the Achilles' heel of atomic spectroscopy?

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One of the oldest adages in analyti- in the interface may hold the key to , tion have been clear for many years to
cal chemistry is that the analysis can the success or failure of the whole ap- most workers in the field. What has
only be as good as the sample. By proach. been less obvious are suitable means
analogy, in atomic spectroscopy the How, then, has this strong diver- to overcome them. Primary weakness-
analysis can only be as good as the gence between the excellent optical es have resulted both from lack of uni-
sample introduction. The sample in- and electronic components of most fied theory and from deficiencies in
troduction process conditions the modern atomic spectrometers, and the existing models of critical processes.
sample as it passes to the flame or rather inadequate sample introduc- Without such models, it is very diffi-
plasma, and so largely determines the tion systems, become so strong? Fur- cult either to optimize existing sys-
accuracy of the analysis. With this in thermore, does there appear to be any tems or to move into new areas. The
mind, it might he thought that sample hope that significant advances in sam- current state of the art is probably the
introduction systems would have oc- ple introduction techniques can be ex- limit of empirical development, and
cupied a central role in research and pected in the next few years? We he- major advances will come only when a
development in atomic spectroscopy lieve that the key to the second ques- sound framework of fundamental
over the past decade. However, even a tion is contained in the answer to the knowledge is in place. Without this
cursory comparison of instruments first. Only by identifying the causes of structure, it will be very difficult even
produced in that time shows that sam- the present blockage in the research to define clear goals for future re-
ple introduction systems have not pipeline can we hope for significant search.
shared in the advances made by opti- advances. In contrast to the rather negative
cal and electronic components. As a The problems of sample introduc- picture painted above, there has been
direct consequence, there has been a strong move in the past few years to
... develop improved sample introduction
relatively little progress over the past
10-15 years in some critical bench- procedures. A comparison of papers
marks. For example, there have been published in the various areas of
no dramatic improvements either in atomic spectroscopy, between 1979
detection limits or in measurement and 1983, bears evidence to this (Fig-
precision. Additionally, the future ure 1).A steady increase in publica-
growth of “speciation” measurements tions that may he considered as about
using atomic spectroscopy may be cur- “sample introduction” is seen. In ad-
tailed. In speciation studies, the sam- dition to broadening the range of sam-
ple introduction system acts as the in- ............. ple introduction techniques available,
terface between the chromatograph strwwntal techniqi these developments may also finally
and the spectrometer. Developments lead to the much-needed improve-
786A * ANALYTICAL CHEMISTRY, VOL. 56. NO. 7. JUNE 1984 0003-2700/84/035 1-786A$01.50/0
0 1984 American Chemical Society
Remrt
Richard F. Browner
School of Chemistry
Georgia Institute of Technology
Atlanta, Cia. 30332
Andrew W. Boorn
Sciex
55 Glen Cameron Road, #202
Thornhill. Ontario L3T tP2, Canada
The
Achilles' Heel
of Atomic Spectroscopy?
menta in some of the analytical bench- lem to be considered is that as ICP troscopy. And this is occurring a: a
marks mentioned earlier. spectroscopists are drawn into studies time when we have barely scraped the
In the remainder of this REPORT we with ICP-mass spectroscopy interfac- surface of establishing acceptable cri-
will attempt to describe some of the ing, the sample introduction criteria of teria for the emission technique! As a
major problems that remain to be con- this new technique may possibly differ redeeming feature of this situation,
fronted in sample introduction. Addi- from those for optical emission spec- there is reason to believe that there-
tionally, current understanding of
-7
basic mechanisms will be reviewed.
The sample introduction routes we t 30
will consider are those most widely
used at present: liquid aerosol, vapor,
25
and dry aerosol (electrothermal) (Fig-
ure 2). In this month's INSTRUMEN- 20
TATION (page 875 A), some recent de- ' !5
velopments in techniques for sample
15
introduction will be described.
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Deflnlng Goals 10
Stated simply, the goals of sample
5
introduction are the following: the re-
producible transfer of a representative
0
portion of sample material to the at- '
Mechanisms
omizer cell, with high efficiency and
L YAppIications
with no adverse interference effects.
Unfortunately, in certain circum-
stances, several of these criteria are d
mutually contradictory. As a further
Instrumentation
.complication, the criteria for optimum
sample introduction in, for example,
L
flame atomic absorption spectroscopy A - % Sample Introduction
(FAAS) differ markedly from those -~ ~
for inductively coupled plasma (ICP) 1979 80 81 82 83
emission spectroscopy. Another prob- Figure 1. Publication trends in atomic spectroscopy, 1979-1983
ANALYTICAL CHEMISTRY, VOL. 56. NO. 7, JUNE 1984 787A
transport term that describes the total
analyte mass transport rate to the at-
omizer. This may he further suhdivid-
ed into components W, and W,,
which are the mass transport rates of
useful and excess analyte, respective-
ly. Here, “useful” analyte is consid-
ered to refer to analyte contained in
drops sufficiently small to produce
useful analytical signal in the atomiz-
\ I er. The “excess” term refers to analyte
- contained in drops too large to pro-
duce useful signal in the atomizer.
Although there is no sharp dividing
line between these two terms, the con-
cept is felt to have considerable prac-
3tornizer tical use. It provides an indication of
&Plasma what drop size the atomizer is able to
accommodate, while giving rise to in-
terference-free analytical signals. For
Figure 2. Sample introduction routes for atomic spectroscopy example, in FAAS using the premixed
air-acetylene flame, the transition
quirements of ICP/MS interfacing ac- ences that they experienced. from useful to excess aerosol, for the
tually may he a little less stringent In flame atomic absorption spec- removal of the classic phosphate-cal-
than those of optical emission spec- troscopy, the guiding principle is that cium interference, occurs in the region
troscopy. For example, height profiles any improvement in transport effi- of three to five pm. The diameter of
should he less important with the MS ciency must not he achieved by intro- the aerosol drop, which contributes
interface, because of the different ducing large aerosol drops to the less than 1%t o the analytical signal, is
sampling mode. Nevertheless, all cur- flame. For a solution of concentration designated dmOx.A lthough numerical
rent knowledge from the emission C (pg/mL), contained in a drop of ini- data for d,,, are presently sparse, this
spectroscopy field will require reeval- tial diameter do (pm), the dry salt parameter should also be a useful
uation for mass spectroscopy applica- crystal of density p (g/cm3), formed benchmark for comparing atomizer
tions. after solvent loss will have diameter d, capabilities.
(pm), given by: Lack of transferability of interfer-
Liquid Sample Introduction
ence and detection limit data between
One of the most attractive aspects laboratories has been a lingering proh-
of liquid sample introduction is its rel- lem in atomic spectroscopy. A major
ative simplicity and reliability. This, source for the discrepancies has been
allied with the fact that a sample dis- For example, a 2-pm-diameter drop of unrealized variations in drop size dis-
solution step is often necessary to pro- 100 pg/mL sodium chloride solution tributions of aerosols generated hy
vide suitable sampling statistics, is will dry to a particle of approximately different instruments. In the past this
probably the reason for the over- 0.07 pm diameter. A 10-pm-diameter has led to the unfortunate situation
whelming use of liquid sample intro- drop of the same solution will give a where lack of data correlation has
duction in all branches of atomic spec- 0.36-pm-diameter particle. been taken as an indication of had
troscopy. Nevertheless, an ICP spec- When nonvolatile matrices are technique. With more objective
troscopist observing 99% of a hard present in high concentration, the va- benchmarks, it should he possible to
won analyte solution passing to waste, porization rate of the analyte will be avoid this type of unproductive con-
or having to wait 60 s between the in- inhibited. In lower temperature flict.
troduction of successive samples to flames, such as air-acetylene, this may There is currently no reliable sam-
the spectrometer, is aware of the need lead to severe vaporization interfer- ple transport data base, even for pa-
for improvement. Table I indicates ences of the calcium-phosphate vari- rameters such as E.. This is a conse-
the relative wastefulness of pneumatic ety. quence of the widespread historical
nebulizers used for liquid sample in- use of indirect methods for measuring
Experimental Sample introduction
troduction. In atomic absorption spec-
Benchmarks
troscopy, no higher than 10%e fficien-
cy can be expected. In ICP spectrosco- While transport efficiency, E”, is a Table 1. Transport Efficiency
py, a typical value is 1%. commonly used criterion for evaluat-
Data for Pneumatic
The most direct approach to over- ing nebulizers and spray chambers, it
coming poor sample transport effi- does not readily lend itself to compari- Nebulizers a
ciency (defined as the percentage of son of different systems. For example,. e .. %
analyte mass reaching the atomizer, the transport efficiency of a system Auxlllary Gas
compared to that aspirated) is to allow often can he increased simply by re- flow, umin
System 0 8.8
more of the pneumatically generated ducing solution uptake rate to the
aerosol to pass directly to the flame or nebulizer with a smaller diameter up- AA-no burner head 5.1 7.4
plasma. Indeed, some early atomic ah- take tube. However, this does not re- AA-with burner head 5.3 6.6
sorption spectrometers used a total sult in any net increase in analyte ICP-wncentric #I 0.6 -
consumption burner placed directly mass transport to the atomizer, or in iCP-concentric #2b 1.4 -
beneath the laminar flame slot as a any signal increase. It has therefore iCP-aossflow 1.5 -
means of high-efficiency sample intro- recently been suggested that a more
* A~~~osoulUstiM .
duction. However, users of these in- useful criterion for assessing nehulizer
ICP concentric nebulizers $I and #2 are
struments will vouch for the poor pre- and spray chamber performance is the differem examples of the same model.
cision and high vaporization interfer- W-parameter (I). W,,, is a mass
788A ANALYTICAL CHEMISTRY, VOL. 56. NO. 7. JUNE 1984
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Flgure 3. Aerosols from pneumatic nebulizers
(top) AA nebulizer (bonan) ICP nebulizer
transport properties. Indirect methods chamber modifies the flow patterns
are generally unreliable, and those considerably. Nevertheless, a compa-
wishing to carry out 6, or W measure- rable range of drop sizes, and equally
ments are referred to recent publica- turbulent gas and aerosol flow pat-
tions for better procedures (2,3).T he terns, will still exist in the more con-
essential point to note is that except strained environment.
where transport efficiencies are excep- The Sauter mean diameter of the
tionally high, i.e., 20% or greater, the aerml produced by pneumatic nebu-
simple arithmetic of calculation en- lizers (e.g., the diameter of the drop
sures that data obtained by indirect whose volume-to-surface-arear atio is
waste collection will have a much the mean of the distribution) has tra-
higher error than data obtained by di- ditionally been described by the equa-
rect aerosol collection. Errors with tion due to Nukiyama and Tanasawa.
ICP systems are especially high, be- The relationship, derived from their
cause of the generally low (0.5-2.0%) c h i cs eries of papers (4), is:
transport efficiencies common to
them. With indirect procedures, it is
easy to obtain e, values with a 500%
positive error (2).
Aerosol Generation and Transport
Processes
where d. (Nm) is the Sauter mean di-
The limitations of pneumatic nebu- ameter, V, the velocity difference he-
lizers are clear from looking at Figure tween gas and liquid flows (ds)u,,
3. Aerosols produced from atomic ab- the surface tension of the liquid (dynl
sorption and ICP nebulizers can be em), p, the liquid density (g/cmS), 7,
seen to have both extremely wide drop the liquid viscosity (poise), and Q,
size ranges, and very turbulent gas and Qg.t he volume flow rates of liquid
flow patterns. The photographs were and gas, respectively (em3/s).
taken with the nebulizers operating in The limitations of this equation are
free air, and the presence of a spray well known, and it is doubtful that it
7SOA ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
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Figure 4. Aerosol modifying processes
is an accurate description of aerosols
generated hy all pneumatic nebulizers.
Nevertheless, it has been shown to
have considerable value, at least for
predicting trends for aerosol genera-
tion in atomic spectroscopy. The im-
portant thing to note is that this equa-
tion refers to the aerosol emerging
from the nebulizer, not to the aerosol
reaching the atomizer. The aerosol
modifying processes that convert the
primary aerosol produced by the neh-
ulizer to the tertiary aerosol arriving
at the atomizer are caused by devices
such as spray chambers, impact heads,
impaction surfaces, etc., placed in the
aerosol path (Figure 4) (I). Aerosol
modifiers may act in a variety of ways,
hut typically remove large drops from
the stream, and allow only drops of
less than a certain size to pass. Impact
beads may also act to generate small
drops by shattering larger drops from
the primary aerosol. This results in
production of the secondary aerosol.
The effectiveness of these aerosol
modifiers in the removal of drops is Figure 5. Drop size distribution curves
792A * ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
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generally expressed as the cutoff di-
ameter of the spray chamber, d,. This .able 111. ICP Signal Enhancements with Organic Solvents a
refers to the drop size, on a drop size I
distribution plot, where the ordinate
mass transport term is reduced to 50%
of its peak value hy the action of the
spray chamber (Figure 5). The transi- :hlwoform 0.4 0.1 2.2 1.1
tion from primary to tertiary aerosol is
MlBK 2.8 0.3 3.2 1.8
accompanied by a large reduction in Xylenes 3.0 0.8 3.2 2.5
mean drop size of the aerosol. The
mass fraction of aerosol contained in *Measured at 15-mm obsewation height, with 1.0Mlmin nebuiizer flow, l2lIminp lasma gas
the larger drops is substantial, and its "^w, l.M/min auxiliary gas flow, and 1.75.kW rf pawr-
loss accounts for the high rate of wast-
age found with pneumatic nebuliza-
tion, as essentially all the large drops
introduction to the plasma, with ei- on the walls of the tubing used for
pass to waste.
ther a cooled spray chamber (9)o r a transport between the vaporizer and
Solvent Vapor lnteractlons with condenser (7). the plasma. For example, volatile ele-
The case of organic solvent intro- ments such as cadmium and zinc may
Flames and Plasmas
duction is a severe one. Nevertheless, he readily lost in the transport pro-
When more aerosol is introduced even aqueous solutions exert a major cess. With ETV, the criterion for opti-
into a flame or plasma, not only the effect on plasma excitation properties. mum sample transport becomes the
mass transport rate of analyte to the Recent studies have indicated that, for production of highly dispersed, partic-
plasma increases, hut also the mass example, a doubling in water aerosol ulate material that may be transport-
transport rate of the accompanying transport rate to an ICP, for a fixed ed readily in the gas stream between
solvent. This may have a significant analyte mass transport rate, can result the vaporizer and the plasma. This is
effect in lowering the temperature of in up to a hundredfold reduction in in marked contrast to the situation in
atomic absorption flames, hut its in- emission intensity (IO). Taken one atomic absorption or emission spec-
fluence is observed most dramatically step further, the high water loading troscopy, where efficient atomization
with the ICP, where high solvent load- from an ultrasonic nebulizer can even in the furnace is essential for accurate
ings may actually extinguish the plas- extinguish a plasma. Consequently, in analysis. With the ICP, the desired
ma discharge. This becomes particu- order to obtain improved detection species are not atomic but molecular,
larly noticeable when volatile organic limits using ultrasonic nehulization, and furthermore preferably con-
solvents are used. Rapid aerosol evap- the solvent loading must he greatly re- densed molecular species. Conse-
oration with volatile solvents results duced. With ultrasonic nehulization, it quently it is desirable that there
in a higher mass transport rate of ana- is possible to increase W (the analyte should he rapid cooling of the vapor
lyte to the plasma, hut also a corre- mass transport rate) to the plasma by after it leaves the vaporizer surface.
sponding higher mass transport rate about 10 times (2). With (partial) This will assist the rapid formation of
of solvent (5).A dditionally, the sol- aerosol desolvation, the signal im- highly dispersed nuclei, with suhse-
vent vapor coming from aerosol evap- provement also can he close to this quent growth by condensation, to give
oration also passes to the plasma. Sev- value. dry aerosol in an easily transportable
eral publications have shown that the The whole area of solvent-plasma, form. The efficient transport of less
effect of organic solvent on the plasma vapor-plasma, and aerosol-plasma in- volatile or carhide-forming species has
is generally to reduce its excitation teraction is in need of thorough inves- been shown to he capable of improve-
properties significantly (6, 7). This tigation. Without a better under- ment by the addition of volatilizing
may result in a substantial net reduc- standing of fundamentals, it will he gases, such as freons, to the argon car-
tion of analytical signal, in spite of the very difficult to make any significant rier stream (15).T his approach is sim-
increase in analyte mass transport improvements in liquid aerosol intro- ilar to the carrier distillation tech-
rate to the plasma. Some selected duction for the ICP. The optimization niques well known to spectrographers.
mass transport data for several sol- of low flow mini- and microtorches While the transport mechanisms as-
vents are shown in Table 11, and signal (11,12) will also require parallel stud- sociated with electrothermal vaporiza-
data in Table 111. ies, as undoubtedly the conditions for tion are much simpler than they are
It is possible to overcome the nega- optimum sample introduction will for liquid sample introduction, the un-
tive effect of increased solvent trans- vary for the different combinations of certainty about optimum plasma exci-
port to the plasma by one of two gas flow and power loading. These tation conditions still remains to he
means: Either oxygen may he added to studies are also relevant to electro- clarified, as mentioned earlier.
the plasma to oxidize the organic thermal sample introduction, as dis-
species present (8),o r excess solvent cussed helow. Vapor Introduction
vapor may he removed before aerosol
Vapor introduction techniques are
Electrothermal Vaporization
well established in AAS for stable, hy-
Electrothermal devices show a great dride-forming species (e.g., arsenic,
deal of promise when used as sample antimony, selenium, tellurium, germa-
'able II. Transport Efficiency
introduction devices for the ICP (13, nium, tin, bismuth, and lead) and also
lata for Various Solventsa 14). Transport mechanisms with elec- for mercury vapor introduction. These
trothermal vaporization (ETV) are techniques have been applied recently
much simpler than those involving liq- to sample introduction with the ICP,
uid sampling. The two mechanisms with considerable success (16,17). Ad-
Xylenes 29.1 14 that can inhibit transport of material ditionally, a more specialized ap-
Nlbobenzene 43.9 15 between the vaporizer and the plasma proach, which involves the production
are first, the formation of nonvolatile of volatile metal chelates from a wide
Water 72.7 1.5
- carbides on the furnace walls (e.g., range of transition elements, has been
a IC" .rDBsflow nebulizer. with silicon, vanadium, etc.) and sec- used successfully with the inductively
ond, the loss of very volatile materials coupled plasma (18).
794,. ANALYTICAL CHEMISTRY, VOL. 56, NO. 7. JUNE 1984
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