Arndt Schimmelmann

Last revision: 05 October 2009

H, C, N, O stable isotope reference materials from Indiana University

WHY DID SOME VALUES CHANGE OVER THE PAST FEW YEARS? Since 2006 we have been recalibrating selected materials using pairs of international standards for anchoring and attenuation of isotopic scales. Also, along with the development of new materials, we repeated measurements of previously available materials, thus resulting in amended average values. See more detailed information in Section (B).

Arndt Schimmelmann, Ph.D.

Indiana University

Department of Geological Sciences

Biogeochemical Laboratories
1001 East 10th Street
Bloomington, IN 47405-1405, USA
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Contents of this web page:

   (A) List of available compounds

   (B) Methods used for characterizing reference materials

   (C) How to order reference materials

   (D) Detailed descriptions of individual reference materials

(A) List of available compounds

* additional repeat measurements are pending. The current average values are sufficiently accurate for calibration of on-line data. We will continue updating average values on this website.



n-Alkanes

available pure and

in mixtures

δD

(mean value in ‰ vs. VSMOW, ± 1s)
(range; # of measurements)

δ¹³C

(mean value in ‰ vs. VPDB, ± 1s)
(range; # of measurements)

Mixture "A3"

(mg in 0.5 mL cyclohexane)

see chromatogram

Mixture "B3"

(mg in 0.5 mL hexane)

see chromatogram

Mixture "C3"

(mg in 0.5 mL hexane)

see chromatogram

C-1    methane CH₄ #1, CAS # 74-82-8 -160.8 ± 2.1‰
from -158.8‰ to -164.2‰; n = 9
-38.25 ± 0.03‰
from -38.23‰ to -38.30‰; n = 6

C-1    methane CH₄ #2, CAS # 74-82-8 -41.3 ± 1.3‰
from -39.7‰ to -42.6‰; n = 4
-37.60 ± 0.03‰
from -37.57‰ to -37.62‰; n = 3

C-12 dodecane C₁₂H₂₆, CAS # 112-40-3 -62.5 ± 2.2‰
from -60.2‰ to -64.7‰; n = 4
-31.99 ± 0.04‰
from -31.94‰ to -32.04‰; n = 6

C-14 tetradecane C₁₄H₃₀, CAS # 629-59-4 -71.7 ± 1.4‰ *
from -69.3‰ to -73.5‰; n = 6
-30.69 ± 0.03‰ *
from -30.67‰ to -30.72‰; n = 3

C-15 pentadecane C₁₅H₃₂, CAS # 629-62-9 -88.4 ± 1.2‰
from -86.7‰ to -90.9‰; n = 10
-29.25 ± 0.01‰
from -29.25‰ to -29.26‰; n = 3

C-16 hexadecane C₁₆H₃₄, CAS # 544-76-3 -76.7 ± 1.7‰
from -74.9‰ to -79.8‰; n = 12
-30.66 ± 0.03‰
from -30.62‰ to -30.70‰; n = 9
0.711 0.141

C-17 heptadecane C₁₇H₃₆, CAS # 629-78-7 -142.4 ± 1.7‰
from -140.4‰ to -143.8‰; n = 5
-31.16 ± 0.03‰
from -31.12‰ to -31.21‰; n = 9
0.710 0.281 0.703

C-18 octadecane C₁₈H₃₈, CAS # 593-45-3 -53.8 ± 2.1‰
from -50.9‰ to -55.7‰; n = 4
-31.11 ± 0.02‰
from -31.08‰ to -31.14‰; n = 8
0.708 0.419

C-19 nonadecane C₁₉H₄₀, CAS # 629-92-5 -118.0 ± 2.0‰
from -116.3‰ to -120.2‰; n = 3
-33.17 ± 0.02‰
from -33.12‰ to -33.19‰; n = 6
0.705 0.560 0.702

C-20 icosane (eicosane) C₂₀H₄₂, CAS # 112-95-8 -52.6 ± 0.8‰
from -51.6‰ to -53.7‰; n = 5
-32.35 ± 0.04‰
from -32.31‰ to -32.39‰; n = 4
0.710 0.701

C-21 heneicosane C₂₁H₄₄, CAS # 629-94-7 -214.7 ± 2.0‰ *
from -211.1‰ to -217.6‰; n = 8
-29.10 ± 0.03‰ *
from -29.07‰ to -29.14‰; n = 7
0.702 0.140 0.700

C-22 docosane C₂₂H₄₆, CAS # 629-97-0 -62.8 ± 1.6‰
from -60.9‰ to -64.9‰; n = 6
-32.87 ± 0.03‰
from -32.84‰ to 32.91‰; n = 5
0.709 0.280

C-23 tricosane C₂₃H₄₈, CAS # 638-67-5 -48.8 ± 1.4‰
from -47.0‰ to -51.2‰; n = 6
-31.77 ± 0.01‰
from -31.76‰ to -31.77‰; n = 5
0.704 0.421 0.700

C-24 tetracosane C₂₄H₅₀, CAS # 646-31-1 -53.0 ± 1.6‰
from -50.7‰ to -54.5‰; n = 4
-33.34 ± 0.02‰
from -33.32‰ to -33.36‰; n = 6
0.711 0.561

C-25 pentacosane C₂₅H₅₂ #1, CAS # 629-99-2 (only available as a component of mixtures A, A2, B, B2, C, C2) -254.7 ± 1.6‰
from -252.9‰ to -256.1‰; n = 4
-28.49 ± 0.01‰
from -28.48‰ to -28.50‰; n = 3

C-25 pentacosane C₂₅H₅₂ #2, CAS # 629-99-2 -125.8 ± 1.6‰
from -123.0‰ to -127.8‰; n = 7
-30.00 ± 0.03‰
from -29.96‰ to -30.04‰; n = 5
0.705

C-25 pentacosane C₂₅H₅₂ #3, CAS # 629-99-2 -254.1 ± 1.5‰
from -252.0‰ to -256.1‰; n = 5
-28.48 ± 0.02‰
from -28.45‰ to -28.51‰; n = 7
0.700 0.700

C-26 hexacosane C₂₆H₅₄, #1, CAS # 630-01-3 -54.9 ± 1.5‰
from -53.2‰ to -56.4‰; n = 5
-33.03 ± 0.03‰ *
from -32.99‰ to -33.06‰; n = 9
0.707 0.141

C-26 hexacosane C₂₆H₅₄, #2, CAS # 630-01-3 -45.9 ± 1.0‰
from -44.4‰ to -46.7‰; n = 5
-32.94 ± 0.01‰
from -32.92‰ to -32.95‰; n = 7

C-27 heptacosane C₂₇H₅₆ #1, CAS # 593-49-7 -227.3 ± 2.0‰
from -225.9‰ to -229.6‰; n = 3
-28.61 ± 0.02‰
from -28.59‰ to -28.65‰; n = 6
0.705

C-27 heptacosane C₂₇H₅₆ #2, CAS # 593-49-7 -178.2 ± 2.5‰
from -173.8‰ to -181.5‰; n = 9
-29.56 ± 0.01‰
from -29.55‰ to -29.57‰; n = 4
0.281

C-28 octacosane C₂₈H₅₈, CAS # 630-02-4 -49.0 ± 1.5‰
from -46.7‰ to -50.0‰; n = 4
-32.21 ± 0.03‰
from -32.16‰ to -32.24‰; n = 8
0.706 0.420

C-29 nonacosane C₂₉H₆₀ #1, CAS # 630-03-5 -179.3 ± 2.7‰ *
from -177.0‰ to -183.0‰; n = 5
-31.08 ± 0.02‰ *
from -31.06‰ to -31.10‰; n = 3
0.704

C-29 nonacosane C₂₉H₆₀ #2, CAS # 630-03-5 -242.6 ± 2.9‰
from -238.3‰ to -245.4‰; n = 5
-30.07 ± 0.01‰
from -30.05‰ to -30.08‰; n = 3
0.561

C-30 triacontane C₃₀H₆₂ #1, CAS # 638-68-6 (only available as a component of mixtures) -46.3 ± 2.1‰
from -42.1‰ to -49.4‰; n = 8
-33.15 ± 0.02‰ *
from -33.13‰ to -33.18‰; n = 9
0.702

C-30 triacontane C₃₀H₆₂ #2, CAS # 638-68-6 -213.4 ± 1.2‰
from -211.8‰ to -215.0‰; n = 8
-29.86 ± 0.01‰
from -29.86‰ to -29.87‰; n = 4
0.700

C-32 dotriacontane C₃₂H₆₆, CAS # 544-85-4 -212.4 ± 1.0‰
from -211.5‰ to -213.3‰; n = 4
-29.47 ± 0.02‰
from -29.45‰ to -29.50‰; n = 6

C-34 tetratriacontane C₃₄H₇₀, CAS # 14167-59-0 -231.8 ± 1.4‰
from -230.0‰ to -233.4‰; n = 4
-29.54 ± 0.02‰
from -29.53‰ to -29.56‰; n = 5

C-35 pentatriacontane C₃₅H₇₂, CAS # 630-07-9 -194.8 ± 0.9‰
from -193.3‰ to -195.7‰; n = 5
-29.84 ± 0.01‰
from -29.84‰ to -29.85‰; n = 3

C-36 hexatriacontane C₃₆H₇₄, CAS # 630-06-8 -246.7 ± 1.4‰
from -245.1‰ to -248.4‰; n = 4
-30.00 ± 0.03‰
from -29.96‰ to -30.04‰; n = 7

C-37 heptatriacontane C₃₇H₇₆, CAS # 7194-84-5 -180.1 ± 1.8‰
from -177.4‰ to -181.5‰; n = 4
-30.24 ± 0.03‰
from -30.21‰ to -30.27‰; n = 4

C-38 octatriacontane C₃₈H₇₈, CAS # 7194-85-6 -102.6 ± 1.3‰
from -101.7‰ to -104.0‰; n = 3
-31.49 ± 0.01‰
from -31.47‰ to -31.50‰; n = 5

C-39 nonatriacontane C₃₉H₈₀, CAS # 7194-86-7 -218.6 ± 2.3‰
from -215.2‰ to -221.7‰; n = 10
-28.68 ± 0.01‰
from -28.67‰ to -28.69‰; n = 4

C-40 tetracontane C₄₀H₈₂, CAS # 4181-95-7 -106.7 ± 0.3‰
from -106.4‰ to -107.0‰; n = 3
-32.20 ± 0.04‰
from -32.16‰ to -32.25‰; n = 4

C-41 hentetracontane C₄₁H₈₄, CAS # 7194-87-8
-206.0 ± 1.7‰ *
from -204.1‰ to -208.3‰; n = 7
-28.97 ± 0.01‰ *
from -28.95‰ to -28.98‰; n = 5

C-44 tetratetracontane C₄₄H₉₀, CAS # 7098-22-8 -199.9 ± 2.0‰
from -197.7‰ to -201.6‰; n = 3
-29.12 ± 0.02‰
from -29.10‰ to -29.15‰; n = 5

C-50 pentacontane C₅₀H₁₀₂, CAS # 6596-40-3 -191.3 ± 1.0‰
from -190.6‰ to -192.0‰; n = 2
-27.79 ± 0.03‰
from -27.77‰ to -27.83‰; n = 6

Fatty Acid Esters

δD

(mean value in ‰ vs. VSMOW, ± 1s)
(range; # of measurements)

δ¹³C

(mean value in ‰ vs. VPDB, ± 1s)
(range; # of measurements)

Mixture "F8"

(mg in 0.5 mL cyclohexane)

see chromatogram

Mixture "L"

(mg in 0.5 mL cyclohexane)

see chromatogram

Decanoic acid methyl ester (C10:0); C₁₁H₂₂O₂, methyl decanoate (solution in hexane, in sealed glass ampoule) CAS # 110-42-9
-215 ± 4‰
from -210.2‰ to -218.2‰; n = 3

-29.67 ± 0.02‰
from -29.65‰ to -29.69‰; n = 3

Tetradecanoic acid methyl ester (C14:0), #1 C₁₅H₃₀O₂, ≥99%, methyl myristate, CAS # 124-10-7
-223.9 ± 1.7‰
from -221.9‰ to -226.0‰; n = 4
-26.69 ± 0.01‰
from -26.68‰ to -26.70‰; n = 3

Tetradecanoic acid methyl ester (C14:0), #14M C₁₅H₃₀O₂, ≥99%, methyl myristate, CAS # 124-10-7
-231.2 ± 1.4‰
from -229.3‰ to -232.3‰; n = 4
-29.98 ± 0.02‰
from -29.96‰ to -29.99‰; n = 3

0.042

Tetradecanoic acid ethyl ester (C14:0), #n14E C₁₆H₃₂O₂, 99%, ethyl myristate, CAS # 124-06-1
-231.2 ± 2.7‰
from -228.1‰ to -234.6‰; n = 7
-29.13 ± 0.03‰
from -29.10‰ to -29.16‰; n = 3
0.049

Hexadecanoic acid methyl ester (C16:0) #1 C₁₇H₃₄O₂, ≥99%, methyl palmitate, CAS # 112-39-0
-227.9 ± 1.6‰
from -225.7‰ to -229.9‰; n = 5
-30.74 ± 0.01‰
from -30.73‰ to -30.75‰; n = 3

Hexadecanoic acid methyl ester (C16:0) #n16M C₁₇H₃₄O₂, ≥99.5%, methyl palmitate,
CAS # 112-39-0
-166.8 ± 1.7‰
from -164.8‰ to -168.6‰; n = 4
-29.90 ± 0.03‰
from -29.87‰ to -29.94‰; n = 3
0.049

Hexadecanoic acid methyl ester (C16:0) #16M C₁₇H₃₄O₂, ≥99%, methyl palmitate,
CAS # 112-39-0
+88.0 ± 1.3‰
from +86.4‰ to +89.8‰; n = 6
-30.48 ± 0.01‰
from -30.47‰ to -30.48‰; n = 4

Hexadecanoic acid ethyl ester (C16:0), #IU 16E C₁₈H₃₆O₂, ≥99%, ethyl palmitate, CAS # 628-97-7
-211.0 ± 1.7‰
from -209.5‰ to -213.5‰; n = 4
-30.92 ± 0.02‰
from -30.90‰ to -30.95‰; n = 3
0.046

Hexadecanoic acid ethyl ester (C16:0), #16E C₁₈H₃₆O₂, ≥99%, ethyl palmitate, CAS # 628-97-7
+275.6 ± 2.1‰
from +273.3‰ to +278.1‰; n = 4
-27.66 ± 0.03‰
from -27.63‰ to -27.69‰; n = 3

Hexadecanoic acid propyl ester (C16:0), #16P C₁₉H₃₈O₂,
≥99%, propyl palmitate, CAS # 2239-78-3
+449.3 ± 2.2‰
from +447.6‰ to +452.2‰; n = 4
-30.03 ± 0.01‰
from -30.02‰ to -30.05‰; n = 4

Hexadecanoic acid butyl ester (C16:0), #16B C₂₀H₄₀O₂, ≥99%, butyl palmitate, CAS # 111-06-8
+502.3 ± 2.9‰
from +498.9‰ to +506.5‰; n = 5
-27.16 ± 0.01‰
from -27.15‰ to -27.17‰; n = 4

Octadecanoic acid methyl ester (C18:0), #n18M C₁₉H₃₈O₂, ~99%, methyl stearate, CAS # 112-61-8
-206.2 ± 1.7‰
from -204.0‰ to -208.2‰; n = 5

-23.24 ± 0.01‰
from -23.23‰ to -23.35‰; n = 4
0.05

Octadecanoic acid ethyl ester (C18:0), #18E C₂₀H₄₀O₂, ~99%, ethyl stearate, CAS # 111-61-5
-214.2 ± 0.7‰
from -213.3‰ to -214.9‰; n = 4
-28.22 ± 0.01‰
from -28.22‰ to -28.24‰; n = 3
0.049

Icosanoic acid methyl ester (C20:0) #2, C₂₁H₄₂O₂, ≥99%
_{(formerly named eicosanoic acid
methyl ester), methyl
icosanoate, CAS # 1120-28-1}
-166.7 ± 0.3‰
from -166.4‰ to -167.1‰; n = 3

-30.68 ± 0.02‰
from -30.66‰ to -30.71‰; n = 3
0.05

Icosanoic acid methyl ester (C20:0) #X, C₂₁H₄₂O₂, ≥99%_{(formerly named eicosanoic acid
methyl ester), methyl icosanoate, CAS # 1120-28-1}
+75.7 ± 1.1‰
from +74.5‰ to +76.8‰; n = 4

-6.91 ± 0.04‰ (¹³C-enriched)
from -6.87‰ to -6.95‰; n = 3

Icosanoic acid methyl ester (C20:0) #20M, C₂₁H₄₂O₂, ≥99%
_{(formerly named eicosanoic acid
methyl ester), methyl
icosanoate,
CAS # 1120-28-1}
+505.5 ± 1.7‰
from +503.5‰ to +506.6‰; n = 3

-28.43 ± 0.02‰
from -28.41‰ to -28.44‰; n = 4

Icosanoic acid ethyl ester (C20:0), #20E
C₂₂H₄₄O₂, ≥99%
_{(formerly named eicosanoic acid
ethyl ester), ethyl icosanoate, CAS #
not available}
+340.8 ± 1.9‰
from +338.7‰ to +342.7‰; n = 4

-24.80 ± 0.01‰
from -24.79‰ to -24.82‰; n = 4

Icosanoic acid ethyl ester (C20:0), #20E2
C₂₂H₄₄O₂, ≥99%
_{(formerly named eicosanoic acid
ethyl ester), ethyl icosanoate, CAS #
not available}
-195.5 ± 1.2‰
from -193.8‰ to -196.6‰; n = 4

-26.10 ± 0.03‰
from -26.08‰ to -26.13‰; n = 3
0.042

Icosanoic acid propyl ester (C20:0), #20P
C₂₃H₄₆O₂, ≥99%
_{(formerly named eicosanoic acid
propyl ester), propyl
icosanoate, CAS # not available}
+191.9 ± 1.6‰
from +190.1‰ to +192.8‰; n = 3

-29.00 ± 0.02‰
from -28.99‰ to -29.02‰; n = 3

Icosanoic acid butyl ester (C20:0), #20B
C₂₄H₄₈O₂, ≥99%
_{(formerly named eicosanoic acid
butyl ester), butyl
icosanoate, CAS #
26718-91-2}
+1.5 ± 1.4‰
from +0.1‰ to +3.3‰; n = 4

-28.64 ± 0.03‰
from -28.62‰ to -28.68‰; n = 4

Tetracosanoic acid methyl ester (C24:0), C₂₅H₅₀O₂, ≥99%, methyl lignocerate,
CAS # 2442-49-1
-179.3 ± 1.7‰
from -177.3‰ to -181.9‰; n = 5
-26.57 ± 0.02‰
from -26.56‰ to -26.59‰; n = 3

Tricontanoic acid methyl ester (C30:0), C₃₁H₆₂O₂, ≥99%, CAS # 629-83-4
-189.4 ± 2.0‰ *
from -187.1‰ to -191.3‰; n = 5

-26.33 ± 0.02‰ *
from -26.31‰ to -26.35‰; n = 5

Other Compounds

for GC-irm-MS

δD

(mean value in ‰ vs. VSMOW, ± 1s) (range; # of measurements)

δ¹³C

(mean value in ‰ vs. VPDB, ± 1s)
(range; # of measurements)

δ¹⁵N

(mean value in ‰ vs. air N₂, ± 1s) (range; # of measurements)

Nicotine #1, C₁₀H₁₄N₂, ≥99%, CAS # 54-11-5

-44.8 ±1.7‰
from -42.4‰ to -46.2‰; n = 4
-29.98 ± 0.01‰
from -29.97‰ to -30.00‰; n = 5
-5.82 ± 0.05‰
from -5.75‰ to -5.88‰
n = 4

Nicotine #2, C₁₀H₁₄N₂, ≥99%, strongly ¹³C-enriched,
CAS # 54-11-5
not determined +7.72 ± 0.02‰
from +7.68‰ to +7.75‰; n = 7
-5.94 ± 0.15‰
from -5.72‰ to -6.18‰
n = 7

Nicotine #3, C₁₀H₁₄N₂, ≥99%, strongly ¹⁵N-enriched, CAS # 54-11-5

-90.0 ± 2.8 ‰
from -87.3‰ to -92.5‰; n = 4
-30.05 ± 0.02‰
from -30.03‰ to -30.07‰; n = 7
+33.62 ± 0.18‰
from +33.40‰ to +33.83‰
n = 7

Nicotine #4, C₁₀H₁₄N₂, ≥99%, medium enrichment in ¹³C and ¹⁵N, CAS # 54-11-5
-105.3 ± 2.1‰
from -103.4‰ to -107.2‰; n = 4
-2.06 ± 0.02‰
from -2.04‰ to -2.08‰; n = 5
+15.49 ± 0.13‰
from +15.31‰ to +15.68‰
n = 7

Nicotine #5, C₁₀H₁₄N₂, ≥99%, CAS # 54-11-5, D-depleted

-162.0 ± 1.4 ‰
from -160.8‰ to -164.6‰; n = 7
-29.63 ± 0.01‰
from -29.61‰ to -29.65‰; n = 5
-6.03 ± 0.04‰
from -5.97‰ to -6.08 ‰
n = 5

cis-1,2-Dichloroethylene #1, C₂H₂Cl₂, CAS # 156-59-2
not determined -22.28 ± 0.01‰
from -22.26‰ to -22.30‰; n = 5
not applicable

cis-1,2-Dichloroethylene #2, CAS # 156-59-2
+728.7 ± 2.4‰
from +726.4 ‰ to +731.4 ‰; n = 4
-22.28 ± 0.01‰
from -22.26‰ to -22.31‰; n = 5
not applicable

n-Butylcyclohexane, ≥99%, CAS # 1678-93-9

-53.3 ± 1.4‰
from -51.5‰ to -55.2‰; n = 6
-24.47 ± 0.01‰
from -24.46‰ to -24.48‰; n = 4
not applicable

t-Butylcyclohexane, ≥99%, CAS # 1678-98-4

-70.6 ± 1.9‰
from -68.1‰ to -72.9‰; n = 6
-26.08 ± 0.03‰
from -26.05‰ to -26.10‰; n = 3
not applicable

5a-Androstane, C₁₉H₃₂ #2, CAS # 438-22-2

-297.4 ± 2.2‰
from -294.7‰ to -299.4‰; n = 5

-31.64 ± 0.01‰
from -31.63‰ to -31.64‰; n = 3
not applicable

Coronene, C₂₄H₁₂, 99%, CAS # 191-07-1

-48.3 ± 0.9‰
from -47.3‰ to -49.3‰; n = 4
-26.81 ± 0.04‰
from -26.77‰ to -26.85‰; n = 4
not applicable

Coumarin, C₉H₆O₂, ≥99.5%, CAS # 91-64-5

+82.3 ± 1.2‰
from +80.9‰ to +83.7‰; n = 4
-35.60 ± 0.01‰
from -35.59‰ to -35.61‰; n = 3
not applicable

Dibenzothiophene, C₁₂H₈S, 99.4%, CAS # 132-65-0 +84.9 ± 1.8‰
from +82.4‰ to +87.5‰; n = 6
-27.68 ± 0.01‰
from -27.66‰ to -27.69‰; n = 4
not applicable

Glyceryl tripalmitate, C₅₁H₉₈O₅, ≥99.0%, CAS # 555-44-2

-215.1 ± 0.9‰
from -214.1‰ to -216.1‰; n = 4
-30.12 ± 0.01‰
from -30.10‰ to -30.12‰; n = 3
not applicable

Phenanthrene, C₁₄H₁₀, ≥99.5%, CAS # 85-01-8 -84.1 ± 1.3‰
from -82.8‰ to -86.2‰; n = 6
-25.39 ± 0.03‰
from -25.36‰ to -25.42‰; n = 6
not applicable

Squalane, C₃₀H₆₂

(2,6,10,15,19,23-hexamethyltetracosane), CAS # 111-01-3

-168.9 ± 1.9‰
from -166.1‰ to -171.2‰; n = 6
-20.49 ± 0.02‰
from -20.46‰ to -20.51‰; n = 6
not applicable

Methanol, CH₃OH, 99.8%, anhydrous,

CAS # 67-56-1

The dD values characterize: (1) bulk hydrogen; (2) methyl hydrogen (calculated after subtracting the OH-hydrogen that was liberated in reactions between MeOH and Na metal). d¹³C was determined in bulk methanol.

5 mL sealed in glass ampoule.

bulk methanol: -112.6 ± 0.8‰

from -111.8‰ to -113.5‰; n = 3

methyl hydrogen: -141 ± 3‰

from -138‰ to -143‰; n = 3

-46.77 ± 0.04‰
from -46.74‰ to -46.82‰; n = 3
not applicable

Ethanol, C₂H₅OH, 82 wt. % (87.32 vol. % ethanol), rest water; CAS # 8024-45-1

Distilled from vodka (C3 plant origin).

5 mL sealed in glass ampoule.

not determined

-27.53 ± 0.02‰
from -27.51‰ to -27.55‰; n = 3
not applicable

Ethanol, C₂H₅OH, 80.7 wt.%, rest water,

CAS # 8024-45-1

Distilled from rum (C4 plant origin).

5 mL sealed in glass ampoule.

not determined

-10.98 ± 0.02‰
from -10.95‰ to -11.00‰; n = 5
not applicable

Derivatizing Compounds

for GC-irm-MS

δD

(mean value in ‰ vs. VSMOW, ± 1s)
(range; # of measurements)

δ¹³C

(mean value in ‰ vs. VPDB, ± 1s)
(range; # of measurements)

Acetic acid anhydride, (C₂H₃O)₂O,

CAS # 108-24-7

99.5%, aliquots of ca. 1 mL sealed in 2005 under argon in brown glass ampoules, stored in freezer.
-133.2 ± 2.1‰
from -131.5‰ to -136.0‰; n = 4
-20.98 ± 0.03‰
from -20.94‰ to -21.01‰; n = 4

Phthalic acid, C₈H₆O₄, CAS # 88-99-3

δD measured in Na-phthalate to exclude exchangeable carboxyl H. δ¹³C measured in free acid.
-95.5 ± 2.2‰
from -93.0‰ to -97.5‰; n = 4
-27.21 ± 0.02‰
from -27.20‰ to -27.23‰; n = 4

Methanol, CH₃OH, 99.8%, anhydrous,

CAS # 67-56-1

The δD values characterize: (1) bulk hydrogen; (2) methyl hydrogen (calculated after subtracting the OH-hydrogen that was liberated in reactions between MeOH and Na metal). δ¹³C was determined in bulk methanol.

5 mL sealed in glass ampoule.

bulk methanol: -112.6 ± 0.8‰

from -111.8‰ to -113.5‰; n = 3

methyl hydrogen: -141 ± 3‰

from -138‰ to -143‰; n = 3

-46.77 ± 0.04‰
from -46.74‰ to -46.82‰; n = 3

Materials for on-line

EA-irm-MS

δD

(mean value in ‰ vs. VSMOW, ± 1s)
(range; # of measurements)

δ¹³C

(mean value in ‰ vs. VPDB, ± 1s)
(range; # of measurements)

δ¹⁵N

(mean value in ‰ vs. N₂ air,
± 1s) (range;
# of measurements)

δ¹⁸O

(‰ vs. VSMOW)

Hexatriacontane, C₃₆H₇₄, CAS # 630-06-8 -246.7 ± 1.4‰
from -245.1‰ to -248.4‰; n = 4
-30.00 ± 0.03‰
from -29.96‰ to -30.04‰; n = 7
not applicable not applicable

Acetanilide #1,, C₆H₅NHCOCH₃, CAS # 103-84-4

not determined

(δD is not stable due to exchangeable
N-linked hydrogen.)
-29.53 ± 0.01‰
from -29.51‰ to -29.54‰; n = 6

+1.18 ± 0.02‰
from +1.16‰ to +1.21‰
n = 4

not determined

Acetanilide #2,, C₆H₅NHCOCH₃, moderately enriched in ¹⁵N, CAS # 103-84-4
not determined

(δD is not stable due to exchangeable
N-linked hydrogen.)
-29.50 ± 0.02‰
from -29.48‰ to -29.53‰; n = 4

+19.56 ± 0.03‰
from +19.53‰ to +19.60‰
n = 7
not determined

Acetanilide #3,, C₆H₅NHCOCH₃, strongly enriched in ¹⁵N, CAS # 103-84-4
not determined

(δD is not stable due to exchangeable
N-linked hydrogen.)
-29.50 ± 0.02‰
from -29.49‰ to -29.52‰; n = 4

+40.57 ± 0.06‰
from +40.52‰ to +40.66‰
n = 6
not determined

Benzoic acid #A, C₆H₅COOH, CAS # 65-85-0
not enriched in ¹⁸O.
not determined

(δD is not stable due to exchangeable
O-linked hydrogen.) -28.81 (Coplen et al., 2006) not applicable
+23.14 ± 0.19‰

(Brand et al., 2009, Rapid Commun. Mass Spectrom. 23: 999-1019)

Benzoic acid #B, C₆H₅COOH, CAS # 65-85-0
enriched in ¹⁸O.
not determined

(δD is not stable due to exchangeable
O-linked hydrogen.) -28.85 (Coplen et al., 2006) not applicable
+71.28 ± 0.36‰

(Brand et al., 2009, Rapid Commun. Mass Spectrom. 23: 999-1019)

Urea #1, CH₄N₂O, ≥99.5%, CAS # 57-13-6

not determined
(δD is not stable due to exchangeable
N-linked hydrogen.)
-34.13 ± 0.03‰
from -34.17‰ to -34.09‰; n = 6

+0.26 ± 0.03‰
from +0.20‰ to +0.28‰
n = 7
not determined

Urea #2,, CH₄N₂O, ≥99.5%, moderately enriched in ¹³C and ¹⁵N, CAS # 57-13-6
not determined

(δD is not stable due to exchangeable
N-linked hydrogen.)
-8.02 ± 0.05‰
from -7.96‰ to -8.08‰; n = 5

+20.17 ± 0.06‰
from +20.09‰ to +20.25‰
n = 6
not determined

Urea #3,, CH₄N₂O, ≥99.5%, strongly enriched in ¹³C and ¹⁵N,
CAS # 57-13-6
not determined

(δD is not stable due to exchangeable
N-linked hydrogen.)
+11.71 ± 0.03‰
from +11.69‰ to +11.76‰; n = 6

+40.61 ± 0.02‰
from +40.58‰ to +40.63‰
n = 7
not determined

Corn starch, (CH₂O)_n, ≥99.5%, CAS # not available. δD is not stable due to exchangeable hydroxyl hydrogen.
not determined

-11.01 ± 0.02‰
from -10.99‰ to -11.03‰; n = 4

not applicable
not determined

Coumarin, C₉H₆O₂, ≥99.5%, CAS # 91-64-5

+82.3 ± 1.2‰
from +80.9‰ to +83.7‰; n = 4
-35.60 ± 0.01‰
from -35.59‰ to -35.61‰; n = 3

not applicable
not determined

Phthalic acid, C₈H₆O₄, CAS # 88-99-3

δD is not stable due to exchangeable carboxyl hydrogen.

not applicable
-27.21 ± 0.02‰
from -27.20‰ to -27.23‰; n = 4

not applicable
not determined

(B) Methods used for characterizing reference materials

Compound-specific hydrogen isotope ratios for organic hydrogen are analytically accessible by high-temperature pyrolysis to elemental hydrogen, and subsequent on-line irm-MS (e.g., Bilke and Mosandl, 2002, Rapid Comm. Mass Spectrom. 16: 468-472 [http://dx.doi.org/10.1002/rcm.599] Sessions et al., 1999, Org. Geochem. 30: 1193-1200 [http://dx.doi.org/10.1016/S0146-6380(99)00094-7]; Scrimgeour et al., 1999, Rapid Comm. Mass Spectrom. 13: 271-274 [http://www3.interscience.wiley.com/cgi-bin/abstract/60500662/START]; Hilkert et al., 1999, Rapid Comm. Mass Spectrom. 13: 1226-1230 [http://www3.interscience.wiley.com/cgi-bin/abstract/62003712/START]). Complete GC-irm-MS and TC/EA-irm-MS systems for on-line measurements are commercially available, but there have been few isotopically defined organic compounds for routine calibration and isotopic quality control. Reference materials ('standards') that reliably establish or confirm isotopic calibrations should ideally be in the same chemical form as the analytes (the expression "isotope standard" should be reserved for international primary standards, such as VSMOW, SLAP, NBS 19, L-SVEC, etc). In contrast, the use of intermittent spikes of introduced elemental "standard" hydrogen gas is fraught with potential problems because it does not take into account D/H fractionations that may occur in the analytical train between injection in the GC and the exit of the pyrolysis reactor.

The Biogeochemical Laboratories at Indiana University, in collaboration with Woods Hole Oceanographic Institution (J. M. Hayes) and Caltech (A. L. Sessions), began the development of isotope reference materials in 1998. We first established the purity of n-alkanes and n-alkanoic acid esters by GC-MS, followed by multiple off-line measurements of D/H and ¹³C/¹²C ratios for each compound. Multiple analyses for each compound were performed via conventional combustion of milligram-amounts of individual compounds in quartz ampoules and cryogenic purification of combustion gases in a vacuum line. Water was converted to elemental hydrogen in contact with uranium, followed by collection of hydrogen gas using a Toepler pump. Gas yields and elemental H/C ratios were routinely monitored manometrically for quality control. Hydrogen and carbon isotopic ratios were determined using Finnigan MAT 252 and Delta Plus XP mass-spectrometers at Indiana University. The hydrogen isotopic calibration employed the conventional normalization to VSMOW (zero per mil) and SLAP (-428 per mil) according to T.B. Coplen (1996; New guidelines for reporting stable hydrogen, carbon, and oxygen isotope-ratio data. Geochimica et Cosmochimica Acta 60: 3359-3360). The same analytical strategy was used in later years to develop additional reference materials, in part in collaboration with Caltech (A. L. Sessions) and K. Freeman's group at Pennsylvania State University. Calibration for carbon isotope ratios relied on stable isotope standards NBS 19 and L-SVEC (both carbonates were digested in 100% phosphoric acid at controlled temperatures). Carbon isotope ratios are reported relative to the VPDB scale where NBS 19 and L-SVEC are defined as exactly +1.95 and -46.6 per mil, respectively (Coplen et al., 2006. New guidelines for δ¹³C measurements. Analytical Chemistry 78 (7), 2439-2441; http://dx.doi.org/10.1021/ac052027c). Calibration for nitrogen was performed using international nitrogen isotope standards IAEA-N-1 and IAEA-N-2 (combusted and processed in the same way as our reference materials, according to the "principle of identical treatment").

(C) How to order reference materials

As a public university, we will supply this reference material as a service to the scientific community rather than as a commercial product. Trial amounts of n-alkane mixtures sealed in glass with sufficient volume for multiple injections are available free of charge within the USA, or for the cost of international shipping. Free trial amounts are also available for acetanilide and some other compounds. To offset our significant investment in pure substances and ongoing analytical effort, we ask for compensation in the amount of $250 for each regular aliquot of reference material (exceptions noted below, e.g. $50 for benzoic acid, $150 for one ampoule of mixture F8). For example, a 0.5 mL aliquot of n-alkane mixture (solution in hexane) sealed under argon in a glass ampoule at U.S. $250 reflects a price of a few cents per injection after appropriate dilution of our product to match the sensitivity of modern GC-irm-MS systems.

For more information, send e-mail to aschimme@indiana.edu, fax 812-855-7899, or send letter to: Arndt Schimmelmann, Indiana University, Department of Geological Sciences, Biogeochemical Laboratories, 1005 East Tenth Street, Bloomington, IN 47405-1405, USA. Our isotope reference materials are research materials offered for use without guarantees and without acceptance of any responsibilities for damages arising from its use or possible failure in any application. It is not sold for profit and is distributed as a service to those engaged in geochemical research. Remedies for any claimed defect in these products will be limited to product replacement or refund of the purchase price. In no event shall Indiana University, the Biogeochemical Laboratories, Woods Hole Oceanographic Institution, Caltech, or Penn State University and their staff be liable for any damages, including but not limited to incidental or consequential damages. By accepting shipment of reference materials you agree to this limitation of liability.

Prior to receiving reference materials, international prospective customers are required to answer a questionnaire that is available on-line: http://php.indiana.edu/~aschimme/questionnaire.pdf. The customer should copy the questionnaire text on his/her institutional letterhead in a way that the official institutional seal, emblem, full name and address are clearly visible on top of the questionnaire. All questions must be answered with yes or no. The questionnaire must be signed, dated, and returned by mail, fax or pdf. We are prohibited by U.S. law from sending reference materials to certain countries.

Shipments of ampoules containing flammable or corrosive liquids must be declared hazardous material. This requires the use of special, IATA-approved, non-reusable safety fibreboard shipping boxes with internal metal or plastic containers (ca. U.S. $15 per container; one container can hold many reference materials). Some countries or regions of countries do not permit the importation or delivery of hazardous materials. International customers are advised to first call their local Federal Express office to inquire about the legality and cost of importing hazardous material. Regardless of destination, we prefer that customers make available their institutional Federal Express account to cover FedEx shipping and customs expenses. Payment for reference materials is required within 30 days after the receipt of reference materials and invoice.

Additional suppliers of reference materials, standards, and isotopes:

NIST web site for stable isotope standards: https://www-s.nist.gov/srmors/tables/view_table.cfm?table=104-10.htm and https://www-s.nist.gov/srmors/.

IAEA web site for stable isotope standards: http://www.iaea.org/programmes/aqcs/database/results/stableisotopes_result.html and http://curem.iaea.org/catalogue/SI/index.html.

Recommended δ-values of many international stable isotope standards and reference materials are listed in http://ciaaw.org/, for example for hydrogen: http://ciaaw.org/Hydrogen.htm.

Other isotopes for academic research may be available at: http://www.isoflex.com; we are not affiliated with this supplier.

(D) Detailed description of individual reference materials

n-Alkanes: Methanes #1 and #2 with a purity of at least 99.5% are sealed at about ambient pressure in 9 mm o.d. Pyrex^® glass ampoules with a volume of at least 10 cm³ per ampoule ($150). Each ampoule has a slender glass appendix at one end that can be easily snapped off for opening of the ampoule, for example under water in order to transfer the gas into another receptacle for sub-sampling. Alternatively, a tube cracker can be used to open the ampoule to a vacuum system (e.g., DesMarais and Hayes, 1976. Tube cracker for opening glass-sealed ampoules under vacuum. Analytical Chemistry, vol. 48, No. 11, 1651-1652). Hexatriacontane (i.e., C36 n-alkane) is available in 100 mg amounts in crimp-sealed glass vials ($250).

n-Alkanes other than methane and hexatriacontane are available as pure substances (typically sealed in glass capillaries, at least 5 milligrams per capillary; $250) and as mixtures that are dissolved in hexane (aliquots of 0.5 mL are sealed under argon in glass ampules; $250 each):

Mixture A3 (i.e. replacement of A and A2) of fifteen n-alkanes (C-16 to C-30), and mixture C3 (i.e. replacement of C and C2) of five n-alkanes (C-17, 19, 21, 23, 25) contain ca. 100 nmol H₂ per compound per mL of solution. This is equivalent to approximately 1.4 mg of each n-alkane per mL of solution. These solutions are suitable for establishing the precision and accuracy of an GC-irm-MS instrument. See chromatograms of mixture A and mixture C. The new mixture A3 is using most of the same original pure n-alkanes as the original mixtures A and A2. The new mixture C3 is using most of the same original pure n-alkanes and the same solvent hexane that that constituted the original mixtures C and C2. Details are listed in the Table above.

Mixture B2 (i.e. replacement of B) of fifteen n-alkanes (C-16 to C-30) contain a five-fold range of concentrations (arranged in three pentads with rising concentrations; see Table above), from 20 nmol H₂ to 100 nmol H₂ per compound per mL of solution. This mixture is designed specifically to test the accuracy of H₃⁺ correction in hydrogen-isotope-IRMS (see Sessions et al., 2001 a and b [http://dx.doi.org/10.1021/ac000488m][http://dx.doi.org/10.1021/ac000489e]). In addition, the solution can be used to measure the H₃⁺ factor under conditions closely matching those experienced by analytes. See a chromatogram of mixture B. In 2005 we prepared a new batch called mixture B2 using the same original pure n-alkanes and the same solvent hexane that that constituted the original mixture B. Mixtures B and B2 are isotopically identical and differ only insignificantly with regard to absolute concentrations of individual n-alkanes.

Typically an n-alkane mixture is injected between samples. The C-16 to C-30 mixtures contain compounds which vary in δD by over 200 per mil, and have been calibrated relative to both VSMOW and SLAP standards. The n-alkanes can therefore be used to normalize isotopic measurements to the accepted VSMOW-SLAP scale. The following example is based on experience and measurements using a customized Finnigan MAT 252, but similar observations are expected from Finnigan Delta+XL and other mass spectrometers (Alex Sessions, pers. comm.). For example, when using a C-16 to C-30 mixture (mixtures A or B), C-18 and C-28 can serve as reference peaks with assigned δD values (see Table above). The other δD values are then calculated by the computer relative to C-18 and C-28. The resulting individual raw δD values determined by GC-irm-MS can be plotted versus those determined by off-line combustion (δD values shown in Table above). Ideally, the points fit well along a straight line with R² larger than 0.999. At WHOI, the slope of the line may vary from 0.92 to 1.01, but it is typically stable to about ± 0.02 over the course of one day. The Y-intercept varies from about -2 to +2 per mil. Thus there is no evidence for substantial internal fractionation in the WHOI instrument, although there is some δD-dependent effect which causes the slope of the line to vary. For each day, a normalization serves to calibrate the instrument with regard to a particular slope and intercept. Additional details are available in the following publications: Sessions A.L., T.W. Burgoyne, and J.M. Hayes, 2001, Correction of H₃⁺ contributions in hydrogen isotope ratio monitoring mass spectrometry. Analytical Chemistry 73: 192-199 [http://pubs.acs.org/isubscribe/journals/ancham/jtext.cgi?ancham/73/i02/abs/ac000489e.html]; Sessions A.L., T.W. Burgoyne, and J.M. Hayes, 2001. Determination of the H₃ factor in hydrogen isotope ratio monitoring mass spectrometry. Analytical Chemistry 73: 200-207 [http://pubs.acs.org/isubscribe/journals/ancham/jtext.cgi?ancham/73/i02/abs/ac000488m.html]. Important update: Wang Y. and A.L. Sessions (2009) Memory effects in compound-specific D/H analysis by gas chromatography/pyrolysis/isotope-ratio mass spectrometry. Analytical Chemistry 80: 9162-9170 [http://dx.doi.org/10.1021/ac801170v]. Several groups reported that the δD value for the first eluted compound (C-16 n-alkane) in mixtures A and B seems to be systematically shifted and should be ignored. The problem is likely attributed to conditioning and/or memory effects. The effect can be mitigated if a C-14 or C-15 n-alkane is coinjected to serve as the initial "throw-away peak" (Nikolai Pedentchouk, pers. comm.). Conditioning, memory effects, and the need for proper reference materials were discussed by Bilke and Mosandl, 2002, Rapid Comm. Mass Spectrom. 16: 468-472 [http://www3.interscience.wiley.com/cgi-bin/abstract/90010602/ABSTRACT].
Fatty Acid Esters include decanoic acid methyl ester (10:0), tetradecanoic methyl and ethyl esters (14:0), hexadecanoic methyl, ethyl, propyl, and butyl esters (16:0), octadecanoic methyl and ethyl esters (18:0), icosanoic methyl, ethyl, propyl, and butyl esters (20:0), tetracosanoic acid methyl ester (24:0), and tricontanoic acid methyl ester (30:0). Individual fatty acid esters are useful in cases when n-alkanes co-elute with analytes following co-injection. While peaks of "standard" H₂ gas can be used to calibrate the δ values of unknown compounds, a more robust approach is to use co-injected organic compounds as the isotopic reference. Decanoic acid methyl ester is dissolved in hexane and sealed under argon in brown glass ampoules (ca. 1 mg C-10 FAME in 0.5 mL hexane per ampoule; $250). All other fatty acid esters are available in 2 to 5 mg amounts as pure substances in crimp-sealed glass vials or in sealed glass capillaries ($250; the amount of ester depends on the supply; many D-enriched esters are in short supply), with the following exception: We have available one type of icosanoic acid methyl ester ('type X') that is artificially enriched in D (+75.7 ‰ vs. VSMOW) and ¹³C (-6.91 ‰ vs. VPDB) and can serve as anchor for 2-point isotopic calibration in GC-irm and EA-irm applications (50 mg for $250 in crimp-sealed glass vial).

We have available a mixture of fatty acid esters dissolved in cyclohexane:

Mixture F8 contains eight n-alkanoic acid esters at concentrations of approx. 0.1 mg per milliliter of solution for each ester. Mixture F8 contains esters with δD values from -166.7 to -231.2 ‰ vs. VSMOW (for details, see Table of participating compounds above). Mixture F8 is available in sealed glass ampoules containing 0.5 mL of solution in cyclohexane under argon ($150 per ampoule). This solutions is suitable for establishing the precision and accuracy of an GC-irm-MS instrument in a similar fashion as with the n-alkane mixtures. In contrast to the n-alkane mixtures, the δD values of participating n-alkanoic acid esters are chosen to minimize the isotopic differences among some of the sequentially eluting peaks. This precaution helps reducing the effect of isotopic peak-to-peak memory and results in a more accurate isotopic calibration. See publication by Wang, Y., Sessions, A.L., (2008) Memory effects in compound-specific D/H analysis by gas chromatography/pyrolysis/isotope-ratio mass spectrometry. Analytical Chemistry 80(23), 9162-9170. [http://dx.doi.org/10.1021/ac801170v]

Other Compounds for GC-irm-MS serve similar purposes as n-alkanes and fatty acid esters. These compounds are available in sealed glass capillaries, crimp-sealed glass vials, or sealed glass ampoules. Ethanols have been distilled from rum (sugar cane, C4 plant) and vodka (potato, C3 plant) by the "U.S. Alcohol & Tobacco Tax and Trade Bureau" (TTB) and contain some water, thus limiting their use to ¹³C/¹²C calibration (5mL, $250). Nicotines are available with different levels of D, ¹³C, and ¹⁵N-enrichment (0.25 mg nicotine in 0.5 mL hexane sealed under argon in glass ampoule, $250 ea.). cis-1,2-Dichloroethylenes are available pure in amounts of at least 50 mg in sealed glass capillaries ($250 ea.). Some substances are toxic, for example nicotine and polyaromatic hydrocarbons.

Derivatizing Compounds for GC-irm-MS are useful for transforming alcohols and acids into derivatized compounds that can be passed over the GC column. An isotopic mass-balance calculation is necessary to arrive at the compound-specific isotope ratios of the original alcohol. Phthalic acid (5 gram), acetic anhydride (1 milliliter), and methanol (5 milliliters) are available as pure substances in sealed glass ampoules or in glass vials. Some substances are highly toxic. In the case of phthalic acid, the D/H ratio was measured using Na-phthalate to exclude the influence of isotopically exchangeable carboxyl hydrogen. The δD value the methyl hydrogen in methanol was determined by first measuring the bulk δD value and the δD value of the hydroxy hydrogen (via producing elemental hydrogen in the reaction of methanol and sodium metal), followed by a mass-balance calculation.

Materials for on-line EA-irm-MS:

n-Alkane C-36: Hexatriacontane, available as pure, powdered compound in crimp-seal glass vials, 100 mg, $250.

Acetanilides: The manufacturers of elemental analyzers are frequently using acetanilide as an internal reference material for molar C/N elemental ratios, but stable isotope ratios of the supplied acetanilide are typically not provided. The Biogeochemical Laboratories at Indiana University purchased pure acetanilide and ¹⁵N-enriched acetanilide. The regular acetanilide #1was powdered using a large, industrial-strength blender. Aliquots of the regular acetanilide #1and the ¹⁵N-enriched acetanilides were melted and subsequently powdered to prepare medium and highly ¹⁵N-enriched acetanilides #2 and #3. Multiple aliquots of each acetanilide were then combusted off-line in individually sealed 'quartz' ampoules, the products CO₂ and N₂ were cryogenically purified and manometrically quantified, and the collected gases were isotopically measured using a Delta Plus XP mass-spectrometer in manual dual-inlet mode. Calibration was performed using international nitrogen isotope standards IAEA-N-1 and IAEA-N-2 for nitrogen (combusted and processed in the same way as acetanilide), and NBS 19 and L-SVEC for carbon stable isotopes. Regular acetanilide #1 is available in 5 g amounts ($250 ea.) and 2 g amounts ($150), whereas each of the ¹⁵N-enriched acetanilides #2 and #3 is available in 2 g amounts ($250 ea.). The user is advised to finely powder the received acetanilides prior to utilizing sub-milligram aliquots.

Benzoic acids: As a reference material for ¹⁸O/¹⁶O, benzoic acid has several advantages over cellulose and many other oxygen-containing compounds; (1) it has been reliably measured isotopically by conventional off-line methods and on-line TC/EA; (b) it is chemically stable and inert, so that its organic oxygen inventory receives no significant addition from elemental atmospheric oxygen species via oxidation over time; (c) it shows no hygroscopicity that could add atmospheric water vapor via adsorption (unlike cellulose); (d) it has been prepared in relatively large quantities with sufficient isotopic homogeneity; (e) it shows no significant toxicity; (h) it does not rapidly exchange isotopically with ambient atmospheric moisture. The IAEA had commissioned the development of two isotopically different benzoic acid oxygen isotope standards (jointly developed by Drs. Willi Brand and Roland Werner at the Max-Planck-Institut für Biogeochemie in Jena, Germany, and by Arndt Schimmelmann at Indiana University; a pdf copy of the final report to the IAEA is available upon request). The artificial enrichment in ¹⁸O in one of the acids did not entail proportional enrichment in ¹⁷O. The algorithm for calculating δ¹³C values hinges on the ¹⁷O abundance, and thus pyrolytically derived CO or CO₂ from our ¹⁸O-enriched benzoic acid would yield incorrect δ¹³C values. The applicability of the ¹⁸O-enriched benzoic acid as a stable isotope standard is thus limited to ¹⁸O/¹⁶O. The use of benzoic acids for carbon isotope ratios was discussed by Coplen et al., 2006 (New guidelines for δ¹³C measurements. Analytical Chemistry 78 (7), 2439-2441; http://dx.doi.org/10.1021/ac052027c). The ¹⁸O-enriched benzoic acid must not be used for calibration of ¹³C/¹²C unless exchange of benzoic-acid-derived carbon-oxides is achieved with excess oxygen via oxidative combustion (Willi Brand, personal communication). Oxygen stable isotope ratios were recently recalibrated (Brand et al., 2009, Rapid Commun. Mass Spectrom. 23: 999-1019). The small supply of benzoic acids retained at Indiana University limits the availability to trial amounts only. Regular users must order their benzoic acids from the IAEA (IAEA 601 and IAEA 602: http://curem.iaea.org/catalogue/SI/SI_008030000.html and http://curem.iaea.org/catalogue/SI/SI_008040000.html and http://curem.iaea.org/catalogue/admin/order_si.html). Each benzoic acid should be powdered by the user prior to use, in order to reduce isotope fractionation among individual crystals.

Ureas with a purity of ≥99.5% are available in 2 g amounts in glass screw-cap vials ($250 ea). There are three types of ureas available. Urea #1 has natural ¹³C and ¹⁵N abundances. Urea #2 is moderately ¹³C and ¹⁵N-enriched, whereas urea #3 is strongly ¹³C and ¹⁵N-enriched. The highest precision can be achieved when each urea is dissolved in water and suitable volumetric aliquots of stock solutions are micro-pipetted into EA capsules. We caution that urea crystals are known to adsorb smaller molecules, for example straight-chain n-alkanes. Off-line combustion experiments with urea stock solutions suggest that CO2 and/or methane from glass blowing flames can be trapped in crystallizing urea and shift bulk δ¹³C values

Corn starch is available in 1 gram amounts in screw-cap glass vials, $150. Coumarin is valuable for hydrogen TC/EA because it has a remarkably high deuterium concentration and can serve as an anchor point for D/H scale calibration. It is available in 100 mg amounts in crimp-sealed glass vials, $250. Phthalic acid: D/H was measured using Na-phthalate to exclude exchangeable carboxyl hydrogen. Phthalic acid is supposed to be used for derivatization of alcohol groups to eliminate exchangeable hydrogen. Phthalic acid itself is not a suitable reference material for hydrogen because it contains exchangeable carboxyl hydrogen. This limitation does not affect its suitability as a carbon isotope reference material. Phthalic acid is available in 3-gram-amounts in screw-cap glass vials, $250.

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