Gas Chromatography Quiz

**Peak area

(Peak width at half peak height)(peak height)

Useful temperature range - important characteristic of a liquid phase.

All liquid stationary phases evaporate or bleed if they are heated to high enough temperature Through trial and error

Capillary vs packed columns

Capillary - open tubular column, diameter 0.2-0.5mm, length 10-100m Packed column - interior diameter 2-4mm, length of 2-3m Capillary gen give better separation + less time Longer length —> more theoretical plates

Flow rates

Capillary column - 20 cm/s for He 10 cm/s for N Packed column - 20-30 ml/min He 3mm 60-70 ml/min 6mm

**GC column material, how chosen, and types, using which and not which ones

Chosen for sample type Polar column, non polar column Choose column most mimics properties of compounds analyzed Stationary phase is sorbet material coated onto *silica* gel Using DX-XXX NOT CARBOWAX

Acetone can be used to what?

Clean GC instruments

**GC carrier gas, common gasses

Common gases: h2, He, N2 Mobile phase Carries sample through column

How Gas chrom works

Components vaporize —> carried by carrier gas into column (where separation occurs) Compounds in mixture partition themselves btw gas phase and liquid phase in column —> an equilibrium that depends on temp, rate of gas flow + solubility of components

Retention time + factors it depends on

Compound always travels through GC column through fixed amount of time Distance from time of injection to time at which peak max occurs is retention time for compound Factors: compound's structure, stationary liquid phase, length of column, carrier gas flow rate, column temperature regimen, solid support, column diameter

**Gas chromatography is useful for:

Compound purity Determining # of components in a system Compound identity

Partition chromatography

Compounds being analyzed absorb on stationary phase (polymer w/ high bp)+ mobile phase (inert gas) doesn't interact with the compound Inert gas carries them down column

Video: Which column are we using and the temperature of instrument

DC200 + 140 C

**Wrong assumption in book

Detector will respond same way to each compound in mixture Not always true - verify by analyzing target compounds at same concentration and verify each peak

**Compounds separated by differences in._________

Differences in interactions with the stationary phase More time spent in mobile phase, the faster compound moves through column

What contributes to relative volatility? Liquid phase provides best if chemically _____

Dipole interactions, van deer walks forces, hydrogen bonding Liquid phase provides best separation if chemically similar to compounds being separated

**Time in stationary and mobile phase

Each ind component will have own equilibrium in phases Dependent on sample compounds affinity for stationary phase GC column allows for thousands of transitions - generally GC columns are meters long vs liquid chrom column 5-350mm long

Only ______ can be injected into gas chromotograph Injection of _____ can destroy effectiveness of GC column Proper amount of _____ is most important in obtaining useful gas chromotogram

Volatile organic compounds Organic or inorganic salts, acids, or bases Sample

Types of capillary columns + common packed columns

Wall coated open tubular column (WCOT) - liquid phase coats interior surface of the tube Support coated open tubular column (SCOT) - liquid phase coats thin layer of solid support that is bonded to the capillary wall Packed columns - porous, iner material that has a large surface area. Ex. Calcined diatomaceous earth Major component silica (siO2) Efficiency of separation increases with decreasing particle size —>increased surface area Increased pressure needed to push mobile phase with smaller particles

Gas Chromatography or Gas-liquid chromatography are used to

- Analyze volatile organic mixtures both qualitativvely and quantitatively - separate complex mixtures

Sources of poor resolution

- Overloading column - too concentrated or volume too large - pushing syringe too slowly -problem with instrument parameters

Sources of lack of peaks

- lack of peaks due to insufficient sample - injecting sample into wrong injection port - clogged syringe

Limitations of Gas chromatography

- only for small amounts of compounds - doesn't identify compounds unless known samples are available Coupling gas chromatograph with mass spectrometer combines separation with identification

Flame ionization detectors (FID)

-Highly sensitive detector system -Used with capillary columns -Compounds leaving column burned in hydrogen air flame -Combustion process produces ions that later the current output of the detector

What affects creation of gas phase

-greater compound's vapor pressure —> greater tendency to go from liquid stationary phase to mobile gas phase -more volatile, more time in gas phase -lower boiling pt, higher vapor pressure go through column and go faster than higher boiling compounds

Why are you seeing additional peaks?

-making a second injection before 1st is complete —> you can't stop the run -observing overlapping and repeating peaks -observing trace impurities

** steps to become a known standard (additional verification)

1) adding target compound to sample to obtain peak enhancement —> spiking Generally after initial analysis 2) spectroscopic means - mass spect A) create own spectral library - library created will match sample environment but takes a lot of time B) use spectral library equipment vendor charge you for - no time but will cost money + not perfect match

*ways Compounds identified by retention times under controlled conditions

1) compare target sample against known standard 2) small amount of target compound is added to sample to obtain PEAK ENHANCEMENT (spiking) 3) spectroscopic means like Mass spectrometry

Basic components of gas chromotograph

1) high pressure pure carrier gas 2) flow controller 3) heated injection port 4) column and column oven (what the sample's components are separated by) 5) detector (reduces electronic signals) 6) recording device or data station

** how does GC work

1) liquid injected + vaporized 2) sample loaded into column by carrier gas 3) select oven temp _ components of sample will spend diff time in stationary + mobile phase 4) first compound to come out is one that spent most time in mobile phase

**GC how to with amounts

1) micro syringe + draw 10 microL of sample 2) use DC-XXX column injection port (XXX is number) DONT USE CARBOWAX 3) hold syringe with one hand. With other hand put plunger btw thumb and pointer - prevent plunger from flying backwards after needle placed in injection port 4) count to 3 [1] put needle at injection port [2] pierce injection port septa with needle + push in until glass of syringe contacts injection port [3] push plunger all the way in + person at comp starts recording detector signal 5)after peak eluding, click button again to end recording + print

Steps for starting GC

1) push plunger with smooth rapid motion, withdraw syringe immediately 2) push start button 3) wait for peaks to appear

**GC parts:

1)Carrier gas 2)Injector - vaporizes sample, turning liquid to gas 3)Column oven - oven helps keep sample volatiles 4) Column - interacts w/ compounds injected to varying degrees & all compounds injected need to interact with stationary phase to get separation 5) Detector (FID/TCD/MS) 6) Computer software

Vid: gas pressure for GC instruent

20 psi

**different column types

Packed column - what we are using Capillary column - more accurate, smaller volume, more expensive, easier to destroy

Relative peak areas

Percentage of compound in a mixture is it's peak area divided by sum of all peak areas

Thermal conductivity detector

Principle: heat conducted away from a hot body at a rate that depends on composition of gas surrounding it Conduction through gas depends on rate at which gas molecules can diffuse to and form metal surface Larger organic molecules are less efficient heat conductors because they diffuse More slowly Change in electrical resistance creates imbalance in circuit that can be recorded **advantages: stability, simplicity, option of recovery of separated material Disadvantage: low sensitivity + can't use with capillary columns

Chromatography + what it plots

Recorded response of the detector's electrical signal as sample passes through it over time Plots intensity of detector response against time Longer compound remains on the column, the broader its peak will be when it passes through the detector

How do you compare compounds?

Retention time peak enhancement mass spectroscopy

Capillary columns stationary phase

Stationary phase - thin, uniform liquid film or thin layer solid Channel through center is left for passage of a carrier gas + molecules of sample

**GC retention time

Time it takes for peak to elite through column + be detected by detector Recording process initiated by signaling software to start recording at injection (use somewhere that allows for instrument control, automation, data processing) we are doing manually 1st peak or dip will be solvent front, unrestrained compounds/air moves at spread of element + not analyze peak

**GC utilizes ______ system and what are they called?

Two phase system Liquid (stationary phase) Gas (mobile phase)

**detector type decided? Which one do we use in this class?

Type decided by type of compounds Ewing analyzed + sensitivity/accuracy needed/ease of use Thermal conductivity detector (TCD used for this classs -current across hot wire changes as gas composition passing wire changes -gas with organic compounds in it will remove less heat form wire, causing wire's resistance to increase