Questions Related to Analytical Instruments

Chromatography is a physical method of separation in which the components to be separated are distributed between two phases, one of which is stationary (stationary phase) while the other (the mobile phase) moves in a definite direction. Chromatography may be preparative or analytical.

A chromatogram is he visual output of the chromatograph in case of an optiomal separation, different peaks or patterns on the chromatogram corresponds to different components of the separated mixture. It is a graph relating concentration of solute leaving a chromatographic column, against time.

HPLC stands for “High Performance Liquid Chromatograph”. Previously it stands for High Pressure Liquid Chromatograph.

The mobile phase carries in GC is a Gas whereas it is Liquid (Solvent/Water) in HPLC.

A Mass Spectrum is intensity Vs m/z (mass-to-charge ratio) plot representing a chemical analysis. Hence, the mass spectrum of a sample is a pattern representing the distribution of components (atoms or molecules) by mass (more correctly mass to charge ratio) in a sample. It is usually acquired using an instrument called a mass spectrometer.

Reversed phase HPLC (RPHPLC) consists of a non-polar stationary phase and a moderately polar mobile phase.

Normal Phase HPLC uses a polar stationary phase and a non-polar mobile phase, and is used when the analyte of interest is fairly polar in nature.

Methanol, Acetonitril etc. are the polar solvents.

Hexane, Dichloromethane, Benzene etc. are some non-polar solvents.

HPLC system with only one solvent pump is called isocratic HPLC.

HPLC system with two solvent pumps is called Binary HPLC.

A Flame Ionization Detector (FID) consists of a hydrogen (H2)/air flame and a collector plate. The effluent from the GC column passes through the flame, which breaks down organic molecules and produces ions. The ions are collected on a biased electrode and produce an electrical signal.

A Flame Ionization Detector (FID) consists of a hydrogen (H2)/air flame and a collector plate. The effluent from the GC column passes through the flame, which breaks down organic molecules and produces ions. The ions are collected on a biased electrode and produce an electrical signal.

The Flame Photometric Detector (FPD) allows sensitive and selective measurements of volatile sulphur and phosphorus compounds. The detection principle is the formation of excited sulphur (S2) and HPO species in a reducing flame. A photo-multiplier tube measure the characteristic chemiluminescent emission from these species. The optical fibre can be changed to allow the photo-multiplier to view light of 394 nm for sulphur measurement or 526 nm for phosphorus.

It’s ‘Electron Capture Detector’.

It’s ‘Nitrogen & Phosphorus Detector’.

m/z ratio denote the quantity formed by dividing the mass of an ion by the unified atomic mass unit and by its charge number (positive absolute value). This has been referred to as a mass-to-charge ratio, although it does not fit this description.

Ultra Violet (UV) detectors measure the ability of a sample to absorb light. This can be accomplished at one or several wavelengths.

Fluorescence detector measures the ability of a compound to absorb then re-emit light at given wavelength. Each compound has a characteristic fluorescence. The excitation source passes through the flow-cell to a photo detector while a mono-chromator measures the emission wavelengths.

Electrochemical detector measure compounds that undergo oxidation or reduction reaction. Usually accomplished by measuring gain or loss of electrons from migrating samples as they pass between electrodes at a given difference in electrical potential.

Retention Time (RT) is the characteristic time it takes for a particulars analyte to pass through the system (from the column in let to the detector) under set conditions.