DIFFERENCE BETWEEN PHOTOMULTIPLIER (PMT) & CHARGED COUPLED DEVICE (CCD) DETECTORS

Some of the most common differences between Photo multiplier (PMT) detectors & Charged Coupled Device detector (CCDs)..


1.       Photomultiplier tubes (PMTs) and Charged coupled devices (CCDs) both give spectra. The difference is the PMT is used with a small slit in front of it to control the bandwidth of light being detected. The CCD takes advantage of the dispersed light fully. The pixel columns will each correspond to a wavelength (resolution and range depend on the grating used). A PMT requires scanning of the Monochromator to collect a spectra. The CCD takes a single snap shot and you have a spectrum. The CCD sensitivity and dynamic range is lower than a PMT.

2.     A photomultiplier tube is a detection device that is made from a glass vacuum tube with a series of metal plate electrodes. A CCD is a solid state detector made from semiconductor materials.

3.     The main difference is one of sensitivity. Generally speaking the better the spectral resolution of the instrument the lower the amount of light reaching the detector and so you need more sensitivity in your detector. A PMT measures a single point in the spectrum at a time whereas with a CCD the complete spectrum is imaged across the CCD and so can be measured all at the same time. 

4.     An instrument with a CCD is usually much faster and cheaper but will not have as good a spectral resolution (the ability to resolve absorbance peaks very close to each other).

5.     CCDs and photomultipliers vary in a number of aspects. One difference is gain, a photomultiplier has gain whereas a CCD does not (hence the multiplier bit of PMT). The PMT gain may be up to 10,000,000 and is available at high speeds and for large area detectors, which means that one can usually get close to the theoretical noise floor. On the other hand, PMTs have poor quantum efficiency compared to CCDs (25% typ against 85% typ) so you can sometimes get better performance with a CCD if you can go slowly enough.

6.     PMTs are also typically single channel devices, although 16 channel linear arrays are available. CCDs are usually linear or 2D arrays.

7.     In a dispersive spectrometer a linear CCD array can capture the entire spectrum in one measurement. A single channel PMT must have the spectrum scanned across it sequentially to produce the entire spectrum.

8.     PMT's are typically preferable to CCD's on spectroscopic application for several reasons. The ability to adjust the gain of each PMT allows a manufacturer to adjust the response of each PMT to the specific signal being measured, so every element you are trying to detect can be analyzed at optimum conditions. Solid state CCD's are a compromise. Every element detected has the same conditions, so most are compromised. 

9.     Also, PMT's can be heated and held at constant temperature (in well made instruments) to prevent drift caused by variation in temperature. If you try to heat a CCD, the noise level will go up, and the signal to noie ratio will degrade as a result. CCD's are sometimes cooled to try to improve their s/n ratio, but usually not cooled enough to really help much due to condensation issues that arise. 

10. A third advantage of PMT's is that they can be used in a vacuum chamber without long term degradation for decades of use. The surface of a CCD will degrade under vacuum over a few (8-15) years. Most manufacturers making CCD based instruments opt for a Nitrogen or Argon flush, rather than vacuum to displace the oxygen from the detector chamber. This method results in decreased performance compared to PMT's, and is used in lower performance less expensive spectrometers.

LASER INDUCED BREAKDOWN SPECTROSCOPY

LIBS, is a spectroscopy technique in which a short laser pulse beam is focused on a target sample. Laser energy ionizes the sample material by heating it,  creating small area of plasma. Excited ions in the plasma state emits light waves which are collected and the spectrum is resolved by a spectrometer and analyzed by suitably calibrated  photon or light detector. Each chemical element has a unique wavelength or signature which can be optically resolved from the obtained spectrum. As  result, the composition of the elements which constitutes in the target sample can be determined. Below provided some of the general information about the technique : i Advantages ii Considerations Spectral coverage vs. resolution Light sensitivity iii. General Applications I. Advantages LIBS is considered one of the most  efficient and user friendly analytical techniques for trace elemental analysis in gases, solids, and liquids. Some of its major advantages include: Real-time measurements: online monitoring and quality control of industrial processes Noninvasive, nondestructive technique: valuable samples can be reused, sensitive materials can be analyzed, suitable for in-situ biological analysis Remote measurements can be done from up to 50 meters distance: can be used in hazardous environments and for space exploration missions on other planets Compact and inexpensive equipment: can be widely used in industrial environments, perfect for field measurements High-spatial resolution: can obtain 2D chemical and mechanical profiles of virtually any solid material with up to 1 µm precision Non or very little sample preparation is required: reduced measurement time, greater convenience, less opportunity for sample contamination Samples can be in virtually any form: gas, liquid, or solids Analysis can be performed with a very small amount of sample (nanograms): very useful in chemistry for characterization of new chemicals and in material science for characterization of new composite materials or nanostructures Virtually any chemical element can be analyzed, such as heavier elements unavailable for X-ray fluorescence Analysis can be done on extremely hard materials like ceramics and superconductors; these materials are difficult to dissolve or sample to perform other types of analysis In aerosols both particle size and chemical composition can be analyzed simultaneously II. Considerations  Spectral Coverage vs. Resolution Compact echelle spectrometers designed for LIBS applications are offered by several manufacturers. In the rare occasion that an application requires even higher resolution, the Acton Series of spectrometers with their long focal lengths are extremely useful. The latest models  use toroid mirrors with improved spectral quality. For a  detector with 1024 horizontal pixels, each of which is 26 m wide, the theoretical field of view is 26.6 mm. But since a standard 25 mm intensifier is used, the field of view is 25 mm. For example, if you decided to utilize a 2400 groove/mm grating in the Acton Series 2500 in order to enhance resolution, the linear dispersion will be 0.6 nm/mm while the spectral coverage will be 0.6*25 = 15 nm. To cover a spectral range between 300 and 600 nm (for example), you will need to perform at least 20 laser shots each time, moving the spectrometer grating to a new position and "gluing" all 20 spectra together. This is a very standard procedure which can be done painlessly and automatically. The only disadvantage to this is that acquisition of one spectrum could take up to a few dozen seconds or longer, which is why the echelle spectrometer has become extremely popular, especially in industrial and field applications where real-time measurements such as online quality control is a must.  Light sensitivity Typically, the laser pulse in LIBS applications lasts for femto- to nanoseconds (10-15 to 10-9 s). Especially in applications where non-invasive and non-destructive analysis is required, a relatively small amount of laser energy is transferred to the sample. Therefore, one laser pulse produces a weak emission signal which is hard or impossible to collect with conventional CCD detectors. That is why intensified CCDs (ICCDs) are widely used in LIBS. To improve the emitting signal on the order of 10-30 times, a scheme with two orthogonal lasers beams is often used. In this dual-scheme, the first and usually more powerful laser pulse ablates and atomizes sample material while the second one heats the ablated material even further, allowing it to improve the intensity of atomic or ionic lines. Factors such as the level of laser excitation energy for both pulses and the time delay between the pulses play a crucial role in achieving signal intensity enhancement. This technique increases the sensitivity of LIBS by  at least one order of magnitude and allows for a greater possible number of applications. If measurement time duration is not an issue, a regular CCD, (1024x1024 pixels, 13 µm pixel size), can be used together with the an spectrometer for LIBS applications. To obtain the reasonable light level required for a non-intensified CCD, long exposure time measurements should be performed. In this case, plasma emission signal is accumulated on the CCD from a multiple laser pulse. However, one should be careful about excessive accumulation of background noise and low signal-to-noise ratio. It is especially important when performing measurements in the open air without an enclosed sample chamber. Since the CCD stays open for a long period of time, all sources of stray light in the room should be eliminated and measurements should be conducted in darkness.  CCD usually proves a more sophisticated system than the  ICCD because intensified CCDs are prone to permanent damage by excessive light levels. Extra care should be taken so as not to expose ICCDs to the bright sources of light like laser reflections. In the case of a regular CCD, it is difficult to damage with excessive light. III. General Applications The fact that LIBS generally requires little-to-no sample preparation, simple instrumentation, and can easily be performed on-the-field in hazardous industrial environments in real-time, it is a very attractive analytical tool. The following are a few examples of real life applications, where LIBS is successfully used: Express-analysis of soils and minerals (geology, mining, construction) Exploration of planets (such as projects using LIBS for analyzing specific conditions on Mars and Venus to understand their elemental composition) Environmental monitoring (Real-time analysis of air and water quality, control of industrial sewage and exhaust gas emissions) Biological samples (non-invasive analysis of human hair and teeth for metal poisoning, cancer tissue diagnosis, bacteria type detection, detection of bio-aerosols and bio-hazards, anthrax, airborne infectious disease, viruses, sources of allergy, fungal spores, pollen). Replacing antibody, cultural, and DNA types of analysis Archeology (analysis of artifacts restoration quality) Architecture (quality control of stone buildings and glasses restoration) Army and Defense (detection of biological weapons, explosives, backpack-based detection systems for homeland security) Forensic (gun shooter detection) Combustion processes (analysis of intermediate combustion agents, combustion products, furnace gases control, control of unburned ashes) Metal industry (in-situ metal melting control, control of steel sheets quality, 2D mapping of Al alloys) Nuclear industry (detection of cerium in U-matrix, radioactive waste disposal)