RELATIVE ACCURACY TESTING 

OF AN X-RAY BASED 

CONTINUOUS MERCURY 

MONITOR

Contract No. DACA42-03-P-0239

(TASK 2)

FINAL REPORT

Prepared for

U. S. Army Corps of Engineers

Engineer Research and Development Center

Construction Engineering Research Laboratory

Submitted By

Cooper Environmental Services

10170 SW Nimbus Ave Suite H5

Portland, Oregon 97223

September 30, 2003
RELATIVE ACCURACY TESTING OF AN XRAY BASED CONTINUOUS MERCURY 

MONITOR

Contract No. DACA42-03-P-0239

(TASK 2)

FINAL REPORT

Prepared by

B.E. Johnsen, J.A. Cooper, M.Nakanishi, S. Fry, D. Barth, P. Ginochio

Cooper Environmental Services

11120 SW 124th Place

Portland, OR 97223

Submitted to

K. J. Hay

U. S. Army Corps of Engineers

Engineer Research and Development Center

Construction Engineering Research Laboratory

P. O. Box 9005, CN-E, Champaign, IL 61826-9005

September 30, 2003
i

EXECUTIVE SUMMARY

Mercury has been identified by the EPA as their air toxic of greatest concern (EPA, 

2000). Accordingly, the EPA and Bush administration have proposed legislation such as

the Clear Skies Act and Utilities MACT which have focused on mercury reduction and 

monitoring. To date, however, limited success with mercury Continuous Emissions 

Monitors (CEMs) has been achieved. Cooper Environmental Services (CES) was 

contracted by the U.S. Army Corp of Engineers to test their Xact continuous mercury

monitor during an EPA sponsored test at a coal-fired power plant (CFPP). The Xact 

extracts a representative emissions sample which is then drawn through a reactive filter. 

The filter is advanced to an x-ray fluorescence analyzer where total mercury mass is 

determined. Results are reported every 20 to 30 minutes with detection limits of better 

than 0.1 micrograms per dry standard cubic meter.

For the tests, CES first evaluated the filter trapping efficiency and precision using a 

Mercury Speciation Cassette (MSC). The MSC consists of a stack of filters which

measures oxidized and elemental mercury. The MSC tests were designed to compare the

capabilities of the filter based approach relative to the EPA’s standard Ontario-Hydro 

(OH) method for measuring mercury. 

In general, the Xact and MSC performed very well relative to the OH, showing accuracy and 

precision that were on the same order as the standard method. Test results showed that:

1) The Xact total mercury relative accuracy was 25%, which was essentially the 

same as the 22% RA between duplicate OH test runs.

2) The MSC total mercury relative accuracy was 12% relative to the OH method, 

substantially less than the 20% required for method equivalency.

3) The MSC fractional mercury speciation was in excellent agreement with OH 

results showing an average of only 4% difference in the percent elemental 

mercury for all 12 OH runs.

4) The MSC relative precision was 30% better than the precision listed in the OH 

method for measurements less than 3 μg/dscm.

This first Xact unit did develop cold spots which caused occasional water condensation 

and inconsistent results. This problem is being mitigated with more uniform heating.

One of the largest problems for all candidate methods was the limited accuracy and 

precision of the EPA standard method at low mercury concentrations. Concentrations for 

all but one OH run were below 3 μg/dscm. The OH has a listed precision of 34% under 

these low concentrations (ASTM, 1999) limiting its ability to evaluate the relative 

accuracy of the mercury CEMs. In addition, mercury was almost exclusively in the 

elemental form for all but one run. Since many CEM approaches are severely impacted 

by oxidized mercury, the tests have limited applicability for CFPP which have significant 

concentrations of oxidized mercury. Although additional testing is needed at varying 

mercury concentrations and speciation levels, the EPA tests demonstrated the feasibility

of the Xact filter based approach to measure mercury in coal-fired power plant emissions.
1

INTRODUCTION

Cooper Environmental Services (CES) was contracted by the U.S. Army Corp of 

Engineers to test their filter-based Xact continuous mercury monitoring approach at an 

EPA sponsored test. The test, under the direction of the Midwest Research Institute 

(MRI), was designed to evaluate various monitoring approaches for measuring mercury

emissions from a coal-fired power plant. MRI’s test occurred in two phases, an initial 

phase in May 2003 and a follow-up phase in July. During each phase, a series of twelve 

EPA standard Ontario-Hydro (OH) reference method runs were conducted to evaluate the 

accuracy of candidate methods (Table 1). 

During the MRI tests, CES conducted a series of grab-sample test runs using its Mercury

Speciation Cassette (MSC). The purpose of the MSC testing during the first phase was to

evaluate CES’ proprietary filter for trapping and speciating capabilities as well as its use

as an alternative to the EPA standard method (OH). During the second phase, CES

conducted precision testing of the MSC and tested its Xact mercury monitor relative to 

the OH reference method. The Xact is a true continuous emissions monitor (CEM) for 

mercury and reports total mercury concentrations every 20 to 30 minutes.

This report summarizes the performances of CES’ MSC and Xact relative to the EPA 

standard Ontario-Hydro method.

MERCURY SPECIATION CASSETTE TESTING

The MSC consists of a filter stack within a Teflon coated stainless steel filter holder 

(Figure 1). The first filter traps particulate and gas phase reactive mercury. This filter is 

followed by a filter for trapping elemental mercury and a back-up carbon impregnated 

filter to ensure that breakthrough was not occurring. The cassettes are small enough to fit 

in the palm of an operator’s hand and can be inserted directly into the stack through a 

port that is 3 inches or larger in diameter. For this test, an in-stack approach was used 

since the stack temperature was about 180° F.

Table 1. MRI Test Arrangement Figure 1. Mercury Speciation Cassette

Phase Date CES Test MRI Test

I May-03 MSC-RA 12 OH Runs

II Jul-03 Xact 12 OH Runs

II Jul-03 MSC-Prec. --
2

The MSC approach offers a number of advantages over traditional standard methods 

including:

 Detection limits of better than 0.1 micrograms/dscm for 50 minutes of sampling

 Sampling, analysis and sample recovery are completed without chemicals

 Sampling can be conducted by one operator

 Cassettes can be changed within five minutes

 Filter samples can be sent via FedEx for next day analysis

 Low per-run analysis cost

 Routine analysis turn-around-time is typically less than one week

 Potential exists for in-field XRF analysis for same day results

 XRF analysis is non-destructive, so filters can be archived and reanalyzed at a 

later date

The results of the MSC testing during phase 1 are illustrated in the regression plot shown 

in Figure 2 and listed in Table 2. A total of 12 OH runs were conducted with each run 

lasting two hours. For each OH run, two 50 minute MSC samples were collected by CES. 

In general, the MSC and OH were in very good agreement with a relative accuracy of 

about 12% (nine valid runs), well within the 20% criteria for alternative methods

specified by EPA’s proposed Performance Specification 12. Three of the 12 OH test runs 

were omitted from all candidate method comparisons. Runs 2 and 6 were omitted because 

of poor OH replication (>40% difference) and run 8 was omitted due to a plant upset and 

highly variable mercury concentrations. For 11 of the 12 runs, the total mercury 

concentration ranged from about one to five μg/m3 with 90% in the form of elemental 

mercury. During run number 8, the stack gas temperatures increased significantly, total 

mercury increased to over 20 μg/m3 and averaged about 14 μg/m3 during the two hour 

OH test run. During this upset period, the elemental mercury concentration dropped from 

about 4 μg/m3 to about 2 μg/m3, but the oxidized mercury concentration increased about 

300 fold; i.e., from about 0.04 μg/m3 to about 12 μg/m3, and represented about 85% of 

the total mercury. The high correlation (r2 = 0.962) and low intercept of the MSC with the 

OH for the nine valid runs indicates good agreement between the two methods.

Figure 2. Correlation of MSC with OH During MRI Phase 1 Testing

y = 0.908x + 0.152

R

2

 = 0.962

0

1

2

3

4

5

6

0 2 4 6

OH (μg/dscm)

M

S

C (μ

g/d

s

c

m)
3

Table 2. Reported MSC and Ontario-Hydro Results for Phase 1 Testing

OH MSC

RUN RUN START STOP OXIDaELEM. TOT. AVG. OH-1 OH-2 AVG. Validb MSC OH % Diff

1A 1-May 13:20 14:15 <0.03 0.74 0.74 >95.9

1B 1-May 14:26 15:19 <0.04 1.42 1.42 >97.2

2A 2-May 10:20 11:11 0.12 2.77 2.89 96.0

2B 2-May 11:20 12:10 <0.08 2.59 2.61 >96.9

3A 2-May 14:41 15:31 0.10 1.63 1.74 94.0

3B 2-May 15:40 16:30 0.19 1.30 1.49 87.4

4A 5-May 10:00 10:50 <0.04 1.38 1.38 >97.1

4B 5-May -- -- -- -- -- NM

5A 5-May 14:10 14:50 <0.05 1.75 1.75 >97.1

5B 5-May 15:00 15:50 <0.04 2.70 2.70 >98.5

6A 6-May 9:28 10:18 <0.04 4.58 4.58 >99.1

6B 6-May 10:27 11:17 <0.04 4.19 4.19 >99.0

7A 6-May 15:27 16:17 <0.04 3.35 3.35 >98.8

7B 6-May 16:26 17:16 <0.04 3.67 3.67 >98.9

8A 7-May 15:21 16:06 19.46 3.62 23.08 15.7

8B 7-May 16:13 17:03 11.80 3.12 14.91 20.9

9A 8-May 9:20 10:10 0.54 4.39 4.93 89.1

9B 8-May 10:17 11:07 0.34 4.22 4.56 92.5

10A 8-May 12:21 13:06 0.29 3.84 4.13 93.0

10B 8-May 13:16 14:06 0.29 4.01 4.30 93.2

11A 9-May 9:22 10:12 0.12 3.41 3.53 96.5

11B 9-May 10:21 11:11 0.18 3.04 3.21 94.5

12A 9-May 13:50 14:40 <0.08 3.57 3.57 >97.8

12B 9-May 14:49 15:39 0.18 4.09 4.27 95.8

-3.6

-2.4

Percent Hg°

99.1

99.2

94.1

96.5

99.2

15.2

98.9

99.4

2.6

-3.1

-8.4

-1.6

-1.0

-0.3

99.0

98.6

-0.4

20.3

94.2

99.5

-3.5

-3.5

5.24

4.26

3.35

3.88

9

10

11

12

5

6

7

8

1

2

3

4

1.88

5.57

4.14

14.27

1.33

4.20

1.64

1.50

4.75

4.22

3.37

3.92

2.23

4.39

3.51

19.00

1.08

2.75

1.61

1.38

DATE

MSC (μg/dscm)

1.36 1.29

5.34 3.05

1.64 --

1.50 1.50

1.88 --

6.91 4.22

4.26 --

4.14 --

14.27 --

3.33 3.36

3.88 --

Yes

Yes

Yes

No

Yes

Yes

TIME OH

Yes

Yes

Yes

No

Yes

Yes

5.24 --

a) Includes particulate and vapor phase oxidized Hg 

b) Valid if OH Duplicates < 30% difference

The MSC relative mercury speciation results (% Hg°) were also in excellent agreement 

with the OH results as shown in Table 2. The 12-run average unsigned mean difference in 

the % Hg° from the two methods was 4.2%. The largest oxidized mercury concentration 

was observed during Run 8 in which the dominating oxidized mercury concentration was 

changing rapidly. In addition, the OH and MSC were both stopped in the middle of the 

run due to the plant upset and the MSC runs covered only 100 of the 120 minutes of the 

OH run during which the mercury concentration was changing rapidly. Thus, the MSC 

clearly demonstrated its responsiveness to changing relative mercury speciation and

demonstrated a high degree of accuracy in the relative species apportionment.

In addition to the stack tests, a series of runs were made comparing the MSC results to a 

direct injection of elemental mercury from a Spectra Gases mercury canister (Table 3, 

Figure 3). The first two tests were conducted using a known concentration of 8 μg/dscm 

with MSC reported concentrations of 8.1 and 8.2 μg/dscm respectively. The MSC results 

are well within the Spectra Gases listed uncertainty of 20%. Following the initial tests, 

another series of three blind (unknown) tests were conducted by MRI with no CES 

personnel present. These blind tests used equivalent certified concentrations of 8, 19 and 

19 μg/dscm respectively with the MSC results closely correlated at 8.4, 18.3, and 17.9 

μg/dscm. The mean MSC recovery for the three blind tests was 98.4% with a standard 

deviation of 5.4% and a range from 94 to 105%. Again, the small percentage difference 

was well within the listed 20% uncertainty of the Spectra canisters. 
4

Table 3. Comparison of MSC Reported 

Mercury Concentrations with Certified 

Spike Concentrations.

MSC Std.

μg/dscm μg/dscm

STD. Known 1-May 8.1 8.0

STD. Known 1-May 8.2 8.0

STD. Blind 8-May 8.4 8.0

STD. Blind 8-May 18.3 19.0

STD. Blind 8-May 17.9 19.0

Average

Standard Deviation

Date % Recovery

101.0

Injection Type

102.5

104.6

96.4

94.3

99.8

4.3

A second series of 11 test runs were conducted using a pair of MSCs during phase 2 of the 

MRI tests (Table 4). Since the May tests had indicated good agreement with the standard 

method for the MSC, the July tests were limited to demonstration of the MSC precision 

relative to the typical OH test run. According to the OH standard operating procedure, the 

OH has an 11% precision when concentrations are above 3 μg/dscm and a 34% precision 

when concentrations are less than 3 μg/dscm. Since only two MSC test runs during phase 2 

were above 3 μg/dscm and all concentrations were below 3.3 μg/dscm, MSC precision could 

not be determined for high mercury concentrations. The MSC precision for the 8 runs below 

3 μg/dscm ranged from 0 to 37% with an average of 24%, significantly better than the OH 

listed precision. It is believed that the MSC precision can be improved to about 5% with 

refinement.

Table 4. MSC Precision Tests During Phase 2 Testing

MSC 

RUN

Total 

(μg/m3)

% 

Diff.

1A 2.22

1B 2.21

2A 2.16

2B* --

3A 2.87

3B 2.43

4A 1.57

4B 2.25

5A 2.10

5B 3.06

6A 3.29

6B 3.01

7A 2.79

7B 3.24

8A 2.33

8B 2.77

9A 2.02

9B 2.55

10A 1.95

10B 2.51

11A 2.05

11B 2.90

21.4

23.8

* 2B Omitted due to water contamination

Avg. (< 3 μg/dscm)

0.4

--

16.7

35.5

37.3

8.8

14.9

17.5

23.0

25.0

34.6

Avg. (all)

0

5

10

15

20

1 2 3 4 5

Run No.

Hg Conc

(micrograms/dscm)

Injected Conc

MSC Results

Figure 3. MSC vs. Certified Mercury 

Concentrations for Spike Injection
5

Overall, the MSC had better precision than the standard OH method under low mercury

conditions, was able to effectively speciate the mercury concentrations, measured the 

standard gases to within 6%, and had a relative accuracy of 12% compared to the OH 

method. However, since the MRI test site had limited opportunities to measure speciated 

mercury and rarely had mercury concentrations above 3 μg/dscm, further testing under a 

variety of stack conditions is recommended.

Xact TESTING

The Xact mercury monitor is designed to continuously measure total mercury

concentrations and report data every 20 to 30 minutes with a detection limit of better than

0.1 μg/dscm (Figure 3). The monitor extracts a representative gas sample and 

concentrates the mercury on a reactive filter. The filter deposit is then advanced to an xray fluorescence (XRF) analyzer which determines mass concentrations. Minimal stack 

gas pretreatment is required, which greatly simplifies the approach. Other advantages of

the Xact include:

 Collection of total Hg including particulate, oxidized and elemental fractions

 Introduction of a fresh filter with each sample – no memory or blank bias

 Since XRF is non-destructive, filters can be archived

 Solid-phase Hg calibration minimizing use of expensive calibration gases

 Since XRF is non-destructive, results can be independently verified by

another method

 Multi-element sensor allows for option of simultaneous monitoring for

chromium, manganese, iron, cobalt, nickel copper, zinc, arsenic, selenium, 

bromine, and lead as well as mercury.

 Smoothly varying theoretically predictable sensitivities as a function of 

atomic number provides additional quality assurance.

Figure 3. Xact Mercury Monitor
6

Xact Performance

During phase 2 testing, typical measured concentrations for both the Xact and the OH 

ranged from one to three micrograms per dry standard cubic meter (μg/dscm). Overall, 

the instrument’s relative accuracy (RA) compared to the OH was 25%, slightly higher 

than the 20% relative accuracy criteria set forth in the proposed performance 

specification (Table 5) and similar to the other CEMs tested by MRI which had RA 

ranging from 15 to 40%.

MRI used duplicate OH trains for all phase 2 testing. On average, the OH duplicate trains 

differed from each other by about 25% with three runs showing a difference of greater 

than 50%. According to the standard method, the OH precision is typically about 34% 

when the mercury concentrations are below 3 μg/dscm range so the MRI precision results 

were typical or slightly better than historical OH precision levels. The magnitude of this 

imprecision, however, is on the same order as the desired relative accuracy. Thus, a large 

uncertainty exists in determining the relative accuracy results. For example, the RA of the 

OH duplicate trains compared to each other was 22%. Any approach being compared 

to the OH could not expect to have a RA better than that of the OH trains relative to 

each other.

The Xact reported concentrations for 10 out of the 12 OH runs conducted during phase 2 

tests. During run 1, the Xact experienced heater problems and developed a leak which was 

not repaired until the run had been completed. During run 7, the instrument was behaving 

appropriately prior to the run and following the run, but was turned off during the run. It is 

not known why the instrument was turned off; however, it was located centrally in a very

crowded room and could have been accidentally turned off by a bystander.

Table 5. Reported Xact and Ontario-Hydro Results for Phase 2 Testing

Xact OH-1 OH-2 OH-Avg.

(μg/dscm) (μg/dscm) (μg/dscm) (μg/dscm)

1 7/24/2003 1010-1210 -- 1.44 1.32 1.38

2 7/24/2003 1420-1620 1.61 3.39 3.38 3.39

3 7/25/2003 0950-1150 2.07 2.27 2.38 2.33

4 7/25/2003 1425-1625 1.61 1.83 2.19 2.01

5 7/28/2003 1120-1320 1.27 1.53 1.26 1.40

6 7/28/2003 1525-1725 1.18 1.79 0.99 1.39

7 7/29/2003 0920-1120 -- 1.46 0.82 1.14

8 7/29/2003 1320-1520 1.69 1.56 1.24 1.40

9 7/30/2003 0915-1115 1.55 1.24 2.25 1.75

10 7/30/2003 1340-1540 1.51 1.17 1.63 1.40

11 7/31/2003 0915-1115 2.30 1.51 1.32 1.42

12 7/31/2003 1325-1525 1.72 1.25 1.20 1.23

1.65 1.70 1.67 1.68

Run Date Time

Avg.
7

In addition to the leak in run 1, the instrument experienced problems with heat tracing 

throughout the tests. The inlet tube immediately prior to the tape was heat traced but not 

thoroughly insulated due to its close proximity to the x-ray tube. The water content of the 

stack was higher than expected which led to periodic condensation in the tube with a 

resultant drop forming on the filter. Since the filter tape is hydrophilic, the droplet would 

result in temporary severe pressure drops and flow control issues. Although the droplets 

impact on pressure was severe, it is not certain that the condensation impacted the tape 

collection efficiency. However, the condensation on the walls could be a potential sink 

for oxidized mercury. For this reason, any future Xact design needs to include a more 

thorough heat tracing of the inlet tube.

Upon initial installation, a series of seven blind mercury standard gas tests were run with 

the Xact. The Xact reported concentrations were within one percent of the certified 

standard concentration. Following OH testing, the Xact was again injected with the 

calibration gas, but the Xact response was significantly lower than expected. It is believed 

that the Xact may have experienced some problems with the calibration gas due to a heat 

tracing problem. On occasion, the Xact also reported blank concentrations of up to one 

μg/dscm. The high blank values could not be duplicated when the instrument was 

returned to CES and are currently being investigated. 

Overall, the prototype Xact was able to effectively measure the mercury for 10 of the 12 runs 

with a relative accuracy of 25%, approaching that of the replicate OH sample trains relative to 

each other (22%). The delivered unit was able to accurately measure the calibration gas and 

overall reported results correlated within uncertainty limits of the OH results. It is 

recommended, however, that the Xact be refined to improve transport and temperature 

control. It is also recommended that further reference method testing be conducted at a source 

with higher mercury concentrations to better evaluate the Xact capabilities and potential 

accuracy.

CONCLUSION AND RECOMMENDATIONS

The MSC was able to effectively measure mercury concentrations with a relative accuracy of 

12% compared to EPA’s standard reference method. The MSC precision was better than the 

OH for the low concentrations present in the stack and the MSC was able to consistently

determine concentrations to within 10% of the gas canister standards. Although only one OH 

run was present with significant oxidized mercury concentrations, the MSC and OH 

percentages of oxidized mercury were in good agreement for that run. Overall, the MSC 

appears to be able to give comparable results to the OH at a fraction of the cost and effort.

The Xact mercury CEM had a relative accuracy of 25% in comparison to the OH method. 

Since the OH method’s precision for the test was also on the order of 25%, the Xact RA 

uncertainty is limited by the uncertainty in the OH method. The Xact was able to 

accurately measure the Spectra gas standard upon initial installation and was able to track 

changes in total mercury concentrations relative to the OH method throughout the test.
8

1) The MSC total mercury relative accuracy was 12%, substantially less than the 

20% required for equivalency.

2) The MSC fractional mercury speciation was in excellent agreement with OH 

results showing an average of only 4% difference for all 12 OH runs.

3) The MSC relative precision was 30% better than the precision listed in the OH 

method for measurements less than 3 μg/dscm.

4) The Xact total mercury relative accuracy was 25%, which was essentially the 

same as the 22% RA between duplicate OH test runs.

This first Xact unit did develop cold spots which caused occasional water condensation 

and inconsistent results. This problem is being mitigated with more uniform heating.

One of the largest problems for all candidate methods was the limited accuracy and 

precision of the EPA standard method at low mercury concentrations. Concentrations for 

all but one OH run were below 3 μg/dscm. The OH has a listed precision of 34% under 

these low concentrations (ASTM, 1999) limiting its ability to evaluate the relative 

accuracy of the mercury CEMs. In addition, mercury was almost exclusively in the 

elemental form for all but one run. Since many CEM approaches are severely impacted 

by oxidized mercury, the tests have limited applicability for CFPP which have significant 

concentrations of oxidized mercury. 

Although the MSC and Xact performed well during the MRI tests, it is recommended that 

they be retested at a site that will provide high enough concentrations for precise OH 

measurements. It is further recommended that:

 The MSC be reengineered for use in higher temperature stacks.

 The MSC undergo testing at a stack with significant quantities of both elemental 

and oxidized mercury.

 The MSC undergo testing at a stack with mercury concentrations greater than 3 

μg/dscm.

 The Xact inlet should be reengineered for improved heat tracing and transport.

 The Xact should undergo testing at a stack with significant quantities of both 

elemental and oxidized mercury.

ACKNOWLEDGEMENTS

CES would like to acknowledge Pall Corporation and MetOne for their assistance with 

equipment development and the U.S. Army Corp of Engineers Construction Engineering 

Research Laboratory for their sponsorship of this project.
9

REFERENCES

EPA, 2000. EPA Fact Sheet Accompanying EPA’s Decision to Regulate Mercury from 

Coal Fired Power Plants. December 14, 2000. Found on the web at 

http://www.epa.gov/ttn/oarpg/t3/fact_sheets/fs_util.pdf

ASTM, 1999. October 21, 1999. Standard Test Method for Elemental, Oxidized, Particle 

Bound and Total Mercury in Flue Gas Generated from Coal-Fired Stationary Sources 

(Ontario-Hydro Method). Found on the web at http://www.astm.org/cgibin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/D6784.htm?E+mystore