无线电台会影响ＮＭＲ

syflu

 When it comes to polluting the environment, humanity has no equals. This applies to all kinds of spaces, including the electromagnetic ether. What follows is a contribution and a warning from Vanni Piccinotti, an engineer based in Florence, Italy, who has decades of experience & [; S% b0 n8 y6 J# Owith installing, testing, and servicing NMR and EPR spectrometers. The # T. M+ L& b) [6 N% O3 Y; Vproblem of RF interference has never been trivial and it keeps slowly but inexorably mounting. The spread of urban areas (combined with the tendency to install MR instruments in those same areas) and the mounting maze of wireless technologies penetrating our daily life spell 6 l! _' ^7 G" N+ u( t) y$ Cnothing good for the future quality of our NMR signals. With concurrent ( m. D# O: m6 G& Q Cprogress in basic sensitivity of the instruments, time might come when - @ i3 w- V/ Q9 z+ k$ @1 f0 C% ?environmental RF noise will be the single major obstacle to further progress. But let us hear what Vanni has to say:

syflu

 

+ v$ w0 ?7 o' U* Q7 z( m" tCross-talk between FM Broadcast Radio Transmitters (88-108 MHz) ; f2 f; F2 ?0 H7 i+ A# Zand NMR Spectroscopy: A recent experience ' P& x- c8 u$ I4 y

Recently I had to install a 400 MHz (9.4 T) NMR Spectrometer. The 7 O5 `8 d* u% F- T/ N) j0 qsystem worked fine and, using an Indirect Detection Probe , met quickly and effortlessly the specifications. But later on, when the customer installed a 13C direct detection Probe, the S/N ratio turned out to be . q9 M. H; t9 J" d/ b/ p5 M- m ~1 lquite low and, on top of it, the sensitivity was subject to erratic and ! v0 |: F8 A& m! H* G4 ?2 z" G8 Wvery large variations from 50:1 to 130:1 (manufacturer's specs give 155:1), without any apparent reason.

Since the nominal 13C observe frequency at 9.4 T is 100,568 MHz, right in the middle of the range of commercial FM broadcasts, I 0 ^% P' r& e$ Y. F; xhave immediately suspected that the spectrometer was picking up one of 7 Y# o" X1 l! e% r8 Ithose radio stations. In fact, using a cheap FM radio receiver, a 9 \( n; W/ ]) Y* | l n. Astrong station was quickly found at 100,60 MHz. At this point, I have connected a simple audio amplifier ending with a loudspeaker to the 4 V& @ V2 H ]2 e3 \0 {output BNC of the observe receiver which was there apparently just for this purpose, and all of us were listening to the radio using a 200.000 $ ?; Q1 E5 y ]( E) iEuro NMR spectrometer, except that the audio quality was really poor, + H) a, y* g$ M7 j0 gmuch worse than from the above-mentioned gadget radio (a shopping mall gift).

The problem is well known from the old times, when the highest : `4 ]$ t7 _; ufield was 2.45 T and the nominal H1 frequency was close to 100 MHz. One + W/ H- Q- }$ S1 Dof the first Italian NMR spectroscopists to experience it, back in 0 @+ D% `/ @2 _) ]1974, was Prof. L.Lunazzi at University of Bologna, on his brand new M2 e1 ~, ]* j8 @, _Varian XL100 spectrometer, and the radio station was Radio San Luchino, : r1 p( H1 L8 F o5 R6 r; h8 o7 Iwell known to anybody living in Bologna, which broadcasts from the top of the nearby Saint Luca hill. 3 ]/ ~) R6 J' P% Q% g& {: l

The obvious solution is to change the magnetic field, and thus : X; W% ~3 u6 |5 Rall resonance frequencies, in order to get out of the modulation / `: m/ M" K! c) F2 ^5 k( s; Genvelope of the interfering transmitter. But this is not always easy, since the range by which one can move the magnetic field changing just ' G' b4 N0 ?2 \& r, d$ rsome software parameters is usually limited to a few tens of kHz in the / w. `: C( z9 y8 u- r) ~frequency domain. If larger variations are required the poor engineer O- ` l* n% a h9 shas to work on the superconducting coils of the magnet, which is a 6 Z n( k, N# n+ ^% {no-trivial job entailing the risk of a total or partial quench. 0 C, U6 W* `9 G% C( u A

Bitter experience shows that persuading the involved radio station to change its operating frequency is a time consuming, frustrating, and apparently quite impossible task.

Being well aware of the problem, my preliminary spectrometer checks always include some blank acquisitions taken before running up the magnet so that there is no chance to observe an NMR signal. The resulting dataset should be pure white noise, without significant spikes. This was done also in this particular installation but, as 3 N& h, u5 w- Y+ @usual, in the days following the energization the magnet drifted a bit, getting closer to the radio station carrier. Furthermore, the usual 13C 2 p1 h$ ]$ h) m) ~spectral widths are quite wide which makes things even worse. Murphy's , k. v' o& S5 s; sLaw has no exceptions! 3 ?) m! Z& J6 B; x7 X! L G3 K

But we are just at the beginning of my real troubles. Before 2 P4 R0 ^2 d; i" j! y1 O# hputting one's hands on the magnet, one should better know how much, in which direction, should the field be moved. I have therefore used a good Spectrum Analyzer (Tektronix model 2710) to check the frequency spectrum around 100 MHz, ready for the worst. And the worst was what I , @0 h9 w0 e' qgot! The band was filled with FM signals, evenly spaced by 250 kHz and + I& g4 Q& _* K D' Iwith modulation envelopes as wide as 100 kHz, so that when I got far : a! S7 D1 U. A% kfrom one station I started receiving the next one; accounting for folding and aliasing effects, there was no chance! The only somewhat free region was at 100,120 MHz, but this implied proton frequency of 4 [/ G, k% L0 t6 Z$ j398.100 MHz. So now the spectrometer is no longer a "400"!

Before installing a spectrometer, you better get a Spectrum Analyzer and check for the presence of RF fields in the instrument + t" W r3 K9 W. d0 eroom, taking care to explore the areas close to the observe frequencies of all the most important nuclei. Don't forget the lock: at 14 T ' Y4 q8 p! L$ L: G& g, c6 f(nominal 1H frequency of 600 MHz) 2H resonates at 92,095 MHz, once again in the FM broadcast band. The lock channel receiver has quite 8 D. B. A. ^8 R- _2 C z2 f* Qnarrow bandpass filters, so hitting a radio is a really bad luck, but it had already happened, resulting in fast lock level variations and totally malfunctioning Gradient Shimming which uses deuterium as observe nucleus!

Needles to say, the extremely high sensitivity of an NMR B4 Y' B' }/ s% n# C1 aSpectrometer shows up. The signal from the guilty radio, as observed on the spectrum analyzer inside the spectrometer room, had very low intensity level of about -70 dBm, some microvolt/meter, but that was / k; v1 P0 x N3 \; }4 x: henough to almost completely hide the quite strong 13C signal from the ASTM sample!

The radio was clearly picked up by the Probe (closing the Preamplifier input with a shielded 50 ohm RF load, all signals 6 }+ F7 _9 g; mdisappear) but, quite surprisingly, there is almost no shielding effect 7 k7 K' Z( ]3 iattributable to the metal body of the magnet, which is after all an 6 l k: p7 o! t" nalmost completely closed cylinder all around the Probe. Most probably a 0 T7 k+ T& \& Y: K5 \good deal of the signal leaks in through the Shim Coils which are mounted very close to the Probe and, together with their connection % {. P7 J" C R/ ocables to the Console, constitute a quite good antenna. * B! b7 W8 @4 k0 d/ f

Too bad the Shim Coils are essential, and effective shielding 9 S' `, O8 {1 |/ q% G4 bof the instrument with a Faraday's cage is always difficult and : m( s8 O/ s) I9 Lexpensive (it may be almost impossible once the spectrometer is 9 o, Z% H. }- f' B$ Einstalled).

Before concluding, let me venture some additional advice based on my experience:

8 v& q% g$ p' @2 e8 f" i= Install the spectrometer in the best shielded room 8 P W$ ~. t$ S/ z# ayou can get; the best choice is once again in the basement, where you have the whole building above the ceiling and its [grounded] + v. J9 v1 R% U& y* `1 q- Xfoundations all around the rest, done in iron-reinforced concrete, amounting to a good Faraday's cage at no extra cost. ) L4 H# U* J" z& }, g

= If possible, avoid top floors. If you can't avoid going upstairs, take a good look out of the window: if you see nearby transmission antennas, get ready for troubles proportional to their dimensions and closeness (to my knowledge, however, mobile telephony antennas cause so far no harm). , S/ ^. R8 T: c

% T4 w6 V% h6 Y& t M# n= I'm sure that an exchange of experiences and/or + X7 Z, o8 \% e6 d* Asuggestions regarding this matter would help a lot to solve many W5 B% o, i# ]* v- X0 Jexisting installation problems and prevent ones yet to come. Stan's Blog is an ideal location and, needless to say, I will be absolutely glad to cooperate. : @& C4 d9 z( M8 Z3 y% ^( B

Vanni Piccinotti, Firenze, 11 April 2008

摘自stan' NMR Blog.

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