Magnetic Peak Switching in Spark Source Mass Spectrometry C. W. Magee and W. W. Harrison Department of Chemistry, University of Virginia, Charlottesville, Va. 22907
The introduction of electrical detection (I, 2) t o spark source mass spectrometry now allows a significant improvement in precision and accuracy using peak switch integration techniques as compared t o photographic detection. Switching from one m a s s t o another c a n be accomplished b y variation of ( a ) the accelerating and electrostatic analyzer voltages or ( b ) the magnetic field strength. The former method has been generally used (3, 4 ) because of problems associated with m a g n e t hysteresis. Barton et al. ( 5 ) h a v e shown that magnetic field control is useful for fast sweeps. Electrostatic peak switching m a y introduce several problems, including possible changes in ion extraction efficiency, variation of energy band pass into the magnetic analyzer, and electron multiplier gain dependence (6) on ion energy. The general electrostatic peak switch mass limitation of M to 2 M before required readjustment of the magnetic field is also a distinct disadvantage when carrying o u t a survey analysis for m a n y elements. Magnetic peak switching eliminates these difficulties because the accelerating and ESA voltages r e m a i n virtually unchanged. An inexpensive magnet control unit was designed and constructed to allow implementation of magnetic peak switching.
EXPERIMENTAL Apparatus. The Magnetic Peak Selector (MPS) interfaced to an AEI MS 702 enables the operator to select any ten masses (or more, with add on capabilities) from 6 to 250 amu and bring them onto the collector slit to within icO.1 amu by a switch selected variation of the magnetic field. The unit is comprised of a magnetic field sensing Hall probe interacting with two associated circuits, one a constant current supply to the Hall probe and the other a magnet control system based on the,Hall voltage. Hall Probe Power Supply. The circuit (Figure 1) is built around an operational power supply (KEPCO Model OPS 100-0.2B) furnishing 100 mA to a high sensitivity Hall probe (F. W. Bell, Model BH-206). The current supply circuit is floating off ground because the Hall output voltage must be referenced to ground and no two leads of the four on the probe may be a t the same potential. The probe current, adjustable by 20% with RV1, is fed through resistor Rq which acts as a ballast against the low probe resistance (-4 n). The voltage drop across Rq is measured with a DVM (HEATH Model EU-805) and serves as a daily current check. The OPS is wired in a constant current mode because the Hall probe resistance changes with a change in magnetic field. While small, this change cannot be neglected compared to the required current regulation of *0.01%. Hall Probe Placement. Probe placement is outside the magnetic analyzer vacuum chamber as previously described (7). Magnet Control Circuit. Figure 2 shows the control circuit for the MPS. The active elements are an integrating null detector followed by a high voltage driver. When coupled to the Hall probe, these amplifiers form a magnetic feedback loop which drives the magnet to produce a Hall voltage equal to a selected R. A. Bingham and R. M . Elliot, Anal. Chem., 43,43,(1971). R. J. Conzemius and H . J . Svec, Talanta, 16,365 (1969). C. A . Evans, R . J. Guidoboni. and F . D . Leipziger, Appl. Spectrosc., 24, 8 5 , (1970). H. J . Svec and R. J . Conzemius in "Advances in Mass Spectrometry," E. Kendrich, Ed., Vol. 4, Institute of Petroleum, Adlard, Dork-
ing, pp 457-64. G . W. Barton, Jr., L. F. Tolman, and R. E. Roulette, Rev. Sci. lnstrum., 31,995 (1960). K. H. Krebs, Forlschr. Phys., 16,419 (1968). C. W. Mageeand W . W . Harrison, Anal. Chem., 45,852 (1973).
reference voltage, but of opposite polarity. Thus, when equilibrium is reached, a null exists a t the summing point of AI. A potential of -6.2 volts is applied to one of the summing inputs of Az (Philbrick, Model 1022 with 2217 power supply). With Rv13 properly adjusted and S3 open, the output of this amplifier will be +60.7 volts, driving the magnet to its highest current of 300 mA. This condition will in turn produce a Hall voltage of approximately -0.42 volt which is fed through SI to one of the summing inputs of integrator AI (Analog Devices, Chopper Stabilized Model 233K). The other summing input is fed by a positive reference voltage formed by the chain R7, RV11, R12, and the s6 selected potentiometer from RV1 to RVlo. RVll (fine) and RV12 (coarse) reference adjusts are set such that when s6 is switched to Set Balance, a reference voltage is produced which forces the magnet current to rise to its maximum rated output of 300 mA to generate a Hall voltage that is sufficiently high to balance the nulling bridge. The maximum magnet current can be reached when S6 is switched to any one of the ten mass selection channels. s6 also has a position to accept a reference from a set of auxiliary channels which can be added through a rear panel connection. Sq enables the magnet to be driven by either the standard AEI scan control unit ( I ) or by the MPS. S1 grounds the inputs of A1 so that the input bias current can be adjusted to zero as determined by integrating with S Z open, and following the output with the panel meter. The meter display switch SJ can alternately be set to monitor the magnet reference voltage. Operation of MPS. With S6 in the Set Balance position, Sz open, S3 closed, and the A1 inputs ungrounded (SI),the MPS is in an equilibrium quiescent state a t maximum magnetic field. If Se is now switched to a mass channel, the positive reference voltage appearing at R2 will be smaller in magnitude than the negative Hall voltage, creating a net negative input current at AI. This off-balance is inverted, integrated, and fed to one summing input (R3) of A2 as a positive ramp voltage which reduces the +60.7-volt output of Az, the magnet reference voltage. The net result is a decrease in magnetic field, reducing the magnitude of the negative Hall voltage, which in turn makes the off-balance at the integrator summing point smaller, slowing down the rate of change of the magnetic field. The system will quickly (within seconds) reach an equilibrium state where the Hall voltage is again equal in magnitude to the reference voltage, representing the magnetic field necessary to bring the preselected mass into register on the collector. The setting of the mass reference voltages (from a table of Hall voltages us. mass) is accomplished by adjustment of potentiometer RV1 through RVlo while monitoring the s6 wiper voltage with the DVM. To within an accuracy of several microvolts, the reference voltages chosen will be equal in magnitude to the Hall voltages reached by the MPS, bringing the desired masses into register on the collector slit. The accuracy of the MPS is limited to approximately *O.l amu by several factors, including residual hysteresis effects, residual input bias current of AI, and stability of the various reference voltages and power supplies.
RESULTS AND DISCUSSION
Current us. Field Switching. T h e relatively infrequent use of magnetic peak switching (8) is i n part due to the difficulty in eliminating magnetic hysteresis effects. A control circuit for magnetic peak switching must be capable of correcting these problems. The magnetic field at a specific current is very dependent on the previous setting of the current. It is, therefore, the magnetic field which must be monitored and controlled, as shown in Figure 3. Three channels of the MPS were set at masses 250, 120, and 30. S t a r t i n g at mass 120, (8) R . J. Conzemius and H. J. Svec, in "Trace Analysis by Mass Spect r o m e t r y , " A . J . Ahearn. Ed., NewYork, N . Y . . 1972,p157.
TO C V N
!4 r
PROBE
Figure 1. Schematic of MPS Hall probe power supply R1 6.2 Kohrns, 10 watts; R 2 900 ohms, 10 watts; 1000.2B; Hall probe-F. W. Bell, Model BH-206
R3 10 ohms, 10 watts; R4 500 ohms, 200 watts; RV, 200 ohms (ten-turn); OPS Kepco, Model
I
+
RV1 3
;
TO G Y M
T E S T JACK
WAS R i r FROM A S C U
kt
TO
AUX
CH
Figure 2. Schematic of MPS control circuit R i , 1 Mohrn; R z , 1 Mohrn; R3, 10 Kohrns; R4, 10 Kohms; RI, 100 Kohrns; Re, 1 Mohrn; R,. 100 Kohrns; C,, 0 . 1 kf; RV1-RV10, 100 Kohrns (ten-turn); R V i i . 10 Kohrns (ten-turn); RV12, 100 Kohrns; RV13, 1 Kohm; M1, 100 @A; A,, Analog Devices, Model 233K; A 2 Teledyne Philbrick, Model 1022
the field was switched to mass 250, then back to mass 120 on the time scale shown. The field returned to within 400 ppm of its previous value as measured by a DVM. The magnet current, however, was lower-165 mA instead of the previous 175 mA. If the field is switched to mass 30 and then back to 120, the field accurately tracks to the preset value even though the current is then back to 175 mA. Thus, if the magnetic field is monitored to align masses, the hysteresis of the magnet is no longer a problem.
Internal Standard Analysis. A further advantage of magnetic peak switching is the ability to compare trace constituents with internal standards using high precision integration techniques. Internal standards in SSMS have normally been used only with photographic detection and electrical scanning because for these techniques the accelerating voltage is left unchanged. However, both these procedures suffer from poor precision, (-f30% re1 std dev). Electrostatic peak switching shows good precision (-5% re1 std dev), but difficulties arise with the use of'
ANALYTICAL CHEMISTRY, VOL. 46, NO. 3, MARCH 1974
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I
I
I
I
x1
I
0
I
1
TIME
Figure 3.
u'
I
I
2
3
in minutes
Magnetic field switching (see text)
internal standards. Sensitivity changes occur, associated with changing the accelerating voltage. Also, a limitation exists in mass of M to 2M without changing the magnetic field. Magnetic peak switching, however, combines the precision of integration techniques with the advantage of a fixed accelerating voltage to allow large numbers of elements to be quickly measured with good accuracy through the use of precise relative sensitivity factors. Table I shows RSFs determined by magnetic peak switching. These data depend not only on the use of the MPS, but also on close monitoring and control of critical experimental parameters, including interelectrode self-shielding and spark gap. Determination of elemental sensitivities relative to an internal standard using magnetic peak switching has been reported b y Hull (9). Table I1 shows an example of magnetic peak switching analysis using an internal standard. NBS low alloy steel No. 462 was used to calculate RSFs for 15 elements us. nickel. NBS No. 461 was then analyzed using these RSFs, taking five 0.01 nC charge accumulations for each element. The large discrepancy for carbon could not be explained. Chromium and niobium are also less than satisfactory. However, all but four of the elements in Table I1 are close to or within the anticipated 10% accuracy range. RSFs may vary somewhat with matrix. Iron, for example, in Table I shows an RSF of 1.54 us. yttrium in an electrode of ashed NBS Bovine Liver compacted in graphite. In an electrode of ashed NBS Orchard Leaves prepared in the same manner, iron shows an RSF of 1.89. The excellent preciszon ( -5%) obtainable with integration methods may be misleading relative to accuracy considerations of a standard and unknown or even to the sample-to-sample use of RSFs. Disadvantages of Magnetic Peak Switching. The magnetic peak selector has been of considerable value in our laboratory. There are, however, certain drawbacks to magnetic switching. The *O.l amu accuracy of the M P S is normally sufficient to put the desired mass on a wide (0.014-in) collector slit, but not accurate enough for precise peak switching analyses. A slight fine tuning (&0.1%) of the accelerating voltage is required to maximize the collector-to-monitor ratio (7). Magnetic switching requires more time than electrostatic switching to change peaks. The settling time is generally between 10 and 20 seconds, as illustrated in Figure 4. (9) C W Hull Ini J Mass Spectrom /on Phys , 3, 293 (1969)
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L
0
20
40
0
TIME
20
40
0
20
43
60
in seconds
Figure 4. Settling characteristics of the MPS when switching from mass 210 to mass 40
Traces are shown of the Hall voltage taken from the MPS test jack while switching from mass 210 to mass 40, approximating a maximum settling time. A slightly underdamped response providing some overshoot yielded the fastest settling time. There exists appreciable time lag of the field behind the current-that is, the field is slower to react than the current. This can be seen in the XI trace of Figure 4. The magnet current started to decrease sharply a t t = 0, but it was 1-2 seconds later before the field started to fall. The X6 and X30 traces show that the field takes one large negative overshoot and then a smaller positive one, equilibrating to a short term (10 minute) stability of better than 100 ppm. Linear Field Scanning. The M P S can also control the magnet for log-ratio scanning, thus producing linear field scans. This is accomplished by inputting a ramp voltage on the auxiliary channel input to S g (Figure 2). The null balance principle of the circuit requires the Hall voltage to be maintained equal to the reference voltage, causing a scan which is linear in magnetic field. This method of scanning has two distinct advantages over conventional exponential scanning. The MPS controls the field rather than the current; the previous condition of the magnet has no effect on dispersion, unlike the current controlled case. With linear field scanning, the dispersion at high mass is greater than for exponential scanning, while less a t the lower mass end. More even dispersion is thus produced. The linear field scan also makes it possible to start a scan a t any point in the mass spectrum, unlike the conventional exponential scan where the starting point is critical (10). A programmed reference voltage from a com(10) Dennis Allenden. AEI Scientific Apparatus Inc personal communication, 1970
NO. 3 , MARCH 1974
Table I. Relative Sensitivity Factors as Determined by Magnetic Peak Switching. RSF = apparent/true relative to Bey Sample
Bovine liver No. 1, day 1 Bovine liver No. 1, day 1 Bovine liver No. 1, day 2 Bovine liver No. 2, day 3 Bovine liver No. 1, day 4
“Mn
66Fe
2.50
1.56
1.50
1.57
4.60
0.679
1.80
2.40
1.57
1.45
1.58
4.81
0.664
1.99
2.25
1.42
1.55
1.72
4.28
0.580
1.84
2.34
1.57
1.51
1.56
4.85
0.479
2.04
2.45
1.56
1.57
1.68
5.32
0.597
2.31
Cert. wt %
Found, w t yo
C P Ti V Cr Mn co
0.15 0.053 (0.01) 0.024 0.13 0.36 0.26 0.34 0.028 ( < O .005) 0.011 0.30 0.022 0.012 (0 .003)
0.313 0.047 0.012 0.021 0.187 0.429 0.269 0.345 0.030 0.001 0.014 0.263 0.023 0.011 0.003
cu
As
Zr Nb MO
Sn W Pb a
75As
ZOBPb
80%
puter interface might be used to drive the MPS in a peak to peak elemental search.
Table 11. Determination of 15 Elements in NBS No. 461 by Magnetic Peak Switching SSMS Using a Nickel Internal Standard. Element
SSZn
W U ’
CONCLUSIONS The ability to bring unequivocally any mass in the spark source spectrum onto the collector slit quickly and unambiguously has led to greater use in our laboratory of the higher precision of peak switching techniques. By comparison, for electrostatic peak switching, “reliable peak identification . . . requires close interaction and decision making by the instrument operator” (8). For our problems, the MPS has provided a solution which is both convenient and inexpensive (total cost including Hall probe is approximately $750). Received for review May 22, 1973. Accepted October 29, 1973. This research was supported by grants from EPA and LEAA.
wNoNi+lat 0.40 (wt %) internal standard.
Single Hollow Fiber Microconcentrator Rashid A. Zeineh Department of Microbiology, Chicago Medical School and the University of Health Sciences, Chicago, 111
Beverly J. Fiorella and Elie P. Nijm School of Associated Medical Sciences, University of Illinois, Chicago. 111.
George Dunea Departments of Nephrology and Hypertension, Cook County Hospital and the .Wektoen institute for Medical Research, and the University of .Yea/th Sciences, Chicago, 111.
The determination and study of various substances in body fluids frequently requires preliminary concentration of the specimen. This requirement is essential when only small volumes are available, ( e . g . , cerebrospinal fluid), or when concentrations are low ( e . g . , proteins in normal urine). An ideal concentrator should be efficient,, easy to operate, cause no denaturation or drying, and allow easy recovery of the concentrate. Concentrating systems currently in use include freeze drying, concentration against a n osmotic gradient, pervaporation in air, precipitation, and suction ultrafiltration (1-5). None of these methods satisfy all the requirements for an ideal concentrator. In this report we describe a single hollow fiber microconcentrator which is simple, efficient, easy to operate, reusable, and which represents a significant improvement over earlier models. (1) G. Schneider and G. Wallenius, Scan. J. Clin. Lab. Invest.. 3, 140 (1951 ) .
EXPERIMENTAL Apparatus. The microconcentrator consists of a single hollow fiber of microtubular semipermeable membrane with an outlet and inlet (Figure 1); a test tube cap with three implanted separate tubular steel outlets; a mini-clamp; connecting tubing; and a Plexiglas stand (Figures 2 and 3). The hollow fiber used in this laboratory is 6 inches long, 0.7-mm inside diameter and has a priming volume of 5 microliters. The tubular membrane is made of acrylic material, has a maximal pore size of 25 Angstroms and is impermeable to substances with molecular weights over 10,000. The ultrafiltration rate by suction is 15 ml per hour.
(2) T. J. Greenwalt, C. J. Van Oss, and E. A . Steane. Arner. J , Clin. Pathol., 49, 472 (1968). (3) J. N. Cummings, J. Neurol. Neurosurg. Psychiat., 16, 152 (1953). (4) F. Miyasato and V. E. Pollak, J . Lab. Clin. Med., 67, 1036 (1966). (5) R. A. Zeineh and 8. J. Fiorella, Amer. J. Med. Techno/, 36, 1 (1970).
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