Atmospheric Environment Vol. 12, pp. 2505-2~18.

0 Pergamon Press Ltd. 1978. Printed in Great Britain. 000+6981/78/1201-25005 $02.00/~

INTERLABORATORY COMPARISON OF FLUOROCARBON MEASUREMENTS

R. A. RASMUSSEN

Atmospheric Trace Gas Laboratory, Department of Environmental Technology, Oregon Graduate Center, 19600 N.W. Walker Road, Beaverton, OR 97005, U.S.A.

(First received 27 February 1978 and injna[form 9 May 1978)

Abstract - Concern for the uncertainty in the precision and accuracy of the past and present halocarbon measurements prompted an informal interlaboratory comparison in 1977. The intercomparison of 16 labs for Fll and F12gavestandarddeviationsfrom themean of 11 to 17%. Thegreatest disagreement for Fll was 20% and 28% and 45% for two labs on F12. Looking carefully at the distribution of the Fll values suggests that about half of the labs maintained agreement at a level of 5%. The 2% precision and 10% accurxy believed to be the state-of-the-art ofmeasurement for the fluorocarbons was not supported by this study. The results indicate some of the difficulties of attemuting to combine data from different investigators for analyzing global burdens, gradients and trends. _

Direct atmospheric measurements of the halocarbons F12, F 11 and Ccl., have been made for almost 8 years beginning with Lovelock’s work at Adrigole, Ireland in 1970 (Pack et al., 1977). More recently the measure- ment of CH,CCl, has been included with the list of species routinely analyzed by gas chromatography using electron capture detectors. The intercomparison of the measurements published by the dozen or more laboratories that have been active in fluorocarbon measurements over the past several years has been difficult due to the lack of a standard reference gas. Recognizing this problem in late 1975, a small informal interlaboratory comparison exercise was conducted by the author (Rasmussen et al., 1976). This work led to the convening of a workshop on Halocarbon Analyses and Measurement Technique held in Boul- der, Colorado, on March 2526,1976. As a result of the workshop, a recommendation was made by the parti- cipants that further collaborative tests should be conducted to better evaluate the state-of-the-art of said measurements. The purpose of a continuing interlaboratory comparison would be (a) to obtain more data to evaluate the accuracy of present measure ments, (b) to provide traceability and intercomparison reference points for reconciling the differences in absolute values reported between laboratories, (c) to develop further the technology needed for handling, storing and distributing a set of working reference gases necessary for insuring the highest possible accuracy and comparability of data, and (d) to obtain the experience required for developing a calibration standard of a known certified accuracy containing F12, Fll, CH,CCl, and Ccl,, at trace concentrations comparable to ambient levels.

laboratories. Essentially, 22 stainless steel canisters filled with the same clean background air were distri- buted. Each canister was filled to approx 30 lb absolute pressure, so the vessels were always at a positive pressure of 15 lb or greater relative to the atmosphere. The procedure greatly reduced the possibility that the samples could become contaminated or changed dur- ing the shipment and related handling and storage time that elapsed during the experiment. The larger volume of available air in these vessels than if eva- cuated canisters filled to ambient pressure had been used also allowed multiple analyses to be performed on the samples by the participating laboratories.

After the 22 canisters had been filled with clean ambient air each one was analyzed three times to determine the actual concentrations of F12, Fll, CH,CCl, and Ccl, before they were shipped to other laboratories. The mean values from the results of the analyses on all 22 samples are given in Table 1 as the submitted values of the halocarbons in the canisters distributed for analyses. The submitted values are essentially the same as the general atmospheric levels of these species in the northern hemisphere obtained during this period as determined in other discrete sampling studies. Therefore, the air contained in the 22 canisters is believed to be representative of northern hemispheric background concentrations for early 1976.

The results from 16 of the laboratories to which samples were sent have been received (Table 2). The laboratories reporting results include 6 in the United States, 2 in Great Britain, 1 in JCR-Italy, 3 in Germany, 1 in Australia, 1 in the Netherlands and 2 in Canada.

In the work reported in this communication, the Overall, the results show fair agreement between data are limited to reporting the random and syste- laboratories. This is suggested by the 11% and 17% matic differences observed between the participating variability for Fll and F12 (Table 1) calculated as the

2505

2506 R. A. hSMUS.SFiN

Table 1. Summary values of halocarbons in samples analyzed for interlaboratory comparison

Submitted sample analysis

F12 Fll CH,CCl, ccl,

PPt v/v PPt v/v PPt v/v PPt v/v

8 250 150 104 150 4.5 1.6 4.0 5.8 2 1.0 4 4

22 22 22 22

Returned results

averaged

R 253 140 88 141 43 16 34 68 17 11 39 48 13 16 9 10

Table 2. Halocarbon results returned from participating laboratories

Sample CAN F12 Fll CH,CCl, ccl, No. No. PPt v/v PPt 'v/v PPt v/v PPt v/v

1 75

2 12

3 11

4 42

5 97

6 77

7 70

8 50

9 15

10 25

11 91

12 35

13 1

14 108

15 71

16 74

243 f 10 2.8%

246* 1.6%

223 f 20 10.8%

250& 2.5

0%

227 9.2%

246 f 12.6 1.6%

322* 19 28.8%

250

0%

362f20 44.8%

249

0%

224 10.4%

243k 4.9 2.8%

204* 16 18.4%

145* 15 3.3%

136 9.3%

120* 10 20.0%

148& 1.5 1.3%

145* 30 3.3%

149 0.7%

15ok 3.6 0%

176& 11 17.3%

120 20.0%

MO* 10

0%

141 6.0%

156 4.0%

124 17.3%

145* 0.5 3.3%

123+ 2.5 18.0%

120* 1.4 20.0%

97 167

100* 10 144+ 3

130

112* 4.3 105+ 3.6

80 120

103k 6 108 + 10

130

37

106

30+ 14

71

126

122

321* 10

Percent difference between participating lab results (PLR) and submitted value (SV):

%= (PLR) - (SV) x loo

(SV) .

Interlaboratory comparison of fluorocarbon measurements 2507

percent standard deviation from the mean values returned (Table 2). It is unfortunate that only half of the laboratories indicated their precision of analyses. A more complete return of the standard deviation of the analyses would have enabled the random error in the data to be better appreciated. Also provided in Table 2 are the percent differences calculated between the individual results of the participating laboratories and the submitted values for Fll and F12. The much greater variability in the CH,CCl, (39%) and Ccl; values (48%) (shown in Table 1) represents both analytical as well as sample stability problems related to wall adsorption phenomena. Because of this sample problem the CH,CCI, and Ccl, data are not dis- cussed in detail. The spread in the fluorocarbon results implies that the measurements made by the inde- pendent laboratories involved in the experiment do not yet have the + loo% maximum systematic error necessary for assessing the global burden and the respective lifetimes of the fluorocarbons from the different data published in the literature. However, in fairness it should be stated that the agreement between laboratories has improved since the Halocarbon Work- shop of March 1976. In fact., the variability in the Fll and F12 measurements has decreased from the respective 15% and 18% values obtained from the first interlaboratory calibration experiment of 1976 to the present 11% and 17% values obtained in 1977. The Fll analyses show an improved agreement, whereas the F12 data show no significant improvement.

Averaging all of the values returned by the parti- cipating laboratories may be unwarranted since there is no reason to expect or want the systematic errors to cancel to provide average values close to the submitted values. Accordingly the very good agreement between the mean F12 values of 250 vs 253 ppt v/v (submitted vs returned average) is not significant. The less good agreement between the mean Fl 1 values of 150 vs 140ppt v/v (submitted vs returned average) is pro- bably more indicative of the degree of intercompara- bility of the results obtained in this experiment. What is important is the need to appreciate the distribution in the values returned compared to the submitted value. This is because these data are more indicative of the systematic errors in the results and accordingly reflect something about the absolute accuracy of the halocarbon concentrations reported by each labo- ratory. These distributions are shown in Figs. 1, 2, 3 and 4. The Fll data in Fig. 1 show an interesting bimodal distribution with an equal number of results between 120-125 ppt v/v as between 145-150ppt v/v and the remaining returns grouped around the sub- mitted value of 150ppt v/v.

Accordingly these figures do more to demonstrate the degree of agreement or disagreement in the results than that determined by averaged values. As expected the results are biased around or away from the submitted value. In fact it is an understanding of the magnitude and probable causes of the variability that is needed if the objective of assessing the differences

pp1 v/v

Fig. 1, Bar graph of Fll determinations.

200 220 240 260 260 300 320 340 360

ppt v/v

Fig. 2. Bar graph of F12 determinations.

3-

::2

?I

0

ppi v/v

Fig. 3. Bar graph of CCI, determinations.

cHfxI, ~Submittad Value

40 60 60 100 120 140 160

ppt v/v

Fig. 4. Bar graph of CH,CCI, determinations.

between labs attempting to do the same experiment is to be accomplished. Since all the labs used essentially the same gas chromatographic technique there is good reason to suspect that the preparation of the standards used in the individual laboratories were responsible for the magnitude of the systematic errors observed. It was intended origbtally to analyze the samples after they were returned. Unfortunately very few of the returned canisters had enough air left in them to conduct this phase of the study. Accordingly, we should be careful

2508 R. A. Rusmvss~~

not to over interpret the limited data base provided by this experiment.

In general the intercalibration experiment has been informative and worthwhile. It has served to get halocarbon measurements in different laboratories intercalibrated with the goal of producing consistent results on the same sample. It has reaffirmed that while agreement between laboratories ranges from poor to excellent, the overall agreement is not as good as required for monitoring or determining, the global burden of the respective species with the purpose’of calculating their atmospheric lifetimes to the degree desired by present models using the data in the literature. It has shown the need to require that the participating laboratories report the number of in- dividual measurements made on a given sample since only about half of the labs participating in the experiment supplied this information. Certainly there is a need for future intercalibration experiments on a more formal basis and the development of standard reference materials that span the compositional range of interest so information on the realiability of the measurements and the nature of the analytical errors can be identified by means of double or muhiple blind samples. The minimum that should be expected from future fluorocarbon comparisons is the mean and the standard deviation from severa! measurements on the common sample made by each laboratory so similar statistics can be prepared from the means returned on

the submitted standard since both the absolute and relative errors are of interest.

Acknowledgements - I would like to gratefully acknowledge the contributions of the many colleagues who interacted in the various aspects of this work. Special thanks are given to Drs. F. Bower, L. Glasgow and R. Prinn for their helpful critique of the original draft. I also want to thank those colleagues at the other laboratories who participated in the analyses Without their contribution, this study would not have been possible.

The participants were: M. DeBortoli, JRC-ISPRA; R. Cicerone, University of Michigan ; M. Ferman, GM Research Lab.: D. Ehhalt. Jiilich: L. Elias. NRC: L. Heidt. J. Krasnec. NCAR ; f. Go&n, NGAA ; R. Guiche&, TN0 ; j. Lovelock; Bowerchalke; S. Penkett, Harwell; P. Fraser, CSIRO; H. Oeser, Hoechst ; Proggmeyer, Battelle-Frankfurt ; H. Schiff, York University ; H. Sit@, SRI ; and T. Thompson, NOAA.

The support for this work was provided by hmds from MCA-76-142.

REFERENCES

Pack C. H., Lovelock J. E., Cotton G. and Curthoys C. (1977) Halocarbon behaviour from a long time series. Atmos- pheric Environment 11, 329-344.

Rasmussen R. A., Pierotti D. J. and Krasnec J. (1976)Analysi.s of halocarbons in the atmosphere: Report of a workshop. Presented at the 69th Annual Meeting of the Air Pollution Control Association paper No. 76-15.1.

Rasmussen R. A. (ed.) Report of the workshop on halocarbon analysis and measurement, Boulder, CO, NASA, Washing ton, D.C., March 25-26, 1976.