25_GC - carrier gas sample introduction heated zones column...

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Unformatted text preview: carrier gas sample introduction heated zones column detector b a c d a - compressed gas cylinder b - pressure regulator c - valve d - filter data system septum Purpose of port is to flash evaporate your sample and introduce it into the column. carrier in TINJ > 50oC above Tcolumn heat source Injection is through a septum. liner column Septum must be stable at the Tinj 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 Removable needle Several needle lengths and angles Sample volumes from < 1 µl an up Body loading Through the barrel plungers When filling, you must insure that there is no air in the syringe. 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 be difficult with a syringe. sample sample vent vent Gas sampling loops and valves offer a high precision (+/- 0.1%) means of introducing gases. Equipment is relative inexpensive and only requires a constant temperature for easy 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. column carrier carrier Packed Columns porous packing open (capillary) Open tubular (capillary) Packed non-porous packing Packed with porous layer liquid coated coated with porous layer packed capillary liquid coated wall Packed length, M bound phase Capillary 0.5 - 5 5 - 100 2-4 0.1 - 0.7 flow, ml/min 10 - 60 10 - 40 4000 capacity 10µg/peak 100ng/peak film thickness, µm 1 - 10 ID, mm 250,000 Porous Layer Open Tube Wall Coated Open Tube conventional Because peaks remain narrower, the sensitivity is improved. 3 - 40 total plates porous layer 0.5 - 15 head pressure, psig bead column Packed Both peaks have an area of 5000 units. Capillary Because the capillary peak is higher, you get a better S/N. 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.) 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) (b) (a) a - initial temperature and time b - ramp (oC/min) c - final hold time and temperature Some GCs will allow for a more complex program. General steps to create a program assuming that the separation is possible. 1. Determine initial temperature and time based on best possible separation of first first few peaks. 2. Repeat 1 for the last few peaks to find the best final temperature and time. 3. Experiment with various ramps to account for the rest of the components. We need a way to measure our eluents as they evolve from the column. Each can be roughly classified based on Destructive vs. nondestructive Virtually every method of directly or indirectly observing eluents as been looked at. We’ll cover some of the more common types. General vs. some discrimination vs. very discriminating Let’s start by reviewing some general concepts such as detection limit and sensitivity. AED High sensitivity - possible selectivity TCD Rapidly respond to concentration changes FID ECD NPD (N) NPD (P) FPD (S) FPD (P) Large linear range Stable with respect to noise and drift Low sensitivity to variations in flow, pressure and temperature Produces an easily handled signal MS (SIM) 10-15 g General purpose Nondestructive Limit of detection Linear range ~ 400 pg/ml carrier ~ 106 Mode of detection Change in resistance of a wire based on variations in the thermoconductivity of the gas evolving from a column. 10-12 g Species hydrogen helium nitrogen ethane water benzene acetone chloroform FTIR MS (Scan) 10-9 g 10-6 g Thermal conductivity 105 cal/cm sec oC 49.93 39.85 7.18 7.67 5.51 4.14 3.96 2.33 10-3 g carrier reference While hydrogen has the largest TC value, helium is commonly used - less reactive. Hydrogen will give a negative peak when helium is the carrier gas. Peak response is a function the the TC value for a species so you must standardize for each eluent of interest. zero control carrier + sample power supply 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 combustible Destructive Limit of detection ~ 5 pg carbon / second Linear range ~ 107 Mode of detection Production of ions in a flame result in a current that can be measured. A make-up gas may be required to maintain an optimum flow - capillary columns Sample components enter at the base of the detector. They mix with hydrogen and enter the flame. Ions are produced that can be measured. collector igniter Compounds with little or no FID response air noble gases No x H2O burner jet hydrogen A make-up gas may also be present if capillary columns are to be used. End of column butane propane ethane methane pentanoic acid butanoic acid propionic acid acetic acid formic acid Response is based on the number of carbon and if other elements like halogens or oxygen are present which reduce combustion. !- are emitted by an 63Ni source. Electrophores will absorb !- , reducing the current. This is the basis for the response NH3 CO CO2 CS2 O2 N2 perhalogenated compounds formic acid formaldehyde Specific - sample must contain a gas phase electrophore Non-destructive Limit of detection ~ 0.1 pg Cl / second Linear range ~ 104 Mode of detection Absorption of ! particles by species containing halogens, nitriles, nitrates, conjugated double bonds, organometallics. anode purge vent anode 63Ni plated surface makeup gas column Relative responses 100 hydrocabons 101 esters, ethers 102 alcohols, ketones, monochlorides, amines 103 monobromides, dichlorides 104 anhydrides, trichlorides 105 - polyhalogenated, mono and diiodo 1 06 ...
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This note was uploaded on 12/06/2011 for the course CHEM 300 taught by Professor Jameshardy during the Fall '10 term at The University of Akron.

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