Infrared spectrophotometric method

Abstract: An optical cell and detector assembly for a spectrophotometric analysing apparatus which comprises a sample cell having walls (12, 14) of optically transparent material defining a sample cavity (20) having closely adjacent the sample cavity (20) a radiation detector (30) and adjacent an opposite wall a lens (36) whereby to focus radiation passing through the cell on to the detector. The apparatus may also comprise optical means (42) for focussing the beam onto the sample cell, chopper means (44) for periodically obscuring the beam, and filter means (48) for selecting one or more wavelengths from the beam. The sample under test is an aqueous emulsion and the filters are chosen to determine the concentration of the non aqueous components of the emulsion by measuring the water displaced by the components.

Description: This invention relates to an infrared spectrophotometric method. Infrared spectrophotometric analyzers are employed for the analysis of emulsions and suspensions, for example in the dairy industry for analysing the fat, protein and lactose content in milk and milk products. An example of such apparatus is described in US patent number 4310763. In particular, Figure 1 of this patent illustrates the optical lay-out of one form of such apparatus known as the "Multispec" instrument. In this apparatus two beams of infrared light, each of a different wavelength are led alternately through an optical cell containing the sample under test to an ellipsoidal mirror from where it is focused on to an infrared detector. The signal from the detector can be analyzed in accordance with well known technology in this field to give a reliable estimate of the component being analyzed in the sample. The above instrument, using the so-called "double-beam-in-wavelength" system analyses milk for fat, protein and lactose and requires filters of suitable wavelength for each component together with reference filters for each sample wavelength. Furthermore, a null-balance system is usually employed involving a relatively expensive attenuator comb and associated servo mechanism and electronics. This reference also discloses the use of a water filter and indicates that this can be used to determine solids or solids non-fat in conjunction with the results obtained from standard methods (e.g. gravimetric and hydrometer). Various factors affect the performance of such apparatus, for example the presence of water vapour in the optical path of the apparatus causes interference by selectively absorbing part of the infrared signal and thus, as well as attempting to reduce the water vapour within the apparatus, for reasons of conserving energy, optical path lengths should be kept to a minimum. Furthermore the presence of large globules in the emulsion or suspension under test scatters infrared radiation, thus causing errors in the optical measurements of the components in the mild sample. As the amount of radiation scatter is proportional to the globules. The necessity for homogenisation of, for example, milk samples greatly increases the cost of a suitable analysing instrument. Ellipsoidal mirrors are relatively expensive pieces of equipment and the use of these tends to increase the cost of the instrument. There is a great need for analysing apparatus which is inexpensive and yet has sufficient accuracy to be useful in dairy industries. US-A-4207469 and US-A-3839633 both describe apparatus for infrared spectrophotometric analysis of aqueous emulsions such as milk employing a number of filters, including one at 4.8 microns which is employed with both the sample and a reference of water to give an indication of the total solids in the sample. The invention seeks to provide a method using economic apparatus for analysing samples which overcomes or reduces the above disadvantages. Our co-pending EP application number 85308178.4 (EP-A-182564) (from which this application is divided), provides an optical cell and detector assembly for a spectrophotometric analysing apparatus which comprises a sample cell having walls of optically transparent material defining a sample cavity having closely adjacent an opposite wall a lens whereby to focus radiation passing through the cell on the detector. The present invention provides a method of infrared spectrophotometric analysis of an aqueous emulsion which comprises providing an apparatus including an infrared source and a detector therefor, optical means for focusing a beam onto a sample cell, chopper means for periodically obscuring the beam, and filter means for selecting one or more wavelengths from the beam, characterised in that the concentration of the non aqueous components of the emulsion is determined by making optical measurements on a sample of the emulsion at a wavelength of 8.64 microns and obtaining the difference between this measurement and a reference measurement on pure water. The present invention may be used with the optical cell detector arrangement of the above mentioned EP-A-182564. Placing the detector closely adjacent, or indeed abutting the sample cell reduces the path length of the radiation, allows the ellipsoidal mirror formerly used to be dispensed with, and, since the temperature of the cell is controlled (for example as described in US 4310736), enables the detector temperature to be monitored and controlled also. In particular, the effect of scattering owing to large globules within the sample under test is very much reduced since even scattered light is likely to be collected by a detector so close to the sample and thus homogenisation of the sample under test may be reduced or eliminated entirely. The detector may even replace part or all of one cell wall. The invention is particularly applicable to the analysis of milk and dairy products. The most important practical measures required here are the amounts of fat and of solids, non-fat (SNF) in the sample. In the apparatus of the invention, by using a "fat" filter such as 5.7 micron filter and a filter at 8.64 microns, these parameters can be simply and accurately determined. One embodiment, includes a microcomputer, small display and keypad, an optics unit, control electronics and a pump/homogeniser. The optics unit, is straightforward, having an infrared source and detector at either end of each light path, with a chopper, filter and cell or cells in between. The chopper has an equal area of "windows" and reflective areas; the speed of said chopper is variable and is controlled by the microprocessor. The filter is located between the chopper and the cell. Different filters may be employed in the apparatus by means of a rotating wheel, the filters being mounted thereon, which rotates so as to place the desired filter into position. The cell is plumbed into the pump/homogeniser. Within the optic unit infrared radiation is emitted from an infrared source. This radiation is incident upon a convex lens which focuses the beam onto a chopper which is driven by a motor. The chopper, with its equal number of reflective areas and windows, alternately transmits then blocks the beam at a rate dependant upon the chopping frequency. The transmitted beam, after having traversed a suitable filter, is then focused by a second convex lens onto a sample cell; the radiation transmitted thereby being incident upon an infrared detector. A resulting A.C. signal is obtained which is indicative of the absorption which has taken place within the cell. The spectrometer operates by changing filters and taking readings with, say, a sample of milk in the cell and comparing these with readings taken, say, with a sample of water in the cell. The readings are a simple ratio of the transmission (absorbency) at one, two or more selected wavelengths of the sample and reference. At one time ratio methods were too inaccurate owing to drift in the sensitivity of the detectors, and null-balance methods have been favoured. However, recent improvements in detector technology enable the ratio method to be used herein with sufficient accuracy and great cost saving. For each sample, the cell is filled by the pump/homogeniser, each filter is moved into position in turn, and measurements are taken. From these measurements the concentrations of components of interest in the sample, for example fat and SNF, or fat, protein and lactose, in milk, are then calculated and displayed. In order to perform this calculation it is necessary to have some reference measurement. For this purpose a distilled water sample is put through from time to time. Since there is a microcomputer with on board clock/calendar in this embodiment, the instrument may itself request a reference sample if one has not been forthcoming for some time. The frequency with which the reference sample is required is dependent upon the stability of temperature and humidity within the instrument. However, before any measurements are made, it is necessary to convert the optical signal arriving at the detector into computable form. This is done in four stages: a) The detector produces a small electronic signal; b) A pre-amplifier amplifies this to a convenient level; c) A programmable gain amplifier under the control of the computer compensates for different signal levels from different filters; and d) A 12 bit analogue to digital converter converts the signal to digital form at a rate in excess of 1000 samples per second. Conventionally a tuned filter and synchronous rectifier (lock-in amplifier) is used, coupled with a low pass filter, and the resultant DC signal is sampled by the computer. By sampling the AC signal and processing it in the computer there are fewer electronic components which clearly means increased reliability. The method also leads to a lower noise level (there are fewer noise generators), lower cost, and an ability to vary the chopping frequency with no hardware changes. The signal is sampled for some time, and between one and two thousand data points are stored. These data are then filtered, using a finite impulse response digital filter with a narrow pass band at the chopping frequency, and the amplitude of the resulting signal is then proportional to the optical signal magnitude. Alternatively a fast Fourier transform may be done and the amplitude at the chopping frequency computed directly. Although fast fourier transforms are time consuming, in this case we require that only one point in the frequency domain is computed, and in certain circumstances this method may be faster than filtering and estimating the amplitude. In either case, the method of ascertaining the optical signal magnitude is by digital signal processing carried out by the computer. In a typical method of operation, for example in the analysis of milk samples, the cell is filled with milk readings taken at 5.7 microns and at 8.64 microns. Similar measurements will have already been made with distilled water in the cell. The difference between the readings for milk and water at 5.7 microns gives a measure of the fat content of the milk sample. The difference between the milk and water readings at 8.64 microns gives a measure of the total solids in the milk. Consequently the difference between these two readings, i.e. total solids and fat, gives a value of the "solids non-fat" of the milk. We have found that 8.64 microns gives good results, with a standard deviation of less than 0.04%. The fat and the SNF levels are by far the most important measures needed in the analysis of milk. The microprocessor within the embodiment is programmed to give a direct read-out of these two levels without the operator having to perform any calculations. One embodiment of the invention has only two lenses and no ellipsoidal or concave mirrors in its optical system. Furthermore, owing to the cell and detector arrangement, a homogeniser may be dispensed with. Thus the cost of the apparatus of this embodiment is only a fraction of that of conventional analysers. The spectrometer operates by changing filters and taking readings with, say, a sample of milk in the sample cell and comparing these with similar readings with, say, a reference of water in the sample cell. A microprocessor may be incorporated as before to compare and integrate the readings automatically without greatly increasing the cost. The readings are a simple ratio of the transmission (absorbance) at one, two or more selected wavelengths of the sample and reference. An advantageous form of this apparatus comprises only one cell. This may firstly be filled with distilled water and measurements taken at, say, 8.64 and 5.7 microns. The cell is then filled with a milk sample and the measurement repeated. Since the measurements on water may be stored by the instrument there is no need to repeat the water measurements with each succeeding milk sample. Thus a single beam at a single wavelength (at any on time) is passed through a single cell. This leads to savings in cost of the optical system needed as compared with double-beam instruments - without incurring penalties in speed of operation owing to the stored water reference information. Instruments embodying either or both aspects of the invention may be made to any desired level of sophistication from very basic (and hence inexpensive), e.g. for use in developing countries, to multi-functional employing, for example, filters for protein and lactose as well as fat and having powerful computing facilities built in. The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a partial sectional view of a cell and a detector assembly in accordance with EP-A-182564. Figure 2 is a simplified view corresponding to figure 1 and illustrating the optical path; and Figure 3 is a spectrometer incorporating the cell and detector. Referring to the drawings, an assembly generally designated 10 comprises a pair of cell walls 12, 14 of optically transparent material, which, for infrared analysis, could be silver iodide, calcium flouride, germanium, zinc-sulphide, or potassium bromide. The cell walls 12, 14 are held within a body 16 by means of bolts 18 in a conventional manner, and defined between them a sample cavity 20. The cavity 20 connects by pasageways 22 and 24 to inlet and exit ports 26, 28 for fluid samples. On one wall 12 of the cell there is mounted a radiation detector 30 having a radiation transparent window 32 and a detecting element 34. Mounted adjacent to the opposite wall 14 of the cell is a lens 36 made of a similarly optical transparent material to that of the cell walls 14, 12. Radiation 38 of the appropriate wave length is, as described more fully hereinafter, directed on to the lens 36 from the remainder of the instrument. The radiation 38 is focussed by the lens 36 through the sample cell and detector window 32 on to the detector element 34. The detector 30 is preferably of a type having a relatively large window 32, for example 10mm diameter or even larger. This ensures that not only can the radiation 38 be focussed within the receiving area of the detecting element 34 but the bulk of any light scattered within the cell will also be collected thus improving the sensitivity of the apparatus. The ellipsoidal mirror is eliminated, and the associated optical path length also. Furthermore, since the body 16 is temperature controlled so as to control the temperature of the cell (as described in USA4317763) the detector 30, being incorporated with the cell, is also temperature controlled so that drift owing to temperature variations of the detector 30 is reduced or eliminated. In operation a reference sample is put in the cell and absorbs varying amounts of radiation at the different wavelengths selected by the filters, and these levels are recorded. Subsequent milk samples absorb more strongly than the water, and so the optical signal at the detector 30, which is an A.C. signal caused by the operation of the chopper, is of a different magnitude dependant upon concentration of components and the filters selected. By computation, estimates may be obtained, from the data so recorded, of the concentration of components of interest within the milk. If desired other filters may be used to give information about protein and lactose. Cross-corrections for interferring components can be carried out using equations known per se and, where a microprocessor is used, these corrections can be programmed in. Furthermore, the simplification embodied in the instrument enables it to be manufactured with very small dimensions. This in turn allows it to be incorporated in plant or equipment for process control. In another embodiment the detector may actually be incorporated into the cell, replacing some or all of the cell exit wall 12. Referring now to figure 3, the remainder of the optical system for a low-cost spectrometer is illustrated. An infrared source 40 is focussed by means of a lens (or lens group) 42 onto a chopper 44 which alternatively allows the beam to pass and blocks it off at a frequency determined by the speed of its drive motor 46. If the detector 30 is a photon detector a high chopper frequency (e.g. 200 c.p.s.) is preferred; but for slower operating detectors very much lower frequencies may be employed (e.g. 10-25 c.p.s.). The detector electronics are tuned to and governed by the chopper speed as known in the art. A replaceable filter 48 is placed between the chopper 44 and the lens 36. In a typical method of operation, in the analysis of milk samples, the cell is firstly filled with water as a reference and the filter 48 is chosen at the Fat "A" waveband, 5.7 microns. Measurements are taken with the beam 38 both on and blocked off (for a "background" level). The filter 48 is then changed to one at 8.64 microns, and the readings repeated. This sequence is then repeated with a sample of milk in the cell. The difference between the readings for milk and water at 5.7 microns (corrected for background) give a measure of the fat content of the milk sample. The difference between the milk and water readings at 8.64 microns (corrected for background) give a measure of the total solids in the milk. Consequently the difference between total solids and fat gives the "solids, non-fat" ("SNF") of the milk. The fat and SNF levels are by far the most important measures needed in the analysis of milk. A microprocessor within the apparatus can be programmed to give a direct read-out of these two levels without the operator having to perform any calculations. Thus sufficiently accurate measurements of the most important parameters for milk analysis can be obtained with an extremely inexpensive instrument. While the invention has been described in relation to instruments useful for analysing milk and dairy products, and the filters chosen have been of wavelengths appropriate to this end-use, it is not so-limited. Other aqueous emulsions can be analyzed and filters will be chosen accordingly. Even in the analysis of milk the particular filter values described are not the only ones which can be used; for example, other filter wavelengths useful for the analysis of milk fat include 3.46 microns and 6.84 microns. The invention described hereabove would be a valuable asset for the dairy industry as it provides a quick and reliable method of ascertaining the quality of milk samples at a relatively inexpensive price.

Claims: A method of infrared spectrophotometric analysis of an aqueous emulsion which comprises providing an apparatus including an infrared source (40) and a detector (30) therefor, optical means (42,36) for focusing a beam onto a sample cell, chopper means (44) for periodically obscuring the beam, and filter means (48) for selecting one or more wavelengths from the beam, characterised in that the concentration of the non aqueous components of the emulsion is determined by making optical meansurements on a sample of the emulsion at a wavelength of 8.64 microns and obtaining the difference between this measurement and a reference measurement on pure water. A method as claimed in claim 1 comprising choosing at least two filters, the second of which selects a wavelength strongly absorbed by one of the components of the sample. A method as claimed in claim 2 wherein a single cell is filled with water for reference measurements then filled with sample for sample measurements. A method as claimed in claim 3 wherein the reference measurements are stored and compared with each successive sample measurement by processor means. A method as claimed in any of claims 1 - 4 wherein a filter selecting a wavelength of 8.64 microns and a filter selecting a wavelength of 5.7 microns are chosen. A method as claimed in any of claims 1 to 5 wherein the AC output from the detector is amplified, different signal levels from different filters are compensated for, and the signal is converted to digital form for computing by an analogue-to digital convertor.

Inventors: SHIELDS JOHN, SHIELDS, JOHN
Applicants: SHIELDS INSTR LTD, SHIELDS INSTRUMENTS LIMITED
Application year: 1985
Main classification: G01N21
IPC codes: G01N21, G01N33