Peristaltic pump

A peristaltic pump comprises a rotatable drum 2 having rollers 5 or cams 5' for squashing a flexible tube 9 against a profiled surface 10 of a presser plate 7. Tube 9 is mounted between supports 14 and 15 on presser plate 7 and is automatically movable between an inoperative and operative (pumping) position by pivotal movement of plate 7 about axis 8 produced by an electro mechanical actuator. A flexible membrane 33 is sandwiched between rollers 5 or cams 5' for eliminating shear forces on the tube 9. The construction of the pump and shape of profile 10 are such as to mimimise pulsations in the output flow. Systems for supplying a sample for analysis to spectroscopic apparatus using the peristaltic pump are also described.

TECHNICAL FIELD 
This invention relates to peristaltic pumps and particularly but not 
exclusively such pumps intended for use in circumstances where the amount 
of material pumped over a given time period needs to be accurately 
controlled. One such circumstance is the presentation to spectroscopic 
apparatus of a sample to be analysed by that apparatus. This invention 
also relates to spectroscopic analysis of substances and is particularly 
concerned with a system and method whereby samples are presented for 
analysis by spectroscopic apparatus. It will be convenient to hereinafter 
describe in the invention relation to spectroscopic apparatus, but it is 
to be understood that a pump according to the invention has other 
applications. 
BACKGROUND 
Peristaltic pumps are well known and comprise a rotatable drum having a 
plurality of rollers located around its periphery, and a flexible tube 
which is held against the drum periphery by a presser plate and through 
which liquid (for example) is pumped. The tube is held by two spaced fixed 
supports, and liquid is caused to move through the tube in response to a 
moving pinch zone caused by the rollers pressing on the tube as the drum 
rotates. 
Pumps of the foregoing kind suffer problems which tend to make them 
unsuitable for use in certain circumstances. One such problem is a 
tendency for the tube to become distorted and to take on a permanent set 
which makes it unsuitable for further work. In operation, the tube is 
pulled over the drum and is attached to the fixed supports so as to have a 
correct amount of longitudinal tension. If the tube is left in the 
operating position for extended periods of time with the pump stationary, 
the aforementioned distortion may occur. Consequently, it is good practice 
to release the presser plate and the tube tension at the end of each 
period of use, but that is often overlooked either by accident or by 
design, particularly as the re-establishment of the pump to an operative 
condition is a tedious process. 
Another problem with peristaltic pumps is that the output flow is a 
pulsating flow, and that tends to make such pumps unsuitable for use in 
some circumstances. For example, the pulsating flow makes such pumps 
unsuitable for use in delivering a sample to be analysed to the nebulizer 
of a spectroscopic instrument. That difficulty can be met by operating the 
pump at a very high speed but such operation is not always possible or 
convenient. 
Another problem in peristaltic pumps that are to be used for accurately 
metering materials and which is a significant component in their cost, is 
that dimensional tolerances in the manufacture of individual components, 
such as the drum and the rollers, and positional tolerances on assembling 
the different parts are necessarily quite small to ensure the requisite 
accuracy in operation of the pump. Also the roller bearings in known pumps 
often corrode and produce unreliable operation. 
DISCLOSURE OF THE INVENTION 
It is an object of the present invention to provide a peristaltic pump in 
which one or more of the aforementioned problems are ameliorated. It is 
another object of an embodiment of the invention to provide a peristaltic 
pump of relatively simple construction and which is relatively inexpensive 
to manufacture. 
A peristaltic pump according to the broadest aspect of the invention is 
characterised in that at least one of the tube supports is movable 
relative to the drum between loaded and unloaded positions rather than 
having a fixed position relative to the drum as in prior constructions. 
Thus, according to this broadest aspect of the invention, there is provided 
a peristaltic pump including a rotatable drum including compressing 
elements around its periphery, a flexible tube extending between two 
spaced supports and a movable presser plate for holding the tube against 
the periphery of the drum between the two spaced supports such that on 
rotation of the drum the compressing elements squeeze the tube against the 
presser plate for moving fluid through the tube, wherein at least one of 
the tube supports is movable relative to the drum to establish an 
operative and an inoperative position for the tube, the operative position 
being when the tube is positioned for pumping. 
Preferably, the movable support is fixed to or formed integral with the 
presser plate so as to move with that plate between the loaded and 
unloaded positions. 
It has been conventional practice in the past to secure the presser plate 
in the pump operative position by means of a spring influenced clamp or 
the like. According to a preferred form of the present invention, the 
presser plate is moved into the operative position by means of an 
electro-mechanical actuator. It is further preferred that the arrangement 
is such that the presser plate is subjected to a control system such that 
it can be moved automatically between operative and inoperative positions 
thereof. Thus, in this preferred arrangement, a peristaltic pump is 
provided in which the tube tension is automatically established as the 
pump is being conditioned for operation and is automatically 
de-established at the end of each operating sequence. 
A further aspect of the invention is that the known rotatable drum and 
rollers assembly in a peristaltic pump according to the invention may be 
replaced by a cam, the profile of which comprises a plurality of camming 
surfaces for squeezing the tube. Thus a pump according to the invention 
will contain tube compressing elements which may be in the form of rollers 
or fixed camming surfaces around the periphery of the drum. 
A further feature in a pump according to the invention is that the presser 
plate may be profiled in such a way as to minimise pulsations and lack of 
stability in the output flow. That profiling is applied to the surface of 
the presser plate which is opposed to the drum and is therefore the 
surface which holds the tube against the drum rollers or camming surfaces. 
Preferably the profiled presser plate surface includes or is composed of 
at least two regions, a pinch region and an expansion region which is 
located downstream of the pinch region in relation to the direction of 
flow through the tube. The pinch region is arranged to cooperate with the 
drum rollers or camming surfaces so that the aforementioned pinch zones 
are created within that region. The expansion region preferably follows 
immediately after the pinch region and is arranged so that the space 
between it and the drum progressively increases in the downstream 
direction. In one arrangement, there is a third region upstream of the 
pinch region, which is called an entrance region and which is arranged so 
that the space between it and the drum progressively decreases in the 
downstream direction. 
The operation of a peristaltic pump is essentially equivalent to moving a 
restriction along the length of the tube so that liquid within the tube 
and in advance of the restriction is pushed through the tube by the 
restriction. There is in fact a plurality of such restrictions which are 
created in sequence by the rollers or camming surfaces as they are moved 
against that section of the tube which is influenced by the presser plate. 
As each roller or cam moves away from that section of the tube, it allows 
the tube to expand and thereby increase the internal volume of the tube 
downstream of the presser plate. That expansion gives rise to pulsations 
in the output flow of the pump. 
Adoption of a presser plate having an expansion region as described above 
enables output flow pulsations to be eliminated, or at least reduced. This 
is achieved by designing the profile of the expansion region so that a 
substantially linear relationship exists between the angular rotation of 
the drum and the increase in the tube internal volume resulting from 
withdrawal of a drum roller or cam from the tube. Ideally the linear 
relationship should be such as to increase the internal volume by the 
volume contained by a single tube pinch over an angular rotation 
equivalent to the angular separation of one roller or camming surface from 
the next, and then the output flow is free of pulsations. In other 
circumstances, a strictly linear relationship may not be possible, but it 
is nevertheless possible to reduce the amplitude of the pulsations to an 
acceptable level by means of profiling as discussed above. 
The presser plate may furthermore be profiled such that over a portion of 
said profile, the gap between the camming surfaces, or rollers of the 
rotating drum, and said portion is less than that which is necessary to 
seal the tube. In operation, the camming surfaces or rollers "over-squash" 
the tube, thus allowing for a lesser degree of accuracy in sizing and 
assembling the pump parts in manufacture, although the primary aim of this 
feature is to allow a lesser degree of relative positioning accuracy of 
the pump parts. 
Another cause of pulsation in the output flow in pump constructions wherein 
the compressing elements are tellers is frictional resistance to rotation 
of the rollers. Each roller rotates in response to a tangential force 
created between it and the pump tube, which is held against movement with 
the drum. That force tends to stretch the tube longitudinally and also 
generates shear forces in the tube because the side of the tube contacting 
the presser plate does not experience the same tangential forces. Such 
shear forces contribute to tube wear and fatigue failure. Also, as each 
roller withdraws from the tube, the tube is able to relax longitudinally 
and thereby cause a change in the internal volume of the tube such as to 
introduce pulsations into the output flow. 
Prior attempts to meet the problem have not been entirely satisfactory. One 
approach has been to reduce frictional resistance to roller rotation by 
mounting the rollers on ball bearings, but that is a complex and costly 
approach which alleviates the problem rather than solves it. Another 
approach has been to drive the rollers through a planetary gear system, 
and that is a very expensive approach which also fails to solve the 
problem. In order to be effective, the rotational speed of the rollers 
must exactly match the traverse speed of the drum over the tube, but such 
matching is seldom achieved. 
According to a further preferred feature of the invention, the 
aforementioned problem is met or at least substantially alleviated by 
interposing a flexible membrane between the drum and the tube such that 
the membrane rather than the tube absorbs the aforementioned shear forces. 
The membrane is anchored upstream of the region within which the tube is 
pinched as previously described, and is preferably composed of a material 
which is not prone to stretch in the longitudinal direction of the tube. 
Suitable materials include polyester such as Mylar and other plastics 
materials, and metal foil, but that is not an exhaustive list. 
In a pump having camming surfaces instead of rollers, the frictional forces 
on the tube will be larger. Thus in this form of pump construction a 
flexible membrane needs to be interposed between the cam and the tube for 
absorbing shear forces that would otherwise act on the tube. It is 
desirable for the contacting surfaces of the cam and the membrane to be 
lubricated and such lubrication may be provided by a lubricating 
substance, for example a silicone grease, placed on the membrane or 
camming surfaces. Alternatively the membrane or camming surfaces may be 
self lubricating. Preferably the membrane is of a substance or constructed 
such that its cam facing surface is very slippery, for example the 
membrane may be a laminate of Mylar and a more slippery plastic. 
When a flexible membrane is disposed between the drum and the tube as 
described above, the tension applied to the tube when it is in its 
operative position is preferably very little (that is, it is close to zero 
or even zero) so that the elasticity of the tube defines a volume of 
liquid between adjacent rollers or cams which is independent of roller (or 
cam) speed. Thus, the tube is preferably held reasonably loosely in its 
operative position and there is minimal or zero stretching of the tube. 
Where the presser arm force is generated by an electromechanical actuator 
such as a stalled DC motor run at a controlled current, friction or other 
effects in the actuator may cause uncertainty in the output torque and 
hence presser plate force. This uncertainty is undesirable and may be 
overcome by cycling the current setpoint about its mean value at a rate 
fast enough to not affect operation of the system, yet slow enough for the 
actuator to respond. 
One technique employed in spectroscopy is to produce a sample solution 
containing the substance of interest, and to introduce that solution into 
a nebulizer which directs an atomized body of the solution into a flame or 
plasma. Preparation of such samples is an exacting and time consuming 
operation, and obtaining a suitable level of dilution is one particular 
problem in that preparation. In addition to the sample solutions, it is 
necessary to prepare standard solutions for comparative reference, and 
that adds further to the time required to conduct the sample analysis. 
There have been several proposals for overcoming or alleviating all or some 
of the foregoing problems. One such proposal is that of Jones as described 
in "Atomic Absorption Newsletter", Vol. 9, No. 1, January-February 1970, 
pages 1 to 5. The Jones proposal involves connecting separate sample 
solution and diluent supplies to the nebulizer through a T junction, and 
delivering the sample to that junction by way of a variable speed syringe 
pump. Fluid flows into the nebulizer at the natural aspiration rate of the 
system. The flow rate of the sample is controlled by the pump speed, and 
the flow of diluent automatically adjusts so that the total flow rate is 
constant. Suitable variation of the pump speed then results in achievement 
of a desired level of dilution in the sample stream presented to the 
nebulizer. 
A deficiency of the Jones proposal is that it utilizes the same pump for 
standard and sample solutions respectively. In particular, it is necessary 
to replace the supply of one solution with the supply source of the other 
when it is required to switch from analysis of one solution to the other. 
It also uses a syringe pump and this involves carryover, slow throughput, 
limited volume and wasted sample problems. 
The use of a peristaltic pump according to the present invention in a 
system for delivering a sample for analysis to spectroscopic apparatus 
offers the advantage that a more even flow of sample into the apparatus is 
achievable than is the case with prior such systems. 
Thus an object of a further aspect of the present invention is to provide 
an improved method and apparatus for preparing and introducing solutions 
for analysis. 
Accordingly the invention also provides a system for delivering a sample 
for analysis to spectroscopic apparatus, including means for supplying a 
stream of sample solution, means for supplying a stream of diluent, means 
for combining the two streams, and means for delivering the combined 
stream into a nebulizer of the spectroscopic apparatus at a substantially 
constant flow rate, wherein the means for supplying a stream of sample 
solution or the means for supplying a stream of diluent includes a 
peristaltic pump according to the invention, and the system is arranged 
such that on variation of the flow rate of the stream of sample solution 
or the stream of diluent, the other stream (of diluent or sample solution) 
is automatically varied to maintain said substantially constant flow rate 
into said nebulizer. 
The invention also provides a method of spectroscopic analysis, wherein a 
system as just described is used to supply a sample to a nebulizer of 
spectroscopic apparatus for calibration of that apparatus and for analysis 
of the sample, the method including supplying streams of sample solution 
from a single on-line source of the sample solution, 
In a method and apparatus according to the further aspect of the invention, 
separate streams of sample solution and diluent respectively are combined 
and then introduced into the nebulizer as a single stream. The flow rate 
of the combined stream into the nebulizer is fixed by the natural 
aspiration rate of the nebulizer, and the cross-sectional size of the 
passage through which the combined stream enters the nebulizer. The sample 
solution and the diluent are fed separately to a junction (that is a 
mixing point) at which the combined stream is formed, and either the 
sample solution or the diluent is delivered to that junction by a variable 
speed peristaltic pump according to the invention. Preferably, the pump 
delivers the sample solution to the junction, and diluent is induced into 
the stream leaving the junction by a pressure differential existing 
between the nebulizer and the junction. As the flow via the peristaltic 
pump changes, the flow of the diluent stream automatically changes to 
compensate. This changing flow results in a changed pressure drop between 
the diluent reservoir and mixing point. Such a pressure change could alter 
the pressure differential at which the nebuliser operates and thus alter 
its uptake rate, affecting overall system performance. To avoid this 
happening, it is desirable to ensure pressure changes at the mixing point 
are small relative to the pressure drop between the mixing point and the 
nebuliser. This can be achieved by ensuring the tubing bore between the 
mixing point and reservoir is substantially larger than between the mixing 
point and nebuliser. To achieve 1% accuracy, a ratio of bore diameter of 
at least 3:1 is desirable, with higher ratio's preferable. 
Assuming the maximum flow from the junction to the nebulizer is fixed at 
F1, and the flow rate of sample solution to the junction is F2, which is 
less than F1, diluent will be induced to flow into the junction at a rate 
equivalent to F1 minus F2. Variation of F2 by appropriate adjustment of 
the pump then automatically results in a variation in the diluent flow 
rate, and consequently the dilution ratio of the stream entering the 
nebulizer. 
In a preferred form of the system and method, standard solution is able to 
be introduced into the aforementioned Junction under the influence of a 
second peristaltic pump according to the invention. Such an arrangement 
enables standard additions to be carried out in a simple and effective 
manner as hereinafter described.

DETAILED DESCRIPTION OF EMBODIMENTS 
The example pump 1 shown in FIG. 1 includes a rotatable drum 2 having a 
body 3 arranged for rotation shout an axis 4, and a plurality of rollers 5 
attached to the drum body 3 so as to extend as a continuous series around 
the periphery of that body 3. Each roller 5 is arranged to rotate relative 
to the drum body 3 about its own individual axis 6. Any suitable drive 
means may be provided to cause rotation of the drum 2. 
A presser plate 7 is mounted on a support (not shown) so as to be rotatable 
about an axis 8 to adopt either a pump inoperative position or a pump 
operative position, which are shown in FIGS. 1 and 2 respectively. A 
flexible tube 9 is interposed between an operative surface 10 of the 
presser plate 7 and the periphery of the drum 2. When the presser plate 7 
is in the operative position as shown in FIG. 2, a portion of the tube 9 
which is trapped between the surface 10 and the rollers 5 is distorted as 
shown in FIG. 3. A pinch zone 11 is thereby created between each roller 5 
and the plate surface 10 so that a body of liquid 12 within the tube 9 is 
trapped between each two adjacent zones 11. As the drum 2 rotates relative 
to the tube 9 the zones 11 move along the length of the tube 9 and thereby 
move the liquid bodies 12 in the same direction. 
The tube 9 may be held against movement with the drum 2 in any suitable 
fashion. It is a characteristic of the particular construction shown 
however, that a section 13 of the tube 9 is held at two supports 14 and 
15, and that the support 14 is attached to or formed integral with a 
portion 16 of the presser plate 7 which is located remote from the plate 
axis 8. The arrangement is such that movement of the plate 7 about the 
axis 8 causes the plate portion 16 to move towards or away from the 
periphery of the drum 2, and the tube support 14 is moved accordingly. In 
the particular arrangement shown, the other tube support 15 is also 
attached to or formed integral with the presser plate 7, but at a location 
adjacent the axis 8 so that there is relatively little movement towards 
and away from the drum 2. 
Any suitable means may be utilised to hold the tube 9 at each of the 
supports 14 and 15. By way of example, two saddles 17 may be secured (for 
example by an adhesive) to the tube 9 against relative movement and 
arranged in spaced relationship such that each is cooperable with a 
respective one of the supports 14 and 15. Each support 14,15 is formed by 
a channel 60 in an end face of presser plate 7 and an intersecting groove 
61 (see FIG. 7). 
Movement of the presser plate 7 about the axis 8 can be achieved through 
use of any suitable drive means. It is preferred however, that an 
electro-mechanical actuator is used for that purpose. By way of example, 
the actuator may include a solenoid 18 as Shown in FIG. 4 which is 
connected between the presser plate 7 and a support 19. In the particular 
arrangement shown in FIG. 4, the solenoid 18 operates when energised to 
move the plate 7 to the pump operative position of FIG. 2, and a spring 20 
functions to move the plate 7 to the pump inoperative position when the 
solenoid 18 is de-energised. In an alternative arrangement which is not 
shown, a spring could move the plate 7 to the operative position and a 
solenoid could operate to move the plate 7 to the inoperative position. 
FIG. 5 shows another possible drive arrangement for the plate 7 in which a 
cam 21 is caused to rotate about an axis 22 by means of a suitable drive 
motor (not shown). The arrangement is such that the rotational position of 
the cam 21 determines the position of the plate 7 relative to the drum 2. 
Any suitable means, such as a spring (not shown), may be used to hold the 
plate 7 in contact with the cam 21. 
A preferred drive arrangement for the plate 7 is shown diagrammatically in 
FIG. 6. According to that arrangement, the plate 7 is mounted directly on 
to the shaft 23 of a drive motor 24, which could be a gear motor or a 
rotatable solenoid, for example. The bearings which support the shaft 23 
determine the pivot axis 8 of the plate 7, and the plate 7 is connected to 
the shaft 23 so that rotation of the shaft 23 causes movement of the plate 
7 between the pump operative and inoperative positions. Such an 
arrangement is extremely simple and involves a minimum number of 
mechanical parts. The arrangement may be such that the drive motor 24 
stalls at the pump operative position, and consequently has the 
characteristic of a current to torque converter when driven at a 
controlled current. Thus, the torque and consequently the force applied by 
the plate 7 to the tube 9, can be controlled by variation of the current 
applied to the motor 24. Any suitable means may be adopted to guard 
against overload of the motor 24. 
It will be appreciated that when the FIG. 6 arrangement is used in the 
assembly of FIGS. 1 and 2, it permits automatic loading and unloading of 
the tube 9, thereby overcoming a major problem with prior peristaltic 
pumps. In particular, the tube 9 is not left in a stressed condition as 
sometimes happens with prior pumps, and therefore has an extended useful 
life, although in a preferred construction of the present invention, the 
tube 9 is only minimally tensioned, as described above. 
The operative surface 10 of the presser plate 10 may be profiled in a 
manner such as to either eliminate pulsations in the output flow, or 
minimise the adverse consequences of any such pulsations. That profiling 
may be such that the surface 10 has two distinct regions, a pinch region 
25 and an expansion region 26, both of which are shown in FIG. 7. In the 
preferred arrangement shown in FIG. 7 however, there is a third region 27 
which will be referred to as the entrance region. The expansion region 26 
is located downstream of the pinch region 25 relative to the direction of 
flow through the tube 9, and the entrance region 27 is located upstream of 
the pinch region 25. 
In the particular arrangement shown in FIG. 7, the pinch region 25 extends 
through an arc of approximately 24.degree., the extremities of which are 
defined by lines 28 and 29 which extend radially from a point 30. It is 
preferred that the surface region 25 is of uniform radius, the center of 
which is located at the point 30. Furthermore, in the installed condition 
of the presser plate 7, the point 30 is preferably substantially 
coincident with the axis 4 about which the drum 2 rotates. The arcuate 
length of the pinch region 25 may be determined by the number of rollers 5 
(that is, 24.degree. is appropriate for a pump having 15 rollers 5) but 
may vary according to requirements, and consequently the 24.degree. extent 
of the arrangement shown should not be understood as critical or 
essential. 
A feature that may be incorporated in the pump is to arrange for the tube 9 
to be "oversquashed" in a portion or all of the pinch region, that is, to 
arrange for the tube walls to be squeezed together slightly beyond the 
limit necessary to effect a seal. This feature admits of greater 
manufacturing and assembly tolerances for the pump parts. 
The expansion region 26 also follows a curved path, but is arranged so that 
the distance between that path and the periphery of the drum 2 
progressively increases in the downstream direction. As previously 
indicated, the profile of the surface region 26 is preferably designed to 
achieve a substantially linear relationship between the angular rotation 
of the drum 2 and the increasing internal volume of the tube 9 which 
follows withdrawal of the rollers 5 from contact with the tube 9. One 
possible approach is to construct a linear expansion profile of the 
surface region 26 from the equation R=RO+KA, where: 
R is the radius of curvature of the surface region 26 at a particular point 
in that region, 
RO is the radius of curvature of the pinch region 25, 
K is a constant defining the rate of expansion of the tube 9, and 
A is the angle of rotation of the drum 2 beyond the point at which the 
surface region 26 commences. 
The extent to which pulsations are reduced depends upon the value selected 
for K. A flow pulsation of approximately 10% is achievable using an 
optimal value for K for the roller system that is employed. 
If a parabolic expansion curvature is selected for the region 26, flow 
pulsations in the region of 8.5% may be achieved, whereas an exponential 
profile can achieve a better result with residual pulsations in the order 
of 5.7%. A satisfactory profile, and the equation for generating that 
profile, can be determined according to individual needs. 
In the particular arrangement shown in FIG. 7, the extremities of the 
expansion region 26 are defined by the lines 28 and 31 which extend 
radially from the point 30. The angular extent of the surface region 26 is 
approximately 66.degree. in the arrangement shown, but a different angular 
extent may be selected. 
The surface region 27 is preferably arranged to achieve progressive 
compression of the tube 9 as the drum 2 advances over the tube section 13. 
Any suitable curvature can be selected for that purpose, Subject only to 
the requirement that the separation between the surface region 27 and the 
drum periphery decreases in the downstream direction. The angular extent 
of the region 27 is defined by the radial lines 29 and 32, and in the 
example shown it is approximately 66.degree., that is, equal to the 
expansion region. 
It is preferred to provide a membrane 33 (FIG. 1) between the drum 2 and 
that part of the tube 9 which is subjected to the influence of the presser 
plate 7. The upstream end 34 of the membrane 32 may be anchored in any 
appropriate fashion so as to be fixed against movement with the drum 2, 
and the anchoring point 35 need not be located as shown in FIG. 1. The 
downstream end 36 of the membrane 33 may be anchored also, but preferably 
in such a way as to allow some movement of that end in the longitudinal 
direction of the tube 9. 
The purpose of the membrane 33 is to absorb shear forces generated by 
frictional resistance to rotation of the rollers 5 and thereby protect the 
tube 9 against longitudinal stretching. It is desirable that the membrane 
33 be sufficiently flexible not to hinder expansion of the tube 9 into the 
region between adjacent rollers 5 as shown in FIG. 3. It is also desirable 
that the membrane 33 be resistant to stretching in the longitudinal 
direction of the tube 9 when subjected to the forces generated by the 
presser plate 7 in the pump operative position. Suitable materials for the 
membrane 33 include plastics films such as polyester. Mylar having a 
thickness in the range of 0.1 millimeter to 0.2 millimeter has been found 
to be satisfactory. 
When the presser plate 7 is in the operative position, the membrane 33 is 
sandwiched between the rollers 5 and the operative surface 10 of the plate 
7. Thus, the membrane 33 supplies the forces required to overcome 
frictional resistance to rotation of the rollers 5, and thereby absorbs 
the associated shear forces. Since the membrane 33 does not stretch, all 
shear and tension forces are eliminated from the tube 9, which is held in 
its operative position between supports 14 and 15 such that virtually a 
zero tension force is applied to it between those supports. That not only 
eliminates flow pulsations induced by longitudinal stretching of the tube 
9, but also leads to longer tube life and more stable flow characteristics 
because of the elimination of shear force fatigue failures. 
The example pump 1' shown in FIG. 8 is similar to the pump described in 
FIGS. 1-7 (note that the same reference numerals, but with a prime, have 
been used in FIG. 8 to denote features that correspond in the two 
embodiments) except that instead of having a rotatable drum and rollers, 
it includes a drum in the form of a cam 3', arranged for rotation about an 
axis 4'. Cam 3' has a plurality of camming surfaces 5' extending as a 
continuous series around its periphery. The number of camming surfaces 5' 
on cam 3' may be chosen to optimise flow linearity, for example, the 
number of surfaces 5' depicted in the FIG. 8 embodiment, namely 15, may be 
increased to reduce the magnitude of each flow pulsation, although the 
frequency of the pulses will be increased. Each surface 5' is circular in 
profile, although it is within the scope of the invention to employ other 
than a circular profile for the camming surfaces. 
A membrane 33' is provided between the cam 3' and that part of the tube 9' 
which is subjected to the influence of the presser plate 7'. As in the 
FIGS. 1-7 embodiment, the purpose of the membrane 33' is to absorb shear 
forces generated by frictional resistance to rotation of the Cam 3' and 
thereby protect the tube 9' against longitudinal stretching. 
Suitable materials for the membrane 33' include plastics films such as 
polyester, particularly materials which are highly slippery so as to 
reduce frictional forces between the camming surfaces 5' and the membrane. 
An example of a particularly suitable material is Ultra High Molecular 
Weight Poly Ethylene (UHMWPE). Membrane 33' may be a laminate of Mylar and 
UMMWPE, and be positioned such that the ITHMWPE faces the camming surfaces 
5' in order to minimise the frictional force between the membrane and 
surfaces 5' as they slide along the membrane. Preferably the membrane is 
such as to consist entirely of the one material. 
It should be noted that as the tube 9 ages, if some extension of it does 
occur due to the cumulative effects of squashing pressure being applied by 
the presser plate and compressing elements, the tube supports 14 and 15 as 
shown in the figures are such as will allow for any such increase in tube 
length while still correctly holding the tube in its operating position. 
According to a preferred feature of the invention a cyclically varying 
holding force is applied to the presser plate 7 or 7' by an 
electro-mechanical actuator. This may be done by varying the electrical 
power supplied to the actuator from a control system (not shown), 
cyclically at a convenient frequency such that the maximum Current 
variation from a mean is that current which approaches that required to 
overcome the internal friction of the actuator. This is known as 
"dithering". 
A peristaltic pump as described is ideally suited for use in spectroscopic 
apparatus for delivering a sample to the nebulizer of such apparatus. An 
example arrangement of that kind is shown diagrammatically by FIG. 9. 
In the FIG. 9 arrangement, a nebulizer 49 is shown which forms part of 
spectroscopic apparatus 38. Nebulizer 49, a sample solution supply 50 and 
a diluent supply 51, are each separately connected to a junction 52. The 
connection between the junction 52 and the nebulizer 49 is by way of a 
feed passage 53 which is preferably of relatively small cross-sectional 
size for a reason hereinafter made clear. The sample solution supply 50 is 
connected to the junction 52 through a delivery passage 54 and a 
peristaltic pump 55 (as previously described in relation to FIGS. 1-7 or 
FIG. 8) having its output side connected to that passage. The diluent 
supply 51 is connected to the junction 52 through a passage 56 which 
preferably has a relatively large cross-sectional size by comparison with 
that of the feed passage 53. 
It is a feature of the arrangement shown that the pump 55 is controllable 
to deliver sample solution through the passage 54 at any of a variety of 
precisely controllable flow rates. 
Because of the relatively small size of the feed passage 53, it exhibits a 
high resistance to flow. The passage 56 on the other hand, exhibits 
relatively low resistance to flow. As a consequence of that difference, 
substantially all the pressure drop in the system illustrated occurs 
between the nebulizer 49 and the junction 52. A nebulizer of the kind used 
in spectrophotometers will usually generate a stable pressure reduction at 
the liquid intake port, and in that event the flow rate through the 
passage 53 will be substantially constant and stable. 
If the pump 55 is operated to deliver sample solution to the junction 52 at 
a flow rate slightly below the natural aspiration rate through the feed 
passage 56, the solution will enter the nebulizer 49 in almost undiluted 
form. Assuming that the solution is found to have a concentration above 
the desirable range, the speed of the pump 55 can be reduced to thereby 
reduce the flow rate through the delivery passage 54 and consequently 
cause increased diluent to be induced into the stream flowing through the 
feed passage 56. Thus, the pump speed can he adjusted to obtain a 
satisfactory dilution ratio in the stream entering the nebulizer 49. 
The sample concentration in the atomized body which is measured by the 
spectrometer, is thereby directly proportional to the rate of flow through 
the delivery passage 54, which is in turn proportional to the speed of 
operation of the pump 55. It is therefore possible to accurately and 
quickly adjust the dilution ratio as required. 
If operation of the pump 55 is stopped for any reason, such as to change 
the sample solution, the nebulizer 49 will continue to aspirate diluent 
from the supply 50. As a result, the nebulizer and associated spray 
chamber do not dry out as occurs in conventional systems, but are 
subjected to rinsing by the clean diluent. That maintains the nebulizer 
and spray chamber in ideal condition for the next sample measuring 
operation. 
Since the diluent will usually have zero absorbance, a system as described 
above can carry out an auto zero function during the time that diluent 
alone is passing through the nebulizer. That has the advantage of 
improving the system performance and simplifying user interaction. 
The system described can also be used for micro sampling. The pump 55 can 
be briefly operated to inject a small quantity of sample solution into the 
diluent stream passing through the feed passage 53. There is no change in 
the flow through the nebulizer during that micro sampling operation, and 
consequently the nebulizer characteristics remain stable. That is in 
contrast with conventional micro sampling involving brief introduction of 
the sampling tube into the sampling liquid while the nebulizer aspirates 
air immediately before and immediately after that introduction of the 
sampling tube. 
The use of a peristaltic pump, because of its flow through characteristics, 
means there are no separate fill or flush stages as would be necessary 
with a syringe pump and which slow down the operating sequence. 
Furthermore, a multi channel peristaltic pump could be used so as to 
simultaneously pump several liquids each of which has a fixed flow rate 
determined by the cross-sectional size of the respective channel (tube) 
through which it flows. By way of example, use of three tubes or channels 
for sample, acidifier and sodium borohydride respectively, makes it 
possible to use the system described for hydride operations. 
FIG. 10 shows a modified form of the system described in relation to FIG. 9 
in which there is added a second pump 57 connected to a supply 58 of 
standard solution and also connected to the junction 52 through a delivery 
passage 59 to a third part of the junction 52. The pump 57 is a variable 
speed peristaltic pump as described in relation to FIGS. 1-7 or FIG. 8 so 
that control of the relative speeds of the two pumps 55 and 57 enables an 
accurate ratio of sample to standard to be maintained. 
The modified system is intended for use in relation to the well known 
standard additions technique, which involves measuring the sample and then 
measuring the sample spiked with an additional known amount of analyte. It 
is possible to determine the concentration of analyte in the original 
(non-spiked) sample by comparing the two measurements. 
Standard additions as carried out in conventional systems has a number of 
deficiencies. By way of example, a linear regression line is usually drawn 
through the spiked and unspiked absorbance readings. That assumes the 
existence of a linear calibration curve, and yet it is known that AA 
calibration curves are not linear at high concentrations of analyte. 
Although several samples spiked to different degrees could he analysed and 
used to compute a non linear calibration curve, the amount of operator 
time and effort necessary to prepare and measure the samples would be 
significant and is therefore seldom done. 
Furthermore, spiking the sample changes the ratio of matrix to analyte 
concentration and therefore changes the effect of the matrix. In order to 
minimise that result, it is desirable that the spike only increase the 
total analyte concentration by a reasonable amount. At the same time, a 
small spike may yield poor precision in the measurement results because of 
the small difference between the spiked and unspiked readings. In 
conventional standard additions analysis however, both the unspiked and 
spiked samples need to be available at the time of the analysis and 
therefore the spiked sample must be prepared without knowing even the 
approximate concentration of the analyte in the sample. This means that 
the change in concentration introduced by the spiking could be very large 
(e.g., 10 or even 100 times for low concentration samples), or conversely 
very small for high concentration samples. In order to overcome that 
problem, it is necessary to first analyse the sample by conventional 
calibration to obtain an approximate analyte concentration, and use that 
information to determine the amount of spiking. Such an approach 
necessarily doubles the operator work load. 
In addition to the foregoing, standard additions using conventional AA 
instrumentation is extremely slow, operator intensive and prone to error. 
Whereas in the case of normal calibration the operator only prepares the 
sample and can analyse the sample in a few seconds, for standard additions 
the operator must prepare and analyse at least one additional solution 
(and often more than one) involving accurate weighing, measuring and 
mixing. The time per sample is increased from a few seconds to several 
minutes. 
A system of the general kind diagrammatically illustrated in FIG. 10 has 
the benefit of overcoming or at least minimising the aforementioned 
problems. That is particularly the case by use of a method as described 
below. 
A preferred method using the system of FIG. 10 involves the following 
steps: 
(a) measure the sample, 
(b) if the measured absorbance is above 50% of the linear range, dilute the 
sample, 
(c) measure the diluted sample, 
(d) add a standard to the sample so as to approximately double the 
absorbance, 
(e) measure the spiked sample, and 
(f) compare measurement (e) with measurement (a) or (c) as appropriate. 
In the event that dilution of the sample is not required, the measurement 
comparison will of course be between the measurement referred to under (a) 
above and the measurement referred to under (e). In the case of a diluted 
sample the comparison will be between the. measurement referred to in (c) 
above and the measurement referred to in (e). 
The method described above has the benefit that the operator is only 
required to present a single sample as normal calibration, and because all 
dilutions and additions are carried out in line it is possible to achieve 
a high throughput, 
It will be apparent from the foregoing description that the system 
according to the present invention provides improvements in both normal 
calibration and standard additions. Operator time is minimised without 
sacrifice of accuracy in the results of the analysis. 
It will be also evident from the foregoing that a pump according to the 
present invention overcomes substantial problems existing in prior 
peristaltic pumps, and in particular provides a peristaltic pump which is 
suitable for use in situations requiring a stable and accurate flow rate. 
Furthermore, a peristaltic pump as shown in FIG. 8 is of more simple 
construction, having fewer moving parts (thus providing a mechanism that 
is less prone to corrosion) and in which the need for very small 
manufacturing and assembling tolerances is reduced. 
Finally, it is to be understood that various alterations, modifications 
and/or additions may be introduced into the constructions and arrangements 
of parts previously described without departing from the spirit or ambit 
of the invention as defined in the appended claims.