I suggest getting a diode array detector to put between the Shimadzu LC and the Sciex 5500 Q-TRAP. If you are planning to use the detector as a stand alone unit, separate from the mass spectrometer, then get an evaporative light scattering detector.

ELSD or CAD, but why not use your MS?

If you are using a QTRAP why do you need another detector ?  Generally lipids are hard to detect with LC detectors due to poor absorbance characteristics and no fluorescence (without derivatization).  Juat use the QTRAP to measure lipids - it will be much easier. 

 Thank you Paul.

the best ditector for HPLC is the refractive index (RI) detector because is the only universal detector for HPLC 

I think so 

good luck

*Mohamed's comment above is misleading and inaccurate.

There is no such thing as a true Universal Detector for HPLC (If there was one, RID would not be it.). RID is close and sometimes you will find misinformation about RID being "universal" (especially in vendor product literature) because it only relies on one physical property for detection (refractive index). However, as your sample's RI approaches the RI of the mobile phase, the sensitivity gets lower until it is finally zero. So that is not a Universal detector at all.

For lipid analysis, there are thousands of papers and articles showing the use of RID, ELSD, CAD and MS units for detection. ELSD is by far the most popular and RID is the simplest to use, least expensive and most limiting (very poor sensitivity).

Try a web search for example application notes and papers.