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The materials examined were silicone, two types of latex, polyethylene, polyurethane (polyether),
polyurethane (polyester), a copolymer of acrylonitrile and butadiene and polyvinyl chloride. The design
of the interface in which they tested the membranes is shown in Figure 9.56. The membrane was
positioned between two stainless steel blocks, and the entire interface was heated by a four element
heater. The temperature of the interface was monitored and controlled by means of a platinum
resistance thermometer. The heaters were carefully located to ensure that the whole block was kept at
an even temperature. The sample column eluent or flow injection sample, entered by a narrow channel
0.01 in. diameter, and then passed over the membrane and out through a larger channel, 0.02 in.
diameter. The larger exit conduit helped reduce the pressure on the membrane, and prevented
mechanical breakdown. The solutes diffused through the membrane under a concentration gradient that
was naturally set up, and on the other side of the membrane, evaporated into the high vacuum of the
mass spectrometer. The vapor then passed down a heated tube into the ion source.

The results indicated that as one might expect, some membranes were more efficient in the transfer of
certain types of materials that others. If the membrane was made of a polymer that was naturally
dispersive in nature (e.g. silicone membranes) then it would transfer dispersive type compounds such as
hydrocarbons efficiently. However, in contrast, the silicone membrane would probably not be so
effective for polar compounds such as methanol. Polar membranes such as the polyurethane or
polyester would be likely transport polar materials such as alcohols more efficiently than hydrocarbons.
It would appear that the membrane interface needs some more development work to be carried out
before it might be considered as a competitor for the electrospray or API interfaces. Nevertheless, it is
an alternative approach which deserves further consideration and, perhaps with further development,
may find use in specific types of analyses.

Finally, in the review of particle beam ionizing techniques, Creaser and Stygall [45] gave a an
interesting chart relating the efficacy of the