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Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry
Handbook of Stable Isotope Analytical Techniques, Volume 1 P.A. de Groot (Editor) 9 2004 Elsevier B.V. All rights reserved.
C H A P T E R 39
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry S. Halasl & T. Durakiewicz2 Uniwersytet Marii Curie-Sklodowskiej, Instytut Fizyki, Pracownia Spektrometrii Mas, P1. M. CurieSklodowskiej 1, 20-031 Lublin, Poland 2 Los Alamos National Laboratories, Condensed Matter & Thermal Physics Group, Mailstop K764, Los Alamos, NM 87545, U.S.A. e-mail: 1 [email protected]; 2 [email protected] 1
39.1 Introduction The quality of isotope ratio measurements made with a mass spectrometer primarily depends on the stability of the ion currents. In this chapter we discuss the most typical sources of the instability of the measured ion currents. At the beginning we will assume that vacuum conditions are good and the magnetic analyzer (this refers to either a permanent magnet or an electromagnet) produces constant field and the high voltage supplied to the ion source is stable. When both conditions are fulfilled simultaneously, then it guarantees a permanent position of the ion beams with respect to the collector slits. Hence, if the ion beams have no tendency to drift, the average ion current should be constant over a long period of time. If this is not the case, then the most likely the ion current variations may be due to the unstable electron beam (in gas ion sources with electron impact ionization) or unstable filament temperature (in thermal ionization sources). These two sources of the ion beam instability can be nearly completely removed by the methods described in sections 39.6 and 39.7. Fluctuations of the ion current of the beams having stable positions with respect to the collector system are usually generated in the ion source. It is characteristic in this case, that the fluctuations observed on any pair of the Faraday cups are correlated. One can easily recognize whether the ion beams fluctuate due to instabilities of high voltage and/or magnetic field or due to variation of the ion production rate. A test of this is to set the major beam at the edge of the collector slit in the position at which the ion current drops to about half of its maximum value (plateau). If the stability of the ion current is apparently worse at this position than at the plateau, then this is due to poor stability of the high voltage supply or, if no permanent magnet is employed, the electromagnet power supply. The high voltage supply may be tested by precision measurement of a portion of the high voltage (between ground and a selected point on the high voltage divider). A precision digital voltmeter (DVM) is necessary for this test. If the DVM reading indicates fluctuations of the high voltage
858
Chapter 39 - S. Halas & T. Durakiewicz
on the ion source, then the high voltage supply should be tested after disconnecting it from the ion source. If the fluctuations disappear, then cleaning of the ion source is mandatory, see section 39.5. The instability of the magnetic field of the analyzer can be detected either by precision measurement of the electromagnet current or by observation of the m / e indicator which displays a value proportional to square of the magnetic field (B2). Simple, but precision m / e indicators are described in section 39.4.
39.2 Instabilities generated in the collector assembly The collector assembly of an isotope ratio mass spectrometer (IRMS) contains two or more Faraday cups mounted on the high quality ceramic insulators. These cups are surrounded by a common electrode for suppressing the secondary electrons. Although the collector assembly of any IRMS is installed inside the high vacuum system, it can be troublesome when a noise is generated by unwanted leaks of the ion currents to ground or by the unstable secondary electron suppressor voltage. The electric leaks are due to presence of various kinds of impurities on the collector ceramics and feedthrough insulators. In order to test the insulation quality of the vacuum feedthroughs and the ceramic spacers of the collector assembly one should observe (record) the noise of the amplifiers on the most sensitive ranges, i.e. at the same conditions as for their zero-setting. Then the amplifiers should be set to the least sensitive range and their inputs disconnected from the collector feedthroughs. Setting the amplifiers to the least sensitive range will help to prevent their high-ohm input circuits from damage during removal. Subsequently, the amplifier box should be closed, or firmly shielded, and the zero-lines recorded again on the most sensitive ranges. If an essential improvement is observed, i.e. the amplitude of the noise is lowered, then the above mentioned insulating surfaces have to be cleaned. First, the accessible surfaces (outside the high vacuum system) are cleaned by washing them with ethyl alcohol and drying. Then the above test is repeated. If the enhanced noise is still observed, then prior to eventual cleaning of the collector assembly and inner surfaces of the feedthroughs, a uniform heating up to 100~ of the flange with the collector assembly is recommended. In most cases such heating performed for a long period (e.g. overnight) significantly reduces the noise due to impurities on the inner insulating surfaces. Nota bene it is a good praxis to keep this part of the vacuum chamber always warm, at 50~ to 60~ except during the measurement time. In the worst case, when the electric leaks are detected after thorough heating of the flange with the collector assembly, the collectors themselves have to be cleaned after venting the analyzer tube. The best method of cleaning of the ceramic spacers is an acid treatment (HC1, HF) followed by boiling twice in distilled water, drying and baking in a quartz tube furnace at 600~ in open air. Deposits on the metallic parts of the collector assembly can be cleaned in the similar manner as the lenses of the ion source, see section 39.5. The second reason for noise in all the Faraday cups can be unstable suppressor voltage, which is supplied to a common electrode (grid) installed in the collector
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry
859
assembly for repelling the secondary electrons. Such a correlated noise can be observed without ion beams. The secondary electrons are produced by high energy ion beams when they pass through the collector slits and hit the surface of the cups. Without any suppressing voltage, for instance if the grid is firmly grounded, it is possible to observe characteristic negative peaks (valleys) adjacent to each positive peak. These valleys are formed due to electrons ejected from the edges of the collector slits. Because the voltage at the suppressing grid should be extremely stable, it is advised to replace any electronic unit (DC supply) by a battery which can be simply arranged from 4 to 6 commercially available 9V batteries. The batteries should be closed in a grounded box and the negative potential of-40V to-60V (depending on the high voltage applied to the ion source) should be connected to the suppressing grid by a shielded cable. In parallel to the output voltage a high-quality RC filter is recommended (e.g. a 1MQ resistor and l gF capacitor). The above comments refer to spectrometers having the suppressing grid installed in the collector system. Some modern machines have a permanent magnet installed for bending of the secondary electron trajectories. A small magnet is able to bend the trajectories of the electrons, but not the ion beams being composed of high energy massive particles. The best combination, used in some modern machines, is magnet and suppressing voltage. 39.3 Conditions for stable work of the ion source
A typical ion source comprises numerous electrodes being kept at precisely selected potentials with respect to the ground (GND). These potentials are set by means of a quality voltage divider which is usually mounted in the same box as the electron emission controller (see section 39.6) or the filament temperature controller, see section 39.7. Firstly, we will consider the electron impact (EI) ion source, which is used in analysis of gases and vapors. The molecules are admitted from an inlet system into a ionization chamber (cage). A schematic diagram of a Nier-type ion source is shown in Figure 39.1. Note that the ionization chamber has the highest potential in the whole mass spectrometer, so a gas has to be introduced to the cage by a piece of insulating tube, usually made of ceramic or quartz glass. The electrons emitted from a hot filament are accelerated by a voltage (from 20 to 100 V, typically 70 V) between the filament and cage. Most of them hit the wall of the ionization cage but a considerable fraction passes through the cage with constant kinetic energy and terminate on a plate (called the electron trap) which is fixed at the opposite side of the cage. The trap potential is somewhat higher (ca. 30 V) than the cage in order to suppress the secondary electrons produced as primary electrons pass through the cage and hit the trap. The electron emission controller keeps the trap current constant by precise adjusting of the filament temperature. The trap current (also called as electron emission current, Ie) ranges from a few microamps to about I mA, typically in IRMS it is set at 50 gA. Two solid-state magnets are usually used to form the magnetic field parallel to the electron beam. In such a magnetic field electrons move on elongated spiral paths, which enhances the ionization efficiency substantially.
860
Chapter 39 - S. Halas & T. Durakiewicz
Schematic diagram of a Nier-type ion source with a typical potential distribution on the electrodes. The extracting plates and beam defining slits are expanded for better insight into construction. Figure 3 9 . 1 -
ca.
If gas pressure in the ionization cage is properly controlled, then only a minor fraction of electrons passing through the cage collides with the molecules causing their ionization and dissociation in the following most common reactions: AB + e ~ A B + + 2e A B + e - - * A + + B + 2e
[39.1] [39.2]
where A and B denote atomic species or fragments a molecule AB. Other species, like doubly charged ions, are produced with significantly lower efficiency; thereby they are not useful in IRMS. The rate of ion production at constant Ie depends on electron energy, i.e. on the electron accelerating voltage. A broad m a x i m u m of ionization efficiency is about 100 V for most of molecular species. The ions formed along the electron beam are driven off by a weak electric field which penetrates into the cage from the space between accelerating and extracting plates, see Figure 39.1. In some EI sources a plate (called as ion repeller) is fixed in the cage in order to enhance the ion extraction. Efficient extraction results in lowering the rate of ion-molecule reactions in the cage, which is particularly important in hydrogen isotope analysis, because species formed in the cage interfere with HD +. The species are formed due to following reactions: H2 + + H ~ H 3 + H2 + H + --* H3 +
[39.3] [39.4]
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry
861
where H2 +, H + and H are formed from H2 by electron beam. Most of ions extracted from the cage should pass subsequently through the slits of the extracting plates. One half of the first set of these plates has a slightly adjustable potential for optimum positioning of the ion beam at the narrowest grounded slits, called beam defining slits. Another adjustment is made by selecting potential (e.g. from - 100 V to + 100 V) of the correction plate. In the case of ideal geometry and clean plates these adjustable potentials will be identical to those of the opposite electrodes. The setting of the adjustable potentials in an ion source (extraction voltage, focusing voltage and correction voltage) should be done alternately several times in order to attain optimum potential distribution. This will result in the most intense and stable ion beam. In thermal ionization mass spectrometers (TIMS), used in IRMS of solids, the electron beam is replaced by an ionizing filament. The analyzed substance is either placed directly on this filament or on a side filament used for its controlled evaporating. Phenomena, evaporation and surface ionization are strongly (exponentially!) temperature dependent. Therefore a high quality temperature controller has to be employed in order to reduce instabilities of the ion beam, see section 39.7. 39.4 m/e indicator A device capable of precisely displaying the m / e ratio of a selected ion beam is extremely useful, not only in identification of impurities in the background of a mass spectrum and in the adjustment of the distance between collector slits, but also in detection of instabilities of the magnetic field produced by an electromagnet. The reason of spatial instability of the ion beam can be easily recognized by use of the m / e indicator, which has a short-term precision of at least 0.01 mass unit.
Halas & Sikora (1987) described a simple dual Hall probe device that indicates m/ e ratio with the required short-term precision of 0.01 mass unit. The principle of its action is as follows. The first Hall probe, H1 in Figure 39.2, is fed by a constant current I1. Thus the output voltage, V1, is proportional to the product V1 ~- IIB
[39.5]
where B is the magnetic field intensity experienced by this Hall probe. The output voltage V1 is repeated by the operational amplifier with 100% feedback coupling. The second Hall probe, H2, is fed by current I2 directly proportional to V1 from a voltageto-current converter. If the second Hall probe experiences the same magnetic field, then its output signal, V2, will be proportional to the product I2B. Then, from equation [39.5] and the proportionality of I2 to V1 we obtain that V2 ~ B 2
[39.6]
which means that V2 will be proportional to the m / e value in the magnetic scanning operation mode.
862
Chapter 39 - S. Halas & T. Durakiewicz
I 11=constant
V2 H2 /
V1
VtoI converter
Figure 39.2 - Schematic diagram of the dual Hall probe method.
By adjustment of the I] value, one can obtain the voltage V2 (in milivolts) exactly equal to m / e ratio, which is very useful. Two small electrically insulated Hall probes may be placed in parallel between the magnet poles to determine the magnetic field intensities. In our m / e indicator two inexpensive CdHgTe thin-film Hall probes were used with 60f~ resistance each. The maximum current applied to feed the probes was less than 20 mA, so that no significant power was dissipated. In the case of mass spectrometers with permanent magnets, B is a constant, and so: m / e ~ 1 / Vacc
[39.7]
where Vacc is acceleration voltage of the ion beam, which is the highest potential in the ion source with respect to ground. Hence, to indicate the m / e ratio it is necessary to reverse the high voltage (or a portion taken from a voltage divider). The reversed voltage can be displayed by means of a so-called double integrating DVM. Such voltmeters are commonly used in laboratories and DVMs with 4.5 digits are commercially available in panel versions with price below 30 US $. These voltmeters compare the measured voltage with a reference voltage produced by a specialized circuit inside the DVM. By reversing the inputs of the "measured voltage" with the "reference voltage" one can simply obtain the reversed voltage on the display. By adjustment of the voltage divider, a selected fraction of the high voltage is reversed in order to obtain the value of m / e ratio on the display. 39.5 Cleaning of the ion source
After a long-term operation of any ion source the insulating ceramics and the lenses became coated by a thin semiconducting layer. The deposits are formed by the decomposition of the analyzed substances as well as metal sputtered off the lenses by the interaction with high speed ions. These layers may produce episodic breakdowns
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry
863
of the high voltage in the ion source or long-term instability of the ion beam due to permanent charging/discharging of the semiconducting layer on the surfaces of the ion lenses. During high voltage breakdowns the ion currents are seen to vary rapidly and with unpredictable recurrence (the characteristic effect is a sudden loss of ion beam signal followed by a return to the previous strength over a matter of seconds). Such an erratic behavior of the mass spectrometer is an immediate signal that the ion source has to be cleaned. The cleaning procedure described below is not difficult and can be done by any competent operator. The ion source can be disassembled from the analyzing tube after venting the instrument through a filter to avoid entry of any solid particles, the most dangerous of which are small ferromagnetic pieces which can be attracted by magnetic field of the analyzer. It is therefore advised that the magnet be removed before venting the analyzer tube. If an electromagnet is used then its power supply should be turned off before venting. Having the ion source in a clean working environment, while the end of the analyzer tube protected against dust by a piece of aluminum foil; disassemble the ionization cage and the lenses. Usually the ionization cage is the most dirty part of a gas ion source. It can be cleaned by removing the deposits by use of a delicate abrasive paper, e.g. 1/600 mm. The lenses can be cleaned in a similar manner. Use of distilled water for periodical rinsing of the parts is recommended. Finally, the parts are rinsed in acetone and boiled in distilled water. At this stage the lenses can be electropolished (see Peele & Brent 1977). The electropolishing is a reverse electrolysis in a viscous strong electrolyte. It is a simple, effective and inexpensive technique. For electropolishing of stainless steel the following recipe for the electrolyte can be used: H2SO4 (1.84 specific gravity) 1000 ml water 370 ml glycerin 1370 ml Stirring, add the acid slowly to the water; avoid overheating. Cool to room temperature, then add the glycerin and stir well. The electrolyte can be used for many cleaning procedures during which the cleaned stainless steel part is immersed as a positive electrode whilst a larger piece of stainless steel foil surrounding that part is connected to the negative pole of a 24V/ 25A rectifier. The use of any other metals or alloys as negative pole is not recommended. The temperature of the electrolyte should be about 60~ The ceramic parts can be cleaned by abrasive paper, rinsed with distilled water, and finally by boiling twice in distilled water. After thorough drying of all the parts in a clean furnace (at 70 to 100~ overnight). Baking of the ceramic parts at 700~ in air may be the next step, if their surfaces still seem to be colorated by a dark contaminant. The ion source can be reassembled using clean tools. Ion sources of modern mass spectrometers often have lenses, which are mounted
864
Chapter 39 - S. Halas & T. D u r a k i e w i c z
on sapphire balls for the best insulation and adjustment. This ensures long-term operation of the source without cleaning and fast outgassing of the surfaces. In contrast, the outgassing of ceramic rods and wobbly ceramic spacers used in older ion sources is time consuming. Sparks due to trapped gas inside ceramic parts may appear when the high voltage supply is turned on too early. The tolerance of the sapphire balls can be limited to _+3 mm, which guarantees identical location of the lenses after cleaning or replacement. 39.6 Stabilization of electron e m i s s i o n current
Fluctuations of the electron emission current transform into fluctuations of the ion current immediately. If the local pressure variations inside the ionization chamber are neglected, the variance of the ion current may be considered as the linear combination of the variance in both electron emission current and ionization cross-section of gas molecules (which varies with electron energy). It is nowadays relatively easy to obtain the electron acceleration voltage (Ve) stabilized with the precision better than 0.01%. Hence, it is only the electron emission current (Ie) that may disturb the ion current stability. There are two general types of electron emission regulators: the series regulator and the switching mode regulator. Regulators, which utilize a switching mode, can offer several advantages over conventional, continuously controlled, series type regulators. In contrast to a well known series-pass regulator of filament current (Halas & Sikora 1990) the comparator in the switching mode circuit has also a positive feedback loop formed by a resistor connecting the noninverting input of an operational amplifier (OA) with its output. This results in pulsation of the output voltage from - 15V to + 15V when the signal at the inverting input is slightly modulated around the reference voltage. Such a design leads to a self-oscillating stabilizer (Durakiewicz 1996) the simplified diagram of which is shown in Figure 39.3. A number of improvements have been made to the original circuit (see Figure 39.4), namely: (1) The electric potential distribution on the filament, cage, ion repeller and trap have been redesigned in order to allow the precise control of the average value of the electron current passing through the cage. The average trap current instead of total emission current is stabilized. (2) The protection of the filament against vacuum break down has been simplified. The respective fragment of electronic circuit comprises only a simple capacitanceresistance filter (C2 = 100 nF and R2 = 2.2 MQ). (3) A modern solid state relay (SSR) for the power supply of the filament was used; thereby the number of electronic components has been reduced. The inverting input of the Schmitt trigger (OA2 with positive and negative feedback loops) is fed from the output of OA1 (trap current-to-voltage converter). Positive feedback loop is sustaining the self-oscillating action of the circuit, whereas the negative feedback is responsible for stabilization. A square wave generated at the output of the trigger passes through the C2-R2 filter to the input of OA3 (voltage follower).
865
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry
Ve
Vr
T
,p
I,F / -)-
I
IeI
"%.
=
9,"
! VREF
'!,.%
[
R3
__.,.r"
I
T Figure 39.3 - Simplified diagram of a self-oscillating electron emission stabilizer. Ie is the electron emission current, Ve is electron acceleration voltage, VREFis the reference voltage, and Vf is the filament supply voltage.
The output voltage from OA3 feeds the light emitting diode of the SSR. Due to the switching action the electron beam is modulated, i.e. the trap current varies periodically between "Hi" and "Lo" values. The switching frequency for a typical filament installed in the ion source ranges from 0.1 to I kHz. The maximum positive voltage is generated on the OA2 output in the case of bad vacuum or any other break in the negative feedback loop. Use of a capacitor (C2 in Figure 39.4:) prevents the filament from burning out; such a protection may not be applied in conventional stabilizers.
(tlA'J
~_T I"-.R1 -.-.-.-] - ~
l +1
Trap l+ Voltage I
CAGE I --
'
+15V ION REPELLER r
TRAP
R~6 ~) ~
r k1 [ ~ P I C A MEN T ~
5 k fa
J R7
klo -15V
I vl
Figure 39.4 - A detailed schematic diagram of the self-oscillating electron emission controller.
866
Chapter 39 - S. Halas & T. Durakiewicz
Although Ie is slightly modulated, its mean value remains extremely stable. Even after a few days of continuous operation the mean Ie value does not vary more than + 1%. The stability is not affected even by large variations of electron acceleration voltage (AV) from 40 to 100 V. The short-term variations of ion current induced by Ie modulation are negligible as long as the ion current detectors have the response time of the order of 100 ms or larger. The long-term stability of the ion current is improved significantly, due to better stability of Ie. The mean value of Ie is not influenced by network supply voltage instabilities of amplitude _+20%. A comparison of results obtained by electron emission stabilizers of various designs is given in Table 39.1. All the designs listed here, except that by Durakiewicz (1996), are generally based on the proportional regulation principle. The advantages of pulsed heating for electron emission control may be summarized as follows: (i) low power consumption - the circuit may be used in a battery operated system (e.g. in space technology), (ii) self-protection against breaks in the negative feedback loop, including the inner part of this loop between filament and electron trap (e.g. due to a bad vacuum), (iii) electron emission current is independent of the electron acceleration voltage and vice-versa, (iv) one pole of the electron acceleration voltage source is connected to the ground of the system, thus Ie does not influence the acceleration voltage itself, (v) the circuit is simple and convenient in operation. The slight modulation of Ie with frequencies above 100Hz is not a drawback in mass spectrometry, where ion current detecting devices with only slow response time are used. Table 39.1 - Comparison of operation parameters of selected electron emission stabilizers. Chapman)
Close & Yarwood (1972)
(1972)
Herbert (1976)
Shaw & Lue (1980)
Halas & Sikora (1990)
Durakiewicz (1996)
operation principle
proportional proportional proportional temperature regulator regulator regulator stabilizer
doubleproportional regulator
selfoscillating
stabilization coefficient
0.1%
relative cost
medium
filament safety
-
0.1%
I low -
0.1%
>1%
1%
0.1%
very high
high
low
low
safe start
-
safe start & loop break
loop break
...........................................................................................................................................................................................................................
Ie/Ue independence
-
, .........................................................................................................................................................................
+
-
~..........................................................................................
-
+
<50%
<80%
l
.................................................................................................................................................................................
energetic efficiency
-
<50%
t...........................................................................................................................................................................................................................
I<50% i
<50%
<30%
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry
39.7 Stabilization of filament temperature In thermal ionization mass spectrometry the ions hot metal with high melting point (usually W, Ta, which can be drawn from the surface at equilibrium well-known Saha-Langmuir formula (Benninghoven 2001)" .+
867
are produced on the surface of a Re, Ir, Pt). The current density, conditions, is determined by the et al. 1987, Valyi 1977, de Laeter
ve
J = 1
g~ E~-q3 + --~exp kT g
[39.8]
where v is the flux density of particles supplied to the surface, e is elementary charge, g+ and gO are statistical weights of ions and neutral particles, respectively, Ei is the ionization energy of the particle, q~is the work function of the metal, k is Boltzmann constant and T is the absolute temperature. The ratio of gO/ g+ is equal 2 and 1/ 2 for alkali and alkaline earth metals, respectively. Numerical values of ionization potentials of elements and statistical weights of ground state of atoms and ions are given in Uns61d (1968). The formula [39.8] is strictly valid only when the surface coverage is small. For negative ions the respective formula is: -
ve
J -
0
[39.9]
1 + g exp q9- A
g
-
kT
where A is the electron affinity. Numerical values for electron affinities of some elements and their oxides can be found in Valyi (1977) and Wachsmann & Heumann (1991, 1992). Inasmuch as both nominator and denominator in formulae [39.8] and [39.9] is strongly temperature dependent, a precise filament temperature stabilization is demanded for production of stable ion beams by the surface ionization process. In the case of a single filament ion source the flux density, v, is driven by the surface diffusion from cooler ends of the filament towards its center. The filament center should be kept at a temperature at which the analyzed atoms can be effectively ionized, which means that the surface coverage is far below the monolayer at the filament center. The filament length may be also crucial in obtaining stable ion currents. It is so because a temperature plateau exists in the case of too long filaments. In such a case, the fluxes of ions are emitted from relatively large surface with high fluctuations due to variations in surface coverage at the plateau region. However, steady conditions for ionic emission can be obtained for relatively short filaments, for which no temperature plateau exists (details on defining long and short filaments may be found in Halas & Durakiewicz, 1998a).
868
Chapter 39 - S. Halas & T. Durakiewicz
The following methods may be used for the filament temperature stabilization: (1) A commonly used method of current stabilization, (2) Filament resistance stabilization, (3) Filament voltage stabilization, (4) Stabilization of power supplied to the filament. Method (1) is relatively easy to apply but it has a significant drawback: the filament temperature depends highly nonlinearly on the heating current. Therefore we describe below the temperature controllers based on methods (2) and (3) which are characterized by the linear dependence of filament temperature on its resistance or on the voltage drop between its ends, respectively. Method (4) will not be considered here because it requires rather complicated electronic circuits. The filament resistance stabilizer can be made on the basis of the Halas-Kaminski bridge (Halas & Kaminski 1995, Halas et al. 1993, Durakiewicz & Halas 1995). A conceptual diagram of the stabilizer is shown in Figure 39.5. The bridge contains a DC excitation voltage source, Ve. The excitation current passes through a diode and the bridge resistors but not through the transistor T. RF and Rv denote, respectively, the resistance of the filament and a variable resistor used for temperature setting. The filament is made of a pure metal (e.g. W, Ta, Re, Pt) which have positive and significantly large temperature coefficients of electrical resistivity. If Rv < RF then the bridge signal supplied to the noninverting sample-and-hold amplifier (S/H) is negative or zero and the transistor does not conduct. The noninverting input of the OA is connected to the output of a S/H amplifier whilst the inverting input of OA is fed by a positive signal from a triangular wave form generator, G. In the reverse situation, i.e. Rv > RF, the imbalance signal generated by the bridge is positive and has a defined magnitude. This signal is amplified by the S/H and held at the S/H output, when the negative potential taken from the output of OA turns to positive. The comparator OA produces a positive output voltage if the generator signal is below the voltage held by S/H. In this fraction of the cycle, transistor T conducts. Hence the power supply, V1, is connected in parallel to RF (through T, which plays the role of a switch). No significant current from V1 can pass through the remaining parts of the bridge due to presence of the diode D. Because V1 considerably exceeds Ve, the excitation current cannot pass through the low-resistance legs of the bridge (Rv and RF) when RF is supplied from V1. This is the reason for using a sample-and-hold amplifier instead of a normal operational amplifier. As result of the action of the circuit shown in Figure 39.5; a series of heating pulses is supplied to RF with frequency driven by the generator but their duration is driven by the output voltage of the S/H amplifier. The bridge is always kept close to the balance state, i.e. RF - Rv. If for some reason (e.g. voltage V1 starts to diminish) RF resistance becomes somewhat lower than Rv, then the bridge imbalance signal becomes somewhat higher, which results in a longer duration of the heating pulses. A complete circuit diagram is described by Halas & Durakiewicz (1998b).
869
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry Figure 39.5 - Conceptual diagram of using the Halas-Kaminski bridge for constant-resistance operation of a filament RF. S / H is a sample-and-hold amplifier; G is a triangular wave generator.
N_
S/H
,," ,,,,-"
o,,,d",.,",,d,,,,,c,,d ,. .,."
R
/'.
;x./
-',.-"%,%
+ ~ "
I
No
",,% ,,, ",. "%",% ", ,, %., "',,,
"de"
/
I G A/V'~]
% ",,%
Ve
---tl
"--,.\.
"%:q I b,l
I",J
"'.....
,,,,,,".--
D ,z' .,."
,.
Vl
lt---~ '1
+
F- 4 ~ T
The use of a simple voltage stabilizer for temperature control of a filament was described by Halas et al. (2001). Voltage may be stabilized typically by use of the same two wires to feed voltage to the filament and measure the voltage value (2-wire method) or, as it is described below, by use of a separate pair of wires for voltage measurement. In such a 4-wires configuration the voltage drop along the supply lines does not influence the value measured by the stabilizer circuit, hence the fluctuation of the ion beam is significantly reduced. The schematic diagram of the circuit is shown in Figure 39.6. The MA741 operational amplifier with the negative feedback loop constitutes the basis of the stabilizer circuit. The reference voltage supplied to the noninverting input of the amplifier may be set either manually by use of the 10k potentiometer, or digitally by use of the digital-to-analog converter (AD7243), or manually by use of the potentiometer. All the fluctuations of the filament voltage are minimized by the negative feedback loop what allows for the temperature stabilization of the filament. In the 4-wires method the resistance variations of filament power supply wires do not affect the filament voltage measurement. The output signal of the MA741 drives the Darlington circuit comprising the 2N3055 and BD439N transistors. This circuit keeps the filament voltage constant and therefore stabilizes its temperature. Since ions have to be formed into a beam, the filament and the whole supply circuit is fixed on the 2kV potential with respect to the ground. Because of the high potential of the source it was necessary to construct the three channel optical relay between the master computer and slave digital-to-analog converter in the filament power supply. In this way the master computer is protected against high voltage. The remaining components identified in the schematic diagram are used to allow smooth switching between the computer and manual control, and to protect the filament against burning during power-on of the supply.
870
Chapter 39
- S. Halas & T. Durakiewicz
SYNC SDIN LK SYNC ~.x.. ~::'S :,, '~' ~ l SDIN \470K D K"I ~.]N SCLK \.,. ", -1- "~ CLR 470KN\470 i i i rlVref d====++5V\ ;,.X, ./ / ..a,k14v \~-- I MAA I12v _ I c12J, liAD7243{ ZlmF 2.2M~ GND T,, ~ , . , ' T 17812 I i ~ +~5 I I [ +5V
'3
~ .~
1",-.t/ I I FI"L IJ'-It 1
_
I MAA
I
I
_
I I ] 7805 l i i35k "+5V 10mP ~ ~ ~mP,ll~P~ 0 ~10 k Vref
=-15V
Vout
Comp_ Adjust"~ == ~I~F
~--_ MA741 L~~__
----~+2kV
///"N\+ 3.8V "
Il
T+V -3x i2 mF
r
2N3055
91
Figure 39.6 - Schematic diagram of the filament temperature controller based on the stabilizer.
wires voltage
)
9N 4-
The voltmeter indicated on the schematic diagram enables the visual control of the filament temperature. This is possible because the temperature-voltage characteristics of the filament are almost linear, as demonstrated by Halas et al. (2001). The voltmeter was calibrated directly in temperature units what allows the operator immediate information on the filament status. The stabilizer is fully computer controlled via the digital-to-analog converter AD7243 (see Halas et al. 2001 for details). In order to demonstrate the superiority of the 4-wires over the 2-wires approach, we have continuously recorded the ion current of 39K+ for 20 minutes. Potassium was loaded onto a platinum filament, 0.05mm x 0.63mm x 10mm, as 50% K3PO4 solution. After drying in air and degassing under vacuum, the filament was incrementally heated to 900K. Ion emission was measured at a filament temperature about 1000K. Comparison of 39K ion currents obtained using the above-described voltage stabilize with 4-wires and 2-wires configuration is shown in Figure 39.7. 39.8 Integrating of the ion currents The ion detection system of an IRMS consists of a collector assembly containing two or more Faraday cups (as mentioned in section 39.2). Although the ion counting
871
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry
Figure 39.7 - Comparison of 39K ion currents obtained by the authors for filament voltage stabilization by use of 2-wires and 4-wires methods, respectively. technique is used in specialized instruments for detection of very weak ion beams (like 3He, 230Th), we will not consider these devices here. The ions collected in each cup produce electric currents, which flow through a high-value resistor (i.e. 109 - 1012 f2) to ground. The voltage produced on the resistor is not amplified but rather repeated by a circuit arranged as a voltage follower, or, more frequently, it is reversed by an amplifier with the resistor forming a negative feedback loop. A typical detector system of an IRMS is shown schematically in Figure 39.8. The ion current is converted to a voltage by an operational amplifier with ultra-low input bias current and the high-value resistor. The output voltages are then converted to trains of short pulses by so-called voltage-to-frequency converters (V/F), the repetiN 109Q I
t---"
V/F Faraday cups
,
Counter
N 1011Q I t---
Stop
V/F
Counter
1J Display
Figure 39.8 - A typical arrangement for digital measurements of current ratios
872
Chapter 39- S. Halas & T. Durakiewicz
tion rate of which is linearly proportional to the input voltage. These pulses are fed to separate counters which are set to zero at the start of measurement. When the major beam counter reaches a count of 106, the minor beam counter is stopped and its value displayed. The value of the full six-digit display is therefore equal to the current ratio. Unfortunately the detection system described above introduces, its own noise, predominantly from the high-value resistors. This is a fundamental phenomenon which cannot be eliminated by technological improvement of the production of resistors (see Felgett & Usher, 1980, for example). Moreover, the resistors also suffer from fluctuations of their value due to variations of the potential drop along the resistor and temperature changes (Habfast, 1960). The key to the improvement of isotope ratio measurements is to replace each highvalue resistor by a capacitor (Jackson & Young 1973, Halas & Skorzynski 1980, McCord & Taylor 1986). This replacement converts the ion detector from an ion "amplifier" to an "integrator" where the voltage on the capacitor raises in time proportionally to the charge collected: T
V - c f l d t --
I.T
C
[39.10]
0
where C is the value of capacitance, I is the ion current and T is the integration period. The value of the capacitor is selected in such a manner that the final voltages are of order of 10 volts. The integration period may be estimated from the following statistical considerations. Let us assume that the 180/160 ratio is measured using CO2 gas. Typical currents obtained by a Nier type ion source for mass 44 and 46 are 2.5 x 10-9A and 1 x 10-11A, respectively. Hence, after time T the number of electrons collected on each capacitor is: n~
I.T
[39.11]
e
where e is the elementary charge 1.602 x 10-19As. According to the general principles of statistics, the relative uncertainty of An/n is equal n-l/2; hence for a desired uncertainty of about 10-5 (i.e. + 0.01%o) the number of ions collected has to be 1010. From equation [39.11] one obtains for the minor beam: T~ 101~
1.602.10-19As 1.6. 102s 10
-11
A
i.e. the integration time has to be of the order of 100 seconds.
[39.12]
873
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry
The obvious inconveniences of the integrating system are that the "---. 2 ""--, 1 "-... capacitors have to be discharged periodically, prior to each charging C cycle, and the system cannot be used I I directly for the instrument adjust" I I ments by continuous monitoring of the ion current. Both difficulties, however, can easily be overcame today by use of the computer controlled high quality reed switches for discharging the capacitors and their instant switching to the resistors. Such a solution was described by Halas & Skorzynski (1980). One pair Figure 39.9 - Schematic diagram of the capacitance/ of reed switches is required for each resistance system. amplifier. The switches are connected in parallel to C and R as shown in Figure 39.9. In the integrating mode switch 2 is closed and switch I is used for periodical discharging of the capacitor. For adjustment of the mass spectrometer, recording the mass spectra, etc., switch I is closed whilst switch 2 is open.
i
R
39.9 Final remarks
Good quality isotope ratio results are worth taking the effort. The ideas and general remarks presented in this chapter are certainly not the only solutions that guarantee success. They are, however, tested by many years of operation and maintenance of several instruments in our Mass Spectrometry Laboratory. In the final section we should make our reader aware, that having the ion beams stable enough to produce satisfactory precision of 0.05 permil or better for standard versus standard measurement does not guarantee identical precision for sample versus standard measurement. If this is not the case, one should check the purity of the sample and/or the gas flow conditions through the inlet system, as well as the linearity of the system. Having stable ion beams, pure samples and a high quality inlet system one has a chance for good and long-term performance provided that settings of the IRMS are favorable. The optimum setting of the ion source should assure maximum ion current and best peak shape at minimum electron beam. The gas flow rate through the capillaries should be selected depending on the geometry of the ion source. The pressure of the analyzed gas (as estimated on the basis of major beam current) during gas flow should not exceed significantly the 100-fold background pressure. Too high a pressure leads to ion scattering by the gas molecules and thereby to peak broadening.
C H A P T E R 39
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry S. Halasl & T. Durakiewicz2 Uniwersytet Marii Curie-Sklodowskiej, Instytut Fizyki, Pracownia Spektrometrii Mas, P1. M. CurieSklodowskiej 1, 20-031 Lublin, Poland 2 Los Alamos National Laboratories, Condensed Matter & Thermal Physics Group, Mailstop K764, Los Alamos, NM 87545, U.S.A. e-mail: 1 [email protected]; 2 [email protected] 1
39.1 Introduction The quality of isotope ratio measurements made with a mass spectrometer primarily depends on the stability of the ion currents. In this chapter we discuss the most typical sources of the instability of the measured ion currents. At the beginning we will assume that vacuum conditions are good and the magnetic analyzer (this refers to either a permanent magnet or an electromagnet) produces constant field and the high voltage supplied to the ion source is stable. When both conditions are fulfilled simultaneously, then it guarantees a permanent position of the ion beams with respect to the collector slits. Hence, if the ion beams have no tendency to drift, the average ion current should be constant over a long period of time. If this is not the case, then the most likely the ion current variations may be due to the unstable electron beam (in gas ion sources with electron impact ionization) or unstable filament temperature (in thermal ionization sources). These two sources of the ion beam instability can be nearly completely removed by the methods described in sections 39.6 and 39.7. Fluctuations of the ion current of the beams having stable positions with respect to the collector system are usually generated in the ion source. It is characteristic in this case, that the fluctuations observed on any pair of the Faraday cups are correlated. One can easily recognize whether the ion beams fluctuate due to instabilities of high voltage and/or magnetic field or due to variation of the ion production rate. A test of this is to set the major beam at the edge of the collector slit in the position at which the ion current drops to about half of its maximum value (plateau). If the stability of the ion current is apparently worse at this position than at the plateau, then this is due to poor stability of the high voltage supply or, if no permanent magnet is employed, the electromagnet power supply. The high voltage supply may be tested by precision measurement of a portion of the high voltage (between ground and a selected point on the high voltage divider). A precision digital voltmeter (DVM) is necessary for this test. If the DVM reading indicates fluctuations of the high voltage
858
Chapter 39 - S. Halas & T. Durakiewicz
on the ion source, then the high voltage supply should be tested after disconnecting it from the ion source. If the fluctuations disappear, then cleaning of the ion source is mandatory, see section 39.5. The instability of the magnetic field of the analyzer can be detected either by precision measurement of the electromagnet current or by observation of the m / e indicator which displays a value proportional to square of the magnetic field (B2). Simple, but precision m / e indicators are described in section 39.4.
39.2 Instabilities generated in the collector assembly The collector assembly of an isotope ratio mass spectrometer (IRMS) contains two or more Faraday cups mounted on the high quality ceramic insulators. These cups are surrounded by a common electrode for suppressing the secondary electrons. Although the collector assembly of any IRMS is installed inside the high vacuum system, it can be troublesome when a noise is generated by unwanted leaks of the ion currents to ground or by the unstable secondary electron suppressor voltage. The electric leaks are due to presence of various kinds of impurities on the collector ceramics and feedthrough insulators. In order to test the insulation quality of the vacuum feedthroughs and the ceramic spacers of the collector assembly one should observe (record) the noise of the amplifiers on the most sensitive ranges, i.e. at the same conditions as for their zero-setting. Then the amplifiers should be set to the least sensitive range and their inputs disconnected from the collector feedthroughs. Setting the amplifiers to the least sensitive range will help to prevent their high-ohm input circuits from damage during removal. Subsequently, the amplifier box should be closed, or firmly shielded, and the zero-lines recorded again on the most sensitive ranges. If an essential improvement is observed, i.e. the amplitude of the noise is lowered, then the above mentioned insulating surfaces have to be cleaned. First, the accessible surfaces (outside the high vacuum system) are cleaned by washing them with ethyl alcohol and drying. Then the above test is repeated. If the enhanced noise is still observed, then prior to eventual cleaning of the collector assembly and inner surfaces of the feedthroughs, a uniform heating up to 100~ of the flange with the collector assembly is recommended. In most cases such heating performed for a long period (e.g. overnight) significantly reduces the noise due to impurities on the inner insulating surfaces. Nota bene it is a good praxis to keep this part of the vacuum chamber always warm, at 50~ to 60~ except during the measurement time. In the worst case, when the electric leaks are detected after thorough heating of the flange with the collector assembly, the collectors themselves have to be cleaned after venting the analyzer tube. The best method of cleaning of the ceramic spacers is an acid treatment (HC1, HF) followed by boiling twice in distilled water, drying and baking in a quartz tube furnace at 600~ in open air. Deposits on the metallic parts of the collector assembly can be cleaned in the similar manner as the lenses of the ion source, see section 39.5. The second reason for noise in all the Faraday cups can be unstable suppressor voltage, which is supplied to a common electrode (grid) installed in the collector
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry
859
assembly for repelling the secondary electrons. Such a correlated noise can be observed without ion beams. The secondary electrons are produced by high energy ion beams when they pass through the collector slits and hit the surface of the cups. Without any suppressing voltage, for instance if the grid is firmly grounded, it is possible to observe characteristic negative peaks (valleys) adjacent to each positive peak. These valleys are formed due to electrons ejected from the edges of the collector slits. Because the voltage at the suppressing grid should be extremely stable, it is advised to replace any electronic unit (DC supply) by a battery which can be simply arranged from 4 to 6 commercially available 9V batteries. The batteries should be closed in a grounded box and the negative potential of-40V to-60V (depending on the high voltage applied to the ion source) should be connected to the suppressing grid by a shielded cable. In parallel to the output voltage a high-quality RC filter is recommended (e.g. a 1MQ resistor and l gF capacitor). The above comments refer to spectrometers having the suppressing grid installed in the collector system. Some modern machines have a permanent magnet installed for bending of the secondary electron trajectories. A small magnet is able to bend the trajectories of the electrons, but not the ion beams being composed of high energy massive particles. The best combination, used in some modern machines, is magnet and suppressing voltage. 39.3 Conditions for stable work of the ion source
A typical ion source comprises numerous electrodes being kept at precisely selected potentials with respect to the ground (GND). These potentials are set by means of a quality voltage divider which is usually mounted in the same box as the electron emission controller (see section 39.6) or the filament temperature controller, see section 39.7. Firstly, we will consider the electron impact (EI) ion source, which is used in analysis of gases and vapors. The molecules are admitted from an inlet system into a ionization chamber (cage). A schematic diagram of a Nier-type ion source is shown in Figure 39.1. Note that the ionization chamber has the highest potential in the whole mass spectrometer, so a gas has to be introduced to the cage by a piece of insulating tube, usually made of ceramic or quartz glass. The electrons emitted from a hot filament are accelerated by a voltage (from 20 to 100 V, typically 70 V) between the filament and cage. Most of them hit the wall of the ionization cage but a considerable fraction passes through the cage with constant kinetic energy and terminate on a plate (called the electron trap) which is fixed at the opposite side of the cage. The trap potential is somewhat higher (ca. 30 V) than the cage in order to suppress the secondary electrons produced as primary electrons pass through the cage and hit the trap. The electron emission controller keeps the trap current constant by precise adjusting of the filament temperature. The trap current (also called as electron emission current, Ie) ranges from a few microamps to about I mA, typically in IRMS it is set at 50 gA. Two solid-state magnets are usually used to form the magnetic field parallel to the electron beam. In such a magnetic field electrons move on elongated spiral paths, which enhances the ionization efficiency substantially.
860
Chapter 39 - S. Halas & T. Durakiewicz
Schematic diagram of a Nier-type ion source with a typical potential distribution on the electrodes. The extracting plates and beam defining slits are expanded for better insight into construction. Figure 3 9 . 1 -
ca.
If gas pressure in the ionization cage is properly controlled, then only a minor fraction of electrons passing through the cage collides with the molecules causing their ionization and dissociation in the following most common reactions: AB + e ~ A B + + 2e A B + e - - * A + + B + 2e
[39.1] [39.2]
where A and B denote atomic species or fragments a molecule AB. Other species, like doubly charged ions, are produced with significantly lower efficiency; thereby they are not useful in IRMS. The rate of ion production at constant Ie depends on electron energy, i.e. on the electron accelerating voltage. A broad m a x i m u m of ionization efficiency is about 100 V for most of molecular species. The ions formed along the electron beam are driven off by a weak electric field which penetrates into the cage from the space between accelerating and extracting plates, see Figure 39.1. In some EI sources a plate (called as ion repeller) is fixed in the cage in order to enhance the ion extraction. Efficient extraction results in lowering the rate of ion-molecule reactions in the cage, which is particularly important in hydrogen isotope analysis, because species formed in the cage interfere with HD +. The species are formed due to following reactions: H2 + + H ~ H 3 + H2 + H + --* H3 +
[39.3] [39.4]
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry
861
where H2 +, H + and H are formed from H2 by electron beam. Most of ions extracted from the cage should pass subsequently through the slits of the extracting plates. One half of the first set of these plates has a slightly adjustable potential for optimum positioning of the ion beam at the narrowest grounded slits, called beam defining slits. Another adjustment is made by selecting potential (e.g. from - 100 V to + 100 V) of the correction plate. In the case of ideal geometry and clean plates these adjustable potentials will be identical to those of the opposite electrodes. The setting of the adjustable potentials in an ion source (extraction voltage, focusing voltage and correction voltage) should be done alternately several times in order to attain optimum potential distribution. This will result in the most intense and stable ion beam. In thermal ionization mass spectrometers (TIMS), used in IRMS of solids, the electron beam is replaced by an ionizing filament. The analyzed substance is either placed directly on this filament or on a side filament used for its controlled evaporating. Phenomena, evaporation and surface ionization are strongly (exponentially!) temperature dependent. Therefore a high quality temperature controller has to be employed in order to reduce instabilities of the ion beam, see section 39.7. 39.4 m/e indicator A device capable of precisely displaying the m / e ratio of a selected ion beam is extremely useful, not only in identification of impurities in the background of a mass spectrum and in the adjustment of the distance between collector slits, but also in detection of instabilities of the magnetic field produced by an electromagnet. The reason of spatial instability of the ion beam can be easily recognized by use of the m / e indicator, which has a short-term precision of at least 0.01 mass unit.
Halas & Sikora (1987) described a simple dual Hall probe device that indicates m/ e ratio with the required short-term precision of 0.01 mass unit. The principle of its action is as follows. The first Hall probe, H1 in Figure 39.2, is fed by a constant current I1. Thus the output voltage, V1, is proportional to the product V1 ~- IIB
[39.5]
where B is the magnetic field intensity experienced by this Hall probe. The output voltage V1 is repeated by the operational amplifier with 100% feedback coupling. The second Hall probe, H2, is fed by current I2 directly proportional to V1 from a voltageto-current converter. If the second Hall probe experiences the same magnetic field, then its output signal, V2, will be proportional to the product I2B. Then, from equation [39.5] and the proportionality of I2 to V1 we obtain that V2 ~ B 2
[39.6]
which means that V2 will be proportional to the m / e value in the magnetic scanning operation mode.
862
Chapter 39 - S. Halas & T. Durakiewicz
I 11=constant
V2 H2 /
V1
VtoI converter
Figure 39.2 - Schematic diagram of the dual Hall probe method.
By adjustment of the I] value, one can obtain the voltage V2 (in milivolts) exactly equal to m / e ratio, which is very useful. Two small electrically insulated Hall probes may be placed in parallel between the magnet poles to determine the magnetic field intensities. In our m / e indicator two inexpensive CdHgTe thin-film Hall probes were used with 60f~ resistance each. The maximum current applied to feed the probes was less than 20 mA, so that no significant power was dissipated. In the case of mass spectrometers with permanent magnets, B is a constant, and so: m / e ~ 1 / Vacc
[39.7]
where Vacc is acceleration voltage of the ion beam, which is the highest potential in the ion source with respect to ground. Hence, to indicate the m / e ratio it is necessary to reverse the high voltage (or a portion taken from a voltage divider). The reversed voltage can be displayed by means of a so-called double integrating DVM. Such voltmeters are commonly used in laboratories and DVMs with 4.5 digits are commercially available in panel versions with price below 30 US $. These voltmeters compare the measured voltage with a reference voltage produced by a specialized circuit inside the DVM. By reversing the inputs of the "measured voltage" with the "reference voltage" one can simply obtain the reversed voltage on the display. By adjustment of the voltage divider, a selected fraction of the high voltage is reversed in order to obtain the value of m / e ratio on the display. 39.5 Cleaning of the ion source
After a long-term operation of any ion source the insulating ceramics and the lenses became coated by a thin semiconducting layer. The deposits are formed by the decomposition of the analyzed substances as well as metal sputtered off the lenses by the interaction with high speed ions. These layers may produce episodic breakdowns
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry
863
of the high voltage in the ion source or long-term instability of the ion beam due to permanent charging/discharging of the semiconducting layer on the surfaces of the ion lenses. During high voltage breakdowns the ion currents are seen to vary rapidly and with unpredictable recurrence (the characteristic effect is a sudden loss of ion beam signal followed by a return to the previous strength over a matter of seconds). Such an erratic behavior of the mass spectrometer is an immediate signal that the ion source has to be cleaned. The cleaning procedure described below is not difficult and can be done by any competent operator. The ion source can be disassembled from the analyzing tube after venting the instrument through a filter to avoid entry of any solid particles, the most dangerous of which are small ferromagnetic pieces which can be attracted by magnetic field of the analyzer. It is therefore advised that the magnet be removed before venting the analyzer tube. If an electromagnet is used then its power supply should be turned off before venting. Having the ion source in a clean working environment, while the end of the analyzer tube protected against dust by a piece of aluminum foil; disassemble the ionization cage and the lenses. Usually the ionization cage is the most dirty part of a gas ion source. It can be cleaned by removing the deposits by use of a delicate abrasive paper, e.g. 1/600 mm. The lenses can be cleaned in a similar manner. Use of distilled water for periodical rinsing of the parts is recommended. Finally, the parts are rinsed in acetone and boiled in distilled water. At this stage the lenses can be electropolished (see Peele & Brent 1977). The electropolishing is a reverse electrolysis in a viscous strong electrolyte. It is a simple, effective and inexpensive technique. For electropolishing of stainless steel the following recipe for the electrolyte can be used: H2SO4 (1.84 specific gravity) 1000 ml water 370 ml glycerin 1370 ml Stirring, add the acid slowly to the water; avoid overheating. Cool to room temperature, then add the glycerin and stir well. The electrolyte can be used for many cleaning procedures during which the cleaned stainless steel part is immersed as a positive electrode whilst a larger piece of stainless steel foil surrounding that part is connected to the negative pole of a 24V/ 25A rectifier. The use of any other metals or alloys as negative pole is not recommended. The temperature of the electrolyte should be about 60~ The ceramic parts can be cleaned by abrasive paper, rinsed with distilled water, and finally by boiling twice in distilled water. After thorough drying of all the parts in a clean furnace (at 70 to 100~ overnight). Baking of the ceramic parts at 700~ in air may be the next step, if their surfaces still seem to be colorated by a dark contaminant. The ion source can be reassembled using clean tools. Ion sources of modern mass spectrometers often have lenses, which are mounted
864
Chapter 39 - S. Halas & T. D u r a k i e w i c z
on sapphire balls for the best insulation and adjustment. This ensures long-term operation of the source without cleaning and fast outgassing of the surfaces. In contrast, the outgassing of ceramic rods and wobbly ceramic spacers used in older ion sources is time consuming. Sparks due to trapped gas inside ceramic parts may appear when the high voltage supply is turned on too early. The tolerance of the sapphire balls can be limited to _+3 mm, which guarantees identical location of the lenses after cleaning or replacement. 39.6 Stabilization of electron e m i s s i o n current
Fluctuations of the electron emission current transform into fluctuations of the ion current immediately. If the local pressure variations inside the ionization chamber are neglected, the variance of the ion current may be considered as the linear combination of the variance in both electron emission current and ionization cross-section of gas molecules (which varies with electron energy). It is nowadays relatively easy to obtain the electron acceleration voltage (Ve) stabilized with the precision better than 0.01%. Hence, it is only the electron emission current (Ie) that may disturb the ion current stability. There are two general types of electron emission regulators: the series regulator and the switching mode regulator. Regulators, which utilize a switching mode, can offer several advantages over conventional, continuously controlled, series type regulators. In contrast to a well known series-pass regulator of filament current (Halas & Sikora 1990) the comparator in the switching mode circuit has also a positive feedback loop formed by a resistor connecting the noninverting input of an operational amplifier (OA) with its output. This results in pulsation of the output voltage from - 15V to + 15V when the signal at the inverting input is slightly modulated around the reference voltage. Such a design leads to a self-oscillating stabilizer (Durakiewicz 1996) the simplified diagram of which is shown in Figure 39.3. A number of improvements have been made to the original circuit (see Figure 39.4), namely: (1) The electric potential distribution on the filament, cage, ion repeller and trap have been redesigned in order to allow the precise control of the average value of the electron current passing through the cage. The average trap current instead of total emission current is stabilized. (2) The protection of the filament against vacuum break down has been simplified. The respective fragment of electronic circuit comprises only a simple capacitanceresistance filter (C2 = 100 nF and R2 = 2.2 MQ). (3) A modern solid state relay (SSR) for the power supply of the filament was used; thereby the number of electronic components has been reduced. The inverting input of the Schmitt trigger (OA2 with positive and negative feedback loops) is fed from the output of OA1 (trap current-to-voltage converter). Positive feedback loop is sustaining the self-oscillating action of the circuit, whereas the negative feedback is responsible for stabilization. A square wave generated at the output of the trigger passes through the C2-R2 filter to the input of OA3 (voltage follower).
865
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry
Ve
Vr
T
,p
I,F / -)-
I
IeI
"%.
=
9,"
! VREF
'!,.%
[
R3
__.,.r"
I
T Figure 39.3 - Simplified diagram of a self-oscillating electron emission stabilizer. Ie is the electron emission current, Ve is electron acceleration voltage, VREFis the reference voltage, and Vf is the filament supply voltage.
The output voltage from OA3 feeds the light emitting diode of the SSR. Due to the switching action the electron beam is modulated, i.e. the trap current varies periodically between "Hi" and "Lo" values. The switching frequency for a typical filament installed in the ion source ranges from 0.1 to I kHz. The maximum positive voltage is generated on the OA2 output in the case of bad vacuum or any other break in the negative feedback loop. Use of a capacitor (C2 in Figure 39.4:) prevents the filament from burning out; such a protection may not be applied in conventional stabilizers.
(tlA'J
~_T I"-.R1 -.-.-.-] - ~
l +1
Trap l+ Voltage I
CAGE I --
'
+15V ION REPELLER r
TRAP
R~6 ~) ~
r k1 [ ~ P I C A MEN T ~
5 k fa
J R7
klo -15V
I vl
Figure 39.4 - A detailed schematic diagram of the self-oscillating electron emission controller.
866
Chapter 39 - S. Halas & T. Durakiewicz
Although Ie is slightly modulated, its mean value remains extremely stable. Even after a few days of continuous operation the mean Ie value does not vary more than + 1%. The stability is not affected even by large variations of electron acceleration voltage (AV) from 40 to 100 V. The short-term variations of ion current induced by Ie modulation are negligible as long as the ion current detectors have the response time of the order of 100 ms or larger. The long-term stability of the ion current is improved significantly, due to better stability of Ie. The mean value of Ie is not influenced by network supply voltage instabilities of amplitude _+20%. A comparison of results obtained by electron emission stabilizers of various designs is given in Table 39.1. All the designs listed here, except that by Durakiewicz (1996), are generally based on the proportional regulation principle. The advantages of pulsed heating for electron emission control may be summarized as follows: (i) low power consumption - the circuit may be used in a battery operated system (e.g. in space technology), (ii) self-protection against breaks in the negative feedback loop, including the inner part of this loop between filament and electron trap (e.g. due to a bad vacuum), (iii) electron emission current is independent of the electron acceleration voltage and vice-versa, (iv) one pole of the electron acceleration voltage source is connected to the ground of the system, thus Ie does not influence the acceleration voltage itself, (v) the circuit is simple and convenient in operation. The slight modulation of Ie with frequencies above 100Hz is not a drawback in mass spectrometry, where ion current detecting devices with only slow response time are used. Table 39.1 - Comparison of operation parameters of selected electron emission stabilizers. Chapman)
Close & Yarwood (1972)
(1972)
Herbert (1976)
Shaw & Lue (1980)
Halas & Sikora (1990)
Durakiewicz (1996)
operation principle
proportional proportional proportional temperature regulator regulator regulator stabilizer
doubleproportional regulator
selfoscillating
stabilization coefficient
0.1%
relative cost
medium
filament safety
-
0.1%
I low -
0.1%
>1%
1%
0.1%
very high
high
low
low
safe start
-
safe start & loop break
loop break
...........................................................................................................................................................................................................................
Ie/Ue independence
-
, .........................................................................................................................................................................
+
-
~..........................................................................................
-
+
<50%
<80%
l
.................................................................................................................................................................................
energetic efficiency
-
<50%
t...........................................................................................................................................................................................................................
I<50% i
<50%
<30%
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry
39.7 Stabilization of filament temperature In thermal ionization mass spectrometry the ions hot metal with high melting point (usually W, Ta, which can be drawn from the surface at equilibrium well-known Saha-Langmuir formula (Benninghoven 2001)" .+
867
are produced on the surface of a Re, Ir, Pt). The current density, conditions, is determined by the et al. 1987, Valyi 1977, de Laeter
ve
J = 1
g~ E~-q3 + --~exp kT g
[39.8]
where v is the flux density of particles supplied to the surface, e is elementary charge, g+ and gO are statistical weights of ions and neutral particles, respectively, Ei is the ionization energy of the particle, q~is the work function of the metal, k is Boltzmann constant and T is the absolute temperature. The ratio of gO/ g+ is equal 2 and 1/ 2 for alkali and alkaline earth metals, respectively. Numerical values of ionization potentials of elements and statistical weights of ground state of atoms and ions are given in Uns61d (1968). The formula [39.8] is strictly valid only when the surface coverage is small. For negative ions the respective formula is: -
ve
J -
0
[39.9]
1 + g exp q9- A
g
-
kT
where A is the electron affinity. Numerical values for electron affinities of some elements and their oxides can be found in Valyi (1977) and Wachsmann & Heumann (1991, 1992). Inasmuch as both nominator and denominator in formulae [39.8] and [39.9] is strongly temperature dependent, a precise filament temperature stabilization is demanded for production of stable ion beams by the surface ionization process. In the case of a single filament ion source the flux density, v, is driven by the surface diffusion from cooler ends of the filament towards its center. The filament center should be kept at a temperature at which the analyzed atoms can be effectively ionized, which means that the surface coverage is far below the monolayer at the filament center. The filament length may be also crucial in obtaining stable ion currents. It is so because a temperature plateau exists in the case of too long filaments. In such a case, the fluxes of ions are emitted from relatively large surface with high fluctuations due to variations in surface coverage at the plateau region. However, steady conditions for ionic emission can be obtained for relatively short filaments, for which no temperature plateau exists (details on defining long and short filaments may be found in Halas & Durakiewicz, 1998a).
868
Chapter 39 - S. Halas & T. Durakiewicz
The following methods may be used for the filament temperature stabilization: (1) A commonly used method of current stabilization, (2) Filament resistance stabilization, (3) Filament voltage stabilization, (4) Stabilization of power supplied to the filament. Method (1) is relatively easy to apply but it has a significant drawback: the filament temperature depends highly nonlinearly on the heating current. Therefore we describe below the temperature controllers based on methods (2) and (3) which are characterized by the linear dependence of filament temperature on its resistance or on the voltage drop between its ends, respectively. Method (4) will not be considered here because it requires rather complicated electronic circuits. The filament resistance stabilizer can be made on the basis of the Halas-Kaminski bridge (Halas & Kaminski 1995, Halas et al. 1993, Durakiewicz & Halas 1995). A conceptual diagram of the stabilizer is shown in Figure 39.5. The bridge contains a DC excitation voltage source, Ve. The excitation current passes through a diode and the bridge resistors but not through the transistor T. RF and Rv denote, respectively, the resistance of the filament and a variable resistor used for temperature setting. The filament is made of a pure metal (e.g. W, Ta, Re, Pt) which have positive and significantly large temperature coefficients of electrical resistivity. If Rv < RF then the bridge signal supplied to the noninverting sample-and-hold amplifier (S/H) is negative or zero and the transistor does not conduct. The noninverting input of the OA is connected to the output of a S/H amplifier whilst the inverting input of OA is fed by a positive signal from a triangular wave form generator, G. In the reverse situation, i.e. Rv > RF, the imbalance signal generated by the bridge is positive and has a defined magnitude. This signal is amplified by the S/H and held at the S/H output, when the negative potential taken from the output of OA turns to positive. The comparator OA produces a positive output voltage if the generator signal is below the voltage held by S/H. In this fraction of the cycle, transistor T conducts. Hence the power supply, V1, is connected in parallel to RF (through T, which plays the role of a switch). No significant current from V1 can pass through the remaining parts of the bridge due to presence of the diode D. Because V1 considerably exceeds Ve, the excitation current cannot pass through the low-resistance legs of the bridge (Rv and RF) when RF is supplied from V1. This is the reason for using a sample-and-hold amplifier instead of a normal operational amplifier. As result of the action of the circuit shown in Figure 39.5; a series of heating pulses is supplied to RF with frequency driven by the generator but their duration is driven by the output voltage of the S/H amplifier. The bridge is always kept close to the balance state, i.e. RF - Rv. If for some reason (e.g. voltage V1 starts to diminish) RF resistance becomes somewhat lower than Rv, then the bridge imbalance signal becomes somewhat higher, which results in a longer duration of the heating pulses. A complete circuit diagram is described by Halas & Durakiewicz (1998b).
869
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry Figure 39.5 - Conceptual diagram of using the Halas-Kaminski bridge for constant-resistance operation of a filament RF. S / H is a sample-and-hold amplifier; G is a triangular wave generator.
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The use of a simple voltage stabilizer for temperature control of a filament was described by Halas et al. (2001). Voltage may be stabilized typically by use of the same two wires to feed voltage to the filament and measure the voltage value (2-wire method) or, as it is described below, by use of a separate pair of wires for voltage measurement. In such a 4-wires configuration the voltage drop along the supply lines does not influence the value measured by the stabilizer circuit, hence the fluctuation of the ion beam is significantly reduced. The schematic diagram of the circuit is shown in Figure 39.6. The MA741 operational amplifier with the negative feedback loop constitutes the basis of the stabilizer circuit. The reference voltage supplied to the noninverting input of the amplifier may be set either manually by use of the 10k potentiometer, or digitally by use of the digital-to-analog converter (AD7243), or manually by use of the potentiometer. All the fluctuations of the filament voltage are minimized by the negative feedback loop what allows for the temperature stabilization of the filament. In the 4-wires method the resistance variations of filament power supply wires do not affect the filament voltage measurement. The output signal of the MA741 drives the Darlington circuit comprising the 2N3055 and BD439N transistors. This circuit keeps the filament voltage constant and therefore stabilizes its temperature. Since ions have to be formed into a beam, the filament and the whole supply circuit is fixed on the 2kV potential with respect to the ground. Because of the high potential of the source it was necessary to construct the three channel optical relay between the master computer and slave digital-to-analog converter in the filament power supply. In this way the master computer is protected against high voltage. The remaining components identified in the schematic diagram are used to allow smooth switching between the computer and manual control, and to protect the filament against burning during power-on of the supply.
870
Chapter 39
- S. Halas & T. Durakiewicz
SYNC SDIN LK SYNC ~.x.. ~::'S :,, '~' ~ l SDIN \470K D K"I ~.]N SCLK \.,. ", -1- "~ CLR 470KN\470 i i i rlVref d====++5V\ ;,.X, ./ / ..a,k14v \~-- I MAA I12v _ I c12J, liAD7243{ ZlmF 2.2M~ GND T,, ~ , . , ' T 17812 I i ~ +~5 I I [ +5V
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Figure 39.6 - Schematic diagram of the filament temperature controller based on the stabilizer.
wires voltage
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The voltmeter indicated on the schematic diagram enables the visual control of the filament temperature. This is possible because the temperature-voltage characteristics of the filament are almost linear, as demonstrated by Halas et al. (2001). The voltmeter was calibrated directly in temperature units what allows the operator immediate information on the filament status. The stabilizer is fully computer controlled via the digital-to-analog converter AD7243 (see Halas et al. 2001 for details). In order to demonstrate the superiority of the 4-wires over the 2-wires approach, we have continuously recorded the ion current of 39K+ for 20 minutes. Potassium was loaded onto a platinum filament, 0.05mm x 0.63mm x 10mm, as 50% K3PO4 solution. After drying in air and degassing under vacuum, the filament was incrementally heated to 900K. Ion emission was measured at a filament temperature about 1000K. Comparison of 39K ion currents obtained using the above-described voltage stabilize with 4-wires and 2-wires configuration is shown in Figure 39.7. 39.8 Integrating of the ion currents The ion detection system of an IRMS consists of a collector assembly containing two or more Faraday cups (as mentioned in section 39.2). Although the ion counting
871
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry
Figure 39.7 - Comparison of 39K ion currents obtained by the authors for filament voltage stabilization by use of 2-wires and 4-wires methods, respectively. technique is used in specialized instruments for detection of very weak ion beams (like 3He, 230Th), we will not consider these devices here. The ions collected in each cup produce electric currents, which flow through a high-value resistor (i.e. 109 - 1012 f2) to ground. The voltage produced on the resistor is not amplified but rather repeated by a circuit arranged as a voltage follower, or, more frequently, it is reversed by an amplifier with the resistor forming a negative feedback loop. A typical detector system of an IRMS is shown schematically in Figure 39.8. The ion current is converted to a voltage by an operational amplifier with ultra-low input bias current and the high-value resistor. The output voltages are then converted to trains of short pulses by so-called voltage-to-frequency converters (V/F), the repetiN 109Q I
t---"
V/F Faraday cups
,
Counter
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Counter
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Figure 39.8 - A typical arrangement for digital measurements of current ratios
872
Chapter 39- S. Halas & T. Durakiewicz
tion rate of which is linearly proportional to the input voltage. These pulses are fed to separate counters which are set to zero at the start of measurement. When the major beam counter reaches a count of 106, the minor beam counter is stopped and its value displayed. The value of the full six-digit display is therefore equal to the current ratio. Unfortunately the detection system described above introduces, its own noise, predominantly from the high-value resistors. This is a fundamental phenomenon which cannot be eliminated by technological improvement of the production of resistors (see Felgett & Usher, 1980, for example). Moreover, the resistors also suffer from fluctuations of their value due to variations of the potential drop along the resistor and temperature changes (Habfast, 1960). The key to the improvement of isotope ratio measurements is to replace each highvalue resistor by a capacitor (Jackson & Young 1973, Halas & Skorzynski 1980, McCord & Taylor 1986). This replacement converts the ion detector from an ion "amplifier" to an "integrator" where the voltage on the capacitor raises in time proportionally to the charge collected: T
V - c f l d t --
I.T
C
[39.10]
0
where C is the value of capacitance, I is the ion current and T is the integration period. The value of the capacitor is selected in such a manner that the final voltages are of order of 10 volts. The integration period may be estimated from the following statistical considerations. Let us assume that the 180/160 ratio is measured using CO2 gas. Typical currents obtained by a Nier type ion source for mass 44 and 46 are 2.5 x 10-9A and 1 x 10-11A, respectively. Hence, after time T the number of electrons collected on each capacitor is: n~
I.T
[39.11]
e
where e is the elementary charge 1.602 x 10-19As. According to the general principles of statistics, the relative uncertainty of An/n is equal n-l/2; hence for a desired uncertainty of about 10-5 (i.e. + 0.01%o) the number of ions collected has to be 1010. From equation [39.11] one obtains for the minor beam: T~ 101~
1.602.10-19As 1.6. 102s 10
-11
A
i.e. the integration time has to be of the order of 100 seconds.
[39.12]
873
Techniques of Ion Current Stabilization in Isotope Ratio Mass Spectrometry
The obvious inconveniences of the integrating system are that the "---. 2 ""--, 1 "-... capacitors have to be discharged periodically, prior to each charging C cycle, and the system cannot be used I I directly for the instrument adjust" I I ments by continuous monitoring of the ion current. Both difficulties, however, can easily be overcame today by use of the computer controlled high quality reed switches for discharging the capacitors and their instant switching to the resistors. Such a solution was described by Halas & Skorzynski (1980). One pair Figure 39.9 - Schematic diagram of the capacitance/ of reed switches is required for each resistance system. amplifier. The switches are connected in parallel to C and R as shown in Figure 39.9. In the integrating mode switch 2 is closed and switch I is used for periodical discharging of the capacitor. For adjustment of the mass spectrometer, recording the mass spectra, etc., switch I is closed whilst switch 2 is open.
i
R
39.9 Final remarks
Good quality isotope ratio results are worth taking the effort. The ideas and general remarks presented in this chapter are certainly not the only solutions that guarantee success. They are, however, tested by many years of operation and maintenance of several instruments in our Mass Spectrometry Laboratory. In the final section we should make our reader aware, that having the ion beams stable enough to produce satisfactory precision of 0.05 permil or better for standard versus standard measurement does not guarantee identical precision for sample versus standard measurement. If this is not the case, one should check the purity of the sample and/or the gas flow conditions through the inlet system, as well as the linearity of the system. Having stable ion beams, pure samples and a high quality inlet system one has a chance for good and long-term performance provided that settings of the IRMS are favorable. The optimum setting of the ion source should assure maximum ion current and best peak shape at minimum electron beam. The gas flow rate through the capillaries should be selected depending on the geometry of the ion source. The pressure of the analyzed gas (as estimated on the basis of major beam current) during gas flow should not exceed significantly the 100-fold background pressure. Too high a pressure leads to ion scattering by the gas molecules and thereby to peak broadening.