Injection Nebulizers

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. 

In addition to concentric pneumatic nebulizers that are used most frequently for the majority of atomic spectrometric systems, a range of other nebulizers also exist. Some nebulizers that are used for plasma instrumentation, e.g. direct injection nebulizers (DIN) or ultrasonic nebulizers (USN), increase... 

Fig. 7. Schematic diagram of a direct injection nebulizer. Reprinted from Zoorob et al. [82] with permission of the Royal Society of Chemistry.

Shum, S.K.C., Neddersen, R. and Houk, R.S. (1992a) Elemental speciation by liquid chromatography-inductively coupled plasma mass spectrometry with direct injection nebulization. Analyst, 117, 577-582. 

Direct injection nebulizers are operated without a spray chamber. Instead, the concentric, pneumatic nebulizer is positioned just below the plasma and 100% of the sample is sprayed into the ICP. The nebulizer replaces the center tube of the ICP torch. The direct injection nebulizers are particularly attractive for the analysis of small sample volumes and for elements, such as Hg, I, and B, that stick to surfaces in the spray chamber and therefore have long washout times when a spray chamber is used. The dead volume of the direct injection nebulizers is small, so they have often been used to couple capillary separations with ICP-MS detec-... 

Two different kinds of direct injection nebulizers are available commercially. The total consumption nebulizer was developed by Greenfield et al. [36] for ICP optical emission spectrometry. The concept for the Cetac direct injection nebulizer (DIN) was developed by Fassel, Houk, and coworkers [35,37]. It has a narrow sample-carrying capillary [30-50 xm inner diameter (i.d.), 0.5 to 1 m long] that extends slightly past the nebulizer gas tube. A second, auxiliary or makeup, nebulizer gas is introduced through another concentric tube outside the nebulizer gas tube. A gas displacement pump (up to 1500 psi) or HPLC pump is used to deliver the sample to the nebulizer through the long, narrow capillary. 

The second type of direct injection nebulizer, called the direct injection high-efficiency nebulizer (DIHEN), is a specific type of the Meinhard HEN [38] that is inserted into the ICP torch in place of the center, injector tube. The main advantage of the DIHEN compared to the Cetac DEN is that a high-pressure pump is not needed to deliver sample to the nebulizer. An unusually low nebulizer gas flow rate (0.25 L/min) and high ICP power (1.5 kW) were found to provide optimal ICP-MS sensitivity when DIHEN is used [38]. 

There are many examples of relatively straightforward use of ICP-MS for the analysis of biological fluids. Antimony has been measured in blood after a 14 1 dilution [236]. Cesium serum levels were found to be elevated in patients with alcohol dementia but not in Alzheimer s disease patients [237]. Cobalt levels in rat serum depended on the form of cobalt [238] ingested. Bismuth levels were measured in human blood and urine by using a direct injection nebulizer [239]. Lead was measured in the blood and blood plasma of smelter workers and the general population [240]. The measurement of trace elements in serum by ICP-MS has been compared to results from neutron activation analysis and proton-induced x-ray emission [241]. Semiquantitative analysis can also be used to obtain a rapid screening of samples [242]. 

Vapor phase dissolution (VPD) is commonly used for surface and contamination analysis of semiconductor wafers [374-379]. HF vapor is used to remove a silicon oxide or native silicon layer. A drop of hydrofluoric acid or deionized water (with a volume of 50 to 200 jxL) is placed on the surface and rolled around the surface to dissolve the metals. The small drop is then analyzed by ICP-MS by using either a direct injection nebulizer, a micronebulizer, or ETV. The ability of ICP-MS to measure several elements rapidly in a small volume of solution is essential. 

Both lead and mercury speciation has been performed by Shum and coworkers [43]. Direct injection nebulization and an ion-pair separation with a microbore LC column were used with ICP-MS detection. A mobile phase of 20 80% v/v ace-tonitrile/water with 5 mM ammonium pentanesulfonate ion pairing reagent at pH 3.4 was used to separate inorganic lead, inorganic mercury, and three organomer-cury species. Detection limits, based on peak area calculations, were 0.2 pg of Pb for all the lead compounds and 7-18 pg of Hg for the mercury compounds. Spiked urine samples were analyzed to evaluate the performance of the method. 

A direct injection nebulizer has been used for CE-ICP-MS [101] with the capillary actually inserted through the central sample introduction capillary of the DEN (Fig. 10.17). This nebulizer is very well suited to the very low flow rates associated with CE and also offers approximately 100% sample transport efficiency. A platinum grounding electrode was used, situated in a three-port connector, which also accommodated the DEN capillary and a makeup buffer flow. The makeup buffer was used so that the DIN operation was independent of the EOF, and the two flows were combined at the capillary exit. The system was used for feasibility studies for As and Se speciation. 

Figure 17 Block diagram of a direct injection nebulizer capillary electrophoresis inductively coupled plasma mass spectrometry (DIN CE-ICP-MS) interface. (From Ref. 101.)...

The potential of CE-ICP-MS for use in speciation studies offers the analyst a useful alternative for the separation of compounds of environmental and clinical importance. Detection limits for CE-ICP-MS are often superior to those achieved with traditional detection methods such as refractive index and uv spectroscopy. The use of low-flow nebulizers, such as the direct injection nebulizer and the high-efficiency nebulizer, which can accommodate the low electroosmotic flow of CE, offers significant advantages in terms of improved sensitivity. 

The key to a successful HPLC-ICP-MS coupling is the interface. In the simplest case the outlet of an HPLC column (4.6 D10 mm) is connected to a conventional pneumatic or crossBow nebulizer. The use of capillary or megabore (0.32D1.0mm) HPLC systems that are becoming popular, especially for RPC, requires the use of micronebulizers, either direct injection nebulizers (DIN) or micronebulizers (e.g., Micromist, DS-5) bttcd with a small-volume nebulization chamber. 

Wiederin, D.R., Smith, F.G. and Houk, R.S., Direct injection nebulization for inductively coupled plasma mass spectrometry. Anal. Chem., 63 (1991) 219. 

Although there has been limited use with CE interfaces, the direct injection nebulizer (DIN) was first described by Shum et al. - and later used by Liu et al. for CE (Fig. 2E). In this design, the nebulizer introduces the sample very near the plasma inside the ICP torch and eliminates the spray chamber assembly. Close to 100% analyte transport efficiency can theoretically be obtained with the DIN, but the nebulizer is restricted to very low liquid flow rate and thus is well matched to CE interfacing. This design does induce local plasma cooling due the lack of desolvation and detection limits are only slightly improved over other nebulizer designs. 

OCN), which is a variation of the pneumatic concentric nebulizer built from flexible capillary mbes, was used in an interface. The OCN has had little application in CE interfaces, owing to its generally lower sensitivity performance when compared to other pneumatic nebulizers used with ICP-MS detection.The direct injection nebulizer (DIN), previously described in The Nebulizer, was used by Liu et al. in a CE interface. The electrophoretic capillary was directly inserted through the central sample introduction capillary of the DIN. A platinum grounding electrode was positioned into a three-port connector. This connector contained the DIN sample introduction capillary as well as a make-up buffer flow. These alternative nebulizers have been successfully used in CE interfaces, but the pneumatic designs dominate the interface systems reported in the literature. 

Fig. 1 Separation of rabbit liver metallothionein using CE-ICP-MS. The protein sample (1 mg/mL dissolved in deionized water) was first subjeeted to CZE with UV detection to optimize CE separation parameters for the major metallothionein isoforms (MT-1 and MT-2) shown in the upper panel. The CE instrument was then coupled to an ICP-MS instrument using a specially modified direct injection nebulizer (CETAC Technologies Inc., Omaha, NB) which enabled the entire capillary effluent from the CE to be directly injected into the ICP plasma torch, thus avoiding postcolumn dilution and band-broadening effects of conventional spray chamber nebulizers. Specific isotopes of cadmium ( Cd) and zinc ( Zn) associated with each isoform peak were monitored as shown by the figures in the lower panel.

The most definitive assessment of the metal composition of metalloproteins comes from the application of element-specific detection methods. CE-ICP-MS provides information not only about the type and quantity of individual metals bound to the proteins but also about the isotopes of each element as well [11,12]. Elemental speciation has become increasingly important to the areas of toxicology and environmental chemistry. Such analytical capability also opens up important possibilities for trace element metabolism studies. Figure 1 depicts the separation of rabbit liver metallothionein containing zinc, copper, and cadmium (the predominant metal) using CE-ICP-MS with a high-sensitivity, direct injection nebulizer (DIN) interface. UV detection (200 nm) was used to monitor the efficiency of the CE separation of the protein isoforms (MT-1 and MT-2), whereas ICP-MS detection made it possible to detect and quantify specific zinc, copper (not shown), and cadmium isotopes associated with the individual isoform peaks. 

Powell MJ, Quan ESK, Boomer DW, et al. 1992. Inductively coupled plasma mass spectrometry with direct injection nebulization for mercury analysis of drinking water. Anal Chem 64(19) 2253-2257. 

Direct injection nebulizers, as they were first described by Kimberley et al. [121], are used without a nebulization chamber. In this way sample losses are low as... 

ICP-AES is often used to determine the concentrations of various elements in a sample. However, an element may be present in a variety of chemical forms or species. By coupling an ICP-AES detector to an ion-chromatographic column, a more complete description of the sample species can be obtained. Such a coupling generally requires a nebulizer to introduce the column effluent into the ICP. Conventional pneumatic nebulizers operate at about 1 mL/min sample flow and may introduce as little as 1 % of the sample into the plasma. A newer direct-injection nebulizer (DIN) operates at sample flow rates only 5 to 10 % that of a conventional nebulizer [2. >). 

The separation of selenite (SeOs ") and selenate (Se04 ) by anion chromatography with ICP-MS detection is a typical example [27]. The connection of the anion exchange column and direct injection nebulizer to the mass spectrometer and chromatographic conditions are similar to those employed with AES detection [26]. It is important to minimize the dead volume between the column and DIN, otherwise significant broadening of the separated peaks can occur. 

D. T. Gjerde, D. Wiederin, F.G. Smith, and B.M. Mattson, Metal speciation by means of microbore columns with direct-injection nebulization by inductively coupled plasma atomic emission spectroscopy, / Chromatogr., 73, 1993. 

M. J. Powell, D. W. Boomer, and D. R. Wiedcrin. Determination of chromium species in environmental. samples using high-pressure liquid chromatography direct injection nebulization and inductively coupled plasma mass spectrometry. Anal. Chern., 67, 2474,1995. 

J.-H. Wang, E.H. Hansen, Interfacing sequential injection on-line preconcentration using a renewable micro-column incorporated in a Tab-on-valve system with direct injection nebulization inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 16 (2001) 1349. 

The detection systems used with HPLC can be broadly divided into three approaches photometry, plasma techniques (ICPAES, ICPMS), and cold vapour atomic absorption and fluorescence spectroscopy (CV-AAS, CV-AFS). The method with the lowest limits of detection (LOD) with sample introduction via a direct injection nebulizer used ICP-MS. An HPLC system coupled to atmospheric pressirre chemical ionization MS was used to identify methyl mercury spiked into a fish tissue CRM (DORM-1, NRCC). This type of system has a significant advantage over elemental detection methods because identification of the species present is based on their structure, rather than matching the analyte s retention time to that of a standard. 

DIN direct injection nebulizer, see also above German DIN norm diuresis excretion of urine DMBA dimethylbenzanthracene DME dropping mercury electrode DMG dimethylglyoxime (analytical reagent)... 

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