EPR Spectrometers at Frequencies Below X-band
10. RESONATOR
2. EPRSPECTROMETERS AT FREQUENCIES BELOW X-BAND
et al. 1999) describe L-band resonators designed for EPR imaging of perfused hearts and living mice. Since the heart preparations and animals involve lossy aqueous solutions, particular attention was paid to minimizing the E field at the sample. For example, the inner edge of the capacitive gap in a 1-loop-2-gap resonator was recessed to decrease the E fringe field (Zweier and Kuppusamy, 1988).
Nitroxide radicals in human skin were imaged using an S-band system (He et al., 2001a). A bridged LGR, which resonated at 2.2 GHz when loaded with the human forearm skin, was used to image nitroxyl radical distribution and metabolism. A 7 mm diameter region of the skin was positioned with a special holder at the end of a 7 mm diameter LGR. Details of this resonator were described by Petryakov et al. (2001). Spectra were obtained of 10.8 moles of nitroxide applied to the surface of the skin of a human forearm. I mm from the end of the resonator, the microwave magnetic field BI was 0.62 GI."JW, and significant EPR signal could be measured up to 6 mm from the end of the resonator. An analogous resonator was used for L-band (1.32 GHz) measurement of a mouse tumor in contact with the resonator, which could sample a volume 10 mm in diameter and 5 mm deep (I1angovan, et al., 2002). He et al. (2001b) described the use of varactor diodes for automatic tuning and automatic coupling of a 700 MHz resonator of a type previously described as transverse electric reentrant (Chzhan et al., 1999).
To avoid the radiation losses inherent in some implementations of the LGR for animals, Sotgiu and coworkers (Momo and Sotgiu, 1984; Sotgiu and Gualtieri, 1985; Sotgiu, 1985; Alecci et al., 1989) created a reentrant form of the LGR in which the return flux was contained within the resonator except for the cylindrical region via which the sample was inserted into the resonator. Essentially, an inductive loop, which may have several branches, surrounds one or more capacitances. This is a flexible design, which presages designs developed in our laboratory for the crossed-loop resonator (Rinard et al., 1996a,b; 2000) and also used in other resonators where it is desirable to avoid a separate shield (Rinard et al., 1999a). Sample regions with 50 mm diameter, sufficient for the study of mice, were produced at 1.34 GHz and 38 mm diameter at 1.68 GHz (Sotgiu, 1985).
Hornak et al. (1991) built a 200 MHz resonator analogous to a version of a LGR that had been used for31pNMR. Called a single turn solenoid, it was made of copper foil (30x82 mm, 0.08 mm thick) on a polyvinyl chloride form and three 20 pF chip capacitors were soldered across the gap. This was placed in a 38 mm diameter, 76 mm long, 0.5 mm thick silver-plated brass shield.
2. EPRSPECTROMETERS AT FREQUENCIES BELOWX-BAND 87 Brivati et al. (1991) tuned the frequency of a ca. 300 MHz LGR by moving a dielectric slab in the gap. This type mechanism was used as part of an automatic coupling control (ACC) circuit.
An innovative crossed-loop resonator (CLR) isolates the EPR signal from source noise and from pulse power in pulsed EPR. At L-band and S-band a CLR has been demonstrated to give results superior to a reflection resonator for CW dispersion spectroscopy, superheterodyne spectroscopy and ESE (Rinard et al., 1996a,b, 2000). The CLR also has been implemented at 250 MHz with a 2.5 cm diameter sample loop (Rinardet al. 2002a).
A 300 MHz resonator used for pulsed EPR (Murugesan et al., 1998;
Devasahayam et al., 2000) was 25 mm diameter and 25 mm long, being constructed of II parallel loops spaced 2.5 mm apart and connected in parallel. The resonator incorporated a parallel resistance to help lower the Q to decrease the ring-down time. Another resonator used for pulsed EPR in the 200-400 MHz range was a short-circuited coaxial line (Rubinson et al., 1998), analogous to that used at 883 MHz (Rubinson et al., 1995). At 300 MHz 10 W incident power yielded B) = 0.9 G. No tuning of the probe was needed to perform relaxation time measurements at 200, 250, 300, and 350 MHz.
To reduce the resonator Q for pulsed 286 MHz EPR, a two-tum coil-shaped polyethylene tube was placed inside a LGR, and water was flowed through the tubing, reducing the Q from 570 to 70 (Yokoyamaet al., 1999).
Using 3 or 5 turns lowered the Q to 44 or 30. This resonator was used to obtain LODESR spectra of a triarylmethyl radical in water with 200 ns 100 W pulses repeated every 2000 ns (average power 10 W) (Yokoyama et al., 1999). Although there might be situations in which it is desirable to lower the Q by adding loss to the resonator , the signal is larger using an overcoupled high-Q resonator than using a resonator with inherently lower Q (Rinardet al., 1994).
An active resonator system was developed for 700 MHz CW EPR (Sato et al., 2002). This is an example of what is called regeneration, as was used in early radio receivers to narrow the bandwidth and block interfering radio signals. Sato et al. showed that it increased the resonator Q, and hence the EPR signal, but it also increased the noise, such that the observed signal-to-noise ratio increased only 50% even though the signal increased a factor of seven.
Giuseppe et al. (2001) designed a one-loop two-gap bridged LGR tuned to 1 GHz for EPR with a coaxial solenoid outside the LGR tuned to 1.52 MHz for NMR detection. The homogeneous RF region was 3 em diameter and 4 em long. A shield surrounded the composite resonator.
A method for estimating the frequency of a bridged loop-gap resonator was described and compared with measurements on prototype resonators in
the ca. 1.2-1.5 GHz range (Hirata and Ono, 1996). Microwave field distributions were calculated for an S-band BLGR (Willeret al., 2000).
Multiple LGRs, magnetically coupled, yielded a 1.3 GHz resonator with wide bandwidth designed for pulsed EPR (Sakamotoet al., 1995). Design equations and test data of a prototype were presented.
There are advantages to using surface coil resonators (Bacic et al., 1989;
Hirataetal., 1995, 2000) for localized in vivo spectroscopy. Although some surface coil probes are a single loop of wire or a flat trace, an LGR or a dielectric resonator (Jiang et al., 1996) can also be used as a surface coil.
Varactor diodes were used to electronically tune the frequency and match of an L-band surface coil resonator (Hirata et al., 2000) . Flexible leads to the loop of a surface-coil-type resonator permit it to be used as an endoscope for in vivo spectroscopy (Ono et al., 1994; Hirata and Ono, 1997; Lin et al., 1997). A surface coil resonator for localized EPR was constructed by cutting a toroidal resonator along a chord to obtain a flat surface (Sotgiu et al.,
I987a).
Analysis (Hirataet al, 1995) and impedance matching (Hirata and Ono, 1997) of surface-coil-type EPR resonators have been described in detail, and their application to studying organs of nitroxide-treated rats was reported (Tada etal., 2000). At 700 MHz, loop diameters of 6 - 18 mm (Hirata et al., 1995) were tested, and the signal strength was largest for a diameter ofca. 8 mm. Impedance matching with varicap diodes provided a tuning range of 50 MHz (Hirata and Ono, 1997). The Q was slightly lowered relative to mechanical matching. Coils with 3, 4, and 10 mm diameter, resonating at about 720 MHz, were used to measure nitroxides in rats (Tada et al., 2000).
The B, field yielded measurable EPR signal up to about 2 mm from the surface of the coil. An exposed rat kidney was inserted into a 10 mm diameter surface coil to image nitroxyl radicals at 700 MHz (Veda et al., 2002). The loaded Q of the coil was about 70 under these conditions.
Some EPR systems also use a birdcage resonator (Bolasetal., 1996), as is common in MRI systems. The LOR yields -..13 larger B, for the same incident power as a birdcage resonator, but the axis of the LGR has to be arranged perpendicular to the axis of the birdcage resonator (Bolas et al., 1996). A birdcage is usually capacitively coupled to the transmission line, but alternatively can be coupled with an inductive loop at some loss of B, homogeneity (Bolas et al., 1996).
A 1.1 GHz "minimal cavity" (a ring resonator 30 mm diameter by 4.5 mm long), without a shield for ease of animal handling, was used to select a limited volume. The absence of a shield also was stated to avoid pickup of low-frequency modulation (Colacicchiet al., 1996).
Resonant inductive coupling was proposed as an improved way of coupling resonators containing lossy electrically conducting samples, and
2. EPR SPECTROMETERSAT FREQUENCIES BELOWX-BAND 89 demonstrated at 200 MHz. (Diodato et al., 1998). In this scheme the inductive coupling loop is itself a resonator with tuning and matching capacitors, and the high-frequency combination of the two resonators was used. This was further elaborated in a resonator in which the resonant coupling loop is between two sections of a LOR (Diodatoet al., 1999).
References to these and other resonator designs are summarized in Table 6.
Table6.Resonator Type of resonator
1-400 MHz
10-120 MHz coil of the Klein and Phelps, 1967 design
27.7 MHz, 680 MHz 29-331 MHz
51 MHz 60 MHz 85 MHz 100 MHz coil
160,288 MHz, 1.12 GHz
200 MHz ca. 200 MHz 200-400 MHz
210 MHz 220 MHz LGR for transmit pulse and 2 pair ofsaddle coils for receiver
225 MHz
Comments
lumped parameter delay line, untuned1tnetwork of coil and 2 capacitors
canbe tunedca. 20 MHz; obtained spectrum of 0.4mMnitroxyl in water at 100 MHz
loop-gap and transmission line resonator, 48.5 mm i.d., Q=222 at 678.6 MHz, Q=96 at 27.66 MHz PEDRI: LGR, birdcage, or Alderman-Grant coil outside an NMRcoil
8-leg, high-pass birdcage, 20 cm diameter, 20 em long, Q=173 coil wound of flat Cu strip helical, transmission and reflection 3 em diameter single-turn Helmholtz pair
PEDRI, split-solenoid NMR coil 85 mm diameter, 67 mm long, and EPR coil of 20 loops connected in parallel 10 mm diameter, 20 mm long
single-turn solenoid, 12 mL volume several birdcage resonators LGR, multiply-tuned; can be set to irradiate simultaneously multiple transitions in a nitroxyl for PEDRI several resonator types compared FT EPR; 59 mm i.d . by 20 mm long l-Ioop 4-gap LGR, A=19J.lT/-JW
tunable LC circuit, 6 turns of#16 wire,0/. inch diameter and I inch long
Reference
Gebhardt and Donnann, 1989
Hatch and Kreilick, 1972 Satoet al., 1997
Lurieet al., 1990,
1991a,b,c, 1992 ; Nicholson et al., 1994a; MUisch et al.,
1999
Lurieet al., 1998 Lloyd and Pake, 1954 Collingwood and White, 1967
Hutchinson and Mallard, 1971
Lurieet al.,1988, 1989
Hornaket al., 1991 Bolaset al., 1996 Alecciet al., 1996
Decorps and Fric, 1969 Alecciet al., 1998a,b
Strandberget al., 1956
Type of resonator Comments Reference
226 MHz PEDRl with 625 kHz NMR Krishnaet al., 2002 transmit saddle coil and diameter
solenoidal receive coil, 5.2 JlT/..JW.
237 MHz LGR, 37 mm diameter, 37 mm long McCallumet al., 1996b 238-250 MHz double split ring, 5.5 em diameter, Smith and Stevens, 1994
10 em long, Q=200 with 100 mL H20 in 42 mm o.d. tube
250 MHz strip-line, 2.58 cm diameter, 3.2 cm Halpernet al., 1989 long; Q =550
250 MHz strip-line, 1.6 cm diameter, 1.5 em Halpernet al., 1995 long, 0.27 GB.for 15 mW
250 MHz Crossed loop resonator Rinardet al., 2002a 267 MHz LGR, 2 em diameter, 4.5 em long, 4 Ardenkjaer-Larsenet al.,
cm diameter shield, Q=380 1998
268 MHz low-temperature NMR coil probe Kimet al., 1996 with 4 capacitors for tuning and
matching was proven with EPR of DPPH
280 MHz ')J4 coaxial line, 2.6 em diameter, Hill and Wyard, 1967 reflection cavity, Q=600
280 MHz orthogonal coils, 5.2 mm o.d. Dijretet al., 1994 sample tube, solenoidal excitation
coil and 2 em diameter Helmholtz detection coil decoupled
geometrically
280 MHz LGR, 4.9 cm i.d., 10 em long, 1- Quaresimaet al., 1992 loop, 2-gap, Q=400 empty, 135 Alecciet al., 1992b with rat
280 MHz Cu tape on PVC form, 70 mm McCallumet al., 1996a diameter, 120 mm long, Q=1300
empty, 25 with rat
280 MHz LGR for pulsed DNP, Cu tape on Alecci and Lurie, 1999 Perspex, 38 mm diameter, Q=173
280 MHzLGR 45 mm diameter Cu loop, 10 mm Yokoyamaet al., 1999 long, Q=570, Q was decreased to
70 with water in polyethylene tube inside LGR
200-400 MHz short-circuited, coaxial line Rubinsonet al., 1998 resonator
300 MHz LODESR ; birdcage, 3.8 em Nicholsonet al., 1994b, diameter, 3 cm long Q=70 ; NMR 1996
coil inside the birdcage
300 MHz 4-turn solenoid, 8 mm diameter Bourget al., 1993 300 MHz parallel coil resonator, 25 mm Devasahayamet al., 2000 ;
diameter, 25 mm long, Q=20-25 for Afeworkiet al., 2000 pulsed EPR
300 MHz ')J2 coaxial transmission cavity , Feher and Kip, 1955 Q=1000-2000
300 MHz half-wave coaxial line, transmission Cook and Stoodley, 1963
2. EPR SPECTROMETERS AT FREQUENCIES BELOWX-BAND 91
Type of resonator Comments Reference
mode, Q=450
300 MHz l-Ioop, l-gap LGR, frequency Brivatiet al., 1991; Stevens tuned by moving dielectric slab in and Brivati, 1994
gap
300, 700, 900 MHz LODESR, bridged LGRs with 4, 2, Yokoyamaet al., 1997a,b or 1 gap, with saddle-type pickup
coils inside the LGR
saddle-type pickup analysis of sensitivity Yokoyamaet al., 1997a,b,
coils 1998
302 MHz coaxial resonator with retarding Medvedevet al., 1976 helical system, Q=300 at room
temperature
310 MHz 1J4 coaxial cavity, matched with Duncan and Schneider, double-stub tuner; Q=1000 at RT, 1965
5000 at liq. He
300,600 MHz coaxial cavity, up to 10mLof Algeret al., 1959 sample
530 to 950 MHz LGR frequency changed in 20 MHz Treiguts and Cugunov, steps by changing capacitance 1995
549,680 MHz reentrant and split-ring Alecciet al., 1989 600-1200 MHz 1J4 coaxial cavity with helical Abdrachmanov and
internal conductor, also used TEJOn, Ivanova, 1973 n=2-4, rectangular cavities
700 MHz flexible surface coil, 6 mm Hirataet al., 1995 diameter,Q;::100
700 MHz flexible surface-coil-type resonator, Hirata and Ono, 1997 uses triaxial cable, 3.8 mm diameter
loop, Q=82
700 MHz flexible surface-coil-type resonator, Linet al., 1997 5 mm diameter
700 MHz active resonator Satoet al., 2002
707 MHz surface-coil-type, 7.5 mm diameter, Onoet al., 1994 Q=391
750 MHz rectangular reentrant resonator, Chzhanet al., 1999 transversely oriented electric field,
Q=550
760-820 MHz bridged LGR, 43 mm diameter , 30 Ishida,et al., 1989 mmlong,Q=1200
883 MHz short-circuited, coaxial line Rubinsonet al., 1995 resonator, Q=69
883 MHz, reentrant LGR: 2-loop, l-gap, 36 Sotgiu and Gaultieri, 1985 mm diameter, for mice, Q=525
890 MHz BLGR, 4.4 em diameter, 1 em long, Satoet al., 2000 0.5 mm Teflon spacers between
loop and bridged shields, Q=510
900 MHz induction coil in coaxial cavity Duncan, 1967 1.0 GHz BLGR with coaxial solenoid for Giuseppeet al., 2001;
simultaneous EPR and NMR, B, Alfonsettiet al., 2001
TyPe ofresonator Comments Reference was uniform over about 25 mm
1001 MHz reentrant coaxial cavity, Q reduced Schmidt, 1972 to 250 by introducing an absorber
into a region of high electric field;
B.0::0.3 G with 1 W incident
1.06 GHz stripline Q=909 empty, 604 with tube of Brown, 1974 water
1.07 GHz minimal 30 mm diameter, 4.5 mm long, Colacicchiet al., 1996
cavity Q=90 empty, 40 with 15 mm
diameter physiological saline, 20-30 with a 20-20-30 g mouse
1.1GHz double split-ring resonator surface Bacicet al., 1989 coil, Cu on quartz tube, 8 mm
diameter, Qo::200
1.1-1.2 GHz LGR, 3.25 ern diameter, 2.5 cm Jianget al., 1995 long, two 1 mm bridged gaps
1.2 GHz dielectric resonator surface probe Jianget al., 1996 1.1-1.3GHz LGR, 2.5 cm diameter, 15 mL Lukiewicz and Lukiewicz,
sample volume 1984
1-1.8 GHz quarter-wavelength stripline Dahlberg and Dodds, 1981 conductor, frequency tunable with
capacitive element
1.2 GHz ceramic 3-loop-2-gap, 2 em diameter sample Chzhanet al., 1993 reentrant loop, 0.14 G per square root watt
1.2 GHz electronically tunable 3-loop-2-gap Chzhanet al., 1995 LGR, piezoelectric adjustment of
capacitance , 2 cm diameter sample loop
1.2GHz toroidal surface coil Sotgiuet al., 1987a
1.25 GHz surface coil BLGR built with Ag Kuppusamyet al., 1998a,b foil on quartz tubes, could sample a
region 10 mm diameter and 5 mm deep
1.25 GHz bridged LGR surface coil, 10 mm Kuppusamyet al., 1998a,b diameter,S mm deep sample
volume
1.34GHz double reentrant LGR: 3-loop, 2- Sotgiu, 1985 gap resonator, for mice, 50 mm
diameter, Q=440; 1.68 GHz, 38 mm diameter, Q=515
1.83 GHz helix, various sizes were tested Nishikawaet al., 1985;
Fujii and Berliner, 1985;
Berliner and Fujii, 1985;
Berliner and Koscielniak, 1991
1.86GHz flat loop surface coil, 4.1 em long, Nishikawaet al., 1985;
0.7 em diameter, 0.8 mm gap Fujii and Berliner ,1985;
2.EPRSPECTROMETERS AT FREQUENCIES BELOW X-BAND 93
TyPe ofresonator L-band
1-2 GHz
L-band L-band
1-2 GHz recessed LGR 2GHz
L-band, S-band
1-4 GHz reentrant cavity
2.1,3.4,6.4, and 9.4 GHz
2.5-2.9 and 2.9-3.4 GHz
3.0-6.3 GHz 3.8-4.4 GHz
S-band LGR ca.4GHz
Comments
interior Al foil shield added to LGR to restrain electric fringe field lumped LC circuit, Q=250 at I GHz and Q=150 at 2 GHz, sample in 3 parallel loops, 8.5 mm diameter and 3 mm apart; useful volume is 0.9 em"; the capacitive term is provided by stub tuners
reentrant, 4.2 mm diameter, 15 mm long sample loop
electronically tunable surface coil designed to minimize E fringe fields; 26 mm diameter, 25 mm long, Q= 1000
quarter wavelength coaxial cavity resonator with spiral internal conductor; Q=200
crossed-loop resonator, dispersion, superheterodyne, ESE at L-band and S-band
Qo::2000, continuously tunable over 1-4 GHz
re-entrant resonators, 4.9 to 8 mm diameter, Q=1650-1800
cylindrical cavity with device for rotating sample at 4.2 K LGR
rectangular TE0I 1cavity, for saturation recovery, single mode and dual-mode cavity for two-frequency SR measurements inductive coupling
cylindrical reentrant cavity made of steatite and silvered internally
Reference
Berliner and Fujii, 1985 Onoet al., 1986 Giordanoet al., 1976
Quineet al., 1996 Hirataet al., 2000 Zweier and Kuppusamy, 1988
Denisov and Kalinichenko, 1965
Rinardet al., I996a,b, 2000
Shing and Buckmaster, 1976
Momo and Sotgiu, 1984 Antipin, 1966
Hyde and Froncisz, 1981 Bowers and Mims, 1959
Hankiewicz at al., 1993;
Romanelliet al., 1994 Gerkin and Szerenyi, 1969