Gas Chromatography b General Design of a Gas Chromatograph b Separation Processes in Gas Chromatography b GC Columns b GC Injectors b GC Detectors

General Design of a Gas Chromatograph

Some of the design “details” b Gas supplies usually have either in-line or instrument mounted traps to remove any water, oxygen, hydrocarbons or other “contaminants” from compressed gases b Gas flows can be controlled using either needle valves or mass-flow controllers (electronic sensors) b Instruments can have multiple injectors, detectors or columns b Injectors and detectors usually have their own temperature controlled zones (small heaters) b The GC oven has a large fan and a vent door to help with rapid cooling of the oven b Data collection (and integration) can be done using a chart recorder, integrator or a computerized data system

Separation Processes in GC b Gas Chromatography as it is usually performed is correctly called gas-liquid chromatography the analyte is in the gas phase in the GC and partitions between the mobile phase (carrier gas) and the liquid stationary phase that is coated on the inside of an open-tubular capillary column or on particles inside a packed columnthe analyte is in the gas phase in the GC and partitions between the mobile phase (carrier gas) and the liquid stationary phase that is coated on the inside of an open-tubular capillary column or on particles inside a packed column b Some packed-column GC uses non-coated solid stationary phases, in which case one is performing gas-solid adsorption chromatography b Capillary, open-tubular column GC is the primary type of GC used in quantitative analysis: higher resolution = greater ability to discriminate between componentshigher resolution = greater ability to discriminate between components smaller capacity of the column is not important as long as sufficient analyte is available for detectionsmaller capacity of the column is not important as long as sufficient analyte is available for detection pg/mL (ppt) to  g/mL (ppm) concentration range for liquid analytespg/mL (ppt) to  g/mL (ppm) concentration range for liquid analytes

The Objective in Chromatography (all types) b Separate your analytes (resolution of 1.5 or better) in the shortest amount of time possible and detect them…. b How can we do this in GC? Use different columns for different analyte typesUse different columns for different analyte types –stationary phase –diameter of column, stationary phase thickness –column length Use different injection types/temperatures to optimize the process of loading the sample on the columnUse different injection types/temperatures to optimize the process of loading the sample on the column Use different temperature (or pressure) programs for the columnUse different temperature (or pressure) programs for the column Select and use a detector that is suitable for the analyte(s) of interestSelect and use a detector that is suitable for the analyte(s) of interest

GC Columns (concentrating on open-tubular capillary columns) b Column “frame” constructed of fused silica tubing b Polyamide coating on the outside gives it strength b Liquid stationary phases coated or bonded to the inside of the tubing b mm + ID, metes in length, stationary phases usually 0.10 to 1.5  m in thickness b Mounted on a wire cage to make them easier to handle

Capillary Column Stationary Phases

Choosing a GC Column… b Is the column compatible with your analytes polar analytes require polar stationary phases so they will spend some of their “time” in the stationary phasepolar analytes require polar stationary phases so they will spend some of their “time” in the stationary phase non-polar analytes require non-polar stationary phasesnon-polar analytes require non-polar stationary phases You usually have to compromise on the stationary phase to get a good column for your analytes (which are probably a mix of polar and non-polar)You usually have to compromise on the stationary phase to get a good column for your analytes (which are probably a mix of polar and non-polar) DB-5, HP-5, EC-5, RTX-5 (95% dimethyl, 5% diphenyl polysiloxane) most common general use column.DB-5, HP-5, EC-5, RTX-5 (95% dimethyl, 5% diphenyl polysiloxane) most common general use column. b Temperature range, solvent and carrier gas compatibility b Sample capacity versus resolution usually determines packed vs.. capillaryusually determines packed vs.. capillary GC’s usually setup for either packed or capillaryGC’s usually setup for either packed or capillary b Let’s say you choose a capillary column, there’s more to think about!

For capillary GC columns…. b Increased length = greater N, therefore a greater R expense is possible band broadening if analytes are on the column too long!expense is possible band broadening if analytes are on the column too long! Increased length leads to longer separations. Do you have the time?Increased length leads to longer separations. Do you have the time? b Increased stationary phase thickness and column diameter provides increased sample capacity and can provide increased resolution tradeoffs are a longer analysis time and more column bleed with thicker stationary phasestradeoffs are a longer analysis time and more column bleed with thicker stationary phases b Is the column compatible with the detector? Thick stationary phases bleed more and will contaminate a mass spectrometer.Thick stationary phases bleed more and will contaminate a mass spectrometer. b For most analytical work, a best “compromise” column is chosen and other variables (temp, etc.) are altered to optimize the separation.

Capillary vs. Packed Columns b Capillary Columns: Higher resolution (R)Higher resolution (R) Greater N and lower HETPGreater N and lower HETP Shorter analysis timeShorter analysis time Greater sensitivityGreater sensitivity Most common in analytical laboratory GC instrumentsMost common in analytical laboratory GC instruments Smaller sample capacitySmaller sample capacity Higher cost/columnHigher cost/column Columns more susceptible to damageColumns more susceptible to damage b Packed Columns Greater sample capacity Lower cost (can make your own) More rugged Most common in process labs or separating/determining major components in a sample (prep GC) Limited lengths helps limit R and N Not compatible with some GC detectors

Temperature Programming in GC b The “simplest” way to alter the separation in GC is to alter the temperature program in the oven. You can also alter the pressure of the carrier gas, but this is less common (much). b Isothermal = constant temperature b Gradient = varied temperature b By altering the temperature, you vary the rate of the reaction for any analyte: they spend more or less time in the stationary phasethey spend more or less time in the stationary phase the greater the difference in the times between analytes, the better the separation!the greater the difference in the times between analytes, the better the separation! Increased differences in the capacity factor (k’)Increased differences in the capacity factor (k’)

b The traps of temperature… If your temperature at a given time is too high, you can cause the peaks to co-eluteIf your temperature at a given time is too high, you can cause the peaks to co-elute –poor resolution vs but a faster separation If your temperature at a given time is too low, you can get still get a good separationIf your temperature at a given time is too low, you can get still get a good separation –adequate resolution, but a separation that takes very long You have to choose a compromise temperature or programYou have to choose a compromise temperature or program

GC Carrier Gases (the mobile phase) b Usually “inert” gases (don’t react with analytes) b Purpose: sweep sample through the columnsweep sample through the column protect column from oxygen exposure at temperatureprotect column from oxygen exposure at temperature assist with function of the detectorassist with function of the detector b Most common: Helium (available relatively pure without extensive purification after it leaves a compressed gas cylinder)Helium (available relatively pure without extensive purification after it leaves a compressed gas cylinder) Nitrogen (usually requires an oxygen and water trap)Nitrogen (usually requires an oxygen and water trap) HydrogenHydrogen –normally used only with flame ionization detectors (FID) since the FID needs it as fuel for the flame –still rarely used due to safety concerns (and chromatographic ones)

GC Injection…. b Samples are injected through a septum: keeps oxygen out of the columnkeeps oxygen out of the column provides a seal to keep the carrier gas pressure up at the head of the columnprovides a seal to keep the carrier gas pressure up at the head of the column –carrier gas flow rate is determined by the pressure or the gas at the opening of the column Many different (mostly proprietary) materialsMany different (mostly proprietary) materials –red rubber (bleeds at about 250 C) –Thermogreen (good up to about 300 C) –High-temperature blue (good a little over 300 C) b The injector is usually lined with a de-activated glass liner prevents metal injector-sample reactions that would alter analytes or damage the metal of the injectorprevents metal injector-sample reactions that would alter analytes or damage the metal of the injector Can be cleaned/replaced regularlyCan be cleaned/replaced regularly

Injection types

b On-Column Injection: used widely in packed-column GC, less in capillary GCused widely in packed-column GC, less in capillary GC sample is deposited directly on the columnsample is deposited directly on the column –Good for thermally unstable compounds –Good for quantitative analysis at low concentrations –all sample is available to travel to the detector –BUT, you can inject only a relatively small amount of sample in capillary GC b Splitless Injection: Sample is vaporized in the injector itself and ALL of the sample is swept onto the column by the carrier gasSample is vaporized in the injector itself and ALL of the sample is swept onto the column by the carrier gas Again, relatively small samples are injected (10  L or less in capillary GC)Again, relatively small samples are injected (10  L or less in capillary GC) Sample spends a large amount of time in the injectorSample spends a large amount of time in the injector Best for trace ( ppm range) concentrations of high boiling point analytes in low boiling point solventsBest for trace ( ppm range) concentrations of high boiling point analytes in low boiling point solvents –extra time in the injector helps volatilize the analytes.

b Split Injection: the injection is split, with only a portion of the sample (usually 1% - 20%) actually making it to the columnthe injection is split, with only a portion of the sample (usually 1% - 20%) actually making it to the column the most common method of injecting samples onto small diameter, open-tubular columns.the most common method of injecting samples onto small diameter, open-tubular columns. –Even if you inject 20  L, only a fraction (adjustable) makes it on to the column Not good for analytes with a wide range of boiling pointsNot good for analytes with a wide range of boiling points –some may be swept out the split vent before they are volatilized b Modern capillary GCs come with a Split/Splitless injectors standard you switch between modes by changing the split vent gas flow and using a different injection liner.you switch between modes by changing the split vent gas flow and using a different injection liner.

GC Detectors b A dozen or more varieties (some obscure) b Must be: sensitive to the analytes of interestsensitive to the analytes of interest compatible with the column, carrier gas, solvent, etc.compatible with the column, carrier gas, solvent, etc. rugged enough to withstand general unattended usedrugged enough to withstand general unattended used –I’ve run our new GC for 36 hours straight without touching it! b Should have a known linear range if the detector response is very linear, you can use a response factor instead of a calibration curve for quantitation!if the detector response is very linear, you can use a response factor instead of a calibration curve for quantitation! b Usually require separate gas supplies (other than the carrier gas), have their own temperature control. b Measure nothing more than a voltage or a current.

FID FPD

Thermal Conductivity (TCD) b The carrier gas has a known thermal conductivity. b As the thermal conductivity of the column eluent (gas flow in) changes, the resistance of the filament changes. b The presence of analyte molecules in the carrier gas alter the thermal conductivity of the gas (usually He) b There is normally a second filament to act as a reference (the carrier gas is split) b Increased sensitivity with decreasing temperature (detector), flow rate and applied current. b Filaments will burn out (oxidized) in the presence of oxygen if hot! Non-destructive

FID b Destructive, sample lost. b Analytes containing C burn in a hydrogen-oxygen flame and produce ions b CHO + ions are collected on a cathode and the current they produce results in the signal b WILL NOT detect non-C containing compounds! b Requires H 2 supply (tank or generator) and O 2 supply (compressed air) b H 2 carrier gas can be used, eliminating the need for a supply for the detector b A makeup gas can also be required!

ECD b Particularly sensitive to halogens nitriles, carbonyls, nitro compounds b Analytes pass through a cell, in which electrons are traveling between a 63Ni electrode and a collector electrode b As analytes with “electron capturing ability” pass through the cell, the flow of electrons is interrupted. b The change in current, due to reduced flow of electrons, is recorded. b EXTREMELY SENSITIVE TO HALOGENS could ruin detector with 1 ppm hexachlorocyclohexane by contaminated it with excess analytecould ruin detector with 1 ppm hexachlorocyclohexane by contaminated it with excess analyte b Widely used for the determination of pesticides, herbicides and PCBs in environmental samples. b Non-destructive