Light-weight flight type double focus and quadruple mass spectrometer models were compared. Data cover size, weight, and power sensitivity required to achieve same resolution sensitivity at given mass number. Comparison was made using mathematical relationships. Analysis was confined to equal ion source area sensitivity variations not more than 40% over mass range

Wilson, S. K.

English

NASA Technical Reports Server

December 1972
	
B72-10702 
NASA TECH BRIEF 
NASA Pasadena Office	 4V 
NASA Tech Briefs announce now technology derived from the U.S. space program. They are issued to encourage 
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Comparative Performance of Double-Focus and
Quadrupole Mass Spectrometers 
Light-weight flight-type models of double-focus 
and quadrupole mass spectrometers have been com-
pared with respect to the size, weight, and power re-
quired to achieve the same resolution and sensitivity 
at a given mass number; the comparison was made 
with the aid of mathematical relationships and the 
analysis was confined to the conditions of equal ion-
source area and sensitivity variations of not more than 
40% over the mass range. 
For the analysis, a mission was considered which 
gave the quadrupole instrument a weight-to-power 
penalty of 226 g/W. To minimize the effective weight, 
the actual weight was made equal to the weight pen-
alty. For alternative missions, different weight penalty 
may be required to obtain the minimum effective 
weight. 
The symmetrical double-focus instrument has the 
least weight and largest ion current; therefore, the 
derived design is based on symmetrical electric and 
magnetic sectors, both taken at an angle of 900 since 
this angle provides the (nearly) shortest ion path 
length. A weight-to-power penalty was not used for 
the design since primary emphasis was on the max-
imum mass number, and maximum power consump-
tion is a function of mass range and not maximum 
mass number. 
When the instruments are used with equivalent ion 
sources and a voltage spread of 1OV, the quadrupole 
has about twice the weight and maximum power. The 
sizes of the two types are about the same. The max-
imum dynamic voltage for the quadrupole is less than
the static voltage for the double-focus mass spectrom-
eter. The maximum ion current for the quadrupole is 
about four times that of the double-focused instru-
ment. (However, the ion source suffers before the ion 
current limit is reached for either instrument.) The 
voltage stability and mechanical tolerance are about 
the same for both instruments. With weight and power 
as bases of comparison, the double-focus mass spec-
trometer seems to be the better. 
In computed data, the weight of the double-focus 
instrument is the magnet weight only, while the 
weight of the quadrupole is the weight of only the 
four rods. The weight of the vacuum envelope for the 
double-focus instrument should be considerably less 
than that of the quadrupole, since it is much smaller; 
however, the combined weight of the magnet yoke 
and the vacuum housing may be about equal to the 
weight of the quadrupole housing. 
Suggested techniques for reducing the magnet 
weight of the double-focus instrument include utiliza-
tion of z-axis focusing, which would permit a longer 
object slit. With an object slit four times as long, the 
weight of the magnet could be reduced by a factor 
of two; maximum voltage would be reduced by a 
factor of 4 and power by a factor of 16. A reduction 
in magnet weight by a factor of 2 might also be 
achieved by using a smaller angle for the magnetic 
sector with an increase in the ion path length. Quad-
rupole weight might be reduced by removal of mate-
rial from the center of the rods or by using lighter 
material such as metallized ceramic. 
(continued overleaf) 
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Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
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Note: 
The following documentation may be obtained 
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Reference:
JPL Technical Memorandum TM 33-456 
(N70-40790), Comparative Performance of 
Double-Focused and Quadrupole Mass 
Spectrometers.
Source: Samuel K.Wilson of 
Caltech/JPL
under contract to

NASA Pasadena Office
(NPO-11689)
. 
I 
B72-10702	 Category 03


Comparative performance of double-focus and quadrupole mass spectrometers

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