Mixing device for the mixing of fluids in an NMR apparatus including a spring attached to the coil affixed to a mixing rod

A mixing device for the mixing of fluid measuring samples in an NMR measuring device, whereby the measuring device includes a rod (10) which is arranged coaxially to the main field magnet coil of the NMR measuring device to be movable in the axial direction and, on one end, exhibits a piston projecting into the measuring sample and, with the assistance of a drive coil (8) arranged at the end facing away from the measuring sample, is moved in the axial direction when current flows through the drive coil (8), wherein a spring element (7) is provided to move the rod (10) into a rest position when no current flows through the drive coil (8), is characterized by the drive coil (8) being arranged, in an axially movable fashion, in the inhomogeneous region of the magnetic field produced by the main field magnet coil coaxial to the main field magnet coil, the drive coil (8), on one axial end, being rigidly connected to the rod (10) and, on the end facing away from the rod (10), being connected, via the spring element (7), to a housing (1) surrounding the NMR measuring apparatus. In this fashion the mixing device does not require magnetic components which would interfere with the homogeneity of the NMR magnetic field and can be arranged compactly in the vicinity of the measuring sample.

BACKGROUND OF THE INVENTION 
The invention concerns a mixing device for the mixing of, in particular, 
fluids and/or gas-phase measuring samples in a nuclear magnetic resonance 
(NMR) measuring apparatus with a high field magnet coil for the production 
of a homogeneous static magnetic field and a measurement volume within the 
measurement sample, whereby the measuring apparatus includes a rod which 
is arranged coaxially to the main field magnet coil, is movable in an 
axial direction, exhibits, on one end, a piston projecting into the liquid 
and/or gas-phase measuring sample, and with the assistance of a drive coil 
arranged at the end facing away from the measuring sample, is moved in the 
axial direction by the flow of current through the drive coil with a 
spring element being provided for to move the rod into a rest position 
when no current flows through the drive coil. 
A mixing apparatus of this kind is, for example, known from the article "A 
High-Pressure Probe for NMR Studies of Homogeneous Catalysis" by Velde and 
Jonas in "Journal of Magnetic Resonance", No. 71, pp. 480-484, 1987. 
The high-pressure NMR measuring apparatus described therein serves for the 
investigation, with the assistance of nuclear magnetic resonance at high 
pressures, of chemical reactions which take place in a saturated gas-fluid 
mixture. From the theory of partial pressures it is known that, the higher 
the applied external pressure, the larger the fraction of gas which can be 
dissolved in the liquid. By means of the resulting chemical reactions, the 
saturated state of the mixture is destroyed and must be constantly 
adjusted. 
The saturated state is an equilibrium state which depends on the 
temperature as well as the pressure. For this reason, for this type of 
measurement, it is necessary to have a variable constant-pressure 
apparatus in the range from 1 to 1000 bar with temperature regulation 
(typically in the region of -20.degree. C. to +180.degree. C.). 
In order to quantitatively record the observed chemical reactions, it is 
necessary for the reactions to take place evenly and simultaneously within 
the entire liquid. It is therefore necessary for the gas and the fluid to 
mix with each other as quickly as possible and as evenly as possible and, 
in fact, before the reaction sets-in. An active mixing process is 
therefore essential for this type of investigation. Without an active 
mixing procedure, the mixing process would only take place by means of 
diffusion and would therefore transpire very slowly. It takes on the order 
of several days to achieve a saturated state through diffusion only. On 
the other hand, an active mixing procedure shortens the necessary mixing 
time by many orders of magnitude; the saturated state is achieved in only 
10 to 60 seconds. 
The mixing method should preferentially take place within the main field 
magnet of the NMR measuring apparatus in order to avoid additional losses 
in time. In this fashion the device is available for further measurement 
as quickly as possible and is therefore capable of exactly recording the 
time development of the chemical reaction. During the NMR measurement 
procedure, which takes place at constant pressure, it is necessary for the 
mixing procedure to be repeated from time to time. This is necessary since 
gas is used-up by the chemical reaction, and therefore new gas must be 
added and mixed-in in order to restore the required saturated state. Only 
a measuring procedure carried out in this fashion leads to reliable, 
reproduceable measurement values of the time development of the reaction. 
This type of reaction time development measurement can be carried out at 
different temperatures and pressures which, in each case, are kept 
constant during the measurement procedure. 
This mixing apparatus itself should, preferentially, be non-magnetic so 
that the necessary extremely high homogeneity of the magnetic field 
produced by the main field coil for the NMR measurement is not 
deteriorated due to the overlap of magnetic fields from the measuring 
apparatus. It is therefore essential that ferromagnetic or permanent 
magnetic materials be, by all means, avoided in the measuring apparatus 
or, when absolutely necessary, be arranged as far away as possible from 
the magnetic center of the NMR measuring apparatus. 
In a known mixing method, the measuring sample to be shaken is initially 
removed from the main field magnet and subsequently is brought in again to 
the main field magnet in order to carry out the NMR measurement. A 
disadvantage of the known method is that the measuring sample begins to 
react following the shaking and before the NMR measurement has begun. In 
addition, the temperature of the measurement sample changes during this 
process and must, subsequently, be restored again to the desired constant 
value. 
In another known measurement method, electric motors with permanent magnets 
are utilized in order to produce a mechanical shaking motion. A method of 
this type as, for example, described in the above cited article in JMR 71, 
480-484 (1987) exhibits, however, a series of serious disadvantages: 
The ferromagnetic and permanent magnetic materials contained in the 
electric motor distort the homogeneous field of the NMR main field magnet. 
In addition, the permanent magnet portion produces additional 
inhomogeneous fields. 
Magnetic fields between the electric motor and the superconducting magnet 
system can lead to mechanical stability problems in the event that the two 
are too close to each other. 
In order to keep the mutual influences between the electric motor and the 
NMR main field magnet as small as possible it is necessary that the 
electric motor be positioned very far away from the magnetic center of the 
NMR main field magnet. This necessarily leads to a large and cumbersome 
mixing apparatus. 
In the event that the electric motor works outside of the main high 
pressure area in which the NMR measuring sample is located, mechanically 
movable high pressure feed-throughs are necessary in order to transmit the 
shaking motion. The latter are extremely difficult to produce, unreliable, 
and nearly impossible to realize at high pressure. On the other hand, an 
electric motor arranged together with the measuring sample in the high 
pressure region likewise leads to serious technical problems. 
In contrast thereto it is the purpose of the present invention to present a 
mixing device of the above mentioned kind with which the homogeneous 
magnetic field of the NMR main field magnet is not encroached upon during 
the course of the NMR measurement having no mechanical forces present 
between the measuring device and the NMR main field magnet and which is 
capable of arrangement in spatial proximity to the NMR measuring magnet to 
work in a particularly reliable fashion in a high pressure region, wherein 
the entire apparatus is capable of particularly compact configuration. 
SUMMARY OF THE INVENTION 
This purpose is achieved in accordance with the invention in a fashion 
which is as surprisingly simple as it is effective in that the drive coil 
is arranged in the inhomogeneous region of the magnetic field produced by 
the main field magnet coil coaxially secured, in a movable fashion, to the 
main field magnet coil with the assistance of the spring element and is 
rigidly connected to the rod. 
Through the configuration of the drive coil in the inhomogeneous portion of 
the NMR magnetic field in the mixing device in accordance with the 
invention, the mechanical forces for driving the movable rod are produced 
with the assistance of the NMR magnetic field, functioning as a stator, by 
the flow of current through the drive coil without the necessity, as in 
conventional electric motors, for magnetic parts. The axial Lorentz force 
acting on the current carrying drive coil in the inhomogeneous magnetic 
field is approximately equal to the product of the coil current, field 
gradient and field strength. "Field" refers to the axial field component 
of the magnet. After switching off the current through the drive coil the 
mixing device does not influence, in any fashion, the homogeneous magnetic 
field in the measurement volume of the NMR apparatus. In this manner the 
mixing device in accordance with the invention can also be arranged in 
close proximity to the measurement volume to facilitate a particularly 
compact construction of the entire installation. 
The utilization of the magnetic field produced by the main field magnet of 
the NMR configuration as a stator field of an iron-free DC current 
rotational motor by appropriate configuration of a rotating coil in the 
magnetic field is per se known in the art through EP 0 136 642 B1. The 
configuration described therein serves, however, only for the production 
of a linear mixing motion in a high pressure spectrometer and is 
categorically different with regard to the mixing device having the above 
mentioned features. 
In a preferred embodiment of the, mixing device in accordance with the 
invention, the housing of the NMR measuring apparatus forms a high 
pressure chamber and the mixing device as well as the measuring sample are 
arranged in the high pressure region of the high pressure chamber. In this 
fashion the decisive advantage of the mixing device in accordance with the 
invention, namely that it can be arranged in proximity to the measuring 
sample without encroaching upon the magnetic field of the main field 
magnet can be utilized particularly effectively for the construction of an 
extremely compact and, with regard to its axial length in comparison to 
conventional configurations, strongly shortened NMR high pressure 
spectrometer. 
An embodiment of the mixing device in accordance with the invention is 
particularly advantageous in which the spring element forms one of the 
necessary two electrical connections for the current supply of the drive 
coil which, in this embodiment, then serves two completely different 
functions simultaneously. 
In particular, in a further embodiment, the spring element can be 
configured as a restoring spring, whereby the drive coil pulls the 
restoring spring against the spring force away from the housing to tension 
same. As soon as current no longer flows through the drive coil the 
restoring spring pulls the drive coil and rod in an axial direction away 
from the measuring sample. 
An embodiment of the mixing device in accordance with the invention is 
particularly preferred in which a measuring coil is provided coaxial to 
the drive coil for the detection of the instantaneous axial position of 
the drive coil. The drive coil then induces, along its moving path, a 
current or voltage signal in the measuring coil which depends on its axial 
position. In this fashion, the mixing motion can be precisely monitored 
from outside. The dimensioning is such that the drive coil can be inserted 
into the measuring coil so that the configuration can be particularly 
compact. 
Also within the purview of the invention is a method for the operation of 
the mixing device in accordance with the invention with which, as 
described above, the drive coil for the production of an axial up-and-down 
motion of the rod is supplied with low-frequency alternating current in 
the range of approximately 0.5 Hz to 10 Hz, preferentially, with 
sinusoidal amplitude. The utilization of a sinusoidal amplitude current 
supply leads to a sine-like deflection function of the drive coil with the 
rod in the axial direction during supply of the drive coil with 
alternating current. Experience has shown that the mixing motion is most 
effective in the above mentioned frequency range of 0.5 Hz to 10 Hz. In 
contrast, for example, to a triangular or tooth-shaped characteristic 
motion, a sine function allows for a particularly gentle catching of the 
mixing motion at the extrema. 
A variation of this method is particularly preferred in which no current is 
sent through the drive coil during an NMR measurement of the measuring 
sample. In this fashion, a negative influence on the homogeneity of the 
NMR magnetic field is, in any event, avoided. 
In a further variation on this method a DC current for the the adjustment 
of the rest position of the drive coil is superimposed on the low 
frequency AC current. 
In an advantageous variation of the method, the instantaneous position of 
the drive coil is detected when current flows through same. In this manner 
it is possible to externally monitor and observe the mixing motion. 
In a particularly preferred improvement of this variation of the method for 
operation of a mixing device in accordance with the invention having a 
measurement coil for the detection of the instantaneous axial position of 
the drive coil as described above, a high frequency alternating current, 
preferentially of the magnitude of approximately 10 kHz, is overlapped 
with the low frequency alternating current through the drive coil with, 
relative to the low frequency alternating current, a low amplitude. By 
means of the geometric configuration of the drive coil and the measuring 
coil both coils, when they are in close proximity to another, are strongly 
magnetically coupled. The high frequency alternating current through drive 
coil which overlaps the low frequency drive current has, due to its 
relatively high frequency in comparison to the inertia of the mixing 
apparatus, practically no influence on the drive process itself. It, 
however, induces a voltage in the measuring coil which depends on the 
axial separation between the measuring and the drive coil. The amplitude 
of the induced high frequency alternating current signal is a measure of 
the axial position of the drive coil and serves for the precise monitoring 
of the mixing motion. 
Further advantages of the invention can be derived from the description and 
the drawing. Likewise, the above mentioned features and those which are to 
be described further below can be utilized in accordance with the 
invention individually or collectively in arbitrary combination. The 
embodiments shown and described are not to be considered as an exhaustive 
enumeration, but have exemplary character only in order to illustrate the 
invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The schematic section of FIGS. 1 and 2 shows an NMR high pressure measuring 
apparatus with a housing 1 configured as a high pressure chamber and an 
access 2, required for purposes of pressure regulation, to which a gas 
compressor 20 is attached. A glass sample receptacle 11 is arranged in the 
lower region of the high pressure chamber and has a sample liquid 12 
located at the homogeneity center of a main field magnet coil (not shown 
in the drawing) surrounding which an NMR coil 13 with connections 6 is 
arranged for extraction of a nuclear magnetic resonance signals from the 
sample. An axially moving rod 10 projects into the glass sample receptacle 
11 with a piston arranged on its lower end within the glass sample 
receptacle 11. The piston mixes the sample fluid 12 as well as gases 
dissolved or mixed therein. It is moved with the assistance of a drive 
coil 8 which is arranged in the inhomogeneous part of the NMR magnetic 
field, functioning as a stator, and is connected to the rod 10 on the end 
located away from the sample. The drive coil 8 is connected to the housing 
1 by means of a restoring spring 7 and is movable together with the rod in 
the axial direction. The connections 4 serve to supply current to the 
drive coil 8. 
A measuring coil 9 is furthermore provided for whose inner diameter is 
larger than the outer diameter of the drive coil 8 so that the drive coil 
8 can be inserted axially into the measuring coil 9. The current or 
voltage induced in the measuring coil 9 through the motion of the drive 
coil 8 is fed outwards through connections 3 so that the up-and-down 
motion of the mixing device can be observed and monitored externally. 
A coiled pipe 5, through which a cooling or warming liquid can flow, is 
provided surrounding the housing 1 of the NMR high pressure measuring 
apparatus for the even temperature control of the measuring region. 
The block diagram shown in FIG. 2 gives the function of the NMR measuring 
apparatus of FIG. 1: 
The pressure within the housing 1 of the NMR measuring cell is held 
constant via an access conduit 2 with the assistance of a gas compressor 
20. A temperature control unit 15 cools or warms the NMR measuring cell 
via a fluid flowing through the coiled pipe 5 and thereby keeps the 
measuring sample at a constant temperature. 
The measuring signals extracted by the NMR coil 13 during the measuring 
phase are introduced via connections 6 to an NMR detector and, thereafter, 
further processed by a computer which is not shown. The fluid sample 12 is 
mixed, between measuring cycles, in the glass sample receptacle 11 with 
the assistance of the above described mixing apparatus. Towards this end a 
current generator 14 supplies current via connections 4 to the drive coil 
8 located in the inhomogeneous portion of the NMR magnetic field. Hereby, 
as described more closely below, DC current is initially supplied to the 
drive coil 8 via a DC current generator 17 in order to adjust its rest 
position. With the assistance of a low frequency oscillator 18 which is 
capable of producing alternating current of frequency in the range of 
approximately 0.5 Hz to 10 Hz, the drive coil 8 is excited into its linear 
up-and-down motion for the purpose of mixing the sample fluid 12. Finally, 
the drive coil 8 is supplied, by means of high frequency oscillator 19, 
with a high frequency signal in the 10-kHz-range of lower amplitude which 
is detected by the measuring coil 9 shown in FIG. 1 and can be introduced 
to a position detector 21 via connections 3 in order to observe the 
instantaneous position of the drive coil 8 and thereby the time variation 
of the mixing motion. 
The mixing device in accordance with the invention, as mentioned above, is 
provided for utilization in NMR experiments with which the fluid mixture 
to be analysed must be thoroughly mixed. It has been developed as an 
optional auxiliary means for introduction into a 400 MHz superconducting 
magnet, schematically indicated in FIG. 3a, with small room temperature 
bore 22 within a cryostat 23. 
The fundamental principle of operation is the utilization of the magnetic 
field gradients which are located about the magnetic center due to the 
directional changes of the magnetic field lines. 
The utilization of magnetic field gradients is possible for any arbitrary 
magnet with the same fundamental construction when the field gradients are 
precisely arranged and when small adjustments are carried out. FIG. 3b 
shows an enlarged section of FIG. 3a with the drive coil 8 in a rest 
position (left) and a deflected position (right) within the inhomogeneous 
magnetic field indicated by the curved field lines. 
The mixer itself can comprise a small PTFE-sieve which is connected to a 
thin PTFE and Berylco rod. This configuration oscillates vertically with 
an amplitude of 25 mm within the glass tube containing the solution which 
is to be analysed by nuclear magnetic resonance. 
FIG. 4a shows an NMR probe head 24 with axially mounted mixing device. The 
upper part of the rod 10 extends through a 103 mm long extension pipe 
which is arranged directly above the NMR probe head 24 and which 
terminates in the intermediate motor region. 
The two principle components, e.g. the extension pipe and the intermediate 
motor region are Berylco cylinders. The inner of these two parts is 
connected to the inside of the NMR probe head 24 so that the entire system 
can be put under pressure. The mixing device can be removed and replaced 
by an upper stopper on the NMR probe head 24. Another possible embodiment 
of the mixing device in accordance with the invention is shown in FIG. 4b. 
The motor is connected by means of a cable to an electronic low frequency 
sine-wave generator. This oscillator 18 was specially constructed for the 
present application and is provided with a dynamic indicator which 
continuously shows the motion of the mixer. 
The mixer, during long experiments, can be started and stopped by remote 
control of a computer. 
The motor comprises the drive coil 8, hung by means of two springs which 
also establish the electrical connections as suggested in FIG. 5a and FIG. 
5b. The central spring 7' comprises insulated copper with a diameter of 
0.2 mm and has a reduced strength to serve only as an electrical 
conductor. Its outer diameter assumes a value of 4.5 mm and its length in 
the rest position of FIG. 5a assumes a value of 44 mm. The outer restoring 
spring 7 comprises (non-insulated) Berylco with a diameter of 0.5 mm. It 
not only establishes an electrical connection but also supports the 
movable motor rod-sieve configuration and has the following dimensions: 
outer diameter 10 mm, 65 windings, length in the rest position 35 mm. 
Motion is achieved by means of a variable sine voltage of low frequency. 
The drive coil 8 is wound from copper wire with a diameter of 0.2 mm and, 
in the embodiment shown, has the following dimensions: inner diameter 6 
mm, outer diameter 10 mm, length 25 mm. 
The force on the drive coil is largest at the location where the product of 
the axial field component times the axial field gradient of the magnet is 
maximum. By way of example, with a particular 400 MHz magnet having 52 mm 
room temperature bore, this location is approximately 220 mm from the 
magnetic center, in the axial direction. The position dependence of the 
forces at this location is relatively flat over a certain length, for 
example, within 5% over a length of 30 mm. 
In order to maximally utilize the force on the drive coil the center of the 
drive coil must, during the mixing procedure, move within the flat portion 
of the force curve. In addition, after switching off the piston mixing 
process, the rod 10 should come to rest on the upper edge of sample fluid 
12. Both these conditions are achieved by positioning the center of the 
drive coil 8, in its rest position, approximately 1 cm above the position 
of maximum force in that, with the assistance of a DC current in the drive 
coil, its center is displaced to the position of maximum force and finally 
in that, with the assistance of a sine-shaped alternating current in the 
drive coil, the mixing motion is produced over a length of approximately 
.+-.1.3 cm. The overlapping of the DC and AC current portions can be 
accomplished with a single signal as shown in FIG. 6. 
The force curves have been measured using a prototype with a spring 7 and a 
drive coil 8 which is attached to a long rod 10 projecting from the bottom 
of the magnet with a diameter of about 0.8 mm. This thin rod extends 
through a glass guiding tube having an inner diameter of 6 mm. 
A DC voltage of 2 V is applied to the coil and the induced motion of the 
rod is measured at different heights by means of this voltage. 
A second stationary pipe measuring coil 9, arranged directly below the 
drive coil 8, is utilized for measuring the motion. 
A signal with high frequency (10 kHz) and small amplitude (100 mV) is 
additively overlapped with the sine signal on the movable drive coil 8. 
This high frequency signal induces a sine signal of 10 KHz in the 
stationary measuring coil 9 whose amplitude is approximately proportional 
to the spatial overlap of the two coils. This type of detection has the 
advantage that it gives a practically continuous linear measurement of the 
displacement. The stationary drive coil 8 comprises two winding layers 
with the following dimensions: diameter 12 mm, wire diameter 0.2 mm, 
length of the coil 25 mm. 
This configuration is very simple and comprises only two coils and four 
electrical connections. There are no ferromagnetic components. The mixing 
device is appropriate for temperatures up to a maximum of 180.degree. C. 
The mixing device is connected to the electronics via a shielded cable 
having 4 conductors. For technical reasons the power ground and the ground 
of the signal detected by the measuring coil 9 are connected to the 
housing of the electronics unit. This ground is, however, not connected to 
the magnet housing 1. Therefore, the signals in the two coils are not 
grounded. They have no common ground due to the large differences in 
current through the two coils. For this reason 4 rather than 3 electrical 
connecting components are necessary. 
The electronics was developed in order to serve two purposes: 
a) To provide for a low frequency sine signal generator with the following 
flexibility: 
--frequencies of 0.5 Hz to 10 Hz 
--amplitudes up to 2 V peak to peak 
--positioning of the drive coil via a continuous offset voltage 
The last adjustment is very important; in this fashion it possible to 
compensate for the friction at the end points of the motion and therefore 
to optimize the working of the mixer. 
b) Since the entire system (under pressure) is completely closed and 
non-visible from the outside it is necessary to know whether or not the 
mixer is working and what the amplitude of its motion is. If the viscosity 
of the product being investigated changes, the amplitude must be adjusted 
while its function is being verified. A motion detector system was 
developed in order to satisfy these requirements. 
It is advantageous to start and stop the motion at the peak of a sine curve 
as shown in FIG. 6 in order to prevent a shaking of the liquid sample 12 
out of the glass sample receptacle 11 or to prevent the building up of 
blockage above the mixture. 
The low frequency oscillator 18 is suitable for digital processing. A 
cosine curve stored in an EPROM memory chip was utilized. A 
digital-to-analog converter followed the memory chip. When the operator 
pushed START the memory was continuously read and therefore delivered a 
sine-shaped signal at the start and stop. This operating mode has the 
advantage that a perfectly synchronized signal is achieved by starting and 
stopping. 
______________________________________ 
Reference Symbol List 
______________________________________ 
1 housing 
2 access to high pressure chamber 
3 measuring coil connections 
4 drive coil connections 
5 coiled temperature control pipe 
6 NMR coil connections 
7 restoring spring 
7' contact spring 
8 drive coil 
9 measuring coil 
10 rod with piston 
11 glass sample receptacle 
12 sample liquid 
13 NMR coil 
14 current generator 
15 temperature control unit 
16 NMR detector 
17 DC current generator 
18 low frequency oscillator 
19 high frequency oscillator 
20 gas compressor 
21 position detector 
22 room temperature bore 
23 cryostat 
24 NMR probe head 
______________________________________