Inductively Coupled Plasma – Mass Spectrometry

Inductively Coupled Plasma – Mass Spectrometry Inductively coupled plasma mass spectrometery (ICP-MS) is an analytical technique that performs elemental analysis with excellent sensitivity. The ICP-MS instrument employs an argon plasma (ICP) as the ionization source and a mass spectrometer (MS), usually with a quadrupole mass filter, to separate the ions produced. It can simultaneously measure most elements in the periodic table and determine analyte concentrations down to the subnanogram per liter, or parts per trillion (ppt), level. It can perform qualitative, semiquantitative, and quantitative analysis, and compute isotopic ratios on water samples, and in waste extracts and digests. In an ICP-MS instrument, liquid samples are introduced by a peristaltic pump to the nebulizer where a sample aerosol is formed. A double-pass spray chamber ensures that a consistent aerosol is introduced to the plasma. Argon gas is introduced through a series of concentric quartz tubes, known as the ICP torch. The torch is located in the center of a RF coil. A Tesla coil ionizes the argon gas and free electrons are accelerated by a 27 MHz radio frequency field. Collisions between the accelerated electrons and the argon gas generate a high-temperature plasma. The sample aerosol is instantaneously decomposed in the plasma to form analyte atoms, some of which are ionized. The ions produced are extracted from the plasma into the mass spectrometer region, which is maintained at a high vacuum (typically 10-6 torr) using differential pumping. The analyte ions are extracted through a pair of orifices, approximately 1 mm in diameter, known as the sampling cone and the skimmer cone. The analyte ions are then focused by a series of lenses into a quadrupole mass analyzer which separates the ions based on their mass-to-charge ratio (m/z). Finally, ions are detected using an electron multiplier, and data at all masses are collected and stored through a computer interface. The mass spectrum generated is extremely simple. Each elemental isotope appears at a different mass; for example, 111Cd would appear at 111 amu whereas 113Cd would appear at 113 amu, with peak intensities directly proportional to the initial concentration of each isotope. Despite the ease of use and excellent sensitivity of this method, quantitative ICP-MS measurements are prone to matrix effects and other interferences that must be considered. For example, the presence of high chloride levels in the sample will result in the formation of 40Ar35Cl+, a molecular ion that interferes with the determination of 75As, the only naturally occurring isotope of arsenic. Other factors, such as the final concentration of an acid used to dissolve the sample, can affect the signal. The method of standard addition can compensate for most of these effects, but this is a time-consuming approach and is not suitable for large numbers of samples. Another strategy that may help is the use of an internal standard element with a mass and ionization energy similar to that of the analyte. A combination of these approaches will be used in this experiment. II. Equipment A. B. C. D. E. F. III. Reagents A. B. C. D. E. IV. Hewlett Packard 4500 Plus ICP-MS seven plastic sample tubes and caps plastic storage bottle 5-mL adjustable volume autopipet 1-mL adjustable volume autopipet 200-µL fixed volume autopipet digested wheat flour sample from GF-AAS experiment internal standard solution containing either 45Sc, 89Y or 118Sn (each 10 µg/mL) reagent standard for assigned element conc. HCl or conc. HNO3, ultrapure 1% HNO3 Procedure A. Design of Standard Addition Protocol 1. The approximate concentration of your assigned element in the wheat flour solution is indicated in Table 1. Develop a standard addition protocol in which you use exactly 5 mL of your unknown plus 200 µL of the recommended internal standard for each sample. Then add 0.0 - 1.8 mL of Millipore water plus the necessary amount of standard stock solution to make the total addition equal to 1.8 mL and the total sample volume 7.0 mL. Plan for 4-5 additions of standard. Determine the concentration of the standard solution needed so that if 1.8 mL of this solution is added to 5 mL of sample the analyte concentration will increase by about 2-fold. B. Preparation of Standard Solution 1. For your assigned element, prepare a standard stock solution having the concentration determined in IV.A.1. Use ultrapure HCl or HNO3, if needed, to dissolve the standard reagent. Store this solution in a plastic reagent bottle that has been cleaned with 1% HNO3 and rinsed thoroughly with deionized water. C. Preparation of Samples for Analysis 1. Prepare each sample in a plastic tube, cap the tube and thoroughly mix. Calculate the concentration of the added standard in each tube and label each tube. 2. Prepare a reagent blank sample in exactly the same fashion with the solvent used in making the standard stock solution. Table 1 Standards Parameters for the Analysis of Wheat Flour by ICP-MS Element Analyte Concentration‡ (ppb) Sr 1 - 10 dissolve SrCO3 in a minimum of HNO3 (conc HNO3 diluted 1:1 with H2O) and dilute with deionized H2O 89 Ba 2 - 20 dissolve BaCO3 in a minimum of HNO3 (conc HNO3 diluted 1:1 with H2O) and dilute with deionized H2O 89 Al 10 - 100 dissolve Al metal in a minimum of HCl (conc HCl diluted 1:1 with H2O) and dilute with deionized H2O 45 Na 10 - 100 dissolve NaCl in deionized H2O 45 Se 1 - 10 wear gloves; dissolve Se in a minimum of conc HNO3 and dilute with deionized H2O 118 Cu 2 - 20 dissolve Cu metal in a minimum of HNO3 (conc HNO3 diluted 1:1 with H2O) and dilute with deionized H2O 118 Fe 50 - 250 dissolve Fe metal in a minimum of HNO3 (conc HNO3 diluted 1:1 with H2O) and dilute with deionized H2O 118 Zn 10 - 100 dissolve Zn metal in a minimum of HCl (conc HCl diluted 1:1 with H2O) and dilute with deionized H2O 118 Mo 1 - 10 dissolve Mo metal in a minimum of conc HNO3 and dilute with deionized H2O 118 Rb 1 - 10 dissolve RbCl in deionized H2O 89 Ca‡‡ 50 - 250 dissolve CaCO3 in a minimum of HNO3 (conc HNO3 diluted 1:1 with H2O) and dilute with deionized H2O 89 Br 5 - 50 dissolve KBr in deionized H2O 118 Cd 0.1 - 2 wear gloves; dissolve Cd metal in a minimum of HNO3 (conc HNO3 diluted 1:1 with H2O) and dilute with deionized H2O 118 ‡ Standard Preparation Internal Standard Y Y Sc Sc Sn Sn Sn Sn Sn Y Y Sn Sn In wheat flour solution. ‡‡ For the calcium analysis, dilute the wheat flour sample 10-fold before proceeding. D. ICP-MS Analysis Your instructor will demonstrate the basic operation of the ICP-MS and the software used for data collection. Under no circumstances should you operate the instrument without the permission of the instructor. For each sample, record the counts measured for your analyte element and the internal standard. V. Data Analysis A. Normalize the signal of the analyte element in each sample by dividing the analyte counts by the internal standard counts for that sample. Subtract the ratio found for the blank from the ratio for each standard and the unknown. Plot this blankcorrected ratio as a function of added standard concentration and fit these data with a linear least-squares line. B. Determine the analyte concentration in your analyzed sample and then in the dilute wheat flour sample. C. Calculate the corresponding concentration (in ppm) of the analyte in the solid wheat flour sample. Report this value. VI. Questions 1. What is the purpose of the internal standard used in this experiment? Be very specific. 2. Indicate one significant change in the experimental procedure that would be needed in order to accurately measure parts per trillion levels of an analyte.