Nebulizer detector

Data for the several flame methods assume an acetylene-nitrous oxide flame residing on a 5- or 10-cm slot burner. The sample is nebulized into a spray chamber placed immediately ahead of the burner. Detection limits are quite dependent on instrument and operating variables, particularly the detector, the fuel and oxidant gases, the slit width, and the method used for background correction and data smoothing. 

A novel interface to connect a ce system with an inductively coupled plasma mass spectrometric (icpms) detector has been developed (88). The interface was built using a direct injection nebulizer (din) system. The ce/din/icpms system was evaluated using samples containing selected alkah, alkaline earths, and heavy-metal ions, as well as selenium (Se(IV) and Se(VI)), and various inorganic and organic arsenic species. The preliminary results show that the system can be used to determine metal species at ppt to ppb level. 

Repetitive routine analysis of a specific sample (e.g., for Quality Control) will usually require a dedicated instrument. Therefore, the chromatograph and, in particular, the detector will be chosen for that specific analysis. Consequently, only one detector will be necessary and the purchase of an armory of detectors on the basis that they might be needed in the nebulous future is not advised. An alternative detector can always be obtained if and when the demand arises. The same argument applies to multi-solvent reservoirs and multi-solvent gradient programmers and other accessories that are not immediately required for the specific analysis in mind. 

As mentioned previously, this detector has been claimed to be universal, i.e., it has been reported that the response of the ELSD is not a function of the nature of the solute. Although this is not, strictly speaking, true, the detector is almost universal. The detector functions by nebulizing column effluent into droplets, which are evaporated in a heated gas stream. A droplet of evaporate, containing some solvent, remains. Light of a wavelength considerably smaller than the evaporate particles, which are 5 to 15 pm in size,161... 

The determination of the total concentrations of metal ions and arsenic in the water samples and in the eluates of solid materials were carried out using ICP-AES (Spectroflame, SPECTRO A.I.) with pneumatic nebulization (cross flow). Anion (S042, Cl ) determinations were done using an ion chromatographic device with IonPac AS12A/AG12A column and a conductivity detector. 

An ELSD converts the HPLC eluent into a particle stream and measures the scattered radiation. It offers universal detection for nonvolatile or semivolatile compounds and has higher sensitivity than the RI detector (in the low ng range) in addition to being compatible with gradient analysis. ELSD is routinely used in combinatorial screening. Response factors are less variable than that of other detectors. An ELSD consists of a nebulizer equipped with a constant temperature drift tube where a counter-current of heated air or nitrogen reduces the HPLC eluent into a fine stream of analyte particles. A laser or a polychromatic beam intersects the particle stream, and the scattered radiation is amplified by a photomultiplier. Manufacturers include Alltech, Polymer Laboratories, Shimadzu, Waters, Sedere, and ESA. 

Solutes were tentatively identified by atmospheric pressure ionization (API)-electrospray-mass selective detector (gas temperature 350°C, flow rate 101/min, nebulizer pressure 30 psi, quadrupole temperature 30°C, capillary voltage 3 500 V). 

Evaporative light scattering is gaining popularity due to its ability to detect analytes on a nonse-lective basis. Basically, this detector works by nebulizing the column effluent, forming an aerosol that is further converted into a droplet cloud for detection by light scattering. This type of detector has been applied to studies of small molecule combinatorial libraries [13,14], carbohydrates [15], and lipids [16,17]. 

FIGURE 7.13 Schematic of an evaporative light scattering detector. The three stages are nebulization, in which column effluent is aerosolized evaporation, in which the mobile phase is vaporized and optical detection, in which the light scattering of the residual solute particles is recorded. Some detectors also include an obstacle in the flow path for droplet discrimination, which leads to a more homogenous distribution of droplet sizes. 

A promising detection principle utilizes similar nebulization procedure as applied in ELS, namely the corona-charged aerosol detectors, CAD. In CAD, the aerosol particles interact with an ionized gas (usually nitrogen). The particles become charged and electrically detected [294]. It has been shown that the response of CAD does not depend on the nature of analyte. On the other hand, the size of the aerosol depends on the mobile phase composition and it has to be calibrated. 

An evaporative light-scattering detector responds to any analyte that is significantly less volatile than the mobile phase.21 In Figure 25-22, eluate enters the detector at the top. in the nebulizer, eluate is mixed with nitrogen gas and forced through a small-bore needle to form a... 

Atomic emission spectroscopy can be employed, generally with an inductively coupled plasma for thermal excitation. The sample is introduced into the plasma as a mist of ultrafine droplets, and the monochromator and detector are set to measure the intensity of an atomic emission line characteristic of the element. This technique is powerful, general, sensitive, linear, and able to measure over 70 elements, and, as a result, is widely used. Response is typically linear over four orders of magnitude in concentration with relative standard deviations of 1 to 3%. In low-salt aqueous solutions, detection limits range from 10 to 1000 nanomolar without preconcentration. Significant problems with saline samples remain, but use of Babington nebulizers alleviates these problems somewhat. 

Fig. 19 Detector block for the light-scattering detector. (1) nebulizer, (2) drift tube, (3) heated copper block, (4) light-scattering cell, (5) glass rod, (6) glass window, (7) diaphragm. (Reproduced from A. Stoly-hwo, H. Colin, and G. Guiochon, J. Chromatogr. 265 1 (1983) with permission.)...

Mass spectrometry (MS) is now an integrated detector for liquid chromatography. This is due to the advent of atmospheric pressure ionization (API) interfaces. In an API interface, the column effluent is nebulized into an atmospheric pressure ion region. Nebulization is performed pneumatically in atmospheric pressure chemical ionization (APCI) by a strong electrical field in electrospray or by a combination of both in ion spray. Ions are produced from the evaporating droplets... 

This detector was first described by Charlesworth (8). Nebulization of the column effluent in a stream of tepid (30-35°C) gas, followed by total vaporization of the solvent in a warm (40-45°C) drift tube, leaves a cloud of particles made of the nonvolatile material contained in the eluent. These particles are carried by the gas stream across a laser beam and then vented. 

Also known as the mass detector, this is an evaporative analyzer in which the mobile phase is removed by nebulization and evaporation prior to the determination of nonvolatile carbohydrates by light scattering (44). Unlike the refractive index detector, it allows gradient elution (eluent is removed before detection) and is more sensitive. The detection limit can go up to a few tens of nanograms injected. 

Maggs [36] developed an electron-capture detector (ECD) which was based 6n the moving wire transport system. This type of detector is now available commercially (Table 3.5). Nota and Palombardi [37] described a system in which the column eluent was continuously nebulized and part of which was directed into the interior of an ECD. 

There is also a standard test method for determination of major and minor elements in coal ash by inductively coupled plasma (ICP)-atomic emission spectrometry (ASTM D-6349). In the test method, the sample to be analyzed is ashed under standard conditions and ignited to constant weight. The ash is fused with a fluxing agent followed by dissolution of the melt in dilute acid solution. Alternatively, the ash is digested in a mixture of hydrofluoric, nitric, and hydrochloric acids. The solution is analyzed by (ICP)-atomic emission spectrometry for the elements. The basis of the method is the measurement of atomic emissions. Aqueous solutions of the samples are nebulized, and a portion of the aerosol that is produced is transported to the plasma torch, where excitation and emission occurs. Characteristic line emission spectra are produced by a radio-frequency inductively coupled plasma. A grating monochromator system is used to separate the emission lines, and the intensities of the lines are monitored by photomultiplier tube or photodiode array detection. The photocurrents from the detector... 

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