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MESI—Membrane

Extraction Sorbent

Interface

by Rick Morehead, Innovations Chemist

moving

parts

MESI is a new sample preparation technique that

uses a permeable membrane to extract volatile

organic compounds (VOCs) from various matrices

and a sorbent trap to concentrate these compounds

prior to analysis by gas chromatography. MESI has

been designed to be an easy to use "solventless"

extraction technique. It can be used for fast, routine

analyses or in long-term or continuous monitoring

operations.

MESI extraction is based on the partitioning of

VOCs across a non-polar membrane approximately

25µm thick. A small section of membrane material

is mounted in a holder that allows one side of the

This product is still in development. If

you would like to discuss potential appli-

cations, please call Technical Service at

800-356-1688 or 814-353-1300, ext. 4,

or contact your local Restek

representative.

membrane to be exposed to the sample matrix. The

other side of the membrane is continuously swept

with a carrier gas. As volatile compounds come in

contact with the membrane surface, migration

through the membrane begins. Compounds are

then picked up by the carrier gas on the other side

of the membrane and passed along to an adsorbent

trap. After the sampling or extraction period is

complete, the trap is very rapidly heated to desorb

all of the compounds that have been collected on

the adsorbent material. Desorbed analytes are

picked up in the flow of the carrier gas and are

transferred to the inlet of the GC analytical column

for separation.

Adsorbent traps used for MESI are made from

small diameter Silcosteel

tubing packed with tradi-

tional trapping materials like Carboxen

, Tenax

other divinylbenzene resins. The capacity of the trap

is related to the type of adsorbent used as well as

the amount of material used. Traps are desorbed by

passing a short pulse of DC current across the trap

and resistively heating the trap.

Figure 1 shows the analysis of a soil sample for the

presence of gasoline range organics (GROs). A 1gm

soil sample was spiked with 1ppm of gasoline and

sampled for 2 minutes. Analytes were extracted by

the membrane and concentrated on the trap, then

were desorbed and analyzed on an Rtx

-1 column.

Good peak shape and response can be achieved by

optimizing the sampling time and the desorption

conditions.

MESI can be used in a variety of sampling applica-

tions. Short turnaround time can be obtained for

higher concentration applications. The cycle time

for heating and cooling the trap can be as short as

thirty seconds, so samples can be collected and

desorbed in rapid succession to perform on-line

monitoring of processes or streams. For lower con-

centration samples, sampling time can be extended

in order to maximize the response for the analytes

of interest. MESI can be used as a bench-top acces-

sory for most GCs or, because of its compact

design, can be used as part of a field portable unit

for on-site analysis.

MESI also can be used with a variety of sample

types. Gas, liquid, and solid samples can be ana-

lyzed by exposing the sample to the membrane

using minor variations in the hardware that holds

the sample.

MESI is a low-cost alternative to other sample

preparation techniques. It is easy to use and auto-

mate, while being sensitive enough for most appli-

cations.

4,5

30m, 0.25mm ID, 0.25µm Rtx

-5 (cat. #10223)

Initial temperature:

40°C (hold 0.5min.) to

100°C @ 5°C/min. to 280°C @ 30°C/min.

(hold 2min.);

Carrier gas:

hydrogen, 5psi

Detector:

FID

1. benzene

2. toluene

3. ethylbenzene

xylene

Figure 1

MESI used to determine hydrocarbons at low ppb levels.

10 min.

great

idea!

GC_EV00395

Questions?

support@restekcorp.com.

The industry’s best technical service

will be glad to help you with

Plus 1

™

service!