CA1193657A

CA1193657A - Double-tuned single coil probe for nuclear magnetic resonance spectrometer

Info

Publication number: CA1193657A
Application number: CA000409902A
Authority: CA
Inventors: Robert A. Mckay
Original assignee: Monsanto Co
Current assignee: Monsanto Co
Priority date: 1981-08-24
Filing date: 1982-08-23
Publication date: 1985-09-17
Also published as: US4446431A; EP0073614A3; DE3268959D1; EP0073614B1; JPS5835451A; EP0073614A2

Abstract

DOUBLE-TUNED SINGLE COIL PROBE FOR
NUCLEAR MAGNETIC RESONANCE SPECTROMETER

ABSTRACT OF THE DISCLOSURE
A double-tuned single coil probe for a nuclear magnetic resonance spectrometer having improved sensitivity is described comprising a double-tuned circuit means in which the low frequency irradiation is fed to a transmission line through an inductor means. The double-tuned circuit means of the invention may be remotely disposed from the magnetic field which results in greater sensitivity.

Description

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_ 1 _ DOUBLE-TUNED SINGLE PROBE FOR
NUCLEAR MAGNETIC RESONA-NCE SPECTROMETER
Th;s invent;on relates to nuclear magnet;c resonance (NMR) spectrometers and more particularly to NMR spectrometers su;table for double-resonance exper;ments.
9ACKGROUND_ F THE IN~VE ION
For some NMR analyses, a sample ;s ;rrad;a~ed w;th radio frequency fields of two different frequencies at relatively high power levels, for example, 300-500 watts. It ;s ;mportant that good coupling to the sample be ach;eved at both ~requencies. The efficiency of pr;or art probes is lim;ted because of the s;ze and magnet;c restrict;ons for probe components. The present invention ;nvolves a double tuned circuit which may be remotely disposed from the magnetic field wh;ch greatly reduces probe des;gn problems because the s;ze and magnetic restrictions for probe components are eliminated which results ;n enhanced sens;t;v;ty and a more eff;c;ent system.
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SUMMARY OF THE INVENTION
In accordance w;th th;s invent;on, an ;mproved double-tuned s;ngle co;l probe for a nuclear magnetic resonance ~NMR~ spectrometer is provided i-n wh;ch all the tun;ng elements can be s;tuated remote from the sample co;l outs;de of the magnet;c f;eld. Th;s perm;ts h;gh power levels w;thout break down of components which is especially advantageous for sol;ds experiments. The ;mproved probe has excellent effic;ency for both ;rrad;a~;on of the sample and detect;on of the ;nduced nuclear resonance s;gnals.
The probe compr;ses a sample r,o;l connected to a double-tuned circu;t means compr;s;ng a h;gh frequency irrad;at;on means and a low frequency ;rrad;at;on means. The two frequenc;es have a rat;o of about at least three to one or 00re and are in the radio frequency (RF) range. The high frequency irrad;at;on means ;s connected to the sample co;l through a trans-m;ss;on l;ne means compr;s;ng a transm;ss;on l;ne of a length of about one half of the h;gh ~requency wavelength. A preferred transm;ss;on l;ne ;s a coax;al cable transm;ss;on line, more preferably a low-loss coaxial cable transm;ss;on l;ne. ~he low frequency ;rrad;at;on means ;s connected to the sample coil through the aforesa;d transm;ssion l;ne means through an ;nductor means connected to the transm;ss;on l;ne at the po;nt along the transm;ss;on l;ne ~here the magn;tude of the ;mpedance for the h;gh frequency ;rrad;at;on ;s at or about m;n;mum.

3~6~i'7 It ;s understood that the probe ;s capable of suffic;ently radiating a sample with both high and low frequency to excite the NMR of the sample nuclei at both frequenc;es and transmitt;ng the signals generated by the sample NMR to a h;gh frequency detect;on means and a low frequency detection means.
An ;mportant feature of the ;nvention ;s an inductor means d;sposed between the h;gh frequency means compris;ng a convent;onal coupl;ng c;rcu;t, and the low frequency means compr;s;ng a conventional coupling circuit. An example of a suitable coupling circuit is a circu;t containing fixed and variable capacitors by which tun;ng can be ach;eved. The ;nductor means, for example, an ;nductor co;l, prov;des an ;nduct;ve ;mpedance to be matched to the ;mpedance of the low frequency means to the ;mpedance of the transm;ss;on l;ne means~ Thus, the low frequency ;rrad;at;on means compr;ses an RF generator connected to the ;nductor means through a capac;t;ve tuning and match;ng c;rcu;t. The h;gh frequency ;rrad;at;on means compr;ses an RF generator connected to the trans-m;ss;on l;ne through a capac;tive tun;ng and match;ng c;rcu;t wh;ch c;rcu;t also conta;ns a low frequency trap.
These and other features of the ;nvent;on w;ll become more apparent by referr;ng to the draw;ngs and descr;pt;ons thereof.

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Fig. 1 is a schematic view ;n block d;agram form of the major components of an NMR spectrometer conta;n;ng the improved probe of the present ;nvent;on.
F;g. 2 ;s a detailed circuit d;agram illustrat;ng an embod;ment of the improved probe of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Fig. 1, there ;s shown a double-tuned NMR spectrometer 1 of the ;nvention~ Spectrometer 1 ;ncludes a sample coil 2 for exciting and detecting the NMR of the sample under analys;s. Signals are transmitted to and from sample coil 2 through trans-mission line segments 3 and 4. Input excitation low frequency (1~) RF ;rradiation is fed to transm;ss;on `I;ne segment 3 through a Gircu;t comprising tuner 11 and f;lter l2. The rema;nder of the lf input RF c;rcu;t compr;ses sw;tch 13 connected to watt meter 14 connected to RF ampl;f;er 15 which amplifier 15 connects through attenuator 16 to low frequency RF generator 17. The portion of the circu;t from sample coil 2 through sw;tch 13 also serves as part of the output low frequency RF detection system/ From sw;tch 13 the low frequency RF NMR output passes through amplifier 18 to RF
quadrature detectors 30.
Input excitation h;gh frequency (hf) RF irradiation is fed to transm;ssion line segment 4 through a c;rcuit compr;s;ng tuner 21 and f;lter 22. Tuners 11 and 21, transm;ss;on l;ne segments 3 and 4, and sample coil 2 compr;se the probe c;rcuit. The remainder of the h-f input RF circu;t comprises switch 23 connected to watt ~ ef~7 meter 24 connected to RF ampl;fier 25 wh;ch ampl;f;er 25 connects through attenuator 26 to h;gh frequency RF generator 27. The port;on of the c;rcu;t from sample co;l 2 through switch 23 also serves as part of the output high frequency RF detection system.
From switch 23 the high frequency RF NMR output passes through amplif;er 28 to RF quadrature detectors 30.
A c;rcu;t also connects tuner 21 through RF pulse mon;tor 29 to contro1 panel and osc;lloscope system 31~
Assoc;ated w;th control panel and osc;lloscope system 31 ;s Mag;c Angle Sp;nner counter and mon;tor 32.
Computer 34 capable of Four;er Transform analys;s ;s connected to RF generators 17 and 27 and RF quadrature detectors 30.
A double-tuned s;ngle co;l probe ;llustrat;ve of an embod;ment of the ;nvent;on ;s shown ;n F;g. 2.
Sample co;l 50 receives and transmits RF signals through a transm;ssion l;ne hav;ng a length d2 f about ~/2, where ~ is the wavelength of the h;gh frequency irrad;at;on. The transm;ss;on l;ne compr;ses segment 51 o~ length dl and segment 52 of length d. Length dl is crit;cal and determ;nes the locat;on of Junct;on Po;nt 8 where the low frequency ;nput and detect;on c;rcu;t connects with the aforesa;d transmiss;on line.
Junct;on Point B must be located along the transmission line where the high frequency impedance is or about min;mum. A method for calculating the location of Junction Po;nt B and the correspond;ng length dl are descr;bed below. A low frequency c;rcu;t su;table for Carbon-13 irrad;at;on compr;ses 22.6 Megahertz ;nput 6~

pOrt 56 adapted for a character;st;c ;mpedance, for example, 50 ohms, and a conven~;onal coupl;ng c;rcu;t comprising grounded capacitor 55 and variable capac;tor 54 wh;ch coupl;ng c;rcu;t ;s connected at po;nt B through h;gh Q ;nductor co;l 53. A Q value as h;gh as poss;ble ;s desirable, but a Q value of at least 200 ;s sat;sfactory. Inductor co;l 53 has suff;c;ent ;nductance so that the impedance at point A looking toward Junct;on po;nt B ;s induct;ve. Thus, the low frequency RF generator connects ;nductor co;l 53 through a capac;t;ve tun;ng and match;ng circu;t. Although inductor co;l 53 ;s ;llustrated ;n ser;es, a su;table alternat;ve compr;ses connect;ng an inductor coil in parallel at po;nt 8 and ;mpedance match;ng by tapp;ng the ;nductor at the appropr;ate po;nt~
A high frequency circuit su;table for proton ;rradiation compr;ses a 90 Megahertz ;nput port 61 adapted for a character;st;c ;mpedance, for example, 50 ohms, and a conventional coupling circu;t compr;sing grounded capacitor 58 and variable capacitor 57. A
trapping circuit for isolat;ng low frequency ;rrad;a-tion comprising variable capacitor 59 in parallel w;th ;nductor co;l 60 ;s connected between the high frequency generator and the coupl;ng c;rcu;t.
Junction Point B where the min;mum ;mpedance occurs can be calculated from the transm;ss;on line equat;on 1.

....

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~3-51-637 z _z Zr cosh rdl_~ ZO sinh rdl (E 1) ZO cosh ~dl + Zr sinh ydl qr Where Zs is the input ;mpedance for a transmiss;on line of length dl, Zr the receiving end impedance, in this case the sample coil S0, ZO the characterist;c impedance of the transm;ssion l;ne segment 51, and =~+j~ where a ;s the attenuat;on constant in Nepers/unit length of the transm;ssion line and ~=2~/A, where ~ is the wavelength of the h;gh frequency irradiation in the transm;ssion line.
As an example, for an NMR spectrometer operat;ng at a h;gh frequency of 90 MHz, a transm;ssion line with ZO=50 ohms (RG-2Z5/U) and a sample coil with an inductance of 0.64 ~h and a Q of 200,Junction Po;nt B
can be calculated. If the sample coil inductor is assumed to be purely reactive and the transmiss;on l;ne lossless (~~0) then Eq. 1 can be s;mplified and dl/~
is equal to .27Z or dl = .272~.
The error ;nvolved by assum;ng that Zs is purely induct;ve and the transm;ssion line lossless, is that a zero null is impl;ed, whereas, actually a finite but low value ex;sts. However~ at Junction Point B
there is a m;n;mum voltage and low impedance which means ;t is the least sensitive spot to connect the low frequency c;rcuit provided the added c;rcuit's impedance is 10 or more t;mes the impedance at point B. Impedance measurements indicated that the impedance at point B is less than about 5 ohms.
Alernat;vely, Junction Point B may be located experimentally by use of an RF ;mpedance meter or volt meter.

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The probe of the ;nvent;on g;ves a high operat;ng Q resulting ;n a good s;gnal-to-noise ratio, provides un;formity of ;rradiation at both frequencies s;nce only a s;ngle coil ;s used, reduces design problems ;n the magnet gap since many of the c;rcu;t componen~s may be located outs;de the magnet;c field, and eliminates break down problems wh;ch occur ;n crossed coil systems.
Although the ;nvent;on has been ;llustrated by typical examples, ;t ;s not l;m;ted thereto. Changes and mod;ficat;ons of the examples of the ;nvent;on here;n chosen for purposes of disclosure can be made which do not cons~itute departure from the spirit and scope of the invention.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. A double-tuned single coil probe for a nuclear magnetic resonance (NMR) spectrometer which probe comprises a sample coil connected to a double-tuned circuit means comprising a high frequency irradiation means and a low frequency irradiation means in which the two frequencies have a ratio of about at least three to one or more and are in the ratio frequency (RF) range, in which the high frequency irradiation means is connected to the sample coil through a transmission line means comprising a trans-mission line of a length of about one half of the high frequency wavelength, and the low frequency irradiation means is connected to the sample coil through the aforesaid transmission line means through an inductor means connected to the transmission line at the point of about minimum impedance for the high frequency radiation.
2. The probe of Claim 1 comprising a probe which sufficiently radiates a sample with both high and low frequency radiation to excite the NMR of the sample nuclei at both frequencies and transmits the signals generated by the sample NMR to a high frequency detection means and a low frequency detection means.
3. The probe of Claim 2 comprising an inductor means which provides an inductive impedance to match the impedance of the low frequency means to the impedance of the transmission line means.
4. The probe of Claim 3 in which the low frequency irradiation means comprises an RF generator connected to the inductor means through a capacitive tuning and matching circuit.
5. The probe of Claim 4 in which the high frequency irradiation means comprises an RF generator connected to the transmission line through a capacitive tuning and matching circuit.
6. The probe of Claim 5 in which the transmission line is a coaxial cable transmission line.
7. The probe of Claim 6 in which the double-tuned circuit means is remotely disposed from the magnetic field.
8. The probe of Claim 7 in which the inductor means is an inductor coil.