Although I cannot tell which analyzer you are specifically referring to, as a testing laboratory, you need to start by understanding the difference between calibration and verification, and what a level of measurement uncertainty and traceability to SI is achievable and acceptable (e.g national or international traceability). This involves having a suitable calibration program and intermediate checks (verification) to meet ISO 17025 clause 6.4 equipment requirements and clause 6.5 Metrological traceability. Note that assuring your results goes beyond the analyzer, it includes all equipment used in the process – e.g analytical balances, glassware, dispensing devices, reference standards to establish the analytical calibration on the instrument.
Regarding your reference to the difference between “external” and “internal” - External is when you contract an ISO 17025 calibration laboratory to perform validations. What you are referring to as “internal” is rather referred to as “intermediate checks” or “verification”. This is because it is possible for laboratories to calibrate internally if calibration requirements are met. Calibration laboratories must have certified reference standards with strong metrological traceability to SI and a fit for purpose, well-documented measurement uncertainty for each test property. When using a calibration laboratory, for example, to calibrate your analytical balances, they need to use a suitable class of reference weights. Depending on the class of balance, a particular class of reference weight must be used to calibrate such a balance, as they have different specifications (agreed technical parameters) resulting in a particular level of measurement uncertainty.
How often you do external calibrations and whether you need to perform intermediate checks (and how often), depends on the process steps and what equipment is used. For analytical balances it is straight forward – a laboratory would use a set of weights that they own, where each piece has metrological traceability to SI, where they were previously calibrated by an external calibration provider (at a suitable frequency, based on risk and need). So here you have reported uncertainties on the calibration certificate that you confirm are acceptable for each piece. Then you perform intermediate checks (verifications) on your balances at suitable time intervals (also based on risk), across the range of use (g) of the balance. For your analyzer, your intermediate checks could be functional, based on the instrument performance, as well as by using standard reference checks against the calibration. This involves using different standard solutions or materials than what was used in setting up the calibration on the analyzer.
For further information see the following:
The article What does ISO 17025:2017 require for laboratory measurement equipment and related procedures? at https://advisera.com/17025academy/blog/2019/07/25/iso-17025-measurement-requirements-of-the-standard/
The ISO 17025 document template: Equipment and Calibration Procedure at https://advisera.com/17025academy/documentation/equipment-and-calibration-procedure//
ILAC G24:2007 Guidelines for the determination of calibration intervals of measuring instruments (note currently under revision) available for download at https://ilac.org/?ddownload=818
You can also refer to another Expert Advice Community Q&A Are intermediate checks required for calibration laboratories? att https://community.advisera.com/topic/are-intermediate-checks-required-for-calibration-laboratories/