Ion-retarding lens which increases the abundance sensitivity of tandem magnetic-analyzer mass spectrometers measures isotopes of low abundance in mass positions adjacent to isotopes of high abundance. The lens increases the abundance sensitivity for isotopes lying farther from high abundance isotopes than the energy cutoff of the lens

Kaiser, K. A.

Stevens, C. M.

English

NASA Technical Reports Server

June 1969
	
Brief 69-10166 
• 4 tsoy cQ.	
AEC-NASA TECH BRIEF 
\LjiJ TIES
AEC-NASA Tech Briefs describe innovations resulting from the research and development program 
of the U.S. AEC or from AEC-NASA interagency efforts. They are issued to encourage commercial 
application. Tech Briefs are published by NASA and may be purchased, 'at 15 cents each, from 
the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Ion-Refrardi ng Lens Improves the Abundance Sensitivity 
, of Tandem Mass Snectrometers 
Retarding Lens
Detector Defining Slit 
Center Slit
 ll 
e.-so.rc.
neticSectors 
Of 30cm Radios 
FIGURE 1. TANDEM MASS SPECTROMETER WITH THE RETARDING LENS IN POSITION 
This information should be of interest to manufac-
turers of mass spectrometers. 
The problem: 
To measure isotopes of very low abundance in mass 
positions adjacent to an isotope of high abundance. 
When the mass spectrum is scanned, "tails" are de-
tected on either side of the mass position of the high-
abundance isotope. These tails are frequently larger 
than the low'-abundance isotopes being measured. One 
method used to reduce these "tails" employs a mass 
spectrometer containing two magnetic analyzers in 
tandem. The resulting "tail," however, is only slightly 
improved over that for a single stage. 
The solution: 
An ion-retarding lens which increases the abund-
ance sensitivity of tandem magnetic-analyzer mass 
spectrometers. The simple and relatively inexpensive 
lens greatly increases the abundance sensitivity for 
those isotopes lying farther from the high-abundance
Fyteu Studs 
Electrodes 
Pyrex Spacers 
	
Scale In CM
	 Spring Loaded
	
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 L_.i I II	 HV Connectors 
0	 2	 4 
FIGURE 2. SECTION DRAWING OF RETARDING LENS

ALL PARTS ARE OF PYRE)( OR TYPE 304 STAINLESS STEEL.) 
isotope than the energy cutoff of the lens. The lens is 
a high-pass energy filter with a cutoff of about 40 
volts below 13,000 volts, which is equivalent to about 
2/3 of a mass unit at mass 250. 
Under typical operating conditions in a 30-cm-
radius, two-stage system, the abundance sensitivity 
improved by a factor of about 10 on the high-mass 
side and about 101 on the low-mass side. 
How it's done: 
The lens is of an einzel or unipotential type with 
thick center electrodes. The lens-focusing properties 
are derived from a two-dimensional potential field with 
an axis of symmetry by numerically integrating a 
paraxial trajectory equation. 
The lens is placed between the spectrometer defin-
ing slit and the detector, the defining slit becoming 
part of the first lens electrode (Figure 1). A section 
drawing of the lens is shown in Figure 2. For an ion-
accelerating voltage of 15 kV and about 40 V below 
(continued overleaf) 
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15 kV (14,960 V) on the center electrodes, the lens 
focuses on the detector which is 8 cm. away. This 
lens, which operates at 15 kV, is only 6 cm long and 
is readily installed in an existing collector system. 
The normal construction procedures used for any-
high-voltage apparatus were followed. Of most im-
portance were: 1) rounding all corners and edges 
that "see" the high voltage, 2) electro-polishing all 
surfaces that "see" the high voltage, and 3) thor-
oughly ion-cleaning the lens before use. 
The performance of the lens was determined by 
1) applying the focus voltage to the center electrodes 
to turn the lens on, or 2) grounding the lens center 
electrodes, thereby effectively turning the lens off. 
These steps were performed alternately while holding 
the other instrument parameters . constant. 
Notes: 
I. There may be some concern about the effect of 
the lens on relative peak intensities which are im-
portant in organic compound identification. 
2. Complete details are available in ANL Report No. 
7393, "Ion-Retarding Lens to Improve the 
Abundance Sensitivity of Tandem Mass Spectro- 
meters." This report is available from the Clear-
inghouse for Federal. Scientific and Technical 
Information, Springfield, Virginia 22151; price, 
$3.00 (microfiche copies, $0.65).
3. Inquiries concerning this information may be di-
rected to: 
Office of Industrial Cooperation 
Argonne National Laboratory 
9700 South Cass Avenue 
Argonne, Illinois 60439 
Reference: B69-10166 
Source: K. A. Kaiser and C. M. Stevens 
Chemistry Division 
Argonne National Laboratory 
(ARG-10365) 
Patent status: 
Inquiries concerning rights for commercial use of 
this innovation may be made to: 
Mr. George H. Lee, Chief 
Chicago Patent Group 
U.S. Atomic Energy Commission 
Chicago Operations Office 
9800 South Cass Avenue 
Argonne, Illinois 60439
S 
S 
0  - 
Brief 69-10166	 0	 Category 02


Ion-retarding lens improves the abundance sensitivity of tandem mass spectrometers

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