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25 gc Document Transcript

  • 1. carrier sample gas introduction heated zones column detector data system b c d a - compressed gas cylinder b - pressure regulatora c - valve d - filter
  • 2. Purpose of port is to flash evaporate your sample septum and introduce it into the column. carrier in TINJ > 50oC above Tcolumn heat source Injection is through a septum. liner Septum must be stable at the Tinj column replaced regularly to maintain seal Syringes are used to introduce a known volume of a liquid or gas samples . Adapters can be used to help control the volume injected.Various styles are available Fixed needle When filling, you must Removable needle insure that there is no Several needle lengths and angles air in the syringe. Sample volumes from < 1 µl an up Body loading Through the barrel plungers
  • 3. Samples must be injected rapidly so that your sample is introduced as a small ‘plug.’ By pulling back on the syringe after injection, you can measure how much sample remains in the needle.Introducing a constant amount of a gas can sample sample be difficult with a syringe. vent ventGas sampling loops and valves offer a high precision (+/- 0.1%) means of introducing gases. columnEquipment is relative inexpensive and only requires a constant temperature for easy carrier carrier use.•  Heart of the separation process.•  Vast number of materials have been evaluated.•  It is usually best to refer to various catalogs as an up to date reference.•  Can be classified by tubing diameter and packing type.
  • 4. Columns Packed open (capillary) Packed Open tubular (capillary) porous non-porous packing packing Packed with coated with porous layer porous layer bead column Porous Wall liquid bound Layer Coated coated phase porous layer Open Open packed liquid coated Tube Tube capillary wall conventional Packed Capillary Because peaks remain narrower, the sensitivitylength, M 0.5 - 5 5 - 100 is improved.ID, mm 2-4 0.1 - 0.7 Packed Both peaks have an areaflow, ml/min 10 - 60 0.5 - 15 of 5000 units.head pressure, psig 10 - 40 3 - 40 Because the capillarytotal plates 4000 250,000 Capillary peak is higher, you getcapacity 10µg/peak 100ng/peak a better S/N.film thickness, µm 1 - 10 0.1 - 8 Unless you’re developing new packing materials or methods, the best starting point is to consult a chromatographic catalog. They provide a wealth of information regarding cost, temperature limits, sample applications. Another factor to consider, you must use the proper column called for by the ‘standard’ method (Ex. A specific EPA method.)
  • 5. With homologues, the retention time increases exponentially with the number of carbon. As tR increases, width increases and the height decreases, making detection impossible after a few peaks have eluted. Since solubility of a gas in a liquid decreases as temperature goes up, we can reduce the retention of a material by increasing Tcolumn. isothermal programmed temperature (c) General steps to create a program assuming that the separation is possible. (b) 1. Determine initial temperature and time based (a) on best possible separation of first first few peaks.a - initial temperature and timeb - ramp (oC/min) 2. Repeat 1 for the last few peaks to find the bestc - final hold time and temperature final temperature and time.Some GCs will allow for a more complex program. 3. Experiment with various ramps to account for the rest of the components.
  • 6. We need a way to measure our eluents as Each can be roughly classified based on they evolve from the column. Destructive vs. nondestructiveVirtually every method of directly or General vs. some discrimination indirectly observing eluents as been vs. very discriminating looked at. Let’s start by reviewing some general conceptsWe’ll cover some of the more common such as detection limit and sensitivity. types. AED High sensitivity - possible selectivity TCDRapidly respond to concentration changes FID ECD Large linear range NPD (N) Stable with respect to noise and drift NPD (P) FPD (S) Low sensitivity to variations in flow, FPD (P) pressure and temperature FTIR Produces an easily handled signal MS (SIM) MS (Scan) 10-15 g 10-12 g 10-9 g 10-6 g 10-3 gGeneral purpose Thermal conductivityNondestructive Species 105 cal/cm sec oCLimit of detection ~ 400 pg/ml carrier hydrogen 49.93Linear range ~ 106 helium 39.85 nitrogen 7.18Mode of detection ethane 7.67 Change in resistance of a wire based on water 5.51 variations in the thermoconductivity of the gas benzene 4.14 evolving from a column. acetone 3.96 chloroform 2.33
  • 7. carrierWhile hydrogen has the largest TC value, reference zero helium is commonly used - less reactive. carrier + control sampleHydrogen will give a negative peak when helium is the carrier gas. power supplyPeak response is a function the the TC value for a species so you must standardize for each eluent of interest. detector block recorder Dual channel detectors require both an analytical column and a blank column. - accounts for response changes due to - variations in temperature - column bleed Single channel TCD systems are available that correct for temperature variations.Specific - sample must be combustibleDestructive Sample componentsLimit of detection ~ 5 pg carbon / second enter at the base ofLinear range ~ 107 the detector. TheyMode of detection mix with hydrogen and enter the flame. Production of ions in a flame result in a current that can be measured. Ions are producedA make-up gas may be required to maintain an that can be measured. optimum flow - capillary columns
  • 8. collector igniter Compounds with little or no FID response air noble gases NH3 CS2 Nox CO O2 burner jet H2O CO2 N2 hydrogen A make-up gas may perhalogenated compounds also be present if formic acid capillary columns formaldehyde are to be used. End of columnResponse is based on the number of carbon Specific - sample must contain a gas phaseand if other elements like halogens or electrophoreoxygen are present which reduce Non-destructivecombustion. Limit of detection ~ 0.1 pg Cl / second Linear range ~ 104 methane ethane butane pentanoic acid propane butanoic acidformic acid acetic acid propionic acid Mode of detection Absorption of ! particles by species containing halogens, nitriles, nitrates, conjugated double bonds, organometallics. !- are emitted by an 63Ni source. anode purge vent Electrophores anode will absorb !- , 63Ni plated surface reducing the current. This is the basis makeup gas for the response column
  • 9. Relative responses100 hydrocabons101 esters, ethers102 alcohols, ketones, monochlorides, amines103 monobromides, dichlorides104 anhydrides, trichlorides105 - polyhalogenated, mono and diiodo106