WO2014027172A1

WO2014027172A1 - Optical cell

Info

Publication number: WO2014027172A1
Application number: PCT/GB2013/000371
Authority: WO
Inventors: Keith Hulme
Original assignee: Starna Scientific Limited
Priority date: 2012-08-16
Filing date: 2013-08-16
Publication date: 2014-02-20
Also published as: GB2504976A; GB201214618D0

Abstract

A cell usable in a cell holder in a photometer, which cell is formed by an incident wall (1) and an exit wall (2) joined by two side walls (3, 4). Two further walls (6, 7) close the cells at each end. A dividing wall (5) is located transversely to the incident and exit walls (1, 2). The dividing wall (5) divides the chamber defined by incident, exit, side and further walls (1, 2, 3, 4, 6, 7) into two compartments which are isolated from one another. The incident and exit walls (1,2) are formed from a single piece of cell material.

Description

Optical Cell

The invention relates to an optical cell for use in a photometer such as a spectrophotometer or a fluorimeter.

Optical cells are used to hold samples for analysis in spectrophotometers and fluorimeters. To facilitate measurements and simplify the problems of alignment and calibration of the respective cells, cells are manufactured to have certain standardised dimensions so that they may be received in a standard cell holder. The use of a standard cell holder greatly simplifies the use of the photometer.

In particular in UV spectroscopy, very, very small differences in thickness, wedge, or raw material make a big difference to the transmission characteristics of a cell. This in turn will have an effect on the uncertainty in any particular measurement, and the effect is especially significant at the limits of the spectral range.

Measurement uncertainty is documented in an uncertainty budget, which is defined as a statement of measurement uncertainty, of the components of that measurement uncertainty, and of their calculation and combination. The uncertainty budget should include the measurement model, estimates and measurement uncertainties associated with the quantities in the measurement model, co-variances, type of applied probability density functions, degrees of freedom, type of evaluation of measurement uncertainty and any coverage factor.

With the known cells in the far UV range, in particular at wavelengths below 200nm, which are typically made of quartz, the magnitude of the errors increase significantly and minor impurities in the quartz affect the results as the transmission properties of the material are a material influence on the results. The uncertainty in a measurement typically increases by a factor of two at these wavelengths and this increase in uncertainty takes the measurement outside of an acceptable uncertainty budget. In metrology, the current standards in uncertainty are governed by BIPM document JCGM 100:2008. DEI 9735599 discloses a differential cuvette for a non-dispersive infrared spectrophotometer, which cuvette comprises a hermetically sealed reference chamber for containing a reference gas and a measuring chamber having an inlet and an outlet.

DD242687 shows a cuvette consisting of a base and a lid, two parallel transparent longitudinal walls and two side walls which create a chamber. The chamber is separated into individual compartments by opaque separating walls mounted perpendicular to the longitudinal walls and positioned in grooves provided in the base, the lid, the longitudinal walls or a combination thereof.

C 101825564 shows an optical detection system and a method for use thereof. According to this document, an optical device is separated into a sample solution chamber and a reference solution chamber. An inlet is provided on each separate chamber.

GB 1267589 discloses a differential cell comprising two diametrically opposed windows arranged to allow light beams to pass through and at least one transparent partition wall with stepped sides positioned between the windows.

Although the documents described above provide cuvettes or cells having at least two compartments, none of the known cells above eliminates variations in transmission characteristics for a standard non-divided cell or for a cell having at least two compartments.

The invention therefore seeks to provide a cell for use in a photometer with improved usability, in particular at the limits of the spectral range, especially in the far UV spectrum.

According to the invention there is provided a cell adapted to be used in a cell holder in a photometer comprising an incident wall and an exit wall, which incident and exit walls are joined by side walls and respective end walls to form a chamber, wherein the chamber is divided by a further wall located intermediate to the end walls, which further wall divides the chamber into two compartments. Preferably, at least one end wall is provided with an opening to enable a compartment to be filled. Preferably, at least one compartment is filled with a reference material. Preferably, the distance between the outer faces of the incident and exit walls is 12.5mm.

By making the cell of two compartments out of one piece, the cell will advantageously virtually eliminate any variations in transmission characteristics for the cell, which enhances the accuracy of the calibration of the cell. As the presence of impurities in the cell material has a significant effect on results, the use of the cell of the invention eliminates the error that would otherwise arise by use of a reference material in a cell made of a different batch of quartz or cell material. It therefore eliminates a variable that in known cells would take the measurements outside an acceptable uncertainty budget. It also eliminates variations in path length that might occur using separate cells.

An exemplary embodiment of the invention will now be described in greater detail with reference to the drawing in which:

Fig. 1 shows a side view of the cell of the invention.

Figure 1 shows a side view of the cell. The cell comprises incident 1 and exit 2 walls, each of which is formed from a single piece of the cell material, which may be quartz or glass by way of example. The cell is formed by side walls 3,4. A dividing wall 5 is located transversely to the incident and exit walls so as to divide the interior of the cell into two compartments, which compartments are thus isolated from one another.

A respective further wall 6, 7 located transversely to the incident and exit walls 1 ,2 and intermediate to the dividing wall 5 and the end of the cell, forms the final closing wall of the respective compartment. The further walls 6,7 are each provided with a small opening, which opening is connected to a respective extension tube 8,9. The extension tube is typically flame sealed to the further wall 6,7.

The extension tube can be sealed with a cap if it is desired to have the compartment re-fillable or alternatively, the tube can be sealed permanently if the compartment is to be filled with a reference material to ensure the characteristics of the reference material. Each end of the cell is closed by a respective cap.

One primary application for the cell of the invention is for sealed solutions and another advantage of having two solutions in one unit is for instance where there is a blank and one sample solution they cannot get the blanks mixed up with blanks from other sets which currently happens where customers have multiple sets. Alternatively the cell can be provided with a screw cap at either or both ends so that a user can introduce their own solutions. The cell would be suitable for use in applications from the far UV through to near infra red, typically from 190nm to 2300nm. The cell of the invention permits measurements to be made within an acceptable uncertainty budget with wavelengths below 200nm

The uncertainty of any given measurement process is constructed by consideration of all the contributing errors to produce the uncertainty budget, which is effectively a +/- tolerance of a mean value. This budget is then multiplied by a coverage factor k (usually k =2 ) which, (if you assume a normal statistical distribution of values) equates to approximately 95 % confidence that the values when measured will fall within the quoted tolerance. This is called the expanded uncertainty budget, and it is this value (together with the confidence interval) that is typically quoted against any measurement value.

If the cell is to be used in a standard cell holder, the external dimension width of the cell will be 12.5mm and the length of the cell is 64.5mm. Each of the walls will typically be 1.25mm thick.

There are some limitations on the instruments with which it can be used because of the beam height of the instruments. The cell of the invention can not only be used for absorption standards in normal UV spectroscopy but the possibility also extends to use for fluorescent standard solutions for fluorimeter applications.

Claims

1. A cell adapted to be used in a cell holder in a photometer comprising an incident wall (1) and an exit wall (2), which incident and exit walls (, 2) are joined by side walls (3, 4) and respective end walls (6, 7) to form a chamber, wherein the chamber is divided by a further wall (5) located intermediate to the end walls (6, 7), which further wall (5) divides the chamber into two compartments

characterised in that the incident wall (1) and exit wall (2) are respectively formed from a single piece of material and in that the two compartments are isolated from one another .

2. A cell according to Claim 1, wherein at least one end wall (6, 7) is provided with an opening to enable at least one of the two compartments to be filled.

3. A cell according to Claim 1 or Claim 2, wherein at least one of the two compartments is filled with a reference material.

4. A cell according to any one of Claims 1 to 3, wherein the distance between an outer face of the incident wall (1) and an outer face of the exit wall (2) is 12.5mm.