dma-80 tips and techniques_dma 8204

Tips and Techniques for DMA-80 with T640/1640 rev. 02 A or higher Rev. 2/6 Milestone Srl

Tips and Techniques for

DMA-80 Optimization

By Stefano Paggio DMA-80 Product Manager


DMA-80 Optimization

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Tips and Techniques for DMA-80 T640/1640 rev. 00/k or higher Rev. 1/6 Milestone Srl

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CONTENTS 1. General Rules

2. Calibration

• How to make a liquid standard solution

• How to store a liquid standard solution

• How to store standard reference materials

• How to optimize the blank

• How to increase the quality of the calibration curve

3. Quality Control

• How to set up a QC program

4. Demonstration

• How to perform a successful demo

• Key advantages of the DMA-80

5. Sample Processing

• Recommendations for preservation of samples

• How to increase the reproducibility

• How to reduce the memory effect

• How to work with unknown samples

• How to work with non-homogeneous samples

• How to increase the recovery of difficult samples

• How to work with difficult matrices

• How to work with liquid samples in auto-sampler mode

• How to work at low concentration

6. Conditioning procedure for new catalyst/amalgamator


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1. GENERAL RULES Before starting to work with the unit, it is necessary to eliminate any contamination coming from environmental dust or from the samples. Before use, a cleaning procedure is always recommended for the metal boats—in the DMA-80 or a muffle at 600°C for a few minutes—and for laboratory tools such as spatula, balance plate, tweezers, etc. It is particularly important to follow this recommendation when working at low concentrations of Hg. For contamination coming from the environmental or the sample, it is important to evaluate how much waiting time is tolerated by the auto-sampler mode. Also, it is a good idea to establish a weekly cleaning routine. 2. CALIBRATION How to make a standard liquid solution. Stock solution of 1000 ppm (1mg/ml) of HgCl2 stabilized in diluted HCl. Standard for atomic absorption. Low value used for 1st cell calibration :

Standard solution

From 5 ppm in ml HCl 36% in ml Deionised

water in ml Blank 0 1 up to 100

0.025 ppm 0.5 1 up to 100 0.05 ppm 1 1 up to 100 0.1 ppm 2 1 up to 100

0.15 ppm 3 1 up to 100


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High value used for 2nd cell of calibration curve :

Standard solution

From 5 ppm in ml HCl 36% in ml Deionised

water in ml 0.2 ppm 4 1 up to 100 0.3 ppm 6 1 up to 100

Standard solution

Stock 1000 ppm in ml HCl 36% in ml Deionised

water in ml 1 ppm 0.1 1 up to 100 2 ppm 0.2 1 up to 100 5 ppm 0.5 1 up to 100 7 ppm 0.7 1 up to 100

10 ppm 1 1 up to 100 The working standards solution must be freshly prepared before every calibration. How to store a liquid standard solution Stock solution of 1000 ppm can be stored for one year if maintained in its original tightly sealed bottle away from sunlight and intense sources of radiation, in a refrigerator at 10°C. Diluted liquid standard solution can be stored for one month if maintained in its original tightly sealed bottle away from sunlight and intense sources of radiation, in a refrigerator at 10°C. If the determined value of the QC solution differs from the known value by > 5%, the standard is discarded. The value may differ if the catalyst is exhausted.


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How to store standard reference materials Standard reference material must be stored in its original tightly sealed bottle away from sunlight, humidity, and intense sources of radiation. Before use, the standard should be thoroughly mixed by rotating the bottle before sampling. If the determined value of the QC solution differs from the specs of known value, the standard is discarded. The value may differ if the catalyst is exhausted. How to do optimize the blanks The blanks are optimized by removing the possible causes of contamination:

Causes Resolution Sample with high

concentration or in “over range.”

Cleaning procedure up to blank absorbance < 0.0010

Program: Max START Temp. 200°C, Drying/Ashing 1 min at 200°C, 2 min up

to 650°C, 1 min at 650°C Metal sample boat corroded, covered

with layer of oxides or with combustion residue that can

retain mercury and then release it in the

next run.

New metal boat or quartz boat*

Traces of Hg originating from new

metal boat


Program: Max START Temp. 200°C, Drying/Ashing 2 min up to 650°C, 1 min

at 650°C The catalyst and

or/and amalgamator exhaust

Change catalyst and amalgamator and repeat calibration


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Oxygen contaminated

Use O2, 99.50% pure for research or replace supplier

Low temperature of catalyst and cuvette

Contact your Milestone service representative to check the temperatures

Contamination from environment

Cleaning procedure up to blank absorbance< 0.0010, if after this the

contamination persist transfer the unit to another laboratory.

* Quartz boat P/N 8338 (10 pieces) completely inert material. How to increase the quality of calibration curves The primary calibration is performed prior to the first use of the instrument, and when spare parts are replaced, such as the catalyst tube, amalgamator, or oxygen cylinder. Calibration of the DMA-80 can be performed with standard solid reference material or standard liquid solution. According to our experience, the best results are achieved with standard liquid solution, because the distribution of Hg is completely homogeneous. Use freshly prepared standard liquid solutions containing different concentrations of Hg that have been stabilized with acid (see “how to make standard liquid solution”). The curve comprises the entire range of interest, working range 0.25–800 ng. An example : Sample amount for each standard : 100 µl Blank, 0.25 ng 5 ng,10 ng, 15 ng, 30ng, 100 ng, 200 ng, 500ng, 700ng and 1000 ng Program: Max START Temp. 200°C, Drying/Ashing 1 min at 200°C, 2 min up to 650°C, 1 min at 650°C The DMA-80 with the new Model 640/1640 terminal offers the opportunity to select different calibration algorithms at the end of


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calibration: for 1st cell (0-20 ng) and for 2nd cell (20-1000ng) linear, s-curve or square calibration curve. For low content of mercury it is possible to force the curve to 0 value. The quality of the calibration curve can be also improved by using quartz boats, because quartz is completely inert, and will not interact with aggressive reagents (acid) present in the liquid standard solution. If metal boats are used to perform the calibration, we recommend always using new boats after the cleaning procedure. In order to reduce error due to operator we suggest using the same volume of standard for each run (100µl) with different concentration, minimum point for calibration 10 (including blank). Acceptance criteria are used for DMA-80 standardization. Samples analyzed shall be within the standard calibration range. Curve linearity shall have r-value >0.995. Standards should be in the appropriate range for each matrix. An initial calibration verification standard is checked once per run after calibration. An initial calibration verification blank is checked once per run after calibration. Verification standard for instrument calibration and standardization: checked every 10 samples and at end of run. A method blank is analyzed with each sample batch, or one per 20 samples (5%), for each matrix. An external reference sample (laboratory control standard) is analyzed with each batch, or one of 20 samples (5%) for each matrix. A matrix spike is analyzed with each sample batch, or one per 20 samples (5%) for each matrix. A sample in replicates (spike in replicates) is analyzed with each sample batch, or one per 20 samples (5%), for each matrix. All values shall be documented and maintained and available for easy reference and inspection.


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3. QUALITY CONTROL How to set up a QC program

The QC program is prepared and overseen by the Laboratory Manager, the QC Chemist, and the Scientific Advisor.

QC protocol Procedure Frequency

Control Standards

The stability of the instrument calibration is

checked during the run by analysis of the control

standards.

Checked every 10 samples

Blanks Method blanks are

analyzed to confirm the absence of contamination.

With each sample batch or one per 20

samples(5%) for each matrix

Duplicates Duplicates are analyzed to check method and stability

and performance



Matrix Spikes

Sample matrix effects are checked by spiking

samples with a known amount of standard and subsequently measuring

the recovery of the analyte in the sample.



Standard Reference Materials

Certified standards are used as control samples to

check the method performance in the matrix.

For each matrix


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All values shall be documented, maintained, and available for easy reference and inspection.

Acceptance Criteria for QC Samples

For more details refer to EPA method 7473.

ACCEPTANCE LIMITS. Control limits shall be listed on charts for different matrices. Acceptable criteria should be set through the use of historical analyses. In the absence of historical data this limit should be set for:

Control standards for checking the stability of the instrument calibration at ±10% of their true value. To do this use two reference standards with range of interest ( see point 3.5 of EPA Method 7473). For each batch of samples processed, a method blank must be carried out. A method blank is prepared by using a volume or weight of reagent water at the volume or weight specified in the preparation method. In absence of project specifics, if the blank is less than 10% of the lowest sample concentration for each analyte, then the method blank would be considered acceptable (see point 9.4 of EPA Method 7473). For each batch of samples processed, a duplicate is carried out to check stability and performance. The acceptable criteria in absence of historical analysis is set at ±10%. Matrix Spike/Matrix Spike duplicated at ± 20% of the spiked for precision and � 20 relative percent difference. After determination of historical data, 20% must still be the limit of maximum deviation for both percent recovery and relative percent difference to express acceptability (see point 9.3 of EPA Method 7473).


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4. DEMONSTRATION How to perform a successful demo Before a demo, it is important to investigate the details of the samples to be processed. First off all, working range: our unit covers from 0.05 ng up to 1000 ng absolute value of Hg. That means for 100 mg of sample, 0.5 ppb up to 10 ppm. Sample status: solid or liquid. For solids (special for inorganic matrix), it is extremely important to have homogeneous distribution of mercury and uniform sample sizes. The best results are achieved using solid samples with a maximum particle size of 100 um. Mix well, and use a pre-concentration step. For liquid samples, the best results are obtained using quartz boats. Quartz boat are completely inert; they have no memory effect, and do not interact with the acid used for stabilizing the samples. In fact, quartz boats produce the best reproducibility (and no memory effect) regardless of matrix type or sample status. Low concentrations of Hg also yield excellent results. Low recovery from an inorganic sample can be increased with a higher decomposition temperature and a longer heating time. Key advantages of the DMA-80 The DMA-80 is the superior direct mercury analyzer because it works at higher temperatures that can guarantee extremely low catalyst memory effect, low memory effect from quartz boats after high concentration of mercury, low blanks, and the best reproducibility compared to other equipment that employs the same mercury vapour generation techniques.


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The speed and flexibility of the DMA-80’s auto-sampler for liquid (by quartz boat) and solid samples is another advantage. Memory effect of catalyst/amalgamator after 1 ppm

Memory effect of quartz boat after 0.1ppm

Memory effect of quartz boat after 1 ppm


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5. SAMPLE PROCESSING Recommendations for preservation of samples Full preservation of samples—domestic sewage, industrial wastes, natural waters, etc.—is a practical impossibility. Regardless of the nature of the sample, complete stability for every constituent can never be achieved. At best, preservation techniques can only retard the chemical and biological changes that inevitably continue after the sample is removed from the parent source. The changes that take place in a sample are either chemical or biological. In the former case, certain changes occur in the chemical structure of the constituents that are a function of physical conditions. Metal cations may precipitate as hydroxides or form complexes with other constituents; cations or anions may change valence states under certain reducing or oxidizing conditions; other constituents may dissolve or volatilize (Hg) with the passage of time. Metal cations may also adsorb onto surfaces (glass, plastic, quartz, etc.), such as, iron and lead. Biological changes taking place in a sample may change the valence of an element or a radical. Soluble constituents may be converted to organically bound materials in cell structures, or cell lysis may result in release of cellular material into solution. The well-known nitrogen and phosphorus cycles are examples of biological influence on sample composition. Therefore, as a general rule, it is best to analyze the samples as soon as possible after collection. This is especially true when the analyte concentration is expected to be in the low range.


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How to increase the reproducibility After calibration, the reproducibility of the unit should be lower than <1.5% for 5 measurements with 0.1 ml of a solution of 100 ppb ug/Kg (10ng). If this goal is not achieved with your samples, the cause may be one or more of the following :

Causes Resolution Solid/liquid sample

contaminated Repeat the test with new sample.

Solid sample with non-homogeneous

distribution of Hg

Mix well and increase the sample amount.

Solid/liquid sample with low amount of Hg

Mix well solid and use pre-concentration step up to satisfactory results.

Old standard liquid solution/liquid solution with

precipitated due to not enough acid.

Prepare fresh standard liquid solution.

Weight/volume not accurate Check balance calibration / Pipette calibration.

DMA-80 unit is not cleaned after high concentration or

“over range”


Process parameter too short Increase: waiting(purge) time/ drying time/ decomposition time.

Leakage of oxygen through the silicone joint between cuvette/amalgamator,

amalgamator/catalyst or catalyst/flange

Repair as shown in the “Maintenance of the DMA-80” manual.

Oxygen contaminated Use only O2, 99.50% pure for research, or replace supplier.

The catalyst and or/and amalgamator exhaust

Change catalyst and amalgamator and repeat calibration.


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Environment contaminated Cleaning procedure up to blank absorbance < 0.0010, if after this the contamination

persists, transfer unit to another laboratory.

How to reduce the memory effect The memory effect is due to high concentration of sample analyzed. In order to reduce it, it is possible operate on different parameters.

1. Reduce the sample amount.

2. Perform a cleaning procedure between each run.

3. Increase the decomposition time.

4. For liquid sample, always use quartz boats (see key advantages).

How to work with unknown samples Unknown samples can contain high concentrations of Hg, so in order to avoid contamination of the catalyst we suggest developing the application with a minimum quantity of sample (<5 mg) and then increasing according the result. Note: 1000 ng is our highest absolute value of mercury detectable. For solid samples with concentrations of Hg higher than our range, it is possible to do a premix from 1:1 to 1:200 with inert material, such as previously decontaminated alumina. For liquid samples with high Hg concentration, it is possible to do a dilution using the same matrix.


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How to work with non-homogeneous samples Non-homogeneous samples can explain poor reproducibility of the results. It is possible to increase the performance with a sample preparation procedure. The problem is due to non-homogeneous distribution of Hg in the matrix, or to different sizes of the same sample—especially for inorganic samples such as soil, sediment, ore, rocks, plastic, etc. To solve the first problem, mixing well should suffice. For the second problem, we need to reduce the sample’s particle size to a powder form, normally below 100 um. Pre-concentration steps may work as an alternative solution. How to increase the recovery of difficult samples It is possible with the DMA-80 unit to increase the recovery from difficult inorganic sample matrices, such as metal oxide, by increasing its decomposition temperature and time.

Matrix Sample Program Recovery 1.Max START Temp. 200°C, Drying/Ashing 2 min up to

650°C, 2 min at 650°C 60%

ZnO

100 mg (size 100

um)

2. Max START Temp. 200°C, Drying/Ashing 200°C, 3 min up

to 750°C, 2 min at 750°C 100.5%

1.Max START Temp. 200°C, Drying/Ashing 2 min up to

650°C, 2 min at 650°C 50 %

MnO

100 mg (size 100

um)

2. Max START Temp. 200°C, Drying/Ashing 200°C, 3 min up

to 750°C, 2 min at 750°C 99.9 %


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How to work with difficult matrices Difficult matrices such as petrochemical products, samples containing solvent, or large amounts (> 100 mg) of organic sample (blood , food, feed, agricultural sample, etc.) can explode without warning, and consequently reduce the sensitivity of the system. For serious problems, and when it is difficult to determine whether the catalyst or amalgamator is the source of reduced sensitivity, we recommend replacing both of them. Dedicated programs must be used for difficult matrices.

Matrix Sample Program Tips and tricks

Low reactivity 20-50 mg

Petrochemical products and

samples containing

solvent

High reactivity 10-20 mg

High reactivity

100 mg

Food, feed, Agriculture

Low reactivity 200 mg (i.e. fresh vegetable)

Reduce the drying temperature and

extend the time if strong

exothermic reaction persists

Use pre-concentration

step to increase signal of low Hg

amount

Food containing oil

100 mg

Max START Temp. 200°C, Drying/Ashing

1min at 200°C, 2 min up to 650°C, 1 min at 650°C

Longer drying step allows a pre-combustion of sample before rapid decomposition at high temperature, reducing the exothermic reaction.


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How to work with liquid samples in auto-sampler mode, in order to avoid evaporation. With liquid samples, metal boats can be used only in single mode, to minimize the loss of mercury due to chemical interaction between the acid matrix and the metal boat. Milestone offers boats made of quartz—a completely inert material— to solve this problem and ensure good results. The following two experiments illustrate the problem, and the effectiveness of this solution. Experiment 1: Metal boat effect with lost of mercury 10 samples of 120 µg/Kg (12ng) have been prepared and run by auto-sampler mode. After starting the autosampler, the waiting metal sample boats react chemically with the liquid standard, and consequently there is a loss of mercury.


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The last boat, run after 50 minutes of waiting, has only 22 µg/Kg or ppb(2.2 ng). More than 80% has evaporated. Experiment 2: Quartz boats (without lost of mercury). 10 samples of 120 µg/Kg (12ng) have been prepared and run in auto-sampler mode.

All the runs have been successfully done without evaporation, good stability and reproducibility, RSD: 1.24 We recommend using the quartz boats for all liquid samples.

Stabilizing Hg samples if used with the autosampler � Up to 15 quartz boats 1% of HCL � Up to 30 quartz boats 3% of HCL � Up to 40 quartz boats 5% of HCL

Reproducibility obtained < 1.5 %


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How to work at low concentrations of Hg With the quartz boats, is possible to optimize the recovery (<1 ng absolute) of Hg without a pre-concentration step.

Matrix Sample Program Rsd

Solid

100 mg (particle size 100 um and

homogeneous)



< 3 %

liquid

100 mg /ul



< 1.5%


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6. Conditioning procedure for new catalyst/amalgamator Introduction The following procedure is to be performed after replacing a new catalyst/amalgamator. Note: before starting the procedure be sure that the catalyst/amalgamator is correctly installed, alignment of actuators and Oxygen flow checked (for more details see operator manual). �

DMA 80 – procedure for catalyst/amalgamator activation Run Reagent/

Sample type

Sample amount

Max START Temperature

Drying/Ashing Time

Drying/Ashing Temperature

Purge Time

1 minute 200°C

2 minutes 650°C

n.1

n.2

Flour

HNO3 5% *

100 mg

200°C

1 minute 650°C

60sec

*: quartz boat (suggested)

Repeat at least 5 times the above procedure, up to absorbance < 0.0030

After conditioning a stability test is required. Repeat 5 times: a fresh aqueous standard of 100ppb, the RSD must be <1.5%. For this evaluation is not important the recovery but only the DMA stability so the previous calibration curve can be used. If the stability test is according to the specs, please proceed making new calibration (see user manual), otherwise repeat the conditioning procedure again.

dma-80 tips and techniques_dma 8204

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