The recently developed technology of triple-quadrupole inductively coupled plasma–mass spectrometry (ICP-MS) offers a unique solution for the removal of spectral interferences in ICP-MS. Arsenic with its single isotope at mass 75 is one of the most difficult elements to measure in complex matrices by ICP-MS. This article discusses how ICP-MS-MS allows the accurate determination of As in complex samples that contain Cl, Ca, Sm, Nd, and Zr.

Arsenic (As) is regulated and routinely monitored in a wide variety of sample types (1), ranging from drinking water through pharmaceuticals, consumer products, food and clinical samples, to contaminated soil, industrial effluent, and trade waste. As well as its historical use as a poison (such as the chemical warfare agent lewisite), arsenic compounds have been widely used as pesticides, herbicides, and insecticides (for example, chromated copper arsenate timber treatment). Arsenic has also been used in pigments, metal alloys, and semiconductor devices, and occurs naturally at high concentration in groundwater in Bangladesh and West Bengal, India. 

The common inorganic forms of As, arsenite (As[III]) and arsenate (As[V]) are toxic to humans; the common organic forms monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) are less toxic, and arsenobetaine (AB, the form most commonly found in seafood) is considered nontoxic (2). Regulatory limits are typically based on "total" As levels, but when the limits are exceeded, separation of the different chemical forms (or "speciation") may be required to assess potential toxicity. 

The method of choice for routine multielement analysis in most applications is inductively coupled plasma–mass spectrometry (ICP-MS), which delivers low detection limits, wide elemental coverage, and wide linear measurement range. However, As is considered a difficult element for quadrupole ICP-MS because of its low detection limit requirements, poor ionization (giving relatively lower sensitivity), and the presence of spectral interferences. 

Various techniques have been used to address these spectral interferences in quadrupole ICP-MS, of which the most widely applicable in recent times has been collision–reaction cell (CRC) technology. While almost all modern quadrupole ICP-MS systems now include CRC technology, even the improved interference-removal capability of CRC does not solve all the potential interferences on As in many complex sample types. However, the recently released technology of triple-quadrupole ICP-MS offers a novel approach that eliminates the spectral interferences that cannot be resolved using quadrupole ICP-MS and ensures accurate low-level analysis of As, even when the sample matrix is complex and variable.  

Calibration Standards and Sample Preparation

All of the sample matrices used for this work were prepared using single-element stock solutions (Spex CertiPrep, Claritas grade). The acid matrix and elemental standard concentrations are shown in the caption for each spectrum and are representative of the acid matrix (dilute HNO

 and HCl) and matrix levels commonly found in ICP-MS samples. 

The sample matrices investigated were 									

Neodymium and samarium (1 ppm each element)

An Agilent 8800 triple-quadrupole ICP-MS system was used for the analysis of As in all the sample matrices. The ICP-MS system uses a sample introduction, plasma ion source, vacuum interface, ion lens, CRC, analyzer quadrupole, and detector system that is essentially the same as that used in quadrupole ICP-MS. However, the triple-quadrupole layout uses an additional quadrupole mass spectrometer (Q1) before the CRC, so the ions that enter the cell can be precisely controlled by tandem mass spectrometry (MS-MS) operation. This eliminates the two primary limitations of CRCs using reaction mode in quadrupole ICP-MS (3): 					

Where the target analyte is unreactive and is measured "on-mass," other analyte ions or matrix ions that enter the CRC may react with the cell gas and create new cell-formed product ions that overlap the original analyte mass.

Where the target analyte itself is reactive, the analyte is measured as a cell-formed product ion, but other analytes or matrix ions may overlap the mass of the analyte product ion.

In contrast to quadrupole ICP-MS, in both of these cases triple-quadrupole ICP-MS can reject the nontarget ions before the cell, so both existing and cell-formed interfering overlaps ions can be avoided. 

The ICP-MS instrument configuration is illustrated in Figure 1. The standard sample introduction for the instrument with a Micromist nebulizer, and standard Ni interface cones was used throughout.  

Figure 1: Schematic of the triple-quadrupole ICP-MS system.			

The ICP-MS system is typically operated in MS-MS mode, but can also be setup with Q1 functioning as an ion guide or bandpass filter (referred to as "single-quad" mode); Single-quad mode on the triple-quadrupole ICP-MS system approximates the operation of a conventional quadrupole ICP-MS system. The measurement of As in the various sample matrices was performed in several acquisition modes to evaluate the performance of the typical modes used on quadrupole ICP-MS, and compare these modes with MS-MS: 					

The operating conditions used for the analysis of all samples are shown in Table I.  

Table I: Acquisition modes used for measurement of As in complex matrices			

Figure 2 illustrates the primary and well-known issue of measuring As at low levels in complex natural samples. The spectrum shown in Figure 2a shows the peak from a 1 ppb (1 μg/L) As standard in 1% HNO

, measured directly at mass 75 in single-quad, no gas mode. The background signals from the argon (Ar) dimer polyatomic ions at mass 76, 78, and 80 can also be seen. The overlaid spectra in Figure 2b show the same mass range for the original standard, plus overlaid spectra of 1 ppb As spikes in a matrix of 5% HCl (in blue) and 5% HCl plus 100 ppm Ca (in green). The polyatomic interferences from ArCl

  cause a large error in the recovery of the 1 ppb As standard.  

Figure 2: Overlaid spectra in single-quad mode with no cell gas, showing (a) 1 ppb As in 1% HNO3 and (b) variable signal at mass 75 in matrices containing 5% HCl and 5% HCl/100 ppm Ca.			

The same spectral comparison for the same sample matrices is shown in Figures 3a and 3b, but with the triple-quadrupole ICP-MS system operated in single-quad mode with He cell gas. Helium collision mode filters out polyatomic ions by kinetic energy discrimination (KED) because of their larger ionic cross-section and higher collision rates. Although the raw signal for As is lower than in no-gas mode (due to analyte ion scattering when the cell is pressurized) the polyatomic interferences from ArCl

  are effectively reduced (Figure 3b), ensuring consistent recovery of the 1 ppb As spike in all three samples (the As peak at mass 75 remains the same in the presence of the Cl and Ca matrices).  

Figure 3: Overlaid spectra in single-quad mode with helium cell gas, showing (a) 1 ppb As in 1% HNO3 and (b) consistent signal at mass 75 in matrices containing 5% HCl and 5% HCl plus 100 ppm Ca.			

Figure 4 again shows the same three overlaid spectra as Figure 2b, but with the addition of a further sample matrix of 1 ppm each of Nd and Sm (in pink). A quadrupole mass spectrometer separates ions on the basis of their mass-to-charge ratio (

), so ions with a double-charge (loss of two electrons) appear at half their true mass; 

 75 in quadrupole ICP-MS, overlapping the 

  analyte ion. While Figure 3b demonstrates that He mode with KED successfully removes polyatomic interferences, Figure 4 shows that He mode is not effective against doubly charged interferences, leading to inaccuracy in the As results in the presence of a Nd or Sm matrix.  

Figure 4: Overlaid spectra in single-quad mode with helium cell gas, showing 1 ppb As in 1% HNO3, 5% HCl, and 5% HCl plus 100 ppm Ca (as in Figure 2b), and variable signal at mass 75 due to Nd++ and Sm++ (1 ppm each element).			

 cell gas on a quadrupole ICP-MS system offers a solution to the problem of doubly charged interferences on As, as O

  reaction product ion in the cell. The reaction process is illustrated schematically in Figure 5. The resulting spectrum showing the AsO

  polyatomic ions, is shown in Figure 6, demonstrating consistent As signal in all the sample matrices.  

Figure 5: Schematic showing the mechanism for measuring As as AsO+ using O2 reaction gas on a conventional quadrupole ICP-MS system: (a) analyte and interfering ions enter the reaction cell, (b) As+ reacts with O2 cell gas to form AsO+ product ion, and (c) quadrupole set to AsO+ product ion mass (m/z 91) rejects original interfering ions 40Ar35Cl+, 40Ca35Cl+, and Nd++ and Sm++.			

 reaction mode on quadrupole ICP-MS does not offer a complete solution for As analysis. As mentioned, one of the principal limitations of reaction mode on quadrupole ICP-MS is that there is no way to control existing on-mass interfering ions that overlap analyte product ions. In the case of As measurement using O

 reaction mode on quadrupole ICP-MS, the AsO

 91) is overlapped by a natural isotope of zirconium (

). In samples that contain even sub-part-per-million levels of Zr, the Zr

  causes a large error in reported results, as illustrated in the spectrum in Figure 7. Even in CRCs where a scanning quadrupole-based cell is used, there is no mechanism for rejecting the 

Figure 6: Overlaid spectra in single-quad mode with O2 reaction cell gas, showing consistent signal for 1 ppb As in 1% HNO3, 5% HCl, 5% HCl plus 100 ppm Ca, and 1 ppm each of Nd and Sm.			

In contrast, on the triple-quadrupole ICP-MS system in MS-MS mode with O

 cell gas, Q1 is set to reject all ions apart from the original mass of the target analyte (As

 75), thereby preventing all other masses from entering the CRC. The AsO

  product ion is still formed by reaction with the O

  is rejected by Q1 and therefore cannot overlap the AsO

  product ion formed in the CRC. This is illustrated schematically in Figure 8, and the resulting MS-MS mode spectrum is shown in Figure 9.  

Figure 7: Overlaid spectra in single-quad mode with O2 reaction cell gas, showing consistent signal for 1 ppb As (as AsO+ at m/z 91) in 1% HNO3, 5% HCl, 5% HCl plus 100 ppm Ca, and 1 ppm each of Nd and Sm (as in Figure 6), and variable signal at m/z 91 due to matrix of 0.5 ppm Zr.			

The consistent spectra shown in Figure 9 illustrate the ability of triple-quadrupole ICP-MS to remove or avoid all of the potential overlaps that occur in the measurement of As (polyatomic ions and doubly charged ions at 

 75, and other analyte or matrix ions that overlap the analyte product ion at 

Figure 8: Schematic showing the mechanism for measuring As as AsO+ using O2 reaction gas on a triple-quadrupole ICP-MS system: (a) Q1 set to m/z 75 rejects all ions except m/z 75, so 91Zr+ at mass 91 is rejected; (b) As+ reacts with O2 cell gas to form AsO+ product ion, 40Ar35Cl+, 40Ca35Cl+, and Nd++ and Sm++ don’t react and stay at m/z 75; and (c) Q2 set to m/z 91, AsO+ product ion mass, rejects original on-mass interferences (still at m/z 75).			

What is less immediately apparent, though no less significant, is the fact that the consistency of the As measurement in O

 mode on the triple-quadrupole ICP-MS system greatly simplifies both method development and routine operation. Triple-quadrupole ICP-MS ensures that all of the common matrix interferences can, for the first time, be avoided under a single consistent set of operating conditions, so analysts no longer need to customize their sample preparation, instrument operating conditions, or analytical methodology based on the sample matrix they are measuring. For many laboratories this offers a major benefit, as routine analysis of variable samples is much less problematic, and data quality is greatly improved.  

Figure 9: Overlaid spectra in MS-MS mode with O2 reaction cell gas, showing consistent signal for 1 ppb As (as AsO+ at m/z 91) in 1% HNO3, 5% HCl, 5% HCl plus 100 ppm Ca, 1 ppm each of Nd and Sm, and 0.5 ppm Zr.			

Accurate Measurement of As in Reference Materials Using ICP-MS-MS in O

The accurate analysis of As in different sample matrices is illustrated in the data presented in Table II, where triple-quadrupole ICP-MS (MS-MS mode; O

 reaction gas) data are compared to the certified values for a wide range of standard reference materials. The matrices ranged from clean water to plant tissue, animal tissue, hair, sediment, and rock samples, covering a wide variety of potential interferences on As. In Table II, both the certified and measured results are reported relative to the solutions measured, to illustrate the wide concentration range measured and the capability of triple-quadrupole ICP-MS to measure As accurately at levels down to less than 200 ppt (200 ng/L).  

Articles

The recently developed technology of triple-quadrupole inductively coupled plasma–mass spectrometry (ICP-MS) offers a unique solution for the removal of spectral interferences in ICP-MS.

ICP–MS-MS Delivers Accurate Trace-Level Arsenic Analysis in Complex Samples

This article describes the use of an in-line, autodilution, and autocalibration sample delivery system coupled to an inductively coupled plasma–mass spectrometry system to analyze a group of over-the-counter pharmaceutical products. The study will follow the method protocol outlined in the recently approved 

 Chapters <232> and <233> on the determination of elemental impurities in the major four drug delivery systems.

The standard method for measuring elemental impurities in pharmaceutical products has been based on a 100-year-old "Heavy Metals Test," described in Chapter <231> of the 

) (1). This test involves precipitation of the metal sulfide in the sample, and compares the intensity and color of the precipitate to a lead standard. Even though this test has been used for more than 100 years, it is well-accepted that it is prone to error because it is not specific to any particular heavy metal, it suffers from quite severe matrix interferences, it is prone to very low recoveries, and it requires a skilled analyst to interpret the color correctly. For this reason, four years ago the USP organization set up an expert committee to investigate the viability of alternative analytical approaches to get around all these problems and replace Chapter <231> with a more robust and reliable instrumental technique.  

In April 2012, USP came out with a brand new inductively coupled plasma–optical emission spectrometry (ICP-OES) and inductively coupled plasma–mass spectrometry (ICP-MS) method to determine a group of metallic contaminants in pharmaceutical products. This method was summarized in General Chapter <232> entitled "Elemental Impurities – Limits" and Chapter <233> entitled "Elemental Impurities – Analytical Procedures,", which will be published in the 

 on December 1, 2012 (2). Pharmaceutical stakeholders will then have until May 2014 to implement these changes and be consistent with the new method described in Chapters <232> and <233>. 

This study focuses on the practical benefits of an ICP-MS system coupled to an in-line, autodilution, and autocalibration sample delivery system to determine a group of toxicologically relevant elements in various pharmaceutical products. It provides an overview of the 

 methodology with particular emphasis on the impurity levels and the recommended analytical procedure. It also analyzes some common pharmaceutical products covering the major drug delivery systems of oral, intravenous, and inhalational, and presents performance figures of merit for the system based on the 

Elemental Impurities in Pharmaceutical Products

Chapter <232> specifies the list of elements and their toxicity limits, defined as maximum daily doses of the different drug categories — oral, parenteral (intravenous injection), inhalation, and large-volume parenteral, whereas Chapter <233> deals with the sample preparation, analytical procedure, and quality control (QC) validation protocol for measuring the elements, including the choice of either inductively coupled plasma–atomic emission spectroscopy (ICP-AES), ICP-MS, or the use of an alternative technique as long as it is fully validated to be acceptable and equivalent to the plasma-based procedures described in the method.  

Chapter <232> Elemental Impurities — Limits

Table I shows the maximum permissible daily exposure (PDE) values (in micrograms per day) for the administration of drug products based on an "average" 50-kg person (3). The toxicity of an elemental impurity is related to its bioavailability. The extent of "chronic" exposure has been determined for each of the elemental impurities of interest for three routes of administration: oral, intravenous (parenteral), and inhalational. When the daily dose of an injection is >100 mL, the amount of element must be controlled through the individual components used to produce the drug product. This is known as the large-volume parenteral (LVP) component limit (in micrograms per gram) and is shown in the last column of Table I.  

Table I: Maximum permissible daily exposure (PDE) values (in Î¼g/day) for the administration of drug products based on an "average" 50-kg person 

This chapter also defines the maximum limit of the elemental impurities based on the material's final use and dosage. This is to ensure the drug manufacturers determine an acceptable level of impurity in the drug based on a typical maximum daily dosage of ≤10 g/day. These values are also used to take into account the sum of all the individual chemicals, active ingredients, and excipient compounds (fillers) used to produce the final drug product, which is intended to be the basis of discussions between the drug manufacturer and the suppliers of the raw materials.  

Chapter <233> Elemental Impurities — Analytical Procedure

This chapter describes the ICP-OES and ICP-MS analytical procedures (including sample preparation), together with the validation protocol for the determination of the elemental impurities in the drug products described earlier (4,5). 

To ensure the method is working correctly, a summary of the QC/QA validation protocols is given below. 

Target (element) limits (known as "J" values) defined as the acceptance value for the elemental impurity being evaluated, based on the weight, number of doses, and frequency of taking or administering the drug.

Calibration using two matrix-matched calibration standards and a matrix-matched blank. For each element, the high standard is twice (2J) the target limit and the low standard is half (0.5J) the target limit.

Samples are diluted so the concentration does not exceed 2× (2J) the target limits.

Samples are analyzed according to the manufacturer's suggestions for instrumental conditions and analyte masses, taking appropriate measures to correct for matrix-induced polyatomic interferences. 

A collision–reaction cell may be used to reduce the polyatomic spectral interferences.

A series of QC validation protocols, including spike recovery, accuracy, precision, and stability tests are followed to ensure the method is generating credible data. 

Sample preparation can be approached in four different ways. However, careful consideration should be given to the choice of acids because of the potential for matrix-induced spectral interferences generated in the ICP. 

Analyze neat, undiluted sample, if the sample is in suitable liquid form.

Dilute the sample in an acidified aqueous solution if the sample is soluble in water.

If the sample is not soluble in water, dilute it in an appropriate organic solvent. 

Use closed-vessel microwave acid digestion for insoluble samples. 

This study was performed using a NexION 300X ICP–MS system (PerkinElmer, Inc.) coupled with a prepFAST (Elemental Scientific Inc.) automated in-line autodilution and autocalibration system. The ICP-MS system has been described in the literature but is basically a single-channel version of PerkinElmer's Universal Cell Technology, which enables the instrument to be used in either the collision (KED) mode or the reaction (DRC) mode using one cell gas, in addition to the standard or normal ICP-MS mode (6).  

Figure 1: Schematic of the sample delivery system (prepFAST) illustrating a two-step process of loading sample into a loop and in-line dilution. During syringe reset in step 1, the loop is rapidly (vacuum) rinsed and loaded with the sample while the nebulizer and spray chamber are rinsed with carrier. In step 2, the sample is injected into the ICP-MS system along a flow path where in-valve addition and mixing of diluent (port 3) and internal standard (port 7) occurs.  

The autodilution system is fully integrated with the ICP-MS system for in-line autodilution of sample and stock standard solutions (7). Solutions are rapidly and reproducibly vacuum loaded from each autosampler location into a sample loop. This process is shown in Figure 1. Dilution, internal standard addition, and mixing all occur in line in the mixing valve. The system's high-resolution syringe-based delivery of all solution flow rates ensures precise and accurate dilution factors while maintaining a constant total sample flow rate. The system is used to autocalibrate the ICP-MS system and perform in-line dilutions of samples. Some of the benefits of this approach for the demands of a high-throughput pharmaceutical laboratory include  

Elimination of the errors associated with manual dilution steps

Reduction of reagent volumes required for high dilution factors

The samples investigated are shown in Table II, representing the three different categories of pharmaceutical products described earlier (oral, inhalation, and large volume parenteral).  

Table II: List of medications evaluated in this study 

For all medications, 1 g of sample was taken and brought into solution with an acid mixture of 2% HNO

 and 0.5% HCl. The arthritis pain medication was crushed into a powder and digested in a Multiwave 3000 microwave digestion system (PerkinElmer, Inc.) and made up to a final volume of 50 mL. The OTC cold and flu remedy was dissolved in the acid mixture, warmed on a hot plate, and made up to 50 mL. Both the allergy medication and the lactated Ringer's solution were already in liquid form, so they were prepared by diluting 1 g to a final volume of 50 mL. The oral medications were further diluted 10-fold in-line, using the autodilution system, to a final dilution factor of 500:1. It's worth emphasizing that if many different medications are being analyzed with large variations in daily dosage, the target values (J) change, requiring additional sample dilution or a new calibration graph. One of the major benefits of the in-line autodilution approach is that it can be done in a fully automated manner, without the need to make additional manual dilutions.  

Table III: ICP-MS instrumental conditions for the analysis of pharmaceutical compounds using USP methodology 

Depending on the target values (J) for the analyte of interest and the sample preparation technique used for the different drug compounds, 

defined calibration solutions at half (0.5J) and twice (2J) the target values are required. Table V (on-line) shows the PDE values (in micrograms per day) for the oral drugs and target concentration values (in micrograms per liter) calculated based on a daily dose of 10 g/day and a final sample dilution of 500:1 (10-fold in-line dilution of 1 g/50 mL). It can be seen that correlation coefficients for all calibration curves generated by in-line dilution (2% HNO

 and 0.5% HCl) of a single stock standard are 0.999 or better. 

Chapter <233> also defines a number of QC–QA protocols to validate the method, which include the following tests:  

Accuracy of the method by spiking the material under investigation at appropriate concentration levels related to the target limits and measuring the % recovery. 

Repeatability of the method by measuring six independent samples of the material under investigation, spiked at the target limits defined and measuring the recovery and precision of the measurements.

This article describes the use of an in-line, autodilution, and autocalibration sample delivery system coupled to an inductively coupled plasma–mass spectrometry system to analyze a group of over-the-counter pharmaceutical products.

ICP-MS with Autodilution and Autocalibration for Implementing the New USP Chapters on Elemental Impurities

Single-reaction-chamber (SRC) microwave digestion was used for the digestion of pharmaceutical samples before inductively coupled plasma–mass spectrometry (ICP-MS) analysis according to draft 

 chapters <232> and <233>. Following the optimization of digestion methodology (vial type, digestion matrix, and temperature program), dietary supplements were digested and analyzed for the "big four" toxic elements. Good quality control data demonstrates the suitability of SRC microwave digestion for this application. 

 chapters <232> and <233> for the measurement of inorganic contaminants in pharmaceutical samples are scheduled to be implemented in early 2014. While samples that are soluble in aqueous and organic solvents may be analyzed directly, a large proportion of samples will require digestion; in fact, digestion may be preferred for inductively coupled plasma–mass spectrometry (ICP-MS) analysis even if the sample is soluble in organic solvent. Closed-vessel digestion is stipulated by 

 and it is expected that microwave digestion will be the predominant digestion technique used; its high pressure and temperature capability offers greater digestion power than hot-plate closed-vessel digestion. 

Single-reaction-chamber (SRC) microwave digestion is a relatively new type of closed-vessel digestion that differs significantly from traditional closed-vessel digestion. A commercially available benchtop SRC digestion system can digest as many as 15 samples simultaneously, at high temperature and pressure. This high temperature and pressure capability enables the complete digestion of virtually every pharmaceutical sample type, producing digest solutions with a very low total organic carbon (TOC) content, which is beneficial for ICP-MS analysis.  

Two sample types, St. John's wort and fish oil, typical of finished product pharmaceuticals, were digested using an SRC digestion system and analyzed for the four toxic 

 elements using collision-cell ICP-MS to evaluate the effectiveness of SRC digestion for this application. Because all samples are digested together in a single chamber with SRC, duplicates and spike recoveries were performed to confirm the retention of volatile elements and the absence of cross contamination.  

The SRC system features a large (typically 1-L) pressurized reaction chamber, which is also the microwave cavity. This enables the intensity and distribution of the delivered microwave energy to be optimized to the shape of the reaction vessel, which ensures even heating and eliminates the need to rotate samples during the digestion program. Samples are placed inside the microwave system together and digested simultaneously. Because the samples are placed inside a pressurized vessel, individual pressure vessels are not needed. Samples are weighed into autosampler-type vials with the appropriate digestion acid and loaded into a rack. Loose-fitting vial caps prevent condensation from the roof of the chamber dripping into the samples (the caps must be loose fitting to ensure pressure equalization within the chamber). The rack is loaded into the chamber, which is then sealed and prepressurized with nitrogen to 40 bar before microwave heating. Prepressurization prevents spitting or boiling of the sample solutions, which prevents cross contamination or loss of volatiles. Because the pressure in the chamber increases with sample temperature, boiling never occurs. An SRC system can operate at very high temperature and pressure — up to 300 °C and 199 bar, which enables the complete digestion of virtually every pharmaceutical sample type, including oils and whole gel caps. Samples with high organic content such as oils generate pressure in the microwave vessel because of the generation of NO

; in traditional microwave digestion, high pressure vessels (typically 100 bar) must be used. The higher pressure capability of an SRC system allows higher sample weights to be digested, including whole gel caps, which is a benefit for pharmaceutical sample analysis. With SRC microwave digestion, because all the samples are digested together under the same conditions, different sample types can be digested together — there is no need to "batch" digestion runs into identical sample types as with traditional microwave digestion. So raw materials, excipients, active pharmaceutical ingredients (API), and the final product can all be digested together in the same run. 

SRC microwave digestion also requires less digestion acid (typically 2–4 mL), which lowers the reagent blank. After completion of the program, the chamber is vented and the rack is removed. Samples are diluted to volume in the vials, and are then ready for aliquotting and measurement. The SRC microwave system used in this work was a benchtop UltraWAVE instrument (Milestone Inc.). Figure 1 shows a flow diagram of the steps in an SRC digestion run.  

Figure 1: Flow diagram of SRC operation. 1 = sample rack is lowered automatically into the microwave chamber, 2 = chamber clamp is secured, 3 = chamber is pressurized with inert gas to prevent boiling, 4 = microwave energy is applied, with all samples under the same temperature and pressure conditions, 5 = chamber is cooled by water cooling, vented, and acid vapors are extracted, 6 = clamp is released and rack automatically rises from chamber.			

Sample vials used in SRC instrumentation are typically available in quartz, TFM (a high temperature polymer from Dyneon), and borosilicate glass. The benefit of glass vials is that they are inexpensive and disposable, which eliminates the need for vial cleaning procedures. The drawback of glass vials is elevated backgrounds (parts-per-billion level) for some elements — namely B, Na, Mg, Al, K, and Ca. However, because these elements are not stipulated in 

 <232>, glass vials can be used. Figure 2 shows the digestion blanks obtained from glass, quartz, and TFM digestion vials for 

 elements. Ru, Os, and the Pt group elements were not measured, but it can be assumed that the vial contribution for these elements is extremely low.  

Figure 2: Digestion blanks obtained from three different vial materials - selected USP elements.			

The microwave program used applied to virtually all sample types: prepressurize with 40 bar nitrogen, ramp to 240 °C over 20 min, and hold for 20 min. Cool-down time was approximately 15 min, giving a total run time of 55 min. If all the samples in the run were easy to digest, the digestion temperature could be reduced to 200 °C, which reduced the cool-down time slightly. Figure 3 shows an actual SRC digestion run. The red line shows the temperature program with the actual temperature achieved during the run superimposed on it. The yellow line shows the applied microwave power, which is automatically adjusted by the instrument software so that the actual digestion temperature follows the program temperature. A proportional integral derivative (PID) controller adjusts the power automatically regardless of the number of samples in the run, acid volume, or sample weight. The blue line shows the pressure achieved during the run. Note that over 95 bar pressure is achieved, which is not possible in conventional closed-vessel digestion. Because the SRC system is capable of very high pressure, higher weights of high organic content sample can be digested, including whole gel caps, which, as we mentioned earlier, is a benefit for pharmaceutical sample analysis. Also, the higher pressure capability allows a higher temperature to be achieved, which gives a more complete digestion. Even for samples such as oils with high organic content, virtually all the organic carbon is decomposed to CO

, giving the sample digest a very low TOC content. This is a benefit for ICP-MS analysis because the presence of carbon in the sample enhances the sensitivity of poorly ionized elements. By removing the organic carbon, the analysis of poorly ionized elements becomes more predictable and reliable.  

Figure 3: Results from a digestion run.			

The digest matrix depends on the sample type and weight. Finished product capsules (St. John's wort) were composited and 0.5 g of sample was digested with 4 mL of HNO

 and 1 mL of HCl. For fish oil gel caps, an entire gel cap (1 g) was digested with 10 mL of HNO

 and 1 mL of HCl. The vial size used was 10 mL, allowing 15 samples to be digested simultaneously. Because all samples are digested together under the same pressure and temperature control, different sample weights and acid chemistries can be digested simultaneously. The only requirement is that the digestion temperature selected must be sufficient to digest the most difficult sample in the batch. 

Samples of St. John's wort capsules and fish oil gel caps were digested as described above. The samples digested completely, forming a clear digestate, which was made up to 100 g with deionized water. The samples were analyzed for the 

 "big four" toxic elements As, Cd, Pb, and Hg using an Agilent 7700x ICP-MS (Agilent Technologies) operated in helium collision mode. The validation of ICP-MS for 

 <232> and <233> has been described previously (1). The use of HCl in the digestion helped to keep the stable Hg in solution while the use of collision mode removed the ArCl interference on As. The addition of HCl also stabilized the platinum group elements when the full 

 element suite was measured. For QC, samples were analyzed in duplicate and a third aliquot was spiked. A reagent blank was also spiked (laboratory fortified blank). Quantitative data, 3-s detection limits, and QC data are shown in Table I.  

Table I: ICP-MS analysis of the "big four" USP analytes in St. John’s wort and fish oil gel caps following SRC digestion			

Quantitative values are expressed in micrograms per gram (parts per million). Duplicate analysis was generally good except for Hg in the fish oil gel caps which suggested possible variability of Hg content between gel caps (that is, the fish oil may have come from different sources). The 3-s detection limits used demonstrate detection limits at the low parts-per-trillion level in the sample as analyzed (dilution factor taken into account). Good recovery was obtained for the spiked samples and also for the spiked reagent blanks. The good recoveries for Hg demonstrate the effectiveness of prepressurization of the SRC system before the digestion sequence. Volatile elements such as Hg were not lost, and no cross contamination of samples occurred because of spitting or boiling.  

SRC microwave digestion offers multiple benefits for sample preparation for trace metals analysis with 

 <232> and <233>. Because of its higher sample capacity, use of disposable vials, and faster cool down time, sample processing throughput is 2–3× higher than conventional closed vessel digestion. The higher digestion quality and low TOC content of the digestates makes analysis by ICP-MS more accurate, and the ability to digest different sample types and higher sample weights together (even entire gel caps) makes it very convenient for pharmaceutical analysis. 

(1) S. Hussain, L. Liba, and E. McCurdy, 

Applications of ICP & ICP-MS Techniques for Today's Spectroscopists,

is with Exova Health Sciences in Santa Fe Springs, California.  

is with Milestone Scientific in Shelton, Connecticut. Please direct correspondence to: 

Single-reaction-chamber (SRC) microwave digestion was used for the digestion of pharmaceutical samples before inductively coupled plasma–mass spectrometry (ICP-MS) analysis according to draft USP chapters <232> and <233>.

The Application of Single-Reaction-Chamber Microwave Digestion to the Preparation of Pharmaceutical Samples in Accordance with USP <232> and <233>

This article discusses how coupling an inductively coupled plasma atomic emission spectrometer with an efficient ultrasonic nebulizer improves trace element detection in a 7.5% (v/v) high-purity sulfuric acid matrix. Elements of interest include Al, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Ti, W, Zn, and Zr. A particular goal is to lower instrument detection limits below 0.1 μg/L for K and Na in this matrix.

Water quality is a critical issue in boiler water reactor nuclear power plants (1). Plant water for steam generation is analyzed after passage through sampling filters to check for indications of corrosion. Corrosion particulate matter can cause numerous problems, including reduced water flow rates, increased pump head pressures, and loss of heat-exchange efficiency as a result of solids buildup on metal surfaces. These problems can all lead to expensive component repairs or replacements. 

The filters in the water filter packs are periodically removed and digested in a sulfuric acid solution to check for indications of corrosion. Inductively coupled plasma–mass spectrometry (ICP-MS) can be readily used for trace-element analysis of sulfuric acid using cell-based quadrupole (2) or sector-field (3) ICP-MS instruments. However, the use of ICP-MS for this application is expensive and requires careful method development. 

This article examines the use of an ultrasonic nebulizer with ICP–atomic emission spectrometry (AES) to improve trace-element detection in a 7.5% high-purity sulfuric acid matrix. The nebulization efficiency of an ultrasonic nebulizer can be up to 10 times higher versus a conventional pneumatic nebulizer as described by Olson, Haas, and Fassel (4) and Bear and Fassel (5). Although non-cleanroom conditions were used, the use of an ultrasonic nebulizer with ICP-AES enabled instrument detection limits for K and Na below 0.10 μg/L. 

An iCAP6500 Duo ICP-AES system (Thermo Scientific) was used in this study. A U5000AT+ ultrasonic nebulizer (CETAC Technologies) was coupled to this spectrometer. 

Reagent blank and standard solutions were prepared from high-purity grade sulfuric acid (Ultrex-II Ultrapure, J.T. Baker). Single-element standards (SCP Science) were used to prepare the working standards and all dilutions were made using 18-MΩ·cm deionized water (Millipore Academic). Solutions were prepared and stored in precleaned fluorinated ethylene propylene (FEP) bottles before analysis.  

Ultrasonic Nebulizer: Principle of Operation

In the ultrasonic nebulizer, a peristaltic pump introduces liquid sample across an oscillating piezoelectric transducer; the oscillations disperse the sample into an aerosol that is swept out of a spray chamber by a flow of argon nebulizer gas from the host ICP-AES instrument. The aerosol passes through a heated "u" tube and an electrothermally cooled condenser. Because of the higher efficiency of the transducer, an integrated condenser is necessary to prevent excess sample solvent from destabilizing or extinguishing the plasma. An integrated drain pump removes the condensed sample solvent and any excess sample liquid from the spray chamber. After passing through the condenser, the dried sample aerosol particles are swept by the nebulizer gas to the ICP-AES instrument for analysis. A schematic of the ultrasonic nebulizer is shown in Figure 1.  

Figure 1: Ultrasonic nebulizer schematic.			

Instrument conditions without and with the ultrasonic nebulizer are given in Tables I and II, respectively. An interface kit with dedicated argon-gas-in and sample-out tubing was used to connect the ultrasonic nebulizer to the ICP torch; removal of the standard ICP nebulizer system and installation of the ultrasonic nebulizer takes approximately 5 min. An important operating parameter change was a reduction of nebulizer gas flow rate from 0.60 L/min (standard nebulizer) to 0.50 L/min (ultrasonic nebulizer); all experiments were performed under non-cleanroom conditions.  

Table I: Operating conditions using a standard nebulizer			

Net analyte intensities for the standard concentric pneumatic nebulizer and the ultrasonic nebulizer are compared in Table III. Intensities are listed for a 100-μg/L multielement spike in 7.5% (v/v) high-purity sulfuric acid. Analyte intensities with the ultrasonic nebulizer are improved 6.8–17.6 times (average factor of 9.9 times) versus the standard nebulizer.  

Table II: Operating conditions using an ultrasonic nebulizer			

A reagent blank and four calibration standards of 20, 50, 100, and 200 μg/L were introduced to the ICP-AES with the standard glass concentric nebulizer and cyclonic spray chamber and then the ultrasonic nebulizer. Instrument detection limits were calculated as three times the standard deviation of the blank concentration; the 100-μg/L standard was run as a check for analyte recoveries.  

Table III: ICP-AES sensitivity comparison for a standard nebulizer vs. an ultrasonic nebulizer in 7.5% (v/v) H2SO4			

Calibration curves for K and Na with the ultrasonic nebulizer are shown in Figures 2a and 2b, respectively. Excellent linearity was obtained, with correlation coefficients of 0.9999 for K at 766.490 nm and Na at 589.592 nm.  

Figure 2: ICP-AES calibration in 7.5% (v/v) H2SO4 using ultrasonic nebulization: (a) K, (b) Na.			

Analyte recoveries for the 100-μg/L standard in 7.5% (v/v) high-purity sulfuric acid using both the standard nebulizer and the ultrasonic nebulizer are in a narrow range within ±5% of 100%. The results are listed in Table IV.  

Table IV: Analyte recoveries obtained using standard and ultrasonic nebulizers in 7.5% (v/v) H2SO4			

As listed in Table V, the ultrasonic nebulizer lowers instrument detection limits for 13 of the 14 measured elements below 0.1 μg/L (tungsten is 0.24 μg/L), for an average improvement of 5.7× versus the standard nebulizer. The instrument detection limits for K and Na are 0.05 μg/L and 0.03 μg/L, respectively.  

Table V: ICP-AES instrument detection limit comparison between the standard nebulizer and the ultrasonic nebulizer in 7.5% (v/v) H2SO4			

The use of an ultrasonic nebulizer with axial-viewing ICP-AES significantly improves instrument detection limits in a high-purity 7.5% (v/v) sulfuric acid matrix because of the higher nebulization efficiency of the ultrasonic nebulizer transducer. Instrument detection limits using ICP-AES are lowered from a factor of 2.5–10 times for 14 different elements; limits for K and Na are both below the application goal of 0.1 μg/L under non-cleanroom conditions.  

Installation of the ultrasonic nebulizer requires removal of the standard nebulizer system with a setup time of 5 min using an interface kit. Modifications to normal ICP-AES operating conditions include a decrease in nebulizer gas flow (from 0.60 L/min to 0.50 L/min) and a modest increase in ICP power (from 1200 W to 1300 W).  

(1) J.J. Wolff, "Ion-Exchange Resins for Use in Nuclear Power Plants," January 2012. 

http://www.purolite.com/Customized/Uploads/PuroliteIonExchangeResinsForUseInNuclearPower.pdf

(2) Agilent ICP-MS Journal Issue 15, Agilent Technologies publication 5988-8983EN, 2003. 

http://www.chem.agilent.com/Library/flyers/Public/5988_8983EN.pdf

(3) Thermo Fisher Scientific Application Note 30105, 2006, 2007. 

https://www.thermo.com/eThermo/CMA/PDFs/Product/productPDF_57328.pdf

(4)  K.W. Olson, W.J. Haas, and V.A. Fassel, 

is with CETAC Technologies in Omaha, Nebraska. Please direct correspondence to: 

This article discusses how coupling an inductively coupled plasma atomic emission spectrometer with an efficient ultrasonic nebulizer improves trace element detection in a 7.5% (v/v) high-purity sulfuric acid matrix.

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Detection of Trace Elements in High-Purity Sulfuric Acid Using Ultrasonic Nebulization with ICP-AES Detection