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Sample Inlets for Plasma Torches, Part C: Solid Inlets

Sample Inlets for Plasma Torches, Part C: Solid Inlets

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110



Mass Spectrometry Basics



Density of argon gas (300 K)



1.78 x 10,3 g/mL



Specific heal of argon gas



0.124 cal/g/K



Flame temperature



5300 K



Flame dimensions (approx)



1T(0.5t )( 2 ~ 1.6 mL



Volume of 1.6 mL argon at 300K



=



1 .6 x 300/5300

0,1 mL



Heat content of flame



0.lxl.78xl0·3xo.124x530o

0.1 cal



Power output of flame (2 rns)



0.1

2x 10. 3



Power input to flame



1 ·2kW



= -50watls



Figure 17.1

A plasma flame commonly has a diameter of about 1 em and a length of about 2-3 em. If this flame is regarded as being

approximately cylindrical, the volume of the flame at about 5300 K is 1.6 ml and at 300 K is 0.1 ml. With a specific heat

for argon of 0.\24 cal/gfK and a density of 1.78 x 10-3 glml (at 300 K), the heat content of the flame is 0.1 cal. However,

since gas flow through the hot flame occurs in a period of about 2 rnsec, the power output of the flame is about 50 W. This

output should be compared with a power input from the high-frequency electromagnetic field of about 1 kW. The seeming

inconsistency between the high temperature and the low heat content arises because of the low number density of hot

particles. (The concentration of electrons and other particles in the hot flame is approximately 10--8 M.)



tions in sample ion yield, often over short periods of time, and these fluctuations affect accurate

measurement of isotope ratios. Thus sample preparation and manipulation are important and, for any

one type of inlet system, judicious choice of inlet conditions and sample preparation by the operator

of the instrument can avoid the worst aspects of the problems just described.



Introduction of Solids

In some cases, it may be convenient to dissolve a solid and present it for analysis as a solution that

can be nebulized and sprayed as an aerosol (mixed droplets and vapor) into the plasma flame. This

aspect of analysis is partly covered in Part B (Chapter 16), which describes the introduction of solutions.

There are vaporization techniques for solutions of solids other than nebulization, but since these require

prior evaporation of the solvent, they are covered here. There are also many solid samples that need

to be analyzed directly, and this chapter describes commonly used methods to do so.

Basically, there is only one method for dealing with solids, and that is to vaporize them in

some way. Because solids vary from highly volatile (e.g., iodine) to highly nonvolatile (e.g., ceramic

materials), it is not surprising that different methods have been devised for vaporizing solid samples.

Although desirable, it is often the case that a solid cannot simply be put into a vaporization chamber.

For example, if a solid has been dissolved first in acid, it is necessary to remove excess acid and/or

solvent from the resulting liquid sample by selective heating so the more volatile components are

vaporized first and any solid residue is vaporized later.

The various heating methods produce a vapor that is a mixture of gas, very small droplets,

and small particles of solid matter (particulates), Before droplets or particulates can coalesce, the

whole vapor is swept into the plasma flame for analysis. Clearly, the closer the heating source is



111



Sample Inlets for Plasma Torches, Part C: Solid Inlets



to the sample flame, the less are the losses due to deposition on surrounding walls in the instrument

or on lead-ins to the flame. However, this does not prevent carriage of vapors over quite long

distances (up to 20 m is possible for some inlets). The more important methods used for introduction

of solids are based on lasers, arcs (and sparks), and conventional electrical heating. In some

instances, the sample can be heated directly by the plasma flame.



Laser Devices (Laser Ablation, LA)

Laser desorption to produce ions for mass spectrometric analysis is discussed in Chapter 2. As

heating devices, lasers are convenient when much energy is needed in a very small space. A typical

laser power is 10 10 W/cm2 • When applied to a solid, the power of a typical laser beam - a few

tens of micrometers in diameter - can lead to very strong localized heating that is sufficient to

vaporize the solid (ablation). Some of the factors controlling heating with lasers and laser ablation

are covered in Figure 17.2.

Typical data for a laser running as a pulsed beam (O-mode) could be:

10 9 watts.cm- 2

15 ns



Power output

Pulse length

laser beam radius



=



0.5 m



10 9 J.s- 1 cm- 2

15 x 10-9 s

0.5 x 10-3 em



=



Area of sample exposed



Typical data for an iron sample could be:

Density of iron

Specific heat of iron



=



1J



8 g.cm-3

0.1 cal.g- 1

4.2 cal



let ~T = the rise in temperatureJK). when an iron sample is heated by

1 laser pulse and a pit of 4 x 10 cm is produced.

Then.



=



the volume of iron ablated

the mass of iron ablated

heat required for ablation



=



But,

heat input by laser in one pulse>

heat input over the ablated area



0.8 x 4 x 10-10 cm 3

2.6 x 10-9rP.

2.6 x 10. 1 x ~T cal



=



3.2 x 10-10 cm 3



10 9 x 15 x 1 O·g x 4.2 cal.cm- 2= 63 eal.cm-2

=

63 x 0.8 x 10-6 cal



5 x 10-5 cal



If,

heat input from laser



And,



=



heat used for ablation,



5 x 10-5



=



2.6 x 10-10 x dT,



~T



=



2 x 10 5 K



At 2% efficiency, estimated ~ T = 4000 K



Figure 17.2

Thesedata are typical of lasers and the sorts of samples examined. The actual numbers are not crucial, but they show how

the stated energy in a laser can be interpreted as resultant heating in a solid sample. The resulting calculated temperature

reached by the sample is certainly too large because of several factors, such as conductivity in the sample, much less than

100% efficiency in converting absorbed photon energy into kinetic energy of ablation, and much less than 100% efficiency

in the actual numbers of photons absorbed by the sample from the beam. If the overall efficiency is 1-2%, the ablation

temperature becomes about 4000 K.



112



Mass Spectrometry Basics



Suffice it to say at this stage that the surfaces of most solids subjected to such laser heating

will be heated rapidly to very high temperatures and will vaporize as a mix of gas, molten droplets,

and small particulate matter. For ICPIMS, it is then only necessary to sweep the ablated aerosol

into the plasma flame using a flow of argon gas; this is the basis of an ablation cell. It is usual to

include a TV monitor and small camera to view the sample and to help direct the laser beam to

where it is needed on the surface of the sample.

With a typical ablated particle size of about l-um diameter, the efficiency of transport of the

ablated material is normally about 50%; most of the lost material is deposited on contact with cold

surfaces or by gravitational deposition. From a practical viewpoint, this deposition may require

frequent cleaning of the ablation cell, transfer lines, and plasma torch.

There are different types of laser, which can be categorized according to the wavelength of the

emitted radiation and by whether or not the lasers are used in a pulsed or continuous mode. Major

characteristics of some commonly used lasers are given in Chapter 18, which should be consulted

for further details. For laser ablation, short-pulsed (Q-mode) or continuous (free-running) mode can

be important for achieving a desired result; however, in a practical sense, ultimate sensitivity of

detection is not strikingly different as pulsed lasers generally give approximately ten-times-lower

sensitivity. Pulsed lasers tend to give less total ablated material, but that material contains a greater

proportion of gas to particulate matter. A proper comparison of the effects of pulsed and free-running

lasers should take into account the total energy absorbed by the sample in unit time.

The degree of focusing of the laser beam is important. A tightly focused laser beam delivers its

energy to a very small area of sample so the density of energy deposition is very high. This method

leads to the formation of a pit in the sample solid where material has been ablated. Successive pulses

deepen the pit. This mode of operation is used to produce a depth profile, viz., a profile of the

composition of a sample throughout it" thickness. When the beam is defocused, the area of sample

irradiated is greater than in the focused mode, and the density of energy deposition is much lower. The

resulting pit is very much shallower and covers a larger area. Therefore, the two kinds of beam, focused

or defocused, have different analytical consequences. In the focused mode, the thickness of a sample

is examined as the laser works its way down an increasingly deeper pit. The defocused mode is mostly

used to survey variations in sample composition across its surface (surface profile). The two methods

of operation (depth and surface profiling) are complementary, and each is useful in its own right.

A further important factor controlling the use of lasers for ablation purposes concerns the

wavelength of the laser light and the sort of material irradiated. For ablation to occur, the sample

should have one or more absorption bands overlapping the laser wavelength. The absorbed photons

are converted rapidly into vibrational and rotational energy in the sample, and, in tum, some of

this internal energy is converted into kinetic energy, leading to ejection of material from the surface

(ablation). If the sample does not absorb light at the laser wavelength, most of the laser beam will

be reflected (or will pass through a transparent sample) without causing any or much heating. If

there is a substantial overlap of laser wavelength and absorption wavelength for the sample, much

of the laser beam will be absorbed, with a concomitant rapid (about 10-7 to 10-8 sec) increase in

temperature and production of ablated material. At suitable infrared wavelengths, most substances

have absorption bands, so the efficiency of energy absorption can be quite high. However, at these

longer wavelengths the energy of each photon is much less than at ultraviolet wavelengths. Therefore, with anyone type of laser, the efficiency and amount of ablation can vary considerably from

sample to sample, but the variation tends to be less at infrared wavelengths.

In considering the use of a laser for ablative sampling in ICPIMS, major criteria that must be

considered include laser power, pulsed or continuous laser operation, pulse repetition rate, focused

or defocused laser modes, depth or surface profiling, and the absorption characteristics of the sample

range. To obtain the greatest ablation yields, all of these factors should be optimized. Of these, the

most difficult is the focusing; therefore, ablation cells normally have some sort of microscope to

observe the surface of the sample continuously and to select the areas to be examined. With modem

technology, it is more convenient and safer to use a charge-coupled television (CCTV) camera to

view the surface and a TV monitor to display the effects of the laser beam on the sample.



Sample Inlets for Plasma Torches, Part C: Solid Inlets



113



Electrical Discharge Ablation

Under the right conditions, an electrical potential placed on two electrodes (anode and cathode)

separated by a short distance (usually I to 5 mm) in a gas at normal pressures will lead to an

electrical discharge as the insulating properties of the gas break down and a current flows between

theelectrodes. The discharge can be intermittent (sparking) or continuous (arcing). These processes

are discussed in greater detail in Chapter 6. Generally, sparking occurs when the insulating propertiesof the gas between the electrodes is beginning to break down but the discharge cannot maintain

itself for long periods. Conditions leading to this behavior are low current flow associated with

highvoltages. Arcing occurs when the discharge becomes self-sustaining (more electrons are formed

than are discharged at the anode) and electrically runs with a high current flow at low voltages.

Sparks last for only a few micro- or milliseconds, but arcs can last for several minutes or more. In

arcs or sparks, electrons flow to the anode, and positive ions bombard the cathode to maintain

overall electrical neutrality.

Because a spark or an arc has a narrow diameter but contains a large number of ions and electrons,

the heating effect caused by the current flow of a spark or arc leads to the electrodes' becoming very

hot over a small area. The anode is usually shaped to a sharp tip to promote the discharge (compare

the effect of a lightning conductor) and is cooled to prevent its temperature from becoming too high;

nothing is then ablated at this point. The heating effect of ion bombardment at the cathode leads to

ablation of electrode material as particulate matter, molten droplets, and vapor (an aerosol). Therefore,

if a sample is included in the cathode material, it too becomes vaporized. The point of contact of the

discharge at the cathode tends to wander over its surface, and therefore any small heterogeneities in

the sample are smoothed during an analysis. As with the laser source discussed in the previous section,

the ablated material is swept into a plasma torch for analysis of the elements present and their isotope

ratios (ICPIMS). The argon gas, which is used to sweep the aerosol into the torch, prevents air getting

to the sample being heated and therefore prevents oxidation or burning of the sample.

One problem with the spark or arc sources lies in sample preparation for nonconducting

materials. If there is sufficient sample and it is conducting, then it can be machined into a cathode

or simply placed on the cathode surface (in electrical contact). Thus, this method of ablation is

very useful for examining metallurgical samples, which are normally conducting. Nonconducting

samples must be thoroughly mixed with a conducting substance such as powdered copper or

graphite, and then pressed into a disc before being placed on the cathode. The added conducting

material must be of ultrahigh purity to prevent the introduction of impurities into the analysis. A

typical generalized spark ablation source is shown in Figure 17.3.



e



/Anode



Argon

_sweep

gas



Sample aerosol

plus argon to _

plasma flame



sample



Electrical

discharge

-Cathode



8

Figure 17.3

A schematic diagram showing the general construction of an arc or spark source. Actual construction details depend partly

on whether samples need to be analyzed automatically. The sample material can be placed on the cathode or can even

compose the whole of the cathode. If graphite is used, the sample needs to be pressed into the shape of a cathode after

admixture with the carbon.



114



Mass Spectrometry Basics



In operation, a spark source is normally first flushed with argon to remove loose particulate

matter from any previous analysis. The argon flow is then reduced, and the cathode is preheated

or conditioned with a short burn time (about 20 sec). The argon flow is then reduced once more,

and the source is run for sufficient time to build a signal from the sample. The spark is then stopped,

and the process is repeated as many times as necessary to obtain a consistent series of analyses.

The arc source operates continuously, and sample signal can be taken over long periods of time.

Calibration of an arc or spark source is linear over three orders of magnitude, and detection

limits are good, often within the region of a few micrograms per gram for elements such as

vanadium, aluminum, silicon, and phosphorus. Furthermore, the nature of the matrix material

composing the bulk of the sample appears to have little effect on the accuracy of measurement.



Electrical Heating (Electrothermal Vaporization, ETV)

Electrothermal heating is basically the use of an electric current to heat any suitable material,

which can be in the form of a wire or filament (direct heating), cup, or shape of a small oven.

Whatever the kind of sample support, the sample to be examined is heated on the wire, in the cup,

or in the oven to a temperature high enough to effect its evaporation (usually between about 100

and 2000°C). Volatile samples are dealt with easily by using a quartz holder, which is then heated

externally. More generally, the sample is placed directly onto a filament prior to heating or put

into a boat, cup, or crucible, which is then heated. Sometimes a sample solution is placed in the

boat, or it is sprayed onto a thin graphite rod or into a graphite tube. After absorption of the sample

solution, solvent is evaporated before residual solid sample is heated to vaporization, and the

resulting aerosol is swept into the plasma flame for analysis. Typically, about 5-10 mg of sample

are needed. Clearly, if the sample is already a solution (analyte dissolved in a solvent), the ETV

device can be used as a liquid inlet (Chapter 16) to examine the solution if the more volatile solvent

is first evaporated at lower temperatures and then the residual analyte is vaporized at higher

temperatures. To achieve variable heating, the electrical supply for ETV analysis is (normally)

continuously variable.

Limits of detection for an ETV source are in the picogram to nanogram range and are often

better than for direct solution introduction via a nebulizer. As with direct-insertion methods (OSI,

see below), graphite supports for solid samples often lead to carbide formation with elements that

resist volatilization. In these cases, a thermal reagent is commonly added to convert the sample

elements into a more volatile form. For example, chlorine- or fluorine-containing compounds, such

as halothanes, can be added to the argon gas (about 2 to 5% by volume), or sodium chloride can

be added directly to the sample preparation so the solids react upon heating. Interferences from

oxide formation is greatly reduced because no solvent is passed to the plasma flame and air (oxygen)

is kept out of the system.



Direct Sample Insertion (DSI)

In principle, DSI is the simplest method for sample introduction into a plasma torch since the

sample is placed into the base of the flame, which then heats, evaporates, and ionizes the sample,

all in one small region. Inherent sensitivity is high because the sample components are already in

the flame. A diagrammatic representation of a DSI assembly is shown in Figure 17.4.

In practice, direct insertion of samples requires a somewhat more elaborate arrangement than

might be supposed. The sample must be placed on an electrode before insertion into the plasma

flame. However, this sample support material is not an electrode in the usual meaning of the term

since no electrical current flows through it. Heating of the electrode is done by the plasma flame.

The electrode or probe should have small thermal mass so it heats rapidly, and it must be stable

at the high temperatures reached in the plasma flame. For these reasons, the sort of materials used



Sample Inlets for Plasma Torches, Part C: Solid Inlets



115



Heat transmitted by

radiation, conduction,

convection

Relatively cool region

(sample introduction)



Argon shield



Is·



I probe

mpe

Argon for plasma



Heat transmitted by

radiation, conduction,

convection

Relatively cool region

(sample introduction)







lnduction



coi'' ----- ~ . )



(b)



!'I



Argon shield



I



S

I

ampe probe

Argon for plasma



Figure 17.4

Diagram illustrating the method of direct insertion of a sample into the plasma flame. (a) Initially, the sample is held on a

wire or in a small graphite or metal boat and then placed just below the flame until conditions have stabilized. (b) The

probe is then raised so the sample holder enters the flame, where ablation occurs, and the sample is simultaneously

fragmented and ionized. The plasma flame is shown in a vertical configuration (instead of the usual horizontal) because

this position eliminates the possibility of the sample's dropping out of the cup used for sample insertion. If there is no

problem of containing or holding the sample, then the flame can be in its more usual horizontal alignment.



for thermal ionization sources in isotope analysis (see Chapter 7) can be used. Graphite is frequently

the material of choice for the probe material, both because of its good thermal properties and

because it can be machined easily. However, graphite (carbon) forms carbides with many elements

at high temperatures, and these can lead to serious interferences in some instances. For example,

the analysis of chromium at m/z 52 is made difficult by the formation of ArC+, which is also at

the same m/z value. Other materials have been used, such as tantalum, tungsten, or molybdenum.

The probe can be in the form of a cup in which to place the sample, or it can be simply a thin

wire loop on which a sample solution has been evaporated, leaving behind a solid residue. Probe

temperatures of 2000 to 3000°C have been measured.

These direct-insertion devices are often incorporated within an autosampling device that not

only loads sample consecutively but also places the sample carefully into the flame. Usually, the

sample on its electrode is first placed just below the load coil of the plasma torch, where it remains

for a short time to allow conditions in the plasma to restabilize. The sample is then moved into the

base of the flame. Either this last movement can be made quickly so sample evaporation occurs

rapidly, or it can be made slowly to allow differential evaporation of components of a sample over

a longer period of time. The positioning of the sample in the flame, its rate of introduction, and

the length of time in the flame are all important criteria for obtaining reproducible results.



116



Mass Spectrometry Basics



Generally, sample sensitivities of several nanograms per gram can be attained, but precision

may not be as good as with other introduction techniques. Volatile elements such as cadmium or

zinc on a probe of small thermal mass are evaporated over a period of about one second, giving a

sharp transient signal, but slower-evaporating elements give wider signals, and sensitivity may not

be as high. This variation in evaporation rate can even be used to achieve excellent sensitivity, as

in the determination of volatile lead in nonvolatile nickel.



Conclusion

Solid samples can be analyzed using a plasma torch by first ablating the solid to form an aerosol,

which is swept into the plasma flame. The major ablation devices are lasers, arcs and sparks,

electrothermal heating, and direct insertion into the flame.



Chapter



18



Lasers and Other Light Sources

Introduction

Before discussing light sources generally, it may be useful to consider some basic characteristics

of light.

Visible light comprises a very small section of the electromagnetic energy spectrum, which

ranges approximately from cosmic rays to radio waves (Figure 18.1). The wavelengths of electromagnetic energies are related to the velocity of light (c) through the formula, c = vA., in which A.

is the wavelength and v is the frequency of the radiation. The highest energies are associated with

the highest frequencies and the lowest energies with lowest frequencies (Figure 18.l ). Wavelengths

follow the inverse order.

The smallest unit (packet) of electromagnetic energy (a photon) is related to frequency by the

formula, E = hv, in which E is the energy and h is Planck's constant. Alternatively, the relation

can be written, E == he/A.. Frequency (v) is a number with units of cycles per second (cps, the

number of times a wavefront passes a given point in unit time, sec') and is given the name Hertz

(Hz). Planck's constant is a fundamental number, measured in lsec or erg-sec.

high frequency

(shalt wavelength)



low frequency

(long wavelength)



t



cosmic psrtictes



ultraviolet



£=h.v

C



microwaves



vIsible



x-rays



=v.A



:. £= h.c/A



infrared



t



ACcurrenl



redioweves



=Planck's constant

c =valocity of light

v =frequency of radiation

h



A. = wavelength of radiation



Figure 18.1

The electromagnetic spectrum ranges from high-energy cosmic rays (high frequency, v) to low-energy alternating current

(low frequency, v). Different parts of the spectrum need different detection systems. The human eye detects light (blue to

red), and the human body can detect heat (infrared), but humans can detect no other sections of the electromagnetic spectrum

without the aid of instruments. Frequency and wavelength are inversely related, so high frequencies imply short wavelengths

and vice versa. The above equations relate energy, frequency, wavelength, and Planck's constant.



117



Mass Spectrometry Basics



118



Units oflight

The units inwhich light ismeasured are summarized briefly here.

The energy emitted by alight source and the energy falling on an object are two ofthe most important measures ofthe

amount oflight. Since the energy of aphoton varies according to its wavelength (E =h.cJI), measurement ofthe numbers

ofphotons emitted orreceived gives a measure ofenergy emitted orreceived. The standard for light sources isemission

at555 nm. At this wavelength, the energy ofone photon is3.6 x 10.19 J.

The standard unit oflight measurement (the light flux) isthe lumen, which isthe amount ofenergy (power, watts) emitted

orreceived (Joules per second). At555 nm, 1lumen = 0.00147 watts = 0.00147 J.s-1. This issometimes called a"Iightwatt".

Since the energy for one photon atthis wavelength is3.6 x 10.19 J, then the number ofphotons represented by 1lumen is

approximately 4 x 1015 per second, radiated orreceived. Thus, the luminous flux (lumens, 1m) gives the power radiating from

an object orthe power received by an object.

The number oflumens indicates the total amount ofpower but gives no indication ofthe "density" ofthat power. This last

measure isgiven by the luminous intensity:

Luminous intensity = lumens/steradians (candelas)

The intensity isthe flux oflight passing through asolid angle of1steradian (1 steradian =the angle subtended at the

surface ofasphere, for which the area illuminated isthe square ofthe radius). Thus, 1steradian (sr) is1cm 2 atthe center

ofasphere 1cm inradius orit can be 1m2 atthe center ofasphere ofradius 1m, and so on. The luminous intensity is

a measure ofthe power density being radiated and the unit isthe candela (cd). For anyone power setting (lumens), the

greater the solid angle, the less the luminous intensity. Alaser beam has an extremely small angle ofdivergence

(10'8 sr) and therefore its luminous intensity isvery high even though the actual power may be low.

Luminance isthe luminous intensity divided by the area ofemission of light (lumenslsteradianlm 2). This isthe power density

emitted per unit area.

The luminance relates tothe luminous intensity radiated from an entire surface ofa light source:

Luminance = lumenslseradianslm 2 =luminous intensity/m 2

Ifthe amount oflight ismeasured over an area ofreceiving surface, the energy falling on the surface ismeasured inlumens

per unit area ("lux" or"phonThus, the number oflux = lumens/m2 and this measures the power received per square meter

ofsurface (energy per second/per unit area) and pbot =lumens/cm2 and measures the light power received per square

centimeter ofsurface.

An intemational "candle" = 1lumen.

Some examples ofapproximate luminances ofvarious light sources are given in Table 18.1.



Figure 18.2

The common units of light intensity or power density of light emitted or received are as shown above. Care should be taken

in comparing luminances. For example, Table 18.1 reveals that a tungsten filament lamp has about a tenth of the luminance

of the sun, but the area of the sun's emitting surface is massively greater than that of a filament lamp, and therefore the

luminous intensity of the sun is massively greater than the luminous intensity of the filament lamp.



Emission of light from various devices has been known for millions of years (sun, lightning,

fireflies). Fires, oil lamps, candles, gas lamps, etc. all use chemical reactions (combustion in air)

to make hot (active) atoms or molecules, which emit light upon cooling. More recently, electrical

production of light has become common, as in tungsten lamps, fluorescent lights, and arc lamps.

The tungsten filament lamp uses electrical resistive heating to make a wire glow white hot, but

fluorescent and arc lights use forms of electrical discharge, similar in principle to the natural

phenomenon of lightning (see Chapter 6). Photodiodes convert electrical energy directly into light

without the need for heating or discharge. In the period 1950 to 1960, a new form of light emission

was developed, for which the acronym LASER was used (light amplification by stimulated emission

of radiation).



119



Lasers and Other Light Sources



TABLE 18.1.

Some Approximate Luminances

of Various Light Sources

Luminance

(candelas/cm-)



Light source

Sun (noon)



160,000



Tungsten filament lamp



12.000



Fluorescent lamp



0.82



Mercury discharge lamp



970



Metal halogen bulb



810



Photoflash light



20,000



TABLE 18.2.

Some Typical Lasers and Their Power Outputs

Lasing

substances a



Physical state



Laser

wavelength

(nm) b



Pulse length or

continuous

wave



Typical

maximum power

output (watts) C



Cr/alumina (ruby)



solid



694



nanoseconds



lOOMW



Ndlglass



solid



1,060



picoseconds



100TW



GalAs



solid



840



continuous



lOmW



Rh6G (dye)



liquid



600



femptoseco nds



10kW

lmW



HeINe



gas



633



continuous



CO 2



gas



10,600



continuous



200W



ArIF



gas



193



nanoseconds



IOMW



a



These designations are popularly used to describe the basis of the laser but are not accurate descriptions of the

chemical states.



b



As the wavelength moves into the infrared region, it is more common to change units from nanometers to

micrometers (microns). For example. 10,600 nm would be written as 10.6 um,



c



The maximum or peak power depends critically on the pulse length. An energy output of I J in one second is

the power of 1 W, but if the 1 J is emitted in a picosecond, the power rises to 1110-12, which is 1012 W = 1 TW,



The amount of light emitted by a source is measured by its luminance or by its luminous intensity,

which are defined in Figure 18.2. Intrinsic light emission relates to the amount of light emitted per

unit area (luminance). Table 18.1 lists approximate luminances for some common light sources.

Almost all of the oldest light sources give light that covers a range of wavelengths and is not

coherent; the light waves are not propagated in phase with each other. The development of the laser

has provided light sources that emit sharply monochromatic, coherent, and intense radiation ranging

from the ultraviolet (UV) to the infrared (IR). Apart from their use in research, lasers have found

important applications in a large range of everyday devices, from CD players to metal plate cutters

and welders. This chapter cannot cover this huge range of applications. Instead, it concentrates on

principles and descriptions of the most commonly used, commercially available lasers. Table 18.2

indicates some of the power outputs obtainable with various types of laser.

Until about the 1990s, visible light played little intrinsic part in the development of mainstream

mass spectrometry for analysis, but, more recently, lasers have become very important as ionization

and ablation sources, particularly for polar organic substances (matrix-assisted laser desorption

ionization, MALDI) and intractable solids (isotope analysis), respectively.



120



Mass Spectrometry Basics



(a)



(b)



,e



~



<:

"'



<:

"'



..!!



oS!



s







J\



A



(C)



(d)



Figure 18.3

The diagrams depict some typical light outputs in relation to wavelength for various sources. (a) The sharp line (single

wavelength) output from a laser or an atomic emission line. (b) The broader but still sharp wavelengths obtained by using

interference filters, rnonochrornators, and diffraction gratings or from molecular emission bands. (c) Broadband light output

covering a wide range of wavelengths from such sources as the tungsten filament lamp. (d) A mixed output of a typical

discharge lamp, having some intense narrow bands from atomic emission superimposed on broadband radiation from

ion/electron recombination.



Some Characteristics of Light as a Waveform

Wavelength

Light sources can emit photons over a wide range of wavelengths (e.g., electric light bulbs) or at

a single wavelength (e.g., lasers). Therefore, a light source can emit a single wavelength, multiple

wavelengths, or broadband radiation (Figure 18.3). Various devices (filters, interference devices,

diffraction gratings, and monochromators) are available for selecting particular wavelengths from

broadband radiation to obtain selectively more monochromatic light.



Intensity

The intensity of a light beam refers to the number of photons it contains passing through a unit area

(flux). The intensity also refers to the amplitude of a multiphoton waveform, which varies with the

number of photons and whether or not the light is coherent or incoherent. The power emitted by a

light source or received by an object (light watts) relates to the energy of each photon (Figure 18.1),

the number of photons, and the time for which light is emitted or received (Figure 18.2).



Coherence

Consider two trains of light waves (Figure 18.4). IT two photon waves are coherent, the waves are in

phase; therefore the wave intensity (amplitude) is doubled at all points. If the waves are not coherent,

the two waveforms are out of step (out of phase), and the amplitude does not equal the maximum

attainable by coherent waves. It is even possible for light waves to overlap in such a way that there

is no light whatsoever; the waves cancel each other (they are totally out of step), and darkness results

(interference). A coherent laser beam can be contrasted with an incoherent incandescent light beam.

In the former, all of the maxima in the waveform occur together in a tightly bundled stream (a light

beam), i.e., the power density is high; for incandescent light, all the waveforms overlap randomly,

and the power density is much lower (Figure 18.4).



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Sample Inlets for Plasma Torches, Part C: Solid Inlets

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