General Chapters: <467> ORGANIC VOLATILE IMPURITIES

467

ORGANIC VOLATILE IMPURITIES

RESIDUAL SOLVENTS LIMITS

For pharmacopeial purposes, residual solvents in pharmaceuticals are defined as organic volatile chemicals that are used or produced in the manufacture of drug substances or excipients, or in the preparation of drug products. The residual solvents are not completely removed by practical manufacturing techniques. Appropriate selection of the solvent for the synthesis of a drug substance or an excipient may enhance the yield, or determine characteristics such as crystal form, purity, and solubility. Therefore, the solvent may sometimes be a critical element in the synthetic process. This General Chapter does not address solvents deliberately used as excipients nor does it address solvates. However, the content of solvents in such products should be evaluated and justified.

Because residual solvents do not provide therapeutic benefit, they should be removed, to the extent possible, to meet ingredient and product specifications, good manufacturing practices, or other quality-based requirements. Drug products should contain no higher levels of residual solvents than can be supported by safety data. Solvents that are known to cause unacceptable toxicities (Class 1, Table 1) should be avoided in the production of drug substances, excipients, or drug products unless their use can be strongly justified in a risk-benefit assessment. Solvents associated with less severe toxicity (Class 2, Table 2) should be limited in order to protect patients from potential adverse effects. Ideally, less toxic solvents (Class 3, Table 3) should be used where practical. The complete list of solvents included in this General Chapter is given in Appendix 1. These tables and the list are not exhaustive. Where other solvents have been used, based on approval by the competent regulatory authority, such solvents may be added to the tables and list.

Testing of drug substances, excipients, and drug products for residual solvents should be performed when production or purification processes are known to result in the presence of such residual solvents. It is only necessary to test for residual solvents that are used or produced in the manufacture or purification processes.

Although manufacturers may choose to test the drug product, a cumulative procedure may be used to calculate the residual solvent levels in the product from the levels in its ingredients. If the calculation results in a level equal to or below that recommended in this General Chapter, no testing of the drug product for residual solvents needs to be considered. If, however, the calculated levels are above the recommended level, the drug product should be tested to ascertain whether the formulation process has reduced the relevant solvent levels to within acceptable amounts. A drug product should also be tested if a residual solvent is used during its manufacture.

See Appendix 2 for additional background information related to residual solvents.

CLASSIFICATION OF RESIDUAL SOLVENTS BY RISK ASSESSMENT

The term “tolerable daily intake” (TDI) is used by the International Program on Chemical Safety (IPCS) to describe exposure limits of toxic chemicals and the term “acceptable daily intake” (ADI) is used by the World Health Organization (WHO) and other national and international health authorities and institutes. The term “permitted daily exposure” (PDE) is defined as a pharmaceutically acceptable intake of residual solvents to avoid confusion of differing values for ADIs of the same substance.

Residual solvents specified in this General Chapter are listed in Appendix 1 by common names and structures. They were evaluated for their possible risk to human health and placed into one of three classes as follows:

Class 1	Residual Solvents: Solvents to be Avoided Known human carcinogens Strongly suspected human carcinogens Environmental hazards
Class 2	Residual Solvents: Solvents to be Limited Nongenotoxic animal carcinogens or possible causative agents of other irreversible toxicity, such as neurotoxicity or teratogenicity. Solvents suspected of other significant but rever- sible toxicities.
Class 3	Residual Solvents: Solvents with Low Toxic Po- tential Solvents with low toxic potential to humans; no health-based exposure limit is needed. [NOTE—Class 3 residual solvents may have PDEs of up to 50 mg or more per day.]*
* For residual solvents with PDEs of more than 50 mg per day, see the discussion in the section Class 3 under Limits of Residual Solvents.

PROCEDURES FOR ESTABLISHING EXPOSURE LIMITS

The procedure used to establish permitted daily exposures for residual solvents is presented in Appendix 3.

OPTIONS FOR DETERMINING LEVELS OF CLASS 2 RESIDUAL SOLVENTS

Two options are available to determine levels of Class 2 residual solvents.

Option 1

The concentration limits in ppm stated in Table 2 are used. They were calculated using equation (1) below by assuming a product weight of 10 g administered daily.

Here, PDE is given in terms of mg per day, and dose is given in g per day.

These limits are considered acceptable for all drug substances, excipients, and drug products. Therefore, this option may be applied if the daily dose is not known or fixed. If all drug substances and excipients in a formulation meet the limits given in Option 1, these components may be used in any proportion. No further calculation is necessary provided the daily dose does not exceed 10 g. Products that are administered in doses greater than 10 g per day are to be considered under Option 2.

Option 2

It is not necessary for each component of the drug product to comply with the limits given in Option 1. The PDE in terms of mg per day as stated in Table 2 can be used with the known maximum daily dose and equation (1) above to determine the concentration of residual solvent allowed in a drug product. Such limits are considered acceptable provided that it has been demonstrated that the residual solvent has been reduced to the practical minimum. The limits should be realistic in relation to analytical precision, manufacturing capability, and reasonable variation in the manufacturing process. The limits should also reflect contemporary manufacturing standards.

Option 2 may be applied by adding the amounts of a residual solvent present in each of the components of the drug product. The sum of the amounts of solvent per day should be less than that given by the PDE.

Consider an example of the application of Option 1 and Option 2 to acetonitrile concentration in a drug product. The permitted daily exposure to acetonitrile is 4.1 mg per day; thus, the Option 1 limit is 410 ppm. The maximum administered daily weight of a drug product is 5.0 g, and the drug product contains two excipients. The composition of the drug product and the calculated maximum content of residual acetonitrile are given in the following table.

Component	Amount in Formulation (g)	Acetonitrile Content (ppm)	Daily Exposure (mg)
Drug substance	0.3	800	0.24
Excipient 1	0.9	400	0.36
Excipient 2	3.8	800	3.04
Drug product	5.0	728	3.64

Excipient l meets the Option 1 limit, but the drug substance, excipient 2, and drug product do not meet the Option 1 limit. Nevertheless, the drug product meets the Option 2 limit of 4.1 mg per day and thus conforms to the acceptance criteria in this General Chapter.

Consider another example using acetonitrile as the residual solvent. The maximum administered daily weight of a drug product is 5.0 g, and the drug product contains two excipients. The composition of the drug product and the calculated maximum content of residual acetonitrile are given in the following table.

Component	Amount in Formulation (g)	Acetonitrile Content (ppm)	Daily Exposure (mg)
Drug substance	0.3	800	0.24
Excipient 1	0.9	2000	1.80
Excipient 2	3.8	800	3.04
Drug product	5.0	1016	5.08

In this example, the drug product meets neither the Option 1 nor the Option 2 limit. The manufacturer could test the drug product to determine if the formulation process reduced the level of acetonitrile. If the level of acetonitrile was not reduced to the allowed limit during formulation, the product fails the requirements of the test.

LIMITS OF RESIDUAL SOLVENTS

Ethylene Oxide

[NOTE—The test for ethylene oxide is conducted only where specified in the individual monograph.] The standard solution parameters and the procedure for determination are described in the individual monograph. Unless otherwise specified in the individual monograph, the limit is 10 µg per g.

Class 1

Class 1 residual solvents (Table 1) should not be employed in the manufacture of drug substances, excipients, and drug products because of the unacceptable toxicities or deleterious environmental effects of these residual solvents. However, if Class 1 residual solvents are used, their levels should be restricted as shown in Table 1, unless otherwise stated in the individual monograph. The solvent 1,1,1-trichloroethane is included in Table 1 because it is an environmental hazard. The stated limit of 1500 ppm is based on safety data.

When Class 1 residual solvents are used in the manufacture of a drug substance, excipient, or drug product, the methodology described in the Identification, Control, and Quantification of Residual Solvents section of this General Chapter is to be applied wherever possible. Otherwise an appropriate validated procedure is to be employed. Such procedure shall be submitted to the USP for inclusion in the relevant individual monograph.

Table 1. Class 1 Residual Solvents

Solvent	Concentration Limit (ppm)	Concern
Benzene	2	Carcinogen
Carbon tetrachloride	4	Toxic and environ mental hazard
1,2-Dichloroethane	5	Toxic
1,1-Dichloroethene	8	Toxic
1,1,1-Trichloroethane	1500	Environmental hazard

Class 2

Class 2 residual solvents (Table 2) should be limited in drug substances, excipients, and drug products because of the inherent toxicities of the residual solvents. PDEs are given to the nearest 0.1 mg per day, and concentrations are given to the nearest 10 ppm. The stated values do not reflect the necessary analytical precision of the determination procedure. Precision should be determined as part of the procedure validation.

If Class 2 residual solvents are present at greater than their Option 1 limits, they should be identified and quantified. The procedures described in the Identification, Control, and Quantification of Residual Solvents section of this General Chapter are to be applied wherever possible. Otherwise an appropriate validated procedure is to be employed. Such procedure shall be submitted to the USP for inclusion in the relevant individual monograph.

NOTE—The following Class 2 residual solvents are not readily detected by the headspace injection conditions described in the Identification, Control, and Quantification of Residual Solvents section of this General Chapter: formamide, 2-ethoxyethanol, 2-methoxyethanol, ethylene glycol, N-methylpyrrolidone, and sulfolane. Other appropriate validated procedures are to be employed for the control of these residual solvents. Such procedures shall be submitted to the USP for inclusion in the relevant individual monograph.

Table 2. Class 2 Residual Solvents

Solvent	PDE (mg/day)	Concentration limit (ppm)
Acetonitrile	4.1	410
Chlorobenzene	3.6	360
Chloroform	0.6	60
Cyclohexane	38.8	3880
1,2-Dichloroethene	18.7	1870
1,2-Dimethoxyethane	1.0	100
N,N-Dimethylacetamide	10.9	1090
N,N-Dimethylformamide	8.8	880
1,4-Dioxane	3.8	380
2-Ethoxyethanol	1.6	160
Ethylene glycol	6.2	620
Formamide	2.2	220
Hexane	2.9	290
Methanol	30.0	3000
2-Methoxyethanol	0.5	50
Methylbutylketone	0.5	50
Methylcyclohexane	11.8	1180
Methylene chloride	6.0	600
N-Methylpyrrolidone	5.3	530
Nitromethane	0.5	50
Pyridine	2.0	200
Sulfolane	1.6	160
Tetrahydrofuran	7.2	720
Tetralin	1.0	100
Toluene	8.9	890
Trichloroethene	0.8	80
Xylene*	21.7	2170
* Usually 60% m-xylene, 14% p-xylene, 9% o-xylene with 17% ethyl benzene

Class 3

Class 3 residual solvents (Table 3) may be regarded as less toxic and of lower risk to human health than Class 1 and Class 2 residual solvents. Class 3 includes no solvent known as a human health hazard at levels normally accepted in pharmaceuticals. However, there are no long-term toxicity or carcinogenicity studies for many of the residual solvents in Class 3. Available data indicate that they are less toxic in acute or short-term studies and negative in genotoxicity studies.

Unless otherwise stated in the individual monograph, Class 3 residual solvents are limited to not more than 50 mg per day (corresponding to 5000 ppm or 0.5% under Option 1). If a Class 3 solvent limit in an individual monograph is greater than 50 mg per day, that residual solvent should be identified and quantified. The procedures described in the Identification, Control, and Quantification of Residual Solvents section of this General Chapter are to be applied wherever possible. Otherwise an appropriate validated procedure is to be employed. Such procedure shall be submitted to the USP for inclusion in the relevant individual monograph.

Table 3. Class 3 Residual Solvents
(limited by GMP or other quality-based requirements in drug substances, excipients, and drug products)

Acetic acid	Heptane
Acetone	Isobutyl acetate
Anisole	Isopropyl acetate
1-Butanol	Methyl acetate
2-Butanol	3-Methyl-1-butanol
Butyl acetate	Methylethylketone
tert-Butylmethyl ether	Methylisobutylketone
Cumene	2-Methyl-l-propanol
Dimethyl sulfoxide	Pentane
Ethanol	1-Pentanol
Ethyl acetate	1-Propanol
Ethyl ether	2-Propanol
Ethyl formate	Propyl acetate
Formic acid

Other Residual Solvents

The residual solvents listed in Table 4 may also be of interest to manufacturers of drug substances, excipients, or drug products. However, no adequate toxicological data on which to base a PDE was found. Specifications for these residual solvents will be provided in the respective individual monograph.

Table 4. Other Residual Solvents
(for which no adequate toxicological data was found)

1,1-Diethoxypropane	Methyl isopropyl ketone
1,1-Dimethoxymethane	Methyltetrahydrofuran
2,2-Dimethoxypropane	Solvent Hexane
Isooctane	Trichloroacetic acid
Isopropyl ether	Trifluoroacetic acid

IDENTIFICATION, CONTROL, AND QUANTIFICATION OF RESIDUAL SOLVENTS

NOTE—The organic-free water specified in the following procedures produces no significantly interfering peaks when chromatographed.

Class 1 and Class 2 Residual Solvents

WATER-SOLUBLE ARTICLES

Procedure A—

Class 1 Standard Stock Solution— Transfer 1.0 mL of USP Class 1 Residual Solvents Mixture RS to a 100-mL volumetric flask, add 9 mL of dimethyl sulfoxide, dilute with water to volume, and mix. Transfer 1.0 mL of this solution to a 100-mL volumetric flask, dilute with water to volume, and mix. Transfer 1.0 mL of this solution to a 10-mL volumetric flask, dilute with water to volume, and mix.

Class 1 Standard Solution— Transfer 1.0 mL of Class 1 Standard Stock Solution to an appropriate headspace vial, add 5.0 mL of water, apply stopper, cap, and mix.

Class 2 Standard Stock Solution— Transfer 1.0 mL of USP Class 2 Residual Solvents Mixture RS to a 100-mL volumetric flask, dilute with water to volume, and mix.

Class 2 Standard Solution— Transfer 1.0 mL of Class 2 Standard Stock Solution to an appropriate headspace vial, add 5.0 mL of water, apply stopper, cap, and mix.

Test Stock Solution— Transfer about 250 mg of the article under test, accurately weighed, to a 25-mL volumetric flask, dissolve in and dilute with water to volume, and mix.

Test Solution— Transfer 5.0 mL of Test Stock Solution to an appropriate headspace vial, add 1.0 mL of water, apply stopper, cap, and mix.

Class 1 System Suitability Solution— Transfer 1.0 mL of Class 1 Standard Stock Solution to an appropriate headspace vial, add 5.0 mL of Test Stock Solution, apply stopper, cap, and mix.

Chromatographic System (see Chromatography

621

)— The gas chromatograph is equipped with a flame-ionization detector, a 0.32-mm × 30-m fused-silica column coated with a 1.8-µm layer of phase G43 or a 0.53-mm × 30-m wide-bore column coated with a 3.0-µm layer of phase G43. The carrier gas is nitrogen or helium with a linear velocity of about 35 cm per second, and a split ratio of 1:5. The column temperature is maintained at 40

for 20 minutes, then raised at a rate of 10

per minute to 240

, and maintained at 240

for 20 minutes. The injection port and detector temperatures are maintained at 140

and 250

, respectively. Chromatograph the Class 1 Standard Solution, Class 1 System Suitability Solution, and Class 2 Standard Solution, and record the peak responses as directed for Procedure: the signal-to-noise ratio of 1,1,1-trichloroethane in the Class 1 Standard Solution is not less than 5; the signal-to-noise ratio of each peak in the Class 1 System Suitability Solution is not less than 3; and the resolution, R, between acetonitrile and methylene chloride in the Class 2 Standard Solution is not less than 1.0.

Procedure— Separately inject (following one of the headspace operating parameter sets described in the table below) equal volumes of headspace (about 1.0 mL) of the Class 1 Standard Solution, Class 2 Standard Solution, and the Test Solution into the chromatograph, record the chromatograms, and measure the responses for the major peaks. If a peak response of any peak in the Test Solution is greater than or equal to a corresponding peak in either the Class 1 Standard Solution or the Class 2 Standard Solution, proceed to Procedure B to verify the identity of the peak; otherwise the article meets the requirements of this test.

Table 5. Headspace Operating Parameters

	Headspace Operating Parameter Sets
	1	2	3
Equilibration temperature ()	80	105	80
Equilibration time (min.)	60	45	45
Transfer-line temperature ()	85	110	105
Carrier gas: nitrogen or helium at an appropriate pressure
Pressurization time (s)	30	30	30
Injection volume (mL)	1	1	1

Procedure B—

Class 1 Standard Stock Solution, Class 1 Standard Solution, Class 2 Standard Stock Solution, Class 2 Standard Solution, Test Stock Solution, Test Solution, and Class 1 System Suitability Solution— Prepare as directed for Procedure A.

Chromatographic System (see Chromatography

621

)— The gas chromatograph is equipped with a flame-ionization detector, a 0.32-mm × 30-m fused-silica column coated with a 0.25-µm layer of phase G16, or a 0.53-mm × 30-m wide-bore column coated with a 0.25-µm layer of phase G16. The carrier gas is nitrogen or helium with a linear velocity of about 35 cm per second and a split ratio of 1:5. The column temperature is maintained at 50

for 20 minutes, then raised at a rate of 6

per minute to 165

, and maintained at 165

for 20 minutes. The injection port and detector temperatures are maintained at 140

and 250

, respectively. Chromatograph the Class 1 Standard Solution, the Class 1 System Suitability Solution, and the Class 2 Standard Solution, and record the peak responses as directed for Procedure: the signal-to-noise ratio of benzene in the Class 1 Standard Solution is not less than 5; the signal-to-noise ratio of each peak in the Class 1 System Suitability Solution is not less than 3; the resolution, R, between acetonitrile and trichloroethylene in the Class 2 Standard Solution is not less than 1.0.

Procedure— Separately inject (following one of the headspace operating parameter sets described in Table 5) equal volumes of headspace (about 1.0 mL) of the Class 1 Sandard Solution, the Class 2 Standard Solution, and the Test Solution into the chromatograph, record the chromatograms, and measure the responses for the major peaks. If the peak response(s) in the Test Solution of the peak(s) identified in Procedure A is/are greater than or equal to a corresponding peak(s) in either the Class 1 Standard Solution or the Class 2 Standard Solution, proceed to Procedure C to quantify the peak; otherwise the article meets the requirements of this test.

Procedure C—

Class 1 Standard Solution, Class 2 Standard Solution, Test Stock Solution, Test Solution, and Class 1 System Suitability Solution— Prepare as directed for Procedure A.

Standard Solution— Transfer an accurately measured volume of the USP Reference Standard for each peak identified and verified by Procedures A and B to a suitable container, and dilute quantitatively, and stepwise if necessary, with water to obtain a solution having a final concentration of 1/100 of the value stated in Table 1 or 2 (under Concentration limit). Transfer 5.0 mL of this solution to an appropriate headspace vial, add 1.0 mL of water, apply stopper, cap, and mix.

Chromatographic System (see Chromatography

621

)— [NOTE—If the results of the chromatography from Procedure A are found to be inferior to those found with Procedure B, the Chromatographic System from Procedure B may be substituted.] The gas chromatograph is equipped with a flame-ionization detector, a 0.32-mm × 30-m fused-silica column coated with a 1.8-µm layer of phase G43 or a 0.53-mm × 30-m wide-bore column coated with a 3.0-µm layer of phase G43. The carrier gas is nitrogen or helium with a linear velocity of about 35 cm per second, and a split ratio of 1:5. The column temperature is maintained at 40

for 20 minutes, then raised at a rate of 10

per minute to 240

, and maintained at 240

for 20 minutes. The injection port and detector temperatures are maintained at 140

and 250

, respectively. Chromatograph the Class 1 Standard Solution, the Class 1 System Suitability Solution, and the Class 2 Standard Solution, and record the peak responses as directed for Procedure: the signal-to-noise ratio of 1,1,1-trichloroethane in the Class 1 Standard Solution is not less than 5; the signal-to-noise ratio of each peak in the Class 1 System Suitability Solution is not less than 3; and the resolution, R, between acetonitrile and methylene chloride in the Class 2 Standard Solution is not less than 1.0.

Procedure— Separately inject (following one of the headspace operating parameters described in Table 5) equal volumes of headspace (about 1.0 mL) of the Standard Solution and Test Solution into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the amount, in ppm, of each residual solvent found in the article under test by the formula:

4(C/W)(rU / rS),

in which C is the concentration, in ppm, of the appropriate USP Reference Standard in the Standard Solution; W is the weight, in g, of the article under test taken to prepare the Test Stock Solution; and rU and rS are the peak responses of each residual solvent obtained from the Test Solution and the Standard Solution, respectively.

WATER-INSOLUBLE ARTICLES

Procedure A—

Class 1 Standard Stock Solution, Class 1 Standard Solution, Class 1 System Suitability Solution, Class 2 Standard Stock Solution, Class 2 Standard Solution, and Chromatographic System— Proceed as directed for Procedure A under Water-Soluble Articles.

Test Stock Solution— Transfer about 250 mg of the article under test, accurately weighed, to a 25-mL volumetric flask, dissolve in and dilute with dimethylformamide to volume, and mix.

Test Solution 1— Transfer 5.0 mL of Test Stock Solution to an appropriate headspace vial, add 1.0 mL of dimethylformamide, apply stopper, cap, and mix.

Test Solution 2— Transfer about 250 mg of the article under test, accurately weighed, to a 25-mL volumetric flask, dissolve in and dilute with 1,3-dimethyl-2-imidazolidinone to volume, and mix. Transfer 5.0 mL of this solution to an appropriate headspace vial, add 1.0 mL of 1,3-dimethyl-2-imidazolidinone, apply stopper, cap, and mix.

Procedure— Separately inject (following one of the headspace operating parameters described in Table 5) equal volumes of headspace (about 1.0 mL) of the Class 1 Standard Solution, the Class 2 Standard Solution, Test Solution 1, and Test solution 2 into the chromatograph, record the chromatograms, and measure the responses for the major peaks. If a peak response of any peak in Test solution 1 is greater than or equal to a corresponding peak in either the Class 1 Standard Solution or the Class 2 Standard Solution, proceed to Procedure B to verify the identity of the peak; otherwise the article meets the requirements of this test. If the peak response for dimethylformamide or N,N-dimethylacetamide in Test Solution 2 is greater than or equal to the corresponding peak in the Class 2 Standard Solution, proceed to Procedure B to verify the identity of the peak; otherwise the article meets the requirements of this test.

Procedure B—

Class 1 Standard Stock Solution, Class 1 Standard Solution, Class 2 Standard Stock Solution, Class 2 Standard Solution, and Class 1 System Suitability Solution— Prepare as directed for Procedure A under Water-Soluble Articles.

Test Stock Solution, Test Solution 1, and Test Solution 2— Proceed as directed for Procedure A.

Chromatographic System— Proceed as directed for Procedure B under Water-Soluble Articles.

Procedure— Separately inject (following one of the headspace operating parameters described in Table 5) equal volumes of headspace (about 1.0 mL) of the Class 1 Standard Solution, Class 2 Standard Solution, Test Solution 1, and/or Test Solution 2 into the chromatograph, record the chromatograms, and measure the responses for the major peaks. If the peak response(s) in Test Solution 1 of the peak(s) identified in Procedure A is/are greater than or equal to a corresponding peak(s) in either the Class 1 Standard Solution or the Class 2 Standard Solution, proceed to Procedure C to quantify the peak; otherwise the article meets the requirements of this test. If the peak response for dimethylformamide or N,N-dimethylacetamide in Test Solution 2 is greater than or equal to the corresponding peak in the Class 2 Standard Solution, proceed to Procedure C to quantify the peak; otherwise the article meets the requirements of this test.

Procedure C—

Class 1 Standard Solution, Class 1 System Suitability Solution, and Class 2 Standard Solution— Proceed as directed for Procedure A under Water-Soluble Articles.

Test Stock Solution, Test Solution 1, and Test Solution 2— Proceed as directed for Procedure A.

Standard Solution, and Chromatographic System— Proceed as directed for Procedure C under Water-Soluble Articles.

Procedure— Separately inject (following one of the headspace operating parameters described in Table 5) equal volumes of headspace (about 1.0 mL) of the Standard Solution, Test Solution 1, and/or Test Solution 2 into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the amount, in ppm, of each residual solvent found in the article under test by the formula:

4(C/W)(rU / rS),

in which C is the concentration, in ppm, of the appropriate USP Reference Standard in the Standard Solution; W is the weight, in g, of the article under test taken to prepare the Test Stock Solution; and rU and rS are the peak responses of each residual solvent obtained from Test Solution 1 or Test Solution 2 and the Standard Solution, respectively.

Class 3 Residual Solvents

If only Class 3 solvents are present, the level of residual solvents is to be determined as directed under Loss on Drying

731

. If the loss on drying value is greater than 0.5%, a water determination should be performed on the test sample as directed under Water Determination

921

. Determine the water by Method Ia, unless otherwise specified in the individual monograph. If a Class 3 solvent limit in an individual monograph is greater than 50 mg per day (corresponding to 5000 ppm or 0.5% under Option 1), that residual solvent should be identified and quantified, and the procedures as described above are to be applied wherever possible. Otherwise an appropriate validated procedure is to be employed. Such procedure shall be submitted to the USP for inclusion in the relevant individual monograph. A flow diagram for the application of residual solvent limit tests is shown in Figure 1.

Fig. 1. Diagram relating to the identification of residual solvents and the application of limit tests.

OTHER ANALYTICAL PROCEDURES

The following procedures, with any necessary variations, are used where specified in the individual monographs.

Method I

A gas chromatograph capable of temperature programming and equipped with a wide-bore, wall-coated open tubular column and a flame-ionization detector is used in the following procedure.

Standard Solution— Prepare a solution, in organic-free water, or the solvent specified in the monograph, containing in each mL, 12.0 µg of methylene chloride, 7.6 µg of 1,4-dioxane, 1.6 µg of trichloroethylene, and 1.2 µg of chloroform. [NOTE—Prepare fresh daily.]

Test Solution— Dissolve in organic-free water, or the solvent specified in the monograph, an accurately weighed portion of the material to be tested to obtain a final solution having a known concentration of about 20 mg of the test material per mL.

Chromatographic System (see Chromatography

621

)— The gas chromatograph is equipped with a flame-ionization detector, a 0.53-mm × 30-m fused silica analytical column coated with a 5-µm chemically cross-linked G27 stationary phase and a 0.53-mm × 5-m silica guard column deactivated with phenylmethyl siloxane. The carrier gas is helium with a linear velocity of about 35 cm per second. [NOTE—When a makeup gas is used, nitrogen is recommended.] The injection port temperature and the detector temperature are maintained at 70

and 260

, respectively. The column temperature is programmed as follows. Initially, the column temperature is maintained at 35

for 5 minutes, then increased at a rate of 8

per minute to 175

, followed by an increase at a rate of 35

per minute to 260

, and maintained at 260

for at least 16 minutes.

Inject the Standard Solution, and record the peak responses as directed for Procedure: a suitable system is one that yields chromatograms in which all of the components in the Standard Solution are resolved; the resolution, R, between any two components is not less than 1.0; and the relative standard deviation of the individual peak responses from replicate injections is not more than 15%.

Procedure— Separately inject equal volumes (about 1 µL) of the Standard Solution and the Test Solution into the chromatograph, record the chromatograms, and measure the peak responses.

Identify, on the basis of retention time, any peaks present in the chromatogram of the Test Solution. The identity and peak response in the chromatogram may be established as being from any of the organic volatile impurities listed in the table shown below or from some other volatile impurity eluting with a comparable retention time as determined by mass spectrometric relative abundance procedures or by the use of a second validated column containing a different stationary phase.

Unless otherwise specified in the individual monograph, the amount of each organic volatile impurity present in the material does not exceed the limit given in the table shown below.

Organic Volatile Impurity	Limit (µg per g)
Chloroform	60
1,4-Dioxane	380
Methylene Chloride	600
Trichloroethylene	80

Method IV

Standard Solution— Prepare as directed for Standard Solution in Method I. Pipet 5 mL of the solution into a vial fitted with a septum and crimp cap, containing 1 g of anhydrous sodium sulfate, and seal. Heat the sealed vial at 80

for 60 minutes.

Test Solution— Transfer 100 mg, accurately weighed, of the material under test to a vial, add 5.0 mL of water, or the solvent specified in the monograph, and 1 g of anhydrous sodium sulfate, and seal with a septum and crimp cap. Heat the sealed vial at 80

for 60 minutes, or as specified in the individual monograph.

Chromatographic System and Procedure— [NOTE—The use of headspace apparatuses that automatically transfer a measured amount of headspace is allowed. Also, the use of a guard column in this headspace procedure is not necessary.] Proceed as directed for Method V, except to inject, using a heated gas-tight syringe, 1 mL of the headspace.

Method V

Standard Solution and Test Solution— Prepare as directed for Method I.

Chromatographic System (see Chromatography

621

)— The gas chromatograph is equipped with a flame-ionization detector, a 0.53-mm × 30-m fused silica analytical column coated with a 3.0-µm G43 stationary phase, and a 0.53-mm × 5-m silica guard column deactivated with phenylmethyl siloxane. The carrier gas is helium with a linear velocity of about 35 cm per second. The injection port and detector temperatures are maintained at 140

and 260

, respectively. The column temperature is programmed according to the following steps. It is maintained at 40

for 20 minutes, then increased rapidly to 240

, and maintained at 240

for 20 minutes.

Inject the Standard Solution, and record the peak responses as directed for Procedure: a suitable system is one that yields chromatograms in which all of the components in the Standard Solution are resolved; the resolution, R, between any two components is not less than 3; and the relative standard deviation of the individual peak responses from replicate injections is not more than 15%.

Procedure— Proceed as directed for Method I, the injection volume being about 1 µL.

Method VI

Standard Solution and Test Solution— Prepare as directed for Method I.

Chromatographic System (see Chromatography

621

)— The gas chromatograph is equipped with a flame-ionization detector. The column and column temperature conditions, as chosen from the list below (see Table 6), are specified in the individual monograph. The carrier gas, linear velocity or flow rate, and detector and injection port temperatures are appropriate to the column dimensions and column temperatures chosen from the list below.

Inject the Standard Solution, and record the peak responses as directed for Procedure: a suitable system is one that yields the chromatograms in which all of the components in the Standard Solution are resolved; the resolution, R, between any two components is not less than 1.0; and the relative standard deviation of the individual peak responses from replicate injections is not more than 15%.

Procedure— Proceed as directed for Method I, the injection volume being about 1 µL.

Table 6. Chromatographic Conditions for Method VI

Chromatographic Conditions	USP Column Designation	Column Size	Column Temperature
A	S3	3-mm × 2-m	190
B	S2	3-mm × 2.1-m	160
C	G16	0.53-mm × 30-m	40
D	G39	3-mm × 2-m	65
E	G16	3-mm × 2-m	70
F	S4	2-mm × 2.5-m	Hold 120 (35 min.) Gradient 120–200(2/min.) Hold 20 min.
H	G14	2-mm × 2.5-m	Hold 45 (3 min.) Gradient 45–120 (8/min.) Hold 15 min.
I	G27	0.53-mm × 30-m	Hold 35 (5 min.) 35–175 (8 /min.) 175–260(35/min.) Hold 16 min.
J	G16	0.33-mm × 30-m	Hold 50 (20 min.) 50–165 (6/min.)

GLOSSARY

Genotoxic carcinogens: Carcinogens that produce cancer by affecting genes or chromosomes.

Lowest-observed-effect level (LOEL): The lowest dose of a substance in a study or group of studies that produces biologically significant increases in frequency or severity of any effects in exposed humans or animals.

Modifying factor: A factor determined by professional judgment of a toxicologist and applied to bioassay data so that the data can be safely related to humans.

Neurotoxicity: The ability of a substance to cause adverse effects on the nervous system.

No-observed-effect level (NOEL): The highest dose of a substance at which there are no biologically significant increases in frequency or severity of any effects in exposed humans or animals.

Permitted daily exposure (PDE): The maximum acceptable intake per day of a residual solvent in pharmaceutical products.

Reversible toxicity: The occurrence of harmful effects that are caused by a substance and that disappear after exposure to the substance ends.

Strongly suspected human carcinogen: A substance for which there is no epidemiological evidence of carcinogenesis but for which there are positive genotoxicity data and clear evidence of carcinogenesis in rodents.

Teratogenicity: The occurrence of structural malformations in a developing fetus when a substance is administered during pregnancy.

APPENDIX 1. LIST OF RESIDUAL SOLVENTS INCLUDED IN THIS GENERAL CHAPTER

Solvent	Other Names	Structure	Class
Acetic acid	Ethanoic acid	CH3COOH	Class 3
Acetone	2-Propanone Propan-2-one	CH3COCH3	Class 3
Acetonitrile		CH3CN	Class 2
Anisole	Methoxybenzene		Class 3
Benzene	Benzol		Class 1
1-Butanol	n-Butyl alcohol Butan-1-ol	CH3(CH2)3OH	Class 3
2-Butanol	sec-Butyl alcohol Butan-2-ol	CH3CH2CH(OH)CH3	Class 3
Butyl acetate	Acetic acid butyl ester	CH3COO(CH2)3CH3	Class 3
tert-Butylmethyl ether	2-Methoxy-2-methylpropane	(CH3)3COCH3	Class 3
Carbon tetrachloride	Tetrachloromethane	CCl4	Class 1
Chlorobenzene			Class 2
Chloroform	Trichloromethane	CHCl3	Class 2
Cumene	Isopropylbenzene (1-Methylethyl)benzene		Class 3
Cyclohexane	Hexamethylene		Class 2
1,2-Dichloroethane	sym-Dichloroethane Ethylene dichloride Ethylene chloride	CH2ClCH2Cl	Class 1
1,1-Dichloroethene	1,1-Dichloroethylene Vinylidene chloride	H2C=CCl2	Class 1
1,2-Dichloroethene	1,2-Dichloroethylene Acetylene dichloride	ClHC=CHCl	Class 2
1,2-Dimethoxyethane	Ethyleneglycol dimethyl ether Monoglyme Dimethyl cellosolve	H3COCH2CH2OCH3	Class 2
N,N-Dimethylacetamide	DMA	CH3CON(CH3)2	Class 2
N,N-Dimethylformamide	DMF	HCON(CH3)2	Class 2
Dimethyl sulfoxide	Methylsulfinylmethane Methyl sulfoxide DMSO	(CH3)2SO	Class 3
1,4-Dioxane	p-Dioxane [1,4]Dioxane		Class 2
Ethanol	Ethyl alcohol	CH3CH2OH	Class 3
2-Ethoxyethanol	Cellosolve	CH3CH2OCH2CH2OH	Class 2
Ethyl acetate	Acetic acid ethyl ester	CH3COOCH2CH3	Class 3
Ethylene glycol	1,2-Dihydroxyethane 1,2-Ethanediol	HOCH2CH2OH	Class 2
Ethyl ether	Diethyl ether Ethoxyethane 1,1¢-Oxybisethane	CH3CH2OCH2CH3	Class 3
Ethyl formate	Formic acid ethyl ester	HCOOCH2CH3	Class 3
Formamide	Methanamide	HCONH2	Class 2
Formic acid		HCOOH	Class 3
Heptane	n-Heptane	CH3(CH2)5CH3	Class 3
Hexane	n-Hexane	CH3(CH2)4CH3	Class 2
Isobutyl acetate	Acetic acid isobutyl ester	CH3COOCH2CH(CH3)2	Class 3
Isopropyl acetate	Acetic acid isopropyl ester	CH3COOCH(CH3)2	Class 3
Methanol	Methyl alcohol	CH3OH	Class 2
2-Methoxyethanol	Methyl cellosolve	CH3OCH2CH2OH	Class 2
Methyl acetate	Acetic acid methyl ester	CH3COOCH3	Class 3
3-Methyl-1-butanol	Isoamyl alcohol Isopentyl alcohol 3-Methylbutan-1-ol	(CH3)2CHCH2CH2OH	Class 3
Methylbutylketone	2-Hexanone Hexan-2-one	CH3(CH2)3COCH3	Class 2
Methylcyclohexane	Cyclohexylmethane		Class 2
Methylene chloride	Dichloromethane	CH2Cl2	Class 2
Methylethylketone	2-Butanone MEK Butan-2-one	CH3CH2COCH3	Class 3
Methyl isobutyl ketone	4-Methylpentan-2-one 4-Methyl-2-pentanone MIBK	CH3COCH2CH(CH3)2	Class 3
2-Methyl-1-propanol	Isobutyl alcohol	(CH3)2CHCH2OH	Class 3
	2-Methylpropan-1-ol
N-Methylpyrrolidone	1-Methylpyrrolidin-2-one 1-Methyl-2-pyrrolidinone		Class 2
Nitromethane		CH3NO2	Class 2
Pentane	n-Pentane	CH3(CH2)3CH3	Class 3
1-Pentanol	Amyl alcohol Pentan-1-ol Pentyl alcohol	CH3(CH2)3CH2OH	Class 3
1-Propanol	Propan-1-ol Propyl alcohol	CH3CH2CH2OH	Class 3
2-Propanol	Propan-2-ol Isopropyl alcohol	(CH3)2CHOH	Class 3
Propyl acetate	Acetic acid propyl ester	CH3COOCH2CH2CH3	Class 3
Pyridine			Class 2
Sulfolane	Tetrahydrothiophene 1,1-diox- ide		Class 2
Tetrahydrofuran	Tetramethylene oxide		Class 2
	Oxacyclopentane
Tetralin	1,2,3,4-Tetrahydronaphthalene		Class 2
Toluene	Methylbenzene		Class 2
1,1,1-Trichloroethane	Methylchloroform	CH3CCl3	Class 1
Trichloroethene	1,1,2-Trichloroethene	HClC=CCl2	Class 2
Xylene*	Dimethybenzene Xylol		Class 2
* Usually 60% m-xylene, 14% p-xylene, 9% o-xylene with 17% ethyl benzene.

APPENDIX 2. ADDITIONAL BACKGROUND

A2.1. Environmental Regulation of Organic Volatile Solvents

Several of the residual solvents frequently used in the production of pharmaceuticals are listed as toxic chemicals in Environmental Health Criteria (EHC) monographs and in the Integrated Risk Information System (IRIS). The objectives of such groups as the International Programme on Chemical Safety (IPCS), the United States Environmental Protection Agency (EPA), and the United States Food and Drug Administration (FDA) include the determination of acceptable exposure levels. The goal is maintenance of environmental integrity and protection of human health against the possible deleterious effects of chemicals resulting from long-term environmental exposure. The procedures involved in the estimation of maximum safe exposure limits are usually based on long-term studies. When long-term study data are unavailable, shorter term study data can be used with modification of the approach, such as use of larger safety factors. The approach described therein relates primarily to long-term or lifetime exposure of the general population in the ambient environment (i.e., ambient air, food, drinking water, and other media).

A2.2. Residual Solvents in Pharmaceuticals

Exposure limits in this General Chapter are established by referring to methodologies and toxicity data described in EHC and IRIS monographs. However, the following specific assumptions about residual solvents to be used in the synthesis and formulation of pharmaceutical products should be taken into account in establishing exposure limits.

Patients (not the general population) use pharmaceuticals to treat their diseases or for prophylaxis to prevent infection or disease.
The assumption of lifetime patient exposure is not necessary for most pharmaceutical products but may be appropriate as a working hypothesis to reduce risk to human health.
Residual solvents are unavoidable components in pharmaceutical production and will often be a part of medicinal products.
Residual solvents should not exceed recommended levels except in exceptional circumstances.
Data from toxicological studies that are used to determine acceptable levels for residual solvents should have been generated using appropriate protocols such as those described, for example, by the Organization for Economic Cooperation and Development (OECD), EPA, and the FDA Red Book.

APPENDIX 3. PROCEDURES FOR ESTABLISHING EXPOSURE LIMITS

The Gaylor-Kodell method of risk assessment (Gaylor, D. W. and Kodell, R. L. Linear Interpolation Algorithm for Low Dose Assessment of Toxic Substance. Journal of Environmental Pathology and Toxicology, 4:305, 1980) is appropriate for Class 1 carcinogenic solvents. Only in cases where reliable carcinogenicity data are available should extrapolation by the use of mathematical models be applied to setting exposure limits. Exposure limits for Class 1 residual solvents could be determined with the use of a large safety factor (i.e., 10,000 to 100,000) with respect to the no-observed-effect level (NOEL). Detection and quantification of these residual solvents should be performed by state-of-the-art analytical techniques.

Acceptable exposure levels in this General Chapter for Class 2 residual solvents were established by calculation of PDE values according to the procedures for setting exposure limits in pharmaceuticals (page 5748 of PF 15(6) [Nov.–Dec. 1989]), and the method adopted by IPCS for Assessing Human Health Risk of Chemicals (Environmental Health Criteria 170, WHO, 1994). These procedures are similar to those used by the U.S. EPA (IRIS) and the U.S. FDA (Red Book) and others. The method is outlined here to give a better understanding of the origin of the PDE values. It is not necessary to perform these calculations in order to use the PDE values presented in Table 2 of this document.

PDE is derived from the no-observed-effect level (NOEL), or the lowest-observed effect level (LOEL), in the most relevant animal study as follows:

The PDE is derived preferably from a NOEL. If no NOEL is obtained, the LOEL may be used. Modifying factors proposed here, for relating the data to humans, are the same kind of “uncertainty factors” used in Environmental Health Criteria (Environmental Health Criteria 170, WHO, Geneva, 1994) and “modifying factors” or “safety factors” in Pharmacopeial Forum. The assumption of 100 percent systemic exposure is used in all calculations regardless of route of administration.

The modifying factors are as follows:

F1 =	A factor to account for extrapolation between species
	F1 =	2 for extrapolation from dogs to humans
	F1 =	2.5 for extrapolation from rabbits to humans
	F1 =	3 for extrapolation from monkeys to humans
	F1 =	5 for extrapolation from rats to humans
	F1 =	10 for extrapolation from other animals to humans
	F1 =	12 for extrapolation from mice to humans

F1 takes into account the comparative surface area to body weight ratios for the species concerned and for man. Surface area (S) is calculated as:

S = kM 0.67,

(2)

in which M = body weight, and the constant k has been taken to be 10. The body weights used in the equation are those shown below in Table A3.-1.

F2 =	A factor of 10 to account for variability between individuals. A factor of 10 is generally given for all organic solvents, and 10 is used consistently in this General Chapter.

F3 =	A variable factor to account for toxicity studies of short-term exposure.
	F3 =	1 for studies that last at least one half-lifetime (1 year for rodents or rabbits; 7 years for cats, dogs, and monkeys).
	F3 =	1 for reproductive studies in which the whole period of organogenesis is covered.
	F3 =	2 for a 6-month study in rodents, or a 3.5-year study in nonrodents.
	F3 =	5 for a 3-month study in rodents, or a 2-year study in nonrodents.
	F3 =	10 for studies of a shorter duration.

In all cases, the higher factor has been used for study durations between the time points (e.g., a factor of 2 for a 9-month rodent study).

F4 =	A factor that may be applied in cases of severe toxicity, e.g., nongenotoxic carcinogenicity, neurotoxicity, or teratogenicity. In studies of reproductive toxicity, the following factors are used:
	F4 =	1 for fetal toxicity associated with maternal toxicity
	F4 =	5 for fetal toxicity without maternal toxicity
	F4 =	5 for a teratogenic effect with maternal toxicity
	F4 =	10 for a teratogenic effect without maternal toxicity

F5 =	A variable factor that may be applied if the no-effect level was not established.

When only a LOEL is available, a factor of up to 10 can be used depending on the severity of the toxicity. The weight adjustment assumes an arbitrary adult human body weight for either sex of 50 kilograms (kg). This relatively low weight provides an additional safety factor against the standard weights of 60 kg or 70 kg that are often used in this type of calculation. It is recognized that some adult patients weigh less than 50 kg; these patients are considered to be accommodated by the built-in safety factors used to determine a PDE. If the solvent was present in a formulation specifically intended for pediatric use, an adjustment for a lower body weight would be appropriate.

As an example of the application of this equation, consider a toxicity study of acetonitrile in mice that is summarized in Pharmeuropa, Vol. 9, No. 1, Supplement, April 1997, page S24. The NOEL is calculated to be 50.7 mg kg–1 day–l. The PDE for acetonitrile in this study is calculated as follows:

In this example,

F1 =	12 to account for the extrapolation from mice to humans
F2 =	10 to account for differences between individual humans
F3 =	5 because the duration of the study was only 13 weeks
F4 =	1 because no severe toxicity was encountered
F5 =	1 because the no-effect level was determined

A3.-1. - Values Used in the Calculations in This Document

Rat body weight	425 g
Pregnant rat body weight	330 g
Mouse body weight	28 g
Pregnant mouse body weight	30g
Guinea-pig body weight	500 g
Rhesus monkey body weight	2.5 kg
Rabbit body weight (pregnant or not)	4 kg
Beagle dog body weight	11.5 kg
Rat respiratory volume	290 L/day
Mouse respiratory volume	43 L/day
Rabbit respiratory volume	1440 L/day
Guinea-pig respiratory volume	430 L day
Human respiratory volume	28,800 L/day
Dog respiratory volume	9000 L/day
Monkey respiratory volume	1150 L/day
Mouse water consumption	5 mL/day
Rat water consumption	30 mL/day
Rat food consumption	30 g/day

The equation for an ideal gas, PV = nRT, is used to convert concentrations of gases used in inhalation studies from units of ppm to units of mg/L or mg/m3. Consider as an example the rat reproductive toxicity study by inhalation of carbon tetrachloride (molecular weight 153.84) summarized in Pharmeuropa, Vol. 9, No. 1, Supplement, April 1997, page S9.

The relationship 1000 L = 1 m3 is used to convert to mg/ m3.

Auxiliary Information— Staff Liaison : Horacio Pappa, Ph.D.

Expert Committee : (GC05) General Chapters 05

USP29–NF24 Page 2580

Pharmacopeial Forum : Volume No. 31(5) Page 1435

Phone Number : 1-301-816-8319