high level due to diode 58 being reversed biased by the positive voltage reference thereof a high level appears at the q output of flip flop 44 to cause a positive output from amplifier 52 which thereby renders fet 36 conductive except for the noise spikes points c d and e in fig2 both fet 36 and diode 34 remain conductive thereafter until the minimum peak point b in fig2 is reached so that the voltage level at the output of the sample and hold means 16 39 substantially follows the decreasing electrical output to the minimum peak although fet 36 is continuously conductive beyond the minimum peak diode 34 becomes reversed biased at each noise spike and the minimum peak with the sample and hold means 16 39 operating to store the lowest electrical output reached prior to each such reverse biased condition whenever the electrical output from the spectrophotometer 12 39 is more negative than the output from the amplifier 26 during the negative slope period prior to the minimum peak a differential results between the inputs of amplifier 42 to cause a signal from the peak anticipation means 20 39 after passing through the minimum peak the electrical output then approaches the positive slope polarity threshold point f in fig2 at which fet 32 in the polarized switching means 18 39 becomes conductive to connect the input of the sample and hold means 16 39 with the output from the comparator means 14 39 while fet 36 becomes nonconductive this positive slope polarity threshold is initiated by amplifier 62 which only develops a negative output when the electrical output from spectrophotometer 12 39 is greater than the voltage level output from the sample and hold means 16 39 by a differential greater than the positive voltage reference.