Recently I've asked your advice on purchasing a multinuclear probe.
The responses were informative and we're sure to ponder over the
details for some time to come. Some of you asked for a summary of
responses; you'll find them below. Since some of you requested
anonymity, only the message bodies are included.
Thank you very much for the time you took to answer us.
Jerzy.
________________________________________________
Jerzy Czaplicki, Ph.D.
Paul Sabatier University
Institute of Pharmacology and Structural Biology
CNRS
205, route de Narbonne
31077 Toulouse
France
Phone: (33) 5 61 17 54 04
Fax: (33) 5 61 17 59 94
Email: cgeorge@ipbs.fr
__________________________________________________________________________
__________________________________________________________________________
You mentioned Doty in your post- have you looked into Nalorac? They make
excellent solution probes, many people think they are better than either
Bruker or Varian.
For our recent 500 purchase, we bought:
a broadband inverse with xyz gradients
a triple H-P-broadband (invrese) with only Z gradient
a 2.5 mm broadband inverse with xyz gradients
I did not consider the gradient effect on sensitivity to be a concern. I
did consider the VT range- that is why one probe has only Z gradient- so
I can have the full VT range if necessary.
Our 500 was only installed last fall, and we are already using all three
gradients to do magic-angle water supression.
My proton 90's (for the 5 mm probes) are 8-9 us.
I bought a Bruker spectrometer and Bruker probes, but thought very hard
about buying Nalorac probes. If I had gone with Varian, I would have
bought Nalorac probes.
Here in the states, is is usually easier to get money at purchase time
than to upgrade later. That is one of the reasons for purchasing the xyz
gradients- in case I needed them later. As I mentioned, we are already
using them.
__________________________________________________________________________
You are going through questions I had myself a while ago; haven't answered
all of them clearly yet, but can share what I've learned so far:
>- Do the XYZ gradients really offer advantages related proportionally to
> the increase in price as compared with the Z gradients? I saw a similar
> question posted recently in this group, but haven't seen any follow-ups.
If dq-cosy on samples in water is important to you, then xyz is worth it.
Otherwise, likely not worth the money. I had heard recently that Bruker now
only offers triple axis gradients? Maybe this isn't true?
>- Specs (pulse widths, sensitivity) are more attractive for fixed frequency
> channels rather than broadband probes. However, even in the optimal cases
> pulse widths are roughly 1.5 - 2 times longer than what we're used to
> (the quote says 12 us on 1H). Does it make sense then to limit oneself to
> a set of fixed channels, if an adjustable probe is available with similar
> characteristics? What do you think of relative merits of, say, HCN probes
> versus HXY ones?
I have not heard of any usable HXY. We do, however have new HCX and HPX
probes (one from Varian, the other from Nalorac). I do think these are very
viable probes, having close to same specs in S/N and pulsewidths. The X
channel, interestingly, on both probes have good pw90 for 13C on the X channel
(better there than the fixed C on the HCX probe!!). S/N suffers about 10%
compared to fixed 1H probes. We do a lot of multinuclear work, though, and
this gives us a lot of flexibility that could not be achieved via fixed freq
probes.
>- Does anyone have any experience with Doty probes? Are they better than
> Bruker's? Better enough to forego a possible discount due to depositing
> a purchase order with the same manufacturer?
Don't know about Doty's liquids probes; would like to hear comments you get
back here. I strongly suggest you look at Nalorac, however. They make great
probes; the best, in my opinion, for certain applications, and competitive in
all areas. We like Brukers probes for multinuclear work.
__________________________________________________________________________
A small bit of advice - I highly recommend Nalorac probes as an
alternative to the Bruker probes --- if you decide to buy a non-Bruker
probe. I am unaffiliated with Nalorac, but I have been highly impressed
with their probes - even though all of our probes are in fact Bruker.
I have used Doty probes in the past and have been concerned with
durability, quality of finish and turn-around time for repairs.
I have found Bruker and Nalorac probes to be of the highest quality.
__________________________________________________________________________
There isn't a perfect probe that can do everything, but rather
you have to look for the best probe for your needs. Try to
decide what the probe is going to be used for. We got
one tuneable leg because we are in a chemistry department and
try to make evgeryone happy. We haven't been limited by single
axis, but I've heard that water suppression is slightly better with triple.
You could try to land a Nalorac probe. They are good, but Bruker
will make that hard for you by discounting their probes with the
console.
__________________________________________________________________________
3D gradients have some advantages over 1D. 3D gradients allow you to select
the direction of the gradient, permitting improved quantum selection using
magic angle gradients. It also has an advantage for diffusion measurement
because convection effects are somewhat weaker in the x-y plane. The main
advantage for 3D gradients is for 3D and 4D NMR where independent quantum
coherence selections are better achieved with orthogonal gradient selection.
My experience with modern broad band versus selective probes is that the
sensitivity and pulse widths are almost identical. If I were in your position,
I would buy a broad band probe.
__________________________________________________________________________
I don't know where you get your specs, but 1.5 year-old Bruker probes have
1H 90's on the order or 7us or less for organic samples or non-salty aqueous
samples (for 500 & 600 MHz); S/N is also higher now; can get 600:1 easily
on ethylbenze (.1% thinwall) at 500MHz, and hopefully it's pushing 700:1 by
now, but one thing I have noticed is that the new proton sensitivity comes
at the expense of the larger outer (13C,15N) coils which sometimes need
300Watts (some even closer to 400Watts) to get the specs they quote:
i.e, 12us on 13C, 35~40us on 15N. Often the AMX's BSV10 heteronuclear
amplifiers are inadequate to put out that type of power.
You want to stay away from broadband tuning probes (especially Bruker),
because you're paying for the flexibility with longer pulsewidths & RF
efficiency at every frequency you really need.
Does it make sense to limit oneself to a set of fixed channels you say?
If you're doing protein NMR, of course; if you need to use it for organic/
inorganic chemists who want to do 113Cd, 65Cu, 29Si occasionally, you
don't have a choice but to get the broadband. For something in between,
Bruker also offers the quad nucleus probes instead of the continuously
variable probes (where you can decouple one fixed frequency and tune
the observe coil to any one or two fixed frequencies at a time with a
capacitor wand); this might be something you want to look into if you're
considering "HXY". You lose very little in performance this way.
I would stay away from Doty probes for solution NMR on biomolecules
until you see in literature completion of work done along the lines of what
you want to do using nothing but Doty probes. For solid state NMR, Doty
is unquestionably one of the top options, but not for liquids.
If you can afford gradients of any kind, they're almost a must for modern
NMR. Your results are so much better with them, you really can't afford
to do without them.
__________________________________________________________________________
We recently purchased a 750 and 600 with XYZ gradients. In our package,
the price differential between Z and XYZ probes weren't a factor. So, we
went with the triple gradients feeling that more experiments would be
developed for triple gradient systems, we could do microimaging with the
triple gradients, and these was no difference in other performance (90's ,
S/N, etc) between the triple and Z gradient probes. This comparison is
between two HCN probes, one Z, one XYZ. We've never had our hands on a HXY
probe.
>- Specs (pulse widths, sensitivity) are more attractive for fixed frequency
> channels rather than broadband probes. However, even in the optimal cases
> pulse widths are roughly 1.5 - 2 times longer than what we're used to
> (the quote says 12 us on 1H). Does it make sense then to limit oneself to
> a set of fixed channels, if an adjustable probe is available with similar
> characteristics? What do you think of relative merits of, say, HCN probes
> versus HXY ones?
>
I think choice of HCN vs HXY depends upon your major usage of the probe,
and whether you can afford two. This may seem obvious, but normally we try
to buy backup probes, and for our HCN probes at 500 MHz we purchased an HPX
probe as the backup. The major use for the HCN is in standard HMQC and
HMBC H,C experiments, with a small sprinkling of H,N experiments. The HPX
has allowed us to do some experiments that we couldn't do prior to owning
this probe, such as 13C{H,31P}.
If your normal mode of operation is solving the structure of labeled
biomolecules, then the HCN probably offers slightly better 90's and
x-nucleus decoupling. If you do a lot of different types of experiments,
then the HXY is your better choice.
As to QUAD vs HCN probes, we have both types here. The QUAD offers much
better sensitivity for C and P, and is the probe of choice for direct
detect 13C spectra. The HCN probes are high-sensitivity for H, so we use
them for the inverse experiments. Again, if you can afford both, go for
it. If not, you have to decide what type of data you generally collect.
__________________________________________________________________________