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\subsection{Environmental Chambers and PLAs}  One of the key development in ESEM is the chamber design since different chambers are usually maintained in different pressures. The two environmental chambers (EC1 ad EC2) separate the column chamber from the sample chamber, functioning as buffer zones to minimized the pressure deviation in each chamber. The first environmental chamber is connected to the column chamber by two pressure limiting apertures (PLA's). The PLA has a tiny opening for electron beams, through which the gaseous molecules will leak from the higher pressure part to the other. The space between two PLAs is pumped to keep the pressure as low as possible.   Suppose the chambers connected by PLAs are kept at pressure $P_0$ and $P_1$ respectively with $P_1 > P_0$. The gap between the two PLAs is pumped so the gas leaking from chamber 1 into the gap will mostly be pumped out, preventing the further leakage into chamber 0.  \subsection{Sample Chamber}  text here