Sample handling system

Details Sample handling system Sample handling system

2001-06-29

2004-10-05

Ludlow, Jan (Department: 1743)

Chemical apparatus and process disinfecting, deodorizing, preser

Analyzer, structured indicator, or manipulative laboratory...

Means for analyzing gas sample

C422S082050, C422S105000, C422S105000, C436S164000

Reexamination Certificate

active

06800251

ABSTRACT:

This invention relates to a sample handling system. More particularly it relates to a sample handling system wherein the sample is a fluid containing particles.
Large scale sample handling systems which are used in laser scattering particle characterisation apparatus typically contain 1 liter of dispersant in which a large amount of sample, typically 2.5-5 grams, is dispersed resulting in millions of particles being carried. The resulting suspension is then continually re-circulated from a storage reservoir to a measurement cell to allow measurement. As a result of the large number of particles the loss of a small percentage of particles from the measured sample by trapping at crevices, seals and by sedimentation does not significantly bias the results of the measurements.
However, as the volume of dispersant in the system decreases so does the number of particles that can be introduced into the system and the impact upon the particle characterisation measurements of losing even a small percentage of the particles is highly significant, as it is imperative that the portion of the sample presented to the laser beam must be representative of the sample as a whole. This is particularly important in that mechanisms for particle loss are size sensitive thus leading to a skewing of the measured particle size distribution.
A drive to miniaturise sample handling systems has arisen, principally from pharmaceutical drug discovery trials where the drug of interest may cost up to £100 k per gram. Therefore, only a small sample will be used in characterisation experiments and as high a recovery rate of the sample as possible is desirable.
Another reason for miniaturisation is the use of exotic, expensive and possibly noxious dispersants, such as dimethyl sulphoxide (DMSO) or tetrahydrofuran (THF), which must be recovered after use. The significance of the dispersant is apparent when considering the “wash down” of the system. The system must be flushed up to 3 times after use in order to prevent cross contamination between measurement sets. Thus, for a 1 liter system up to 4 liters of dispersant must be used for each measurement set. It is therefore desirable to improve upon the basic features of large-scale sample handling systems (a schematic of which is shown in
FIG. 1
) whilst gaining additional benefits from the miniaturisation of the system.
Current small-scale sample handling systems have a total volume of approximately 150 ml per fill volume and use 0.2 to 0.5 grams of particles per sample. These sample handling systems have a number of further biases associated with them that can skew the measurement results including limitations upon the density of particles that can be measured as it requires a high pump power to keep heavy particles moving in a uniform random suspension. As particle size increases the volume increases as the cube of the particle diameter whereas the viscous drag forces, which maintain the particle in suspension, vary as the square of the diameter. Therefore, the particle size density drag force relationship is very important, for example 100 &mgr;m silicon particles will typically settle from suspension in water in a second, thereby severely limiting the available data acquisition period.
A number of solutions to this problem have been applied to small volume systems including where a syringe is used to inject the sample directly into the measurement cell and measurements are made before the particles can settle out. Another solution to the problem of settling out is the use of a rotating stirrer bead (flea) on the base of a narrow sample cuvette (as shown in FIG.
2
). This has the problem that only the region of the sample near to the flea is adequately stirred with dead volumes away from the flea receiving only poor agitation. There is also only a very weak agitation in the vertical direction with such motion only arising from the interactions of the fluid with the walls of the cuvette. This arrangement can also result in stratification of the particles within the cuvette with coarse materials being more concentrated in the lower half of the cell, for example. Additionally the flea is optimised to operate in a circular beaker not a cuvette of narrow rectangular cross-section. Therefore an excessively long light path would need to be used if the cuvette were to be optimised for the use of a flea.
One proposed alternative method of maintaining the suspension of particles is the use of a horizontally rotating paddle in a cuvette (as shown in FIG.
3
). This system does have disadvantages associated with it including regions where there is poor agitation, typically in the corners of the cell where deposits of coarse particles can form. A further problem is that the particles follow preferred trajectories that are dictated by their particle size. This can result in the formation of strata within the cell, the strata containing different particle size populations by virtue of the preferred trajectories and thus skewing measurement results. Also at higher speeds of rotation of the paddle the system acts as a centrifuge throwing larger particles out to the sides of the cell and depleting those particles from the central volume, where measurements are usually taken, which again skews measurement results.
The use of a manual pre-stir and measurement after particles have settled into Brownian motion and sedimentation is known, however, this is only feasible with very fine particles as the time taken for a measurement is typically longer than the time taken for the particles to settle in such a system.
Many systems use an ultrasonic transducer in order to disperse the particles within the suspension and also a degree of agitation of the particles occurs due to the sonication. There are two usual forms of ultrasonic transducer, the first being a limpet style of transducer which is attached to the outer surface of the tank wherein the suspension is stored, the second type of ultrasonic transducer is an in-line probe which is in effect immersed in the solution.
The use of a limpet style transducer attached to the sample tank in a large volume system typically does not give a high degree of coupling of the sonication energy into the sample. This arrangement is inefficient as only a small amount of displacement is caused for a large energy input.
The in-line probe type of transducer (see for example
FIG. 4
) yields excellent coupling of the sonication energy into the sample. However, there are regions around the probe and its entry point into the flow path, which will not be well flushed with liquid and could present a potential source of particle trapping and therefore biasing of the system. There is also the problem of large potential power losses at the seal between the internal and external parts of the probe.
Also as the transducer only covers a small area of the tank in a large volume system it is possible that a large proportion of the sample may bypass the ultrasonic transducer and thereby avoid being agitated.
The majority of sample handling systems have a tank in which a large volume of sample, i.e. particulate matter and dispersant, are stored. As it is difficult to achieve uniform agitation of particles with any appreciable density or size variation there is a tendency to size separation within the tank. As a result of this separation it is difficult to find a level at which the outlet to the pump is free of any bias. The sample return from the measurement cell can also significantly bias a tank system, as it is possible that coarse material “short circuit” the tank and pass straight back into the pump inlet thus the material passing through the measurement cell will be unrepresentative of the true bulk nature of the sample and appear overly coarse.
The sample drain also presents a number of problems as current drains utilise the fact that the pump cavity floor is typically the lowest part of the flow path and thus the pump cavity floor is made so that it can drop away and the sample can drain from this point. Provided that the actuator closing the drain

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