The UV-Visible spectroscopy is one of the more classical techniques of spectroscopic analysis of a substance. It can be applied to all materials in the form of gas, liquid or solid; for this reason, specific considerations must be considered case by case. The analysis can be qualitative and quantitative.
Just because the technique lends itself to the analysis of all types of samples, care and time spent for the preparation of the sample may depend very much on the sample and the type of analysis desired. A quantitative analysis always requires a mass calibration to preliminarily point and a remarkable precision in the preparation. In this case the preparation represents 98% of the analysis. For a qualitative measure reflectance of a solid sample, virtually no preparation is necessary;interpretation of the data can be however very difficult, if not almost impossible.
Dispersive instruments UV-Visible generally cover the range 190-900 nm, and are best used; They may cost too much content. If instead there is a need to broaden the spectral range in the NIR, the cost considerably increases, but the range is usually 190-3000 nm. For the analysis of samples in reflectance is required as an integrating sphere accessory.measureThe UV-Visible and UV-Visible-NIR spectra are moderately difficult to interpret and poorly selective. For this, apart from some cases, the technique is not preferable for the recognition of unknown samples; infrared spectroscopy is definitely more suitable.The UV-Visible can be very useful for the quantification of substances, especially if dissolved in a liquid or in a gas. The assessment must be done on a case by case basis and in advance.
Infrared Spectroscopy (FTIR)
Infrared spectroscopy is the most used for the recognition of unknown substances and provides, in many cases, the possibility for quantitative analysis. It is applicable to almost any type of samples using appropriate accessories. It also allows the analysis of samples at the microscopic level or the size of the sample parts up to more or less 10 micron. The spectra can be obtained at any temperature and pressure with appropriate accessories. The analysis time is very short with the use of tools to Fourier transform (FTIR)); the greater the sample preparation time, especially in case of quantitative analysis. The on-line analysis of process is only possible in some cases.
Gas: there are cells for gas; the analysis is always both qualitative and quantitative.
Liquids: there are various methods of analysis, by deposition on a support, the calibrated cell of liquid, the analysis in reflectance; They should be evaluated depending on the needs.
Solids: the sampling method of a solid can be very different; a sample can be cast film and analyzed as such in transmittance or reflectance, or incorporated into transparent matrices, or, with appropriate accessories, analyzed in reflectance without any preparation.generally speaking, is needed to develop a certain manual skill to obtain high quality spectra; there are 'tricks' on different sample preparation which greatly improve the data and allow you to obtain quantitative analysis even where it seems impossible.The accessories related to the preparation is always necessary.
The choice is very wide but, just for this, should be carried out under the guidance of industry experts to meet the unique needs without unnecessary costs (it is useless to think of a FTIR microscope if it ever will use!). For many applications the ability to have a library of spectra as wide as possible in the field of interest is an important factor. The care of the equipment is crucial for its durability. It is recommended that a standard room temperature and humidity and a flushing with inert gas or dry air. A compact, simple, flexible, with resolution up to 1 cm-1 and a library of suitable spectra can still be a good starting point.
The measurement is generally composed of:
Sample preparation: represents from 20% to 99% of the quality of the result.
Measurement: usually 1% analysis (just a few mouse clicks and Know yourself the right software tool)
interpretation of data: from 20% to 99% of the correctness of the result.
Regarding the measurement, the most care should be stored in good maintenance of the instrumentation and in its periodic control.
Dedicated NIR spectrometers have specific applications but still allow qualitative and quantitative analysis routine even in difficult environments such as on-line process control using long probes even tens or hundreds of meters.
In this case the calibration is the basis of the measure, but it is previously necessary, the assessment of the technical requirements: not everything can be measured with the NIR.
Raman spectroscopy can provide a very high number of information comparable to those achievable with the infrared spectroscopy. Some key points:positives: The sample requires no preparation and is analyzed as such.
Raman spectra are rich in information
Raman spectra can be performed on-line and in difficult conditions, with long, remote probes even hundreds of meters. You can use portable spectrometers for the execution of measures "in situ."downsides: It is a technique not yet widely diffuse: there are very few specialists and some instruments still suffer from 'craftsmanship'.
The technique is not applicable in the presence of fluorescence (to be checked on their materials).
Virtually no preparation is needed. Consequently there is no need for any particular accessory.Instrumentation. The instrumentation is divided in conventional Raman and FT-Raman, which can also be used as an accessory of a FTIR. In general it is expensive and the choice is undoubtedly lower than the infrared spectrometers. There are not hygroscopic parts as for the FTIR instruments and hence is not required particular conditioning. The use of the equipment, however, requires well-trained staff.
With current CCD arrays the measure has become faster, as for the FTIR. Sample preparation take few minutes, but the interpretation of the spectra is a subject for specialists.
It is not based, like its predecessors, on interaction of electromagnetic radiation with matter, however, we speak of the mass spectra as a measure in a series of signals due to the masses of the analyzed components. It can be very useful in the identification of unknown substances and is a very sensitive technique for small amounts of substance. Therefore it lends itself to the development of trace detection with quantitative methods: methods are, however, well defined and the right method is crucial.
The treatment and the sample preparation are critical and may require long time.
You should always know how to treat a given sample for the analysis and in what conditions must be analyzed.
It varies depending on the required sensitivity. Usually the components to be analyzed are separated chromatographically.
Costs are important and using instrumentation requires a certain specialization, in addition to a perfect cure.
The measure may take a few hours; any change of conditions may take even longer times.
The interpretation of mass spectra requires a thorough knowledge of the art and the methods that are being used..
Dr. Maurizio Veronelli - Specialist in Polymer Science
RDLab137 - Milano
Last Revision: 03/03/2018