MZTLTI-ELEMENTAL TRACE ANALYSIS OF SOILS BY INDUCTIVELY ...

Element

peak area

ipeak height

suggested values

Figure 9: Difference Between Peak Area and Peak Height
For V, Cr, La and Mo in NIST 2710

error bars represent the standard deviation

peak height caldations (Figure 9)- As a result, peak area calculations were used for the
remainder of thiswork.
3.3 Vanadium and Chromium

In previous work in particdarthat by persaudl1,V and Cr have been two very

difncult elements to determine accurately by slurry nebulization using ICP-MS when
present in a heterogeneous material such as soil. Persaud et aLl1 used a 1-mm injector
torch cumbined with a 15% nitrogen-argon mixed-gas plasma and found the results for V
and Cr were systematidy low comparedto c e e e d values. This negativebias was
attri'butedto incomplete atomization/ionization of the slurry particles.

Here, different studies were conducted for all the elements studied, whïch involveci
utilization of a 1-mminjector torch at optimal nebulizer gas flow rate (NGF'R) with
optimal and non-optimal x-directional sarnpling position, which were 1-4mm and 1.7 mm
respectivelyftom the samplerto the top of the load coil. A 2-mm injector torch was also
studied at optimal and non-optimal NGFRs and a 5% nitrogen-argon mixed-gas plasma at
an optimum N G R

Both 2710 and 2711 soil slurries at 1%( d v ) did not yield acceptable results for
V, as expected, with the 1-mm plasma torch at the optimal sarnpling position. Results for
V with standard deviationsare listed in Tables 6 and 7 and Figures 10 and 11show
cornparisons for soils 27 10 and 2711. The precision, otherwise lmown as the relative
standard deviation (RSD)for this method was faU as demonstrated by Figures 12 and 13,
ranging between 15% and 16%. While using the 1-mm injector torch, it was extremely
difEcult to achieve a signal that was stable and reproducible for alI elements determineci, as
demonstrated with Cr (Figure 14). This effect may aise f?om incomplete atomizationand
ionization of V and Cr as previously docurnented. Results were poorer at the less

Experimental Method

Figure 10: Concentration of V, Cr and La For Soi1 2710
According To Each Experimental Method

-s.t. - short torch x = 1.4 mm sampling, s.t. 1.7 short torch x = 1.7 mm sampling

1.t. - wide torch, -0.08, -0.05, -0. I O - wide torch for -0.08 L/min, -0.05 L/min, -O. I O L/min
NGFR respectively, 5% N - 5% N2-ArMixed-Gas Plasma

a.v. - accepted values, error bars represent the standard deviation

optimal sampling position with the 1-mm torch injector for both soils (Tables 6,7 and 8).

Table 6= QuantitativeResults For Each Element in NIST 2710

According To Each Operating Condition @ d g )

zmm 2-mm %mm Z m m

Optimum 4.06 Umin -0.ûû3 Umin 4.10 Umin 1-mm 1-mm S%Mùed

NGFR NGFR NGFR NGFR x = l A mm x =l.7 mm Gas Plasma

42.3 +/- 1.9 51.2 +/- 1.3 41-1+/-0.8 19.3 +/- 6.6 40.9 +/-6.4 10+/- 11 87-1+/-7-4

26.9 +/-O. 1 1-9+/- 0.5 25-1+/- 0.2 l2.3+/-3.0 36+/-11 23+/-4 3.4+/-1.5 .
29.8 +/- 1.0 11+/- 8 36.2 +/-O S 21.5+/-28 19.6+/-0-8 33.1+/-3.8 19,2+/-0.8

4440 +/-480 220 +/- 160 4900 +/- 18 570 +/- 300 370 +/- 190 4120 +/- 1300 210 +/- 100
7.4 +/- 0.9 2.2 +/- 1.2 14-0+/- 0.4 2.8 +/- 0.7 27 +/-35 13+/-3 2.6+/-1.6

380+/- 420 3560 +/- 13( 1230+/-44 7940 +/- 220 380 +/- 1830 780 +/- 1050 5000+1-

Table 7: Quantitative Results for elements in NIST 2711According
-
To Each Operating Condition @g/g)

2-mm 2-mm 2-mm 2-mm 5% Mlred

Optlmm 4.05 Umin 4.08 Umin 4.10 Llmin 1-mm 1-mm Gas

Element NGFR NGFR NGFR NGFR x = 1.4 mm x = 1.7 mm Plasma
v 37.5+/-0.7 7.4+/-4.6 28.8+/-1.8 19.0+/-4.8 40.6+/-6.1 1.5+/-2.3 79,4+/-3.1

Cr 27.4+/-1.1 23.0+/-3.3 24.4+/-1.0 13.2+/-4.0 35.9 +/- 10.9 26.1 +/- 1.9 2.5 +/- 1.5

La 16.7+/-0.1 14.3+/-0.1 15.4+/-0.1 15.8+/-0.2 19.1 +/-OS 15.9+/-0.1 13,9+/-0.2

Pb 630 +/-57 920 +/- Il0 950 +/- 120 290 +/- 36 180+/- 32 1090+/- 98 960 +-; 24

M o D.49+/-0.29 6.0+/-5.3 1.8+/-0.8 0.54-W-O.1C 26 +/- 35 0.39 +/-0.14 3.3 +/- 0.6

Zn 40 +/- 1120 860 +/- 570 440 +/- 140 7750 +/- 53C 370 +/- 1830 1180+/- 250 44200 +/-

1200

Table 8: Certitied and SuggestedValues For Elements in NIST 27 10 and 2711

Element NX3T 27101 udg NIST 271V udg

V (certined) 76.6 +/- 2.3 81-6+/- 2-9

Cr 39 47

La 34 40
Pb (certifieci)
5532 +/- 80 1162+/- 31

Mo 19 1.6
Zn (certifEed)
6952 +/- 91 350.4 +/- 4.8

There are two large merences between the two torches used in these
experimentq namely the injector intemal dimeter and torch length. The short torch has

Experimental Method

Figure 11: Concentration o f V, Cr and La For Soi1 2721
According To Each Experimental Method

s-t-- short torch x = 1.4 mm sarnphg, s.t. 1.7 - short torch x = 1.7 mm sarnpling
1.t. - wide torch, -0.08,-0.05,-0.1O - wide torch for -0.08L/min, -0.05Wmin, -0.10 L/min

NGFR respectively, 5% N - 5% N2-Ar Mixed-Gas Plasma

a.v. - accepted values, error bars represent the standard deviation

an hjector intemal diameter of 1-mm, while the long torch has an injector intemal
diameter of 2-mm. The 1-mm injector torch is 2-mm long fiom the injector tip, while the
2-mm injectortorch is 4.2-mm long fiom the injector tip. The longer length of the 2-mm
injeetortorch prevents samphg "deep" in the plasma compareci to the shorter torch. As a
result, the sarnplingposition caanot be optimized with the longer torch, while it c m be
with the 1-mm torch injector.

To increase the residence thte and the mean particle droplet size of the aerosol
reaching the plasma, the 2-mm injector torch was employed. The fist series of
experimentswith the 2-mm injector used the optimal nebulizer gas flow rate (NGFR).
Results for vanadium were sign%cantly improved for both 2710 and 2711being (42.3 +/-
2.0) pglg and (37.5 +/- 0.7)pg/g respectively. Generaily, the 2-mm injector torch
provided better signal stability and reproducibility (Figure 12 and 13) as demonstrated by
the much improved precision of 4.1 % and 4.7% for soil slurries 2710 and 2711.
However, accordingto the certified values listed in Table 8, the results obtaïned were stiU
not comparable, requiring M e r improvement.

Unfortunately, there is very Little literature available for cornparability of results
using a 2-mm injector torch. There is some work availableusing a 3-mm Unjector torch.
Ambrose and coworkersgused a 3-mm injector to determinetrace elements in soil, V in
particular. Initial experiments ushgthe standard Scott double-pass spray chamber were
disappointhgfor the goupg. The Scott double-pass spray chamber was replaced with a
single p a s spray chamber, but problems were encountered with formation of condensation
at the base of the injector, redting in plasma instabilityg. M e a d a reduced-volume spray
chamber was used that is longer that the standard Scott double pass spray chamber and
has a bulbous endg. Ambrose and coworkersgfound this modifiecl spray chamber worked

Experimental Method

Figure 12: Relative StandardDeviationof V,Cr and La For Soil2710

According To Each Experimental Method -

s-t. - short torch x = 1.4mm sampling, s.t. 1.7 - short torch x = 1.7 mm samphg

1.t. - wide torch, -0.08, -0.05, -0.10 - wide torch for -0.08 Llmin, -0.05 L/rnin, -0.10 Llrnlli

NGFR respectively, 5% N - 5% N2-Ar Moced-Gas Plasma

Experimentai Method

Figure 13: Relative StandardDeviation of V, Cr and-LaFor Soi1 2711
According To Each Experirnental Method

-S-t.- short torch x = 1.4mm sampling, S.t. 1.7 short torch x = 1.7 mm sampling
-1-t. wide torch, -0.08,-0.05,-0.10- wide torch for -0.08L/min, -0.05Umin, -0.10Umin

-NGFR respectively, 5% N 5% N2-ArMixecCGas Plasma

and yielded good results for V,such as (130 +/- 5) pg/g compared to a certifiai value of

(140 +/- 8) pg/g.
To further increase the residencet h e in the 2-mm injectortorch, three less

optimal NGFRs were selected, namely -0.08 L/min, -0.1 Llmin and -0.05 L/min fiom the
optimum setting. The result ofthese decreases are shown in Tables 6 and 7.

Table 9: Relative StandardDeviation (%) For NIST 2710

For EachElement Under DXEerent Operating Conditions

2-mm 2-mm 2-mm 5% Mked
- -2-mm
1-mm 1-mm Optimum -0.05 Umin 0.08 Umin 0.1 Umin Gas

NGFR Piasma

34.2 8.4
NGFR 1 1NGFR
1.8
2.5

26

71

52-5

3.6

74-4

Table 10: Relative Standard Deviation (%) For NIST 27 1 1
For Each Element Under Dserent Operating Conditions

2-mm -2-mm 2-mm -2-mm 5%
Optimum 0.1
1-mm 1-mm -0.05 Umin 0.08 Umin Umin Mised
Element x =1.4 mm r = 1.7 mm NGFR NGlFR Gas
NGFR NGFR 25.1
V 147 15 1-8 30-4 Plasma
30.3 3-8 61.2 6.2 3.9
Cr 7.2 0.2 1.5
La 0.37 2.5 48.9 14.1 4 60.1
Mo 34.6 133 27 16.5
Zn 21.6 489 0.83 O. 13 1.1
17.6 9.1 6.9
Pb 9 87.4 47.2 12
12.6
66.5 32.8 2.7

12.1 12.9 2.5

From Tables 6 and 7, it is clear for 2710 at -0.08 L/mh there is very little
merence compared to results at the optimum NGFR. However, with less than -0.10
Lhin there is a signiscant &op in detected concentration. During experimentation,the

signal was sporadic and reproduciity of the peaks was poorer compared to the -0.08
Umin drop, as noted by the RSD in tables 9 and 10. A shorter residence time
(-0.05 L/min) was also tned which only provided better result for soil2710 only, while
results for soil2711 were quite poor. These differencesin results may be attniuted to the
nature of the soil material,

In an attempt to M e r improvethe quantitationof V in a heterogeneous material,
a 5% Ntrogen-argon mùred-gas plasma was employed with the 2-mm injector torch at an
oprimal NGFR. The results obtained using this technique (Tables 6,7and 8) were

excellent for 2711 and good for 2710 when compared to the cemfied values of (76.6+/-
2.3) pglg and (8 1.6 +/-2.9) pgig for 27 I l and 2710 respectively. A student t test was

performed to compare the experimental results with the certified values. For both 2710
and 2711, the experimental t values were much lower than t nom the table at the 95%
confidence intenal, indicatingthere is no statistical merence between the experimental
values and the certifieci values. As a result, utilization of a 5% N2-Ar mixed-gasplasma
with a 2-mm injector, is a method for accurate quantitation of V at trace levels in soil.

For V it is obvious the results obtained for NIST 2711are not as dose in
agreement as those forNIST 27 10 since values are either higher or lower compared to

certifieci values. This deviation in results between soii samples has also been demonstrated
by Persaud and coworkersll and Ambrose and coworkersgfor the same element.

Despite accurate results obtained for V, the opposite was tme for Cr when
compared to the 39pglg and 47 pg/g suggested values for soils 2710 and 2711
respectively, as shown in figures 10and 11. Unexpectedly, the best result obtained for Cr,
compared to al1 of the work completed here, was the 1-mm injector at optimal sampling

Figure 14: FI For Cr From Two Replicates ofNIST 2711
Using The 1-mmInjectorAt x = 1.4 mm Sampling

Figure 15: FI Peak For Cr From Two Replicates ofNIST 2711
Using The 2-mm Injector At Optimal NGFR

position and optimalNGFR This 1-mm injector torch experiment yielded a result of (36
+/- 11) gg/g for each of 2710 and 2711. These results far surpass those obtained using
the 1-mm torch and a 15% mixed-gas plasma11. However, the reîiabilïty of these results is
questionable, rnainly fi-omthe difficulty in peak reproduaibility, proven by the low
precision ranging between 30-3 1%. Surprisingly, the mixed-gas plasma did not provide
adequate results for Cr. In fact, results were more than ten times lower than that achieved
using the 1-mmtorch.

To demonstrateifresidence time is the only problem, or ifthe injector i n t d

diameter also needs to be Iarger, the sampling position of the 1-mm injector torch was
moved back to 1.7 mm fiom 1.4 mm. Using Cr as an example, results for this experiment

were poorer, (23 +/- 4) pg/g and (26.1 +/- 1-9)&g for 271O and 2711 soils respectively,

compared to the optimum samplùig position. Precision for this experiment ranged
between 7.2%and 19% (Figures 12 and 13). Therefore, the intemal diameter of the
injectortorch does need to be wider to improve atomizattion efficiency, in addition to
residence tirne.

Jarvis and williams8dem~mtratedthe use of the 3-mm injector torch for the
deternination of Cr in a soil slurxy- The increased intemal diameter, acwrding to Jarvis
and ~ 1 1 l i a . mres~u~lts~in a decrease in linear velocity and corresponding jncrease in
residence t h e , therefore improving atomization efficiency. Using the 3-mm torch, the

best example of a result for Cr was (12.0 +/- 0.4)pg/g compared to a reference material
of 10pg/g with a RSD of 40/08. However, not aU quantitative resuits tiom Jarvis and
wfiams8were consistent beîween soil samples. For example, two soils reference
materials were quantified at 13.1pg/g and 384gg/g respedvely compared to certified

values of 53pglg and 405pg/g% OveraU, quantitativeresults in thiswork for Cr (Tables
6, 7 and 8) showed much srnalier deviation between soi1samples compared to Jarvis and
williamss.

Using the optimum NGFR and -0.08 L/mh NGFR with a 2-mm injector torch
provided improved quantitative results (figures 13 and 14) comparedto that obtained with
mixed-gas and betterRSD fiom 0-6% to 4-0% (figures 15 and 16). A decrease in NGFR

of- 0.1 L h i n dramaticaliy reduced wncentrations found for both soils d o m to 19pg/g-

Concentrations for Cr detection significantiy decrease with a decreasedNGFR beyond
-0.08L/min, and therefore, it may be conchded that a 0.08 L/min drop f?om the optimum
may be the lower Iilnit to which an increased residence time may be usefhlly a~hieved.

The shorter residence time associated with -0.05 L/mh NGFR yielded much lower

results for both soils compared to the -0.08 L/min NGFR .

3.4 Lanthanum and Molybdenum
From the study by Persaud et al.11, La was the easiest element to determine using

ICP-MS, 1-mm injector torch and mixed-gas. In this work, the same has dso been proven
to be true. As expected, determinationusing a 1-mm injector torch at an optimum
sampling position did not yield results for both soils comparableto suggested values
(Figures 10 and I 1).

At the less optimal sampling position (1-7mm) and a 1-mm hjector torch, results

for 27 10 were excellent (33.1 +/- 3-8)pg/g compared to the optimal sarnpling position

and suggestedvalue of 34pglg. This experiment provided the best quantitativeremit for
soi1 2710. The relative standard deviation for this method was fair at 12%, owing stillto
the instability effécts posed by the 1-mm injector torch.

The remit for soi12710 was (29.8+/- 1-0)pglgfor an optimum NGFR and 2-mm
torch. Using a -0.08 Umin NGFR a value of(36.2 +/-0.5)pg/g and 1.3% RSD (figures

10, 11, 12and 13) was found for soii 2710. Generally, ail results were much improved
f?om Persaud et al. who detennined La in the same soit to be (25 +/- 2) pg/g.

A lower NGFR than -0-10L/min resulted in a much decreased concentration,
Thereby, -0.08 Umui was consideredto be the lower lirnit- Results for a NGFR of

-0.05 L h i n was considerablypoorer for soi12710 behg (11.3 +/- 8.0) pg/g.

Results for soil2711 did not foilow the same trend as those for soil 2710- The
same deviation between soil samples was also observed by Persaud and coworkersl! The

best resuit was obtained using the 2-mm torch with an optimal NGFR. Comparingboth

torch injectors, the highest concentration was obtained using the 1-mm injector torch at
the optimum sampling position.

A 5% N2-Armixed-gas plasma yielded the lowest result for both soil slumies. The
combination of a longer plasma torch and a mixed-gasplasma unfortunately Mecs fiom a
greatly decreased sensitivity. As a result, the percentage of Mrogen could not be
increased any fùrther than 5% or else the sensitivity wodd decrease by 50% with an
additional 5% increase in percent nitrogen and the maximum operating sensitivity would
be less than 10,000 countds for 100pg/L compared to the normal sensitivity ranghg
between 600,000 to 800,OO countds for 100pg/L during continuous nebukition of
solutions. Tables 11 and 12 Iist the sensitivities for al1 elements determinecl and the
varying operating conditions.

As stated in previous workll, a 0.1% ( d v ) soil sluny concentration provided too
low a Mo concentrationfor accurate quantitative and semi-quantitativeanalysisby an Elan

500 ICP-MS instrument. For tbis determination, the soi1s1urry concentration was

increased to 1.0% ( m h ) for accurate quantitativeanalysis. Figure 16shows the most
accuate quantitativeanalysis is possible utilking the -0.08 Umin NGFR. Results were

(1-76+/- 0.83)pglg for 2711slwy compared to the suggestedvalue of 1.6pglg.

The 1-mm injector torch at both the optimal and non-optimal sarnplingpositions
provided poor quantitative values for Mo in soil2711.

Soi1sluny 2710 was best adyzed using the l e s optimal value of-0.08Umin with

the 2-mm injector torch (14.0 +/-0.3) &g, wMe the 1-mminjector torch at the
non-optimal samphg position (1.7 mm) yielded a closer quantitative result, (13 +/- 3)
pg/g, than that achieved at the optimal sampling position. Kowever, the poor RSD of

22% (Figure L7), compared to the better less optimal value suggests that an improved
result might be obtained with a wider bore injector such as a 3-mm injector torch.

Jarvis and wrlliams8also deterrnined Mo using a different rnethod with 13
reference materials. For the majori~of samples, Mo could not be accurately determined8.
For example, results ranged f?om (0.46,24.6, 0.83 and 0.75) pg/g compared to reference
values of (1, 34.4, 1.5 imd4) gg/g respectively. In contrast, deviationswere not as large
in this work

3.5 Sensitivity
As seen fiom Tables 11 and 12, of all the operating conditions, the mUred-gas

plasma have the lowest sensitivities for V,Cr, Zn and Pb . The sensitivities for La are
comparable between the mixed-gas plasma and the 1-mm injector torch at 1.7mm fiom
the sampler to the top of the load mil. While for Mo, the lowest sensitivity was obtained
ushg the 1-mm injector at 1.4 mm fkom the sampler to the top of the load c d . The
majority of elements, with the exception of Mo, Zn and Pb, had the best sensitivity using
the 2-mm injector at an optimalNGFR The optimal semitivity for Mo was achieved

Experimental Method

Figure 16: Concentrationo f Mo For Soi1 2710 and 271 1
According To Each Experimental Method

-set.- short torch x = 1.4m m samphg, s-t. 1.7 short torch x = 1.7 mm sampling

1.t. - wide torch, -0.08, -0.05, -0.1O - wide torch for -0.08 L/min, -0.05 L/min, -0.10 L/min
NGFR respectively, 5% N - 5% N2-Ar Mixed-Gas Plasma

a.v. - accepted values, error bars represent the standard deviation

Expenmental Method

Figure 17: Relative Standard Deviation of Mo For Soi1 2710 and 27 11
According To Each Experimental Method

- -s.t. short torch x = 1.4 mm sampling, SA. 1.7 short torch x = 1.7mm sampling

1.t.- wide torch, -0.08, -0.05, -0.10 - wide torch for -0.08 L/min, -0.05 L/mi~,-0.10 L/min
NGFR respectively, 5% N - 5% N+ Mured-GasPlasma

using either a 1-mm hjector at 1.7 mm distance fîom the smpler to the load c d or a
2-mm injector at -0.05 Umin nom the optimum NGFR Best sensitivityvalues for Zn and
Pb were obtained with a 2-mm injector at -0.05 Umin fiom the optimumNGFR and the
1-mm injector at 1.7mm from the sampler.

Table 11: Log of SensitiviityFor Elements Quantïfïed For The 1-mmInjector and 2-mm
Injector At The Optimum and -0.05 Umîn From The Optimum NGFRExperiments

Element 2-mm 2-mm
Optimam 4.05 Umin
v
NGFR NGFR
Cr
La 13.4 12-1

Mo 13-2 13.1
Zn 13.5
14.5
Pb 13.6
12.7
10.7 10.8

12.3 12.3

Table 12: Log of Sensitivity For Each Element Quantifiecl For The 2-mm Injector At
-0.08 L/mh and -0.10 L/mh From The Optimum NGFR and 5% Mixed-Gas Plasma

Element 2-mm 2-mm 5% Mixed
V -0.08Umin 4.10 Umin Gas

NGFR NGFR Piasma
11.5
13.3 13.2

In terms of injector width, if the sensitivïty of the 2-mm injector torch at an
optimumNGFR is compared to the 1-mm injectortorch at 1.4 mm fiom the sampler tip to
the top of the load mil, it is clear that for aU elements, the 2-mm injector has the best
sensitivity. Also cornparhg the 2-mminjector torch at -0.08 Umin fkom the optimum

NGW which provided the best quantitative results for the majority of elements, with the
1-mm injector torch at a distance of 1.4 mm fiom the sampler to the top ofthe load coil, it
is also clear in this case, that the 2-mm injector provided better sensitivitiesfor ali the
elements.

In ternis of increased residence tirne provided by the 2-mm injector, the optimum
NGFR sensitivïtiesare comparable to the NGFR experiment that was -0.08 L/min from
the optimumNGFR A NGFR of -0.10 Umin fiomthe optimumNGFR compared to the
optimal NGFR showed a small differencein sensitivity in contrast to the -0.08 L/&
experiment. However, the 5% mixed gas plasma suffered in sensitivity compared to both
the optimal and non-optimal NGFRs.

3.6 Zinc and Lead
Lead and zinc have been two of the most difEcult elementsto quantitate accurately

in this project. As shown in Figure 18for Pb, the most accurate detedion method for
soils 2710 and 2711using a 2-mminjector torch is the less optimal value of -0.08Wmin
NGFR compared to certiiïed values. Results for -0.08 L/min NGFR were (4900 +/- 18)

pg/g and (950 +/- 122)pglg for 2710 and 2711respectively. In fact, the result for 2710

was in better agreement with the certined value than that obtained by Persaud and
coworkersu (Table 7). The ciifference fkom the certifiai value for Persaud and
coworkersll and the work completed here was 1218pg/g and 632pglg respectively. The

result nom Persaud and coworkersll, (1263 +/- 71) pg/& for Pb in 2711 was comparable

to this work. However, Pb value for 2711using a 5% mixed-gas plasma was better (1090

+/- 98)pg/g in contrast to Persaud and coworkersll.

Experimental Method

Figure 18: Concentration of Pb For Soi1 2710 and 2711
According To Each Experimentd Method

SA. - short torch x = 1.4 mm sampling, s.t. 1.7 - short torch x = 1.7 mm sampling
1.t. - wide torch, -0.08,-0.05,-0.10 - wide torch for -0.08Umin, -0.05 Wmin, -0.10 Umin

-NGFR respectively, 5% N 5% N2-ArMixed-Gas Plasma

a.v. - accepted values, error bars represent the standard deviation