ES&T
New Solvent-Free Sample Preparation Techniques Based on Fiber and Polymer Technologies
I
n the analysis of organic envi ronmental pollutants, the analytes of interest usually must first be separated from samples be cause most analytical instruments are incapable of handling matrices. Traditional sample preparation methods, such as liquid-liquid ex traction and Soxhlet extraction, are time consuming, difficult, or expen sive to automate. Moreover, these t e c h n i q u e s require high-purity toxic organic solvents, which are expensive and hazardous to the en vironment. Under increasing regu latory pressure, reducing or elimi nating the use of organic solvents has become an irreversible trend. Consequently, there is great interest in new solvent-free sample prepara tion 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 extrac tion. The latter two are limited to the analysis of volatile analytes; the former requires large quantities of high-purity C0 2 and is not easily made portable. Furthermore, all these techniques require several steps to proceed from sample prep aration to analysis. Another tech nique, solid-phase extraction, re quires the use of small quantities of organic solvent but can suffer from both the plugging of cartridges when analyzing samples with sus pended solids and breakthrough volumes that are highly matrix de pendent. This paper describes two simple, solvent-free sample preparation techniques based on new fiber and polymer technologies: solid-phase microextraction (SPME) [1-3) and membrane extraction with a sorbent interface (MESI) (4). In SPME a fine, fused silica fiber coated with a poly-
ANNA A. B O Y D - B O L A N D MENG CHAI YU Z . L U O ZHOUYAO ZHANG ΜΙΝ J . YANG J A N U S Z B. P A W L I S Z Y N University of Waterloo Waterloo, Ontario, Canada N2L 3G1
TADEUSZ
GÓRECKI
Technical University of Gdansk Gdansk, Poland meric stationary phase (Figure la) 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 lb). 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$04.50/0 © 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, Κ is defined as the ra tio 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 depen dent on the type of coating and its volume, just as in liquid—liquid ex t r a c t i o n p r o c e d u r e s w h e r e the choice of solvent and the quantity used affect sensitivity. In SPME, the concentration and extraction steps occur simulta neously because all the analytes ex tracted into the coating are intro duced into the gas chromatograph. The selection of a suitable coating is therefore important in order to ob tain maximum extraction of ana lytes. Currently two phases are available, one nonpolar [poly(dimethylsiloxane)] and one more po lar [poly(acrylate)], enabling analy ses of a wide variety of analytes. Table 1 summarizes the published applications of SPME. The extraction and injection pro cesses 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
FIGURE 1
(a) Solid-phase microextraction (SPME) and (b) membrane extraction with a sorbent interface (MESI) SPME device
Sampling environment
1
Sorbent interface
Sample vial
Coated fiber
Carrier gas out Heating coil
(a) SPME
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 equi librium. Exposure times of less than the time required to reach equilib rium can be used without loss of precision and often without loss of sensitivity, if the Κ values are high. After the extraction step, the SPME device is removed from the sample and inserted into the injec tion port of a gas chromatograph. Because of the small dimensions and cylindrical geometry of the fi ber, conventional injection ports easily accommodate direct fiber in jection 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 pol lutants in a sample is linearly re lated to the amount extracted by the fiber coating after a fixed time. 570 A
(b) MESI
Through external or internal cali bration, the concentration of or ganic compounds can be deter m i n e d from the GC d e t e c t o r ' s response for a fixed extraction time or at any time after equilibrium has been r e a c h e d . A l t e r n a t i v e l y , through a modification in the SPME device to allow cooling of the fiber coating, samples can be heated while the fiber coating is main tained at a low temperature. This sampling approach substantially in creases the amount of analytes ex tracted 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 monitor ing of volatile organic compounds (VOCs) in various environmental matrices. The MESI procedure (Fig ure lb) is quite different from that of SPME. The membrane acts as a bar rier to ensure the selectivity, and the sorbent interface acts to concen
Environ. Sci. Technol., Vol. 28, No. 13, 1994
trate analytes, enabling high sensi tivity. The hydrophobicity of the membrane keeps moisture and high molecular weight compounds from entering the GC column, hence pro longing its life. The system can be operated in either batch or multi plex modes. Because of the through put and other advantages, the multi plex GC method offers significantly better detection and time efficiency than the batch method [11). In the batch mode, the stripped analytes accumulate at the interface for a period of time before an opti mized heating pulse is applied to the sorbent trap to desorb all the compounds into the gas chromato graph for analysis. The cycle of trapping and heating is repeated for continuous monitoring. The sensi tivity 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 mem brane extraction module continu ously pass through the sorbent inter face where analyte concentrations in the carrier gas are modulated by a
both the extraction module and the sorbent interface include no moving parts, the MESI system is very rug ged and requires little or no mainte nance 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 effi ciency, and overall simplicity, but also because the column capacity is no longer limited by solvent. Col umns with very small internal di ameters and t h i n n e r stationary phases can be used, which greatly improves the separation efficiency and provides the potential for ex tremely 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 de vice is small and portable and acts as a storage device for analytes ex tracted in the field. It is thus most useful for spot sampling. MESI can be used for long-term field monitor ing or "sniffing," where the mem-
TABLE 1
Published applications of SPME Fiber coating
Thickness (mm)
Target analyte
Matrix
Method LOD
Reference
Poly(dimethylsiloxane)
100.00
Poly(acrylate)
15.00 95.00
VOCs VOCs VOCs PAHs/PCBs Phenols Pesticides
Gas Water Soil Water/soil Water Water
Sub ppbv 0 . 0 0 1 - 3 ppb 0 . 0 0 2 - 1 ppb Low pptr 0 . 0 0 4 - 0 . 2 ppb Low pptr
6 7 8 9 10
* Boyd-Boland, Α.; Pawliszyn, J., unpublished results.
random sequence of thermal desorption pulses. The column responds to each concentration pulse indepen dently, 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 car rier gas for the analysis of volatile analytes is typically N2 or He, but any inert gas can be used. The sol vent-free analysis of semivolatile analytes requires a high-pressure membrane module; high-density C0 2 is used as the stripping phase.
Applying the technologies Automation is of great impor tance in the development of new an alytical techniques. The advantages of a u t o m a t e d m e t h o d s i n c l u d e greater sample t h r o u g h p u t , in creased time efficiency, and re duced labor costs. Automation of SPME and MESI procedures is sim ple but different from other analyti cal techniques. Because the SPME device is basi cally a syringe, the whole analysis process from sampling to injection to separation to quantitation is eas ily automated with the use of a modified GC a u t o s a m p l e r . For MESI, automation is achieved by placing the system on line and by activating trapping, desorption, and analysis with a computer. Because
FIGURE 2
GC-MS of diesel exhaust using solid-phase microextraction
uj—I
10:00
1 ρ-,—|—|—,—,—,—j—,—,—f—τ
1 1 ,—,—ρη—,—|—,—,—|
1 ,
I
1 "f^ I
I
1 I
|
I
I—I
t
|—I
f—I
I
1 I
1 1—I—]—1—I—I—I
15:00
1 Ι
Υ
Γ
20:00
Time (min) (a) Total ion chromatogram; (b) selected ion plot of mass 128; (c) selected ion plot of mass 178; (d) selected ion plot of mass 202.
Environ. Sci. Technol., Vol. 28, No. 13, 1994
571 A
FIGURE 3
Chromatogram of a solid-phase microextraction extraction of a 4-mL aqueous solution of 22 nitrogen-containing herbicides (100 ppb each) 100%
-ι
2
9 4
21 18 Έ £ ο
Ι
8
2
1
°
ï 19 3
1
5
13
11
t
L· L jll JUJU y [... M JlflT Jfl ^, «„
brane is left exposed to an area and 15:00 of analytes is 20:00 FIGURE 4 25:00 30:00 35:00 the concentration Chromatogram of an aqueous BTEX solution sampled from Time (min) monitored as a function of time. An Peak assignment: 1. Eptam; 2. Sutan; 3. Vernam; 4. Tillam; headspace, 5. Ordram; 6. Propachlor; using a 7.fiber Ro-neet; with 8. Trifiuralin; a 15-mm-thick 9. Balan; 10. Simazine; 11. Atrazine; example of this type of application 12. Propazine: 13. Tolban; 14. Terbacil; 15. Sencor; 16. Bromacii; 17. Dual; 18. Paarlan; 19. coating Prowl; 20. Oxadiazon; 21. Goal; 22. Hexazinone. poly(dimethylsiloxane) 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 3 and SPME are ideally suited for coupling with portable gas chro5 matographs. 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. Τ Γ Ί τ
• I
Recent applications of SPME SPME has been used to analyze volatile and semivolatile organic 572 A
Environ. Sci. Technol., Vol. 28, No. 13, 1994
Column: 4 m χ 0.25 mm χ 0.25 mm SPB-5; oven temperature: 45 'C; carrier gas: helium, ~220 cm/s; 100 ppb BTEX mixture. Peak assignment: 1. benzene; 2. toluene; 3. ethylbenzene; 4. m,p-xylene; 5. o-xylene.
FIGURE 5
Quantitative extraction of BTEX, using membrane extraction with a sorbent interface
UL υ
Uvr s
ju s
s)
|lO 20 30 40 ( )
|ίθ 20 30 40 < > (ίθ 20 30 40
