Converting Helium Carrier Gas GC Methods Nitrogen and Hydrogen

Transcription

1 Converting Helium Carrier Gas GC Methods Nitrogen and Hydrogen Jim McCurry Agilent Technologies Wilmington, DE USA 1

2 Market Situation The world of He supply is not reliable, prices are increasing and customers are seeking alternative carrier gases Agilent Restricted Page 2 2

3 The Carrier Gas Decision No Is the chemist willing to convert to alternative gasses? Yes GC Is the Application based on GC or GC/MS? GC/MS Does the current GC method have too much resolution? No Yes He Conservation Consider migration to N 2 Consider migration to H 2 GC/MS specific H 2 considerations Agilent Restricted Page 3 3

4 Migration to H 2 : Considerations for GC & GC/MS No Is the chemist willing to convert to alternative gasses? Yes GC Is the Application based on GC or GC/MS? GC/MS Does the current GC method have too much resolution? No Yes He Conservation Consider migration to N 2 Consider migration to H 2 GC/MS specific H 2 considerations Agilent Restricted Page 4 4

5 H 2 : Considerations for GC & GC/MS Application H 2 safety Sources of hydrogen Plumbing modifications Chromatographic method migration Method revalidation Agilent Restricted Page 5 5

6 H 2 Safety GC, GC/MS: Both offer H 2 enabled features Agilent H 2 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 Page 6 6

7 Source of Hydrogen H 2 Generator preferred Very clean H 2, > % 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 H 2 Cylinder Consider gas clean filter Possible to add safety device Agilent Restricted Page 7 7

8 H 2 : Plumbing Modification Considerations Tubing: Use chromatographic quality stainless steel tubing Do not use old tuning (H 2 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 Page 8 8

9 Migration to N 2 for GC Application No Is the chemist willing to convert to alternative gasses? Yes GC Is the Application based on GC or GC/MS? GC/MS Does the current GC method have too much resolution? No Yes He Conservation Consider migration to N 2 Consider migration to H 2 GC/MS specific H 2 considerations Agilent Restricted Page 9 9

10 Use N 2 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 Page 10 10

11 Helium Carrier Gas Alternatives Hydrogen, nitrogen, argon, argon/methane Detector considerations can influence choice Hydrogen and nitrogen are the most common alternatives 11

12 Helium Carrier Gas Alternatives Important Theoretical Considerations Relating To Peak Efficiency Calculating Efficiency Efficiency & Carrier Gas Linear Velocity Start Inert Gas Solute t m t ' R t R Let's relate n to the length of the column. Plates per meter (N) = n L Time or L Height equivalent to a theoretical plate (HETP) = n We would like to know the actual time the component spends in the stationary phase. n = Thus, the more efficient the column, the bigger the "N" the smaller the "HETP. t ' R t R ' = t - R t m = Corrected Retention Time. n = effective theoretical plates. t ' R W h 2 HETP MOLECULAR DIFFUSION HETP = A + B { } } µ ( opt µ) B µ + C µ C RESISTANCE TO MASS TRANSFER A EDDY DIFFUSION Efficiency is a function of the carrier gas linear velocity or flow rate. The minimum of the curve represents the smallest HETP (or largest plates per meter) and thus the best efficiency. "A" term is not present for capillary columns. Plot of HETP versus linear velocity is know as the Van Deemter plot. The linear velocity value at the minimum of the curve is the optimum value for achieving the best efficiency. 12

13 Helium Carrier Gas Alternatives Let s Make This Easy Calculating Efficiency Efficiency & Carrier Gas Linear Velocity Start Inert Gas Solute t m t ' R t R Let's relate n to the length of the column. Plates per meter (N) = n L Time or L Height equivalent to a theoretical plate (HETP) = n We would like to know the actual time the component spends in the stationary phase. n = Thus, the more efficient the column, the bigger the "N" the smaller the "HETP. t ' R t R ' = t - R t m = Corrected Retention Time. n = effective theoretical plates. t ' R W h 2 HETP MOLECULAR DIFFUSION HETP = A + B { } } µ ( opt µ) B µ + C µ C RESISTANCE TO MASS TRANSFER A EDDY DIFFUSION Efficiency is a function of the carrier gas linear velocity or flow rate. The minimum of the curve represents the smallest HETP (or largest plates per meter) and thus the best efficiency. "A" term is not present for capillary columns. Plot of HETP versus linear velocity is know as the Van Deemter plot. The linear velocity value at the minimum of the curve is the optimum value for achieving the best efficiency. 13

14 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 N 2, 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

15 The Tyranny of Van Deemter Why Nitrogen Gets a Bad Rap for Capillary GC HETP (mm) N 2 C 17 at 175º C k' = 4.95 OV m x 0.25 mm x 0.4µ He Nitrogen 58 cm/s 2.4 ml/min Helium 58 cm/s 2.5 ml/min Hydrogen 58 cm/s 1.7 ml/min.6.4 H Average Linear Velocity (cm/sec) R = 1.17 R = 1.37 N 2 actually provides the best efficiency, but at a slower speed Most helium methods have too much resolution Lower N 2 efficiency at higher flows can still provide good enough resolution Most GC methods now use constant flow N 2 efficiency losses with temp programming are not as severe 15

16 Helium Carrier Gas Alternative Test Case: ASTM D6584 for Free and Total Glycerin in Biodiesel Istd 2 Istd Glycerol 2. Monoglycerides 3. Diglycerides 4. triglycerides 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 o C for 1 min. Oven Ramp 1: 15 o C/min to 180 o C Oven Ramp 2: 7 o C/min to 230 o C Oven Ramp 3: 30 o C/min to 380 o C, hold 10 min. Detector: FID with 25 ml/min N 2 makeup 16

17 Configure Inlet for Carrier Gas in Chemstation 17

18 Set the Control Mode: Flow or Holdup Time 18

19 Wider Retention Time Variation Using the Same Flow min Helium Flow: 3.00 ml/min P: 7.63 psi T r : min. µ: cm/s min Hydrogen Flow: 3.00 ml/min P: 3.85 psi T r : min. µ: cm/s min Nitrogen Flow: 3.00 ml/min P: 7.09 psi T r : min. µ: cm/s

20 Same Holdup Time (T r ) Gives Consistent Retention Times min Helium Flow: 3.00 ml/min P: 7.63 psi T r : min. µ: cm/s min Hydrogen Flow: 2.64 ml/min P: 3.43 psi T r : min. µ: cm/s min Nitrogen Flow: 2.94 ml/min P: 6.98 psi T r : min. µ: cm/s

21 Monoglyceride Resolution Good Enough Using Nitrogen Carrier Mono1 Mono2 area: 1049 Mono3 Helium Flow: 3.00 ml/min T r : min. Mono1 Mono2 area: 1050 Mono3 Hydrogen Flow: 2.64 ml/min T r : min. Mono1 Mono2 area: 1049 Mono3 Nitrogen Flow: 2.94 ml/min T r : min

22 ASTM D Quantitative Results For Alternative Carrier Gas Weight Percent Helium Hydrogen Nitrogen Glycerin Monoglycerides Diglycerides Triglycerides Total Glycerin

23 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, C 8 aromatics and C 9 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

24 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 80/100 Chromosorb PAW, 22 x 1/16 stainless Only one difference in instrument requirements non-polar capillary column D µm methyl silicone, 30m x 0.53mm D µm methyl silicone, 30m x 0.53mm 24

25 Configuration and Operation for D4815 and D5580 Primary flow S/S EPC split/splitless inlet TCEP column HP-1 capillary column Secondary flow PCM EPC variable restrictor FID TCD 25

26 Instrument Conditions D4815 Method D5580 Method carrier gas nitrogen nitrogen Inlet Split/Splitless Split/Splitless inlet temperature 200 Deg C 200 Deg C inlet presuure 9 psi (constant P) 25 psi (constant P) TCEP column flow 5 ml/min 10 ml/min split vent flow 70 ml/min 100 ml/min split ratio 15:1 100:1 PCM pressure program 13 psi for 14 min 23 psi for 12.1 min 99 psi/min to 40 psi 99 psi/min to 40 psi HP-1 column flow 3 ml/min 10 ml/min FID temperature 250 deg C 250 deg C oven temperature 80 deg C isothermal 60 C for 6 min 2 C/min to 115 C 115 C for 1.5 min Run time 16 min 35 min 26

27 Analysis of MtBE and Ethanol in Gasoline using N 2 Carrier Gas MtBE DME(IS) benzene valve reset ethanol DME(IS) benzene aromatic and heavy non-aromatic hydrocarbons

28 ASTM Precision Specifications D4815 Precision Measures Repeatability Reproducibility Compound Mass % Spec Observed Spec Observed Ethanol Ethanol MtBE MtBE Accuracy Evaluation MtBE mass % Sample known found SRM2294 # SRM2294 # AccuStd Check

29 D5580 Requires Two Runs per Sample for Complete Aromatic Analysis Good resolution with nitrogen carrier gas T T3 4 Method D5580A 1. benzene 2. toluene 3. 2-hexanone (IS) 4. C 8 aromatics, C 9 plus aromatics, and non-aromatic hydrocarbons Method D5580B 1 T hexanone (IS) 2. ethylbenzene 3. m,p-xylene 4. o-xylene 5. C 9 plus aromatics T

30 Detector Response Precision for a Pump Gasoline Sample 5 Runs Over 5 Days FID Response (area counts) Run # Method MtBE Benzene Toluene Ethylbenzene m,p-xylene o-xylene A B A B A B A B A B Avg Std. Dev %RSD

31 Correcting Occasional Resolution Problems Initial oven temp = 60 o C m,p-xylene T2 = 1.85 min. ethylbenzene Initial oven temp = 50 o C m,p-xylene T2 = 1.85 min. ethylbenzene Some combinations of TCEP and HP-1 columns show less resolution using N 2 carrier Fastest and easiest solution is to lower the initial oven temperature 31

32 Using Nitrogen With High Resolution Capillary Columns Test Case: EN14103 for Total FAME and Methyl Linolenate Content in B100 Biodiesel Expand method to include animal fat derived biodiesel Replace C17 FAMEs ISTD with C19 FAME ISTD Expand method to include tropical oil (palm) derived FAMEs Quantify C6 to C24 FAMEs and methyl linolenate (C18:3) Requires column temperature program to resolve all FAMEs Method updated to provide better precision Verify C19:0 ISTD purity 32

33 EN14103:2011 GC Configuration and Operating Conditions Standard 7890A GC Hardware G3440A Agilent 7890A Series GC Option psi split/splitless Inlet with EPC control Option 211 Capillary FID with EPC control G4513A Agilent 7693 Autoinjector 19091N-133 HP-INNOWax Column, 30 m x 0.25 mm ID x 0.25 mm Split/Splitless Inlet Temperature Split flow 250 o C 100 ml/min. Column Flow He or H 2 at 1 ml/min. constant flow 60 o C for 2 min. 10 Column temperature o C /min. to 200 o C 5 o C/min. to 240 o C Hold 240 o C for 7 min. Flame ionization detector 250 o C 33

34 EN14103 FAME Identification Standard Helium at 1 ml/min Constant Flow Peak # Name RT (min.) Peak # Name RT (min.) 1 methyl hexanoate C6: methyl arachidate C20: methyl myristate C14: methyl eicosonate C20: methyl myristoleate C14: methyl eicosadienoate C20: methyl palmitate C16: methyl arachidonate C20: methyl palmitoleate C16: methyl eicosatrienoate C20: methyl stearate C18: methyl behenate & C22: methyl oleate (9) C18: methyl eicosapentaenoate C20:5 8 methyl oleate (11) C18: methyl erucate C22: methyl linoleate C18: methyl lignocerate C24: methyl linolenate C18: methyl nervonate C24: methyl docosahexaenoate C22:

35 EN14103 FAME Retention Time Standard Comparison of He and N 2 Carrier Gas pa Helium at 1 ml/min Constant Flow min Nitrogen at 1 ml/min Constant Flow min 35

36 EN14103 Soya B100 Biodiesel Comparison of Helium and N 2 Carrier Gas C19:0 (IS) Helium at 1 ml/min Constant Flow min Nitrogen at 1 ml/min Constant Flow min 36

37 EN14103 FAME Content in Biodiesel Comparison of He and N 2 Carrier Gas FAME Content (wt%) Helium Nitrogen Sample Run 1 Run 2 r Run 1 Run 2 r r (spec) SRM Rapeseed Coconut Rape/Coco (50/50) Helium C18:3 (wt%) Nitrogen Sample Run 1 Run 2 r Run 1 Run 2 r r (spec) SRM Rapeseed Coconut Rape/Coco (50/50)

38 If You MUST Use Helium Consider Conservation No Is the chemist willing to convert to alternative gasses? Yes GC Is the Application based on GC or GC/MS? GC/MS Does the current GC method have too much resolution? No Yes He Conservation Consider migration to N 2 Consider migration to H 2 GC/MS specific H 2 considerations Agilent Restricted Page 38 38

39 Reduce Helium Consumption 7890 GC Gas Saver reduces split flow to conserve helium consumption Leak checking Turn off the system to avoid long term idle Not always recommended for some GC columns or traps Agilent Little Falls Site was able to reduce consumption by 50% Agilent Restricted Page 39 39

40 Summary Migrating your GC method from helium carrier gas can be easily done For resolution critical methods, H 2 offer the best alternative Agilent GC and GC/MS systems have many built-in safety features For many GC applications, N 2 offers a cheap, easy alternative without any safety worries Most existing helium methods have too much resolution N2 can be used without changing any of the existing GC conditions keep the holdup time the same as the original method 2-D methods have high resolution built-in, so N 2 is ideally suited as a carrier gas Valve-based or Deans switch, not GCxGC If helium alternative is not an option, consider conservation 40