4. Chemical analysis

4.1  Determination of sulphate, nitrate, chloride, ammonium, sodium, potassium, calcium, and magnesium with ion chromatography

4.1.1  Scope and Application

Ion chromatography can be used for the determination of the ions in the following samples:

The pretreatment of the samples before analysis is described together with the sampling in the preceding Sections. Special conditions for the different sample matrices are given in these Sections.

The concentration range of the method is typically 0.01–10 µg/ml.

4.1.2  Principle

A small volume of the sample, typically less than 0.5 ml, is introduced into the injection system of an ion chromatograph. The sample is mixed with an eluent and pumped through a guard column, a separation column, a suppressor device and a detector, normally a conductivity cell.

The separation column is an ion exchange column which has the ability to separate the ions of interest. The separation column is often preceded by a shorter guard column of the same substrate as in the separation column to protect the separation column from overloading and particles. Different types of separation columns, eluents and suppression devices have to be used for anions and cations respectively. Each ion is identified by its retention time within the separation column. The sample ions are detected in the detection cell, and the signals produced (chromatograms) displayed on a strip chart recorder or a PC equipped with the necessary software for measurement of peak height or area.

The ion chromatograph is calibrated with standard solutions containing known concentrations of the ions of interest. Calibration curves are constructed from which the concentration of each ion in the unknown sample is determined.

4.1.3  Interferences

Any species with a retention time similar to that of the main ions could interfere. With the exception of NO2-, precipitation or filter extracts do normally not contain such species. Large amounts of one of the ions may interfere by reducing the peak resolution of the next ion in the elution sequence. Sample dilution can then be necessary.

In some systems the so-called negative water dip in the start of the chromatogram may interfere with the Cl- determination. This can be avoided by adding a small amount of concentrated eluent to all samples and calibration standards to match the eluent concentration.

When analyzing alkaline impregnated filters and denuders, the sample matrix may influence the shape of the chromatogram peak and give wrong results if comparisons are made with calibration standards made from pure water solutions. In some cases this can be avoided by using the peak area instead of the peak height, but using the peak area in the low concentration range may often fail.

It is strongly recommended to match the calibration solutions with the sample matrix. For some samples, e.g. extracts from impregnated filters, the sample matrix may cause a slight distortion of the chromatogram. This may cause erroneous results if the calibration solution does not have a similar ionic composition. If ordinary calibration solutions are used, it must regularly be checked if this causes a problem by using control samples with known concentration of the ions with the same matrix as in the samples. One way of doing this is to extract unexposed impregnated filters with the normal calibration standards and with the same volume of water as the samples. Analyses of these samples should not give deviating results from the calibration standard concentrations.

Samples that contain particles larger than 0.45 µm and reagent solutions that contain particles larger than 0.20 µm require filtration to prevent damage to the instrument columns and flow systems. If the sample is left undisturbed in the sample tube for some days before analysis, these problems can be avoided by simply place an in-line filter in the tubing in front of the columns.

The presence of air bubbles in the columns, tubing or conductivity detector cell will cause baseline and peak variability. Using boiled solutions as eluents will help to minimize the introduction of air.

4.1.4  Instrumentation

Different commercial instruments are available using different columns and suppressor devices. Two main types of instruments using different suppressor techniques, chemical and electronic suppression, are on the market. One example of specific equipment for each of these two types is given below. The examples below do not exclude a use of other commercial equipment which allow the analyses to be carried out with the required accuracy and precision.

4.1.4.1  The Dionex (Dionex Corporation, Sunnyvale, CA, USA) system

Modern versions of Dionex instruments are usually equipped with injection valve, pump constructed from inert material (both gradient and isocratic pumps are available), separation column, suppressor system and a conductivity detector (in some cases a UV/Vis absorbance detector may be used). The instruments may be operated with manual injection or automated using an autosampler. The chromatograms are recorded on a strip-chart recorder, an integrator or direct on a PC-based Chromatography Workstation.

The chemical suppressor in the Dionex system has undergone significant improvements during the last years, as ordinary packed ion exchange columns which had to be chemically regenerated, have been replaced initially by hollow fibre suppressors and then by micro-membrane suppressors with higher suppression capacity and a smaller dead volume. The last versions of these suppressors are equipped with a self-regenerating system based on electrolysis of water from the eluent itself.

Table 4.1.1 shows the guard columns, separation columns and suppressors which are recommended for the different sample types in 1994.


Table 4.1.1: Columns and suppressors recommended by Dionex in 1994.

 

Samples

Separation/ 
Guard columns

Suppressor

Anions

All types mentioned above

AS9-SC/AG9-SC

AMMS-II or ASRS, 4mm

 

All samples excluding KOH-impregnated filters

AS4A/AG4A

AMMS-II or ASRS, 4mm

Cations

(Both monovalent and divalent)

 

 

 

Aerosol filters and precipitation

CS12/CG12

CSRS, 4mm


Producers of ion chromatographs also specify the eluent to be used and its concentration. Therefore no specific instructions regarding eluents are given in this manual. The column is delivered with a test chromatogram showing the separation of the different ions and the retention times. When installing a new column, it should be checked if the performance is as stated in the test chromatogram.

For other details on running the instruments, reference is made to the appropriate Instrument Manual.

4.1.4.2  The Waters (Waters Association, Milford, MA, USA) system

The Waters system is an electronically suppressed system, i.e. without a chemical based device to reduce the conductivity of the eluent, but with the possibility to subtract the conductivity of the eluent.

The following description of one possible instrument set-up and column choice is given by the Air Quality Department of Finnish Meteorological Institute (FMI):

Equipment

Pump Waters HPLC pump Model 501
(with pulsation suppression)
Injector and autosampler Waters Model 712 WISP and Waters Model 717 96 or 48 samples analyzed sequentially
Detector  Waters Model 431
Microcomputer  NEC 486/66i, 20/240 MB
Software Waters Maxima 820 and Baseline

Conditions for anions (precipitation, aerosol filters and alkaline impregnated filters)

Eluent  Borate/Gluconate
In-line filter Waters Guard Pak (0.22 µm)
Column Precipitation and aerosol filters: Waters IC-Pak A HR (4.6 x 75 mm, 6µm, 30 ± 3 µeq/ml)
Impregnated filters: Waters IC-Pak A (4.6 x 50 mm, 10µm, 30 ± 3 µeq/ml)
Flow rate IC-Pak A HR: 1.0 ml/min
IC-Pak A: 1.2 ml/min
Injected volume  20-200 µl
Run time  Appr. 16 min (Cl-, NO3-, SO4--)

Conditions for cations (precipitation samples)

Eluent EDTA/HNO3
In-line filter Waters (0.22µm, Cat no. 32472, Millipore)
Column Waters IC-Pac C M/D (3.9 x 150 mm, 5 µm, 2.0 ± 0.2 meq/ml)
Flow rate 1.0 ml/min
Injected volume 20-200 µl
Run time Ca. 18 min (NH4+, Na+, K+, Mg++, Ca++)

Detection limits

The analytical detection limits (in mg/l) obtained at FMI with the described equipment, defined as 2x (peakheight of lowest standard/height of baseline noise), are given in Table 4.1.2.

Table 4.1.2: Detection limits for Waters systems at FMI.

 

Detection limits

Lowest calibration standard

Cl-

0.010

0.05

NO3--N

0.010

0.05

SO4---S

0.020

0.05

NH4+-N

0.002

0.02

Na+

0.002

0.02

K+

0.006

0.02

Mg++

0.003

0.02

Ca++

0.005

0.02

 

More practical hints on the use of the Waters system written by Anni Reissell, FMI are available from the CCC.

4.1.5  Reagents and standards

All reagents must be of recognized analytical grade. The water used for dilution should be deionized and filtered. The water should have a resistance > 10 MW/cm and not contain particles larger than 0.20 µm. The sample, calibration standards and reagent solution bottles should be made of polyethylene or polypropylene. For the anions, borosilicate glass may also be used.

4.1.5.1  Eluent solutions

The chemicals and concentrations to be used are normally given by the manufacturers of the different separation columns.

4.1.5.2  Stock standard solutions

Stock standard solutions e.g. 1000 mg (based on the element)/litre, may be purchased as certified solutions from different manufacturers or NIST (National Institute for Standards and Technology, USA), or prepared from salts or oxide dried in the prescribed way, dissolved and diluted to 1000 ml as listed in Tables 4.1.3 and 4.1.4:

Table 4.1.3: Preparation of stock standard solutions. The salt amount indicated gives 1000 mg of the anions per litre.

Salt

Weight (g)

Drying temp. °C

Drying time (hours)

NaCl

1.6485

150

1

Na NO3

6.0679

105

2

Na2SO4

4.4299

105

24


Table 4.1.4: Preparation of stock standard solutions. The salt amount indicated gives 1000 mg of the cations per litre.

Salt

Weight (g)

Drying temp. °C

Drying time (hours)

NH4Cl

3.8190

105

1

NaCl

2.5421

150

2

KCl

1.9067

105

1

CaCO3

2.4971

180

1

MgO

1.6581


 
The CaCO3 should be added to approximately 600 ml of water. Then add concentrated hydrochloric acid (HCl) slowly until the entire solid has dissolved, and dilute to 1000 ml with water.

The MgO should be dissolved in 10 ml concentrated nitric acid (HNO3) before diluting to 1000 ml with water.

The other salts should be dissolved directly in water.

These stock standards are stable for at least 1 year.

4.1.5.3  Calibration solutions

Five calibration solutions and one zero standard (blank, normally water) are needed to generate a suitable calibration curve. The range to be used will depend on the concentration range for the different samples.

One example is given for each of the ion types:

0, 0.5, 1.0, 2.5, 5.0 and 10.0 ml of each of the anion stock standards are transferred with calibrated pipettes to 1000 ml volumetric flasks and diluted to volume with deionized water. These calibration standards will contain 0, 0.5, 1.0, 2.5, 5.0 and 10.0 mg/l respectively calculated on the basis of Cl, NO3-N and SO4-S.

0, 0.5, 1.0, 2.5, 5.0 and 10.0 ml of each of the cation stock standards are trans­ferred with calibrated pipettes to 1000 ml volumetric flasks and diluted to volume with deionized water. These calibration standards will contain 0, 0.5, 1.0, 2.5, 5.0 and 10.0 mg/l respectively calculated on the basis of NH4-N, Na, K, Ca and Mg.

If control samples have shown the necessity to match the matrix in the calibration solutions with the sample matrix (see 4.1.3 Interferences), addition of the matrix must be done before diluting to volume.

The calibration standards may be stored for 3 months in acid-cleaned polyethylene or polypropylene containers in a refrigerator. Special attention should be paid to control contamination from ammonia in the laboratory air.

4.1.6  Procedure

The ion chromatograph should be operated according to the manufacturers description.

The calibration solutions and control samples should be used as described in Section 5.

The width of the retention time window used to make identifications should be based on measurements of actual retention time variations of standards over the course of a day. Three times the standard deviation of a retention time can be used to calculate a suggested window size for each ion.

However, it is important to use the experience of the analyst in the interpretation the chromatograms.

4.1.7  Calculation of the results

The concentration of the different ions in the sample solutions are found by using the calibration curve manually or directly from a computer or integrator. To calculate the air concentrations for air samples from these values, use the appropriate formulas given in the actual sections on sampling.

4.1.8  References

Small, H. (1989) Ion Chromatography. New York, Plenum Press.


Last revision: November 2001