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Converting GC Methods from Helium to Nitrogen Carrier

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Converting GC Methods from Helium to Nitrogen Carrier Converting Helium Jim McCurry Carrier Gas GC Agilent Technologies Methods Nitrogen and Wilmington, DE 18951 USA Hydrogen 1 October 12, 2012 Market Situation . The world of He supply is not reliable, prices are increasing and customers are seeking alternative carrier gases Agilent Restricted Page2 2 October 12, 2012 The Carrier Gas Decision No Is the chemist willing to convert Yes to alternative gasses? GC Is the Application based on GC/MS GC or GC/MS? Does the current GC method No have too much resolution? Yes He Conservation Consider migration to N2 Consider migration to H2 GC/MS specific H2 considerations Agilent Restricted Page3 3 October 12, 2012 Migration to H2: Considerations for GC & GC/MS No Is the chemist willing to convert Yes to alternative gasses? GC Is the Application based on GC/MS GC or GC/MS? Does the current GC method No have too much resolution? Yes He Conservation Consider migration to N2 Consider migration to H2 GC/MS specific H2 considerations Agilent Restricted Page4 4 October 12, 2012 H2: Considerations for GC & GC/MS Application H2 safety Sources of hydrogen Plumbing modifications Chromatographic method migration Method revalidation Agilent Restricted Page5 5 October 12, 2012 H2 Safety GC, GC/MS: Both offer H enabled features 2 • Agilent H2 safety letter and safety manuals available • GC four levels of safety design • Safety Shutdown • Flow Limiting Frit • Oven ON/OFF Sequence • Explosion Test • Newer version 6890, 7890 GC and 5773, 5975 MSD offer greater safety than older versions of GC and GC/MSD. Agilent Restricted Page6 6 October 12, 2012 Source of Hydrogen H2 Generator – preferred • Very clean H2, >99.9999% available • More consistent purity • Built-in safety considerations • Make sure to buy a good one with a low spec for water and oxygen • Parker’s H-MD are used in LFS and SCS H2 Cylinder • Consider gas clean filter • Possible to add safety device Agilent Restricted Page7 7 October 12, 2012 H2: Plumbing Modification Considerations Tubing: • Use chromatographic quality stainless steel tubing • Do not use old tuning (H2 is known as scrubbing agent) • Especially don’t use old copper tubing (brittleness is a safety concern) Venting: • Connect split vent and septum purge vent to exhaust Leak checking: • Recommend G3388B leak detector Agilent Restricted Page8 8 October 12, 2012 Migration to N2 for GC Application No Is the chemist willing to convert Yes to alternative gasses? GC Is the Application based on GC/MS GC or GC/MS? Does the current GC method No have too much resolution? Yes He Conservation Consider migration to N2 Consider migration to H2 GC/MS specific H2 considerations Agilent Restricted Page9 9 October 12, 2012 Use N2 as carrier gas Many HPI methods suited to nitrogen • Readily available and less expensive gas • No safety concern • Suitable for simple routine analysis (w. enough resolution) • 2-D GC ideally suited to nitrogen – Resolution issues solved by using 2 different columns Potential issues • Reduced separation resolution • Not suitable for GC/MSD and certain GC detector applications Agilent Restricted Page10 10 October 12, 2012 Helium Carrier Gas Alternatives • Hydrogen, nitrogen, argon, argon/methane • Detector considerations can influence choice • Hydrogen and nitrogen are the most common alternatives 11 October 12, 2012 Helium Carrier Gas Alternatives Important Theoretical Considerations Relating To Peak Efficiency Calculating Efficiency Efficiency & Carrier Gas Linear Velocity We would like to know the actual time the Efficiency is a function of the component spends in the stationary carrier gas linear velocity or flow B Solute phase. HETP = A + Cµ rate. µ t ' 2 + Start t ' = t - R R R t m n = The minimum of the curve 5.545 W h represents the smallest HETP (or Inert Gas largest plates per meter) and thus tR = Corrected Retention Time. t m t ' ' the best efficiency. "A" term is not R Time HETP t MOLECULAR present for capillary columns. R DIFFUSION B{ } C RESISTANCE TO MASS TRANSFER Let's relate “n” to the length of the column. n = effective theoretical n plates. Plates per meter (N) = or } A EDDY DIFFUSION L L Height to a theoretical plate (HETP) = µ ( opt equivalent n µ) Thus, the more efficient the column, the bigger the "N" the smaller the • Plot of HETP versus linear velocity is know as the Van Deemter "HETP“. plot. • The linear velocity value at the minimum of the curve is the optimum value for achieving the best efficiency. 12 Helium Carrier Gas Alternatives Let’s Make This Easy Calculating Efficiency Efficiency & Carrier Gas Linear Velocity We would like to know the actual time the Efficiency is a function of the component spends in the stationary carrier gas linear velocity or flow B Solute phase. HETP = A + Cµ rate. µ t ' 2 + Start t ' = t - R R R t m n = The minimum of the curve 5.545 W h represents the smallest HETP (or Inert Gas largest plates per meter) and thus tR = Corrected Retention Time. t m t ' ' the best efficiency. "A" term is not R Time HETP t MOLECULAR present for capillary columns. R DIFFUSION B{ } C RESISTANCE TO MASS TRANSFER Let's relate “n” to the length of the column. n = effective theoretical n plates. Plates per meter (N) = or } A EDDY DIFFUSION L L Height to a theoretical plate (HETP) = µ ( opt equivalent n µ) Thus, the more efficient the column, the bigger the "N" the smaller the • Plot of HETP versus linear velocity is know as the Van Deemter "HETP“. plot. • The linear velocity value at the minimum of the curve is the optimum value for achieving the best efficiency. 13 Helium Carrier Gas Alternatives Let’s Make This Easy • Goal: change carrier gas while keeping other method conditions the same – use the same column – use the same oven program – adjust column flow or holdup time velocity to: • same peak elution order • same peak retention times • For N2, test resolution of key components • adjust GC conditions (temp, flow) if needed • Easier method revalidation – minimal changes to timed integration events – minimal changes to peak identification table • Use tools built into the Agilent Chemstation to guide us through the process 14 October 12, 2012 The Tyranny of Van Deemter Why Nitrogen Gets a Bad Rap for Capillary GC N HETP (mm) 2 C17 at 175º C k' = 4.95 Nitrogen Helium Hydrogen 1.2 OV-101 58 cm/s 58 cm/s 58 cm/s 2.4 mL/min 2.5 mL/min 1.7 mL/min 25m x 0.25 mm x 0.4µ 1.0 .8 He .6 H .4 2 .2 10 20 30 40 50 60 70 80 90 Average Linear Velocity (cm/sec) R = 1.17 R = 1.37 • N2 actually provides the best efficiency, but at a slower speed • Most helium methods have too much resolution – Lower N2 efficiency at higher flows can still provide “good enough” resolution • Most GC methods now use constant flow – N2 efficiency losses with temp programming are not as severe 15 October 12, 2012 Helium Carrier Gas Alternative Test Case: ASTM D6584 for Free and Total Glycerin in Biodiesel Istd 2 1. Glycerol Istd 1 2. Monoglycerides 3. Diglycerides 2 4. triglycerides 3 1 4 5 10 15 20 25 30 35 COC Inlet: Oven Track Mode Pre-column: Ultimetal 2m x 0.53mm ID Column: Ultimetal DB5HT, 15m x 0.32mm ID x 0.1 df Column Flow: Helium at 3.0 mL/min (50 deg C) Column Pressure: 7.63 psi constant pressure mode Initial Column Temp: 50 oC for 1 min. Oven Ramp 1: 15 oC/min to 180 oC Oven Ramp 2: 7 oC/min to 230 oC Oven Ramp 3: 30 oC/min to 380 oC, hold 10 min. Detector: FID with 25 mL/min N2 makeup 16 Configure Inlet for Carrier Gas in Chemstation 17 Set the Control Mode: Flow or Holdup Time 18 October 12, 2012 Wider Retention Time Variation Using the Same Flow 23.818 min Helium Flow: 3.00 mL/min P: 7.63 psi Tr: 0.472 min. µ: 52.97 cm/s 23.469 min Hydrogen Flow: 3.00 mL/min P: 3.85 psi Tr: 0.420 min. µ: 59.50 cm/s 23.705 min Nitrogen Flow: 3.00 mL/min P: 7.09 psi Tr: 0.464 min. µ: 53.84 cm/s 18 19 20 21 22 23 24 19 October 12, 2012 Same Holdup Time (Tr) Gives Consistent Retention Times 23.818 min Helium Flow: 3.00 mL/min P: 7.63 psi Tr: 0.472 min. µ: 52.97 cm/s 23.862 min Hydrogen Flow: 2.64 mL/min P: 3.43 psi Tr: 0.472 min. µ: 52.97 cm/s 23.776 min Nitrogen Flow: 2.94 mL/min P: 6.98 psi Tr: 0.472 min. µ: 52.97 cm/s 18 19 20 21 22 23 24 20 October 12, 2012 Monoglyceride Resolution “Good Enough” Using Nitrogen Carrier Mono2 area: 1049 Helium Flow: 3.00 mL/min Mono1 Mono3 Tr: 0.472 min. Mono2 area: 1050 Hydrogen Flow: 2.64 mL/min Mono1 Mono3 Tr: 0.472 min. Mono2 area: 1049 Nitrogen Flow: 2.94 mL/min Mono1 Mono3 Tr: 0.472 min. 16.5 17 17.5 18 18.5 19 19.5 21 October 12, 2012 ASTM D6584 - Quantitative Results For Alternative Carrier Gas Weight Percent Helium Hydrogen Nitrogen Glycerin 0.015 0.014 0.013 Monoglycerides 0.226 0.216 0.223 Diglycerides 0.114 0.115 0.110 Triglycerides 0.071 0.085 0.098 Total Glycerin 0.097 0.095 0.098 22 October 12, 2012 Analysis of Oxygenates and Aromatics in Gasoline Using 2-D Gas Chromatography ASTM Method D4815 – Oxygenated Additives – Ethers and alcohols from 0.1 wt% to 15 wt% – Usually only one or two additives in a sample ASTM Method D5580 – Aromatics in Gasoline – Measures benzene, toluene, C8 aromatics and C9 plus aromatics – Two injections per sample for complete analysis Preliminary separation removes light hydrocarbons from sample – Polar TCEP micro-packed columns retains polars and aromatics – Back flush TCEP column to non-polar capillary column (HP-1) to complete analysis 23 October 12, 2012 GC System for D4815 and D5580 Similarities between each method – Same GC hardware requirements • Inlets, detectors, plumbing and valve configuration – Same separation scheme • 2-D separation using polar TCEP micro-packed primary column • 20% TCEP on
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