PMC

Cross plots of (a) Ph/nC₁₈ vs Pr/nC₁₇, (b) C₂₇/C₂₉ vs Pr/Ph, (c) TAR vs Pr/Ph, and (d) 4-MSI vs C₂₄Te/C₂₃TT. (Note: C₂₇/C₂₉ = (5α(H),14α(H),17β(H) C₂₇ sterane 20S + 5α(H),14β(H),17β(H) C₂₇ sterane 20S + 5α(H),14β(H),17β(H) C₂₇ sterane 20R + 5α(H),14α(H),17α(H) C₂₇ sterane 20R)/(24-ethyl-5α(H), 14α(H), 17α(H)–C₂₉ sterane 20S + 24-ethyl-5α(H), 14β(H), 17β(H)–C₂₉ sterane 20R + 24-ethyl-5α(H), 14β(H), 17β(H)–C₂₉ sterane 20S + 24-ethyl-5α(H), 14α(H), 17α(H)–C₂₉ sterane20R); TAR = (nC₂₇ + nC₂₉ + nC₃₁)/(nC₁₅ + nC₁₇ + nC₁₉); 4-MSI = the ratio of C₃₀ 4-methylsterane to C₂₉ regular steranes; and C₂₄Te/C₂₃TT = C₂₄ tetracyclic terpane/C₂₃ tricyclic.)

Multiple reaction monitoring (MRM) gas chromatography–mass spectrometry (GC–MS) ion chromatograms showing sterane distributions (M⁺ → 217 Daion transitions) for hydrogenation products from the demosponge Rhabdastrella distincta and two rhizarian protists (A. lobifera and C. reptans) suppliedto us by Nettersheim et al.. While our MRM analyses confirmed the sterane distributions for A. lobifera, we could not reproduce the C₃₀ sterane signalor compound distributions reported for C. reptans. a, For C₃₀ steranes, repeat MRM analyses of R. distincta, even at low sample concentrations usinghigh dilution factors, consistently yielded a well-defined 26-mes signal. In contrast, C. reptans principally generated noise even at optimum solution concentrations. A similar absence of robust C₃₀ sterane signal for C. reptans was also observed using GC–triple quadrupole (QQQ)-MS analysis performed at Massachusetts Institute of Technology. b, The total C₂₇–C₃₀ sterane distribution for C. reptans is dominated by C₂₈ and C₂₉ compounds, similar to green algae, as found previously for chlorarachniophyte protists. c, Any trace C₃₀ sterane signal hidden within the noise may only be present at levels <0.002% of total steranes, which is at least two orders of magnitude lower than the percentage abundances routinely reported in steroid assays. Compound assignments for any putative C₃₀ steranes at these trace levels is problematic and not commonly practiced. A dilution factor of 1 represents the optimum concentration for maximizing C₂₇–C₂₉ sterane signal.

MRM-GC-MS (414→217 Da) chromatograms of the C₃₀ sterane distributions in representative Tonian sedimentary rocks (b-c, this study) compared with a (a) ES0083 Lower Cambrian Reference Oil from Eastern Siberia (). ES0083 has a C₃₀ sterane distribution with three different resolvable series: 24-n-propylcholestane (n/blue peaks), 24-isopropylcholestane (i/green peaks), and 26-methylstigmastane (m/red peaks; ; ). One novel C₃₀ sterane series (#/black peaks) was detected in the least mature rocks of the Chuar Group from Sixtymile Canyon section. The new C₃₀ sterane series is found in the (c) solvent extract (free) phase of sample SWE2 and also in the (b) kerogen (bound) phase. Sample (d) v12 shows the absence of any detectable C₃₀ steranes within the Visingsö Group outcrops. Peaks labeled “X” represent cross talk from other compound classes (usually hopanes). The αααR isomer is indicated by a label to show the difference in compound elution times between all four C₃₀ sterane series (in increasing elution order of 24-npc, 24-ipc, 26-mes with the new C₃₀ sterane compound eluting last)

MRM-GC-MS (M⁺→217 Da) chromatograms of the free (left) and kerogen-bound (right) C₂₆–C₃₀ steranes from sample SWE2 (Chuar Group, Sixtymile Canyon locality). Sterane distributions and relative abundance comparisons between the bitumen and kerogen phases of sedimentary organic matter confirm that the biomarkers are syngenetic with the host sedimentary rocks. Compared to the free phase, the sterane distributions from the bound phase show a slightly less mature compound distribution (e.g., 21-nor < 27-nor, C₂₇ αααR > αααS) due to steric protection from the kerogen network. The “regular” sterane series show the four stable isomer peaks (the αααS + R and αββR + S isomers), while “βαS” and “βαR” indicate the two main diasterane isomers, which are rearranged steranes prominent in the free biomarker distributions. 21-nor = 21-norcholestane; 27-nor = 27-norcholestane; E = ergostane; C = cryostane; # = C₃₀ series with unknown side chain chemistry. Cross talk (x) from the more abundant hopanes can appear in the C₃₀ sterane (414→217 Da) trace when C₃₀ sterane abundances are low/absent