera ture conditions (Table 2). 10:00 15:00 Accuracy and precision. Because SPME is primarily an equilibrium technique, cali (b) bration is necessary for quantitation. When air samples are analyzed, calibra tion is quite easy. Because sampling oc Έ curs either in an open space or in a largevolume air sample, the amount of analytes ο 2,4-Dichlorophenol c extracted by the fiber coating is, as ex g K=47 "(β pressed by Equation 2, linearly related to the partition coefficient, Kis, which is de termined by humidity and temperature. The concentration of analytes in air can o-Xylene thus be determined directly from the re K=5A / sponse of the gas chromatograph detec jJL il.L tor after correction for humidity and tem 10:00 15:00 perature (21). Retention time (min) For relatively clean aqueous samples (< 1% organics) such as drinking water or Figure 2. Total ion current GC/MS chromatogram of BTEX and phenol other well-defined matrices, external cal mixtures in water. ibration works very well and is usually car (a) Extracted with a poly(dimethylsiloxane) coating; (b) extracted with a polyacrylate coating. K is ried out by spiking a known amount of the coating/water partition coefficient of the analyte, which drops by more than 2 orders of target analytes into a clean matrix and magnitude for o-xylene and increases by more than an order of magnitude for 2,4-dichlorophenol when changing the coating from nonpolar poly(dimethylsiloxane) to polyacrylate. then performing SPME. Analyte concen tration in unknown samples can be accu rately determined by comparing the de be released into the headspace and then active compounds can also be added to soil tector signal with the calibration curve. Multilevel validation for drinking water concentrated in the fiber coating. Some samples to facilitate the release of ana shows excellent agreement of the SPME soil matrices, such as sand, have rela lytes and improve sensitivity. For exam tively weak analyte/matrix interactions, ple, we have found that the amount of vola and purge and trap (22). which results in easy headspace SPME For complex soil samples, external cali tile hydrocarbon compounds extracted sampling for volatile compounds. Others, from soil matrices by SPME increases sig bration may not work well because of ma however, such as clay with its high metal nificantly when the sampling is carried trix effects, and standard addition or in content and large surface area, have out at higher temperatures with 10% water ternal standards must be used (18,23,24). strong analyte/matrix interactions, mak For internal standards to work well, their content (20). ing headspace sampling difficult. For ther partition coefficients must be similar to Heating solid samples helps to release mally stable analytes, heating the sample analytes into the headspace and facilitates those of the target analytes. Isotopically is a convenient and efficient way to re the extraction process during SPME labeled analogues of target analytes are lease analytes from the matrix to the sampling. However, as the temperature in the best internal standards for SPME be headspace and improve sensitivity. A small creases, thefibercoating begins to lose cause their chemical and physical prop amount of water and/or other surfaceerties are similar to those of their unlaits ability to adsorb analytes. Thus,
Il ,
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beled counterparts (18). Because spiked analytes may not interact with the matrix as strongly as native ones, experimental conditions must be carefully adjusted to ensure the effective release of native analytes into extracting media (17). If analytes have very large partition coefficients, because of strong affinity for the coating or because of heating the sample with simultaneous cooling of the coating, exhaustive extraction can be achieved. Although calibration is no longer necessary, it is usually used anyway to confirm the extraction efficiency. Precision in SPME is very good because it is a single-step method and therefore the random sources of error associated with transfer of analytes are minimized. Precision is typically ~ 5% RSD for manual operation and can be as low as 1% using an autosampler. Speed of extraction. The rate of extraction is determined by the efficiency with which the sample is agitated. When sonication is used, the equilibration time is close to 1 min (15). For more practical agitation methods such as magnetic stirring, the equilibration time is typically 2-60 min, depending on the agitation rate and the partition coefficient. At equilibrium, the SPME sampling method has the maximum sensitivity, but for practical purposes, extraction time can be shortened depending on sensitivity requirements. Temperature also has an effect on extraction time because mass transport of analytes from the sample matrix into the fiber coating is faster at higher temperatures. Heating, such as microwave heating, can also generate convective currents in samples that improve transfer of analytes from the matrix to the extracting phase.
250 °C in a fraction of a second followed by focusing at the front of the gas chromatographic column (14). The injection band can be further shortened by using an injector that can generate a heating pulse instead of constant heating; through an internal heating device inside thefiber;or by directly passing the current through a fiber made of metal (wire), as shown in Figure 3. For compounds with very high molecular weights (such as perylene), however, carry-over may be a problem because current coatings cannot be heated to temperatures above 300 °C (18). Desorption from the fiber can also be accomplished using an appropriate solvent, a condensed medium such as compressed C02, or an aqueous solution. For example, the selective extraction of bismuth ions from afibercoated with an ionexchange polymer can be accomplished using acidic potassium iodide solution as a complexing reagent (25). Applications
Thus far, SPME has been used in environmental, food, and drug studies. The SPME device has been used to monitor both indoor and outdoor air pollution. It can differentiate analytes dissolved in air from those adsorbed on particulates by passing the air sample rapidly around thefiber,similar to a dénuder. For an environment with known humidity and temperature, the concentrations of pollutants can be accurately calibrated against an instrument signal. The SPME device can also produce integrated response if used together with a diffusive barrier such as a membrane that converts SPME into a passive sampler. SPME with a polytdimethylsiloxane)
coating has been used successfully to extract VOCs from drinking water by placing the fiber directly into water samples. SPME can provide detection limits that are equivalent to or better than those required by the Environmental Protection Agency, and it has very good precision ( ~ 5%) as well as a wide dynamic linear range (more than 3 orders of magnitude). Through headspace sampling, SPME has also been used to extract organic compounds from wastewater, sludge, soil, and other complex matrices. Sub-part-perbillion detection limits and good precision have been achieved when sampling at room temperature from those complex matrices (20). Even with thin coatings [15-pm poly(dimethylsiloxane)], SPME can be used to extract semivolatile compounds such as PAHs and polychlorinated biphenyls from aqueous samples (18). Data quality, in terms of precision and accuracy, is similar to that obtained with EPA Method 1625 when the isotope dilution technique is used. SPME has also been applied for sampling phenols in water by using a polyacrylate coating; the detection limit, linear range, and precision are all better than or equivalent to EPA method specifications. Recent results show that SPME can also be used to efficiently extract herbicides and pesticides from an aqueous matrix. SPME eliminates the use of solvent not only in extraction, but also during injection, which greatly improves chromatographic separation efficiency (26). Solventfree injection should result in the use of columns with thinner layers of stationary phase and smaller internal diameters. The cylindrical geometry and small diameter of the fiber can fit into an injector with a
Desorption
Thermal desorption of analytes from an SPME coating is very effective in most cases. As the temperature increases, the coating/gas partition coefficients decrease and the ability of the coating to retain analytes diminishes very quickly. The constant flow of carrier gas within a gas chromatograph injector also facilitates the removal of analytes from the coating. For volatile and some semivolatile compounds, analytes can be desorbed from the coating at a temperature between 150 °C and
Table 2. Percentage of spiked BTEX compounds adsorbed by SPME fiber coating from clay under various extraction conditions Analyte
Room temperature
50 °C
50 °C/ 15% H20
250 "CI H&C»
170 °C/ 5% H20/H&C
Benzene Toluene Ethylbenzene m,p- Xylene o- Xylene
0.01 0.01 0.002 0.003 0.003
0.08 0.06 0.06 0.07 0.08
0.11 0.14 0.21 0.22 0.24
27 11 6 11 7
64 84 91 95 100
3
Simultaneous heating of the sample and cooling of the SPME device.
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Report Future development
Modified syringe
Injector nut Carrier gas inlet Metallic fiber To capillary column or mass spectrometer Figure 3. Device for electrically heating a SPME fiber made from a metallic material.
very small volume or can fit directly into a column. Analytes can be thermally desorbed very quickly; this generates a very narrow band of analytes in the column. With a small internal diameter column and a thin stationary phase, the separation ef ficiency can be enhanced and fast GC sep arations can be made in a very compact instrument. Thus SPME has the potential to become a very useful sampling tool for on-site environmental analysis when cou pled with a high-efficiency, portable, isother mal gas chromatograph. Combined with a smaller injector capable of pulse heating, SPME can be used to introduce an ex tremely narrow band of analytes into a gas chromatographic column and facilitate separations in a very short time (a few sec onds) (27). SPME can also be used to extract target analytes from food and drug samples. It has been used for accurate determination of the caffeine content in coffee and tea (24) as well as volatile impurities in drugs. SPME methods are being developed for determination of illegal drugs in urine sam ples or alcohol content in blood. Headspace SPME is also being investigated for flavor analysis in food and beverages. On line monitoring of organic compounds in a flowing stream is also possible by using SPME as an interface between the flow ing stream and a gas chromatograph (15). 852 A
SPME is a newcomer in the field of sol vent-free sample preparation. To date, the. potential of the technique has been dem onstrated primarily for organic analysis in environmental samples by coupling with GC. Initial data also indicate that the tech nique can be applied to inorganic analy sis when ion-exchange coatings are used. Obviously, by directly coupling SPME to an atomic absorption, inductively coupled plasma, spark, or glow discharge instru ment, SPME applications can be extended to a wide range of inorganic compounds. As fiber coatings with molecular recogni tion capabilities such as caging molecules are developed, specific extraction can be achieved. The cylindrical geometry of SPME is ideally suited as a sample introduction tool for MS. By fast desorption of analytes from the fiber coating, analytes can be rap idly introduced into an ionization cham ber, generating a very narrow band. A peak width of < 0.5 s has been achieved in coupling SPME with an ion trap mass spectrometer. The narrow peak increases the peak height and improves the S/N, resulting in a detection limit of 0.15 pg with a S/N of 100. SPME/MS can lead to detection of trace target analytes such as drugs at the femtogram level. With bioaffinity coatings, SPME could also be used to extract proteins and other biologically significant species directly from body fluids or single cells, label them with appropriate reagents, and then desorb them into a solvent for HPLC analy sis, into a buffer for capillary electrophore sis, or directly into a mass spectrometer. SPME could also be used with matrixassisted desorption; the coating could be used to extract proteins and to act as the desorption medium. The silica fiber used in the SPME device is an optical fi ber designed to deliver the laser desorp tion pulse (27), which can be used in matrix-assisted desorption (28). A very promising application of SPME is its combination with electrochemistry, particularly for inorganic speciation. The conducting polymer or metal coating can be used to concentrate reduced target ana lytes before introduction into an appropri ate instrument. For example, a goldcoated platinum electrode/fiber is capable of differentiating between free mercury
Analytical Chemistry, Vol. 66, No. 17, September 1, 1994
ions and complex mercury species. The development of new sorbents will be very important for establishing SPME as a leading technique for sample prepara tion. The development of thermally sta ble polar sorbents is especially important for the determination of polar compounds. Because the sorbent is the heart of SPME, advances in this area will greatly extend the application of SPME.
This work was supported in part by the Natu ral Sciences and Engineering Research Council of Canada, Supelco Canada Ltd., Supelco Inc., Varian, Dow Chemical, and Imperial Oil.
References (1) Noble, D. Anal. Chem. 1993, 65,693 A. (2) Charalambous, G. Analysis of Food and Beverages, Headspace Technique; Aca demic Press: New York, 1978. (3) Grob, K.; Zucher, F. J.J. Chromatogr. 1976,117,285. (4) Robbat, Α.; Liu, T.; Abraham, Β. Μ. Anal. Chem. 1992, 64,1477-83. (5) Hawthorne, S. Anal. Chem. 1990, 62, 633A-642A (6) Pratt, K. F.; Pawliszyn, J. Anal. Chem. 1992, 64,2101-10. (7) Kotiaho.T.; Lauritsen, F. R; Choudhury, T. K.; Cooks, R. G.; Tsao, G. T. Anal. Chem. 1991, 63, 875 A. (8) Yang, M. J.; Harms, S.; Luo, Y. Z.; Pawl iszyn, J. Anal. Chem. 1994, 66,1339^6. (9) Yang, M. J.; Pawliszyn, J. Anal. Chem. 1993, 65, 2538-41. (10) Poole, C. F.; Schuette, S. A. J. High Résolut. Chromatogr. 1983, 6, 526-49. (11) Hagen, D. R; Markell, C. G.; Schmitt, G; Blevins, D. B. Anal. Chim. Acta 1990, 236,157-64. (12) Kraut-Vass, A;Thoma,J./ Chromatogr. 1991,538,233-40. (13) Arthur, C. L; Pawliszyn, J. Anal. Chem. 1990, 62,2145. (14) Louch, D.; Motlagh, S.; Pawliszyn, J. Anal. Chem. 1992,64,1187-99. (15) Motlagh, S.; Pawliszyn, J. Anal. Chim. Acta 1993,284, 265-73. (16) Zhang, Z.; Pawliszyn, J. Anal. Chem. 1993, 65,1843-52. (17) Pawliszyn, J . / Chromatogr. Sci. 1993,32, 31. (18) Potter, D.; Pawliszyn, J. Environ. Sci. Technol. 1994,28,298-305. (19) Buchholz, K.; Pawliszyn, J. Anal. Chem. 1994, 66,160-66. (20) Zhang, Z.; Pawliszyn, J.J. High Résolut. Chromatogr. 1993,16, 689-92. (21) Chai, M.; Arthur, C; Belardi, R; Pratt, K. Analyst 1993,118,1501-05. (22) MacGillivary, B.; Fowlie, P.; Pawliszyn, J. / Chromatogr. Sci., in press. (23) Page, B.; Lacroix, G.J. Chromatogr. 1993, 648,199-211. (24) Hawthorne, S.; Miller, D.; Pawliszyn, J.; Arthur, C.J. Chromatogr. 1992, 603,18591.
(25) Otu, E.; Pawliszyn, J. Mikrochim. Acta 1993,112,41-46. (26) Arthur, C. L; Chai, M.; Pawliszyn, J./. Microcol. Sep. 1993, 5, 51-56. (27) Pawliszyn, J.; Liu, S.Anal. Chem. 1987, 59,1475-78. (28) Cisper, M.; Earl, W.; Nogar, N.; Hemberger, P. Anal. Chem. 1994, 66,18971901.
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Janusz Pawliszyn is an associate professor of analytical chemistry at the University of Waterloo (Waterloo, Ontario, Canada N2L 3G1). He received a chemical engineering degree and an M.S. degree in analytical chemistry from the Technical University of Gdansk and a Ph.D. from the University of Southern Illinois. His research interests in clude development of methods for eliminat ing organic solvents from the sample prepa ration step, concentration gradient tech niques, and CCD imaging methods.
MinJ. Yang (left) received his B.Sc. degree in 1990 and his M.Sc. degree in 1992 from the University of Waterloo, where he is currently completing work for his Ph.D. in optimization and application of membrane extraction with a sorbent interface. His re search interests include design and imple mentation of automated sample prepara tion systems and detector signal-processing methods for GC. Zhouyao Zhang (right) received his B.Sc. degree in chemistry from the University of Science and Technology (China) in 1984. He received his M.Sc. degree in 1992 from the University of Waterloo, where he is cur rently pursuing a Ph.D. in analytical chemistry.
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