Biodiesel is a renewable fuel produced from vegetable oil, animal fats, and waste cooking oils. Feedstocks such as soy, canola, mustard, sunflower, coconut, palm, and cottonseed oil as well as beef tallow and fish oils have been used to manufacture biodiesel.

As a result of the world's energy demands, the use of biodiesel is increasing rapidly. Blends of biodiesel with conventional petroleum diesel represent a common use of biodiesel. In the United States, B20 (a blend of 20% biodiesel and 80% petroleum diesel) is recognized as an alternative diesel fuel. Biodiesel has a number of advantages, including:  

Reduction of toxic exhaust emissions — lower amounts of hydrocarbon (HC), carbon monoxide (CO), and particulate emissions (PM)

Greenhouse gas savings — carbon dioxide (CO

) produced by burning biodiesel is used by subsequent crops used to produce the fuel

Fast biodegradability — approximately 4–5 times as fast as petroleum diesel

Higher flashpoint than petroleum diesel — 150 °C versus 70 °C

Can be used in diesel engines with little or no modification

Because the presence of contaminants can lead to operational problems, the American Society for Testing and Material (ASTM) and European Standard (EN) have developed standards to which pure biodiesel (B100) can be tested. The ASTM Standard is D-6751, shown in Table I.  

Physical, chromatographic, and spectroscopic methods are used to apply the standard. Here we look at the use of the FuelPro Biodiesel Metals Analyzer (Teledyne Leeman Labs, Hudson, New Hampshire) to apply the ASTM standard. 

Accurate elemental analysis of biodiesel requires analytical methodology that is both sensitive and selective. The analyzer meets these requirements and can determine up to 70 different elements in a sample. The analyzer permits close monitoring of elemental content throughout processing, from the raw oil to the finished product. Trace metal analysis is an important part of quality control as well as quality checks of the finished products. 

The effects of the biodiesel not meeting the specifications of the D-6751 standards follow:  

Phosphorus — Phosphorus has been shown to damage the ability of after-treatment systems to reduce exhaust emissions as intended. The influence of phosphorus is cumulative; as a result, very low levels of contamination over the significant amount of fuel consumed by an engine can lead to unexpected deterioration of the after-treatment system.

Alkali and alkaline metals — Sodium and potassium hydroxides are utilized as catalysts, and magnesium and calcium as absorbents in the production of biodiesel and should be removed through the biodiesel production process. These residual metals can form deposits in fuel injection system components and poison emission control after-treatment systems.

Sulfur — Sulfur levels in fuel are regulated by various governmental agencies to assure compatibility with emission standard requirements. In the U.S., there are currently three sulfur grades: S5000, S500, and S15, for both D1 and D2 petroleum diesel fuel. Biodiesel blends must not exceed the applicable maximum sulfur levels as defined for petroleum diesel.

Biodiesel samples were prepared by diluting 1:10 with kerosene. The analyzer was calibrated with standards prepared by diluting Plasma-Pure biodiesel stock standards. Standard concentrations were at the 0.00, 10.0, and 20.00 ppm levels for Na, K, Cu, Mg, P, and S. 

The analyzer's preprogrammed method was followed for the set-up and analysis of biodiesel. 

The values obtained as a result of the analysis of a B100 biodiesel sample are shown in Table II. All concentrations are given in ppm. The concentrations in the original sample are listed in the column labeled "Final Concentration (ppm)" ("ND" indicated that the analyte was not detected). The column labeled "Spike recovery (%)" contains the spike recovery data for a 3-ppm spike of all the analyte elements. (A separate sample was spiked with approximately 9 ppm sulfur to determine its recovery since the multielement stock standard contains sulfur as a matrix element.) The spike data show excellent recoveries, indicating that the 1:10 dilution is sufficient to eliminate any viscosity effects that may affect accuracy.  

The results indicate that this biodiesel sample passes the ASTM D-6751 standard. The result for sulfur indicates that it successfully passes the S-15 low sulfur designation that all highway diesel fuels had to meet from 2007 onward. 

Table III contains typical detection limits obtained in the oil matrix. The detection limits were determined by taking three replicates measurements in blank oil and multiplying the standard deviation by three.  

Biodiesel is analyzed effectively using this method, and the detection limit capability of the instrument meets the requirements of the ASTM and EN Standards against which biodiesel must be measured. 

Samples also are prepared effectively by dilution with a suitable solvent. Excellent recoveries are obtained from spiked biodiesel samples. This indicated that the method is suitable for the analysis of biodiesel fuels and that matrix interferences are not a problem. 

 is with Teledyne Leeman Labs, Hudson, New Hampshire. 

(2)  Specification for Biodiesel (B100) 0 ASTM D6751. 

Articles

The author looks at methods used to detect the presence of contaminants in biodiesel.

The Determination of Phosphorus, Sulfur, Sodium, Potassium, Calcium, and Magnesium in Biodiesel

Inductively coupled plasma–mass spectrometry (ICP-MS) has been established as the most appropriate method for routine environmental and geological applications, owing to its extended elemental coverage, superior sensitivity, and rapid analytical capabilities. Nevertheless, the technique's association with spectroscopic interferences remains, despite the fact that in recent years the introduction of collision–reaction cell (CRC) instrumentation has solved this shortcoming. This article will discuss the ICP-MS method, its usability in environmental and geological analysis and relevant regulations, and will build the case for the CRC approach as the most efficient solution addressing the limitations of ICP-MS. An experiment illustrates the flexibility and power of collision cell technology–based ICP-MS to analyze a variety of common environmental and geological sample matrices.

The inductively coupled plasma–mass spectrometry (ICP-MS) technique is based upon the coupling of an inductively coupled plasma, producing ions, and a mass spectrometer that separates and detects the ions. Argon gas (Ar) is used for the production of a plasma or gas consisting of ions, electrons, and neutral particles. Following dispersion into the stream of argon gas, samples are carried to the inductively coupled plasma, where their elements are atomized and ionized. The resulting ions are then passed through a series of apertures (cones) into a high-vacuum mass spectrometer where they are separated according to their mass-to-charge ratio (

) and counted. The isotopes of the elements are identified by their 

. The intensity of a specific peak in the mass spectrum is proportional to the amount of that isotope in the original sample.  

ICP-MS in Environmental and Geological Analysis

ICP-MS is a popular method, routinely used in both standard and high-sensitivity modes for fast and accurate measurements of trace (parts-per-million to parts-per-billion) and ultratrace (parts-per-billion to parts-per-quadrillion) elemental concentrations in a wide range of geological and environmental applications. The increasing popularity of the technique is attributed to the important benefits that it offers, including its multielemental capability compared with conventional elemental techniques, high sensitivity, superior detection limits, increased dynamic range, and isotopic ratioing capabilities. ICP-MS can be used to analyze both solid and liquid samples, requiring only small sample quantities thanks to its analytical range from the parts-per-trillion to the parts-per-million regions.  

Used in conjunction with varying nebulization and calibration techniques, ICP-MS is applicable in both liquid and solid materials. This highly sensitive and powerful technique performs reliable trace element distribution analysis of geological materials; identification of ultratrace cesium (Cs) and rubidium (Rb) in hydrothermal pore waters; and monitoring of transition, trace, and heavy metal contamination in environmental water samples. Further applications include preliminary investigation into the determination of osmium (Os) in sea water, lead isotopic ratio determination in mud cores taken from salt marshes, and analysis of uptake fluxes of trace redox-sensitive elements from sea water in sediments. Regardless of the sample type, it is important that it is free of particulate matter, as even the smallest particle can disrupt sample flow and block or clog the sample tips within the spray chamber. 

Having recognized the unparalleled sensitivity, precision, and reliability of ICP-MS, environmental protection regulatory bodies across the world have mandated the use of ICP-MS for a variety of applications. 

The U.S. Environmental Protection Agency (EPA) National Environmental Methods Index currently lists 11 approved ICP-MS methods by agencies within the United States. The most commonly used methods are those that have been developed by the EPA covering the use of ICP-MS for monitoring various elements in drinking water (Method 200.8), wastewater and solid waste (SW-846 Method 6020), and low-level trace in drinking water (Method 1638). In addition, the EPA has recognized ICP-MS as the only approved multielement method for determining arsenic and uranium in drinking water. The U.S. Department of Agriculture, Food Safety and Inspection Service, Office of Public Health Science has introduced Method CLG-TM3.01. This method specifies the use of ICP-MS for the analysis of cadmium (Cd) and lead (Pb) at parts-per-billion levels in beef, pork, and poultry muscle, liver, and kidney.  

Although very powerful and widely recognized by regulatory agencies, ICP-MS presents a few limitations, including spectroscopic interferences. 

Interferences are caused by polyatomic species that have similar mass to the target species and that stem from the sample matrix, Ar, or entrained gas. Solvents, reagents, glassware, and other sample processing hardware also can yield artifacts or interferences to sample analysis.  

There are other types of interferences including isobaric elemental interferences that are caused by isotopes of different elements forming atomic ions with the same nominal 

. Ions consisting of more than one atom or charge trigger the formation of isobaric molecular and doubly charged ion interferences, respectively. Physical interferences are associated with the sample nebulization and transport processes as well as with ion-transmission efficiencies. Memory interferences or carryover occur when there are large concentration differences between samples or standards that are analyzed sequentially.  

Interferences limit the ability of ICP-MS to determine certain elements of interest while also increasing maintenance requirements and reducing the reliability and quality of the data produced. Eliminating interferences provides numerous advantages, including significantly improved detection limits for interfered analytes, analyte confirmation by isotope ratio measurement, and superior analytical confidence in complex matrices. 

The combination of ICP-MS and collision–reaction cell (CRC) instrumentation has demonstrated a unique capability of significantly reducing and even eliminating interferences.  

CRC instrumentation employs either passive collisional interactions in which no chemical reaction occurs, or reactive interactions in which the charge or structure of either the analyte or interferent are changed chemically. Collision-based instruments use an inert collision gas, helium (He), to reduce the kinetic energy of the polyatomic interferent, thus preventing it from entering the quadrupole analyzer. Reaction-based analyzers use a range of reactive gases, including hydrogen, methane, and ammonia, to chemically shift one member of the analyte–interferent pair to another mass.  

Reaction and collision cells share the same technology, but where they differ is the regime established inside. Reactive chemistry can be applied to polyatomic species that react with the gas and consequently are either eliminated or modified. Additionally, reactive chemistry can modify analyte ions and analyze them at masses different from their natural isotope mass.  

Collision instruments can separate all kinds of overlapping molecular and polyatomic analyte ions from monoatomic ions if they differ in kinetic energy. This process is based upon collisions using inert gases like He and is called kinetic energy discrimination (KED). Collision instruments using KED have the advantage of being universally able to reject all polyatomic interferences in any matrix and are therefore well suited to multielemental analysis in complex or unknown matrices.  

Using ICP-MS in conjunction with state-of-the-art collision–reaction instrumentation can reduce spectral interferences efficiently and to negligible levels while the analytes remain relatively unaffected. Accurate analysis at low concentration levels can be performed even in difficult matrices. There are a number of spectral interferences that can be reduced with reactive chemistry, while others are better treated under KED conditions. For highest flexibility and best detection power in sample analysis, experiments should switch between several cell regimes. 

An experiment was developed to illustrate the flexibility and power of collision cell technology (CCT)-based ICP-MS to analyze a variety of common environmental and geological sample matrices. 

A Thermo Scientific XSERIES 2 ICP-MS system equipped with third-generation CCTED CRC instrumentation (Thermo Fisher Scientific, Bremen, Germany) was configured with an SC2 FAST automated sample introduction system (Elemental Scientific Inc., Omaha, Nebraska). The schematic in Figure 1 outlines the principle of the sample introduction system, which relies on a six-port switching valve to load and inject samples.  

This system significantly reduces sample uptake and washout times and dramatically decreases matrix load that reaches the plasma. The ICP-MS instrumental parameters are shown in Table I. A universal H

 in He gas mixture was used in the collision–reaction cell for the suppression of interferences.  

The experimental methodology performed to test the high-throughput setup for environmental samples was based upon conditions outlined in the Multi-Media, Multi-Concentration, Inorganic Analytical Service for Superfund Method ILM05.4. This method was introduced by the EPA within the Contract Laboratory Program (CLP) for water and soil/sediment environmental analysis.  

To improve the determination of analytes with a high ionization potential (such as Be, As, and Se), a stream of methane was added to the nebulizer gas. This carbon-loading enhances sensitivity for these analytes (Figure 2) and improves long-term stability by the reduction of matrix deposition on the ICP-MS interface.  

A sequence of 500 samples including a calibration and integrated QC (Table II) was performed for 23 analytes.  

A sample turnaround of approximately 83 s, including uptake, analysis, and wash, provided an overall batch acquisition time of 12 h for the entire 500 samples. Five samples (an interference check solution [ICSA], the same solution with a 20-ppb spike [ICSAB], NRC SLRS-3 river water reference material with varying spike concentrations, NIST 1643c reference material, and a detection limit check solution [CRI]) were classed as unknown samples and looped continuously throughout the experiment. The mean results for periodic analysis (every 20 unknowns) of the laboratory control standards (LCS) are shown in Table III.  

The stability of the results for samples SLRS-3 + spike and NIST 1643c are shown in Figure 3.  

Rock samples weighing 0.5 g were digested using an Aqua Regia mix (12.5 mL 30%) and diluted an additional 10 times before analysis. A sequence of 478 samples, including a calibration and integrated QC, was performed for 30 analytes. A sample turnaround, including uptake, analysis, and wash, of approximately 80 s resulted in an overall acquisition time of 11 h for the 478 samples. A standard operating procedure similar to the CLP method was followed, and a well-characterized rock was used as an LCS in the QC protocol along with a continuing calibration verification (CCV) and a continuing calibration blank (CCB). Five samples (an ICSA, the same solution with a 20-ppb spike, a rock sample [GEOMS], GEOMS spiked with varying analyte concentrations, and a CRI) were classed as unknown samples and looped continuously throughout the experiment. Results for the determination of analytes in the unknown samples over the 11-h period are shown in Figure 4.  

When used with a universal gas mixture for all analytes, CCT-based ICP-MS has been demonstrated to be a multielemental technique with high-throughput capabilities. With methane addition, the analysis can benefit from an increase in sensitivity for analytes with a higher ionization potential, such as Be and Se, which are often at lower concentrations in environmental samples, as well as benefiting from an improvement in long-term stability. Throughput and stability are further improved and necessary maintenance is reduced thanks to the configured sample introduction system, which cuts uptake and washout time and introduces less matrix into the plasma over time. 

CCT-based ICP-MS is a proven multielemental technique, capable of performing fast and accurate analyses of an extended range of common environmental and geological sample matrices. Through the use of a segmented flow sample introduction system, matrix deposition on the ICP-MS interface is reduced significantly, leading to increased first-time pass QC analyses and longer periods between necessary maintenance. Spectral interferences associated with the traditional ICP-MS method are reduced considerably, and sample throughput is increased. 

Julian Wills, Shona McSheehy, Tomoko Oki,

 are with Thermo Fisher Scientific, Bremen, Germany. 

 is with Thermo Fisher Scientific, Winsford, UK. 

The authors discuss the ICP-MS method, its usability in environmental and geological analysis and relevant regulations, and how to address its limitations.

Complete Multielemental Analysis of Environmental and Geological Samples Using CCT-Based ICP-MS

Elemental analysis of food substances presents a challenge because of the wide variety of food types and range of concentrations that need to be analyzed. The complexity of food matrices requires digestion to both break down the matrix and to convert solids into liquids for analysis. ICP-MS has the ability to measure both trace and elevated analyte levels during the same analysis, while the use of cell-based ICP-MS systems can remove the effects of interferences, allowing lower levels to be determined. This article discusses the analysis of a variety of food matrices with a single digestion procedure and instrumental method. 

Food safety is an area of growing concern as a result of pollution, which can enter the food supply and be passed along to consumers, leading to health problems. Therefore, it is important that all food types are monitored for contaminants and toxic elements. It is also equally important to monitor levels of nutritionally significant elements, which are present at elevated levels. Much work has been done on the analysis of food substances with inductively coupled plasma–mass spectrometry (ICP-MS) (1–9). 

To establish analytical conditions for the determination of metals in a variety of food types, several food standard reference materials have been analyzed for both elevated and trace elemental content using ICP-MS. Emphasis was placed on finding a single set of sample preparation and analytical conditions that would be applicable to a number of different food matrices.  

Table I: Standard reference materials analyzed 

The samples analyzed in this study were all standard reference materials from NIST (Gaithersburg, Maryland) and represent a variety of food types. Table I shows the materials analyzed. Approximately 0.5 g of sample was digested in triplicate (along with a blank) with 4 mL of Optima-grade HNO

 (Fisher Scientific, Pittsburgh, Pennsylvania) and 2 mL of Optima-grade H

 (Fisher Scientific) in a Multiwave 3000 microwave digestion system (Anton Paar, Graz, Austria). Precleaned HF-100 vessels were used in a 16-position rotor. A pressure–temperature sensor was used to monitor the pressure and temperature of the sample in position 1 of the rotor. The external infrared sensor provided the temperatures for each individual sample in the rotor. The digestion program consisted of 30 min of heating and 15 min of cooling, as shown in Table II. The samples were dissolved completely, resulting in clear solutions that were diluted with 1% HNO

Mixed element calibration standards of appropriate concentrations were prepared from PerkinElmer Pure multielement and single element standards (PerkinElmer LAS, Shelton, Connecticut) in 1% HNO

. The following internal standard mix was spiked into each sample, blank, and standard: Sc, Ga, In, and Tm.  

All analyses were performed on an ELAN DRC II (PerkinElemerSciex, Concord, Ontario, Canada); instrumental parameters appear in Table III.  

Most elements were analyzed in standard mode (that is, no reaction gas in the cell). However, those elements that suffered from interferences were measured in dynamic reaction cell (DRC) mode using oxygen and ammonia as the reaction gases. Because no elements were certified in the sucrose sample, spike recoveries of common elements were measured.  

Table V: Analysis of NIST 8433: Corn Bran 

Sodium and potassium were measured in standard mode, but with a different Rpa (DC voltage) than other elements. Both of these elements were present at high concentrations in the final solution and are either monoisotopic (Na) or greater than 90% natural abundance at the primary isotope (

K). As a result, it could be difficult to measure these elements in the same sample along with elements present in the low microgram-per-liter range. If the sample is not diluted adequately, Na and K could saturate the detector. If the sample is diluted too much, the other trace elements cannot be measured.  

Table VI: Analysis of NIST 8436: Durum Wheat Flour 

To solve this problem, the signals from sodium and potassium were attenuated selectively without affecting the sensitivity of other elements (10). This was accomplished by increasing the RPa on the reaction cell quadrupole only for sodium and potassium. Varying the RPa value changes the amount of the attenuation, effectively extending the dynamic range. Table IV shows the reaction cell parameters for the analytes measured in DRC and extended dynamic range modes.  

Table VII: Analysis of NIST 8414: Bovine Muscle Powder 

All quantitative measurements were made against aqueous, external three- or five-point calibration curves.    

Table VIII: Analysis of NIST 2976: Mussel Tissue 

Tables V–X show results for the various food matrices analyzed. The majority of results are within the certified ranges, demonstrating the robustness and applicability of the methodology for various food types.  

Table IX: Analysis of NIST 1549: Nonfat Milk Powder 

This work has demonstrated the ability to analyze a variety of food samples accurately for elements present at both trace and nutritional levels. A single microwave digestion procedure and ICP-MS method were used for all samples, demonstrating their robustness and flexibility. The ability to attenuate the intensities of specific elements selectively without affecting other isotopes allows for the analysis of both trace and high-level analytes to be performed using a single dilution. The analytical results show that microwave digestion followed by ICP-MS analysis effectively measures both toxic and nutritional elements in a variety of food matrices.  

 are with PerkinElmer, Waltham, Massachusetts. 

(2)  M. Resano, E. Garcia-Ruiz, L. Moens, and F. Vanhaecke, 

(3)  R.B. Myors, R. Hearn, and L.G. Mackay, 

(4)  M. Shah, R.G. Wuilloud, S.S. Kannamkumarath, and J.A. Caruso, 

(5)  C. Fragniere, M. Haldimann, A. Eastgate, and U. Krahenbuhl, 

(6)  C.R. Quetel, J.P. Snell, Y. Aregbe, L. Abranko, Zs. Jokai, C. Brunori, R. Morabito, W. Hagan, S. Azemard, E. Wyse, V. Fajon, M. Horvat, M. Logar, O.F.X. Donard, E. Krupp, J. Entwisle, R. Hearn, M. Schantz, K. Inagaki, A. Takatsu,P. Grinberg, S. Willie, B. Dimock, H. Hintelmann, J. Zhu, E. Blanco Gonzalez, G. Centineo, J. Ignacio Garcia Alonso, A. Sanz-Medel, and E. Bjorn, 

(7)  D. Hammer, M. Nicolas, and D. Andrey, 

(8)  W. Dufalilly, L. Noel, and T. Guerin, 

(9)  M.A. Knopp, F. Chan, and K.R. Neubauer, Analysis of Food Substances with Dynamic Reaction Cell ICP-MS, PerkinElmer Field Application Report, 2004. 

(10) F. Abou-Shakra, "Extending the Dynamic Range of the ELAN DRC by Selective Attenuation of High Signal" PerkinElmer Field Application Report 007437_01 (2005). 

Elemental analysis of food substances presents a challenge because of the wide variety of food types and range of concentrations that need to be analyzed. This article discusses the analysis of a variety of food matrices with a single digestion procedure and instrumental method.

The Analysis of Food Substances by ICP-MS

A new approach to the control of spectral overlap interferences in inductively coupled plasma–mass spectrometry (ICP-MS) is described. In this technique, called the collision reaction interface (CRI), gas is injected directly into the sampled plasma as it passes through the tips of the ICP-MS interface cones. The introduced gas collides and reacts with potentially interfering ions, removing them from the plasma before any ions are extracted by the ion optics. The capabilities of the CRI technique are illustrated with reference to the determination of Se under "hot plasma" conditions, of 52Cr in the presence of 2-propanol under "hot plasma" conditions, and of a range of trace metals in certified urine samples, with no matrix matching.

Some common interferences in inductively coupled plasma–mass spectrometry (ICP-MS) are caused by ions having the same mass-to-charge ratio (

) as the isotope ions of interest (1). The most common techniques for controlling these interferences involve either the use of "cool plasma" conditions (2) or of various pressurized multipole ion guides (3), in which the ion beam is exposed to a gas after it has been extracted from the plasma by the ion optics. As shown in Figure 1, the collision reaction interface (CRI) (4) works by injecting a gas into the plasma as it passes through the tip of one or both of the interface cones.  

Collisions between gas molecules and plasma ions change the kinetic energy of the ions and can induce reactions such as charge transfer, proton transfer, collisional excitation–dissociation, electron–ion reaction, and ion–molecule reaction. Ions that normally would cause spectral interferences are more susceptible to such processes than are the analyte ions. Consequently, interfering ions can be selectively removed from the plasma before any ions are extracted into the ion optics. The high plasma density in the interface apertures leads to a high collision frequency. Hydrogen and helium are used as CRI gases; they provide efficient interference attenuation, and there is no need to use expensive, toxic, and corrosive gases such as ammonia. Cleaning the CRI is as simple as the routine cleaning of the interface cones in a conventional ICP-MS instrument. 

Examples of the Removal of Interfering Ions with the Collision Reaction Interface

  is always present under "hot plasma" conditions. It interferes with the determination of selenium, because the three most abundant isotopes of Se have the same nominal mass as Ar

  is suppressed under "cool plasma" conditions (2), but these are not useful for Se determinations because of the high ionization energy of this element (941 kJ/mol, 9.75 eV). When H

 is injected into the sampled plasma through the CRI, Ar

 molecules. A chemical reaction then occurs, in which a proton is transferred from a H

, a neutral H atom and a neutral Ar atom. The product ion ArH

 molecule and a proton is transferred from the ion to the molecule, forming a neutral Ar atom and an H

 = 3 amu/unit electronic charge) does not interfere with any isotopes of interest in ICP-MS, and in any case is likely to be destroyed by reaction with plasma electrons to form H

, where the CRI gas flow is increasing stepwise. Because no Se was present in the test solution, the measured signal for 

  is due entirely to the interfering ion 

. Also shown in Figure 2 is the signal ratio of 

. The time scan was carried out with a test solution (Var-IS-1) containing 10 μg/L of internal standard elements (Bi, In, Li, Sc, Tb, and Y). During the time scan, the H

 gas was injected into the plasma through the CRI skimmer cone tip. The H

 flow rate was stepped from 0, 20, 50, 80, 100, to 120 mL/min with about 50 s between each step.  

The efficiency of interference reduction–removal using CRI is demonstrated clearly in Figure 2, in which the signal for the interfering species Ar

) is decreasing progressively with increasing H

 gas flow rate, while the signal to interferent level (indicated by the ratio 

) is improving continuously. As seen in Figure 2, at a H

 flow rate of around 120 mL/min, the interference from Ar

  has been removed completely, and the sensitivity for the analyte 

  is still maintained at a high level (over 50,000 c/s per 1 μg/L of indium). 

  interferes with the major isotope of chromium (

Cr, natural abundance 83.79%). When He is injected into the sampled plasma through the CRI, interaction between the electron clouds of the He atoms and those of polyatomic ions can make a large polyatomic interfering ion such as ArC

  rotationally and vibrationally excited. In subsequent collisions, an excited ArC

  ion can receive sufficient energy to bring about its dissociation, removing the 

  interference from the determination of 

Figure 3 shows interference reductions in three-dimensional graphs, where the signals for 

  were measured at various CRI gas flow rates. The test solution contained 10 μg/L Co in a matrix solution of 1% (v/v) concentrated HNO

, 1% (v/v) concentrated HCl and 1% (v/v) 2-propanol. Helium was used as the CRI gas. Because no Cr was present in the test solution, the 

  signal is due to interfering polyatomic ions such as 

. As shown in Figure 3a, this signal for interfering polyatomic ions decreased with increasing He flow at the tip of the skimmer cone. With He flows over 120 mL/min, the signal for interfering ions was reduced to a minimum. In contrast, the He flow at the tip of the sampler cone had little effect. The efficiency of interference reduction is demonstrated in Figure 3b, where the signal ratio of 

  (that is, the ratio of the signal from a real analyte to that from interfering ions) improved with increasing He flow rate at the skimmer. Again, the He flow at the sampler had no significant effect.  

The results in Figure 3 are typical of experimental results obtained so far, which all show that gas injected into the sampler cone is much less effective at reducing or removing interfering ions. This is not surprising, because the concentration of plasma ions in the sampler is about 1000 times less than in the skimmer. It is therefore possible to obtain a much larger ratio of CRI gas to plasma ions at the skimmer than at the sampler for the same gas flow. In Figure 3, the maximum gas flow into the sampler was 10 times that into the skimmer; use of greater gas flows was considered impractical.  

Table I lists typical detection limits (DL) measured in the CRI mode under "hot plasma" conditions, determined as the concentration corresponding to three times the standard deviation of 10 replicates of a blank solution (1% [v/v] concentrated HNO

). These measurements were made under routine analytical laboratory conditions, not "clean-room" conditions.  

Table I: Typical detection limits for selected isotopes 

Figure 4 shows the long-term signal stability in a high total dissolved solids matrix in the CRI mode over a period of 5 h, using a solution containing 0.1% (w/v) (1000 mg/L) NaCl, spiked with 1 μg/L of various analytes. The results were extremely stable over 5 h. The relative standard deviations (RSD) of the measured signals for most analytes did not exceed 5% over the 5 h. Similar results also were obtained using He as the CRI gas.  

An Application of the CRI: Determination of Trace Metals in Urine 

The work was carried out on a Varian 820-MS ICP-MS system (Varian, Palo Alto, California), which includes the CRI. Table II shows conditions for the urine analysis.  

Table II: ICP-MS conditions for urine analysis 

As an example of the use of the CRI in a real analysis, we analyzed two certified urine samples for a range of trace metals, using calibration solutions prepared in dilute nitric acid, without matrix matching.  

Urine metal control materials, UMC L-1 and UMC L-2, were reconstituted according to the procedure supplied with the materials. For trace metals determination with the ICP-MS system, the reconstituted urine solution was further diluted by a factor of 10, by adding 1 mL of the urine into a precleaned tube containing 9 mL of 2% (v/v) concentrated HNO

Diluted (1:10) urine solution was used for direct ICP-MS analysis. Calibration solutions were prepared in 2% (v/v) concentrated HNO

High-purity nitric acid and hydrochloric acid (both AR Select Plus, Mallinckrodt Baker, Phillipsburg, New Jersey), 2-propanol (SupraSolv for gas chromatography, Merck KGaA, Darmstadt, Germany), and deionized water (18 MΩ cm, Millipore Milli-Q, Billerica, Massachusetts) were used in the preparation of sample and calibration solutions. All labware, new or used, was cleaned thoroughly by acid washing and rinsing, and the clean containers were left filled with 2% (v/v) concentrated HNO

 until use. Three multielement calibration solutions were prepared for the urine analysis, at concentration levels of 1 μg/L, 5 μg/L, and 10 μg/L, by diluting a 10-mg/L multielement stock solution (SPEX CertiPrep, Inc., Metuchen, New Jersey) with 2% (v/v) concentrated HNO

. An internal standard solution, containing 20 μg/L of 

Bi, was prepared by diluting a 100-mg/L internal standard stock solution (Var-IS-1, Inorganic Ventures, Inc., Lakewood, New Jersey) with 2% (v/v) concentrated HNO

. The diluted internal standard solution was added to the nebulizer through a "Y-piece" using the third channel of the ICP-MS system sampling pump. Two urine metals control materials (Lyphochek urine metals control, Level-1 (LOT 69101) and Level-2 (LOT 69102), Bio-Rad Laboratories, Irvine, California) were  then selected as the sample for the direct trace element determination in urine. 

The analyte concentrations measured in two certified urine metal-control materials using the ICP-MS system are listed in Table III. The measured values shown in Table III are the average of two repeat measurements.  

Table III: Metal element concentrations measured in Lyphochek urine metals control, Level 1 and Level 2 

These results show excellent agreement between measured and certified values. The measured concentrations for Ni in UMC L-1 and Se in UMC L-2 are slightly lower than the certified range. This difference is likely to be due to the different techniques used for the analysis, because the certified values for both Ni and Se were obtained using atomic absorption spectroscopy rather than ICP-MS. 

The collision reaction interface provides a simple, but very effective, approach to removing common interferences in ICP-MS analysis. This unique approach reduces spectral overlap interferences from common polyatomic ions on elements such as As, Se, Cr, V, and Fe, thus achieving lower detection limits for these elements under "hot plasma" conditions, even for samples (such as urine) that have complex matrices. The CRI does not pass the extracted ion beam through a pressurized multipole; instead, gases are injected directly into the plasma through the tips of the interface cones to remove interfering ions by collisions or chemical reactions. This simple and innovative approach reduces or removes interfering ions before any ions are extracted from the sampled plasma into the ion optics. The CRI approach also provides operational advantages including low maintenance; easy operation; and no use of toxic, corrosive, or expensive gases such as ammonia. 

 are with Varian, Inc., Palo Alto, California. 

(3)  S.D. Tanner, V.I. Baranov, and D.R. Bandura, 

(4)  I. Kalinitchenko, United States Patent 7,329,863 (2008).  

(5)  V.G. Anicich, An index of the literature for bimolecular gas phase cation-molecule reaction kinetics, JPL Publication 03-19, National Aeronautics and Space Administration, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, 2003, pp. 506–510. 

The authors discuss a new approach to the control of spectral overlap interferences in inductively coupled plasma–mass spectrometry.

Simple and Effective Control of Spectral Overlap Interferences in ICP-MS

There are many choices for components that constitute the "front end" of an inductively coupled plasma (ICP) spectrometer, including a number of developments over the past several years. In this article, the goal is to describe the options available and critique the pros and cons of each. In the end, the performance of the spectrometer depends upon the proper selection for each stage of the sample introduction system.

The sample introduction system includes all components involved in carrying the sample from the autosampler to the spectrometer, as shown in Figure 1.  

The following sections will address each component sequentially following the flow of sample through the system. 

The autosampler probe normally is prescribed by the specific brand and model of autosampler. However, there are a few considerations to bear in mind. The internal bore of the probe should be consistent and small to eliminate long fill and void times. It is best if the internal bore of the probe is continuous with the capillary attached to it to achieve laminar flow. Otherwise, pressure changes can occur and result in degassing of the sample, which can cause air bubbles to form in the sample line. Periodic release of air bubbles during integration can result in imprecision. Also, some autosampler probes are available with a built-in filter on the end to prevent particulates from reaching the nebulizer.  

This compressible tubing is wrapped around the pump rollers, which when moving, push the sample (and waste) in the proper direction. There is a wide variety of tubing materials to choose from, and the choice depends upon the sample characteristics. Commonly used tubing materials include the following:  

PVC Standard; also known as Tygon ST (Saint-Gobain Corporation, Valley Forge, Pennsylvania). This is the least expensive tubing and is used for most aqueous samples.

Solva; also known as Tygon HC or solvaflex. This is commonly used for petrochemicals, especially the measurement of wear metals in used engine oils (typically diluted in kerosene or xylene).

TygonMH (Saint-Gobain). This is a special formulation of PVC that is more expensive than PVC Standard but stands up well to a wide variety of solvents.

Fluran; also known as Viton (DuPont). Fluran is also a relatively expensive tubing material. It has often been used for organic solvents, but its strength is its resistance to strong acids and bases.

Table I is a general guide to help you choose the most suitable material. A more complete listing of tubing and compatibilities can be found elsewhere (1).  

Table I: Compatibility of pump tubing material with various solvents 

After you have decided on the tubing material, you must choose the proper internal diameter (i.d.), which is denoted by a universal color combination for the pump tags on the tubing (for example, two orange tags always indicate 0.89 mm i.d.). In combination with the pump size and speed, the tubing internal diameter determines the sample flow rate. As always, there are tradeoffs. Small internal diameter tubing combined with a higher pump speed is preferable to achieve a consistent signal with minimal pulsations from the rollers. On the other hand, the faster pump speed will cause the tubing to wear faster. One end of the pump tubing is connected to the autosampler probe and the other to the nebulizer capillary. It is sometimes difficult to insert the capillary into the end of the pump tubing if the internal diameter is small. If this is the case, flared-end tubing is available to facilitate installation. This is particularly important for the internal standard line if an in-line addition method is in use. 

The nebulizer capillary carries sample from the pump tubing to the nebulizer. Perfluoroalkoxy (PFA) and tetrafluoroethylene (TFE) are good choices for the material because they are "non-sticky" and available in high purity. To achieve fast washout times, both the length and the internal diameter of the tubing should be minimized as much as is practical.  

The nebulizer is one of the most critical components of the system. Its job is to convert the sample into a consistent and dense aerosol, one that is composed of small droplets. To select the most appropriate nebulizer for a specific application, you should consider a number of characteristics:  

Tolerance to total dissolved solids (TDS)

Commonly employed nebulizers include the following:  

The most commonly used nebulizer for both ICP-OES and ICP-MS is the concentric glass, so named because the sample line is concentric within the gas line. It delivers consistent aerosol and achieves high precision. The major drawback is its tendency to clog when running samples with particulates or high total dissolved solids. Advances in nebulizer design have clearly improved its tolerance to complex samples (Figure 2).  

Cross-flow nebulizers are constructed so that the sample and gas lines meet at right angles. They were once commonly used but today are used primarily for samples containing hydrofluoric acid because of their inert construction. However, concentric polymer nebulizers are also compatible with HF and deliver excellent performance, thus explaining their growth in popularity.  

Babington-style nebulizers include V-Groove nebulizers and function by pumping the sample over a gas orifice. This design can handle samples with very high salt content and even particulates. Unlike concentric nebulizers, Babington nebulizers are not self-aspirating and are therefore more susceptible to pump pulsations.  

Ultrasonic nebulizers pump the sample onto an oscillating piezoelectric transducer, thereby increasing sample transport efficiency, usually in the range of 20% at 1-mL/min uptake rates (2). For clean samples, this can result in significant lowering of detection limits. One drawback is the high price tag. Also, their detection limit advantage is reduced greatly with complex samples because the matrix transport efficiency is increased as well.  

Micro flow nebulizers are normally a subcategory of concentric nebulizers and operate at sample uptake rates well below 1 mL/min, typically in the range of 0.050 to 0.200 mL/min. Sample transport efficiencies at these low uptake rates equal or exceed those of ultrasonic nebulizers (3). These are a good choice when sample volume is limited or waste disposal is expensive (for example, radioactive).  

The function of the spray chamber is to filter the aerosol produced by the nebulizer so that only the smallest droplets reach the plasma. The most commonly used spray chambers in plasma spectrometry are the cyclonic, barrel (or Scott type), and conical configurations (Figure 3).  

Notice that the cyclonic and Scott-style spray chambers are available as both single and double pass (or baffled) configurations. The double pass acts as a secondary filter, further reducing the mean droplet size. The conical spray chamber uses a built-in impact bead to create a finer mist. In all three designs, gravity is used to remove the larger droplets, which are then diverted to the drain. Only in the cyclonic design is centrifugal force also employed to remove the larger droplets, thus reducing re-nebulization and enhancing precision. For this and other reasons, cyclonic spray chambers have gained significantly in popularity. Cyclonic spray chambers are also available with reduced volumes to speed washout times when used with micro flow nebulizers. Both cyclonic and Scott-type chambers are commercially available with inert construction to accommodate HF. 

For some applications, it is desirable to heat or chill the spray chamber or simply to prevent the temperature from varying (4). For these applications, the above spray chambers are available in jacketed configurations that allow an external supply to pump temperature-controlled liquid through the chamber housing. A more elegant solution is a standalone Peltier-driven device, which replaces the existing chamber. Examples of both of these are shown in Figure 4.  

The function of the torch is to convert the sample aerosol into atoms or ions. This is accomplished by injecting the aerosol into a contained argon plasma, where desolvation, atomization, excitation, and ionization take place. All torches have at least three flows. The coolant gas is directed between the outer tube and intermediate tube and sheaths the outer tube from the plasma. Insufficient coolant flow will quickly destroy the torch. The auxiliary gas is directed between the intermediate tube and the injector. This flow is much lower and is used to control the height of the plasma above the injector. The sample itself (along with the nebulizer gas) goes through the injector tube and must achieve enough velocity to puncture the plasma without unduly decreasing the residence time of the atoms (or ions). There are three categories of torch design distinguished by the degree of demountability (Figure 5).  

The single piece quartz torch is the general use torch and the least expensive with respect to initial investment. However, the torch must be completely replaced if any component is damaged and lacks versatility. The semi-demountable torch has a demountable injector, which allows different orifice size injectors to be installed without acquiring a new torch. For example, when analytes are in high salt matrices, a wider bore orifice reduces the likelihood of problematic salt buildup at the injector tip. Conversely, for organic solvents, a smaller-bore orifice reduces the plasma load. Also, the injector can be replaced with one made of a material other than quartz for specific applications, as follows:  

In this article, the author describes the components available for the "front end" of an inductively coupled plasma spectrometer and discusses the pros and cons of each.

Special Issues-10-01-2008

The Determination of Phosphorus, Sulfur, Sodium, Potassium, Calcium, and Magnesium in Biodiesel

Complete Multielemental Analysis of Environmental and Geological Samples Using CCT-Based ICP-MS

The Analysis of Food Substances by ICP-MS

Simple and Effective Control of Spectral Overlap Interferences in ICP-MS

Selecting the Best ICP Sample Introduction System