Extracting data from an array.............................................................................

First of all, I would recommend using System.Collection.Generic.List with proper type argument instead of ArrayList which became obsolete with introduction of generics; this type force you into doing type cases, which is inherently error-prone.

Instead of the type double[2] you use (as I understand, one element is Mass, another is Intensity), you need to use explicit structure with explicit names:

use List = System.Collection.Generic.List<Element>;

struct Element { //give it a semantic name instead, which I don't know, what is it?
    internal Element(double mass, double intensity) { Mass = mass; Intensity = intensity; } 
    internal double Mass;
    internal double Intensity;
} //struct Element

//...

//I abbreviated type name in "using" for convenience:
List masslistArray = new List();

//...
masslistArray[0] = new Element(6, 55); 

Create another array in the same way of the same type or of the similar type but with the element type different similar structure. Then you can access elements of index (within available Length (this is the instance property of List)) or adding elements via Add method. Using indexed access you can assign elements element-to-element.

If you want to have different element types for different array, but there is a semantic way to convert it component-by-component, you can create an implicit convertions operator, for example:

struct Point { 
    //...
} //struct Point

struct Element { 
    public static implicit operator Point(Element value) {
        return new Point(value.Mass, 0, /* something like that... */ );
    } //operator Point
} //struct Element

Without the operator you would have to assign between different types one per component basis (each double member separately, for example), the operator will allow to assign array elements directly.

For arrays of the same type you can also use Array.Copy to make slices and copy part of one array onto another one.

—SA