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I need help with parameters on a Varian system for Pt-195 NMR. I am having trouble with signal to noise. From what I've read the sensitivity should be better than carbon but I'm not seeing that at all. Any suggestions?

asked May 03 '12 at 20:16

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I haven't done 195Pt in a while, but I remember it wasn't too bad. One thing to consider is the very big chemical shift range. It can be almost 1 MHz at 11.7T (500 MHz 1H). This can mean a lot of searching if you have no idea where your peaks are. The chemical shifts are also very temperature dependent, about 1 ppm per degree. So, good temperature control is critical if you want narrow lines. Also, if you have to go searching for peaks, remember that your probe may not have that kind of bandwidth. You may have to record the range in portions, re-tuning the probe between regions. You will also need short pulses if you want to excite a wide spectral window.

Start with a good standard sample. 1.2 M Na2PtCl6 is what is normally used. This has a IUPAC "chi" value of 21.496 784 MHz. Multiply this by the frequency in MHz of TMS /100 to get the frequency of the reference. On our 500 instrument TMS is exactly 500.13 MHz, so the Pt reference would be 107.511 866 MHz. You should see this sample in 1 scan at 10 mm, maybe a few scans at 5 mm.

In liquids, the T1 values are typically 1s or less, so you do not need a huge relaxation delay.

Let me know if you are still not having any luck seeing the signal.

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answered May 07 '12 at 08:48

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Kirk Marat
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updated May 09 '12 at 07:44

Thank you, you have confirming I am on the right track, am using a 500 MHz instrument that has a probe with more than enough bandwidth and have been able to get a good sharp peak and good signal to noise on a K2PtCl6 solution. - Chemist30# (May 11 '12 at 19:06)

I will try varying the pulse width to see if that helps with peak resolution for more difficult samples. I am using a short relaxation delay, I realized a long delay was not necessary from my initial runs. I don't have enough points to vote but I would give your answer a thumbs up if I could - Chemist30# (May 11 '12 at 19:06)

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Glad your seeing signals. I'm not sure what you mean by: "I will try varying the pulse width to see if that helps with peak resolution for more difficult samples." Varying the pulse width will not help resolution. Pulse width will effect excitation bandwidth. You will want a fairly short pulse in order to excite a fairly wide bandwidth (chemical shift range). If you use a 10 us pulse, then you actually have a null in the excitation envelope at +/- 100 kHz from the transmitter. For 195Pt, the 90 degree pulse is probably just slightly longer than for 13C. If you have a 10 us 90, then a 30 to 45 degree flip angle will give good excitation over several hundred kHz. If you have a very narrow chemical shift range (and know where it is), then you can centre the transmitter on the shift range and use a longer pulse - up to 90 deg.

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answered May 16 '12 at 07:46

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Kirk Marat
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One other consideration: In Hz, the 195Pt peaks are usually fairly broad, consistent with their fast relaxation time. To improve S/N, you may wish to apply a very steep line-braodening function, perhaps 10 or 20 Hz, to improve S/N.

I've also found that the large breadth of the 195Pt spectrum confers a large baseline roll and posed phasing problems if the transmitter is not close to the peak. I suggest two things: 1) use an initial scan to scope out approximately where the peak is, then move your transmitter near the peak for your high-quality acquisition, 2) backwards linear-predict a few points (4 or 5).

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answered May 21 '12 at 07:59

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jkurutz
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