Liquid chromatography / Mass Spectroscopy (LC / MS) is a technique which combines high performance liquid chromatography HPLC, a powerful analytical separation technique with mass spectroscopy ,a powerful analysis & detection technique. There are two common atmospheric pressure ionization (API) LC/MS process: Electrospray Ionization (ESI) & Atmospheric Pressure Chemical Ionization (APCI). Both are soft ionization technique. Both of these processes are compatible with most chromatographic separations. API processes are also compatible with most mobile phase solvents (volatile) & volatile buffers.
         HPLC can be combined with Photo Diode Array detector (PDA).
         HPLC seperations can be detected by PDA & Mass spectrometer as different detectors.
Ion Trap MS/MS (MS n) Function :
         A. Ion Injection & accumulation
         B. Isolation( Ejection of masses above & below parent ion)
         C. Fragmentation( CID of precursor of all product ions)
         D. Ejection of product ions to detector
         Ion trap facility has it's own advantage over other tandem mass as in this method you can select your interested ion peak and accumulate for fragmentation for further MS study so this will be without any interference from other ions results in higher sensitivity. This advantage is exploited for small impurities profiling studies.

PDA Detector :
         It has three dimensional projection over entire UV-VIS range.
         It is non-distructive detector so it can be put in series like HPLC-PDA Mass Spectrometer.