Comment pubs.acs.org/ac
Comment on “Intracavity OptoGalvanic Spectroscopy Not Suitable for Ambient Level Radiocarbon Detection” Daniel Murnick,* Junming Liu, and Mark DeGuzman Department of Physics, Rutgers University, Newark, New Jersey 07102, United States
Anal. Chem. 2015, 87, 9025−9032. Difficulties are apparent in the first set of experiments presented in Figure 2 of Paul and Meijer. The time variation in the transversely irradiated 12CO2 ratios as well as the ICOGS 14 CO2 ratios indicates possible problems with their experimental system. These ratios should be constant and should exhibit a linear Allan variance. Possible experimental variables influencing their measurements but not carefully controlled by the authors are temperature, pressure, and gas flow rate drifts. Most importantly, without a well-understood external reference cell, the 14CO2 laser wavelength was unstabilized. The figure clearly shows inconsistent results. Depending on chopping rate, pressure, temperature, and exact laser power and lock point used, the expected separation in the case studied can be small but should be constant. The 14C concentration depends on the 12 CO2 ratio and requires a rigorous deconvolution algorithm. In the absence of a well-characterized and stable reference cell, all dependent variables must be constant to quantitatively compare samples with the configuration used. The data displayed simply indicates that that was not the case for the experiments described. The independent batch mode experiments using enriched 14 CO2 described in the Paul and Meijer work are interesting but have little relationship to the title or conclusions of their paper. Again, there is no reference cell used and, significantly, there is no 12CO2 monitoring shown. As the experiments are done in batch mode, there must be time dependent dissociation present that does not come to equilibrium for possibly many hours. This would have been obvious with 12CO2 monitoring. Nevertheless, with a more careful analysis of their data, some valid conclusions might have been drawn. The graphs and corresponding raw data shown in Figure 1 were kindly sent to us in mid 2014 by one of the authors, Dipayan Paul, and are similar to Figure 3 of Paul and Meijer. There is a significant pressure change observed over the course of the measurements that is less obvious in Figure 3 of Paul and Meijer due to a scale change. No pressure dependent studies were carried out in their work, but the sensitivity to pressure is documented in their references 7 and 10. Their laser is stabilized on the peak of the ICOGS signal, which is not necessarily the peak of the 14CO2 resonance, obviously not for the P(20) line where, as the authors’ note, it is due to a very close accidental coincidence in 12CO2. The laser-off feed through signal, given as 0.4 mV, should have been subtracted as a vector from the total signal for proper analysis. The total background also consists of ionization from high lying states and off resonant enhancement of other absorbing transitions. For 100% CO2, the ionization
n a recent paper in Analytical Chemistry, Paul and Meijer1 reported a series of survey experiments with a new intracavity optogalvanic system. Data obtained were heuristically reduced in an attempt to calibrate the system for radiocarbon analysis. In our opinion, the title of the paper does not describe the work reported and the conclusions reached are not supported by the data presented. No theoretical explanations are given for the manner in which their system was designed nor for the method by which their data were analyzed. In this Comment, some of the major inconsistencies in their work are pointed out and it is shown that with a more careful analysis of their data their conclusions can be refuted. In 2008, a new intracavity optogalvanic spectroscopy, given the acronym ICOGS, was introduced.2 Though the basic physics of the technique was described at that time, the experiments reported were complex and required a thorough understanding of gas laser and glow discharge physics to properly carry out. Due to gaps in a full theoretical understanding, it was noted then that many variables had to be carefully controlled with respect to both the laser and the sample discharge cell in order to obtain consistent results. In particular, it was pointed out that measurements using an external reference discharge cell were crucial for control and stabilization of laser and system variables. Typically, if one is to claim that a particular experiment is incorrect, it is necessary to provide a valid theoretical basis for that claim and/or to demonstrate unequivocally that the reported results could not be reproduced under identical conditions. Paul and Meijer do neither. Their work begins by stating that a new two internal cell laser system is to be used, eliminating the crucial external reference cell, containing highly enriched radiocarbon, which had been used in the original ICOGS system. It then makes the logically inconsistent assumption that an internal reference with natural abundance radiocarbon is equivalent to an enriched external cell. In order to determine the effect of the internal cell, it must first be compared with an external cell including benchmark tests such as stability and Allan variance measurements. This was not done. Further, as in previous studies2,3 and the authors’ own work, it was shown that the total ICOGS radiocarbon signal with natural abundance radiocarbon present has a high background component such that the analysis technique they describe is nonquantitative and can only be used to test stability. Further, the so-called reference cell signal can not be used to lock the laser to the center of the 14CO2 resonance but, rather, to near the peak of the laser gain profile. In the set of experiments with enriched 14CO2 reported by Paul and Meijer, the sample itself is used as its own reference. No control or benchmark experiments are presented to validate their system or analysis.
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© 2016 American Chemical Society
Published: March 25, 2016 4571
DOI: 10.1021/acs.analchem.5b04943 Anal. Chem. 2016, 88, 4571−4573
Comment
Analytical Chemistry
Figure 1. Data obtained with the system of Paul and Meijer, similar to that of Figure 3 of ref 1 and analyzed, in 2014, as described in the text.
level background along with the nonresonant CO2 far wing background can be found from the signal vector for dead CO2. Then, by subtracting that vector, the remaining 14CO2 vector, amplitude, and phase can be determined. We have attempted to do that with the data set supplied by Dr. Paul in 2014, sharing the results shown in Figure 2 with the authors at that time. Figure 2a shows the 14CO2 amplitudes, and in Figure 2b are the corresponding phases of the resultant vector at the chopping frequency when all calculable backgrounds are subtracted. Note that contrary to the conclusions of Paul and Meijer there is a clear and consistent separation in amplitude from dead (taken as zero 14C amplitude and plotted at a 14C ratio of 10−15) to modern and beyond for all wavelengths studied. The points at 10−9 concentration, in the data set provided, are above the trend lines, possibly due to variations in discharge system parameters for what we were told was the first measurement in the series. The phases for all but the P(20) line are the same for all concentrations, as expected, except for the highest concentration where some of the ICOGS theory4 assumptions may need modification. The P(20) phase variation is likely due to the large effect of the nearby 12CO2 background resonance and the fact that the laser stabilization is not at the proper wavelength. It would be interesting to carry out similar vector decompositions for the data summarized in Figure 5 of Paul and Meijer. It should be noted that the statistical uncertainties for the data of Figures 1 and 3 of Paul and Meijer are significantly smaller than what is shown in Figure 5 of their work. Paul and Meijer state that is because they have averaged three independent measurements and show the standard deviation about the mean. The fact that independent measurements vary significantly from run to run confirms to us that the lack of a reference cell and corresponding control of crucial experimental variables by Paul and Meijer is the true cause for their variability. As the
Figure 2. (a) Amplitude of 14CO2 contribution to the total ICOGS signal at the fundamental chopping frequency as determined by proper background subtraction from the data of Figure 1. (b) Corresponding phase of the 14 CO2 contribution. In both cases, the scale for the P(20) line is on the right. 4572
DOI: 10.1021/acs.analchem.5b04943 Anal. Chem. 2016, 88, 4571−4573
Comment
Analytical Chemistry uncontrolled variables mainly effect the background and the heuristic analysis method used by Paul and Meijer ignores background contributions, the variability in the numbers they quote for power normalized total signal are not surprising. In summary, we would conclude that Paul and Meijer’s work, though confusing, has made a contribution to ICOGS science for analysis of radiocarbon. Their hypothesis that an internal reference cell with contemporary levels of 14CO2 can be substituted for an external reference cell with enriched radiocarbon is unproven. Contrary to the conclusions in Paul and Meijer, their survey measurements with enriched 14CO2 without an external reference cell or proper stabilization of their 14CO2 laser still shows clear validity for the ICOGS technique reported in ref 1 when analyzed carefully. Their work may lead to advancement of the development of intracavity optogalvanic instrumentation for radiocarbon detection.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We wish to thank Dipayan Paul for generously sharing his raw data with us prior to publication. Our work is supported by the US National Science Foundation under grant 1454918.
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REFERENCES
(1) Paul, D.; Meijer, H. A. J. Anal. Chem. 2015, 87 (17), 9025−9032. (2) Murnick, D. E.; Dogru, O.; Ilkmen, E. Anal. Chem. 2008, 80 (13), 4820−4. (3) Persson, A.; Eilers, G.; Ryderfors, L.; Mukhtar, E.; Possnert, G.; Salehpour, M. Anal. Chem. 2013, 85 (14), 6790−6798. (4) Kimble, H. J. IEEE J. Quantum Electron. 1980, 16 (4), 455−461.
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DOI: 10.1021/acs.analchem.5b04943 Anal. Chem. 2016, 88, 4571−4573
