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A novel energy analyzer with a simple-structured Bessel-box has been successfully developed and incorporated into ionization gauges and quadrupole mass spectrometers (QMS) to improve their measurement of the total and partial pressures,respectively.We found that the energy analyzer may not only effectively separate the gas phase ions and electron stimulated desorption (ESD) ions produced in the ionizer,but also screen the ion collector from irradiation of the soft X-rays and VUVphotons generated in the ionizer.The screening effects considerably better the trace impurity detection limit of the QMS down to a few tens ppb in the pressure range of 10-3 Pa.The modified ionization gauge is capable of measuring pressure ranging from 10-11 to 10-6 Pa in the pulse counting mode and its upper limit may extends to 10-3 Pa in DC ion current mode with typical sensitivities factor of(6.7±0.2)×10-3 Pa-1 for N2 and (2.3±0.2)×10-3 Pa-1 for H2,respectively.No ESD ions was observed in the simple-looking spectrum obtained with the modified QMS.We also found that the energy spectra of fragment ions depend strongly on species of parent ions.