Download Getting Started - Center for Structural Biology

6.9 Adjusting Shims
Table 19. Hexadecimal Codes for Shim Groups
Z1C
Z1
Z0
—
XY
XZ
Y1
X1
—
—
—
—
—
—
—
Z5
Z4
Z3
Z2C
Z2
ZXY
XZ2
ZX2Y2
YZ2
Y3
X3
YZ
X2Y2
8 4 2 1 8 4 2 1 8 4 2 1 8 4 2 1 8 4 2 1 8 4 2 1 8 4 2 1
• q recalls an algorithm’s internal parameters so that shimming starts quickly. q is a
background autoshim that keeps the magnetic field at an optimum during experiments
of long duration. Shimming is performed at the time wshim instructs. Only the portion
of the shim methods following the letter q is executed after the experiment’s first
increment. Any shim method may follow q; however, the sz1 (Z1 only) and szq (Z1,
Z2) are the most effective. Multiple shim methods may follow q, but time effectiveness
is reduced.
Methods may be entered into the shimmethods file using a text editor such as vi. The
macros newshm and stdshm provide an interactive method of defining shim methods.
Note that unlike normal text files, which have unrestricted size, the maximum text file size
for a shim method is 128 characters.
The following examples show complete user-defined shim methods:
• szq:cmm,rn,stz:cmm,ry,szq:cmm; means shim Z1C, Z2C with medium to
medium criterion, turn off spinner, shim X, Y, XZ, YZ, Z1 with medium to medium
criterion, turn on spinner, and then shim Z1C, Z2C again with medium to medium
criterion throughout.
• d50,szq:cmm,d150,sza:cmm; means to sample every 0.5 seconds while
shimming Z1C, Z2C, and then to sample every 1.5 seconds while shimming all Z
gradients. Use medium to medium criterion throughout.
• t60,szq:cmm,t240,sza:cmm; means shim Z1C, Z2C for 60 seconds
maximum, then shim all Z gradients for a maximum of 4 minutes. Use medium to
medium criterion throughout.
• f,ry,t600,szq:cmm; means turn on spinner and FID shim Z1C, Z2C with
medium to medium criterion for 10 minutes maximum (not available on GEMINI
2000).
• t60,sza:cmm,q,t30,sz1:cmm; (with wshim='f20') means initially shim
on all Z gradients for 60 seconds, then shim Z1. After every 20 FIDs, shim Z1 for
30 seconds.
• sza:cmm,q,t30,szq:cmm;(with wshim='f10') means initially shim on all Z
gradients (with no time out) and then perform a Z1, Z2 shim for 60 seconds every
10 FIDs.
Interactive Autoshim
To enter the interactive Autoshim mode, from the Acquisition window, do the following:
1.
Open the SHIM display
2.
Use the mouse to make a series of choices for computer shimming, perhaps the Z1
and Z2 gradients, and select a criterion, such as L > M.
3.
Click on the auto button in the SHIM display to initiate Autoshim.
01-999083-00 A1298
Getting Started: VNMR 6.1B
143