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Sample Inlets for Plasma Torches, Part B: Liquid Inlets

Sample Inlets for Plasma Torches, Part B: Liquid Inlets

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104



Mass Spectrometry Basics



1 em



.:



Density of argon gas (300 K)



1.78 x 10-3 glmL



Specific heat of argon gas



0.124 cal/g/K



Flame temperature



5300K



Flame dimensions (approx)



1T (0.5)2 x 2 = 1.6 mL



Volume of 1.6 mL argon at 300K



1.6 x 300/5300

0.1 mL



Heat content of flame



0.1 x 1.78 x 10-3 x 0.124 x 5300

0.1 cal



Power output of flame (2 ms)

Power input to flame



0.1



2x10- J



= 50 watts

-



1 - 2 kW



Figure 16.1

A plasma flame commonly has a diameter of about 1 cm 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 m!. With a specific heat

for argon of 0.124 cal/g/K and a density of 1.78 x 10-3 g/ml (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 lO-s M.)



secondary processes faster than they can be replaced by the primary generation process, then

the plasma process ceases and the flame goes out. Fluctuations in flame temperature and performance lead to significant variations 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 anyone 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.



Liquid Inlets

Suitable inlets commonly used for liquids or solutions can be separated into three major classes,

two of which are discussed in Parts A and C (Chapters 15 and 17). The most common method of

introducing the solutions uses the nebulizer/desolvation inlet discussed here. For greater detail on

types and operation of nebulizers, refer to Chapter 19. Note that, for all samples that have been

previously dissolved in a liquid (dissolution of sample in acid, alkali, or solvent), it is important

that high-purity liquids be used if cross-contamination of sample is to be avoided. Once the liquid

has been vaporized prior to introduction of residual sample into the plasma flame, any nonvolatile

impurities in the liquid will have been mixed with the sample itself, and these impurities will appear

in the results of analysis. The problem can be partially circumvented by use of blanks, viz., the

separate examination of levels of residues left by solvents in the absence of any sample.



105



Sample Inlets for Plasma Torches, Part B: Liquid Inlets



Heat transmitted by

radiation, conduction,

convection

Relatively cool region

(sample introduction)



1



S

ib

ampeproe

Argon for plasma



Argon shield



Heat transmitted by

radiation, conduction,

convection

Relatively cool region

(sample introduction)



Ind~tio:b~;I/ ~

Argon shield



. (



1



S



ample probe

Argon for plasma



Figure 16.2

Diagram illustrating the method of direct insertion of a sample into a plasma flame. 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. The probe is then

moved up 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 normal horizontal alignment.



Direct Insertion Methods (Direct Solids Insertion, DSI)

DSI is discussed in Part C (Chapter 17), since the approach usually requires an initial evaporation

of solvent from a solution by moderate heating in a gas stream so as to leave the solute (the

analytical sample). The resulting residual sample is then heated strongly to vaporize it. Typically,

a solution is placed onto a heat-resistant wire or onto a graphite probe, and then the solvent is

allowed to evaporate or is encouraged to do so by application of heat, directly or indirectly.

The residual solid on its metal or graphite support is placed just below the plasma flame, which

is allowed to stabilize for a short time. The probe and sample are then driven into the hightemperature flame, which causes vaporization, fragmentation, and ionization (Figure 16.2).

Because the heat capacity of the flame is relatively small, the sample holder and sample should

have as Iowa thermal mass as possible so as not to interfere with the operation of the flame.

With the direct-insertion method, samples appear transiently in the flame; therefore, if a wide

range of elements is to be examined, the mass spectrometer should be one that can span a wide

mlz range in the short space of time the sample takes to pass through the flame (quadrupole,

time-of-flight). Further details of the DSI technique are discussed in Part C (Chapter 17).



106



Mass Spectrometry Basics



Electrothermal Heating Methods (Electrothermal

Vaporization, ETV)

This topic is described in Part C (Chapter 17) because application of heat to remove the solvent

from a solution results in a residual solid (or other relatively nonvolatile material), which must then

be heated strongly to effect vaporization to form an aerosol. The latter is swept into the plasma

flame with a flow of argon gas. The ETV method involves placement of a sample solution on a

wire, into a boat or cup, or in an oven, where the solvent is evaporated by application of moderate

heat. The resulting solvent vapors are routed from the plasma flame, or the evaporation is done

slowly to avoid extinguishing the flame. After evaporation of the solvent, the flame is allowed to

stabilize if necessary, and then the sample holder (probe) is heated strongly by electrical means to

vaporize the residual solid sample as an aerosol, the droplets of which are led into the flame by

argon gas flowing through a transfer line. In one variation of this method, the sample on its sample

holder (a wire) is placed close to the flame in the sample inlet tube, and the wire is then heated

electrically to drive off first the solvent and then the sample. (This apparatus is similar to that

shown in Figure 16.2.)

Further details of the ETV technique are described in Part C (Chapter 17).



Nebulizer Methods

By far the most widely used method of introducing a liquid sample into a plasma flame is by

splitting the liquid into a stream of droplets (nebulization), which is led along a transfer line and

then into the center of the plasma flame. During transfer, the droplets evaporate and become very

small, often consisting of only residual analyte. Once in the flame, the small drops of sample are

fragmented and ionized. The process can be used to generate a transient or a continuous signal.

The nebulization concept has been known for many years and is commonly used in hair and

paint spays and similar devices. Greater control is needed to introduce a sample to an YCP instrument. For example, if the highest sensitivities of detection are to be maintained, most of the sample

solution should enter the flame and not be lost beforehand. The range of droplet sizes should be

as small as possible, preferably on the order of a few micrometers in diameter. Large droplets

contain a lot of solvent that, if evaporated inside the plasma itself, leads to instability in the flame,

with concomitant variations in instrument sensitivity. Sometimes the flame can even be snuffed out

by the amount of solvent present because of interference with the basic mechanism of flame

propagation. For these reasons, nebulizers for use in ICP mass spectrometry usually combine a

means of desolvating the initial spray of droplets so that they shrink to a smaller, more uniform

size or sometimes even into small particles of solid matter (particulates).

Nebulizers can be divided into several main types. The pneumatic forms work on the principle

of breaking up a stream of liquid into droplets by mechanical means; the liquid stream is forced

through a fine nozzle and breaks up into droplets. There may be a concentric stream of gas to aid

the formation of small droplets. The liquid stream can be directed from a fine nozzle at a solid

target so that, on impact, the narrow diameter stream of liquid is broken into many tiny droplets.

There are variants on this approach, described in the chapter devoted to nebulizers (Chapter 19).

Thermospray nebulizers operate on the principle of converting a thin stream of liquid into

a rapidly expanding vapor through application of heat. As the liquid stream to be nebulized

reaches the end of a capillary tube, it meets a short region of tubing that has been preheated to

such a temperature that the solvent begins to boil rapidly. The resultant expanding gas stream

mixes with unvaporized liquid to give a fine spray of droplets from the end of the capillary tube.

These sorts of nebulizers are used to introduce solution eluants from liquid chromatography

instruments, particularly those using the very narrow nanobore columns. (Thermospray is



Sample Inlets for Plasma Torches, Part B: Liquid Inlets



107



described in Chapter 11.) Electrospray forms another mode of nebulization suitable for liquid

chromatography and is discussed in detail in Chapter 8.

Droplets can be produced ultrasonically. Application of a rapidly oscillating electric potential

(200 to 1000 kHz) to certain types of inorganic crystal causes a face of the crystal to oscillate at

a similar rate (piezoelectric effect). For use in piezoelectric devices, the moving face of the crystal

is usually protected by fastening a thin metal plate to it. Since erosive and corrosive effects are

frequently common with such devices, chemically and physically robust metals such as titanium

are used to provide this facing to the piezoelectric crystal. Any thin stream of liquid directed at a

steep angle onto the rapidly oscillating surface of a piezoelectric crystal is subjected to a standing

acoustic wave, which breaks the stream into droplets. The rate at which droplets are produced is

much greater than for pneumatic nebulizers, and a desolvation chamber is necessary to avoid overly

large amounts of solvent and sample entering the flame and causing instability.

Once an aerosol has been produced, it is usually in the form of vapor (gas) from the solvent

plus droplets of the original solution and sometimes also particulate (solid) matter. The droplets

usually cover a wide range of sizes, which are not static but vary as the droplets proceed toward

the plasma flame. As the aerosol is swept toward the flame by a flow of argon gas, small droplets

grow bigger by collision with others, but, overall, droplets become smaller as the solvent evaporates.

Thus, the initial aerosol produced at the nebulizer will have changed its size distribution by the

time it meets the plasma flame. Most of these changes will have occurred within about 100 to 200

msec. The changes are frequently assisted by incorporation of a desolvation chamber between the

nebulizer and the flame.



Desolvation Chambers

The solutions introduced into an ICPIMS instrument are commonly water-based (acids, alkalies,

salts, or sea water) but can also be organic based, as with effluents from liquid chromatography

columns. The two types of solvent cause different problems for a plasma flame, but high concentrations of either need to be avoided. A plasma flame has low thermal capacity and depends critically

on a continuous formation of electrons and ions. If the flame is diluted by large amounts of vapor

or by the presence of certain elements, its temperature is reduced, with a subsequent reduction in

ionization efficiency. At best, a variable efficiency of ionization causes problems with the measurement of accurate abundances of ions, and, at worst, the flame may be extinguished altogether. Apart

from these problems, inside the flame, water can give rise to interferences due to oxide formation

within droplets before evaporation is complete. This effect is particularly marked for the elements

vanadium, molybdenum, lanthanum, cerium, thorium, and uranium. For organic solvents, flame

efficiency can even be increased by chlorinated solvents which give better efficiencies than do

hydrocarbons. An excess of organic solvent can lead to a buildup of carbon deposits on the sampling

cone situated at the tip of the plasma flame (see Chapter 14, "Plasma Torches").

These factors make it necessary to reduce the amount of solvent vapor entering the flame to

as Iowa level as possible and to make any droplets or particulates entering the flame as small and

of as uniform a droplet size as possible. Desolvation chambers are designed to optimize these

factors so as to maintain a near-constant efficiency of ionization and to flatten out fluctuations in

droplet size from the nebulizer. Droplets of less than 10 jnn in diameter are preferred. For flow

rates of less than about 10 Ill/min issuing from micro- or nanobore liquid chromatography columns,

a desolvation chamber is unlikely to be needed.

The simplest desolvation chambers consist simply of a tube heated to about 150 aC through

which the spray of droplets passes. During passage through this heated region, solvent evaporates

rapidly from the droplets and forms vapor. The mixed vapor and residual small droplets or particulates of sample matter are swept by argon through a second cooled tube, which allows vapor to



108



Mass Spectrometry Basics



Condensate



Figure 16.3

In a typical desolvation chamber, the initial sample solution is nebulized by some form of spraying device. The resultant

aerosol - a mix of gas, vapor, and droplets having a wide range of sizes - is swept through a heated tube by a flow of

argon gas. The tube is typically held at about 150°C. Here, much of the solvent is vaporized from the droplets, which are

greatly reduced in size to become small multimolecular aggregates (very small droplets). This mixture of small gaseous

solvent molecules and the larger analyte particles or aggregates passes into a cool region, where the solvent molecules,

diffusing more rapidly from the stream of gas and droplets, condense onto the walls of the tube and are run off as liquid

waste. The remaining small droplets and particulates (together with traces of solvent) pass on into the center of the plasma

flame, where fragmentation and ionization of sample occurs. In other devices, the desolvation chamber consists of a

membrane through which the small solvent molecules can diffuse but the larger droplets and particulates cannot; the materials

retained on the membrane are passed on to the flame.



condense on its walls and to be run off to waste (Figure 16.3). This second tube can be maintained

at a temperature of about 0 to -lO°C. Other, more elaborate systems that subject the sample to

alternate heating and cooling treatments are used to remove almost all of the solvent.

A second form of desolvation chamber relies on diffusion of small vapor molecules through

pores in a Teflon" membrane in preference to the much larger droplets (molecular agglomerations),

which are held back. These devices have proved popular with thermos pray and ultrasonic nebulizers,

both of which produce large quantities of solvent and droplets in a short space of time. Bundles

of heated hollow polyimide or Naflon'" fibers have been introduced as short, high-surface-area

membranes for efficient desolvation.



Conclusion

Solutions can be examined by ICPIMS by (a) removing the solvent (direct and electrothermal

methods) and then vaporizing residual sample solute or (b) nebulizing the sample solution into a

spray of droplets that is swept into the plasma flame after passing through a desolvation chamber,

where excess solvent is removed. The direct and electrothermal methods are not as convenient as

the nebulization inlets for multiple samples, but the former are generally much more efficient in

transferring samples into the flame for analysis.



Chapter



17



Sample Inlets for Plasma

Torches, Part C: Solid Inlets

Introduction

To examine a sample by inductively coupled plasma mass spectrometry (TCPIMS) or inductively coupled

plasma atomic-emission spectroscopy (ICP/AES), it must be transported into the flame of a plasma

torch. Once in the flame, sample molecules are literally ripped apart to form ions of their constituent

elements. These fragmentation and ionization processes are described in Chapters 6 and 14. To introduce

samples into the center of the plasma flame, they must be transported there as gases or finely dispersed

droplets of a solution or as fine particulate matter (aerosol). The various methods of sample introduction

are described here in three parts - A, B, and C; Chapters 15, 16, and 17 - to cover gases, solutions

(liquids), and solids. Some types of sample inlets are multipurpose and can be used with gases and

liquids or with liquids and solids, but others have been designed specifically for only one kind of analysis.

However, the principles governing the operation of inlet systems fall into a small number of categories.

This chapter deals specifically with substances that are normally solids at ambient temperatures.



Problems of Sample Introduction

The two major difficulties facing the analyst/mass spectrometrist concern firstly how to get the

whole of the sample into the plasma flame efficiently and secondly how to do so without

destabilizing or extinguishing the flame. Although plasma flames operate at temperatures of 6000

to 8000 K, the mass of gas in the flame is very small, and its thermal capacity is correspondingly

small (Figure 17.1).

Therefore, if a large quantity of sample is introduced into the flame over a short period of time,

the flame temperature will fall, thus interfering with the basic ionization processes leading to the

formation and operation of the plasma. Consequently, introduction of samples into a plasma flame

needs to be controlled, and there is a need for special sample-introduction techniques to deal with

different kinds of samples. The major problem with introducing material other than argon into the

plasma flame is that such additives can interfere with the process of electron formation, a basic factor

in keeping the flame self-sustaining. If electrons are removed from the plasma by secondary processes

faster than they can be replaced by the primary generation process, then the plasma process ceases

and the flame goes out. Fluctuations in flame temperature and performance lead to significant varia109



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



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