New Solvent-Free Samnle Based on Fiber and Polymer Technolomies
I
n the analysis of organic environmental pollutants, the analytes of interest usually must first be separated from samples because most analytical instruments are incapable of handling matrices. Traditional sample preparation methods, such as liquid-liquid extraction and Soxhlet extraction, are time consuming, difficult, or expensive to automate. Moreover, these techniques require high-purity toxic organic solvents, which are expensive and hazardous to the environment. TJnder increasing regulatory pressure, reducing or eliminating the use of organic solvents has become an irreversible trend. Consequently, there is great interest in new solvent-free sample preparation techniques that also are less time consuming and more field portable. Current solvent-free extraction procedures include supercritical fluid extraction, dynamic purge and trap, and static headspace extraction. The latter two are limited to the analysis of volatile analytes; the former requires large quantities of high-purity CO, and is not easily made portable. Furthermore, all these techniques require several steps to proceed from sample preparation to analysis. Another technique, solid-phase extraction, requires the use of small quantities of organic solvent but can suffer from both the plugging of cartridges when analyzing samples with suspended solids and breakthrough volumes that are highly matrix dependent. This paper describes two simple, solvent-free sample preparation techniques based on new fiber and polymer technologies: solid-phase microextraction (SPME) (2-3) and membrane extraction with a sorbent interface (MESI) ( 4 ) .In SPME a fine, fused silica fiber coated with a poly-
ANNA A, BOYDiBOLAND MENG CHAl YU 2, L U O ZHOUYAO ZHANG M I N J, Y A N G JANUS2 B, PAWLISZYN University of Waterloo Waterloo, Ontario, Canada N2L 3G1
TADEUSZ GbRECKl Technical University of Gdansk Gdansk, Poland
meric stationary phase (Figure l a ) is used to extract and concentrate analytes directly from a sample. In MESI a carrier gas stream flows through a hollow fiber membrane, a sorbent interface, and then the gas chromatograph (Figure I b ) . Analytes diffuse from the sample into the membrane and are carried to the sorbent, where they are concentrated and subsequently thermally desorbed for analysis. For air or water analysis, the devices are placed directly into the sample from which the analytes of interest are to be extracted. For more complex samples such as wastewater, sludge, or soil, sampling is carried out in the headspace above the sample matrices ( 5 ) .
Solid-phase microextraction SPME is predominantly operated as an equilibration sampling technique, in contrast to solid-phase extraction methods that assume total extraction. For practical purposes, the coated fiber is contained in a specially designed syringe that operates just like a conventional syringe. The syringe’s needle protects
0013-936X/94/0927-569A$O4.50/00 1994 American Chemical Society
the fiber when septa are pierced and allows for simple exposure to the sample or GC injection port. A coated fiber can typically be used for up to 100 injections. The analytes partition into the coating as a function of their partition coefficient, K, analogous to the partitioning that occurs from an aqueous phase to an organic solvent in liquid-liquid extraction processes. Thus, K is defined as the ratio of the concentration of the analyte in the coating at equilibrium to the concentration of an analyte in the sample at equilibrium. Under these conditions, analytes having a greater affinity for the coated phase than for their matrix (e.g.,air, water, or soil) partition into the coated phase and concentrate there until equilibrium is established. Thus the sensitivity of the method is dependent on the type of coating and its volume, just as in liquid-liquid extraction procedures where the choice of solvent and the quantity used affect sensitivity. In SPME, the concentration and extraction steps occur simultaneously because all the analytes extracted into the coating are introduced into the gas chromatograph. The selection of a suitable coating is therefore important in order to obtain maximum extraction of analytes. Currently two phases are available, one nonpolar [poly(dimethylsi1oxane)l and one more polar [poly(acrylatell , enabling analyses of a wide variety of analytes. Table 1 summarizes the published applications of SPME. The extraction and injection processes are very simple. Initially, the coated fiber is exposed to a stirred sample or the headspace above a stirred sample for an appropriate amount of time. The exposure time is dependent upon the analyte and
Environ. Sci. Technol., Vol. 28,No. 13, 1994 569 A
( a ) Solid-phase microextractioir ( % ! W E . .4nd 'ti) memhra,ie extraction with interface INIESI)
the matrix; it can be either the time taken to reach equilibrium (which can vary from a minute to an hour) or a fixed time before reaching equilibrium. Exposure times of less than the time required to reach equilibrium can be used without loss of precision and often without loss of sensitivity, if the K values are high. After the extraction step, the SPME device is removed from the sample and inserted into the injection port of a gas chromatograph. Because of the small dimensions and cylindrical geometry of the fiber, conventional injection ports easily accommodate direct fiber injection without modification. The analytes are thermally desorbed from the coated phase directly onto the GC column. All analytes that have partitioned into the coated phase are therefore transferred to the gas chromatograph, resulting in high sensitivity. Analytes are then focused at the head of the column before separation. The concentration of target pollutants in a sample is linearly related to the amount extracted by the fiber coating after a fixed time.
Through external or internal calibration, the concentration of organic compounds can be determined from t h e GC detector's response for a fixed extraction time or at any time after equilibrium has been r e a c h e d . Alternatively, through a modification in the SPME device to allow cooling of the fiber coating, samples can be heated while the fiber coating is maintained at a low temperature. This sampling approach substantially increases the amount of analytes extracted by the coating, and can lead to quantitative extraction of many organic compounds, eliminating the need for calibration. Membrane extraction with a sorbent interface MESI was developed to allow rapid routine analysis and longterm, on-line continuous monitoring of volatile organic compounds (VOCs) in various environmental matrices. The MESI procedure (Figure Ib] is quite different from that of SPME. The membrane acts as a barrier to ensure the selectivity, and the sorbent interface acts to concen-
570 A Environ. Sci. Technol., Vol. 28,No. 13. 1994
il si)! hei!,
trate analytes, enabling high sensitivity. The hydrophobicity of the membrane keeps moisture and high molecular weight compounds from entering the GC column, hence prolonging its life. The system can be operated in either batch or multiplex modes. Because of the throughput and other advantages, the multiplex GC method offers significantly better detection and time efficiency than the batch method ( 2 1 ) . In the batch mode, the stripped analytes accumulate at the interface for a period of time before an optimized heating pulse is applied to the sorbent trap to desorb all the compounds into the gas chromatograph for analysis. The cycle of trapping and heating is repeated for continuous monitoring. The sensitivity of the MESI system in the batch mode depends on the length of the trapping time at the sorbent interface. In the multiplex mode, the stripped analytes from the membrane extraction module continuously pass through the sorbent interface where analyte concentrations in the carrier gas are modulated by a
,
ublished applications of SPME Thlckneos Ta et (mm) Jyte
1 Paly(dirnethylsilaxane) 100.00 )ly(acrylate)
x
VOCs
vocs vocs
MethodLOD
Relennce
Subppbv
Water
0.001-3 ppb
6 7
Soil
0.002-1 ppb
8
15.00
PAHdPCBs Waterhi1 Law pptr
95.00
Phenols Pesticides
Water Water
9
0.004-0.2 ppb 10
Low pptr
* Boyd-Boland, A,: Pawliszyyn, J., unplblishd results.
random sequence of thermal desorption pulses. The column responds to each concentration pulse independently, and component peaks from different pulses overlap to form a multiplex chromatogram at the GC detector. The analysis results are computed by calculating the crosscorrelation between the random desorption sequence and the detector signal. MESI is currently applicable only to the analysis of volatile and semivolatile nonpolar analytes. The carrier gas for the analysis of volatile analytes is typically N, or He, but any inert gas can be used. The solvent-free analysis of semivolatile analytes requires a high-pressure membrane module: high-density CO, is used as the stripping phase.
I
Applying the technologies Automation is of great importance in the development of new analytical techniques. The advantages of automated methods i n c l u d e greater sample throughput, increased time efficiency, and reduced labor costs. Automation of SPME and MESI procedures is simple but different from other analytical techniques. Because the SPME device is basically a syringe, the whole analysis process from sampling to injection to separation to quantitation is easily automated with the use of a modified GC autosampler. For MESI, automation is achieved by placing the system on line and by activating trapping, desorption, and analysis with a computer. Because
both the extraction module and the sorbent interface include no moving parts, the MESI system is very rugged and requires little or no maintenance for prolonged periods of use, making it ideal for installation in a process line. The use of organic solvents has been completely eliminated in all aspects of both of these procedures. This is significant not only in terms of reduced cost, improved efficiency, and overall simplicity, but also because the column capacity is no longer limited by solvent. Columns with very small internal diameters a n d thinner stationary phases can he used, which greatly improves the separation efficiency and provides the potential for extremely rapid chromatography. A further advantage of the methods is the one-step sample preparation in which analytes are extracted and concentrated simultaneously, thus eliminating losses that can occur during conventional preconcentration techniques. Both SPME and MESI are suitable for field monitoring. The SPME device is small and portable and acts as a storage device for analytes extracted in the field. It is thus most useful for spot sampling. MESI can be used for long-term field monitoring or “sniffing,” where the mem-
FIGURE2
GGMS of diesel exhaust using solid-ob-
microextraction
0.46% (b)
(a) Total ion chromatogram;(b)sdectad Ion plot of mass 128; (c) select4 ion plot of mass 178: (d) selected lon plol Of mass 202.
Envimn. Sd. Technol.. Val. 28, NO. 13,1994 571 A
iGURE
:hromatogram of a solid-phase microextractionextraction of a 4-mL aqueous solution of 22 iitrogen-containing herbicides (100 ppb each) 100%
1 21
I
18
1
1 15100
2o:oo
25:OO
30:OO
35:OO
Time (min) . .
Peak assignment: 1. Eptam: 2. Sulan: 3. Vernam; 4. Xllam; 5. Ordram; E. Propachlor; 7. Ro-neel: E. Trifluralin; 9. Baian; 10. Simazine; 11. Atrazine; 12. Propazine: 13. Tolban: 14. Terbacil; 15. Sencor: 16. Bromacil; 17. Dual; 16. Paarlan; 19. Prowl; 20. Oxadiaon; 21. Goal: 22. Hexainone.
hrane is left exposed to an area and the concentration of analytes is monitored as a function of time. An example of this type of application is monitoring VOCs in laboratory air during a workday by directly exposing the membrane to the air. A disadvantage of this technique is the memory effect that can occur when all the analytes are not desorbed from the sorbent before the next analysis takes place. The solvent-free nature of these processes ensures that both MESI and SPME are ideally suited for coupling with portable gas chromatographs. This enables rapid and simple analysis of samples directly in the field, eliminating the need for their transportation. Also, the nature of the two processes permits their automation from sampling through analysis, with virtually no human intervention. Recent applications of SPME SPME has been used to analyze volatile and semivolatile organic 572 A
Environ. Sci. Technol., Vol. 28.No. 13, 1994
FIGURE 4
Chromatogram of an aqueous BTEX solution sampled from headspace, using a fiber with a lbmm-thick poly(dimethylsi1oxane) coating 4
ive e.
5
tion I
10
,
15
ising mem
le ex1
25
?nt interfa
D n wi
35
Total analysis time (min) le: 4. m, p-xylene; 5. c-xylene
compounds in the gas phase. The method requires minimal sample preparation time and is suitable for on-site air monitoring, either by direct exposure of the fiber to air or by exposure of the fiber to a sample collected in a gas sampling bulb. The technique has been applied to both spot sampling and integral exposure over time (Meng, C.; Pawliszyn, J., unpublished results). An example of this type of analysis is the identification of polycyclic aromatic hydrocarbons (PAHs) in diesel exhaust. We placed an SPME device directly in the exhaust emitted from a diesel vehicle and obtained the chromatogram shown in Figure 2. The compounds present in the exhaust-including normal and branched alkanes, alkyl benzenes, alkyl n a p h t h a l e n e s , a n d some PAHs-were identified by GC-MS. The chromatogram shows the presence of naphthalene, phenanthrene and anthracene, and fluoranthene as indicated by the mass peaks 128, 178, and 202,respectively. SPME w i t h a poly(acry1ate)coated fiber has been used to successfully determine parts per trillion levels of nitrogen-containing herbicides and ppb levels of organophosphates. The poly(dimethy1si-
1oxane)-coated fiber has enabled ppb determination of organochlorine pesticides. The chromatogram shown in Figure 3 reveals that SPME detected 22 nitrogen-containing herbicides, each at 100 ppb, in an aqueous solution. Partitioning from the aqueous phase to the coated fiber was most successful for analytes falling into the nitroaniline and thiocarbamate groups, as reflected by the greater peak areas.
inserted into the sample vial to react with analytes. As analytes react with the derivatizing reagent their concentration is depleted in the fiber coating, thus enabling more analytes to partition into the fiber for further reaction. Because the derivatizing reagent is present in excess of the analytes of interest, the amount of derivatized product formed is dependent on the initial concentration of the analytes of interest.
In situ derivatization Direct SPME analysis of very polar analytes, such as fatty acids, is difficult even with a polar fiber. By modifying matrices with salts and strong acids, the polarity and solubility of some analytes are decreased and extraction is substantially enhanced. However, this approach is limited and time consuming if large quantities of samples need to be analyzed. Therefore, a more efficient method is required. In situ derivatization has the potential to be an effective alternative. In situ derivatization is a relatively simple process. The fiber is placed in the derivatizing reagent until a sufficient quantity has been adsorbed by the coating, then it is
SPME and MESI with high-speed GC
High-speed GC is potentially attractive in fields such as process analysis or environmental monitoring. A successful separation requires that the injection bandwidth be small in comparison to chromatographic band broadening, and that all other contributions to band broadening, such as interfaces, detector volume, and electronics, be negligible. It is very difficult to achieve this goal by using regular sample introduction techniques. Theoretical considerations ( I ) indicate that desorption of volatile analytes from the fiber coating can be accomplished in a very short time (< 1SI. With a properly designed in-
Environ. Sci. Technol.. VoI. 28.No. 13, 1994 573 A
jector, SPME can be used as a sample introduction technique for highspeed separations. Regular GC injectors usually cannot be used because of their geometry or heating characteristics: therefore a dedicated injector has been designed and built ( 1 2 ) . For MESI, highspeed GC can he accomplished simply by using a low thermal capacity sorbent interface that allows for extremely rapid desorption. Figure 4 shows the separation of a 100-ppb BTEX (benzene, toluene, ethylbenzene. xylene) mixture sampled from the headspace by SPME. The separation is completed in 12 s; the entire process from sampling through separation takes 2 min. The overall precision of the method is very good, and the relative standard deviation of peak areas ranges from 1.3% for m , p xylene to 6.3% for toluene (seven determinations). Quantitative extraction by MESI of BTEX from the headspace of a sample vial containing a known quantity is shown in Figure 5. As time progresses, the quantity extracted decreases until all the BTEX has been removed. By integrating the peaks at each time and summing them, we can determine the amount of each analyte present in the mixture.
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Conclusions Both the SPME and MESI techniques are useful and viable alternatives to methods requiring organic solvents. The two techniques are relatively new; the full scope of their applications is unknown. Currently the techniques are limited only by the types of coatings and membranes commercially available. A wide range of analytes can be determined from almost any matrix, and published applications range from analyses of food to groundwater. Routine analysis of large numbers of samples is facilitated by the ability to automate both techniques and to couple them with extremely rapid chromatography. The potential for direct on-line analysis with MESI makes it a powerful tool for industrial applications. Headspace methods provide an ideal alternative for analyzing volatile and semivolatile analytes in various environmental samples. The advantages include elimination of matrix effects and prolonged lifetime of the fiber and membrane. Both SPME and MESI can be used in this way. T h e development of s i m p l e , 574 A
rapid. solvent-free extraction procedures such as SPME and MESI, together with the development of portable gas chromatographs. will enable rapid and simple analysis of volatile and semivolatile analytes directly in the field.
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Zhouyao Zhang Ill received his 5%. degree from the University o Science and Technology [China] andhis'M.S. d e g r e e p , m the vniversity of Woterloo, where e I S pursuing his Ph.D.
A n n a Boyd-Bolond is ii postdoctoral fellow in Ianusz Pmvliszyn's laborataries at the University of IVoterloo. Her research interests include developing n e w m e t h o d s and applications f o r SPME. She received her B.Sc. degree and Ph.D. from the Inorgonic Chemistry Department of the University of Sydney (Austroliaj.
Mcng Choi I / ) received her B.Sc. degree f r o m the Deportment of Chemistry,
Zhengzhou University [People's Repub/ic of China] and an M A . degree from the University of Waterloo, Canada. Her research focuses on the development of the SPME method for the analysis of environmental oir pollutants.
Yu 2. Lou (r) is a technicion working with the Pawliszyn group on the optimization and application ofmembrone extraction with a sorbent interfoce. He received his B S c . degree from Southwest Normal University ond his M.Sc. from Sichuan University (both in China).
Environ. Sci. Technol.. Vol. 28,No. 13, 1994
Janusz B. Pawliszyn (11 is ossocinte professorofanolyiical chemistryat the Lrniversify of Waterloo. His reseorch interests include developing methods for eliminating organic solvents from the sample preparation step, concentration gradient techniques, and CCD imaging methods. Powliszyn received chemicol engineering and M.S. degrees in anolyiical chemistry from the Technical Universify of Gdansk; his PhD. is from the University of Southern Illinois. Tadeusz G6recki Ir) is a tutor and research assistant ot the Technical University of Gdansk, Poland, currently working as the post-doctoral fellow at the University of Waterloo. His reseorch interests include development of methods for sample preparation. analysis of trace organic water and air pollutonts, ond lnstrumentol aspects of GC. He received his chemicol engineering, M.S., ond Ph.D. degrees from the Technical University of Gdansk.
