Different variations of Electromagnetic radiation are used in analytics. These methods are often gathered together under the term “Spectroscopy”. They do all have something in common though. It is the change in the ray after its having struck the sample that is used for the analysis.
The Electromagnetic Spectrum
The “Spectrum” is the term given to the entire range of electromagnetic waves with their various wavelengths. For historical reasons the spectrum is arbitrarily subdivided into regions. These usually extend across a very broad range of wavelengths. If one arranges the waves according to increasing frequency or decreasing wavelength, the spectrum begins at the low frequency or the many thousand kilometer long wavelength end, respectively. Next to these are the radio waves with longwave at 10 km and 30 kHz and above. Next to the ultra-short waves (FM), which extend only to 10 m and 300 MHz, are microwaves extending to just 1 mm and 30 GHz. After the terahertz range comes infrared or thermal radiation (reaching 3,0 μm and 385 THz) and then, visible to the human eye, light waves: Red (640 - 780 nm, 384 - 468 THz), Orange (600 - 640 nm, 468 - 500 THz), Yellow (570 -600 nm, 500 - 526 THz), Green (490 – 570 nm, 526 - 612 THz), Blue (430 - 490 nm, 612 - 697 THz) and Violet (380 - 430 nm, 697 - 789 THz). Next we have UV rays extending to 50 nm and 300 PHZ, x-rays to 1 nm and 30 EHz and then Gamma rays to 10 pm and 30 EHZ.
The regions of frequency used in analytics extend from terahertz spectroscopy to Mössbauer spectroscopy (zetahertz region, gamma rays).After infrared spectroscopy via Raman and NIR spectroscopy come colorimetry and photometry in the visible regions of light in the spectrum, from 190nm to 1100nm (UV – VIS to Near Infra-Red). This is the area of focus of the “Photometry Compendium”, a cross media project between “GIT Laborfachzeitschrift” and “WTW” (Wissenschaftlich -Technische Werkstätten), a producer of photometry devices. It is our mutual goal to bring together all information that is needed to apply this technology competently.
Similarly to other spectroscopic methods, photometry uses the decrease in intensity of the original light compared to the intensity after its passage through a sample, to calculate the so called Transmission T: (T = I/I0)
As photometry takes place mainly in the visible region of the spectrum and the adjacent frequencies, it utilizes the change in intensity of the source light being shone through a sample, usually through the visible colouring of a liquid.
When this colour is directly linked to the concentration of a substance that is being sought after, one can identify the concentration of this substance by a known means (thickness layer) using the decrease in the intensity of the light.
The colour can be a characteristic of the sample. Such as in the routine analysis of quality assessment and control of wine, where the colour of the wine itself is called for. However, a colour reaction, which produces the concentration related colouring for a sought after substance, is often used too. Such tests are available for hundreds of different substances in various levels of concentration and measured quantities. Some UV-VIS photometric methods also use the absorption of light in the non-visible region such as in measuring the concentration of DNA, which absorbs at 260 nm, or proteins, which are measured at 280 nm.
A lamp produces polychromatic light - wolfram and halogen lights for the visible region, deuterium and mercury lights for the UV region and in recent years increasingly LEDs. The light is directed through a slit onto a monochromator (e.g. a prism or a diffraction grating) and then through another slit and a measuring cuvette filled with the liquid that is to be assessed. The dimensions of the measuring cuvette are known, therefore the distance that the light travels through the sample is also known. Any light that passes through the sample falls on a detector which determines its intensity and compares it to the intensity of the light at its origin. The result is the so called, extinction, the decrease in light intensity caused by the sample. Single beam photometers have certain disadvantages, however. The brightness of the bulb and the sensitivity of the detector change as time goes by affecting the accuracy of the measurements and they therefore require frequent recalibration. With a double beam photometer, the light is directed via a mirror and then either alternately through the sample and through a reference without a colour reaction or, a known amount of the light is diverted through a semi-transparent mirror and onto a second beam path. Both beams fall on their respective detectors. Fluctuations in light intensity can be compensated for, however not any changes in the detectors. These can only be ruled out by using a single detector which receives the beams of light alternately.
The range of possible applications for the modern spectrophotometer is vast.
The number and diversity of test methods available for determining the concentration of the most diverse analytes and the range of concentration covered by chemistry alone are extensive. On top of these is a series of applications, which measures either a change in the concentration ratio over a period of time or a change in other parameters of the test, using multiple measurements of one or several different wavelengths. Besides this are the already mentioned applications, in which optical characteristics of the substance that is to be determined can be used directly. Photometry is necessary for wide range of industries, too. The classic area of application, the chemical or biological analytical laboratory is only one player of several, nowadays.
An example of photometry having a direct influence on man’s everyday existence is in the quality assurance of drinking water supplies and the food industry. Environmental analysis and the monitoring of processes in sewage treatment plants carry the same level of importance for all. Applications for a photometer can be found in the routine analytics of open waters through environmental protection and fisheries, colour measurement in the printing and dyeing industries, the determination of colour nuances in wine making or in field analysis in geology, pharmaceuticals, medicine and, and, and…
Photometry is one of the most important methods in analytics and each person in Germany has, at least, indirect contact with this technology. Although the technology is simple in principle, there is a lot that one must know if one wants to achieve good results with a photometer. The collaborative project between Wiley-VCH and WTW, “www.photometry-compendium.com” is being launched in this issue of G.I.T. Laboratory Journal and on the Microsite with the same address and will give users a comprehensive insight into photometry. A straightforward instruction manual, tips for use, the most frequent mistakes made and much more will be addressed in G.I.T. Laboratory Journal. There will be demonstration videos and reinforcing back ground information. Visit the microsite and don’t miss the next issues.