NBSIR 77-1211

Evaluation of X-ray

Fluorescence Analysis for the

Determination of Arsenic,

Vanadium, Cadmium, Lead and

Mercury in Various Matrices

P. A. Pella, R. L. Myklebust, M. M. Darr, and K. F. J. Heinrich

National Bureau of Standards

Department of Commerce

Washington, D.C. 20234

October 1976

Final Report

Issued June 1 977

Prepared for Environmental Protection Agency

Research Triangle Park, North Carolina 2771

1

NBSIR 77-1211

EVALUATION OF X-RAY

FLUORESCENCE ANALYSIS FOR THE

DETERMINATION OF ARSENIC,

VANADIUM, CADMIUM, LEAD AND

MERCURY IN VARIOUS MATRICES

P. A. Pella, R. L. Myklebust, M. M. Darr, and K. F. J. Heinrich

National Bureau of Standards

Department of Commerce

Washington, D C. 20234

October 1976

Final Report

Issued June 1977

Prepared for Environmental Protection Agency

Research Triangle Park, North Carolina 2771

1

O* *

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch. Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Evaluation of X-ray Fluorescence Analysis for the

Determination of Arsenic, Vanadium, Cadmium, Lead and

Mercury in Various Matrices

ABSTRACT

Limits of detection for arsenic, vanadium, cadmium,

lead and mercury have been determined with a wavelengthdispersive or energy-dispersive x-ray spectrometer, or with

both, in various matrices consisting of cupric oxide, ferric

oxide, lead oxide, coal and fly ash.

1. INTRODUCTION

The limits of detection have been determined for arsenic

in cupric oxide and in ferric oxide, for vanadium in cupric

oxide, ferric oxide, lead oxide, coal, and in fly ash, for

cadmium in cupric oxide, for lead in cupric oxide and in

ferric oxide, and for mercury in coal. Limits of detection

for mercury in coal have already been determined and reported

in NBSIR 75-675 and are included as an addendum to this report.

Measurements were made with either a wavelength- dispers ive

x-ray spectrometer or an energy-dispers ive instrument, or,

in some cases, with both. Samples were prepared by adding

known amounts of NBS Standard Reference Materials to the

matrix of interest and grinding to obtain a homogeneous

sample. To obtain concentrations of an element in the 10

to 100 ppm range, successive dilutions of the sample with

the matrix material followed by grinding were required.

2. EXPERIMENTAL

The energy-dispersive instrumental arrangement used in

this work differed to some extent from that described

previously [1] . A commercially available system was used

consisting of a tungsten target x-ray tube for exciting

various secondary emitters. These consisted of titanium,

1
Table

1.

Instrumental

Conditions

<L> E

440o0 0>• HtO3 3 o i i to 1 to to 1 i 1 t—t o3 i i 0 1 0 0 1 i 1 X > X X X

LO 3 w to in CO CO to r-H 3 -H 0 0 i <D i 1 CD 0 0 C3

II K-l < X X i X i 1 X X X 0E 3 3 to •rH 0 •rH 3 > 3 o o o o o o o

•H 0 O o o o i o o i o o 0 X E 0 lo in LO i 04 o> i LO 04 • >

•H (!) E •rH E—< CO E X

LO04

to 0 Eh O •rH a. • 0 • CO U H-) 0 3 X 3

rH 0 3 o o •rH i •rH •H i O o X 3 a CO -H s ri. i 2 2 i S 43 E 43 E C4 E E- X X

do 3) oo 3 3 • •

CL) < < 3 LO 0 3 to < < < E E < < rH E X S3 E E E LO LO E £ r • rH 0 o O LO o • • r—H 3 + 3

X) -H r-H 04 rH to to rH rH t—3 • 35 3 i i 33 ,

3 3)

E—1 -rH i #» i CJ DO 0 3

33 > > > > > > > 3 +4 0

4= 3 44 44 44 44 44 44 rH rH 3) o o O LO O O rH O 3 CJ •rH CJ to to ro 04 04 to to X 3 <4H 3 > SP O

to r-H C3

DO r\ ' ' •rH S3 S3 co 3 o 3!

•rH *rH Tj

X E <4H 0 3 S3 o o o o o to E 3 x 3 0 0 o 1 o o 1 1 o o i < 3 0

0) 35 E 0 LO 1 LO LO 1 1 LO LO i rH •rH 0 > O -H 0 LO 43 3

•rH

U

r1 CO • • • 3 3

(O CO o O 3 £ O 0

(1) f—i CD CD o X 2 a DO 03 3 3 rH 0

in DO 3 •rH •rH rH ,—

3 E • rH 03 C0 X 43 Uh 43 Uh o r3 •rH a Eh X •rH 1 X •H 1 1 Ph •rH i C+H 3 DO 3

CQ EhX 1 o3X 1 1 o3X i DO 3

43 CJ Eh Eh to 3 •rH DO

4-> CJ CJ 3 0 35 3

DO rH 3 •rH S3 CD rH 3

0) CO < < < < < 3 3 0 3

r—H

(3 E E E E E <D •rH 3 3 0 X O LO o0

LO LO £ ' ' •rH

O > 03 "H Tf H- OH d- 0

o3 Eh H-> 1 1 1 i 3) E 3 1 • rH *S 1 1 1 r i <D E 0 rH X 03 > > > > > 3 3 3

S3 i 34 i 44 i 44 44 44 3 CM 3 3 O O LO O r- H-> OO Eh O O lo to o •rH 3

CJ S 3- S 3- 34 3" cj *3 S d- 3 rH E 0

<D 0 3 E • • X to 43 to 3 3 •H

to to to 0 O 3

o3 o to < o 3 •H 35 0 0 o OJ O rH O O CN1 •rH 3 3 E

rH 33 0 O (HI 03

X 3 3 CD rH 3 O •rH Oh CJ X 3 0) O T—t CJ CJ Uh E 0 3 E CJ Uh CJ Uh X • rH 0

o3 3 S3 3 3 3 3 rH to 0

CO •rH •rH S3 (3 3 S3 •rH •rH • rH 3 r-H >

•rH •rH •rH •rH i o o •rH CO CO 3) 43) 43 X CJ 2 X < < > > > > CJ Ph 34 3 43 0 31 2
nickel, molybdenum and tin, which were very effective because

these monochromatic x-ray lines could be selected to provide

the most efficient excitation conditions for analysis of the

elements in the sample. A 16-position sample holder was also

constructed at NBS in order to obtain good reproducibility

on replicate measurements. The instrumental conditions used

for each type of spectrometer are summarized in table 1.

Samples were prepared by addition of known amounts of

NBS-SRM's or pure reagents to the various matrices of

interest followed by grinding in a mortar and pestle. In

table 2 is a summary of the reagents employed. Successive

dilution of the samples with the matrix was performed to

obtain concentrations of an element in the 10-100 ppm range.

One-gram samples of the powders were then added to Somar*

cups and subsequently analyzed.

NBS-SRM No.

83c

1052b

1053a

1059b

Table 2. Known Samples

Compound Name

Arsenic trioxide

13.0% V in Bis (1 -phenyl - 1 , 3-butanediono)

oxovanadium (IV)

24.8% Cd in cadmium cyclohexanebuty rate

36.7% Pb in lead cyclohexanebutyrate

The organo-metallic compounds were selected because of

their relatively low concentrations of the element of

interest. These required fewer dilutions to be made in order

to obtain concentrations in the low ppm range. The copper

oxide matrix was prepared from oxidation of copper metal

*Certain commercial equipment, instruments or materials are

identified in this paper in order to adequately specify the

experimental procedure. In no case does such identification

imply recommendation or endorsement by the National Bureau

of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the

purpose

.

3
(NBS-SRM 45d) . The ferric oxide and lead oxide were

analytical reagent grade, and the coal and fly ash were

NBS-SRM’s 1632 and 1633, respectively.

3. MEASUREMENTS AND RESULTS

The limit of detection is often used to characterize the

sensitivity of a particular analytical technique for certain

trace analysis problems. Because of the numerous and sometimes inconsistent definitions of a detection limit that

appear in the literature (see reference 2 for example), it

is important to specify how it is defined. For the purpose

of this report, the method proposed by Currie [2] is used.

An equation can be written for a detection limit based on

Poisson counting statistics such as

4.65/N-

= r

°

t £ sTD N c „ n -N

b I D o

( 1 )

where is the lowest limit of detection theoretically

obtainable of the element of interest for a single measurement

in concentration units (ppm) ; Cg^ is the known concentration

(ppm) of that element in the standard; N

q

is the total number

of counts corresponding to the background or blank in some

known t ime \ interval ; and Ng^ is the total number of counts

(i.e., gross counts) from the standard in the same time interval. Substitution of x-ray intensities for the background and

net signal intensities of the standards in the above equation

yields a concentration level which should lead to a detected

result with 95% confidence. This equation is identical to

the one which appears in a previous report [1] except for the

numerical constant 4.65. This constant is used when as few

as two measurements are made of the background and sample

(e.g., paired observations). The quantity ( N sTD~ N

o^

/

C STD is

usually referred to as the sensitivity in ppm“l and is tabulated along with x-ray intensity data in table 3.

4
Table 3. Data Summary

Wavelength- Dispers ive

Samples

Cone

.

ppm

Net peak counts (av)

Background

counts (av) n

e

s ( count s)

f 8

1 .95 N 1/2

0 Sens it ivity

ppm' 1

As(CuO) 10,000

250

100

so

10

19,533

9,398

5,299

1,930

33, 522 10 200 1.833 183

97

As(Fc

2 03

) 200

50

-- -- -- -- --

V(CuO) so 21,893 173,752 3 1200 2.920 417 438

V(Fe

2 03

) 100

SO 13, 538 7,558

18,740 5 61 2.132 137 143

V (coal) 17.5

7.0

-- -- -- -- -- --

V (fly ash) 703S

14

-- -- -- -- -- -- --

V(PbO) 200 -- -- -- -- -- -- --

Cd(CuO) 250 3,229 22,191 7 127 1.943 149 13

Pb(CuO) 125 5,319 35,409 3 136 2.920 188 42.5

Pb(Fe

2 0

3

) 200 100

50

Energy -Dispersive

As(CuO) 10,000

250

100

50

10

23,718

643

260

140

41

94 3 12.9 2.920 9.7 2.6

As(Fe

2 03

) 200

50 629

242 236 3 13.7 2.920 15.4 4.0

V(CuO) 50 4,901 5,500 4 56 2.353 74 98.0

V(Fe

2 03

) 100

so .. .. _ _ _ _ _ _

V (coal) 17.5

7.0 10,2172,766

4,223 4 84 2.353 65 536

V (fly ash) 7035

14

38,030

28,849

14,554

11,193 4 280 2.353 106 543

V(PbO) 200 2,515 1,472 3 56 2.920 38 12.6

Cd(CuO) 250 -- -- -- -- -- -- --

Pb(CuO) 125 245 105 3 13 2.920 10. 2 1.8

Pb(Fe

2 03

) 200 100

SO

861 364

83 311 9 18 1.860 16.8 4.0

e n is the number of replicate measurements of the background.

^estimated standard deviation of a single measurement of the background.

^Student’s t based on (n-1) degrees of freedom.

5
A limit of detection based on measurement data can also

be calculated according to reference [2], if the variability

of the blank is known or can be measured. If n replicate

measurements of the blank are made and the standard deviat ion ( i . s) of a single measurement is computed, then an

e

. , exper imenta

according to the equation

experimental limit of detection (C ) can be calculated

C A U

r 2/2 t s exp

L STD N std -N

q

( 2 ) *

where t is Student’s t [3] based on n-1 degress of freedom. Limits of detection for arsenic, vanadium, cadmium, and

lead were calculated according to the above equations and

are tabulated for comparison in table 4.

It is interesting to compare the variability of the

background tabulated in table 3 for each type of spectrometer.

Of course, if the variability is computed according to

1/2 equation 1, where it is equal to N

q

, then the only contribution to the random error is assumed to be due to the

Poisson statistics of error. The limit of detection calculated

in this manner represents the lowest limit theoretically obtain

able. If the standard deviation is computed such as in

equation 2, then the total contribution to the random error

is included in the calculation of the detection limit and will

necessarily include experimental sources of random error (e.g.,

the effect of ambient temperature variations). It should be

emphasized, however, that systematic error is not included in

these calculations and should be considered separately.

Comparison of the computed standard deviation (i.e.,

1/2 column 6, table 3) with the N

q

values via the chi-square

test indicated that the Poisson error is the primary contribution to the random error in most cases. The chi-square

: The factor 2/2 is due to comparison of the unknown with the

blank, and that Lp-2L^.

6
Table

4.

Limits

of

Detection

for

As,

V,

Cd,

and

Pb

in

Various

Matrices

Calculated

from

Equations 1

and 2 £ p p o o o X # • • 0 v£> 1 LO CJ p rg LO 1 OJ •H

LO p- LO E • • • P O vO i ai

I—

P rg 1 r—

M

CJ

p / \ ,

,—\ /•

—\ • H rH rsj rH (NI

r'd cj>

75 < E

&. P X P tn

<u lo i cj p

•iH

po E to i •H

Ph LO i M

r—( (On 3 M /- N /—<v P

CJ

p rH eg •H

•H

v

—

V ' P

+-> po E u

a.p

O 00 LO o *=d“ to

x

p • • • • • • c 0 to to rg l 1 rH i *K to i vO o

cj

p * eg l 1 1 i to 75

•rH 7) •Ho

VO rH

p LO lo LO o E • • • • • • 75 P to ^d" l 1 o i o i *d" TJ

i—i p. i 1 1 i rH 0

+-> 0

cj

p f—\ /—\ f N f—\ /—\ ,—\

/*

—\ /—\ / N f—\ U

•H r-H eg rH rg rH rsi rH rg rH rg

X

v

—/ wv

—

0

T3P E 3

Ph Ph vO o O X Ph • • • 0 p 0 o o l oo > W> CJ p rH ^d“ i rg •h jc; •H

00 to cr>

75

u P 0 ctf 0 > P

P-H

75 75 4H s • • • •H P O

Ph co r- l r- 1 1 1 TJ 0

rH

p rH l r-H i l 1 l P X

•P Hp 75 +-) CJ p /—

/—\ ,—\

+-> "H -H

•H rH eg r-H rg W)"P iH V / X P 1 -H 0 XP rH CO CTj 0 p -H > 0 P X IT) aj P <3 •H to < £ 0 > P O rH

•M o rg cd

X o , \/ \ 1 3 0 o rH rO rH CM

CJ Uh CJ Uh CU

v—/'

—'-K 7
values were exceeded for V in CuO and for V in fly ash,

and indicate that the background variability can be attributed

to other sources.

Limits of detection for arsenic and lead in coal, and

in fly ash could not be accurately determined because of the

AsKa-PbLa x-ray line interferences. Efforts to obtain samples

of coal and fly ash containing lower concentrations of As

and Pb were not successful. Poor results were obtained

for cadmium because the background in all cases was very

high and the signal level was generally too low with both

types of x-ray spectrometers. Vanadium in ferric oxide,

however, could not be measured accurately with the energydispersive spectrometer because of the interference of the

vanadium Ka line with the escape peak caused by the FeKa

x-ray line.

4 . CONCLUSION

The statistical limits of detection for arsenic, vanadium,

cadmium and lead were determined in various matrices consisting of copper oxide, iron oxide, lead oxide, coal and fly

ash. In general, the lowest limits of detection were obtained

for vanadium in the various matrices with an energyv dispersive spectrometer. These results can be explained on

the basis of the generally low background in the x-ray

energy region of vanadium and the high excitation efficiency

of the nickel secondary emitter. In contrast, a high

detection limit for cadmium (i.e., 70 ppm) in copper oxide

was obtained.

8
5. REFERliNCI-S

[1]

, Myklebust, R. L. , Darr, M. M. and Heinrich, K. F. J.,

NBSIR 75-675 (1974) .

[2]

. Currie, L. A., Anal. Chem. 4^0, 586 (1968).

[3]

. Natrella, M. G., Experimental Statistics, NBS Handbook

91, table T-5 (1963)

USCOMM-NBS-DC
.

.