Inductively coupled plasma–mass spectrometry (ICP-MS) is renowned for its phenomenal sensitivity as an elemental analyzer. It is the most sensitive elemental spectrometric technique, with detection limits in the low parts-per-trillion or even parts-per-quadrillionrange for many elements. This article examines the reasons for this sensitivity and considers how it might eventually be further improved.

There are basically three types of commercially available inductively coupled plasma–mass spectrometry (ICP-MS) instruments that differ mainly in the type of mass analyzer they use. They are quadrupole, magnetic sector field (also called high resolution ICP-MS), and time of flight. Of these, quadrupole ICP-MS is by far the most commonly used, and this article will be limited to this type of instrument. Most of the relevant principles, however, generally apply to other mass analyzer types as well. 

The ICP-MS instrument consists of four main components. The inductively argon coupled plasma ICP serves as the high temperature ion source. The ICP essentially is the same as that used in ICP optical emission instruments with one important difference — the plasma in optical devices is primarily an atom source whereas in a mass spectrometer it produces ions. The interface and ion optic components extract the ions from the plasma at atmospheric pressure and focus them into the mass analyzer, which is at high vacuum. The mass analyzer (quadrupole in this discussion) separates the ions based on their mass-to-charge ratio (

) and delivers them sequentially to the detector. The detector is an electron multiplier that generates an electrical pulse to the counting circuitry for each ion that strikes it. Most newer quadrupole ICP-MS instruments also include a collision–reaction cell between the interface and analytical quadrupole. The function of this cell is to remove polyatomic (molecular) interferences before mass analysis. Some loss of signal can occur in this cell due to collisions between analyte ions and gas in the cell. 

The argon ICP is a highly efficient ion source. Most metals are ionized at 80 to >95% efficiency, while a few high ionization potential metals such as mercury have ionization efficiencies as low as 40%. Even nonmetals such as sulfur, phosphorus, silicon, and all of the halogens except fluorine are sufficiently ionized to allow for highly sensitive measurement. Only five elements cannot be directly measured by ICP-MS: hydrogen, helium, fluorine, neon, and argon. The first four are not ionized because their first ionization potentials are higher than that of argon, and the last argon is not measurable in an argon plasma. 

Let's examine the actual efficiency of a typical quadrupole ICP-MS system. We'll use 1 part per trillion (ppt) as an example detection limit, though some elements will be lower, some higher. A 1-ppt (w/v) concentration  is equivalent to 1 ng/L or 10

  g/L.  The equivalent number of atoms (ions) will depend on the atomic weight. Applying the Avogadro constant (6.02 X 10

  atoms of Be (atomic mass 9) or ~2.53 X 10

  atoms of U (atomic mass 238) per liter. Using a convenient nebulizer flow rate of 1 mL/min would result in between 1.1 X 10

  atoms per second per ppt being introduced to the instrument at the nebulizer. ICP-MS sensitivity is commonly measured in terms of millions of counts per second at the detector per ppm introduced to the nebulizer (Mcps/ppm, also called MHz/ppm). This value varies depending on the element as well as instrument design and operating conditions but is typically ~100–500. So if we work back to our 1-ppt example, this translates to 100–500 cps/ppt.  Using the calculation above for the number of atoms introduced to the nebulizer (we'll use an average value of 10

 ) for a 1-ppt solution, we can calculate the efficiency of generating a pulse at the detector from a given atom introduced to the nebulizer as 100/10

 . About one in a million atoms introduced to the nebulizer actually reach the detector. Where do the rest go? If we examine all of the components of a typical ICP-MS system (Figure 1), we will see that there are inefficiencies (some intentional, some unavoidable) at each stage. 

Figure 1:  Block diagram depicting the functional components of a modern quadrupole ICP-MS system in order from left to right.			

Table I shows the efficiencies that can be assigned to the various stages of ICP-MS analysis in very general terms. 

Multiplying the efficiencies in Table I results in an overall estimated efficiency of about 0.00002% (1 in half a million), which is in general agreement with the estimate based on measured response. So, the most sensitive elemental analyzer we have is only about 0.00002% efficient. What if anything can we do about that? And, do we really need to do anything? There are two schools of thought on whether we need to further improve ICP-MS sensitivity. The first school, typified by the commercial service laboratories, would generally say that 1-ppt detection limits are good enough for most applications, and that in fact it is very difficult to routinely produce blanks at that level for many elements. Therefore the actual method detection limits are not instrument limited but are limited by the cleanliness of the sample preparation and laboratory environment. 

Table I: Approximate efficiencies for the various components and processes of a modern quadrupole ICP-MS instrument			

The other school may be looking for sample detection limits below 1 ppt but more commonly is trying to compensate for very small samples or analyte loss during sample preparation or dilution. These are typically research laboratories that can better afford to spend the time necessary to produce sufficiently clean blanks. In any event, the quest for more sensitivity is a real one and has resulted in significant improvements over the years.  

Early instruments typically were capable of <1 MHz/ppm sensitivity compared with 100–500 MHz/ppm or higher today. A close look at Table I shows a number of areas where there is significant room for improvement as well those where there is not much to be gained. Most of the opportunities lie in the processes of transporting the sample into the mass spectrometer.  

The development of high efficiency or direct injection or total consumption nebulizers in which there is no spray chamber or spray chamber drain permits virtually 100% of the nebulized sample to be introduced to the ICP torch. However limitations in the ability of current ICP plasmas to tolerate increased sample load limit these applications to very low flow, and therefore the total amount of sample introduced to the plasma may not be significantly higher. As a result, total consumption nebulizers have more utility in applications where the sample flow rate or volume is very small rather than in improving instrument sensitivity.  

Sampling and skimming efficiency are also quite low (~2% each), again for practical reasons. The sampler and skimmer cones make up the interface between the hot plasma operating within atmospheric pressure and the cool, high-vacuum regions of the mass spectrometer. Through two stages of pressure reduction, the pressure is reduced from ~1 bar outside the sampler cone to ~10

  bar immediately behind the skimmer. This reduction is achieved through the removal of primarily argon from the plasma and to a much lesser degree sample matrix components, mainly from water. A further 10-fold reduction in pressure is achieved via an entrance aperture into the mass analyzer section that is pumped by high-vacuum turbomolecular pumps. Considering the 10

 -fold reduction in pressure across the interface, the 4 X 10

  loss of sample isn't too bad. Increasing the efficiency of the interface region (sampler and skimmer) requires larger cone orifices, which in turn require significantly larger vacuum pumps to maintain the necessary vacuum. This can be (and is) done, but the cost of the larger pumps is high. Behind the skimmer, most of the plasma gas has been removed, and the neutral plasma has been largely converted to a positively charged ion beam through removal of electrons at the skimmer or first lens. It is at this point, the ion lens region, that the best opportunity for increasing the efficiency exists. The function of the ion lenses is to focus the ion beam into the mass spectrometer with minimal loss while preventing the transmission of photons and neutral particles that can cause elevated background (noise) if they reach the detector. The ion lenses take advantage of the fact that positively charged ions can be focused and steered by electrostatic fields that have no effect on photons and neutrals. Because of this, the detector can be positioned so that it is not in line of sight of the plasma and the ions steered to the detector.  

Two common approaches are taken. The first is to insert a disk in the direct ion path between the interface and quadrupole. This "photon stop" or "shadow stop" serves to physically block light and neutral particles from reaching the detector. This approach was used on early R&D and first-generation instruments in the 1980s. The analyte ions must be defocused around the stop and then refocused behind it. This is a simple but inefficient process that causes significant discrimination against low-mass ions and results in loss of sensitivity in general.  

Alternatively, more-modern instruments that locate the quadrupole and detector off axis from the initial ion optics can achieve the same reduction in photons and neutrals while maintaining higher ion transmission across the mass range. This is achieved by using special ion optical components, often called chicane or Omega (Agilent Technologies) lenses, which serve to deflect the ion beam while maintaining ion focus as much as possible. Improvements in ion optics have been responsible for much of the increase in ICP-MS sensitivity over the years, and there is still room for more improvement. Beyond the ion lenses lies the collision cell in instruments so equipped.  

The collision cell is a small pressurized cell containing a multipole ion guide to guide the ions through the cell as efficiently as possible. A number of forces within the cell are acting to disrupt the orderly flow of ions from the cell entrance to the cell exit. These are mainly space charge; that is, the mutual repulsion of positively charged ions confined to a small space and collisional scattering due to collisions with the gas in the cell. The multipole's function is to keep the ions focused in the center of the cell and transmit them to the cell exit, while permitting efficient collisions with the cell gas.  

Three common multipole configurations are used. They are quadrupole (four rods), hexapole (six rods) and octopole (eight rods). Each type of multipole has its strengths and weaknesses.  However as a collision cell ion guide, the octopole has the benefits of highest simultaneous transmission efficiency across the mass range and a wider potential well, which permits more of the space between the rods to be used for collisional purposes. As a result, a collision–reaction cell based on an octopole ion guide can be smaller in volume, reducing gas flow requirements and speeding up cell evacuation. After the ions reach the quadrupole, there is still some opportunity for improvement in sensitivity.  

The quadrupole mass filter operates by creating a hyperbolic RF field in which stable ion trajectories are dependent on the mass-to-charge ratio of the ion and the amplitude of an applied RF and DC voltages. The transmission efficiency of the quadrupole is similar to other mass spectrometers in that mass resolution is gained at the expense of sensitivity. Therefore, sensitivity can be improved somewhat if the mass resolution is decreased —that is, the mass peaks are widened. The danger in doing this is related to a characteristic called abundance sensitivity.  Abundance sensitivity is the net signal from an isotope present at mass M divided by the signal at mass M+1 or M–1 when there is no isotope present at either M+1 or M–1. This value typically is ~10

  and is somewhat higher on the low mass (M–1) side. The net effect is that a weak peak adjacent to a strong peak will be affected by the tailing from the larger peak. Better (higher) abundance sensitivity values can be achieved through the use of quadrupole rods adhering to the ideal hyperbolic cross-sectional shape rather than the less expensive and more common cylindrical shape.  

Continual research related to optimizing these components of modern ICP-MS instruments has resulted in significant improvements in both high sensitivity and reduced background when compared with early instruments. This trend obviously will continue, though to what end and through what means is difficult to say. After all, there are still a lot of atoms unaccounted for in that 1-ppt solution. 

 is with Agilent Technologies, Bellevue, Washington. 

Articles

Factors Determining Sensitivity in ICP-MS

Recently, the world's attention has turned to the toxic element content found in the painted surfaces of consumer products. The U.S. Consumer Product Safety Commission (CPSC) has responded to this interest by lowering the maximum permissible lead levels allowed in paint from 600 mg/kg to 90 mg/kg (16 CFR part 1303). In this work, the determination of leachable toxic elements from a variety of paint samples by inductively coupled plasma–optical emission spectroscopy is described. Paint samples included indoor and outdoor house paint, paint scraped from plastic and wooden toys, and paint scraped from the handle of screwdrivers. Toxic elements were leached from paint samples using either 0.07 M hydrochloric acid or 4% acetic acid solutions in accordance with protocols outlined in ASTM F963-08 and ASTM C738-94, respectively. Results between extraction methods were compared and detection limits in each acid matrix were calculated.

Recognition of the adverse effects of human exposure to toxic elements leached from painted surfaces has increased significantly over the last two decades. These elements are used commonly in paint pigments and as paint additives to increase the speed with which paint dries after application and to improve its durability and resistance to mildew corrosion. Lead(II) chromate (PbCrO

) are responsible for paint colors referred to as "chrome yellow," "white lead," and "red lead," respectively. 

Lead and other toxic elements also are used in the toy manufacturing process and can be concentrated particularly in those toys made out of plastic. Painted toys sometimes contain lead-based paint that gives them a durable, brightly colored outer surface, as illustrated in Figure 1. Plastic toys contain additional amounts of lead to increase the flexibility of the material and to make it more resistant to warping from heat and sunlight. 

Figure 1: Yellow-painted plastic toy car.			

When the paint is in good condition, homes and toys pose little or no health risk. The concern arises when these paints peel or crack as they age and deteriorate or when a painted surface is scraped, sanded, or heated. Exposure to high concentrations of these toxic elements could be a risk if the paint chips are ingested or if dust from the paint is inhaled. 

In response to this potential health threat, a number of regulatory agencies have set maximum limits on the concentrations of toxic elements in paint and in various consumer products. The U.S. Consumer Product Safety Commission (CPSC) set the maximum allowable lead content in paint to 0.009% (90 mg/kg) as outlined in the Code of Federal Regulations (16 CFR part 1303). This limit, reduced from 0.06% (600 mg/kg), went into effect in August 2009 (1). 

The European Union set limits on the maximum allowable concentrations of toxic elements that can be leached from toys and those limits are outlined in EN 71, part 3 (2). These regulations have been adopted by the U.S. CPSC and have been incorporated into guidelines set by the International Organization for Standardization (ISO) regarding toy safety (ISO 8124 part 3). A summary of those guidelines is outlined in Table I. Toy samples that are tested under these regulations must be prepared for analysis according to the hydrochloric acid leaching procedure outlined in test method ASTM F963-08 (3). 

Table I: Limits of element migration from toy materials as outlined in ASTM F963-08			

The U.S. Food and Drug Administration (FDA) set guidelines for the maximum amount of cadmium and lead that can be leached from a variety of ceramic and flatware materials. All items that fall under these regulations are intended for use with food and leachable limits for cadmium and lead vary depending upon the end use of the material (see Table II). The FDA refers to a leaching procedure with acetic acid, outlined in test method ASTM C738-94 (4), for testing materials that are regulated under FDA guidelines. 

Table II: Maximum allowable concentrations of Pb and Cd that can be leached from ceramics, as outlined in ASTM C738-94			

Both extraction methods mentioned here are designed to leach toxic elements from paint in a manner that mimics the human digestion process. Total dissolution methods provide accurate and reproducible results for the determination of toxic elements in paint samples and consumer products; however, these methods are not desirable for this application as results do not accurately reflect the concentration of each element that is accessible to the human body upon ingestion of the material. 

This article will demonstrate the ability of inductively coupled plasma–optical emission spectroscopy (ICP-OES) to analyze lead-based paints. The sensitivity and large linear dynamic range of the approach will be used to determine a wide range of elements in several different paint samples. 

A Dual View Prodigy High Dispersion ICP spectrometer (Teledyne Leeman Labs, Hudson, New Hampshire) equipped with an 88-position autosampler was used to generate the data for this article. The sample introduction system consisted of a four-channel peristaltic pump, a glass cyclonic spray chamber with a center knockout tube, a single-piece quartz torch, and a Seaspray concentric nebulizer (Glass Expansion, Pocasset, Massachusetts). Instrument conditions used for all data collection are listed in Table III. 

Table III: Instrument parameters used for all sample analyses			

All solutions were prepared using >18 MΩ cm

  water (Barnstead, Dubuque, Iowa) and either reagent grade hydrochloric acid (VWR, West Chester, Pennsylvania) or reagent-grade glacial acetic acid (Aldrich, St. Louis, Missouri). Multielement calibration standards were prepared by dilution from single-element stock solutions (PlasmaPure Standards, Teledyne Leeman Labs) containing 1000 μg/g of As, Ba, Cd, Cr, Hg, Pb, Sb, and Se. All dilutions were performed on a w/w basis. Standards were prepared at relatively low concentrations over a short range to cover the low concentrations expected in solution following extractions of the paint samples. Standards were prepared in two acid matrices that were carefully matched to those in the prepared paint samples (0.07 M HCl and 4% CH

COOH). The concentrations used for calibration are listed in Table IV. 

Table IV:  Calibration standard concentrations, prepared in both 0.07 M HCl and 4% acetic acid matrices			

Various paint samples were prepared for analysis according to extraction methods outlined in ASTM F963-08 and ASTM C738-94. Paint samples included red and blue paint from rubber balls, yellow paint from a plastic toy car, blue paint from a wooden spinning top, red paint from trees associated with a train set, red paint from a children's plastic workshop set, yellow paint from the handle of a screwdriver, indoor latex house paint, oil-based exterior paint scraped from the deck and outdoor walls of a house, and a certified reference material, Trace Metals in Paint Chips (RTC CRM013-050, lot# AW13). 

For those samples extracted according to ASTM F963-08, approximately 100 mg of each paint sample was weighed into a 20-mL polypropylene beaker and the exact weight was recorded. To each beaker, 5 mL of 0.07 M HCl was added and all beakers were heated to 37 °C on a hot plate for 2 h in the dark. All samples were agitated for the first hour of heating and were allowed to sit without agitation for the second hour of heating. After cooling, samples were filtered through 0.45-μm filter paper and the final volume was recorded.  

For samples extracted according to ASTM C738-94, approximately 100 mg of each paint sample was weighed into a 50-mL polypropylene volumetric centrifuge tube (VWR, West Chester, Pennsylvania) and the exact weight was recorded. To each tube, 10 mL of 4% CH

COOH was added and all tubes were allowed to sit for 24 h in darkness. Samples were then filtered through 0.45-μm filter paper and the final volume was recorded. 

After the plasma was ignited, the instrument was allowed to warm up for 15 min before the torch observation height was set using standard 5 (see Table IV). Following calibration, a detection limit study was performed to determine the instrument's detection limits for the elements of interest in 0.07 M hydrochloric acid and 4% acetic acid matrices. The instrument detection limit (IDL) for each analyte was calculated to be the concentration equal to three times the standard deviation of 10 replicate measurements of the calibration blank. 

Table V: Detection limits in 0.07 M HCl and in 4% acetic acid matrices			

Results for IDLs in both acid matrices and are reported in Table V. Results in Table V indicate that the detection limits are more than sufficient for determining whether the analytes of interest are present at concentrations that exceed the regulatory requirements outlined earlier. 

Table VI: Results for paint sample analyses following extraction with 4% acetic acid			

Results for the analysis of paint samples following acetic acid and hydrochloric acid extractions are listed in Tables VI and VII, respectively. Numbers in parentheses represent %RSD values for the corresponding measured analyte concentration, both in units of milligrams per kilogram unless otherwise specified. Results in both tables are listed in units of milligram per kilogram and reflect the concentration in the original solid material. Results in Table VIII have been corrected using the analytical correction factors outlined in Section 8.3.4.3 of ASTM F963-08. The correction factors are as follows: As, 60%; Ba, 30%; Cd, 30%; Cr, 30%; Hg, 50%; Pb, 30%; Sb, 60%; and Se, 60%. 

Table VII: Results for paint sample analyses following extraction with 0.07 M HCl			

Results from Table VII indicate that the blue wooden top contains lead at a concentration that exceeds limits regulated by the U.S. CSPC. If all samples tested in this work were regulated under U.S. CSPC guidelines for migratory elements, the yellow screwdriver handle, the indoor paint sample, and both outdoor paint samples would exceed regulatory limits for lead. The yellow screwdriver handle would also exceed limits for chromium. 

Table VIII: Comparison of acetic and hydrochloric acid extraction procedures based on results for Pb and Cd			

Results for several samples from Tables VI and VII along with results from the certified reference material are listed in Table VIII for comparison between the two extraction procedures. Results for the certified reference material are included for evaluating the efficiency with which cadmium and lead can be extracted from material containing a known concentration of each analyte (certified values for cadmium and lead in the CRM sample are 37.8 μg/g and 643 ?g/g, respectively). Numbers in parentheses represent %RSD values for the corresponding measured analyte concentration, and all results and RSD values are listed in units of milligram per kilogram unless otherwise specified. Because the acetic acid extraction specifically targets cadmium and lead, only results for cadmium and lead were selected for comparison. 

Results in Table VIII demonstrate the significant variability between extraction methods as well as variability within each extraction method. Results for duplicate preparations of the certified reference material further illustrate the challenges associated with obtaining reproducible results and indicate the reason that certified reference materials are not used for validating these extraction methods. Results indicate that the hydrochloric acid extraction procedure generally is more aggressive than the acetic acid procedure; however, a total dissolution method is needed to obtain results for cadmium and lead that would fall within the accepted concentration range for certified reference materials. 

The determination of eight toxic elements in paint and in painted surfaces of consumer products following two different acid extractions has been presented. The extraction methods were designed to simulate the ingestion of paint flaked from the surface of a consumer product and results would reflect the concentration of each element that the human body could leach out during digestion.  

Samples containing concentrations of lead and cadmium that exceed FDA regulated limits can be determined using both the acetic and hydrochloric acid extraction methods, regardless of the variability exhibited in the results from each extraction. The hydrochloric acid extraction is recommended for testing products regulated under U.S. CSPC guidelines. Hydrochloric acid was a more aggressive leaching solution and would provide a more conservative estimate of the potential for exposure to toxic elements upon ingestion of the product in question.  

Maura Mahar, Manuel Almeida, and Peter Brown

 are with Teledyne Leeman Labs, Hudson, New Hampshire.  

(1) Consumer Product Safety Commission, Code of Federal Regulations, 16 CFR part 1303, Ban of Lead-Containing Paint and Certain Consumer Products Bearing Lead-Containing Paint. 

(2) European Standard on Safety of Toys, EN 71 part 3, Migration of Certain Elements. 

(3) American Society for Testing and Materials (ASTM) International, ASTM F963-08, Standard Consumer Safety Specification for Toy Safety. 

(4) American Society for Testing and Materials (ASTM) International, ASTM C738-94(2006), Standard Test Method for Lead and Cadmium Extracted from Glazed Ceramic Surfaces. 

Analyis of lead paint in toys using ICP-OES

Determination of Toxic Elements Leached from Toys and Household Structures by Inductively Coupled Plasma–Optical Emission Spectroscopy

Because of the significance of clean water for human health and agriculture, water analysis is critical for determining both elements present at high levels as well as trace contaminants. Several types of waters must be analyzed: water for human consumption must be monitored for both nutritional and toxic components, wastewater discharge must be analyzed to verify the effectiveness of treatment in removing toxic components, and lake and river water should be analyzed to help determine mobility of elements through the ecosytem. 

A variety of techniques can be used for these analyses. For example, elements that typically are present at elevated levels (such as Na, Mg, K, and Ca) can be measured easily by inductively couple plasma–optical emission spectrometry (ICP-OES), while elements normally present at trace levels are best measured with techniques optimized for low-level analysis, including ICP-mass spectrometry (MS), graphite furnace atomic absorption spectrometry (AAS), or hydride generation AAS. Among these techniques, ICP-MS and ICP-OES are uniquely suited to analyze both low and elevated levels, with ICP-MS allowing lower levels to be measured than ICP-OES. 

The growing popularity of cell-based ICP-MS has opened new possibilities for water analysis. Because cell-based ICP-MS eliminates the effects of interferences, it is now possible to measure elements at lower levels, usually limited by the cleanliness of the reagents and laboratory environment. The ability to measure lower levels allows regulated limits to be lowered. Recently, the U.S. Environmental Protection Agency has lowered the acceptable limit of arsenic in drinking water from 50 μg/L to 10 μg/L.  

Another aspect of cell-based ICP-MS is the ability to suppress signal sensitivity, thus allowing higher levels to be measured, essentially extending the dynamic range of the instrument. This capability allows elements present at high levels (such as Na, Mg, K, and Ca) to be analyzed by ICP-MS with less dilution. Depending upon the configuration of the ICP-MS system, the suppression of signal sensitivity can be applied on a per-isotope basis without affecting the sensitivity of other isotopes, which allows both major and trace levels to be measured from the same sample in a single method. 

This article will focus on the analysis of drinking and saline waters for both major and minor elements in a single analysis. All water samples were analyzed by ICP-MS and ICP-OES to confirm the results. 

Samples were analyzed without any pretreatment. Drinking waters were analyzed directly, and saline waters were diluted 12.5 times with 1% (v/v) HNO

 (Suprapur, Merck KGaA, Darmstadt, Germany). 

Internal standards were used to compensate for possible matrix effects during sample introduction. An internal standard mix (Y, Rh, and Re) was added on-line by merging flows of the sample and internal standard mix.  

External calibration curves were applied using standards ranging from 5 μg/L to 400 mg/L, depending upon the element and expected levels. Elements likely to be present at low concentrations (that is, As, Se, and Pb) used calibration curves established with 5, 10, 50, and 100 µg/L standards, while high-level elements (that is, Na, Mg, K, and Ca) used standards at milligram-per-liter concentrations. 

Table I: ICP-MS instrumental parameters			

All analyses were performed on an ELAN DRC-e ICP-MS system (PerkinElmer, Inc., Shelton, Connecticut) equipped with a Peltier-cooled spray chamber and a Meinhard C-type glass concentric nebulizer. In a single method, the elements were grouped into three modes: standard mode, extended dynamic range mode (that is, standard mode with the application of bandpass parameters), and dynamic reaction cell (DRC) mode using oxygen as a reaction gas. Instrumental parameters are reported in Table I, and dynamic reaction cell conditions appear in Table II. 

Table II: Reaction cell parameters for ICP-MS analyses			

The samples also were analyzed for comparison by ICP-OES, using an Optima 5300 DV system (PerkinElmer, Inc.). Using dual-view capabilities, elements present at elevated levels were measured in radial mode and trace-level elements were determined axially. Several wavelengths were used for each element to check for possible spectral interferences; the preferred wavelengths, along with analysis mode and instrumental conditions, are reported in Tables III and IV. 

Table III: ICP-OES instrumental parameters			

The ability to suppress signal intensity selectively on a per-isotope basis without affecting the sensitivity of other isotopes is accomplished by varying the reaction cell bandpass parameters (specifically RPa) when the reaction cell is not filled with a gas. This allows a high degree of tuneability of isotopic sensitivities to best meet the desired analytical levels. 

Table IV: Wavelengths and analysis mode for ICP-OES analyses			

Figure 1 shows calibration curves for Na, Mg, K, and Ca up to 400-, 100-, 40-, and 200-mg/L levels, respectively. Correlation coefficients for these calibration curves were at least 0.9999, which demonstrates linearity and allows the accurate analysis of elevated levels of these elements in samples. 

Tables V and VI show both the ICP-MS and ICP-OES results for drinking waters and saline waters, respectively. The agreement between all techniques for elements present at both trace and elevated levels in both water types confirms the concentrations. The effectiveness of extending the dynamic range is evidenced by the results for Na and Mg in the saline waters: The concentrations from both ICP-MS and ICP-OES are nearly identical. 

Table V: ICP-MS and ICP-OES results for drinking waters			

This work has demonstrated the ability of DRC-ICP-MS to analyze elements present at both trace and elevated levels in drinking and saline water samples. The capability to suppress signal intensity selectively without the use of a cell gas allows the dynamic range of DRC-ICP-MS to be extended and matrix elements to be measured. All results were confirmed using ICP-OES. Although both ICP-OES and DRC-ICP-MS give similar results for these samples, it should be noted that these are complementary techniques with each having its strengths and weaknesses for various sample types and analyte levels. 

Table VI: ICP-MS and ICP-OES results for saline waters			

 is Senior Inorganic Specialist with PerkinElmer, Inc., Shelton, Connecticut. 

(1) T.J. Wilkinson, D.L. Perry, M.C. Martin, W.R. McKinney, and A.A. Cantu, 

(2) S.D. Mansfield, E. de Jong, R.S. Stephens, and J.N. Saddler, 

(3) A. Kher, M. Mulholland, E. Green, and B. Reedy, 

ICP-MS for the analysis of drinking water.

ICP-MS Determination of Micro and Macro Elements in Waters

Recent technological advances can improve profitability for environmental service laboratories. A programmable temperature spray chamber serves two purposes. First, by maintaining a stable temperature, a more consistent instrument response is achieved. Secondly, adjustment of the temperature can be used to control sample transport and thus have a pronounced effect on limits of detection. A discrete sample-introduction accessory can be combined with the programmable temperature spray chamber to reduce both carryover and the sample cycle time to lower the cost of analysis.

The analytical requirements for the measurement of trace metals in environmental applications are demanding. Whether using inductively coupled plasma–optical emission spectrometry (ICP-OES) or inductively coupled plasma–mass spectrometry (ICP-MS), the laboratory must demonstrate low detection limits, stability, accuracy, high precision, and low carryover. Because these analyses are typically carried out in commercial service laboratories, economics also plays a significant role, making metrics such as the number of samples run per hour important criteria. In this article, we will discuss components and parameters that facilitate and expedite the analysis of wastewater and drinking water for metals of environmental concern. 

Table I: Analytical requirements of various U.S. EPA methods for water (Î¼g/L) 

Regulatory agencies in many countries specify maximum contaminant levels (MCL) that must be achieved in drinking waters. These agencies also specify instrument detection limits required to analyze samples. U.S. EPA Method 200.7 documents the criteria and instrumentation for analyzing water and waste by ICP-OES. More specifically, EPA Method 200.5 describes a method for analyzing drinking water by axially viewed ICP-OES. EPA Method 200.8 is an ICP-MS method for drinking water that also has been approved for use with wastewater. The latest statement of work (SOW) from the EPA Contract Laboratory Program, ISMO 1.1, which was written as a defensible method for the analysis of wastewater, lists higher limits of detection. Table I lists a sample of element detection limits from a number of these documents. Table I shows that in the case of drinking water by ICP-OES, the required detection limits are very demanding — and only certain ICP manufacturers claim instrument detection limits (IDLs) low enough to achieve them, although most are quite close. For wastewaters by ISMO 1.1, the levels are easily achievable with all axially viewed ICP systems. However, there is an advantage to using a radially viewed ICP system to perform these tests because radially viewed ICP is more robust, less prone to matrix interferences, and has greater linearity. 

Table II: Experimental conditions for the data shown in Figure 2 

Improving detection limits by as little as 50% can be very beneficial for axially viewed instruments to attain drinking water IDLs and for radially viewed instruments to achieve wastewater IDLs. It has been shown that spray chamber temperature has a proportional effect on transport efficiency (1). A programmable-temperature spray chamber capable of both heating and cooling has been described previously (2). Known as the IsoMist (Glass Expansion, Pocasset, Massachusetts), this device utilizes Peltier-effect heat exchange to achieve a range of –10 to +60 °C with a stability within 0.1 °C. It is often assumed that a standard sample uptake rate of 1–2 mL/min maximizes the degree of solvent loading in the plasma and that higher sample transport would lead to an unstable plasma. However, we conducted a study of 18 elements using the programmable-temperature spray chamber to control sample loading (Figure 1) and found that by increasing the temperature (and thus sample transport), significant gains in intensity were achieved without destabilizing the plasma. This work was performed on an Optima 2100 DV ICP-OES instrument (PerkinElmer, Shelton, Connecticut). Table II gives the experimental parameters and Figure 2 shows the spray chamber mounted onto the torch of the instrument. Figure 3 shows that the average intensity gain closely tracks the mass transport resulting in a 48% enhancement at 35 °C compared with results obtained at room temperature (20 °C). It also shows no increase in the suppression due to high matrix (1000 ppm Na and K and 500 ppm Ca) at higher temperature–transport. This is significant because axial ICP-OES can be extended to lower IDLs, which would make it capable of achieving the requirements for drinking water. Also, increased temperature can be used to lower the IDLs of radially viewed ICP-OES so that they can meet the requirements for wastewater analysis. 

Figure 1: Effect of spray chamber temperature on individual element intensities. 

Figure 2: Programmable-temperature spray chamber mounted onto the ICP-OES instrument. 

Figure 3: Effect of spray chamber temperature on average intensity, mass transport, and matrix suppression. 

Because the accuracy of any relative analytical method depends (among other factors) upon the stability of the analytical instrument subsequent to standardization, anything that improves stability also improves accuracy. Therefore, the methods cited in this article all have requirements for stability in the form of a periodic calibration check. An initial calibration check run after standardization must be within 5% of the true value according to Method 200.7. Additional checks must be run at least every 10 samples. A check consists of two solutions, a standard and a blank. The standard must be within 10% of its nominal value for all elements to continue running samples. If it fails it can be repeated once, and if it still fails, the instrument must be restandardized and all of the initial checks must be rerun along with all of the samples run subsequent to the previous successful check. This is a very time-consuming and labor-intensive process, so it is imperative that failures do not occur. Table III lists the starting checks required after each standardization.

Table III: Initial check solutions subsequent to standardization according to ISMO 1.1 

Although ICP-OES instruments are quite stable over the long term, they are susceptible to signal fluctuations due to changes in the temperature of the sample introduction system. In fact, Method 200.5 recommends waiting for 30 min after plasma ignition before beginning standardization to allow the instrument to reach thermal equilibrium. The manager of a large environmental service laboratory went further still, recommending a full 1-h wait and also warning never to open the door to the laboratory's kitchen area during a run for fear of temperature drift. Figure 4 compares the signal stability of a range of element lines over a long-term run both with and without temperature stabilization. While in both cases all elements stayed within the 10% allowance, the temperature-stabilized results were markedly more consistent. Both of these runs were performed in a laboratory with excellent temperature control. Because it takes only a few minutes for the programmable-temperature spray chamber's temperature to stabilize near ambient, there is no need to wait 30 or 60 min to begin standardization after the plasma has been initialized. 

Figure 4: Comparison of intensity drift with and without spray chamber temperature control. 

The relationship between speed and productivity is obvious, but carryover is also tied to productivity due to the requirements of the methodology. For example, EPA Method 200.7 mandates that a blank be run following each calibration check and that the carryover not exceed the IDL. To ascertain that this criterion is attained, rinse times of 45–60 s are typical following each sample analysis. Recently, discrete sampling techniques have been described that dramatically reduce the required rinse time (3). One such device is the Niagara Plus sample-introduction accessory (Glass Expansion, Melbourne, Australia), which is shown in Figure 5. It consists of two components, a customized valve and a positive displacement pump. The valve was designed specifically for this application and incorporates several features that uniquely benefit this application as follows:  

Very low sweep volume to minimize carryover

Rugged PEEK rotor to withstand nasty samples without degradation

Replaceable stator seal assembly for improved maintenance

Stackable two-way valves to accommodate multiple configurations

Figure 5: Photo of the sample introduction accessory (Niagara Plus). The valve assembly is on the right and the positive displacement pump is on the left. 

The positive displacement pump is used to fill the sample loop reproducibly and quickly. Figure 6 is a schematic of the sample introduction accessory showing both the filling and injecting modes of operation. Table IV shows the typical time saving achieved using the accessory. Note that the rinse time on the instrument method could be set to zero because the sample loop is being rinsed during the uptake delay and autosampler movement. We tested this device for carryover on a PerkinElmer Elan 9000 ICP-MS system. The conditions are outlined in Table V, and the results are shown in Figure 7. For all elements, carryover was less than the IDL. In this case, not only was carryover minimized, but the analysis cycle was reduced by 62% (from 3.5 min to 80 s). For a commercial laboratory to increase its sample throughput by almost a factor of three without acquiring additional spectrometers is a huge economic gain. An additional economic benefit to the laboratory is the increased lifetime of consumables such as nebulizers, torches, and interface cones due to the decreased exposure to sample matrix. 

Figure 6: Flow diagram for the sample introduction accessory. Left-hand diagram: filling sample loop and rinsing the nebulizer and spray chamber. Right-hand diagram: aspirating sample and rinsing the autosampler probe. 

Figure 7: Results for carryover using the sample introduction accessory with ICP-MS immediately after running a 40-Î¼g/mL standard. 

Table IV: Typical time saving with Niagara Plus (s) 

The reliability of trace-metal measurements in environmental applications has improved significantly over the past decade due to both refinements in instrumentation and the propagation of well-defined methodology. In this article, we have focused on a number of ways to help commercial environmental laboratories increase efficiency and lower the cost of analysis. We believe that not only will these techniques become entrenched in environmental laboratories in the developed world, but they will also facilitate increased environmental assessment in the undeveloped and developing world. 

Table V: Experimental conditions for the data in Figure 7

 are with Glass Expansion, Pocasset, Massachusetts.  

(3)  P. Field, M. LaVigne, K.R. Murphy, G.M. Ruiz, and R.M. Sherrell, 

Environmental applications of ICP and ICP-MS

Optimizing Productivity for Environmental Applications of ICP

Reduced sulfur emission standards now enforce low sulfur fuels in the European Union and North Amercia and require oil refineries to produce low sulfur diesel fuels. Gas chromatography–inductively coupled plasma mass spectrometry (GC–ICP-MS) is a sensitive, compound-specific technique that allows for quantification of sulfur species at the part-per-billion level in fuels. The information that GC–ICP-MS provides can help the industry understand sulfur chemistry and develop and improve sulfur remediation strategies, and thus, comply with reduced sulfur fuel regulations.

Fuel combustion contributes a significant amount of sulfur to the atmosphere, resulting in ongoing concerns with respect to the concentrations of atmospheric sulfates, sulfuric acid, and the subsequent formation of acidic rain. Elevated atmospheric sulfur concentrations are also known to induce respiratory and cardiovascular illnesses in humans, and there are significant risks associated with long-term human exposure to sulfur pollution in the environment. 

Reformulated gasoline, a gas blended to burn cleaner, has been long established as a means of reducing smog-forming and toxic emissions. In the U.S., reformulated gasoline is currently used in 17 states as well as in the District of Columbia. Approximately 30% of gasoline sold in the U.S. is reformulated, ensuring that about 75 million people are breathing cleaner air (1). 

The U.S. Environmental Protection Agency (U.S. EPA) has introduced a set of regulations to enforce lower total sulfur concentrations in petroleum and diesel fuels with the goal of reducing subsequent sulfur emissions to the atmosphere. Legislation also exists to regulate the quality and use of reformulated fuel. 

In January 2001 and in June 2004, the U.S. EPA finalized the Highway Diesel (2) and Nonroad Diesel (3) rules, respectively, which mandate the use of lower sulfur fuels in diesel engines. The rules impose a total sulfur limit of 15 ppm. The aim is to reduce harmful emissions by 90% or more yielding significant, long-term benefits for public health and the environment. It has been estimated that by 2030, the reductions in sulfur emissions from nonroad diesel engines will annually prevent 12,000 premature deaths, 8900 hospitalizations and one million work days lost (4). 

Further to these rules, the U.S. EPA Regulation of Fuels and Fuel Additives (5) prescribes requirements for the control and prohibition of fuels and additives for use in motor vehicles and motor vehicle engines. The regulation enforces 30 ppm maximum sulfur content in gasoline. It also requires that gasoline sold in certain areas be reformulated. This also is mandated by the Clean Air Act, which requires that reformulated gasoline be sold in the nine largest metropolitan areas of the country with the most severe summertime ozone levels (6).  

In accordance with legislation, there is now an increasing requirement for the petrochemical industry to realize effective analytical methodologies for sulfur removal in both raw and processed petrochemical samples. 

The petrochemical industry has long recognized the need to monitor and control total sulfur concentrations in petroleum and diesel fuels before supply. The majority of the sulfur content in gasoline results from the incorporation of the fluid catalytically cracked (FCC) naphtha stream. As a result, the production of low-sulfur FCC naphtha can enable the production of fuels with reduced sulfur content. Another issue that the petrochemical industry faces is the contamination of low-sulfur fuel from common pipelines in the distribution system that also distributes high-sulfur fuels such as jet fuel. To ensure compliance with the maximum content levels set by regulations, the desulfurization process must be optimal. Therefore, an analytical technique offering the capability to determine which sulfur species have been effectively removed in the process is needed. 

Gas chromatography (GC) coupled with atomic emission detection (AED) and with sulfur chemiluminescence detection (SCD) have been used extensively for the analysis of sulfur species and total sulfur concentrations in various petrochemical fuels. However, the sensitivity of these techniques is considered insufficient for monitoring fuels containing reduced sulfur concentrations. As a consequence, a more sensitive analytical technique is required to satisfy current and future fuel-analysis requirements. 

GC coupled with element-specific inductively coupled plasma–mass spectrometry (ICP-MS) offers an alternative elegant analytical solution for sensitive analyses of sulfur species in fuels. ICP-MS is a multielement, multi-isotopic technique and has shown good performance for the direct analysis of trace elements in gasoline (AN 40768). 

Commercial GC–ICP-MS instrumentation has evolved from use in academic research to use in routine and commercial research.  

Capabilities of third-generation collision–reaction cell design are exploited within GC–ICP-MS instrumentation and a powerful reactive chemistry approach attenuates the required isobaric and gas-based polyatomic species to achieve interference-free analysis. All carbon-and plasma-based polyatomic interferences are removed, allowing accuracy within 10% and detection limits in the nanogram-per-kilogram range for most elements. 

Overall, GC–ICP-MS methodology provides a highly sensitive and selective approach for quantitative sulfur speciation analyses. It offers a complete analytical solution to refineries and the petrochemical industry at large for the characterization and quantification of sulfur species in fuels. The following experiment was developed to demonstrate the use of GC–ICP-MS for the analysis of sulfur species in FCC naphtha and commercially available gasoline samples. 

A Thermo Scientific TRACE GC Ultra gas chromatograph and a Thermo Scientific AS3000 autosampler were coupled with a Thermo Scientific XSERIES 2 ICP-MS detector via a GC transfer line. The ICP-MS detector was configured with a dual-mode sample introduction system to facilitate simultaneous introduction of both liquid and gaseous samples through the ICP-MS nebulizer–spray chamber arrangement and GC sample introduction systems, respectively. The ICP-MS detector also was initially autotuned and performance-tested using its integral software tools. 

The GC–ICP-MS methodology used for this study facilitated quantification of intrinsic sulfur species using external calibration and a species-independent calibration (SIC) technique. Total sulfur concentrations also were inferred from the sum of the intrinsic sulfur species concentrations. GC–ICP-MS analysis was performed during continuous aspiration of an aqueous blank solution through the nebulizer–spray chamber arrangement, maintaining robust wet plasma conditions throughout the analysis. 

Ions derived from the processed chromatographic eluent were extracted from the plasma and focused into the collision–reaction cell of the ICP-MS detector. The collision–reaction cell was pressurized with oxygen throughout the analysis and operated under non-kinetic energy discrimination (KED) conditions to promote conversion of sulfur (

  species subsequently were transferred to the quadrupole for detection and quantitation. This analytical approach shifted the 

 S analyte away from common gas-based polyatomic interferences and also converted any 

  ions, which would otherwise form an isobaric interference with 

  species to facilitate interference-free analysis.  

The major sulfur-containing species encountered in fuel were mercaptans, thiophenes, benzothiophenes, sulfides, and disulfides. Standards of thiophene, 2-methylthiophene, 2-ethylthiophene, benzothiophene, and 2-methyl benzothiophene were used to optimize the GC separation methodology and to generate fully quantitative external calibration curves. A noticeable broadening of the thiophene peak in commercial gasoline compared with the standard chromatography probably was due to coelution of benzene (Figure 1). 

Wherever possible, sulfur species were identified in the FCC naphtha and gasoline samples using spiking experiments with known standards. However, similar sensitivity responses were observed for each thiophene standard — that is, as calculated from each fully quantitative external calibration curve. This observation is illustrated in Figure 2, which shows a sulfur calibration in the region of 0.1–10 ppm using calibration standards derived from five sulfur species. 

GC–ICP-MS facilitated similar calibration sensitivities for different sulfur species because the inductively coupled plasma processed the eluted sulfur species similarly, regardless of the organic or inorganic functionality. This SIC technique can be exploited further to enable semiquantification of uncharacterized sulfur species and, as such, to avoid the necessity for multiple species-specific fully quantitative calibrations. 

Table I presents fully quantitative data for the five calibrated sulfur species standards and semiquantitative data for uncharacterized species using SIC. The gasoline and FCC naphtha samples at a concentration of 5 ppm were found to contain total sulfur concentrations of 26 ppm and 389 ppm, respectively. The GC-ICP-MS methodology was validated by calculating the recovery (89–103%) of standards spiked into the diluted FCC naphtha sample. Limits of detection (3σ) for the five thiophene analogues were calculated in the 5–10 ppb range. 

Table I: Full and semiquantitative data and figures of merit			

Sulfur emissions have been found to pollute the atmosphere and lead to the formation of acidic rain while also causing respiratory, cardiovascular, and other severe illnesses in humans. These toxic emissions are produced by fuel combustion, necessitating the use of cleaner, environment-friendly fuels. Reformulated gasoline is capable of reducing smog-forming and toxic sulfur emissions. Aiming to reduce harmful emissions and ensure the safety of humans and the environment, the U.S. EPA specifies maximum allowable concentrations of sulfur in fuel as well as mandating the use of reformulated fuel.

To comply with the stringent legislation, petrochemical companies need to implement a powerful, dependable analytical method. GC–ICP-MS increasingly is being recognized within the petrochemical industry as the method of choice for highly sensitive and selective analyses of sulfur species in fuels. The technique uses a simple, automated, and interference-free analytical approach that generates accurate results and enables effortless regulatory compliance. 

 is with Thermo Fisher Scientific, Bremen, Germany. She acknowledges W.M. Geiger, Consolidated Sciences, Inc., for the FCC Naphtha sample and valuable input.

(1) EPA website, Reformulated Gas, Basic Information, 

(2) EPA website, Federal Register / Vol. 66, No. 12 /January 18, 2001/Rules and Regulations, PART 80—REGULATION OF FUELS AND FUEL ADDITIVES, 

www.epa.gov/otaq/regs/hd2007/frm/frdslreg.pdf

(3) EPA website, 40 CFR Parts 9, 69, et al., Control of Emissions of Air Pollution, From Nonroad Diesel Engines and Fuel; Final Rule, 

(4) EPA website, Nonroad Diesel Equipment, Clean Air Nonroad Diesel –Tier 4 Final Rule, 

(5) Electronic Code of Federal Regulation website, Title 40: Protection of Environment, PART 80—REGULATION OF FUELS AND FUEL ADDITIVES, 

http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr;rgn=div5;view=text;node=40%3A16.0.1.1.9;idno=40;sid=e9b47ca0afa90e604adec25e65fcac88;cc=ecfr#40:16.0.1.1.9.4.47.2

(6) EPA website, Federal Register Environmental Documents, Regulation of Fuels and Fuel Additives: Standards for Reformulated and Conventional Gasoline, 

www.epa.gov/EPA-AIR/1996/November/Day-13/pr-23839DIR/Other/fuel.txt.html

GC-ICP-MS for the analysis of gasoline and petroleum

Using GC–ICP-MS for Speciation of Sulfur in Reformulated Fuels

Safer drinking water. Lead paint in toys. Cleaner-burning gasoline. These could be the headlines on your favorite website on any given day of the week, as each of these issues has captured the attention of the media and the world at large in recent years. They've affected the daily lives of each and every one of us at some point, whether we fished through our children's toys after the most recent lead paint scare, or boiled water like many in New York City did during a recent contamination. 

However, these life-affecting issues have one more important thing in common, and that is the fact that they are covered here in the November 2009 edition of 

 annual ICP/ICP-MS supplement. Disseminating research that makes the world a better place has long been at the core of 

editorial mission, and there may be no better example of this than the supplement you are now reading. Here you will find practical, real-world examples of spectroscopy improving — and even saving — lives. For many of us, when a law is passed mandating lower levels of toxins in drinking water or cleaner-burning gasoline, we assume it just happens overnight. But here in this supplement, you will find the actual research behind the headlines, and see the everyday work that must be done once laws are enacted and regulations passed. 

As always, we hope you enjoy this supplement and find it useful in your everyday work. We are proud to bring you this research from the front lines of materials analysis, and perhaps the next time our nightly cable news programs bring us a breaking story on a lead paint scare, or contaminated drinking water, or legislation aimed at reducing emissions, we will remember these individuals doing the real-world research that makes the world a better place. 

Special Issues-11-01-2009

Factors Determining Sensitivity in ICP-MS

Determination of Toxic Elements Leached from Toys and Household Structures by Inductively Coupled Plasma–Optical Emission Spectroscopy

ICP-MS Determination of Micro and Macro Elements in Waters

Optimizing Productivity for Environmental Applications of ICP

Using GC–ICP-MS for Speciation of Sulfur in Reformulated Fuels

The Research Behind the Headlines