The environment that is required for the ICP-MSICP-QQQ instrument depends on the application.
If you want to analyze ultra trace levels (ppt) of Na, Al and Fe, a clean room is required. For semiconductor applications, better than class 1,000 is preferable for the Agilent ICP-MS itself and better than class 100 is preferable for the sample preparation facility.
For other applications such as environmental, biological, botanical and geological, a clean room is not required. However, it is recommended that the ICP-MSICP-QQQ is located in a separate area to avoid cross-contamination. In particular, the sample preparation facility used for open vessel digestions must be isolated.
People are one of the main sources of contamination. Wash hands before experiments and wear clean gloves.
The sample bottles and volumetric flasks used are also very important. A new sample bottle may contain high concentrations of Na, Al, K, Fe, Zn, Sn and Pb. Bottles and flasks must be washed prior to use in a 5%(v/v) HNO3 solution for at least one night. They should be kept in a 5% (v/v) HNO3 solution while not in use, and washed with plenty of pure water just before use.
Nalgene PMP volumetric flasks are recommended for the preparation of standard solutions, polyethylene bottles for stock standard solutions and samples, and PFA bottles for pure water and acids. PTFE is difficult to clean because of its higher hydrophobicity.
The following procedure for PTFE and quartz cleaning has been suggested by an acid manufacturer in Japan:
Since the normal sample type for ICP-MSICP-QQQ is an aqueous solution, water quality is critical. Water is also used to make standard solutions and dilute samples. The suitability for ICP-MSICP-QQQ applications of three types of pure water: distilled water, deionized water and sub-boiling water is discussed below.
Distilled water is made by boiling and condensing water. This is a classical and easy way to obtain pure water. However, the quality of this water is not good enough for ICP-MSICP-QQQ applications because some volatile elements are vaporized with water. Pb, B and Zn are common impurities in distilled water.
Deionized water is obtained by passing water through ion exchange columns. This technique effectively reduces impurities in water. Modern pure water systems use this technique, for example the Millipore Milli-Q series deionized water units. Water purity is normally expressed in terms of resistivity and 18.3 MWcm is theoretically the maximum resistivity for pure water. However, there are non-conductive impurities in pure water and resistivity is only one of the indicators of water quality.
Some impurities such as Mg, Al, Ca, B may be detected in the filter used in the pure water system. We recommend you remove filters that are not necessary to analyze inorganic components. For details, contact the manufacturer of your pure water system.
The purity of ultrapure water seems to be better as the size of the system is larger. Use the largest ultrapure water system that is available. The systems that circulate pure water can produce water of high purity.
Do not store the ultrapure water from a pure water system in a tank. Collect the ultrapure water in a clean and well-used container and use it immediately. Note that a tube or storage tank that is connected to the outlet of the ultrapure water system decreases the purity of the ultrapure water.
Sub-boiling water is a type of distilled water, but there is a difference in the boiling procedure. The water is not completely boiled, but heated usually by an infrared lamp. Deionized water is used as the source water and heated up to about 40 to 50ºC. The vaporized water is recovered by a condenser. Since the water vapor pressure is very low at this temperature, the amount of pure water recovered is only about 2 L/day with a normal system. In order to obtain high purity water, the material used for the sub-boiling system is important. There are two types of material used: PTFE and Quartz.
This system can be used for purifying not only water, but also acids used in the semiconductor industry.
Boron is a very difficult element to remove because of its low dissociation in water and high volatility.
To reduce the boron background from the sample introduction system, use the inert sample introduction kit. Boron memory comes from the glass parts of the sample introduction system, even from quartz.
Which method should be used to analyze impurities in pure water? Since ICP-MSICP-QQQ is not an absolute analytical technique, standard samples have to be analyzed and pure water is used as a blank solution. Therefore, a water sample must be concentrated prior to analysis. The preconcentration can be done by evaporating the water, taking care not to evaporate volatile elements with low boiling points. Use a high purity closed quartz vessel to avoid contamination.
Elements are very unstable in pure water and are adsorbed onto the inner surface of vessels, transport tubes and the peristaltic pump tube. Therefore a small amount of nitric acid should be added to the water: at least 0.1% as nitric acid is recommended. 100 µL conc. HNO3 in 100 mL of sample gives about a 0.06% nitric acid concentration. A concentration of 0.1% is not high enough to form a stable solution, therefore the sample should be analyzed directly after preparation. At least 1% HNO3 acid is required to make a solution that is stable for more than a few minutes.
Many samples are digested by acids and alkalis. There are many grades chemicals. Refer to the certified values that manufacturers provide with the reagent to select a suitable one.
Since the final solution to be analyzed often contains acids and alkalis, analysis of these chemicals is very important. The following section gives some basic information about these chemicals.
Main polyatomic ions due to acid are shown in the table.
m/z |
Element |
HNO3 |
HCl |
H2SO4 |
|---|---|---|---|---|
20 |
Ne(90.5%) |
OH2 |
|
|
21 22 23 24 25 |
Ne(0.27%) Ne(9.2%) Na(100%) Mg(79.0%) Mg(10.0%) |
OH3 |
|
|
26 27 28 29 30 |
Mg(11.0%) Al(100%) Si(92.2%) Si(4.7%) Si(3.1%) |
CO, N2 N2H, COH NO |
|
|
31 32 33 34 35 |
P(100%) S(95.0%) S(0.75%) S(4.2%) Cl(75.8%) |
NOH O2 O2H O2 O2H |
Cl |
S SH, S S, SH SH |
36 37 38 39 40 |
S(0.02%), Ar(0.34%) Cl(24.2%) Ar(0.06%) K(93.2%) Ar(99.6%), K(0.01%), Ca(96.9%) |
Ar ArH Ar ArH Ar |
ClH Cl ClH
|
S SH
|
41 42 43 44 45 |
K(6.7%) Ca(0.65%) Ca(0.14%) Ca(2.1%) Sc(100%) |
ArH ArH2
CO2 CO2H |
|
|
46 47 48 49 50 |
Ti(8.2%) Ti(7.4%) Ca(0.19%), Ti(73.7%) Ti(5.4%) Ti(5.2%), V(0.25%), Cr(4.4%) |
NO2
ArN |
ClN
|
SN SN SO, SN SO SO |
51 52 53 54 55 |
V(99.8%) Cr(83.8%) Cr(9.5%) Cr(2.4%), Fe(5.8%) Mn(100%) |
ArC, ArO
ArN ArNH |
ClO, ClN ClOH ClO ClOH
|
SO
|
56 57 58 59 60 |
Fe(91.8%) Fe(2.2%) Fe(0.29%), Ni(68.3%) Co(100%) Ni(26.1%) |
ArO ArOH
|
|
|
61 62 63 64 65 |
Ni(1.1%) Ni(3.6%) Cu(69.2%) Ni(0.91%), Zn(48.6)% Cu(30.8%) |
|
|
SO2, S2 SO2, S2 |
66 67 68 69 70 |
Zn(27.9%) Zn(4.1%) Zn(18.8%) Ga(60.1%) Zn(0.62%), Ge(20.5%) |
ArN2
ArNO |
ClO2
ClO2
|
SO2, S2
SO2, S2
|
71 72 73 74 75 |
Ga(39.9%) Ge(27.4%) Ge(7.8%) Ge(36.5%), Se(0.87%) As(100%) |
Ar2
Ar2
|
ArCl
ArCl
ArCl |
ArS ArS ArS
|
76 77 78 79 80 |
Ge(7.8%), Se(9.0%) Se(7.6%) Se(23.5%), Kr(0.36%) Br(50.7%) Se(49.8%), Kr(2.3%) |
Ar2 Ar2H Ar2 Ar2H Ar2 |
ArCl
|
ArS
SO3 |
81 |
Br(49.3%) |
Ar2H |
|
SO3H |
Reference: H. Kawaguchi and T. Nakahara, "Plasma Source Mass Spectrometry (in Japanese)", Japan Scientific Societies Press, 1994, p. 51. |
||||
Analyte signals will be stable up to the concentrations described within this section. However, direct analysis of minerals acids or alkalis at high concentration levels, for long periods, may cause corrosion damage to the sample introduction area.
HNO3 is the most suitable acid for ICP-MSICP-QQQ because there are fewer interferences due to the acid itself and it is a strong oxidizing agent. Typical impurities in HNO3 are Pb, Sn and Zn, which are volatile elements and are carried through the distillation process. To get better quality HNO3, a sub-boiling system is commonly used, e.g. TAMAPURE guarantees impurity levels lower than 10 ppt. Typical ICP-MSICP-QQQ interferences are N2, NO with Si and ArN with a minor isotope of Fe. HNO3 solutions up to 40% can be introduced.
HCl is also a very good acid to digest metal oxides and metals, but is rarely used alone for digestion. HCl is a reducing agent and not used for digesting organic materials. Typical impurities in HCl are As, Sb and Sn because of their volatility. To get better quality HCl, the sub-boiling system is used.
Typical ICP-MSICP-QQQ interferences are ClO and ArCl with V, Cr, As and Se. Therefore, HCl should be avoided when analyzing these elements.
Platinum group elements are stable in an HCl matrix solution. Chlorides of As, Sb, Sn, Se, Ge and Hg are readily lost during open vessel digestions at higher temperatures. Utilization of HCl should be avoided, if possible, for ICP-MSICP-QQQ, as HCl has to be removed from the sample by evaporation. Up to 18% HCl solutions can be introduced into the ICP-MSICP-QQQ.
HF is an acid which dissolves silica and attacks Ni interface cones, therefore a special sample introduction system must be used. The inert sample introduction kit consists of a crossflow nebulizer, a polypropylene spray chamber and a Pt injector torch. Interferences due to HF aren't critical; ArF would interfere with Co although formation of ArF is insignificant.
Fluorides of B, Si, As and Sb might be lost along with HF, while salts of Ca and K are poorly soluble in HF. HF can be removed from the sample by evaporation. Up to 30% HF can be introduced into ICP-MSICP-QQQ.
H2O2 is a strong oxidizing agent and used with other acids for digestion instead of HClO4. The H2O2background shows a similar background to water and H2O2is one of the best chemicals for ICP-MSICP-QQQ analysis. A 30% H2O2solution can be analyzed. Up to 60% H2O2solutions exist, but higher concentrations of H2O2are extremely dangerous, therefore the ultrasonic nebulizer should not be used for higher concentrations of H2O2. Since the dissociation constant of H2O2is low in water, a small amount of HNO3, to give about 0.1%, should be added.
H2SO4 is an oxidizing agent and used for the digestion of organic materials and geological materials with other acids. Since the boiling point of H2SO4 is extremely high, 338 , it is hard to decompose in the plasma. It adheres to the interfaces and the lenses, causing deterioration of Ni and Cu interfaces and instability of signal. Pt interfaces have to be used. In addition, H2SO4 forms polyatomic ions; SO, SO2 and S2 interfering with major isotopes of Ti and Zn and minor isotopes of V and Cr. The quality of H2SO4 is not as good as other acids because the distillation requires higher temperatures.
Sulfates of Ba, Ca, Pb and Sr have very low solubility, and those of Ag, As, Ge, Hg, Re and Se are volatile and might be lost during open vessel digestion. H2SO4 cannot be removed from the sample by evaporation unlike HCl and HF, therefore utilization of H2SO4 is not recommended for ICP-MSICP-QQQ. The maximum recommended concentration that should be aspirated for long periods of time is 1% with the concentric nebulizer (CN), crossflow nebulizer (CF) and babington nebulizer (BN), and 5% with the micro flow nebulizer.
H3PO4 is not commonly used for digestion, but used as a buffer agent with other acids. Its extremely high boiling point limits its application in ICP-MSICP-QQQ, as H3PO4 cannot be decomposed in the plasma, and it deteriorates Ni interfaces. In addition, H3PO4 forms several polyatomic ions, HxPyOz. Therefore utilization of H3PO4 should be avoided. The maximum concentration that can be analyzed is less than 0.1% with CN, CF and BN.
HClO4 is one of the strongest oxidizing agents and it reacts explosively with organic materials. Therefore, organic materials should be pretreated with HNO3 or a mixture of HNO3 and HClO4.
Removal of HClO4 is more difficult than for HCl, and ArCl and ClO will interfere with As, Se and V. Therefore, H2O2 is preferred rather than HClO4 as an oxidizing agent for ICP-MSICP-QQQ samples.
Aqua regia is commonly used for the digestion of metals and alloys, especially for precious metals such as Au, Pt and Pd. Since aqua regia includes HCl, there are polyatomic interferences due to Cl. An HCl matrix is generally removed by evaporation.
NaOH is not commonly used for digestion and is rarely analyzed by ICP-MSICP-QQQ. Alkali solutions dissolve glass slightly at room temperature, therefore the inert introduction system has to be used. Higher (%) concentrations of Na may cause matrix suppression and interfere with m/z 63 on Cu.
LiBO2 is the most commonly used reagent for the alkali fusion technique which is applied to the digestion of refractory compounds such as geological and metallurgical samples.
However, higher concentrations of Li and B cause matrix suppression. The microwave digestion that is described later will be used instead of the alkali fusion technique.
NH4OH is one of the commonly used alkali solutions in semiconductor manufacturing for cleaning organic compounds.
Because of its constituents, N, H and O, the polyatomic interferences are similar to those caused by water in the plasma. However, 5% is the maximum recommended concentration that can be analyzed with the CN and CF because of higher vapor pressures at room temperature.
To stabilize signals of analytes and neutralize samples, adding HNO3 might be better.
TMAH is also commonly used in semiconductor manufacturing for cleaning organic compounds, and is now also used for the digestion of blood and biological materials.
Because of its constituents, C, H, N and O, there are few significant polyatomic interferences except ArC which interferes with Cr.
Because of its strong alkaline character, the inert sample introduction system has to be used, and 5% is the maximum concentration that can be used with the CF and BN because of higher vapor pressures at room temperature.