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index.Residual Gas Analyzers.
Gas Analysis Systems.
Membrane Probes for Liquids using the carrier gas technique.
INTRODUCTION.
A semi-permeable hydrophobic membrane permits the diffusion of gases and volatiles but is highly impermeable to the solvent molecules.
CARRIER GAS.
The membrane probe, seen schematically in Figure 1, can be used in two basic modes :-
1. With air as carrier gas
2. With a non air constituent as carrier gas
Two Swagelock type connectors are available for making tubing connections to the probe and a 10 metre length of suitable tubing is supplied together with a comprehensive spares kit.
If analysis is required for volatiles such as alcohols, acids, aldehydes etc., the simplest approach is to connect one side of the probe to any inlet of the Spectramass Multiport Inlet Manifold with the adaptor provided. In this way air is sucked in through the other port and collects any gases/volatiles diffusing through the membrane, thus acting as a carrier gas. The resulting gas mixture is analysed by the mass spectrometer in the normal way by selecting the peaks of interest and ignoring the peaks due to air.
Clearly, if it is an air component that you are interested in, then this approach will not work and a carrier gas will need to be provided which does not interfere with the gas(es) of interest. Helium is the carrier gas normally employed.
If this technique is to be employed then the flow regulator of the carrier gas cylinder MUST be capable of fine tuning so that the flow rate can be carefully set and controlled. Too high a flow rate may damage the membrane and too low a rate may mean that the response to gas changes is unacceptably long.
RESPONSE TIMES.
The response time can be judged by conducting a simple experiment such as breathing over the membrane and watching for the increase in the mass 44 peak(carbon dioxide) which should be monitored in Peak Select (Channel) Mode with a single peak selected.
Other tests might include immersing the membrane in beakers of water and carbonated soft drink or alcohol solutions, to time how long it takes to see the change when moving from one solution to the other.
It should be possible to find a combination of Precision Code, Gain and carrier gas flow rate to give response times of just a few seconds. This delay can be compensated for in your DQC 2000 software by an entry in the "C/O delay" column for each stream.
CALIBRATION.
A calibration curve can be produced prior to an experiment with the membrane probe, using a stepwise dilution approach of the volatile or gas of interest.
Figure 2 is an example of such a calibration curve using isopropyl alcohol.
Firstly, it is important to know a unique peak at which to monitor the component of interest. This may not always be the base (largest) peak of the mass spectrum. In our example we selected m/e 45. The peak at m/e 31 is characteristic of all alcohols and can be used where mixtures of alcohols are encountered.
For volatiles, temperature is an important parameter to consider, so all solutions should be brought to the operating temperature of the fermentation prior to analysis.
The background pressure at m/e 45 should be measured first. This is the partial pressure that exists in the vacuum system itself and its value must be subtracted from each subsequent measurement so that the partial pressure due to the concentration of isopropyl alcohol can be calculated. A "blank" solution of distilled water at the correct temperature can be used for determination of the background level.
Start with a solution of 100mls of alcohol in 1 litre of distilled water as the strongest solution then progressive dilutions of 50:50 in distilled water were made, and the corresponding partial pressure measured, until the level was no longer detectable above the background. The calibration curve can now be used for estimations of isopropyl alcohol (at the same temperature) by reading off the concentration at the partial pressure detected.
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