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 

October 18, 2012 
Collision Cell MDL Study Results i 10/18/2012 

Table of Contents 

Page

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

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

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

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

Pooled MLs ............................................................................................................................................... 3 

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

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

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

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

References ..................................................................................................................................................... 5 
Collision Cell MDL Study Results 1 10/18/2012 

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 the analytes EPA expects to be subject to regulation. 

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. The four 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 

CSC provided each of the four 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. 
Collision Cell MDL Study Results 2 10/18/2012 

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. Each laboratory provided its own 

reagent water for use as the clean reference matrix for its MDL study aliquots. 

CSC and EAD held a kick-off conference call with all of the 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. 

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

electronic data deliverable (an Excel file) from each laboratory 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 four 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 four 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. 

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

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. 
Collision Cell MDL Study Results 3 10/18/2012 

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) 

xത = 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 (4 in this study) 

The pooled MDL was calculated using the formula: 

where: 

2.49 = the 99th percentile t-statistic with 24 degrees of freedom (i.e., the total number of MDL replicates 

combined across all laboratories, minus the number of laboratories). 

The pooled MDL values were rounded to two significant figures. 

Pooled MLs 

The pooled ML was derived from the pooled MDL by multiplying sw, 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. 

MLpooled  sw x10

n 1

(x x)

s

n

i 1

2

i









m

s

m

i 1

2

i

w

s  

MDLpooled  sw x 2.49
Collision Cell MDL Study Results 4 10/18/2012 

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 

In some instances, the ML rounds down to 1, 2, or 5, and in other cases, it rounds up. For example the 

cadmium MDL of 0.11 would yield an unrounded ML value of 0.3368, which rounds down to 0.2, rather 

than up to 0.5, because it is below 0.35, which is the mid-point between 0.2 and 0.5. 

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 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 all 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) 

Matrix Analyte 

Maximum 

MDL 

Maximum 

ML 

Pooled 

MDL 

Pooled 

ML 

Current 

Procedure MDL 

FGD Arsenic 0.52 2 0.24 1 0.61 

FGD Cadmium 0.11 0.2 0.05 0.2 0.59 

FGD Chromium 0.17 0.5 0.08 0.2 NA 

FGD Lead 0.31 1 0.13 0.5 0.36 

FGD Selenium 1.1 5 0.49 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) 

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 
Collision Cell MDL Study Results 5 10/18/2012 

CSC Recommendations 

As noted at the outset, 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. 

Both of these primary goals were achieved. 

CSC recommends that EAD use the maximum MDL and maximum ML values for the FGD matrix to 

support the regulatory effort. The maximum values are the “worst-case” results from the study, and the 

other laboratories in the study were able to achieve better sensitivity. The data from this study illustrates 

that all of the laboratories could achieve MDL values at least that low, and the maximum ML values 

represent the corresponding quantitation limits from the study. For arsenic and selenium, the metals of 

greatest concern to EAD, those maximum values are: 

Analyte Maximum MDL (μg/L) Maximum ML (μg/L) 

Arsenic 0.52 2 

Selenium 1.1 5 

However, EAD’s secondary goals for this 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. Such revisions are likely to include adding example performance data to 

the tables at the end of the procedure document; such performance data may be used by laboratories and 

others in evaluating the utility of the procedure for various aqueous matrix types. 

Given those 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. 

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

laboratories had MDLs above the pooled value and some laboratories had 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. Their in-house MDLs might be above or below the pooled 

values. However, as long as the MDLs were adequate to support whatever decision was to be made by 

using the procedure, that laboratory’s performance of the method would be considered acceptable. 

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

maximum values. 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 

four laboratories. The text would further clarify that the example limits are not requirements that a given 

laboratory must meet, or that the example limits are to be used as QC limits. 

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.