Intensive General Chemistry

Chemical separations II

Isabelle Vu Trieu ilv2@columbia.eduhttp://ull.chemistry.uakron.edu

Gas versus Liquid chromatography

Phase 1

Phase 2

A A A

A

A

A

A

AAA

A

Partition constant: K = aA(stationary)/ aA(mobile)

Mobilephase

Stationaryphase

Gas or Liquid

Chromatography - Principles

Retention time (tr) : time required for a given component to emerge from the column

Retention volume (Vr) : volume of mobile phase required for a given component to emerge from the column

Vr = tr x F

Flow rate

A typical chromatogram

Retention time

Hold-up timeAdjusted retention time

Qualitative / quantitative analysis

Qualitative– “Blind” method

– Indicate the presence /absence of a substance (not what it is)

– Only one piece of information (Tr)

Quantitative– Analysis based on:

• Peak height

• Peak area

– Calibration & standards• External standard

• Internal standard

GC instrumentation

Carrier gas

Injection port

Columns

wide bore

Higher capacity Better resolution

capillaryOpen (coated)

Detectors

Thermal conductivity– General purpose

– Non destructive

– Detect the change in resistance of a wire based on variations of the thermoconductivity of the gas evolving from the column

Flame ionization– Specific (combustible

sample only)

– Destructive

– Measure the current due to the production of ions when burning the sample in a flame

HPLC equipment

Sample injector

Sampling Loop system:

Columns:

Normal phase– Stationary phase is

polar• Ex: silica, alumina

– Mobile phase is non-polar

• Ex: hexane

Reverse-phase– Stationary phase is

non-polar• Ex: carbon chain

bonded to silica

– Mobile phase is polar• Ex: water, methanol

Columns are packed, typically 10-20 cm long, 5mm id

Detector: