NBSIR 75-675

Evaluation of X-Ray Fluorescence

Analysis for the Determination of

Mercury in Coal

Robert L. Myklebust, Martha M. Darr. and Kurt F. J. Heinrich

National Bureau of Standards

Department of Commerce

Washington, D. C. 20234

December 1974

Final Report

Prepared for Environmental Protection Agency

Research Triangle Park North Carolina 27711

NBSIR 75-675

EVALUATION OF X-RAY FLUORESCENCE

ANALYSIS FOR THE DETERMINATION OF

MERCURY IN COAL

Robert L. Myklebust, Martha M. Darr, and Kurt F. J. Heinrich

National Bureau of Standards

Department of Commerce

Washington, D. C. 20234

December 1974

Final Report

Prepared for Environmental Protection Agency

Research Triangle Park, North Carolina 27711

•"•tAO Of *

U S. DEPARTMENT OF COnHMERCE. Rogers C.B. Morton. Secretary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts, Director

livaluation of X-ray !• 1 uorcscciice

for the Determination of Mercury

Ana lysis

in (;oal

ABSTRACT

Limits of detection Cor mercury in coal have been determined on both a wa ve 1 eng th- d i s per s i ve and an ene rgy - dispers i ve

x-ray spectrometer. They are between 2 and 3 ppm under best

conditions for both spectrometers. fechniques for reducing

the background intensity measured by the energy - dispers ive

system are discussed along with methods of preparing coal

specimens for analysis in both instruments.

^ • INTRODUCTION

The mercury content of natural coal ranges between 0.05

and 0.4 parts per million (ppm) according to both atomic

absorption analysis and neutron activation analysis [1].

bven though the mercury concentration is so low, the large

amount of coal burned each year liberates tons of mercury

into the environment. Many different analytical procedures

are needed to monitor both the sources and the emissions of

toxic materials such as mercur>'. The primary objective of

this work is to experimentally ascertain the sensitivity,

by x-ray fluorescence analysis, for the detection of mercury

in coal.

IVe have selected two instrumental arrangements for

testing the limit of detection of mercury in coal by x-ray

fluorescence. ilic data thus produced should serve as a guide

to tiie configuration of apparatus most appropriate and sensitive for trace determinations by x-ray spectrometry.

The first { ene rgy - d ispers i ve J arrangement consisted

mainly of an x-ray tube and a Si (Li J detector with a multichannel pulse height analyzer. Primary x-rays emitted

by the tube excite the specimen which emits an x-ray spectrum

characteristic oi the elements in the specimen. The secondary

x-rays are analyzed by the Si (Li) detector which produces

electrical pulses proportional in height to the energy of the

1
respective x-ray photons. After amplification, these pulses

are sorted and stored by the multichannel analyzer. Since

this system simultaneously collects all of the x-ray lines

emitted by the specimen, the concentration of every element

for which there is an x-ray line may be determined at the

same time. Iherefore, this system is useful for the analysis

of the major constituents in a mu 1 t

i

- e 1 emen t specimen. The

scattering by the specimen of continuous radiation generated

in the x-ray tube produces a background which makes it difficult to detect elements present in trace quantities. To

reduce this scattering and thus the background, either secondary x-ray targets or x-ray filters can be introduced between

the x-ray tube and the specimen. ' '"' v-n-v

The second ( wa ve 1 ength - d i sper s i ve J arrangement is a commercial s ing 1 e - channe 1 x-ray spectrometer with crystal

dispersion. Primary x-rays emitted by an x-ray tube are

also used to excite the specimen. The crystal spectrometer

is aligned so that the characteristic x-ray line to be measured is diffracted by the analyzing crystal into the scintillation detector. The pulses from the detector are amplified

and counted. Since this detector is not flooded with x-rays

other than the line of interest, and has a considerable loss

in intensity due to diffraction by the analyzing crystal, a higher power may be used on the x-ray tube than with the

energy-dispersive system. The peak signal measured with this

system is generally higher than that measured with the energydispersive system.

Since the peak signal and background signal are measured

consecutivel >' with the wave 1 ength - dis per s i ve system, any

instrumental drift limits the useful length of time for measurements. Drift is not a limitation with the energy-dispersive

system since both the peak signal and background signal may

be simultaneously measured. We found that the limit of

detection for mercury iij. coal is about 2 ppm with either

2
system. However, to obtain similar results, the energydispersive system must operate for about ten times as long

as the wavelength-dispersive system. These results are in

agreement with the conclusions reached by (lilfrich et al. [2]

who made similar comparisons of equipment for the analysis

of air pollution samples, and with the conclusions of Leyden

[3J on environmental water analysis.

The tochniciues employed in this work are now being

applied to determine the limits of detection of other elements

(Pb, As, CaI and V) in coal and in several other materials, such

as fly ash, l'e,0^, PbO and CuO. We are also trying to further

improve botli of our s\stems.

2. EXPERIMtiNTAL

2 . 1 Lne rgy - 1)1 spe r s i ve System

The primar\ x-rays emitted by the x-ray tube contain both

characteristic x-ray lines from the target (Mo, Pt, etc.)

and continuous radiation. The most efficient excitation of

an x-ray line in a specimen is obtained when the energy of

a strong primar\' x-ray line from the tube is slightly higher

than the excitation potential of the x-ray line in the specimen. The continuum generated by the tube also excites x-ray

lines in the specimen; however, part of it is scattered by

the specimen, particularly from one of low atomic number, such

as coal. This scattered radiation produces a high x-ray background (figure 2a). Since, at a given signal intensity, the

'limits of detection of any element are lowest when the lineto - background ratio is highest, it is desirable to decrease

the background. Ihree methods of separating the continuum

component from primary x-rays can be employed:

1. Use the x-ray tube to excite a secondary target

which in turn excites the specimen (figure 2b).

2. Use a filter between the x-ray tube and the specimen

to preferentially absorb the continuum (figure 2c).

3
3. Use a crystal monochroma tor to diffract a primary

x-ray line from the tube onto the specimen.

The first two methods have been applied to measure the

limit of detection for mercury in coal. With the energydispersive system, we have produced efficient excitation of

both the HgLu and UgL^^ x-ray lines while reducing background

radiation. The first apparatus constructed used a silver

anode x-ray tube to excite a pure secondary molybdenum target

(see fig. IJ. X-rays from this target are in an efficient

energy interval (12-15 keV) and do not cause line interferences. A Si(LiJ solid-state detector and multichannel pulseheight analyzer (MCA) are used for detection.

We have replaced the original device by a more permanent

and versatile configuration. Since molybdenum tubes are more

common for x-ray fluorescence than silver tubes, a molybdenum

anode x-ray tube has been substituted for the silver anode

tube, and a pressed pellet of Y^O^ has replaced the molybdenum

secondary target (fig. 2b). The HgLa x-ray line is effectively

excited by the yttrium K-lines from the Y^O^ target which have

an energy of about 15 keV and are strongly excited by the

molybdenum tube.

In another modification to the system, the Y^O^ pellet

was placed directly in front of the window of the x-ray tube

(fig. 2c). In this arrangement, most of the x-ray radiation

generated by the tube is absorbed in the 1 mm thick "^2^3

pellet. The YKa x-rays generated in the pellet are of sufficient intensity to efficiently excite the HgLa x-ray line

in the specimen.

2 . 2 Wave length- Dispers ive System

The wavelength-dispersive system used was a standard commercial x-ray fluorescence unit. For the measurement of

the llgLa^ x-ray line, a LiP crystal and a scintillation

detector were used. Since a molybdenum anode x-ray tube

4
5
\

Figure 2. Instrumental arrangements of the energydispersive x-ray fluorescence equipment,

(a) direct excitation of specimen by x-ray

tube; (b) excitation of specimen by secondary

target; (c) excitation of specimen by x-rays

filtered to remove continuum.

6
produces energies in the range efficient for the excitation

of the mercury lines without generating any interfering

x-ray lines, this tube was selected for the work.

In addition to the molybdenum tube, platinum, tungsten

and chromium tubes were available. The chromium anode tube

could not be used since it does not emit an x-ray line of

sufficient energy to excite HgLa. Both the platinum anode

and tungsten anode tubes had to be ruled out because of

interferences with the analytical x-ray lines of mercury.

A brief comparison of the tried configurations is shown in

table 1.

Table 1 Comparison of Equipment

X-ray tube

Secondary fluorescer

Filter

fluorescent x-ray

1 ine

Cry s tal

Detector

Read - out

Line measured

EDS is Energy- Dispersive System

WDS is Wavelengtli- Dispersive System

EDS^-1 EDS- 2 EDS- 3 WDS^

Ag Mo Mo Mo

Mo Y2O3 Y2O3

Y2O3

MoK YK YK MoK

u a a a LiF

Si(Li) Si(Li) Si(Li) Scintillation

MCA MCA MCA Scaler

HgLa HgLa HgLa HgLa

7
2 . 3 Specimen Preparation

One NBS Standard Reference Material (SRM 1630)** was

available for the determination of mercury in coal. This

SRM contained 0.15 ppm of mercury, and was used as a base

material to which known amounts of mercury were added. For

example, a 100 mg specimen was prepared by placing 100 mg of

SRM 1630 powder (-325 mesh) on a sheet of 6 pm Mylar* and

adding 0.1 ml of trich loroethy lene which, upon drying, causes

the powder to form a cake. After it had dried, 0.001 to

0.025 ml of a solution of mercuric eye lohexanebuty rate (NBS

SRM 1064) in toluene was added to the cake.

Several different methods of mounting specimens for

x-ray fluorescence analysis were tried.

1. The specimen was confined to a diameter of 13 mm

and held between two sheets of Mylar.

2. The specimen was confined to a diameter of 25 mm

and held between a sheet of Mylar and a filter paper backing.

3. The specimen was confined to a diameter of 25 mm and

held between two sheets of Mylar.

4. The specimen was pressed into a pellet 12.5 mm in

diameter and heJd between Mylar sheets.

Homogeneity tests were made on three 500 mg specimens

mounted by method 3 and on two 500 mg specimens mounted by

method 4 (all doped at the 50 ppm Hg level). In the case

of method 3, the specimens were prepared and the mercury

measured. They were then stirred while in their same mounts

and remeasured. No significant difference in the mercury

*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.

**Availabie from The Office of Standard Reference Materials,

National Bureau of Standards, Washington, I). C. 20234 .

8
measurements was observed. Measurements on both sides of

two different pellets mounted by method 4 also showed no

significant differences.

Coal specimens containing 10, 20 and 50 ppm of mercury

were mounted by the four techniques above. Specimens of

20 mg, 100 mg and 500 mg were mounted by methods 1, 2 and 3.

For method 4, 100 mg , 300 mg and 500 mg specimens were

pressed. The 100 mg specimens were very thin and difficult

to handle.

3. MEASUREMENTS AND RESULTS

The theoretical limit of detection will be defined as that concentration which yields a signal three times as high

as the background [4]. Individual pulses arrive at a statistically varying rate, and the number of pulses accumulated

in fixed time intervals, for a given specimen and stable

instrumental conditions, varies according to a Poisson distribution. Therefore, the theoretical standard deviation

of the measured background is equal to the square root of

the number of pulses collected from the background.

If N is the total number of counts collected at the

o background, the theoretical limit of detection (C^^) for one

measurement can be calculated with the following equation:

3/Nr

C = C - ^tl std N ^,-N std o where C ^ , is the concentration of the measured element std

in the standard, and N is the number of counts collected ' s td

from" the standard in the same time interval from that element

in the standard. This equation is based on the fact that at

low concentrations, the calibration curves are linear in x-ray

fluorescence analysis.

If multiple measurements are made on the blank and on

the specimen, the experimental limit of detection may be

9
caJculated from the standard deviation (a) of the mean of

the blank determinations. If we have n measurements of

the background (X^J, o may be calculated with the following

equat ion

^ \ . n z xr - ( Z X. ) .

' _ 1=1 1=1

If the mean of the background counts is estimated as:

- 1 1=1

v/e can calculate an interval from X-to//ii to X+to//n which

we expect, with a certain confidence, to bracket the true

background. I'he factor t is a function of the number of

measurements and of the confidence level desired [5]. There

fore, for an element to be detected, the line signal must

exceed tlie mean of the background measurement by more than

t o//n. The true limit of detection may then be calculated

with the following equation:

,.-

^ C = C ^ ^'"^

el std N ,

• std o Several experiments were performed on the wavelengthdispersive apparatus to determine the experimental limit of

detection for mercury in coal. A 200 mg specimen containing

0.125 weight percent of mercury was mounted by method 2 discussed earlier and a wavelength scan was made to check for

any possible interferences (see fig. 3). As can be seen in

this figure, the background for HgLa is much lower than the

background for llgL3^. for this reason, the HgLa x-ray line

was chosen for all analyses.

10
11
Optimum operating conditions for the molybdenum x-ray

tube were determined by measuring the peak signal-to-background ratio on a SOU mg coal specimen containing 10 ppm

mercury mounted by method 2, and on a similar coal specimen

without mercury. On the basis of the results listed in table

2, all subsequent wavelength- dispersive measurements were made

at 4 5 keV and 40 mA. '

.

. / . Table 2 100-Second Measurements of 10 ppm HgLu in Coal with Different

Operating Conditions for the Molybdenum Anode X-ray Tube

Wave length- Ui f fract ion (LiF) Technique

Theore tica

keV mA

Background

on Blank HgLa Peak

Limit of

Detection

50 - 4 0 1 6 584 1524 2 . 5 ppm

45 40 13410 1508 2 . 3 ppm

4 5 30 10 289 950 3.2 ppm

35 3 5 8090 585 4.6 ppm

Three 500 mg specimens containing 10, 20 and 50 ppm

mercury were prepared and mounted by method 2. The results

of 100-s measurements are summarized in table 3. The lowest

achievable theoretical limit of detection for mercury is

2-3 ppm. The 50 ppm specimen and the blank were each measured

five times. The experimental limit of detection for a 991

confidence interval was 7.5 ppm mercury for the mean of five

measurements; however, when the blank was measured 15 times,

this limit -of detection decreased to 1,6 ppm.

The various spec imen - mount ing procedures are compared

in table 4. Each point is the result of a single 100 s count.

Since methods 1 and 3 give the best results, they appear to

be the best techniques for sample preparation.

12 ,
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15
With the energy-dispersive apparatus, shown in figure 1,

several different preparations were also tried. Method 1 was selected for these measurements. Specimens of 500 mg

,

200 mg , 100 mg and 20 mg were tried (all doped with mercury

at the 50 ppm level). The best signal-to-background ratio

was achieved with the 100 mg specimen. The theoretical limit

of detection with this specimen was 2.6 ppm mercury for a one-hour measurement. '

''

" After the energy-dispersive system was modified, specimens with a larger diameter could more easily be used. Coal

specimens of 500 mg mounted by method 3 were found to give

the highest s igna 1 - to- background ratio for mercury (fig. 4).

lo obtain similar limits of detection from both energydispersive and wavelength-dispersive systems, sufficient

counts must be accumulated by each system to give equivalent

statistical results. Since the energy-dispersive system collects information on the whole x-ray spectrum, only a small

portion of the signal is within the peak of interest while

the wavel ength- dispers ive system collects information on only

the peak of interest [2]. Therefore, with the energydispersive system, counts must be accumulated for a longer

period of time than with the wave length- dispers ive system.

The theoretical limit of detection can be improved by

counting for longer periods of time. For a 50 ppm mercury

specimen, the theoretical limit of detection was 3.9 ppm

for a 500 s counting time. When the time was doubled to

1000 s, the limit of detection dropped to 2.5 ppm; however,

to further lower the limit of detection requires very long

counting times (see fig. S) . The experimental limit of

detection for the average of seven measurements for 500 s each was calculated to be 2.2 ppm at a 99^ confidence interval.

There were no significant differences in the results

obtained from either of the two experimental arrangements

of the Y^O, pellet. However, significantly lower backgrounds

14
Figure 4. biie rgy - d i spe rs i ve spectra of 0.125 weight

percent mercury in coal. " (a) direct excitation by Mo X ray tube; (b J cxc i t a tion by

Y2O5 secondary target; (cj excitation by

VtO^ filtered radiation.

15
oo

CM ou oo o

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I/)

LU

z:

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6h "Jdd

X e

•H

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16
were obtained by carefully collimating the x-rays to reduce

scattered x-rays in the specimen chamber. (Collimators were

placed between the secondary fluorescer and the specimen,

and between the specimen and the detector (fig. 2b).

4 . CONCLUSIONS

Ihe lowest limit of detection determined by both the

wave length- d i spers i vc and the energy-dispersive systems is

approximately 2 ppm mercury in coal. For such results, the

counting time with the energy-dispersive system must be

increased by a factor of ten over that with the wavelengthdispersive system. However, with the energy- dispersive

system, the concentrations of other elements may be simultaneously determined along with mercury. With a wavelengthdispersive system, the number of elements simultaneously

determined is limited to the number of crystal spectrometers

on the system.

At the present time, neither of these x-ray fluorescence

techniques is sensitive enough to determine mercury in natural

coal; however, these techniques may be satisfactory for other

materials. Besides coal, the limit of detection for mercury

will be determined in fly ash, ¥e20^, l^bO and CuO. Several

other elements (such as lead, arsenic, cadmium and vanadium)

are also important in these materials, and will be determined.

Along with these measurements, we will continue to examine

further techniques for improving our limits of detection.

17
5. REFERENCES

[IJ Schlesinger, M. D. and Schultz, H. , U.S. Bureau of

Mines Report of Investigation 7609 (1972).

[2] Gilfrich, J. V., Burkhalter, P. G., and Birks, L. S.

,

Anal. Chem. 4^, 2002 (1973).

[5] Leyden, U. E., Amer. Lab. Nov. , p. 24 (1974).

[4] MUller, R. 0., Spec trochemica 1 Analysis by X-ray

Fluorescence , Plenum Press, New York, pp. 106-129

< "(1972) .

[5J Natrella, M. G., NBS Handbook 91, pp. 2-2 to 2-3 (1963).

USCOMM-NBS- DC

18
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. inUU.K A 1 ION OK KKI'OK I'NO. NBSIR 75-675

2. Ciov't Accession

No.

3. Recipient's Accession No.

4. TITI.F AND SUB 1 11 l.i- Evaluation of X-ray Fluorescence Analysis for

the Determination of Mercury in Coal

5. Publication Date December 1974

6. F^crforming Organization ( ode

7. AUTMOR(S)

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

8. Performing Organ. Report No.

9. PERFORMINC. ORCANIZA I ION NAMI- AND ADDRESS

NATIONAL BUREAU OF StANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

i U. rroject/ 1 ask/ work Unit iNo.

11. C;ontract /Grant No.

12. Sponsoring Organization Name and ( omplete Address (Street, City, State, ZIP)

Same as No. 9.

13. Type of Report & Period

f overed Final

15. SUPPLEMKNTAKY NOTES

16. ABSTRACT (A 200-word or less factual summary of most tugpificant inlormation. If document includes a si^ificant

bibliography or literature survey, merttion it here.)

Limits of detection for mercury in coal have

been determined on both a wavelength-dispersive and

an energy- dispersive x-ray spectrometer. They are

between 2 and 3 ppm under best conditions for both

spectrometers. Techniques for reducing the background intensity measured by the energy- dispersive

system are discussed along with methods of preparing

coal specimens for analysis in both instruments.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) fiackground; morcury in coal; trace analysis;

x-ray fluorescence; x-ray spec trography

.

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