Ultraviolet Analyzers

The measurements by ultraviolet analyzers can utilize (1) single-beam (2) split-beam (3) dual-beam, single-detector (4) dual-beam, dual-detector (5) flicker photometer (6) photodiode and (7) retroreflector designs. The standard errors of these measurements are 2% FS, whereas it is 1% FS for the fiber-optic diode-array designs. These analyzers can handle process pressures up to 50 barg (750 psig) and temperatures up to 450°C (800°F). 

Ultraviolet analyzers have been discussed in previous subsections in connection with the measurement of the concentrations of specific materials. Therefore, only a summary of their features is provided here. Table 3.43 provides a list of UV-absorbing substances. 

Taylor WB, LeBlanc JC, WhuUams DW, Herbert MA, Johns HE. (1972) Intensified ultraviolet analyzing lamp for flash photolysis. Rev Sci Instrum 43 1797-1799. 

On-line composition is usually measured by gas chromatographs. Other analyzers include infrared and ultraviolet analyzers, mass spectrometers, boiling point analyzers, wet-chemical analyzers, flash point analyzers, and refractive index analyzers. 

Temperature measurement Thermocouples Bimetal thermometer Resistance thermometer Radiation pyrometer Infrared measurement Conductivity pH Viscosity Infrared analyzer Mass spectrometer Radiation analyzer Chromatographic analyzer Ultraviolet analyzer Photonic measurement... 

To make progress in further analyzing the first-order results obtained above, it is useful to eonsider the wavelength X of the light used in most visible/ultraviolet, infrared, or mierowave speetroseopie experiments. Even the shortest sueh wavelengths (ultraviolet) are eonsiderably longer than the spatial extent of all but the largest moleeules (i.e., polymers and biomoleeules for whieh the approximations we introduee next are not appropriate). 

EPA Method 6C is the instrumental analyzer procedure used to determine sulfur dioxide emissions from stationaiy sources (see Fig. 25-30). An integrated continuous gas sample is extracted from the test location, and a portion of the sample is conveyed to an instrumental analyzer for determination of SO9 gas concentration using an ultraviolet ( UV), nondispersive infrared (NDIR), or fluorescence analyzer. The sample gas is conditioned prior to introduction to the gas analyzer by removing particulate matter and moisture. Sampling is conducted at a constant rate for the entire test rim. 

ICP-OES is one of the most successful multielement analysis techniques for materials characterization. While precision and interference effects are generally best when solutions are analyzed, a number of techniques allow the direct analysis of solids. The strengths of ICP-OES include speed, relatively small interference effects, low detection limits, and applicability to a wide variety of materials. Improvements are expected in sample-introduction techniques, spectrometers that detect simultaneously the entire ultraviolet—visible spectrum with high resolution, and in the development of intelligent instruments to further improve analysis reliability. ICPMS vigorously competes with ICP-OES, particularly when low detection limits are required. 

The product may be analyzed by gas chromatography on an 8 mm. x21S cm. column heated to 220-240° and packed with Dow-Corning Silicone Fluid No. 550 suspended on 50-80 mesh ground firebrick. The chromatogram obtained with this column exhibits a single major peak. The ultraviolet spectrum of an ethanol solution of the product has a maxium at 250 m>i (s = 17,200). 

Reactions Producing Fluorescent Radiation In the reaction chamber of the ultraviolet fluorescent analyzer reactions producing fluorescence are... 

Construction and Operation of Analyzer The construction principle of a sulfur dioxide analyzer based on the ultraviolet fluorescence principle is shown in Fig. 13.48. Undesired wavelengths are removed from the irradiating beam as tar as pos.sible using filters. The irradiating light 214 nm) is focused by a lens at the center of the reaction chamber. 

The sulfur dioxide analyzer based on the ultraviolet principle is a sensitive instrument. Its detection limit can be less than one ppbv (parts per billion by volume). When used in emission measurements, the sample gas IS normally diluted prior to the measurement using a diluting stack sampler. 

Ultraviolet (UV) analyzer An instrument using the wavelength of light to determine the properties of a gas or vapor. 

The ultraviolet absorption spectrum of hydrogen was analyzed by Dieke and Hopfield (8). They identified the three lowest... 

The predominant method of analyzing environmental samples for methyl parathion is by GC. The detection methods most used are FID, FPD, ECD, and mass spectroscopy (MS). HPLC coupled with ultraviolet spectroscopy (UV) or MS has also been used successfiilly. Sample extraction and cleanup varies widely depending on the sample matrix and method of detection. Several analytical methods used to analyze environmental samples for methyl parathion are summarized in Table 7-2. 

Headspace analysis has also been used to determine trichloroethylene in water samples. High accuracy and excellent precision were reported when GC/ECD was used to analyze headspace gases over water (Dietz and Singley 1979). Direct injection of water into a portable GC suitable for field use employed an ultraviolet detector (Motwani et al. 1986). While detection was comparable to the more common methods (low ppb), recovery was very low. Solid waste leachates from sanitary landfills have been analyzed for trichloroethylene and other volatile organic compounds (Schultz and Kjeldsen 1986). Detection limits for the procedure, which involves extraction with pentane followed by GC/MS analysis, are in the low-ppb and low-ppm ranges for concentrated and unconcentrated samples, respectively. Accuracy and precision data were not reported. 

The concentrations of nitrosamines were reduced to undetectable levels by ultraviolet treatment of the amine solutions and were not increased by addition of 2 ppm NaN02> indicating that the nitrosamines were present originally in the amines and were not formed in the GC injection port. Similar concentrations were found when the amine samples were analyzed using the column extraction method. Direct injection is appropriate for analysis of relatively simple mixtures, if adequate precautions are taken ( ), but can result in significant artifact formation in more complex systems (42). 

A fully automated instrumental procedure has been developed for analyzing residual corrosion inhibitors in production waters in the field. The method uses ultraviolet (UV) and fluorescence spectrophotometric techniques to characterize different types of corrosion inhibitors. Laboratory evaluations showed that fluorescence is more suitable for field application because errors from high salinity, contamination, and matrix effect are minimized in fluorescence analysis. Comparison of the automated fluorescence technique with the classic extraction-dye transfer technique showed definite advantages of the former with respect to ease, speed, accuracy, and precision [1658],... 

A variety of formats and options for different types of applications are possible in CE, such as micellar electrokinetic chromatography (MEKC), isotachophoresis (ITP), and capillary gel electrophoresis (CGE). The main applications for CE concern biochemical applications, but CE can also be useful in pesticide methods. The main problem with CE for residue analysis of small molecules has been the low sensitivity of detection in the narrow capillary used in the separation. With the development of extended detection pathlengths and special optics, absorbance detection can give reasonably low detection limits in clean samples. However, complex samples can be very difficult to analyze using capillary electrophoresis/ultraviolet detection (CE/UV). CE with laser-induced fluorescence detection can provide an extraordinarily low LOQ, but the analytes must be fluorescent with excitation peaks at common laser wavelengths for this approach to work. Derivatization of the analytes with appropriate fluorescent labels may be possible, as is done in biochemical applications, but pesticide analysis has not been such an important application to utilize such an approach. 

Extracts of all matrices were analyzed by reversed-phase HPLC using ultraviolet detection at a wavelength of 266 nm (Soekhoe and Kerstens, 1995). The limit of detection (LOD) was 10 mg/L for all matrices. Recovery was > 90% and "between days" CV of the analytical chemical method was < 10%. 

---