1.1 History

Air and wipe samples containing metal and metalloid particulate have always been analyzed at the OSHA Salt Lake City Analytical Laboratory using atomic absorption or emission spectrometry (8.1). Constituents in bulk samples have been determined semi-quantitatively using this technique.

1.2 Principle

Air samples of the workplace are taken using calibrated sampling pumps with cassettes containing either mixed cellulose ester (MCE) or polyvinyl chloride (PVC) filters. These samples are prepared in the laboratory using concentrated (concd) acids or extracted with deionized water if a soluble fraction is required. The sample solution is diluted to a known volume after any necessary matrix modifiers are added. The sample is then aspirated into the flame of an atomic absorption or emission spectrophotometer (AAS or AES) and the molecules in the sample solution are subjected to the following processes:

1) nebulization
2) desolvation
3) liquefaction
4) vaporization
5) atomization
6) excitation (atoms converted from "ground" to excited state)
7) ionization

The absorption or emission of light occurring during processes 5 and 6 is then measured at the characteristic wavelength for the element of interest.

For absorption, a hollow cathode lamp or an electrodeless discharge lamp (EDL) is used as the light source. A double beam spectrophotometer is normally used where the lamp radiation alternately passes through and around a flame into which the sample is being aspirated. The sample is atomized and the metal or metalloid atoms absorb light from the source at their characteristic wavelengths. This absorption is proportional to the concentration of the element present in the sample solution. A monochromator isolates the characteristic radiation of the element being analyzed. A photosensitive device then measures the intensity of the transmitted radiation from the two light paths to determine the amount of absorbance occurring in the flame.

For emission, a light source is not used. The sample is introduced into the flame, atomized and excited, and then the light emission from excitation is isolated and measured. The intensity of the light emitted is proportional to the concentration of the element present.

The following flames are used in this method for absorption or emission:

a) Air/Acetylene mixture (Air/C₂H₂)
b) Nitrous oxide/Acetylene mixture (N₂O/C₂H₂)
c) Air/Hydrogen mixture (Air/H₂)

The use of a specific flame is dependent on the respective element's analytical stability, sensitivity, and interferences.

1.3 Advantages and Disadvantages

1.3.1 This analytical method is specific for the element to be determined and does not distinguish different compounds. When an analysis for a compound is requested, an elemental analysis is performed on the sample. A gravimetric factor is then applied to calculate the compound value (Note: For some compounds, additional analytical procedures (i.e. ion chromatography or X-ray diffraction) can be used to confirm the presence of the particular compound.

1.3.2 The analysis will also not differentiate between different particle size ranges, such as dusts and fumes.

1.3.3 Metallic analytes having Permissible Exposure Limits (PELs) designated as the soluble form (i.e., iron soluble salts, nickel, etc.) can be analyzed using this method. Samples for soluble analytes are extracted with deionized water and an elemental analysis is performed on the extract.

1.3.4 Some compounds may not dissolve using the digestion procedures described herein. In these cases, an alternative digestion method should be used.

1.3.5. Several elements can be determined from the same filter sample using this method; however, digestion procedures may solubilize only certain metals. If a combination of metals is requested on the same filter, all of the metals must be soluble in the digestion procedure used.

1.3.6 The equipment used is inexpensive and does not require specialized training.

1.4 Use of Metal and Metalloid Compounds in Industry

Metals, their alloys, and compounds are used in a wide variety of industries. In certain operations (e.g., welding, smelting, grinding, etc.), particulate matter containing metals and their compounds may be released into the workplace atmosphere. These substances pose a potential health hazard to workers exposed to them (8.2-8.4). Further documentation regarding industrial use, toxicity, and physical properties may be found in NIOSH criteria documents for the particular substance.

2.1 The qualitative detection limits listed in Table 2 were taken from reference 8.5. The analytical detection limits (8.1.) listed were determined from routine laboratory analyses using the definition listed in Appendix A. These limits are approximate since they are dependent on instrument performance and optimization, sample characteristics, and the range of standards analyzed.

2.2 The upper linear range for each element is also given in Table 2. These ranges were taken from reference 8.6. Instrument response is linear to greater concentrations if an alternate wavelength is used; however, the detection limit may also increase. Samples can be diluted to bring the concentration of the element(s) within the linear range. The upper linear range for most elements is usually found near 0.25 to 0.30 absorbance units (ABS).

2.3 The sensitivity for each element is also listed in Table 2. These values are for a nebulizer which has been optimized to give an ABS of 0.25 for an aqueous solution containing 5 µg/mL Cu (8.6.). The actual sensitivity obtained will depend on the particular instrument and flame used, the sample matrix, and instrument operating parameters.

4.1 Chemical or condensed phase interferences occur when the element of interest combines with another species in the flame, thus altering the number of atoms available for emission or absorption. This can result in either a positive or negative bias (usually negative) in the results obtained. Chemical interferences can be controlled by using a hotter flame, or by the addition of a releasing agent which inhibits the reaction between the metal and the interfering species.

4.2 Matrix interferences occur when the physical characteristics (viscosity, surface tension, etc.) of the sample and standard solutions differ considerably. This may occur when samples contain large amounts of dissolved salts or acid, when different solvents are used for samples and standards, or when the temperatures of samples and standards are appreciably different. To control this, samples and standards must be matrix matched, or the sample must be diluted until any matrix effect becomes insignificant.

4.3 Ionization interferences occur when the flame temperature is sufficiently high to ionize the atoms of interest. This changes the absorption spectrum of the analyte and effectively removes atoms from the flame, causing a loss of sensitivity. Ionization interferences are controlled by adding large amounts (usually >0.1%) of an easily ionized metal such as Na, K, Cs, or rubidium (Rb). The excess electrons released in the flame greatly reduces the degree of ionization of the metal being determined.

4.4 Spectral interferences occur when an element other than the one analyzed absorbs at the same wavelength. This causes a positive bias in the results obtained when the interfering element is present in the samples. In this case, an alternate line should be used. Spectral interferences also occur when a multielement hollow cathode lamp is used which contains elements with absorbing wavelengths close to one another and the analytical slit width used is wide enough to allow the wavelengths of more than one element to pass. If the sample contains two or more of these elements, a positive bias will occur. To resolve this, a single element lamp, an alternate wavelength, or in certain cases, a narrower slit width can be used.

4.5 Background absorption interferences include flame absorption, molecular absorption, and light scattering:

a) Flame absorption is most severe below 250 nm. This absorption can be controlled by careful optimization of fuel and oxidant flow rates. Other mechanisms of control are: Use of flames which are more transparent at these wavelengths (i.e., Air/H₂ or argon/hydrogen flames), or deuterium arc background correction (DABC).

b) Molecular absorption is controlled by using hotter flames to break down molecular species or by DABC.

c) Light scattering occurs at shorter wavelengths when samples have a large salt content; this is controlled using DABC.

4.6 Large amounts of silicates or other particulates may interfere and may also cause aspiration problems (8.7). If present, they should be removed by filtration. The particulate should then be re-digested and analyzed to ensure the analyte(s) of interest have been completely extracted.

4.7 This analytical method is normally not compound-specific. Compounds are only determined as the element, and a significant positive bias can occur when any sample has additional analytes containing the same element. Other analytical procedures may be necessary to identify a specific compound. An assessment of the industrial operation sampled may also provide information regarding the potential existence of other analytes that could cause a positive bias.

4.8 Potential interferences for several of the elements determined by this method are listed in Appendix B.

5.1 Equipment - Air Filter Samples

5.1.1 Mixed cellulose ester (MCE) filters (0.8 µm pore size), cellulose backup pads, and cassettes, 37-mm diameter (part no. MAWP 037 A0, Millipore Corp., Bedford, MA). Filters and cassettes having a 25-mm diameter can also be used.

5.1.2 Gel bands (Omega Specialty Instrument Co., Chelmsford, MA) for sealing cassettes.

5.1.3 Sampling pumps capable of sampling at 2 liters per minute (L/min).

5.1.4 Assorted flexible tubing.

5.1.5 Stopwatch and bubble tube or meter for pump calibration.

5.2 Equipment - Wipe Samples

5.2.1 Smear tabs (part no. 225-24, SKC Inc., Eighty Four, PA), or wipe filters (Whatman no. 41 or no. 42 filters, Whatman Labsales Inc., Hillsboro, OR).

5.2.2 Deionized water.

5.2.3 Scintillation vials, 20-mL (part no. 74515 or 58515, Kimble, Div. of Owens-Illinois Inc., Toledo, OH) with polypropylene or Teflon cap liners. Metal cap liners should not be used.

5.3 Equipment - Bulk Samples

5.3.1 High-volume sampling pump with appropriate sized MCE collection filters.

5.3.2 Scintillation vials, 20-mL (same as Section 5.2.3).

5.4 Sampling Procedure - Air Filter Samples

5.4.1 Place a MCE filter and a cellulose backup pad in each two- or three-piece cassette. Seal each cassette with a gel band.

5.4.2 Calibrate each personal sampling pump with a prepared cassette in-line to approximately 2 L/min.

5.4.3 Attach prepared cassettes to calibrated sampling pumps (the backup pad should face the pump) and place in appropriate positions on the employee or workplace area.

5.4.4 Collect the samples at approximately 2 L/min for the recommended sampling times (unless otherwise noted):

Time Weighted Average Samples	240 to 480 min
Short-Term Exposure Limit Samples	15 min
Ceiling Samples	5 min*

The analytical sensitivity of a specific analyte may dictate the use of a different sampling time.

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* When determining compliance with the Ceiling PEL for sodium hydroxide, take 15-min samples.
 

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5.4.5 Place plastic end caps on each cassette after sampling. Attach an OSHA-21 seal around each cassette in such a way as to secure the end caps.

5.5 Sampling Procedure - Wipe Samples

Certain analytes may have a skin designation (See Table 1).

5.5.1 Wear clean, impervious, disposable gloves when taking each wipe sample.

5.5.2 Moisten the wipe filters with deionized water prior to use.

5.5.3 If possible, wipe a surface area covering 100 cm².

5.5.4 Fold the wipe sample with the exposed side in.

5.5.5 Transfer the wipe sample into a 20-mL scintillation vial and seal with vinyl or electrical tape. Securely wrap an OSHA-21 seal length-wise from vial top to bottom.

5.6 Sampling Procedure - Bulk Samples

5.6.1 In order of laboratory preference, bulk samples may be one of the following:

1) a high-volume (>1,000 L) filter sample of the workplace area,
2) a representative settled dust (rafter) sample,
3) a sample of the bulk material in the workplace.

5.6.2 If possible, transfer the bulk material or filter into a 20-mL scintillation vial and seal with vinyl or electrical tape. Securely wrap an OSHA-21 seal length-wise from vial top to bottom.

5.7 Shipment

5.7.1 Submit at least one blank sample with each set of air or wipe samples. Blank filter samples should be handled in the same manner as other samples, except that an air or wipe sample is not taken.

5.7.2 The type of bulk sample should be stated on the OSHA 91A and cross-referenced to the appropriate air sample(s). Bulk samples should be shipped with Material Safety Data Sheets (if available) and should be sent separately from air samples. Check current mailing restrictions and ship bulks to the laboratory by an appropriate method.

5.7.3 Send all samples to the laboratory with the OSHA 91A paperwork requesting the specific analyte(s) of interest. If analysis of a mixture of different elements or compounds is necessary, contact the lab to ascertain which analytes can be analyzed together.

6.1 Safety Precautions

6.1.1 Care should be exercised when handling any acidic solutions. Acid solution contact with work surfaces should be avoided. If any acid contacts the eyes, skin, or clothes, flush the area immediately with copious amounts of water. Medical treatment may be necessary.

6.1.2 All work with concd acids is potentially hazardous. Always wear safety glasses and protective clothing. Prepare all mixtures, samples, or dilutions in an exhaust hood. To avoid exposure to acid vapors, do not remove any beakers from the hoods until they have returned to room temperature.

6.1.3 Extra care should be used when handling perchloric acid (HClO₄). Perchloric acid should only be used in a hood that has been approved for HClO₄ use. In this hood:

a) Organic reagents should not be used or stored near HClO₄.

b) A water washdown system for the ducts and work surface must be installed and periodically used.

c) Precautions should be taken to ensure that explosions or spontaneous ignition of sample material from HClO₄ is prevented.

Working with HClO₄ is very hazardous. Be sure to wear safety glasses, a labcoat, and gloves. Always add nitric acid (HNO₃) with HClO₄. When digesting backup pads or other samples with HClO₄, watch them carefully since there is a chance they could ignite. Always keep HNO₃ nearby when using HClO₄. In the event of sample media ignition, quickly douse the sample with a small portion of HNO₃.

6.1.4 Care should be exercised when using laboratory glassware. Chipped pipettes, volumetric flasks, beakers, or any glassware with sharp edges exposed should not be used.

6.1.5 Pipetting is always performed using an automatic pipet or pipette bulb, never by mouth.

6.1.6 Before using any instrument, the operator should consult the Standard Operating Procedure (SOP) (8.8) and any instrument manuals.

6.1.7 Since metallic elements and other toxic substances are vaporized during flame operation, it is imperative that an exhaust hood is installed and used directly above the burner chamber of the spectrometer. Always ensure the exhaust system is operating before proceeding with the analysis.

6.2 Equipment

6.2.1 Atomic absorption spectrophotometer consisting of a(an):

Nebulizer and burner head.

Pressure-regulating devices capable of maintaining constant oxidant and fuel pressures.

Optical system capable of isolating the desired wavelength of radiation.

Adjustable slit.

Light measuring and amplifying device.

Display, strip chart, or computer interface for indicating the amount of absorbed or emitted radiation.

Deuterium Arc Background Corrector. This is usually required for determinations at short (<250 nm) wavelengths.

Light source for absorption:

a) Hollow cathode lamp for the specific element or multielement (Note: Please see specific limitations of multielement lamps in Appendix B)

b) Electrodeless Discharge Lamp (EDL) for the specific element. This type of lamp may provide better sensitivity and detection limits for some elements, especially Se, Sn, and Sb. If used, a separate EDL power supply is usually necessary.

6.2.2.Oxidant: Compressed, filtered air free from water, oils and other contaminants.

6.2.3 Nitrous oxide (N₂O).

6.2.4 Fuel (Use flash arrestors when using flammable gases. Consult with the manufacturer for appropriate use.):

a) Acetylene, commercially available acetylene dissolved in acetone.
CAUTION: Do not use grades of acetylene that contain solvents other than acetone. These solvents may damage PVC tubing in some instruments. Do not use acetylene when the tank pressure drops below 520 kPa (75 psi).

b) Hydrogen is used as the fuel in the determination of certain elements.

6.2.5 Pressure regulators, Two-stage.

6.2.6 Flash arrestors (model 6103, Matheson Gas Products, East Rutherford, NJ).

6.2.7 Glassware

a) Conical beakers, 125- and 250-mL

b) Volumetric flasks, Class A: 10-, 25-, 50- and 100-mL

c) Pipettes, Class A: Assorted sizes

6.2.8 Forceps.

6.2.9 Exhaust hood and hotplate, or microwave digestion system (model no. MDS-81, CEM Corp., Matthews, NC).

6.2.10 Filtering apparatus consisting of MCE filters, 0.45-µm pore size, 47-mm diameter (cat. no. HAWP 047 00, Millipore Corp., Bedford, MA) and filtering apparatus (cat. no. XX15 047 00, Millipore).

6.2.11 Analytical balance (0.01 mg).

6.3 Reagents (All chemicals should be reagent grade or better. Many of the chemicals listed below are only used in specific instances. Specific reagents are listed within the additional procedures in Table 4 and also in Table 5.)

6.3.1 Deionized water (DI H₂O) with a specific conductance of less than 10 µS.

6.3.2 Ammonium fluoride (NH₄F) solutions (used for specific insoluble compounds, see AP 6, Table 4).

a) Ammonium fluoride, 1 M: Dissolve 37.04 g NH₄F and dilute to 1 L in DI H₂O. Store in a polyethylene bottle.

b) Ammonium fluoride, 0.1 M in 4% HNO₃: Carefully add 40 mL concd HNO₃ and 100 mL of the 1 M NH₄F solution to 500 mL DI H₂O and dilute to 1 L in a polyethylene volumetric flask. Store in a polyethylene bottle since acidic solutions of NH₄F may form small amounts of HF and etch glass containers.

6.3.3 Hydrogen peroxide (H₂O₂), 30% (used for digestions of Cr, see AP 5, Table 4).

6.3.4 Mineral acids (used for digestions)

CAUTION: Refer to Section 6.1.2. before using acids

a) Hydrochloric acid (HCl), concd (36.5 to 38%).

b) Hydrofluoric acid (HF), concd (49%).

c) Nitric acid (HNO₃), concd (69 to 71%).

d) Perchloric acid (HClO₄), concd (69 to 72%). Please see Section 6.1.3 before using HClO₄.

e) Sulfuric acid (H₂SO₄), concd (95 to 98%).

f) Acid mixture for platinum digestions: Prepare a mixture of HCl/HNO₃ by slowly and carefully adding 82  mL concd HCl to 18 mL concd HNO₃ (CAUTION: Do not store this solution; dispose of properly after use).

6.3.5 Mineral acids (used for dilutions or cleaning glassware)

CAUTION: Refer to Section 6.1.2. before using acids.

a) Nitric acid, 1:1 HNO₃/DI H₂O mixture: Carefully add a measured volume of concd HNO₃ to an equal volume of DI H₂O.

b) Nitric acid, 4% v/v: Carefully add 40 mL concd HNO₃ to 500 mL DI H₂O and dilute to 1 L.

c) Nitric acid 10% v/v: Carefully add 100 mL of concd HNO₃ to 500 mL of DI H₂O and then dilute to 1 L.

d) Nitric and hydrochloric acid v/v mixture (4% HNO₃ / X% HCl, where X% is listed below): Carefully add the appropriate amount of concd HCl to 500 mL of DI H₂O:

4% HCl	40 mL
16% HCl	160 mL
32% HCl	320 mL

Then carefully add 40 mL concd HNO₃ and dilute to 1 L with DI H₂O.

6.3.6 Chemical or ionization interference suppressants

a) Aluminum ion, 5,000 µg/mL: Dissolve 69.52 g aluminum nitrate (Al(NO₃)₃·9H₂O) and dilute to 1 L in DI H₂O.

b) Potassium ion, 5,000 µg/mL: Dissolve 9.54 g potassium chloride (KCl) in DI H₂O and dilute to 1 L.

c) Sodium ion, 5,000 µg/mL: Dissolve 12.71 g sodium chloride (NaCl) in DI H₂O and dilute to 1 L.

6.3.7 Stock standard solutions

Commercially available aqueous standards are used. Expiration dates for standards should be followed. If there is no expiration date, dispose of after 1 year. As an alternative, standards can be prepared using the procedures described in the SOP (8.8) or instrument manufacturer manuals (i.e., 8.6, 8.9, 8.10).

6.4 Glassware Preparation

6.4.1 Place the conical beakers in an exhaust hood and add approximately 10 mL of a 1:1 HNO₃/DI H₂O mixture in each 125- or 250-mL conical beaker. Apply moderate heat until refluxing occurs. Decant the acid mixture into a waste container and allow the beakers to cool before removing from the hood. Rinse the beakers thoroughly with DI H₂O.

6.4.2 Rinse all volumetric flasks with 10% v/v HNO₃ and then rinse thoroughly with DI H₂O.

6.5 Working Standards

6.5.1 Dilute stock standard solutions to the appropriate ranges using a diluent that will match the sample matrix. Use information in Tables 1 and 2 as guides for the ranges; use Table 5 for matrices. The standard concentrations should bracket the expected sample concentrations and the standard/sample matrices should match.

6.5.2 Store standards in appropriate containers. Protect Ag standards from light by storing them in actinic or brown plastic bottles. Store standards containing NH₄F in polyethylene containers.

6.6 Sample Preparation

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Note: Always prepare blank samples with every sample set. Prepare an additional blank media sample any time an extra procedure is used (i.e. wiping out the particulate contained inside a cassette with an MCE filter or preparing a contaminated backup pad). This blank media should be from the same manufactured lot as the prepared filter or backup pad.
 

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6.6.1 Preparation of air and wipe samples

Open the filter cassette or scintillation vial, carefully remove the sample filter with forceps, and place in a labeled conical beaker. Use 125-mL beakers for air samples and smear tabs; use 250-mL beakers for large wipe samples. If the cassette or vial contains loose dust, carefully pour the dust into the beaker. Always rinse the cassette with DI H₂O and pour the water into the beaker and wipe out the cassette with a moistened, clean filter and place this filter in the sample beaker.

If the backup pad appears to be discolored, it may be due to leakage of air around the filter during sampling. In these cases, the pad should also be prepared and analyzed. 

6.6.2 Preparation of bulk samples

Review any available material safety data sheets to determine safe bulk handling. The safety data may also offer a clue as to the aliquot amount needed for adequate detection of the element(s) of interest.

Measure by volume or weight an appropriate aliquot of any liquid bulk sample.

Weigh the appropriate amount of any solid bulk sample.

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Note: Aliquot amounts of bulks are dependent on the analytical sensitivity, detection limit, and solubility of the material used. If uncertain, a 20- to 50-mg aliquot of a solid material can be taken as a starting point. Make sure the aliquot taken is representative of the entire bulk sample. If necessary, use a mortar and pestle to grind any nonhomogenous particulate bulk samples in an exhaust hood.
 

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After measuring, transfer the aliquot to a 250-mL conical beaker.

6.6.3 Extraction or digestion - all samples

Consult Tables 4 and 5 to determine the reagents used during extraction or digestion for each element to be analyzed. Some elements (Ba, Sn, etc.) or compounds are not digested with concd HNO₃, but are prepared using alternate procedures (APs) listed in Table 4. These elements or compounds and their AP numbers are:

Ag	AP 1	LiH	AP 7	Te	AP 1
Al (soluble)	AP 2	MgO	AP 3	TiO2	AP 8
Al (pyro powders)	AP 3	Na cmpds	AP 7	Tl (soluble)	AP 2
Au	AP 4	Ni (soluble)	AP 2	Y	AP 3
Ba (soluble)	AP 2	Mo (soluble)	AP 2	Zr	AP 6
Ca cmpds	AP 3	Mo (insoluble)	AP 3
Cr (II or III)	AP 2	Pb	AP 1
Cr (metal)	AP 5	Pt (metal)	AP 4
CsOH	AP 7	Sb	AP 1
Fe (soluble)	AP 2	Se	AP 1
Hf	AP 6	Sn (inorganic)	AP 4
KOH	AP 7	SnO	AP 4

For the element or compounds listed above, follow the APs recommended and then proceed with Section 6.6.2. For other elements or compounds, follow the procedures a, b, or c listed below:

a) All MCE air filters and smear tabs requiring HNO₃ digestion

Place the beakers in an exhaust hood and add 3 to 5 mL concd HNO₃ to cover the filter. Place the beakers on a hot plate and heat the samples until about 1 mL remains. Add a second portion of approximately 1 to 2 mL of concd HNO₃. Apply heat until the appropriate amount of HNO₃ remains in the beaker (1 mL of HNO₃ will give a 4% HNO₃ matrix when diluted to 25 mL final volume).

b) Large wipe, PVC filters, or backup pads

Place the beakers in an exhaust hood and add the following amount of concd HNO₃ to the beakers:

Large wipes and backup pads 10 to 15 mL PVC filters 3 to 5 mL

Place the beakers on a hot plate and heat the samples until about 1 mL remains. Add 2 mL of concd HClO₄ along with a second portion of 2 mL HNO₃, heat the sample, and then remove when about 1 mL remains. (Note: Please see Section 6.1.3. before using HClO₄.) As an alternative, an extraction of the backup pad or wipe sample using only HNO₃ may be used. Add HNO₃ to the media, digest on a hotplate, and continue to add HNO₃ until the solution becomes clear. Remove the beaker from the hotplate when the appropriate amount of HNO₃ remains.

c) Bulk samples

Add 10 to 30 mL HNO₃, place the beaker on a hot plate, and digest the bulk sample until the material dissolves and the appropriate amount of solution remains (about 1 mL if diluting samples to 25 mL, 2 mL if 50 mL final volume, etc. After dilution this will give a final volume of 4% HNO₃). If necessary, use other acids, or use a microwave digestion system to facilitate digestion [For further information regarding microwave digestion, see the Standard Operating Procedure (8.11.)].

6.6.4 Filtration - all samples

1) Samples Previously Extracted:

Samples extracted with DI H₂O should normally be filtered. If particulate is present, filter the extract through a 0.45-µm MCE filter. Save the extract as the soluble portion. If necessary, digest the particulate on both filters using procedure (a) above or the applicable AP to prepare the remaining insoluble material for additional analyses. To control for potential contamination, prepare blank samples in the same fashion as the filtered samples.

2) Samples Previously Digested:

If particulate matter is present after digesting, cool the sample, add approximately 10 mL DI  H₂O, then filter the solution through a 0.45-µm MCE filter. Save the filtrate. Repeat digestion procedure (a) above for the filter containing the particulate.

6.6.5 Dilution - all samples

Allow all digested samples to cool to room temperature in an exhaust hood before proceeding. Additional sample or filtrate treatment may be required for certain elements. Perform any special sample treatments recommended in Table 5, and then quantitatively transfer each sample and each filtrate solution to individual volumetric flasks. Add any reagents necessary to achieve the final solution concentrations listed in Table 5 for specific analytes. Dilute to volume with DI H₂O and then mix well. Solution volumes are dependent on the following factors:

a) The amount of sample the industrial hygienist has collected (air volume and/or filter loading).

b) The detection limit of the analytical method.

c) The PEL of the analyte.

d) The number of analytes requested.

Air samples are normally diluted to 25 mL unless one or more of the above factors suggests an alternate volume should be used. For routine analysis, at least 1/10 of the OSHA PEL should be detectable. Final solution volumes can be estimated using the following equation:
 

FV Factor =

0.1 × PEL × air volume QnDL × GF

Where:

PEL	=	Permissible Exposure Limit (in mg/m3)
Air Vol	=	Air Volume taken (in L)
QnDL	=	Quantitative Detection Limit (in µg/mL)
GF	=	Gravimetric Factor (if required - some factors are listed in Table 6)

Quantitative detection limits are listed in Table 2.

The FV factor assists in determining the final volume. Sample solution volumes normally used are: 5-, 10-, 25-, 50-, or 100-mL. Final volumes of 50- and 100-mL are normally reserved for wipe or bulk samples. If possible, FV should always be larger than the final solution volume. For example, if a sample has a 200-L air volume, a PEL of 0.05 mg/m³, a GF of 1, and a QnDL of 0.09 µg/mL, then:

     FV Factor = 11.1

and a final volume should be 10-mL. Due to the limited amount of solution available for analysis and the potential for sample loss during transfer, 5-mL solution volumes are only used when absolutely necessary.

6.7 Instrument Setup and Analysis

6.7.1 Set up the AAS or AES according to the SOP (8.8) or the manufacturer's instructions. Use the flame and wavelength recommended in Table 7. If alternate conditions are necessary, consult the instrument manufacturer's manual for other settings and operating procedures. Install an EDL or hollow cathode lamp for the element of interest and allow it to warm up for 10-20 min or until the energy output stabilizes. Optimize conditions such as lamp position, burner head alignment, fuel and oxidant flow rates, etc. See the SOP (8.8) or specific instrument manuals for details.

6.7.2 Aspirate and measure the ABS of a standard solution for the element of interest. The standard concentration should be within the linear range for the element. Compare the ABS to an expected sensitivity value (Note: Some values are listed in Table  7; these were adapted from reference 8.6 or obtained at the OSHA laboratory). Then aspirate the smallest standard to be used and assure the ABS reading is above the background level of the instrument.

6.7.3 Make any adjustments necessary for the particular analysis, such as: scale expansion, burner head rotation, background correction, or alternate wavelength.

6.7.4 Aspirate and measure the ABS of a prepared standard solution, then determine the baseline by aspirating DI H₂O and measuring the ABS.

6.7.5 Analyze standards, samples, and blanks. Repeat the baseline determination after each solution is analyzed. The baseline readings will assist in correcting any instrument drift. If more than one solution has been prepared for a sample (i.e. filtrate and sample, or soluble and insoluble portions), analyze each for all requested elements. Standards must bracket the sample concentrations. Analyze a standard after every four or five samples. Standard readings should be within 10 to 15% of the readings obtained at the beginning of the analysis.

6.7.6 If any samples exceed the linear range, they should be diluted. When diluting a sample, be sure that the diluted sample has the same matrix as the original sample and standards. If a number of samples must be diluted, it may be more advantageous to use a less sensitive wavelength.

6.8 Analytical Recommendations

6.8.1 When a fresh standard is prepared, analyze the old and new standards and compare results to verify the new standard is correct. If two or more stock solutions are available for working standard preparations, rotate the preparation from one stock solution to the next to verify the quality.

6.8.2 Keep a permanent record of all standard preparation and comparison data. Assign and follow expiration dates for all standards.

6.8.3 Always analyze blank samples along with the other samples. Treat blanks in the same fashion as samples, including any filtration steps.

6.8.4 When analyzing for Ag, carry-over from a large concentration sample or standard to the next sample can occur, causing erroneous readings. To remedy this, aspirate 4% HNO₃ instead of water between samples.

6.8.5 In this method, many different matrices are used to digest and keep analytes in solution. Occasionally, during multiple element analysis of the same sample, matrix effects can occur if standards are not matrix-matched with samples. Also, it is sometimes necessary to prepare samples in a matrix substantially different from recommendations. If these conditions occur, one or two standards should be prepared in the same matrix to determine any matrix effects. A reagent blank should also be prepared and analyzed to determine any effect on the background signal. If a significant difference is noted in the analytical signals for the two different matrices, a full set of standards should always be prepared in the sample matrix and analyzed with the samples.

7.1 Subtract each baseline ABS from the corresponding standard ABS, and plot the net ABS versus the standard concentrations. Using a least squares method, determine the equation for the best curve fit.

7.2 Subtract each baseline ABS from the corresponding sample or blank ABS, and use the standard curve to calculate the concentration of each analyte in µg/mL.

7.3 Calculate the concentration for each air sample as:
 

C =	[(A × SA × D × GF) - (B × SB × GF)] air volume

Where:
 

C	=	analyte (mg/m3)
A	=	concn of analyte in the sample solution (µg/mL)
B	=	concn of analyte in the blank solution (µg/mL)
SA	=	sample solution volume (mL)
SB	=	blank solution volume (mL)
D	=	dilution factor (if any)
GF	=	gravimetric factor (if any; see Table 6)
Air Vol	=	air volume sampled (L)

7.4 For wipe or bulk samples, calculate the total amount (in µg) of analyte in each sample using the equation above. An air volume is not used. Convert bulk sample analytes to % composition using:

analyte % (w/w) =

(C) (100%) (sample wt) (1,000 µg/mg)

(Bulk Samples)

Where:
 

C	=	analyte amount (µg)
Sample wt	=	aliquot (in mg) of bulk taken in Section 6.6.

7.5 Reporting Results to the Industrial Hygienist

For those samples only extracted with DI H₂O, report the sample results as the soluble fraction of the sample.

If more than one solution exists for a sample, and it is not necessary to report results separately, then combine these results. An example is a sample that was filtered due to insoluble particulate. The results from the filtrate plus results from the second particulate digestion are added together.

7.5.1 Report air sample results as mg/m³ analyte.

7.5.2 Report wipe sample concentrations as total micrograms or milligrams analyte.

7.5.3 Report bulk sample results as approximate percent by weight analyte (note: Sample results for bulk liquids may be reported as approximate percent by volume if volumetric aliquots were taken during sample preparation.) Due to differences in sample matrices between bulks and standards, bulk results are approximate.

8.1 Occupational Safety and Health Administration Analytical Laboratory: OSHA Manual of Analytical Methods edited by R.G. Adler (Method No. I-1). Salt Lake City, UT. 1977.

8.2 Clayton, G.D. and F.E. Clayton, ed.: Patty's Industrial Hygiene and Toxicology. 3rd ed. New York: John Wiley and Sons, 1978.

8.3 American Conference of Governmental Industrial Hygienists: Documentation of the Threshold Limit Values and Biological Exposure Indices. 5th Ed. Cincinnati, OH: American Conference of Governmental Industrial Hygienists, 1986.

8.4 National Institute for Occupational Safety and Health: The Industrial Environment--Its Evaluation and Control. Washington, DC: Government Printing Office, 1973.

8.5 Slavin, S., W.B. Barnett, and H.L. Kahn: The Determination of Atomic Absorption Detection Limits by Direct Measurement. Atomic Absorption Newsletter 11: 37-41 (1972).

8.6 Perkin-Elmer Corp.: Analytical Methods for Atomic Absorption Spectrophotometry. Norwalk, CT: Perkin-Elmer Corp., 1973 and revised edition, 1982.

8.7 National Institute for Occupational Safety and Health: NIOSH Manual of Analytical Methods. 2nd ed. (Method no. 173) Cincinnati, OH: National Institute for Occupational Safety and Health, 1977.

8.8 Occupational Safety and Health Administration Technical Center: Standard Operating Procedure for Atomic Absorption. Salt Lake City, UT. In progress (unpublished).

8.9 Fisher Scientific Company: Atomic Absorption Methods Manual. Waltham, MA: Fisher Scientific Co., 1977.

8.10 Instrumentation Laboratory Inc.: Atomic Absorption Methods Manual. Wilmington, MA: Instrumentation Laboratory Inc., 1975.

8.11 Occupational Safety and Health Administration Analytical Laboratory: Standard Operating Procedure for Microwave Digestions by D. Cook. Salt Lake City, UT. 1989 (unpublished).

8.12 "Air Contaminants; Final Rule": Federal Register 54:12 (19 Jan. 1989). pp. 2923-2960 and also 54:127 (5 July 1989). pp. 28054-28061.

8.13 Occupational Safety and Health Administration Analytical Laboratory: OSHA Laboratory Quality Control Division Data by B. Babcock, Salt Lake City, UT, 1989 (unpublished).

8.14 Slavin, Walter: Atomic Absorption Spectroscopy. New York: Interscience Publishers, 1968.

8.15 Ediger, R.D.: Atomic Absorption Analysis with the Graphite Furnace using Matrix Modification. Atomic Absorption Newsletter. 14(5): 127-130 (1975).

Analyte Suspected to be Present	Final Concentration
CsOH	DI H2O/1,000 µg/mL potassium ion
LiH, Na cmpds	DI H2O
KOH	DI H2O/1,000 µg/mL sodium ion

1. Air/Acetylene mixture, lean, blue flame
2. Air/Acetylene mixture, very lean, blue flame
3. Nitrous oxide/Acetylene mixture, rich, red flame
4. Nitrous oxide/Acetylene mixture, slightly rich, red flame
5. Air/Hydrogen mixture