LC – MS/MS offers a range of benefits to the analytical chemist and such features are highlighted in the present article. However, before listing of such benefits you should appreciate the elementary differences between the two techniques.
Single quadrupole mass spectrometer
A single quadrupole mass spectrometer comprises of a single quadrupole assembly Q1 used for scanning and filtering ionized molecules through ramping of DC and RF voltages across diagonally opposite quadrupole rods. A complete ramp results in acquisition of full scan mass spectra also referred to as Multiple Ion monitoring (MIM). This affords quantitative estimation of the sample constituents. In the Selected Ion Monitoring (SIM) mode a fixed combination of DC and RF voltages is applied which permits passage of selected m/z ratio ions to pass through to the detector and all other ions are filtered out.
Triple quadrupole mass spectrometer
Triple quadrupole mass spectrometer comprises of three quadrupoles Q1, Q2 and Q3. Quadrupoles Q1 and Q3 serve as mass filters and the middle quadrupole Q2 functions as a collision cell. Specific precursor ions of interest isolated by Q1 on reaction in the collision cell collide with neutral gas molecules such as nitrogen to fragment as product ions which reach the third quadrupole assembly Q3 which allows only a specific m/z ratio ions range to pass through to the detector. In other words a triple quadrupole system is useful for fine tuning the separation further and improving resolution of ions segregated by single quadrupole instruments.
Advantages of LC – MS/MS
The fine tuning of resolved ions diminishes the interference from co – eluting compounds present in the sample. In other words this reduces time and effort in carrying out tedious HPLC separations. Higher signal-to-noise (S/N) ratios permit quantification at lower levels of quantification.
Analysis for specific analytes in complex matrices is made possible using LC – MS/MS requiring less matrix clean-up. No preliminary derivatization is essential thereby reducing cost on expensive derivatizing reagents, time saving resulting in high sample throughputs and low maintenance due to non-introduction of harsh reagents into the system.
Improvement in selectivity
Improved resolutions in triple quadrupole separations lead to higher selectivity. Multiple reaction monitoring using LC-MS/MS offers many-fold selectivity improvements over single ion monitoring (SIM) using a single quadrupole LC- MS.
Wide linearity range
A wide linear range is desirable for quantitative estimations. Triple quadrupole using typically ion spray ionization results in 3-4 times larger linear range when using MRM mode. This is in addition to higher accuracy and reproducibility of results achieved in a single quadrupole instrument.
LC – MS/MS due to its superior performance features is an expensive proposition but with a choice of several ionization techniques it permits handling of low volatility thermally labile compounds with varying polarities in several application areas such as pharmaceuticals, foods and flavours, cosmetics, sports medicine, explosives, etc. The application potential is particularly promising in bioanalytical studies, therapeutic drug monitoring, clinical monitoring, forensic toxicology and proteomics.