Sample introduction direct injection nebulizers

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. 

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. 

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. 

Organolead speciation analysis has also been achieved by the use of modified silica capillaries following nebulization with a direct injection nebulizer (DIN), which is very suitable for the introduction of liquid samples into ICP-MS. 

Concentric nebulizers date back more than a century, to Gouy [75], and in principle are selfaspirating. They can be made of metal, plastic, or glass. In atomic absorption spectrometry. Pt-lr capillaries are often used to allow the aspiration of highly acid solutions. In plasma atomic spectrometry, concentric glass nebulizers (Meinhard) 74] are well known. They use a rather lower gas flow (1-2 L/min) than atomic absorption types (up to 5 L/min). In both cases, aspiration rates are 1-2 mL/min and aerosol efficiencies 2-5%. Direct injection nebulizers are a useful alternative for the introduction of small-volume samples [76]. 

Descriptions of other sample introduction systems, including ultrasonic nebulization (USN), direct injection nebulization (DIN), and electrothermal vaporization (ETV), can be found in the literature [64—66]. 

The sample introduction from an LC column to ICP-MS is performed by a nebulizer. The usual nebulizers are pneumatic nebulizers, such as Meinhard, crossflow, or microconcentric nebulizers (MCNs). Additionally, there is the ultrasonic nebulizer (USN), the direct-injection nebulizer (DIN), and the hydraulic high-pressure nebulizer (HHPN). The nebuli-zation efficiency depends on nebulizer type and is typically low for Meinhard and crossflow nebulizers (only around 1—5 %, [23]), whereas it is high for the DIN and USN. 

There are many other nonstandard sample introduction devices such as laser ablation, ultrasonic nebulizers, desolvation devices, direct injection nebulizers, flow injection systems, and electrothermal vaporization, which are not described in this chapter. However, because they are becoming more and more important, particularly as ICP-MS users are demanding higher performance and more flexibility, they are covered in greater detail in Chapter 17. 

They were initially developed over 15 years ago and found some success in certain niche applications that could not be adequately addressed by other nebulization systans, such as introducing samples from a chromatography separation device into an ICP-MS or the determination of mercury by ICP-MS, which is prone to severe memory effects. Unfortunately, they were not considered particularly user-friendly, and as a result became less popular when other sample introduction devices were developed to handle microUter sample volumes. More recently, a refinement of the direct injection nebulizer has been developed, called the direct inject high-efficiency nebulizer (DIHEN), which appears to have overcome many of the limitations of the original design. The advantage of the DIHEN is its ability to introduce microliter volumes into the plasma at extremely low sample flow rates (1-100 pL/min), with an aerosol droplet size similar to a concentric nebnlizer fitted with a spray chamber. 

Alternative Sample Introduction Techniques Nonstandard sampling accessories like laser ablation systems, flow injection analyzers, electrothermal vaporizers, cooled spray chambers, desolvation equipment, direct injection nebulizers, and automated sample delivery systans and dilu-tors are considered critical to enhancing the practical capabilities of the technique. Their use has increased significantly over the past few years as ICP-MS is being asked to solve more and more diverse application problems. This chapter reflects the increased interest in sampling accessories, especially in the area of specialized sample introduction and desolvation devices to reduce the impact of conunon interferences. 

The sample introduction system can simply be re placed by a high-efficiency one, such as an ultrasonic nebulizer coupled to a desolvation system. Analyte transport efficiencies of 10-20% are typically achieved using such a system. Other high performance nebulization systems include direct injection nebulization, which introduces 100% of the sample directly into the plasma (i.e. no spray chamber is needed), thermospray, hydraulic high-pressure nebulization and monodisperse dried microparticulate injection. 

Todoli j. L. and Mermet J. M. (2001) Evaluation of a direct injection nebulizer (DIHEN) by comparison with a high efficiency nebulizer (HEN) coupled to a cyclonic spray chamber as a liquid sample introduction system for ICP-AES, J. Anal. At. Spectrom. 16 514-520. 

The LC-ICP-MS experiments in the authors laboratory have employed an ultrasonic nebulizer, which is roughly 10% efficient [20, 21] but has a substantial dead volume in the gas phase. When the technique progresses to the use of better separations of more complex mixtures, sample introduction devices with less opportunity for broadening, such as the direct injection nebulizer [22, 23], may become necessary. 

The liquid sample introduction system most commonly used on an ICP-MS is very similar to that used on a flame Atomic Absorption Spectrometer or an ICP-OES. Liquid samples can be directly injected using a pneumatic nebulizer and a spray chamber. 

Figure 28-5 Continuous sample introduction methods. Samples are frequently introduced into plasmas or flames by means of a nebulizer, which produces a mist or spray. Samples can be introduced directly to the nebulizer or by means of flow injection (FIA) or high-performance liquid chromatography (HPLC). In some cases, samples are separately converted to a vapor by a vapor generator, such as a hydride generator or an electrothermal vaporizer.

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