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Inductively Coupled Plasma–Mass Spectrometry



By Genevieve Kahrilas

Inductively Coupled Plasma–Mass Spectrometry (ICP-MS) is a versatile analytical instrument capable of detecting and measuring the concentrations most elements on the periodic table (see figure) at parts-per-trillion (ppt) detection limits [1]. Analyte is typically in aqueous media, but organic solvents may also be used. The working analytical range spans nine orders of magnitude, making this instrument suited for both trace metal analysis as well as measurement of relatively high concentrations of analyte. The instrument is capable of both quantitative analysis and semi-quantitative broad-spectrum scans for elements, making ICP-MS a suitable technique for a large number of analytical needs.


ICP-MS is capable of analyzing all elements which are colored at detection limits as indicated by the legend.  From PerkinElmer.
ICP-MS is capable of analyzing all elements which are colored at detection limits as indicated by the legend. From PerkinElmer.


Theory and Principles



This YouTube video from Agilent Technologies provides a good (albeit overdramatic) overview of what occurs inside an ICP-MS instrument. Additionally, the PerkinElmer technical note The 30-Minute Guide to ICP-MS provides an excellent summary of ICP-MS and its capabilities.









Briefly, in ICP-MS, the analyte is aerosolized and introduced into an argon inductively coupled plasma. The plamsa generates heat of 6,000 - 10,000 ºC which singly or double ionizes solution components, including the analyte. The analyte-argon plasma beam is then focused by skimmer cones into the mass spectrometer, typically a quadrupole. The quadrupolefilters out all ions except for those of a preselected m/z ratio, and therefore only the desired analyte reaches the detector for quantification. Selection of analyte based on m/z ratio means the ICP-MS easily performs isotope analysis (see: "Accurate and precise determination of isotopic ratios by MC-ICP-MS: A review " by Lang, 2009). Because the plasma generates positively charged ions, it can be very difficult to analyze elements which prefer negative ionization states (such as I and Br).

The PerkinElmer ELAN DRC II ICP Mass Spectrometer



The ICP-MS in discussion here is the ELAN DRC II by PerkinElmerSCIEX Instruments, located in the soil testing lab on the third floor of the Natural and Environmental Sciences building (NESB). Contact the soil testing lab to sign up for use of this instrument. It may be helpful to potential users of this instrument to take one of PerkinElmer's online training courses focusing on its use, applications, and basic maintenance.

Safety


This section highlights key points found within the ELAN ICP-MS Series safety manual [2].
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Where warning labels are affixed to the ELAN DRC II


General Tips

  • In general, keep your work area clean, and do not consume food or drink when working with heavy metal samples. Also ensure that your work area is well-ventilated as toxic combustion products (e.g. ammonia, metal vapor, and ozone) can be generated by the ICP system.

UV Light Warning

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Caution symbol

  • The plasma torch used by the ICP-MS emits UV radiation. Never view the ICP torch directly while in operation without UV protective eyewear.
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Electric shock hazard symbol

High Voltage Warning

  • The ICP-MS generates extremely high voltages. Do not disable safety devices and interlocks, and it is recommended that you call a PerkinElmer service specialist if the ICP-MS needs serious maintenance. Avoid tampering with anything labeled with a caution symbol or the electric shock hazard symbol.

Radio Frequency Hazard
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Radio frequency radiation symbol

  • The ICP-MS generates high amounts of radio frequency energy which is hazardous if allowed to escape. Do not tamper with shielded enclosures and/or the RF power supply as marked with the radio frequency radiation symbol. Do not disable the safety interlocks which otherwise prevent you from operating the system without all shields, covers, and doors in place.

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Hot surface symbol

Hot Surfaces Hazard

  • The torch components and connected interfaces become extremely hot, and remain hot for some time after the plasma has been shut off. Avoid touching components marked with the hot surface symbol during and after operation of the ICP-MS.

Drain System Hazards
  • The drain vessel supplied with the ELAN is made of high density polyethylene (HDPE) and gathers the effluent from the ICP sample introduction system. Never use a glass drain vessel (which may break), and never enclose the drain vessel (which may cause a build-up of hazardous gases leading to explosion or fire).

Exhaust Fume Hazards

  • The exhaust fumes created by the ICP-MS are both hot and potentially corrosive, depending on the samples that you are running. Take precautions such as extra ventilation, specialized clothing, and appropriate eye-wear to keep yourself protected.

Gas Cylinder Hazard

  • The ELAN uses argon (Ar) gas in its operation. Gas cylinders are highly pressurized, and can be extremely dangerous if mishandled. Take extreme care when working with cylinders and follow the OSHA guidelines on compressed gas and equipment.

ICP-MS Systems


A brief overview of the ICP-MS systems and their purpose in the chain of events leading to measurement, followed by more specific descriptions of the major systems. Much of this information is found within the ELAN DRC II Hardware guide [3]. Please refer to Appendix B for part orders if reordering becomes necessary.

The ELAN DRC II system consisted of an instrument, a computer, and a printer. The peristaltic pumps along the side of the instrument are responsible for delivery of aqueous analyte solution to the nebulizer. The nebulizer then creates a fine aerosol from the analyte solution which passes through a spray chamber before being introduced into the ICP torch. Rising temperatures dehydrate the analyte solution, forming a solid which then sublimates into a gas before entering the ICP. Once in the plasma, high temperatures of 6,000 ºC singly ionize the solution components, which continue on through skimmer cones and entering vacuum pressures. The ion beam then passes through ion lenses which pass only ionized species into the universal cell, which may run in collision or reaction mode to reduce sample interferences. Only then does the ion beam enter the mass analyzer (a quadrupole) whereupon ions not of the selected m/z ratio collide with the rods and do not pass through to the dynode detector. When analyte strikes the detector, a series of dynodes multiplies the signal which is then read and interpreted with the appropriate software, which runs on a Windows operating system.

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Simplified diagram of how the subsystems in the ELAN DRC II fit together.
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The ELAN DRC II major subsystems.

Instrument Specifications

  • System Power: 200-240 V, 50/60 Hz, 1600 volt amperes
  • RF Generator Power: 200-240 V, 50/60 Hz, 4400 volt amperes
  • Dimensions:
    • Width: 99 cm
    • Height: 77 cm
    • Depth: 73 cm
    • Weight: 295 kg
  • Elevation: The maximum elevation at which the ELAN may safely operate is 2,000 m above sea level. Note the elevation of the soil testing lab is at approximately 1,525 m.
  • Room Temperature: 20 ±2 ºC (68 ±3.6 ºF) for best results. The system is able to tolerate temperatures from 15 to 30 ºC safely.
  • Relative Humidity: Between 35% and 50% for optimum performance. A range between 20% and 80% is tolerated. Note that Fort Collins occasionally drops below and rises above this tolerated humidity range.
  • Quality of Environment: to minimize contamination of samples and especially for ultra-trace quantification, the ICP-MS must reside in a dust-free environment (class 1000 environment). It must also be free of smoke and corrosive fumes, not prone to vibration, out of any direct sunlight, and not located near heat sources such as radiators.
  • Coolant requirements: if not using PerkinElmer coolant for the recirculating cooling system, the alternative must meet these specifications:
    • Sediment and hardness free
    • pH between 6.5 and 8.5
    • Total heavy metals, most notably copper, < 1 ppm.
  • Argon Specifications:
    • Flow rate: 415 ±7 kPa (60 ±1 psi) at 20 L/min
    • Quality criteria: >99.996% purity, <5 ppm oxygen, <1 ppm hydrogen, <20 ppm nitrogen, <4 ppm water.
    • Either liquid or gaseous argon can be used.
  • Dynamic Reaction Cell (DRC) Gas Specifications: Typically, the DRC gas used is anhydrous ammonia. DRC gas must be delivered to the instrument at 48 kPa (7 psi). Must meet electronic or semiconductor grade specifications: 99.999% pure, liquid form.

Sample Introduction System

The sample introduction system includes all components responsible for analyte delivery to the ICP torch. The main systems involved include the autosampler, the peristaltic pump, the nebulizer, the spray chamber, and the quartz injector.
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Block diagram depicting the parts of the sample introduction system.

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Depiction of the parts which comprise the sample introduction system.
  • Autosampler: The autosampler automates delivery of analyte-containing solution to the tubing leading towards the peristaltic pump. The autosampler used with the ELAN DRC II is the FIAS-400MS and is completely computer-controlled via software. The FIAS-400MS has its own reference manual should problems occur.
  • Peristaltic Pump: A peristaltic pump uses positive displacement to pump fluids through associated tubing. The pump on the ELAN has three channels, is capable of variable speeds, and is under computer control. Speed can vary between 0.2 and 2.5 mL/min, capable of changing in 0.1 mL/min increments, and uses 0.76 mm tubing.
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A close-up of the peristaltic pump.

  • Nebulizer: A nebulizer aerosolizes the analyte solution. It is a quartz concentric design. Perfectly capable of handling 50% (v/v) solutions of hydrochloric, nitric, and phosphoric acid on a regular basis. Can also tolerate concentrations of sodium hydroxide up to 3.0% (w/v).
  • Spray Chamber: A spray chamber removes larger particles from the analyte mist prior to injection. The ELAN uses a quartz cyclonic spray chamber. Part # WE02-5221.
  • Injector: The injector delivers sample to the ICP torch. Made of quartz, there are a few sizes of injectors depending on your needs. A 0.85 mm inner diameter (I.D.) injector is recommended for use with organic solvents. Otherwise, the 1.5 mm or the standard 2.0 mm I.D. injectors will suit most needs.

ICP System

Ionization prior to m/z separation is made possible by generation of ICP. The torch is comprised of two concentric quartz tubes through which argon flows at nonequivalent rates. The argon feeding the plasma flows through a gap between the quartz middle and outer tubes, which is surrounded by an RF coil near its end. Plasma formation occurs inside the outer quartz tube where it is surrounded by the coil.
  • Frequency: 40 MHz free-running.
  • Power: 1600 watts.
  • Coil: 3-turn 1/8 in copper tubing
  • Automatic Ignition: Plasma ignition is created via software control window. Manual ignition is also possible by using the instrument front panel.

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The ICP torch and mount assebly with ball joint injector.


Sampler and Skimmer Cones

The interface (in reference to the depressurized region between the ionization scheme and the mass spectrometer) contains two cones responsible for narrowing the ion beam: a sampler and a skimmer cone. The sampler cone is blunt shaped with a central orifice of 1.1 mm diameter and is responsible for letting only the central portion of the ion beam to pass. Past the sampler cone, the pressure drops to approximately 4 torr and the ion beam then enters the skimmer cone. This sharp-angled cone has a central orifice of 0.9 mm diameter and is responsible for further narrowing the inbound ion beam. New components for this system may be purchased at the PerkinElmer online store.

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Detailed diagram of the sampler and skimmer cones.

Vacuum System

The vacuum system utilized by the ELAN is comprised of two main components: the roughing pump and the turbomolecular pump. The roughing pump is a rotary vane vacuum pump and is directly attached to the region immediately behind the sampler cone. This pump provides vacuum pressure of approximately 4 torr and is responsible for pumping away the majority of the gas stream from the plasma torch. The turbomolecular pump on the ELAN depressurizes the region behind the skimmer cone. This holds the ion optics region at a pressure of 8 x 10^-4 torr and the mass filter region to 1 x 10^-5 torr.
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The roughing and backing pumps' location on instrument.

Mass Spectrometer and Ion Optics

The mass spectrometer used by the ELAN DRC II is a quadrupole, and it is responsible for m/z selection of the analyte ion. Before entering the quadrupole for mass analysis, however, the ion beam is focused into a more narrow beam via a single cylinder ion lens. This cylinder lens operates within a voltage range of -24 V to +20 V as set by digital-to-analog converters controlled by the computer software.
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An exploded view of the cylinder lens.
The focused ion beam then passes into the dynamic reaction cell (DRC) which utilizes a reaction gas (typically anhydrous ammonia) to reduce analyte interferences. Its operation requires both radio frequency and direct current voltages, which are provided by the power supply and controlled via computer software. By interrupting the sequence of reactions that would otherwise result in an interfering signal, the DRC greatly decreases matrix effects during analysis. (See section: How does DRC work? in the PerkinElmer technical document "Expanding the capabilities of the ICP-MS."

Only then does the ion beam enter the quadrupole for m/z filtering. Four parallel conductive rods operate in vacuum pressures below 2 10^-5 torr use AC and DC voltages to cause ions of incorrect m/z values to collide with the rods; only the ions having the selected m/z value are stabilized and reach the detector. The rods in the ELAN DRC II are gold-metallized rods held in place by ceramic mounted collars, such that the coefficient of thermal expansion between the mating parts is matched and the system will not suffer from heat effects.

Detector System

The ELAN DRC II uses a dual mode detector which senses ions passed by the quadrupole, multiplies the signal, and sends it to the computer for data processing. This detector is capable of measuring both analog and pusle counts simultaneously, maximizing the ion signal count rate (the maximum pulse count detectable by the instrument is 2 x 10^6 counts). The detector is located within the vacuum chamber and is comprised of 26 dynodes responsible for the electron multiplication process. As the ions pass through the quadrupole, they strike the first angled dynode, and continue along a curved path which aidds in reducing background noise from neutrals and stray electromagnetic radiation from the ion source. Secondary electrons are created by the first dynode which descend and multiply through the remaining dynodes. Should the ion pulse count exceed the detector's limits, the pulse section of the detector is disabled and only 13 dynodes are used; this extends the dynamic range of the detector to enable analysis of higher concentrations of analyte.

Software System

The software responsible for the operation of the ICP-MS is called the ELAN 3.4 ICP-MS Instrument Control Software and runs on an attached Windows OS computer. The Software Reference Guide contains detailed information on how to use this software to achieve the best analytical results possible.

Methods and SOP's



  • The method development gude for the ELAN DRC II by the soil testing lab. When you are ready to create your own experiment on the ICP-MS, follow these standard operating procedures (SOPs) to turn the instrument on and ready it before using the software to select or create a method.
  • Calibration techniques may be found on page 40 of the "Software Reference Guide." Tuning and Optimization may be found on page 85.
  • "Determination of Trace Elements in Waters and Wastes by Inductively Coupled Plasma-Mass Spectrometry" by Long et. al. (1994). This details the EPA method for quantification of trace quantities of the elements Al, Sb, As, Ba, Be, Cd, Cr, Co, Cu, Pb, Mn, Hg, Mo, Ni, Se, Ag, Tl, Th, U, V, and An dissolved in water.
  • "Interference Removal and Analysis of Environmental Waters Using the ELAN DRC-e ICP-MS" by Neubauer and Wolf (2004). This paper, sponsored by PerkinElmer, lists the ELAN DRC II's known interferences and how to amend your method to avoid false positives.
  • Refer to the "Software Reference Guide," page 23, for more specifics on developing your own method.


Matrix Interferences



Occasionally, the Ar gas from the ICP will react with other molecules found within the matrix to produce polyatomic species which occur at the exact correct m/z ratio to produce false positive detection of a given analyte. A full list of interferences may be viewed in the document "A Table of Polyatomic Interferences in ICP-MS" by Ray and Weidmeyer and also "Interference Removal and Analysis of Environmental Waters Using the ELAN DRC-e ICP-MS" by Neubauer and Wolf (2004).
Interferences.jpg


The dynamic reaction cell (DRC) has the ability to reduce or eliminate many of these interferences. Please refer to the hardware manual and/or read
Reaction cells and collision cells for ICP-MS: a tutorial review by Scott D. Tanner et. al. See page 145 of the Software Reference Guide for specific instructions on how to correct interferences with the ELAN software.

Brine Analysis

High levels of salt may confound the analysis process. Some methods in the literature that may prove useful:
"Determination of trace metals in sea water by ICP-MS after matrix separation" by Sekhar et. al. (2003).
"Direct determination of trace elements in sea-water using reaction cell inductively coupled plasma mass spectrometry" by Louie et. al. (2002).

Troubleshooting and General Maintenance




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Under the lid of the ELAN DRC II

Unexplainable bad spectra or lack of response to analyte from the ELAN DRC II can result from one of many minor problems which commonly occur with regular use of the instrument. This section serves as a growing, living chronicle of the most common problems with the ELAN, associated symptoms, and how to quickly fix the issue. Whenever a new problem is encountered with the ELAN DRC II, please take pictures of the process and add them here such that others may benefit. This section is meant to supplement (not replace!!) the Hardware Guide, and the two should be used in conjuncture for best results.

Note that any consumables needed for the ELAN DRC II may be bought at PerkinElmer's online catalogue.

Tubing Problems


The ELAN uses an elaborate set of tubes to suck up sample for analysis. Clogged tubes, though rare, may occur with inadequate filtering of samples prior to analysis. Alternatively, the tubes may develop holes and breaks from the pumping system (as the tubes age, they become brittle). If you discover a puddle forming under the tubes, you most likely have a hole or break in the tubing system. To resolve this issue, replace the affected tube with a fresh tube located the adjacent cabinet. If supply is exhausted, more tubes may be bought from PerkinElmer. All hoses and tubes should be detached from the system after use every day to prevent premature formation of holes and breaks. Section 4-30 of the Hardware Guide includes detailed instructions for pump and tube maintenance.
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Tubing system responsible for analyte delivery to nebulizer.

Clogged Sampler and Skimmer Cones


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What very, very dirty sampler and skimmer cones looks like.

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Notice the salt build-up.



The ELAN DRC II uses two platinum cones to concentrate the plasma/analyte beam prior to mass separation, called the sampler and skimmer cones. When the samples you run are too concentrated or, alternatively, have salt concentrations in your matrix which are especially high, the ICP-MS cones may become dirty and/or clogged. When this happens, open the lid of the instrument and carefully remove the cone for cleaning. (Please refer to page 4-35 of the ELAN DRC II Hardware guide.) If dirty, the cone will have visible build-up coating it. Using a 5% nitric acid solution and a cotton swab, carefully clean off the residual build-up from the cone components. If the cone is especially clogged or dirty, use a spare cap to soak the tip in nitric until the clog is alleviated. The cones should appear bright, metallic, and shiny once cleaned. Allow the cones to dry overnight before operating the instrument again. Section 4-35 and 4-36 of the Hardware Guide detail how to remove, clean, and reinstall the cones and O-rings.


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Using a cotton swab and 5% nitric acid solution to gently clean the platinum inlet port.

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Soaking the platinum cone tip only in 5% nitric acid solution.


Melted ICP Torch


The ICP torch on the ICP-MS is located within a coiled wire adjacent to the cones (figure). Its proper function is dependent on having adequate exhaust ventilation, which has been known to be periodically non-functional in the soil sciences laboratory. Unfortunately, staff and scientists are not always made aware when these outages occur. If the ventilation system is not working and the ICP-MS is turned on, the very hot plasma torch exhaust will accumulates around the torch, causing the plasma torch to melt almost immediately. Should this happen, the torch will appear visually melted upon inspection, and will have to be replaced. ICP torch maintenance is detailed in section 4-15 of the Hardware Guide.
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The plasma torch of the ELAN DRC II.
Examples of destroyed ICP torches (taken from online source).
Examples of destroyed ICP torches (taken from online source).


References Cited



1.PerkinElmer Instruments. The 30-Minute Guide to ICP-MS; Electronic Document. www. perkinelmer. com/Catalog/CategoryPage. htm: 2001. http://www.perkinelmer.com/PDFs/Downloads/tch_icpmsthirtyminuteguide.pdf
2.ELAN ICP-MS Series Safety Manual; 016628 F; PerkinElmerSCIEX Instruments: 2001.
3.ELAN DRC II Hardware Guide; 1014467B; PerkinElmerSCIEX Instruments: 2005