Results of the ICP/MS Collision Cell Method Detection Limit Studies in the 

Synthetic Flue Gas Desulfurization Matrix

Prepared for

Marion Kelly and Lemuel Walker

U.S. Environmental Protection Agency

Office of Water

Office of Science and Technology

Engineering and Analysis Division (4303T)

1200 Pennsylvania Avenue, NW

Washington, DC 20460

Prepared by

CSC

6101 Stevenson Avenue

Alexandria, VA 22304

Prepared under

EPA Scientific and Technical Support Contract EP-C-12-008

January 16, 2013
Collision Cell MDL Study Results i 1/16/2013

Table of Contents

Background...................................................................................................................................................1

Page

Study Implementation...................................................................................................................................1

Derivation of Detection and Quantitation Limits..........................................................................................2

Pooled MDLs............................................................................................................................................3

Pooled MLs...............................................................................................................................................4

Summary of Study Results............................................................................................................................4

Synthetic FGD Matrix...............................................................................................................................4

Reagent Water...........................................................................................................................................4

CSC Recommendations................................................................................................................................5

References.....................................................................................................................................................5
Collision Cell MDL Study Results 1 1/16/2013

Results of the ICP/MS Collision Cell Method Detection Limit Studies in the 

Synthetic Flue Gas Desulfurization Matrix

Background

The U.S. Environmental Protection Agency (EPA) Engineering and Analysis Division (EAD), within the 

Office of Water (OW) has developed a draft procedure for the use of a collision cell accessory with an 

inductively coupled plasma and mass spectrometer (ICP/MS) in the analysis of effluent samples from flue 

gas desulfurization (FGD) processes at steam electric generating facilities (Reference 1). The collision 

cell accessory has the potential to reduce interferences in ICP/MS analyses from non-target analytes 

present in typical wastewaters from this industry, thus allowing more accurate and precise measurement 

of the concentrations of target analytes .

The focus of the present study was to characterize method detection limits (MDLs) and minimum levels 

(MLs) of quantitation for the target analytes in both reagent water and in a synthetic matrix that is 

designed to mimic the composition of FGD wastewaters. The target analytes for this study were arsenic, 

cadmium, chromium, lead, selenium and thallium.

The primary goals of this study were to:

• Obtain MDL data, from at least three laboratories, that reflect the capabilities of the draft EAD 

ICP/MS collision cell procedure in FGD wastewaters, and 

• Use the MDL data to derive an ML for each target analyte to support EAD’s efforts to regulate 

discharges from FGD systems. 

Secondary goals were to:

• Obtain MDL data from laboratories using ICP/MS instrumentation and collision cell accessories other 

than those used in the initial development of the procedure,

• Obtain laboratory feedback regarding the draft procedure itself, including information about 

modifications to the operating conditions that may be needed for specific ICP/MS instrumentation or 

collision cell accessories.

Study Implementation

EPA tasked CSC to assist in designing and implementing this study. CSC supported the effort by 

procuring a sufficient quantity of a synthetic matrix designed to mimic the composition of FGD 

wastewaters, procuring MDL study services from three qualified laboratories, and providing study 

coordination and implementation support. Details of the study are described in a formal study plan 

reviewed and approved by EAD (Reference 2).

CSC issued purchase orders to three laboratories (two commercial and one utility) to perform MDL 

studies as outlined in 40 CFR Part 136 Appendix B. EAD worked with an instrument vendor to obtain 

similar MDL data from the vendor’s in-house laboratory. After completion of the MDL studies by those 

four laboratories, CSC and EAD worked with the commercial laboratory that developed the collision cell 

procedure for EAD to obtain an additional set of MDL data in the FGD matrix.

The five laboratories were:

• Agriculture and Priority Pollutants Laboratories, Inc. (APPL) - commercial

• Hampton Roads Sanitation District (HRSD) - utility

• Southwest Research Institute (SwRI) - commercial

• Perkin Elmer (PE) - instrument vendor

• TestAmerica Laboratories - Denver - commercial, and developer of the draft procedure
Collision Cell MDL Study Results 2 1/16/2013

CSC provided each of the five laboratories with one 500-mL volume of the synthetic FGD matrix 

described in the study plan. CSC tracked each shipment of the solution, confirmed condition upon receipt 

by each laboratory, and used the receipt dates to establish due dates for the study results.

This synthetic matrix was prepared at twice the nominal concentrations of the constituents described in 

the draft collision cell procedure so that it could be used to support another EAD study of the collision 

cell instrumentation. Each laboratory participating in the MDL study diluted this solution by a factor of

two to create the reference matrix for its FGD MDL study aliquots. 

The first four laboratories provided their own reagent water for use as the clean reference matrix for their 

MDL study aliquots. The fifth laboratory was not tasked with performing an MDL study in reagent 

water; this laboratory only performed an MDL study in the FGD matrix.

CSC and EAD held a kick-off conference call with the first four laboratories on August 28, 2012. CSC

and EAD responded to several rounds of follow-up questions after the call. Answers to all questions from 

laboratory participants were routed to all laboratories via email as quickly as practical, in order to keep 

the study on schedule. CSC also worked with the standards vendor and all of the laboratories to refine the 

available information on the background levels of some of the target analytes that were found in the 

synthetic matrix, so that each laboratory could adjust the concentrations of these analytes spiked into the 

MDL study aliquots. CSC and EAD held a conference call with the fifth laboratory on November 3, 

2012, to discuss their participation and share the information about the background levels in the synthetic 

FGD matrix that had been provided to the other four laboratories earlier.

As data were delivered to CSC, the results were reviewed in two stages. The first stage was to review 

each laboratory’s electronic data deliverable (an Excel file) with the results for each target analyte in each 

matrix. CSC confirmed the standard deviations calculated by the laboratories for each analyte and the 

resulting MDL values. No calculation errors occurred.

The second stage of the review process involved a closer examination of the raw data and QC results 

supporting the MDL sample results. The results from each laboratory were reviewed against the QC 

specifications in the laboratory statement of work and the study plan. Because the draft collision cell 

procedure is still under development, some deviations from the draft procedure’s QC limits were 

anticipated. Examples include the QC limits for the internal standards employed in the procedure. The 

draft QC limits for samples specified internal standard responses of 60 - 125% of the responses in the 

calibration blank, and some responses were above and below those limits. No gross deviations occurred, 

and any minor deviations were noted, but were not a priori grounds for exclusion of the results from the 

study data set. The data from the MDL study samples may be used by EAD to refine the QC 

specifications for the final procedure at a later date.

The remainder of this document describes the results of the study, including CSC’s derivation of pooled 

MDLs and MLs for ICP/MS collision cell analyses of the synthetic FGD matrix and reagent water using 

the results from all the laboratories. CSC’s recommendations for the uses of the results are provided at 

the end of the document.

Derivation of Detection and Quantitation Limits

CSC combined the data from all the laboratories and calculated the following limits:

• the maximum MDL for each analyte

• the maximum ML for each analyte

• the pooled MDL for each analyte

• the pooled ML for each analyte.
Collision Cell MDL Study Results 3 1/16/2013

The maximum MDL was simply the highest MDL value for the analyte reported by any of the five 

laboratories in the study. CSC rounded the maximum MDLs to two significant figures (i.e., two decimal 

places for values less than 1, and one decimal place for values between 1 and 10).

The maximum ML is derived from that maximum MDL value by multiplying standard deviation of the 

results from that laboratory by 10 and rounding the result to the nearest value of 1, 2, or 5 times 10n

, 

where n is a positive or negative integer. This rounding approach is described in EAD’s formal definition 

of the ML (Reference 3) and is designed to facilitate preparation of calibration standards at practical 

concentrations. 

As noted above, all five laboratories provided MDL data for the FGD matrix, and the first four 

laboratories were tasked to provide MDL data for the reagent water matrix. The derivations of the pooled 

MDLs and pooled MLs are described in detail below.

Pooled MDLs

The pooled MDL for each of the six target analytes was calculated from the aggregated results from all 

four laboratories. Each laboratory prepared and analyzed seven MDL study aliquots of the synthetic FGD 

matrix and seven more aliquots of reagent water. Each laboratory reported the results for each aliquot, 

along with their calculated single-laboratory MDL value for each matrix.

For each matrix, CSC confirmed the reported standard deviations of the seven MDL replicates (in 

concentration units) for each analyte in each laboratory using the formula:

where:

s = standard deviation

xi = result for MDL aliquot i (in concentration units)

x� = the mean of all seven MDL aliquots

As noted above, there were no calculation errors. CSC then calculated the pooled within-laboratory 

standard deviation, sw, for each analyte and matrix using the formula:

where:

si = the standard deviation of the seven MDL replicates (in concentration units) for laboratory i, and

m = the number of laboratories (5 for the FGD matrix and 4 for reagent water)

The pooled MDL was calculated using the formula:

where:

tm = the 99th percentile t-statistic for the total number of MDL replicates combined across all

laboratories, minus the number of laboratories. For the FGD matrix, with five laboratories, 

tm = 2.457, and for the reagent water matrix, with four laboratories, tm = 2.492.

The pooled MDL values were rounded to two significant figures for the purposes of this report.

n 1

(x x)

s

n

i 1

2

i

−

−

=

∑=

m

s

m

i 1

2

i

w

∑s = =

pooled w m MDL = s x t
Collision Cell MDL Study Results 4 1/16/2013

Pooled MLs

The pooled ML was derived by multiplying the pooled within-laboratory standard deviation, sw, from the 

pooled MDL calculation, by 10: 

The pooled ML was then rounded to the number nearest value of 1, 2 or 5 times 10n

, where n is a positive 

or negative integer.

Summary of Study Results

The tables below summarize the results of the method detection limit (MDL) studies conducted in the 

synthetic flue gas desulfurization (FGD) matrix and reagent water.

Synthetic FGD Matrix

The values for the current procedure MDL are those that TestAmerica determined in 2009, using an 

earlier version of the collision cell procedure and a different version of the synthetic FGD matrix; these 

values are included above simply for reference. TestAmerica did not determine the MDL for chromium 

at that time because of problems with the background concentration of this metal in the FGD matrix used 

at that time.

Reagent Water

As part of the overall study design, the first four laboratories also determined MDLs in reagent water. 

The goal of that effort was to illustrate the capabilities of the collision cell procedure in a clean reference 

matrix with no interferences. The table below summarizes the results from those four laboratories. In all 

instances, the reagent water values are below the corresponding FGD values, illustrating the potential 

effects of the FGD matrix interferences on method sensitivity. Because the current draft procedure 

document does not include MDLs determined in reagent water, that column has been omitted.

Summary of Method Detection Limits and Minimum Levels in the

Synthetic Flue Gas Desulfurization Matrix (in μg/L, 5 labs)

Matrix Analyte

Maximum 

MDL

Maximum 

ML Pooled MDL

Pooled 

ML

Current Procedure 

MDL

FGD Arsenic 0.80 2 0.35 1 0.61

FGD Cadmium 0.19 0.5 0.08 0.2 0.59

FGD Chromium 0.32 1 0.13 0.5 NA

FGD Lead 0.31 1 0.14 0.5 0.36

FGD Selenium 1.1 5 0.44 2 0.31

FGD Thallium 0.07 0.2 0.04 0.2 0.23

Summary of Method Detection Limits and Minimum Levels in Reagent Water (in μg/L, 4 labs)

Matrix Analyte Maximum MDL Maximum ML Pooled MDL Pooled ML

RW Arsenic 0.07 0.2 0.04 0.2

RW Cadmium 0.02 0.1 0.01 0.05

RW Chromium 0.05 0.2 0.03 0.1

RW Lead 0.13 0.5 0.05 0.2

RW Selenium 0.8 2 0.32 1

RW Thallium 0.02 0.05 0.01 0.05

ML s x10 pooled = w
Collision Cell MDL Study Results 5 1/16/2013

CSC Recommendations

The primary goals of this study were to obtain MDL data from at least three laboratories that reflect the 

capabilities of the draft EAD ICP/MS collision cell procedure in FGD wastewaters, and use those MDL 

data to derive an ML for each target analyte.

CSC calculated both pooled and maximum values for the MDLs and MLs for the target analytes. The

pooled MDL and ML values provide a more accurate estimate of method sensitivity than the maximum 

values. By their very nature, the pooled limits are somewhat like an average of what was observed in the 

study. Some laboratories in this MDL study achieved MDLs above the pooled value and some 

laboratories achieved MDLs below the pooled value. Results from laboratories with ICP/MS collision 

cell experience using the procedure in the future would be expected to follow the same pattern; some 

laboratories with some laboratories achieving MDLs above the pooled values and others below the pooled 

values.

For the purpose of developing effluent limitations guidelines and standards for discharges of FGD 

wastewater, CSC recommends that EAD use the maximum MDL and maximum ML values for the FGD 

matrix. The maximum values are the “worst-case” results for each of the target analytes from the MDL 

study, meaning that the other laboratories in the study were able to achieve better sensitivity. The data 

from this study illustrates that all of the laboratories in this study (which were all identified as welloperated laboratories with experienced staff and operating the appropriate analytical instrumentation) 

could achieve MDL values at least that low, and the maximum ML values represent the corresponding 

quantitation limits from the study.

EAD’s secondary goals for this MDL study and the related method performance study being conducted 

separately by TestAmerica include gathering data that may be used to revise and improve the collision 

cell procedure itself. To address these secondary goals, CSC recommends that EAD include the pooled 

MDL and pooled ML values for both the FGD and reagent water matrices in the revised procedure, along 

with the maximum MDL and maximum ML values for the FGD matrix. Supporting text in the procedure 

and the tables can explain that the pooled values are indicative of the MDLs and MLs that may be 

achieved using the procedure, based on a study involving five laboratories. 

References

1. Draft FGD ICP/MS Collision Cell Procedure for Trace Element Analysis in Flue Gas Desulfurization 

Wastewaters, June 2012.

2. Study Plan for Developing Method Detection Limits and Minimum Levels of Quantitation for Selected 

Analytes Using EAD’s Draft Collision Cell ICP/MS Procedure, July 24, 2012.

3. Federal Register, Guidelines Establishing Test Procedures for the Analysis of Pollutants; 

Measurement of Mercury in Water (EPA Method 1631, Revision B); Final Rule, 64 FR 30417, 

June 8, 1999.