There is one issue you need to be careful with in snipping portions of sample to analyze: The mass of one portion may be different than another. And, given that you are dealing with partitioning between the collar and the air in a vial, the ratio of the quantity of air to the quantity of collar may have some impact on what you measure. A variation on your experiment might be to take a jar that is large enough to hold the collar with room to measure head space. Put a hole in the cap that will just admit the SPME sampling device - and perhaps seal the opening with a septum. Put the entire collar in the jar, equilibrate it and the SPME fiber for an hour or so, and run the GC/MS run and then take the collar out of the jar and let it be exposed to the environment to which you normally have it exposed and then at the end of the week use the same jar, collar, SPME fiber etc. and measure again the same way. There is no variability in how much sample you use - it is the same sample, just aged, and there is no change in ratio of solid to gas in the sampling vial. And if there is any variation in application of the pheromone along the material in the collar, it does not matter. You need to do the equilibration with the SPME fiber for the same length of time and at the same temperature each time - because how much pheromone makes it out of the collar depends on diffusion rates (time) and then how much makes it out of the air and into the fiber again depends on rates (time again). And both equilibrium and rate change with time and temperature.
This leaves the issue of instrument calibration. The sensitivity of the mass spec can change over time. You need a reference sample to check or calibrate the GC/MS. You can make up a solution of the pheromone in a suitable solvent and put a few microliters in to a headspace vial to measure the response. Note that the solvent can change the partitioning into the SPME fiber, so this will demonstrate consistency of response for a given level of pheromone, but cannot be depended on for measurement of gas phase concentration without further proof.
One other thing to consider: If you put the collar onto an animal, the collar will pick up volatile compounds from the animal, which will become part of the partitioning process. And these compounds can swamp the fiber, reducing the response of the pheromone. But as long as the material you are sampling is consistent except for the volatilization of the compound of interest, your experiment should give you a good idea of the decrease in concentration of the pheromone over time. There is debate among some about the utility of SPME for quantification. And the safest method is to have an internal standard that partitions exactly like the analyte of interest. An isotope labeled internal standard is the best way to do this. And, if the partitioning of the analyte is changed by an excess of other stuff changing the partitioning, the internal standard is affected the same way and the ratio gives you the information you need. But, we’ll assume that other volatile components are not changing – at least for now…