PM Cation Analysis 

Revision 7 

Date: August 25, 2009 

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Standard Operating Procedure for
 

PM2.5 Cation Analysis
 

Environmental and Industrial Sciences Division
 

RTI Intemational*
 

Research Triangle Park, North Carolina
 

Prepared by: Date: ¥/~/oq

Approved by:----H4-h''---W'--It~'-':::....:-f-~''-''-'-'=--f__--

Reviewed by :-~"",¥,-~..=::....:...--."--....,..e..-;;I-::"":,,,::::-r--- Date: ~ftt;P,T / 7 

Ii1RTI 

INTERNATIONAL 

* RTI International is a trade name of Research Triangle Institute. 
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Contents 

Section Page 

1.0 Procedural Section ...............................................................................................................3 

1.1 Purpose and Applicability........................................................................................3 

1.2 Summary of Method ................................................................................................3 

1.3 Health and Safety Warnings ....................................................................................4 

1.4 Cautions ...................................................................................................................4 

1.5 Interferences.............................................................................................................4

1.6 Personnel Qualifications ..........................................................................................4 

1.7 Apparatus and Materials ..........................................................................................4 

1.8 Ion Chromatography Reagents ................................................................................5 

1.9 Calibration Standards...............................................................................................6 

1.10 Quality Control Solutions ........................................................................................6 

1.11 Quality Assurance Solutions....................................................................................7 

1.12 Sample Collection....................................................................................................8 

1.13 Sample Handling......................................................................................................8 

1.14 Filter Extraction Procedure......................................................................................8 

1.15 IC Procedure ..........................................................................................................10 

1.16 Calculations and Data Reduction...........................................................................11 

2.0 Quality Control and Quality Assurance.............................................................................11 

3.0 Reference ...........................................................................................................................11 

Tables 

Table Page 

1. Accuracy Values for QA/QC Samples and Spiked Pm2.5 Filter Extracts ........................ 3 

2. Precision Values for PM2.5 Filter Extracts, Extracting Solution, and QC Samples......... 4 

3. Configurations of Ion Chromatographs Used for Cation Analysis.................................. 5 

4. Preparation of Cation Calibration Standards ................................................................... 7 

5. Preparation of Anion Quality Control Samples............................................................... 8 

6. Preparation of Anion Quality Assurance Samples .......................................................... 8 
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Standard Operating Procedure for PM2.5 Cation Analysis 

1.0 Procedural Section 

1.1 Purpose and Applicability 

This document outlines procedures for the extraction and subsequent determination of 

ammonium, sodium, and potassium ions in filter extracts. Analytical procedures outlined are 

specific to the ion chromatographs used in RTI’s ion analysis laboratories.

1.2 Summary of Method 

Cations in solution are separated when passed through a surface-sulfonated ion-exchange resin 

due to the differing affinities of the cations for the active sites on the resin. After separation, the 

cations pass through a suppressor column, which exchanges all anions for OH- ions. Species are 

detected and quantified as their hydroxides by a conductivity meter. The eluent is sulfuric acid, 

which yields deionized water when passed through the suppressor column. 

In a laboratory evaluation of the accuracy of the method, spiked PM2.5 filter extracts and quality 

assurance/quality control (QA/QC) samples were analyzed for sodium, ammonium, and 

potassium ions. The accuracy (expressed as % recovery) achieved using the subject method is 

presented in Table 1. 

To test the precision of the method, PM2.5 filter extracts were analyzed in duplicate, and the 

blank extracting solution and a low-level QC sample were analyzed seven times each. The 

results are summarized in Table 2. 

Table 1. Accuracy Values for QA/QC Samples and Spiked Pm2.5 Filter Extracts 

Analyte 

QA/QC Sample 

Average % Recovery* (range) 

Spiked Extract 

Average % Recovery* (range) 

NH4

+

 99.0 (91.0 - 108.0) 

n = 59 

99.4 (96.4 - 105.9) 

n = 14 

Na+ 104.2 (100.0 - 107.7) 

n = 20 

101.8 (97.0 - 105.1) 

n = 5 

K+ 103.0 (96.6 - 106.3) 

n = 20 

99.2 (97.0 - 102.7) 

n = 5 

*% Recovery = (concentration found/concentration expected) H 100. 
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Table 2. Precision Values for PM2.5 Filter Extracts, Extracting Solution, and QC Samples 

Sample Type 

Analyte 

PM2.5 Filter Extracts 

Average RSD** 

(range) 

Blank Extracting 

Solution Average 

(Std Dev), 

n = 7 

0.05 ppm 

QC Sample 

RSD, n = 7 

NH4

+

 0.6 (0.1 - 1.6) 

for 13 duplicates 

0.000 (0.000) 2.0 

Na+ 8.0 (0.6 - 19.8) 

for 17 duplicates 

0.001 (0.003) 13.3 

K+ 1.1 (0.1 - 3.1) 

for 17 duplicates 

0.000 (0.000) 2.0 

** RSD = relative standard deviation (the standard deviation divided by the average value and expressed as a 

percentage) 

1.3 Health and Safety Warnings 

The PM2.5 ion analysis operations do not involve unusual risks from electrical equipment or 

chemical exposures. Standard RTI laboratory health and safety precautions will be followed. 

1.4 Cautions 

Laboratory personnel should always wear clean clothes and wash hands thoroughly before 

performing filter handling and analysis procedures. The use of gloves is required and will 

minimize the potential for laboratory contamination. 

1.5 Interferences 

Large amounts of cations eluting close to the ions of interest will result in an interference. No 

interferences have been observed in extracts analyzed by RTI to date. If interferences are 

observed, several steps to increase separation can be taken, such as reducing eluent strength 

and/or flow rate, or replacing the guard and/or separator column. 

1.6 Personnel Qualifications 

Personnel employed to perform ion analysis operations will have at least an associate's degree in 

a laboratory science and will be trained by a supervisor before being allowed to process client 

samples for the PM2.5 program. 

1.7 Apparatus and Materials 

 Disposable centrifuge tubes with screw caps, 50 mL(polypropylene) 

 Calibrated Rainin electronic pipette (10-mL capacity) and adjustable Eppendorf 

pipette (10-100μ) 

 Tweezers 
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 Ultrasonic bath fitted with epoxy-coated test tube rack to hold centrifuge tubes 

 Mechanical shaker 

 Ion chromatograph complete with workstation (See Table 3.) 

 Pressurized eluent reservoirs 

 Volumetric flasks in an assortment of sizes 

 Dionex autosampler vials with filter caps 

 Coldroom at ≤4o C 

 Refrigerators. 

Table 3. Configurations of Ion Chromatographs Used for Cation Analysis 

Instrument ID C1 C2 C3 C4 

Dionex Model DX-500 DX-600 ICS-2000 DX-600 

Chromatography 

Module 

LC20 (no 

temperature unit) LC30 None (built in 

temperature unit) LC30 

Pump IP20 IP25 IS2000 IS25 

Conductivity 

Detector 

CD20 

DS1A Cell 

CD20 

DS3 Cell Built-in DS6 cell CD25 

DS3 Cell 

Autosampler AS40 AS40 AS40 AS40 

Software 

Windows 2000 

Dionex PeakNet 

5.2 

Windows 2000 

Dionex PeakNet 

5.2 

Windows XP 

Dionex 

Chromeleon 

Windows 2000 

Dionex 

Chromeleon 

Guard Column none none none none 

Separator Column CS12A CS12A CS12A CS12A 

Suppressor 

Column 

CSRS-300 auto 

regenerator 

CSRS-ultra auto 

regenerator 

CSRS-ultra auto 

regenerator 

CSRS-ultra auto 

regenerator 

Other - - 

Built-in eluent 

regenerator (not 

used) 

EG50 eluent 

generator (not 

used) 

1.8 Ion Chromatography Reagents 

Use ACS reagent grade chemicals and 18.2MΩ-cm deionized water for the preparation of all 

solutions. 

1. Concentrated Eluent Stock Solution: 5N H2SO4, purchased from VWR Scientific 

2. Working Eluent, 22mN Sulfuric Acid: Dilute 4.4 mL 5N H2SO4 to 1 liter using 

deionized water. Sonicate for 15 minutes just prior to use to de-gas the solution. 
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1.9 Calibration Standards 

Use ACS reagent-grade chemicals and 18.2MΩ-cm deionized water for the preparation of all 

solutions. Dry the salts used for the preparation of calibration standards at 105EC for 2 hours 

and cool in a desiccator immediately before use. 

1. Calibration Standard Stock Solution, 1000 mg/L each NH4

+

, Na+, and K+: 

Dissolve 2.9654 g NH4Cl, 2.5422 g NaCl, and 2.2284 g K2SO4 in 1 liter deionized 

water. 

2. Standard Solution A: Dilute 10 mL stock solution to 100 mL with deionized 

water (100 mg/L NH4

+

, Na+, K+). 

3. Standard Solution B: Dilute 10 mL Standard Solution A to 100 mL with 

deionized water (10 mg/L NH4

+

 , Na+, K+). 

4. Using Standard Solutions A and B, prepare calibration standards with deionized 

water in 100-mL volumetric flasks, as shown in Table 4. Prepare fresh calibration 

standards weekly. 

1.10 Quality Control Solutions 

Use ACS reagent-grade chemicals and 18.2MΩ-cm deionized water for the preparation of all 

solutions. Dry the salts used for the preparation of calibration standards at 105EC for 2 hours 

and cool in a desiccator immediately before use. Quality control (QC) solutions must be 

prepared independent of the calibration solutions. 

1. Sodium QC Stock Solution, 1000 mg/L Na+

: Dissolve 1.2711 g NaCl in 500 mL 

of deionized water 

2. Ammonium QC Stock Solution, 1000 mg/L NH4

+

: Dissolve 1.4827 g NH4Cl in 

500 mL of deionized water 

3. Potassium QC Stock Solution, 1000 mg/L K+

: Dissolve 1.1142 g K2SO4 in 500 

mL of deionized water 

4. QC-Intermediate Solution, 100 mg/L each Na+

, NH4

+

, and K+: Pipette 10 mL of 

1000 mg/L Na+

, 10 mL of 1000 mg/L NH4

+

, and 10 mL of 1000 mg/L K+ into a 

100-mL volumetric flask and dilute to the mark with deionized water. 

5. QC Samples: Using the QC-intermediate solution, prepare QC samples with 

deionized water in 100-mL volumetric flasks, as shown in Table 5. Prepare fresh 

QC samples as needed. 
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Table 4. Preparation of Cation Calibration Standards 

Standard 

NH4

+

, Na+, K+

(mg/L each) 

mL of Standard 

Solution/100 mL 

STANDARD SOLUTION A 

1 25.0 25.0 

2 10.0 10.0 

3 3.0 3.0 

STANDARD SOLUTION B 

4 1.0 10.0 

5 0.3 3.0 

1 mg/L STANDARD (Standard 4) 

6 0.1 10.0 

7 0.05 5.0 

1.11 Quality Assurance Solutions 

Use commercially prepared, NIST-traceable solutions to prepare an intermediate QA solution 

with known concentrations of Na+, NH4

+

, and K+. Solutions can be purchased from CPI 

(www.cpichem.com) or GFS Chemicals (www.gfschemicals.com). 

1. QA-Intermediate Solution, 100 mg/L each Na+, NH4

+

, and K+: Pipette 10 mL of 

1000 mg/L Na+, 10 mL of 1000 mg/L NH4

+

, and 10 mL of 1000 mg/L K+ into a 

100-mL volumetric flask and dilute to the mark with deionized water 

2. Prepare QA samples with deionized water in 100-mL volumetric flasks, as shown 

in Table 6. Prepare fresh QA samples as needed. 
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Table 5. Preparation of Anion Quality Control Samples 

QC Sample ID 

mL QCIntermediate Final Volume, mL 

(volumetric flask size)

Na+, NH4

+

, and K+ 

Conc (mg/L) 

RTI 2 ppm QC 2.0 100 2.0 

RTI 5 ppm QC 5.0 100 5.0 

Table 6. Preparation of Anion Quality Assurance Samples 

QA Sample ID Dilute 

Final Volume, 

mL 

(volumetric flask 

size) 

Na+, NH4

+

, and K+ 

Conc (mg/L) 

GFS 4.0 ppm QA 4.0 mL QA-Intermediate 100 4.0 

GFS 0.4 ppm QA 10 mL GFS 4.0 ppm QA 100 0.4 

1.12 Sample Collection 

Sample collection is not applicable to this SOP because samples are acquired by the state agency 

responsible for exposing the filters. 

1.13 Sample Handling 

Note: Additional information on this topic can be found in the SOP Sample Receiving, 

Shipping, and Archiving Procedures for the PM2.5 Chemical Speciation Program, RTI 

International, Center for Environmental Measurements and Quality Assurance, 1999. 

RTI will provide chain-of-custody documentation with all sample shipments to track and ensure 

that samples are collected, transferred, stored, and analyzed by authorized personnel; sample 

integrity is maintained during all phases of sample handling and analysis; and an accurate written 

record is maintained of sample handling and treatment from the time of its collection, through 

the laboratory analytical process, to the eventual relinquishing of all data to the client. 

Upon initial receipt of filters, RTI will prepare a Filter Inventory Sheet containing the filter 

identification numbers, box numbers, date received, date inspected, and number of filters 

rejected. This form will allow laboratory personnel to select and use the filter boxes in the 

proper sequence. 

1.14 Filter Extraction Procedure 

1.14.1 Nylon Filters 

Note: Filters to be analyzed for anions and cations or for cations only will be extracted 

with deionized water. 

To extract the filters, the analyst will do the following: 
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1. Remove filters to be extracted from the freezer and allow them to equilibrate to 

room temperature. 

2. Using gloved hands and tweezers, place each filter in a polypropylene centrifuge 

tube that has been labeled with the sample ID printed on a durable (waterresistant) label. 

3. Label a 50-ml extraction tube as AReagent Blank DI H2O.@ 

4. Add 25.0 mL of deionized water to each tube using a calibrated automatic pipette. 

5. Screw the cap tightly on the centrifuge tube. 

5. Ensure that the filter is completely submerged in the extraction solution. 

6. Place the batch of centrifuge tubes in an epoxy-coated wire test tube rack and 

place the extraction rack in the ultrasonic bath. Make sure that the water level of 

the ultrasonic bath is higher than the extraction solution level but below the screw 

cap. Sonicate for 60 minutes. 

CAUTION: Monitor the bath temperature during sonication. The temperature should 

not exceed 27EC. Add ice initially to lower the temperature and add ice as 

necessary during the sonication to maintain an acceptable temperature.

7. Install the extraction racks on the mechanical shaker and shake overnight in a 

cold room (#4EC) at approximately 60 cycles per minute. 

8. Record the date of extraction on the RTI Sample Log Form. 

9. Store the extracts in a refrigerator until analysis. 

1.14.2 Teflon Filters 

1. Remove filters to be extracted from the freezer and allow them to equilibrate to 

room temperature. 

2. Using gloved hands and tweezers, place each filter in polypropylene centrifuge 

tube that has been labeled with the sample ID printed on a durable (waterresistant) label. 

3. Label a 50-ml extraction tube as AReagent Blank DI H2O.@ 

4. Remove the caps from all 50-ml extraction tubes. To prevent contamination, 

place the caps in an upside-down position. 

5. Using an Eppendorf 100-Fl pipette, wet the entire surface of each Teflon filter 

with 100 Fl of nanopure ethanol. This is done by very slowly pipetting the 

ethanol on the center of the filter. Capillary action will distribute the ethanol over 

the entire surface. The AReagent Blank DI H2O” tube will not contain a filter. 

Add the 100 Fl of ethanol directly to the bottom of the tube. 

Note: Before proceeding, visually inspect each filter to be sure that the entire 

filter surface is wet. 

6. Using a calibrated automatic pipette, add 25.0 ml of deionized water to each 

extraction tube. The deionized water must have a resistance of at least 18.2MΩcm. 
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7. Recap all extraction tubes tightly to prevent leakage during the extraction 

procedure. Be sure that the exposed area of the filter is completely immersed in 

the extraction solution. 

CAUTION: Monitor the bath temperature during sonication. The temperature 

should not exceed 27EC. Add ice initially to lower the temperature, 

and add ice as necessary during the sonication to maintain an 

acceptable temperature. 

8. Place the extraction rack in the ultrasonic bath. Make sure that the water level of 

the ultrasonic bath is higher than the extraction solution level, but below the 

screw cap. Sonicate for 60 minutes. 

9. Install the extraction racks on the mechanical shaker and shake overnight in a 

cold room (#4EC) at approximately 60 cycles per minute. 

10. Record the date of extraction on the RTI Sample Log Form. 

11. Store the extracted filters in the refrigerator prior to analysis. 

1.15 IC Procedure 

1. Fill the eluent reservoirs with eluent. 

2. Start the eluent flow, activate the self-regenerating suppressor, and allow the 

baseline to stabilize. 

3. Inject four eluent blanks to flush the system and to ensure that the system is 

operating properly. 

4. Using the calibration schedule, perform the daily multipoint calibration over the 

appropriate range followed by the GFS 4.0 ppm QA sample. If the observed 

value for any cation differs by more than 10% from the known value, identify and 

correct the problem before analyzing samples. 

5. Load the filter extracts into the autosampler vials according to the schedule 

prepared for that day. The daily schedule includes duplicate samples, spiked 

samples, and QA/QC samples. 

6. Begin the analysis run, occasionally checking to ensure that the system is 

operating properly. 

7. Examine the data at the end of the run. If the NH4

+

, Na+, or K+ concentration of 

any extract exceeds the upper end of its calibration curve, dilute the extract 

appropriately and analyze that day or include with the samples to be analyzed the 

following day. 
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1.16 Calculations and Data Reduction 

For ion chromatographs using Dionex PeakNet® sofrware, peak areas are automatically entered 

into the computer where calculations are performed using a quadratic fit to the calibration data. 

The quadratic fit yields the following: 

yi = axi

2

 + bxi + c 

where 

 y = the calculated cation concentration, μg/L 

 x = the instrument response 

For ion chromatographs using Chromeleon® software, NH4

+

 peak areas are automatically 

entered into the computer where calculations are performed using a cubic fit to the calibration 

data. The quadratic fit yields the following: 

yi = axi

3

 + bxi

2

 + cxi + d 

where 

 y = the calculated NH4

+

 concentration, μg/L 

 x = the instrument response 

The cubic fit for NH4

+

 is used at the recommendation of Dionex. Na+ and K+ concentrations are 

calculated using a quadratic fit as described above. 

The calibration curve from 0.05 to 10.0 ppm is used for the calculation of the extract NH4

+

, Na+, 

and K+

 concentrations. If a cation concentration exceeds 10 ppm, the extract is diluted 

appropriately (usually 5-fold) to bring the cation concentration into the calibration range and 

reanalyzed. 

2.0 Quality Control and Quality Assurance 

Compare the regression parameters (a, b, c, and correlation coefficient) for the standard curves 

with those obtained in the past. If they exceed the control limits, stop the analysis and identify 

the problem. 

Analyze QC samples (see Section 1.13) at the beginning of every analytical run. Compare the 

results with those obtained during previous QC tests. If the observed concentration of any ion 

differs from the known value by greater than 10%, stop the analysis until the problem is 

identified and corrected. Analyze a duplicate sample, a QA/QC sample, and a spiked sample 

after at least every 20 field samples. 

3.0 Reference 

DRI Document No. 8068.1F4, Appendix D, Section 4.2.