1. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 1 (19) Surface chemical analysis of precision weights for the design of optimised cleaning methods. Speaker: Ulf Jacobsson Authors: Ulf Jacobsson and Peter Sjövall SP Swedish National Testing and Research Institute

2. NSCLI 2004 Workshop & Symposium 2 (19) Contents Background The TOF-SIMS instrument Sheet and wire weights Cleaning methods Results Summary Discussion References Acknowledgements

3. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 3 (19) Background for this work A set of high precision mass standards at SP did not have the expected stability. An initial TOF-SIMS analysis showed relatively strong signals from silicon oils (polydimethyl siloxane, PDMS). A method to possibly remove the contaminants was suggested. A comparison with a different weight set was done to see if PDMS-signals could be found on this set as well.

4. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 4 (19) sample The principle of TOF-SIMS spectroscopy Ion source TOF analyzer Time Of Flight Secondary Ion Mass Spectroscopy. Heavy ions Ga + 25 keV are accelerated towards the sample. Secondary ions sputtered out of the surface can be analyzed. Analysis area 100 x 100 µm 2. Mass loss in the order of 1-10 ng.

5. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 5 (19) The TOF-SIMS instrument, interior The sample holder is small, only weights smaller than 1 g can be examined. Sample holder

6. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 6 (19) Some notes about SIMS spectroscopy TOF-SIMS is here used as a semi-quantitative technique. The signal is related to the absolute amount of a substance in a complicated way making it difficult to evaluate spectrums without a reference standard. In a SIMS spectrum all fragments of, for example, long hydrocarbon chains are displayed. Data fitting algorithms are necessary. Work is under way to make quantitative, traceable SIMS measurements with the help of reference standard materials.

7. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 7 (19) Sheet and wire weights OIML-shaped sheet and wire weights in the range 100-500 mg.

8. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 8 (19) Cleaning methods  Light brushing, gently blowing with air  Chamois leather  Ethanol and chamois leather  Water or ethanol in ultra sound bath  Steam jet cleaning (part of the BIPM method)  Soxhlet extraction Several weight cleaning methods can be use depending on the severity of the contamination: The methods above are powerful but unspecific, no consideration is taken about the nature of the pollutants.

9. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 9 (19) Another suggested cleaning method  Immerse the weights in n-heptane to remove silicon oils  Agitate vigorously  Rinse in ethanol to remove other contaminants  Agitate vigorously  Dry with a lint free cloth  Wait 5 days before calibrating the weights

10. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 10 (19) Experiment sequence The experiments were performed during the timescale of a year to observe the build up of contaminants. In connection with four events TOF-SIMS spectroscopy was used with notation below: BC1Before cleaning 1 AC1After cleaning 1 Wait about one year for sheet and wire weights BC2Before cleaning 2 AC2After cleaning 2 In this work normalized spectrums are used, making the total signal level for an element or compound over all measurements add up to 1.

11. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 11 (19) Results 1 TOF-SIMS spectrums Example of a spectrum of positive ions showing number of counts versus mass for some elements and compounds on a weight. DOP - plasticizers P - PDMS

12. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 12 (19) Results 2 TOF-SIMS spectrums Normalized TOF-SIMS signal intensities for a sheet weight.

13. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 13 (19) Results 3 TOF-SIMS spectrums Normalized TOF-SIMS signal intensities for a wire weight.

14. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 14 (19) Results 4 Mass stability

15. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 15 (19) Summary Two weight sets were examined One set containing wire weights, was used for field calibrations. One set containing sheet weights, remain stored in the laboratory. Studied with TOF-SIMS before and after cleaning. Peaks from PDMS-oils lower after cleaning. Peaks from fatty acids not affected. No significant mass changes were observed.

16. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 16 (19) Discussion Possible future use: Tailor specific cleaning methods for weights. Characterisation of surfaces of unstable weights. Possible to make an “image” of the abundance of a particular substance with a lateral resolution of 1µm. Depth profiling where the beam “drills” through contamination and oxide layers.

17. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 17 (19) References of recent work in the field Davidson S., Severn I., and Bayliss D., “Mass standards – a high precision study of commonly used methods for cleaning stainless steel weights” Meas. Sci. Technol., 13, 1178-1182, 2002 Davidson S., ”A review of surface contamination and the stability of standard masses”, Metrologia, 40, 324 – 338, 2003 B. R. Chakraborty, D. E. Lehman and N. Winograd, "Time-of- Flight Static Secondary Ion Mass Spectrometry Analysis of Surface Contamination on Pt/Ir Standard Mass Material", Rapid Commun. in Mass Spec. 12, 1261 – 1265, 1998

18. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 18 (19) Acknowledgements This work has been supported by National Metrology of the Swedish Ministry of Industry, Employment and Communication grant No 38:10. Mr Tommy Pettersson, EXAKT Vågteknik AB for providing wire weights for the experiments Dr Lars Cedheim,SP for suggesting a method to remove PDMS oils

19. SP Swedish National Testing and Research Institute NSCLI 2004 Workshop & Symposium 19 (19) Surface chemical analysis of precision weights for the design of optimised cleaning methods. Authors: Ulf Jacobsson and Peter Sjövall SP Swedish National Testing and Research Institute PO. Box 857, SE-50115 Borås, SWEDEN Email: ulf.jacobsson@sp.se Phone: +46 (0)33 165299