CHAPTER 3 Inductively Coupled Plasma—Atomic Emission …

Loading...

Taking too long?

Reload document

| Open in new tab

CHAPTER 3

Inductively Coupled Plasma—Atomic Emission Spectrometry

3.1

Introduction and History

Greenfield et al. developed plasma-based instruments in the mid 1960s

about the same time flame-based instruments such as FAAS and FAES (Chapter

2) became prominent (Analyst, 89, 713-720, 1964). These first plasma-based

instruments used direct current (DC) and microwave-induced (MI) systems to

generate the plasma. Interference effects and plasma instability limited the utility

of plasma instruments during analysis; consequently flame-based spectrometry

instruments (such as FAAS) dominated the analytical market for metals analysis

and remain effective today.

The limitations of the first plasma instruments were overcome by utilizing

an inductively coupled plasma (ICP) instead of DC or MI generated plasma. ICP

optical systems became popular in the 1980s due to their decreased cost, lower

time investment during analysis, and labor saving advantages. FAAS/FAES

instruments require a unique radiation source (lamp) for the approximately 35

elements they can measure. Because the lamp must be changed between each

element of interest, FAAS/FAES techniques analyze a single element at a time

and are unable to easily analyze metalloids. ICP optical systems, by contrast,

can analyze about 60 different elements at the same time with a single source

(the plasma). The most common instruments today are inductively coupled

plasma—atomic emission spectrometers (ICP-AES) and inductively coupled

plasma—mass spectrometers (ICP-MS). ICP-AES will be discussed in this

chapter while ICP-MS will be the subject of the next chapter.

3.2 Atomic Emission Spectrometry Theory

The operation of an ICP-AES system relies upon the same interaction of

molecules with electromagnetic radiation that was presented in Chapter 1. The

two emission systems, FAES and ICP-AES, differ in the way atomic species are

created and excited. Because of the relatively low temperatures (~2000-2500 C)

in a flame-based system, not all of the atoms or elements present in the sample

are excited, particularly if they exist in a polyatomic compound. Some elements

readily form non-emitting and refractory oxides that result in an underestimation

of their concentration. In plasma-based systems the temperature is considerably

hotter (~6000 to 10 000 K) that results in more effective excitation of atoms

(generally greater then 90%) of approximately 60 elements including some

nonmetals. This intense heat prevents polyatomic species from forming, thus

increasing the detection limits for many elements. Atoms are excited, and in

many cases ionized, by the intense heat of the plasma, and the emission of a

photon occurs via resonance fluorescence (normal valance electron relaxation by

photon emission). While plasma-based systems eliminate many problems, they

are not free of interferences due to the excitation and subsequent emission of

spectral lines for every element in the sample as well as the Ar added to facilitate

plasma generation. The spectral overlay that results from these possible

emissions is overcome in modern instruments with specialized sequential

monochromators (Section 3.4.4). ICP-AES, compared to FAAS/FAES, offers

high selectivity between elements, high sensitivity, a large dynamic range,

especially as compared to FAAS that is limited by Beer’s law, lower detection

limits, multi-element detection, and fewer matrix interferences.

3.3 Components of an Inductively Coupled Plasma—Atomic Emission

Spectrometry System (ICP-AES)

3.3.1 Overview:

An ICP-AES system can be divided up into two basic parts; the inductively

coupled plasma source and the atomic emission spectrometry detector. Figure

3-1 shows the common components of an ICP-AES system from the late 1980s

to the 1990s. The inductively coupled plasma source has mostly been

unchanged since its invention with the exception of innovation in monochromator

type, which enables greater suppression of interference phenomena.

Modifications of this common system will be explained in the following sections.

Sample solutions include digested soil or other solid material or natural water.

Typically the sample solution is acidified up to 2-3% in HNO3 to prevent

adsorption of metals onto polypropylene sample bottle or onto instrument tubing

or glassware prior to introduction into the plasma. In Figure 3-1, the sample is

introduced to the nebulizer chamber via a peristaltic pump and tygon tubing

attached to an automatic sampler. A peristaltic pump operates by sequentially

compressing flexible tubing with evenly spaced and rotating rollers that pull/push

the liquid through the system. The rate of sample introduction into the plasma

changes as the rotation rate of the peristaltic rollers increases or decreases.

Flow of sample and Ar gas through the small aperture of the nebulizer creates

very small droplets that form a mist of µm-sized particles in the nebulizer

chamber. Larger sample droplets collect on the chamber walls and are removed

through a drain, while smaller particles travel with the Ar flow and enter the torch.

Evaporation, atomization, and excitations/ionizations occur in the plasma at

temperatures reaching 10 000 K. Ar not related to the sample is also excited and

ionized because this gas both carries the sample aerosol and confines the

location of the plasma to prevent damage to the rest of the instrument. As the

excited/ionized atoms leave the hot portion of the plasma, excited valence

electrons relax and emit a photon characteristic of the electron transition. This

photon is specific to the element but does not yield any information about the

isotopic state of the element, unlike in mass spectrometry (Chapter 4). Visible

and UV radiation emitted from the sample constituents enters the

monochromator through a small slit where the wavelengths are separated by

grating(s) and/or prism(s) before being captured and measured by a wide variety

of detectors.

Because spectral interferences may still occur, the choice and

configuration of the monochromators in the instrument is important and has been

the target of innovation. In Figure 3-1, the most common form of a

monochromator (a Rowland circle) and detector (photomultiplier; PMT) is shown:

The Rowland system utilizes a concave Echellette-style grating monochromator

to separate the various emission lines and simultaneously focus individual

wavelengths on to a series of slits, with each slit aligned to allow a specific

wavelength of radiation to pass to a detector. The standard detector, a

photomultiplier tube (PMT), was discussed in Section 2.2.9. Some systems use

multiple PMTs at fixed locations to monitor each wavelength simultaneously

(Figure 3-1) whereas other systems use a single PMT and move it to different

locations to detect each wavelength. Data from these detectors are processed

by a computer because multiple wavelengths are measured in an ICP-AES

system at the same time.

Figure 3-1. Overview of a Basic Inductively Coupled Plasma—Atomic Emission

Spectrometry (ICP-AES) from the 1990s.

3.3.2 Sample Introduction and Optimization

The predominate form of sample matrix in ICP-AES today is a liquid

sample: acidified water or solids digested into aqueous forms. Given the

automated nature of the ICP analysis, all modern systems are purchased with

automatic samplers where a computer-controlled robotic sampling arm takes

liquids from each sample via a peristaltic pump from plastic tubes located in

specific locations in a sampling tray. Liquid samples are pumped into the

nebulizer and sample chamber via a peristaltic pump as shown below. Then the

samples pass through a nebulizer that creates a fine mist of liquid particles.

Larger water droplets condense on the sides of the spray chamber and are

removed via the drain (pumped out of the chamber also by the same peristaltic

pump) while finer water droplets move with the argon flow and enter the plasma.

Nebulizers help ensure that the sample enters into the plasma at a uniform flow

rate and specific droplet size. Droplets that are great than 5 µm in diameter are

likely to interfere with plasma stability.

Figure 3-2. An Overview of Sample Introduction and the Nebulizer Chamber.

(The nebulizer shown here is a pneumatic style, described below.)

While there are numerous types of nebulizers for a variety of specific

applications, the three most commonly types are the (1) pneumatic, (2)

ultrasonic, and (3) grid. Because argon is used in generating the plasma

(discussed below) it is most often used as the gas in these various nebulizers,

but other gases can be used. The most common pneumatic nebulizer for

samples containing low concentrations of total dissolved solids is the concentric

nebulizer shown in Figure 3-3, but higher suspended solids and dissolved solids

samples are commonly introduced to the plasma via the Babington nebulizer.

Figure 3-3. Diagram of a Pneumatic Concentric Nebulizer.

Figure 3-4. Diagram of a Pneumatic Babington Nebulizer.

Figure 3-5. Diagram of a Pneumatic Cross-Flow Nebulizer.