Reduced drift, high accuracy stable carbon isotope ratio measurements using a reference gas with the Picarro 13 CO 2 G2101-i gas analyzer

Transcription

1 Reduced drft, hgh accuracy stable carbon sotope rato measurements usng a reference gas wth the Pcarro 13 CO 2 G2101- gas analyzer Chrs Rella, Ph.D. Drector of Research & Development Pcarro, Inc., Sunnyvale, Calforna rella@pcarro.com Summary: The Pcarro G2101- gas analyzer provdes exceptonally low drft measurements rght out of the box, allowng feld measurements of the stable sotope rato 13 CO 2 wth a precson of 0.3 perml n fve mnutes, and a drft of about 1-2 perml n a 24 hour perod. For more demandng applcatons, t s possble to reduce the drft dramatcally by referencng the measurements to a secondary or workng gas standard. In ths whte paper we descrbe a method for employng a workng standard to mprove the measurement precson to below 0.1 perml. If the workng standard s also traceable to a prmary reference scale, ths precson can translate nto an overall accuracy of better than 0.1 perml. Performance of the G1101- and G2101- wth and wthout a reference gas The Pcarro G2101- s a laser-based analyzer that employs cavty rngdown technology to measure the 13 CO 2 stable sotope abundance. The stable sotope abundance s measured by measurng two ndependent spectral absorpton lnes n the near-nfrared regon of the spectrum, one for 12 C 16 O 2 and one for 13 C 16 O 2. The rato of the peak heghts of the lnes s a measure of the rato of the concentratons of each of the two sotopologs. The strength of the absorpton features are ntrnsc propertes of the molecules that depends only on temperature, pressure, and the composton of the background gas mxture. Temperature and pressure are very well-controlled n the G1101- and the G Therefore, the ablty to measure the spectra repeatably over tme wll lead to repeatable, low-drft sotope rato measurements. Pcarro s WS-CRDS technology produces ncredbly stable spectra, but even wth these nstruments there s always some small resdual drft, whch s the order of 1 2 perml. In prncple, some or all of ths drft could be elmnated through use of a workng standard, dependng on the source of the drft. To nvestgate the potental advantages of operatng wth a workng reference, we performed a smple experment wth two gas

2 bottles (Fgure 1). The 3-way valve was swtched every two mnutes. The average sotope rato from each two mnute segment was calculated, after dscardng the short segment of tme near the transtons to allow the nstrument to settle fully. The experment ran for four days. Fgure 1: schematc for testng the accuracy of the nstrument usng a workng standard. Note: the G1101- was upgraded to be able to hand fast concentraton transents, a package whch comes standard n the G In Fgure 2 we dsplay the raw data from the full four day test, as well as a detaled zoom of a partcular pont n tme. Note the drft n the concentraton of bottle #2. Ths drft was caused by the fact that the bottle was nearly empty (less than 100 p.s..) and was no longer delverng a stable concentraton to the analyzer.

3 Fgure 2: concentraton data from the four day test on two bottles. Each step s two mnutes n duraton. Analyzng each two mnute segment gves Fgure 3, showng the resultng carbon doxde concentraton from each of the two bottles. The drft of bottle #2 s clear from ths fgure (note the change n scale between bottle #1 and bottle #2). Gven the drft of bottle #2, we wll consder bottle #1 as the workng standard, though ths dstncton s not mportant for ths experment.

4 Fgure 3: concentraton values for each 2-mnute segment. Note the break n the vertcal axs, and the change n scale. Now, we nvestgate the performance of the reported stable sotope rato. Frst, we look at the reported values from bottle #1, n Fgure 4. Note that above 20 mnutes, there s lttle to no beneft from averagng the standard devaton never drops below 0.22 perml, even wth a four hour average. Ths lmt s due to systematc drft n the nstrument over the four day measurement perod. The data for bottle #2 follows a smlar dependence.

5 Fgure 4: reported delta from bottle #1, wth multple averages We now plot the dfference between the reported delta from of the two bottles. The dfference s calculated usng the followng expresson: 2 = δ ( δ + 1 δ ) + where n s the delta reported from the n th bottle (ether #1 or #2), and - s the measurement of bottle #1 mmedately precedng the th measurement of bottle #2, and + s the measurement mmedately after. Usng ths procedure, we can derve the dataset shown n Fgure 5. As expected, the data are about 1.4 tmes noser for 4 mnute averages, but the precson rapdly mproves. The precson at 20 mnutes s already an mprovement over the sngle bottle data, and at two hours, the overall precson has decreased to below 0.1 perml. Interestngly enough, ths should be a worst case scenaro for the performance of ths method the drft observed n bottle #2 on the concentraton data s lkely also to be observed to some extent n the sotope rato data. It s a dstnct possblty that the overall +0.1 perml drft over the duraton of the experment s due to ths bottle drft.

6 Fgure 5: reported dfference of the sotope ratos between bottle #1 and bottle #2. We present the results of the rollng average analyss for the sngle bottle data and dfference, n Fgure 6. The precson s defned as the standard devaton of the entre four days of data. The blue dashed lne n the fgure follows a smple square root law for the mprovement of the precson. The sngle bottle does not contnue to mprove past about 5-10 mnutes, whereas the referenced data contnues to mprove even past 100 mnutes. Agan, t s mportant to remember that ths performance may be lmted by the fact that bottle #2 was drftng. Further valdaton of the method s requred.

7 Fgure 6: precson reported from the four days of measurements for the sngle bottle data and the referenced two-bottle data. Summary Usng a smple workng standard as a reference, t s possble to acheve sub- 0.1 perml precson usng the G If the workng standard s also traceable wth hgh accuracy to a prmary reference scale, ths precson can translate nto an overall accuracy of better than 0.1 perml. Acknowledgments The author would lke to thank Professor Ralph Keelng of the Scrpps Insttuton of Oceanography for suggestng ths lne of research.