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XCEM QA APPLIED TO EVALUATION 

OF XCEM/M29 MERCURY 

CONCENTRATION DIFFERENCES 

MEASURED DURING RELATIVE 

ACCURACY TESTS

1. INTRODUCTION

Mercury represents a distinctly different hazardous element from the others included in 

this test in that it is more likely to exist in the vapor phase and has been shown to exhibit 

distinctly different transport properties. These properties appear to have contributed to 

differences in the mercury concentrations measured by the different methods compared in 

this test. This appendix discusses the details associated with both the adjustments that 

were made to the XCEM calibration factors and losses of mercury from particulate 

deposits after sample collection.

The XCEM mercury results were, on average, 18% greater than the predicted 

concentration, 27% greater than the M29 results, 37% greater than the laboratory XRF 

measurements of the XCEM deposit spots, and 45% greater than the CVAA 

measurements on the XCEM deposit spots. Subsequent investigations of these 

differences strongly suggest that the XCEM results are the best estimate of the actual 

mercury concentrations in the stack. It is hypothesized that the large differences between 

the XCEM mercury results and the results from the other methods is due to the loss of 

mercury from the filter deposits after the M29 and XCEM samples were collected and 

after the XCEM original measurements were made. This hypothesis is described in more 

detail in the following subsection. The experimental measurements are described in 

Subsection C and the results are summarized and discussed in Subsection D. The 

hypothesized model is then evaluated relative to the available data in Subsection E.

A model has been developed to explain the apparent high XCEM mercury concentrations 

relative to M29. In this model, mercury is lost from the particulate fraction of the M29 

and XCEM filters after the initial XCEM measurements were made. That is, a substantial 

fraction of mercury was associated with the particulate fraction, and the mercury in this 

particulate was unstable. Since all of the other analytical methods relied on aged 

samples, the mercury concentration results were low due to vaporization of particulate 

mercury prior to laboratory analysis. As discussed in the following subsection, a 

substantial amount of experimental data is available, all of which is supportive of this 
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hypothesized model. Most of this experimental data was developed only because of the 

unique characteristics of the XCEM and the fact that the XCEM elemental measurement 

is nondestructive and the spectra from each of the original measurements were archived.

2. AA

a. Experimental 

The initial XCEM measurements were based on the analysis of filter deposits, which 

were a combination of both absorbed vapor phase mercury species and surface deposits 

of particulate mercury species. Each deposit was analyzed using three different X-ray

excitation conditions, resulting in spectra that were used to determine concentration. All 

spectra were archived in the XCEM computer. As such, the deposit was available for 

further analysis and testing, and the original spectra was available for comparison with 

subsequently developed spectra.

Following the validation testing at TEAD, several tests were conducted by CES to better 

understand the mercury concentration differences between M29 and the XCEM.

 All of the XCEM deposit spots corresponding to all of the M29 runs were 

reanalyzed by CES’s laboratory QuanX XRF analyzer six weeks after the initial 

validation test. In addition, ten XCEM filter deposit spots were analyzed over a 

period of two months to evaluate mercury stability. Spectra from these tests were 

compared to archived spectra from the original validation test.

 The XCEM deposit spots corresponding to M29 Runs 5 and 6 were analyzed first 

by laboratory XRF and then by cold vapor atomic absorption by an independent 

laboratory (Columbia Analytical Services).

 The elemental concentrations in the MSE-TA spiking solutions corresponding to 

M29 Runs 5 and 6 were analytically determined by HKM labs in Butte, MT. 

3. AAA

a. Results and Discussion

The mercury results for the original validation test are summarized in Table C1. There is 

a clear bias of about 26% between the XCEM results and the M29 results. Normally, the 

M29 results would be accepted as the best estimate of stack mercury concentration and it 

would be assumed that the candidate method (XCEM) was in error. However, because of 

the total quality assurance associated with the XCEM and the fact that the XRF analysis 

is non-destructive, it is possible to conduct further analyses to evaluate the potential cause 

of this difference. Additionally, the laboratory analysis results for each component of the 

M29 sampling trains (i.e. probe, filter, and back half) were determined separately, and are 

available for interpretation. 

Table C.1 Mercury Reported Concentrations During Year 2002 Method 29 Validation Testing.
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PRD M29 XC QN CA PRD XC QN CA

1 324 332 367 257 0.98 1.10 0.77

2 325 334 381 251 0.97 1.14 0.75

3 333 294 365 262 1.13 1.24 0.89

4 322 327 368 301 0.98 1.13 0.92

5 323 318 379 308 274 1.02 1.19 0.97 0.86

6 329 280 378 288 254 1.18 1.35 1.03 0.91

7 327 285 392 302 1.15 1.37 1.06

8 325 306 406 294 1.06 1.33 0.96

9 324 309 405 287 1.05 1.31 0.93

10 328 292 395 282 1.13 1.35 0.97

11 326 295 397 286 1.11 1.35 0.97

12 322 293 389 267 1.10 1.33 0.91

AVG. 326 305 385 282 264 1.07 1.27 0.93 0.88

SD 3.3 19 14 19 13.8 0.07 0.10 0.09 0.03

μg/DSCM

RUN

Normalized to M29

i. Loss of Mercury From the XCEM Filter

a. Laboratory XRF Measurements (QN)

Two months after the validation testing, CES’s laboratory QuanX XRF analyzer 

reanalyzed the original XCEM deposit samples with good replication for all elements 

except mercury (see Table 5 in main body of text).

A series of ten spots on the XCEM filter tape were reanalyzed between on 6/21/02 and 

7/11/02. The spots, which represent 10 XCEM runs, showed a consistent mercury loss of 

about 30% over the three-week period. Other sources of error such as shifts in geometry

or instrument instability were eliminated as possible systematic sources of error by noting 

that the other elements were replicated within experimental error to a few percent.

b. XCEM Measurements (XCEM)

To confirm this loss of mercury, five XCEM deposit spots were re-analyzed by the 

TEAD XCEM, which was still operating with the same conditions and calibration factors 

as used during the M29 tests about two months earlier. The X-ray spectra from the 

original analysis of XCEM Run Numbers 939 to 943 (M29 Run Number 4) are compared 

in Figure C1. This comparison clearly shows that there is good agreement for the two 

closest analyte peaks for zinc and lead, but a substantial reduction in the peak intensity

for the mercury L-alpha analyte line. A comparison of the XCEM mercury

concentrations measured during the M29 testing (5-14-02) with those measured with the 

XCEM on 7-25-02 indicate a 31.4 ± 0.4 μg/m3 reduction in measured concentration. 

Although this reduction might be associated with possible systematic errors such as 

sample positioning, this possible source of error was eliminated by comparing other 

elements such as zinc and lead as well as the ratio of mercury to these elements. The 
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reduction in mercury to zinc ratio, for example, was 29.1%, which is in good agreement 

with the mercury-measured reduction.

Clearly, mercury was lost from the XCEM deposit after the sample was collected and 

analyzed by the XCEM. As discussed in the following subsection, the available data 

strongly suggests that a substantial portion of the M29 mercury was also lost from the 

M29 PM filter deposits prior to analysis.

Figure C.1 Comparison of XCEM X-Ray Spectra Measured During Method 29 

Testing and Seventy-One Days Later.

ii. Instability of 

M29 Mercury PM 

Deposit Highly 

Likely

Although there is no direct evidence of the loss of mercury from the M29 samples, there 

is ample indirect evidence that a similar loss to that of the XCEM deposit would be 

expected. This indirect evidence is discussed in the following three subsections.

c. Train Location of Mercury Deposits

The recovery and analysis of the M29 probe, filter, nitric acid impingers, sulfuric acid 

impingers and the hydrochloric acid rinse were kept separate for both the May 2001 and 

May 2002 M29 tests. The results are summarized and compared in Table C2. It is 

interesting to note that during the 2001 testing, only about 1% of the mercury was 

deposited on the quartz fiber filter. This is typical of most stack measurements of 

mercury, and as such there is little concern for the stability of the PM deposited on the 

filter. However, this is not the case for the 2002 mercury measurements. During these 

latter tests, 18% of the mercury was deposited on the filter during runs 1 through 5 with 

no lead in the mercury-nickel-zinc spiking solution. After adding lead to the spiking 

solution, the percent of mercury depositing on the filter increased to 29% for the 

remaining M29 runs. 

Table C2. Comparison of the Location in M29 Sampling Trains Where the Mercury 

Was Deposited During the 2001 and 2002 Tests.

0

2000

4000

6000

8000

10000

12000

14000

 

C

o

u

nts

XCEM 051402 XCEM 072502

Zinc

Mercury

Lead
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YEAR RUNS FILT. PROBE M2B M3A M3B M3C TOTAL

2001 1-13 1.1 0.1 93 0.4 0.3 5.2 100

2002 1-12 24.7 0.1 71 0.2 0.1 4.1 100

2002 1-5 18.2 0.2 77 0.3 0.1 4.7 100

2002 6-12 29.4 0.1 67 0.2 0.0 3.7 100

2001 1-13 6.0 0.2 414 2.2 1.0 26 450

2002 1-12 166 0.8 484 1.6 0.4 28 681

2002 1-5 129 1.1 551 1.9 0.7 34 718

2002 6-12 191 0.6 437 1.4 0.2 24 654

Percent on 

Filter

Mass on 

Filter (μg)

It is interesting to note that during the 2001 M29 tests, the reported M29 mercury

concentrations were 12% greater than the predicted concentrations; similar to the 18% 

(12% with corrected solution concentrations) measured this year with the XCEM. On the 

other hand, the 2002 M29 results are 6% less than the predicted concentrations rather 

than 12% greater like last year.

These results are certainly consistent with the hypothesis that M29 mercury results are 

low because there was a loss of mercury from the M29 quartz fiber filter between the 

time it was collected on the filter and the time it was analyzed. It is also consistent with 

the fact that many of the mercury compounds and amalgams of mercury are relatively 

unstable and have relatively high vapor pressures. Thus, it should not be surprising that 

if there is a substantial portion of the mercury on the filter as there was during the 2002 

tests, there might be a potential for significant loss due to volatilization. It also needs to 

be noted that the filters were not stored in a controlled environment from the time they

were collected until they were delivered to the analytical laboratory several days later. 

During this time, they may have been exposed to relatively high ambient temperatures 

while being transported through the western desert from Tooele, UT to California in the 

back of a closed panel truck.

d. Correlation with Percent Mercury on M29 Filter

The percent difference between the M29 mercury results and the XCEM results is 

significantly correlated with the percent mercury on the M29 filters as is illustrated in 

Figure C2. That is, the percent difference between the two methods (percent loss from 

the M29 filter) is dependent on the fraction of the total mercury measured in the M29 

train that is on the filter. This observation is consistent with the proposed hypothesis for 

the difference between the various methods, that is loss of mercury from the M29 filter. 

Figure C2. Percent Difference Between M29 and the XCEM vs. Percent of Mercury 

on the M29 Filter
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y = 1.25x - 4.3223

R

2

 = 0.8055

0

10

20

30

40

50

0 10 20 30 40

Percent of Hg on M29 Filter

Percent Difference Between

M29

a

n

d

X

C

E

M

ii. Precision

The difference in the mercury concentrations was not likely due to imprecision in either 

of the two measurements. Mercury was one of two elements spiked by MSE-TA which 

were clearly not present in the background stack emissions. Thus, the variability in the 

mercury concentration was due primarily to variability in the spiking and stack flow 

rates. The mercury precision as measured by the XCEM was 3.8% and 6.1% for M29, 

which includes the variability in the above two parameters as well as the measurement 

method variability.

iii. Mercury to Nickel Ratio

Nickel was another element that clearly was not present in the background, and it was in 

the same spiking solution as the mercury. Thus, the ratio of mercury should not vary

significantly since the concentrations for these two elements was kept constant for these 

latest tests. The XCEM measured mercury to nickel ratio was 1.37 ± 0.015 (1.1% 

relative) and close to both the M29 ratio, 1.41 ± 0.116 (8.3 % relative), and the predicted 

concentration ratio based on the measured solution concentration ratio of 1.33. It is 

interesting to note that the XCEM precision is significantly better than the M29 ratio 

precision, both of which should be relatively independent of factors other than the 

individual method. 

iv. Low Columbia Analytical Services Results

The XCEM deposit samples submitted to Columbia Analytical Services for CVAA 

analysis were extracted 41 days after they were analyzed by CES. Based on an assumed 

linear loss rate, the expected mercury concentrations at the time of extraction would be 

about 10% lower than measured by the CES XRF analyzer. The observed difference was 

14%.
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4. OTHER POSSIBLE EXPLANATIONS

OTHER FACTORS THAT COULD CONTRIBUTE TO DIFFERENCES IN 

XCEM AND M29 MERCURY MEASUREMENTS

1. XCEM MEASUREMENTS

a. Systematic Errors

i. Impacting All Elements

1. Volume 

a. Meter: Could contribute either a positive or 

negative artifact. Three NIST traceable 

meter/regulator compared well during test. 

Estimated uncertainty is 3% of the reported flow 

rate.

b. Volume Adjustment: The calculated volume was 

adjusted by a factor of 1.025 (increased by 2.5%) to 

compensate for incomplete drying of the air, which 

was determined in CES’s laboratory prior to 

shipping the XCEM. The uncertainty in this 

adjustment factor is estimated an ±0.020. Because 

of the actual conditions during the testing at the 

Army incinerator, it is thought that this adjustment 

is possibly too large for the actual incinerator 

conditions. Thus, it is possible that the recorded 

concentrations reported by the XCEM were high by 

1 to 2% due to this adjustment factor.

c. Leak: This source of error would contribute a 

negative artifact to the XCEM results. The XCEM 

results were greater than the M29 results. As such, 

this source of error could not explain the observed 

difference. In addition, the pressure drop across the 

filter is monitored during the testing and recorded in 

the system. No alarms were noted and none were 

indicated upon review of the data. This source of 

possible error is expected to contribute well less 

than 1% to the uncertainty in the flow measurement. 

2. Thin Film to Spot Calibration Factor

a.

3. Deposit Positioning (XYZ)

a. Direction of tape flow (X): The accuracy of tape 

movement in this direction is ±0.025 mm. The 
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XRF beam intensity is insensitive to deposit spot 

position in this direction to ± 1mm. The uncertainty

in concentration due to positioning error in this 

direction is expected to be substantially less than 

1%. This error would bias the results low because it 

would tend to place the deposit at a non-optimal 

position.

b. Perpendicular to tape flow direction and in the same 

plain (Y): The large axis of the X-ray beam is 

aligned in this direction. As such, the XCEM is less 

sensitive to deposit positioning in this direction. 

The alignment of the sampling plunger and the 

analysis beam spot are accurate to ±0.050 mm. The 

XRF beam intensity is insensitive to deposit spot 

position in the Y direction to ±1.5 mm. 

Misalignment of the tape has no impact on the 

intensity as long as the spot is on the tape. Thus, 

error in the alignment of the deposit and analysis 

position in this direction is expected to be 

insignificant; i.e., substantially less than 1%. This 

error would bias the results low because it would 

tend to place the deposit at a non-optimal position.

c. Height of tape above tube and detector (Z): 

Differences between the tape deposit and calibration 

standards position in the Z direction can contribute 

systematic error to all of the elements. It is not 

subject to error in the X and Y directions because 

the standards are larger than the analysis spot. The 

XCEM has been designed to position the tape 

deposit in the middle of the XRF analysis plain. In 

this position, the beam is relative insensitive to 

position in the Z direction at the ±0.1 mm range. 

The error due to differences in position of the tape 

deposits and the standards has been empirically

determined to be less than 0.5% at the 95% 

confidence level. In this case, the results could be 

biased either high or low.

d. Shape of deposit: Misalignment of the XCEM 

plunger and the supporting button can cause 

deviations from a circular deposit. In general, the 

XRF intensity is insensitive to deposit geometry so 

long as the spot is smaller than the beam diameter. 

However, because there are small variations in 

sensitivity within the beam area, error can be 

introduced due to variations in deposit geometry. It 

has been empirically determined that there is less 
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than a 3% difference in sensitivity for a deposit the 

size of the XCEM deposit, 6 mm diameter, and a 3 

mm diameter spot (one fourth the area). From 

observation of the small deviations of spot 

geometry for actual deposits during these tests, it is 

estimated that this source would contribute no more 

than about 1% uncertainty to the reported XCEM 

results. These deviations, however, would be such 

that it would bias the XCEM results high.

ii. Impacting One or More Elements

1. Transport

a. Hg

b. PM

c. Hg/Ni

2. Calibration Errors

a. Uncertainty in Standard Concentration

i. Stated Concentrations

ii. Sensitivity Curves

b.

3. Spectral Analysis

a. Energy calibration

4. Analyte Line Interferences

a. Zn and Pb on Hg

b. Ba on Cr

c. Pb on As

5. Background/Blank Interferences

a. Cr

6. aa

7.

b. Random Errors

i. Impacting All elements

1. Tape positioning

2. Tape Efficiency

ii. Impacting One of More Elements

1. Contamination

2. Counting Statistics

3. Signal to Noise

c. Quality Assurance and Control Measures During Tests

i. XCEM Stability: The stability of the XRF component of the 

XCEM was monitored with each analysis, using Pd peak intensity

from a Pd rod permanently positioned in the X-ray beam. This 

intensity varied from 95% to 105% of the mean during the M29 

tests compared. Counting statistics was responsible for most of 

this variability, and no drift trends were apparent. 

ii. Daily Zero and Span Checks: The XCEM has the capability for 

automated zero and span checks. These were not conducted during 
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this series of tests. Instead, measurements of NIST and Army 

standards were made at the beginning and end of each day of 

testing. 

iii. NIST and Army Standard Measurements

1. NIST standards (SRM 1832 and 1833) were analyzed daily

during the tests. They were usually run before and after 

each days runs. Recovery for Ti, V, Mn, Fe, Cu, Zn, and 

Pb were 100±2% of the NIST value. Thin film NIST 

standards for Cr, Ni, As, Cd, Sn, Sb, Ba and Hg are not 

available. Hg was calibrated using its relationship to Pb 

and other L-line standards, and its stability was checked 

using a HgAg standard, which gave replicate analysis 

results to within ±3%. 

2. The other non-NIST standard elements were checked with 

Army secondary standards calibrated at CES laboratories. 

Repeated measurements with these standards yielded 

recoveries of 100±3%.

iv. Energy Calibration: The XRF analyzer component of the XCEM 

was calibrated each morning to minimize impacts of small drifts in 

peak position. Errors associated with peak drift would be 

exhibited in standard replication, which was typically less than 

±3%. 

d. Summary of Potential XCEM Errors

e. Aa

f.

2. METHOD 29 MEASUREMENTS

a. M29 Train Replication

b. Velocity

c. Loss 

d. Contamination

e. Laboratory Analysis

f. Recovery Efficiency

3. AAA

5. CONCLUSION

The weight of evidence clearly indicates that the XCEM mercury deposit was unstable as 

well as the M29 PM deposit on the quartz fiber filter. This instability was most likely the 

cause of the difference between the mercury measured by these two methods and the 

other methods. It is highly likely that if these losses had not occurred, the XCEM would 

have passed the PS10 relative accuracy tests.
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It is recommended that in future M29 testing, the filters be immediately sealed and 

cooled to at least 0°C, stored at below freezing temperatures and digested as soon as 

possible after sampling.