Performance Specification XX: 

Performance Specifications for a Quantitative 

Reference Aerosol Generator 

June 9, 2005

DRAFT 

Submitted to: 

The Environmental Protection Agency 

Research Triangle Park, NC 

Prepared by: 

Cooper Environmental Services, LLC 

10180 SW Nimbus Blvd Suite J6 

Portland, Oregon 97223
Performance Specification XX: 

Performance Specifications for a Quantitative 

Reference Aerosol Generator 

June 9, 2005

DRAFT 

Submitted to: 

The Environmental Protection Agency 

Research Triangle Park, NC 

Prepared by: 

C. A. Yanca, D.C. Barth, J.A. Cooper, 

B.E. Johnsen, M.P. Nakanishi, K.A. Petterson, 

Cooper Environmental Services 

10180 SW Nimbus Blvd Suite J6 

Portland, Oregon 97223 
Performance Specification XX: 

Performance Specifications for a Quantitative 

Reference Aerosol Generator 

Prepared by:___________________________________ Date:_________

 Catherine A. Yanca, Environmental Scientist 

Reviewed by: ___________________________________ Date:_________ 

 Krag A. Petterson, Environmental Scientist 

Reviewed by: ___________________________________ Date:_________ 

 Bruce E. Johnson, QA Manger 

Approved by: ___________________________________ Date:________ 

 John A. Cooper, Director 
Performance Specifications for a QRAG: V1.0 Page i of iii 

Executive Summary 

This performance specification describes the criteria that a Quantitative Reference Aerosol 

Generator (QRAG) must meet to be considered valid for EPA compliance and/or other 

regulatory applications. A QRAG should be capable of generating an aerosol containing an 

analyte whose concentration is traceable to the National Institute of Standards and 

Technology (NIST) or other equivalent reference. It quantitatively combines solids and/or 

liquids with a carrier gas to form a stable flow of aerosol with known reference 

concentration. Before it can be used with regulatory applications, it must be certified 

according to the requirements of PS-XX. To be certified, you are required to do the following 

on an annual basis: 

• You must ensure that all key components of the QRAG and the analyte standard are 

process traceable to NIST or other equivalent reference. 

• You must test the stability of the QRAG, which is measured by the percent relative 

standard deviation (PRSD) over a 12-hour test period. The PRSD must be less than 

5%. 

• You must determine the concentration of the QRAG’s generated aerosol with a 

reference method that measures the concentration of the emissions over the proposed 

applicable concentration range. The generated aerosol concentration and the 

reference method measured concentration must have a correlation coefficient (r)

greater than 0.95, a slope equal to 1.00 ± 0.15, and an intercept less than 1/10th of the 

applicable regulated concentration. 
Performance Specifications for a QRAG: V1.0 Page ii of iii 

Table of Contents 

Executive Summary................................................................................................................. i 

List of Symbols .......................................................................................................................iii 

1.0 Using PS-XX................................................................................................................... 1 

1.1 What is the purpose and applicability of Performance Specification XX? ............... 1 

1.2 What is PS-XX?......................................................................................................... 1 

1.3 What special definitions apply to PS-XX? ................................................................ 2 

2.0 Interferences, Safety, and Equipment Requirements................................................. 3 

2.1 Are there any potential interferences for my QRAG? ............................................... 3 

2.2 What do I need to know to ensure the safety of persons using PS-XX? ................... 3 

2.3 What equipment and supplies do I need? .................................................................. 3 

2.4 What reagents and standards do I need?.................................................................... 4 

3.0 Performance Evaluation and Documentation Procedures......................................... 5 

3.1 What pre-test performance evaluation procedures do I follow?................................ 5 

3.2 How do I monitor and document my QRAG’s performance?................................... 5 

3.3 What concentration range must my linearity tests span?........................................... 5 

4.0 Performance Criteria and Calculations....................................................................... 6 

4.1 What performance criteria must my QRAG meet? ................................................... 6 

4.2 What calculations and data analyses are required?.................................................... 6 

5.0 Other Requirements and Information......................................................................... 9 

5.1 What quality control and assurance procedures are required by PS-XX?................. 9 

5.2 What standardization procedures must I use?............................................................ 9 

5.3 What pollution prevention procedures must I follow? .............................................. 9 

5.4 What waste management procedures are required by PS-XX? ................................. 9 

5.5 What references apply to PS-XX?............................................................................. 9 

 
Performance Specifications for a QRAG: V1.0 Page iii of iii 

List of Symbols 

b0 = The y intercept 

1 b = Slope 

PRSD = Percent relative standard deviation (given as a percent) 

r = The correlation coefficient 

SD = Standard deviation 

x = Data set for the QRAG generated aerosol concentrations

i x = Value of the ith data point for variable x

x = Average of the values in data set x

y = Data set for the reference method aerosol concentrations 

i y = Value of the ith data point for variable y

y = Average of the set of values of variable y
Performance Specifications for a QRAG: V1.0 Page 1 of 9 

1.0 Using PS-XX 

1.1 What is the purpose and applicability of Performance Specification XX? 

The purpose of Performance Specification XX (PS-XX) is to establish the initial performance 

requirements for evaluating the acceptability of a quantitative reference aerosol generator 

(QRAG) prior to using the aerosol it generates as a reference for compliance or other 

regulatory applications. Such applications may include, (1) instrument, method or procedure 

calibration, (2) evaluation or certification for initial acceptance, and (3) continuing quality 

assurance audits or other applications. 

1.1.1 Application 

PS-XX applies to you if you will be using a QRAG to validate an instrument, method 

or procedure that will be used in a compliance or other regulatory application. 

1.1.2 Compliance 

You must comply with PS-XX prior to using any equipment that depends on the 

QRAG for calibration and/or validation. 

1.1.3 Other Data 

All components of the QRAG must be certified as NIST-traceable or equivalent; you 

must obtain the necessary documents of NIST or equivalent certification before using 

the QRAG. 

1.2 What is PS-XX? 

This performance specification describes the criteria that a QRAG must meet to be 

considered valid for use in a compliance or other regulatory application. It should be capable 

of generating an aerosol of known analyte concentration. A QRAG combines solids and/or 

liquids with a carrier gas to form an aerosol stream with a NIST-traceable or equivalent 

analyte concentration that is stable over extended periods. 

1.2.1 Basic Requirements 

You must have your QRAG certified according to the requirements of PS-XX before 

its aerosol emissions can be used in a compliance or other regulatory application. To 

be certified, you are required to do the following on an annual basis: 

• You must ensure that all key components of the QRAG and standards are 

process traceable to NIST (Subsection 4.1.1). 

• You must test the stability of the QRAG, which is measured by the percent 

relative standard deviation (PRSD) of a 12-hour test run. The PRSD must be 

less than 5% (Subsection 4.1.2). 
Performance Specifications for a QRAG: V1.0 Page 2 of 9 

• You must determine the concentration of the QRAG’s generated aerosol with a 

reference method over a range of concentrations that exceeds the range over 

which it will be used (Subsection 3.3). The generated aerosol concentration 

and the reference method measured concentration must have a correlation 

coefficient (r) greater than 0.95, a slope equal to 1.00 ± 0.15, and an intercept 

less than 1/10th of the applicable regulated or compliance concentration 

(Subsection 4.1.3). 

1.3 What special definitions apply to PS-XX? 

Aerosol: A suspension of solids and/or liquids in a gas. 

Analyte: A species (element, metal, etc.) of interest in an analytical method. 

Analytically NIST Traceable Parameter: A parameter whose value is traceable to NIST 

through a measurement that is NIST traceable1

. 

Applicable Concentration Range: The range of concentrations over which a QRAG must 

be able to produce an aerosol of NIST-traceable or equivalent analyte concentration to be 

used for a given application. 

Certified Concentration Range: The range of concentrations for which the QRAG is 

certified. 

Key Component: A piece of equipment or instrument that measures any parameter used to 

determine the generated aerosol concentration. 

Linearity: The degree to which two parameters are linearly related as determined by the 

value of the slope and correlation coefficient of a least squares regression analysis. 

NIST: The National Institute of Standards and Technology, Gaithersburg, MD. 

Particulate: A particle of solid or liquid matter. 

Process NIST Traceable: A parameter whose value is traceable to NIST through a 

procedure, the steps of which are each NIST traceable1

. 

Quantitative Reference Aerosol Generator (QRAG): An aerosol generator which emits an 

aerosol of known analyte concentration that is NIST or equivalent traceable. 

Sampling System: A collection of integrated components designed to collect a portion of an 

aerosol that accurately represents an analyte concentration. 

Stability: Percent relative standard deviation of a value measured over an extended period of 

time. 

Standard: Something established by authority, custom, or agreement for use as a rule or 

basis for comparison in measurement. 
Performance Specifications for a QRAG: V1.0 Page 3 of 9 

Standard deviation: A parameter that indicates the way in which a probability function or a 

probability density function is centered around its mean and that is equal to the square root of 

the moment in which the deviation from the mean is squared. 

Traceability: A property of a result of a measurement or the value of a standard whereby it 

can be related to stated references, usually national or international standards, through an 

unbroken chain of comparisons all having stated uncertainties2

. 

Traceability to NIST: A documented procedure by which a measured response is related to 

a standard with an accuracy defined by and certified by the National Institute of Standards 

Technology (NIST). 

Transport/Interface Component: That portion of a test arrangement that connects a 

QRAG to the point where the aerosol will be evaluated. 

Uncertainty: An indication of the degree of reliability of a value usually expressed 

statistically (probability) as a multiple of the standard deviation of a set of measurements. 

Unless otherwise stated, the uncertainty in a mean value of a series of measurements will be 

the standard deviation. In the case of values based on calculations, the uncertainty in the 

value is equal to the propagated uncertainty based on the uncertainties in the parameters used 

in the calculations. 

2.0 Interferences, Safety, and Equipment Requirements 

2.1 Are there any potential interferences for my QRAG? 

There are no interferences in this procedure. 

2.2 What do I need to know to ensure the safety of persons using PS-XX? 

People using PS-XX procedures may be exposed to hazardous materials, operations, and 

equipment. This performance specification may not address all of the safety problems 

associated with its use. It is your responsibility to establish appropriate safety and health 

practices and determine the applicability of regulatory limitations prior to performing these 

procedures. You must consult your QRAG manual and adhere to its specific safety 

precautions. 

2.3 What equipment and supplies do I need? 

Your QRAG must be able to continuously emit an aerosol of predictable concentration. You 

should select a QRAG and a transport system that is appropriate for your specific application. 

The PS-XX requires that you include, at minimum, the equipment described in subsections 

2.3.1-2.3.2. 
Performance Specifications for a QRAG: V1.0 Page 4 of 9 

2.3.1 The QRAG System 

A QRAG consists of the following five major components: 

• Analyte Control: This component quantitatively transports the analyte and/or 

analyte containing carrier to the point of aerosolization, controls the conditions 

within which it is delivered, and determines the rate at which it is delivered. 

• Gas Control: This component quantitatively transports the aerosolizing and/or 

carrier gas to the point of aerosolization, controls the conditions within which it 

is delivered, and determines the rate at which it is delivered 

• Aerosolization: This component combines the analyte and/or analyte carrier 

with the aerosolizing gas to form a particulate and gas mixture or aerosol under 

quantitatively controlled conditions. 

• Particulate Size Separator: This component separates small particulates more 

likely to have high transport efficiency from larger particles. The separator 

quantitatively recycles the large particulates that are removed from the aerosol 

and controls the conditions under which this process takes place. 

• Total Flow Control: This component controls the total flow rate of the aerosol 

emerging from the particulate size separator component and controls the 

conditions under which the final QRAG aerosol is emitted. 

2.3.2 Reference Measurement System 

For the purpose of certifying the QRAG emissions, you must use either an EPA

method or an approved alternative or conditional method. It must be capable of 

measuring the concentration of analyte in the QRAG emissions over the certified 

concentration range and test period with an accuracy and precision adequate to evaluate 

the accuracy and precision of the candidate QRAG. 

2.4 What reagents and standards do I need? 

2.4.1 Analyte Standard 

The QRAG generates an aerosol with NIST-traceable or equivalent accuracy based on 

the accuracy of the analyte-containing standard. An adequate supply of standard 

required by a test plan should be prepared prior to testing. The primary requirements 

of these standards are that (1) the accuracy of the analyte concentrations be NISTtraceable (2) the standards be stable over the period of their use. 

2.4.2 Gas 

You must select a gas to use for creating the analyte aerosol. The chosen gas must 

not react with the analyte or with any component in the aerosol that would generate 

either analyte or analyte interferants. 
Performance Specifications for a QRAG: V1.0 Page 5 of 9 

3.0 Performance Evaluation and Documentation Procedures 

3.1 What pre-test performance evaluation procedures do I follow? 

3.1.1 Installation 

You must install your QRAG according to the manufacturer’s written instructions and 

in a manner that allots an appropriate amount of space for the QRAG to be connected 

to the reference method measurement system. 

3.1.2 Operation 

You must operate your QRAG according to the manufacturer’s written instructions 

and specifications. 

3.2 How do I monitor and document my QRAG’s performance? 

You must monitor you QRAG’s performance according to the instructions written by the 

manufacturer, with special attention given to safety precautions. You must also document 

the QRAG’s performance in accordance with the manufacturer’s instructions. 

3.3 What concentration range must my linearity tests span? 

To satisfy the linearity criteria, your tests must span the full applicable concentration range 

by meeting the following requirements: 

• 25% of the data points must be within the upper 66% of the applicable concentration 

range 

• 35% of the data points must be between 33% and 66% of the applicable concentration 

range. 

• 15% of the data points must be within bounds of the lower 33% of the applicable 

concentration range 

You must run linearity tests at a minimum of 4 different concentrations, including a blank. 

You must ensure that aerosol concentrations required for testing are within (1) the detectable 

concentration range of the equipment to be calibrated or evaluated and (2) the QRAG’s 

minimum and maximum limits as determined by the manufacturer. Minimum of 12 data 

points 
Performance Specifications for a QRAG: V1.0 Page 6 of 9 

4.0 Performance Criteria and Calculations 

4.1 What performance criteria must my QRAG meet? 

Your QRAG must meet the following performance criteria to be certified as a quantitative 

reference aerosol generator. You must demonstrate that your QRAG meets these criteria on 

an annual basis at a minimum. 

4.1.1 NIST-Traceability 

You must ensure that the generated aerosol concentration of the QRAG is process 

traceable to NIST or equivalent. This process traceability is established by 

confirming the traceability of the QRAG’s parameters1,2. You must obtain 

documentation demonstrating that the QRAG’s key components have NIST-traceable 

or equivalent certification and that the certification is current. But changes, bumps 

needs to be validated before use. Dan is concerned that in this early stages of use, 

there is significant room for developing leaks and other problems between when it 

was last certified and when it might be used. He feels for a while we need to 

demonstrate that it is working prior to its use. This isn’t too much of a problem if we 

use the XFM to certify it prior to use. E. G. install the QRAG and start generating an 

aerosol. Then collect a XFM sample or series of samples and fedx them back to the 

lab for analysis.

4.1.2 Stability Criteria 

You must determine the percent relative standard deviation (PRSD) of the analyte 

concentration in the QRAG aerosol over a 12-hour tests period. The PRSD must be 

less than 5% (see also Section 4.2.3). Specify minimum # of measurements need 

instructions on how to do the tests and how many data points do we need.

4.1.3 Linearity Criteria 

You must demonstrate that a linear relationship exists between the QRAG generated 

aerosol analyte concentration and the reference method measured concentration. This 

relationship must meet the following criteria (see also Section 4.2.4):

• The correlation coefficient (r) must be greater than 0.95. 

• The slope must be equal to 1.00 ± 0.15%. 

• The intercept must be less than 1/10th the applicable compliance or other 

regulated concentration. Dan was somewhat confused with this section and how it 

related to bias, etc.

4.2 What calculations and data analyses are required? 

4.2.1 Arithmetic Mean 

Calculate the arithmetic mean of a data set as follows.
Performance Specifications for a QRAG: V1.0 Page 7 of 9 

= ∑ i x

n

1

x

 Equation 1 

Where: 

x = Arithmetic mean 

n = Number of data points 

xi = Value of each data point 

4.2.2 Standard Deviation 

n 1

( x x ) SD

2

i

−

∑ − = Equation 2 

Where: 

SD = Standard deviation 

x = Arithmetic mean 

n = Number of data points 

xi = Value of each data point 

4.2.3 Stability 

To measure the stability of your QRAG, you must conduct a 12-hour test run using a 

reference or other EPA approved method to measure the analyte concentration in the 

emitted aerosol. You must take a minimum of 6 measurements during this time period. 

Calculate the PRSD using the following equation: 

*100%

x

SD PRSD ⎟

⎠

⎞ ⎜

⎝

⎛ = Equation 3 

Where: 

PRSD = Percent relative standard deviation 

SD = Standard deviation 

x = Arithmetic mean 

4.2.4 Linearity 

To determine the linearity of your QRAG, you must measure the concentration of its 

aerosol emissions with the reference or other EPA approved method over the 

applicable concentration range (Section 3.3). You must test the QRAG at the 
8 

following aerosol concentrations at a minimum: one concentration must be at or above 

the upper end of the concentration range, two concentrations must be between this 

concentration and a blank, and one test must be a blank. You must calculate the 

QRAG’s generated aerosol concentration using equations defined by the manufacturer. 

You must determine the slope, correlation coefficient, and the intercept using a least 

squares linear regression of the aerosol concentration as measured by the reference or 

other EPA approved method and the QRAG reference aerosol analyte concentration. 

The relationship must be in the form of a straight line as follows: 

1 0 y = b x + b Equation 4 

Where: 

y = reference method measured aerosol concentrations 

x = predicted generated aerosol concentrations 

b0 = y intercept 

1 b = slope 

To calculate the slope, b1: 

( )( )

∑( )

∑ −

− − = 2

i

i 1

1

x x

x x y y b Equation 5 

Where: 

i x = value of the ith data point for data set x

x = average of the values in data set x

i y = value of the ith data point for set of data y

y = average of the values in data set y

To calculate the intercept, b0: 

b y b x 0 = − 1 Equation 6 

In addition to determining the slope and intercept of the linear relationship, you must 

calculate the correlation coefficient, r , according to the following equation:

( )( )

∑( )∑( )

∑

− −

− − = x x y y

x x y y

r

i 1

i 1 Equation 7 
Performance Specifications for a QRAG: V1.0 Page 9 of 9 

4.2.5 Uncertainty 

It is also recommended that you report the uncertainty of the QRAG’s generated aerosol 

concentration. This uncertainty should be determined using standard propagation of error 

techniques3

 relevant to the parameters used to calculate aerosol analyte concentration. The 

uncertainty values for the parameters are those listed as certified uncertainties by the 

manufacturers of the key components and/or NIST or equivalent. This procedure should 

be consistent with those used by NIST4,5 ,6. How is this used? Is it confusing? A 

distinction needs to be made between propagated uncertainty, measured error and how 

each is used and for example type of illustrations. Do we need a sample calculation or a 

discussion of how this uncertainty is calculated? Should we make the analogy with the 

uncertainty associated with a cal gas? 

5.0 Other Requirements and Information 

5.1 What quality control and assurance procedures are required by PS-XX? 

You must ensure that the QRAG components are NIST-traceable after installation and before 

using the generated reference aerosol in a compliance or other regulated application. You must 

obtain valid NIST certifications for all key components and standards as well as document all 

equipment calibrations with NIST standards. You must also ensure that the QRAG is 

functioning according the manufacturer’s specifications prior to aerosol generation 

5.2 What standardization procedures must I use? 

The QRAG generated aerosol analyte concentration is based on parameters such as solution 

concentration, gas flow, rate of solution mass loss, and rate of vapor loss. You must document 

and demonstrate that each of the parameters affecting the final concentration of aerosol 

emissions is process traceable to a NIST standard and certification of the standard is current. 

5.3 What pollution prevention procedures must I follow? 

The excess aerosol generated by the QRAG should be trapped in a filter and disposed of 

according to appropriate regulations. Unused hazardous supplies should also be disposed of 

according to regulation. 

5.4 What waste management procedures are required by PS-XX? 

Any hazardous materials used should be handled according to their Material Safety Data Sheets. 

5.5 What references apply to PS-XX? 

1. Scott Specialty Gases. Scott Gas Mixtures Data: Gas and Liquid Mixture Basics. 

Plumsteadville, PA: Scott Specialty Gases; 2004. 
Performance Specifications for a QRAG: V1.0 Page 10 of 9 

2. Bureau International des Poids et Mesures, International Electrotechnical Commission, 

International Federation of Clinical Chemistry and Laboratory Medicine, International 

Organization for Standardization, International Union of Pure and Applied Chemistry 

International Union of Pure and Applied Physics, International Organization of Legal 

Metrology [BIPM, IEC, IFCC, ISO, IUPAC, IUPAP, OIML]. International Vocabulary 

of Basic and General Terms in Metrology (VIM). Geneva, Switzerland: BIPM, IEC, 

IFCC, ISO, IUPAC, IUPAP, OIML; 1993. Available from: ISO, Geneva, Switzerland; 

ISBN 92-67-01075-1. 

3. McClave, J.T., F.H. Dietrich, II, and T. Sincich. Statistics, 7th Ed. Upper Saddle River, 

NJ: Prentice Hall; 1997. 

4. Scace, G.E. and J.T. Hodges. Uncertainty of the NIST Low Frost-Point Humidity 

Generator. Gaithersburg, MD: National Institute of Standards and Technology; 2004. 

5. American National Standards Institute/National Conference of Standards Laboratories 

[ANSI/NCSL]. U.S. Guide to the Expression of Uncertainty in Measurement.

Washington, D.C: ANSI/NCSL; 1997. Available from: NCSL International, Washington, 

D.C; Z540-2-1996. 

6. Taylor, B.N. and C.E. Kuyatt. Guidelines for Evaluating and Expressing the Uncertainty 

of NIST Measurement Results. Gaithersburg, MD: National Institute of Standards and 

Technology [NIST]; 1994. Available from: NIST, Gaithersburg, MD; TN 1297.