replacement - How to stop Replace duplicating a list when using Table?

I have a 3 dimensional list of data pairs in the form:

{{{{x11,y11},{x12,y12}},{x1...,y1...}},{{x21,y21},{x22,y22}},{x2...,y2...}}... etc

Incidentally the data originates from a Gas Chromatograph, and is ordered by: [[Channel (1-4), Experiment Number (1 - some number), Peak Number (1 - some number)]], and the pairs represent {Retentiom time, Peak area}.

I want to use the replace function to search the data under the condition that the retention time falls within some range, when this condition is satisfied for the pair: {x_,y_}, x is replaced by the name of the gas that elutes e.g. CH4, C2H4 etc., and y is replaced by the peak area multiplied by a response factor.

The retention time window condition is defined for each channel and for each species. Therefore, I made a list of species and channels containing the time windows, the species name and response factor: e.g.

species={CH4,C2H4,etc...}
CH4={CH4ch1,CH4ch2,CH4ch3,CH4ch4}

CH4ch1={{retentiontimeMin,retentiontimeMax},{"speciesname",responsefactor}}

The code that I am using is as follows:

Table[gcData[[channel, All, All]] /. {x_, y_} /; 
 species[[l, channel, 1, 1]] < x < 
  species[[l, channel, 1, 2]] -> {species[[l, channel, 2, 2]], 
 y*species[[l, channel, 2, 1]]}, {channel, 1, 4}, {l, 1, 7}] 

Where gcData is the list of data, and l is the species number.

My problem is that when I use replace I get my original list of data, duplicated 7 times (the number of species I am searching for), where in each iteration the species I am searching for is correctly identified and replace, but on the subsequent iteration I do not see the originally replaced species.

A sample section from the resultant data is shown below:

{{{{"CH4", 34423708},
   {1.581, 240631},
   {1.741, 63594},
   {2.192, 5568}}}
 {{{1.406, 34423708},
   {"C2H6", 0.292896},
   {1.741, 63594},
   {2.192, 5568},}}},
 {{{1.406, 34423708},

   {1.581, 240631},
   {"C2H4", 0.0772031},
   {2.192, 5568},}}}}

How do I stop it duplicating the list?