BACKGROUND

Measurement of volatile organic compounds (VOCs) in water usually involves a separation step in which the organics are first extracted from the matrix. Popular analytical techniques used for VOCs analysis are

• Purge and trap,
• Headspace analysis, and
• Solid phase micro-extraction.

All these methods first isolate the organics from water prior to analysis by GC or GC/MS.

Membrane extraction has emerged as a promising technique for separating VOCs from water. Separation is based on organic pervaporation through the membrane while the aqueous phase is blocked. Recently we have developed a measurement technique based on membrane permeation that can be used for analyzing sub ppb levels of VOCs and also for continuous on-line monitoring. This technique is referred to as Gas Injection membrane extraction (GIME).

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WORKING PRINCIPLE

The schematic diagram of GIME-GC system is shown in Fig. 1. The membrane module is fitted with multiple membrane fibers. An inert carrier gas (such as N₂) continuously flows through the membrane module. A 6-port sampling valve is used to introduce a fixed volume of water sample onto the gas which injects the sample into the membrane module. As the sample flows inside the membrane fibers, the organics permeate through the membrane. They are stripped by a stream of N₂ gas that flows countercurrent on the other side of the membrane, and pneumatically transported to the microtrap. Once membrane extraction is completed, the microtrap is electrically heated to desorb the VOCs into the instrument for analysis.  

In GIME, there is no mixing between the carrier gas and the aqueous sample, thus eliminating sample dispersion within the carrier. The gas also cleans the membrane surface after the sample is pushed through the membrane module. This reduces the static aqueous boundary layer effects that cause long lag time in the extraction profile. Fig. 2 is a comparison of membrane extraction profile of GIME with that of aqueous elution, which shows that GIME extraction is much faster while maintaining the same sensitivity. A chromatogram obtained by GIME from a water sample containing ppb level of VOCs as specified in EPA method 602 is shown in Fig. 3.

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ADVANTAGES OF GIME

• GIME eliminates boundary layer effects and sample dispersion commonly encountered when a liquid carrier is used to elute a sample through a membrane. This results in a faster membrane extraction, which increases sample throughput and is highly desirable for on-line monitoring.  
• The sample is introduced as an injection pulse into the membrane module rather than a continuous stream. This eliminates any steady state requirements and significantly reduces analysis time.  
• Detection limits for most VOCs are at sub ppb levels.  
• Simple, inexpensive instrumentation, and fast analysis.  
• Capability of continuous, on-line analysis as well as analysis of individual samples.

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REFERENCES

1.      Gas Injection Membrane Extraction For Fast On-Line Analysis Using GC Detection, Kou, D.; San Juan, A.; Mitra, S. Anal. Chem. 73(22), 5462-5467 (2001). 

2.      On-Site And On-Line Analysis Of Chlorinated Solvents In Ground Water Using Pulse Introduction Membrane Extraction Gas Chromatography (PIME-GC).  San Juan, A.; Guo, X; Mitra, S. J. Sep. Sci.  24(7), 599-605 (2001). 

3.     Theoretical analysis of non-steady-state, pulse introduction membrane extraction with a sorbent trap interface for gas chromatographic detection, Guo, X.; Mitra, S. Anal. Chem. 71(20), 4587-4593  (1999). 

4.      Enhancement of Extraction Efficiency and Reduction of Boundary Layer Effects in Pulse Introduction Membrane Extraction. Guo, X.; Mitra, S. Anal. Chem. 71, 4407-4413 (1999). 

5.      Pulse introduction membrane extraction for analysis of VOCs in individual aqueous samples and for continuous on-line monitoring, X. Guo and S. Mitra, J. of Chromatogr. A, 826, 39-47 (1998).

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