▁spect ro scop ic ▁signals ▁are ▁discussed , ▁som etime ▁also ▁two ▁dimensional ▁correlation ▁spect ro sc opy ▁is ▁used ▁and ▁refers ▁to ▁the ▁same ▁technique . ▁ ▁In ▁ 2 D ▁correlation ▁analysis , ▁a ▁sample ▁is ▁subject ed ▁to ▁an ▁external ▁perturb ation ▁while ▁all ▁other ▁parameters ▁of ▁the ▁system ▁are ▁kept ▁at ▁the ▁same ▁value . ▁This ▁perturb ation ▁can ▁be ▁a ▁system atic ▁and ▁controlled ▁change ▁in ▁temperature , ▁pressure , ▁p H , ▁chemical ▁composition ▁of ▁the ▁system , ▁or ▁even ▁time ▁after ▁a ▁catal yst ▁was ▁added ▁to ▁a ▁chemical ▁mixture . ▁As ▁a ▁result ▁of ▁the ▁controlled ▁change ▁( the ▁perturb ation ), ▁the ▁system ▁will ▁under go ▁variations ▁which ▁are ▁measured ▁by ▁a ▁chemical ▁or ▁physical ▁detection ▁method . ▁The ▁measured ▁signals ▁or ▁spect ra ▁will ▁show ▁system atic ▁variations ▁that ▁are ▁processed ▁with ▁ 2 D ▁correlation ▁analysis ▁for ▁interpretation . ▁ ▁When ▁one ▁cons iders ▁spect ra ▁that ▁consist ▁of ▁few ▁bands , ▁it ▁is ▁quite ▁obvious ▁to ▁determine ▁which ▁bands ▁are ▁subject ▁to ▁a ▁changing ▁intensity . ▁Such ▁a ▁changing ▁intensity ▁can ▁be ▁caused ▁for ▁example ▁by ▁chemical ▁re actions . ▁However , ▁the ▁interpretation ▁of ▁the ▁measured ▁signal ▁becomes ▁more ▁tricky ▁when ▁spect ra ▁are ▁complex ▁and ▁bands ▁are ▁heavily ▁over la pping . ▁Two ▁dimensional ▁correlation ▁analysis ▁allows ▁one ▁to ▁determine ▁at ▁which ▁positions ▁in ▁such ▁a ▁measured ▁signal ▁there ▁is ▁a ▁system atic ▁change ▁in ▁a ▁peak , ▁either ▁continuous ▁rising ▁or ▁drop ▁in ▁intensity . ▁ 2 D