1 / 85

Chapter 11 Atomic Mass Spectrometry

Atomic Mass Spectrometry. Atomic mass spectrometry offers A number of advantages over the atomic optical spectrometric methods that we have thus far considered, including (1)detection limits that are, for many elements, as great as three orders of magnitude better than optical methods; (2)

clarke
Download Presentation

Chapter 11 Atomic Mass Spectrometry

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

    1. Chapter 11 Atomic Mass Spectrometry

    2. Atomic Mass Spectrometry Atomic mass spectrometry offers A number of advantages over the atomic optical spectrometric methods that we have thus far considered, including (1)detection limits that are, for many elements, as great as three orders of magnitude better than optical methods; (2) remarkably Simple . spectra that are usually unique and often easily interpretable (3) the ability to measure atomic isotopic ratios. Disadvantages include (1) instrument costs that are two to three times that of optical atomic instruments, (2) instrument drift that can be as high as 5% to 10% per hour (3) certain types of interference effects that are discussed later.

    3. 11A Atomic Mass Spectrometry

    4. SOME GENERAL FEATURES OF ATOMIC MASS SPECTROMETRY An atomic mass spectrometric analysis 1 involves the following steps: (1) atomization, (2) conversion of a substantial fraction of the atoms formed in step I to a stream of ions (usually singly charged positive ions), (3) separating the ions formed in step 2 on the basis of their mass-to-charge ratio (m/z), where m is the mass number of the ion in atomic mass units and z is the number of fundamental charges that it bears, and (4) counting the number of ions of each type or measuring the ion current produced when the ions formed from the sample strike a suitable transducer.

    5. 11A-1 Atomic Masses in Mass Spectrometry

    6. Atomic Masses in Mass Spectrometry Atomic and molecular masses are generally expressed in atomic mass units (amu), or daltons (Da).

    7. Uses of Mass Spec forms ions, usually positive, study charge/mass ratio very characteristic fragmentation pattern in charge/mass ratio data easier to interpret than IR and/or NMR provides accurate MW of sample used to determine isotopic abundances

    8. Nearly all elements in the periodic table can be determined by mass spectrometry More selective and sensitive than optical instruments Simple spectra Isotope ratios Much more expensive instrumentation

    9. 11A-2 Mass-to-Charge Ratio

    10. Mass-to-Charge Ratio The mass-to-charge ratio of an ion is the unitless ratio of its mass number to the number of fundamental charges z on the ion. Thus: For 12C1H4+ m/z = 16.0313/1 =16.0313 For 13C1H42+ m/z = 17.0346/2 = 8.5173

    11. 11A-3 Types of Atomic Mass Spectrometry

    12. Types of Atomic mass spectrometers

    13. 11B mass spectrometer

    14. mass spectrometer A mass spectrometer is an instrument that produces ions and separates them according to their mass-to-charge ratios, m/z. Most of the ions we will discuss are singly charged so that the ratio is simply equal to the mass number of the ion. Several types of mass spectrometers are currently available from instrument manufacturers. In this chapter, we describe the three types that are used in atomic mass spectrometry: the quadrupole mass spectrometer, the time-af-flight mass spectrometer, and the double-focusing mass spectrometer.

    16. Atomic mass spectrometer

    17. 11 B-1 Transducers for Mass Spectrometry

    18. Transducers for Mass Spectrometry Several types of transducers are commercially available for mass spectrometers. The electron multiplier is the transducer of choice for most routine experiments.

    19. electron multiplier

    20. electron multiplier Figure 11-2a is a schematic of a discrete-dynode electron multiplier designed for collecting and converting positive ions into an electrical signal. This device is very much like the photomultiplier transducer for ultraviolet-visible radiation, with each dynode held at a successively higher voltage. When energetic ions or electrons strike the Cu-Be surfaces of the cathode and the dynodes, bursts of electrons are emitted.

    21. The Faraday Cup

    22. The Faraday Cup Figure 11-3 is a schematic of a Faraday cup collector. The transducer is aligned so that ions exiting the analyzer strike the collector electrode. This electrode is surrounded by a cage that prevents the escape of reflected ions and ejected secondary electrons. The collector electrode is inclined with respect to the path of the entering ions so that particles striking or leaving the electrode are reflected from the entrance to the cup. The collector electrode and cage are connected to ground through a large resistor. The charge of the positive ions striking the plate is neutralized by a flow of electrons from ground through the resistor.

    24. 11 B-2 Quadrupole Mass Analyzers

    25. Quadrupole Mass Analyzers The most common type of mass spectrometer used in atomic mass spectroscopy is the quadrupole mass analyzer shown in Figure 11-6. This instrument is more compact, less expensive, and more rugged than most other types of mass spectrometers. It also has the advantage of high scan rates so that an entire mass spectrum can be obtained in less than 100 ms.

    27. Mass Analyzer (Quadrupole)

    28. Ion Trajectories in a quadrupole

    29. Ion Trajectories in a quadrupole To understand the filtering capability of a quadrupole, we need to consider the effect of the dc and ac voltages on the trajectory of ions as they pass through the channel between the rods. Let us first focus on the pair of positive rods, which are shown in Figure 11-7 as lying In the xz plane. In the absence of a dc voltage, ions in the channel will tend to converge in the center of the channel during the positive half of the ac cycle and will tend to diverge during the negative half. This behavior is illustrated at points A and B in the figure. If during the negative half cycle an ion strikes the rod, the positive charge will be neutralized, and the resulting molecule will be carried away. Whether a positive ion strikes the rod depends on the rate of movement of the ion along the z-axis, its mass-to-charge ratio. And the frequency and magnitude of the ac signal.

    30. Ion trajectories in a quadrupole A pair of positive rods (as lying in the xz plane). In the absence of a dc potential: Positive half of the ac cycle: Converge (ion in the channel will tend to converge in the center of the channel during the positive half of the ac cycle). Negative half of the ac cycle: Diverge (ions will tend to diverge during the negative half).

    33. Scanning with a quadrupole Filter

    34. Scanning with a quadrupole Filter To scan a mass spectrum with a quadrupole instrument, the ac voltage V and the dc voltage U are increased simultaneously from zero to some maximum value while their ratio is maintained at slightly less than 6. The changes in voltage during a typical scan are shown in Figure 11-9. The two diverging straight lines show the variation in the two dc voltages as a function of time.

    35. Scanning with a quadrupole Filter

    36. 11 B-3 Time of Flight Analyzers

    37. Time of Flight Analyzers In time-of-flight (TOF) instruments, positive ions are produced periodically by bombardment of the sample with brief pulses of electrons, secondary ions, or laser generated photons. These pulses typically have a frequency of 10 to 50 kHz and a lifetime of 0.25 ms. The ions produced in this way are then accelerated by an electric field pulse of 103 to 10' V that has the same frequency as, but lags behind, the ionization pulse. The accelerated particles pass into a field-free drift tube about a meter long (Figure 11-10).

    38. Time of Flight Analyzers non-magnetic separation detector - electron multiplier tube instantaneous display of results

    39. 11 B-4 Double-Focusing Analyzers

    40. Double-Focusing Analyzers As shown in Figure 11-11, a double-focusing mass spectrometer contains two devices for focusing a beam of ions: an electrostatic analyzer and a magnetic sector analyzer.

    41. Mattacuh-Herzog type double-focusing mass spectrometer

    42. 11C INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY

    43. INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY Since the early 1980, ICPMS has grown to be one of the most important techniques for elemental analysis because of its low detection limits for most elements, its high degree of selectivity, and its reasonably good precision and accuracy. In these applications an ICP torch serves as an atomizer and ionizer.

    44. Instruments for ICPMS 11 C-1

    45. Instruments for ICPMS Figure 11-12 shows schematically the components of a commercial ICPMS systemI2 A critical part of the instrument is the interface that couples the ICP torch, which operates at atmospheric pressure with the mass spectrometer that requires a pressure of less than 10-4 torr.

    46. ICP-MS: a handy tool!

    47. Typical mass spectrum

    48. 11C-2 Atomic Mass Spectra and Interferences

    49. Atomic Mass Spectra and Interferences One of the advantages of using mass spectrometric detection with ICPs as opposed to optical detection is that mass spectra are usually much simpler and easier to interpret than corresponding optical spectra, This property is especially true for those elements such as the rare earths that may exhibit thousands of emission lines.

    51. Spectral Interferences

    52. Spectral Interferences? Refractory oxide As a result of incomplete dissociation of the sample matrix or from recombination in the plasma tail MO+, MO2+, MO3+ Doubly charged ions

    53. Spectral Interferences? Isobaric overlap Due to two elements that have isotopes having substantially the same mass 40Ar+ and 40Ca+ Poly atomic Due to interactions between species in the plasma and species in matrix or atmosphere 56Fe and 40Ar16O 44Ca and 12C16O16O.

    54. Isobaric interferences?

    55. Matrix Effects

    56. Matrix Effects In ICPMS, matrix effects become noticeable at concomitant concentrations of greater than about 500 to 1000 mg/mL. Usually these effects cause a reduction in the analyte signal, although under certain experimental conditions signal enhancement is observed.

    57. Applications of ICPMS 11C-3

    58. Applications of ICPMS ICPMS can be used for qualitative, semiquantitative, and quantitative determination of one or more elements in samples of matter.

    59. Qualitative and Semiquantitative Applications

    60. Qualitative and Semiquantitative Applications

    61. Detection Limits

    62. Detection Limits One of the main attractions of ICPMS lies with the lower detection limits attainable with mass spectrometric detection than with optical detection. These limits in many cases equal and sometimes exceed those that can be realized by electrothermal atomic absorption methods.

    63. Detection Limits

    64. quantitative Analyses

    65. quantitative Analyses The most widely used quantitative method of ICPMSuses a set of calibration standards for preparing a calibrationcurve. Simple aqueous standards are usually adequate if the unknown solutions are sufjiciently dilute - less than 2000 g/mL of total dissolved solid.

    66. quantitative Analyses

    67. How good this is?

    68. Isotope Ratio Measurements

    69. Isotope Ratio Measurements The measurement of isotope ratios is of considerable importance in several fields of science and medicine. For example, archeologists and geologists use such data to establish the age of artifacts and various types of deposits. Chemists and clinicians use isotopically enriched materials as tracers in various types of studies. The outcome of these studies is based on isotope ratio measurements.

    70. SPARK SOURCE MASS SPECTROMETY 11 D

    71. SPARK SOURCE MASS SPECTROMETRY In SSMS. the atomic constituents of a sample are converted by a high-voltage (-30 kv), radio-frequency spark to gaseous ions for mass analysis. The spark is housed in a vacuum chamber located immediately adjacent to the mass analyzer. The chamber is equipped with a separate high-speed pumping system that quickly reduces the internal pressure to about 10-8 torr after sample changes. Often, the sample serves as one or both electrodes. Alternatively. it is mixed with graphite and loaded into a cup-shape electrode. The gaseous positive ions formed in the spark plasma arc drawn into the analyzer by a de voltage.

    72. spectra 11 D-1

    73. spectra Like ICP mass spectra, spark source mass spectra are much simpler than atomic emission spectra, consisting of one major peak for each isotope of an element as well as a few weaker lines corresponding to multiply charged ions and ionized oxide and hydroxide species. The presence of these additional ions creates the potential for interference just as in ICPMS.

    74. Qualitative Applications 11 D-2

    75. Qualitative Applications SSMS is a powerful tool for qualitative and semiquantitaative analysis. All elements in the periodic table from 7Li through 238Ucan be identified in a single excitation. By varying data-acquisition parameters, it is possible to determine order of magnitude concentrations for major constituents of a sample as well as for constituents in the parts-per-billion concentration range. Interpretation of spectra does require skill and experience, however, because of the presence of multiply charged species, polymeric species, and molecular ions.

    76. Quantitative Applications A radio-frequency spark is not a very reproducible source over short periods. As a consequence. it is necessary to integrate the output signals from a spark for periods that range from several seconds to hundreds of seconds if good quantitative data are to be obtained .The detection system must be capable of electronic signal integration,

    77. GLOW DISCHARGE MASS SPECTROMETY 11 E

    78. GLOW DISCHARGE MASS SPECTROMETRY As shown in Section 8C-2, a glow-discharge source is used as an atomization device for various types of atomic spectroscopy.In addition to atomizing samples, it also produces a cloud of positive analyte ions from solid samples. This device consists of a simple two electrode closed system containing argon at a pressure of 0.1 to 10torr. A voltage of 5 to 15 kv from a pulsed dc power supply is applied between the electrodes, causing the formation of positive argon ions, which are then accelerated toward the cathode.

    79. Methods for elemental surface analysis 11 F

    80. Methods for elemental surface analysis Two atomic mass spectrometric methods are often used to determine the elemental composition of solid surfaces: secondary-ion mass spectrometry and laser microprobe mass spectrometry.

    81. Advanced Analytical Chemistry – CHM 6157 ® Y. CAI Florida International University Updated on 9/13/2006 Chapter 3 ICPMS Isotope Dilution Isotope dilution is a super internal standard addition method on the basis of isotope ratios. Add a known amount (spike) of a stable enriched isotope of the element considered, which has at least two stable isotopes 1 and 2, to the sample Measure the isotope ratio of isotopes 1 and 2 in the Spike, the unspiked sample and finally the spiked sample. The concentration of the element of interest can then be deducted from these isotopic ratios and from the amount of spike added.

    82. Advanced Analytical Chemistry – CHM 6157 ® Y. CAI Florida International University Updated on 9/13/2006 Chapter 3 ICPMS Advantages: Simplified chemical and physical separation procedures Elimination (reduction) of matrix effects Elimination of the effect of instrumental drift

    83. Advanced Analytical Chemistry – CHM 6157 ® Y. CAI Florida International University Updated on 9/13/2006 Chapter 3 ICPMS Theory In principle, any element with at least two isotopes that can be measured is suitable for determination by isotope dilution. The two selected are designed 1 and 2. Three solutions will be used: Sample (s) Standard (t) Spiked sample (m)

    84. Advanced Analytical Chemistry – CHM 6157 ® Y. CAI Florida International University Updated on 9/13/2006 Chapter 3 ICPMS 1ns is the number of moles of isotope 1 in the sample. 2ns is the number of moles of isotope 2 in the sample. 1nt is the number of moles of isotope 1 in the standard. 2nt is the number of moles of isotope 2 in the standard. Rs is the ratio of isotope 1 to isotope 2 in the sample solution. Rt is the ratio of isotope 1 to isotope 2 in the standard. Rm is the ratio of isotope 1 to isotope 2 in the spiked sample.

    85. Mass Analyzer Double-Focusing Analyzers higher resolution, need higher amplification 2 magnets or 1 magnet & 1 electrostatic field

More Related