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Background of the workshop on residual gas analyzer calibration
Vacuum/volume 35/number Printed in Great Britain
0042-207X85$3.00+ .OO @ 1985 Pergamon Press Ltd
12fpages 629 to 636/l 985
Abstracts from a workshop residual gas analysers” 7-9 May 1985 Gaithersburg,
Maryland,
Background of the workshop on residual gas analyzer calibration Strictly speaking, residual gas analysers ‘(RGAs) are mass spectrometers that have been optimized for sensitivity and ultrahigh vacuum compatibility so that they may be used to determine the gas constituents of a vacuum system at its lowest pressures. More generally, the term has come to identify ‘low cost’ mass spectrometers, now almost exclusively of the quadrupole type, that can be attached as an accessory to vacuum systems and used to determine gas composition at ‘elevated’ pressures as well as at base vacuums. In recent years the cost of these instruments has come down significantly and their operation automated to varying degrees so that they can be used by someone not skilled in the art. As a consequence, their use in both science and industry has grown significantly. In many of these applications there is a desire for quantitative data, in some cases with errors as small as 10, or even 1%. Unfortunately, very little work has been done in the way of systematically evaluating the absolute accuracy of the variety of RGAs available; their design is complex and a large number of variables are subject to change; and they are
Quadrupole mass spectrometers: design considerations and performance Peter H Dawson, Division of Microstructural Sciences, National Research Council of Canada, Ottawa, Ontario, Canada KlA OR6 The interrelationships of the various elements of quadrupole system design-source, lens system, analyser, detector, data system-are discussed with the emphasis on how these influence RGA performance, particularly with regard to stability, reproducibility, linearity and mass discrimination.
The limitations of the quadrupole mass spectrometer when used for gas analysis J H Leek, Department of Electrical Engineering and Electronics, The University of Liverpool, Liverpool M9 3BX, UK Three design parameters control the ‘quality’ of a quadrupole mass spectrometer with a given specification. These parameters
* Sponsored by the US National Bureau of Standards and the Fusion Technology Division of the American Vacuum Society.
on calibration
of
USA
increasingly used where the facilities and people required to monitor their performance are not available. Therefore, it was felt desirable to hold a workshop to examine the calibration of RGAs. The questions to be addressed were: the need for calibration; observed instrument performance; and possible calibration techniques. The workshop was held 7-9 May 1985, in Gaithersburg, Maryland, and was co-sponsored by the National Bureau of Standards and the Fusion Technology Division of the American Vacuum Society. Attendance was encouraged from all areas of vacuum technology. The 80 attendees agreed that while no definitive conclusions had been reached, the session was a very useful start in addressing these issues and should be followed by a similar meeting in 2 yr time. Charles R Tilford US Department of Commerce, NBS, Gaithersburg, Maryland 20899, USA
are: (i) the maximum voltage available from the ac power supply (I’,,,); (ii) the length (1) and (iii) the diameter (d) of the ‘rods’ in the quadrupole filter. A rule of thumb guide is that the quality of any instrument is proportional to the product d x 1x V,. (The cost is of course also approximately proportional to this product.) For example for two instruments with the same V, and d and designed to have the same sensitivity, resolution and mass range the one with the longer rods is likely to perform better in a given situation. The ion injection energy would be lower for the shorter instrument. This would dictate a high sensitivity source. Both the low injection energy and the high energy source increase the possibility of non-linear unstable operation. Small instruments specially made for residual gas analysis must inevitably sacrifice some desirable operating characteristics in order to meet the requirements of small size, simplicity and low cost. For example they may become distinctly non-linear at the high pressure end of the operating range. This effect is very serious in many commercial residual gas analysers where behaviour becomes complex at lo- ’ torr even though manufacturers claim linearity to at least 5 x lo-’ torr. Further, sensitivity may not be independent of pressure even in the low pressure regime 1O-7-1O-5 torr. There are significant differences in performance for instruments with similar specifications. It would help in the choice of instruments for a particular application if a standard method of presenting performance data could be agreed to by manufacturers. (For example quoting ‘operating pressure range’ would imply rigorous definitions of ‘linearity’ and minimum detectable partial pressure.) 629
0042-207X85$3.00+ .OO @ 1985 Pergamon Press Ltd
12fpages 629 to 636/l 985
Abstracts from a workshop residual gas analysers” 7-9 May 1985 Gaithersburg,
Maryland,
Background of the workshop on residual gas analyzer calibration Strictly speaking, residual gas analysers ‘(RGAs) are mass spectrometers that have been optimized for sensitivity and ultrahigh vacuum compatibility so that they may be used to determine the gas constituents of a vacuum system at its lowest pressures. More generally, the term has come to identify ‘low cost’ mass spectrometers, now almost exclusively of the quadrupole type, that can be attached as an accessory to vacuum systems and used to determine gas composition at ‘elevated’ pressures as well as at base vacuums. In recent years the cost of these instruments has come down significantly and their operation automated to varying degrees so that they can be used by someone not skilled in the art. As a consequence, their use in both science and industry has grown significantly. In many of these applications there is a desire for quantitative data, in some cases with errors as small as 10, or even 1%. Unfortunately, very little work has been done in the way of systematically evaluating the absolute accuracy of the variety of RGAs available; their design is complex and a large number of variables are subject to change; and they are
Quadrupole mass spectrometers: design considerations and performance Peter H Dawson, Division of Microstructural Sciences, National Research Council of Canada, Ottawa, Ontario, Canada KlA OR6 The interrelationships of the various elements of quadrupole system design-source, lens system, analyser, detector, data system-are discussed with the emphasis on how these influence RGA performance, particularly with regard to stability, reproducibility, linearity and mass discrimination.
The limitations of the quadrupole mass spectrometer when used for gas analysis J H Leek, Department of Electrical Engineering and Electronics, The University of Liverpool, Liverpool M9 3BX, UK Three design parameters control the ‘quality’ of a quadrupole mass spectrometer with a given specification. These parameters
* Sponsored by the US National Bureau of Standards and the Fusion Technology Division of the American Vacuum Society.
on calibration
of
USA
increasingly used where the facilities and people required to monitor their performance are not available. Therefore, it was felt desirable to hold a workshop to examine the calibration of RGAs. The questions to be addressed were: the need for calibration; observed instrument performance; and possible calibration techniques. The workshop was held 7-9 May 1985, in Gaithersburg, Maryland, and was co-sponsored by the National Bureau of Standards and the Fusion Technology Division of the American Vacuum Society. Attendance was encouraged from all areas of vacuum technology. The 80 attendees agreed that while no definitive conclusions had been reached, the session was a very useful start in addressing these issues and should be followed by a similar meeting in 2 yr time. Charles R Tilford US Department of Commerce, NBS, Gaithersburg, Maryland 20899, USA
are: (i) the maximum voltage available from the ac power supply (I’,,,); (ii) the length (1) and (iii) the diameter (d) of the ‘rods’ in the quadrupole filter. A rule of thumb guide is that the quality of any instrument is proportional to the product d x 1x V,. (The cost is of course also approximately proportional to this product.) For example for two instruments with the same V, and d and designed to have the same sensitivity, resolution and mass range the one with the longer rods is likely to perform better in a given situation. The ion injection energy would be lower for the shorter instrument. This would dictate a high sensitivity source. Both the low injection energy and the high energy source increase the possibility of non-linear unstable operation. Small instruments specially made for residual gas analysis must inevitably sacrifice some desirable operating characteristics in order to meet the requirements of small size, simplicity and low cost. For example they may become distinctly non-linear at the high pressure end of the operating range. This effect is very serious in many commercial residual gas analysers where behaviour becomes complex at lo- ’ torr even though manufacturers claim linearity to at least 5 x lo-’ torr. Further, sensitivity may not be independent of pressure even in the low pressure regime 1O-7-1O-5 torr. There are significant differences in performance for instruments with similar specifications. It would help in the choice of instruments for a particular application if a standard method of presenting performance data could be agreed to by manufacturers. (For example quoting ‘operating pressure range’ would imply rigorous definitions of ‘linearity’ and minimum detectable partial pressure.) 629