Direct injection nebulization

Small samples (25-200 mm ) may be introduced using the technique of pulse nebulization (also known as discrete sample nebulization, direct-injection cup nebulization, gulp sampling, and Hoescht cup nebulization). This technique may also be employed for higher concentrations than normally nebulized. A cup or funnel made of an inert material (e.g., polytetrafluoroethylene) is attached to the nebulizer tubing and the sample is put into the cup as a discrete aliquot using a micropipette. The sample is totally consumed and the transient peak signal recorded. 

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. 

Becker JS, Dietze H-J, McClean JA, Montaser A (1999) Ultratrace and isotope analysis of long-hved radionuclides by inductively coupled plasma quadrupole mass spectrometry using a direct injection high efficiency nebulizer. Anal Chem 71 3077-3984... 

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. 

Singh J, McLean JA, Pritchard DE, et al. 1998b. Sensitive quantitation of chromium-DNA adducts by inductively coupled plasma mass spectrometry with a direct injection high-efficiency nebulizer. Toxicological Sciences 46 260-265. 

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. 

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