Biology Reference
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low sensitivity, and sample throughput has been
increased with software automation and robotic
assistance. Improved probe technology has
resulted in an increase of approximately 40% in
sensitivity measurement for the ethylbenzene
standard every three years. 17 Superior magnet
shielding technology to reduce magnetic stray
scale RF coils is applicable to mass or volume
limited samples. Bruker
'
s 1.7 mm OD Micro-
CryoProbe
NMR requires only 30 m L of sample
volume. Cryogenic probes can also be used with
a
flow cell and combined with the separation
power of liquid chromatography for liquid chro-
matography (LC)-NMR and LC-NMR-MS (mass
spectrometry). 19 Use of high-pressure liquid
chromatography (HPLC)-NMR has also been
reported. 20
Sensitivity gain at room temperature opera-
tion is also realized from miniaturization of RF
coils designed for liquid-state applications. The
CapNMR
field strength
magnet designs have reduced the required
minimum footprint necessary for high
field and more compact, higher
eld
systems. This has enabled researchers to equip
their laboratories with magnets as high as 800
MHz within the typical 12-foot laboratory ceiling
and take advantage of higher sensitivity and
spectral dispersion.
Cryogenically cooled probes offer a less
expensive solution for increasing sensitivity.
Thermal noise from electronics is dramatically
reduced by cooling the radio frequency (RF) coils
and preampli
probe from Protasis uses a 5 m Lor
10 m L
flow cell with observe volume of 2.5 m L
and 5 m L, respectively, and has similar mass
sensitivity as a cryogenic probe. 21 Recently
a dual-sample probe was introduced with two
independent
flow paths and detection
elements. 22 Gökay et al. recently reviewed the
use of micro
ers with cold helium gas
(
10 e 25 K). This step can offer a three- to four-
fold increase in sensitivity compared to conven-
tional room temperature probes for low
conductivity organic solvents. This increase
translates to a factor of 16 in experimental time
reduction for the same signal-to-noise ratio.
Bruker
ow probe NMR and its use in
conjunction with high-pressure liquid chroma-
tography, capillary electrophoresis, and gas chro-
matography. 23 According to the authors, the
mass sensitivity of capillary NMR is up to
100-fold greater compared to conventional
5 mm NMR probes.
w
s nitrogen-cooled cryogenic probe,
offering a factor of 2 to 3 sensitivity gain, is
a less expensive alternative. The gain in sensi-
tivity is reduced for aqueous samples, which
have higher conductivity, and is further reduced
with increasing salt content. To minimize signal
degradation from salt content, a salt-tolerant
cryogenic probe has been designed to align
a rectangular or oval NMR tube inside the probe
to minimize interference from the electric
'
SAMPLE PREPARATION FOR NMR
ANALYSIS
Sample collection, storage, and preparation
are crucial
c work-
d whether genomics, proteomics, or metabolo-
mics d and, if done incorrectly, can lead to
results that are biased or later invalidated. A
few researchers have employed NMR analysis
to look for the effects of sample storage. Laurid-
sen et al. observed that sodium azide is not
needed as a preservative as long as the urine
samples are stored below e 25 C. 24 Beckonert
et al. compared the NMR spectra of fresh raw
urine and the same urine stored at e 40 Cfor
stages
for any scienti
eld
and has resulted in a 20% to 35% gain in sensi-
tivity. 17,18 However, these specialty shaped
tubes are expensive and are not suited for
projects with a high number of samples. An
alternative is to use dual 3 mm outer diameter
(OD) tubes or smaller diameter tubes such as
a single 3 or 4 mm OD tube. The combination
of cryogenic probe technology with micro-
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