Over twenty years have passed since ICP Mass Spectrophotometers (ICP-MS) was first introduced by R.S. Houk, A.L. Gray et al. in 1980, then put on the market in 1983, and is now widely used in various fields. This is especially so in the semi-conductor industry, as ICP mass spectrometry is used as analysis method for quality control of high-purity material, where demands increase with the times. Also, it is expected that the method can be applied to analysis of trace amounts of hazardous metals, and recently with various legislations in the environmental field, ICP-MS is used to respond to the stricter environmental and drainage standards.
ICP-MS offers the following features:
As shown in Figure 1, ICP-MS consists of an ion source (ICP), a sampling interface, ion lens, a mass spectrophotometer and a detector.
Figure 1: ICP-MS Structure
The ion source, ICP is an ideal ionization source for mass spectrometry, and can ionize over 90% of many elements. Ions produced in the ICP are led through the sampling interface to the mass analysis unit. The sampling interface unit consists of two metallic cones, the sampling cone (orifice radius about 0.5 to 1mm) and the skimmer cone (orifice radius about 0.5 to 1mm), and a rotary gear pump ventilates between the two into several hundreds Pa condition. The path of the ions pulled through by the sampling cone and the skimmer cone converge into the mass spectrophotometer through the ion lens. The ion lens and the mass spectrophotometer unit are ventilated to 10-3 and 10-4 Pa respectively, by the turbo molecular pump. The ions sorted by mass with the mass spectrophotometer are detected by the ion detector.
One problem with ICP-MS is the spectral interference that occurs when the spectrum of ions or molecular ions with the same mass number as the objective element overlap and interfere. Spectral interference can be categorized as follows:
Especially in the case of 1., where Argon (Ar) contained in plasma gas is a main cause, interferes evenly with any sample. Accordingly, measurement of elements interfered by Ar molecular ions is conducted in a high background condition, making infinitesimal concentration measurements extremely difficult.
Chart 1: Major Ar molecular ions
Chart 1 shows major elements affected by molecular ions of Argon origin. K, Ca and Fe are especially affected, as the Ar molecular ion levels for these elements range from tens to hundreds of ppb when converted to the concentration for each element, and ppt-order analysis under these conditions are nearly impossible. Cool Plasma Measurement addresses the problem of infinitesimal concentration analysis for elements affected by Ar molecular ions. As its name suggests, Cool Plasma refers to the lower than normal temperature of the plasma. Ar molecular ions are difficult to be produced in a cool plasma state and the background becomes as low as possible. As a result, the lower detection limit improves. Chart 2 shows detection limit (DL) and the background equivalent concentration (BEC) under cool plasma conditions. The background level is reduced to 1ppt or lower, making ppt-order analysis possible.
| Element | Mass Number | DL(ppt) | BEC(ppt) |
|---|---|---|---|
| Na | 23 | 0.05 | 0.07 |
| Al | 27 | 0.05 | 0.03 |
| K | 39 | 0.18 | 0.57 |
| Ca | 40 | 0.19 | 0.71 |
| Fe | 56 | 0.28 | 0.54 |
| Cu | 63 | 0.09 | 0.08 |
DL: Concentration calculated by multiplying the repeated measurement result of the blank by 3
BEC: The blank value converted to concentration
Chart 2: Detection Limit and Background with Cool Plasma
Environmental samples such as stream water and lake water contain many matrix components in addition to the measured elements. Therefore, many problems occur when measuring these matrix components with ICP-MS.
One is the spectral interference mentioned in the Cool Plasma description. Cool plasma can reduce molecular ions of Argon origin, but at the same time increases the molecular ions of elements contained in the sample. Also, because there is a strong desensitization effect due to the matrix, it cannot be practically used for environmental samples. Therefore, spectral interference must be reduced using other approaches. There are several forms to molecular ions and molecular ions of oxides have an especially large effect. A large percentage of oxide ion are produced from the oxygen of water (H2O) contained in sample. Therefore, reducing the water content of a sample can significantly lower the production of oxides. Also, plasma conditions and sampling interface shape in the vacuum unit can dramatically change the production rate of oxides, so optimizing these conditions can lower the production of oxides.
SPQ9000 employs a trace amount nebulizer (lowers water content), a spray chamber cooling (drains water), a plasma torch for environmental samples (sets plasma conditions to make production of ions difficult) and cones for environmental samples (reduce molecular ion production) to make measurements with little spectral interference possible.
Chart 3: Stream Water Analysis
Chart 3 shows a standard stream water measurement on sale from Japan Society for Analytical Chemistry
Hazardous elements such as arsenic, chromium and bromine have varying toxicity based on their chemical form. Measurement with ICP-MS can only be used to acquire information on total concentration, not toxicity. Recently, techniques that combine ICP-MS with chromatography equipments such as Ion Chromatography (IC) and High-Performance Liquid Chromatography (HPLC) have received attention. In these cases, ICP-MS is used as the detector for the chromatography equipment, enabling higher sensitivity than using chromatography equipment alone. Here, we will introduce an example of a simultaneous analysis of bromate ions and bromic ions in tap water using a combination with IC.
Bromic ions themselves are not hazardous, but if ozone treatment is used to disinfect tap water, a byproduct, bromate ion is produced. Bromate ions are hazardous, so it is important to determine how much bromine is contained as bromate ions. DX-500 by Dionex Corporation was used as IC.
Figure 2 shows the measurement results of bromic and bromate ions when ICP-MS is combined with IC.
Figure 2: Measurement Results of Bromic and Bromate Ions When Combined with IC
| IC | IC+ICP-MS | IC+ICP-MS | |
|---|---|---|---|
| Injection Rate (µL) | 200 | 200 | 500 |
| Bromic Ions | 0.8 | 0.09 | 0.02 |
| Bromate Ions | 0.5 | 0.11 | 0.02 |
unit: µg/L
Chart 4: Detection Limit When IC and ICP-MS Are Connected
When the injection rate was increased to 500uL, the detection limit was over 20 times better when compared to using IC alone.
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