The 30-Minute Guide to ICP-MS

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ICP Mass Spectrometry The 30-Minute Guide to ICP-MS First…A Little History ICP-MS offers many benefits to What Can Be Measured ICP-MS (Inductively Coupled laboratories performing trace metal With an ICP-MS? Plasma Mass Spectrometry) is not a determinations. For laboratories using flame or new technique at all. In fact, it is ICP-MS offers detection limits furnace AA or ICP-OES, ICP-MS over 20 years old! The first papers equal to or better than those offers the opportunity to achieve on ICP-MS were published in the attainable using Graphite higher productivity and obtain 1980s and PerkinElmer SCIEX Furnace Atomic Absorption lower detection limits. Let’s look at introduced the first commercially (GFAA) with much higher what an ICP-MS instrument can do available ICP-MS instrument in productivity. and how it does it. 1983. Since that time, many The ICP-MS instrument ICP-MS can easily handle both improvements and refinements measures most of the elements in simple and complex sample have been made to each generation the periodic table. The elements matrixes. of ICP-MS instrumentation. Today, shown in color in Figure 1 can be PerkinElmer SCIEX is on its fifth ICP-MS has detection limit analyzed by ICP-MS with detection generation ICP-MS and the capabilities that are superior limitsa at or below the part per technique is used daily by many to those obtained in Inductively trillion (ppt)b range. Elements that laboratories as a routine analytical Coupled Plasma Optical are in white are either not measur- tool. Emission Spectrometry (ICP- able by ICP-MS (the upper right OES). hand side) or do not have naturally occurring isotopes. The

a The detection limits are based on a 98% confidence level (3 standard deviations). b Identifying a single part per trillion of an element in a solution is analogous to locating a single white raisin in a house (2,700 sq. ft) full of regular raisins.

Detection Limit Ranges

Figure 1. Elements determined by ICP-MS and approximate detection capability. www.perkinelmer.com lines shown for each element in these other inorganic techniques for Data handling and system Figure 1 depict the number and elemental determination is that ICP- controller – controls all aspects relative abundance of the natural MS can determine the individual of instrument control and data isotopes for that element – this is isotopes of each element. This handling to obtain final concen- sometimes referred to as the allows ICP-MS to perform isotope tration results isotopic fingerprint of the element. ratio and isotope dilution measure- Most samples analyzed by ICP- The naturally occurring isotopes of ments. MS are liquids. However, solid each element all have the same samples can be analyzed using atomic number (number of protons A Quick Overview lasers or heated cells to vaporize in the nucleus), but differ by the An ICP-MS consists of the following the sample. Gas samples can be atomic mass. This is the result of components (see Figure 2): measured by direct introduction the different number of neutrons Sample introduction system – into the instrument. The most present in the nucleus of each consists of the peristaltic pump, common sample introduction isotope. nebulizer, and spray chamber system used on an ICP-MS consists ICP-MS shares some compo- and provides the means of of a nebulizer and spray chamber. nents, including the quadrupole getting samples into the instru- The nebulizer converts the liquid mass spectrometer, vacuum system, ment samples into very small droplets. and detector, with other types of ICP torch – generates the plasma These droplets are carried through mass spectrometers such as LC/MS which serves as the ion source of the spray chamber and into the tube and GC/MS. However, the plasma the ICP-MS, converting the or injector that is the center channel ion source used in ICP-MS is much analyte atoms to ions of the torch and then into the different than the lower-energy ion plasma. The plasma ionizes the sources used in LC/MS and GC/MS Interface – links the atmospheric elements present in the droplets. instruments. The extreme high pressure ICP ion source and the These ions then pass through the temperature of the plasma ion high vacuum mass spectrometer interface and the ion lens. After source completely breaks apart the Vacuum system – provides high being focused by the ion lens, the molecules present in a sample. As a vacuum for ion optics, quadru- ions are separated by their mass-to- result, the ICP-MS detects only pole, and detector charge ratio in the mass spectrome- elemental ions. This makes ICP-MS Lens – focuses ions into a beam ter and measured by the detector. much more comparable to other for transmission into the quadru- Once the detector measures the inorganic techniques, such as AAS pole ions, the computerized data system and ICP-OES, for elemental analy- Quadrupole – acts as a mass is used to convert the measured sis. In addition, the standard and filter to sort ions by their mass- signal intensities into concentra- sample preparation techniques, to-charge ratio (m/z) tions of each element and generate sample introduction systems, and a report of the results. The dotted potential interferences are all very Detector – counts individual ions blue arrow in Figure 2 shows the similar to those in AAS and ICP- passing through the quadrupole direction of travel of the sample OES. One advantage of ICP-MS over

Figure 2. Components of an ICP-MS system. through the ICP-MS. The next few Other types of nebulizers are also atoms travel through the plasma, pages briefly describe the function useful for introducing the liquid they absorb more energy from the of each part of the ICP-MS in more sample to the ICP-MS instrument. plasma and eventually release one detail. The specific type of nebulizer used electron to form a singly charged can be selected based on the sample ion. The singly charged ions exit Sample Introduction type and volume. the plasma and enter the interface The sample introduction system on region. an ICP-MS is very similar to that The ICP Torch - Making Ions Similar processes occur in the used on a flame AA or ICP-OES. The plasma generated in the ICP flame or plasma of AA and ICP When measuring metal content in torch creates a very hot zone that instruments, respectively. The a liquid sample, the liquid must be serves a variety of functions. At a difference is that ICP-MS measures in a form acceptable to the instru- temperature of approximately the ions themselves and the other ment’s plasma. This is achieved by 6000°C, the plasma is about 10 techniques measure light instead. introducing the sample to a nebuliz- times hotter than a pizza oven, er such as a cross-flow type shown three times hotter than a welding The Interface – in Figure 3. A cross-flow nebulizer torch, and equal to the temperature Sampling Ions works on the same principle as the at the surface of the sun. The Placing a plasma, operating at old hand-pumped insecticide plasma is generated by passing 6000oC, near an ion focusing sprayer: argon through a series of concentric device, operating near room 1. A flow of gas is passed at right quartz tubes (the ICP torch) that are temperature, is like placing the angles over the end of a tube wrapped at one end by a radio Earth about a half-mile away from containing a liquid. frequency (RF) coil. Energy sup- the sun. In addition to a large plied to the coil by the RF generator temperature difference, the plasma 2. The flow of gas shears the liquid couples with the argon to produce operates at a pressure that is much into very small droplets forming the plasma. higher than the vacuum required by an aerosol. During their voyage into the the ion lens and mass spectrometer 3. The droplets then pass through a plasma, shown in Figure 4, the portions of the instrument. spray chamber that eliminates all liquid droplets containing the The interface allows the plasma droplets except those that are the sample matrix and the elements to and the ion lens system to coexist c right size and velocity for be determined are dried to a solid, and the ions generated by the introduction into the plasma. and then heated to a gas. As the

Aerosol (fine droplets) Argon Nebulizer Flow

Liquid Sample

Figure 3. Cross-flow nebulizer. Figure 4. Form of sample in ICP-MS.

c One micron diameter (1 X 10-6 meters) is considered an ideal diameter for introduction into the plasma. plasma to pass into the ion lens The Vacuum System – the quadrupole region. Since the regiond. The interface (shown in Provides Correct Operating ions generated in the plasma are Figure 5) consists of two inverted Pressure nearly all positively charged, they funnel-like devices called cones. The distance from the interface to have a natural tendency to repel The sampler cone is located next to the detector of an ICP-MS is each other. In order to get as many the plasma and the skimmer cone is typically one meter or less. If an ion of the ions as possible into the located several millimeters behind is to travel that distance, it cannot quadrupole for mass separation and the sampler cone. Each cone has an collide with any gas molecules. eventual measurement, it is neces- opening of approximately one- This requires removal of nearly all sary to keep the ion beam from millimeter in diameter at the apex of the gas molecules in the space diverging. This is achieved by that permits the ions to pass between the interface and the passing the ions through a charged through. The cones are typically detector to create a vacuum. This metallic cylinder that acts as a made of nickel or platinum and are task is accomplished using a focusing lens. Since the charge on mounted into a water-cooled metal combination of turbomolecular the lens is the same as the charge on housing to prevent damage from the pumps and mechanical roughing the ions, the ions are repelled (like heat of the plasma. The region pumps called the vacuum system. pushing the positive poles of two between the two cones is evacuated The turbomolecular pumps work magnets together) back toward each to a pressure of a few Torre by a like jet turbines and are capable of other to form a focused ion beam. mechanical roughing pump. The rapidly pumping a chamber to a The small disk located between the holes in the cones or orifices must pressure of 1 x 10-5 Torr. The skimmer cone and the cylinder lens be large enough to prevent un- roughing pump connected to the is the shadow stop. This device vaporized materials from clogging interface area (where the cones are keeps the photons and unionized them. The orifices must also be located) removes most of the excess materials emitted from the plasma small enough to maintain a consis- sample matrix ions and gasses. from moving downstream where tent vacuum on the other side of the Routine maintenance on the they could have an adverse effect interface. As samples pass through vacuum system generally consists on the performance of the ICP-MS. an ICP-MS instrument, materials of changing the oil in the roughing Again, focusing ions in a mass will eventually deposit on these pumps every 2 to 3 months. spectrometer is like focusing light cones and will need to be removed in an optical spectrometer. Howev- by occasional cleaning. How often The Lens System - Focusing er, the optical spectrometer uses an the cones require cleaning is an Ions optical lens to bend the light beam, important maintenance item that The ion lens is positioned immedi- while the mass spectrometer uses must be considered when selecting ately behind the interface. It is an electrical field to focus the ion an ICP-MS. responsible for focusing ions into beam.

The Quadrupole - Separating Ions The mass spectrometer separates the singly charged ions from each other by mass, serving as a mass filter. Three main types of mass spectrometers are used in commer- cial ICP-MS systems: quadrupole, time-of-flight, and magnetic sector. The quadrupole is the type most commonly used in routine analytical instrumentation. A quadrupole consists of 4 rods approximately 20 cm in length and 1 cm in diameter arranged as shown in Figure 6. The quadrupole mass spectrome- Figure 5. The ICP-MS interface. ter works by allowing only one mass to pass through to the detector d "An Improved Interface for Inductively Coupled Plasma-Mass Spectrometry (ICP-MS)”, D. J. Douglas and J. B. French, Spectrochimica Acta,Vol. 41B, No 3, pp. 197-204, 1986. e The Torr is a unit of pressure equal to 1-760th of an atmosphere. The Torr is named after the physicist Evangeslista Torricelli (1608-1647) who is credited with inventing the barometer. at any given time. The quadrupole only one mass goes through the Data Handling and System actually sorts on the mass-to-charge quadrupole at a time. Controller (often referred to as m/z) ratio of the Another way to think of this is All ICP instruments require com- ions. The quadrupole does this by to compare it to AA or ICP-OES. puters and sophisticated software setting up the correct combination Instead of using an optical grating to control the plasma and mass of voltages and radio frequencies to in a monochrometer to separate out spectrometer as well as perform guide the ions with the selected m/z the wavelength of light for the calculations on the data collected. between the four rods of the element being determined, the mass The following is an overview of the quadrupole. Ions that do not have spectrometer separates the ions on main software features provided by the selected m/z pass out through the basis of their mass and charge. the ELAN software on all the spaces between the rods and are PerkinElmer SCIEX ICP-MS ejected from the quadrupole. The The Detector - instruments. mass spectrometer can move to any Counting Ions Instrument Status and Control m/z needed to measure the ele- The ions exiting the mass spectrom- All the parts of the ICP-MS dis- ments of interest in the sample eter strike the active surface of the cussed above are under software analyzed. For example, to measure detector and generate a measurable control. The software makes sure sodium, which has a single isotope electronic signal. The active surface each part of the instrument is at mass 23, the mass spectrometer of the detector, known as a dynode, working properly and can provide can be set to allow ions with m/z = releases an electron each time an the operator with useful informa- 23/1 to pass. For copper, which has ion strikes it. In Figure 7, the ion tion regarding the instrument an isotope at mass 63, the mass exiting the quadrupole strikes the status. For example, there are over spectrometer can be set to pass ions first dynode which releases elec- 70 check points on the ELAN ICP- with m/z = 63/1. If a doubly trons and starts the amplification MS that are monitored by the charged ion were formed, for process. The electrons released system to ensure the instrument is +2 example Ba , the mass spectrome- from the first dynode strike a always running in a safe and ter would need to be set for a m/z of second dynode where more elec- reliable manner. 69f to allow this ion to go through. trons are released. This cascading Instrument Optimization Even though the quadrupole of electrons continues until a All the routine optimization mass spectrometer only allows one measurable pulse is created. By procedures performed on the ELAN m/z to pass through the rods at any counting the pulses generated by are controlled by the computer. Just given time, the voltage settings on the detector, the system counts the open the optimization window for the rods can be changed rapidly. ions that hit the first dynode. The quadrupole on the ELAN® Compared with AA and ICP-OES, Series ICP-MS from PerkinElmer the detectors used in ICP-MS are SCIEX can see from m/z = 1 to m/z not much different than the photo- = 240 in less than 0.1 seconds. This multiplier tubes used as the is called the scan speed of the detectors in these optical instru- quadrupole. This is the reason ICP- ments. Instead of detecting the light MS can determine so many differ- emitted by an analyte, the actual ent elements quickly even though analyte ion itself is being detected.

Signal

Figure 6. Quadrupole mass filter. Out

Figure 7. ICP-MS detector. f Barium has a major isotope at mass 138 and the charge is 2, so m/z=138/2=69. the parameter to be optimized and solution containing as few as three identified and the concentration the software will do the optimiza- elements, a high-quality semi- estimated by comparing the tion automatically. quantitative analysis for 82 ele- measured signal to a stored Data Handling and Calculation ments can be performed in just a response file for that element. The software also translates the ion few minutes. Semi-quantitative Quantitative Analysis counts measured by the detector analysis provides a fingerprint of The ICP-MS accurately determines into useful information. The ICP- the elements present in a sample how much of a specific element is MS instrument can provide data in and the approximate concentrations in the material analyzed. In a one of four ways – semi-quantita- of each element. This information typical quantitative analysis, the tive analysis, quantitative analysis, can help determine what standards concentration of each element is isotope dilution analysis, and are necessary for quantitative determined by comparing the isotope ratio analysis. The ELAN analysis. Additionally, semi- counts measured for a selected software offers the user flexibility quantitative analysis can provide isotope to an external calibration in how analytical methods, calibra- valuable information on what other curve that was generated for that tions, and reports are configured. elements are present in a sample element. Liquid calibration stan- Results can be generated using that could cause interferences and dards are prepared in the same customized report formats or easily potentially affect the results. In the manner as used in AA and ICP-OES transferred to a Laboratory Informa- example shown in Figure 8, the analysis. These standards are tion Management System (LIMS) or concentration of zinc has been analyzed to establish the calibration other data handling system. determined to be 76 ppb. The curve. The unknown samples are software does this by comparing the Semi-quantitative Analysis then run and the signal intensities measured spectrum of the unknown For some analyses, it is not neces- are compared to the calibration sample (in blue) to the known sary to calibrate the ICP-MS for curve to determine the concentra- isotopic fingerprints for each each element. After the instrument tion of the unknown. Figure 9 element. When a match is obtained has been calibrated using a single shows an example calibration curve (shown in red), the element is for the determination of lead.

Figure 8. Semi-quantitative analysis. Isotope Ratio analyses and is often used to certify operation of the ICP-MS. Since ICP-MS instruments measure standard reference materials. On-line Help and Pathfinder™ specific isotopes of an element, the Automated Quality The ELAN software contains ratio of two or more isotopes can Control Checking context-sensitive help to assist the readily be determined. Isotope ratio The software can also monitor analyst in using the software. There determinations are used in a variety calibration and check standard is also an operational guide called of applications including geological responses and take actions to Pathfinder that walks new users dating of rocks, nuclear applica- correct any problems with the through the creation of analytical tions, determining the source of a analysis using the automated methods, running samples, and contaminant, and biological tracer quality control checking feature. In reporting results. studies. addition, check standard and spike ElanRemote™ Isotope Dilution recoveries can be calculated and Using ElanRemote, a service Isotope dilution experiments can compared to acceptable limits. The engineer or technical specialist can also be performed by ICP-MS. In user can specify what action to remotely access the instrument. isotope dilution, the sample is carry out for quality control This can help eliminate costly on- spiked with an enriched isotope samples that are outside established site service and support calls, by of the element of interest. The limits – from recalibration and re- allowing support personnel to enriched isotope acts as both a running samples to flagging remotely monitor the instrument calibration standard and an internal samples and continuing the and diagnose the problem. Many standard. Because the enriched analysis. The quality control times, service and technical support isotope has the same chemical and software in the ELAN software calls can be avoided by making physical properties as the analyte adheres to the stringent U.S. EPA adjustments via the remote connec- element, it is the best possible Quality Control Criteria. The tion. If a service call is necessary, internal standard. For this reason, automated quality control checking ElanRemote ensures the service isotope dilution is recognized as software is a key to obtaining good engineer will arrive with the correct being the most accurate type of all quality data from unattended parts to repair the instrument.

Figure 9. Quantitative calibration curve for lead at 10, 20, and 100 ppb. Summary elemental composition of samples. References There are many similarities between ICP-MS generally has fewer interfer- 1. "Applications and Technology of a ICP-MS and the other analytical tools ences than ICP-OES and is much New ICP-MS Spectrometer," used in the laboratory, such as faster than AA and Graphite Furnace Atomic Spectroscopy, Special Atomic Absorption and ICP Optical AA for the determination of multiple Issue, Vol. 16, No. 1, Jan/Feb, Emission Spectrometry. The ICP-MS analytes per sample. ICP-MS detec- 1995. is another way to determine the tion limits are generally much lower 2. "Method Development Strategies elemental content of samples. ICP- than those that can be achieved by for ICP-MS," Ruth Wolf and Zoe MS accomplishes this by counting ICP-OES and GFAA. Quadrupole Grosser, American Environmental the number of ions at a certain mass ICP-MS instruments are capable of Laboratory, February 1997. of the element instead of the light measuring as many as 35 elements in emitted by the element, as in optical a sample in two to three minutes. 3. "Mass Spectrometry of Inductively techniques. Sample preparation for Because the spectrometer and all the Coupled Plasmas," R.S. Houk, ICP-MS is very similar to that used in accessories are under computer Analytical Chemistry, Vol. 58, No, AA and ICP-OES, and in many cases control, the system can literally 1, 1986. is identical. Standards are analyzed operate 24 hours per day, 7 days per 4. "An Improved Interface for ICP- to generate a calibration curve and week, and analyze over 300 samples MS," D.J. Douglas and J. B. French, the signals from unknown samples per 24-hour day. In short, no other Spectrochimica Acta Part B, Vol. are compared against the calibration technology can provide the low 41B, No.3, 1986. curve to determine the concentration detection limits and the high produc- of each metal in the sample. The tivity for elemental analysis offered software reports data and results for by ICP-MS. quantitative, semi-quantitative, This guide is intended as a quick isotope ratio, or isotope dilution general overview of ICP-MS. For analyses. those interested in more details ICP-MS has many advantages over regarding the design and operation other technologies, such as AA and of ICP-MS instruments, additional ICP-OES, for determining the details can be found in the scientific literature (1 - 4).

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