fig1 shows the calibration process according to a first aspect of the invention . firstly , a number of prepreg samples are prepared at step 10 . these prepreg samples are stored for a set time under set environmental conditions , as indicated by step 12 . the samples are stored for a spread of different times and under different environmental conditions in order to provide a wide sample base for the calibration process . the range of conditions and number of samples required to give appropriate accuracy to the method is decided using standard statistical design of experiments techniques , as will be understood and commonly used by the skilled person . the design of experiments calculations may be performed using computer programs such as “ jmp ” available from sas ( www . jmp . com — as available in july 2014 ) and “ r - project ”, which is an open - source project ( www . r - project . org as available in july 2014 ). this results in a collection of variously aged prepreg samples 14 . each of these samples is then examined by an infrared spectrometer 16 , and also chemically tested for reactivity and tack tested 18 . therefore , for each sample , an infrared spectrum is obtained , together with a reactivity and tack measurement . these measurement results are stored in a database , and statistical analysis is performed to correlate infrared spectra with the physical properties of the sample , as shown in step 20 . the analysis of the samples includes classifying samples as either “ not acceptable ”, “ just acceptable ”, or “ acceptable ” depending on the measured physical properties and whether the sample is still suitable for use or whether it should be scrapped , as indicated by step 22 . in this case , “ not acceptable ” indicates a material is no longer suitable for use ; “ just acceptable ” indicates that a material is currently suitable for use but will no longer be suitable for use in the near future , or that the material is suitable for use , but with some loss of functionality ; and “ acceptable ” indicates that a material is suitable for use . the output may be in the form of different lights being illuminated on the testing device , or a screen associated with the testing device indicating the results , either as words and / or colours . as the final step 24 in the calibration process , the infrared spectrometer is programmed with the results data such that future analysis of measurement data may be performed automatically , as described further with respect to fig2 . fig2 shows a method according to a second embodiment of the invention . an infrared spectrometer , calibrated as described with reference to fig1 , is used to take a measurement of a sample of reactive material , as shown at step 26 . the measurement spectra is then analysed by the spectrometer at step 28 , where a comparison is made to the stored spectra results . the analysis of the spectra results in the determination of which category the reactive material falls into , and the spectrometer outputs a user readable indication of the category at step 30 . once an infrared spectrometer has been calibrated correctly , infrared spectrometers with the same specifications , for example the same make and model of infrared spectrometer , may be programmed with the appropriate calibration data without requiring the actual calibration process to be repeated . fig3 shows a spectrometer according to a third aspect of the invention . the spectrometer 32 is a handheld spectrometer comprising an infrared measurement device 34 , a control unit 36 , and a memory unit 38 , and a display 40 . the infrared measurement device is of common construction as would be understood by a skilled person . an example infrared measurement device is the hand - held ir spectrometer , 4100 exoscan series ftir ™, available from agilent technologies , santa clara , usa . the memory unit 38 includes a database of correlations between infrared spectra and the measurement results obtained during the calibration process as described with reference to fig1 . in order to take a measurement , a user holds the infrared measurement device up to a material and activates the device . the measurement results are analysed by the control unit 36 and compared to the database of results . depending on the results of the analysis , the display 40 indicates what category the material falls into , displaying the term “ not acceptable ”, “ just acceptable ”, or “ acceptable ”. in alternative embodiments the category may be indicated by a colour being displayed , for example a selection of red , amber , or green . such a visual reference will be easily understood by an operator and may make the spectrometer easier to use . whilst the present invention has been described and illustrated with reference to particular embodiments , it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein . by way of example only , certain possible variations will now be described . the method may be applied to assess thermal , ultra violet , and / or chemical damage to chemically reactive materials . the method may also be used to test the chemical characteristics of a material compared to a specific expected performance of that material , for example mechanical performance . where in the foregoing description , integers or elements are mentioned which have known , obvious or foreseeable equivalents , then such equivalents are herein incorporated as if individually set forth . reference should be made to the claims for determining the true scope of the present invention , which should be construed so as to encompass any such equivalents . it will also be appreciated by the reader that integers or features of the invention that are described as preferable , advantageous , convenient or the like are optional and do not limit the scope of the independent claims . moreover , it is to be understood that such optional integers or features , whilst of possible benefit in some embodiments of the invention , may not be desirable , and may therefore be absent , in other embodiments . while at least one exemplary embodiment of the present invention ( s ) is disclosed herein , it should be understood that modifications , substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure . this disclosure is intended to cover any adaptations or variations of the exemplary embodiment ( s ). in addition , in this disclosure , the terms “ comprise ” or “ comprising ” do not exclude other elements or steps , the terms “ a ” or “ one ” do not exclude a plural number , and the term “ or ” means either or both . furthermore , characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise .