Abstract: In an optogalvanic spectroscopy system, a plasma is exposed to light at a wavelength associated with a particular species in the analyte. The light is amplitude modulated. The amplitude of a modulated component in the signal representing electrical impedance of the plasma is detected in a phase independent manner, so that the detected amplitude is substantially independent of the phase of the modulated component in the impedance signal relative to the amplitude modulation of the light. The system has reduced sensitivity to changes in the spectrum of the applied light.

Abstract: A feedback sample and hold circuit bank (10) of 24 stages (12) is described. The bank (10) is of the type having an operational amplifier common to all the stages (12) and having its output connected to each stage (12) through a sampling switch (S.sub.S) to the input of a buffer (B). A holding capacitor (C.sub.H) is connected between input of the buffer (B) and ground potential. A correction coupling capacitor (C.sub.CC) and a correction holding capacitor (C.sub.CH) are connected respectively in series between the input of the buffer (B) and ground. A correction switch S.sub.C is connected between the amplifier output (14) and the common node (22) of the correction capacitors (C.sub.CC, C.sub.CH). When the sampling switch (S.sub.S) opens for the hold condition, the correction switch (S.sub.C) briefly remains closed to permit the amplifier (A) to correct the hold voltage for switching charge feedthrough error from the sampling switch (S.sub.S) by charging the correction holding capacitor (C.sub.CH).