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AA or ICP ? W h i c h Te c h n i q u e I s t h e Best for Yo u r L a b o r a t o r y ?

The information here is very simplistic and is meant as an overview of AA and ICP. More detailed documents related to AA and ICP are available.

AA vs. ICP

AA and ICP are both techniques for determining what elements and at what concentrations are in a solution. Atomic Absorption (AA) is one process, while ICP (Inductively Coupled Plasma) uses the process of atomic or ionic emission. Thus, AA uses absorption of light and ICP uses emission of light. A simple example of emission can be seen when a pot of water boils over on to a flame while cooking. The flame will change color and emit a yellowish-orange color from sodium (Na) or salt in the tap water. Another example of the emission processes are fireworks. Each different, bright color in the sky is the result of emission from different elements in the periodic table. Every element can have this absorption/emission process depending on the energy. The color or wavelength can help determine what element is present. The intensity or brightness helps determine the concentration.

AA Figure 1 is the basic configuration of an AA unit. The unit consists of a light source called a hollow cathode lamp (HCL). Generally, every element to be analyzed requires its own HCL. Multi-element HCLs exist, but with these it is best for the elements to have characteristics that are similar like sodium (Na) and potassium (K), which are Group 1 alkali metals, or magnesium (Mg) and calcium (Ca), which are Group 2 alkaline-earth metals.

Figure 1: Basic Configuration of an AA

The atomizer can be a flame

or graphite furnace unit.

Flame is typically used for measuring in the part per million range (ppm or mg/L), while graphite furnace is for measuring in the part per billion range (ppb or ug/L). The monochromator is used to separate the colors or wavelengths of light so the detector can measure the intensity. The monochromator uses a grating to isolate a particular wavelength, for example, copper at 324.8 nm. AA is typically a single-element technique, meaning one element is measured at a time. Thus, in a multi-element analysis, all samples are measured for

copper, and then all samples are measured for iron, etc. The signal processor and the display are your computer and monitor, respectively. Flame AA generally uses two different types of flames as can be seen in Figure 2. An air/acetylene flame has a temperature of about 2300 oC, while the nitrous oxide/acetylene flame has a temperature of about 3000 oC. As can be seen in the periodic table below, elements with a dark green triangle in the bottom right-hand corner use an air/acetylene flame, while elements with a dark blue triangle in the bottom right-hand corner use a nitrous oxide/acetylene flame. The burner head for the air/acetylene flame is standard, while the burner head for the nitrous oxide/acetylene flame is optional. There are two other techniques used for specific elements. The techniques utilize either a hydride vapor generator (HVG) or a mercury vapor unit (MVU), which are optional accessories. The HVG elements have a light green triangle in the bottom right-hand corner. Those elements include arsenic (As), selenium (Se), and antimony (Sb), as well as mercury (Hg). The MVU can also analyze mercury as shown in light blue. The difference between the HVG and MVU is that the HVG can be used with an autosampler, but the MVU is strictly manual.

Figure 2: Periodic Table with AA Method of Analysis

ICP Figure 3 is the basic configuration of an ICP. The sample is introduced to the plasma (10,000 o K) by a nebulizer. The standard nebulizer is a pneumatic nebulizer which uses gas (in this case argon) to turn the liquid into an aerosol. Flame AA uses a similar pneumatic nebulizer. There is also an optional ultrasonic nebulizer for increasing sensitivity (see Detection Limits below).

Figure 3: Basic Configuration of an ICP

The ICP uses a monochromator, like AA, to separate wavelengths of light. However, the ICPE9000 also uses a prism to separate the order of light, resulting in a two-dimensional spectrum of light as shown in Figure 4. The database has over 110,000 wavelengths/orders of light. Thus, all elements are simultaneously measured at once, unlike AA. In the echellegram the wavelength increases from left to right and the order increases from top to bottom. A CCD camera captures a picture similar to that of a photographic camera but it is much more sophisticated.

Figure 4: Echellegram of Spectra from an ICP

Shimadzu’s ICPE-9000 utilizes the axial view as standard, shown in Figure 5. The radial view is optional. The axial is about 10 times more sensitive than the radial view with aqueous solutions. Why use the radial view? To increase the linear dynamic range or how many orders of magnitude that can be measured. An order of magnitude is a factor of 10; for example, having a calibration curve from 1 to 10 ppm is one order of magnitude. A calibration curve from 1 to 100 ppm is two orders of magnitude, etc. Also, high salt or organic matrices utilize the radial view because the salts and carbon will clog the sample orifice and coat the mirrors, respectively. The ICPE-9000 also enables dual view for measuring in both axial and radial views. Axial View

Figure 5: Axial and Radial Views of an ICP

Radial View

Comparative Specifications Detection Limits

Shown in Table 1 are detection limits for Shimadzu’s AA-7000 and ICPE-9000.The detection limits for AA-7000 include both flame and graphite furnace, with units of concentration reported in ppm and ppb, respectively. The ICPE-9000 has two sets of data as well using the standard pneumatic concentric nebulizer and the optional ultrasonic nebulizer. The units of concentration for both ICP techniques are in ppb. Model AA-7000 AA-7000 Method Flame Furnace element DL(3σ）(ppm) DL(3σ）(ppb) Ag 0.005 0.04 Al 0.1 As Hydride 0.1 Au 0.02 0.04 Ba 0.2 Be 0.01 Bi 0.1 0.1 Ca 0.01 0.03 Cd 0.003 0.004 Co 0.02 0.1 Cr 0.01 0.02 Cu 0.01 0.03 Fe 0.03 0.08 K 0.002 0.03 Mg 0.001 Mn 0.01 0.01 Mo 0.3 Na 0.001 0.01 Ni 0.02 0.2 Pb 0.05 Sb Hydride 0.2 Se Hydride 0.2 Si 1 Sn 1 0.5 Sr 0.02 0.02 Ti 2 V 0.3 Zn 0.002 0.01

ICPE-9000 ICPE-9000 Std Nebulizer Ultrasonic DL(3σ）(ppb) DL(3σ）(ppb) 0.3 0.04 0.5 0.2 4 3 0.5 0.04 0.01 0.01 0.02 0.002 2 0.4 0.005 0.0002 0.1 0.02 0.2 0.03 0.3 0.06 0.4 0.03 0.1 0.05 0.4 0.02 0.005 0.0005 0.03 0.01 0.5 0.08 0.3 0.02 0.3 0.1 2 0.3 3 5 4 0.8 0.5 0.1 1 0.3 0.007 0.001 0.1 0.01 0.2 0.02 0.2 0.02

Table 1: Detection Limits for Shimadzu AA-7000 and ICPE9000

Automation

Autosamplers can be used for both AA and ICP. The AA-7000 has 60 sample positions and 8 reagent positions. The ICPE-9000 has a similar 60 samples/8 reagents sampler as well as a larger 240-position sampler. Although an autosampler can be used, most laboratories would not let a flame AA be operated overnight unattended as they feel this is unsafe. Graphite furnace AA and ICP can operate overnight for use 24 hours/day, seven days/week. Also, the HVG can be automated and used with both flame AA and ICP.

Linear Dynamic Range

Shown in table 2 is the linear dynamic range (LDR), the working range or the area used for the calibration curve. Below is a generalization and the LDR varies from element to element, wavelength to wavelength, and even order to order in the case of ICP. Table 2: Linear Dynamic Range by Method

Method Furnace AA Hydride AA Flame AA ICP Axial ICP Radial ICP Dual View

Linear Dynamic Range 2.5 3 4 6 7 10

Sample Throughput

Flame analysis takes about 5-10 seconds without an autosampler. Adding an autosampler increases analysis time because there is more tubing the sample and the rinse must travel thru. Consequently, time could double or triple. Adding a hydride vapor generator in between the AA and autosampler adds more tubing, causing analysis time to be between 30 and 60 seconds. Graphite furnace AA is typically about 2-3 minutes because of various heating stages called 1) drying, 2) ashing, 3) atomization, and 4) cleaning depending on the samples. ICP measurement time is typically 30-60 seconds. Again, will the sampling be manual or automated? Adding an autosampler will increase analysis time 30 to 60 seconds. As a general rule, if analyzing 6 elements or less, then AA is the best method. For analyzing 7 elements or more, move to ICP. Also, the scientist must determine how many samples per day will need to be analyzed. As the sample volume grows and the number of elements increases then ICP may be the more appropriate technique.

Interferences

There are many types of interferences in AA and ICP but most can be corrected. Chemical interferences are more common in AA, but many can be eliminated with chemical or matrix modifiers. Because ICP has a very high temperature at 10,000 oK all the chemical bonds are broken and there are no chemical interferences.

Ionization interferences can be a problem in Group II elements when using a nitrous oxide/acetylene flame for AA. By adding Group I elements such as Cs or K, which are easily ionized and thus a source of electrons, to the samples, the Group II element will shift the equilibrium from the ionic form to the atomic form. Thus, the technique is called atomic absorption, not ionic absorption. ICP already contains a very large number of electrons, meaning ionization interferences are essentially eliminated. Spectral interferences in AA are limited because of the large line spacing of the absorption profile. In AA the “slit width” or bandwidth is set in the nanometer (nm) range. With ICP, as mentioned above, there is a database of a least 110,000 lines, which may overlap wavelengths or contain a line very close to the target element. The ICPE-9000 has a resolution of 5 picometers (pm), which is about 1000 times smaller than that used with AA. The advantage of the ICPE-9000 database is “Method Diagnosis Assistant” software, which can analyze overlapping emission lines from different elements and use another wavelength for the target element(s) in the sample. The “Method Diagnostic Assistant” is for post-run analysis. There is also a “Method Development Assistant” used prior to analysis to avoid these issues and to select the correct wavelengths upfront. Physical interferences are related to the solvent and matrix which ultimately affect droplet size. AA has fewer physical interferences because the droplet sizes are larger and the nebulizer efficiency is higher at about 5%. ICPs require smaller droplets and less sample (water vapor) or the plasma will be extinguished. A typical pneumatic nebulizer is only about 1% efficient. ICPs have many different types of nebulizers available depending on the solvent and matrix. Torch orientation also plays a role in the physical interferences with respect to ICP as shown in Figure 6. For samples containing high solids or salt matrices, plasma gas flow can be disrupted, resulting in poor reproducibility. A radial orientation means the solids coming out of the injector tip (red circle) will travel in the direction of gravity and lay on the bottom of the quartz torch, and plasma gas flow will be altered. The ICPE-9000 utilizes a vertical orientation, minimizing interference of the flow dynamics of the plasma.

Figure 6: Torch Type Orientation for ICPs

Wavelength Range

Both AA and ICP cover the necessary wavelength ranges for most of the elements of interest. The AA-7000 has a wavelength range from 190-900 nm for atomic absorption. The ICPE-9000 has a wavelength range from 167-800 nm for emission. Since the wavelength is below the vacuum UV region (i.e. below 200 nm), the system uses a vacuum as the name implies. Many ICPs on the market try to cut cost by eliminating the vacuum and having the customer pay daily by supplying a purge gas. The advantage of using a vacuum is the pressure can be measured with a Pirani gauge. With purge systems it is difficult to tell if the system is fully purged because there is no method to measure. If the purge is not 100% at the start of operation, this will result in drift.

Background Correction

The AA-7000 utilizes two types of background correction: deuterium (D2) and Self-Reversal (SR), which cover 190-430 nm and 190-900 nm, respectively, or the entire analytical range. The AA-7000 can perform a 1 point shifted wavelength correction. The ICPE-9000 can utilize a 1 or 2 point shifted wavelength correction. Shifting the background to the left, right, or both from the target wavelength corrects for the background.

Cost

Initial purchase price has flame AA the least expensive, followed by graphite furnace AA, followed by axial viewing ICP, and then dual-view ICP.

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