Dean’s Analytical Chemistry Handbook 2nd Edition by Pradyot Patnaik 2

(BQ) Part 2 book Analytical chemistry handbook has contents: Mass spectrometry, radiochemical methods, magnetic susceptibility, electroanalytical methods, thermal analysis, refractometry, elemental analysis of organic compounds,...and other contents.

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SECTION 10

MASS SPECTROMETRY

Table 10.1 Isotopic Abundances and Masses of Selected Elements 10.8

10.6.4 Characteristic Low-Mass Neutral Fragments from the Molecular Ion 10.12

Table 10.3 Mass Spectral Data (Relative Intensities) for the C 1 to C 3 Alcohols 10.25

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Source: DEAN’S ANALYTICAL CHEMISTRY HANDBOOK

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10.2 SECTION TEN

Mass spectrometry is the analytical technique that provides the most structural information for the leastamount of analyte material It provides qualitative and quantitative information about the atomic andmolecular composition of inorganic and organic materials and their chemical structures As an analyt-ical technique it possesses distinct advantages:

1 Increased sensitivity over most other analytical techniques because the analyzer, as a mass-charge

filter, reduces background interference

2 Excellent specificity from characteristic fragmentation patterns to identify unknowns or confirm

the presence of suspected compounds

3 Information about molecular weight.

4 Information about the isotopic abundance of elements

Mass spectrometry often fails to distinguish between optical and geometrical isomers and the

positions of substituent in o-, m- and p- positions in an aromatic ring Also, its scope is limited in

identifying hydrocarbons that produce similar fragmented ions

Sec 20 and in the environmental analysis of trace organic pollutants is highlighted in Sec 21

Functionally, all mass spectrometers have these components (Fig 10.1): (1) inlet sample system, (2) ion source, (3) ion acceleration system, (4) mass (ion) analyzer, (5) ion-collection system, usu-ally an electron multiplier detector, (6) data-handling system, and (7) vacuum system connected tocomponents (1) through (5) To provide a collision-free path for ions once they are formed, the pres-sure in the spectrometer must be less than 10–6torr

10.1.1 Inlet Sample Systems

Gas samples are transferred from a vessel of known volume (3 mL), where the pressure is measured,into a reservoir (3 to 5 L) Volatile liquids are drawn through a sintered disk into the low-pressurereservoir in which they are vaporized instantly Oftentimes a nonvolatile compound can be convertedinto a derivative that has sufficient vapor pressure

The gaseous sample enters the source through a pinhole in a piece of gold foil For analyticalwork, molecular flow (where the mean free path of gas molecules is greater than the tube diameter)

is usually preferred However, in isotope-ratio studies viscous flow (where the mean free path is

FIGURE 10.1 Components of a mass spectrometer (From Shugar and Dean, The Chemist’s Ready

Reference Handbook, McGraw-Hill, New York, 1990.)

MASS SPECTROMETRY

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smaller than the tube diameter) is employed to avoid any tendency for various components to flowdifferently from the others.

Ionization methods in mass spectrometry are divided into gas-phase ionization techniques and ods that form ions from the condensed phase inside the ion source All ion sources are required toproduce ions without mass discrimination from the sample and to accelerate them into the mass ana-lyzer The usual source design has an ion withdrawal and focusing system The ions formed areremoved electrostatically from the chamber Located behind the ions is the repeller, which has thesame charge as the ions to be withdrawn A strong electrostatic field between the first and secondaccelerating slits of 400 to 4000 V, which is opposite in charge to the ions, accelerates the ions totheir final velocities

meth-10.2.1 Electron Ionization

The electron ionization source is a commonly used ionization method The ionizing electrons fromthe cathode of an electron gun located perpendicular to the incoming gas stream collide with the sam-ple molecules to produce a molecular ion A source operating at 70 V, the conventional operatingpotential, also has sufficient energy to cause the characteristic fragmentation of sample molecules.Some compounds do not produce a molecular ion in an electron ionization source This is a dis-advantage of this source

A mass spectrometer is calibrated in the electron ionization mode Perfluoroalkanes are oftenused as markers because they provide a peak at intervals of masses corresponding to CF2groups

mag-Often the primary reason for using this technique is to determine the molecular weight of a

com-pound For this purpose a low-energy reactant, such as tert-C4H9+(from isobutane) is frequently used

In the first step the reagent gas is ionized by electron ionization in the source Subsequent reactionsbetween the primary ion and additional reagent gas produce a stabilized reagent gas plasma When

a reagent ion encounters a sample molecule (MH), several products may be formed:

MH2+by proton transfer

M+by hydride abstraction

MH+by charge transferPractically all the spectral information will be clustered around the molecular ion, or one mass unitlarger or smaller, with little or no fragmentation This type of ionization is desirable when an analy-sis of a mixture of compounds is needed and the list of possible components is limited The generalabsence of carbon–carbon cleavage reactions for the chemical ionization spectra means that theyprovide little skeletal information

MASS SPECTROMETRY 10.3

1B Munson, “Chemical Ionization Mass Spectrometry,” Anal Chem 49:772A (1977).

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Negative chemical ionization2can be conducted with hydroxide and halide ions For these ies the charges on the repeller and accelerating slits in the ion source are reversed with the repellerhaving a negative charge.

stud-10.2.3 Other Ionization Methods

The less frequently used ionization methods receive only brief mention here For more details sult the references cited

con-Field ionization3and field desorption4are techniques used for studying surface phenomena, such

as adsorbed species and trapped samples, and the results of chemical reactions on surfaces; they arealso suitable for handling large lipophilic polar molecules

Fast atom bombardment5and plasma (californium-252) desorption6techniques deal rather tively with polar substances (usually of higher molecular weight) and salts Samples may be bulksolids, liquid solutions, thin films, or monolayers

effec-In thermal ionization the sample is put on a filament substrate (a metal ribbon), which is heated in

the mass spectrometer source until the sample evaporates (ca 2000°C) Filament-loading procedurestend to be element-specific Both positive and negative ions are produced, and thermal ionization usu-ally results in the formation of long-lived, stable ion beams Thermal ionization is appropriate forinorganic compounds that have ionization potentials in the range from 3 to 6 eV On the other hand,the technique is inefficient for organic compounds because their ionization potentials usually rangefrom 7 to 16 eV

Laser desorption methods7–9produce a microplasma that consists of neutral fragments togetherwith elementary molecular and fragment ions Suitable mass spectrometers are limited to time-of-flight and Fourier-transform spectrometers

The recent development of electrospray ionization10has extended the range of masses amenable

to study by mass spectrometery to above several hundred kilodaltons, and commercial instrumentsare available

The function of the mass analyzer is to separate the ions produced in the ion source according to theirdifferent mass–charge ratios The analyzer section is continuously pumped to a very low vacuum sothat ions may be passed through it without colliding with the gas molecules The energies and veloc-

ities v of the ions moving into the mass analyzer are determined by the accelerating voltage V from the ion source slits and the charge z on the ions of mass m:

(10.1)1

2

12

1 1 2

2 2 2

3 3 2

2R C Dougherty, “Negative Chemical Ionization Mass Spectrometry,” Anal Chem 53:625A (1981).

3M Anbar and W H Aberth, “Field Ionization Mass Spectrometry,” Anal Chem 46:59A (1974).

4W D Reynold, “Field Desorption Mass Spectrometry,” Anal Chem 51:283A (1979).

5M Barber et al., “Fast Atom Bombardment Mass Spectrometry,” Anal Chem 54:645A (1982).

6R D MacFarlane, “Californium-252 Plasma Desorption Mass Spectrometry,” Anal Chem 55:1247A (1983).

7R J Cotter, “Lasers and Mass Spectrometry,” Anal Chem 56:485A (1984).

8E R Denoyer et al., “Laser Microprobe Mass Spectrometry: Basic Principles and Performance Characteristics,” Anal.

Chem 54:26A (1982).

9D M Hercules et al., “Laser Microprobe Mass Spectrometry: Applications to Structural Analysis,” Anal Chem 54:280A

(1980).

10C M Whitehouse et al., Anal Chem 57:675 (1985).

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10.3.1 Magnetic-Deflection Mass Analyzer

In a single-focusing magnetic-sector mass analyzer, the ion source, the collector slit, and the apex ofthe sector shape (usually 60°) are colinear Upon entering the magnetic field, the ions are classified

and segregated into beams, each with a different m/z ratio.

(10.2)

where H is the strength of the magnetic field and r is the radius of the circular path followed by the

ions Since the radius and the magnetic field strength are fixed for the particular sector instrument,

only ions with the proper m/z ratio will pass through the analyzer tube without striking the walls,

where they are neutralized and pumped out of the system as neutral gas molecules Focusing isaccomplished by changing either the electrostatic accelerating voltage or the magnetic field strength;

often the former is allowed to diminish while the spectrum is scanned Each m/z ion from light to

heavy is successively swept past the detector slit at a known rate The detector current is amplifiedand displayed on a strip-chart recorder Since the ion paths are separated from one another, therecorder signal will fall to the baseline and then rise as each mass strikes the detector The height ofthe peaks on the chart will be proportional to the number of ions of the corresponding mass–charge ratio

A magnetic-sector mass analyzer has a mass range of 2500 Da at 4-kV ion accelerating voltage.Mass resolution is continuously variable up to 25 000 (10% valley definition) Metastable peaks thataid in structural elucidation are also recorded

10.3.2 Double-Focusing Sector Spectrometers

Because single-focusing mass analyzers are not velocity focusing for ions of a given mass, theirresolving power is limited In double-focusing mass spectrometers an electrostatic deflection field isincorporated between the ion source and the magnetic analyzer Resolving power lies in the range of

100 000 Additional focusing is achieved with quadrupole lenses placed before the electrostatic fieldand between the electrostatic and magnetic fields

10.3.3 Quadrupole Mass Analyzer

In the quadrupole mass analyzer, ions from the ion source are injected into the quadrupole array,shown in Fig 10.2 Opposite pairs of electrodes are electrically connected; one pair at +Udcvolts andthe other pair at −Udcvolts An rf oscillator supplies a signal to each pair of rods, but the signal to thesecond pair is retarded by 180° When the ratio Udc/Vrfis controlled, the quadrupole field can be set

to pass ions of only one m/z ratio down the entire length of the quadrupole array When the dc and rf

amplitudes are changed simultaneously, ions of various mass–charge ratios will pass successivelythrough the array to the detector and an entire mass spectrum can be produced

Registration of negative ions, as from a chemical ionization source, is possible with two electronmultipliers, one for positive and one for negative ions

Scan rates can reach 780 Da ⋅ s–1before resolution is significantly affected The quadrupole mass

analyzer is ideal for coupling with a gas chromatograph Practical m/z limits are 4000 Da.

10.3.4 Time-of-Flight Spectrometer

In the time-of-flight (TOF) mass spectrometer, the ions leave the source as discrete ion packets bypulsing the voltage on the accelerating slits at the exit of the ion source Upon leaving the accelerat-ing slits, the ions enter into the field-free region (drift path) of the flight tube, 30 to 100 cm long,

m z

H r V

= 2 22

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with whatever velocity they have acquired [Eq (15.1)] Because their velocities are inversely portional to the square roots of their masses, the lighter ions travel down the flight tube faster thanthe heavier ions The original ion packet becomes separated into “wafers” of ions according to theirmass–charge ratio The wafers are collected sequentially at the detector.

pro-A spectrum can be recorded every 10 s This makes the TOF mass spectrometer suitable for kinetic studies and for coupling with a gas chromatograph to examine effluent peaks

10.3.5 Ion-Trap Mass Spectrometer

A quadrupole ion-trap consists of three electrodes; two end-cap electrodes normally are held at groundpotential and between them a ring electrode to which an rf potential, often in the megahertz range, isapplied to generate a quadrupole electric field These components can be held in the palm of the hand.Ionization in ion traps is commonly achieved by electron ionization, which occurs within the trap.Chemical ionization uses the variable time scale of the ion trap first to generate reagent ions via elec-tron impact and then allows these reagent ions to react with the vaporized analyte molecules Bothionization methods are limited to gaseous samples

Desorption ionization methods enable mass spectrometry application to fragile nonvolatile pounds, which can be implemented by forming ions in an external source by fast ion bombardment

com-or secondary ion mass spectrometry, and then injecting them into the trap Although trapped ions can

be mass-analyzed by several methods, a mass-selective instability scan is used most commonly In

this procedure, a change in operating voltages is used to cause trapped ions of a particular m/z ratio

to adopt unstable trajectories By scanning the amplitude of the rf voltage applied to the ring

elec-trode, ions of successively increasing m/z are made to adopt unstable trajectories and to exit the ion

trap, where they can be detected by using an externally mounted electron multiplier Other methodsfor mass analysis have been described.11

11R G Cooks et al., Chem Eng News 1991(March 25):26.

FIGURE 10.2 Quadrupole mass analyzer (From Shugar and Dean, 1990.)

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10.3.6 Additional Mass Analyzers

Space precludes more than mention of the more sophisticated mass analyzers, such as the transform (ion-trap) mass spectrometer,12,13tandem mass spectrometers,14triple quadrupole massspectrometer,15and inductively coupled plasma–mass spectrometer.16Triple quadrupole instrumentsare now routinely used in protein structure determinations, pesticide residue analysis, and drugmetabolism studies

Fourier-10.3.7 Resolving Power

The most important parameter of a mass analyzer is its resolving power Using the 10% valley

defi-nition, two adjacent peaks (whose mass differences are ∆m) are said to be separated when the

val-ley between them is 10% or less of the peak height (and the peak heights are approximately equal).For this condition, ∆m/m equals the peak width at a height that is 5% of the individual peak height.

A resolution of 1 part in 800 adequately distinguishes between m/z values 800 and 801 so long

as the peak intensity ratio is not greater than 10 to 1 However, if one wanted to distinguish betweenthe parent peaks of 2,2-naphthylbenzothiophene (260.0922) and 1,2-dimethyl-4-benzoylnaphthalene(260.1201), the required resolving power is

In the electron multiplier the ion beam strikes a conversion dynode, which converts the ion beam to

an electron beam A discrete dynode multiplier has 15 to 18 individual dynodes arranged in a tian blind configuration and coated with a material that has high secondary-electron-emission prop-erties A magnetic field forces the secondary electrons to follow circular paths, causing them to strikesuccessive dynodes

vene-A microchannel plate is a solid-state electron multiplier composed of a hexagonal packed array of millions of independent, continuous, single-channel electron multipliers all fusedtogether in a rigid parallel array With channel densities on the order of 106per cm2, these devicesare one of the highest pixel density sensors known Pore diameters range from 10 to 25 mm Theinside of each pore, or channel, is coated with a secondary-electron-emissive material; thus eachchannel constitutes an independent electron multiplier The onset of ion feedback within thechannel can be staved off by curving each channel in the plate but at the cost of considerable spatial distortion

close-m m

13A G Marshall and L Schweikhard, Int J Mass Spectrom Ion Proc 118/119:37 (1992).

14J V Johnson and R A Yost, “Tandem Mass Spectrometry for Trace Analysis,” Anal Chem 57:758A (1985).

15 R A Yost and C G Enke, “Triple Quadrupole Mass Spectrometry for Direct Mixture Analysis and Structure Elucidation,”

Anal Chem 51:1251A (1979).

16R S Houk, “Mass Spectrometry of Inductively Coupled Plasma,” Anal Chem 58:97A (1986).

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10.4.2 Faraday Cup Collector

The Faraday cup collector consists of a cup with suitable suppressor electrodes, to suppress secondary-ion emission, and guard electrodes It is placed in the focal plane of the mass spectrometer

WITH MOLECULAR STRUCTURE

10.5.2 Natural Isotopic Abundances

The relative abundances of natural isotopes produce peaks one or more mass units larger than the

parent ion (Table 10.1a) For a compound C wHxNyOz, there is a formula that allows one to

calculate the percentage of the heavy isotope contributions from a monoisotopic peak P Mto

TABLE 10.1 Isotopic Abundances and Masses of Selected Elements

(a) Abundances of some polyisotopic elements, %

(b) Selected isotope masses

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17J H Beynon and A E Williams, Mass and Abundance Tables for Use in Mass Spectrometry, Elsevier, Amsterdam, 1963.

Tables of abundance factors have been calculated for all combinations of C, H, N, and O up to mass

500.17

Compounds that contain chlorine, bromine, sulfur, or silicon are usually apparent from prominentpeaks at masses 2, 4, 6, and so on, units larger the nominal mass of the parent or fragment ion For

example, when one chlorine atom is present, the P+ 2 mass peak will be about one-third the

inten-sity of the parent peak When one bromine atom is present, the P+ 2 mass peak will be about thesame intensity as the parent peak The abundance of heavy isotopes is treated in terms of the bino-

mial expansion (a + b) m , where a is the relative abundance of the light isotope, b is the relative dance of the heavy isotope, and m is the number of atoms of the particular element present in the

abun-molecule If two bromine atoms are present, the binomial expansion is

(10.5)

Now substituting the percent abundance of each isotope (79Br and 81Br) into the expansion:

gives

which are the proportions of P:(P + 2):(P + 4), a triplet that is slightly distorted from a 1:2:1

pat-tern When two elements with heavy isotopes are present, the binomial expansion

is used

Sulfur-34 enhances the P + 2 peak by 4.22%; silicon-29 enhances the P + 1 peak by 4.71% and the P+ 2 peak by 3.12%

10.5.3 Exact Mass Differences

If the exact mass of the parent or fragment ions is ascertained with a high-resolution mass

spec-trometer, this relationship is often useful for combinations of C, H, N, and O (Table 10.1b):

(10.6)

One substitutes integral numbers (guesses) for z (oxygen) and y (nitrogen) until the divisor becomes

an integral multiple of the numerator within 0.0002 mass unit

For example, if the exact mass is 177.0426 for a compound containing only C, H, O, and N (notethe odd mass which indicates an odd number of nitrogen atoms), thus

y

(a+b) (m c+d)n

0.255+0.500+0.250( 0 505)2+2 0 505 0 495( )( ) ( + 0 495)2

(a+b)2=a2+2ab+b2

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when z = 3 and y = 1 The empirical formula is C9H7NO3since

10.5.4 Number of Rings and Double Bonds

The total number of rings and double bonds can be determined from the empirical formula(CwHxIzNy) by the relationship

when covalent bonds comprise the molecular structure Remember the total number for a benzenering is 4 (one ring and three double bonds); for a triple bond it is 2

10.5.5 General Rules

1 If the nominal molecular weight of a compound containing only C, H, O, and N is even, so is the

number of hydrogen atoms it contains

2 If the nominal molecular weight is divisible by 4, the number of hydrogen atoms is also divisible

by 4

3 When the nominal molecular weight of a compound containing only C, H, O, and N is odd, the

number of nitrogen atoms must be odd

10.5.6 Metastable Peaks

A further means of ion characterization is achieved by monitoring specific fragmentations of a sen parent ion This approach involves monitoring of metastable peaks that correspond to fragmen-tation that occurs in the first field-free region of a double-focusing mass spectrometer (also of a 60°sector instrument) The field-free region is between the exit of the ion source and the entrance to themass analyzer Signal detection is dictated by the mass-to-charge ratios of both parent and daughter

cho-ions Metastable peaks m* appear as a weak, diffuse (often humped-shaped) peak, usually at a

non-integral mass The one-step decomposition process takes the general form

Original ion → daughter ion + neutral fragment (10.7)The relationship between the original ion and daughter ion is given by

MASS SPECTROMETRY

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which provides this information:

200+→ 172++ 28The probable neutral fragment lost is either CH2= CH2or CO

The mass spectrum is a fingerprint for each compound because no two molecules are fragmented andionized in exactly the same manner on electron-impact ionization When mass spectra are reported,the data are normalized by assigning the most intense peak (denoted as base peak) a value of 100.Other peaks are reported as percentages of the base peak

A very good general survey for interpreting mass spectral data is given by Silverstein et al.18

10.6.1 Initial Steps in Elucidation of a Mass Spectrum

1 Tabulate the prominent ion peaks, starting with the highest mass.

2 usually only one bond is cleaved In succeeding fragmentations a new bond is formed for each

additional bond that is broken

3 When fragmentation is accompanied by the formation of a new bond as well as by the breaking

of an existing bond, a rearrangement process is involved These will be even mass peaks whenonly C, H, and O are involved The migrating atom is almost exclusively hydrogen; six-memberedcyclic transition states are most important

4 Tabulate the probable groups that (a) give rise to the prominent charged ion peaks and (b) list the

neutral fragments

10.6.2 General Rules for Fragmentation Patterns

1 Bond cleavage is more probable at branched carbon atoms: tertiary > secondary > primary Thepositive charge tends to remain with the branched carbon

2 Double bonds favor cleavage in a beta position to the carbon (but see rule 6).

3 A strong parent peak often indicates a ring.

4 Saturated ring systems lose side chains at the alpha position carbon Upon fragmentation, two ring

atoms are usually lost

5 A heteroatom induces cleavage at the bond in the beta position to it.

6 Compounds that contain a carbonyl group tend to break at this group; the positive charge remains

with the carbonyl portion

7 For linear alkanes, the initial fragment lost is an ethyl group (never a methyl group), followed by

propyl, butyl, and so on An intense peak at mass 43 suggests a chain longer than butane

8 The presence of Cl, Br, S, and Si, can be deduced from the unusual isotopic abundance

pat-terns of these elements These elements can be traced through the positively charged

18R M Silverstein, G C Bassler, and T C Morrill, Spectrophotometric Identification of Organic Compounds, 5th ed.,

Wiley, New York, 1991.

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fragments until the pattern disappears or changes due to the loss of one of these atoms to aneutral fragment

9 When unusual mass differences occur between some fragment ions, the presence of F (mass

dif-ference 19), I (mass difdif-ference 127), or P (mass difdif-ference 31) should be suspected

10.6.3 Characteristic Low-Mass Fragment Ions

Mass 30 = Primary aminesMasses 31, 45, 59 = Alcohol or etherMasses 19 and 31= Alcohol

Mass 66 = Monobasic carboxylic acidMasses 77 and 91= Benzene ring

10.6.4 Characteristic Low-Mass Neutral Fragments from the Molecular Ion

Table 10.2 is condensed, with permission, from the Catalog of Mass Spectral Data of theAmerican Petroleum Institute Research Project 44 These, and other tables, should be consulted forfurther and more detailed information

Included in the table are all compounds for which information was available through the C7pounds The mass number for the five most important peaks for each compound are listed, followed

com-in each case by the relative com-intensity com-in parentheses The com-intensities com-in all cases are normalized to the

n-butane 43 peak taken as 100 Another method for expressing relative intensities is to assign the

base peak a value of 100 and express the relative intensities of the other peaks as a ratio to the basepeak Taking ethyl nitrate as an example, the tabulated values would be

Ethyl nitrate 91(0.01)(P) 46(100) 29(44.2) 30(30.5) 76(24.2)

The compounds are arranged in the table according to their molecular formulas Each formula isarranged alphabetically, except that C is first if carbon occurs in the molecules, followed by H if itoccurs The formulas as then arranged alphabetically and according to increasing number of atoms

of each kind, all C4compounds being listed before any C5compounds, and so on

Mass (H O) From alcohols, aldehydes, ketonesMass (F) and (HF) Fluorides

Mass (HCN) Aromatic nitriles or nitrogen heterocyclesMass Indicates either CHO or C H

Mass Indicates either CH O or NOMass (HS) and 34 (H S Thiols

Mass CH CO via rearrangement from a methyl ketone or an aromatic

acetate or an aryl -NHCOCH groupMass C H or CH CO

42

4345

2

2 5 2

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MASS SPECTROMETRY 10.13 TABLE 10.2 Mass Spectra of Some Selected Compounds

Mass numbers (and intensities) of:

CBrClF2 Difluorochlorobromomethane 164(0.23) 85(86) 87(27) 129(17) 131(16) CBr2F2 Difluorodibromomethane 208(1.7) 129(70) 131(68) 79(18) 31(18) CCl2F2 Difluorodichloromethane 120(0.07) 85(33) 87(11) 50(3.9) 101(2.8) CCl3F Fluorotrichloromethane 136(0.04) 101(54) 103(35) 66(7.0) 35(5.8)

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TABLE 10.2 Mass Spectra of Some Selected Compounds (Continued )

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MASS SPECTROMETRY 10.15 TABLE 10.2 Mass Spectra of Some Selected Compounds (Continued)

C4H7ClO2 2-Chloroethyl acetate 122(0.0) 43(162) 73(43) 15(36) 27(29)

C4H7ClO2 Ethyl chloroacetate 122(0.96) 29(130) 27(41) 77(37) 49(29)

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TABLE 10.2 Mass Spectra of Some Selected Compounds (Continued)

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C5H12O Methyl sec-butyl ether 88(2.0) 59(142) 29(50) 27(27) 41(25)

C5H12O Methyl tert-butyl ether 88(0.02) 73(119) 41(33) 43(32) 57(32)

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MASS SPECTROMETRY 10.19 TABLE 10.2 Mass Spectra of Some Selected Compounds (Continued)

MASS SPECTROMETRY

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C6H12O Ethenyl isobutyl ether 100(5.8) 29(73) 41(65) 57(58) 56(40)

C6H14O Ethyl sec-butyl ether 102(1.5) 45(150) 73(76) 29(51) 27(39)

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Nearly all these spectra have been recorded using 70-V electrons to bombard the sample molecules

Secondary-ion mass spectrometry (SIMS) is used for the analysis of surface layers and their position to a depth of 1 to 3 nm A focused ion beam strikes the sample surface and releases sec-ondary ions, which are detected by a mass spectrometer Typical instrumentation might involve aplasma-discharge source coupled with a quadrupole mass analyzer The plasma discharge also serves

com-as a sputtering device to remove successive layers of sample for profiling the material

The SIMS technique affords qualitative identification of all surface elements and permits fication of isotopes and the structural elucidation of molecular compounds present on a surface.Detection sensitivity is in parts per million SIMS is also useful for analyzing nonvolatile and ther-mally labile molecules, including polymers and large biomolecules

identi-19K F J Heinrich and D E Newbury, eds., Secondary Ion Mass Spectrometry, NBS Spec Publ No 427, U.S Government

Printing Office, Washington, D.C., 1975.

20R J Day, S E Unger, and R G Cooks, “Molecular Secondary Ion Mass Spectrometry,” Anal Chem 52:557A (1980).

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10.8 ISOTOPE-DILUTION MASS SPECTROMETRY (IDMS)

Stable isotopes can be used to “tag” compounds and thus serve as tracers to determine the ultimatefate of the compound in chemical or biological systems and also as an analytical method A number

of stable isotopes in sufficiently concentrated form are available for studying organic and inorganicsystems: H, B, C, N, O, S, and Cl In principle, IDMS is applicable to all 60 elements that have morethan one available stable isotope.21 These isotopes complement the relatively larger number ofradioactive isotopes The isotope-dilution method (Sec 11.2.3) can be employed equally well withstable isotopes.22IDMS is based on the addition of a known amount of enriched isotope (called thespike) to a sample After equilibration of the spike isotope with the natural element in the sample,

MS is used to measure the altered isotopic ratio(s) It is only necessary to know the ratio of isotopespresent in the added sample of the substance, the ratio present in the final sample isolated from themixture, and the weight of the added sample

The measured ratio (R m) of isotope A to isotope B can be calculated as follows:

(10.9)

where A x and B x are the atom fractions of isotopes A and B in the sample, A s and B sare the atom

fractions of isotopes A and B in the spike, C x and C sare the concentrations of the element in the

sam-ple and spike, respectively, and W x and W sare the weights of the sample and spike, respectively Theconcentration of the element in the sample can then be calculated:

(10.10)

Because IDMS requires equilibration of the spike isotope and the natural isotope(s), the samplemust be dissolved If the sample does not completely dissolve, if the spike or sample isotopes are selec-tively lost before equilibration, or if contamination occurs in the dissolution process, the measured iso-topic ratios will not reflect the accurate ratio of added spike atoms to sample atoms for that element.Thermal ionization is the ionization method of choice for precise and accurate IDMS Precision andaccuracy are typically 0.1% or better Other useful types of mass spectrometry include electron ioniza-tion with thermal probes, spark source, secondary ions, resonance ionization, and field desorption.The isotope-ratio mass spectrometer, a less expensive adaptation of the usual mass spectrometer,

is available for work in this field In the modified instrument the ion currents from two ion beams—for example, the ion beams from 32SO2and34SO2, are collected simultaneously by means of a dou-ble exit slit and are amplified simultaneously by two separate amplifiers The larger of the twoamplified currents is then attenuated by the operator until it exactly balances the smaller signal fromthe other amplifier The ratio of the two signals is determined from the attenuation required This is

a null method and practically eliminates the effect of other variables in the system

Sensitivity and specificity are the major advantages of mass spectrometry as a quantitative analyticaltechnique An ion incorporating the intact molecule (molecular ion peak) is most characteristic.Production of molecular ions (or at least high-mass fragment ions) is favored by the use of low-energyelectron-impact ionization or by the use of chemical ionization By judicious choice of reagent gases,

21“Relative Abundance of Naturally Occurring Isotopes,” in J A Dean, ed., Lange’s Handbook of Chemistry, 14th ed.,

McGraw-Hill, New York, 1992, pp 4.53 to 4.56.

22J D Fassett and P J Paulsen, “Isotope Dilution Mass Spectrometry for Accurate Elemental Analysis,” Anal Chem.

61:643A (1989).

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the latter ionization method provides the opportunity for selective ionization of certain components

of complex mixtures Detection of analytes with high electron affinities through negative chemicalionization provides another useful technique

The flow of each kind of molecule through the leak in the inlet system is molecular; that is, therate of flow is proportional to the partial pressure of the species behind the leak and independent ofthe presence of other kinds of molecules Consequently, the intensities of the various ion beams fromthe source are also proportional to the partial pressures of the substance behind the leak If two or

more species yield ion beams having the same m/z ratio, the beam intensities, which are usually sured in arbitrary units and conventionally called peak heights, are additive Thus, for a mixture of x number of substances at a total pressure of P0in the reservoir behind the leak, x peaks are selected

mea-for measurement Spectra are recorded on pure samples of each component From inspection of theindividual mass spectra, analysis peaks are selected on the basis of both intensity and freedom frominterference It possible, monocomponent peaks (perhaps molecular-ion peaks) are selected.Computation is simplified if the components of the mixture give at least one peak whose intensity isentirely due to the presence of one component

From the mass spectrum of each pure compound, the sensitivity is obtained by dividing the peakheight of each significant peak by the pressure of the pure compound in the sample reservoir of themass spectrometer From the simplified case in mixtures when the intensity of one peak is entirelydue to the presence of one component, the height of the monocomponent peak is measured anddivided by the appropriate sensitivity factor to give its partial pressure Then division by the totalpressure in the sample reservoir at the time of analysis yields the mole fraction of the particularcomponent

If the mixture has no monocomponent peaks, simultaneous linear equations are then set up fromthe coefficients (percent of base peak) at each analysis peak For example, the significant portion ofthe mass spectral data is given in Table 10.3 for individual C1to C3alcohols Using mass peaks at

32, 39, 46, and 59, four equations are written:

TABLE 10.3 Mass Spectral Data (Relative Intensities) for the C 1 to C 3 Alcohols

Percent of base peak (italic)

MASS SPECTROMETRY

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Next substitute the values for the mixture peaks of the unknown into the above equations.

16.23x2= M46= 1100, x2= 67.78

4.00x3+ 5.52x4= M39= 3000, x3= 50.50

9.61x3+ 3.58x4= M56= 2300, x4= 506.9

68.03x1+ 1.14(67.78) + 2.25(50.50) = M32= 600; x1= 6.01Division by the appropriate sensitivity factor (Table 10.1) yields the partial pressure of each compo-nent (× 10–3torr):

Methanol 6.01 ⫼ 8.76 = 0.686Ethanol 67.78 ⫼ 17.98 = 3.7701-Propanol 50.50 ⫼ 26.51 = 1.9052-Propanol 506.90 ⫼ 23.47 = 21.60Finally, each partial pressure is divided by the total pressure (27.96 × 10–3torr), yielding the frac-tional mole content which, when multiplied by 100, gives the percent in the sample:

Methanol 0.686 ⫼ 27.96 = 0.0245 (or 2.45%)Ethanol 3.770 ⫼ 27.96 = 0.1348 (or 13.48%)1-Propanol 1.905 ⫼ 27.96 = 0.0681 (or 6.81%)2-Propanol 21.60 ⫼ 27.96 = 0.7725 (or 77.25%)The sum of the partial pressures should equal the total sample pressure A discrepancy would indi-cate an unsuspected component or a change in operating sensitivity It must be emphasized that sen-sitivities are not precisely reproducible from instrument to instrument or from time to time, and thatcalibrations with pure substances must be run on the same instrument under the same conditions and

at as nearly as possible the same time as the mixture analysis

The combination of chromatographic and mass spectrometric techniques makes available the benefitsand advantages of both analytical fields The chromatograph does the separating and the mass spec-trometer does the identifying and quantitation Benefits include near-universal analyte response, lowdetection limits, and high information content provided from the mass spectra of organic compounds.Since infrared is nondestructive, it is possible to combine the three instruments into a GC-FTIR-MS.23

10.10.1 GC-MS

There are three requirements for a GC-MS interface: (1) The volume of gas from the gas graph must be reduced to that compatible with the inlet of the mass spectrometer and, furthermore,this pressure reduction should be accomplished with reducing the analyte concentration (2) Thespectra of analytes should be obtained “on the fly,” often on the order of milliseconds (3) A final

chromato-10.26 SECTION TEN

23C L Wilkins, Science 222:291 (1983).

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requirement is a data system capable of handling the volume of data generated by a fast-scanningmass spectrometer.

The direct type of interface with open tubular columns involves extending the end of the opentubular column from the gas chromatography directly into the ion source of the mass spectrometer.The GC flows are low enough, and the vacuum pumping high enough that the vacuum required bythe MS can be maintained without any other interfaces Two disadvantages prevail: all the columneffluent is deposited in the ion source of the MS, causing it to become contaminated rather quickly,and the GC column cannot be changed without shutting down the MS because there is no way to iso-late one from the other

In the open split interface, the space between the GC column and the MS inlet is maintained atabout atmospheric pressure by the use of a second source of gas and a separate vacuum The amount

of purge gas can be controlled to enable the column to be disconnected without shutting down the

MS and undesirable sample components can be removed before they enter the MS

Elemental speciation methods in which GC, LC, and SFC are coupled with ICP mass etry have been reported.24The glow discharge source can replace the ICP unit

spectrom-10.10.2 LC-MS 25,26

The problem encountered when interfacing LC with MS is the mismatch between the mass flowsinvolved in conventional HPLC which are two or three orders of magnitude larger than can beaccommodated by conventional MS vacuum systems Another problem is the difficulty of vaporiz-ing involatile and thermally labile molecules without degrading them excessively

10.10.2.1 Thermospray Method.27,28 The HPLC effluent is fed into a microfurnace maintained

at up to 400ºC that protrudes into a region of reduced pressure (approximately 1 torr) The heat ates a supersonic, expanding aerosol jet that contains a mist of fine droplets of solvent vapor andsample molecules The droplets vaporize downstream and the excess vapor is pumped away Ions ofthe sample molecules are formed in the spray either by direct desorption or by chemical ionizationwhen used with polar mobile phases that contain buffers such as ammonium acetate A convention-

cre-al electron beam is used to provide gas-phase reagent ions for the chemiccre-al ionization of solute ecules

mol-10.10.2.2 Aerosol-Generation Interface.29 The high-pressure effluent from the HPLC columnpasses through a small-diameter orifice to form a fine liquid jet that breaks up under natural forces

to form uniform drops that are dispersed with a gas stream introduced at a right angle to the liquidflow The solvent evaporates in a desolvation chamber A two-stage aerosol-beam separator, whichconsists of two nozzle and skimmer devices, reduces the column pressure to the pressure in the ionsource

10.10.2.3 Electrospray Interface.30 A voltage of 2 to 3 kV is applied to the metal capillary tip,which is typically 0.2 mm o.d and 0.1 mm i.d and located 1 to 3 cm from a large planar counterelectrode This counterelectrode has an orifice leading to the MS sampling system The very highelectric field imposed causes an enrichment of positive electrolyte ions at the meniscus of the solution at the capillary tip At a sufficiently high field, the cone is not stable and a liquid filament

24N P Vela, K K Olson, and J A Caruso, Anal Chem 65:585A (1993).

25T R Covey et al., Anal Chem 58:1451A (1986).

26A L Yergey, C G Edmonds, I A S Lewis, and M L Vestal, Liquid Chromatography/Mass Spectrometry, Plenum, New

York, 1990.

27C R Blakeley and M L Vestal, Anal Chem 55:750 (1983).

28L Yang, G J Ferguson, and M L Vestal, Anal Chem 56:2632 (1984).

29R C Willoughby and R C Browner, Anal Chem 56:2626 (1984).

30P Kebarie and L Tang, Anal Chem 65:972A (1993).

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with a diameter of a few micrometers, whose surface is enriched on positive ions, is emitted fromthe cone tip At some distance downstream the filament becomes unstable and forms a fine mist ofpositively charged droplets The charged droplets shrink by solvent evaporation and repeated dropletdisintegrations, leading to very small, highly charged droplets capable of producing gas-phase ionswhich enter the MS.

When coupling capillary electrophoresis to MS, electrical connection is established at the plated CE capillary terminus Various designs feature sheath flow (coaxial) interface, a liquid-junctioninterface, and a sheathless interface.31

gold-10.10.2.4 Mechanical Transport Interface. In the moving-belt interface, chromatographiceffluent is deposited or sprayed onto a continuous moving belt that is woven from ultrafine quartzfiber The belt passes below an infrared heater that evaporates most of the mobile phase before thebelt reaches the entrance slit to the first of two successive vacuum locks The pressure is reduced

to 0.1 torr before the belt moves into a chamber where a flash vaporizer vaporizes the sample intothe ion source The belt then exits the chamber, passes over a final heater that cleans the belt byevaporation of any residue, and then moves out through the locks to recycle In some versions thebelt passes through a wash bath after leaving the vacuum locks The moving-belt interface can beused with magnetic sector or quadrupole instruments and in either the electron or chemical ion-ization mode

NIST/EPA/NIH Mass Spectral Database, NIST Standard Reference Data, Gaithersburg, Maryland, 1992.

Continuously upgradable data compatible with PCs.

Watson, J T., Introduction to Mass Spectrometry, Raven, New York, 1985.

31R D Smith et at., Anal Chem 65:574A (1993).

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SECTION 11

RADIOCHEMICAL METHODS

11.1

Table 11.6 Isotopes Measured by Liquid-Scintillation Counting 11.20

Table 11.8 Characteristics of Scintillation Solvents 11.22

Table 11.9 Estimated Sensitivities by Neutron Activation Analysis 11.28 Table 11.10 Elements Detectable with 14-MeV Neutrons 11.31

Source: DEAN’S ANALYTICAL CHEMISTRY HANDBOOK

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11.1 INTRODUCTION

Radioactivity is the spontaneous disintegration of an atom that is accompanied by emission of ation There are many radioactive elements that are isotopes (having the same atomic number but dif-ferent atomic mass) of nonradioactive elements An atom of a radioactive isotope has the samenumber of orbital electrons as an atom of its nonradioactive counterpart and, in general, will behavechemically and biologically like the nonradioactive species Therefore experimental and diagnostic

radi-as well radi-as analytical procedures can utilize atoms of radioactive isotopes radi-as tracers The differencebetween the radioactive and the nonradioactive atoms of identical elements is the number of neutrons

in the nucleus, the number of protons and electrons being the same for all (Some elements havemore than two isotopes.)

11.1.1 Modes of Radioactive Decay

All radioactive nuclides exhibit the phenomenon of radioactive decay with emission of one or more ofseveral types of radiation The modes of decay with associated radiations are summarized in Table 11.1

In many cases, the product nucleus is left in an excited state after a decay [a, b, electron capture (EC)].

Stability is achieved by emission of gamma radiation from the nucleus Sometimes this process takesplace very quickly; otherwise gamma radiation may be delayed, perhaps by many days The latter sit-

uation is known as an isomeric transition, and the nucleus is said to be in a metastable state, denoted

by m after the mass number.

Alpha, gamma, and x radiations have discrete energies; by suitable methods one can detect these

energies as photopeaks Beta particles, either a very energetic negatively charged electron (negatron) or

a positively charged electron (positron), have a continuous distribution of kinetic energies up to a

max-imum energy Emax, the value given in tables of nuclides For example, the energy of the beta particlesemitted in the decay of 32P lie in a continuum from 0 to 1.71 MeV; the average energy is 0.69 MeV

All positron decays are accompanied by annihilation radiation resulting from destruction of the

positrons The energy of this radiation is 0.511 MeV, and two such photons arise from the tion of each positron Every electron-capture and internal-conversion process is followed immedi-ately by emission of x rays

destruc-A positron-emitting atom may decay by capturing one of its own orbital K electrons (K capture

or internal conversion) The excess energy is emitted as g radiation The daughter element (one atomic

number less than its parent) has a vacant K orbital; x radiation characteristic of the daughter is ted when L- and M-level electrons fall into the K level.

emit-Radioactive decay of a nucleus often results in the formation of a daughter nucleus in an excitedunstable state As the daughter nucleus changes from the excited state to a state of lower energy, ener-

gy is released, usually as electromagnetic radiation in the form of g radiation The change is termed

an isomeric transition (IT) because the nucleus decays with no change in atomic mass or number.

A selection of radionuclides and their characteristics is given in Table 11.2 Radionuclides of ments occurring naturally in biological organisms, such as 3H,14C,32P, 35S, and 131I, are used forbiological and medical research as tracers or in labeled compounds wherein one or more radioactiveatoms have replaced stable atoms of the same element in the compound

ele-11.2 SECTION ELEVEN

TABLE 11.1 Modes of Radioactive Decay

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RADIOCHEMICAL METHODS 11.3 TABLE 11.2 Table of Nuclides

Explanation of column headings

Nuclide Each nuclide is identified by element name and the mass number A, equal to the sum of the numbers

metastable isotope An asterisk preceding the mass number indicates that the radionuclide occurs in nature The digits in parentheses following a numerical value represent the standard deviation of that value in terms of the final listed digits.

Half-life The following abbreviations for time units are employed: y = years, d = days, h = hours, m = minutes, and s = seconds.

Natural abundance The natural abundances listed are on an “atom percent” basis for the stable nuclides present in naturally occurring elements in the earth’s crust.

Thermal neutron absorption cross section Simply designated cross section, it represents the ease with which a given nuclide can absorb a thermal neutron (energy less than or equal to 0.025 eV) and become a

other than (n,g), it is so indicated.

Major radiations In the last column are the principal modes of disintegration and energies of the radiations

in million electronvolts (MeV) in parentheses Symbols used to represent the various modes of decay are

α, alpha particle emission K, electron capture

γ , gamma radiation x rays)

useful for identification purposes For detailed decay schemes the literature should be consulted.

Natural Cross Element A Half-life abundance, % section, b Radiation, MeV

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11.4 SECTION ELEVEN

TABLE 11.2 Table of Nuclides (Continued)

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RADIOCHEMICAL METHODS 11.5 TABLE 11.2 Table of Nuclides (Continued)

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0.055, 0.097, 0.119, 0.146, 0.164, 0.22, 0.291, 0.32)

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