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Using Nitrogen Instead of Helium As Carrier Gas • Using Nitrogen instead of Helium as carrier gas, while maintaining exact the same separation efficiency, retention times, peak elution order, without changing the oven temperature conditions. Jaap de Zeeuw Restek Corporation, Middelburg, The Netherlands Copyrights: Restek Corporation Considerations for using a certain carrier gas • Availability / delivery • Price Helium supply is under pressure • Purity • Speed of analysis • Type of detection system (PDD, MS..) • Sensitivity (TCD) Copyrights: Restek Corporation What are the alternatives? Nitrogen or Hydrogen “Slow” carrier gas “ Fast” carrier gas Cheap Cheap Can be generated Can be generated Safety If there is “enough” If there is “Just enough” resolution resolution Copyrights: Restek Corporation Both carrier gases are relatively Cheap Both gases can be generated using generators • Unlimited supply • Low cost • Relative small investment For use as carrier gas need to make sure the purity is very high; Consider Gas filtration Copyrights: Restek Corporation Van Deemter curve for different gases optimum gas velocities Van Deemter Plot 1.0 N2 Slow He 0.6 H2 FAST HETP (mm) HETP 0.2 10 20 30 40 50 60 70 Average Linear Velocity [cm/sec] Flow [mL/min] Copyrights: Restek Corporation Impact of optimum velocity on analysis time Copyrights: Restek Corporation 6 Operating Nitrogen at higher velocities Average linear velocity [cm/s] Using Nitrogen at the same linear velocity as Helium will result in an increase of HETP by approx. a factor 2 Copyrights: Restek Corporation Impact on Resolution A factor 2 lower efficiency means only a factor 1.4 lower resolution.. R = resolution Nth = theoretical Plate number α = selectivity Nth is linear with Column length K = retention factor Resolution is not.. Loss of efficiency by a factor 2 only results in a loss of 1.4 on resolution Copyrights: Restek Corporation Impact on Resolution A factor 2 lower efficiency means only a factor 1.4 lower resolution.. Loss of efficiency by a factor 2 only results in a loss of 1.4 on resolution Nth = 200.000 Nth = 100.000 Nth = 50.000 Copyrights: Restek Corporation Maintain the separation efficiency using N2 Using same column and nitrogen under OPTIMAL conditions (12 cm/s) Impact • 2 x slower then Helium, analysis time will be 2x longer • Peaks will be lower (sensitivity) • Need to change temperature program for same peak elution order Copyrights: Restek Corporation Copyrights: Restek Corporation 29 How to maintain the separation efficiency using N2.. Using a smaller diameter column Using the SAME carrier gas we know that: Replacing a 30m x 0.25mm for a 20m x 0.15mm, Analysis times are approximate 2x shorter.. Copyrights: Restek Corporation Using a 0.15mm ID instead of 0.25mm, under HELIUM 30m x 0.25mm Rxi-5Sil MS, 28 min df = 0.25 μm 0 10 20 20m x 0.15mm Rxi-5Sil MS, 15 min df = 0.15 μm 0 0 0 2 4 6 8 10 12 14 Tim e (m in) Copyrights: Restek Corporation How to maintain the separation efficiency using N2.. Using a smaller diameter column Replacing a 30m x 0.25mm for a 20m x 0.15mm Operate the 0.15mm at a higher velocity to get the SAME analysis time.. We know we will loose some “efficiency”.. Copyrights: Restek Corporation H2 Loss of efficiency using N2 a higher velocity He This is not as bad as expected N2 Under optimum, N2 already generated higher efficiency; Using smaller diameter columns: - The optimum is at higher velocity - The slope of the curve reduces Copyrights: Restek Corporation Calculating the impact Using method translator Helium Nitrogen 30/0.25 20/0.15 32 21.5 cm/s Can use the SAME oven program Same analysis time Copyrights: Restek Corporation Work done with Pro EZ-GC Modeler; Helium, 30m x 0.25mm Copyrights: Restek Corporation Work done with Pro EZ-GC Modeler; Nitrogen, 20m x 0.15mm Copyrights: Restek Corporation Overlay of 2 chromatograms… Copyrights: Restek Corporation The Practical test Copyrights: Restek Corporation Challenge • Take POLAR column : Stabilwax • Use COMPLEX mixture : Fragrances • Compare 30m x 0.25mm with 20m x 0.15mm Operate with same temperature programming using pre-selected flow / linear velocities Two situations: Efficiency and Speed optimized flow setting for He Copyrights: Restek Corporation Optimized Flows for He and N2 Efficiency Speed optimized Optimized 0.25mm He 1.4 mL/min 2.0 mL/min 0.15mm N2 0.27 mL/min 0.38 mL/min Actual Flows used to use the same program and obtain same analysis time 0.15mm N2 0.36 ml/min 0.52 mL/min Copyrights: Restek Corporation Efficiency optimized flow Copyrights: Restek Corporation Copyrights: Restek Corporation 30m x 0.25mm x 0.25µm Stabilwax He constant 1.4 mL/min 40°C (0.1 min), 6.4°C/min to 250°C Efficiency-optimized flow Men’s After Shave Optimal heating rate 6.81 min 31.23 min 20.2m x 0.15mm x 0.15µm Stabilwax N2 constant 0.36 mL/min 40°C (0.1 min), 6.4°C/min to 250°C Men’s After Shave 6.75 min 31.24 min Copyrights: Restek Corporation Detail of separation Copyrights: Restek Corporation Efficiency optimized flow Copyrights: Restek Corporation Copyrights: Restek Corporation Note the Pressures and flows used For Helium 15.27 psi 1.40 mL/min For Nitrogen 17.42 psi 0.36 mL/min Very small difference 4 times lower flow Copyrights: Restek Corporation Cost of helium and Nitrogen Helium $300 per cylinder Nitrogen $ 60 per cylinder Save 400% on carrier gas costs.. Copyrights: Restek Corporation Another example: Pesticides using an optimized selective phase Copyrights: Restek Corporation 20m x 0.15mm x 0.15µm Rtx-CLPesticides: Nitrogen 30m x 0.25mm x 0.25µm Rtx-CLPesticides: Helium Agilent 6890 GC-ECD with split/splitless inlet Restek Sky® Liner Split, Precision® with Wool, 4mm Organochlorine pesticides, 1 µL fast autosampler injection 250°C, split ratio 50 20m x 0.15mm x 0.15µm Rtx-CLPesticides Nitrogen, constant flow 0.41 mL/min Holdup time 1.37 min 150°C (0.1 min), 14°C/min to 320°C (0.76 min) 30m x 0.25mm x 0.25µm Rtx-CLPesticides Helium, constant flow 1.42 mL/min Holdup time 1.38 min 150°C (0.1 min), 14°C/min to 320°C (0.76 min) Copyrights: Restek Corporation Nitrogen Helium 0.15mm ID 0.25mm ID Efficiency- optimized flow ~ Matched holdup times Same oven program! Copyrights: Restek Corporation 20m x 0.15mm x 0.15µm Rtx-CLPesticides N2 constant 0.41 mL/min 150°C (0.1 min), 14°C/min to 320°C 3.65 min Organochlorine Pesticides Mix 10.58 min 30m x 0.25mm x 0.25µm Rtx-CLPesticides He constant 1.52 mL/min 150°C (0.1 min), 14°C/min to 320°C Organochlorine Pesticides Mix 3.63 min 10.59 min Copyrights: Restek Corporation 20m x 0.15mm x 0.15µm Rtx-CLPesticides N2 constant 0.41 mL/min 150°C (0.1 min), 14°C/min to 320°C 3.65 min Organochlorine Pesticides Mix 10.58 min 30m x 0.25mm x 0.25µm Rtx-CLPesticides He constant 1.52 mL/min 150°C (0.1 min), 14°C/min to 320°C Organochlorine Pesticides Mix 3.63 min 10.59 min Copyrights: Restek Corporation 20m x 0.15mm x 0.15µm Rtx-CLPesticides N2 constant 0.41 mL/min cis-Chlordane Heptachlor trans-Chlordane 4,4’-DDE epoxide Endosulfan I 6.55 7.05 min min 30m x 0.25mm x 0.25µm Rtx-CLPesticides He constant 1.52 mL/min 6.56 7.06 min min Copyrights: Restek Corporation Example 3: PAH on Rxi-17Sil MS Copyrights: Restek Corporation Medium polar phase: Rxi-17Sil MS GC-FID • EU 15+1 PAH Standard, 1 µL • Precision split liner with wool, 4 mm ID • 275°C, split ratios 50/250 for 0.25/0.15mm • GC oven program • 200°C (1.7 min) • 3.75°C/min to 260°C • 35°C/min to 350°C (10 min) • Flame Ionization Detector, 360°C Copyrights: Restek Corporation 30.2m x 0.25mm x 0.25µm Rxi-17Sil MS He at 1.4 mL/min constant flow Oven: 200°C (1.7 min), 3.75°C/min to 260°C, 35°C/min to 350°C (10 min) EU 15+1 PAH Standard 5-Methylchrysene Benzo[a]pyrene 19.80 min 21.38 min Benz[a]anthracene Benzo[ghi]perylene 18.57 23.87 Dibenzo[a,h]pyrene min min Benzo[c]fluorene 28.66 min 14.15 min Copyrights: Restek Corporation 20.0m x 0.15mm x 0.15µm Rxi-17Sil MS N2 at 0.32 mL/min constant flow Oven: 200°C (1.7 min), 3.75°C/min to 260°C, 35°C/min to 350°C (10 min) EU 15+1 PAH Standard 5-Methylchrysene 19.81 Benzo[a]pyrene min 21.39 min Benz[a]anthracene 18.61 Benzo[ghi]perylene min 23.95 Dibenzo[a,h]pyrene min Benzo[c]fluorene 28.93 min 14.22 min Copyrights: Restek Corporation 30.2m x 0.25mm x 0.25µm Rxi-17Sil MS He at 1.4 mL/min constant flow Oven: 200°C (1.7 min), 3.75°C/min to 260°C, 35°C/min to 350°C (10 min) [k] [j] Benzo fluoranthenes [b] 20.83 20.75 min min RS = 1.2 RS = 1.4 Copyrights: Restek Corporation 20.0m x 0.15mm x 0.15µm Rxi-17Sil MS N2 at 0.32 mL/min constant flow Oven: 200°C (1.7 min), 3.75°C/min to 260°C, 35°C/min to 350°C (10 min) [b] [j] Benzo fluoranthenes [k] 20.83 20.75 min min RS = 1.2 RS = 1.4 Copyrights: Restek Corporation Example 4: Dioxin analysis using Waters APGC T-QS comparing 30m/0.25 under He with 20m/0.15 under N2 • Splitless injections of 1.0µL with inlet temperature of 290°C and purge time of 2.8 minutes • GC column flow of 0.66mL/min for 2.8 minutes, reducing to 0.33 mL/min thereafter • Initial oven temperature of 130°C, hold for 2.8 minutes, ramps as follows: • Ramp 1: 25°C/min to 220°C, hold 0 minutes • Ramp 1: 4°C/min to 280°C, hold 0 minutes • Ramp 1: 14°C/min to 310°C, hold 4 minutes • Total time 27.5 minutes Copyrights: Restek Corporation 30m x 0.25mm Rxi-5Sil MS 20m x 0.15mm Rxi-5Sil MS HELIUM NITROGEN Copyrights: Restek Corporation Data generated by Rhys Jones, Waters UK Copyrights: Restek Corporation Robustness study Repeated injections of used motor oil, follow EPA 8081B Copyrights: Restek Corporation 4,4’-DDT Endrin DDT Breakdown Mix • Nitrogen carrier gas – 0.41 mL/min constant flow EPA Method • Split injection 8081B: – Split ratio 50 “If degradation – Inlet split flow 20.5 mL/min of either DDT or Endrin • Higher flow through inlet exceeds Endrin minimizes sample contact 15%..
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