Abstract: The invention comprises a mass spectrometer adapted for the determination of isotopic ratios which has several ion-collecting means disposed along its mass focal plane. Separate current amplifier means are provided for each ion collecting means and comprise an electrometer amplifying element, and an input resistor disposed in an inner box, itself disposed within a sealed evacuated housing. Means are provided for cooling the amplifier element to a temperature substantially below 20.degree. C. Means are provided for maintaining the temperature of at least the input resistor substantially constant. Cooling the amplifier element, typically to between 0.degree. and 5.degree. C., results in an unexpected reduction in low frequency noise which allows the time required to determine an isotope ratio to be reduced, thereby substantially increasing sample throughout.