Ch24. Gas Chromatography (GC)

Transcription

1 Ch24. Gas Chromatography (GC) 24.1 What did they eat in the year 1000? From 13 C content of cholesterol in ancient bone 13 C : 1.1%, 12 C: 98.9% 13 C/ 12 C ratio types of plants Bones of 50 people in Barton-on-Humber extracted CO 2 13 C/ 12 C ratio : 21~24 ppth diet of local plants 13 C/ 12 C ratio ~28ppth Separation Process in GC 24.2 MP: gas He, H 2, N 2 Sample : must be in GASEOUS state SP: Liquid GLC (partition) : nonvolatile liquid bonded to column wall Solid GSC (adsorption) To maintain gaseous

2 24-1. Separation Process in GC Open Tubular Column (OTC) :made with fused silica (SiO 2 ) WCOT: wall coated, 0.1~5μm film SCOT: support coated, solid particles PLOT: porous layer, large surface area Separation Process in GC 24.4 Column dimensions: 0.1~0.53 mm I.D., 15~100m length (30m common) Porous Layer

3 24-1. Separation Process in GC 24.5 Effect of Column diameter narrower the column, higher resolution higher operating pressure less sample capacity DB-1 (0.25 μm thick) -15m WCOT at 95 o C, He, 34cm/s Separation Process in GC 24.6 Effect of Column Length The longer the column, the higher resolution (Rs) DB-1 (0.25 μm thick) μm WCOT at 95 o C, He, 34cm/s

4 24-1. Separation Process in GC 24.7 Effect of stationary phase thickness on OTC performance d t r, sample cap, Rs Under the same const u x but good only for volatile samples DB-1, 0.32 mm-id,15m WCOT at 40 o C, He, 38cm/s Separation Process in GC 24.8 Choice of Liq St. Phase Like dissolves like For nonpolar solutes Nonpolar column -When column ages, SP bakes off SiOH exposed tailing increases - O2 exposure to column degradation

5 24-1. Separation Process in GC Packed Column fine particles of solid support coated with nonvolatile Liq SP solid particles itself higher sample cap. But broader peaks longer retention times less resolution Both types are packed in stainless steel column 3-6mm id, 1-5m long Used for prep sep Separation Process in GC Retention Index Relative retention times of polar and nonpolar solutes change as polarity of SP changes Nonpolar, PDMS Strongly polar poly(ethylene glycol) 1 μm thick

6 24-1. Separation Process in GC solute elutes with an increasing order of BP from nonpolar SP : when polar SP(24-9b) interaction between SP-sample H-bonding - dipole interaction - nonpolar alcohol ketone ankane Kovats Retention Index: I Octane: I=800 logt r '(unk) logtr '(n) I = 100 n + (N n) Nonane: I=900 logtr '(N) logtr '(n) N: larger alkane number, n: smaller alkane number Separation Process in GC Temperature and Pressure Programming Temp. programming : T raise during separation increase vaporization pressure decrease t r of late eluting comp. Pressure programming : increase inlet pressure u x tr : good for thermally unstable solute 1.6mmx6m packed

7 24-1. Separation Process in GC Carrier gas He: most common, compatible with most detectors N 2 : gives lower D.L. than He in FID because diffusion of solute at different, MP affects broadening Diffusion in MP: H 2 > He > N 2 Drawbacks of H 2 - react with catalytically with unsat. compound on metal surface - not good with MS since vac oil is destroyed by H2 - >4% vol forms explosive mixture in air 6. Guard column & Retention Gap Guard column : protect main column, made with same diameter, short length accumulate nonvolatile substabces 0.25mmx25m, OV101 WCOT Sample Injections sandwich injection tech SPLIT injection: routine for introducing small sample volume into OTC - usually analyte of interest constitute >0.1% of sample 1μL injection Only 0.2~2% of sample to the detector Split ratio 50:1 ~ 600:1

8 24-2. Sample Injections Splitless injection : best for trace levels of high BP solute in low boiling Solvents <0.01% of samples: large volume ~2μL in 2s solvent trapping: initial col. Temp below 40 o C the BP of solv. sample condenses at the beginning of column raise T cold trapping : 150 o C lower than BP of solute of interest All others (other solute & solv) evaporated but high BP solute remain at beginning of column Sample Injections Injections of a solution cont. 1 vol% MIBK(bp118oC) & 1 vol% p-xylene (bp138) in dichloromethane (bp40) on a BP-10 moderately polar Cyanopropyl phenyl methyl silicone OTC (0.22mmx10m, 0.25μm-thick) 3. On-column injection : best for thermally unstable solutes & high-boiling solv. injections on column directly no loss

9 24-3. Detectors Qualitative : MS & FT-IR peak can be compared with library spectra. - compare retention times or add standard of same comp. Quantitative : area of peak concentration - always needed to add internal stds. 1. Thermal Conductivity Detectors (TCD) most common, simple, universal not sensitive (no good for <0.53 mm id col) detect the heat transfer of a substance He (common carrier in TCD) : second highest T.C. (H 2 1 st ) when analyte passes detector (less He flows) T increases voltage increases sensitivity 1 flowrate, ΔT(bet. Filament & block) Detectors Flame Ionization Detector (FID) eluate burnt on H 2 & air : C CH radical CHO + ~ 1/10 5 C produces an ion response to organic compounds : ~ 7 orders of magnitude D.L. ~ 100 times smaller than TCD insensitive to nonhydrocarbons : H 2, He, N 2, O 2, CO, O 2, CO 2, H 2 O, NH 3, H 2 S,

10 24-3. Detectors Detectors Electron Capture Detector (ECD) sensitive to X-containing molecules, conjugated carbonyls, nitriles, insensitive to hydrocarbons, alcohols, ketones. carrier : N 2 or 5% CH 4 in air ionization by high E electrons (β-rays) from 63 Ni : analyte with high EA capture e - signal extremely sensitive ~ MS SIM mode

11 24-3. Detectors Other detectors 1) Nitrogen-Phosphorous detector (NPD) : modified FID - alkaline flame detector : N, P selective. Response 10 4 ~ 10 6 : greater than carbon, : drug, pesticide, herbicide : NO 2-, CN -, PO 2- produced when analyte contact Rb 2 SO 4- containing glass bead at burner tip create currents 2) Flame Photometric Detector (FPD) : optical emission from phosphorous, sulfur, lead : eluate pass FID, excited atom emit λ P=536nm, S=394nm Detectors FID for natural gas Sulfur chemiluminescence detector Organosulfur compds too dilute to be detected in FID.

12 24-3. Detectors GC-MS : If affordable, MS is the best SIM at m/z78 Lowest detection limit by 10 2 ~ Detectors Selected Reaction Monitoring (SRM) Extract from orange peel pesticide fensulfothion : precursor m/z 293 selected by Q1 264 Q3

13 24-3. Detectors CID of fensulfothion 24-4 Sample Preparation extracting analyte from a complex matrix pre-concentrating very dilute analyte removing or masking interfering species derivatizing analytes into detectable more on Ch.28 1) Solid phase microextraction : extract compounds from liq air without using any solvent Syringe for solid-phase microextraction.

14 24-4 Sample Preparation Solid-phase microextraction and desorption of analyte from the coated fiber Into a GC 24-4 Sample Preparation GC of Warfare Nerve Agents sampled by SPME for 30min. from seawater Spiked with 60 nl of each agent per liter (60ppb v/v) NPD detection (D.L. 0.5ppb) 0.32mm x 30 m with 0.1μm thick (Phenyl) 0.05 (methyl) 0.95 polysiloxane

15 24-4 Sample Preparation ) Purge and Trap : removing volatile analytes from liquids or solids & concentrating analytes & introducing into GC