fig1 shows a simple prior art system 10 to transfer sample from a trap 11 in which the sample has already been retained into a mass spectrometer 12 . the system is also provided with a gas supply 13 , an exhaust 14 , a heater 15 and valves 16 , 17 . valve 16 is a leak valve that admits a small portion of the flow into the mass spectrometer ( limited by the capacity of the high vacuum pumps required for the mass spectrometer ). heater 15 heats the trap 11 to release the analyte from the absorbent within the trap , valve 17 is opened to allow transport gas from the gas supply 13 to be admitted to the trap 11 . this pushes the analyte out of the trap 11 , as a relatively concentrated “ plug ” of gas , and transports it to the mass spectrometer inlet valve 16 . region 18 is normally at , or a little above , atmospheric pressure during this process . fine control of the timing of opening of valve v 2 allows the sample plug to be stopped at the inlet valve 16 , if time is needed for the analysis . there are also possible variations that use a pump instead of excess pressure in the gas supply to drive the transport gas . it should be noted that the concept of a transport gas only works if the diffusion of the analyte is relatively slow compared with the gas velocity . at the opposite extreme , if the diffusion rate is very fast , the analyte would behave essentially independently of any other gas in the system and there would be no sample plug . u . s . pat . no . 5 , 142 , 143 uses a variation of the method described in fig1 . this document teaches explicitly the use of a transport gas at a reduced pressure , having reduced the trap pressure prior to desorption , in an attempt to further increase the concentrating power . fig2 shows a typical double membrane concentrator 20 , as implemented by the applicant in its existing portable battery powered mass spectrometer . the sample to be analysed , usually air at or near atmospheric pressure , is drawn from an inlet 21 past a first membrane 22 by sampling pump 23 . some sample gas permeates through the 44 opened to vent region 43 . the trap 41 can now be replaced with a fresh one , for further analysis to take place . the design used to implement this schematic seeks to minimise the distance between the trap and the inner membrane 47 so that analysis time is as short as possible . the better the vacuum in the region 43 before the heating starts , the higher the concentrating ratio and the faster the diffusion will be . the limit will be determined by the size of pump used and the room temperature out - gassing rate . once the pressure is so low that the mean free path of gas molecules is longer than the tube diameter , there will be diminishing returns in the sense that further pressure reduction will not speed up the diffusion process much further . note that in contrast to the prior art , there is no sample “ plug ” to be pushed or entrained into the mass spectrometer . the concentrating power of the trap depends on the ratio of the sampled volume when loading the trap to the whole volume of region 43 , as well as on the pressure ratio . thus it is desirable to minimise the volume of the region 43 . the inner membrane 47 experiences very little mechanical load because of the low pressure difference in this setup . this means that there is the possibility to use much thinner material which will speed up the process of diffusion through the membrane . there may be more choice to the membrane in other ways too if mechanical strength is less of an issue . there are of course many variations that could be envisaged . example 1 : the plumbing to load the trap could be included . example 2 : multiple traps could be processed at once to speed the overall cycle ; one trap would be pumping , another being analysed and a third in the sample phase and so on . also the membrane 47 could be replaced by an alternative flow restrictor . an arrangement that would include the sampling is depicted in fig5 . the top of the trap is now sealed with a valve 49 , which , when open , would allow the vacuum pump to draw air through the trap to load it . whilst fig4 and 5 show a theoretical arrangement , an actual example is shown in fig6 . the device 50 includes a trap 51 which is connected , by means of a quick release seal 52 , to a passageway 53 which is in communication with chamber 54 in body 55 . the trap 57 is surrounded by a heater 56 which can be operated to raise the temperature of the trap , for example , to initiate desorption of the trapped material . the upper end , in fig5 , of the trap is sealed by means of end seal 57 . in this example , the carrier gas and analyte have already been passed through the trap , which has then been connected to the body 55 for analysis . main body 55 is provided with an ultra high vacuum isolation valve 58 which seals chamber 54 from a membrane 50 . the chamber 54 is also connected , via passage 59 to a vacuum pump ( not shown ). the inner membrane 60 , which may be selectively or non - selectively permeable , is located between valve 58 and a passageway 61 which leads to , in this example , a mass spectrometer . the mass spectrometer is operated at a lower pressure to that found in chamber 54 , as described with regard to fig4 , thereby enabling diffusion of the analyte from trap 51 into the spectrometer for analysis . the present invention provides a solution to the problems associated with the double membrane concentrator and an improvement to conventional trap and desorb concentrators . the first membrane is removed and a conventionally engineered trap put in its place . the sample , at atmospheric pressure , is loaded onto the room temperature trap with the uhv valve protecting the inner membrane and spectrometer . after sampling , the trap can be flushed with dry gas if necessary to reduce water vapour . then the intermediate vacuum space , which now includes the trap , is evacuated with the existing miniature pump from 1 bar to less than 1 mbar , increasing the potential concentrating power of the trap by a factor of 1000 over and above the volume ratio of sampled air to trap volume . the pump is sealed off and the trap is heated to release the compounds of interest . there is no carrier gas to push the analyte towards the inner membrane , however , at these reduced pressures , diffusion rates are much higher , owing to the longer mean free path , therefore the gas at the inner membrane rapidly reaches an equilibrium where the partial pressures of the compounds of interest are proportional to the absolute quantities that were absorbed onto the trap . it should be noted that this implies that no special measures are necessary to achieve a precise pressure during the evacuation phase and that the conductance from the intermediate pressure region , including the trap , into the mass spectrometer is still very small compared with the trap volume , so the partial pressures of analyte remain relatively stable whilst an equilibrium is established across the inner membrane and during the measurement . the concentrating power of the present invention can easily be made much greater than that of a single membrane . therefore , in this example , the overall sensitivity of the instrument is increased by over an order of magnitude even though one of the concentrating membranes has been removed and even for compounds which work well with the membrane . for polar compounds , the gains may be a great deal higher again . in many cases , it may not even be necessary to have an inner membrane ; a flow restrictor with the correct conductance could be used instead . the trap material is more easily engineered for a wide range of applications as the diffusivity is not a factor , in contrast to membrane requirements . the desorb phase may be a simple step change in temperature , designed to desorb all compounds of interest , or it may be some controlled time profile such as ramp . this can be used to achieve a greater degree of discrimination between compounds which might have similar mass spectra but different temperature dependence in the partition coefficient . automation of the trap , evacuate , desorb process could be achieved simply in a variety of ways . if the positive displacement pump has a well defined pumping speed , there is the possibility of using it both for sampling and for the evacuation . in this case , a single electrically operated valve in place of the end seal might be all that is required for a simple cycle that does not require a dry flush . a variety of valves and other equipment developed for gas chromatography could easily be adapted for use in more complex schemes . an additional advantage of a trap based sampling system in that the traps may be loaded with very simple apparatus quite separately from the analysis . even when using a portable gas analyser on - site , it may be convenient to load a set of traps at locations around a site without having to carry the mass spectrometer to each point and then bring the traps back to a central point , perhaps a vehicle , for analysis . this mode of operation is compatible with the present invention . although the discussion has concerned gas analysis , the gas in question may be head space or sparging gas that has been in contact with liquids or even solid material . therefore it can be seen that the invention might be employed , for example , for water or soil analysis . some trap materials may even be suitable for exposure directly to fluids . the combination of remote loading of the traps , together with the ability to deal with humid samples at very high sensitivity may make this a particularly suitable concentrator for use in breath analysis . sampling could be achieved by breathing a known volume directly through the trap . of course the present invention may be used with any analyser that operates with a suitable vacuum and may , in particular , be used with a mass spectrometer that employs further pressure steps for other purposes , for example a differentially pumped ion source .