You need to make sure that your cells are still viable (living) during the time that you keep the cells in the serum free media. If cells start to die or break up, your secretome analysis will be "contaminated" with information/proteins arising from within the cells.
Serum starvation can be used to study the secretome, but extended periods (>24 hours) are not ideal. Use serum-reduced medium, perform time course experiments, include controls, assess cell health, and normalize data to obtain reliable results. Consult with an LC-MS expert to optimize sample preparation and data analysis for your secretome study.