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  <xs:element id="ck0001137360_C000053007Member" abstract="true" name="C000053007Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0001137360_TexasRiskMember" abstract="true" name="TexasRiskMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0001137360_SpecialTaxBondRiskMember" abstract="true" name="SpecialTaxBondRiskMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0001137360_ICEIntermediateAMTFreeBroadNationalMunicipalIndexreflectsnodeductionforfeesexpensesortaxesIndexMember" abstract="true" name="ICEIntermediateAMTFreeBroadNationalMunicipalIndexreflectsnodeductionforfeesexpensesortaxesIndexMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0001137360_S000019191Member" abstract="true" name="S000019191Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0001137360_C000053008Member" abstract="true" name="C000053008Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0001137360_ICELongAMTFreeBroadNationalMunicipalIndexreflectsnodeductionforfeesexpensesortaxesIndexMember" abstract="true" name="ICELongAMTFreeBroadNationalMunicipalIndexreflectsnodeductionforfeesexpensesortaxesIndexMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0001137360_S000038973Member" abstract="true" name="S000038973Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0001137360_C000119796Member" abstract="true" name="C000119796Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0001137360_IllinoisRiskMember" abstract="true" name="IllinoisRiskMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0001137360_ICE112YearHighYieldCrossoverMunicipalBondIndexreflectsnodeductionforfeesexpensesortaxesIndexMember" abstract="true" name="ICE112YearHighYieldCrossoverMunicipalBondIndexreflectsnodeductionforfeesexpensesortaxesIndexMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0001137360_S000019192Member" abstract="true" name="S000019192Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0001137360_C000053009Member" abstract="true" name="C000053009Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0001137360_PreRefundedMunicipalSecuritiesRiskMember" abstract="true" name="PreRefundedMunicipalSecuritiesRiskMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0001137360_ICEShortAMTFreeBroadNationalMunicipalIndexreflectsnodeductionforfeesexpensesortaxesIndexMember" abstract="true" name="ICEShortAMTFreeBroadNationalMunicipalIndexreflectsnodeductionforfeesexpensesortaxesIndexMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
</xs:schema>
