Atomic MASS Spectrometry PDF

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ATOMIC MASS SPECTROMETRYIntroductionAtomic mass spectrometry is a versatile and widely used tool for identifying the elements present in samples of matter and for determining their concentrations. Nearly all the elements in the periodic table can be determined by mass spectrometry. Atomic mass spect...

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ATOMIC MASS SPECTROMETRY Introduction Atomic mass spectrometry is a versatile and widely used tool for identifying the elements present in samples of matter and for determining their concentrations. Nearly all the elements in the periodic table can be determined by mass spectrometry. Atomic mass spectrometry offers following advantages over atomic optical spectrometric methods – (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; and (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, and (3) certain types of interference effects that are discussed later.

1.

Some General Features of Atomic Mass Spectrometry

An atomic mass spectrometric analysis contains the following steps: (1) atomization, (2) conversion of a substantial fraction of the atoms formed in step 1 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 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. The data from mass spectrometry are usually presented as a plot of relative intensity or ion abundance versus m/z. 1.1

Atomic Masses in Mass Spectrometry

Mass spectrometers discriminate among the masses of isotopes. We therefore, review briefly some terms related to atomic (and molecular) masses. Atomic and molecular masses are generally expressed on the atomic mass scale, which is based on a specific isotope of carbon. One unified atomic mass unit on this scale is equal to 1/12 the mass of a neutral

12 6C

atom. The unified atomic mass unit is given the symbol u. One unified atomic mass

unit is commonly termed one dalton (Da). Thus, one unified atomic mass unit, or Da, is: Mass of one atom

12C 6

=

12 g C12/mol C12 6.022142 × 1023 atoms C12 /mol C12

= 1.992646 × 10−23 g/atom C12

= 1.992646 × 10−26 kg/atom C12 Atomic Mass Spectrometry

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The unified atomic mass unit is then: 1u = 1Da =

1 (1.992646 × 10−23 g) 12

= 1.6605387 × 10−24 g = 1.6605387 × 10−27 kg The relative atomic mass of an isotope such as

35 17Cl

is then measured with respect to the mass of

12

the reference 6C atom. Chlorine-35 has a mass that is 2.914071 times greater than the mass of the carbon isotope. Therefore, the atomic mass of the chlorine isotope is: Atomic mass

35 17Cl

= 2.914071 × 12.000000 Da = 34.968853 Da

Because 1 mol of

12 6C

35

weighs 12.000000 g, the molar mass of 17 Cl is 34.968853 g/mol.

In mass spectrometry, we are often interested in the exact mass m of particular isotopes of an element or the exact mass of compounds containing a particular set of isotopes. Thus, we may need to distinguish between the masses of compounds such as: 12C1H4

m = (12.000000 x 1) + (1.008 x 4) = 16.03200 Da

13C1H4

m = (13.000000 x 1) + (1.008 x 4) = 17.03200 Da

12C1H3 2H1

m = (12.000000 x 1) + (1.008 x 3) + (2.0160 x 1) = 17.0400 Da

Normally, exact masses are quoted to three or four figures to the right of the decimal point because typical high-resolution mass spectrometers make measurements at this level of precision. In other contexts, we use the term nominal mass, which implies a whole-number precision in a mass measurement. Thus, the nominal masses of the three isomers just cited are 16, 17, and 17 Da, respectively. The chemical atomic mass, or the average atomic mass (A), of an element in nature is given by the equation: 𝑛

𝐴 = 𝐴1 𝑝1 + 𝐴2 𝑝2 + ⋯ + 𝐴𝑛 𝑝𝑛 = ∑ 𝐴𝑛 𝑝𝑛 𝑖=1

Where 𝐴1 , 𝐴2 , … , 𝐴𝑛 are the atomic masses in daltons of the 𝑛 isotopes of the element and 𝑝1 , 𝑝2 , … , 𝑝𝑛 are the fractional abundances of these isotopes in nature.

Atomic Mass Spectrometry

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The average, or chemical, molecular mass of a compound is then the sum of the chemical atomic masses for the atoms appearing in the formula of the compound. Thus, the chemical molecular mass of CH4 is 12.011 + (4 x 1.008) = 16.043 Da. The atomic or molecular mass expressed without units is the mass number. 1.2

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

12C1H +, 4

m/z = 16.032/ = 16.032. For

13C1H 2+, 4

m/z =

17.032/2 = 8.516. 1.3

Types of Atomic Mass Spectrometry

Table 1 lists the most important types of atomic mass spectrometry. Table 1―Types of Atomic Mass Spectrometry

Inductively coupled plasma mass spectrometry (ICPMS) is the most popular out of all the techniques. The first three techniques are hyphenated methods, involving combinations of two instrumental techniques that produce analytical results superior in some way to the results from either of the original individual methods.

2.

Mass Spectrometers

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. The three most important types of instruments used in atomic mass spectrometry are: the (1) quadrupole mass spectrometer, the (2) time-of-flight mass spectrometer, and the (3) double-focusing mass spectrometer.

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The block diagram in Figure 1 shows the principal components of all types of mass spectrometers.

Figure 1―Components of a Mass Spectrometer The purpose of the inlet system is to introduce a micro amount of sample into the ion source where the components of the sample are converted into gaseous ions by bombardment with electrons, photons, ions, or molecules. Alternatively, ionization is accomplished by applying thermal or electrical energy. The output of the ion source is a stream of positive (most common) or negative gaseous ions that are then accelerated into the mass analyzer, which separates ions according to their mass-to-charge ratio. The transducer converts the beam of ions into an electrical signal that can then be processed, stored in the memory of a computer, and displayed or stored. The vacuum system maintain a low pressure in all of the components except the signal processor and readout. Low pressure ensures fewer collisions in the mass spectrometer to produce and maintain free ions and electrons. 2.1

Mass Analyzers

2.1.1 Quadrupole Mass Analyzer The most common type of mass analyzer used in atomic mass spectroscopy is the quadrupole mass analyzer shown in Figure 2. This instrument is more compact, less expensive, and more rugged than most other types of mass analyzers. It also has the advantage of high scan rates so that an entire mass spectrum can be obtained in less than 100 ms. Atomic Mass Spectrometry

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Figure 2―A Quadrupole Mass Analyzer The heart of a quadrupole analyzer is the four parallel cylindrical rods that serve as electrodes. Opposite rods are connected electrically, one pair being attached to the positive side of a variable dc source and the other pair to the negative terminal. In addition, variable radio-frequency ac voltages, which are 180° out of phase, are applied to each pair of rods. To obtain a mass spectrum with this device, ions are accelerated into the space between the rods by a potential difference of 5 to 10 V. Meanwhile, the ac and dc voltages on the rods are increased simultaneously while maintaining their ratio constant. At any given moment, all of the ions except those having a certain m/z value strike the rods and are converted to neutral molecules. Only ions having a limited range of m/z values reach the transducer. Quadrupole mass spectrometers with ranges that extend up to 3000 to 4000 m/z are available from several instrument manufacturers. These instruments easily resolve ions that differ in mass by one unit. Generally, quadrupole instruments are equipped with a circular aperture, rather than a slit (see Figure 2), to introduce the sample into the dispersing region. The aperture provides a much greater sample throughput than can be tolerated in magnetic sector instruments, in which resolution is inversely related to slit width. 2.1.2 Time-of-Flight (TOF) Mass Analyzer In TOF, the time required for positive ions to travel from an ionization source to a detector is measured. As shown in Figure 3, in TOF instruments, during ionization, positive ions are produced periodically by bombardment of the sample with brief pulses of electrons, secondary ions, or lasergenerated photons. Atomic Mass Spectrometry

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The ions produced in this way are then accelerated into a field-free drift tube (1 – 2m in length) by an electric field pulse of 103 to 104 V. Separation of ions by mass occurs during the transit of the ions to the detector located at the end of the tube. Because all ions entering the tube have the same kinetic energy, their velocities in the tube vary inversely with their masses with the lighter ions arriving at the detector earlier than the heavier ones.

Figure 3―A Time-of-Flight Mass Analyzer The equation governing TOF mass spectrometry is: 𝑡𝐹 = Atomic Mass Spectrometry

𝐿 𝑚 = 𝐿√ 2𝑧𝑒𝑉 𝑣 Dr AN Gounden

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Where 𝑡𝐹 the flight is time and 𝐿 is the distance from the source to the detector. Typical flight times are 1 to 50 μs. TOF instruments offer several advantages over other types of mass spectrometers, including simplicity and ruggedness, ease of accessibility of the ion source, and virtually unlimited mass range. They suffer, however, from limited resolution and sensitivity. TOF instruments also require fast electronics because ions often arrive at the transducer only fractions of microseconds apart. Several instrument manufacturers offer TOF instruments, but they are less widely used than are magnetic sector and quadrupole mass spectrometers. The linear TOF mass analyzer (Figure 3a) has relatively low resolving power that is limited by flighttube length, acceleration field strength, and the spatial and velocity distributions of the ion packet produced by the ionization source. Use of an ion mirror, or reflectron, to reflect ions can compensate for differences in kinetic energy because more energetic ions penetrate deeper into the reflectron and take a slightly longer path than less energetic ions. This results in ions of the same m/z, but with differing kinetic energies, arriving at the detector at essentially the same time, thus improving the resolving power. Also, with a reflectron, a longer flight path can be achieved in a shorter distance. Several different geometries of reflectron mass analyzers have been proposed. A very popular design is the right angle or orthogonal acceleration geometry shown in Figure 3b. In this system, a continuous beam of ions is produced and extracted by a pulsed electric field into one arm of the flight tube. The design minimizes energy dispersion and provides high resolving power. The reflectron mass analyzer is also capable of acquiring spectra very rapidly (μs) with good sensitivity. The inferior resolution and reproducibility of TOF mass analyzers often make them less satisfactory relative to quadrupole and magnetic analyzers for many applications. ICP-TOF mass spectrometers, for example, typically are an order of magnitude poorer in sensitivity and in detection limits than comparable quadrupole systems. Several advantages partially offset these limitations, however, including simplicity and ruggedness, ease of accessibility to the ion source, virtually unlimited mass range, and rapid data-acquisition rate. Recently, a new type of mass spectrometer, called a distance-of-flight instrument, has been introduced and characterized. In this type of spectrometer, the m/z values of the ions produced are determined by the distance the ions travel in a fixed time interval. The mass spectrum is calculated from the instrument response function and the positions of the ions obtained with a position-sensitive detector. The distance-of-flight technique has shown the ability to detect ions simultaneously over a wide range of m/z values.

Atomic Mass Spectrometry

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2.1.3 Double-Focusing Mass Analyzer Figure 4 shows a double-focusing mass spectrometer. It contains two devices for focusing a beam of ions, (1) an electrostatic analyzer and (2) a magnetic sector analyzer.

Figure 4―Mattauch-Herzog-type double-focusing mass spectrometer. The ions from a source are accelerated through a slit into a curved electrostatic field that focuses a beam of ions having a narrow band of kinetic energies onto a slit that leads to a curved magnetic field. In this field, the lightest ions are deflected the most and the heaviest the least. The dispersed ions then fall on a photographic plate or an array transducer and are thus recorded. Resolution >105 has been achieved with instruments based on this design. As shown in Table 1, analyzers of this type are used in several types of atomic mass spectrometry. 2.2

Transducers for Mass Spectrometry

There are several types of transducers available for mass spectrometers. The electron multiplier is the transducer of choice for most routine experiments. 2.2.1 Electron Multipliers Figure 5a shows a schematic of a discrete-dynode electron multiplier designed for collecting and converting positive ions into an electrical signal. This device is similar to 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. Atomic Mass Spectrometry

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The electrons are then attracted to the next dynode down the chain until, at the last dynode, a huge number of electrons appear for every ion that strikes the cathode. Electron multipliers with up to twenty dynodes are available that typically provide a current gain of 107.

Figure 5―(a) Discrete-dynode electron multiplier (b) Continuous-dynode electron multiplier.

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Figure 5b illustrates a continuous-dynode electron multiplier. The device is shaped like a cornucopia and is made of glass heavily doped with lead to give the material a small conductivity. A voltage of 1.8 to 2 kV applied across the length of the transducer produces a voltage gradient from one end to the other. Ions that strike the surface near the entrance eject electrons that are then attracted to higher-voltage points farther along the tube. These secondary electrons skip along the surface, ejecting more electrons with each impact. Transducers of this type typically have gains of 105, but in certain applications gains as high as 108 can be achieved. In general, electron multipliers are rugged and reliable and are capable of providing high-current gains and nanosecond response times. These transducers can be placed directly behind the exit slit of a magnetic sector mass spectrometer, because the ions reaching the transducer usually have enough kinetic energy to eject electrons from the first stage of the device. Electron multipliers can also be used with mass analyzers that use low-kinetic-energy ion beams (that is, quadrupoles), but in these applications the ion beam exiting the analyzer is accelerated to several thousand electron volts prior to striking the first stage. 2.2.2 The Faraday Cup Figure 6 is a schematic of a Faraday cup collector.

Figure 6―Faraday cup detector. In this transducer, the 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. The resulting voltage drop across the resistor is amplified by a high-impedance amplifier. The response of this transducer is independent of the energy, the mass, and the chemical nature of the ion. Atomic Mass Spectrometry

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The Faraday cup is inexpensive and simple mechanically and electrically; its main disadvantage is the need for a high-impedance amplifier, which limits the speed at which a spectrum can be scanned. The Faraday cup transducer is also less sensitive than an electron multiplier, because it provides no internal amplification. 2.2.3 Array Transducers An array transducer positioned in the focal plane of a mass spectrometer (see Figure 4) provides several significant advantages over single-channel detection; the most important is the simultaneous detection of multiple resolution elements. Others include greater duty cycle, improved precision using ratio measurements and internal standards, and improved detection of fast transient signals.

3.

Inductively Coupled Plasma Mass Spectrometry (ICPMS)

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. For solutions, sample introduction is accomplished by a conventional or an ultrasonic nebulizer. For solids, one of the other sample-introduction techniques, such as spark, laser ablation, or glow discharge, are used. In these instruments, positive metal ions, produced in a conventional ICP torch, are sampled through a differentially pumped interface linked to a mass analyzer, usually a quadrupole. Spectra produced in this way, which are remarkably simple compared with conventional ICP optical spectra, consist of a simple series of isotope peaks for each element present. These spectra are used to identify the elements present in the sample and for their quantitative determination. Usually, quantitative analyses are based on calibration curves in which the ratio of the ion count for the analyte to the count for an internal standard is plotted as a function of concentration. Analyses can also be performed by the isotope dilution technique. 3.1

Instruments for ICPMS

Figure 7 shows schematically the components of a commercial ICPMS system. 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 -2 Pa

(10-4 torr). This coupling is

accomplished by a differentially pumped interface coupler that consists of a sampling cone, which is a water-cooled nickel cone with a small orifice (...