help with calculation from dataframe with multiple entries per sample

Hi 

I have a dataframe similar to:

Sample<-c(1,1,1,2,2,2,3,3,3)

Time<-c(1,2,3,1,2,3,1,2,3)

Mass<-c(3,3.1,3.4,4,4.3,4.4,3,3.2,3.5)

mydata<-as.data.frame(cbind(Sample,Time,Mass))

(
  Sample Time Mass
1      1    1  3.0
2      1    2  3.1
3      1    3  3.4
4      2    1  4.0
5      2    2  4.3
6      2    3  4.4
7      3    1  3.0
8      3    2  3.2
9      3    3  3.5

where for each sample, I've measured mass at different points in time. 

I now want to calculate the difference between Mass at Time 2 and 3 for each unique Sample and store this as a new variable called "Gain2-3". So in my example three values of 0.3,0.1,0.3 would be calculated for my three unique samples and these values would be repeated in the table according to Sample. I am thus expecting:

mydata #after adding new variable

  Sample Time MassGain2-3
1      1    1  3.00.3
2      1    2  3.1 0.3
3      1    3  3.4 0.3
4      2    1  4.0 0.1
5      2    2  4.3 0.1
6      2    3  4.4 0.1
7      3    1  3.0 0.3
8      3    2  3.2 0.3
9      3    3  3.5 0.3

mydata$gain2.3 <- with( mydata, ave( Mass , Sample, FUN=function(x) (x[3]-x[2]) ) )
mydata

Does anyone have any suggestions as to how to do this? I've looked at the various apply functions but I can't seem to make anything work. I'm fairly new to R and would appreciate specific suggestions.