Gas Diffusion Techniques
All gas diffusion methods are based on interfacing the donor stream containing the analyte, with an acceptor stream containing a reagent. The porous hydrophobic membrane serves as a barrier that allows only the gaseous species passed through. This makes the gas diffusion methods very selective, since the non volatile species will not reach the detector.
Flow scheme of gas diffusion unit and its photo shown from acceptor side. Unit dimensions are 4cmW, 2cmD and 3cm H.
The instrumental setup can be configured in two ways:
The continuous programmable flow method where both donor and acceptor stream are continuously pumped (FIAlab 2500 or FIAlab 2600).
The stopped flow technique where both donor and acceptor stream are controlled by syringe pumps (FIAlab 4002sp, microSI and FIAlab 3600)
The gas diffusion unit for continuous flow has diffusion path 2 cm long and 2 mm wide. The acceptor stream is monitored by a flow cell with 10 cm long light path. The flow injection system is operated at a programmable flow rate of 0.75mL/min per channel while the sample passes through the diffusion unit and at a flow rate of 3.0mL/min during flush period. The injection volume is 300 microliters of sample and sampling frequency is 40 injections/hour.
The stop flow method uses the sandwich gas sensor, where the injected analyte is in contact with the gas diffusion membrane during the stopped flow period. It yields higher sensitivity and consumes far less of sample and reagents then continuous flow method.(micro SI, FIAlab 3200 and FIAlab 4002sp)
The Sandwich Gas Diffusion Sensor uses a pair of optical fibers that monitor the acceptor solution adjacent to the diffusion membrane.
The stop flow FI technique is robust, since it uses syringe pumps, and therefore suitable also for continuous monitoring. Sensitivity of this method is easily adjusted by selecting the duration of the stopped flow time, while using the same experimental setup and reagent concentrations. The Sandwich Sensor provides response in real time, since it monitors the kinetics of the diffusion and of the subsequent chemical reaction and uses smaller volumes of sample than the continuous flow method.
All FIAlab gas diffusion modules use robust polymer made hydrophobic membrane.
FIAlab 2500. Flow injection system configured for gas diffusion based assay and the side view of the diffusion unit designed for continuous programmable flow method.
2 ppm to 40 ppmN at sampling rate 40s/hr.
The schematic of Sandwich Gas Sensor and its photo shown from the donor side
FIAlab 4002sp. Syringe based flow system configured for gas diffusion stop flow based assay. The schematic of Sandwich Gas Sensor and its photo shown from the donor side. 0.1ppm to 50ppm N at sampling rate 30 to 60s/hr.
Examples of Gas Diffusion Based Assays
Analyte
|
Monitored Species
|
Matrix
|
Method of Detection
|
Reference
|
Ammonia
|
Water
|
Fluorescence
|
18.866
|
|
Marine Water
|
Fluorescence
|
18.136
|
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Seafood
|
pH/Spectrophotometry
|
16.597, 5.074
|
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TMA
|
Fish
|
pH/Spectrophotometry
|
11.328, 11.059
|
|
Ammonia, TMA
|
Seafood
|
pH/Spectrophotometry
|
13.200, 13.050
|
|
Ammonia
|
Fermentation
|
Spectrophotometry
|
6.662
|
|
Ammonia
|
Industrial Waste
|
Spectrophotometry
|
6.493
|
|
Ammonia
|
Milk, Meat
|
Conductivity
|
17.938, 1.537
|
|
Ammonia
|
Soil Digests
|
Spectrophotometry
|
12.020
|
|
Carbon Dioxide
|
Water
|
pH/Spectrophotometry
|
18.069
|
|
Carbon Dioxide
|
Gas
|
pH/Spectrophotometry
|
13.779
|
|
Carbon Dioxide
|
Water
|
pH/Spectrophotometry
|
13.719
|
|
Carbon Dioxide
|
Beer, Soft Drinks
|
pH/Spectrophotometry
|
6.387
|
|
Sulphite
|
Sulphur Dioxide
|
Fruit Juices
|
Amperometry
|
19.484
|
Sulphite
|
Sulphur Dioxide
|
Wine
|
Spectrophotometry
|
18.384, 15.654
|
Sulphite
|
Sulphur Dioxide
|
Wine
|
Conductometry
|
8.822
|
Chlorine
|
Chlorine
|
Water, Free Chlorine
|
Spectrophotometry
|
15.432, 11.820
|
Chlor Dioxide
|
Chlorine
|
Water
|
Spectrophotometry
|
9.815, 9034
|
Ozone
|
Ozone
|
Residual Ozone, Water
|
15.041, 839
|
|
Ethanol
|
Ethanol
|
Liquors
|
Spectrophotometry
|
16.229, 15.783
|
Cyanides
|
Hydrogen Cyanide
|
Waste Water @ Ppb Level
|
spectrophotometry
|
18.163, 18.113, 15.133
|
Xanthates
|
Carbon Disulphide
|
Flotation Bath
|
UV Spectrophotometry
|
19.157
|
NOTES: 1) Reference numbers refer to Hansen’s Bibliography
2) See Manuals & Methods for more information.
|
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Other Applications
In principle any compound with high vapor pressure can be assayed by gas diffusion method. The samples,
introduced into the donor channel can be either liquids or gases.
The gas diffusion modules can also be used for separation after hydride generation with subsequent electrochemical, atomic absorption ( AA) or inductively coupled plasma (ICP) detection.
Limitations and Interferences
The body of gas diffusion module and of gas sensor is fabricated from Plexiglass and will not withstand
organic solvents. Plese enquire for other available materials ( ULTEM, STEEL).
High concentrations of surfactants will flood membrane and destroy its gas permeation ability.
For further details on programmable flow technique and stop plow technique consult Manual Gas Diffusion Methods and Protocols ( Ammonia)
