Analysis of nitrogen isotope ratios in amino acids by GC/C/IRMS

Transcription

1 Analysis of nitrogen isotope ratios in amino acids by GC/C/IRMS 27 March 2013 Yoshito Chikaraishi The initial version was released by YC in May The present version reflects feedbacks from Svenja Kruse (Univ. of British Columbia) and Shawn A. Steffan (Univ. of Wisconsin-Madison), Elizabeth Bosak (Univ. of Wisconsin-Madison) in March 2012.

2 Instrument Instruments in my lab: GC Agilent 6890N Injector Gerstel PTV Column Agilent HP Ultra-2 (50m, 0.32mm, 0.52µm) Flow rate (He) 1.4ml/min Oven program 40ºC (2.5 min) -15ºC/min (0 min) - 3ºC/min (0 min) - 6ºC/min - 220ºC (17.3min) Interface GC combustion III (oxidation at 1000ºC, reduction at 550ºC) IRMS Delta plus XP Standard mix of authentic amino acids: after HCl hydrolysis and derivatization, derivatives are stored at 20ºC, in DCM (Currently, I use new standard mix. See page 10 ). Compound Source Known Amino acid Pv/OiPr derivatives δ 15 N MW N(wt%) mg/10ml MW N(wt%) ng/µl ngn/µl nmol/µl L-alanine Ala CERKU glycine Gly CERKU L-valine Val BioGeos L-leucine Leu CERKU L-norleucine Ile BioGeos L-aspartic acid Asp BioGeos L-serine Ser BioGeos L-glutamic acid Glu BioGeos L-phenylalanine Phe BioGeos L-hydroxyproline Hyp BioGeos

3 Procedure for δ 15 N analysis 1. Set up GC/IRMS (e.g., connect GC and IRMS, focusing) 2. Check N 2 leaks for both back-flush and straight mode 3. Check stability by On/OFF of Ref. gas 4. Check reproducibility by measuring δ 15 N-known authentic AAs (amino acid standards), as Pv/OiPr derivatives 5. Make up a sequence file 6. Measure both authentic AAs and samples of interest 7. Check baseline separation on the chromatograms 8. Normalize the measured values (i.e., δ 15 N, vs ref.gas) to international scale (i.e., δ 15 N, vs Air)

4 1. Focusing Check these 9 boxes To find the best (optimum) position, apply the following steps: 1. After peak center, open auto focus. 2. Enter 50 and 500 for the step width in these two boxes 3. Click OK 4. After peak center, enter 25 and 250 in these two boxes, and click OK 5. After peak center, enter 12 and 120 in these two boxes, and click OK 6. After peak center, 6 and 60 in these two boxes, and click OK 7. After peak center, enter 3 and 30 in these two boxes, and click OK 8. After peak center, enter 2 and 20 in these two boxes, and click OK 9. After peak center, enter 1 and 10 in these to boxes, and click OK (twice)

5 3. Check stability by On/OFF of Ref. gas Generally, 1σ <

6 4. Check reproducibility by δ 15 N of known authentic AAs Set the isotope value of the ref. gas to 0 on the PC software, measure the authentic AA derivatives, and check the linearity of the relationship between the measured δ 15 N (, vs ref. gas) and known δ 15 N (, vs air) values Ref. N 2 Ref. N 2 m/z 28 Intensity (mv) Ala Gly Val Leu Nle Asp Ser Glu Phe Hyp Retention time (min) 25 If you do not get a good linear relationship, please check the following points: 1. the condition of the ox./red. furnace 2. the condition of GC column 3. dirt in inlet/liner δ 15 N, vs Air y = x (R 2 = 0.999) 4. deterioration of capillary connection δ 15 N, vs refs

7 5. Make up a sequence file For example, Line GC method Sample Num. Injection vol. (µl) Sample name IRMS Method 1 Ultra-2 AA 1 1 STD Pv/OiPr Ultra-2_AA_15N 2 Ultra-2 AA 2 1 Sample-A Ultra-2_AA_15N 3 Ultra-2 AA 3 1 Sample-B Ultra-2_AA_15N 4 Ultra-2 AA 4 1 Sample-C Ultra-2_AA_15N 5 Ultra-2 AA 5 1 Sample-D Ultra-2_AA_15N 6 Ultra-2 AA 1 1 STD Pv/OiPr Ultra-2_AA_15N 7 Ultra-2 AA 6 1 Sample-E Ultra-2_AA_15N 8 Ultra-2 AA 7 1 Sample-F Ultra-2_AA_15N 9 Ultra-2 AA 8 1 Sample-G Ultra-2_AA_15N 10 Ultra-2 AA 9 1 Sample-H Ultra-2_AA_15N 11 Ultra-2 AA 10 1 Sample-I Ultra-2_AA_15N 12 Ultra-2 AA 1 1 STD Pv/OiPr Ultra-2_AA_15N

8 8. Normalization of measured values to the international scale -1 When you set the isotope value of the ref. gas to 0 on the PC software, you have measured values ( vs ref. gas). Known δ 15 N Measured value = δ 15 N, vs ref. gas 1st 2nd 3rd 4th 5th Average 1σ Ala Gly Val Leu Nle Asp Ser Glu Phe Hyp δ 15 N, vs Air y = x (R 2 = 0.999) δ 15 N, vs refs You can normalize δ 15 N to the international scale (i.e., δ 15 N, vs Air) by using the equation: y = x (in this case). The slope 1.0 means that we need this linear normalization instead of an one-point normalization. Known δ 15 N δ 15 N, vs Air 1st 2nd 3rd 4th 5th Average 1σ Δ Ala Gly Val Leu Nle Asp Ser Glu Phe Hyp Average Std.dev σ = Precision Δ (= average known) = Trueness In this way, Accuracy (= Mean of Δ) should be Zero ± something.

9 8. Normalization of measured values to the international scale -2 With the equation: (δ 15 N, vs Air) = (measured δ 15 N, vs refs) (in this case), you can calculate normalized δ 15 N (, vs Air) for the samples of interest. Measured value = δ 15 N, vs ref. gas Sample-A Sample-B Sample-C Sample-D Sample-E Sample-F Sample-G Ala Gly Val Leu Ile Pro Ser too small Met too small too small too small too small too small Glu Phe δ 15 N, vs Air Sample-A Sample-B Sample-C Sample-D Sample-E Sample-F Sample-G Ala Gly Val Leu Ile Pro Ser 0.9 n.d Met n.d. 3.0 n.d. n.d. n.d. n.d Glu Phe

10 New standard mix We currently developed wide-range (from 25.9 to ) isotopic reference materials of amino acids, which are commercially available from Indiana University ( and SI science ( Compoud Source known δ 15 N Amino acid Pv/OiPr derivatives MW N(wt%) mg/10ml MW N(wt%) ng/µl ngn/µl nmol/µl L-alanine Ala Indiana Univ glycine Gly SI science L-valine Val BioGeos L-leucine Leu SI science L-norleucine Nle SI science L-aspartic acid Asp SI science L-methionine Met SI science L-glutamic acid Glu SI science L-phenylalanine Phe Indiana Univ L-hydroxyproline Hyp SI science Known δ 15 N δ 15 N (, vs Air) by GC/IRMS δ 15 N1st δ 15 N2nd δ 15 N3rd δ 15 N4th Average 1σ Δ Ala Gly Val Leu Nle Asp Ser Glu Phe Hyp Average Std.dev δ 15 N ( vs Air) determined by GC/IRMS y = R 2 = Known δ 15 N ( vs Air)